STABILITY OF DRUGS AND DRUG PRODUCTS - HEC
Transcript of STABILITY OF DRUGS AND DRUG PRODUCTS - HEC
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STABILITY OF DRUGS
AND DRUG PRODUCTS
Iqbal Ahmad TI MSc (KU) PhD (London) DSc (KU)
Royal Society Fellow Professor Department of Pharmaceutical Chemistry
Former Professor and Chairman Department of Pharmaceutical Chemistry
Faculty of Pharmacy University of Karachi
Muhammad Ali Sheraz B Pharm M Phil PhD (BMU)
Associate Professor and Chairman Department of Pharmacy Practice
Sofia Ahmed B Pharm M Phil PhD (BMU)
Associate Professor and Chairperson Department of Pharmaceutics
Faculty of Pharmaceutical Sciences Baqai Medical University Karachi
HIGHER EDUCATION COMMISSION ISLAMABAD - PAKISTAN
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or by any means ndash including but not limited to electronic mechanical photocopying recording
or otherwise or used for any commercial purpose what so ever without the prior written permission
of the publisher and if publisher considers necessary formal license agreement with publisher
may be executed
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develop authorship cadre among teaching and researcher community of higher education
institutions in the country For information please visit wwwhecgovpk
HEC ndash Cataloging in Publication (CIP Data)
Iqbal Ahmad Dr Chemical and Photo stability of Drugs and Formulated Products
1 Drug Stability
61686 - dc23 2016
ISBN 978-969-417-189-0
First Edition 2016
Copies Printed 500
Published By Higher Education Commission ndash Pakistan
Disclaimer The publisher has used its best efforts for this publication through a rigorous system of
evaluation and quality standards but does not assume and hereby disclaims any liability to any person
for any loss or damage caused by the errors or omissions in this publication whether such errors or
emissions result from negligence accident or any other cause
Copyrights Higher Education Commission
Islamabad
Lahore Karachi Peshawar Quetta
3
Dedicated to
Professor Dr Syed Sabir Ali (Late)
Founder Dean
Faculty of Pharmaceutical Sciences
Baqai Medical University Karachi
4
CONTENTS
LIST OF FIGURES xi LIST OF TABLES xii LIST OF ABBREVIATIONS xiii FOREWORD xv PREFACE xvi ACKNOWLEDGMENTS xvii ABOUT THE AUTHORS xviii
1 INTRODUCTION 1 11 DEFINITION OF STABILITY 1 12 TYPES OF STABILITY AND DOSAGE FORMS 1 13 FACTORS INFLUENCING STABILITY 2 131 External Factors 2 132 Internal Factors 2 14 IMPORTANT TYPES OF STABILITY 2 141 Chemical Stability 2 142 Photostability 3 143 Physical Stability 3 144 Solid State Stability 3 145 Microbiological Stability 4 15 ANALYTICAL METHODS 4 16 STABILITY EVALUATION 4 17 STABILITY TESTING 4 18 FORCED DEGRADATION STUDIES 5 19 STATISTICAL APPLICATIONS 5 110 ROLE OF PHARMACIST 5 111 LITERATURE ON DRUG STABILITY 6 112 CONTENTS OF MONOGRAPH 6 REFERENCES 7 2 CHEMICAL KINETICS 13 21 INTRODUCTION 13 22 BASIC KINETIC PRINCIPLES 13 221 Reaction Rate 13 222 Molecularity and Order of Reaction 14 2221 Molecularity 14 2222 Order 14 223 Half-Life and Shelf-Life of Drug 14 2231 Half-life (t12) 14 2232 Shelf-life (t90 or t95) 14 2233 Expiration dating 14 23 KINETICS OF CHEMICAL REACTIONS 15 231 Zero-Order Reaction 15 232 Pseudo Zero-Order Reaction 15 233 First-Order Reaction 16 234 Pseudo First-Order Reaction 17 235 Second-Order Reaction 17 236 Determination of Reaction Order 18 2361 Substitution method 18 2362 Graphical method 18
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2363 Half-life method 18 24 COMPLEX CHEMICAL REACTIONS 19 241 Reversible First-Order Reaction 19 2411 Example of calculation of equilibrium constant and rate constants for a
reversible first-order reaction 20
242 Parallel Reactions 21 2421 Parallel reactions involving the formation of two products 21 2422 Parallel reactions involving the formation of three products 22 243 Consecutive Reactions 23 244 Enzyme Catalysed Reactions 24 25 FACTORS AFFECTING DRUG DEGRADATION REACTIONS 25 251 Temperature 25 252 Q10 Values 26 2521 Q ΔT calculation 27 2522 Shelf-life calculation 27 253 Nonisothermal Prediction of Rate of Degradation 27 254 pH 28 255 Catalysis 28 2551 Specific acid-base catalysis 28 2552 General acid-base catalysis 30 256 Ionic Strength Effect (Primary Salt Effect) 31 257 Solvent 31 258 Oxygen 32 2581 Oxidizable drugs 32 2582 Oxidation reactions 33 259 Surfactant 34 2510 Moisture 34 2511 Problems 34 REFERENCES 37 3 CHEMICAL STABILITY 43 31 INTRODUCTION 43 32 STUDY OF THE CHEMICAL STABILITY OF A DRUG 43 33 CHEMICAL DEGRADATION REACTIONS 44 331 Hydrolysis 44 3311 Hydrolysis of esters 44 3312 Hydrolysis of amides 45 3313 Hydrolysis by ring opening 46 332 Oxidation 48 333 Decarboxylation 50 334 Elimination 50 335 Isomerization 51 336 Dimerization 51 337 Epimerization 52 338 Dehydration 52 339 Dehydrogenation 53 3310 Dehalogenation 53 34 CHEMICAL STABILITYDEGRADATION STUDIES 53 341 Aqueous Solution 53 342 Pharmaceutical Preparations 54 REFERENCES 57 4 PHOTOSTABILITY 61 41 INTRODUCTION 61 42 PHOTOSTABILITY AND RELATED ASPECTS 61 421 Photostability 61
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422 Effects of Photoinstability 61 4221 Chemical and physical changes 61 4222 Biological effects on administration 62 4223 Light induced side effects through interaction with endogenous substances 62 423 Objectives of Photostability Studies 62 424 Industrial Awareness on Photostability 62 43 PHOTOCHEMISTRY 63 431 Basic Laws of Photochemistry 63 432 Stages of Photochemical Reactions 63 433 Role of Photochemistry in Photostability Studies 63 44 PHOTOCHEMICAL REACTIONS 63 441 Regions of UV Visible and Sunlight Radiation 64 442 Important Chemical Functions for Photoreactivity in Organic Molecules 64 443 Photophysical Processes 64 45 PRIMARY PHOTOCHEMICAL REACTIONS 65 451 Flash Photolysis 65 452 Laser Flash Photolysis 66 453 Two-Laser Flash Photolysis 66 454 Time-Resolved Spectroscopy 67 455 Excited State Reactions 67 456 Photosensitized Reactions 68 4561 Type I Free radical mechanism 68 4562 Type II Mechanism involving singlet oxygen 68 46 PHOTODEGRADATION REACTIONS 69 461 Photooxidation Reactions 70 4611 Photooxidation of benzaldehyde 70 4612 Photooxidation of ascorbic acid 71 462 Photoreduction Reactions 71 4621 Photoreduction of riboflavin 71 463 Photodealkylation Reactions 72 4631 Photodealkylation of riboflavin 72 464 Photoaddition Reactions 72 4641 Photoaddition of riboflavin 72 465 Photoaquation Reactions 72 4651 Photoaquation of cyanocobalamin 72 466 Photodegradation of Moxifloxacin 73 4661 Acid solution 73 4662 Alkaline solution 73 467 Other Photodegradation Reactions 75 468 Photochemical Interactions 75 4681 Interaction of riboflavin with ascorbic acid 75 4682 Interaction of nicotinamide with ascorbic acid 75 4683 Interaction of α-tocopherol with ascorbic acid 76 4684 Interaction of nicotinamide with riboflavin 76 4685 Interaction of ascorbic acid with cyanocobalamin 76 REFERENCES 78 5 PHYSICAL STABILITY 83 51 INTRODUCTION 83 52 ANALYTICAL TECHNIQUES IN THE STUDY OF PHYSICAL STATE 83 521 Thermal Methods 83 5211 Thermogravimetric analysis (TGA) 83 5212 Differential scanning calorimetry (DSC) 83 5213 Differential thermal analysis (DTA) 83 5214 Microcalorimetry 83 5215 Isothermal calorimetry 84
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5216 Dilatometry 84 5217 Hot-stage microscopy 84 522 Spectroscopic Methods 84 5221 Vibrational spectroscopy 84 5222 Fourier transform infrared (FTIR) spectroscopy 84 5223 Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) 84 5224 Solid state nuclear magnetic resonance (SSNMR) spectroscopy 84 5225 Dynamic light scattering (DLS) 84 5226 X-ray powder diffraction (XRPD) 85 5227 Single crystal X-ray diffraction (XRD) 85 523 Other Techniques 85 5231 Polarized light microscopy 85 5232 Particle electrophoresis 85 53 CHANGES IN PHYSICAL STABILITY 85 54 FACTORS AFFECTING PHYSICAL STABILITY 86 541 Internal Factors 86 542 External Factors 86 543 Amorphous State 86 544 Dosage Forms 87 5441 Solid dispersions 87 5442 Semi-solid dispersions 87 5443 Creams 89 5444 Liposomes 90 5445 Proteins 90 545 Crystalline State 90 546 Polymorphism 91 5461 Conformational polymorphism 92 5462 Solvatomorphism 92 5463 Packing polymorphism 92 5464 Pseudopolymorphism 93 5465 Forced polymorphism 93 547 Characterization of Polymorphs 93 548 Pharmaceutical Implications 95 REFERENCES 96 6 SOLID STATE STABILITY 101 61 INTRODUCTION 101 62 TOPOCHEMICAL REACTIONS 102 63 CHEMICAL DEGRADATION REACTIONS 103 631 Solvolysis 103 632 Oxidation 104 633 Deamidation 105 634 Pyrolysis 106 635 Photolysis 106 64 FACTORS AFFECTING STABILITY IN THE SOLID-STATE 107 641 Moisture 107 642 Temperature 107 65 DRUG INTERACTIONS 108 66 KINETICS OF SOLID STATE DEGRADATION 109 67 SOLID STATE STABILITY STUDIES 110 671 Structural Studies 110 672 Kinetic Studies 111 673 Effect of Excipients 114 674 Effect of Aging 114 REFERENCES 116
8
7 FORCED DRUG DEGRADATION 121 71 INTRODUCTION 121 72 OBJECTIVES 122 73 FACTORS INVOLVED IN DEGRADATION 122 731 Degradation Conditions 122 732 Degradation Limits 122 733 Method of Analysis 123 74 FORCED DEGRADATION STUDIES IN THE DEVELOPMENT OF DRUGS 124 75 ANALYTICAL TECHNIQUES USED IN FORCED DEGRADATION
STUDIES 124
76 DRUG DEGRADATION STUDIES 124 REFERENCES 128 8 PACKAGING EFFECTS ON DRUG STABILITY 131 81 INTRODUCTION 131 82 DEFINITIONS 131 83 TYPES 132 831 Primary Packaging Material 132 832 Secondary Packaging Material 132 84 FUNCTIONS 132 85 SELECTION 133 86 PACKAGING STUDIES 133 861 Solid Dosage Forms 133 862 Liquid Dosage Forms 134 87 STABILITY PREDICTION IN PACKAGED PRODUCTS 136 88 STABILITY TESTING 136 REFERENCES 137 9 STABILIZATION 139 91 INTRODUCTION 139 92 PREVENTION OF DEGRADATION REACTIONS 139 921 Common Degradation Reactions 139 9211 Hydrolysis 139 9212 Oxidation 140 9213 Photolysis 140 922 Prevention of Degradation Reactions Involving Steric Structural Variations 141 9221 Cyclization 141 9222 Dimerization 142 9223 Epimerization 142 9224 Racemization 143 9225 Polymerization 143 93 METHODS OF STABILIZATION 143 931 Temperature Control 143 932 Cyclodextrin Complexation 144 933 Polymer Complexation 144 934 Use of Stabilizers 145 935 Liposomal Formulation 145 94 CHEMICAL AND PHOTOSTABILIZATION STUDIES 145 941 Chemical Stabilization 145 9411 Amorphous drugs 145 9412 Binary co-amorphous mixtures 146 9413 Solid dosage forms 147 9414 Liquid dosage forms 147 942 Photostabilization 149 9421 Solid and semisolid dosage forms 149 9422 Liquid dosage forms 150
9
REFERENCES 151 10 STABILITY OF HERBAL DRUGS AND PRODUCTS 157 101 INTRODUCTION 157 102 DEFINITIONS 157 1021 Herbal Drugs 158 1022 Processed Herbal Drugs 158 1023 Herbal Drug Preparations 158 1024 Herbal Drug Extracts 158 103 QUALITY CONTROL METHODS 159 1031 Herbal Products 159 1032 Essential Oils 159 1033 Herbal Extracts 159 104 FINGERPRINT ANALYSIS OF HERBAL DRUGS 160 105 STORAGE 165 106 PHOTOSENSITIVITY REACTIONS OF HERBS 165 107 STABILITY OF HERBAL DRUGS AND PRODUCTS 165 1071 Photodegradation of Herbal Drugs 165 1072 Chemical Degradation of Herbal Drugs 168 108 STABILITYDEGRADATION STUDIES OF HERBAL DRUGS IN
FORMULATIONS 169
109 STABILITY TESTING OF HERBAL PRODUCTS 170 1010 HERB-DRUG INTERACTIONS 171 REFERENCES 173 11 STABILITY-INDICATING ASSAY METHODS 179 111 INTRODUCTION 179 112 DEFINITIONS 179 113 DEVELOPMENT OF ANALYTICAL METHODOLOGY FOR ASSAY OF A
DRUG COMPOUND 179
114 DEVELOPMENT OF ANALYTICAL METHODOLOGY FOR STABILITY-INDICATING ASSAY OF A DRUG COMPOUND
180
115 STABILITY-INDICATING SPECTROMETRIC ASSAY METHODS 181 1151 One-Component Assay 181 1152 Multicomponent Assay 181 11521 Two-component assay (additive absorbencies) 181 11522 Three-component assay (additive absorbencies) 182 1153 Advantages 183 1154 Applications 183 116 STABILITY-INDICATING THIN-LAYER CHROMATOGRAPHIC (TLC) AND
HIGH-PERFORMANCE THIN-LAYER CHROMATOGRAPHIC (HP-TLC) ASSAY METHODS
184
117 STABILITY-INDICATING HIGH-PERFORMANCE LIQUID CHROMATOGRAPHIC (HPLC) ASSAY METHODS
194
1171 Development of HPLC Stability-Indicating Assay Methods 194 1172 Applications 194 11721 Drug mixture 194 11722 Stress testingforced degradation studies 194 118 VALIDATION OF STABILITY-INDICATING ASSAY METHODS 194 1181 Linearity 196 1182 Range 197 1183 Accuracy 197 1184 Precision 198 11841 Repeatability 199 11842 Intermediate precision 199 11843 Reproducibility 199
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1185 Specificity 200 1186 Sensitivity 202 11861 Limit of detection (LOD) 202 11862 Limit of quantification (LOQ) 202 1187 Robustness 202 REFERENCES 204 12 REGULATORY ASPECTS OF STABILITY TESTING 209 121 INTRODUCTION 209 122 OBJECTIVES 210 1221 The Development Phase 210 1222 The Approval Phase 210 1223 The Post-Approval Phase 210 123 DESIGN OF STABILITY STUDIES 211 1231 Stress Testing 211 1232 Selection of Batches 211 1233 Container Closure System 211 1234 Test Procedure and Test Criteria 212 1235 Frequency of Testing 212 12351 Long term or real-time studies 212 12352 Accelerated studies 212 12353 Intermediate studies 212 124 STORAGE CONDITIONS 213 1241 General Case 213 1242 Drug Substance or Drug Product Intended for Storage in a Refrigerator 214 1243 Drug Substance or Drug Product Intended for Storage in a Freezer 215 1244 Drug Products Packaged in Impermeable Containers 215 1245 Drug Products Packaged in Semi-Permeable Containers 215 1246 Drug Substance or Drug Product Intended for Storage Below -20 oC 216 125 PHOTOSTABILITY 216 1251 Light Sources 216 1252 Testing Criteria 217 1253 Presentation of Samples 218 1254 Post Exposure Sample Analysis 218 1255 Recommendation for Handling and Packaging 218 126 EVALUATION OF TEST RESULTS 218 127 STABILITY REPORT 219 128 STATEMENTS AND LABELING 219 129 STABILITY COMMITMENT 219 1210 ONGOING STABILITY STUDIES 220 1211 IN-USE STABILITY TESTING 220 1212 VARIATIONS 221 REFERENCES 223 INDEX 225
11
LIST OF FIGURES
21 Zero-order plot of A versus time 15 22 First-order plot of log A versus time 16 23 Second-order plot of 1[A] versus time 18 24 RatendashpH profile for photodegradation of moxifloxacin in aqueous solution 29 25 RatendashpH profile for the photolysis of riboflavin in aqueous solution 29 26 RatendashpH profile for the photolysis of cyanocobalamin in aqueous solution 30 41 Photooxidation of benzaldehyde 70 42 Chemical structures of riboflavin and photoproducts 43 Proposed pathway for the photodegradation of MF in acid solution 73 44 Proposed pathway for the photodegradation of MF in alkaline solution 74 45 Proposed pathway for the photodegradation of MF in alkaline solution 74 61 Solid crystalline state dimer of 4-aminosalicylic acid a and c indicate
directions in the arrangement of crystals 103
71 Stress conditions used for the degradation of drug substances and drug products
123
111 Chemical structure of riboflavin 180 112 Hydrolysis of aspirin 185 113 Hydrolysis of procaine HCl in alkaline solution 186 114 Alkaline hydrolysis of riboflavin at pH 110 187 115 Alkaline hydrolysis of formylmethylflavin at pH 110 188 116 Thermolysis of aqueous soluiton of reserpine (pH 30) at 60degC 189 117 Hydrolysis and photooxidation of sulfacetamide in aqueous solution 190 118 Photolysis of riboflavin at pH 70 191 119 Photoaddition reaction of riboflavin at pH 70 192 1110 Photolysis of ascorbic acid in the presence of riboflavin at pH 40 193 1111 Calibration curve of sulfacetamide sodium in aqueous solution pH 70 196 1112 Overlay UV spectra of sulfacetamide sodium in aqueous solution pH 70 197 1113 Illustration for accuracy and precision 199 1114 HPLC Chromatogram of carvedilol and its photodegradation products 200 1115 Absorption spectra of riboflavin (mdashmdash) cyclodehydroriboflavin (hellip)
formylmethylflavin (------) at pH 20 in KClndashHCl buffer 201
121 Flow chart for photostability testing of drug products 218
12
LIST OF TABLES
11 Types and criteria for acceptable levels of stability 2 12 Types of pharmaceutical dosage forms 2 21 Order of reactions half-life and shelf-life equations 19 22 Q10 factors for 10deg interval and Ea values 26 23 RatendashpH profiles for the degradation of drugs 28 61 Moisture content of commonly used tablet excipients at 25degC on
storage at different relative humidities (RH) 107
62 A comparison of the apparent zero-order rate constants (k0) for the degradation of various vitamin A derivatives at 50degC and their melting points
108
71 Widely used conditions for conducting forced degradation 123 72 Application of analytical techniques in forced degradation studies 124 81 Packaging preservation of product stability against environmental
factors 132
82 t90 Values of drugs in plastic and glass containers 135 91 Stabilization of drug substances in solid statesolid dosage forms 148 92 Stabilization of drug substances in liquid dosage forms 149 101 Analytical methods for the study of herbal drugs 160 102 Sensitivity and storage of some herbal drugs and products 161 103 Herbs causing skin sensitivity on exposure to sunlight 165 104 Some light sensitive drugs and products 165 105 Photodegradation of herbal drugs by sunlight 166 106 Storage conditions for stability testing of drug substances 171 107 Adverse effects of herbs and herbal products 172 111 Analytical parameters for the validation of sulfacetamide sodium 196 112 Accuracy and precision of sulfacetamide sodium by the UV
spectrometric method at 95 confidence interval 198
113 Comparison of tolfenamic acid recovery by titrimetric UV and FTIR spectrometric methods
198
121 Mean climatic conditions calculated data and derived storage conditions
210
122 Stability protocol design using bracketing 213 123 Stability protocol design using matrixing 213 124 General case (drug substance or drug product) 214 125 Drug substance or drug product intended for storage in a refrigerator 214 126 Drug substance or drug product intended for storage in a freezer 215 127 Drug products packaged in semi-permeable containers 215
13
LIST OF ABBREVIATIONS
Abbreviation Name a Absorptivity A Absorbance AAS Atomic absorption spectrometry ANN Artificial neural networks Arg Arginine ATR Attenuated total reflectance BP British Pharmacopoeia CD Circular dichroism CDs Cyclodextrins CE Capillary electrophoresis CDRF Cyclodehydroriboflavin CMF Carboxymethylflavin C Concentration Cp Heat capacity CRH Critical relative humidity COSY Correlation spectroscopy CTAB Cetyltrimethylammonium bromide Cys Cystine 2D NMR 2 dimensional nuclear magnetic resonance spectroscopy DFT Density function theory DLS Dynamic light scattering DRIFTS Diffuse reflectance infrared Fourier transform spectroscopy DSC Differential scanning calorimetry DTA Differential thermal analysis EMC Equilibrium moisture content EP European Pharmacopoeia F Flavin FDA Federal Drug Authority FMF Formylmethylflavin FMO Front molecular orbital fs Femto second FTIR Fourier transform infrared GC Gas chromatography GCndashMS Gas chromatography-mass spectrometry GE Gel electrophoresis GMP Good Manufacturing Practice HOMO Highest occupied molecular orbital HPLC High performance liquid chromatography HPndashTLC High performance thin-layer chromatography HSQC Heteronuclear single quantum coherence spectroscopy ICH International Conference on Harmonization ISO International Organization for Standardization
IR Infrared K Kelvin klxndashh Kilolux hour LASER Light amplification by stimulated emission of radiation LC Liquid chromatography LDPE Low density polyethylene
14
LF Lumiflavin LCndashMS Liquid chromatography-mass spectrometry LCndashMSTOF Liquid chromatography-mass spectrometrytime-of-flight LOD Limit of detection LOQ Limit of quantification Lys Lysine LUMO Lowest unoccupied molecular orbital MASndashSSNMR Magic angle spinning solid state nuclear magnetic resonance MC Methyl cellulose MCC Microcrystalline cellulose ML Mole per litre MEP Molecular electrostatic potential Minndash1 Per min MS Mass spectrometry MSMS Tandem mass spectrometry Msndash1 Mole per second Mndash1 sndash1 Per mole per second NA Nicotinamide NCE New chemical entity NF Norfloxacin NIR Near infrared NMR Nuclear magnetic resonance NSAIDs Nonsteroidal anti-inflammatory drugs PDA Photodiode array PEG Polyethylene glycol PDI Polydispersity index PLS Partial least squares PSD Particle size distribution ps Pico second PVA Polyvinyl alcohol PVP Polyvinyl pyrrolidone QndashTOFndashESIndashMSMS Quadrupole time-of-light electrospray ionization tandem mass
spectrometry RF riboflavin RH Relative humidity RPndashHPLC Reversed phase high performance liquid chromatography sndash1 Per second SER Serine SSNMR Solid state nuclear magnetic resonance SLN Solid lipid nanoparticles SN ratio Signal to noise ratio Tg value Glass transition temperature TGA Thermogravimetric analysis TLC Thin-layer chromatography Tm Melting point Tmc Critical mobility temperature Trp Tryptophan UPLC Ultra performance liquid chromatography USP United States Pharmacopeia UV Ultraviolet Vis Visible WHO World Health Organization Wm2 Watt per square meter XRPD X-ray powder diffraction XRD X-ray diffraction
15
FOREWORD
The stability of drugs and drug products is a subject of great importance for the assessment of the quality efficacy and safety of the products The knowledge of various aspects of stability is essential for the drug development process Stability testing provides information about the factors that affect the expiration dating of drug products
The authors have wide experience of teaching and research in the field and their efforts to present various aspects of the subject in the form of a monograph are commendable No attempts have been made to write monographs in specialized areas of pharmaceutical disciplines in this country This monograph meets the requirements of MPhil PhD courses in drug stability in various universities and would be of great help to postgraduate students in finding the relevant information in a unified source An understanding of the advanced concepts and their applications would assist in the development of different dosage forms
The monograph is a valuable contribution to the existing literature in the field of drug stability and would also be useful to teachers pharmacists and R amp D personnel in pharmaceutical industries
Prof Dr Zahida Baqai MBBS MRCOG FRCOG FICS FCPS Ph D
Vice Chancellor Baqai Medical University
16
PREFACE
This monograph has been prepared to meet the requirements of M PhilPh D courses in drug stability and related fields taught in the Faculties of Pharmaceutical Sciences in Pakistan It covers a wide range of topics related to drug stability with chapters on general introduction and those concerning chemical kinetics chemical stability photostability physical stability solid-state stability forced drug degradation packaging effects on stability stabilization stability of herbal drugs and products stability-indicating assay methods and regulatory aspects of stability testing Each chapter provides a brief introduction to the topic definitions of the terms used theoretical background relevant literature and discussion of the material An understanding of various aspects of drug stability is essential for the development of different dosage forms A prominent feature of each topic is the inclusion of current and previous research studies to apprise the students of the developments being made in the field to enable them to design their own research projects in a specific area of the subject The monograph would be helpful to the teachers in providing a suitable background of various aspects of drug stability and to workers engaged in quality control stability testing and drug development in pharmaceutical industries
This monograph on drug stability is the only one of its kind prepared on the subject for postgraduate students The authors have made the best of efforts in the selection compilation and presentation of the subject material However any inadvertent errors and omissions are regretted The authors would be grateful for pointing out any errors or shortcomings in the text
Iqbal Ahmad
Muhammad Ali Sheraz
Sofia Ahmed
August 2016
17
ACKNOWLEDGMENT
The authors are very grateful to Professor Dr Syed Fazal Hussain of the Faculty of
Pharmaceutical Sciences Baqai Medical University Karachi Professor Dr Anwar Ejaz Baig of the
Department of Pharmaceutics Faculty of Pharmacy Ziauddin Medical University Karachi
Professor Dr Faiyaz H M Vaid of the Department of Pharmaceutical Chemistry Faculty of
Pharmacy amp Pharmaceutical Sciences University of Karachi Professor Dr Usmanghani Khan
Consultant Herbion Pakistan (Pvt) Ltd Karachi Professor Dr Iqbal Azhar Dean Faculty of
Pharmacy amp Pharmaceutical Sciences University of Karachi and Dr Saif-ur-Rehman Khattak
Director Central Drugs Laboratory Karachi Drug Regulatory Authority of Pakistan for their kind
help and valuable suggestions for the improvement of the monograph The authors are very grateful
to Dr Saif-ur-Rehman Khattak for contributing a chapter on regulatory aspects of drug stability
They are also thankful to Mr Zubair Anwar Ph D scholar for literature search and computer work
One of the authors (IA) is highly appreciative of the patience and support of his wife Shamim
Iqbal during the preparation of this monograph
The authors express their heartfelt gratitude to the Higher Education Commission
Government of Pakistan for the publication of this monograph
18
ABOUT THE AUTHORS
Dr Iqbal Ahmad is Professor of Pharmaceutical Chemistry and Director Postgraduate Studies at the Faculty of Pharmaceutical Sciences Baqai Medical University Karachi He previously served as Professor and Chairman Department of Pharmaceutical Chemistry at the Faculty of Pharmacy University of Karachi He obtained a Ph D degree in Pharmaceutical Chemistry from the University of London and conducted Postdoctoral research at North E Wales Institute of Higher Education UK and Department of Biochemistry University of Arizona USA He has the privilege of working with Professor Lord George Porter Nobel Laureate at Imperial College London on a Royal Society Fellowship He has vast experience of teaching and research extending over a period of 50 years and has to his credit more than 200 publications including 2 books and 12 chapters He has supervised more than 60 students for M Pharm M Phil and Ph D degrees at the University of Karachi and Baqai Medical University He was awarded the D Sc degree in Pharmaceutical Chemistry by the University of Karachi and Tamgha-e-Imtiaz by Government of Pakistan in 2014 for his outstanding academic and research contribution
Dr Muhammad Ali Sheraz is Associate Professor and Chairman of the Department of Pharmacy Practice at the Faculty of Pharmaceutical Sciences Baqai Medical University Karachi He obtained a Ph D degree in Pharmaceutics from Baqai Medical University and conducted Postdoctoral research at the University of Sheffield UK on a fellowship awarded by Higher Education Commission of Pakistan He is a HEC approved supervisor for M Phil and Ph D studies He has published more than 60 research papers and has co-authored 6 chapters and a book published in USA He has so far supervised 5 students for M Phil degree He is also the Editor in Chief of the Baqai Journal of Health Sciences
Dr Sofia Ahmed is Associate Professor and Chairperson of the Department of Pharmaceutics at the Faculty of Pharmaceutical Sciences Baqai Medical University Karachi She obtained a Ph D degree in Pharmaceutics from Baqai Medical University and then conducted Postdoctoral research at the University of Sheffield UK on a fellowship awarded by Higher Education Commission of Pakistan She is a HEC approved supervisor for M Phil and Ph D studies and has published more than 60 research papers She has also co-authored 6 chapters and a book published in USA She has supervised 3 students for M Phil degree
CHAPTER ndash 1
19
INTRODUCTION The stability of drug substances and drug products is a subject of great interest to
pharmacists drug manufacturers and regulatory agencies Stability is a critical quality attribute a measure of good manufacturing practices (GMP) and an integral part of drug development process It is of fundamental importance among all the characteristics of a drug product since any physical and chemical change with time may affect the quality efficacy and safety of the product Stability is a regulatory requirement for the registration of drug products in most of the countries This is necessary to ensure that a safe and effective product is available to the patient throughout its shelf-life
Drug substances are susceptible to chemical physical and microbiological degradation under different conditions due to their sensitivity to environmental factors that may lead to a change in the chemical structure or the physical state This could have serious consequences on their biological efficacy and safety It is essential for the manufacturer to ensure the quality of the product under the conditions to which it is exposed during manufacture transportation and storage
Stability studies are necessary for the selection of suitable packaging materials and storage conditions to avoid chemical and physical changes and interactions between the drug and the excipients Pharmaceutical products included in the pharmacopoeias must be stored under specified conditions to maintain quality attributes during the shelf-life period Preventive measures are necessary for the storage of drug products under adverse climatic conditions (ie high temperature and high humidity) Stability considerations are important in the development of therapeutically effective dosage forms
Stability studies are required for all finished products by the manufacturer including the products that are reconstituted or diluted with saline solution or 5 dextrose solution before use The reconstituted or diluted solutions of a product also need to be subjected to stability assessment over the recommended storage period The compatibility of drugs in admixtures and the stability of preservativesstabilizers used should also be investigated This would provide information on drug interactions and the efficacy of preservativesstabilizers during the shelf-lives of the products
The cost of treating a new chemical entity (NCE) through the drug development process involving discovery toxicology clinical development and commercialization ranges from $ 800 million to $ 12 billion In the optimization of the drug development process a deep understanding of the key factors affecting the stability profile of the drug product and the execution of an effective stability program are important in the commercialization of the product (Huynh-Ba 2009)
11 DEFINITION OF STABILITY
Stability is considered as the period of time under specific storage conditions and in a specific container-closure system that a product will retain within predefined limits all of its original characteristics The United States Pharmacopeia (USP 2016) defines stability as the extent to which a product retains within specified limits and throughout its period of storage and use (ie its shelf-life) the same properties and characteristics that it possessed at the time of its manufacture
12 TYPES OF STABILITY AND DOSAGE FORMS
The different types of stability (ie chemical physical microbiological therapeutic and toxicological) and their criteria for acceptable levels (USP 2012) are given in Table 11 The stability of drug substances depends on the dosage forms of the product and their susceptibility to environmental conditions The various pharmaceutical dosage forms are described in Table 12 These may undergo a chemical andor physical change during manufacture storage and use affecting their stability
Table 11 Types and criteria for acceptable levels of stability
20
Type of
stability
Conditions maintained throughout the shelf-life of the drug
product
Chemical Each active ingredient retains its chemical integrity and labeled
potency within the specified limits
Physical The original physical properties including appearance palatability
uniformity dissolution and suspendability are retained
Microbiological Sterility or resistance to microbial growth is retained according to
the specified requirements Antimicrobial agents that are present
retain effectiveness within the specified limits
Therapeutic The therapeutic effect remains unchanged
Toxicological No significant increase in toxicity
Table 12 Types of pharmaceutical dosage forms
Dosage form Phase Example
Solid One or more than one solid Tablets capsules lozenges pills
granules powders suppositories
Semisolid One or two liquid and one solid Ointments gels pastes
Liquid Liquid Solutions parenterals syrups
elixirs drops gargles
Emulsion Liquidliquid or liquidsolid Creams
Inhaler Solidgas or Liquidgas Aerosols
13 FACTORS INFLUENCING STABILITY
Several factors are involved in altering the chemical and physical characteristics of drug substances and drug products These factors may influence the stability of different dosage forms during manufacture and storage and can be described as
131 External Factors
These include temperature light moisture oxygen carbon dioxide and microbial contaminants
132 Internal Factors
These include pH solvent medium polarity buffer species ionic strength particle size metal contaminants and drug-drug drug-excipients and drug-container interactions
The external factors can be controlled by using suitable packaging materials and appropriate storage conditions The effect of internal factors can be minimized by the selection of optimum formulation conditions to achieve an acceptable level of stability The shelf-life of the packaged product can then be determined under the recommended storage conditions
14 IMPORTANT TYPES OF STABILITY
141 Chemical Stability
The chemical reactions undergone by drug substances in liquid dosage forms and affecting the stability of a product include hydrolysis (eg esters amide imides) oxidation (eg ascorbic acid epinephrine vitamin A) epimerization (eg tetracyclines moxalactam etoposide) isomerization (eg cytarabine amphotericin B cyclosporine A) decarboxlyation (eg 4-aminosalicylic acid etodolac) dehydration (eg glucose erythromycin prostaglandin E1 and E2) and others
21
The screening of degradation products for their potential toxicity is part of the safety evaluation program Computer-assisted technologies are now being used for the prediction of toxicological behavior of pharmaceutical degradation products (Jamrogliewicz 2016)
142 Photostability
The photostability of drug substances and drug products is an important factor in the assessment of the overall stability of solid and liquid dosage forms A large number of pharmacopoeial drugs are sensitive to light and their formulated products may be degraded during manufacture storage and administration This could result in the loss of potency change in efficacy and adverse biological effects Knowledge of the photochemical behavior of drugs under stipulated light exposure conditions could provide guidance for handling packaging and labeling of the products The use of suitable packaging material can provide protection to the products from photodegradation Opaque and amber colored containers are suitable for light protection in the UV and visible region The important photodegradation reactions of drugs include photooxidation (eg ascorbic acid) photoreduction (eg riboflavin) photoaquation (eg cyanocobalamin) photocyclization (eg meclofenamic acid) photodealkylation (eg chloroquine) photodecarboxylation (eg amino acids) photoisomerization (eg aztreonam) photodimerization (eg primaquine) photo-induced hydrolysis (eg sulfacetamide) and photo-induced ring cleavage (eg norfloxacin)
143 Physical Stability
The physical stability of drug products takes into consideration the physical changes occurring in the products These changes depend on the physical properties of the drugs such as melting point particle size polymorphic behavior texture and morphology
The physical stability of liquid dosage forms is affected by changes in appearance alteration in viscosity discoloration precipitation polymorph formation (low solubility) drug adsorption (container surface) and microbial growth
The changes in the physical stability of solid dosage forms involve polymorphic transition solvation and desolvation salt and salt exchange amorphization and reversion to crystalline form and moisture adsorption These changes may lead to the physical destabilization of the product
144 Solid State Stability
The solid state stability deals with the physical and chemical transformations occurring in the solid state (or solid dosage forms) under the influence of factors such as moisture and temperature or during storage with time (such as polymorphic transitions) The physical changes in the solid state have been discussed by Santos (1999) and involve
Particle size growth and surface area changes
Precipitation from solution at refrigerated temperatures
Degree of hydration
Deliquescence or softening
Crystallization of amorphous material
Solid state transitions
The chemical degradation of drugs in the solid state and in the solid dosage forms occurs in the presence of moisture and at high temperature It involves reactions such as solvolysis (eg acetylsalicylic acid) oxidation (eg ascorbic acid) decarboxylation (eg carbenicillin sodium) deamidation (eg peptides) pyrolysis (eg fluconazole) and photolysis (eg furosemide) The solid state degradation of drugs is affected by properties such as melting point crystalline state and hygroscopic character of the drug
145 Microbiological Stability
22
The microbiological stability of drug products is essential for the efficacy and safety of the products The sterility or resistance to microbial growth should be maintained throughout the shelf-life period The efficacy of the preservative should remain unaltered within the specified limit The multidose aqueous preparations contain a preservative to protect against spoilage during use The preservative has no influence on the vulnerability of the product to contamination (ie the access of organisms into it that largely depends on the container design) However a good design minimizes the level of organisms introduced during use and operates in harmony with an effective preservative to protect the consumer (Hodges 1999) A pathogen-contaminated product can result in severe consequences both for the consumer and the manufacturer and therefore adequate preservative activity is vital for the product For regulatory approval it is necessary to show adequate preservative performance at the time of manufacturing as well as later during the shelf-life period The requirements for the biological assessment of preservative activity have been discussed by Hodges (1999)
15 ANALYTICAL METHODS
Many regulatory agencies require an assessment of the individual and the total limits of degradation products in the specifications of the drug products This can be achieved by the application of a stability-indicating assay method such as high-performance liquid chromatography (HPLC) for the determination of the intact drug as well as its degradation products The method should be validated to ensure the desired specificity for a particular system It can also be applied to assess the stability of drug products manufactured in several dosage forms with variable strengths and stored in different packaging Aubry et al (2009) have discussed the development of stability-indicating assay methods
16 STABILITY EVALUATION
The purpose of stability studies is to establish based on testing a minimum of three batches of the drug substance and evaluating the stability information (including as appropriate results of the physical chemical biological and microbiological tests) a re-test period applicable to all future batches of the drug substance manufactured under similar circumstances The degree of variability of individual batches affects the confidence that a future production batch will remain within specification throughout the assigned re-test period (ICH Guideline 2003)
The design of the stability studies of drug products should be based on the evaluation of all factors that may cause a physical chemical andor biological change during the recommended storage period It should include the assay of the drug and degradation products and measurement of change in pH color appearance etc for liquid dosage forms and hardness dissolution moisture content etc for solid dosage forms and any other tests depending on the dosage form
An understanding of the stimuli causing the degradation of drugs and the mode of their degradation is helpful in the evaluation of the stability of drug products The various stimuli that lead to the degradation of a drug include oxygen temperature humidity and light The pH of the medium buffer content metal contaminants etc also plays a part in the degradation process The evaluation of the stability of drugs enables the development of safe and effective dosage forms selection of suitable packagings establishment of appropriate storage conditions and assignment of shelf-lives
17 STABILITY TESTING
Stability testing is an integral part of drug development process and is an essential requirement for the registration of drug products The ICH (2003) WHO (2009) and FDA (1998 2014) have provided guidelines for the stability testing of new drug substances and products which involve long term intermediate and accelerated stability studies The purpose of stability testing is to provide evidence on how the quality of a drug substance or drug product varies with time under the influence of a variety of environmental factors such as temperature humidity and light and to establish a reset period for the drug substance or a shelf-life for the drug product and recommended storage conditions (ICH 2003) Similar ICH guideline is also available for
23
photostability testing of new drug substances and products The photostability testing should be an integral part of stress testing and should be conducted on at least one primary batch of drug product if appropriate The purpose of photostability testing is to evaluate the intrinsic photostability characteristics of new drug substances and products to demonstrate that the light exposure does not result in an unacceptable change The standard conditions for photostability testing are described in ICH Q1B guideline (ICH 1996)
18 FORCED DEGRADATION STUDIES
Forced degradations studies of new drug substances and drug excipients involve a degradation process at conditions that are more severe than those of the accelerated or stress conditions These studies are required for the establishment of the degradation pathways characterization of the degradation products determination of intrinsic stability of drug substances elucidation of the mechanism of degradation reactions and development of stability-indicating assay methods Pharmaceutical industry conducts forced degradation studies on drugs during the preformulation stage to select appropriate active ingredients and excipients to characterize degradation products to assess compatibility of ingredients and to conduct formulation development Different aspects of the forced degradation of pharmaceuticals have been reviewed by Reynolds (2004)
19 STATISTICAL APPLICATIONS
Statistics plays an important role in the stability studies of drug products (Carstensen et al 1992 Helboe 1992 Lin et al 1993 Chow and Liu 1995 Chen et al 1997) Statistical methods have been proposed for the design and analysis of stability studies (Nordbrock 1992 Carstensen et al 1992 Fairweather et al 1995 Chen et al 1997) and for testing and classification of stability data with multiple factors (Chow and Shao 1989 1990 Chen et al 1995 Golden et al 1996 Ahn et al 1997)
Statistical treatment of stability data provides information on the effect of batch-to-batch variations dosage unit to dosage unit variations small scale-production scale process variations packaging variations and strength variations on the expiration dating The ICH (1994) guideline for industry on stability testing of new drug substances and products has recommended the use of bracketing and matrixing as an experimental design for testing the stability data to obtain expiration dating of the products Bracketing involves the design of a stability schedule such that only samples on the extremes of certain design factor eg strength package size are tested at all time points as in a full design The design assumes that the stability of any intermediate level is represented by the stability of extremes tested Matrixing involves the design of a stability schedule such that a selected subset of the total number of possible samples for all factor combination is tested at a specific time point At a subsequent time point another subset of samples for all factor combinations is tested The design assumes that stability of each subset of samples tested represents the stability of all samples at a given time point (ICH 2003)
110 ROLE OF PHARMACIST
The pharmacist has to play an important role in ensuring the quality efficacy and safety of the products dispensed under his supervision He should be aware of the factors involved in the destabilization of drugs under adverse climatic conditions and evolve a strategy to overcome them He should ensure that the products meet the acceptable criteria of stability under the prescribed storage conditions during their shelf-life period It is the time period during which a drug product is expected to remain within the approved shelf-life specifications provided that it is stored under the conditions defined on the container label It is also referred to as expiration dating period (ICH 2003)
111 LITERATURE ON DRUG STABILITY
Extensive literature on various aspects of drug stability is available Some of the important sources are as follows
24
Books
Windheuser (1970) Connors et al (1986) Rubinstein (1989) Albini and Fasani (1998) Mazzo (1999) Carstensen and Rhodes (2000) Yoshioka and Stella (2000) Tonnesen (2004) Baertschi (2005) Piechocki and Thoma (2007) Huynh-Ba (2009) Trissel (2009) Grimm et al (2011) Loftsson (2014)
Chapters
Schwartz and Nelson (1966) Ho (1972) Simonelli and Dresback (1972) Lintner (1973) Hashmi (1973) Lachman et al (1986) Racz (1989) Lim et al (1993) Grimm (2000) Matthews (2000) Valvani (2000) Pugh (2002) Tonnesen (2002) Hawely and Van Arendonk (2002) Guillory and Poust (2002) Ghosh (2005) Fasani and Albini (2005) Ahmad and Vaid (2006) Florence and Attwood (2006) OrsquoDonnell and Bokser (2006) Singh (2006) Jackson and Lowey (2010) Sinko (2011) Govindarjan (2014)
Reviews
Macek (1960) Garrett (1967) Maudling and Zoglio (1970) Tingstad and Dudzinski (1973) Allen (1974) Carstensen (1974) Madsen et al (1974) Zoglio et al (1975) Amirjahed (1977) Mollica et al (1978) DeRitter (1982) Carstensen and Rhodes (1984) Ahmad (1985) Sugden (1985) Greenhill and McLelland (1990) Wessels et al (1997) Singh (1999) Singh and Bakshi (2000ab) Tonnesen (2001) Boreen et al (2003) Glass et al (2004) Waterman and Adami (2005) Phalekar et al (2008) Panda et al (2013) Bajaj et al (2012) Baertschi et al (2013 2015) Vinodi et al (2015) Ahmad et al (2016)
Pharmacopoeias
British Pharmacopoeia (2016) United States Pharmacopeia (2016) European Pharmacopoeia (2015) and other pharmacopoeias
Regulatory Aspects
Food and Drug Administration Guidelines (FDA) (1987 1998) International Conference on Harmonization (ICH) Guidelines (1996 1997 2002 2003) World Health Organization (WHO) Guideline (2009)
112 CONTENTS OF MONOGRAPH
This monograph presents an overall view of different aspects of drug stability to cover the course contents for M Phil Ph D program in different disciplines of pharmaceutical sciences Chapters 2 3 4 5 and 6 are devoted to chemical kinetics chemical stability photostability physical stability and solid state stability Chapters 7 8 and 9 deal with forced drug degradation packaging effects on stability and stabilization The last three chapters 10 11 and 12 cover stability of herbal drugs and products stability-indicating assay methods and regulatory aspects of stability testing
25
REFERENCES
Ahmad I Stability indicating assays in pharmaceutical quality control In Ahmad T Khalid R editors Proceedings International Seminar on Policies Management and Quality Assistance of Pharmaceuticals Quality Control Authority Ministry of Health Government of Pakistan Islamabad Pakistan 1985 pp 256ndash264
Ahmad I Vaid FHM Photochemistry of flavins in aqueous and organic solvents In Silva E Edwards AM editors Flavin Photochemistry and Photobiology The Royal Society of Chemistry Cambridge UK 2006 Chap 2
Ahmad I Ahmed S Anwar Z Sheraz MA Sikorski M Photostability and Photostabilization of drugs and drug products Int J Photoenergy 2016 ID8135608
Ahn H Chen J Lin TD A two-way analysis of covariance model for classification of stability data Biomedical J 199739559ndash576
Albini A Fasani E editors Drug Photochemistry and Photostability Royal Society of Chemistry Cambridge UK 1998
Allen NS Photofading and light stability of dyes and pigmented polymers Polym Degrad Stab 197444357ndash374
Amirjahed AK Simplified method to study stability of pharmaceutical preparations J Pharm Sci 197766785ndash789
Aubry A-F Tatersall P Ruan J Development of stability indicating methods In Huynh-Ba K editor Handbook of Stability Testing in Pharmaceutical Development-Regulations Methodologies and Best Practices Springer New York USA 2009 Chap 7
Bajaj S Singla D Sakhuja N Stability testing of pharmacetucial products J App Pharm Sci 201202129ndash138
Baertschi SW editor Pharmaceutical Stress Testing Predicting Drug Degradation Taylor amp Francis New York USA 2005
Baertschi SW Clapham D Foti C Jansen PJ Kristensen S Reed RA Templeton AC Tonnesen HH Implication of in-use photostability proposed guidance for photostability testing and labeling to support the administration of photosensitive pharmaceutical products Part 1 Drug products administered by injections J Pharm Sci 20131023888ndash3899
Baertschi SW Clapham D Foti C Kleinman MH Kristensen S Reed RA Templeton AC Tonnesen HH Implication of in-use photostability proposed guidance for photostability testing and labeling to support the administration of photosensitive pharmaceutical products Part 2 Topical drug product J Pharm Sci 20151042688ndash2701
Boreen AL Arnold WA McNeill K Photodegradation of pharmaceuticals in the aquatic environment A review Aquatic Sci 200365320ndash341
British Pharmacopoeia Her Majestyrsquos Stationery Office London UK 2016
Carstensen JT Franchini M Ertel K Statistical approaches to stability protocol design J Pharm Sci 199285303ndash308
Carstensen JT Rhodes CT Expiration dating for pharmaceuticals Drug Dev Ind Pharm 198410 Nos 8 and 9
Carstensen JT Rhodes CT editors Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000
Carstensen JT Stability of solids and solid dosage forms J Pharm Sci 1974631ndash14
Chen J Ahn H Tsong Y Shelfndashlife estimation for multi-factor stability studies Drug Info J 199731573ndash587
26
Chow Sc Shao J Test for batch-to-batch variation in stability analysis Statistics in Medicine 19898883ndash890
Chen J Hwang JS Tsong Y Estimation of shelf-life with mixed effects models J Biopharm Stat 19955131ndash140
Chow SC Shao J Estimating drug shelf-life with random batches Biometrics 1990471071ndash1079
Chow Sc Liu JP Statistical Design and Analysis in Pharmaceutical Sciences Marcel Dekker New York 1995
Connors KA Amidon GL Stella VJ Chemical Stability of Pharmaceuticals A Handbook for Pharmacists 2nd ed John Wiley New York USA 1986
DeRitter E Vitamins in pharmaceutical formulations J Pharm Sci 1982711073ndash1096
European Pharmacopoeia European Pharmacopoeial Convention and the European Union Strasbourg France 8th edition 2015
Fairweather W Lin TD Kelly R Regulatory design and analysis aspects of complex stability studies J Pharm Sci 1995841322ndash1326
Fasani E Albini A Photostability stress testing In Baertschi SW editor Pharmaceutical Stress Testing Predicting Drug Degradation Taylor amp Francis New York NY USA 2005
FDA Guidance for Industry Draft Stability Testing of Drug Substances and Drug Products 1998
FDA Guideline for submitting documentation for the stability of human drugs and biologics Food and Drug Administration Rockville MD USA 1987
FDA (CDER) Guidance for Industry ANDAS Stability testing of drug substances and products Questions and Answers May 2014
Florence AT Attwood D Drug stability Physiochemical Principles of Pharmacy 4th ed Pharmaceutical Press London UK 2006 Chap 4
Garrett ER Kinetics and mechanism in stability of drugs In Bean HS Beckett AA Carless JE editors Advances in Pharmaceutical Sciences Academic Press London UK vol 2 1967 Chap 1
Ghosh TK Chemical kinetics and stability In Ghosh TK Jasti BR editors Theory and Practice of Contemporary Pharmaceutics CRC Press New York USA 2005 Chap 8
Glass BD Nwak CS Brown ME The thermal and photostability of solid pharmaceuticals J Therm Anal Calorim 2004771013ndash1036
Golden MH Cooper D Riebe M Carswell K A matrixed approach to long-term stability testing of pharmaceutical products J Pharm Sci 199686240ndash244
Govindarjan R Chemical reaction kinetics and drug stability In Dash AK Singh S J Tolman editors Pharmaceutics Basic Principles and Application in Pharmacy Practice Academic Press New York USA 2014 Chap 5
Greenhill JV McLelland MA Photodecomposition of drugs Prog Med Chem 19902751ndash121
Grimm W Harnischfeger G Tegtmeier M Stabilitatsprufuing in der Pharmazie 3rd ed Thieme Stinttgart Germany 2011
Grimm W A rational approach to stability testing and analytical development for NCE drug substance and drug products marketed product stability testing In Carstensen JT Rhodes CT editors Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000 Chap 13
Guillory K Poust RI Chemical kinetics and drug stability In Banker GS Rhodes CT editors Modern Pharmaceutics Marcel Dekker New York USA 2002 Chap 6
27
Hashmi MH Stability of vitamins in pharmaceutical preparations In Assay of Vitamins in Pharmaceutical Preparations John Wiley amp Sons New York USA 1973 Chap 2
Hawely LC Van Arendonk MD Expiration dating In Swarbrick J Boylan JC editors Encyclopedia of Pharmaceutical Technology 2nd ed Marcel Dekker New York USA 2002 pp 1211ndash1219
Helboe P New designs for stability testing programs Matrix or factorial designs Authorities viewpoint on the predictive values of such studies Drug Info J 1992 26629ndash634
Ho NFH Predicting drug stability of parenteral admixtures In Francle DE Whitney HAK editors Perspectives in Clinical Pharmacy Drug Intelligence Publications Hamilton IL USA 1972 pp 421ndash437
Hodges N Assessment of preservative activity during stability studies In Mazzo DH editor International Stability Testing Interpharm Press Buffalo Grove Illinois USA 1999 Chap 7
HuynhndashBa K editor Handbook of Stability Testing in Pharmaceutical Development Regulations Methodologies and Best Practices Springer New York USA 2009 Chap 1
ICH Harmonized Tripartite Guideline ICHndashQ1A Stability testing of new drug substances and products Genva Switzerland 1994
ICH Harmonized Tripartite Guidelines Q1A (R2) Stability testing of new drug substances and products (second revision) 2003
ICH Harmonized Tripartite Guidelines Q1B Photostability testing of new drug substances and products Genva Switzerland 1996
ICH Harmonized Tripartite Guidelines Q1C Stability testing for new drug forms Genva Switzerland 1997
ICH Harmonized Tripartite Guidelines Q1D Bracketing and matrixing designs for stability testing of drug substances and drug products Genva Switzerland 2002
Jackson M Lowey A Formulation and stability Handbook of Extemporaneous Preparations a Guide to Pharmaceutical Compounding Pharmaceutical Press London UK 2010
Jamrbgilwicz M Consequences of new approach to chemical stability tests to active pharmaceutical ingredients Front Pharmacol 2016717
Lachman L DeLuca P Akers MJ Kinetic principles and stability testing In Lachman L Lieberman HA Kanig JL editors The Theory and Practices of Industrial Pharmacy 3rd ed Lee amp Fabiger PA USA 1986 Chap 26
Lim KK Lin TD Kelly RE Stability of drugs In Buncher CR Tsay JY editors Statistics in the Pharmaceutical Industry 2nd ed Marcel Dekker New York USA 1993 pp 419ndash444
Lintner CJ Pharmaceutical product stability In Cooper MS editor Quality Control in Pharmaceutical Industry vol 2 Academic Press New York USA 1973
Loftsson T Drug Stability for Pharmaceutical Scientists Elsevier Amsterdam The Netherlands 2014
Macek TJ Stability problems with some vitamins in pharmaceuticals Am J Pharm 1960433ndash455
Madsen BW Anderson RA Herbison-Evans D Sneddon W Integral approach to nonisothermal estimation of activation energies J Pharm Sci 197463777ndash781
Matthews BR Regulatory aspects of stability testing in Europe In Carstensen JT Rhodes CT editors Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000 Chap 17
Maudling HV Zoglio MA Flexible nonisothermal stability studies J Pharm Sci 197059333ndash337
28
Mazzo DJ editor International Stability Testing Interpharm Press Buffalo Grove Illinois USA 1999
Mollica JA Ahuja S Cohen J Stability of pharmaceuticals J Pharm Sci 197867443ndash465
Nordbrock E Statistical comparison of stability study design J Biopharm Statist 1992291ndash113
OrsquoDonnell PB Bokser AD Stability of pharmaceutical products In Hendrickson R editor Remington the Sciences and Practice of Pharmacy 21st ed Lippincott Williams amp Wilkins Baltimore MD USA 2006 Chap 5
Panda A Kulkarni S Tiwari R Stability studies An integral part of drug development process IJPRBS 2013269ndash80
Phalekar NR Harinarayana D Madgulker AR Improvement of photostability in formulation a review Asian J Chem 2008205095ndash5108
Piechocki JT Thoma K editors Pharmaceutical Photostability and Stabilization Technology Informa Healthcare New York USA 2007
Pugh J Kinetics and product stability In Aulton ME editor Pharmaceutics the Science of Dosage Form Design Churchill Livingston London UK 2002 Chap 7
Racz I Drug stability Drug Formulation John Wiley amp Sons New York USA 1989 Chap 2
Reynolds DW Forced degradation of pharmaceuticals Am Pharm Rev 2004756ndash61
Rubinstein MH editor Pharmaceutical Technology Drug Stability Ellis Horwood Limited John Wiley amp Sons New York USA 1989
Santos I Drug substance solid state characterization and stability In Mazoo DG editor International Stability Testing Interpharm Press Buffalo Grove Ilinois USA 1999 Chap 8
Schwartz MA Nelson E Drug Stability In Martin EW editor Husarsquos Pharmaceutical Dispensing 6th ed Mack Publishing Easton PA USA 1966 Chap 20
Simonelli AP Dresback DS Principles of formulation of parenteral dosage forms (stability considerations) In Francle DE Whitney HAK editors Perspectives in Clinical Pharmacy Drug Intelligence Publications Hamilton IL USA 1972 pp 408ndash412
Singh S Bakshi M Guidance on conduct of stress tests to determine inherent stability of drugs Pharm Tech 2000a31ndash14
Singh S Bakshi M Guidance on conduct of stress tests to determine inherent stability of drugs Pharm Tech Asia 2000b324ndash36
Singh S Drug stability testing and shelfndashlife determination according to international guidelines Pharm Tech 19992368ndash88
Singh S Stability-testing during product development In Jain NK editor Pharmaceutical Product Development CBS Publishers New Delhi India 2006 Chap 9
Sinko PJ Chemical kinetics and stability In Martins Physical Pharmacy and Pharmaceutical Sciences 6th ed Lippincott Williams amp Wilkins New York USA 2011 Chap 14
Sugden JK Photostability of cosmetic material Int J Cosmet Sci 19857165ndash173
Tonnesen HH editor Photostability of Drugs and Drug Formulations 2nd ed CRC Press Boca Raton FL USA 2004
Tonnesen HH Formulation and stability testing of photolabile drugs Int J Pharm 20012251ndash14
Tonnesen HH Photodecomposition of drugs In Swarbrick J Boylan JC editors Encyclopedia of Pharmaceutical Technology Taylor amp Francis New York NY USA 2nd ed 2002 pp 2197ndash2203
29
Tingstad J Dudzinski J Preformulation studies II Stability of drug substances in solid pharmaceutical systems J Pharm Sci 1973621856ndash1860
Trissel LA Trisselrsquos Stability of Compounded Formulations American Pharmacists Association Washington DC USA 4th ed 2009
United States Pharmacopeia 39 ndash National Formulary 34 United States Pharmacopoeial Convention Rockville MD USA 2016
Valvani SC Industrial stability testing in United States and computerization of stability data In Carstensen JT Rhodes CT editors Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000 Chap 15
Vinodi VB Budhwaar V Manda A Photochemical fate of pharmaceuticals An updated review IJPRBS 2015454ndash70
Waterman KC Adami RC Accelerated aging prediction of chemical stability of pharmaceuticals Int J Pharm 2005293101ndash125
Wessels P Holz M Emi F Krumners K Ogorka J Statistical evaluation of stability-data for pharmaceutical products for specification setting Drug Dev Ind Pharm 199723427ndash439
Windheuser JJ editor The dating of Pharmaceuticals University of Wisconsin Press Madison WI USA 1970
WHO Technical Report Series No 953 Annex 2 Stability testing of active pharmaceutical ingredients and finished pharmaceutical products 2009
Yoshioka S Stella VJ Stability of Drugs and Dosage Forms Kluwer Academic New York USA 2000
Zoglio MA Maudling HV Streng WH Vincek C Nonisothermal kinetic studies III rapid nonisothermal-isothermal method for stability prediction J Pharm Sci 1975641381ndash1383
30
31
CHAPTER ndash 2
CHEMICAL KINETICS 21 INTRODUCTION
Chemical kinetics deals with the quantitative study of the rates of chemical reactions and the factors that influence them The study of the stability of drug products involves the evaluation of the kinetics of chemical degradation reactions of drugs in dosage forms This is necessary to predict the shelf-life of the product under defined storage conditions The drug substances possess diverse chemical structures and may follow one or more than one modes of degradation with different orders of reaction under various conditions The most commonly occurring degradation reactions of drugs include oxidation hydrolysis and photolysis These reactions may occur during manufacturing storage and use of drug products The practicing pharmacist should be aware of the effects of these processes on the potency loss andor toxicity development in the product to ensure the safety of the patient
An accurate assessment of the potency loss of a drug in a product can be made by the application of a stability-indicating assay method that is also capable of determining the degradants and related compounds The assay data are then subjected to kinetic treatment to determine the shelf-life of the product and to establish the expiration dating A change in formulation parameters to improve the stability of the product may require a reconsideration of the packaging and storage conditions This would again need an establishment of the reset period or shelf-life under the proposed storage conditions
The determination of the rates of degradation reactions undergone by drug substances and the prediction of shelf-lives and expiration dates of drug products requires a sound knowledge of the fundamental principles of chemical kinetics The kinetic parameters could be useful in the elucidation of the mechanisms of degradation reactions and thus enable to adopt appropriate measures for the stabilization of the product Several excellent accounts of the subject with applications to the degradation kinetics of drug substances and drug products are presented in books (Lintner 1973 Lachman and DeLuca 1986 Carstensen 2000 Pugh 2002 Ghosh 2005 Guillory and Poust 2002 Wigent 2006 Florence and Attwood 2006 Sinko 2011) monographs (Windheuser 1970 Connors et al 1986 Laidler 1987 Carstensen and Rhodes 2000 Yoshioka and Stella 2000 Loftsson 2014) and reviews (Macek 1960 Garrett 1967 Mollica 1978 Connors 1981 DeRitter 1982 Waterman and Adami 2005 Ahmad et al 2016a) This chapter deals with a brief description of the fundamental principles of chemical kinetics their application to drug stability and the factors affecting the stability of pharmaceuticals
22 BASIC KINETIC PRINCIPLES
221 Reaction Rate
The rate of a reaction is defined as the change in concentration of a reactant or products per unit time Consider a reaction in which two reactants A and B react to yield products C and D
aA + bB cC + dD (21)
where a b c d represent the number of molecules A B the reactants and C D the products The rate of this reaction can be expressed as
Rate = ndash 1 d[A]
= ndash 1 d[B]
= 1 d[C]
= 1 d[D]
(22) a dt b dt c dt d dt
32
where d[A] d[B] d[C] and d[D] each is an infinitesimal change in the concentration of A B C and D during the infinitesimal time dt A decrease in concentration is denoted by a minus sign
The relationship between the rate of reaction and the concentration of reactants is given by eq (23)
Rate = k [A]a [B]b (23)
where k is the rate constant
If the concentration is expressed in ML the rate constant is expressed as moles per unit time for example Msndash1 or Mndash1sndash1
The rate of a reaction is directly proportional to the concentration of the reactants each concentration being raised to a certain power usually equal to the number of molecules a and b of the reactants A and B respectively
222 Molecularity and Order of Reaction
2221 Molecularity
Molecularity is defined as the number of molecules taking part in a chemical reaction A reaction in which only one reactant molecule in involved is called a Unimolecular reaction if two reactant molecules are involved it is called a bimolecular reaction and if three reactant molecules are involved it is called a termolecular reaction
2222 Order
The order of a reaction is defined as the sum of exponents of the concentrations of reactants involved in the rate equation It can also be defined with respect to a single reactant For example in eq (23) the reaction order with respect to A is a and with respect to B is b If a + b = 1 it is a first-order reaction if a + b = 2 it is a second-order reaction and if a + b = 3 it is a thirdndashorder reaction
The molecularity and the order are the same for a simple onendashstep reaction
223 Half-Life and Shelf-Life of Drug
2231 Half-life (t12)
The half-life (t12) of a reaction is defined as the time required for the drug concentration to decrease to 50 of its initial concentration The value of t12 is a function of the rate constant of the reaction
2232 Shelf-life (t90 or t95)
The shelf-life (t90 or t95) of a product is defined as the time required for the product after manufacture to decrease to the minimum acceptable level of the label claim (ie 90 or 95) It is also a function of the reaction rate constant
2233 Expiration dating
A drug product should comply with the official standards of identity strength quality and purity during the expiration dating period This period can be defined as ldquothe time interval that a drug product is expected to remain within an approved shelf-life specification provided that it is stored according to label storage conditions and that it is in the original container closure systemrdquo The expiryexpiration date is the actual date period on the containerlabel of a drug product designating the time during which a batch of a drug product is expected to remain within the approved shelf-life specifications if stored under defined conditions and after which it must not be used (Hawely and Van Arendonk 2002)
The expiration dating of drug products manufactured in a particular country is to be determined according to the storage conditions prevailing in the climatic zone of that country In Pakistan this applies to the conditions prescribed for Climate Zone IVa (hot and humid)
33
23 Kinetics of Chemical Reactions
231 Zero-Order Reaction
In a zero-order reaction the rate of disappearance of a reactant A remains constant and is independent of concentration as shown by eq (24)
ndashd[A] = k0 (24) dt
The integrated form of eq (24) is
A = A0 ndash k0t (25)
where
A is the amount of A0 remaining at time t
A0 is the initial concentration of A and
k0 is the zero-order rate constant
The rate constant k0 of a zero-order reactions can be determined from the slope of a linear plot of A versus t (Fig 21)
Fig 21 Zero-order plot of A versus time
The half-life (t12) of a zero-order reaction is directly proportional to the initial concentration of the reactant and is inversely proportional to the rate constant (Table 21)
Example Fading of color of dyes (Garrett and Carper 1955)
232 Pseudo Zero-Order Reaction
In certain pharmaceutical systems such as suspensions the drug is degraded by a first-order reaction (Section 233) However the drug present in the solid form in the suspension dissolves in the aqueous phase and thus a constant concentration of the drug is maintained in the solution In this case although the reaction is first-order with respect to the drug it follows zero-order reaction
The rate of the reaction can be expressed as
34
ndashd[A] = k1 [A] = k0 (26) dt
where
k1 is the first-order rate constant
A is the concentration of dissolved drug and
k0 is the zero-order rate constant (The rate constant k0 is determined as mentioned in section 231)
Examples
1 Hydrolysis of aspirin in aqueous suspensions (Blaug and Wesolowski 1959)
2 Hydrolysis of amoxicillin in aqueous suspensions (Zia et al 1977)
233 First-Order Reaction
In a first-order reaction the rate of disappearance of the reactant A is proportional to the concentration of A at time t as given by eq (27)
ndashd[A] = k1 A (27) dt
The integrated form of eq (27) can be expressed by eq (28)
ln A ndash ln A0 = ndashk1t (28)
or
ln A = ln A0 ndash k1t (29)
Rearranging and converting logarithms to the base 10
k1 =
2303 log
A0 (210)
t A
In a first-order reaction there is an exponential decrease in the concentration of the reactant A with time
The rate constant k1 for a first-order reaction can be obtained from the slope of a linear plot of log A versus t (Fig 22)
Fig 22 First-order plot of log A versus time
35
The t12 of first-order reaction is independent of the initial concentration of the reactant and depends on the rate constant (Table 21) First-order reactions are the most widely occurring reactions in the degradation of drugs in formulated products
Examples
1 Hydrolysis of aspirin (Edwards 1950)
2 Oxidation of ascorbic acid solutions (Blaug and Hajratwala 1972)
3 Photolysis of riboflavin in aqueous solutions (Ahmad et al 2004a)
234 Pseudo First-order Reaction
A chemical reaction in which one of the reactants is present in large excess shows an order that is different from that of the actual order This type of reaction is called pseudo first-order reaction
Consider a reaction
A + B Products
This is a second-order reaction and the rate is given by eq (211)
Rate = k [A] [B] (211)
Since [B] is present in large excess its concentration is considered constant during the course of the reactions and eq (211) can be expressed as
Rate = kprime [A] (212)
where
kprime = k [B]
Thus the reaction is first-order with a rate constant kprime and is termed as an apparent or pseudo first-order reaction
This applies to the hydrolysis of drugs in aqueous solution in which the amount of water is in large excess and does not alter during the course of reaction
Example Hydrolysis of ethyl acetate (Pugh 2002)
235 Second-order Reaction
In a second-order reaction two molecules react to yield the products
A + B Products
The rate of the reaction is proportional to the concentration of the two reactants
ndashdA =
dB = k2 [A] [B] (213)
dt dt
where
k2 is the second-order rate constant and
A and B are the concentrations of the two reactants
In a simple case if [A] = [B] each having the same molar concentration or two [A] molecules react the rate of the reaction can be expressed as
ndashdA = k2 [A]2 (214)
dt
The integrated form of eq (214) is
36
1 ndash
1 = k2t (215)
[A] [A0]
or
1 =
1 + k2t (216)
[A] [A0]
The rate constant k2 for a second-order reaction can be determined from the slope of a linear plot of 1 [A] versus t (Fig 23)
The t12 of a second-order reaction is inversely proportional to the initial concentration of the reactant and the rate constant (Table 21)
Examples
1 Hydrolysis of esters in alkaline solution (Pugh 2002)
2 Thermolysis of formylmethylflavin in acid solution (Ahmad and Vaid 2008)
3 Photolysis of formylmethylflavin in organic solvents (Ahmad et al 2006)
Fig 23 Second-order plot of 1[A] versus time
236 Determination of Reaction Order
The order of a reaction can be determined by the following methods
2361 Substitution method
The concentration data obtained on the degradation of a drug at various time intervals during a kinetic study are substituted in the integrated forms of the equations for zero- first- and second-order reactions and the values of the rate constant k are calculated The reaction is of the order for which the rate equation gives a constant value of k within the experimental error limits
2362 Graphical method
The graphical method of the determination of the order of a reaction involves the plotting of concentration or a function of concentration data for the reactant A versus t A linear plot of A versus t represents a zero-order reaction a linear plot of log A versus t represents a first-order reaction and a linear plot of 1[A] versus t represents a second-order reaction
2363 Half-life method
The half-life period (t12) of a reaction is expressed as
37
1
(217) [A]nndash
1
where
n is the order of reaction For a second-order reaction it is assumed that A = B
For a reaction carried out at two different initial concentrations A1 A2 the t1frac12 and t2 frac12 are given by the relation
t1frac12 =
A2 nndash1 (218)
t2 frac12 A1
Rearranging and converting to the log form gives
n = log [t1frac12 t2 frac12]
+ 1 (219) log [A2A1]
The t12 values are determined from plots of A versus t at two different concentrations and finding the values of t at A12 and A22 The substitution of the values of t12 and the initial concentrations in eq (219) gives the order of reaction
Table 21 Order of reaction half-life and shelf-life equations
Order Integrated rate equation (linear form)
Units of k
Half-life (t12) Shelf-life (t90)
Zero A = A0ndashk0t
conc timendash1
(eg Msndash
1)
05 A0 k0 = A0 2k0 01 A0 k0
First log A = log A0ndashk1t 2303
timendash1
(eg sndash1)
2303 log A0
k1 050A0
2303 log
A0
k1 090A0
Second 1 A = 1 A0 + k2t
concndash1 timendash1
(eg Mndash
1sndash1)
1 A0 k2 ndash
24 Complex Chemical Reactions
The degradation reactions of many drugs may not follow zerondash firstndash or second-order kinetics These reactions may include more than onendashstep with the same or different pathways and could be complex involving reversible parallel or consecutive reactions
241 Reversible First-Order Reaction
A reversible first-order reaction may be represented as
A Bk
k
Where k and kˊ are the first-order rate constants for the forward and reversible reactions respectively
If the initial concentration of A is a units and that at time t is (andashx) units the concentration of B at time t is x units
38
The net rate of reaction at time t is expressed as
dx = k (andashx) ndash kˊx (220)
dt
At equilibrium
dx = 0 (221)
dt
and
k (andashxe) = kˊxe (222)
where xe is the value of x at equilibrium
therefore
k = K
xe (223)
k andashxe
where K is the equilibrium constant of the reaction and can be calculated from the analytical data (concentration) as a function of time Substituting the value of kˊ obtained from eq (222) into eq (220) gives
dx = k (andashxe) ndash
kxe (andashxe)
dt xe
= ka
(xe ndashx) (224) xe
Integrating eq (224) between the limits of t = 0 and t = t and x = 0 and x = x gives
kat = ln
xe (225)
xe xendash x
It is seen from eq (222) that kaxe = k + kˊ and substitution of this value is eq (225) gives
(k + kˊ)t = ln
xe xendash
x
and
A graph of t versus log (xendash x) gives a straight line of slope ndash 2303 (k + kˊ) which can be used to calculate the values of k and kˊ using the values of equilibrium constant (K) for the reactions (Griffiths and Thomas 1963) In view of the complexity of reversible reactions the solution to a problem is presented
2411 Example of calculation of equilibrium constant and rate constants for a reversible first-order reaction (Griffiths and Thomas 1963)
t = 2303
log
xe
(k + kˊ)
xendash x
t = 2303
log xendash 2303
log (xendash x) (226) (k + kˊ)
(k + kˊ)
39
Problem
The acid catalyzed conversion of a hydroxyl acid into lactone has been carried out in 01 M HCl solution at 20degC The initial concentration of the acid was 1820 units and the concentration of the lactone as a function of time was
Time (min) 0 20 35 50 65 80 100 infin
Lactone conc (units) 0 240 365 491 609 710 805 1330
Calculation
A graph of t versus log (xendash x) gives a straight line with a slope
Form the
experimental data xe = 1330 and a = 1820
since
k = K = 271 and k = 271 k
k
substituting the value of k in eq (227) gives
371 kˊ = 960 times 10ndash3 minndash1
kˊ = 259 times 10ndash3 minndash1 (first-order rate constant for the forward reaction)
k = 701 times 10ndash3 minndash1 (first-order rate constant for the reversible reaction)
and
K = 271 (equilibrium constant for the reaction)
Example
Hydrolysis of triazolam in aqueous solution (Konishi et al 1982)
242 Parallel Reactions
Many drugs degrade simultaneously by two or more pathways The major reaction pathway depends on the experimental conditions
2421 Parallel reactions involving the formation of two products
Consider the degradation of a molecule A into products B and C by parallel first-order reactions
where
k1 and k2 are the rate constants for the formation of the products B and C respectively
The rate of the reactions can be expressed as
ndash2303 = ndash240
(k + kˊ)
and k + kˊ = 960 times 10ndash3 minndash1 (227)
and the equilibrium constant K = xe
= 1330
= 271 a ndash xe 490
40
ndashd[A] = k1A + k2A = (k1+ k2) [A] = kobs (228) dt
where kobs is the overall rate constant and is the sum of the rate constants k1 and k2 for the individual reactions Using the concentration of the products B and C the values of the two rate constants can be determined
k1 =
[B] (229)
k2 [C]
kobs = k1 (1 + [C] [B]) = k2 (1 + [C] [B]) (230)
Examples
1 Simultaneous photolysis and photoaddition reaction of riboflavin in aqueous solutions (Ahmad et al 2004b)
2 Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution (Ahmad et al 2010)
2422 Parallel reactions involving the formation of three products
Consider the degradation of a molecule A into products B C and D involving parallel first-order reactions
The method of calculation involved in the determination of the first-order rate constants k1 k2 and k3 for these reactions has been reported (Frost and Pearson 1964 Ahmad et al 2016b)
Considering A B C and D to represent the corresponding concentrations during the reactions and A0 as the initial concentration the overall rate of the reaction can be expressed as
ndashdA = k1A + k2A + k3A = (k1 + k2 + k3) A (231)
dt
= kobs A
kobs = k1 + k2 + k3
and
ln (A0)
= kobs t (232) (A)
or
A = A0 endashkt (233)
The reaction is simple first-order as far as the loss of A is concerned
dB = k1A = k1 A0 endashkt
dt
and
B =
ndashk1 A0 endashkt + constant kobs
41
or
B = B0 + (k1 A0k) (1 ndash endashkt) C = C0 + (k2 A0k) (1 ndash endashkt) D = D0 + (k3 A0k) (1 ndash endashkt)
(234)
If
B0 = C0 = D0 the equations simplify and CB = k2k1
and
DB = k3k1 (235)
or
B C D = k1 k2 k3
The product concentrations occur in constant ratio to each other These are independent of the time and the initial concentration of the reactant and can be used for the calculation of the three rate constants
Examples
1 Liquid-phase pyrolysis of α-pinene (Fuguitt and Hawkins 1947)
2 Photodegradation reactions of riboflavin in aqueous solution(Ahmad et al 2016b)
243 Consecutive Reactions
The simple form of a consecutive reaction can be expressed as
A B C (236)
Where k1 and k2 are the first-order rate constants for the degradation of A to B an intermediate leading to the formation of C as the final product
The rate of degradation of A is given by the eq (237)
ndashd[A] = k1 [A] (237)
dt
The rate of change of [B] is expressed by the eq (238)
ndashd[B] = k1 [A] ndash k2 [B] (238)
dt
and the rate of formation of [C] by eq (239)
ndashd[C] = k2 [B] (239)
dt
The integrated form of eq (237) is
[A] = [A0] endashk1t (240)
A combination of eq (238) and eq (239) gives
ndashd[B] = k1 [A0] endashk1t ndash k2 [B] (241)
dt
[B] = [k2 ndash k1]
(endashk1t ndash endashk2t) (242) k1[A0]
Since
[A0] = [A] + [B] + [C] (243)
k1 k2
42
[C] = [A0] ndash [A] ndash [B] (244)
or
[C] =
[A0] 1+ 1
k2 endashk1t ndash endashk2t) (245) [k1 + k2]
Using the Eqs (240) (242) and (245) the values of the rate constants k1 and k2 and the concentration of the final product C can be obtained
Example
Effect of borate buffer on the photolysis of riboflavin in aqueous solution (Ahmad et al 2008)
244 Enzyme Catalyzed Reactions
Enzyme catalyzed reactions occur in biological system and proceed as follows
1 Formation of a complex between the enzymes (E) and the substrate (S)
E + S ESk
k
(246)
2 Breakdown of the complex to form the products (P) and regeneration of the enzyme
ES P + Ek
(247)
These reactions can be described by the application of Michaelis-Menton equation Consider a fraction of enzyme molecules (α) that is involved in the formation of the complex The rate of complex formation (eq (246) forward reaction) would be proportional to the concentration of the free enzyme (1ndashα) [E]0 and also to the concentration of the substrate
where [E]0 is the total concentration of the enzyme
Therefore
v = k (1ndashα) [E]0 [S] (248)
The rate of the reverse reaction is proportional to the concentration of complex (α) Therefore
v = k α [E]0
At equilibrium
v = v
and
k (1ndashα) [E]0 [S] = k α [E]0
Therefore
α
=
k
[S] (249) (1ndashα)
k
Since k k = K equilibrium constant for the reaction (eq (246)) eq (249) can be expressed as
α
=
k
[S] (249) (1ndashα)
k
α = K [S]
(250) 1 + K [S]
43
Assuming that the reaction (eq (249)) is quite slow for the equilibrium (eq (248)) to be undisturbed the rate of reactions v˶ being proportional to the concentration of the complex would be
v˶ = k˶ α [E]0
=
k˶ K [S] [E]0
1 + K [S]
=
k˶ K [S] [E]0 (251)
Km + [S]
where Km = 1K and is called Michaelis constant It is the dissociation constant of the enzyme-substrate complex
Eq (251) may be rearranged as
v˶ (Km + [S]) ndash k˶ [S] [E]0
Therefore
v˶ =
k˶ [E]0 ndash
v˶ (252)
[S] Km Km
A plot of v˶ [S] versus v˶ should be a straight line of slope ndash 1 Km The intercept on the vt
axis is v˶ [E]0 the rate when α = 1 It indicates the maximum rate when in the presence of a high concentration of the substrate the enzyme is completely in the complex form Under these conditions the rate of the reaction if proportional to the concentration of the complex is independent of substrate concentration and attains a limiting value (Griffiths and Thomas 1963)
25 FACTORS AFFECTING DRUG DEGRADATION REACTIONS
In the evaluation of the stability of drug substance and drug products it is necessary to consider the factors that affect the rate of degradation under various conditions and hence the shelf-life of the product This information could be useful in achieving the stabilization of the product
251 Temperature
Collision between molecules initiates a chemical reaction and higher the number of collision per unit time higher is the rate of reaction The number of collisions increases with an increase in temperature and hence the rate of reaction A two to three times increase in the rate of many reactions with a 10 degree increase in temperature has been observed The energy of activation (Ea) is the minimum amount of energy required for a reaction to occur In drug products an increase in temperature T leads to an increase in degradation The relationship between T and the rate constant k for the degradation of a drug is given by the Arrhenius equation
k = AendashEa RT (253)
or ln k = ln A ndash Ea RT (254)
where
k is the reaction rate constant of any order
A is the frequency factor
Ea is the activation energy
R is the gas constant (8314 J molndash1Kndash1) and
T is the absolute temperature in K
44
A plot of ln k versus the reciprocal of T would be linear with a slope equal to ndashEaR and an intercept on the vertical axis equal to ln A This plot can be used to determine the rate constant for the degradation of the drug at any temperature (eg 298 K) and hence the product shelf-life at room temperature (25degC)
The value Ea can also be calculated by determining k at two temperatures T1 and T2 using the equation
log k2
= Ea
(T2 ndash T1)
(255) k1 2303R T1 T2
Eq (255) can be applied to the determination of the rate constant at one temperature using the values of Ea and the rate constant at another temperature
A study of the thermal degradation 78-dimethyl-10-formylmethylisoalloxazine a riboflavin analog in acid solution at 40ndash60degC has been conducted The values of activation energy (Ea) and the frequency factor (A) for the reaction have been determined as 150 kcalmol (628 kJmol) and 243times1010 sndash1 respectively (Ahmad and Vaid 2008)
252 Q10 Values
Connors et al (1986) introduced the Q10 method to determine the shelf-lives of drugs stored at different temperatures The method can be used to estimate the effect of 10degC rise in temperature on the degradation of drugs The Q10 is defined as ldquothe factor by which the rate constants increase with a 10degC increase in temperaturerdquo and is expressed as
Q10 =
k(T1 + 10) (256)
k T1
It is related to the activation energy Ea
Q10 = exp ndash
Ea 1
ndash
1
(257) R
T + 10
T
Thus Q10 is directly proportional to Ea and is inversely proportional to temperature Using eq (257) the Q10 value can be calculated from the known value of Ea
According to Connors et al (1986) it is assumed that Ea is constant and would be the same for any interval of temperature (for example 20ndash30degC) The Ea values for drug degradation reactions are usually in the range of 12ndash24 kcalmole The values of Ea corresponding to three values of Q10 are given in Table 22
Table 22 Q10 factors for 10deg interval and Ea values
Q10 (20ndash30degC) Ea (kcalmol) kJmol
20 122 508 30 194 808 40 245 1021
The values of Q10 = 2 3 or 4 represent low average and high estimates of Q10 when Ea is unknown and show that the rate of degradation of the majority of drugs increases by a factor of two to four for a 10o increase in temperature in the range of 20ndash30degC
For a given change in temperature ΔT = T2 ndash T1 Q ΔT can be calculated as
Q ΔT =
k (T ndash ΔT) = Q10
(ΔT10) (258) kT
If the shelf-life at one temperature T1 (t90 (T1)) is known the shelf-life at a second temperature can be calculated as
t90 (T1) = ak (T1) (259)
45
where a is a constant depending on the order of reaction
Since
T2 = T1 + ΔT t90 (T2) = a k (T1 + ΔT) (260)
and combining this with eq (258)
t90 (T2) = a k T1
Q10 (ΔT10) (261)
Since
t90 (T1) = a k(T1)
t90 (T2) = t90 (T) Q10 (ΔT10) (262)
2521 QΔT calculation
1 Calculate the factors by which rate constants may change for (a) a 20 to 40degC temperature change and (b) a 20 to 0degC temperature change
Solution
Apply eq (258)
(a) Q + 20 = Q102010
= 40 90 160 for Q10 = 2 3 4 respectively
The values indicate that the rate increases between 4-fold and 16-fold probably with an average increase of about 6-fold
(b) When ΔT = ndash20
Q ndash20 = Q10ndash2010
= 14 19 116 for Q10 = 2 3 4 respectively
The above values show that the rate decreases to between 14 and 116 of the initial rate
2522 Shelf-life calculation
The shelf-life of a reconstituted product is 100 h on storage in a refrigerator (5degC) What is the shelf-life if the product is stored at room temperature (25degC)
Solution
Apply eq (262)
t90 (25) = 1002 (25ndash5)10 = 25 h
253 Nonisothermal Prediction of Rate of Degradation
The evaluation of the stability of drugs can also be carried out by nonisothermal kinetics (Hadjiioannou et al 1993) The degradation rates are obtained by conducting an experiment in which the temperature is programmed to change at a predetermined rate The temperature and time are related as
1T = 1T0 + αt (263)
where
T0 is the initial temperature and α is a reciprocal rate constant
The Arrhenius eq (255) for time 0 and time t can be expressed as
log kt = log k0 + Ea
(T2 ndash T1)
(264) 2303R T1 T2
46
Substitution of eq (263) after rearrangement of eq (264) gives
log kt = log k0 + Ea
(αt) (265) 2303R
As temperature is a function of time t kt is determined by a change in a range of temperature The slope of the line for eq (265) is ndashEaα 2303 and the intercept at time zero is log k0 Using the values of k0 and Ea and substitution of these values into the Arrhenius equation (eq (264)) would give the value of the rate constant at room temperature The method of programmed temperature is used for the prediction of shelf-lives of drug products
254 pH
The pH of a solution has great influence on the rate of hydrolytic degradation reactions of drugs in liquid dosage forms Several studies have been conducted to evaluate the effect of pH on the stability of drugs (Connors et al 1986) and to determine the optimum pH range for the stabilization of the product The influence of pH on the hydrolysis of drugs is due to the catalytic effect of H+ and OHndash ions (specific acid-base catalysis) or different cationic and anionic buffer species (general acid-base catalysis)
The effect of pH on the rate of degradation of a drug can be expressed in terms of ratendashpH profiles These profiles can be used to determine the pH of maximum stability (pHmax) of the drug in a liquid dosage form The different types of ratendashpH profiles for the degradation of drugs are reported in Table 23 (Connors et al 1986)
Table 23 RatendashpH profiles for the degradation of drugs
Type of profile Interpretation Relationships
V-shaped Specific acid and base catalysis pHmin = frac12 pKw + frac12 log kH+ kOHndash Sigmoid curve One ionizable group affecting the
rate pHinft = pKa (for k vs pH plot)
Bellndashshaped curve
Two ionizable groups affecting the rate
pHmax = frac12 (pK1 + pK2)
255 Catalysis
2551 Specific acidndashbase catalysis
The degradation rate constant kobs for a specific acidndashbase catalyzed reaction involving H+ and OHndash ions can be expressed as
kobs = k0 + kH+ [H+] + kOHndash [OHndash ] (266)
where
k0 is the rate constant of the uncatalyzed reaction
kH+ is the rate constant for the specific acidndashcatalyzed reaction and
kOHndash is the rate constant for the specific basendashcatalyzed reaction
The specific acid-base catalyzed reactions are second-order reactions However at fixed pH where H+ and OHndash ions are constant the reaction apparently follows first-order kinetics
A plot of kobs versus pH of the solution (ratendashpH profile) for the specific acid-base catalyzed photodegradation of the fluoroquinolone moxifloxacin is shown in Fig 24 (Ahmad et al 2014a) The values of rate constants in the alkaline range are nearly twice compared to those determined in the acid range indicating that OHndash ions exert a greater catalytic effect on the reaction than that of the H+ ions The kobs has a minimum value at pH 75 at which the drug is most stable
47
Fig 24 RatendashpH profile for photodegradation of moxifloxacin in aqueous solution
The ionization of a drug in aqueous solution may have considerable effect on the rate of degradation of a drug For example riboflavin (vitamin B2) is a dipolar molecule with pKa values of 17 and 102 In the acid pH range the photolysis of protonated riboflavin is catalyzed by H+ ion and in the alkaline pH range the anionic form of the molecule is subjected to degradation by OHndash ion catalysis Riboflavin shows a bell-shaped log kndashpH profile to exhibit the variations in the rate as a function of pH (Fig 25) (Ahmad et al 2004a)
Fig 25 RatendashpH profile for the photolysis of riboflavin in aqueous solution
Cyanocobalamin (vitamin B12) undergoes photolysis by zero-order kinetics in aqueous solution It has a pKa values 35 and its protonated form is degraded faster than the neutral form which is stable Thus a pH range of 6ndash7 is most suitable for the stability of cyanocobalamin in vitamin preparations (Fig 26) (Ahmad et al 1992) Such ratendashpH profiles are necessary to determine the pH range for the optimum stability of drugs in liquid dosage forms
00
100
200
300
400
500
600
700
00 20 40 60 80 100 120 140
pH
kob
stimes
10
4 (m
in-1
)
00
40
80
120
160
200
240
280
320
360
00 20 40 60 80 100 120 140
pH
ko
bstimes
10
2 (m
in-1
)
48
Fig 26 RatendashpH profile for the photolysis of cyanocobalamin in aqueous solution
2552 General acid-base catalysis
Buffers are normally used to maintain the desired pH of liquid dosage forms However the buffer species may act as proton donors (eg H2PO4
ndash) or proton acceptors (eg HPO42ndash) and thus
catalyze the degradation of drugs present in the formulation It is therefore necessary to choose a buffer system that has no or minimum effect on the stability of the drugs General acidndashbase catalysis refers to the catalysis of drug degradation that is carried out by the acidic or basic species other than the H+ or OHndash ion In buffer catalyzed reaction the activation energy is lowered which leads to a greater number of collisions of molecules to accelerate the rate of reaction
The kobs for a general acidndashbase catalyzed reaction involving the monoanion or the dianion or both anions of H3PO4 in the degradation of a drug can be written as
kobs = k0+kH+[H+]+kH2O+kOH
ndash[OHndash]+kH2PO4ndash[H2PO4
ndash]+kHPO42ndash[HPO4
2ndash] (267)
= k0 + kˊ [B]T
where
k0 kH+ and kOH
ndash are the rate constants as described in eq (267)
kH2PO4ndash is the rate constant for the reaction catalyzed by H2PO4
ndash ions
kHPO42ndash is the rate constant for the reaction catalyzed by HPO4
2ndash ions
kˊ is the overall rate constant for the reaction catalyzed by all the species and
BT is the total buffer concentration
The values of kH2PO4ndash and kHPO42ndash can be calculated by the method described by Florence
and Attwood (2006)
A plot of kobs versus BT gives an intercept of k0 and a slope of kˊ The values of buffer catalyzed rate constants can be determined by rearrangement of eq (267) in a linear form
kˊ =
(kobs ndash k0) =
kH2PO4ndash [H2PO4
ndash] + kHPO4
2ndash
[BT ndash [H2PO4ndash
] (268)
BT BT BT
Another plot of kˊ versus the fraction of acid buffer component [H2PO4ndashBT] will give an
intercept at [H2PO4ndashBT] = 0 which is equal to kHPO4
2ndash The value kˊ at [H2PO4ndashBT] = 1 gives the rate
constant kH2PO4ndash Thus the catalytic effect of the individual buffer species on the rate of degradation
of a drug can be evaluated The phosphate acetate and carbonate buffer catalyzed photodegradation reactions of riboflavin have been reported (Ahmad et al 2005 2011 2014b)
000
060
120
180
240
300
360
420
00 20 40 60 80 100 120 140
pH
ko
bs
times10
7 (m
in-1
)
49
256 Ionic Strength Effect (Primary Salt Effect)
The rate of a drug degradation reaction involving two ionic species A and B with charges ZA and ZB respectively is affected by other ionic species such as electrolytes (eg NaCl) present in the solution The effect of ionic strength on the rate of a reaction can be expressed as
log k = log k0 + 2 AZAZBradicmicro (269)
where
k0 is the rate constant in an infinitely dilution solution (micro = 0)
A is a constant for a given solvent and temperature (eg 05 for water at 25degC) and micro is the ionic strength of the solution
A plot of log k versus radicmicro gives a straight line with a slope of 102ZAZB (for water at 25degC) and an intercept of log k0 An increase in the ionic strength would decrease the rate of reaction between oppositely charged ions and increase in the rate of reaction between similarly charged ions In the case of barbituric acid the degradation in alkaline solution occurs by the attack of OHndash ions on the monoanion species of the molecule An increase in the ionic strength of the solution leads to an increase in the rate of degradation (Garrett et al 1971) If one of the reactants is a neutral molecule ZA ZB = 0 In this case the rate constant should be independent of the ionic strength in dilute solutions
Eq (269) can be applied to the reactions conducted at an ionic strength less than 001 If the ionic strength is higher than this value (ie up to 01) a modified form of eq (269) can be used for drug degradation reactions
log k = log k0 = 2 QZAZB radicmicro1 + radicmicro (270)
Studies of the effect of ionic strength on the degradation rate of benzyl penicillin (Carstensen 1970) carbencillin (Zia et al 1974) betamethasone-17 valerate (Khattak et al 2012) and riboflavin (Ahmad et al 2016b) have been reported
257 Solvent
Solvents can influence the rates of degradation of drugs in liquid dosage forms since these may contain mixtures of water and organic solvents such as ethanol propylene glycol and glycerin The organic solvents are often used to increase the solubility of drugs and in certain cases to enhance the stability of the formulations The addition of organic solvents may alter characteristics such as the dielectric constant (a measure of solvent polarity) and the viscosity of the medium Variations in the dielectric constant of a solvent can results in a change in the free energy ΔG and hence the rates of the reaction Reactions between ions and dipoles are facilitated by the solvents of high dielectric constant These involve the formation of polar intermediate states and thus proceed with an enhanced rate of reaction
The effect of solvent dielectric constant on the rate of a reaction can be expressed by eq (271)
log kobs = log kε = infin ndash KZAZB
(271) ε
where
kobs is the rate constant of the reaction
kε = infin is the rate constant of the reaction in a solvent of infinite dielectric constant
K is a constant for a given reaction at a given temperature
ε is the dielectric constant of the reaction and
ZAZB are the charges on A and B ions
50
The rate constant k increases with an increase in the dielectric constant for ions of same charge and decreases for ions of different charge
A plot of log kobs versus 1ε should be linear with a positive slope for reactant ions of opposite sign and a negative slope for reactant ions of like signs
A decrease in dielectric constant results in a decrease in the rates of anion-dipole reactions and an increase in the rates of cation-dipole reactions For example the rate constant for the hydrolysis of chloramphenicol in water-propylene glycol mixtures has been found to increase with a decrease in the dielectric constant due to H3O+ ion-dipole reaction in the presence of perchloric acid (Marcus and Teraszka 1959)
Several studies have been conducted to evaluate the effect of solvents on the kinetics of degradation of drugs including riboflavin (Ahmad et al 2015a) norfloxacin (Ahmad et al 2015b) levofloxacin (Ahmad et al 2013) moxifloxacin (Ahmad et al 2014a) β-lactams antibiotics (Hou and Poole 1969) aspirin (Bakar and Niazi 1983) and indomethacin (Ghanem et al 1979)
258 Oxygen
2581 Oxidizable drugs
Many drugs are susceptible to oxidation and undergo degradation in solid or liquid dosage forms in the presence of oxygen during processing or storage Examples of these drugs include 5-aminosalicylic acid ascorbic acid captopril cholecalciferol epinephrine hydroquinine fumagillin hydrocortisone ergocalciferol 6-mercaptopurine methyldopa morphine phenylbutuzone prednisolone promethazine spironolactone sulpyrine tetrazepam vitamin A and sulfides (Yoshioka and Stella 2000 Connors et al 1986) Antibiotics steroids vitamins fats and oils also undergo oxidative degradation by molecular oxygen Molecular oxygen in the ground state exists as a diradical or in the triplet state (3O2) It can be excited by UV light to form the singlet state (1O2)
triplet state singlet state
(272)
Singlet oxygen is a strong oxidizing agent and is more reactive than triplet oxygen It is
involved in many photosensitized oxidation reactions (Smith et al 1973)
hv
51
Example
Oxidation of ascorbic acid (Blaug and Hajratwala 1972)
Protection from oxygen can be achieved by storing the drug products in an anaerobic atmosphere by purging the solution with nitrogen addition of antioxidants and removal of metal ions that initiate catalytic reaction An oxidizable drug may be stabilized by the use of a compound of lower reduction potential Eo than the drug The oxidative degradation of a drug may be minimized by adjusting the pH of the medium to a value where a reversible redox process may occur Solid dosage forms eg tablets can be protected from oxygen by film coating and suitable packaging
2582 Oxidation reactions
The majority of drugs exist in the reduced state and are thus susceptible to oxidation The absorption of UV and visible light may lead to photodegradation The chemical and photooxidation reactions involve one electron change in the molecule The oxidation-reduction reactions occur simultaneously and involve transfer of electrons For example the oxidation of iron can be expressed by eq (273)
Fe2+ Fe3+ + endash (273)
In organic compounds the oxidation state of carbon atom is given by the number of bonds between carbon and oxygen The oxidation state of carbon compounds increases with the number of these bonds Consider the oxidation of methane
CH4 CH3OH CH2O HCOOH CO2 (274)
The oxidation of hydroquinone to quinone in aqueous solution involves the reaction of the ionized form of the molecule depending on the pH of the solution (Connors et al 1986)
OH
OH O-
O-
+ 2HO2
O
O
+H2O2
OH
O
O
+ H2O
(275)
The mechanism of oxidation of sodium sulfite (an antioxidants) in the presence of a metal ion (M+) catalyst involves several steps and is described by Connors et al (1986) as follows
SO32ndash + M+ SO3
ndash + M (276)
SO3ndash + O2 SOndash
5 (277)
SOndash5 + HSO3
ndash HSO5ndash + SO3
ndash (pH le 7) (278)
SOndash5 + SO3
2ndash SO52ndash + SO3
ndash (pH le 7) (279)
SO3 2ndash + HSO5
ndash HSO4ndash + SO4
2ndash (pH le 7) (280)
SO3 2ndash + SO5
2ndash 2SO42ndash (pH le 7) (281)
SO3ndash + SOndash
5 S2O6
2ndash + O2 (282)
SOndash5 + inhibitor nonreactive products (283)
where
eq (276) is the initial step of the reaction eqs (277)ndash(279) are the propagation steps eqs (280) and (281) are the oxidation steps giving the ultimate oxidation product SO4
2ndash and eqs (282) and (283) are the termination steps
The pH dependence of the reaction is due to the amount of fractions of SO32ndash and HSO3
ndash ions present at a particular pH
52
259 Surfactant
Surfactants are compounds that are capable of lowering the surface tension or interfacial tension between the two liquids or between a liquid and a solid Surfactants may act as detergents wetting agents emulsifiers foaming agents and dispersants They may inhibit the rates of degradation reactions and thus improve the stability of drugs Several studies have been carried out to evaluate the effect of surfactants on the stability of drugs in pharmaceutical systems Some of these studies are presented as follows
An early study of the effect of surfactants on the rate of hydrolysis of esters using benzocaine has been conducted It has been found that the rate of hydrolysis of benzocaine in alkali-stable nonionic surfactants varies with the concentration of the surfactant The hydrolysis takes place both in the micelle and in the aqueous phase Anioinc and cationic surfactants stabilize the drug to base catalysis with an eighteen-fold increase in half-life in 5 lauryl sulfate solution (Reigelman 1960)
The effect of surfactant micelles on the aqueous stability of β-lactam antibiotics has been studied by determining the apparent binding constants of the micellar-antibiotic complex as a function of solution pH and ionic strength using dynamic dialysis method The interaction of these antibiotics in the nonionic and anionic micelles of polyoxyethylene-23-lauryl ether and sodium lauryl sulfate showed large differences in the binding constants of undissociated and ionized species of pencillins Acid degradation of pencillins is protected in micellar solutions of the above two surfactants (Tsuji et al 1982) The forced degradation of aqueous paliperidone solutions under photolytic stress conditions on exposure to sunlight for 72 h has shown major degradation by HPLC in the presence of cationic and nonionic surfactants at concentration exceeding critical micellar concentration (CMC) (Marothu et al 2015)
The solid lipid nanoparticles (SLN) have been found to undergo enzymatic degradation by pancreatic lipase at different rates in the presence of surfactants The degradation of SLN depends on the length of fatty acid chains in the glycerides and the surfactant used for the production of SLN It has been found that longer the fatty acid chain the slower the degradation The surfactant accelerates (eg cholic acid sodium salt) or hinders (eg Poloxamer 407 a hydrophilic non-ionic surfactant) the degradation of SLN due to steric factors (Olbrich and Muller 1999)
The emulsion stability of surface active (eg phenobarbital) and non surface active (eg benzocaine) drugs in triphasic systems in the presence of the ionic surfactant cetyltrimethylammonium bromide (CTAB) and the nonionic surfactant Brij 97 (polyoxyethylene 10 oleoyl ether) has been studied by droplet size analysis using photon correlation spectroscopy The droplet size of CTABndashstabilized emulsion system has been found to be bigger than that of the Brij 97-stabilized system because of the relatively small dense interfacial packing of the cationic surfactant CTAB forms a complex with the drugs that increases the stability of the emulsion (Chidambaram and Burgess 2000)
2510 Moisture
Moisture present in the surroundings may be adsorbed on the surface of solid drugs or solid formulations and cause dissolution of the active ingredient This may affect the drugs susceptible to hydrolytic degradation for example aspirin an ester and sulfacetamide an amide The hygroscopic content of the solid dosage forms may be detrimental in promoting hydrolytic reactions
Moisture may play the role of a catalyst is drug degradation reactions Water may participate as a reactant in degradation processes such as hydrolysis isomerization or other bimolecular reactions In these reactions the rate of degradation of the drug is a function of the concentration of water H+ ions or OHndash ions and may be expressed as
ndashd[A] = kH
+ [H+] [A] + kH2O [H2O] [A] + kOHndash [OH+] [A] (284)
dt
53
Examples of effect of moisture on the kinetics of degradation of drugs include ascorbic acid (Yamamoto and Kawai 1959) thiamine salts (Yamamoto and Inazu 1959a) aspirin (Yamamoto and Inazu 1959b) ranitidine HCl (Teraoka et al 1993) and vitamin A (Carstensen et al 1966)
Moisture can change the physical characteristics of tablets such as disintegration and hardness and thus may facilitate the degradation of active ingredients (Ahmad and Shaikh 1994a 1994b) Relationships between moisture content and degradation of a drug (Kornblum and Sciarrone 1964) and moisture uptakes of tablets a function of storage time (Ahmad and Shaikh 2003) have been reported
2511 Problems
Zero-Order Reactions
1 The degradation of a dye in liquid preparations follows zero-order kinetics at 25degC The rate of the reaction is 73times10ndash7 absorbance units per min
Calculate
a) The half-life of a preparation with an initial absorbance of 0240 at 450 nm
b) The predicted life of the preparation at 25degC When the absorbance of the solution is 0100
Answer
a) 114 days
b) 133 days
2 The first-order rate constant k1 for the degradation of a drug at pH 50 is 2times10ndash7 sndash1 The solubility of the drug is 1 g100 ml For a suspension of the drug containing 25 g100 ml calculate
a) Zero-order rate constant k0
b) Shelf-life in solution (zero-order dependent)
Answer
a) k0 = 220times10ndash7 g dL sndash1
b) t90 = 132 days
c) t90 = 61 days
First-Order Reactions
3 A drug product (100 mgmL) becomes ineffective after 25 degradation The drug content was found to be 82 mgmL If the drug is degraded by first-order
Calculate
a) The expiration date on the label and
b) The half-life of the product
Answer
a) t75 = 174 months
b) t12 = 00165 month
4 A drug product undergoes degradation by first-order Using the following assay data calculate the rate constant and the half-life
54
Time (month) 0 2 4 6 12 18 24
concentration
100 895 774 680 455 309 210
Answer
a) k = 00651 month
b) t12 = 165 months
Second-Order Reactions
5 The saponification of ethyl acetate by NaOH was carried out at 25degC The initial concentration of ethyl acetate and NaOH were 00100 M The concentrations of NaOH after 50 min was determined as 000600 M Calculate the second-order rate constant and half-life of the reaction
Answer
a) k = 103 Mndash1 minndash1
b) t12 = 971 min
a The reaction of a drug A with a reagent B was carried out at equal concentrations of the reactants The decrease in the concentrations of A was determined spectrometrically as follows
t (s) 0 100 200 300 400 500
[A] times 103
M
500 327 240 192 159 140
Prepare a graph of A versus t and determine the order of reaction using the half-life method
Answer
Second-order reaction
6 The second-order rate constants k2 for the alkaline hydrolysis of aspirin at 30 40 and 50degC are 00572 0106 and 0192 Mndash1 sndash1 respectively What is the activation energy (Ea) in kcal molendash1 and kJ molendash1 and the frequency factor A in sndash1 for the reaction
Answer
Ea = 120 kcal molendash1 or 502 kJ molendash1
A = 267times107 sndash1
7 The first-order rate constant for the degradation of a drug at 80degC was determined as 96times10ndash7 sndash1 If the activation energy Ea for the degradation is 245 kcal molendash1 what is the rate constant at 60degC
Answer
k2 = 118times10ndash7 sndash1
8 The hydrolysis of a drug is independent of pH in the range of 2ndash7 in ortho-phosphate buffer The first-order rate constant in the pH range was determined as 626times10ndash6 sndash1 at 80degC The activation energy Ea of the reaction at pH 60 is 24 kcal molendash1 Calculate the shelf-life at 25degC in ortho-phosphate buffer
55
Answer
t90 = 35 months
Q10 Calculations
9 Calculate the Q10 factors by which the rate constants may change for a change of a 10deg around room temperature (20ndash30degC) for two reactions with activation energies of 120 and 240 kcal molendash1
b) Calculate the factors by which the above rate constants may change for a 25 to 50degC change
Answer
a) Q ΔT = 54
b) Q ΔT = 300
10 The expiration period for a reconstituted product (Q10 = 20) is 72 h when stored in a refrigerator at 5degC Calculate the expiration period when the product is stored at room temperature
Answer
t90 (25deg) = 18 h
11 An aqueous drug solution stored at room temperature (25degC) showed a shelf-life of 10 days Find the shelf-life when the solution is stored at 15degC (cold room) and at 8degC (refrigerator) if the Q10 value is 20
Answer
t90 (15deg) = 20 days
t90 (5deg) = 40 days
The shelf-life will be increased from 10 days to 40 days on storing the solutions in refrigerator
The problems included in this section have been selected from text books (Connors et al 1986 Hadjiioannou et al 1993 Sinko 2011 Florence and Attwood 2006 Loftsson 2014)
56
REFERENCES
Ahmad I Shaikh RH Prediction of shelf-life of packaged paracetamol tablet formulations Pak J Pharmacol 1994a11 53ndash58
Ahmad I Shaikh RH Effect of temperature and humidity on the disintegration time of packaged paracetamol tablet formulations Pak J Pharm Sci 1994b71ndash7
Ahmad I Shaikh RH Effect of moisture on the stability of packaged paracetamol tablet formulations Pak J Pharm Sci 20031613ndash16
Ahmad I Vaid FHM Thermal degradation of 78-dimethyl-10-formylmethylisoalloxazine in acid solution a kinetic study J Chem Soc Pak 20085 688ndash691
Ahmad I Fasihullah Q Vaid FH A study of simultaneous photolysis and photoaddition reactions of riboflavin in aqueous solution J Photochem Photobiol B Biol 2004b7513ndash20
Ahmad I Fasihullah Q Vaid FH Effect of phosphate buffer on photodegradation reactions of riboflavin in aqueous solution J Photochem Photobiol B Biol 2005 178229ndash234
Ahmad I Fasihullah Q Vaid FH Photolysis of formylmethylflavin in aqueous and organic solvents Photochem Photobiol Sci 20065680ndash685
Ahmad I Hussain W Fareedi AA Photolysis of cyanocobalamin in aqueous solution J Pharm Biomed Anal 1992109ndash15
Ahmad I Ahmed S Sheraz MA Vaid FH Effect of borate buffer on the photolysis of riboflavin in aqueous solution J Photochem Photobiol B Biol 20089382ndash87
Ahmad I Ahmed S Anwar Z Sheraz MA Sikorski M Photostability and Photostabilization of drugs and drug products Int J Photoenergy 2016a ID8135608
Ahmad I Fasihullah Q Noor A Ansari IA Ali QN Photolysis of riboflavin in aqueous solution a kinetic study Int J Pharm 2004a280199ndash208
Ahmad I Ahmed S Sheraz MA Vaid FH Ansari IA Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution Int J Pharm 2010390174ndash182
Ahmad I Anwar Z Ahmed S Sheraz MA Bano R Hafeez A Solvent effect on the photolysis of riboflavin AAPS PharmSciTech 2015a161122ndash1128
Ahmad I Anwar Z Ali SA Hasan KA Sheraz MA Ahmed S Ionic strength effects on the photodegradation reactions of riboflavin in aqueous solution J Photochem Photobiol B Biol 2016b157113 ndash119
Ahmad I Sheraz MA Ahmed S Kazi SH Mirza T Aminuddin M Stabilizing effect of citrate buffer on the photolysis of riboflavin in aqueous solution Results Pharma Sci 2011111ndash15
Ahmad I Bano R Sheraz MA Ahmed S Mirza T Ansari SI Photodegradation of levofloxacin in aqueous and organic solvents A kinetic study Acta Pharm 201363221ndash227
Ahmad I Bano R Musharraf SG Ahmed S Sheraz MA ul Arfeen Q Bhatti MS Shad Z Photodegradation of moxifloxacin in aqueous and organic solvents a kinetic study AAPS PharmSciTech 2014a151588ndash1597
Ahmad I Anwar Z Iqbal K Ali SA Mirza T Khurshid A Khurshid A Arsalan A Effect of acetate and carbonate buffers on the photolysis of riboflavin in aqueous solution a kinetic study AAPS PharmSciTech 2014b15550ndash559
Ahmad I Bano R Musharraf SG Sheraz MA Ahmed S Tahir H ul Arfeen Q Bhatti MS Shad Z Hussain SF Photodegradation of norfloxacin in aqueous and organic solvents A kinetic study J Photochem Photobiol A Chem 2015b3021ndash10
Blaug SM Hajratwala B Kinetics of aerobic oxidation of ascorbic acid J Pharm Sci 197261556ndash562
57
Blaug SM Wesolowski JW The stability of acetylsalicylic acid in suspension J Am Pharm Assoc Sci Ed 195948691ndash694
Bakar SK Niazi S Stability of aspirin in different media J Pharm Sci 1983721024ndash1026
Carstensen JT Aron ES Spera DC Vance JJ Moisture stress tests in stability programs J Pharm Sci 1966 55561ndash563
Carstensen JT Rhodes CT Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000
Carstensen JT Kinetic salt effect in pharmaceutical investigations J Pharm Sci 1970591140ndash1143
Carstensen JT Solution kinetics kinetic pH profiles In Carstensen JT Rhodes CT editors Drug Stability Principles and Practice 3rd ed Marcel Dekker New York USA 2000 Chaps 2 and 3
Chidambaram N Burgess DJ Effect of cationic surfactant on transport of surface-active and non-surface-active model drugs and emulsion stability in triphasic systems AAPS PharmSciTech 20002E28
Connors KA Amidon GL Stella VJ Chemical Stability of Pharmaceuticals A Handbook for Pharmacists 2nd ed John Wiley New York USA 1986
Connors KA The study of reaction kinetics J Parenteral Sci Tech 198135186ndash190
De Ritter E Vitamins in pharmaceutical formulations J Pharm Sci 1982711073ndash1096
Edwards IJ The hydrolysis of aspirin A determination of the thermodynamic dissociation constant and a study of the reaction kinetics by ultra-violet spectrophotometry Trans Faraday Soc 1950 46 723ndash735
Florence AT Attwood D Physiochemical Principles of Pharmacy 4th ed Pharmaceutical Press London UK 2006 Chap 4
Frost AA Pearson RG Kinetics and Mechanism John Wiley amp Sons Inc New York USA 1964 Chap 8
Fuguitt RE Hawkins TE Rate of the thermal isomerization of α-pinene in the liquid phase J Am Chem Soc 194769319ndash322
Garrett ER Carper RF Predictions of stability in pharmaceuticals I Color stability in a liquid multisulfa preparations J Am Pharm Assoc Sci Ed 195544515ndash518
Garrett ER Kinetics and mechanism in stability of drugs In Bean HS Beckett AH Carless JE editors Advances in Pharmaceutical Sciences vol 2 Academic Press London UK 1967 Chap 1
Garrett ER Bojarski JT Yakatan GJ Kinetics of hydrolysis of barbituric acid derivatives J Pharm Sci 1971601145ndash1154
Griffths PJE Thomas JDR Calculations in Advanced Physical Chemistry Edward Arnold London UK 1963 Chap 9
Ghanem AH Hassan ES Hamdi AA Stability of indomethacin solubilized system Pharmazie 197934406
Ghosh TK Chemical kinetics and stability In Ghosh TK Jasti BR editors Theory and Practice of Contemporary Pharmaceutics CRC Press London UK 2005 Chap 8
Guillory JK Poust PI Chemical kinetics and drug stability In Banker GS Rhodes CT Modern Pharmaceutics 4th ed Marcel Dekker New York USA 2002 Chap 6
Hadjiioannou TP Christian GD Koupparis MA Macheras PE Quantitative Calculations in Pharmaceutical Practice and Research VCH Publishers New York 1993 Chap 7
58
Hawley LC Van Arendonk MD Expiration dating In Swarbrick J Boylan JC editors Encyclopedia of Pharmaceutical Technology 2nd ed Marcel Dekker New York 2002 pp 1211ndash1219
Hou JP Poole JW β-lactams antibiotics their physicochemical properties and biological activities in relation to structure J Pharm Sci 196960503ndash532
Khattak SR Shaikh D Ahmad I Usmanghani K Sheraz MA Ahmed S Photodegradation and stabilization of betamethasone-17 valerate in aqueousorganic solvents and topical formulations AAPS PharmSciTech 201214177ndash182
Konishi M Hirai K Mori Y Kinetics and mechanism of the equilibrium reaction of triazolam in aqueous solution J Pharm Sci 1982711328ndash1334
Kornblum SS Sciarrone BJ Decarboxylation of p-aminosalicylic acid in the solid state J Pharm Sci 196453935ndash941
Lachman L DeLuca P Kinetics principles and stability testing In Lachman L Lieberman HA Karring JL editors The Theory and Practice of Industrial Pharmacy 3rd ed Lea amp Febiger Philadelphia USA 1986 Chap 26
Laidler KJ Chemical Kinetics 3rd ed Harper amp Row New York USA 1987
Lintner CJ Pharmaceutical product stability In Cooper MS editor Quality Control in Pharmaceutical Industry vol 2 Academic Press New York USA 1973 pp 141ndash238
Loftsson T Drug Stability for Pharmaceutical Scientists Academic Press London UK 2014
Macek TJ Stability problems with some vitamins in pharmaceuticals Am J Pharm 196023150ndash161
Marcus AD Taraszka AJ A kinetic study of the specific hydrogen ion catalyzed solvolysis of chloramphenicol in water-propylene glycol system J Am Pharm Assoc Sci Ed 19594877ndash84
Marothu VK Nellutla A Gorrepati M Majeti S Mamidala SK Forced degradation studies and effect of surfactants and titanium dioxide on the photostability of paliperidone by HPLC Ann Pharm Fr 201573289ndash296
Mollica JA Ahuja S Cohen J Stability of pharmaceuticals J Pharm Sci 197867443ndash465
Olbrich C Muumlller RH Enzymatic degradation of SLN-effect of surfactant and surfactant mixtures Int J Pharm 199918031ndash39
Pugh J Kinetics and product stability In Aulton ME editor Pharmaceutics The Science of Dosage Form Design 2nd ed Churchill Livingstone London UK 2002 Chap 7
Riegelman S The effect of surfactants on drug stability I J Am Pharm Assoc Sci Ed 196049339ndash343
Sinko PJ Martinrsquos Physical Pharmacy and Pharmaceutical Sciences 6th ed Lippincott Williams amp Wilkins Philadelphia PA USA 2011 Chap 14
Smith LL Teng JI Kulig MJ Hill Fl Sterol mechanism XXIII Cholesterol oxidation by radiation induced processes J Org Chem 1973381763ndash1765
Teraoka R Otsuka M Matsuda Y Effects of temperature and relative humidity on the solid-state chemical stability of ranitidine hydrochloride J Pharm Sci199382601ndash604
Tsuji A Miyamoto E Matsuda M Nishimura K Yamana T Effects of surfactants on the aqueous stability and solubility of beta-lactam antibiotics J Pharm Sci 1982711313ndash1318
Waterman KC Adami RC Accelerated aging prediction of chemical stability of pharmaceuticals Int J Pharm 2005293101ndash125
59
Windheuser JJ The Dating of Pharmaceuticals University of Wisconsin Press Madison WI USA 1970
Wigent RJ Chemical kinetics In Hendrickson R editor Remington The Science and Practice of Pharmacy 21st ed Lippincott Williams amp Wilkins Philadelphia PA USA 2006 Chap 19
Yamamoto R Inazu K Studies on the stability of dry preparations VI Relation between atmospheric humidity or the moisture content and stability of diluted preparations of various thiamine salts Yakuzaigaku 1959a19113ndash117
Yamamoto R Inazu K Studies on the stability of dry preparations X Relation between atmospheric humidity and stability of diluted preparations of acetylsalicylic acid [in Japanese] Yakuzaigaku 1959b19117ndash119
Yamamoto R Kawai S Studies on the stability of dry preparations VII Relation between atmospheric humidity and the stability of ascorbic acid sodium ascorbate and their diluted preparations Yakuzaigaku 195919 35ndash39
Yoshioka S Stella VJ Stability of Drugs and Dosage Forms Kuluwer Academic New York USA 2000 Chap 2
Zia H Teharan M Zargarbashi R Kinetics of carbencillin degradation in aqueous solutions Can J Pharm Sci 1974 9112ndash117
Zia H Shalchian N Borhanian F Kinetics of amoxicillin degradation in aqueous solutions Can J Pharm Sci 19771280ndash83
60
61
CHAPTER ndash 3
CHEMICAL STABILITY
31 INTRODUCTION
The stability is an essential quality characteristic of drug products It is considered as the most important factor in relation to a drug substance for development into a therapeutically active dosage form The assessment of the chemical and physical stability of a product is carried out during the preclinical formulation studies process development and packaging evaluation The efficacy and safety of a product is based on the stability characteristics of the active ingredients and excipients
Knowledge of the specific chemical functional groups of a drug molecule may enable the prediction of its degradation pathways and a possible approach to its stabilization The selection of an appropriate packaging system is necessary to ensure the chemical and physical stability of the product during the storage period and use The assessment of the stability of drug substances and drug products is a mandatory requirement by regulatory agencies
The chemical stability of drug products involves the assessment of the chemical integrity and labeled potency of all the ingredients and that any change should be within the specified limits Several accounts of the chemical stability of drug substances and drug products are available including monographs (Connors et al 1986 Carstensen and Rhodes 2000 Yoshioka and Stella 2000 Baertschi 2005 Loftsson 2014) books (Lintner 1973 Racz 1989 Guillory and Poust 2002 Florence and Attwood 2006 OrsquoDonnell and Bokser 2006 Sinko 2011) and reviews (Carstensen 1974 Mollica et al 1978 De Ritter 1982 Grit and Crommelin 1993 Bastin et al 2000 Waterman and Adami 2005 Blessy et al 2014)
32 STUDY OF THE CHEMICAL STABILITY OF A DRUG
The study of the chemical stability of a drug substance requires a consideration of the following factors
The solubility of the drug in aqueous and organic solvents
The spectral characteristics of the drug molecule
The ionization behavior (pKa) of the drug molecule
The sensitivity of the drug to environmental factors excipients and medium characteristics
Chemical degradation pathways
Structural characteristics of chemical degradants
Toxicity of chemical degradants
A validated stability-indicating method for the assay of intact drug and degradants
If a drug undergoes hydrolysis oxidation or photolysis reaction the following sequence of steps is involved in this study
Verification of degradation by a certain mode of reaction using chromatographic and spectrometric techniques Thin layer chromatography (TLC) pattern and UV and visible spectral changes provide an indication of the degradation of the compound
62
Separation isolation purification and characterization of the degradation products using chromatographic (eg HPTLC HPLC electrophoresis) spectrometric (UV-vis FTIR NMR MS GCMS LCMS) and other techniques
Separation of degradation products from the parent compound by appropriate extraction methods and confirmation by HPLC and UV-vis spectrometry The separation may also be achieved directly by HPLC
Development of a specific (stability-indicating) analytical method for the assay of the intact drug in the presence of degradation products and any interfering substances present in degraded solutions
Evaluation of the kinetics of degradation of the drug and determination of its shelf-life (t90)
Establishment of stability protocol for the drug product under specified storage conditions (ie temperature relative humidity light exposure) according to ICH Guidelines
Screening of the degradation products for their potential toxicity
33 CHEMICAL DEGRADATION REACTIONS
The drug substances are chemical entities that possess diverse molecular structures and different functional groups They may undergo degradation reactions in aqueous and organic solvents through various pathways depending upon the factors causing degradation The major modes of drug degradation are
Hydrolysis
Oxidation
Decarboxylation
Elimination
Isomerization
Dimerization
Epimerization
Dehydration
Dehydrogenation
Dehalogenation
These reactions are described as follows
331 Hydrolysis
Hydrolytic degradation in aqueous solution and in liquid dosage forms is among the most common reactions destabilizing the drugs that contain ester amide imide carbamate lactone nitrile and carbohydrate groups A large number of drugs are susceptible to acid andor alkaline hydrolysis such as aspirin paracetamol sulfacetamide indomethacin procaine digoxin riboflavin lincomycin chloramphenicol penicillins cephalosporins and benzodiazepenes The pH of the medium plays an important role in the hydrolysis of drugs (see Section 254)
3311 Hydrolysis of esters
The ester compounds undergo hydrolysis through nucleophilic attack of water or OHndash ions on the ester group
63
Acetylsalicylic acid (Aspirin)
Aspirin (31) is the most common example of the hydrolytic degradation of an ester compound It undergoes hydrolysis in aqueous solution to form salicylic acid (32) and acetic acid (33) The reaction is accelerated with an increase in temperature (Fersht and Kirby 1967)
O
OH
O
OCH3
O
OH
OH
+ CH3
O
OH
H2O
(31) (32) (33)
Procaine
The most important reaction involved in the degradation of procaine (34) is hydrolysis It leads to the formation of 4-aminobenzoic acid (35) and diethylaminoethanol (36) The rate of the reaction is influenced by the ionization of the molecule (pKa 805) (Higuchi et al 1950)
C2H5
NH2
O O CH2 CH2 N
C2H5
H2O
NH2
O OH
+C2H5
OH CH2 CH2 N
C2H5
(34) (35) (36)
3312 Hydrolysis of amides
Compounds containing an amide bond are less susceptible to hydrolysis compared with those containing an ester bond This is because of the fact that the carbonyl carbon of the amide bond has a lower electrophilic character
Paracetamol
Paracetamol (37) is hydrolyzed in aqueous solution to form 4-aminophenol (38) and acetic acid (33) (Koshy and Lach 1961)
NHCOCH 3
OH
H2O
NH2
OH
+ CH3
O
OH
(37) (38) (33)
Sulfacetamide
Sulfacetamide (39) in aqueous solution is hydrolyzed to form sulfanilamide (310) and acetic acid (33) (Meakin et al 1971) Sulfanilamide (310) undergoes oxidation to yield 44rsquo-azobenzenedisulfonamide (311) which is further oxidized to 44rsquo-azoxybenzenedisulfonamide (312) on exposure to light These reactions are accompanied by the formation of a yellow to reddish brown color (Ahmad and Ahmad 1981 1989 Ahmed et al 2016)
64
+ CH3
O
OH
SO2NHCOCH 3
NH2
H2O
H2NO2S
NH2 (39) (310) (33)
H2NO2S
NH2
oxidation
SO2NH2
N
N
SO2NH2
SO2NH2
N
N
SO2NH2
oxidation
O
(39) (311) (312)
3313 Hydrolysis by ring opening
The hydrolysis of a drug molecule by ring opening could occur by the cleavage of the CndashN bond
Riboflavin
Riboflavin (vitamin B2) (313) undergoes base-catalyzed hydrolysis by cleavage of the isoalloxazine ring to give 12-dihydro-67-dimethyl-2-keto-I-D-ribityl-quinoxaline-3-carboxylic acid (β-keto acid) (314) and 67-dimethyl-4-D-ribityl-23-dioxo-1234-tetrahydroquinoxaline (flavor-violet) (315) (Surrey and Nachod 1951 Ahmad et al 1973) The degradation reaction is accompanied by the loss of absorption of riboflavin at 445 nm and is accelerated by an increase in temperature
65
CH2OH
CH2
OCH3N
NNH
CH3
O
OH H
OH H
OH H
CH2OH
CH2
CH3N
N
O
CH3
OH H
OH H
OH H
COOH
CH2OH
CH2
CH3N
NH
O
CH3
OH H
OH H
OH H
O
OH-O2
OH-
(314) (315)
(313)
66
Norfloxacin
Norfloxacin (316) a fluoroquinolone antibacterial agent is susceptible to hydrolytic degradation by piperazine ring cleavage in the alkaline solution to form the products 317 and 318 The reaction occurs in the presence of light (Ahmad et al 2015)
F
O
N
O
OH
CH3
N
NH2
+
hv
OH-
F
O
N
O
OH
CH3
NH
NH3
+
-CH2CH2NH2
F
O
N
O
OH
CH3
NH3
+
(318)
332 Oxidation
The oxidative degradation of drugs is another widely occurring reaction in the presence of oxygen or an oxidizing agent Exposure of the drug to atmospheric oxygen during manufacturing storage or use can adversely affect the drug content by oxidation reactions (see Section 2582) Many drugs undergo oxidation including ascorbic acid vitamin A glucose morphine hydrocortisone methyldopa aldehydes phenols unsaturated compounds thiols phenothiazenes and polyenes The pH of the medium may affect the rate of oxidation reactions as a result of ionization and change in the redox potential of the species involved
Ascorbic acid
The degradation of ascorbic acid (vitamin C) (319) in aqueous solution under aerobic conditions results in the oxidation of the molecule to dehydroascorbic acid (320) The dehydroascorbic acid undergoes hydrolysis to form diketogulonic acid (321) in alkaline solution (Blaug and Hajratwala 1972)
(316) (317)
67
OOHOHC
CH2OHOHOH
oxidation
OOHOHC
CH2OH
O O
HOOC
C
C
CHOH
CHOH
CH2OH
O
O
Hydrolysis
(321)
Morphine
The major degradation reaction of morphine (322) in aqueous solution is oxidation by air and light The oxidation products of morphine include pseudomorphine (noxydimorphine) (323) and morphine N-oxide (324) (Yeh and Lach 1961)
OH
N
OH
O
CH3
O
OH
N
OH
O
CH3
N
O
OH OH
CH3
oxidation
O
(323) (324)
(319) (320)
(322)
68
Phenols
Phenols (325) undergo facile oxidation reactions The hydroxyl group is strongly electron donating to the phenyl ring which is oxidizable Abstraction of the proton gives a stable radical which then reacts with molecular oxygen The deprotonation of phenol at higher pH to the phenolate anion strongly catalyzes auto-oxidation reaction (base-catalyzed auto-oxidation) The phenolate anion is an effective nucleophile that can react with electrophilic species at either the oxygen or the ortho or para positions Phenolic compounds are oxidized in the presence of Fe3+ or Cu2+ ions (Baertschi 2005)
OH O O
CH
O
CH
O
CH-H
OH-
O-
-e- oxidative reactions especially at ortho and
para positions of phenolic compounds
333 Decarboxylation
A drug possessing a carboxyl group may degrade by decarboxylation reaction under certain conditions
4-Aminosalicylic acid
The major degradation reaction of 4-aminosalicylic acid (326) in aqueous solution is decarboxylation which leads to the formation of 3-aminophenol (327) The reaction is faster in the acid medium compared to that of the alkaline medium in which the molecule is in the ionized form (Connors et al 1986)
OH
O OH
NH2
-CO2
OH
NH2
(326) (327)
334 Elimination
Elimination reactions involve the removal of two or more substituent from a molecule either in one step or two steps The one step mechanism is known as E2 reaction (bimolecular) and the two step mechanism is known as E1 reaction (unimolecular)
Trimelamol
Trimelamol (N2N4N6-trimethylol-N2N4N6-trimethylmelamine) (328) a synthetic carbinolamine-containing antitumor drug undergoes degradation by two major pathways One degradation pathway involves the loss of hydroxylmethylene groups by elimination of
(325)
69
formylaldehyde to form the parent compound trimethylmelamine (329) The products have been determined by HPLC to evaluate the kinetics of the reaction (Jackson et al 1991)
CH3
HOH2C CH3
N
CH3
N
N
N
N
CH2OH
N
CH2OH
CH3
H CH3
N
CH3
N
N
N
N
H
N
H
-HCHO
(328) (329)
335 Isomerization
Isomerization reactions involve the transformation of one molecule into another having exactly the same atoms but with a different arrangement
Cephalosporins
Cephalosporins are known to undergo isomerization of the double bond involving the ∆3 position (330) to the ∆2 position (331)
NH2S
N
O
2
3
S1
4
N
X
COOX
O
NH
N
OCH3
NH2S
N
O
2
3
S1
4
N
X
COOX
O
NH
N
OCH3
∆3-isomer ∆2-isomer
isomerization
(330) (331)
336 Dimerization
It is a chemical reaction in which two molecular subunits are joined resulting in the formation of a dimer
Nalidixic Acid
Nalidixic acid (332) undergoes dimerization on thermolysis by decarboxylation to form a dimer (333)
COOH
O
N NCH3
CH2 CH3
-CO2
thermolysis
CH3
CH2
N NO
N NCH3
CH2 CH3
O
CH3
(332) (333)
70
337 Epimerization
The epimerization process involves the changing of one of the chiral center in a molecule to form another molecule called epimer An epimer molecule differs from the other molecule (its diastereomer) by only one chiral center Epimers are not mirror images of each other and have multiple sterogenic centers
Ergotamine
Ergotamine (334) undergoes acid-catalyzed reversible epimerization at Cndash8 and Cndash2prime positions of the molecule (335) in the absence of air and light The epimerization at Cndash8 occurs in the lysergic acid part of the molecule at pH 38 in the temperature range of 30ndash60degC The reaction at Cndash2prime takes place in the cyclic tripeptide part of the molecule at pH 36 in the temperature range of 50ndash80degC (Ott et al 1966) Both isomers are detectable in ergotamine tartrate parenteral solutions
O
N
8
NH
H
H NH O
2
O
N
N
OH
O
H
CH2HCH3
CH3
O
N
8
NH
H
H NH O
2
O
N
N
OH
O
H
CH2HCH3
CH3
(334) (335)
338 Dehydration
Dehydration is a chemical reaction that involves the loss of a water molecule from the reacting molecule
Glucose
Glucose (336) undergoes dehydration reaction to form 5-(hydroxymethyl)-2-furaldehyde (337) on heating with hydrochloric acid (Wolfrom et al 1948)
O
OH OH
CH2OH
OH OH OHOH2C CHO
(336) (337)
Batanopride Hydrochloride
In acidic media (pH 2ndash6) batanopride hydrochloride (338) an antiemetic drug is degraded by intramolecular cyclization followed by dehydration to form 23-dimethylbenzofuran (339) (Nassar et al 1992)
71
O
O
O
CH3
CH3
NHCH2CH2N
H5C2
H5C2
NH2
Cl
ONHCH2CH2N
H5C2
H5C2
Cl
NH2
O
CH3
CH3
(338) (339)
339 Dehydrogenation
Dehydrogenation is a chemical reaction that involves the removal of hydrogen from a molecule
2- Aminofluorene
2-Aminofluorene (340) undergoes oxidative dehydrogenation to 2-nitro-9-fluorenone (341) in acetonitrile using potassium iodide-tert-butyl hydroperoxide (KIndashTBHP) as catalytic system at 80 0C (Kumar et al 2011)
NH2
KI-TBHP
NO2
O
(340) (341)
3310 Dehalogenation
A reaction involving the removal of a halogen atom from a molecule
Norfloxacin
Norfloxacin (316) undergoes defluorination in neutral aqueous solution to form the product (342) (Fasani et al 1999)
N
O
F
N
NH
COOH
CH3
N
O
N
NH
COOH
CH3
hv
-F-
(316) (342)
34 CHEMICAL STABILITYDEGRADATION STUDIES
Several studies have been conducted to evaluate the chemical stability and degradation of drug substances and drug products Some of these studies are presented as follows
341 Aqueous Solution
A kinetic study of the alkaline hydrolysis of 78-dimethyl-10-(formylmethyl)isoalloxazine (FMF) (343) an intermediate product in the photodegradation of riboflavin has been conducted in the pH range 9 to 12 FMF leads to the formation of lumichrome (LC) (344) and lumiflavin (LF) (345) in alkaline solution with second-order rate constants of 0348 and 0063 Mndash1 sndash1 at pH 9 and 0068 and 0132 Mndash1 sndash1 at pH 12 respectively FMF and the hydrolytic products LC and LF have
72
been identified chromatographically and determined by a multicomponent spectrometric method LC and LF were extracted from the degraded solutions with chloroform at pH 20 and determined by a two-component method at 356 and 445 nm FMF was determined directly at 385 nm in the aqueous phase The molar concentrations of these compounds were used to evaluate the kinetics of the reaction (Ahmad et al 1980)
High-performance liquid chromatography (HPLC) has been used to study the chemical stability of 5-aza-2prime-deoxycytidine in the whole pH range It undergoes fast reversible degradation to form N-(formylamidino)-Nprimeβ-D-2-deoxyribofuranosylurea which further degrades to 1-β-D-2prime-deoxyribofuranosyl-3-guanylurea in alkaline solution The kinetics of the degradation reactions has been studied The degradation of 5-aza-2prime-deoxycytidine in alkaline solution is similar to that 5-aza-cytidine The intermediate product in the reaction is most stable in the neutral solution stored at low temperature (Lin et al 1981)
Cefoxitin sodium undergoes specific acid-base catalyzed hydrolysis of the ester group and the β-lactam ring in aqueous solution The apparent first-order rate constants for the hydrolytic reaction at pH 3 to 9 have been determined Under these pH conditions cefoxitin sodium shows about 10 loss in two days at 25degC The amorphous form of the drug is less stable than the crystalline form (Oberholizer and Brenner 1979)
The chemical stability of ranitidine hydrochloride in aqueous solution at different pH values and temperatures has been studied using a HPLC method The percent degradation of the drug increases with a decrease in pH of the medium and an increase in temperature The results indicate that the degradation of ranitidine is a specific acid-catalyzed reaction (Teraoka et al 1993)
CHO
OCH3
CH2
N
NNH
N
CH3
O
OCH3 N
NNH
NH
CH3
O
OCH3
CH3
N
NNH
N
CH3
O
OH-
OH-
(344) (345)
342 Pharmaceutical Preparations
Insulin preparations stored at different temperatures have been found to undergo hydrolytic degradation The degradation is rapid in acid media as a result of deamidation at residue AsnA21 and is slow in the neutral media due to deamidation at residue AsnB3 The degradation rate of insulin at residue B3 varies with temperature and preparation A reduction in B3 transformation has been observed for crystalline insulin compared to that of the amorphous form In certain
(343)
73
crystalline suspensions cleavage of the peptide bond A8ndashA9 takes place The hydrolytic degradation of insulin involves the participation of an imide intermediate in the reaction Preparations containing rhombohedral crystals along with free zinc ions undergo hydrolysis of the peptide chain only (Brange et al 1992a) The storage of insulin preparations at 4ndash45degC leads to the formation of covalent high molecular weight products mainly the covalent insulin dimers In the preparations containing protamine covalent insulin-protamine products are formed The formation of oligo compounds and polymers also takes place at gt25degC by parallel or consecutive reactions Temperature exerts a pronounced effect on the formation of different products in insulin preparations The dimer formation occurs between molecules within hexameric units present in all types of insulin preparations and the formation of dimers is greater in preparations containing glycerol (Brange et al 1992b)
The lyophilized proteins and peptides contain sugars and polyols as bulking agents and lyoprotectants but the reducing sugars have been found to react with proteins The recombinant human relaxin in lyophilized preparations reacts with glucose used as excipient to undergo fast covalent modification The LCMS and tryptic mapping of the protein showed that one degradation pathway involves covalent adduct formation of glucose with the side chain amino groups of the protein (ie Lys and Arg) by Maillard reaction The other pathway leads to Ser degradation from C-terminal of the β-chain of proteins The latter reaction occurs predominantly in the solid state and involves the reaction of glucose with Ser hydroxyl group and hydrolysis of TrpndashSer amide bond through a cyclic intermediate product Mannitol (polyhydric alcohol) and trehalose (nonreducing sugar) do not undergo such reactions with relaxin (Li et al 1996)
A study has been carried out to determine 1) the relation between chemical stability aging state and global molecular motion and 2) the molecular mobility in multicomponent systems It also envisaged to find out whether annealing a glass below its transition temperature (Tg) has any effect on its chemical stability and to determine if the degradation rate couples with global relaxation times determined by calorimetric method andor with T1 and T1rho relaxation times determined by solid state NMR spectrometry In this study the chemical degradation of lyophilized aspartamesucrose and aspartametrehalose (110 ww) preparations has been investigated to evaluate the impact of annealing on their chemical stability by the application of stretched time kinetics The results supported the hypothesis that molecular mobility for structural relaxation is affected by thermal transitions Such an effect is critical for chemical stability and annealing results in the stabilization of the preparations (Luthra et al 2008)
Pseudolatexes of biodegradable polyesters poly (D L-lactide) and poly (ε-caprolactone) are used as aqueous coating material for sustained release dosage forms A study has been conducted out to determine the effect of surfactant temperature pH and particle size on the hydrolytic degradation of these polymers in the form of colloidal dispersions The nonionic surfactant has no effect on the stability of the dispersion Storage of dispersions in unbuffered solution for one year at 5degC showed small changes in molecular weight of the polymers Rapid hydrolytic degradation of the dispersions was observed at 37degC The polymers stored at pH 165 at 37degC underwent enhanced degradation while these were stable at pH 165 at 5degC for 4 months (Coffin and McGinety 1992)
The effect of spray drying and processing conditions on the residual moisture content and biochemical stability of inhalation protein powders has been investigated The mannitol-formulated powders of a humanized monoclonal antibody (anti-IgE) and recombinant human deoxyribonuclease (rhDNase) have been prepared by spray drying and the residual moisture and moisture uptake determined by thermal gravimetric analysis and gravimetric moisture sorption isotherm respectively The main degradation product of the protein the protein aggregate observed on long-term storage was determined by size exclusion HPLC The results showed that spray-dried powders with about 3 moisture equivalent to freeze-dried powder could be obtained by high temperature spray-drying At a RH of air lower than 50 during processing and storage the powders maintain aerosol performance (fine particle fraction) The powders on storage under dried conditions show better long-term biochemical stability of the proteins (Maa et al 1998)
74
The effect of surface charge on the degradation kinetics of methyl paraben used as a model solute in oil-in-water emulsions has been studied The surface charge is varied by adding phosphatidylglycerol (anionic surfactant) or stearylamine (cationic surfactant) to a intravenous lipid emulsion that was stabilized using egg phospholipid The rates of hydrolytic degradation (pH 80) in aqueous phase oil phase interface and aqueous micellar phase have been determined using a four-phase kinetic model The degradation rate in aqueous phases depends on zeta potential as a result of surface charge on the pH of microenvironment of oil drops (surface activity) The rate of hydrolysis of methyl paraben depends on the pH of microenvironment and on the pH of the bulk The hydrolysis rate is inversely proportional to the partition coefficient of methyl paraben The surface charge effect is greater with a small partition coefficient and smaller with a large partition coefficient (Pongcharoenkiat et al 2002)
A study has been conducted to determine the impact of drying methods on the stability of dried vaccine preparations A sucrose-based preparation of a live attenuated virus vaccine of parainfluenza strain as such and that containing a surfactant was dried by freeze drying spray drying and foam drying methods Differential scanning calorimetry specific surface area analysis and electron microscopy were used to characterize the dry powders The preparations were stored at 4 25 and 37degC and the rate constants for degradation were determined The spray dried preparation showed the highest specific surface area (~282 m2gndash1) in the absence of surfactant and the foam dried preparation showed the lowest specific area (~ 01 m2gndash1) in the presence and absence of surfactant Electron microscopic measurements indicated the highest surface coverage in spray dried preparation and lowest in foam dried preparation without surfactant The vaccine showed highest stability at 25 and 37degC in foam dried preparation with surfactant and lowest stability in spray dried preparation without surfactant (Abdul-Fallah et al 2007)
The chemical stability of rabeprazole sodium (proton-pump inhibitor) in simulated intestinal fluid (pH 68) in the presence of certain excipients such as Brij 58 (nonionic surfactant) Poloxamer 188 (nonionic copolymer) Cremophor RH40 (solubilizer) Gelucire 4414 (nonionic surfactant) and PEG 6000 at 37 and 60degC has been studied The main degradation product thioether-rabeprazole has been identified by LCMS and rabeprazole and its degradation product determined by HPLC Rabeprazole degrades by first-order kinetics and the rate constants at 37 and 60degC are 075 and 278 hndash1 respectively without the presence of excipients The addition of excipients has been found to improve the stability of rabeprazole The greatest stabilizing effect has been observed in the presence of Brij 58 which reduced the rate constants for degradation at 37 and 60degC to 022 and 053 hndash1 respectively It has been concluded that the presence of suitable excipients in rabeprazole preparations enhances its stability in intestinal tract resulting in maximum bioavailability (Ren et al 2008)
The effect of pH suspending agents and temperature on the suspensions of ibuprofen powder and microspheres has been studied by an accelerated stability protocol using a HPLC method The suspensions were found to be stable in different suspending agents on storage for a period of 3 months at 23 37 and 45degC The dissolution stability of microspheres prepared from an optimized formulation (17 drug loading) showed that suspensions of ceresine wax microspheres stored at 37degC give faster release of the drug than that at 23degC The microsphere suspensions in syrup stored at 37degC showed faster dissolution rates than those suspended in methyl cellulose This could be due to an interaction between microsphere constituents and syrup Microcrystalline wax microsphere suspensions give better dissolution stability than those of ceresine wax microspheres At higher pH the drug release is faster from suspended microspheres The dissolution stability of microsphere is not significantly affected by the particle size (Adeyeye and Price 1993)
75
REFERENCES
Abdul-Fallah AM Trnong-he V Yee L Pan E Ao Y Kalonica DS Pikal MJ Drying-induced variations in physico-chemical properties of amorphous pharmaceuticals and their impact on stability II Stability of a vaccine Pharm Res 200724715ndash727
Adeyeye CM Price JC Chemical dissolution stability and microscopic evaluation of suspensions of ibuprofen and sustained release ibuprofen-wax microspheres J Microencapsul 199714357ndash377
Ahmad I Bano R Musharraf SG Sheraz MA Ahmed S Tahir H ul Arfeen Q Bhatti MS Shad Z Hussain SF Photodegradation of norfloxacin in aqueous and organic solvents a kinetic study J Photochem Photobiol A Chem 20153021ndash10
Ahmad I Beg AE Zoha SMS Studies on degradation of riboflavin and related compounds II Multicomponent spectrophotometric determination of thermal degradation products of riboflavin J Sci Univ Kar 1973284ndash91
Ahmad I Rapson HDC Heelis PF Phillipis GO Alkaline hydrolysis of 78-dimethyl-10-(formylmethylflavin)isoalloxaine A kinetic study J Org Chem 198045731ndash733
Ahmad T Ahmad I Degradation studies on sulphacetamide eye-drops Part 1 Pharmazie 198136619ndash621
Ahmad T Ahmad I Photo-oxidation of sulphanilamide to azo and azoxy compounds Pak J Pharm Sci 198921ndash5
Ahmed S Anwar N Sheraz MA Ahmad I Stability of sulfacetamide in pharmaceutical preparations a review In Berhardt LV editor Advances in Medicine and Biology Nova Science Publishers Inc New York USA 2016 Chap 8
Baertschi SW editor Pharmaceutical Stress Testing Predicting Drug Degradation Taylor amp Francis New York USA 2005 Chap 3
Bastin RJ Bowker MJ Slater BJ salt selection and optimization procedures for pharmaceutical new chemical entities Org Proc Res Dev 20004427ndash435
Blaug SM Hajratwala B Kinetics of aerobic oxidation of ascorbic acid J Pharm Sci 197261556ndash562
Blessy M Patel RD Prajapati PN Agarwal YK Development of forced degradation and stability indicating studies of drugs A review J Pharm Anal 20144159ndash165
Brange J Langkjaer L Havelund S Voslashlund A Chemical stability of insulin 1 Hydrolytic degradation during storage of pharmaceutical preparations Pharm Res 1992a9715ndash726
Brange J Havelund S Hougaard P Chemical stability of insulin 2 Formation of higher molecular weight transformation products during storage of pharmaceutical preparations Pharm Res 1992b9727ndash734
Carstensen JT Rhodes CT Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000
Carstensen JT Stability of solids and solid dosage forms J Pharm Sci 1974631ndash14
Coffin MD McGinity JW Biodegradable pesudolaxates The chemical stability of poly(DL-lactide) and poly (ε-caprolactone) nanoparticles in aqueous media Pharm Res 19929200ndash205
Connors KA Amidon GL Stella VJ Chemical Stability of Pharmaceuticals A Handbook for Pharmacists 2nd ed John Wiley New York USA 1986
De Ritter E Vitamins in pharmaceutical formulations J Pharm Sci 1982711073ndash1096
76
Fasani E Barberis Negra FF Mella M Monti S Albini A Photoinduced CndashF Bond Cleavage in Some Fluorinated 7-Amino-4-quinolone-3-carboxylic Acids J Org Chem 1999645388ndash5395
Fersht AR Kirby AJ The hydrolysis of aspirin Intramolecular general base catalysis of ester hydrolysis J Am Chem Soc 1967894857ndash4863
Florence AT Attwood D Drug stability Physiochemical Principles of Pharmacy 4th ed Pharmaceutical Press London UK 2006 Chap 4
Grit M Crommelin DJ Chemical stability of liposomes implications for their physical stability Chem Phys Lipids 1993643ndash18
Guillory K Poust RI Chemical kinetics and drug stability In Banker GS Rhodes CT editors Modern Pharmaceutics Marcel Dekker Inc New York USA 2002 Chap 6
Higuchi T Havinga A Busse LW The kinetics of the hydrolysis of procaine J Am Pharm Assoc Sci Ed 195039405ndash410
Jackson C Crabb TA Gibson M Gibson R Godgery R Saunders R Thurston DE Studies on the stability of trimelamol a carbinolamine-containing antitumor drug J Pharm Sci 199180245ndash251
Koshy KT Lach JL Stability of aqueous solutions of N-acetyl-p-aminophenol J Pharm Sci 196150113ndash118
Kumar RA Maheswari CU Ghantasala S Jyothi C Reddy KR oxidative dehydrogenation of 3H-quinazolin-4-ones abd 4H-31-benzoxazin-4-ones via benzylic oxidation and oxidative dehydrogenation using potassium iodide-tert-butyl hydroperoxide Adv Synth Catal 2011353401ndash410
Li S Patapofi TW Overcarhier D Hsu C Nguyen TH Borchardt RT Effect of reducing sugars on the chemical stability of human relaxin in the lyophilized state J Pharm Sci 199685873ndash877
Lin KT Momparler RL Rivard GE High-performance liquid chromatographic analysis of chemical stability of 5-aza-2-deoxycytidine J Pharm Sci 1981701228ndash1232
Lintner CJ Pharmaceutical product stability In Cooper MS editor Quality Control in Pharmaceutical Industry vol 2 Academic Press New York USA 1973
Loftsson T Drug Stability for Pharmaceutical Scientists Elsevier Amsterdam The Netherlands 2014
Luthra SA Hodge IM Utz M Pikal MJ Correlation of annealing with chemical stability in lyophilized pharmaceutical glasses J Pharm Sci 2008975240ndash5251
Maa YF Nguyen PA Andya JD Dasovich N Swerney TD Shire SJ Hsu CC Effect of spray drying and subsequent processing conditions on residual moisture content and physicalbiochemical stability of protein inhalation powders Pharm Res 199815768ndash795
Meakin BJ Tansey IP Davies DJ The effect of heat pH and some buffer materials on the hydrolytic degradation of sulphacetamide in aqueous solution J Pharm Pharmacol 197123252ndash261
Mollica JA Ahuja S Cohen J Stability of pharmaceuticals J Pharm Sci 197867443ndash465
Nassar MN House CA Agharkar SN Stability of batanopride hydrochloride in aqueous solutions J Pharm Sci 1992811088ndash1091
OrsquoDonnell PB Bokser AD Stability of pharmaceutical poducts In Hendrickson R editor Remington the Sciences and Practice of Pharmacy 21st ed Lippincott Williams amp Wilkins Baltimore MD USA 2006 Chap 5
77
Oberholtzer ER Brenner GS Cefoxitin sodium solution and solid-state chemical stability studies J Pharm Sci 197968863ndash866
Ott H Hofmann A Frey AJ Acid-catalyzed isomerization in the peptide part of ergot alkaloids J Am Chem Soc 1966881251ndash1256
Pongcharoenkiat N Narsimhan G Lyons RT Hein SL The effect of surface charge and partition coefficient on the chemical stability of solutes in ow emulsions J Pharm Sci 200291559ndash570
Racz I Drug stability Drug Formulation John Wiley amp Sons New York USA 1989 Chap 2
Ren S Park MJ Sah H Lee BJ Effect of pharmaceutical excipients on aqueous stability of rabeprazole sodium Int J Pharm 2008 350197ndash204
Sinko PJ Chemical kinetics and stability In Martins Physical Pharmacy and Pharmaceutical Sciences 6th ed Lippincott Williams amp Wilkins New York USA 2011Chap 14
Surrey AR Nachod FC Alkaline hydrolysis of riboflavin J Am Chem Soc 1951732336ndash2338
Teraoka R Otsuda M Matsuda Y Effect of temperature and humidity on the solid-state chemical stability of ranitidine hydrochloride J Pharm Sci 199382601ndash604
Waterman KC Adami RC Accelerated aging prediction of chemical stability of pharmaceuticals Int J Pharm 2005293101ndash125
Wolfrom ML Schuetz RD Cavalieri LF Chemical interaction of amino compounds and sugars III The conversion of D-glucose to 5-(hydroxymethyl)-2-furaldehyde J Am Chem Soc 194870514ndash517
Yeh SY Lach JL Stability of morphine in aqueous solution III Kinetics of morphine degradation in aqueous solution J Pharm Sci 19615035ndash42
Yoshioka S Stella VJ Stability of Drugs and Dosage Forms Kluwer Academic New York USA 2000
78
79
CHAPTER ndash 4
PHOTOSTABILITY
41 INTRODUCTION
The sensitivity of many drug substances and drug products to light (British Pharmacopoeia 2016 United States Pharmacopeia 2016 European Pharmacopoeia 2015) can lead to photochemical degradation resulting in potency loss altered efficacy and undesirable biological effects This is an indication of the low quality of manufactured products There are several modes of photodegradation of drugs depending on the chemical structure spectral characteristics and photoreactivity of the compound as well as the irradiation wavelengths and the stress conditions employed The degradation products may be inactive andor toxic to the physiological system The study of photodegradation reactions can provide useful information for the stabilization of drug products The evaluation of the photostability of drugs and drug products is an essential component of formulation development Photostability testing has to be conducted on the drug substances and drug products according to the guideline of International Conference on Harmonization (ICH 1996) This ensures the quality strength and freedom from any phototoxic effects on the administration of photolabile drugs
In the study of the photostability of drugs and their products it is necessary to develop a knowledge of the principles of photochemistry to understand the mode of degradation to interpret the stability data to achieve reliable results and to draw meaningful conclusions The study of the photodegradation reactions of drug substances is a prerequisite to evaluation of the photostability
Several monographs (Albini and Fasani 1998 Tonnesen 2004 Piechocki and Thoma 2007) detailed accounts (Connors et al 1986 Greenhill and McLelland 1990 Moan 1996 Beijersbergen van Henegouwen 19811997 Beaumont 1999 Carstensen 2000 Yoshioka and Stella 2000 Tonnesen 2002 Fasani and Albini 2005 Ahmad and Vaid 2006) and reviews (Sugden 1985 Tonnesen 2001 Boreen et al 2003 Kullavanijaya and Lim 2005 Vinod et al 2015 Ahmad et al 2016a) have been published on the photochemistry photostability phototoxicity photoprotection and photostability testing of drugs and drug products for the benefit of research workers involved in the field
42 PHOTOSTABILITY AND RELATED ASPECTS
421 Photostability
The photostability of a drug may be defined as the response of a pharmaceutical compound or a formulated product on exposure to radiation from sunlight ultraviolet (UV) light or visible light (or UV-visible light) in the solid and liquid state that leads to a chemical or physical change
The response of the drug to light absorption and excitation can be considered in terms of photodegradation (photolysis) reactions through the formation of free radicals or photosensitization reactions by intermolecular energy transfer These reactions involve primary (photochemical) and secondary (chemical) processes to give the final products (Ahmad et al 2016a)
422 Effects of Photoinstability
The photoinstability of a drug may lead to the following changes
4221 Chemical and physical changes
Loss of potency and efficacy
80
Alteration in physical characteristics
Appearance of color turbidity or sediment
Evolution of gas and obnoxious smell
Formation of toxic photoproducts
Photocatalytic effects of metal contaminants
Variation in dissolution profile
Loss of package integrity
4222 Biological effects on administration
Change in bioavailability
Toxicity of photoproducts
Adverse biological reactions
4223 Light induced side effects through interaction with endogenous substances
Phototoxic reactions (chlorpromazine psoralen tetracycline)
Photoallergic reactions (4-aminobenzoic acid sulfonamides thiazides)
Photosensitization reactions (oestrogens chloramphenicol ethanol)
These aspects have been discussed by Tonnesen (2004) Beijersbergen van Henegouwen (1997) Moan (1996) Epstein and Wintroub (1985) Harber et al (1982) Moyal and Fourtanier (2004) and Ahmad et al (2016a)
The pharmacist should advise patients to avoid sunlight and use protective creams to minimize the effects of light exposure
423 Objectives of Photostability Studies
The photosensitivity and photoinstability of a large number of drugs and adjuvants require a knowledge of their photochemical behavior to evaluate
Intrinsic photostability characteristics
Physical and chemical changes on exposure to light
Photodegradation pathways and mechanisms
Prediction of shelf-life of drug products
Need for measures during manufacturing labeling packaging transportation and storage to overcome the effects of light exposure
Need for modification of formulation parameters
Efficacy of stabilizing agents in photostabilization
Designing of appropriate packaging
Light induced biological effects
424 Industrial Awareness on Photostability
There is a need to create awareness of the importance of photostability studies and photostability testing of drugs and drug products among the technical workers of pharmaceutical industry This would motivate them to conduct such studies and testing on their photosensitive products and to adopt appropriate measures in industrial processes to overcome photostability problems This is necessary to safeguard the interest of the consumer
81
In view of the ICH QIB Guideline (1996) it is essential to conduct photostability studies and photostability testing on new drugs and drug products to ensure product quality This requires a knowledge of the following aspects
Solubility of the drug in aqueous and organic solvents
Spectral characteristics of the drug molecule
Sensitivity of the drug molecule to UV and visible light
Mode of photodegradation and nature of photoproducts
A validated stability-indicating assay method to determine the contents of intact drug and photoproducts in degraded material
Drug photoreactivity and stabilization
43 PHOTOCHEMISTRY
Photochemistry is the study of the chemical and physical effects of light absorption and electronic excitation resulting from the interaction of electromagnetic radiations with matter The electronic excitation of molecules takes place by the absorption of light in the UV and visible region
431 Basic Laws of Photochemistry
Grottus and Draper Law It states that only the absorbed light is photochemically active
Stark and Einstein Law It states that a molecule absorbs one quantum of light and from the resulting excited state all the primary processes arise
Noyes et al (1956) have defined the primary photochemical process as ldquoThe primary photochemical process comprises the series of events beginning with the absorption of a photon by a molecule and ending either with the disappearance of that molecule or with its conversion to a state such that its reactivity is statistically no greater than that of similar molecules in thermal equilibrium with their surroundingsrdquo
432 Stages of Photochemical Reactions
The majority of photochemical reactions proceed in stages that involve
Absorption of electromagnetic radiation by a molecule and promotion to electronically excited states
Occurrence of primary photochemical reactions through the electronic excited states
Occurrence of secondary (dark) reactions by the transformation of excited state species (eg free radicals) to stable products
433 Role of Photochemistry in Photostability Studies
The photodegradation reactions of drug substances may follow several pathways to form different products The elucidation of the mechanisms leading to these pathways requires a thorough understanding of the nature and type of the photochemical reactions involved This would largely depend on the presence of certain functional groups physical characteristics (light absorption pKas solubility etc) and photoreactivity of the compound The assessment of the photostability of pharmaceutical compounds is based on the study of all those factors that determine the rates and mechanisms of the underlying photochemical reactions
44 PHOTOCHEMICAL REACTIONS
The photochemical reactions of organic molecules including a vast majority of drugs occur by the absorption of specific wavelengths in the UV and visible region (200ndash700 nm) with energies up to about 300 kcalmole This leads to the excitation of π and nonbonding (n) electrons in molecules containing chromophoric groups (eg C=C C=O C=N) by πndashπ and nndashπ electronic transitions The asterisk () denotes the anti-bonding molecular orbitals with the electron in the
82
excited state This may be followed by the loss of energy by heat light emission (fluorescence and phosphorescence) or free radical formation The stabilization of the unpaired electron in the excited state by delocalization would facilitate the formation of free radicals The free radicals may react to form stable products The photochemical degradation of organic compounds involves various mechanisms depending upon the chemical nature and photoreactivity of the compounds In the study of photochemical reactions a strict control of experimental conditions including the wavelengths and intensity of light is required to obtain reliable results This is particularly important in kinetic studies The number of photons absorbed by the reactants can be determined by chemical actinometry The details of the photochemistry of organic (Horspool and Armesto 1992 Horspool and Lenci 2004 Turro et al 2010) and drug molecules (Beijersbergen van Henegouwen 1981 Albini and Fasani 1998 Tonnesen 2004 Ahmad and Vaid 2006) are well documented
441 Regions of UV Visible and Sunlight Radiation
The regions of UV visible and sunlight radiation involved in the photochemical reactions are
UVA 320ndash400 nm
UVB 290ndash320 nm
UVC 200ndash290 nm
Visible 400ndash700 nm
Sunlight It includes UVA UVB and visible radiations
The majority of photochemical reactions of drug molecules take place by UVA UVB and visible radiations
442 Important Chemical Functions for Photoreactivity in Organic Molecules
The presence of following chemical functional groups in organic molecules (Albini and Fasani 1998) is necessary for the occurrence of photochemical reactions
C = C double bond (oxidation isomerization)
C = O carbonyl group (reduction fragmentation)
C6H5NO2 nitroaromatic group radical (intermolecular hydrogen abstraction rearrangement to nitrile esters)
C6H4Cl2 aryl chloride (homolytic heterolytic dechlorination)
Compounds containing a weak CndashH bond (photo-induced fragmentation via a hydrogen atom transfer or electron-proton transfer)
Sulfides alkenes polyenes and phenols (highly reactive with singlet oxygen photochemically formed from ground state triplet oxygen)
Triplet oxygen (3O2) Singlet oxygen (1O2)
443 Photophysical Processes
The various photophysical processes involved in the absorption and dissipation of light energy (Eq 41) ndash (Eq 47) have been described by Moore (2004) This may be followed by photochemical processes to form free radicals and subsequently the final products (Eq 48) ndash (Eq 411)
hv
hv
83
Absorption Ao 1A (excited singlet state) (41)
Internal conversion 1A Ao (ground singlet state) (42)
Fluorescence 1A Ao + hvacute (43)
Photoionization 1A Abull+ + endash (44)
Intersystem crossing 1A 3A (excited triplet state) (45)
Internal conversion 3A Ao (ground singlet state) (46)
Phosphorescence 3A Ao + hvPrime (47)
Radical formation 3A + Ao Abull+ + Abullndash (ionic radicals) (48)
Abull+ Abull (oxidized radical) (49)
Abullndash AHbull (reduced radical) (410)
Final products 2AHbull AH2 + Ao (411)
A molecule in the ground state (Ao) on the absorption of photons of specific wavelengths in the UV or visible region is promoted to the excited singlet state (1A) in which the electron spins remain anti-parallel (Eq 41) The molecule in the excited singlet state with life time of nanosecond can dissipate its energy by different physical process and thus deactivated This could happen by internal conversion (ic) (Eq 42) a non-radiated transition to the ground state or by photon emission (fluorescence) to return to the ground state (Eq 43) The excess energy in an excited state can also be dissipated as heat on collision with neighboring molecules by vibrational relaxation (VR) Since in the excited singlet state a reduction in the ionization potential of the molecule takes place it is easy to remove the electron in the excited state than it is in the ground state of the molecule This occurs in the presence of an electron acceptor as a result of photoionization (Eq 44) particularly in the molecules having an anionic state Another process that can occur from the excited singlet state is by intersystem crossing (isc) to the metastable excited triplet state (3A) in which the electron spins are parallel (Eq 45) The isc has a high efficiency for molecules that are photochemically active The excited triplet state with life-times of the order of microsecond to seconds has a greater probability of interaction with other molecules and undergoes photochemical reaction Alternatively it can return to the ground state by another ic (Eq 46) or by the emission of phosphorescence (Eq 47) Further photochemical processes involving the excited triplet state may lead to the formation of cationic and anionic radicals (Eq 48) which can be converted to neutral oxidized radicals (Eq 49) and neutral reduced radicals (Eq 410) The neutral free radicals may react to form the final products (Eq 411) The excited triplet state is a more powerful electron donor or acceptor than the ground state of a molecule All these processes mostly occur within a span of nanoseconds to seconds
45 PRIMARY PHOTOCHEMICAL REACTIONS
The study of primary photochemical reactions of molecules involving electronically excited states their interactions (eg electronproton transfer) and decay processes have been conducted using flash photolysis and related techniques These are described in the following sections
451 Flash Photolysis
The technique of flash photolysis was developed to study fast reactions by Norrish and Porter (Porter 1950) who were awarded Nobel Prize in chemistry for this discovery in 1967 This
The Principal author (IA) has the privilege of working with Professor Lord George Porter on laser flash photolysis studies of redox reactions of photosystem II D1D2 cytochrome b559 reaction centers of higher plants at Imperial College London on a Royal Society Fellowship during the period 1989ndash1990
hv
isc
-H+
+H
ic
ic
84
technique took considerable time for its further development instrumentation and applications in the study of excited state reactions
Flash photolysis involves the exposure of a compound in solution to an intense flash of light of very short duration (of the order of microseconds 10ndash6s) to initiate a chemical reaction by producing excited state species (ie singlets and triplets) and thereby free radicals that lead to stable products
The technique has the limitations of nonuniform light intensity and the duration of flash These factors have been overcome by the development of laser flash photolysis
452 Laser Flash Photolysis
This technique uses laser (Light Amplification by Stimulated Emission of Radiation) flashes to generate excited states of a molecule and to study the formation and decay of excited singlet and triplet states and free radicals A laser beam is produced by supplying excitation energy to a substance (eg rod of ruby) to produce a large number of excited state atoms that can release spontaneous energy with the emission of photons
Pulsed lasers provide emission of radiation for periods of very short duration [microsecond (micros) to picoseconds (ps)] to detect excited state species and to follow photochemical processes having half-lives up to picoseconds (ps 10ndash12 s) to femtoseconds (fs10ndash15 s)
Laser flash photolysis is one of the most effective methods of studying the rates of reactions of transient species such as the excited singlet and triplet states free radicals and ions formed in chemical and biological systems The technique is used for the study of very fast photochemical reactions occurring up to the fs time scale The 1999 Nobel Prize in chemistry was awarded to Professor Ahmad Zewail of California Institute of Technology for the study of very fast chemical reactions using ultra-short laser flashes on the time scale of fs where these reactions actually occur
453 Two-Laser Flash Photolysis
The technique of two-laser flash photolysis is used to study the fate of bimolecular reactions involving an upper excited state of a molecule (A) in solution competing with a fast unimolecular deactivation process of a lower excited state (A) A high-intensity radiation source is required to produce high concentrations of A and A species by using two lasers of different wavelengths sequentially (Turro et al 2010)
The first laser gives rise to the lower excited state
A A (412)
This is achieved by the excitation of the ground singlet state (So) of a molecule to the excited singlet state (S1) S1 is converted to the lower excited triplet state (T1) by intersystem crossing (isc)
S1 T1 (413)
The second laser selectively excites T1 into an upper level by the process
T1 T2 (414)
This is achieved by the excitation of T1 to the upper triplet state (T2) termed above as A The use of a tunable laser allows the selection of the photons of the second wavelength corresponding to the absorption maximum of A Examples of the application of this technique include the study of the αndashcleavage of carbonyl (eg benzil) and halo-aromatic compounds (eg 2-bromonaphthalene) through a biphotonic process The photostability of 2-bromonaphthalene shows that T1 is not reactive towards CndashBr bond cleavage When the molecule undergoes two-laser (ie two photon) flash photolysis it results in the loss of bromine atom The details of this technique and its applications to the study of photochemical degradation of organic compounds are given by Turro et al (2010)
hv
isc
hv
85
454 Time-Resolved Spectroscopy
The absorption spectra of transient species (ie excited singlet and triplet states) formed in the photochemical reactions with life-times in the time scale of micros to fs are determined using time-resolved spectroscopic techniques
The details of all these techniques and their applications to the study of the kinetics and mechanisms of photochemical reactions of drug substances are presented by Navaratnum (2004) Some examples of the applications of laser flash photolysis in drug degradation studies include flavin electron transfer reactions (Ahmad and Tollin 1981a) flavin triplet quenching and semiquinone formation (Ahmad and Tollin 1981b) primary processes in the laser flash photolysis of L-ephedrine (Navaratnum et al 1983) primary processes in the photochemistry of fenbufen (Navaratnum and Jones 2000) primary photophysical properties of ofloxacin (Navaratnum and Claridge 2000) laser and flash photolytic studies on the effect of solvents and solutes on excited singlet and triplet states of NNNprime-tetramethyl paraphenylene diamine (TMPD) (Richards and Thomas 1970) laser flash photolysis of tolmetin (Sortino and Scaiano 1999a) photophysical properties of rufloxacin (Sortino et al 1999b) photodegradation of formylmethylflavin (Heelis et al 1980) laser flash photolysis studies of electron transfer between semiquinone and fully reduced free flavin and horse heart cytochrome c (Ahmad et al 1981) and the cytochrome c-cytochrome oxidase complex (Ahmad et al 1982) multiple radical pair states in photosystem 2 reaction centers (Booth et al 1991) electron transfer oxidation of tryptophan and tyrosine (Lu and Lin 2002) timendashresolved thermal lensing and phosphorescence studies on photosensitized molecular oxygen formation (Redmand and Braslavsky 1988) and nanosecond time-resolved resonance raman and absorption studies of the photochemistry of chlorpromazine (Sarata et al 2000)
455 Excited State Reactions
Moore (2004) has discussed the reactions occurring directly from the excited states (singlet and triplet) and are briefly described in this section
An excited state molecule (A) can react with another molecule (Q) to form a complex called exciplex (AQ) in the excited state The molecule Q is a quencher (deactivator of an excited state) of fluorescence (by deactivation of excited single state) or phosphorescence (by deactivation of excited triplet state) Quenching normally takes place by intermolecular energy transfer or electron transfer between A and Q
If the molecule A has a high concentration then in the excited state (A) it can interact with another molecule in the ground state to form AA species called an excimer In both cases the formation of an exciplex and an excimer gives rise to a bathochromic shift in the fluorescence emission of the molecule (A) The polycyclic aromatic hydrocarbons exhibit excited state complexes Exciplex formation may occur in concentrated solutions of drug molecules or in solid-state mixtures leading to electron transfer to the drug molecule the quencher or both Photoaddition reactions have been reported to proceed via exciplex formation with the quencher molecule chemically bound to the drug molecule An example of these reactions includes the photoaddition of riboflavin (RF) in the presence of divalent ions such as HPO4
2ndash or SO42ndash ions
These anions form a complex with RF in the excited state [RF HPO42ndash] and catalyze the
photoaddition reaction They also quench the fluorescence of RF The photoaddition of RF leads to the formation of cyclodehydroriboflavin (CDRF) (Schuman Jorns et al 1975 Ahmad et al 2004a 2005 2006) according to the following reaction
RF+HPO42ndash [RFHPO4
2ndash] CDRF (415) complex
A molecule in the excited state is considered as a more powerful electron donor or acceptor than it is in the ground state It can react with a quencher molecule in the following manner
A+ Q AQ A+bull + Qndashbull (416)
86
A+ Q AQ Andashbull + Q+bull (417)
The Eqs (416) and (417) represent the oxidative and reductive quenching of A respectively The quencher molecule is reduced or oxidized in the reaction The electron transfer processes may involve the formation of an exciplex in the presence of a quencher in polar solvents An example of these reactions is the electron transfer to the flavin excited triplet state (3F) (Eq 418) conversion of [Fndashbull] and [F+bull] radicals to neutral reduced [FHbull] (Eq 419) and oxidized radicals [Fbull] (Eq 420) and the oxidation of the flavin semiquinone (FH) by oxidized flavin radical (F+) (Eq 421) oxidized 26-dimethylphenol radical (PHO) (Eq 422) and by 25-dichlorobenzoquinone (QN) (Eq 423) studied by laser flash photolysis The bimolecular rate constants for triplet quenching by 26-dimethylphenol and flavin semiquinone yields are dependent on dielectric constant and viscosity of the medium (Ahmad and Tollin 1981a) The triplet quenching and electron transfer reactions of flavins (F) are expressed as follows
3F + F Fndashbull + F+bull (418)
Fndashbull + H+ FHbull (419)
F+bull + ndashH+ Fbull (420)
FHbull + F+bull 2F + H+ (421)
FHacute + PHObull F+ PHOndash + H+ (422)
FHacute + QN F+ QNndashbull + H+ (423)
456 Photosensitized Reactions
Photosensitization is the involvement of molecular species called photosensitizer to bring a photochemical change by light absorption and electron or energy transfer to another molecular species which does not absorb light The photosensitizer does not directly participate in the photochemical reaction The majority of these reactions occur by photosensitized oxidation These reactions involve two mechanisms termed as Type I and Type II depending upon the nature of oxidation
4561 Type I Free radical mechanism
In this mechanism the excited state (singlet or triplet) of the sensitizer (S) reacts with the substrate to give free radicals through hydrogen atom or electron transfer
4562 Type II Mechanism involving singlet oxygen
This mechanism involves the reaction of the excited state of the sensitizer (S) with molecular oxygen (3O2) to give rise to singlet oxygen (1O2) The singlet oxygen reacts with the substrate to give oxidation products
The Type I and Type II reactions may occur independently or simultaneously depending on the chemical nature and the reactivity of the substrate and the sensitizer reaction medium oxygen content and the affinity of the sensitizer and the substrate These factors have been discussed by Henderson and Dougherty (1992) Under specific experimental conditions one type of photosensitized reaction may dominate the other type An example of photosensitized reaction would be described with reference to riboflavin (vitamin B2) (RF) It strongly absorb in the visible region at 444 nm (British Pharmacopoeia 2016) and participates in a number of photosensitized reactions involving different substrates (eg SH) by Type I and Type II mechanisms (Ahmad and Vaid 2006 Silva and Quina 2006 Garcia et al 2006) Silva and Quina (2006) have described these reactions and are presented with further explanations
Type 1 mechanism
RF 1RF formation of excited singlet state (424)
1RF 3RF transformation to excited triplet state (425)
3RF+SH RFndashbull+ SH+bull formation of radical ions (426)
hν
isc
87
RFndashbull+ SH+bull RFHbull + Sbull formation of free radicals (427)
RFndashbull+ O2 RF+ O2ndashbullformation of superoxide radical anion (428)
2RFHbull RF+RFH2 formation of oxidized and reduced molecules
(429)
RFH2 +O2 RF+ H2O2 formation of hydrogen peroxide (430)
H2O2 + O2ndashbull OHndash+OHbull+O2 formation of hydroxyl ion and radical
(431)
Sbull+H2O2O2ndashbullOHndash Sox formation of oxidation products of substrate
(432)
SH+bull+H2O2O2ndashbullOHndash Sox formation of oxidation products of substrate
(433)
Type 2 mechanism
RF 1RF formation of excited singlet state (434)
1RF 3RF transformation to excited triplet state (435)
3RF+3O2 RF+1O2 formation of singlet oxygen by energy transfer (436)
SH+1O2 SOOH formation of oxidation products of substrate (437)
In the above equations RF 1RF and 3RF represent RF molecule in the ground state excited singlet state and excited triplet state respectively RFndashbull RFHbull and RFH2 are the radical anion the free radical and the reduced form of RF SH is the reduced substrate and SH+bull Sbull and Sox represent the radical cation free radical and oxidized form of the substrate respectively
46 PHOTODEGRADATION REACTIONS
A large number of drug substances are sensitive to light and undergo photodegradation by various pathways on exposure to light These reactions may proceed through free radical intermediates and could involve more than one step to form the final products The major modes of photodegradation reactions are as follows
Photoaddition (eg riboflavin)
Photoaquation (eg cyanocobalamin)
Photodealkylation (eg chloroquine)
Photodecarboxylation (eg amino acid)
Photodehalogenation (eg meclofenamic acid)
Photodimerization (eg primaquine)
Photoelimination (eg mefloquin)
Photodehydrogenation (eg nifedipine)
Photo-induced hydrolysis (eg sulfacetamide)
Photoisomerization (eg chlordiazepoxide)
Photooxidation (eg ascorbic acid)
Photopolymerization (eg 2-hydroxyethyl methacrylate)
Photo-induced rearrangement (eg benzydamine)
oxidation
isc
hν
88
Photoreduction followed by oxidation (eg riboflavin)
Photoinduced ring cleavage and other reactions (eg fluoroquinolones)
In some photodegradation reactions more than one pathway may be involved such as in the case of sulfacetamide hydrolysis is followed by oxidation in the case of riboflavin reduction is followed by oxidation and in the case of furosemide oxidation is followed by reduction The photodegradation of drug substances may also occur by simultaneous (parallel) reactions to give two or three products or by consecutive reactions involving an intermediate species to give the final product These reactions may involve zero first or second-order kinetics (see Chapter 2) Several examples of the photodegradation reactions of durg substances involving different mechanisms have been reported (Albini and Fasani 1998 Carstensen 2000 Yoshioka and Stella 2000 Fasani and Albini 2005 Tonnesen 2002 2004 Ahmad and Vaid 2006 Sinko 2006) The photostability and related aspects of drug substances and drug products have been dealt by many workers (Lintner 1973 Tonnesen 1991 2001 2002 2004 Tonnesen and Karlson 1995 1997 Tonnesen and Moore 1993 Albini and Fasani 1998 Fasani and Albini 2005 Piechocki and Thoma 2007 Bhalekar et al 2008) The phototoxic phototherapeutic and photosensitization effects of drugs have been reviewed by Magnus (1976) Beijersbergen van Henegouwen (1981) and Moan and Juzenas (2004) Examples of some photodegradation reactions are presented in this section
461 Photooxidation Reactions
4611 Photooxidation of benzaldehyde
The photooxidation of drugs by UV radiation involves a free radical mechanism This has been studied with reference to the photooxidation of benzaldehyde (Moore 1976) In the free radical chain process a sensitizer (eg benzophenone) abstracts a hydrogen atom from the drug molecule (Eq 438) The free radical of the drug reacts with a molecule of oxygen (Eq 439) The chain reaction in propagated by removing a hydrogen atom from another molecule of oxidant a hydroperoxide (Eq 440) The hydroperoxide then reacts further by a nonradical mechanism to form inert products (Eq 441) The scheme showing initiation propagation and termination steps in the chain reaction involved in the photooxidation of benzaldehyde is presented in Fig 41
CHO + hv CO
+ H
CO
+ O2
CO3
CO3
+ CHO CO3H CO
+
2CO3
inert products
Initiation
Propagation
Propagation
Termination
Fig41 Photooxidation of benzaldehyde
(438)
(439)
(440)
(441)
89
4612 Photooxidation of ascorbic acid
Ascorbic acid (vitamin C) (AH2) (41) on UV irradiation undergoes photooxidation to dehydroascorbic acid (A) (43) through the ascorbyl radical anion (42) according to the reactions shown below (Eq 442)
OH
OO
OH OH
OH
H
hv-e--2H+
+e-+2H+
OH
OO
O O-
OH
H
-e-
+e-
OH
OO
O O
OH
H
(41) (42) (43)
The photochemical reactions involved in the photooxidation of AH2 may be described by a general scheme (Ahmad et al 2016b Sheraz 2009) as follows
AH2 [1AH2] (443)
[1AH2] [3AH2] (444)
[3AH2] + AH2 AHbull+ + AHbullndash (445)
AHbull+ AHbull (446)
AHbullndash AHbull (447)
AH2 AHndash + H+ (448)
AHbull + AHbull AH2 + A (449)
AHbull + O2 A + HO2bull (450)
HO2bull + AHndash AHbull + H2O2 (451)
According to this scheme the ground state AH2 molecule is promoted to the excited singlet state [1AH2] by the absorption of a photon of UV light (Eq 443) The [1AH2] state may undergo intersystem crossing (isc) to form the excited triplet state [3AH2] (Eq 444) This state may react with a ground state AH2 molecule to produce cationic [AHbull+] and anionic [AHbullndash] ascorbyl radicals (Eq 445) These radicals may be converted to neutral radicals by gaining (Eq 446) or losing an electron (Eq 447) AH2 is ionized in water to form an ascorbyl ion [AHndash] (Eq 448) The ascorbyl radicals [AHbull] may react to give AH2 and dehydroascorbic acid [A] molecules (Eq 449) The [AHbull] radicals can be oxidized to form peroxyl [HO2
bull] radicals (Eq 450) which on interaction with AHndash ions may form [AHbull] radicals and H2O2 (Eq 451) [AHbull] may further take part in the reaction
462 Photoreduction Reactions
4621 Photoreduction of riboflavin
A detailed study of the photoreduction reactions of riboflavin (RF) (44) in aqueous solution has been made by Ahmad et al (1981a 1990 2004b 2006 2008 2011 2013 2014a) and other workers (Cairns and Metzler 1971 Heelis 1982 1991 Holzer et al 2005 Insinka-Rak et al 2012 2014 Sheraz et al 2014) RF on light absorption is promoted to the excited singlet state [1RF] (Eq 452) followed by its conversion to the excited triplet state [3RF] (Eq 453) which leads to the formation of leucodeuteroflavin [RFH2] by intramolecular photoreduction (Eq 454) [RFH2] is oxidized to formylmethylflavin (FMF) (45) as an intermediate product in the reaction (Eq 455) FMF is hydrolyzed to lumichrome (LC) (46) in acid solution (Eq 456) and to LC and lumiflavin (LF) (47) (Eq 457) in alkaline solution (Ahmed et al 1980 2004b) It is also oxidized to carboxymethylflavin (CMF) (48) The rate of photodegradation of RF is faster at higher pH due to the sensitivity of RF excited triplet state [3RF] to alkaline hydrolysis The chemical structures of RF
(442)
hv
+e
-e
90
and photoproducts are shown in Fig 43 The mechanism of photodegradation of RF by photoreduction (Ahmad and Vaid 2006) is represented by the following reactions
RF [1RF] (452)
[1RF] [3RF] (453)
[3RF] RFH2 (454)
RFH2 FMF + side-chain products (455)
FMF LC + side-chain products (456)
FMF LC + LF + side-chain products (457)
463 Photodealkylation Reactions
4631 Photodealkylation of riboflavin
It has been suggested that RF may be degraded by photodealkylation reaction which may lead to the formation of LC directly through the excited singlet state [1RF] (Song 1971)
RF [1RF] LC (458)
464 Photoaddition Reactions
4641 Photoaddition of riboflavin
RF also undergoes photodegradation in the presence of divalent ions such as HPO42ndash and
SO42ndash ions by the photoaddition reaction to form cyclodehydroriboflavin (CDRF) (49) The
appearance of the peak around 410 nm in the absorption spectra of photodegraded solutions of RF is due to the formation of CDRF in the reaction (Ahmad et al 2004a) The photoaddition of RF occurs via the RFndashHPO4
ndash2 complex which creates sterically favorable condition for C (9)(2primeα) interaction (Eq 459) (Schuman Jorns et al 1975) The involvement of excited singlet state [1RF] in this reaction has been suggested on the basis of quenching experiment The presence of HPO4
2ndash ions may facilitate the reorientation of Cndash2prime hydroxyl group to affect photoaddition The autoxidation of dihydroflavin intermediate leads to the formation of CDRF (Eq 460) The photoaddition of RF is expressed by the following reactions
RF RFndashHPO42ndash [1RF] (459)
complex
[1RF] [Dihydroflavin] CDRF (460)
The kinetics of simultaneous photoreduction and photoaddition reactions of RF has been studied by Ahmad et al (2004a)
465 Photoaquation Reaction
4651 Photoaquation of cyanocobalamin
Cyanocobalamin (vitamin B12) is sensitive to light and its photochemical conversion to hydroxocobalamin (vitamin B12b) takes place in aqueous solution (Connors et al 1986 Ahmad et al 1992) The photolysis of B12 takes place according to the following reaction
[Co3+ CN] [Co3+ OH] + CNndash (461) B12 B12b
[Co3+ OH] [Co3+ OH2]+ irreversible oxidation products B12b B12a (462)
In the photolysis process the CNndash group with its full complement of electrons is replaced by a water molecule without causing any change in the valency of cobalt (Eq 461) B12b exists in equilibrium with aquocobalamin (B12a) in aqueous solution (Eq 462) This reaction takes place by the absorption of light leading to πndashπ transition in the corrin ring The photolysis reaction is pH dependent with the lowest rate in the pH range of 6ndash7
O2
hv
H+ OHndash
autoxidation
hv H2O
OHndash pKa= 78
H+
HPO42ndash hv
91
466 Photodegradation of Moxifloxacin
Moxifloxacin (MF) (410) is an important fluoroquinolone antibacterial agent It undergoes several photodegradation reactions under acid and alkaline conditions (Ahmad et al 2014b) These reactions are described as follows
4661 Acid Solution
MF (410) on UV excitation undergoes hydroxylation of the piperidine ring to form the products (411 412) The product (412) is then degraded by photooxidation of the pyrrole ring in the diazabicyclononane side chain give the products (413 and 414) The product (414) undergoes further reaction by the cleavage of the diazabicyclononane side chain to produce the quinolone derivative (415) as the final product The rate and extent of formation of these products depends on the pH and acid-base equilibria in the region (Fig 44)
4662 Alkaline Solution
MF (410) on light absorption undergoes hydroxylation and photooxidation of the pyrrole ring to form product (411) and on oxidation of piperidine ring in the side chain to give the product (414) This is followed by cleavage of the diazabicyclononane side chain of the product to form quinolone derivative (415) as in the case of acid solution However the detection of only three products in alkaline solution indicates that the reaction is faster in the alkaline solution compared to that of the acid solution This could be due to the greater reactivity of any intermediates involved in the process to form the detected products The mode of photodegradation of MF is similar in acid and alkaline media as a result of the specific acid-base catalysis in the whole pH range (Fig 45)
CH2OH
OCH3
CH2
N
NNH
N
CH3
OHH
OHH
OHH
O
CHO
OCH3
CH2
N
NNH
N
CH3
O
CH2OH
(HOHC) 2
OCH3 N
NNH
N
CH3
CH2
CHO
O
OCH3 N
NNH
NH
CH3
O
COOH
OCH3
CH2
N
NNH
N
CH3
O
OCH3
CH3
N
NNH
N
CH3
O
Fig 43 Chemical structures of riboflavin and photoproducts
(44) (45)
(46) (47)
(48) (49)
92
N
O
CH3
O O
OH
F
N
NH2
+
N
O-
O O
OH
F
N
NH2
+
+N
O
CH3
O O
OH
F
N
NH2
+
OH
N
O
CH3
O O
OH
F
N
NH2
+
O
O N
O
CH3
O O
OH
F
N
NH2
+
ON
O
CH3
O O
OH
F
NH3
+
hv
hydroxylation
hydroxylationoxidation
oxidation
clevage of diazabicyclononane
side chain
Fig 44 Proposed pathway for the photodegradation of MF in acid solution
N
O
CH3
O O
OH
F
N
NH2
+
N
O-
O O
OH
F
N
NH2
+ OH
N
O
CH3
O O
OH
F
NH3
+
hv
hydroxylation
hydroxylationoxidation
clevage of diazabicyclononane
side chain
O N
O
CH3
O O
OH
F
N
NH2
+
O
Fig 45 Proposed pathway for the photodegradation of MF in alkaline solution
(410) (411) (412)
(413)
(414)
(415)
(410)
(411)
(414)
(415)
93
467 Other Photodegradation Reactions
The details of other photodegradation reactions of drugs (photodealkylation photodecaroxylation photodehalogenation photodimerization photoelimination photodehydrogenation photo-induced hydrolysis and photoisomerization) are described by Ahmad et al (2016a)
468 Photochemical Interactions
Many drugs present in combination in a product may undergo chemical interactions to affect the stability of the individual components The photochemical interactions of ascorbic acid (AH2) with riboflavin (RF) nicotinamide (NA) and αndashtocopherol (TP) in cream formulations have been studied by Ahmad et al (2012) and are described in this section
4681 Interaction of riboflavin with ascorbic acid
The interaction of RF with the ascorbyl ion (AHndash) may be represented by the following reactions proposed by Silva and Quina (2006)
[RF] [1RF] (452)
[1RF] [3RF] (453)
[3RF] + AHndash RFndash bull + AHbull (463)
AHbull + O2 A + HO2ndash (464)
HO2ndash+ AHndash H2O2 + AHbull (465)
RF on the absorption of a photon of light is promoted to the excited singlet state [1RF] (Eq 452) and may undergo intersystem crossing (isc) to form the excited triplet state [3RF] (Eq 463) The [3RF] may react with the ascorbyl ion [AHndash] to generate the ascorbyl radical (AHbull) (Eq 463) (Kim et al 1993) The ascorbyl radical reacts with oxygen to give dehydroascorbic acid [A] and peroxyl radical (HO2
ndash) (Eq 464) This radical may interact with ascorbyl ion to generate further ascorbyl radicals (Eq 465) These radicals may again take part in the sequence of reactions to form A The role of RF in this reaction is to act as a photosensitizer in the oxidation of AH2 to A
4682 Interaction of nicotinamide with ascorbic acid
NA is known to form a 11 complex with ascorbic acid (Guttman and Brooke 1963) The complexation of NA and AH2 may result from the donor-acceptor interaction between AH2 (donor) and NA (acceptor) as observed in the case of tryptophan and NA (Florence and Attwood 2006) The interaction of NA and AH2 can be expressed by the following reactions
NA [1NA] (466)
[1NA] [3NA] (467)
[3NA] + AH2 NAH + AHbull (468)
2 AHbull A + AH2 (448)
2NAH + O2 2NA + H2O2 (469)
In the presence of light NA is promoted to the excited singlet state [1NA] (Eq 466) and is then converted to the excited triplet state [3NA] by intersystem crossing (isc) (Eq 467) The interaction of [3NA] with AH2 may cause reduction of NA [NAH] to form the ascorbyl radicals [AHbull] (Eqs 468) which are oxidized to dehydroascorbic acid [A] (Eq 448) The NAH may be oxidized to NA and H2O2 (Eq 469)The proposed reactions suggest that on photochemical interaction AH2
undergoes photosensitized oxidation in the presence of NA indicating that the photostability of ascorbic acid is affected by NA
isc
isc
hv
94
4683 Interaction of αndashtocopherol with ascorbic acid
TP is an unstable compound and its oxidation by air results in the formation of an epoxide which than produces a quinone that is inactive (Connors et al 1986) TP is destroyed by sunlight and artificial light emitting the wavelengths in the UV region (Ball 2006) It interacts with other antioxidants such as ascorbic acid (AH2) according to the following reactions
TPndashObull + AH TP + AHbull (470)
2AHbull A + AH2 (448)
TP + AHbull TPndashObull + AH2 (471)
The tocopheroxyl radical (TPndashO) reacts with AH2 to regenerate TP and form the ascorbyl radical (AHmiddot) (Eq 470) This radical undergoes further reactions as described by equations (Eq 448) and (Eq 471) (Traber 2007) It may disproportionate back to A and AH2 (Eq 448) or react with TP to produce again the TPndashO radical and AH2 (Eq 471) Thus in the presence of light TP leads to the oxidation of AH2 which is ultimately regenerated in the reaction (Davies et al 1991) Ascorbic acid has the ability to act as a co-antioxidant with the TPndashO to regenerate TP (Packer et al 1979 2002) In this manner AH2 and TP act synergistically to function in a redox cycle to stabilize AH2
4684 Interaction of nicotinamide with riboflavin
The photochemical interaction of NA with RF has been studied by Ahmad et al (2016c) and is represented by the following reactions
RF [1RF] (452)
[1RF] [3RF] (453)
[3RF] + RFox RFHbull +RFoxbull (472)
2RFHbull RFox + RFH2 (473)
RFH2 FMF + side chain products (455)
FMF LC + side chain products (456)
FMF LC + LF + side chain products (457)
RFH2 + NA FMF + NAH (474)
2NAH + O2 NA + H2O2 (475)
The RF in the ground state absorbs light and is excited to the singlet state [1RF] (Eq 452) which may be converted to the excited triplet state [3RF] by intersystem crossing (isc) (Eq 453) The interaction of [3RF] with a ground state [RF] molecule leads to the formation of a semiquinone radical [RFHbull] and an oxidized [RFox
bull] radical (Eq 472) The disporportination of two semiquinone radicals results in the formation of an oxidized [RF] and a reduced [RFH2] molecule (Eq 473) [RFH2] is oxidized to give formylmethylflavin [FMF] (Eq 455) which undergoes hydrolysis to yield lumichrome [LC] lumiflavin [LF] and side chain products (Eq 456 and 457) NA (electron acceptor) may undergo photochemical interaction with a [RFH2] molecule to form [FMF] and a reduced [NAH] (Eq 474) The [NAH] molecule is oxidized to NA (Eq 475) In this manner NA accelerates the rate of photodegradation of RF in aqueous solution
4685 Interaction of ascorbic acid with cyanocobalamin
The study of the photochemical interaction of ascorbic acid [AH2] with cyanocobalamin
[Co3+ CN] has been conducted by Ahmad et al (2016d) The reactions involved in the interaction
can be expressed as follows
[Co3+ CN] 1[Co3+ CN] (476)
1[Co3+ CN] 3[Co3+ CN] (477)
hv
isc
oxidation
hydrolysis
hv
hv
isc
95
AH2 AHndash + H+ (478)
3[Co3+ CN] + AHndash [Co2+] + AH + CNndash (479)
AH Abull ndash + H+ (480)
3[Co3+ CN] + Andash [Co2+] + A + CNndash (481)
[Co2+] [Co3+ OH] (482)
[Co2+] Corrin ring cleavage oxidation products (483)
AH + AH AH2 + A (484)
The ground state B12 molecule [Co3+ CN] absorbs light and is promoted to the excited
singlet state 1[Co3+ CN] (Eq 476) This may be converted to the excited triplet state 3[Co3+ CN] by
intersystem crossing (isc) (Eq 477) The formation of a corrin triplet has been observed on the
basis of phosphorescence quenching AH2 on ionization gives ascorbyl ions (AHndash) (Eq 478) The 3[Co3+ CN] may react with AHndash
ions and reduced to B12r form [Co2+] along with a AH radical (Eq
479) AH may deprotonate to form Andash anion radical (Eq 480) The 3[Co3+ CN] could also react
with the Andash anion radical to form [Co2+] and a A radical (Eq 481) The [Co2+] form of B12 can
either be oxidized to B12b [Co3+OH] (Eq 482) andor undergo oxidative degradation to corrin ring
cleavage products (Eq 483) depending on AH2 concentration Two AH may combine to give a
reduced [AH2] and an oxidized [A] molecule (484)
O2
O2 OHndash
96
REFERENCES
Ahmad I Rapson HD Multicomponent spectrophotometric assay of riboflavine and photoproducts J Pharm Biomed Anal 19908217ndash223
Ahmad I Tollin G Solvent effects of flavin electron transfer reactions Biochemistry 1981a205925ndash5928
Ahmad I Tollin G flavin triplet quenching and semiquinone formation by aliphatic αndashsubstitutes acetic acids Intermediates in flavin sensitized photocarboxylation Photochem Photobiol 1981b34441ndash445
Ahmad I Vaid FHM Photochemistry of flavin in aqueous and organic solvents In Silva E Edwards AM editors Flavin Photochemistry and Photobiology The Royal Society of Chemistry Cambridge UK 2006 Chap 2
Ahmad I Hussain W Fareedi AA Photolysis of cyanocobalamin in aqueous solution J Pharm Biomed Anal 1992109ndash15
Ahmad I Cusanovich MA Tollin G Laser flash photolysis studies of electron transfer between semiquinone and fully reduced free flavins and horse heart cytochrome c Proc Natl Acad Sci USA 1981786724ndash6728
Ahmad I Cusanovich MA Tollin G Laser flash photolysis studies of electron transfer between semiquinone and fully reduced free flavins and the cytochrome c-cytochrome oxidase complex Biochemistry 1982213122ndash3128
Ahmad I Fasihullah Q Vaid FH A study of simultaneous photolysis and photoaddition reactions of riboflavin in aqueous solution J Photochem Photobiol B 2004a7513ndash20
Ahmad I Fasihullah Q Vaid FH Effect of phosphate buffer on photodegradation reactions of riboflavin in aqueous solution J Photochem Photobiol B Biol 200578229ndash234
Ahmad I Fasihullah Q Vaid FH Effect of light intensity and wavelengths on photodegradation reactions of riboflavin in aqueous solution J PhotochemPhotobiol B Biol 20068221ndash27
Ahmad I Rapson HDC Heelis P Phillips GO Alkaline hydrolysis of 78-dimethy140-(formylmethyl) isoalloxezine a kinetic study J Org Chem 198045731ndash733
Ahmad I Shad Z Qadeer K Bano R Effect of stabilizers on the chemical and photodegradation of ascorbic acid in aqueous solution Baqai J Health Sci 2016b19(1)3ndash11
Ahmad I Ahmed S Sheraz MA Vaid FH Effect of borate buffer on the photolysis of riboflavin in aqueous solution J Photochem Photobiol B Biol 20089382ndash87
Ahmad I Ahmed S Anwar Z Sheraz MA Sikorski M Photostability and photostabilization of drugs and drug products Int J Photoenergy 2016a Article ID 8135608
Ahmad I Fasihullah Q Noor A Ansari IA Ali QN Photolysis of riboflavin in aqueous solution a kinetic study Int J Pharm 2004b280199ndash208
Ahmad I Sheraz MA Ahmed S Bano R Vaid FH Photochemical interaction of ascorbic acid with riboflavin nicotinamide and alphandashtocopherol in cream formulations Int J Cosmet Sci 201234123ndash131
Ahmad I Ahmed S Sheraz MA Vaid FH Ansari IA Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution Int J Pharm 2010390174ndash182
Ahmad I Ahmed S Sheraz MA Aminuddin M Vaid FH Effect of caffeine complexation on the photolysis of riboflavin in aqueous solution a kinetic study Chem Pharm Bull 2009571363ndash13770
Ahmad I Mirza T Iqbal K Ahmed S Sheraz MA Vaid FHM Effect of pH buffer and viscosity on the photolysis of formylmethylflavin a kinetic study Aust J Chem 201365579ndash585
Ahmad I Ahmed S Sheraz MA Anwar Z Qadeer K Noor A Evstigneev MP Effect of nicotinamide on the photolysis of riboflavin in aqueous solution Sci Pharm 2016c84289ndash303
Ahmad I Sheraz MA Ahmed S Kazi SH Mirza T Aminuddin M Stabilizing effectof citrate buffer on the photolysis of riboflavin in aqueous solution Results Pharma Sci 2011111ndash15
97
Ahmad I Anwar Z Iqbal K Ali SA Mirza T Khurshid A Khurshid A Arsalan A Effect of acetate and carbonate buffers on the photolysis of riboflavin in aqueous solution a kinetic study AAPS PharmSciTech 2014a15550ndash559
Ahmad I Bano R Musharraf SG Ahmed S Sheraz MA ul Arfeen Q Bhatti MS Shad Z Photodegradation of moxifloxacin in aqueous and organic solvents a kinetic study AAPS PharmSciTech 2014b151588ndash1597
Albini A Fasani E Drugs Photochemistry and Photostability The Royal Society of Chemistry Cambridge UK 1998
Ball GFM Vitamins in Food Analysis Bioavailability and Stability CRC Press Boca Raton Florida USA 2006 Chap 15
Beaumont TG Photostability testing In Mazoo DT editor International Stability Testing Interpharm Press Buffalo Grove Illinois USA 1999 Chap 2
Beijersbergen van Henegouwen GMJ Photochemistry of drugs in invitro and invivo In Breimer DD Speiser D editors Topics in Pharmaceutical Sciences Elsevier Biomedical Press North-Holland 1981 pp 233ndash256
Beijersbergen van Henegouwen GMJ Medicinal photochemistry phototoxic and phototherapeutic aspects of drugs Adv Drug Res 19972979ndash170
Bhalekar MR Harinarayana D Madglukar AR Improvement of photostability in formulation A review Asian J Chem 2008205095ndash5108
Booth PJ Crystall B Ahmad I Barber J Porter G Klug DR Observation of multiple radical pair states in photosystem 2 reaction centers Biochemistry 1991307573ndash7586
Boreen AL Arnold WA McNeill K photodegradation of pharmaceuticals in the aquatic environment A review Aquat Sci 200365320ndash341
British Pharmacopoeia Monograph on Riboflavin Her Majestyrsquos Stationary Office London UK 2016 Electronic version
Cairns WL Metzler DE Photochemical degradation of flavins VI A new photoproduct and its use in studying the photolytic mechanism J Am Chem Soc 1971932772ndash2777
Carstensen JT Catalysis complexation and photolysis In Carstensen JT Rhodes CT editors Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000 Chap 5
Connors KA Amidon GL Stella VJ Chemical Stability of Pharmaceuticals A Handbook for Pharmacists 2nd ed Wiley New York USA 1986 pp 95ndash96
Davies MB Austin J Partridge DA Vitamin C Its Chemistry and Biochemistry The Royal Society of Chemistry Cambridge 1991 Chap 7
Epstein JH Wintroub BU Photosensitivity due to drugs Drugs 19853042ndash57
European Pharmacopoeia European Pharmacopoeial Convention and the European Union Strasbourg France 8th edition 2015
Fasani E Albini A Photostability stress testing In Baertschi SW editor Pharmaceutical Stress Testing Predicting Drug Degradation Taylor amp Francis Boca Raton Florida 2005 Chap 10
Florence AT Attwood D Drug stability Physiochemical Principles of Pharmacy 4th ed Pharmaceutical Press London 2006 pp 411
Garcia NA Criado SN Massad WA Riboflavin as a visible light sensitizer in the aerobic photodegradation of ophthalmic and sympahtomimetic drugs In Silva E Edwards AM editors Flavin Photochemistry and Photobiology The Royal Society of Chemistry Cambridge 2006 Chap 4
Greenhill JV McLelland MA Photochemistry of drugs in vitro and in vivo In Ellis GP West GB editors Progress in Medicinal Chemistry Elsevier Amsterdam The Netherlands 1990
Guttman DE Brooke D Solution phase interaction of nicotinamide with ascorbic acid J Pharm Sci 1963 Oct52941ndash5
98
Harber LC Kochevar IE Shalita AR Mechanism of photosensitization to drugs in human In Regan JD Parrish JA editors Science of Photomedicine Plenium Press New York NY USA 1982 pp 323ndash347
Heelis PF Philips GO Ahmad I Rapson HDC The photodegradation of formylmethylflavinndasha steady state and laser flash photolysis Photochem Photophys 19801125ndash130
Heelis PF The photophysical and photochemical properties of flavins (isoalloxazines) Chem Soc Rev 19821115ndash39
Heelis PF The photochemistry of flavins In Muller F editor Chemistry and Biochemistry of flavoenzymes Vol 1 CRC Press Boca Raton FL USA 1991 pp 171ndash193
Henderson BW Dougherty TJ How does photodynamic therapy work Photochem Photobiol 199255145ndash157
Holzer W Shirdel J Zirak P Penzkofer A Hegemann P Deutzmann R Hochsmuth E Photo-induced degradation of some flavins in aqueous solution Chem Phys 200530869ndash78
Horspool WH Armesto D Organic Photochemistry A comprehensive Treatment Ellis Horwood New York USA 1992
Horspool WH Lenci F editors Handbook of Organic Photochemistry and Photobiology CRC Press Boca Raton Florida USA 2004
Insińska-Rak M Golczak A Sikorski M Photochemistry of riboflavin derivatives in methanolic solutions J Phys Chem A 20121161199ndash1207
Insińska-Rak M Sikorski M Riboflavin interactions with oxygenndasha survey from the photochemical perspective Chemistry 20142015280ndash15291
ICH Harmonized Tripartite Guideline ICH Q1B Photostability of Testing of New Drug Substances and Products Geneva Switzerland 1996
Kim H Kirschenbaum LJ Rosenthal I Riesz P Photosensitized formation of ascorbate radicals by riboflavin an ESR study Photochem Photobiol 199357777ndash784
Kullavanijaya P Lim HW Photoprotection J Am Acad Dermatol 200552937ndash958
Lintner CJ Pharmaceutical product stability In Cooper MS editor Quality Control in the Pharmaceutical Industry vol 2 Academic Press New York USA 1973 pp 161ndash162
Lu CY Liu YY Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study Biochim Biophys Acta 2002157171ndash76
Magnus IA Drug and chemical photosensitization In Magnus IA editor Dermatological Photobiology Blackwell Scientific Publication Oxford UK 1976 Chap 16
Moan J Benefits and adverse effects from the combination of drugs and light In Tonnesen HH editor Photostability of Drugs and Drug Formulations Taylor amp Francis London UK 1996 pp 173ndash188
Moan J Juzenas P Biological effects of combination of drugs and light In Tonnesen HH editor Photostability of Drugs and Drug Formulation 2nd ed CRC Press Boca Raton Florida USA 2004 Chap 9
Moore DE Antioxidant efficiency of polyhydric phenols in photooxidation of benzaldehyde J Pharm Sci 1976651447ndash1451
Moore DE Photophysical and photochemical aspects of drug stability In Tonnesen HH editor Photostability of Drugs and Drug Formulation 2nd ed CRC Press Boca Raton Florida USA 2004 Chap 2
Moore DE Photophysical and photochemical aspects of drug stability In Tonnesen HH editor Photostability of Drugs and Drug Formulation 2nd ed CRC Press Boca Raton Florida USA 2004 Chap 2
Moyal D Fourtanier A Acute and chronic effects of UV on skin In Rigel DS Weiss RA Lim HW Dover JS editors Photoaging Marcel Dekker New York NY USA 2004 pp 15ndash32
99
Navaratnam S Claridge J Primary photophysical properties of ofloxacin Photochem Photobiol 200072283ndash290
Navaratnam S Land EJ Parsons BJ Ahmad I Phillips GO Primary processes in the laser flash photolysis and pulse radiolysis of l-ephedrine Photochem Photobiol 198338153ndash159
Navaratnam S Jones SA Primary process in the photochemistry of fenbufen in acetonitrile J Photochem Photobiol A Chem 2000132283ndash290
Navaratnam S Photochemical and photophysical methods used in study of drug photochemistry In Tonnesen HH editor Photostability of Drugs and Drug Formulation 2nd ed CRC Press Boca Raton Florida USA 2004 Chap 12
Noyes Jr WA Porter GB Jolley JE The primary photochemical process in simple ketones Chem Rev 19565649ndash94
Packer JE Slater TF Willson RL Direct observation of a free radical interaction between vitamin E and vitamin C Nature 1979278737ndash738
Packer L Traber MG Kraemer K Frei B The Antioxidant Vitamins C and E AOCS Press Illinois USA 2002 Chap 1
Piechocki JT Thoma K Pharmaceutical Photostability and Stabilization Technology Informa Healthcare New York 2007
Porter G Flash photolysis and spectroscopy a new method for the study of free radical reactions Proc R Soc A 1950200284ndash300
Redmond RW Braslavsky SE time resolved thermal lensing and phosphoresence studies of photosensitized molecular oxygen formation Influence of the electronic configuration of the sensitizer on sensitization efficacy Chem Phys Lett 1988148523ndash529
Richards JT Thomas JK Laser and flash photolysis studies on the effects of various solvents and solutes on the excited singlet and triple states of NNNN1N1-tetramethyl paraphenylone diamine (TMPD) Trans Faraday Soc 19701056201ndash6205
Sarata G Sakai M Takahashi H Nanosecond time resolved resonance Raman and absorption studies of the photochemistry of chlorpromazine and related phenothiazine derivatives J Raman Spectrosc 200031785ndash790
Schuman Jorms M Schoumlllnhammer G Hemmerich P Intramolecular addition of the riboflavin side chain Anion-catalyzed neutral photochemistry Eur J Biochem19755735ndash48
Sheraz MA Formulation and stability of ascorbic acid in liquid and semisolid preparations Ph D thesis Baqai Medical University Karachi Pakistan 2009
Sheraz MA Kazi SH Ahmed S Mirza T Ahmad I Evstigneev MP Effect of phosphate buffer on the complexation and photochemical interaction of riboflavin and caffeine in aqueous solution a kinetic study J Photochem Photobiol A Chem 201427317ndash22
Silva E Quina FH Photoinduced processes in the eye lens Do flavin really play a role In Silva E Edwards AM editors Flavin Photochemistry and Photobiology The Royal Society of Chemistry Cambridge UK 2006 Chap 7
Sinko PJ Martinrsquos Physical Pharmacy and Pharmaceutical Sciences 5th ed Lippincott Williams amp Wilkins Baltimore Maryland USA 2006 pp 425ndash428
Song PS Chemistry of flavins in their excited states In Kamin H editor Flavins and Flavoprotein University Park Press Baltimore USA 1971 pp 37ndash61
Sortino S Scaiano JC Laser flash photolysis of tolmetin a photodiabetic decarboxlyation with a triplet carbon ion as the key intermediate in the photodecomposition Photochem Photobiol 1999a69167ndash172
Sortino S Marconi G Giuffrida S De Guidi G Monti S Photophysical properties of rufloxacin in natural aqueous solution Photochem Photobiol 1999b70731ndash736
Sugden JK Photostability of cosmetic material Int J Cosmet Sci19857165ndash173
Tonnesen HH Photostability of Drugs and Drug Formulations CRC Press Boca Raton Florida USA 2nd ed 2004
100
Tonnesen HH editor Photostability of Drugs and Drug Formulations 2nd ed CRC Press Boca Raton Florida USA 2004
Tonnesen HH Karlsen J Photochemical degradation of components in drug formulations A discussion of experimental conditions PharmEuropa 19957137ndash141
Tonnesen HH Karlsen J A comment on photostability testing according to the ICH guidelines calibration of light sources PharmEuropa 19979735ndash736
Tonnesen HH Moore DE Photochemical degradation components in drug formulation Pharm Technol 1993527ndash33
Tonnesen HH Formulation and stability testing of photolabile drugs Int J Pharm 2000221ndash14
Tonnesen HH Introduction Photostability testing in drugs and drug formulationsndashwhy and how In Tonnesen HH editor Photostability of Drugs and Drug Formulations 2nd ed CRC Press Boca Raton Florida USA 2004 Chap 1
Tonnesen HH Photochemical degradation of components in drug formulations Part I An approach to the standardization of degradation studies Pharmazie 199146263ndash265
Tonnesen HH Formulation and stability testing of photolabile drugs Int J Pharm 20012251ndash14
Tonnesen HH Photodecomposition of drugs In Swarbrick J Boylan JC editors Encyclopedia of Pharmaceutical Technology vol 3 Marcel Dekker New York USA 2002 pp 2197ndash2203
Traber MG Vitamin E In Zempleni J Rucker RB McCormick DB Suttie JW editors Handbook of Vitamins 4th ed Taylor amp Francis CRC Press Boca Raton Florida USA 2007 Chap 4
Turro NA Ramamurthy V Scaiano JC Modern Molecular Photochemistry of Organic Molecules University Science Book Sausalito California USA 2010 pp 531ndash535
United States Pharmacopeia 39 ndash National Formulary 34 United States Pharmacopoeial Convention Rockville MD USA 2016
Vinodo VB Budhwaar V Nanda A Photochemical fate of pharmaceuticals an updated review IJPRBS 2015454ndash70
Yoshioka S Stella VJ Stability of Drugs and Dosage Forms Kluwer AcademicPlenium Publishers New York USA 2000 pp 28ndash32 105ndash107135ndash137
101
CHAPTER ndash 5
PHYSICAL STABILITY
51 INTRODUCTION
Drug substances and drug products may undergo physical and chemical changes during storage as a result of environmental factors and chemical interactions Physical stability significantly contributes to the chemical stability of the products It is necessary to consider the physical stability of pharmaceuticals in addition to chemical stability and the stability in biological fluids (in vivo) to not only ensure their quality during the shelf-life period but also to maintain their organoleptic properties for consumer acceptance The physical properties of a drug such as melting point particle size and solubility depend on its physical state (eg crystalline or amorphous) and any change in this property could affect the physical stability of the material Physical instability may be considered as any change in the physical state of a formulation during preparation or storage The study of a change in the physical characteristics of drugs and excipients gives an indication of variations in the quality attributes of the product Physical stability is a key factor in product integrity in the dosage forms It may alter the dissolution profile and bioavailability of the drug The dissolution rate may be considered as a measure of physical stability Changes in physical stability may influence the chemical stability of drugs and lead to an acceleration of the degradation processes in the products Therefore appropriate measures should be taken to maintain the physical stability of the products
52 Analytical Techniques in the Study of Physical State
Various analytical techniques have been used for the characterization of the physical state of drug substances and excipients and to study the effect of any variations on their stability These techniques have also been applied to the quantitative analysis of active ingredients and are briefly described as follows
521 Thermal Methods
5211 Thermogravimetric analysis (TGA)
It involves the measurement of change in sample weight as a function of temperature andor time A thermobalance continuously records the weight loss or gain of a sample as a function of time It is used to determine the thermal stability of a material and the fraction of volatile components present
5212 Differential scanning calorimetry (DSC)
It is a modern and accurate technique used in the analysis of solid formulations DSC involves the measurement of difference in heat capacity between the sample and a reference as a function of temperature or temperature It can be used to monitor the energy released or absorbed through chemical reactions occurring during the heating process
5213 Differential Thermal Analysis (DTA)
It involves the measurement of difference in temperature between the sample and a reference as a function of temperature The changes on heating the sample include melting phase transition sublimation and decomposition
5214 Microcalorimetry
It is used to study the kinetics of chemical degradation of drug substances The heat flow produced in a degradation reaction follows a certain order of reaction The thermal conductivity
102
detector can detect small amount of degradation at room temperature such as that involved in the slow solid-state degradation of drugs
5215 Isothermal calorimetry
All physical and chemical processes are accompanied by heat exchange with their surroundings In this technique the sample is maintained under isothermal conditions within a microcalorimeter When a chemical reaction occurs a temperature gradient is formed between the sample and its surroundings The resulting heat flow between the sample and its surroundings is measured as a function of time The technique is used for the characterization and stability assessment of different physical forms of a drug or a product
5216 Dilatometry
Dilatometry is a thermoanalytical method used to measure the shrinkage or expansion over a controlled temperature range (up to 1000degC) It is used to measure the rate of chemical reactions such as changes in molar volume in polymerization reactions and rates of phase transformations
5217 Hot-stage microscopy
It involves the measurement of changes in a crystal on temperature variation and provides useful information on solid-state transitions
522 Spectroscopic Methods
5221 Vibrational spectroscopy
Vibrational spectroscopy is a collective term used to describe infrared (IR) and Raman spectroscopy It involves the measurement of vibrational energy levels associated with the chemical bonds in a compound It is used for the characterization and structure determination of drug substances and to study the interactions occurring within a sample
5222 Fourier transform infrared (FTIR) spectroscopy
FTIR spectroscopy is used as a finger print technique for the characterization of the polymorphs of a compound It can also be used to determine the quality of a sample composition of a mixture and the nature of molecular interactions
Attenuated total reflectance (ATR) is used in conjunction with FTIR (ATRndashFTIR) spectroscopy to enable the samples of a drug to be examined directly in the solid or liquid state ATR uses the property of total internal reflection resulting in an evanescent wave (that tends to vanish) A beam of infrared light is passed through the ATR crystal in such a way that it reflects it at least once off the internal surface in contact with the sample
5223 Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)
It is a technique that collects and analyzes IR radiations scattered by fine particles and powders Sampling is fast and easy because little or no sample preparation is required It is used for the identification of raw material elucidation of crystal properties of polymorphs and quantitative analysis of drug mixtures
5224 Solid state nuclear magnetic resonance (SSNMR) spectroscopy
The solid state NMR spectroscopy is a well established technique for the characterization of the exact structure and differentiation of materials such as the polymorphs and solvates It is also used for the study of their interactions with adsorbed species (eg carbon dioxide water)
5225 Dynamic light scattering (DLS)
DLS is used to measure the size of particles at the submicron level It monitors the Brownian motion of particles suspended in a liquid with light scattering The larger the particle the slower the Brownian movement is observed It is also used to measure the zeta potential (surface
103
charge) of a particle and to determine the molecular weight of organic compounds DLS is also referred to as Photon Correlation Spectroscopy (PCS)
5226 X-ray powder diffraction (XRPD)
It measures the average spacing between the layers or rows of atoms in a molecule It is used for the characterization and identification of polycrystalline phases of a compound The main use of this technique is to identify components in a sample by a searchmatch method on comparison with known diffraction patterns The technique can also be applied to the quantitation of different phases
5227 Single crystal X-ray diffraction (XRD)
XRD is used to determine the orientation and the structural features of a single crystal for characterization
523 Other Techniques
5231 Polarized light microscopy
It is a very useful technique in the study of polymorphism for particle characterization such as the size shape and aggregation
5232 Particle electrophoresis
It is a widely used technique for the measurement of zeta potential An electric field is applied across a sample (eg suspension) which induces charged particles to move The velocity of the particle is dependent on its zeta potential The measurement of zeta potential is necessary for the prediction of formulation stability and interactions It can also be used as a simple method of quality control
This chapter deals with a brief description of the various aspects of physical stability of drugs and drug products Detailed accounts of the physical stability of drug substances (Yoshioka and Stella 2000) extemporaneous preparations (Jackson and Lowey 2010 Haywood and Glass 2013) and physical testing of drug products (Carstensen 2000) are available for further study Several reviews have been published on the physical stability of amorphous and crystalline states ( Berglund et al 1990Saleki-Gerhardt et al 1994 Hancock and Zografi 1997 Craig 1999 Yu et al 2001 Vippagunta et al 2001 Babu et al 2012) solid dispersions (Qian et al 2010 Kalia and Poddar 2011 Kapoor et al 2012 Kumavat et al 2013) emulsions (Zografi 1982) nanoparticles (Wu et al 2011) and physical transformations (Morris et al 2001 Zhou et al 2009 Bhattacharaya and Syrayanarayanan 2009)
53 CHANGES IN PHYSICAL STABILITY
Drug substances and adjuvants are usually manufactured in the solid form and exist in the amorphous state or in different crystalline states (polymorphs) The amorphous drug in most cases is not stable and may gradually change to a thermodynamically more stable crystalline form It may also undergo hydration or dehydration process during storage The changes in the physical stability of liquid dosage forms may lead to a change in appearance formation of precipitates formation of polymorphs of low solubility flocculation and sedimentation drug adsorption on to the container surface and microbial growth The change in physical stability of solid dosage forms may affect characteristics such as appearance (eg color shape) mechanical strength (eg tablet hardening softening) content uniformity (eg suspensions) and dissolution rate and bioavailability The major cause of all these factors is phase transition occurring in the material It may involve polymorphic transition solvation and desolvation salt and salt exchange and amorphization and devitrification (reversion to crystalline form) Phase transition can occur through solid state melt solution or solution mediated mechanisms Pharmaceutical processes including comminution compaction granulation drying and coating may lead to partial or complete phase transition resulting in the physical destabilization of the material
104
54 FACTORS AFFECTING PHYSICAL STABILITY
Most of the multicomponent systems used in the formulation of drug products do not assume a state of thermodynamic equilibrium and therefore undergo transitions to achieve a state of equilibrium These transitions are accompanied by a decrease in the free energy of the system and are influenced by the following factors
541 Internal Factors
Formulations of drug substances often contain additives and excipients and may involve potential drugndashdrug and drugndashexcipients interactions and compatibility problems These could lead to changes in the physical and chemical stability of the system Two or more drugs present in a product may also react with each other and thus cause a change in the physicochemical characteristics of the product
542 External Factors
The storage of pharmaceutical products at high temperature may cause transmission of the thermal activation energy to the system to make it thermodynamically unstable This may lead to physical changes such as those observed in appearance crystalline structure consistency viscosity homogeneity dispersion firmness and disintegration of solids The changes in the physical state may also include drying of semisolid dosage forms liquefaction of eutectic mixtures of powder due to low melting point and cementing of tablets etc
Solid dosage forms on storage under humid conditions may lead to the absorption of moisture resulting in changes in the mechanical strength of the tablets The change in mechanical strength is a function of moisture uptake of the tablet the moisture permeability of the package and the humidity conditions employed Physical stability of solid pharmaceuticals is also affected by the plasticizing effect of water probably due to an increase in molecular mobility Amorphous drugs (eg indomethacin nifedipine lamotrigine mesylate) show decreased values of glass transition temperature (Tg) and increased crystallization on the absorption of moisture Tg and NMR relaxation-based critical mobility temperature (Tmc) both are useful parameters for the measurement of molecular mobility Tmc of an amorphous drug is generally lower than Tg indicating that the glassy pharmaceutical solids show significant molecular mobility even at temperature below Tg (Yoshioka and Stella 2000)
543 Amorphous State
The non-crystalline state of solids is known as the amorphous state These solids do not possess long-range order characteristics of a crystal and have no unit cells They appear to behave like super-cooled liquids that show the arrangement of a molecule in a random order similar to that of the liquid state Solids in the amorphous state exhibit properties that are different from those of the crystalline state of the same substance They do not have a melting point as observed in the case of crystalline materials possessing a crystal lattice that collapses on melting
The Tg is characteristic of the amorphous solids It represents the temperature at which an amorphous material changes its physical character from a glass-like solid into a more mobile rubber like state Tg is a measure of the stability of the amorphous state of a drug The physical stability of amorphous solids increases with an increase in Tg The use of an amorphous drug in a dosage form leads to an increase in the rate of dissolution and consequently the bioavailability of the drug (Bauer 2009)
The drugs and adjuvants in the amorphous state have generally greater solubility in water than that of the crystalline state which has a lower ground state free energy (∆G) compared to the amorphous state Therefore the drugs in the later state would convert to the thermodynamically more stable crystalline state on storage According to Yoshioka and Stella (2000) this change may lead to drastic variations in release characteristics of the drug which would alter its clinical efficacy and toxicological effect Examples of conversion of amorphous state of drugs to crystalline state during storage include nifedipine (Uekama et al 1992) oxyphenbutazine (Matsuda and Kawaguchi 1986) and furosemide (Matsuda et al 1992) The characteristics and significance of
105
the amorphous state in pharmaceutical systems have been reviewed in detail by Hancock and Zografi (1997) whereas their preparation characterization and stabilization has been discussed by Yu (2001)
Mahlin and Bergstrom (2013) studied the physical stability of the amorphous state of fifty structurally diverse drugs using DSC and XRPD methods The thermal properties and molecular weight of drugs were used to develop predictive methods of physical stability Drugs with molecular weight greater than 300 gmole were expected to transform into their amorphous state by spray drying and melt-cooling technology The Tg and molecular weight were used to predict the physical stability of the material on storage for one month for the glass-forming compounds A strong sigmoid relationship has been found between the crystallization temperature and physical stability of the drugs These observations are useful in the development of amorphous formulations of drugs
The amorphous state stability of ketoprofen and flurbiprofen has been studied by thermal analysis The amorphous forms of these compounds were obtained by super-cooling of the melt in a DSC apparatus and subjected to storage for sixty days The measurement of the enthalpy (∆H) the distribution of molecular relaxation times (Tm) and Tg of the amorphous samples indicated that flurbiprofen has greater physical stability at any aging temperature compared to that of ketoprofen The values of ∆H and Tm of flurbiprofen were found to be greater than those of ketoprofen Both amorphous drugs were suggested to be classified as ldquofragilerdquo (Hoti et al 2012)
A study has been conducted to evaluate commonly calculated parameters of the amorphous state of different drugs in term of their predictive capabilities of physical stability It included the determination of configurational heat capacity (Cp) and rate dependence of Tg of the material The amorphous samples were heated at 1 Kmin from 50degC below to 30degC above the Tg The ∆Cp was calculated as the difference between Cp of the amorphous and the crystalline states and the configuration thermodynamic properties entropy (∆S) enthalpy (∆H) and Gibbs free energy (∆G) were also determined The results indicated that all the drugs are fragile glass formers however variations in the degree of fragility with a group of drugs (acetaminophen cefuroxime axetil donepezil HCl indomethacin lacidipine nifedipine salsalate simvastatin tolbutamide and troglitazone) were observed Below the Tg fragility showed no linear correlation with amorphous stability and the strong glass formers could form more stable glasses It has been observed that below Tg no clear relationship between the various factors and physical stability exists Above Tg ∆S showed the largest correlation with stability however the stability above Tg can not necessarily be related to the physical stability below Tg and therefore ∆S may only serve as a limited predictive parameter of physical stability (Graeser et al 2008)
Lobmann et al (2011) developed a co-amorphous system to enhance the physical stability and dissolution rate of drug substances It was applied to a combination of non-steroidal anti-inflammatory drugs (NSAIDs) naproxen and indomethacin The co-amorphous binary phase of these drugs was prepared at molar ratios of 21 11 and 12 by quench cooling and the physical stability was studied at 27715 and 29815 K under dry conditions using XRPD analysis FTIR was used to detect molecular interaction between the two drugs and DSC to assess Tg The results indicated that naproxen in combination with indomethacin is converted to the co-amorphous form FTIR spectra suggested the formation of a heterodimer between the two drugs A sample at 11 ratio of the drugs remained in the amorphous form while those at 12 and 21 ratios resulted in the recrystallization of these drugs upon storage The dissolution testing of the co-amorphous form showed an increase in the dissolution rate of both drugs with a synchronized release for the 11 blend This approach can be adopted to overcome the problem of formulation of poorly-soluble crystalline drugs so as to increase their solubility and dissolution rate
544 Dosage Forms
The physical stability of the amorphous drugs in various dosage forms has been studied by several workers and is presented in the following sections
106
5441 Solid dispersions
The physical stability of four alcohol-free spironolactone suspensions stored at 4 25 and 40degC over a period of sixty days has been studied The rheological behavior particle size variation and optical characteristics were used to predict long-term physical stability of the suspensions All the suspensions were coarse dispersion with particle size greater than 1 microm Sedimentation was found to occur in one suspension and flocculation of the drug in the other three suspensions (syrup base) The high viscosity of these suspensions made it difficult to achieve homogeneous redispersion It affected their dissolution profile that was the slowest in comparison to the other suspension There was no change in color or odor of the suspensions during storage at all the three temperatures A very slight increase in particle size distribution (PSD) was observed after the storage period (Bernal et al 2014) PSD is a critical parameter that affects the bioavailability and pharmacokinetics of the product (Kulshreshtha et al 2010) Optical analysis of the suspensions was carried out to detect destabilization of the suspensions This technique is used to assess the physical stability of the system without diluting or modifying the formulation (Gonzalez-Mira et al 2010)
The evaluation of the physical stability of clopidogrel oral suspension indicated isomeric conversion of the active Sndashform of the drug to the inactive Rndashenantiomer A 15 and 30 conversion of the Sndashform to Rndashenantiomer took place after storage for four days at 4 and 25degC respectively On the basis of these results an expiry date of one month under refrigeration conditions and two week at 25degC has been recommended (Mihaila et al 2012)
The amorphous solid dispersions of quinapril HCl and citric acid (11 to 16) prepared by colyophilization have been subjected to degradation in citrate buffer at 80degC and Tg values measured by DSC The rate of reaction showed low dependence at pH 249 on the Tg of the sample However the rate decreased on an increase in Tg value at pH ge 275 The rate of degradation was found to increase with pH at a constant Tg value of about 70degC The degradation of quinapril HCl is a function of the relative concentrations of quinapril and its zwitterionic form At higher pH values with a higher concentration of zwitterionic form the rate of the reaction is strongly affected by the Tg of the mixture and hence the molecular mobility At the lower pH with a higher concentration of the non-ionized quinapril molecule the degradation is less sensitive to Tg probably due to a different mechanism involved (Li et al 2002)
Solid dispersions have been shown to possess a potential to increase the release rates of poorly water soluble drugs Most of the drug candidates in pharmaceutical pipeline today are either poorly soluble or water-insoluble To meet this challenge various processes have been developed to increase the solubility dissolution rate and bioavailability of active ingredients of pharmaceutical formulations belonging to Biopharmaceutics Classification System (BCS) II and IV Out of the various formulations (solid dispersion nanoformulations lipid formulations cyclodextrin complexes etc) solid dispersion is a vital area of research in the development of pharmaceutical formulations Specifically it involves the development of formulations with a high drug loading andor containing drugs with a high tendency to crystallize (indicated by high melting point of gt 290degC) (Brough and Williams 2013) Such dispersion is basically a simple two-component system of drug polymer interaction in the solid state where the drug and the polymer act as a solute and solvent respectively The ultimate success of a solid dispersion is determined by its performance on dissolution after oral administration The general strategy behind almost all the solubilization technologies is the so called ldquospring- and ndash parachutesrdquo concept (Guzman et al 2007) According to this concept for solid dispersions drug should first dissolve along with the soluble polymer matrix to create a supersaturated solution (ldquothe springrdquo) after which super-saturation should be maintained long enough for drug absorption (ldquothe parachutesrdquo) to take place
The major problem with most of the solid dispersions for oral use is that they form a supersaturated drug solution when they come in contact with the aqueous environment of GIT Drugs in this state have a tendency to precipitate rapidly before being absorbed (causing poor bioavailability) Recently such work has been carried out to evaluate suitable polymers that are able to maintain a supersaturated drug concentration in vivo for an extended period of time to allow optimal absorption Fortunately few polymers specifically some cellulose derivatives are known
107
to possess this ability and they include hydroxypropyl methyl cellulose (HPMC) and hydroxypropyl methyl cellulose acetate succinate (HPMC AS) vinyl polymers such as polyvinylpyrrolidone (PVP) and polyvinyl pyrrolidone-co-vinyl acetate (PVPVA) (Xu and Dai 2013) The mechanism how the polymer prolong drug super-saturation is still not fully understood
A study has been carried out to evaluate the effect of certain formulation parameters ie solvent evaporation temperature drug PVP ratio and PVP molecular weight on the physical stability of the amorphous solid dispersion of piroxicam The results showed that the evaporation temperature has the highest effect in inhibiting the nucleation of piroxicam while piroxicamPVP ratio has the highest effect in decreasing the crystal growth The influence of increasing evaporation temperature and piroxicamPVP ratio are in the same order of magnitude to increase the physical stability of dispersions The PVP molecular weight showed a minor effect in decreasing the crystal growth of piroxicam in PVP matrix The studies were carried out using polarized light microscopy (Wu et al 2011)
Yang et al (2010) developed a kinetic model to predict the physical stability of amorphous drugndashpolymer solid dispersions on recrystallization The kinetics of recrystallization was determined by DSC for amorphous efavirenzndashPVP solid dispersion stored at controlled temperature and relative humidity The kinetic model was used to determine the recrystallization rate constant and the microscopic geometry of crystal growth Temperature was found to affect the drug recrystallization rate constant according to the Arrhenius relationship while the rate constant increased linearly with relative humidity PVP content inhibited the recrystallization process by increasing the crystallization activation energy and decreasing the equilibrium crystallinity
FTIR spectroscopic imaging has been applied to study the physical stability of solid dispersions of poorly water-soluble drugs in polyethylene glycol (PEG) and their dissolution in water The amorphous nifedipine was found to crystallize within PEGndash8000 for formulations containing 10 drug The crystallization of the drug within the polymer matrix reduced its rate of dissolution FTIR imaging in the ATR mode provided information on the mechanism of the dissolution of nifedipine from solid dispersions in water-soluble polymers which is helpful for the optimization of manufacturing of these formulations (Chan and Kazarian 2004)
The effect of thermal methods (eg melt method) on the polymorphic changes in the formulation of solid dispersion of candesartan cilexetil with polyethylene glycol 8000 has been studied DSC XPRD FTIR and HPLC have been used to evaluate the polymorphic changes in the final formulation of the drug DSC indicated the shift of endothermic peak of the formulation toward the lower temperature XPRD showed the relative degree of crystallinity as 0645 FTIR indicated a shift in the peaks of the drug due to polymorphic changes HPLC showed the in vitro release of candesartan cilexetil from the solid dispersion within 10 min It has been concluded that the preparation of this formulation at high temperature may result in polymorphic changes in the drug (Thirupathi et al 2014)
5442 Semisolid dispersions
The physical stability of a semisolid dispersion of piroxicam into hard gelatin capsules prepared with Gelucire 4414 (a methyl acetate derivative) labrasol and excipients such as microcrystalline cellulose (MCC) mannitol and lactose (αndashmonohydrate) has been studied The master dispersion containing only Gelucire 4414 20 ww and labrasol 80 ww was stored at 5plusmn3degC in a refrigerator while the modified dispersion with the excipients (2 ww) were kept at 25plusmn2degC 60plusmn5 RH in a climatic chamber for one year Dissolution tests were carried out in media at different pH on the freshly prepared dispersions and on those stored for three six and twelve months FTIR and DSC studies confirmed the existence of piroxicam in the amorphous state in all the dispersions under the specified storage conditions for one year (Karatas and Bekmezci 2013)
5443 Creams
The colloidal stability of alcoholic emulsion creams stored for six months at ambient temperature has been studied It was found that the size of fat droplets significantly affects the stability of creams during storage Dispersion of about 80 of the lipid fraction by pressure
108
homogenization to the size below 20 microm prolonged the stability of the system by two months The enrichment of the creams with 01 each of lecithin and sodium caseinate imparted full physical stability to the creams during the storage period as a result of an increase in lipid phase dispersion and delay in coalescence The addition of oxidized sterol (05 ) decreased the mobility of dispersed phase droplets and protected against coalescence and cream formation A decrease in conductance of the creams was also observed during storage the largest decrease (from 99 to 72 microScm) being in the presence of oxidized starch which increased the viscosity of the emulsion There was no correlation between emulsion conductance and droplet size in the dispersed phase (Tarko and Tuszynski 2007 Floury et al 2000)
The physical stability of theophylline cream (ow) is affected by the presence of preservatives The addition of preservatives in the cream stored for forty nine days at room temperature did not show a change in physical characteristics While the cream without preservatives exhibited a few signs of dryness and color change The growth of microorganisms in the cream can cause separation of fatty material and thus affect its physical stability Good homogenization technique may decrease the effect of heat on the stability of the cream (Abdul Hussain et al 2009)
Physical changes in the water-in-oil creams of ascorbic acid have been observed when
stored at 301degC for 3 months The creams showed changes in color creaming and phase separation (breaking) with time All the physical changes were found to be affected by the formulation factors such as type of emollient and humectant pH and viscosity of the medium and specific gravities of the liquids used in the formulation of the creams (Sheraz et al 2014)
5444 Liposomes
The physical stability of uncoated and chitosan-coated liposomes (1 ww soy lecithin) incorporating polyphenol-rich grape seed extract (01 ww) has been studied Both types of liposomes showed good physical stability during storage for eight days It was confirmed by the measurement of particle diameter using DLS and the determination of polydispersity index (PDI) values that did not change during storage PDI is considered as an indicator of the broadness of particle size distribution The uncoated liposomes showed the smallest PDI (02) indicating that the solution was monodispersed whereas the coated liposomes were found to be polydispersed After the storage period uncoated liposomes (empty) showed a broad particle size distribution as a result of the oxidative degradation of unsaturated fatty acids in phospholipids The measurement of zeta-potential of all the liposomes using particle electrophoresis did not show any change during storage Zeta-potential is a measure of the surface charge of the particles and affects the repulsive colloidal interactions It gives an indication of the physical stability of coated liposomes (Gibis et al 2013 Laye et al 2008 Panya et al 2010) In another study carried out on the physical stability and drug release of cholesterol derivatives in liposomes revealed a positive charge at a pH between 3 and 10 as indicated by zeta-potential It was further revealed that cholesterol liposomes have better physical stability compared to that of cholesterol without liposomes (Yang et al 2013)
5445 Proteins
The development of protein pharmaceuticals involves the study of their physical stability under normal and stress conditions According to Chang and Yeung (2010) the physical stability of the majority of proteins can be expressed in terms of resistance to unfolding forces because aggregation andor precipitation can occur when the structural change results in a less soluble conformational state Conformational changes in proteins occur as a result of the conversion of their biologically active forms to non-active andor inactive conformations The resistance to unfolding (thermodynamic stability) depends on various forces that contribute to the folding of proteins These forces result from covalent bonds electrostatic interaction hydrophobic interactions hydrogen bonds and van der Waal interactions (Dill 1990 Guo et al 2006)
545 Crystalline State
Crystalline state of the matter is the state in which the molecules are packed in a defined order that is repeated throughout its particles in the system The physical stability of solid drugs is influenced by their crystalline state The crystalline drugs have lower ground state free energy and
109
higher free energy of activation (∆G) with lower reactivity ∆G gives the difference in free energy between the reactant state and the activated state The different crystalline forms of the same drug (polymorphs) have different free energies and may undergo transition from one form to the other Polymorphic transition in drugs may occur during storage under different conditions of temperature and humidity and thus alter their critical properties such as the solubility and the dissolution rate This may affect the efficacy of the drug in a formulation
Crystalline drugs although are known to be more stable but possess a problem of low solubility and bioavailability Therefore often various methods are employed to modify the physical state properties of the active crystalline drug and enhance its solubility and bioavailability These methods may include alteration in crystal structure drugndashpolymer complexation formation of solid dispersions formulation of drug with lipophilic bases particle size reduction salt formation etc The techniques used for such alteration may include solvent evaporation solidification of melt mechanical activation of crystalline mass rapid precipitation from solution ie during spray drying or freeze-drying etc One problem often encountered during the preparation of amorphous solids from their crystalline form is their reconversion to the parent state In order to avoid such problem often hydrophilic polymers are added in the dispersions Hydrophilic polymers are known to retard recrystallization of amorphous forms by their antiplasticizing effect Such polymers may also forms a complex with the drug and increase its Tg Storage of amorphous blends below their Tg and protection from plasticizers such as moisture can retard the chances of recrystallization Huang and Dai (2014) have reviewed the various aspects of solid dispersion and drugndashpolymer interaction for poorly soluble drugs
Tolfenamic acid is a crystalline drug that belongs to the fenamate family of NSAIDs Its amorphous form has been prepared either by freeze-drying with chitosan (Ahmed et al 2013) and polyacrylic acid (Sheraz et al 2015) or by solvent evaporation technique with polyurethane (Istanbullu et al 2013) It has been found that the transformation of crystalline state to amorphous form is not only limited to the technique employed but also depends on the properties such as the ratio and molecular weight of the polymer pH of the medium and storage conditions employed In case of tolfenamic acid the molecular weights of chitosan and polyacrylic acid showed to affect the conversion from the crystalline state to the amorphous form whereas low molecular weight polymers showed better conversion than the high molecular weight polymers Similarly pH has also been shown to play an important role in the transformation of physical state properties of tolfenamic acid The pH values near to the pKa values of the polymers have shown better conversion into the amorphous state with lesser amount of the polymer required This could be due to the better miscibility of the drug with the polymer thus resulting in better interaction between the two compounds (Ahmed et al 2013 Sheraz et al 2015) In the case of solvent evaporation technique used for the preparation of films of tolfenamic acid with polyurethane it was observed that the solvent employed for the evaporation also affects the degree of conversion from the crystalline to the amorphous state More amorphous tolfenamic acid was formed in films where only tetrahydrofuran was used as compared to films prepared with a mixture of tetrahydrofuran and ethanol (Istanbullu et al 2013) No recrystallization was observed in any of the samples prepared with chitosan polyacrylic acid or polyurethane when stored in a desiccator for a period of 3 months
Many water-soluble crystalline compounds after micronization have poor physical stability on exposure to moisture It results in caking and severe aggregation which can be detrimental to the performance of their pharmaceutical products It has been observed that micronization gives rise to amorphous regions into the crystalline material that cannot be determined by the XRPD method These amorphous regions transform due to surface sintering and recrystallization at relative humidity well below the deliquescent point The characterization of micronized solids can be carried out using microcalorimetry (Bystrom 1990)
546 Polymorphism
Polymorphism can be defined as the existence of a solid material (eg drug substance) in more than one form or crystalline structure known as polymorph The polymorphs can be classified into two types as monotropes (a polymorph unstable at all temperature and pressures eg glyceryl
110
stearates) and enantiotropes (a polymorph reversibly converted into another form by changing temperature or pressure eg sulfur) This classification is based on the stability of the polymorphs over a range of temperatures or pressures below their melting points at a constant atmospheric pressure The transition temperature is expressed as the temperature at which two polymorphs possess identical free energies (∆G) can coexist together and have same solubilities in a solvent In a certain temperature range below the solid melting temperatures a polymorph having the lower free energy that corresponds to the lower solubility is considered as the thermodynamically stable form
The crystalline structure of a compound exerts a profound effect on its solid-state properties For a given material the heat capacity conductivity volume density viscosity surface tension diffusivity crystal hardness shape and color refractive index electrolyte conductivity melting and sublimation properties latent heat of fusions heat of solvation solubility dissolution rate enthalpy of transition phase diagrams stability hygroscopicity and rate of reactions are all affected by the nature of the crystal structures The differences in solid state properties of alternate crystal forms could give rise to measurable differences in the properties of pharmaceutical systems (Brittain 1999 2002a) Various aspects of polymorphism of pharmaceutical compounds have been discussed by Borka and Haleblian (1990) Brittain (1999) and Singhal and Curatalo (2004)
The different types of polymorphism are described as follows
5461 Conformational polymorphism
The polymorphism resulting from different conformers of the same molecule is called conformational polymorphism For example the existence of probucol a cholesterol lowering drug into two polymorphic forms The molecular symmetry of the molecule is lost in the structure of form 1 The less symmetrical conformer (form 2) is more stable with lower activation energy than form 1 (Gerber et al 1993)
5462 Solvatomorphism
A solvatomorph can be defined as a crystalline solid in which solvent molecules are included in the structure through the existence of positional substitution at positions that are site specific and related to other solvent molecules through translational symmetry It may also involve the incorporation of water into a crystal lattice For example ampicillin crystallizes in one trihydrate and at least two anhydrate forms The transition temperature for the two forms in the presence of water is 42degC where as the trihydrate is formed when crystallization is conducted below this value and the anhydrates are formed on crystallization at temperatures exceeding 42degC (Boles and Girven 1976)
5463 Packing polymorphism
Polymorphism that exists as a result of difference in crystal packing of molecules is termed as packing polymorphism Single-crystal X-ray crystallography has been used to determine the structures of organic molecules The structure of resorcinol (13-dihyrdoxybenzene) showed that the crystalline material corresponded to that ordinarily formed at room temperature was termed as the αndashform (Robertson 1936) Later it was found that the αndashform undergoes transformation into a denser crystalline form when heated about 74degC The structure of this form (denoted as βndashform) was completely different from that of the αndashform The crystal structures of the two forms showed that resorcinol is locked into a single confirmation and that each form is characterized by a different form of hydrogen bonding The αndashform exhibits a relatively open architecture maintained by a spiraling array of hydrogen bonding that ascends through the various plains of the crystal The effect of the thermally induced phase transformation is to remove the open arrangement of the αndashform by a more compact and parallel arrangement of the molecule to give the βndashform The crystal change leads to an increase in the crystal density on passing from the αndashform to the βndashform The molecular packing existing in the βndashform is typical of hydrocarbon than that of a hydroxylic compound such as resorcinol (Robertson et al 1938)
111
5464 Pseudopolymorphism
The pharmaceutical compounds are often crystallized using different solvents During this process the solvent molecules may be incorporated into the crystal lattice in a fixed ratio This leads to the formation of co-crystals termed as solvates If the crystallization is carried out using water the crystals are termed as hydrates These crystalline forms of the compound are called pseudopolymorphs and the phenomenon is known as pseudopolymorphism
5465 Forced polymorphism
In the study of the polymorphs an attempt is made to induce or force polymorphism in drug substances It does not imply that any polymorphic forms observed will necessarily be present during the drug development processes or on stability evaluation Under forced polymorphism the forms obtained may not appear during the manufacturing process or stability evaluation However if these forms are found during drug development it would be easier to detect them and to determine their influence on the physical stability of the drug and the product
547 Characterization of Polymorphs
The polymorphs of crystalline pharmaceutical solids can be identified by using various analytical techniques (Haleblian 1975 Stagner and Guillory 1979 Ford and Timminis 1989 Wu et al 1994 Brittain 1995 1997 1999 Threlfali 1995 Bougay 2001) The most widely used techniques for the physical characterization of solid materials (polymorphs and solvates) include crystallography microscopy thermal analysis solubility determination vibrational spectroscopy and NMR spectrometry (Brittain 2002b) The major criterion for the existence of different types of polymorphs is the observation of semiequivalence of their crystal structures by XRPD analysis A very important method for the characterization of polymorphs is microscopy It can show variations in the habits of different crystal structures and this is useful in the characterization of polymorphs (Haleblian 1975) The hot-stage microscopy and thermal microscopy are extensively used techniques for the characterization of polymorphs and solvates These techniques involve the observation of changes during the heating and cooling of a few mg of the substance or any crystalline material on a microscopic slide (McCorne 1957) The thermal microscopic studies of a large number of pharmaceuticals have been conducted (Kuhnert-Brandstalter 1971)
Thermal analysis methods have been used for the characterization of polymorphs (Ford and Timminis 1989) These methods involve the determination of a physical property of the drug substance as a function of an externally applied temperature In these methods the physical property and the sample temperature are automatically measured and the sample temperature is varied at a predetermined rate These techniques are widely used in pharmaceutical industry for the characterization of compound purity polymorphism and excipients compatibility (Giron 1986) The most commonly used methods of thermal analysis include TGA DSC and DTA These techniques provide information on phase transformation as a function of temperature (Brittain 2000)
The relative stability of the polymorphic forms of drugs can also be studied by thermal analysis Melting temperatures of the compounds can be used to establish the relative order of the stability of their polymorphic forms and any interphase conversions involved In the case of auranofin the anhydrous polymorphic form of the molecule is most stable as indicated by the melting point and heat of fusion data (Lindenbaum et al 1985) DTA thermograms of the two forms of chloroquin diphosphate have shown that one form is pure but the other form is a mixture of two polymorphs (van Aerde et al 1984) A DTA study of the dissolution of three crystalline forms of spironolactone in conjugation with XRPD showed differences in the behavior of the drug (Salole and Al-Sarraj 1985) DSC analysis of the polymorphic compounds has the advantage that the area under DSC peak is directly proportional to the heat absorbed or evolved on heating The peak area integration gives the enthalpy of the reaction (∆H) and throws light on the thermodynamic behavior of the system (Brittain 2002b)
FTIR spectrometry has been used to differentiate and characterize the polymorphic forms of drug substances The spectra of the two forms of ranitidine HCl show difference in the region above 3000 cmndash1 and in the regions 2300ndash2700 cmndash1 and 1570ndash1620 cmndash1 (Cholertou et al 1984)
112
The different crystalline forms of zenoterone have been found to give characteristic absorption bands in the IR region which can be used for the identification of these forms (Rocco et al 1995) The polymorphic changes in tolfenamic acid has been studied using FTIR spectrometry (Jabeen et al 2012 Mattei and Li 2012 Sheraz et al 2015)
The ATRndashFTIR spectrometry has been used for the identification and quantitation of two polymorphs of aprepitant (an antagonist) for chemotherapyndashinduced emesis The spectra of the powdered samples of the polymorph pair were obtained over the wavelength range 700ndash1500 cmndash
1 Significant spectral differences between the two polymorphs at 1140 cmndash1 have been observed that indicate that this technique can be used for definitive identification of the polymorphs The quantification of the polymorphic form of the drug was carried out using a calibration plot of peak ratio of the second derivative of absorbance spectra against the weight percent of form II in the mixture The polymorphic purity results obtained by ATRndashFTIR spectrometry were in good agreement with the prediction made by XRPD analysis (Helmy et al 2003)
DRIFTS coupled with partial-least-squares (PLS) data analysis has been applied for the determination of the components of solid state mixtures of ephedrine and pseudoephedrine The cross-validated standard errors of prediction of 074 wt in the concentration range of 0ndash50 wt and 011 wt in the concentration range 0ndash50 wt have been obtained (Dijiba et al 2005) The technique coupled with artificial neural networks (ANNs) in two versions (ANN-raw and ANN-pca) support vector machines (SVMs) lazy learning (LL) and PLS regression has been used to quantify carbamazepine crystal forms in ternary powder mixtures (I III and IV) The analysis has been carried out in the IR spectral regions of 675ndash1180 and 3400ndash3600 cmndash1 The results indicate that all the selected algorithms perform better than the PLS regression with a root mean squared error of prediction (RMSEP) of 30ndash82 (Kipouros et al 2006)
The two polymorphs of famotidine have been determined by DSC and FTIR microspectroscopy The results show that the raw material of the drug consists of form B The intensity of the IR absorption band of the B form at 3505 cmndash1 gradually decreases with the grinding time while two new IR bands at 3451 and 1671 cmndash1 for famotidine form A slowly appear The peak intensity ratio of 34513505 cmndash1 linearly increases with the grinding time suggesting that the grinding process could induce polymorphic transformation of famotidine from form B to form A by a zerondashorder process (Lin et al 2006)
The two polymorphic forms (I and II) of fluconazole have been prepared by crystallization in dichloromethane and characterized using DSC TGA XRPD solubility and DRIFTS DRIFTS has also been used to study the kinetics of the transformation of polymorph II (metastable form) to polymorph I (stable form) under different isothermal temperatures The application of 18 solid-state reaction models showed that the Prout-Tompkins model provides the best fits for transformation The activation energy (Ea) value derived from the rate constants of the model was found to be 329 kJ molndash1 (Obaidat et al 2010)
Solid state NMR (SSNMR) spectrometry has been employed for the qualitative differentiation of polymorphs or solvates The technique shows differences in their molecular conformation as a result of crystallographic vibrations The crystal structure of one form of fosinopril sodium shows a most stable phase which is different from that of its metastable phase (Brittain et al 1993) The SSNMR spectrometry has also been applied to determine the phase composition of anhydrate and dihydrate forms of carbamazepine (Suryanarayanan and Widemann 1990) The SS13CndashNMR spectra of the polymorphs of furosemide show a greater molecular mobility and disorder in its form II compared with the rigid and uniformly ordered structure of form I (Doherty and York 1988)
The polymorphic form of clopidogrel hydrogen sulfate (HSCL) (an antiplatelet agent) in solid dosage forms can be verified by SSNMR spectrometry Such structural characterization of the polymorph could assist in the development of new pharmaceutical formulations containing HSCL and also in the identification of its counterfeit drugs (Pindelska et al 2015) The micro- or nano crystalline proteins can be studied by magic-angle spinning (MAS)ndashSSNMR spectroscopy The technique is used to provide atomic-resolution insight into the structure of the molecule when single crystals cannot be studied by XRD method Slight differences in the local chemical
113
environment around the proteins including the cosolvent and the buffer indicate whether single crystal is formed by a protein It has been observed that several formulations of the microcrystals of the protein GBI give very high quality of SSNMR spectra The polymorphs of the protein have been characterized by XRPD and NMR assignments have been made The technique has potential utility in the study of the formulation of industrial and therapeutic proteins (Schmidt et al 2007)
The applications of SSNMR spectrometry in the characterization of pharmaceutical solids including drug substances and solid dosage forms have been reviewed (Tishmack et al 2003) This technique is generally used for1) studying structure and conformation 2) analyzing molecular motions (relaxation and exchange spectrometry) 3) assigning resonances (spectral editing and two-dimensional correlation spectrometry) and 4) measuring internuclear distances
548 Pharmaceutical Implications
The physical stability of drug substances (amorphous or crystalline) and drug products involves the study of variations in their physical state over a period of time Most of the drug substances are crystalline in nature and may occur in the form of different polymorphs The study of polymorphism crystallization and characterization of the polymorphs is an important aspect of preformulation work in drug development The investigation of the solid state properties and their changes in drug substances could enable the selection of a polymorph that is thermodynamically most stable The polymorphs of drug substances can show variations in solubility and dissolution rates that could result in nonequivalent bioavailability of their polymorphic forms It is therefore necessary to evaluate polymorphism in drug substances to ascertain the role of their polymorphic forms in the development of formulations A drug may exist in more than one polymorphic form one of which may be more stable than the others and could be preferred for the formulation of a product However if a metastable form has higher solubility better release characteristics and reasonable stability over a period of time it may be used for development work
The poorly water-soluble drugs are generally formulated in their amorphous state This state possesses a higher internal energy enhanced molecular motion and better thermodynamic properties than those of the crystalline state These characteristics lead to enhanced solubility as well as dissolution rate However the amorphous drugs tend to crystallize during manufacturing storage or administration It is therefore necessary to apply methods for the stabilization of amorphous drugs to take advantage of their enhanced solubility and dissolution rate in the formulation of solid dosage forms
114
REFERENCES
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Ahmed S Sheraz MA Rehman IH Studies on tolfenamic acid-chitosan intermolecular interactions effect of pH polymer concentration and molecular weight AAPS PharmSciTech 201314870ndash879
Babu NJ Sanphui P Nangia A Crystal engineering of stable temozolomide cocrystals Chem Asian J 201272274ndash2285
Bauer JF Pharmaceutical solids-the amorphous phase J Valid Technol 200963ndash68
Berglund M Bystrom K Persson B Screening chemical and physical stability of drug substances J Pharm Biomed Anal 19908639ndash643
Bernal NP Calpena AC Mallandrich M Ruiz A Clares B Development physical-chemical stability and release studies of four alcohol-free spironolactone suspensions for use in pediatrics Dissol Technol 201419ndash30
Bhattacharya S Suryanarayanan R Local mobility in amorphous pharmaceuticals-characterization and implications on stability J Pharm Sci 2009982935ndash2953
Boles MO Girven RJ The structures of ampicillin A comparison of the anhydrate and trihydrate forms Acta Cryst 1976B322279ndash2284
Borka L Haleblian JK Crystal polymorphism of pharmaceuticals Acta Pharm Jugosl 19904071ndash94
Bugay DE Characterization of the solid-state spectroscopic techniques Adv Drug Deliv Rev 20014843ndash65
Brittain HG Morris KR Bugay DE Thakur AB Serajuddin ATM Solid-state characterization of focinopril sodium polymorphs J Pharm Biomed Anal 1993111063ndash1069
Brittain HG Physical Characterization of Pharmaceutical Solids Marcel Dekker Inc New York USA 1995
Brittain HG Polymorphism in Pharmaceutical Solids Marcel Dekker Inc New York USA 1999 pp 331ndash361
Brittain HG Effect of mechanical processing on phase composition J Pharm Sci 2002a 911573ndash1580
Brittain HG Polymorphism pharmaceutical aspects In Swarbrick J Boylan JC editors Encyclopedia of Pharmaceutical Technology 2nd ed Marcel Dekker Inc New York USA 2002b pp 2239ndash2249
Brittain HG Spectral methods for the characterization of polymorphs and solvates J Pharm Sci 199786405ndash412
Brittain HG X-ray diffraction II Using single-crystal X-ray diffraction to study polymorphism and solvatomorphism Spectroscop 200015 34ndash39
Brough C Williams RO 3rd Amorphous solid dispersions and nano-crystal technologies for poorly water-soluble drug delivery Int J Pharm 2013453157ndash166
Bystrom K Microcalormetric testing and physical stability of drugs in the solid state TA Brochure Lukens Drive New Castle UK 1990
Carstensen JT Physical characteristics of solids In Carstensen JT Rhodes CT editors Drug Stability Marcel Dekker Inc New York USA 2000 Chap 8
Chan KLA Kazarian SG FTIR spectroscopic imaging of dissolution of solid dispersion of nifedipine in poly(ethylene glycol) Mol Pharm 2004 1331ndash335
115
Chang BS Yeung B Physical stability of protein pharmaceuticals In Jameel F Hershenson S editors Formulation and Process Development Strategies for Manufacturing Biopharmaceuticals John Wiley amp Sons Inc New York USA 2010 pp 69ndash104
Cholertou TJ Hunt JH Klinkert G Martin-smith M Spectroscopic studies on ranitidine-its structure and the influence of temperature and pH J Chem Soc Perkin Trans 19842 1761ndash1766
Craig DQ Royall PG Kett VL Hopton ML The relevance of the amorphous state to pharmaceutical dosage forms glassy drugs and freeze dried systems Int J Pharm 1999179179ndash207
Dijiba YK Zhang A Niemczyk TM Determinations of ephedrine in mixtures of ephedrine and pseudoephedrine using diffuse reflectance infrared spectroscopy Int J Pharm 200528939ndash49
Dill KA Dominant forces in protein folding Biochemistry 1990297133ndash7155
Doherty C York P Furosemide crystal forms solid-state and physicochemical analyses Int J Pharm 1988 47141ndash155
Floury J Desrumaux A Lardierres J Effect of high pressure homogenization on droplet size distributions and rheological properties of model oil-in-water emulsion Innov Food Sci Emerg Technol 20001127ndash134
Ford JL Timmins P Pharmaceutical Thermal Analaysis Techniques and Applications Ellis Horwood Chichester UK 1989
Gerber JJ Caira MR Lotter AP Structures of two conformational polymorphs of the cholesterol-lowering drug probucol J Cryst Spect Res 199323863ndash869
Gibis M Rahn N Weiss J Physical and oxidative stability of uncoated and chitosan coated liposomes containing grape seed extract Pharmaceutics 20135421ndash433
Giron D Applications of thermal analysis in the pharmaceutical industry J Pharm Biomed Anal 19864755ndash770
Gonzalez-Mira E Egea MA Garcia ML Souto EB Design and ocular tolerance of flurbiprofen loaded ultrasound-engineered NLC Colloids Surf B Biointerfaces 201081412ndash421
Graeser KA Patterson JE Rades T Evaluation of thermodynamic and kinetic parameters as predictors of physical stability of the amorphous state AAPS Annual Meeting 2008 Atlanta GA USA
Guzmaacuten HR Tawa M Zhang Z Ratanabanangkoon P Shaw P Gardner CR Chen H Moreau JP Almarsson O Remenar JF Combined use of crystalline salt forms and precipitation inhibitors to improve oral absorption of celecoxib from solid oral formulations J Pharm Sci 2007962686ndash2702
Guo J Harn N Robbins A Dougherty R Middaugh CR Stability of helix-rich proteins at high concentrations Biochemistry 2006458686ndash8696
Haleblian JK Characterization of habits and crystalline modification of solids and their pharmaceutical applications J Pharm Sci 1975641269ndash1288
Hancock BC Zografi G The characteristics and significance of the amorphous state in pharmaceutical systems J Pharm Sci 1997861ndash12
Haywood A Glass BD Liquid dosage forms extemporaneously prepared from commercially available products considering new evidence on stability J Pharm Pharm Sci 201316441ndash445
Helmy R Zhou GX Chen YW Crocker L Wang T Wenslow Jr RM Vailaya A Characterization and quantitation of aprepitant drug substance polymorphs by attenuated total reflectance Fourier transform infrared spectroscopy Anal Chem 200375605ndash611
116
Hoti E Qiendro G Censi R Matino PD Malaj L Investigation of the drug stability at the amorphous state using thermal analysis J Chem Chem Eng 20126646ndash650
Huang Y Dai WG Fundamental aspects of solid dispersion technology for poorly soluble drugs Acta Pharm Sin B 2014418ndash25
Istanbullu H Ahmed S Sheraz MA Rehman IH Development and characterization of novel polyurethane films impregnated with tolfenamic acid for therapeutic applications Biomed Res Int 20132013178973
Jabeen S Dines TJ Leharne SA Chowdhry BZ Raman and IR spectroscopic studies of fenamates-conformational differences in polymorphs of flufenamic acid mefenamic acid and tolfenamic acid Spectrochim Acta A Mol Biomol Spectrosc 201296972ndash985
Jackson M Lowey A Formulation and stability Handbook of Extemporaneous Preparations a Guide to Pharmaceutical Compounding Pharmaceutical Press London 2010
Kapoor B Kaur R Kaur S Behl S Solid dispersion An evolutionary approach for solubility enhancement of poorly water soluble drugs Int J Recent Adv Pharm Res 201221ndash16
Kalia A Poddar M Solid dispersions An approach towards enhancing dissolution rate Int J Pharm Pharm Sci 201139ndash29
Karataş A Bekmezci S Evaluation and enhancement of physical stability of semi-solid dispersions containing piroxicam into hard gelatin capsules Acta Pol Pharm 20137883ndash897
Kiporros K Kachrimanis K Nikolakakis I Tserki V Malamataris S Simultaneous quantification of carbamazepine crystal forms in ternay mixtures (I III and IV) by diffuse reflectance FTIR spectroscopy (DRIFTS) and multivariate calibration J Pharm Sci 2006952419ndash2431
Kuumlhnert-Brandstaumltter M Thermomicroscopy in the Analysis of Pharmaceuticals Pergamon Press Oxford UK 1971
Kulshreshtha AK Singh ON Wall GM editor Pharmaceutical Suspensions From Formulation Development to Manufacturing Springer New York USA 2010 pp 39ndash65
Kumavat SD Chaudhari YS Badhe M Borole P Shenghani K Physical stability of amorphous solid dispersions a review Int J Pharm Arch 20132129ndash136
Laye C McClements DJ Weiss J Formation of biopolymer-coated liposomes by electrostatic deposition of chitosan J Food Sci 200873 N7ndashN15
Li G Guo Y Zografi G Effect of a citrate buffer system on the solid-state chemical stability of lyophilized quinapril preparations Pharm Res 20021920ndash26
Lin S-Y Cheng W-T Wang S-L Thermodynamic and kinetic characterization of polymorphic transformation of famotidine during grinding Int J Pharm 200631886ndash91
Lindenbaum S Raittie E Zuber G Miller M Ravin L Polymorphism of auranofin Int J Pharm 198526123ndash132
Lobmann K Lactinen R Grohganz H Gordon KZ Strachin C Rades T Coamorphous drug systems enhanced physical stability and dissolution rate of indomethacin and naproxen Mol Pharm 201181919ndash1928
Mahlin D Bergstroumlm CA Early drug development predictions of glass-forming ability and physical stability of drugs Eur J Pharm Sci 201349323ndash332
Matsuda Y Kawaguchi S Physiochemical characterization of oxyphenbutazine and solid state stability of its amorphous form under various temperature and humidity conditions Chem Pharm Bull 1986341289ndash1298
Matsuda Y Otsuka M Onone M Tatsumi W Amorphism and physicochemical stability of spray-dried fruosemide J Pharm Pharmacol 1992 44627ndash633
117
Mattei A Li T Polymorph formation and nucleation mechanism of tolfenamic acid in solution an investigation of pre-nucleation solute association Pharm Res 201229460ndash470
McCorne WC Fusion Methods in Chemical Microscopy Interscience New York USA 1957
Morris KK Griesser UJ Eckhardt CJ Stowell JG Theoretical approach to physical transformations of active pharmaceutical ingredients during manufacturing processes Adv Drug Deliv Rev 20014891ndash114
Mihalia B Elhi D Rozek T Milnc R Chiral stability study of oral liquid clopidogrel formulations in infants J Pharm Prac Res 201242106ndash110
Obaidat RM Alkhamis KA Salem MS Determination of factors affecting kinetics of solid-state transformation of fluconazole polymorph II to polymorph I using diffuse reflectance Fourier transform spectroscopy Drug Dev Ind Pharm 2010 36570ndash580
Panya A Laguerre M Lecomte J Villeneuve P Weiss J McClements DJ Decker EA Effects of chitosan and rosmarinate esters on the physical and oxidative stability of liposomes J Agric Food Chem 2010585679ndash5684
Pindelska E Szeleszczuk L Pisklak DM Mazurek A Kolodziejski W Solid-state NMR as an effective method of polymorphic analysis solid dosage forms of clopidogrel hydrogensulfate J Pharm Sci 2015104106ndash113
Qian F Huang J Hussain MA Drug-polymer solubility and miscibility Stability consideration and practical challenges in amorphous solid dispersion development J Pharm Sci 2010992941ndash2947
Robertson JM The structure of resorcinol a quantitative X-ray investigation Proc Roy Soc Lond 193615779ndash99
Robertson JM Ubbelohde AR A new form of resorcinol I Structure determination by X-rays Proc Roy Soc Lond 1938167122ndash135
Rocco WL Morphet C Laughlin SM Solid-state characterization of zanoterone Int J Pharm 199512217ndash25
Saleki-Gerhardt A Ahlneck C Zografi G Assessment of disorder in crystalline solids Int J Pharm 1994101237ndash247
Salole EG Al-Sarraj H Spironolactone crystal forms Drug Dev Ind Pharm 198511855ndash864
Schmidt HL Sperling LJ Gao YG Wylie BJ Boettcher JM Wilson SR Rienstra CM Crystal polymorphism of protein GB1 examined by solid-state NMR spectroscopy and X-ray diffraction J Phys Chem B 200711114362ndash14369
Sheraz MA Khan MF Ahmed S Kazi SH Khattak SR Ahmad I Factors affecting formulation characteristics and stability of ascorbic acid in water-in-oil creams Int J Cosmet Sci 201436494ndash504
Sheraz MA Ahmed S Rehman IU Effect of pH polymer concentration and molecular weight on the physical state properties of tolfenamic acid Pharm Dev Technol 201520352ndash360
Singhal D Curatolo W Drug polymorphism and dosage form design a practical perspective Adv Drug Deliv Rev 200456335ndash347
Stagner WC Guillory JK Physical characterization of solid iopanoic acid forms J Pharm Sci 1979681005ndash1009
Suryanarayanan R Weidmann TS Quantitation of the relative amounts of anhydrous carbamazepine and carbamazepine dihydrate in a mixture by solid-state nuclear magnetic resonance Pharm Res 19907184ndash187
Tarko T Tuszynski T Influence of selected additives on colloid stability of alcohol emulsion creams Pol J Food Nutr Sci 20075717ndash24
118
Tishmack PA Bugay DE Byrn SR Solid-state nuclear magnetic resonance spectroscopyndashpharmaceutical applications J Pharm Sci 2003 92441ndash474
Thirupathi A Reddy A Narayana A Meshram S Sampathi S Solid state characterization of the polymorphic changes in candesartan cilexetil solid dispersion with poly ethylene glycol 8000 J Pharm Sci Res 2014627ndash32
Threlfali TL Analysis of organic polymorphs Analyst 19951202435ndash2460
Uekama K Ikegami K Wang Z Horiuchi Y Hirayana F Inhibitory effect 2ndashhydroxypropylndashβndashcyclodextrin on crystalndashgrowth of nifedipine during storage Superior dissolution and oral bioavailability compared with polyvinyl pyrrolidone Kndash30 J Pharm Pharmacol 1992 4473ndash78
Van Aerde Ph Remon JP De Rudder D van Sevem R Braeckman P Polymorphic behavior of chloroquine diphosphate J Pharm Pharmacol 198436190ndash191
Vippagunta SR Brittain HG Grant DJW Crystalline solids Adv Drug Deliv Rev 2001483ndash26
Wu L Zhang J Watanabe W Physical and chemical stability of nanoparticles Adv Drug Deliv Rev 201163456ndash469
Wu JX Yang M van den Berg F Pajander J Rades T Ranlanen J The influence of solvent evaporation rate on the physical stability of solid dispersion-a fast screening approach The electronic conference on pharmaceutical sciences (ECPS) MDPI AG Basel Switzerland 2011 (wwwsciforumnet)
Wu L-S Torosian G Sigvardson K Gerard C Hussain MA Investigation of mocicinze hydrochloride polymorphs J Pharm Sci 1994 831404ndash1406
Xu S Dai WG Drug precipitation inhibitors in supersaturated formulations Int J Pharm 201345336ndash43
Yang J Grey K Doney J An improved kinetics approach to describe the physical stability of amorphous solid dispersions Int J Pharm 201038424ndash31
Yang B Sheng-Yong G Jin-Ye W 35th Annual International Confernce Organized by Engneering in Medicinal Biology Society (EMBC) 3ndash7 July 2013 Osaka Japan
Yoshioka S Stella VJ Physical stability of drugs substances Stability of Drugs and Dosage Forms Kluwer Academic Plenum Publishers New York USA 2000 Chap 3
Yu L Amorphous pharmaceutical solids preparation characterization and stabilization Adv Drug Deliv Rev 20014827ndash42
Zhou D Understanding physiochemical properties for pharmaceutical product development and manufacturing II Physical and chemical stability and excipients compatibility J Valid Technol 200936ndash47
Zografi G Physical stability assessment of emulsions and related disperse systems a critical review J Soc Cosmet Chem 198233345ndash358
119
CHAPTER ndash 6
SOLID STATE STABILITY
61 INTRODUCTION
The chemical degradation of drug substances in the solid state is a subject of great interest to pharmaceutical scientists because a large number of drugs have been developed as solid dosage forms Several workers have dealt with the stability of drugs in the solid state (Connors et al 1986 Byrn et al 1999 Santos 1999 Carstensen 1974 2000 Yoshioka and Stella 2000 Bastin et al 2000 Guillory and Poust 2002 Florence and Attwood 2006 Zhou et al 2009) and many reviews have been published (Koenigbauer et al 1992 Lai and Topp 1999 Phipps and Mackin 2000 Huang and Tong 2004 Yoshioka and Aso 2007 Zhou 2009) to understand the characteristics and any transitions in the solid state The chemical degradation of drugs in the solid state and in solid dosage forms is more complex than that occurring in the liquid media The formulation of a drug in a complex matrix in solid dosage forms creates the possibility of interaction between the drug and the excipients that may give rise to incompatibility and stability problems In addition to this external factors such as moisture and temperature also affect the stability of solid drugs and dosage forms Guidelines on stability studies have been provided by regulatory authorities (ICH 2003 WHO 2009 FDA 2014 EMEA 2003)
An understanding of the solid state properties of a drug and their impact on its stability is an essential component of the drug development process The characterization of the solid states of a drug and the selection of the best form in term of stability and performance is the first step before subjecting the active pharmaceutical ingredient (API) to further studies The physical state thermal behavior and polymorphism are important characteristics that affect the stability of a drug in a formulation
The stability of a solid drug may often depend on the state in which it is present in a dosage form Drugs in the amorphous state have the advantage of higher solubility faster dissolution and greater bioavailability However the stability of the amorphous drugs is lower than those that exist in the crystalline state Drugs in the amorphous state may be affected by moisture which leads to plasticization of the amorphous form resulting in a decrease in the stability of the drug These aspects have been discussed in chapter 5 Moisture may also participate in the degradation reactions (such as hydrolysis hydration isomerization etc) to destabilize the drug
The chemical stability of amorphous drugs can be improved if binary molecular mixtures (solid molecular dispersions) of the drugs are prepared using excipients such as polyvinyl pyrrolidone (PVP) (61) a proton acceptor which forms hydrogen bonding with the drug to stabilize it On the other hand dextrans (62) that act as proton acceptor as well as proton donor can be used to stabilize a drug that possess both characteristics
(61) (62)
120
Solid state degradation reactions may involve phase transformation dehydration desolvation and chemical degradation by oxidation cyclization solvolysis hydrolysis deamidation etc Knowledge of these reactions in a pharmaceutical system would enable the pharmaceutical scientist to take necessary steps to prevent these reactions and thus enhance the stability of the drugs in solid dosage forms
The degradation of a drug in the solid state has been found to mainly occur in the solution phase involving the solvent layers in contact with the solid phase The solvent may come from various sources as described by Connors et al (1986)
A melt from the drug or an ingredient of the formulation with a low melting point
Residual moisture or solvent from wet granulation
Moisture absorbed onto the excipients such as starch lactose microcrystalline cellulose
Adsorbed atmospheric moisture
A solvate or hydrate losing its bound solvent with time or temperature variations
The solid state degradation of drug substances may also occur at high temperatures in the absence of water vapors
The design of the solid state degradation study of a drug requires knowledge of the important variables (such as particle size of the crystal stresses created in the crystal trace impurities in the crystal) involved in the degradation The experimental setup should have provision for the control of temperature and water vapor pressure during the degradation reaction along with a method of proper homogenous sampling of the degraded material in the mixture to ensure the accuracy of results The degradation of the polymorphic forms of a drug may occur differently due to a difference in their physical characteristics All these factor may influence the results of a solid state degradation study
The experimental methods used in a kinetic study of solid state drug degradation involve the application of techniques such as reflectance spectroscopy X-ray diffraction thermal methods microscopy dilatometry gas pressure-volume analysis and other techniques (see Chapter 5) The treatment of solid state reaction data temperature effects on solid state reactions and application of Arrhenius equation equilibria involved in solid state degradation and use of vanrsquot Hoff equation for a drug in the hydrate form in equilibrium with its dehydrated form have been described (Monkhouse and Van Campen 1984)
62 TOPOCHEMICAL REACTIONS
The chemical reactions occurring by deformations in the solid crystalline state are termed as topochemical reactions These reactions have specific requirements to occur and depend on the order of molecular packing in a crystal lattice A thermal or photo-induced molecular rearrangement (ie bond angle and distance) in the solid state would lead to a chemical reaction in the crystal lattice The nature and magnitude of this reaction would depend on the intensity of the external stimuli In topochemical reactions the products are different from those formed in the liquid state The chemical reactivity in the solid state is determined by the crystal structure of a compound Any defects or strains in the crystal surface produce sites of high energy that are involved in the initiation of a chemical reaction Crystalline disorders are the main cause of the susceptibility of a solid compound to chemical degradation
The degree of crystallinity of a drug may be affected by manufacturing processes (milling granulation compaction etc) This would influence the reactivity of the material The rate of a chemical reaction in the solid state may be enhanced by an increase in the surface area as a result of smaller particle size of the crystals This would increase the magnitude of crystal defects and hence an increase in the rate of reaction
4-Aminosalicylic acid undergoes dimerization in the crystalline state and occurs in the form of a dimer as shown in Fig 61 It involves the formation of a hydrogen bond between carboxyl
121
groups and an intramolecular hydrogen bond between hydroxyl group at 2 position and oxygen atom of the carboxyl group (Pothisiri and Carstensen 1975)
Fig 61 Solid crystalline state dimer of 4-aminosalicylic acid a and c indicate directions in the arrangement of crystals
63 CHEMICAL DEGRADATION REACTIONS
The chemical degradation of drug substances in the solid state may occur by the following reactions
631 Solvolysis
It is a major reaction occurring in the solid state degradation of drugs by the participation of a solvent It also includes the hydrolysis of a compound such as acetylsalicylic acid (aspirin) (63) to give salicylic acid (64) and acetic acid (65) The acceleration of the reaction with time has been attributed to the formation of the degradation products These products lower the pH of the sorbet moisture layer that further catalyses the degradation of aspirin It undergoes acid catalysis at low pH (Yang and Brooke 1982)
122
O
OH
O
OCH3
O
OH
OH
+ CH3
O
OH
H2O
(63) (64) (65)
It also involves the decarboxylation of a compound such as 4-aminosalicylic acid (66) at high temperature to form 3-aminophenol (67) The reaction occurs both in the absence and presence of moisture (Kornblum and Sciarrone 1964)
OH
O OH
NH2
heat
OH
NH2
+ CO2
(66) (67)
Generally the greater the solvation in the crystal the lower is the solubility and dissolution rate Thus solvated and non-solvated forms of poorly soluble drugs may exhibit differences in bioavailability Anhydrous form of ampicillin is absorbed to a greater extent from hard gelatin capsule or aqueous suspension than the trihydrate form of ampicillin (Hill et al 1972)
632 Oxidation
It involves the reaction of a drug in the solid state with molecular oxygen The reaction can proceed slowly by auto-oxidation in the presence of oxygen Unsaturated fats undergo auto-oxidation to initially form hydroperoxides which on further oxidation give low molecular weight fatty acids These acids impart the typical odor to fats
The auto-oxidation of a compound occurs through the initiation propagation and termination steps to form the oxidation products It involves the participation of free radicals and oxygen to complete the reaction The various steps in auto-oxidation may be described by the following equations
Initiation
A A (61)
A + SH AndashH + S (62)
Propagation
S+ O2 SOO (63)
SOO+ SH SOOH + S (64)
Termination
S + S SndashS (65)
S+ SOO SOOS (66)
123
In this process the free radicals may be formed by hemolytic cleavage of the chemical bond of an initiator (A) (61) The free radical of a compound (SH) may be formed on the abstraction of a hydrogen atom by the free radical formed in the initiation step (A) (62) The newly formed free radical (S) then reacts with oxygen to produce a peroxy free radical (SOO) (63) This free radical abstracts a hydrogen atom from another molecule of SH which is in turn oxidized to produce a hydroperoxide (SOOH) along with the formation of another free radical of the compound (S) (64) The chain reaction is an auto-oxidation (autocatalytic) process that continues until the termination of the free radicals ((65) and (66)) The oxidation of several molecules of a compound can be caused by the participation of a single free radical in the reaction
Examples of solid state oxidation of drugs include ascorbic acid (68) (Willson et al 1996) and excipient-induced oxidation of a cyclic heptapeptide (69) in lyophilized formulation The reducing sugar impurities in mannitol act as oxidizing agent in the reaction (Dubost et al 1996)
OO
OH
OH
OH
OH OH
NH
NH
O
NH NH
OH
O
NH
O NH
O
NH
NH2
O
O
S
SNH
O N
NH2 O
(68) (69)
633 Deamidation
Peptide and protein drugs are often formulated in the solid state to achieve stabilization However these agents can undergo degradation and inactivation during storage These reactions are affected by temperature moisture content excipients and the physical state of the formulation (amorphous versus crystalline) A major reaction undergone by peptides and proteins is deamidation of amino acid moieties (Lai and Topp 1999) The deamidation of L-asparagine (610) in polypeptides by a nonenzymatic reaction has been studied (Li et al 2005a Yang and Zubarev 2010) It gives rise to L-succinimide (611) followed by the formation of L-aspartate (612) and other compounds
NH2
NH
O
O
CH3NH
CH3
NH
O
O
NH
CH3 O
O
NH
CH3
OH
NHCH3
deamidation hydrolysis
(610) (611) (612)
The effect of sucrose and mannitol on the deamidation kinetics of some model peptides has been studied (Li et al 2005b) An automatic computerized technique for the quantitative determination of the deamidation rates of proteins has been developed It has been found that a large number of proteins undergo deamidation reactions (Robinson 2002)
124
634 Pyrolysis
Pyrolytic degradation of a compound involves thermally induced bond rupture in the solid state in the absence of a solvent or moisture Fluconazole (613) crystals undergo pyrolytic degradation at 290degC Pyrolysis-GCMS study of the reaction has shown the formation of hexafluorobenzene (614) as a degradation product On pyrolysis at 500 and 750degC the degradation products of fluconazole have been detected in gaseous state The nature of pyrolysis products depends on the temperature used for the reaction (Moura et al 2010)
290 oC
F
FF F
OH
N
NN
N
N N
(613) (614)
Another example of the pyrolytic degradation of a drug is polymethyl silsesquioxane It is degraded in the presence of nitrogen at 900degC to give silica silicon oxycarbide and traces of amorphous carbon (Ma et al 2002)
635 Photolysis
A drug in the solid state may undergo photolytic degradation on exposure to light in the presence or absence of a solvent A solid dosage form like a tablet or capsule may also be affected by light In this case photolytic degradation may occur on the surface of the formulation It may be accompanied by the appearance or fading of color Examples of solid state photolytic degradation of drugs include the polymorphic forms of furosemide (615) (De Villiers et al 1992) and indomethacin (616) (Matsuda et al 1980) Photodegradation assessment of ciprofloxacin moxifloxacin norfloxacin and ofloxacin in the presence of excipients from tablets has been made by HPLCndashMSMS and DSC methods (Hubicka et al 2013)
N
H3CO
O
Cl
CH3
OH
O
O
NH
O
OH
Cl
SO
O
NH2
(615) (616)
The photodegradation of colors added to the tablets has been studied It has been found that this occurs due to a surface phenomenon that results in the fading of colors of tablets to a depth of about 03 mM Prolonged light exposure does not affect deeper into the tablet coating and thus the drug content remains stable (Lachman et al 1961) The photostability of indomethacin in gelatin capsules depends on the capsule shell thickness and on the concentration of opacifier (titanium dioxide) The gelatin films and indomethacin tablets on exposure to a 400 W mercury vapor lamp for 2 h developed color A good linear relationship has been found between color difference values and square root of exposure time at different concentrations and thicknesses The rate of coloration is directly proportional to the transmission of films over the wavelength range involved in the photodegradation of indomethacin (Mastsuda et al 1980)
125
64 FACTORS AFFECTING STABILITY IN THE SOLID STATE
641 Moisture
The presence of moisture and water content in a dosage form can affect its stability An increased exposure of the dosage form to atmospheric moisture or from that of the excipients has been found to produce deleterious effect on the stability of active ingredients Attempts should be made to select excipients in accordance with the chemical nature of the drug to be formulated and to minimize the exposure of the dosage form to excessive moisture during manufacturing and storage The moisture content of some excipients commonly used in tablet formulations is reported in Table 61
The majority of the excipients reported in Table 61 possess a considerable amount of moisture content at higher RH This moisture may be present in the loosely bound or strongly bound form If this moisture comes in contact with the drug it would be destabilized Tingstad and Dudzinski (1973) have studied the effect of moisture on the solid state stability of drugs To minimize the effect of moisture they suggested the use of highly sealed containers determination of the amount of water present in the dosage form and use of a separate sealed ampoule for each assay and water determination This would avoid the disturbance of water equilibration when the container is opened Genton and Kesselring (1977) have found a linear relationship between log k for the solid state degradation of nitrazepam and the RH The stability of drugs in solid dosage forms can be studied by subjecting them to the temperature and RH conditions recommended in ICH guideline (ICH 2003)
Table 61 Moisture content of commonly used tablet excipients at 25degC on storage at different relative humidities (RH) (Callahan et al 1982)
Excipient (USP NF grade)
Equilibrium moisture content (EMC) at 25degC
RH
33 75 100
Anhydrous calcium phosphate lt01 lt01 70 Spray dried lactose 05 10 215 Magnesium stearate 31 35 ndash Microcrystalline cellulose 37 81 ndash Polyethylene glycol 3350 lt03 20 622 Pregelatinized starch 78 147 364 Corn starch 80 144 165 Povidone 122 278 ndash
642 Temperature
Temperature is known to affect the stability of drugs in solid dosage forms However other factors may complicate the evaluation of results under the following conditions (Connors et al 1986)
Humidity is not simultaneously controlled
One of the ingredients the drug or the excipients has a low melting point
One of the ingredients has loosely bound water and alterations in temperature change the degree of binding of the water to the excipients
One of the ingredients of the dosage form is present in the form of a hydrate or solvate that loses its bound solvent on changes in temperature
The solid dosage form is stored in different types of containers open or closed and permeable or hermetic that may affect the stability in different ways
The thermal degradation of vitamin A esters and other derivatives in the solid state has been studied by observing changes in crystallinity by melting point determination It has been
126
concluded that the degradation of these compounds depends on their melting point and that the stability increases with an increase in the melting point (Table 62) The degradation at 50degC follows an apparent first-order kinetics (Guillory and Higuchi 1962) It has been suggested that the degradation occurs only in the liquid phase on the surface of the crystal The fraction of the drug that undergoes degradation is a function of the melting point of the crystalline solid and can be expressed by Eq (67)
log X1 = 2303 R
(1T ndash 1Tm) (67) ndash∆Hf
where X1 is the mole fraction of the compound in the melt form or the amount of the liquid phase ∆Hf is the molar heat of fusion R is the gas constant T is temperature and Tm is the melting point of the pure solid compound in degree Kelvin Under these conditions the rate of the degradation reaction is proportional to X1 and a linear relationship is observed between log k and Tm
ndash1 where k is the apparent zero-order rate constant
Table 62 A Comparison of the apparent zero-order rate constants (ko) for the degradation of various vitamin A derivatives at 50degC and their melting points
Vitamin Aa derivatives ko times 102 mol hndash1 Melting point (degC)
Acetate 270 57ndash58 Phthalimide-N-acetate 480 111ndash112
Nictomate 250 93ndash94 345-Trimethoxybenzoate 140 85ndash86
Succinate triphenyl guanidine salt 076 140ndash1405 Benzhydrazone 038 181ndash182
a All the compounds do not follow the same pattern which may be due to differences in their crystal structure and other factors
65 DRUG INTERACTIONS
The drug may undergo drug-drug and drug excipient (additive) interactions in a solid dosage form The stability of a drug can be affected by the excipients which may act as surface catalysts alter the pH of the moisture layer or directly react with the drug The potential influence of excipients on the drug bioavailability is well known which results by virtue of the formation of poorly soluble non-absorbable drug-excipient complexes for example between tetracyclines and dicalcium phosphate used as a diluent Thus the excipients in solid dosage forms must comply with the requirements of FDA monograph (21 CFR 3301 (e)) regarding the stability of solid dosage forms
Racz (1989) has dealt with the drug-additive and additive-additive interactions in detail These interactions may vary with the nature of the additives The different types of additives used in the formulation of solid dosage forms include anion-active (negatively charged) additives (acrylic acid polymers such as carbapol 934 ionic hydrocolloids sodium alginate) cation-active (positively charged) additives (quaternary ammonium salts benzalkonium chloride cetylpyridinium chloride) amphoteric additives (proteins gelatin) and non-active additives (polyvinyl pyrolidone (PVP)) methyl cellulose (MC) and derivatives polyethylene glycols (PEG) polyvinyl alcohol (PVA) and starches) Drug interactions with different additives may decrease the stability by chemical degradation or improve the stability (for example by complexation) The drug-drug interaction may occur in combination products Aspirin (617) has been found to undergo reaction with acetaminophen (paracetamol) (618) to form its acetyl ester (619) and salicylic acid (620) (Koshy et al 1967)
127
O
OH
O
OCH3
+
NH
OH
O
CH3
+
NH
O
CH3
O
O
CH3
O
OH
OH
(617) (618) (619) (620)
66 KINETICS OF SOLID STATE DEGRADATION
The kinetics of thermal degradation of a compound in the absence and presence of moisture or solvents in the solid state has been described by Ng (1975) and Carstensen (1974) and is summarized by Connors et al (1986)
Most of the thermal degradation reactions can be expressed by Eq (68)
dxdt = k α 1ndashp (1ndashα) 1ndashq (68)
where α is the concentration of fractional degradation and k is a composite rate constant which includes a term for N0 the number of potential degradation nuclei sites on the solid The quantities p and q are the parameters related to the mechanism of the degradation reaction with limits in the range of 0ndash1
Eq (68) has been developed on the basis of the fact that the degradation of a compound initially occurs at nuclei (stress points imperfections dislocation points) on the crystal surface The rate of degradation (dxdt) is directly proportional to α the fractional degradation This is because of the fact that the actual act of degradation induces an increase in the rate of degradation through an increase in stress or dislocation in the crystal that results in an increase in the number of nuclei undergoing degradation The dependence of the rate of degradation on the increase in the crystal stress is given by p
If p = 0 then α 1ndashp = α which shows that the rate is directly proportional to α The term (1ndashα)1ndashq is used to describe the degradation behavior of the drug If both p and q are unity Eq (68) is reduced to Eq (69) indicating an overall zero-order kinetics
dxdt = k (69)
If p = 0 and q = 1 Eq (69) is expressed in the form of Eq (610)
dαdt = kα (610)
In this case the degradation has been found to follow as apparent exponential kinetics If p and q both are equal to zero the reaction can be expressed by Eq (611)
dαdt = k α (1ndashα) (611)
Eqs (68)ndash(611) originally developed for the study of kinetics of thermal degradation of drugs can also be applied to moisture dependent reactions
The single component drugs in the solid state in a pharmaceutical system undergo degradation by zero-order or first-order reaction It is difficult to determine the exact order of a reaction unless the reaction is carried out up to an adequate number of half-lives to find whether it follows zero-order or first-order kinetics (see Chapter 2)
The kinetics aspects of chemical degradation of solids and solid dosage form have been discussed by Florence and Attwood (2006) Yoshioka and Stella (2000) and Carstensen (2000)
128
67 SOLID STATE STABILITY STUDIES
Several studies on various aspects of solid state stability have been conducted to investigate the structural features of different solid state forms kinetics of degradation and effect of excipients on the stability of drug substances Some of these studies are presented in the following sections
671 Structural Studies
Multistep methods have been developed for the screening of physical and chemical stability andor reactivity of new drug candidates The physical reactivity test is used to provide information on the existing solid-state form in relation to the thermodynamically stable form A method to find the most stable form of the drug has been described In the case of polymorphism a search for additional polymorphs is made and different solid phases are characterized Special tests for the characterization of hydrates and anhydrous forms are reported (Berglund et al 1990)
Thermoanalytical methods and non-thermal methods such as XRPD and IR spectrometry have been used to study the structural changes of crystalline forms of moxifloxacin HCl (621) stored under different conditions of relative humidity (0 40 75 and 90 RH) for a period of one month After the storage period at 0 and 40 RH there was no change in the crystalline structure of the drug showing good physical stability of the material However in samples stored at 75 and 90 RH a hydrated crystalline form has been identified (Julio et al 2015) The study indicated that crystalline forms of moxifloxacin are not stable at higher RH
N
O
OH
O
F
OCH3
NNH
H
H
HCl
(621)
A solid-state stability study of the β-lactam antibiotic meropenem (622) has been conducted using UV FTIR and Raman spectrometry The optimum molecular geometry harmonic vibrational frequencies IR intensities and Raman scattering characteristics have been determined according to the density-functional theory (DFT) The differences between the observed and scaled wave number values of peaks in the FTIR and Raman spectra made it possible to detect non-degraded and degraded samples of the drug Molecular electrostatic potential (MEP) front molecular orbitals (FMOs) and the gap potential between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been determined to enable the interpretation of the results (Criclecka-Piontek et al 2013)
NO
O
OH
H HCH3
S
NH
O
N
CH3
CH3
O
OH
(622)
129
672 Kinetic Studies
Isothermal calorimetry has been applied to determine the rate of solid state room temperature degradation of drug substances This technique involves measurement of the rate of heat output of a compound at several elevated temperatures and determination of its rate of degradation at a single temperature as well as the activation energy The solid state stability of phenytoin triamterene digoxin tetracycline theophylline and diltiazem has been studied by this method (Koenigbauer et al 1992)
The chemical stability of ranitidine HCl (623) in the solid state at various temperatures has been studied by a HPLC method The critical relative humidity (CRH) of the bulk powder was found to be ~ 67 RH The amount of water adsorbed by the powder above the CRH is proportional to the RH value The percent degradation of the drug powder at 60ndash70 RH is higher than that observed above 70 RH Ranitidine HCl powder is unstable around the CRH (Teraoka et al 1993)
ON
CH3
CH3 S
NH
NO2
NHCH3
(623)
The evaluation of the effect of lsquomicroenvironmental pHrsquo on the stability and dissolution of solid dosage forms has gained considerable importance The microenvironmental pH has shown significant impact on the stability of drugs which are affected by the pH of the solution The kinetics of degradation of such drugs is dependent on the microenvironmental pH of the solid material The use of pH modifiers is an effective method to modulate the microenvironmental pH so as to improve the stability of solid dosage forms The selection of the appropriate pH modifier its concentration and method of incorporating the modifier is important to enhance the stability of the drug (Badawy et al 2007)
The degradation kinetics of asparagine (610) in two model peptides has been studied at 50degC at pH 7 in the presence and absence of 5 sucrose (624) or mannitol (625) and at 50degC and 30 RH in solid samples lyophilized from the solutions Solid formulations have been characterized using Karl Fischer coulometric titration TGA DSC FTIR and solid-state NMR spectrometry Asparagine showed similar pseudo first-order rates for deamidation in solution and in the absence of sucrose and mannitol The addition of 5 sucrose or mannitol was found to decrease the rates up to 17 The model peptides degraded 2 to 80 fold more slowly in the solid formulations of sucrose and mannitol than those in 5 solutions of these carbohydrates Mannitol formulations were found to be largely amorphous immediately after lyophilization with some crystalline like structures while sucrose formulations remained amorphous after lyophilization and storage Sucrose stabilized the peptides against deamidation in the solid state (Li et al 2005b)
NH2
NH
O
O
CH3NH
CH3
(610)
130
(624)
OHOH
OH
OH OH
OH
(625)
Differential scanning calorimetry (DSC) has been used to study the kinetics of thermal degradation of several derivatives of glycine (ie 35-disubtiutied tetrahydro-2H-135-thiadiazine-2-thione THTT and derivatives) (626) in the solid state to serve as an amino acid and peptide drug model The two DSC peaks indicated the melting and degradation of the compounds respectively The Augin Benret and Kissinger equations were used to determine the activation energy of the degradation reaction carried out up to 300degC the activation energy of melting and the enthalpy (∆H) of the compounds The study has been used to evaluate the relative stability of the compounds and the most stable prodrug that possesses the highest activation energy and the longest shelf-life (Abdol-Elrahman et al 2002)
N
S
N
CH3
S
O
OH
(626)
The stability of 135-triazine (627) a corticotrophin releasing factor inhibitor has been studied in solid formulations and the structure of degradants elucidated by LCMS and NMR spectrometry The degradation of 135-triazine involves hydrolysis of the triazine ring and hydroxy substitution of amino group on the triazine ring followed by its hydrolysis The stability of the compound is dependent on the manufacturing process and degradation is more rapid in amorphous regions formed during the process The degradation rate in tablet formulations is enhanced under high humidity (Badawy et al 2009)
N
N
N
NH2 NH2
NH2
(627)
131
The stability of freeze-dried liposomes of different lipid composition containing trehalose as a lypoprotectant has been investigated The dry cakes of liposomes were exposed to different temperatures for 30 min and the retention of carboxyfluorescein and average vesicle size after rehydration were examined by DSC FTIR was employed to study the acyl chain order and interaction between trehalose molecules and phospholipid head groups All lipid compositions of liposomes showed induction of leakage suppression of onset bilayer melting transition temperature (Tm) and enhancement of the interaction between sugar and phospholipids below the glass tranisition temperature (Tg) These changes were accompanied by melting transition of the bilayers It has been concluded that for liposomes freeze-dried in trehalose the temperature range of bilayer melting is a better indicator than the Tg for maximum temperature exposure of liposomes for short period of time (30 min) (Van Winden and Crommelin1999)
Cyclodextrin (CD) (628) has been used to prepare inclusion complexes with drugs in the solid state The drugndashCD complexes have greater stability and potential advantage in dosage form design such as layered formulations The 2D heteronuclear and homonuclear correlation solid-state NMR (SSNMR) involving 1H 13C 19F and 31P nuclei has been used to investigate drugndashCD interactions in these complexes that involve dipolar interactions between nuclei within the drug and CD molecules The technique provides information on the inclusion of drug within the CD cavity in powder samples of drug complexes of dipivoxil voriconazole dexamethasone and prednisolone SSNMR can be used for the characterization and quantitative analysis of solid drugs and their complexes (Vogt and Strohmier 2012)
(628)
The chemical degradation pathways of amorphous solids can be determined by the relative mobilities of potential reactants The molecular dynamic simulations of amorphous glasses of PVP containing small amounts of water ammonia and a small peptide (629) over a period of 100 ns have been used to monitor aging process of PVP segments and embedded solutes Tg values have been obtained by observing changes in slopes of the volumetemperature profiles and the internal energytemperatures profiles for the inherent structures on cooling at different rates Determination of molecular trajectories below Tg show temporal and spatial heterogenicity in the polymer and solute mobility with each molecule showing different relaxation behavior for translational rotational andor conformational motions The data have been used to study the degradation of the peptide by deamidation (Xiang and Anderson 2004)
NH
R O R
NHNH
O R
NHNH
O
NH2
O R O R O R
OH
(629)
132
673 Effect of Excipients
It is important to understand the role of excipients with a high affinity for water in a formulation exposed to moisture In this context the effect of polyvinyl pyrrolidone (PVP) and RH on the solid state stability of anhydrous theophylline has been studied by moisture uptake XRPD HPLC and FTIR spectrometry The physical mixtures of anhydrous theophylline and PVP were stored at room temperature at various humidities and the physical and chemical changes monitored A hypothesis has been presented to explain the role of amorphous polymeric excipients and the associated mobility of water The mechanism of protection of hydration of theophylline (630) by PVP involves a desiccant action The efficiency of this action is dependent upon the amount of water in the system and the kinetics of reaching the equilibrium moisture content (EMC) (Kesavan and Garnet 1996)
N
NNH
N
CH3
O
O
CH3
(630)
The effect of amorphous bulking agents on the chemical stability of freeze-dried drugs has been studied PVP dextrans of different molecular weight and lactose have been used as bulking agents and sucrose as an acid sensitive compound Lyophiles of the bulking agent and sucrose at 101 (ww) ratio were examined by XRPD DSC and Karl Fisher titration The amount of sucrose inversion in lyophiles stored at 60degC was determined by HPLC It has been observed that the bulking agent has a major impact on both the solid-state acidity (measured by Hamrnett acidity function) and the degradation rate The values of degradation rate constants are higher for dextran lyophiles (more acidic) that those of PVP and sucrose (less acidic) The Hamrnett acidity function can be used to predict the order of stability of acid-sensitive drugs in lyophiles prepared with different bulking agents (Lu et al 2009)
674 Effect of Aging
The term ldquoagingrdquo is used to express the physical instability of pharmaceutical dosage forms It is a process through which changes in the disintegration andor dissolution properties of dosage forms are caused by delicate alterations in the physicochemical characteristics of the inert or active ingredients in the dosage forms As the disintegration and dissolution of the drugs may be rate-determining steps in their absorption any changes in these processes due to aging of dosage forms could affect the bioavailability of the product (Guillory and Poust 2002) Several studies have been carried out on the aging of the excipients and solid dosage forms Some of these studies are presented in this section
The physical aging of PVP K25 on storage has been studied by positron lifetime spectroscopy and scanning electron microscopy The transition of PVP K25 from glassy state (at 25degC 55 RH) to completely plasticized wet rubbery state (at 25degC 75 RH) is not uniform A slow anomalous structure is formed on storage at 65 RH It has been found that the actual water content and storage conditions determine the size distribution of free volume holes in the material Under high humidity conditions a hydrogen bound ldquonetworkrdquo is formed between the polymer chains and the water molecules (Suvegh and Zelko 2002)
The influence of aging on the release of salbutamol sulfate from oral formulations (lipid matrices) prepared with Gelucirereg as a lipid excipient has been studied The release profiles of the drug from the capsule showed dependence on the type of Gelucires indicating a fast release from Gelucire 3510 a slow release from Gelucire 4607 and a slower release from Gelucire 4809 Differential scanning colorimetric studies of the physical state of the drug in different matrices have
133
shown aging effects on storage It has been concluded that a higher effect of aging on the capsules is indicated by a faster rate of dissolution (San Vicente et al 2000)
The effect of aging on acetaminophen tablets prepared by wet granulation using povidone or pregelatinized starch as binder on storage at 40degC 52 RH and 40degC 94 RH for eight weeks has been studied At 40degC 52 RH the tablets showed an increase in hardness and at 40degC 94 RH a decrease in hardness The pregelatinized starch granulated tablets showed a lower effect of changes in hardness by humidity than the povidone granulated tablets The disintegration of tablets with both of these binders slowed down with an increase in humidity A considerable slow down in the dissolution of the tablets was observed at 40degC 94 RH compared to that at 40degC 52 RH The tablets containing pregelatinized starch were less affected by humidity than those containing povidone (Sarisuta and Parrott 1988)
The tablets prepared by wet granulation have been found to be affected by the moisture content of granules on aging The evaluation of changes in hardness disintegration and drug release of tablets prepared by direct compression of different bases with variable moisture content has been made Tablet with high initial moisture content showed an increase in hardness on storage depending upon the physical properties of the base and the absolute moisture content Hardness increase resulted in an increase in disintegration time and a decrease in drug release The moisture uptake of tablets enhanced the disintegration time as well as the drug release The tablets prepared with lactose as a base with variable initial moisture content were highly resistant to any changes on storage (Molokhia et al 1987)
The effect of aging on the stability of glibenclamide (GB) β-cyclodextrin (CD) systems and CDndashcomplexed GB tablets has been investigated using IR spectrometry and X-ray diffraction analysis The results indicated that the physicochemical properties of the tablets are not affected even after storage for four years However the crystallinity of the physical mixture of GBCD decreases with aging The effect of aging on the dissolution of GB in tablets can be overcome by preparing a GBCD complex in the tablet dosage form (Babu and Pandit 1999)
The effect of humidity aging on hardness disintegration and dissolution of Ca3 (PO4)2ndash
based tablet with variable moisture content has been evaluated It has been found that a decrease in the disintegration time an increase in the dissolution rate and no change in the hardness of the tablets with higher initial moisture content occurs on aging under low humidity On the contrary a decrease in hardness an increase in disintegration and a decrease in dissolution rate of tablets with lower initial moisture content occurs on aging under high humidity conditions The physical characteristics of the tablets are affected by the moisture content of tablet granulation at compression time and moisture uptake on aging during storage (Chowhan and Amaro 1979)
The influence of aging on the dissolution of phenylbutazone tablets has been studied The dissolution rate of old tablet batches has been found to decrease gradually with aging A similar effect is produced by subjecting the tablet to higher temperatures This effect may be related to the subcoat layer of the sugar coating of the tablet that strongly adheres to the tablet core and thus causes a slowdown in its disintegration (Barrett and Fell 1975)
134
REFERENCES
Abdol-Elrehman MI Ahmed MO Ahmed SM aboul-Fadl T El-Shorbazi A Kinetics of solid state of glycine derivatives as a model for peptides using differential scanning calorimetry Biophys Chem 200297113ndash120
Babu JR Pandit JK Effect of aging on the dissolution stability of glibenclamideβ-cyclodextrin complex Drug Dev Ind Pharm 1999251215ndash1219
Badawy SI Hussain M Zhao FQ Huang Y Palaniswamy V Degradation pathways of a corticotropin-releasing factor antagonist in solution and solid states J Pharm Sci2009 982636ndash2647
Badawy SI Hussain MA Microenvironmental pH modulation in solid dosage forms J Pharm Sci 200796948ndash959
Barrett D Fell JT Effect of aging on physical properties of phenylbutazone tablets J Pharm Sci 197564335ndash337
Bastin RJ Bowker MJ Slater BJ Salt selection and optimization procedures for pharmaceutical new chemical entities Org Proc Res Dev 20004427ndash435
Berglund M Bystroumlm K Peacutersson B Screening chemical and physical stability of drug substances J Pharm Biomed Anal 19908639ndash643
Byrn SR Pfeiffer RR Stowell JG Solid-State Chemistry of Drugs 2nd ed SSCI Inc West Lafayette Indiana USA 1999
Callahan JC Cleary GW Elefant M Nash RA Equilibrium moisture content of pharmaceutical excipients Drug Dev Indus Pharm 19828355ndash369
Carstensen JT Solid state stability Drug Stability Principles and Practices Carstensen JT Rhodes RT editors Marcel Dekker New York USA 2000 Chap 6
Carstensen JT Stability of solids and solid dosage forms J Pharm Sci 1974631ndash14
Connors KA Amidon GL Stella VJ Solid state decomposition Chemical Stability of Pharmaceuticals A Handbook for Pharmacists 2nd ed John Wiley New York USA 1986 Chap 6
Chowhan ZT Amaro AA The effect of low- and high-humidity aging on the hardness disintegration time and dissolution rate of tribasic calcium phosphate-based tablets Drug Dev Ind Pharm 19795645ndash562
Criclecka-Piontek J Paczkowska M Lewandowska K Barszcz B Zalewski P Garbacki P Solid-state stability study of meropenem ndash solutions based on spectrophotometric analysis Chem Cent J 2013798
De Villers MM van der Watt JG Lotter AP Kinetic study of the solid-state photolytic degradation of two polymorphic forms of furosemide Int J Pharm 199288275ndash283
Dubost DC Kaufman MJ Zimmerman JA Bogusky MJ Coddington AB Pitzenberger SM Characterization of a solid state reaction product from a lyophilized formulation of a cyclic heptapeptide a novel example of an excipient-induced oxidation Pharm Res 1996131811ndash1814
EMEA (European Medicines Evaluation Agency) Guideline on stability testing stability testing of existing active substances and related finished products European Agency for the Evaluation of Medicinal Products (EMEA) London UK 2003
FDA (CDER) Guidance for Industry ANDAS Stability testing of drug substances and products Questions and Answers Silver Spring MD USA May 2014
135
Florence AT Attwood D editors Drug stability Physicochemical Principles of Pharmacy Pharmaceutical Press London UK 2006 Chap 4
Genton D Kesselring UW Effect of temperature and relative humidity on nitrazepam stability in solid state J Pharm Sci 197766676ndash680
Guillory K Higuchi T Solid state stability of some crystalline vitamin A compounds J Pharm Sci 1962 51100ndash105
Guillory K Poust RI Chemical kinetics and drug stability in Banker GS Rhodes CT editors Modern Pharmaceutics Marcel Dekker New York USA 2002 Chap 6
Hill SA Seager H Taskis CB Comparative dissolution rates of anhydrous and trihydrate forms of ampicillin J Pharm Pharmacol 197224152ndash153
Huang LF Tong WQ Impact of solid state properties on developability assessment of drug candidates Adv Drug Deliv Rev 200456327ndash334
Hubicka H Zmudzki P Talik P Zuromska-Witek B Kozek J Photodegradation assessment of ciprofloxacin moxifloxacin norfloxacin and ofloxacin in presence of excipients from tablets by HPLCndashMSMS and DSC Chem Cent J 2013a71ndash12
Hubicka U Zmudzki P Talik P Zuromska-Witek B Krzek J Photodegradation assessment of ciprofloxacin moxifloxacin norfloxacin and ofloxacin in the presence of excipients from tablets by UPLCndashMSMS and DSC Chem Cent J 2013b73ndash12
ICH Harmonized Tripartite Guidelines Q1A (R2) Stability testing of new drug substances and products (second revision) Geneva Switzerland 2003
Julio TA Garcia JS Bonfilio R Trevisan MG Solid state stability and solubility determination of crystalline forms of moxifloxacin hydrochloride Int J Pharm Pharm Sci 20157200ndash214
Kesavan JG Peck GE Solid-state stability of theophylline anhydrous in theophylline anhydrous-polyvinylpyrrolidone physical mixtures Drug Dev Ind Pharm 199622189ndash199
Koenigbauer MJ Brooks SH Rullo G Couch RA Solid-state stability testing of drugs by isothermal calorimetry Pharm Res 1992 9939ndash944
Kornblum SS Sciarrone BJ Decarboxylation of p-aminosalicylic acid in the solid state J Pharm Sci 196453935ndash941
Koshy KT Troup AE Duvall RN Conwell RC Shankle LL Acetylation of acetaminophen in tablet formulations containing aspirin J Pharm Sci 1967 561117ndash1121
Lachman L Weinstein S Swartz CJ Urbanyi T Cooper J Color stability of tablet formulations III Comparative light fastness of several water-soluble dyes and their corresponding lakes J Pharm Sci 196150141ndash144
Lai MC Topp EM Solid state chemical stability of proteins and peptides J Pharm Sci 199988489ndash500
Li B Gorman EM Moore KD Williams T Schowen RL Topp EM Borchardt RT Effects of acidic N + 1 residues on asparagine deamidation rates in solution and in the solid state J Pharm Sci 2005a94666ndash675
Li B OMeara MH Lubach JW Schowen RL Topp EM Munson EJ Borchardt RT Effects of sucrose and mannitol on asparagine deamidation rates of model peptides in solution and in the solid state J Pharm Sci 2005b941723ndash1735
Lu E Ewing S Gatlin L Suryanarayanan R Shalaev E The effect of bulking agents on the chemical stability of acid-sensitive compounds in freeze-dried formulations sucrose inversion study J Pharm Sci 2009983387ndash3396
136
Ma J Shi L Shi Y Luo S Xu J Pyrolysis of polymethylsilsesquioxane J Appl Polym Sci 2002851077ndash1086
Matsuda Y Itooka T Mitsuhashi Y Photostability of indomethacin in model gelatin capsules effect of film thickness and concentration of titanium dioxide on the coloration and photolytic degradation Chem Pharm Bull 1980282665ndash2671
Molokhia AM Al-Shora HI Hammad AA Aging of tablets prepared by direct compression of bases with different moisture content Drug Dev Ind Pharm 1987131933ndash1946
Monkhouse DC Van Campen L Solid state reactions-theoretical and experimental aspects Drug Dev Ind Pharm 1984 101175ndash1276
Moura EA Correia LP Pinto MF Procopio JVV de Souza FS Macedo RO Thermal characterization of the solid state and raw material fluconazole by thermal analysis and pyrolysis coupled to GCMS J Thermal Anal Calori 2010100289ndash293
Ng W-L Thermal decomposition in the solid state Aust J Chem 1975281169ndash1178
Phipps MA Mackin LA Application of isothermal microclorimetry in solid state drug development Pharm Sci Technol Today 200039ndash17
Pothisiri P Carstensen JT Solid-state decomposition para-substituted salicylic acids J Pharm Sci 1975641931ndash1935
Robinson NE Protein deamidation Proc Natl Acad Sci USA 2002995283ndash5288
Teraoka R Otsuka M Matsuda Y Effects of temperature and relative humidity on the solid-state chemical stability of ranitidine hydrochloride J Pharm Sci 199382601ndash604
Tingstad J Dudzinski J Lachman L Shami E Simplified method for determining chemical stability of drug substances in pharmaceutical suspensions J Pharm Sci 1973621361ndash1363
Santos I Drug substances Solid state characterization and stability In Mazzo DJ editor International Stability Testing Interpharm Press Inc Buffalo Grove Illinois USA Chap 9
San Vicente A Hernaacutendez RM Gascoacuten AR Calvo MB Pedraz JL Effect of aging on the release of salbutamol sulfate from lipid matrices Int J Pharm 200020813ndash21
Sarisuta N Parrott EL Effects of temperature humidity and aging on the disintegration and dissolution of acetaminophen tablets Drug Dev Ind Pharm 1988141877ndash1881
Van Winden EC Crommelin DJ 1999 Short term stability of freeze-dried lyoprotected liposomes J Control Release 1999 5869ndash86
Vogt FG Strohmeier M 2D solid-state NMR analysis of inclusion in drug-cyclodextrin complexes Mol Pharm 201293357ndash3374
Wilson RJ Beezer AE Mitchell JC Solid state reactions studied by isothermal microcalorimetry The solid state oxidation of ascorbic acid Int J Pharm 199613245ndash51
WHO Technical Report Series No 953 Annex 2 Stability testing of active pharmaceutical ingredients and finished pharmaceutical products World Health Organization 2009
Xiang TX Anderson BD A molecular dynamics simulation of reactant mobility in an amorphous formulation of a peptide in poly(vinylpyrrolidone) J Pharm Sci 200493855ndash876
Yang H Zubarev RA Mass spectrometric analysis of asparagine deamidation and aspartate isomerization in polypeptides Electrophoresis 2010 31 1764ndash1772
Yang W-H Brooke D Rate equation for solid state decomposition of aspirin in the presence of moisture Int J Pharm 1982 11271ndash276
Yoshioka S Aso Y Correlation between molecular mobility and chemical stability during storage of amorphous pharmaceuticals J Pharm Sci 200796960ndash981
137
Yoshioka S Stella VJ Chemical stability of drugs substances Stability of Drugs and Dosage Forms Kluwer AcademicPlenium Publishers New York USA 2000 Chap 2
Suumlvegh K Zelkoacute R Physical aging of poly(vinylpyrrolidone) under different humidity of conditions Macromolecules 200235795ndash800
Zhou D Porker W Zang GCZ Drug stability and stability studies In Qui Y Chen X Zhang G Liu L Porker W editors Developing Solid Oral Dosage Forms Pharmaceutical Theory amp Practice Academic Press San Diego PA USA 2009 Chap 9
Zhou D Physical and chemical stability and excipient compatibility J Valid Technol 200936ndash47
138
139
CHAPTER ndash 7
FORCED DRUG DEGRADATION 71 INTRODUCTION
A consideration of the chemical stability of drug substances is of fundamental importance to the formulator since it affects the quality efficacy and safety of drug products It is necessary to know any change in the chemical stability of a drug substance with time under the influence of environmental factors such as air moisture heat light and chemical factors such as pH solvent buffer This can be achieved by performing stress testing or forced degradation studies on drugs and drug products The ICH (International Conference on Harmonization) Q1A (R2) Guideline (ICH 2003) states the object of stress testing of new drug substances as follows
ldquoStress testing of the drug substance can help identify the likely degradation products which can in turn help establish the degradation pathways and the intrinsic stability of the molecule and validate the stability indicating power of the analytical procedures used The nature of the stress testing will depend on the individual drug substance and the type of drug product involvedrdquo
The ICH guideline makes further recommendation as follows
ldquoStress testing is likely to be carried out on a single batch of the drug substance It should include the effect of temperatures (in 10degC increments (eg 50degC 60degC etc) above that for accelerated testing) humidity (eg 75 RH or greater) where appropriate oxidation and photolysis on the drug substance The testing should also evaluate the susceptibility of the drug substance to hydrolysis across a wide range of pH values when in solution or suspension Photostability testing should be an integral part of stress testing However it may not be necessary to examine specifically for certain degradation products if it has been demonstrated that they are not formed under accelerated or long term storage conditions Results from these studies will form an integral part of the information provided to regulatory authoritiesrdquo
The requirements for ICH Q1B Guideline (ICH 1996a) on photostability testing are as follows
ldquoForced degradation testing studies are those undertaken to degrade the sample deliberately These studies which may be undertaken in the development phase normally on the drug substances are used to evaluate the overall photosensitivity of the material for method development purposes andor degradation pathway elucidationrdquo
Forced degradation of new drug substances and drug excipients is considered as a degradation process that has been carried out at conditions that are more severe than those of the accelerated conditions It enables the establishment of the degradation pathways determination of the chemical structure of the degradation products assessment of intrinsic stability of drug substances and the validation of stability-indicating assay methods An understanding of the chemical reactivity of molecules during forced degradation studies is helpful in the development of different dosage forms Forced degradation studies are also considered as integral part of the drug development process The pharmaceutical industry performs forced degradation studies on drugs during the preformulation stage to enable the selection of appropriate active ingredients and excipients product characterization compatibility assessment assay development and formulation development and optimization Since these studies provide information on the mode of degradation of a drug and the products formed it can be used for the improvement of the manufacturing process and in the design of suitable packaging Forced degradation studies require the application of stability-indicating assay methods for the accurate determination of the intact drug in the presence of degradation products (see Chapter 11) Data
140
on forced degradation studies are required for New Drug Application (NDA) for registration purpose The development methodology application and regulatory aspects of forced degradation studies of drug substances and drug products have been reviewed (Reynolds 2004 Reynolds et al 2002 Ngwa 2010 Maheswaran 2012 Singh and Rehman 2012 Roge et al 2013 Singh et al 2013 Jain and Basniwal 2013 Charde et al 2013 Hotha et al 2013 Blessy et al 2014 Deokate and Gorde 2014 Shete et al 2014 Rawat and Pandey 2015 Tamizi and Jouyban 2016)
72 OBJECTIVES
The objectives of forced degradation studies of new drug substances and drug products have been described by Blessy et al (2014) and are as follows
Establishment of degradation pathways of drug substances and drug products
Differentiation of degradation products of the drug in the drug products from those formed from the non-drug product in a formulation
Determination of the chemical structures of degradation products
Determination of the intrinsic stability of a drug substance in a formulation
Elucidation of the degradation mechanism of the drug substances and drug products such as oxidation hydrolysis thermolysis photolysis isomerization
Development of the stability-indicating assay method of the drug substances
Understanding of the chemical properties of drug molecules
Development of more stable formulations
Determination of degradation profiles of the drug substances similar to that observed in a formal stability study under ICH conditions
Solution of stabilityndashrelated problems of drug substances
They may also include
Identification of impurities related to drug substances or excipients
73 FACTORS INVOLVED IN DEGRADATION
731 Degradation Conditions
The stress conditions selected for the forced degradation study of a drug or product should be considered according to its degradation behavior during manufacturing storage and use (Jenke 1996) A general protocol of forced degradation conditions used for drugs or products (Blessy et al 2014) is presented in Fig 71 The normally used stress conditions in a forced degradation study include acidbase hydrolysis oxidation thermolysis and photolysis to achieve 10 degradation These conditions have been described by Ngwa (2010) and are summarized in Table 71
732 Degradation Limits
It is important to set the limits of degradation of a drug or product that are acceptable in forced degradation studies The degradation limits of 5 10 and 20 have been considered as acceptable for the validation of chromatographic methods (Carr and Wahlich 1990 Jenke 1996 Reynolds et al 2002) Protocols for forced degradation studies of drugs and products could be different as a result of variations in drug concentrations matrices and other factors A maximum period of 14 days of stress testing in acidbase solution and 24 days in peroxide solution has been recommended to produce stressed samples in a forced degradation study (Klick et al 2005) It is necessary to avoid over-stressing of a sample that may lead to the formation of a secondary
141
degradation product not observed in formal studies on the shelf-life stability of a drug On the other hand under stressing a sample may not produce sufficient amount of the degradation products (Maherwaran 2012) These factors should be taken into consideration in the design of a forced degradation study to achieve the desired objectives
Forced degradation study
Drug Substance Drug product
Solid SolutionSuspension Solid SolutionSuspension
Semisolid
Photolytic
Thermal
ThermalHumidity
AcidBase hydrolysis
Oxidation
Photolytic
Thermal
Oxidative
Photolytic
Thermal
ThermalHumidity
Oxidative
Photolytic
Thermal
ThermalHumidity Fig 71 Stress conditions used for the degradation of drug substances and drug products
Table 71 Widely used conditions for conducting forced degradationa
Reaction Condition Storage
Hydrolysis Control drug (water) 40degC 60degC
01 M HCl 01 M NaOH Acid Base control (without drug) pH 2 4 6 8 Oxidation 3 H2O2 25degC 60degC H2O2 control Thermolysis Heat (stability chamber) 60degC 60degC 75 RH 80degC 80degC 75 RH Heat control Room temperature Photolysis Light 1 (ICH option 1) ndash Light 2 (ICH option 2) ndash Light control ndash
a The studies can be carried out at a drug concentration of 1mgml (Bakshi and Singh 2002) This concentration is considered sufficient to detect even the minor degradation products by analytical methods such as HPLC The sampling during a degradation reaction may be done at suitable intervals depending upon the nature and the rate of reaction for 1 to 7 days or more
733 Method of Analysis
In forced degradation studies it is necessary to use an assay method that is stability-indicating A stability-indicating method is a validated quantitative analytical method used to determine the concentration changes in a drug or product with time without interference from degradation products impurities and excipients (FDA 2000 ICH 2005) The specificity of the method can be confirmed by its application to samples that have undergone stress testing The development of stability-indicating assay methods for application to pharmaceutical systems has been discussed by many workers (Ahmad 1985 Bakshi and Singh 2002 Ruan et al 2006
142
Smela 2005 Aubry et al 2009 Annapurna et al 2012) A detailed treatment of the stability-indicating assay methods has been presented in Chapter 11
74 FORCED DEGRADATION STUDIES IN THE DEVELOPMENT OF DRUGS
Hawe et al (2012) have discussed the relevance of forced degradation studies in different phases of drug development with recommendations for selecting suitable conditions The ICH Q5C guideline (ICH 1996b) states that forced degradation studies can help in 1) the assessment whether accidental (or intended) exposure to conditions other than those proposed for example during transportation or storage is deleterious to the product and 2) the evaluation of analytical method as indicator of product stability
Forced degradation studies are important in formulation development to identify the stable formulation The degradation behavior of a drug or a product under particular stress conditions such as temperature or light can be correlated with the proposed storage conditions The stability of test formulations under specified forced degradation conditions could be compared and the most stable and robust formulations may be selected for further development
75 ANALYTICAL TECHNIQUES USED IN FORCED DEGRADATION STUDIES
Forced degradation studies can only be carried out with the help of suitable analytical techniques for the characterization of the degradation products and for the assay of the drug and degradation products under the intended stress conditions The application of these techniques is essential for the detection and determination of the degradation products to assess the stability of the drug substance or drug products The various analytical techniques used during the forced degradation studies are summarized in Table 72 The application of various chromatographic methods in forced degradation profiling of a large number of drugs has been reported by Jain and Basniwal (2003)
Table 72 Application of analytical techniques in forced degradation studies
Technique Type
Spectroscopy Ultraviolet infrared Raman nuclear magnetic resonance mass fluorescence circular dichroism
Chromatography Size exclusion HPLC (reversed phase ion exchange) HPLCndashmass spectrometry (HPLCndashMS) UPLC UPLCndashmass spectrometry (UPLCndashMS)
Electrophoresis Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) capillary electrophoresis capillary electrophoresisndashmass spectrometry (CEndashMS)
76 FORCED DEGRADATION STUDIES
Several forced degradation studies have been carried out on drug substances and drug products Some of these studies are presented in this section
A forced degradation study of rebamipide in bulk and in tablet dosage form has been conducted The drug and tablet (extract) solutions were subjected to acid and alkaline hydrolysis hydrogen peroxide oxidation thermolysis and photolysis and the drug contents were determined using a newly developed validated stability-indicating RPndashHPLC assay method A comparison of the conventional degradation using reflux and microwave assisted degradation showed that the microwave radiation can be used to enhance force degradation under hydrolytic conditions The drug was stable to acid hydrolysis and oxidative thermolytic and photolytic degradation However on alkaline hydrolysis rebamipide (71) underwent amide bond (CndashN) cleavage to form 4-chlorobenzoic acid (72) and (2-oxo-12-dihydroquinolone-4-yl) alanine (73) (Sonawane and Gide 2011)
143
O
NH
O
O OH
NH
Cl
alkalinehydrolysis
O
OH
Cl
+
NH
O
NH2
O OH
(71) (72) (73)
The forced degradation behavior of lumivudine (74) under stress conditions of hydrolysis (acid base neutral) oxidation thermolysis and photolysis according to ICH guideline Q1 A (R2) (ICH 2003) has been studied Lumivudine is stable in neutral solution and unstable in acid and alkaline solutions It undergoes extensive oxidative degradation and is stable to heat and light Five degraded products of the drug have been separated by LC and identified by LCndashMSTOF methods (Bedse et al 2009)
NH2
N
N
O
S
OH
O
(74)
The chemical structures of the forced degradation products of tamsulosin (75) an α1ndashadrenorecpetor antagonist have been determined by a gradient HPLC combined with quadrupole time-of-flight electrospray ionization tandem mass spectrometry (LCQndashTOFndashESIndashMSMS) method Tamsulosin was found to degrade under hydrolytic (base and neutral) oxidative thermolytic and photolytic conditions Twelve degradation products of the drug have been identified in the study (Namdev et al 2014)
NH
CH3
NH2O
S
O
OCH3
OO
OH
(75)
The forced degradation of clobetasol 17-propionate (76) under different stress conditions ie acid base neutral hydrolysis oxidation thermolysis and photolysis has been studied using a validated stability-indicating RPndashHPLC method The drug undergoes extensive degradation in strong base and under oxidative conditions (Fauzee and Walker 2013)
144
O
OH
F
H
H
O
Cl
CH3
CH3O
CH3
OCH3
(76)
The forced degradants of carisbamate (77) have been separated by a RPndashHPLC method and characterized by ESIndashMS 1H and 13C NMR MSMS and 2D NMR (Cosy and HSQC) spectrometry These products result from acidbase hydrolysis hydrogen peroxide oxidation thermolysis and photolysis under stress conditions (Rao et al 2013)
Cl
O
O
NH2
OH
(77)
The stability of crystolepine HCl (78) under various stress conditions (acid alkali neutral light dry heat and oxidation at different temperature) has been studied The drug is highly sensitive to oxidative conditions and is stable in acid and neutral solutions Exposure to light and dry heat at 60degC for 12 h did not affect the drug concentration in the samples (Kuntworbe et al 2013)
CH3
N-
N+
(78)
The dry heat forced degradation of buserelin (79) a GnRH agonist peptide drug used in cancer therapy has been carried out in the solid state by exposing the powder to high temperatures for prolonged periods The assay of the drug and its degradants was performed by a stability-indicating UPLCndashphotodiode array (PDA) method The statistical evaluation of different solid state kinetics models indicated the application of the Ginstling-Brounshtein model to the data No significant degradation was observed under hot melt extrusion conditions ie 5 min at 100degC and 125degC (DrsquoHondt et al 2014)
145
NH
O
O
NH
O
N
NH
NH
ONH
NH
OH
NH
OOH
O
NH
O CH3
ONH
OCH3
CH3NH O
N
O
NH
CH3
N
NH2NH2
CH3
CH3
(79)
146
REFERENCES
Ahmad I Stability indicating assays in pharmaceutical quality control In Ahmad T Khalid R editors Proceedings International Seminar on Policies Management and Quality Assistance of Pharmaceuticals Quality Control Authority Ministry of Health Government of Pakistan Islamabad Pakistan 1985 pp 256ndash264
Annappurna MM Mohapatro C Narendra A Stability-indicating liquid chromatographic method for the determination of lentrozole in pharmaceutical formulation J Pharm Anal 20122298ndash305
Aubry A-F Tattersall P Ruan J Development of stability indicating methods In Huynh-Ba K editor Handbook of Stability Testing in Pharmaceutical Development Regulations-Methodologies and Best Practices Springer New York USA 2009 Chap 7
Bakshi M Singh S Development of validated stability-indicating assay methodsndashcritical review J Pharm Biomed Anal 2002281011ndash1040
Bedse G Kumar V Singh S Study of forced decomposition behavior of lamivudine using LC LCndashMSTOF and MS J Pharm Biomed Anal 2009 4955ndash63
Blessy M Patel RD Prajapati PN Agrawal YK Development of forced degradation and stability indicating studies of drugsndasha review J Pharm Anal 20144159ndash165
Carr GP Wahlich JC A practical approach to method validation in pharmaceutical analysis J Pharm Biomed Anal 1990 8613ndash618
Charde MS Kumar J Welankiwar AS Chakole RD Review development of forced degradation studies of drugs Int J Adv Pharm 20132 34ndash39
DrsquoHondt M Fedorova M Peng C-Y Gevaert B Taevernier L Hoffmann R Spiegeleer BD Dry heat forced degradation of buserelin peptide kinetics and degradant profiling Int J Pharm 2014 46748ndash49
Deokate UA Gorde AM Forced degradation and stability testing strategies and analytical perspectives PharmaTutor 2014261ndash74
Fauzee AF Walker RB Forced degradation studies of clobetasol 17-propionate in methanol propylene glycol as bulk drug and cream formulations by RPndashHPLC J Sep Sci 201336849ndash856
FDA Guidance for Industry Analytical Procedures and Methods Validation Chemistry Manufacturing and Controls Documentation Guidance Food and Drug Administration USA 2000
Hawe A Wiggenhorn M Van De Weert M Garbe JHO Mahler H-C Jiskoot W Forced degradation of therapeutic proteins J Pharm Sci 2012101895ndash913
Hotha KK Reddy SPK Raju VK Ravidranath LK Forced degradation studies practical approach-overview of regulatory guidance and literature for the drug products and drug substances Int Res J Pharm 2013478ndash85
ICH Harmonized Tripartite Guideline Validation of Analytical procedures Text and Methodology Q2(R1) International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use Geneva Switzerland 2005
ICH Harmonized Tripartite Guideline ICHndashQ1A (R2) Stability testing of new drug substances and products (second revision) Geneva Switzerland 2003
ICH Harmonized Tripartite Guideline ICHndashQ1B Photostability testing of new drug substances and products Geneva Switzerland 1996a
ICH Harmonized Tripartite Guideline ICHQ5C Stability testing of biotechnologicalbiological products Geneva Switzerland 1996b
147
Jain D Basniwal PK Forced degradation and impurity profiling recent trends in analytical perspectives J Pharm Biomed Anal 2013 8611ndash35
Klick S Muijselaar PG Waterval J Eichinger T Korn C Gerding TK Debets AJ van de Griend CS van den Beld C Somsen GW De Jong GJ Toward a generic approach for stress testing of drug substances and drug products Pharm Technol 200548ndash66
Jenke DR Chromatographic method validation a review of current practices and procedures II Guidelines for primary validation parameters J Liq Chromatogr Relat Technol 199619737ndash757
Kuntworbe N Alany RG Brimble M Al-Kassas R Determination of pKa and forced degradation of the indoloquinoline antimalarial compound cryptolepine hydrochloride Pharm Dev Technol 201318866ndash876
Maheswaran R FDA perspectives scientific considerations of forced degradation studies in ANDA submissions Pharm Tech 20123605
Namdev D Borkar RM Baju B Kalariya PD Rahangdale VT Gananadhamu S Srinivas R Identification of forced degradation products of tamsulosin using liquid chromatographyelectrospray ionization tandem mass spectrometry J Pharm Biomed Anal 201488245ndash255
Ngwa G Forced degradation as an integral part of HPLC stability-indicating method development Drug Deliv Technol 20101005
Rao RN Ramakrishna K Sravan B Santhakumar K RPndashHPLC separation and ESIndashMS 1H and 13C NMR characterization of forced degradants including process related impurities of carisbamate method development and validation J Pharm Biomed Anal 20137749ndash54
Rawat T Pandey IP Forced degradation studies for drugs substances and drug products-scientific and regulatory considerations J Pharm Sci Res 20157238ndash241
Reynolds DW Facchine KL Mullaney JF Alsante KM Hatajik TD Motto MG Available guidance and best practices for conducting forced degradation studies Pharm Technol 200248ndash56
Reynolds DW Forced degradation of pharmaceuticals Am Pharm Rev 2004756ndash61
Roge AB Tarte PS Kumare MM Shendarkar GR Vadvalkar SM Forced degradation study an important tool in drug development Asian J Pharm Res 20133198ndash201
Ruan J Tattersall P Lozano R Shah RR The role of forced degradation studies in stability indicating HPLC method development Am Pharm Rev 2006946ndash53
Shete S Dhale C Joshi S Hole R Forced degradation study to stability indicating method World J Pharm Pharm Sci 20143863ndash873
Singh R Rehman ZU Current trends in forced degradation study for pharmaceutical product development J Pharm Educ Res 2012354ndash63
Singh S Junwal M Modhe G Tiwari H Kurmi M Parashar N Sidduri P Forced degradation studies assess the stability of drugs and products Trends Anal Chem 2013 4971ndash88
Smela JW Regulatory considerations for stability indicating analytical methods in drug substance and drug product testing Am Pharm Rev 2005851ndash54
Sonawane S Gide P Optimization of forced degradation using experimental design and development of a stability-indicating liquid chromatographic assay method for rebamipide in bulk and tablet dosage form Sci Pharm 20117985ndash96
Tamizi E Jouyban A Forced degradation studies of biopharmaceuticals selection of stress conditions Eur J Pharm Biopharm 20169826ndash46
148
149
CHAPTER ndash 8
PACKAGING EFFECTS ON STABILITY 81 INTRODUCTION
The regulatory authorities and pharmaceutical manufacturers have to pay great attention to the stability of drug products to ensure their shelf-lives and delivery to the consumer with the highest quality attributes One of the essential components in this effort is the packaging of the products The design of a package for a particular product depends on the stability profile of the product The packaging development and integrity plays a major role in maintaining the stability of the individual solid and liquid dosage forms The stability of a product depends on the efficacy of the packaging material to preserve its chemical and physical characteristics during the storage period
The container-closure system is an important component of packaging that can affect the stability of a product The packaging material and system must be suitable for its intended use and should adequately protect the product from deterioration and tampering It should be compatible with the dosage form and should be composed of materials that are considered safe for use specially the child resistant containers (WHO 2009) The photostability characteristics of new drug substances and products should be evaluated to show that on exposure to light the product and packaging materials do not result in any undesirable change (ICH 1996)
An understanding of the factors influencing drug stability and the application of modern packaging techniques could enable the development of suitable packaging materials An appropriate design of a stability program including different batches of a finished product in various strengths and involving different types of packaging material can be implemented by the application of factorial designs such as matrixing and bracketing Several authors have dealt with the subject of pharmaceutical packaging and its various aspects (Croce et al 1986 Harburn 1991 Dean et al 2000 Byett 2002 Soroka 1996 2002 Sinha et al 2006 Yam 2009) and a number of reviews have been published on the selection role and design of packaging and its effect on the chemical and photostability of drug products (Akala 2010 Reed et al 2003 Templeton et al 2005 Waterman and MacDonald 2010 Sacha et al 2010 Sabah et al 2014) Guidelines on the packaging of pharmaceutical products are also available for the industry (FDA 1995 WHO 2002 United States Pharmacopeia 2016 British Pharmacopoeia 2016)
82 DEFINITION
Several definitions of packaging are described in the literature
World Health Organization (2002)
ldquoPackaging may be defined as the collection of different components (eg bottle vial closure cap ampoule blister) which surround the pharmaceutical product from the time of production until its userdquo
Soroka (1996)
ldquoPackaging is a coordinated system of prepared goods for transport distribution storage sales and use It is a complex dynamic scientific aesthetic and business function which in its most fundamental form contains protects preserves provides convenience and informs the concerned people within the acceptable environmental constraintsrdquo
Sinha et al (2006)
150
ldquoPharmaceutical packaging is a means of providing protection presentation identification information convenience compliance and compatible unit that maintains the integrity and stability
of the productrdquo
83 TYPES
The packaging material used for pharmaceutical products can be divided into two types
831 Primary Packaging Material
It consist of bottles containers ampoules vials tubes etc and contains the product It provides protection to the product from any change against environmental factors The primary packaging material should be compatible and should not undergo any chemical interaction with the product resulting in leaching of the components absorption of the material and adsorption of the drug on the surface of container The primary packaging for a product (eg container and closure) must be suitable for the specific purpose because it is in direct contact with the product
832 Secondary Packaging Material
It consists of cartons boxes drums etc to hold the primary packagings of the product The secondary packaging material provides protection against external factors as well as the physical and mechanical stress during transportation and shipment It also provides strength for stacking in the ware house
84 FUNCTIONS
Packaging is an integral part of the drug development process in particular the stability assessment program It plays a significant role in protecting the drug product from chemical and physical changes caused by the environmental factors These changes may occur under ambient or accelerated storage conditions The various functions of the primary packaging material to preserve product stability against environmental factors are given in Table 81
A major route of the chemical degradation of drugs in liquid preparations involves hydrolytic processes as a result of change in pH for example in esters and amides This can be controlled by the use of Type 1 glass containers (borosilicate glass highly resistant to chemical action) for parenterals or Type II glass containers (chemically treated soda glass high hydrolytic resistance) for acid or neutral aqueous preparations (British Pharmacopoeia 2016) The choice of these containers depends on the nature of the preparation and its sensitivity to hydrolytic degradation The light sensitivity of drug substances (eg vitamins steroids alkaloids fluoroquinolones NSAIDs) and drug products requires an effective packaging system for protection against photochemical damage The various Pharmacopoeias (British Pharmacopoeia 2016 United States Pharmacopeia 2016 European Pharmacopoeia 2015) have prescribed conditions for the use of containers (eg light-resistant) and storage (eg protected from light) for light-sensitive drugs and products These can be protected from light by the use of opaque or amber colored containers Amber glass is effective against UV and visible light (lt470 nm) The opaque secondary packaging also provides protection from light
Table 81 Packaging preservation of product stability against environmental factors
Factor Packaginga
Ultraviolet visible and sunlight (200 ndash 800 nm)
Light-resistant containers (opaque or amber)
Temperature Plastic containers (heat resistant)
Atmospheric gases (oxygen carbon dioxide)
Hermeticair-tight containers
Moisturevapors tightair-tight containers Particulate matter well-closedtight containers Microbial containments tight containers
151
a The secondary packaging also provides a certain degree of protection from light heat moisture gases microbes etc
85 SELECTION
The selection of packaging material for a drug product is based on its efficacy and performance characteristics to preserve the quality strength and safety of the product It takes into consideration the nature of the product the chemical and physical characteristics of the product the protective requirements of the product and its marketing needs It should be compatible with the product possess high protective efficacy against environmental factors be chemically non-reactive and non-toxic and should have aesthetic appeal The use of substandard packaging material could lead to the destabilization of the product as a result of different stress conditions drug-container interactions and chemical degradation (Sabah et al 2014) According to Sinha et al (2006) the choice of the packaging material for a pharmaceutical product depends on the following factors
Dosage form desired eg syrup tablet creams
Degree of required protection
Compatibility of the material with the dosage forms
Presentation and aesthetics
Consumer convenience eg size weight of dosage form
Method of filling
Method of sterilization
Method of distribution-hospital pharmacy retailer
Capacity of packaging-small bulk for pharmacies OPD
Required shelf-life
86 PACKAGING STUDIES
Several studies have been conducted to evaluate the effect of packaging on the stability of drug products during storage under different conditions (eg humidity temperature light) These studies have been helpful in the design of suitable packaging material to meet a particular requirement in order to achieve optimum stability of the product Some of these studies are presented in the following sections
861 Solid Dosage Forms
The effect of packaging on the storage of strip sealed carbamazepine tablets (Tegretol and Tegral) under different temperaturehumidity conditions has been studied The tablets in original strips were stored at 40deg 50deg and 60degC for 6 3 and 1 month respectively at 75 RH The tablets removed from their strips and placed in closed bottles were also exposed to 40degC 97 RH for 5 min daily for 1 month The tablet performance was examined by dissolution using a HPLC method The results indicated that the stress conditions used do not affect the stripped Tegretol tablets while dissolution of tegral tablets stored at 50ndash60degC 75 RH for 6 months is increased The tablets stored at 40degC 97 RH for 1 month showed similar effects They were hardened and contents dissolved to the extent of about 7 in 60 min Finlepsin tablets dispersed in bottles were also exposed to 25deg and 40degC 97 RH for 5 min daily by removing the caps for 1 month Under these conditions the effect of humidity has been found to be more drastic at 40degC than that of 25degC with a decrease in dissolution rate All the brands of carbamazepine tablets stored under different stress conditions remained chemically stable (Al-Zein et al 1999)
The efficacy of different packages used to evaluate the chemical stability of the tablets of a new moisture sensitive drug stored under accelerated conditions for 6 months has been
152
determined The values of equilibrium moisture content (EMC) have been found to be 23 24 and 29 at 25degC 60 RH 30degC 60 RH and 40degC75 RH respectively The permeation of the containers (mg per blister per day) increased in the order cold-form aluminium blister (0001) aclar blister (0008) cyclic olefin blister (0040) and polyvinyl chloride blister (0259) The drug contents after storage of the product in different packages for 6 months at 40degC 75 RH were cold-form aluminium blister (100) high density polyethylene container (99) aclar blister (97) cyclic olefin blister (91) and polyvinyl chloride blister (84) The stability of the packaged product has been predicted on the basis of EMC degradation rate of unpackaged product and moisture barrier characteristics of the packages (Allinson et al 2001)
The physical and chemical stability of fixed dose combinations (FDC) of the anti-tuberculosis drugs rifampicin isonizaid pyrazinamide and ethambutol stored for 3 months under ICH accelerated conditions (40degC 75 RH) in packaged and unpackaged tablets forms has been studied After three months the unpackaged tablets showed severe physical and chemical changes compared to those observed in the packaged products An interaction between isoniazid and rifampicin in the stored tablets was also found that could affect the potency loss of these drugs It has been suggested that these tablets should be packed in moisture barrier packaging to eliminate the effect of moisture on the stability of the products (Bhutani et al 2004)
A study has been carried out to determine the effects of temperature and humidity on the stability of aspirin and ascorbic acid in different tablet matrices stored in various packagings The stability of the drugs was found to vary according to a relation between the hardness and moisture sorption of the tablets The packaging used were evaluated on the basis of the parameters 1) the ratio of residual concentration of the tabletted drug in closed containers to the ratio of residual concentration of the drug in exposed tablets (stability ratio) and 2) the moisture uptake of tablets in closed containers The results indicated the superiority of cellophane and aluminium foil strip single dose packagings as moisture barriers to well-closed glass or plastic container under the intermediate-to-high water vapor pressure conditions employed during the storage (Lee et al 1965)
The stability of paracetamol tablets packed in polycoated paper cellophane PVCACaluminium foil and PVC PVDC aluminium foil has been studied under different storage conditions for six months The shelf-lives of the tablets at 25degC 75 RH in these packages were 182 171 191 and 230 months respectively Thus PVC PVDC aluminium foil offered best protection to the tablets compared to the other packagings (Ahmad and Shaikh 1993) Paracetamol tablets in these packagings showed an increase in disintegration time on increasing the temperature from 25ndash45degC at 75 RH The tablets packaged in PVC PVDC aluminium foil were least affected by changes in disintegration time during storage (Ahmad and Shaikh 1994a) The tablets stored in these packagings at 25degC 75 RH showed a loss in hardness from 5 to 10 and at 45degC 75 RH from 10 to 39 (Ahmad and Shaikh 1994b) The PVC PVDC aluminium foil packaging gave best protection to tablets against moisture on storage under different RH conditions (Ahmad and Shaikh 2003)
862 Liquid Dosage Forms
The stability of various injection dilutions of taxol (03 06 09 and 12 mgml in 50 polyoxyethylated castor oil and 50 dehydrated ethanol ) in infusion solutions (5 dextrose or 09 sodium chloride solution) stored in 100 ml glass bottles polyvinyl chloride (PVC) infusion bags and polyolefin containers at 20ndash23degC for 24 hours has been studied The drug content was determined by a stability-indicating HPLC method and the clarity was observed by visual inspection The drug did not show any loss in 24 hours All the solutions became hazy initially Solutions in PVC bags developed greater haze with time compared to those in glass and polyolefin containers The haze in solutions stored in PVC bags was identified as being due to the leaching of di(2-ethylhexyl) phthalate (DEHP) used as a plasticizer The formation of DEHP was not observed in solutions stored in glass and polyolefin containers The results indicated that the taxol solutions stored in different containers are chemically stable over a period of 24 hours (Waugh et al 1991)
153
The chemical degradation of ceftazidine in intravenous solutions (40 mgml) stored in 100 ml polypropylene (PP) bags and polyvinyl chloride (PVC) bags and in glass containers filled with 5 dextrose or 09 sodium chloride solution at 20 and 35degC for 20 hours has been studied Ceftazidine and its main degradation product pyridine were assayed by a HPLC method The degradation of the drug was greater in PP and PVC bags than that in the glass bottles Solutions stored in PP bags were more stable compared to those in PVC bags The results showed that glass containers are better than the PP and PVC bags for the storage of ceftazimide solutions at different temperatures (Arsene et al 2002)
The stability of beclofen (10 mgml) diltiazem HCl (12 mgml) dipyridamole (10 mgml) and flecainide acetate (20 mgml) in extemporaneously compounded oral liquids has been determined These liquids were prepared in a 11 mixtures of Ora-Sweet and Ora-Plus and Ora-Sweet SF and Ora-Plus (Paddock Laboratories USA) and cherry syrup and stored in 100 ml amber and clear polyethylene terephthalate containers three each at 5 and 25degC in the dark for 60 days The drug content of each preparation was determined by a stability-indicating HPLC method At the end of the storage period it was found that beclofen diltiazem HCl dipyridamole and flecainide acetate solutions retained an average of 92 of the initial concentration at both 5 and 25degC There was no change in appearance odor or pH of the solutions All the containers were found to provide good stability to these drugs in oral liquid preparations (Allen and Erickson 1996)
The stability of a number of drugs in under filled plastic and glass containers has been evaluated The drugs were reconstituted according to the manufacturersrsquo instructions and then added to 50 ml dextrose injection (5) in PVC bags and glass partial-filled bottles All admixtures were stored at 25degC unprotected from light and the drug content determined over 24 hours by a HPLC method Methotrexate leucovorin calcium cytarabine dactinomycin mithramycin vinblastin sulfate cyclophosphamide and dacarbazine were stable (10 or no change in 24 hours) in plastic and glass containers Doxorubicin and fluorouracil were found to be more stable in plastic containers than the glass containers The t90 values of the drugs are reported in Table 82
It has been suggested that carmustine and bleomycin sulfate should be administered only in glass containers in which these drugs are more stable Mitomycin dissolved in 09 NaCl injection is more stable in plastic container while it is not stable in 5 dextrose injection (Benvenuto et al 1981) The stability of carboplatin (32 mgml in 5 glucose infusion solution) stored in polyethylene polypropylene and glass containers at 25 40 and 60degC has been studied using a HPLC method The degradation of carboplatin follows an apparent first-order kinetics that does not depend on the nature of the container The application of Arrhenius equation indicated a lt2 loss in the concentration of the drug at room temperature in one month (Prat et al 1994)
Table 82 t90 Values of drugs in plastic and glass containers
Drug Container t90 value (h)
Doxorubicin glass 40 Fluorouracil glass
plastic 7 43
Vincristine sulfate glass 10 Bleomycin sulfate plastic 07 Carmustine plastic 06
A comparison of the adsorption effects of antineoplastic drugs on low density polyethylene (LDPE) containers glass containers and PVC bags has been made The therapeutic doses of common cytotoxic drugs carboplatin carmustine cytarabine dacarbazine fluorouracil gemcitabine melphalan methotrexate and vinorelbine were placed in the containers filled with 09 isotonic sodium chloride solution and 5 dextrose infusion solution The containers were stored in the dark at 4 and 25degC for 168 hours and the drug contents were determined by a HPLC method Carmustine did not adsorb in LDPE and glass containers at 4degC However a little loss in the concentration was observed at 25degC A greater loss of the drug was noted in PVC bag Dacarbazine and melphalan also showed a loss in the concentration that was independent of the
154
type of container The other drugs did not show any loss in concentration The stability of the drugs in these containers appeared in the order glass lt LDPE lt PVC (Beitz et al 1999)
The stability of the antineoplastic drug docetaxel in infusion solutions has been studied after 1) reconstitution of the injection concentrate and 2) further dilution in 09 sodium chloride and 5 dextrose solution on storage in polyolefin containers and PVC bags The HPLC analysis indicated that reconstituted docetaxel solutions were stable to the extent of 95 or more for four weeks at 4 and 25degC The diluted solutions (03 mgml and 09 mgml) were also stable at a level of 95 or more for four weeks in polyolefin containers at 25degC However docetaxel in dilute solutions stored in PVC bags showed precipitation after the 5th day The leaching of DEHP from PVC bags by docetaxel infusion solutions with time was also observed (Thiesen and Karmer 1999)
The photostability of a compound (lyophilized product reconstituted solution (0365 mgml in 20 ml of 33 dextrose03 NaCl)) has been studied at 25degC under combined UV-visible light (81 klx visible and 43 Wm2 UV light) using a photostability chamber The concentration of the active material and degradant (formed by photoisomerization) was determined by a stability-indicating HPLC method The results showed that the lyophilized product and the reconstituted solution degrade to the extent of 009 and 029 per klxndashh respectively The drug solution before lyophilization (manufacturing) and post lyophilization (secondary packaging) degraded to the extent of 0017 and 0014 per klxndashh respectively The amount of combined UV-visible light exposure to achieve 01 photodegradation of reconstituted solution in amber-vial was 35 h and of lyophilized product was 108 h (Templeton et al 2005)
The antihypertensive 14ndashdihydropyridine drugs are sensitive to light and are dispensed in solid dosage forms However the solutions of these drugs have been stabilized by using photoprotective polyethylene terephthalate (PET) containers The solutions of felodipine in blue PET containers are completely stabilized for 6 h when exposed to stress irradiation conditions using a Xenon lamp On the contrary the t90 of the drug in glass containers has been found to be 24 min The study shows that the polymeric containers are effective as packaging material for the photoprotection of liquid preparation of these drugs (DeLuca et al 2016)
87 STABILITY PREDICTION IN PACKAGED PRODUCTS
A consideration of the stability of drug products must take into account the packaging since it affects the shelf-life of the product Packaging plays several roles in improving or worsening the shelf-life The packaging effects on the stability of the product include 1) altering the movement of volatilegaseous materials between inside and outside of the package and 2) providing leachable and extractable impurities into a dosage form Packaging slows down the equilibration of the external humidity with the active ingredient inside the packaging The water-impermeable packaging (eg glass bottles foil-foil blisters) prevents and transfer of moisture to the product In this case the equilibrium relative humidity (ERH) inside the packaging will be a function of the moisture content of the drug product as packaged and the adsorption tendency of the product at a give temperature In the case of water-permeable packaging (eg plastic bottles and blisters) moisture will enter or leave the package at a rate that depends on the moisture vapor transmission rate (MVTR) that is a function of the packaging material the thickness of the package the surface area of the package and the difference between RH inside and outside the packaging As the moisture difference between external and internal environments becomes closer the moisture transfer rates slow down (Waterman 2009)
88 STABILITY TESTING
The stability testing should be conducted on the dosage form packaged in container-closure system proposed for marketing (including as appropriate any secondary packaging and container label) Any studies carried out on the drug product outside its immediate container or in other packaging material can form a useful part of the stress testing of the dosage form or can be considered as supporting information respectively (ICH 2003)
155
REFERENCES
Ahmad I Shaikh RH Stability of paracetamol in packaged tablet formulations Pak J Pharm Sci 1993 637ndash45
Ahmad I Shaikh RH Effect of temperature and humidity on the disintegration time packaged paracetamol tablet formulations Pak J Pharm Sci 1994a71ndash7
Ahmad I Shaikh RH Effect of temperature and humidity on hardness and friability of packaged paracetamol tablet Pak J Pharm Sci 1994b769ndash78
Ahmad I Shaikh RH Effect of moisture on the stability of packaged paracetamol tablets formulations Pak J Pharm Sci 2003 1613ndash16
Akala EO Effect of packaging on stability of drugs and drug products In Gad SC editor Pharmaceutical Manufacturing Handbook Regulations and Quality Pharmaceutical Sciences Encyclopedia John Wiley amp Sons Inc Hoboken New Jersey USA 2010 pp641ndash686
Al-Zein H Riad LE Abd-Elbary A Effect of packaging and storage on the stability of carbamazepine tablets Drug Dev Ind Pharm 199925223ndash227
Allen LV Jr Erickson MA 3rd Stability of baclofen captopril diltiazem hydrochloride dipyridamole and flecainide acetate in extemporaneously compounded oral liquids Am J Health Syst Pharm 1996532179ndash2184
Allinson JG Dansereau RJ Sakr A The effects of packaging on the stability of a moisture sensitive compound Int J Pharm 200122149ndash56
Arsegravene M Favetta P Favier B Bureau J Comparison of ceftazidime degradation in glass bottles and plastic bags under various conditions J Clin Pharm Ther 20022205ndash209
Beitz C Bertsch T Hannak D Schrammel W Einberger C Wehling M Compatibility of plastics with cytotoxic drug solutions-comparison of polyethylene with other container materials Int J Pharm 1999185113ndash121
Benvenuto JA Anderson RW Kerkof K Smith RG Loo TL Stability and compatibility of antitumor agents in glass and plastic containers Am J Hosp Pharm 1981 381914ndash1918
British Pharmacopoeia Her Majestyrsquos Stationary Office London UK 2016 Electronic version
Byett J A Handbook of Packaging Technology John Wiley New York USA 2002
Bhutani H Mariappan TT Singh S The physical and chemical stability of anti-tuberculosis fixed-dose combination products under accelerated climatic conditions Int J Tuberc Lung Dis 200481073ndash1080
Croce CP Fisher A Thomas RH Packaging material sciences In Lachman L Lieberman HA Kanig JL editors The Theory and Practice of Industrial Pharmacy 3rd ed Lee amp Fabiger PA USA 1986 Chap 24
Dean DA Evans ER Hall IH Pharmaceutical Packaging Technology Taylor amp Francis London UK 2000
De Luca M Ioele G Spatari C Ragno G Photostabilization studies of antihypertensive 14-dihydropyridines using polymeric containers Int J Pharm 2016505376ndash382
European Pharmacopoeia European Pharmacopoeial Convention and the European Union Strasbourg France 8th edition 2015
FDA Guidance for industry container closure systems for packaging human drugs and biologicals Maryland USA 1995
Harburn K Quality Control of Packaging Materials in Pharmaceutical Industry Marcel Dekker Inc New York USA 1991
156
ICH Harmonized Tripartite Guideline ICHndashQ1A (R2) Stability testing of new drug substances and products (second revision) Geneva Switzerland 2003
Lee S DeKay HG Banker GS Effect of water vapor pressure on moisture sorption and the stability of aspirin and ascorbic acid in tablet matrices J Pharm Sci 1965541153ndash1158
Prat J Pujol M Girona V Muntildeoz M Soleacute LA Stability of carboplatin in 5 glucose solution in glass polyethylene and polypropylene containers J Pharm Biomed Anal 1994 1281ndash84
Reed RA Harmon P Manas D Wasylaschuk W Galli C Biddell R Bergquist PA Hunke W Templeton AC Ip D The role of excipients and package components in the photostability of liquid formulations PDA J Pharm Sci Technol 200357351ndash368
Sabah A Ahmad I Arsalan A Arif A Tanwir S Abbas A Ahmed FR Features functions and selection of pharmaceutical packaging materials Int J Pharm and Neutra Res 201411ndash12
Sacha GA Saffell-Clemmer W Abram K Akers MJ Practical fundamentals of glass rubber and plastic sterile packaging systems Pharm Dev Technol 2010156ndash34
Soroka W Fundamentals of Packaging technology Institute of Packaging Professionals Naperville Illinois USA 1996
Soroka W Fundamental of Packaging Technology Institute of Packaging Professionals Naperville Illinois USA 2002
Sacha GA Safell-Clammer W Abram K Akers MJ Pharmaceutical product development fundamentals Practicals fundamentals of glass rubber and plastic sterile packaging systems Pharm Dev Tech 2010156ndash34
Sinha VR Kumria R Katare OP Pharmaceutical packaging In Jain NK editor Pharmaceutical Product Development CBS Publishers amp Distributors New Delhi India 2006 Chap 11
Templeton AC Xu H Placek J Reed RA Implications of photostability on the manufacturing packaging storage and testing of formulated pharmaceutical products Pharm Technol 200568ndash86
Thiesen J Kraumlmer I Physico-chemical stability of docetaxel premix solution and docetaxel infusion solutions in PVC bags and polyolefine containers Pharm World Sci 199921137ndash141
United States Pharmacopeia 39 ndash National Formulary 34 United States Pharmacopoeial Convention Rockville MD USA 2016
Yam KL Encyclopedia of Packaging Technology John Wiley amp Sons New York USA 2009
Waterman KC Understanding and predicting pharmaceutical product shelf-life In Huynh-Ba K editor Handbook of Stability Testing in Pharmaceutical Development Springer New York USA 2009 Chap 6
Waterman KC MacDonald BC Packaging selection for moisture protection for solid oral drug products J Pharm Sci 2010994437ndash4452
Waugh WN Trissel LA Stella VJ Stability compatibility and plasticizer extraction of taxol (NSCndash125973) injection diluted in infusion solutions and stored in various containers Am J Hosp Pharm 1991481520ndash1524
WHO Guidance on packaging for pharmaceutical products World Health Organization Technical Report Series No 902 2002
157
CHAPTER ndash 9
STABILIZATION
91 INTRODUCTION
Drug substances are sensitive to environmental factors and drug products may undergo chemical and physical degradation reactions during formulation production storage and shipment The degradation may be prevented and the shelf-lives of the products may be enhanced by adopting appropriate stabilization methods The application of these methods would depend on the nature of the dosage form type of the degradation reaction and the sensitivity of the active ingredient to factors such as oxygen moisture temperature light pH buffer ionic strength solvent etc The common approaches to minimize degradation and to achieve stabilization of drug products include
Optimization of formulation parameters (eg excipients particle size pH solvent buffer) with respect to the stability of the drug
Control of drug-drug and drug-excipient interactions
Control of environmental factors
Use of appropriate stabilizers and coating agents
Nitrogen purging during production and packaging for oxidizable products
Use of appropriate packaging material
Use of recommended storage conditions
The various methods for the stabilization of drug products have been described by Connors et al (1986) Racz (1989) Thoma (1996) Yoshioka and Stella (2000) Eccleston (2002) Imp-Ensep (2002) Sinko (2011) and reviewed by Connors et al (1997) Yu (2001) Challa et al (2005) Piechocki and Thoma (2007) Rasheed et al (2008) Bhattacharya and Syrayanarayanan (2009) Laitinen et al (2013) Sheraz et al (2015) and Ahmad et al (2016)
92 PREVENTION OF DEGRADATION REACTIONS
921 Common Degradation Reactions
9211 Hydrolysis
The hydrolytic reactions can be minimized by
Adjustment of pH to an optimum value
Use of buffers at a low concentration
Use of appropriate cosolvents
Example Hydroylsis of chlordiazepoxide
Chlordiazepoxide (91) is hydrolyzed by ring cleavage to form a benzophenone derivative (93) through the participation of a lactam intermediate (92) by specific acid-base catalysis
158
N
N
NHCH 3
Cl
NH
NCl
O
O
NH2
O
H2O
O
(91) (92) (93)
The stabilization of chlordiazepoxide can be achieved by the adjustment of pH to an optimum value of 20 It can also be stabilized by using a separately packaged solvent to prepare a solution by reconstitution of the lyophilized compound at the time of the use of the product (Maulding et al 1975)
9212 Oxidation
The oxidation reactions can be minimized by
Exclusion of oxygen
Adjustment of pH (increase in oxidation potential by decrease in pH)
Use of antioxidants
Example Oxidation of ascorbic acid
Ascorbic acid (94) is easily oxidized to dehydroascorbic acid (95) in aqueous solutions in the presence of air The rate of oxidation is increased with pH due to the formation of ascorbyl ion (AHndash) as an intermediate in the reaction
OH
OO
OH OH
OH
H
OH
OO
O O
OH
H
oxidation
(94) (95)
The stabilization of ascorbic acid can be achieved by the adjustment of pH to 20ndash30 to form the nonionized species of the molecule (Blaug and Hajratwala 1972)
9213 Photolysis
The photolysis (photodegradation) reactions can be minimized by
Use of amberopaque containers
Use of stabilizers
Use of UV and visible absorbers
Example Photooxidation of menadione
Menadione (vitamin K3) (96) undergoes photooxidation in aqueous solution on irradiation with UV light to form 2-methyl-23-epoxy-14-naphthaquinone (97)
159
CH3
O
O
hv
pH 6-12
CH3
O
O
O
(96) (97)
Menadione can be stabilized in aqueous solution by the adjustment of the pH in acidic region It can also be stabilized in the presence of various electron donors The stability of the drug increases with an increase in the concentration of the electron donor (Vire et al 1980)
922 Prevention of Degradation Reactions Involving Steric Structural Variations
In addition to common degradation reactions described above other chemical transformations involving steric structural variations may occur during the manufacture and formulation of drug substances
9221 Cyclization
Example Cyclization of diclofenac sodium
Diclofenac sodium (98) undergoes intramolecular cyclization in acid solution in which the molecule exists in the nonionized state The H3O+ ion-catalyzed degradation of the intermediate product (99) leads to the formation of a lactam as the final product (910) (Palomo et al 1999)
O
C
NH
OH
ClClCl Cl
O-
NH+
OH
Cl Cl
N+
OH
Cl Cl
N O
+H+
H3O+
(910) (99)
(98)
160
The rate determining step in the reaction is the H3O+ ion-catalyzed degradation of the intermediate product (99) A change in pH towards the neutral region would lead to the stabilization of the molecule
9222 Dimerization
Example Dimerization of amoxicillin
Amoxicillin (911) a penicillin derivative undergoes dimerization reaction at higher concentrations in the pH range of 80ndash100 while hydrolysis of the molecule occurs at lower concentrations In the dimerization reaction two molecules of amoxicillin undergo interaction One molecule undergoes nucleophilic attack of the α-amino group of the side chain on β-lactam carbonyl group of the other molecule to form a dimer (912) (Bundgaard 1977)
CH3S
OH
CH CO
NH2
NH CH
ON
CH3
COO-
+CH3S
OH
CH CO
NH2
NH CH
ON
CH3
COO-
CH3S
CH3S
OH
CH CO
NH2
NH CH
CO
NH
CH3
COO-
NH
CH
OH
CO NH
CO
N
CH3
COO-
Dimerization
(912)
The dimerization of amoxicillin can be prevented by a decrease in concentration and increase in pH of the solution The injectable preparations of amoxicillin may be used in a buffered lyophilized form to prevent the dimerization
9223 Epimerization
Example Epimerization of tetracycline
Tetracycline (913) solutions undergo epimerization to form 4-epitetracycline (914) during storage The epimer is more toxic than the tetracycline (Yuen and Sokolski 1977)
(911)
161
N(CH 3)2OH
OH
OH O OH
OH
O
CH3H
CONH 2
H
(H3C)2N
OH
OH
OH O OH
OH
O
CH3
CONH 2
(913) (914)
Epimerization of tetracycline can be minimized by adjusting the pH values of the solutions below 20 or above 90
9224 Racemization
Example Racemization of epinephrine
The optically active compounds such as epinephrine (915) can be converted from one form to the other (916) in aqueous solution under certain conditions (Hellberg 1955)
NH
OH
OH
OH
H
CH3
OH
OH
NH
OH
H
CH3
(915) (916)
The racemization of epinephrine can be prevented by adjusting the pH of the solution in the pH range of 35ndash55 Complexation of adrenaline with boric acid also leads to an increase in the stability of the drug
9225 Polymerization
Example Polymerization of ampicillin
The concentrated aqueous solutions of sodium ampicillin (917) (10ndash25 wv) for IM administration undergo change to form high molecular weight ampicillin polymers when stored at room temperature and pH values in the range of 80ndash100 (Racz et al 1989)
O
NH
NH2
NO
S CH3
HH
H
HO
OH
CH3
(917)
The polymerization of ampicillin can be prevented by maintaining the pH of the solution in the pH range of 30ndash60
93 METHODS OF STABILIZATION
The important methods for the stabilization of drug substances are as follows
931 Temperature Control
The rates of chemical degradation reactions involving drug substances are normally proportional to the number of collisions per unit time An increase in the number of collisions with
162
an increase in temperature results in an enhanced degradation of drugs The relation between the rate constant of degradation and the temperature is given by Arrhenius equation (see Section 251) The higher the activation energy Ea (energy required to transfer a molecule from the ground state to the transition state) the more difficult is for a molecule to undergo degradation An increase in the stability of drug products can be achieved by a decrease in temperature above the frozen conditions The storage conditions of drug substances and drug products are given in the pharmacopoeias
932 Cyclodextrin Complexation
Cyclodextrin (CDs) are cyclic oligosaccharides that consist of six (αndashCD) seven (βndashCD) or eight (γndashCD) units of dextrose These have lipophilic inner cavities and hydrophilic out surfaces The chemical structure of CDs (628) is presented in section 672 CDs may interact with a large number of drugs and form noncovalent inclusion complex This may lead to the stabilization of drug molecules CDs may interact with a large number of drugs and form noncovalent inclusion complex This may lead to the stabilization of drug molecules CDs complexation could also improve the water solubility and bioavailability of drugs The application of CDs in drug stabilization (Loftsson and Brewester 1996 Yoshioka and Stella 2000) in drug delivery (Challa et al 2005 Rasheed et al 2008) and as excipients (European Medicine Agency 2014) have been reported
CDs have been found to improve the stability of a number of labile drugs against dehydration hydrolysis oxidation and photolysis resulting in an increase in the shelf-life of these drugs (Loftsson and Brewster 1996) The rate of degradation of labile drug can be slowed down by inclusion into the CD cavity The rate of a drug (11 complex) in CD solution is the average of the degradation rates of the free drug and the drugndashCD complex (Rasheed et al 2008)
Equilibrium binding of the drug and CD to form a 11 complex can be expressed as
Drug + CD DrugCD complex
For a 11 complex the following equation can be used to determine the equilibrium binding or association constant K from the slope of the linear portion of the curve
Kab =
S0 (1ndashslope)
Slope
Where S0 is the intrinsic solubility of the drug under the condition of the study The binding constants for many drugndashCD complexes range from 100 to 20000 Mndash1 A 1100 dilution of the solution as an injection or dilution in the stomach and intestine can reduce up to 70 of the complex drug to free drug (Challa et al 2005) Since the hydrolysis of CDs encapsulated drugs is slower than the free drugs The stability of the drugndashCD complex (ie magnitude of stability of constant of complex) is an important factor in the stabilization of the drug (Kang et al 2003 Ma et al 2000 Dwivedi et al 1999)
Some examples of the CD complexation effect on the improvement of the stability of drugs include thermal stability of diclofenac sodium in the solid state by βndashCD (Cwiertnia et al 1999) photostability of promethazine by HPndashβndashCD or DMndashβndashCD (Lutika 2002) Photostability of doxorubicin by HPndashβndashCD or HPndashγndashCD (Brewester et al 1992) shelf-life (4 years) of glibenclamide by βndashCD (Babu and Pandit 1999) Stability against hydrolysis of benzylpenicillin by HPndashβndashCD (Pope et al 1991) and stability against intramolecular cyclization of quinaril in solid state by βndashCD of HPndashβndashCD (Li et al 2002)
933 Polymer complexation
The polymer-based amorphous solid dispersions offer a major development in the formulation of poorly water soluble drugs Polymers are inert hydrophilic drug carrier matrices that have the advantage of greater stability and solubility The drug-polymer interactions involve hydrogen bonding higher structural relaxation time and delayed crystallization kinetics that results
163
in the stabilization of the drug The polymer may also cause steric and structural effects to cause a greater stability to the drug (Thompson et al 2006 Kothari et al 2015 Baghel et al 2016)
The physical stability and molecular mobility of drug-solid dispersions are affected by drug-polymer hydrogen bonding interactions A study of solid dispersion of nifedipine with polyvinylpyrrolidone (PVP) hydroxypropylmethyl cellulose (HPMC) and poly(acrylic acid) (PAA) has indicated that the hydrogen bonding structural relaxation time and crystallization kinetics are in the order PVPgt HPMCgt PAA PVP showed the highest amount of drug hydrogen bonding to the polymer and the highest resistance to crystallization (Kothari et al 2015)
Multiple nanosuspensions of drugs such as azodicarbonamide fenofibrate griseofulvin ibuprofen and phenyl butazone have been stabilized by using the combination of a nonionic cellulosic polymer (hydroxymethyl cellulose) and an anionic surfactant (sodium dodecyl sulfate) The physical stability of wet-milled drug nanosuspensions is enhanced when the surfactant concentration is optimum to overcome the Ostwald ripening (Li et al 2011) The reduction of size of pharmaceutical suspensions to microm and nm scale to achieve increased dissolution rate creates the problem of particles agglomeration This has been overcome by the use of biocompatible polymers These polymers stabilize the suspensions by imparting surface-active steric and kinetic stability through an increase in the viscosity and change to non-newtonian rheological properties (Romanski et al 2011)
The peptide drugs are known to undergo chemical degradation by hydrolytic reactions The degradation can be prevented by complexation of the peptide with a polymer such as PVP It has been suggested that the stabilization of the peptide is due to the formation of the secondary structure in which the polymer exerts steric effect depending on its size and structural effect leading to an increase in the distance between reacting atoms within the peptide (Thompson et al 2006)
934 Use of Stabilizers
Stabilizers are generally used to protect a drug from chemical degradation in a dosage form These include antioxidants complexing agents and chelating agents The most commonly used antioxidants are sodium sulfite sodium bisulfite sodium metabisulfite α-tocopherol ascorbic acid acetylcysteine butylated hydroxytoulene (BHT) butylated hydroxyanisole (BHA) propyl gallate in a combination of 01ndash02 The complexing agents include caffeine and cyclodextrins The common chelating agent for metal ion contaminants is sodium editate
935 Liposomal Formulation
Liposomes are microscopic and submicroscopic phospholipid vesicles having a bilayer membrane structure These drug delivery systems provide protection against chemical photochemical and biological degradation The stability of drugs in liposomes is affected by liposomal composition entrapment efficacy and drug-lipid interactions (Michaelis et al 2005) Several drugs have been stabilized against chemical and photodegradation by entrapment in liposomes such as riboflavin (Loukas et al 1995ab Ahmad et al 2015a) doxorubicin (Bandak et al 1999) fluoroquinolones (Vazquez et al 2001 Budai et al 2008 Ahmad et al 2016) amlodipine (Ragno et al 2003) barnidipine (Ioele et al 2014) tretinoin (Ioele et al 2005) and local anesthetics (Habib and Rogers 1987 1989)
94 CHEMICAL AND PHOTOSTABILIZATION STUDIES
The chemical and photostabilization of different drugs and dosage forms have been studied by several workers Some examples of these studies are presented in the following sections
941 Chemical Stabilization
9411 Amorphous Drugs
There is an increasing number of new therapeutically active pharmaceutical compounds with low water solubility This has created problems in the formulations of their oral dosage forms The formation of stabilized amorphous forms of poorly water soluble compounds can help to
164
increase the solubility dissolution rate and bioavailability of these compounds The stabilization of amorphous drugs and related aspects have been reviewed by many workers (Laitinen et al 2013 Kawabata et al 2011 Qian et al 2010 Bhattacharaya and Syrayanarayanan 2009 Janssen and Van der Mooler 2009 Yu 2001 Leirner and Dressman 2000 Serajuddin 1999 Craig et al 1999 Hancock and Zografi 1997)
The formulation of solid polymer dispersions is considered as the best method for the stabilization of amorphous drugs and the enhancement of their dissolution rate However alternative methods of stabilization of amorphous drugs have been suggested (Laitinen et al 2013) These methods are based on the formulation of co-amorphous mixtures of small molecules and the use of mesoporous silicon and silicon-based carriers These approaches have been found to be useful in the stabilization of amorphous drugs
The amorphous state of a drug is unstable (eg nifedipine furosemide novobiocin) on thermodynamic considerations since it tends to revert back to the crystalline state with time It has been observed that the storage of amorphous material at Kauzmann temperature (TK) (the temperature at which entropy of the supercooled liquid is equal to that of the crystalline material) gives good physical stability to the material TK is taken as the maximum temperature for the storage of amorphous formulations (Yu 2001 Kaushal et al 2004 Kaushal and Bansel 2008)
It has been observed that the storage of unstable amorphous drugs (eg nifedipine furosemide novobiocin) at TK provides good physical stability to such drugs (Graeser et al 2009)
Several factors affect the crystallization of amorphous state (Marsac et al 2006 Kushal and Bansel 2008 Bhugra and Pickel 2008 Grzykowska et al 2010) and include
Thermodynamic (configurational entropy enthalpy or Gibbs free energy ∆G)
Kinetic (molecular mobility glass transition temperature (Tg) or structural relaxation time is an indication for this)
Molecular (eg hydrogen binding) interactions
Moisture content
Method and condition of preparation
The main factor governing the physical stability of the amorphous state of a drug is molecular mobility The highest physical stability is shown by the compounds that have high Tgs high configurational entropy barriers high TKs and low molecular motilities (Zhou et al 2002 Laitinen et al 2013)
9412 Binary Co-Amorphous Mixtures
It is well known that the addition of certain excipients such as surfactants anti-plasticizers and other inhibitors of crystallization can lead to the stabilization of amorphous drugs The binary amorphous systems have been found to possess a potential for the improved stability of drugs Small molecules such as citric acid sugars urea and nicotinamide have been used as carriers for the stabilization of amorphous drugs in solid dispersions (Lu and Zografi 1998 Ahuja et al 2007 Masuda et al 2012) The forces involved in complex formation include van der Waals forces dipole-dipole interactions hydrogen bonding Coulomb forces and hydrophobic interactions (Yoshioka and Stella 2000)
Examples of the binary co-amorphous mixtures include indomethacinranitidine citric acid acyclovircitric acid paracetamolcitric acid anhydrate and naproxencimetidine These mixtures are stabilized by hydrogen bonding interactions and possess a greater shelf-life than that of the drug alone Indomethacin has been stabilized with cimetidine by salt formation that also leads to an increase in the stability of the drug (Laitinen et al 2013)
165
9413 Solid Dosage Forms
∆9-tetrahydrocannabinol hemisuccinate (THCndashHS) has been stabilized in polymeric matrix systems using a hot-melt process at low temperature The addition of vitamin E succinate to the prodrug greatly reduced the degradation of THCndashHS during the polyethylene matrix production at 80degC A combination of vitamin E succinate and Noveon AAndash1 (a high molecular weight acrylic acid polymer cross linked with divinyl glycol) gives best stabilization to the prodrug system during production and storage at 4degC The degradation of THCndashHS is minimized in the acidic medium (Munjal et al 2006)
The stabilization of certain enzymes has been achieved by chemical modification α-Amylase has been stabilized by covalent linkage to the anionic polysaccharide carboxymethyl cellulose The modified enzyme has improved thermal and pH stability compared to the native enzyme The conjugate shows more resistance to the action of denaturing agents such as urea and sodium dodecyl sulfate (Villalonga et al 1999) Acetylcholinesterase is inactivated on chemical modification by thiosulfinate allicin on reaction with the buried cysteine (Cys 231) Circular dichroism (CD) spectral measurements have shown that the inactivation of the enzyme can be reversed by reaction with glutathione The half-life of allicin modified enzyme at room temperature is ~100 min The transition of the modified enzyme can be prevented by divalent cations Ca2+ Mg2+ and Mn2+ for gt24 h at room temperature Differential scanning calorimetry (DSC) has been used to confirm the stabilization of the modified enzyme by divalent cations (Millard et al 2003)
The interactions between drugs (eg indomethacin) and the surface of excipients such as Neusilin (a synthetic magnesium alumina metasilicate) on amorphization by co-grinding the mixture imparts physical stability to the drug during storage (Bahl and Bogner 2006) The silanol rings present on the surface of Neusilin make it a potential proton donor as well as proton acceptor The hydrogen bonding between silanol rings and the drugs are also involved in the stabilization of drugs including quinapril HCl acelofenac and other acidic drugs (Gupta et al 2003 Hailu and Bogner 2009)
The control of the environmental factors in the stabilization of some drug substances in the solid state or solid dosage forms is given in Table 91
9414 Liquid Dosage Forms
Cyanocobalamin (vitamin B12) is degraded in liquid multivitamin preparations by interaction with other vitamins The degradation of cyanocobalamin can be greatly reduced by the addition of complex cyanides (eg potassium ferrocyanide potassium cobalcyanide potassium cuprocyanide) or iron salts (eg ferrous sulfate ferrous gluconate ferric ammonium citrate) The stabilization of cyanocobalamin is more effective with complex cyanides than with the iron salts under aerobic conditions The complex cyanides are also effective in protecting cyanocobalamin against UV light (Zuck and Conine 1963)
The stabilization of cyanocobalamin in liquid multivitamin preparations can also be achieved by the use of α-hydroxynitriles of their esters (918) The stabilizing effect is due to the degradation of α-hydroxynitrile into hydrogen cyanide and the corresponding aldehydye α-hydroxynitrile also protects cyanocobalamin from degradation on exposure to UV light for short periods of time (Conine and Zuck 1963)
OH
R1
R2
CN
(918)
A study of the degradation of cyanocobalamin and hydroxocobalamin (vitamin B12b) in the presence of ascorbic acid at pH 1ndash8 has been conducted Cyanocobalamin is degraded to hydroxocobalamin which is further degraded to corrin ring oxidation products Both of these
166
compounds degrade by an apparent first-order kinetics and t12 values range from 137 to 1375 h and 25 to 875 h respectively The second-order rate constants for the interaction of cyanocobalamin and hydroxocobalamin with ascorbic acid are 005 to 028times10ndash2 and 110 to 3008times10ndash2 Mndash1sndash1 indicating a greater effect of ascorbic acid on the degradation of the later compound Both compounds can be stabilized in the presence of ascorbic acid in the acid region around pH 20 Cyanocobalamin is stable in the absence of ascorbic acid in the pH range 6 to 7 that is suitable for pharmaceutical formulations (Ahmad et al 2014)
The use of antioxidants and stabilizers in the stabilization of drug substances in the dosage forms is given in Table 92
Table 91 Stabilization of drug substances in solid statesolid dosage forma
Drug Drug dosage form Preventioncontrol
Paracetamol in combination with aspirin and codeine phosphate
Tablets Moisture and alkali
4-aminosalicylic acid solid Moisture and elevated temperature
Amoxicillin Crystalline powder High humidity and temperature Amphotericin Powder Light and air Ampicillin Powder Interconversion of
hydroxyethyl and unhydrated forms humidity and temperature
Ascorbic acid Solid High moisture content Aspirin Solid Minimizing contact with water
basic substances (eg carboxylic salts) and nucleophiles (eg amines and hydroxyl groups)
Aztreonam Lyophilized powder Moisture light and high temperature
Diethylpropion HCl solid Moisture and light
a Selected from monographs in Connors et al (1986)
167
Table 92 Stabilization of drug substances in liquid dosage forma
Drug substance Degradation reaction
pH of maximum stability
Addition of Antioxidant stabilizer
Paracetamol Hydrolysis 50ndash70 4-Aminosalicylic acid
Decarboxylation 92ndash97 Na2S2O5 to prevent color formation
Aminobarbital Hydrolysis Low pH Ampicillin Hydrolysis 58 Addition of alcohol to lower
dielectric constant of solution to enhance stability
Aspirin Hydrolysis 25 Atropine Hydrolysis 35 5-azacytidine Hydrolysis 25 EDTA NaHSO3
Aztreonam Hydrolysis 60 Benzylpenicillin Hydrolysis 675 Improved stability in
suspension form Carbenicillin Hydrolysis 65 Cephradine Hydrolysis 20ndash50 Chlordiazepoxide Hydrolysis 20ndash35 Protection from light Cholecalciferol Oxidation Ethylgalate BHT improved
stability in syrup form Clindamycin Hydrolysis 40 Cyanocobalamin Cyclization 45ndash50 EDTA citric acid cysteine Cytrabine Deamination 69 Diazepam Hydrolysis 50 Enhanced stability in mixed
aqueous solution Erythromycin Hydrolysis 70ndash75 5-Flurouracil Hydrolysis 90 Meperidine Hydrolysis 40 6-Mercaptopurine Oxidation 20ndash80 Protection from light and
moisture Methotrexate Hydrolysis 70 EDTA α-Methyldopa Oxidation 50ndash60 Protection from light Morphine Oxidation 30ndash50 Protection from light
NaHSO3 Na2S2O5 EDTA Nystatin Oxidation 70 BHA BHT propyl gallate Oxazepam Hydrolysis 50ndash60 Phenylbutazone Hydrolysisoxidation 60ndash70 Procaine Hydrolysis 30ndash40 Polysorbate 80 Promethazine Oxidation 20ndash30 EDTA Sulfacetamide Hydrolysis 50ndash90 Thiamine HCl Oxidation 20 EDTA
a Selected from monographs in Connors et al (1986)
942 Photostabilization
9421 Solid and Semisolid Dosage Forms
The photosensitive drugs such as danurubicin dihydroergotamine haloperidol furosemide nifedipine and nitrofurazone may undergo photodegradation when their dosage forms are exposed to light during the manufacturing process and handling by the end user The photostabilization of these drugs may be achieved by the application of the principle of photoprotection by spectral overlay It involves the use of suitable colorants or excipients absorbing daylight in the region that corresponds to the absorption characteristics of the individual drug This is applicable to tablets or topical dosage forms (Thoma and Klimek 1991)
168
The photostabilization of light sensitive drug products such as nifedipine tablets can be achieved by film coating with TiO2 (9ndash29) to impart opacity Scanning electron microscopy used to evaluate film thickness of the tablets (2ndash15 weight increase) showed the thickness in the range of 24ndash145 nm The uncoated and film coated tablets were exposed to 44 klux light for 21 days The results showed that the coated tablets (29 TiO2) at a thickness of 145 microm provided good protection to the drug from photodegradation compared to that of the uncoated tablets (Bechard et al 1992)
The uncoated sorivudine and nifedipine tablets have been stabilized against photodegradation using iron oxides that absorb UV-light The 10 mg wet granulated tablets containing 02 yellow iron oxide were exposed to room light or 400 foot-candle light for a fixed period of time The assay of the drugs in the tablets showed that the uncoated tablets containing iron oxide were more stable in light compared to those with no stabilizer The addition of 02 iron oxides (black yellow and red) to the uncoated tablets showed greater protection from light (gt11) compared to a film coated tablet A combination of yellow and red iron oxides was more effective as light protectant that the use of single stabilizer (Desai et al 1994)
Boric acid (BA) has been used to stabilize ascorbic acid (AH2) against UV light in ow cream formulations prepared using different humectants and emulsifiers The apparent first-order rate constants (kobs) for the photodegradation of AH2 in creams range from 042ndash120times10ndash3 minndash1 compared with those of 059ndash130times10ndash3 minndash1 in the absence of BA The second-order rate constants for the interaction of AH2 and BA are in the range of 261ndash602times10ndash3 Mndash1 minndash1 indicating the inhibitory effect of BA on the degradation of the vitamin The nature and amount of the humectant and emulsifier and the physical properties of creams influence the extent of stabilization The Photostabilization of AH2 in creams involves the formation of a complex between AH2 and BA (Ahmad et al 2015b)
9422 Liquid Dosage Forms
The photosensitive drugs can be stabilized by complex formation between the drug and certain agents Riboflavin has been stabilized by complexation with caffeine The complex form of riboflavin is stable in aqueous solution around pH 60 and is suitable for pharmaceutical formulations (Ahmad et al 2009) Caffeine complexation involves the formation of stacking complexes (Evstigneev et al 2005) The complex formation between the ribityl side chain of riboflavin and boric acid leads to the photostabilization of the vitamin in aqueous solution (Ahmad et al 2008)
169
REFERENCES
Ahmad I Ahmed S Sheraz MA Vaid FH Effect of borate buffer on the photolysis of riboflavin in aqueous solution J Photochem Photobiol B 20089382ndash87
Ahmad I Ahmed S Sheraz MA Aminuddin M Vaid FH Effect of caffeine complexation on the photolysis of riboflavin in aqueous solution a kinetic study Chem Pharm Bull 2009571363ndash1370
Ahmad I Ahmed S Anwar Z Sheraz MA Sikorski M Photostability and photostabilization of drugs and drug products Int J Photoenergy 2016 ID 573267
Ahmad I Arsalan A Ali SA Sheraz MA Ahmed S Anwar Z Munir I Shah MR Formulation and stabilization of riboflavin in liposomal preparations J Photochem Photobiol B Biol 2015a153358ndash366
Ahmad I Sheraz MA Ahmed S Kazi SH Khan MF Vaid FHM Effect of boric acid on the photostabilization of ascorbic acid in creams containing different humectants and emulsifiers Lat Am J Pharm 2015b342021ndash2026
Ahmad I Arsalan A Ali SA Bano R Munir I Sabah A Formulation and stabilization of norfloxacin in liposomal preparations Eur J Pharm Sci 201691208ndash215
Ahmad I Qadeer K Zahid S Sheraz MA Ismail T Hussain W Ansari IA Effect of ascorbic acid on the degradation of cyanocobalamin and hydroxocobalamin in aqueous solution a kinetic study AAPS PharmSciTech 2014151324ndash1333
Ahuja N Katare OP Singh B Studies on dissolution enhancement mathematical modeling of drug release of a poorly water-soluble drug using water-soluble carriers Eur J Pharm Biopharm 20076526ndash38
Babu RJ Pandit JK Effect of aging on the dissolution stability of glibenclamidebeta-cyclodextrin complex Drug Dev Ind Pharm 1999251215ndash1219
Bahl D Hudak J Bogner RH Comparison of the ability of various pharmaceutical silicates to amorphize and enhance dissolution of indomethacin upon co-grinding Pharm Dev Tech 200813255ndash269
Bandak S Ramu A Barenholz Y Gabizon A Reduced UV-induced degradation of doxorubicin encapsulated in polyethyleneglycol-coated liposomes Pharm Res 199916841ndash846
Baghel S Cathcart H OReilly NJ Polymeric amorphous solid dispersions a review of amorphization crystallization stabilization solid-state characterization and aqueous solubilization of biopharmaceutical classification system class II drugs J Pharm Sci 20161052527ndash2544
Bechard SR Quarishi O Kwong E Film coating effect of titanium dioxide concentration and film thickness on the photostability of nifedipine Int J Pharm 199287133ndash139
Bhattacharya S Syrayanaryanan R Local mobility in amorphous pharmaceuticals-characterization and implications on stability J Pharm Sci 2009982935ndash2953
Bhugra C Pikal MJ Role of thermodynamic molecular and kinetic factors in crystallization from the amorphous state J Pharm Sci 2008971329ndash1349
Blaugh SM Hajratwala B Kinetics of aerobic oxidation of ascorbic acid J Pharm Sci 197261556ndash562
Brewster ME Loftsson T Estes KS Lin JK Effect of various cyclodextrins on solution stability and dissolution rate of doxirubicin hydrochloride Int J Pharm 199279289ndash299
Bundgaard H Polymerization of penicillins II Kinetics and mechanism of dimerization and self-catalyzed hydrolysis of amoxycillin in aqueous solution Acta Pharm Suec 19771447ndash66
170
Budai M Grof P Zimmer A Papai K Klebovich I Ludanyi K UV light induced photodegradation of liposome encapsulated fluoroquinolones an MS study J Photochem Photobiol A Chem 2008198268ndash273
Challa R Ahuja A Ali J Khar RR Cyclodextrins in drug delivery An updated review AAPS PharmSciTech 20056E329ndashE356
Chung JE Yokoyama M Yamato M Aoyagi T Sakurai Y Okano T Thermo-responsive drug delivery from polymeric micelles constructed using block copolymers of poly(N-isopropylacrylamide) and poly(butylmethacrylate) J Control Release 199962115ndash127
Conine JW Zuck DA Stabilization of vitamin B12 II α-hydroxynitriles J Pharm Sci 19635263ndash66
Connors KA Amidon GL Stella VJ Chemical Stability of Pharmaceuticals 2nd ed John Wiley New York USA 1986 pp 76 97 132ndash133
Connors KA The stability of cyclodextrin complexes in solution Chem Rev 1997971325ndash1357
Craig DQM Royall PG Katt VL Hopton ML The relevance of amorphous state to pharmaceutical dosage forms glassy drugs and freeze dried systems Int J Pharm 1999 179179ndash207
Cwiertnia B Hladon T Stobiecki M Stability of diclofenac sodium in the inclusion complex with beta-cyclodextrin in the solid state J Pharm Pharmacol 1999511213ndash1218
Desai DS Abdelnasser MA Rubitski BA Varia SA Photostabilization of uncoated tablets of sorivudine and nifedipine by incorporation of synthetic iron oxides Int J Pharm 199410369ndash75
Dwivedi AK Kulkarni D Khanna M Singh S Effect Of cyclodextrins of the stability of new antimalarial compound N1-3-acetyl-45-dihydro-2 furanyl-N4-(6-methoxy8-quinolinyl)-14-pentane diamine Ind J Pharm Sci 199961175ndash177
Eccleston GM Emulsions and microemulsions In Swarbrick J Boylan JC Encyclopedia of Pharmaceutical Technology 2nd ed Marcel Dekker Inc New York USA 2002 pp 1077ndash1078
European Medicines Agency Background review for cyclodextrin used as excipients London UK 2014
Evstigneev MP Rozvadovskaya AO Santiago AAH Mukhina YV Veselkov KA Rogova OV Davies DB Veselkov AN A 1H NMR study of the association of caffeine with flavin mononucleotide in aqueous solutions Rus J Phys Chem A 200579573ndash578
Grzybowski K Pluch M Grzybowski A Wojnarowska Z Hawelek L Kolodziejczyk K Ngai KL Molecular dynamics and physical stability of amorphous anti-inflammatory drug celecoxib J Phys Chem B 201011412792ndash12801
Graeser KA Patterson JE Zeitler JA Gordon KC Rades T Correlating thermodynamic and kinetic parameters with amorphous stability Eur J Pharm Sci 200937492ndash498
Gupta MK Vanwert A Bogner RH Formation of physical stable amorphous drugs by milling with Neusilin J Pharm Sci 200392502ndash517
Habib MJ Rogers JA Stabilization of local anesthetics in liposomes Drug Dev Ind Pharm 1987131947ndash1971
Habib MJ Rogers JA Stabilization of local anesthetics in liposomes In Rubinstein MH editor Pharmaceutical Technology Drug Stability Ellis Horwood Limited John Wiley amp Sons New York USA 1989 Chap 2
Hancock BC Zografi G Characteristics and significance of the amorphous state in pharmaceutical systems J Pharm Sci 1997861ndash12
171
Hailu SA Bogner RH Effect of the pH grade of silicates on chemical stability of coground amorphous quinapril hydrochloride and its stabilization using pH-modifiers J Pharm Sci 2009983358ndash3372
Hellberg H A procedure for estimating the racemisation of adrenaline or noradrenaline in dilute solution by means of an ion exchanger J Pharm Pharmacol 19557191ndash197
Huang Y Dai WG Fundamental aspects of solid dispersion technology for poorly soluble drugs Acta Pharm Sin B 2014418ndash25
ImpndashEmsap W Preratakul O Siepmann J Disperse systems In Banker GS Rhodes CT editors Modern Pharmaceutics 4th ed Marcel Dekker Inc New York USA 2002 Chap 9
Ioele G Cione E Risoli A Genchi G Ragno G Accelerated photostability study of tretinoin and isotretinoin in liposome formulations Int J Pharm 2005293251ndash260
Ioele G De Luca M Ragno G Photostability of barnidipine in combined cyclodextrin-in-liposome matrices Future Med Chem 2014635ndash43
Janssen S Van der Mooter G Review Physical chemistry of solid dispersions J Pharm Pharmacol 2009611571ndash1586
Kang J Kumar V Yang D Chowdhury PR Hohl RJ Cyclodextrin complexation influence on the solubility stability and cytotoxicity of camptothecin an antineoplastic agent Eur J Pharm Sci 200215163ndash170
Kaushal AM Bansel AK Thermodynamic behavior of glassy state of structurally related compounds Eur J Pharm Biopharm 2008691067ndash1076
Kaushal AM Gupta P Bansel AK Amorphous drug delivery systems molecular aspects design and performance Crit Rev Ther Drug Carrier Syst 200421133ndash193
Kawabata Y Wada K Nakatani M Yamada S Onoue S Formulation design for poorly water-soluble drugs based on biopharmaceutics classification system basic basic approaches and practical applications Int J Pharm 20114201ndash10
Kothari K Ragoonanan V Suryanarayanan R The role of drug-polymer hydrogen bonding interactions on the molecular mobility and physical stability of nifedipine solid dispersions Mol Pharm 201512162ndash70
Laitinen R Lobmann K Struchan CJ Grohganz H Rades T Emerging trends in the stabilization of amorphous drugs Int J Pharm 201354365ndash79
Leirner C Dressman J Improving drug solubility for oral delivery using solid dispersion Eur J Pharm Biopharm 2000 5047ndash60
Li J Guo Y Zografi G The solid-state stability of amorphous quinapril in the presence of beta-cyclodextrins J Pharm Sci 200291229ndash243
Loftsson T Brewster ME Pharmaceutical applications of cyclodextrins 1 Drug solubilization and stabilization J Pharm Sci 1996851017ndash1025
Loukas YL Jayasekera P Gregoriadis G Characterization and photoprotection studies of a model γ-cyclodextrin included photolabile drug entrapped in liposomes incorporating light absorbers J Phys Chem 1995a2711035ndash11040
Loukas YL Jayasekera P Gregoriadis G Novel liposome-based multicomponent systems for the protection of photolabile agents Int J Pharm 1995b11785ndash94
Lu Q Zografi G Phase behavior of binary and tertiary amorphous mixtures containing indomethacin citric acid and PVP Pharm Res 1998151202ndash1206
Lutika A Investigation of interaction of promethazine with cyclodextrins in aqueous solution Acta Pol Pharm 20025945ndash51
172
Ma DQ Rajewski RA Vander Velde D Stella VJ Comparative effects of (SBE)7m-beta-CD and HP-beta-CD on the stability of two anti-neoplastic agents melphalan and carmustine J Pharm Sci 200089275ndash287
Marsac PJ Konno H Taylor LS A comparison of the physical stability of amorphous felodipine and nifedipine systems Pharm Res 2006232306ndash2316
Masuda T Yosihaski Y Yonemochi E Fujii K Uekusa H Terada K Cocrystallization and amorphization induced by drug-excipient interaction improves the physical properties of acyclovir Int J Pharm 2012422160ndash169
Maulding HV Nazareno JP Pearson JE Michaelis AF Practical kinetics III Benzodiazepine hydrolysis J Pharm Sci 197564278ndash284
Michaelis M Zimmer A Handjou N Cinatl J Cinatl J Jr Increased systemic efficacy of aphidicolin encapsulated in liposomes Oncol Rep 200513157ndash160
Millard CB Shynrov VL Newstead S Shin I Roth E Silman I Weiner L Stabilization of a metastable state of Torpedo californica acetylcholinesterase by chemical chaperones Protein Sci 2003122337ndash2347
Munjal M ElSohly MA Repka MA Chemical stabilization of a ∆9-tetrahydrocannabinol prodrug in polymeric matrix systems produced by a hot-melt method role of microenvironment pH AAPS PharmSciTech 20067E1ndashE11
Palomo ME Ballesteros MP Frutos P Analysis of diclofenac sodium and derivatives J Pharm Biomed Anal 19992183ndash94
Piechocki JT Thoma K editors Pharmaceutical Photostability and Stabilization Technology Informa Healthcare New York USA 2007
Pope E Loftsson T Bodor N Solubilization and stabilization of a benzylpenicillin chemical delivery system by 2-hydroxypropyl-beta-cyclodextrin Pharm Res 199181044ndash1049
Qian F Huang J Hussain MA Drug-polymer solubility and miscibility stability considerations and practical challenges in amorphous solid dispersion development J Pharm Sci 2010992941ndash2947
Racz I Drug Formulations John Wiley New York USA 1989 Chap 2
Rasheed A Ashok Kumar CK Sravanthi VVNSS Cyclodextrins as drug carrier molecules A review Sci Pharm 200876567ndash598
Ragno G Cione E Garofalo A Genchi G Ioele G Risoli A Spagnoletta A Design and monitoring of photostability systems for amlodipine dosage forms Int J Pharm 2003265125ndash132
Rios-Doria J Carie A Costich T Burke B Skaff H Panicucci R Sill K A versatile polymer micelle drug delivery system for encapsulation and in vivo stabilization of hydrophobic anticancer drugs J Drug Deliv 20122012951741
Romanski FS Muzzio FJ Tomassone MS Important factors in the size reduction of polymer-stabilized drug particle suspensions using high-pressure homogenization J Pharm Innov 2011697ndash106
Serajuddin ATM Solid dispersion of poorly water-soluble drugs early promises and recent breakthroughs J Pharm Sci 1999881058ndash1066
Sheraz MA Khan MF Ahmed S Kazi SH Ahmad I Stability and stabilization of ascorbic acid Formulation 20151022ndash25
Sinko PJ Chemical kinetics and drug stability In Martinrsquos Physical Pharmacy and Pharmaceutical Sciences 6th ed Lippincott Williams amp Wilkins Philadelphia USA 2011 Chap 14
173
Thoma K Photodecomposition and stabilization of compounds in dosage forms In Tonnesen HH editor Photostability of Drugs and Drug Formulations Taylor amp Francis London UK 1996 Chap 6
Thoma K Klimek R Photostabilization of drugs in dosage forms without protection from packaging materials Int J Pharm 199167169ndash175
Thompson S Sinha S Topp E Camarda KV A molecular design approach to peptide drug stabilization Molecular Simulation 2006 32 291ndash295
Villalonga R Gomoz L Rasmfrez HL Villalonga ML Stabilization of α-amylase by chemical modification with carboxymethylcellulose J Chem Technol Biotechnol 199974635ndash638
Vire JC Patriaarche GJ Christian GD Electrochemical study of the degradation of vitamins k group Pharmazie 198035209ndash212
Yoshioka S Stella VJ Stability of Drugs and Dosage Forms Kluwer AcademicPlenium Publishers New York USA 2000 Chap 2
Yu L Amorphous pharmaceutical solids preparation characterization and stabilization Adv Drug Deliv Rev 20014827ndash42
Yuen PH Sokoloski TD Kinetics of concomitant degradation of tetracycline to epitetracycline anhydrotetracycline and epianhydrotetracycline in acid phosphate solution J Pharm Sci 1977661648ndash1650
Vaacutezquez JL Berlanga M Merino S Domegravenech O Vintildeas M Montero MT Hernaacutendez-Borrell J Determination by fluorimetric titration of the ionization constants of ciprofloxacin in solution and in the presence of liposomes Photochem Photobiol 20017314ndash19
Zhou D Grant DJW Zhang GGZ Law D Schmitt EA Physical stability of amorphous pharmaceuticals importance of configurational thermodynamic quantities and molecular mobility J Pharm Sci 20029171ndash83
Zuck DA Conine JW Stabilization of vitamin B12 1 Complex cyanides J Pharm Sci 19635259ndash63
174
175
CHAPTER ndash 10
STABILITY OF HERBAL DRUGS AND PRODUCTS 101 INTRODUCTION
Interest in herbal drugs from natural sources has grown in recent years and herbal products are being used as alternative and complementary medicines worldwide Herbal drugs have been utilized from time immemorial and are still part of modern medicine Some of the active ingredients derived from natural products and used as drugs are anthaquinones (cascara) artemesinin (artemisia) atropine (nightshade) colchicine (autumn crocus) digitoxin (foxglove) diosgenin (Mexican yam) morphine (opium poppy) podophyllin (mayapple) quinine (cinchona bark) reserpine (Indian snakeroot) taxol (Pacific yew) vincristine (periwinkle) and many antibiotics (Der Marderosian and Riedlinger 2006)
The herbal products may contain either a single or a mixture of several herbal ingredients often five to ten or more in a single formulation in the solid semisolid or liquid dosage form This creates the possibility of medium effects and interactions between different active ingredients and with excipients to cause the degradation of the individual components This may lead to stability problems affecting the potency and efficacy of active ingredients individually as well as the overall biological activity of the products Therefore proper care is required in the handling drying and storage of herbal drugs to maintain their potency safety and efficacy
The commonly used dosage forms of herbal drugs include powdered material spray dried extract powdered material and freeze-dried powdered material either used as such or mixed with excipients to formulate as tablets and capsules Other dosage forms include creams ointments semisolid preparations liquid preparations syrups liquid extracts tinctures etc The whole or powdered bulk material on drying in the oven or under sunlight as such or encapsulated may be affected by environmental factors such as air moisture heat light and microbes and thus lose potency The most common reaction undergone by chemical constituent of herbal drugs is oxidative degradation However the hydrolytic photolytic or other modes of degradation may also occur depending on the nature of the drug The herbal products are standardized to ensure the presence of the desired amount of active ingredients in the single or polyherbal formulation manufactured in different dosage forms before marketing
Herbal drugs are playing an important role in the treatment of a wide range of ailments They are generally considered safe however some of the herbal drugs may contain toxic constituents with undesirable side effects The importance and use of herbal drugs have been described by Majno (1975) Fransworth et al (1985) Bukhari et al (1987) Gilani et al (1992) Bisset (1994) Barl (1997) Duke and Martinez (1994) Roberts and Tyler (1997) Bouldin et al (1999) Fabricant and Fransworth (2001) Phillips (2002) Sagar et al (2003) Bodeker et al (2005) Barnes et al (2007) Tapas et al (2008) Andreescu et al (2008)
102 DEFINITIONS
A plant drug or herbal medicine has been defined by WHO (1993) as ldquoa plant-derived material or preparation with therapeutic or other human health benefits which contains either raw or processed ingredients from one or more plants In some traditions materials of inorganic or animal origin may also be presentrdquo
Herbal drugs processed herbal drugs herbal drug preparations and herbal drug extracts have been defined by British Pharmacopoeia (2016) as follows
176
1021 Herbal Drugs
ldquoHerbal drugs are mainly whole fragmented or broken plants parts of plants algae fungi or lichen in an unprocessed state usually in dried form but sometimes fresh Certain exudates that have not been subjected to a specific treatment are also considered to be herbal drugs Herbal drugs are precisely defined by the botanical scientific name according to the binominal system (genus species variety and author)rdquo
Whole describes a herbal drug that has not been reduced in size and is presented dried or undried as harvested for example dog rose bitter funnel or sweet funnel Roman chamomile flower
Fragmented describes a herbal drug that has been reduced in size after harvesting to permit ease of handling drying andor packaging for example cinchona bark rhubarb passion flower
Broken describes a herbal drug in which the more-fragile parts of the plant have broken during drying packaging or transportation for example belladonna leaf matricaria flower hop strobile
Cut describes a herbal drug that has been reduced in size other than by powdering to the extent that the macroscopic description in the monograph of the herbal drug can no longer be applied When a herbal drug is cut for a specific purpose that results in the cut herbal drug being homogenous for example when cut for herbal teas it is a herbal drug preparation
Herbal drugs are obtained from cultivated or wild plants Suitable collection cultivation harvesting drying fragmentation and storage conditions are essential to guarantee the quality of herbal drugs
1022 Processed Herbal Drugs
ldquoProcessed herbal drugs are obtained by subjecting herbal drugs to traditional processing methods Processed herbal drugs are defined precisely by the botanical scientific name according to the binomial system (genus species subspecies variety and author) and plant partrdquo
Processed Herbal Drugs are obtained by subjecting herbal drugs to specific types of processing according to traditional processing methods These traditional processing methods have the potential to alter the physical characteristics andor chemical constituents of herbal drugs Traditional processing methods may require the addition of processing aids to the herbal drug for example honey vinegar wine milk and salt The additional processing aids used should be of a suitable quality or of pharmacopoeial quality where a monograph exists The method of traditional processing is provided under the production section in individual monographs
1023 Herbal Drug Preparations
Herbal drug preparations are defined as the homogenous products obtained by subjecting herbal drugs to treatments such as extraction distillation expression fractionation purification concentration or fermentation
1033 Herbal Drug Extracts
Herbal drug extracts are liquid (liquid extraction preparations) semisolid (soft extracts and oleoresins) or solid (dry extracts) preparations obtained from Herbal drugs using suitable solvents
An extract is essentially defined by the quality of the herbal drug by its production process (extraction solvent(s) method of processing etc) and by its specifications
Standardized extracts are adjusted to a defined content of one or more constituents with known therapeutic activity This is achieved by adjustment of the extract with inert excipients or by blending batches of the extract
Quantified extracts are adjusted to one or more active markers the content of which is controlled within a limited specified range Adjustments are made by blending batches of the extract
177
Other extracts are not adjusted to a particular content of constituents For control purposes one or more constituents are used as analytical markers The minimum content for these analytical markers is given in an individual monograph in British Pharmacopoeia (2016)
103 QUALITY CONTROL METHODS
The application of quality control methods in the assessment of quality determination of the potency of active ingredients and detection of impurities is necessary to assure good manufacturing practices quality control safety efficacy and stability of the herbal products The following quality control methods are normally applied to the herbal material
1031 Herbal Products
Tests of identity purity foreign matter loss on drying water content pH heavy metals total ash acid insoluble ash extractable matter swelling index bitterness value microbial contamination and assay of active ingredients
1032 Essential Oils
Test of identity (chromatographic profile) relative density refractive index optical rotation fatty acids and resinified oils freezing point acid value peroxide value foreign esters and residue on evaporation
1033 Herbal Extracts
Test of identity relative density water content solvent content loss on drying dry residue residual solvents heavy metals microbiological quality aflatoxins B1 ochratoxin A pesticide residues and assay of active ingredients
Quality control methods for herbal drugs have been described by WHO (1998) British Pharmacopoeia (2016) EMEA (1998 2005) Eskinazi et al (1999) Capasso et al (2000) and Ahmad and Usmanghani (2003) and discussed by Barl (1997) Bauer (1998) De Smet (1999) Tsai (2001) Gaedcke and Steinhoff (2002) Mukherjee (2002) Phillips (2002) Liang et al (2004) Wani (2007) Kunle (2012) Zhang et al (2012) Bele and Khale (2013) and Azmir et al (2013)
The analytical methods used for the isolation separation characterization determination and stability studies of herbal drugs degradation products and contaminants are given in Table 101
104 FINGERPRINT ANALYSIS OF HERBAL DRUGS
Herbal drugs are cultivated in a certain region or different areas of the world Even though herbal drugs may belong to the same species the quality and efficacy may be different according to the growing conditions such as climate and soil based on the geographical origin Therefore rapid sensitive and accurate analytical methods are required to determine the correct contents of the major constituents of herbal drugs and to discriminate them according to the geographical origin (Woo et al 1999)
Fingerprint analysis is an accepted method for the assessment of the quality of Traditional Chinese Medicines (TCM) or herbal drugs by WHO (2001) A fingerprint can be considered as a chemical profile that represents the chemical composition of the samples of TCM or the herbal drugs Chromatographic fingerprint analysis using CE GC GCndashMS HPTLC HPLC and HPLCndashMS (Peishan 2001 Drasar and Moravcova 2004 Gong et al 2004 Xie 2005 Lu et al 2005 Xie et al 2006 2007 Yin and Qian 2007 Chen et al 2007 Jiang et al 2007) has been applied to the authentication of plant species origin of Chinese herbs quality standards to ensure consistency and stability of herbal drugs assessment of raw material and in-process assay and the detection of adulterants in herbal drugs
The absorption spectra such as ultraviolet (UV) (Ni et al 2009) infrared (IR) (Cao et al 2002 Xu et al 2002 Zhou et al 2003 Xu et al 2005 Yang et al 2009) near infrared (NIR) (Woo et al 1999 Scafi and Pasquini 2001 Laasonen et al 2002 Sun et al 2010) Fourier transform infrared (FTIR) (Ayiguli et al 2006 Chen et al 2007Jin et al 2008 Li et al 2006 Pei et al 2008 Wu et al 2008 Cai et al 2009 Cheung et al 2009) nuclear magnetic resonance
178
(NMR) (Kang et al 2008) and mass spectra (MS) (Cai et al 2002) fingerprint analyses have been used as effective techniques for the identification of cultivation areas of herbs counterfeit drugs and drugs in multicomponent matrices pattern recognition for discrimination of herbal drugs and processing and quality control of herbal drugs Small changes in test samples may be detected by variations in fingerprints for the differentiation of herbal drugs
105 STORAGE
The plant material used as drugs is normally stored in glass containers or as alcoholic or aqueous extracts to preserve the active ingredients and enhance shelf-life The herbal drugs should be stored under appropriate storage conditions to ensure potency efficacy and safety The British Pharmacopoeia (2016) prescribes storage conditions for herbal drugs and products in terms of protection from light moisture and heat and use of well-filled air tight and light-resistant containers The recommended storage for herbal drugs is at a temperature le25degC and when frozen at or below ndash18degC In the case of certain drugs (eg Sterculia granules) storage in a dried place is recommended and for certain drugs (eg Tolu balsam) storage in powder form is not recommended The storage conditions of some herbal drugs and products are given in Table 102
Table 101 Analytical methods for the study of herbal drugs
Methods Applications
Extraction Methods Liquidndashliquid phase Liquidndashsolid phase
Extraction of hydrophobic components in the organic phase Separation of mixtures of different molecules
Chromatographic Methods Thin-layer chromatography (TLC) High-performance thin-layer chromatography (HPTLC) High-performance liquid chromatography (HPLC) (normal and reversed phase) Gas-liquid chromatography (GLC)
Separation isolation and determination of constituents of plant materials and herbal drugs
Electrophoresis Capillary electrophoresis (CE) Gel electrophoresis (GE)
Separation isolation and determination of constituents of plant materials and herbal drugs
Spectroscopic Methods Ultraviolet spectrometry (UV) Visible spectrometry (Vis) Infrared spectrometry (IR) Fourier transform spectrometry (FTIR) Nuclear magnetic resonance spectroscopy (NMR) Mass spectrometry (MS) GCndashmass spectrometry (GCndashMS) HPLCndashmass spectrometry (HPLCndashMS) Spectrofluorimetry Circular dichorism (CD) Optical rotatory dispersion (ORD) X-ray Diffractometry (XRD) Atomic absorption spectrometry (AAS)
Structural and quantitative analysis Structural and quantitative analysis Structural analysis Structural analysis Structural analysis Elemental analysis
179
Table102 Sensitivity and storage of some herbal drugs and productsa (British Pharmacopoeia 2016)
Herbal Drug Major Constituents Method of Analysis
Sensitivity
Storage
Essential oils Terpenes (mono- and sesqui-terpenes)
GLC Light Well-filled air tight containers protected from light
Herbal Teas Light Protected from light
Barbados aloes
Hydroxyanthracene derivatives
Spectrometric 512 nm
Air tight containers
Cape aloes Hydroxyanthracene derivatives
Spectrometric 512 nm
Air tight containers
Angelica sinensis root
Z-ligustilide LC Moisture Protected from moisture
Star Anise oil Pseudoisoeugenol 2-methylbutyrate
GLC Heat Temperature le25degC
Azadirachta indica leaf
Tetranortriterpenoids (salannin azadirachtin-A)
LC Moisture Protected from moisture
Belladona leaf powder
Alkaloids (hyoscyamine)
Acid-base titration
Air tight containers
Berberis aristata
Berberine LC Moisture Protected from moisture
Fresh Bilberry Anthocyanins (chrysanthemin)
Spectrometric 528 nm
Heat When frozen at or below ndash 18degC
Black current syrup
Redox titration
Light Well-filled container protected from light
Caraway oil Essential oil (β-myrcene carvone)
GLC Heat Temperature le25degC
Cardamom oil Essential oil (β-myrcene carvone)
GLC Light Well closed container protected from light
Cassia oil Essential oil (trans-cinnamaldehyde trans-2-methoxy-cinnamaldehyde coumarin eugenol
GLC Heat Protected from heat
Cinnamon bark oil
Essential oil (cineole linalol β-caryophyllene trans-cinnamic aldehyde eugenol
GLC Heat Protected from heat
Ceylon cinnamon leaf oil
Cineloe linalol β-caryophyllene trans-cinnamic aldehyde eugenol
GLC Heat Protected from heat
180
Clove oil Essential oil (β-caryophyllene eugenol acetyl eugenol)
GLC Heat Protected from heat
Colophony Thymol linalol Do not reduce to a powder
Table 102 continued Coriander oil Essential oil
(terpenes and camphor)
GLC Heat Temperature le25degC
Digitalis leaf Cardenolic glycosides Spectrometric 540 nm
Moisture Protected from moisture
Dill oil Carvone Titration with KOH in 90 ethanol
Light Well-filled containers protected from light
Eucalyptus oil Essential oil (α-α-pinene limonene 18-cineole)
GLC Heat Temperature le25degC
Bitter fennel Essential oil (anethole fenchone)
GLC Moisture Protected from moisture
Bitter-fennel fruit oil
Essential oil (α-pinene trans-anethole fenchone estragol)
GLC Heat Temperature le25degC
Bitter-fennel Herb oil
Essential oil (α-pinene limonene fenchone trans-anethole)
GLC Heat Temperature le25degC
Sweet fennel Essential oil (Anethole)
GLC Moisture Protected from moisture
Fig Moisture Protected from moisture
Ipecacuanha root powder
Moisture Protected from moisture
Ispaghula husk granules
Moisture Protected from moisture
Juniper oil Essential oil (α- and β-pinene sabinene β-myrcene limonene terpinen-4-ol
GLC Heat Temperature le25degC
Lavender oil Essential oil (18-cineole 3-octanone linalol linalyl acetate α-terpineol)
GLC Heat Temperature le25degC
Lemon oil Essential oil (β-pinene sabinene limonene γ-terpinene geranial)
GLC Heat Temperature le25degC
Terpeneless lemon oil
Aldehydes Titration with KOH in 60 ethanol
Light Well-filled container protected from light
181
Liquorice root for use in TCMb
Glycyrrhizic acid LC Moisture Protected from moisture
Mandarin oil Essential oil (α- and β-pinene β-myrcene limonene γ-terpinene)
GLC Heat Temperature le25degC
Table 102 continued
Matricaria oil Essential oil (chamazulene α-bisabolol bisabolol oxides)
GLC Heat Temperature le25degC
Dementholized mint oil
Essential oil (limonene cineole menthone isomenthone menthol pulegone carvone)
GLC Heat Temperature le25degC
Myrrh Tincture
Anethole thymol Do not store in plastic container
Nutmeg oil Essential oil (α-and β-pinene sabinene car-3-ene limonene γ-terpinene myristicine terpinen-4-ol)
GLC Heat Protected from heat
Orange oil Essential oil (aldehydes)
Keep in well-filled container protected from light
White Peony root
Paeoniflorin LC Moisture Protected from moisture
Peppermint oil
Essential oil (limonene 18-cineole menthone menthol menthofuranisomenthone pulegone)
GLC Heat Temperature le25degC
Gastro-resistant peppermint oil capsules
Peppermint oil (limonene 18-cineole menthone menthol menthofuran isomenthone pulegone)
GLC Light Protected from light
Peru balsam Esters (benzyl benzoate benzyl cinnamate)
Gravimetric Light Protected from light
Phellodendron amurense bark
Berberine palmatine LC Moisture Protected from moisture
Phellodendron Chinese bark
Berberine LC Moisture Protected from moisture
182
Dwarf Pine oil Essential oil (α-and β-pinene camphene car-3-ene β-myrcene limonene β-phellandrene terpinolene p-cymene)
GLC Heat Inert containers Temperature le25degC
Rosemary oil Essential oil (α- and β-pinene camphene β-myrcene limonene cineole camphor borneol)
GLC Heat Temperature le25degC
Table 102 continued
Sage oil Essential oil (linalol linalyl acetate α-terpineol germacrene)
GLC Heat Temperature le25degC
Scutellariae baicalensis root
Baicalin LC Moisture Protected from moisture
Senna fruit Hydroxyanthracene glycosides
Spectrometric 515 nm
Moisture Protected from moisture
Senna granules
Sennsides LC Moisture Air tight containers
Senna leaf Hydroxyanthracene glycosides
Spectrometric 515 nm
Moisture Protected from moisture
Spearmint oil Essential oil (limonene cineole menthone isomenthone menthol carvone)
GLC Light Keep in a well-closed containers protect from light
Sterculia granules
Gum (volatile acid)
Acid-base titration
Moisture Stored in a dry place
Stramonium leaf
Alkaloids (hyocyamine and hyocine)
Acid-base titration
Moisture Protected from moisture
Prepared stramonium
Alkaloids (hyocyamine hyocine)
Acid-base titration
Moisture Stored in air tight containers
Thyme oil Essential oil (β-myrcene γ-terpinene p-cymene linalol thymol carvacrol)
GLC Heat Temperature le25degC
Tolu balsam Oleo-resin (cinnamic acid)
Acid-base titration
Do not store in powder form
Turpentine oil Essential oil (α-and β-pinene camphene car-3-ene limonene longifolene β-caryophyllene)
GLC Heat Temperature le25degC
183
Withania somnifera root
Withaferin A and withanolide A
LC Moisture Protected from moisture
a For many herbal drugs and products storage conditions are not mentioned
bTCM Traditional Chinese Medicine
106 PHOTOSENSITIVITY REACTIONS OF HERBS
The term photosensitivity is used to describe an adverse biological reaction occurring as a result of the action of sunlight on a herbal product The reaction may be phototoxic photoallergic or photosensitization Herbs can cause photosensitivity reactions to skin on exposure to sunlight (Ernst 2000 Ernst et al 1998 Palanisamy et al 2003) These reactions result in inflammation of the skin with redness similar to sunburn and other disorders (Table 103)
Table 103 Herbs causing skin sensitivity on exposure to sunlight
Herb Reaction caused to skin
Essential oils (lime lemon orange bergamot grapefruit ginger cumin angelica root) used topically in aromatherapy
redness hyperpigmentation blistering
St Johnrsquos wort itching erythema Kava drink (Pacific island)
photosensitive dermatitis
Yohimbe (containing yohimbine alkaloid) photosensitive dermatitis ingestion
Some light sensitive herbal drugs are reported in Table 104
Table 104 Some light sensitive drugs and products
Natural Compounds Amphotericin ammoidin atropine emetine cyanocobalamin ephedrine ergocalciferol erythromycin folic acid reserpine retinol riboflavin steroids
Plant Products Essential oils fixed oils ginseng dry extract peru balsam cardamom fruit podophyllum resin
Animal Products Hard fat
107 STABILITY OF HERBAL DRUGS AND PRODUCTS
Stability is an important consideration in the assessment of the quality efficacy and safety of herbal products The chemical constituents of the herbal ingredients of these products may undergo various degradation reactions during production storage and use The stability of herbal products has been reviewed by Thakur et al (2011) Deepa and Kannappan (2008) Bankoti et al (2012) Dawar et al (2013) Hou et al (2013) and Noor-ul-Basar et al (2013) Some stability studies of herbal drugs and products are reported in the following sections
1071 Photodegradation of Herbal Drugs
Many herbal drugs are sensitive to sunlight and artificial light and undergo photodegradation to form inactive or toxic products (Table 105)
184
Table 105 Photodegradation of herbal drugs by sunlight
DrugMaterial Use Photoproducts
Triclosan antimicrobial agent
dibenzodichlorodioxin (more toxic)
Fenpropathrin pyrethroid insecticide
decarboylated and ester bond cleavage products
Azadirachtin (Azadirachdica indica fruit) (Neem plant)
insecticide photodegradation products involving tigolyl moiety
Chlorophyll (leaves of higher plants eg barley)
oxidative products (hematinic acid methyl ethyl maleimide methyl vinyl maleimide dialdehyde)
Membrane proteins (containing tryptophan)
food material
photooxidation products (formation indicated by loss of tryptophan fluorescence at 290 nm)
Reserpine oxidative products (3-dehydroreserpine and lumireserpine)
Ephedrine 25-diphenyl-34-dimethyl oxazolidine Riboflavin formylmethylflavin lumichrome
lumiflavin
Some examples of the photodegradation of herbal drugs are as follows
Ephedrine
Ephedrine (101) in aqueous solution on exposure to sunlight and UV light in the presence of air is oxidized to benzaldehyde (102) which condenses with the unreacted ephedrine to form 34-dimethyl-25-diphenyl-13-oxazolidine (103) This product is biologically inactive (Khan et al 1975)
CH3OH
CH3
NH
hv
O
CH3OH
CH3
NH
+
O
O
N
CH3 CH3
(101) (102) (103)
Reserpine
Reserpine (104) in aqueous solution and chloroform on exposure to light undergoes photooxidation to form 34-dehydroreserpine (105) which is further oxidized to 3456-tetradehydroreserpine (lumireserpine) (106) (Wright and Tang 1972)
(101) (102)
185
OO
CH3
ONH
NH
H
O
O
CH3
O
CH3
O
O
O
CH3
CH3
CH3
H hv
OO
CH3
ONH
NH+ H
H
O
O
CH3
O
CH3
O
O
O
CH3
CH3
CH3
H
OO
CH3
ONH
NH+ H
H
O
O
CH3
O
CH3
O
O
O
CH3
CH3
CH3
H
Riboflavin
Riboflavin (vitamin B2) (107) on photodegradation in aqueous solution gives rise to formylmethylflavin (108) as an intermediate product which is hydrolyzed to lumichrome (109) and lumiflavin (1010) (Ahmad and Rapson 1990 Ahmad et al 2004)
N
NNH
N O
O
CH3
CH3
CH2
C OHH
C OHH
C OHH
CH2OH
N
NNH
N O
O
CH3
CH3
CH2
CHO
N
NNH
N O
O
CH3
CH3
CH3
N
NNH
NH
O
O
CH3
CH3
hv
H+OH-
OH-
(104) (105)
(106)
(107) (108)
(109) (1010)
186
Quinine
Quinine (1011) on UV irradiation in aqueous solution leads to the formation of 6-methoxy-quinoline-4-ylmethyl-oxonium (1012) as the final photoproduct (Yadav et al 2013)
N
O
CH3OH
N
H
CH2
H
N
HO+H2
O
CH3
hv
(1011) (1012)
1072 Chemical Degradation of Herbal Drugs
Many drugs are sensitive to air and pH and undergo chemical degradation by oxidation hydrolysis and other reactions in aqueous solution Some examples of the chemical degradation of drugs are as follows
Erythromycin
Erythromycin (1013) in acidic solutions undergoes acid-catalyzed dehydration reaction by the loss of one molecule of water to form anhydroerythromycin (1014) (Atkins et al 1986)
O
CH3
CH3
CH3
H5C2
O
CH3
CH3
CH3
O
OH
OH
O
O
OH
O
CH3 OCH3
OH
CH3
O
OH
CH3
N(CH 3)2
O
CH3
CH3
H5C2
O
CH3
CH3
CH3OH
O
O
O
CH3 OCH3
OH
CH3
O
OH
CH3
N(CH 3)2
O
O
CH3
H+
-H2O
(1013) (1014)
Digitoxin
Digitoxin (1015) is degraded by acid-catalyzed hydrolysis at pH 1 to 2 to give digitoxigenin (1016) and other products (Peters et al 1978)
OO
O
OO
OH O
O O
OH
OH
OH
OH
CH3
CH3
CH3CH3
CH3
H
H
H
OH
O O
OH
CH3
CH3
H
H
HH+
(1015) (1016)
187
Morphine
Morphine (1017) in aqueous solution undergoes oxidation in the presence of air to give pseudomorphine (oxydimorphine) (1018) and other products (Yeh and Lach 1961)
OH
N
OH
O
CH3
O2
O
OH OH
N
CH3
O
N
O
CH3
OH
(1017) (1018)
Atropine
Atropine (1019) is degraded by H+ ion catalyzed hydrolysis in aqueous solution to form tropine (1020) and tropic acid (1021) (Kirchhoff et al 2004)
O
CH2OH
O
NCH3H
OH
NCH3H
O
CH2OH
OH
+H+
(1019) (1020) (1021)
Pilocarpine
Pilocarpine (1022) undergoes hydrolysis in alkaline solution to form isopilocarpic acid (1023) (Bundgaard and Hansen 1982 Zoppi et al 2012)
OH-
N
NCH3
OH
H3CH2C
O
CH2OHO N
NCH3
H3CH2C
O
(1022) (1023)
Ginseng Saponins
The ginseng saponins ginsenosides Rg1 Re and Rb1 have been found to degrade under mild acidic conditions to form prosapogenins which have been identified by 13CndashNMR spectroscopy Rg1-prosapogenins II is a mixture of ginsenoside Rh1 and its Cndash20 isomer formed by the hydrolysis and epimerization at Cndash21 Rg1-prosapogenin III is a Cndash25 26 hydrated derivative of Rg1-prosapogenin II Re-prosapogenin II has been shown to be a mixture of ginsenoside Rg2 and its Cndash20 epimer and Re-prosapogenin III as a Cndash25 26 hydrated derivative of Re-prosapogenin II (Han et al 1982)
108 STABILITYDEGRADATION STUDIES OF HERBAL DRUGS IN FORMULATIONS
Acid-base titrimetry and NMR spectroscopy have been used to study the kinetics of OH ion-catalyzed hydrolysis and epimerization reactions of pilocarpine in ophthalmic solutions The pseudo-first-order rate constants and activation energies of the reactions have been determined
188
Epimerization of pilocarpine is the major pathway of degradation of the drug that involves the formation of a carbanion stabilized by resonance with the enolate hybrid The rate of epimerization to isopilocarpine is temperature dependent that may affect the stability of pilocarpine in ophthalmic solutions on sterilization by heat (Nunes and Brochmann-Hanssen 1974)
There is a high demand for optically pure drugs for the preparation of stable herbal formulations with a chiral quality of the desired isomer In a study the effects of cyclodextrins (CDs) and derivatives on the kinetics of racemization and hydrolysis of (ndash)-(S)-hyoscyamine and (ndash)-(S)-scopolamine has been investigated The stability tests involved the chromatographic determination of the enantiomer composition and degradation products All CDs except αndashCD have been found to slow down the racemization and hydrolytic reactions of these alkaloids depending on the pH and temperature The drugndashCD complexation results in the inhibition of the OHndash ion andor H2O attack on the drug molecule to cause the degradation reactions The formation of a soluble 11 drugndashCD complex has been confirmed by NMR spectroscopy (Blaschke et al 1993)
The effects of microwave (12 and 15 min at 1100 W) and conventional heating (36 and 45 min at 230 OC) on refined and virgin olive oil have been studied The amount of oxidative and hydrolytic degradation of different oils has been determined by high performance size-exclusion chromatography The results show that the formation of polar compounds of triglyceride oligopolymers and oxidized triglycerides are more than 26 after the most intense treatment The microwave heating results in a higher amount of oxidative degradation The polar compounds have an adverse effect on human health (Caponio et al 2002)
The epimerization of ergot alkaloids in rye flour after baking cookies and then subjecting them to an in vitro digestion model using salivary gastric and duodenal juices has been studied The toxic (R)-epimers and inactive (S)-epimers of several ergot alkaloids were determined by a HPLC method with fluorescence detection A 2ndash30 degradation of different alkaloids has been observed with an increase in epimeric ratio towards the (S) epimer The degradation to the (R)-epimer was found to increase after the digestion of cookies The results show selective toxification of ergotamine and ergosine in duodenal juice (intestinal tract) which should be taken into consideration in use of the product (Markel et al 2012)
The microemulsions are used to simultaneously deliver flavor oils and lipophilic bioactive compounds in beverages In this context the delivery of β-carotene in microemulsions formulated with peppermint oil and a blend of Tween 20 and sunflower lecithin has been studied The poorly water-soluble and oil-soluble β-carotene dissolved in microemulsions with particle size less than 10 nm was found to be stable during storage at room temperature for 65 days The addition of β-carotene does not change the flow properties and Newtonian viscosity of microemulsions The degradation of β-carotene in these emulsions during storage and thermal treatment at 60ndash80degC follows first-order kinetics The antioxidant property of peppermint oil and excess of lecithin protects β-carotene from degradation These microemulsions may have applications in the manufacture of transparent beverages (Chen and Zhong 2015)
Other studies on the stability of herbal formulations include the stability of terpenes in lemon oil (Nguyen et al 2009) herbal capsules with different ingredients (Bankoti et al 2012) ointments containing eucalyptus oil (Dawar et al 2013) herbal antihypertensive formulations containing reserpine (Deore et al 2013 Sandhya et al 2014) and herbal cream containing embelin (Bele and Khale 2011)
109 STABILITY TESTING OF HERBAL PRODUCTS
Stability testing is carried out to provide evidence of variations in the quality of drug products with time under the influence of environmental factors such as temperature humidity and light for a period of 6 to 12 months These studies are necessary to recommend storage conditions and to assign a shelf-life to the product The storage conditions used for the stability testing of drug products (ICH 2003) are given in Table 106 The standard conditions for the photostability testing of drug substances and products are described in ICH (1996) (see Chapter 12 Section 127)
189
Table 106 Storage conditions for stability testing of drug substances
Study Storage condition Minimum time period covered by data at submission
Long-terma 25plusmn2degC 60plusmn5 RH or 30plusmn2degC 65plusmn5 RH
12 months
Intermediateb 30plusmn2degC 65plusmn5 RH 6 months Accelerated 40plusmn2degC 75plusmn5 RH 6 months
a It is up to the applicant to decide whether long term stability studies are performed at 25plusmn2degC 60plusmn5 RH or 30plusmn2degC 65plusmn5 RH
b If 30plusmn2degC 65plusmn5 RH is the long-term condition there is no intermediate condition
The stability testing of pharmaceutical active ingredients (Vipul and Devesh 2012) stability studies of ayurvedic health supplements (Deepa and Kannappan 2012) and Unani (herbal) formulations (Noor-ul-Basar et al 2013) have been conducted
1010 HERBndashDRUG INTERACTIONS
The popularity of herbal products in the treatment of diseases is increasing worldwide However it requires an understanding of the potential interactions between herbs and prescribed drugs if administered concurrently The likelihood of herb-drug interactions could be higher than drug-drug interactions since the drugs usually contain a single chemical ingredient while herbal products contain mixtures of pharmacologically active constituents (Fugh-Berman and Ernst 2001) Many herbs and drugs are therapeutic at one dose and toxic at another Herb-drug interactions could lead to an increase or decrease in the pharmacological and toxicological effects of the either component In some cases synergistic therapeutic effects may affect the dosing of long-term medications eg herbs that decrease glucose concentration in diabetes could cause hypoglycemia on combination with conventional drugs (Fugh-Berman 2000)
Clinical studies have shown that the use of St Johnrsquos wort with certain drugs lowers their serum concentrations eg digoxin (Johne et al 1999) phenprocoumon (Maurer et al 1999) indinivir (Piscitelli et al 2000) and amitriptyline (Roots et al 2000) The chewing of Latha edulis (Khat) affects the pharmacokinetics of single-dose ampicillin and reduces its bioavailability in the system (Attef et al 1997) The plasma concentrations of prednisolone are increased by the use of liquorice (Chen et al 1990) which also potentiates the vasoconstrictor response of hydrocortisone (Teeluksingh et al 1990) Denshen interferes with platelet function and decreases the elimination of warfarin Ginkgo and garlic are also known to interfere with the platelet function and cause bleeding even in the absence of treatment with anticoagulants (Chan et al 1995) The interactions of herbal supplements containing coumarin derivatives and possessing antiplatelet and anticoagulant properties with aspirin and other non-salicylate non-steroidal anti-inflammatory drugs (NSAIDs) (eg ibuprofen flurbiprofen diflunisal naproxen Ketorolac ketoprofen and meclofenamate) results in the reduction of platelet aggregation (Abebe 2002) The interaction of St Johnrsquos wort with conventional drugs has been reviewed in detail (Mills et al 2004)
The interaction of herbal drugs and conventional drugs need further studies to understand the mechanisms of their interactions It would be advisable not to use both types of drugs concurrently to avoid any adverse effects Some of the adverse effects of herbal products are reported in Table 107 The adverse effects of herbal medicines have been dealt by De Smet (1995 2004) De Smet et al (1997) and Tyagi and Delanty (2003) The interactions between these drugs and conventional drugs may also cause potency loss of either or both of the drugs and thus affect their efficacy and bioavailability
190
Table 107 Adverse effects of herbs and herbal products
Herb Active constituents
Drug Adverse effects
Karela Chlorpropamide decreased glucose concentration in blood
Liquorice (Glycyrrhiza glabra)
Glycyrrhazin Glycyrrhetinic acid
Prednisolone Hydrocortisone Oral contraceptives
decrease plasma clearance potentiation of cutaneous vasodilator response hypertension edema
Salbokinto (Asian herbal mixture)
Prednisolone increased prednisolone concentration
Shankahapushpl (Ayurvedic mixed herb syrup)
Phenytoin decreased phenytoin concentration
Tamarind Aspirin Increased aspirin bioavailability
Yohimbine Tricyclic antidepressants
hypertension
191
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Ahmad I Rapson HD Multicomponent spectrophotometric assay of riboflavine and photoproducts J Pharm Biomed Anal 19908217ndash223
Ahmad I Usmanghani K Analysis of Medicinal Compounds and Plant Drugs Research Institute of Indusyunic Medicine Karachi Pakistan 2003 Chapter 5
Andreescu C Mulsant BH Emanuel JE Complementray and alternative medicine in the treatment of bipolar disorderndasha review of the evidence J Affect Disorders 200811016ndash26
Atkins PJ Herbert TO Jones NB Kinetic studies on the decomposition of erythromycin A in aqueous acidic and neutral buffers Int J Pharm 198630199ndash207
Attef OA Ali AA Ali HM Effect of Khat chewing on the bioavailability of ampicillin and amoxycillin J Antimicrob Chemother 199739523ndash255
Ayiguli T Zhou Q Dong XO Sun SQ Study on the identification of standard and false Gancao by Fourier transform infrared spectroscopy Guang Pu Xue Yu Guang Pu Fen Xi 2006261238ndash12341
Azmir J Zaidul ISM Rahman MM Sharif KM Mohamed A Sahena F Jahurul MHA Ghafoor K Norulaini NAN Omar AKM Techniques for extraction of bioactive compounds from plant materials a review J Food Eng 2013117426ndash436
Bankoti K Rana MS Bharadwaj MK Accelerated stability study of herbal capsules IOSR J Pharm 201221ndash6
Barl B Quality analysis and standardized extracts of medicinal herbs Proceedings of the Prairie Medicinal and Aromatic Plants Conference (PMAP) Manitoba Canada March 9ndash12 1997
Barnes J Anderson LA Phillipson JD Herbal Medicine 3rd ed Pharmaceutical Press London UK 2007 pp 1ndash23
Bauer R Quality criteria and standardization of phytopharmaceuticals Can acceptable drug standards be achieved Drug Inform J 199832101ndash110
Bele AA Khale A Standardization of herbal drugs an overview IRJP 2011256ndash60
Bisset NG Herbal Drugs and Phytopharmacetucials CRC Press Boca Raton FL USA 1994
Blaschke G Lamparter E Schuler J Racemization and hydrolysis of tropic acid alkaloids in the presence of cyclodextrins Chirality 1993578ndash83
Bodeker C Bodeker G Ong CK Grundy CK Burford G Shein K WHO Global Atlas of Traditional Complementary and Alternative Medicines World Health Organization Geneva Switzerland 2005
Bouldin AS Smith MC Garner DD Szeinbach SL Frate DA Croom EM Pharmacy and herbal medicine in the US Soc Sci Med 199949279ndash289
British Pharmacopoeia Vol IV Herbal Drugs Herbal Drug Preparations and Herbal Medicinal Products Her Majestyrsquos Stationery Office London UK 2016
Bundgaard H Hansen SH Hydrolysis and epimerization kinetics of pilocarpine in basic aqueous solution as determined by HPLC Int J Pharm 198210281ndash289
Bukhari AQS Kapadia Z Ahmed S Khan MI Ahmed SI The medicinal concept and efficacy of herbs Pak J Pharmacol 1987455ndash63
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Cai Z Lee FSC Wang XR Yu WJ A capsule review of recent studies on the application of mass spectrometry in the analysis of Chinese medicinal herbs J Mass Spectrom 2002371013ndash1024
Cai F Sun SQ Yan WR Niu SJ Li XE Identification and analysis of genuine and false Flos Rosae Rugosae by FTIR and 2D correlation IR spectroscopy Guang Pu Xue Yu Guang Pu Fen Xi 2009292429ndash2433
Cao F Zhou Q Sun SQ Study on the identification of standard and false Tianma by two-dimensional infrared correlation spectroscopy Med Instrum 2002419ndash21
Caponio F Pasqualone A Gomes T Effects of conventional and microwave heating on the degradation of olive oil Eur Food Res Technol 2002215114ndash117
Capasso R Izzo AA Pinto L Bifulco T Vitobello C Mascolo N Phytotherapy and quality of herbal medicines Fitoterapia 200071S58ndashS65
Chan K Lo AC Yeung JH Woo KS The effects of Danshen (Salvia miltiorrhiza) on warfarin pharmacodynamics and pharmacokinetics of warfarin enantiomers in rats J Pharm Pharmacol 199547402ndash406
Chen H Zhong Q Thermal and UV stability of β-carotene dissolved in peppermint oil microemulsified by sunflower lecithin and tween 20 blend Food Chem 2015174630ndash636
Chen C Zhang H Xiao W Yong ZP Bai N High-performance liquid chromatographic fingerprint analysis for different origins of sea buckthorn berries J Chromatogr A 20071154250ndash259
Chen JB Zhou Q Sun SQ Yu L Xu KY Study on quality control of traditional Chinese medicine ginseng injection with Fourier transform infrared spectroscopy Spectrosc Spectral Anal 2007271493ndash1496
Chen MF Shimada F Kato H Yano S Kanaoka M Effect of glycyrrhizin on the pharmacokinetics of prednisolone following low dosage of prednisolone hemisuccinate Endocrinol Jpn 199037331ndash341
Dawar N Arora M Naved T Tyagi VK Stabilities studies of formulations containing eucalyptus Indo Global J Pharm 20133174ndash180
De Smet PAGM Health risks of herbal remedies Drug Safety 19951381ndash93
De Smet PAGM Keller K Hansel R Chandler RF Adverse Effect of Herbal Drugs Springer-Verlag Heidelberg Germany 1997
De Smet PAGM Overview of herbal quality control Drug Inform J 199933717ndash724
De Smet PAGM Health risks of herbal remedies an update Clin Pharm Ther 2004761ndash17
Deepa P Kannappan N Comparative stability study of formulated ayurvedic health supplement and marketed product Der Pharma Chemica 201242068ndash2072
Deore SL Mohod MA Baviskar BA Khadabadi SS HPTLC validated stability indicating assay method for marketed herbal antihypertensive formulations Pharm Methods 2013411ndash15
Der Marderosian AH Riedlinger JE Complementary and alternative medicinal health care In Hendrickson R editor Remington the Science and Practice of Pharmacy Lippincott Williams amp Wilkins New York USA 21st ed 2006 Chap 132
Drasar P Moravcova J Recent advances in analysis of Chinese medical plants and traditional medicines J Chromatogr B Analyt Technol Biomed Life Sci 20048123ndash21
Duke JA Martinez RV Handbook of Ethnobotanicals (Peru) CRC Press Boca Raton FL USA 1994
193
EMEA Quality of Herbal Medicinal Products Guidelines European Agency for the Evaluation of Medicinal Products (EMEA) London UK 1998
EMEA Guidelines on Quality of Herbal Medicinal ProductsTraditional Medicinal Products EMEACVMP81400 Review European Agency for the Evaluation of Medicinal Products (EMEA) London UK 2005
Ernst E Adverse effects of herbal drugs in dermatology Brit J Dermatol 2000143923ndash929
Ernst E Rand JI Barnes J Stevinson C Adverse effects profile of the herbal antidepressant St Johns wort (Hypericum perforatum L) Eur J Clin Pharmacol 199854589ndash594
Eskinazi D Blumenthal M Fransworth N Riggins CW Botanical Medicine Efficacy Quality Assurance and Regulation Mary Ann Libert New York USA 1999
Fabricant DS Fransworth NR The value of plants used in traditional medicine for drug discovery Environ Health Perspect 200110969ndash75
Fransworth NR Akerele O Bingel AS Soejarto DD Guo Z Medicinal plants in therapy Bull World Health Organization Geneva Switzerland 198563965ndash981
Fugh-Berman A Herb-drug interactions Lancet 2000355134ndash138
Fugh-Berman A Ernst E Herbal-drug interactions review and assessment of report reliability Br J Clin Pharmacol 200152587ndash595
Gaedcke F Steinhoff B Quality assurance of herbal medicinal products In Herbal Medicinal Products Medpharm GmbH Scientific Publishers Stuttgart Germany 2003 pp 36ndash66 81ndash88
Gilani AH Molla N Rahman AU Shah BH Phytotherapy ndash the role of natural products in modern medicine J Pharm Med 19922111ndash119
Gong F Liang YZ Fung YS Chau FT Correction of retention time shifts for chromatographic fingerprints of herbal medicines J Chromatogr A 20041029173ndash183
Han BH Park MH Han YN Woo LK Sankawa U Yahara S Tanaka O Degradation of ginseng saponins under mild acidic conditions Planta Med 198244146ndash149
Hou D Song J Shi L Ma X Xin T Han J Xiao W Sun Z Cheng R Yao H Stability and accuracy assessment of identification of traditional chinese materia medica using dna barcoding a case study on flos lonicerae japonicae BioMed Res Int 2013 Article ID 549037
ICH Harmonized Tripartite Guideline ICH Q1B Photostability of Testing of New Drug Substances and Products Geneva Switzerland 1996
ICH Harmonized Tripartite Guideline ICHndashQ1A (R2) Stability testing of new drug substances and products (second revision) Geneva Switzerland 2003
Johne A Brockmoumlller J Bauer S Maurer A Langheinrich M Roots I Pharmacokinetic interaction of digoxin with an herbal extract from St Johns wort (Hypericum perforatum) Clin Pharmacol Ther 199966338ndash345
Jiang F Tao Y Shao Y Fingerprinting quality control of Qianghuo by high-performance liquid chromatography-photodiode array detection J Ethnopharmacol 2007111265ndash270
Jin ZX Xu SY Sun SQ Zhou Q Analysis of Acanthopanax senticosus harms for different parts using Fourier transform infrared spectroscopy Guang Pu Xue Yu Guang Pu Fen Xi 2008282859ndash2863
Kang J Choi MY Kang S Kwon HN Wen H Lee CH Park M Wiklund S Kim HJ Kwon SW Park S Application of a 1H nuclear magnetic resonance (NMR) metabolomics approach combined with orthogonal projections to latent structure-discriminant analysis as an
194
efficient tool for discriminating between Korean and Chinese herbal medicines J Agric Food Chem 20085611589ndash11595
Khan U Ahmad I Zoha SMS Studies on the quantitative determination and photodegradation of ephedrine Pak J Sci Ind Res 197518229ndash230
Kirchhoff C Bitar Y Ebel S Holzgrabe U Analysis of atropine its degradation products and related substances of natural origin by means of reversed-phase high-performance liquid chromatography J Chromatogr A 20041046115ndash120
Kunle OF Egharevba HO Ahmadu PO Standardization of herbal medicinesndasha review Int J Biodiver Conser 20124101ndash112
Li YM Sun SQ Zhou Q Tao JX Noda I Study of traditional Chinese animal drugs using FT-IR and 2D-IR correlation spectroscopy Spectrochim Acta A Mol Biomol Spectrosc 200663565ndash573
Liang Y-Z Xie P Chan K Quality control of herbal medicines J Chromatogr B 200481253ndash70
Lu GH Chan K Liang YZ Leung K Chan CL Jiang ZH Zhao ZZ Development of high-performance liquid chromatographic fingerprints for distinguishing Chinese Angelica from related umbelliferae herbs J Chromatogr A 20051073383ndash392
Majno GM Healing Hand Man and Wound in the Ancient World Harvard University Press Cambridge MA USA 1975
Markel S Dib B Maul R Koppen R Koch M Nehls I Degradation and epimerization of ergot alkaloids after baking and in vitro digestion Anal Bioanal Chem 20124042489ndash2497
Maurer A Johne A Bauer S Interaction of St Johnrsquos wort extract with phenprocoumon Eur J Clin Pharmacol 199955A22
Mills E Montori VM Wu P Gallicano K Clarke M Guyatt G Interaction of St Johns wort with conventional drugs systematic review of clinical trials BMJ 200432927ndash30
Mukherjee PW Quality Control of Herbal Drugs An Approach to Evaluation of Botanicals Business Horizons Publishers New Delhi India 2002
Nguyen H Campi EM Jackson WR Patti AF Effect of oxidative deterioration on flavor and aroma components of lemon oil Food Chem 2009112388ndash393
Ni L-J Zhang L-G Hou J Shi W-Z Guo M-L A strategy for evaluating antipyretic efficacy of Chinese herbal medicines based on UV spectra fingerprints J Ethnopharmco 200912479ndash86
Noor-ul-Basar S Rani S Zaman R A review on stability studies of Unani formulations JPSI 201321ndash8
Nunes MA Brochmann-Hanssen E Hydrolysis and epimerization kinetics of pilocarpine in aqueous solution J Pharm Sci 197463716ndash721
Palanisamy A Haller C Olson KR Photosensitivity reaction in a woman using an herbal supplement containing ginseng goldenseal and bee pollen J Toxicol Clin Toxicol 200341865ndash867
Pei LK Sun SQ Guo BL Huang WH Xiao PG Fast quality control of Herba Epimedii by using Fourier transform infrared spectroscopy Spectrochim Acta A Mol Biomol Spectrosc 200870258ndash264
Peishan X A feasible strategy for applying chromatography fingerprint to assess quality of Chinese herbal medicine Trad Chinese Drug Res Clin Pharmacol 200103
Peters U Funcke C Hausamen TU Staib W Quantitative studies on acid hydrolysis of digitoxin ArzneimittelForschung 197828750ndash752 Pharm J 2002269459ndash460
195
Phillips G The importance of standardization techniques for herbal medicines Pharm J 2002269459ndash460
Piscitelli SC Burstein AH Chaitt D Alfaro RM Falloon J Indinavir concentrations and St Johns wort Lancet 2000355547ndash548
Roberts JE Tyler VE Tylerrsquos Herbs of Choice The Therapeutic Use of Phytomedicinals The Haworth Press New York USA 1997
Roots I Johne A Schmider J Interaction of a herbal extract from St Johnrsquos wort with amitriptyline and its metabolites Clin Pharmcol Ther 200067PIIIndash69
Scafi SH Pasquini C Identification of counterfeit drugs using near-infrared spectroscopy Analyst 20011262218ndash2224
Sagar BPS Tyagi K Zafar R Failures and successes of herbal medicines The Indian Pharmacist 20030717ndash24
Sandhya MB Smita GM Gangane PS HPLC stability indicating method for Marketed herbal antihypertensive formulations containing rauwolfia serpentine Indian J Natural Sci 201441448ndash1458
Sun S Chen J Zhou Q Lu G Chan K Application of mid-infrared spectroscopy in the quality control of traditional Chinese medicines Planta Med 2010761987ndash1996
Tapas AR Sakarkar DM Kakde RB Flavonoids as nutraceuticals Trop J Pharm Res 200871089ndash1099
Tsai T-H Analytical approaches for traditional Chinese medicines exhibiting antineoplastic activity J Chromatogr B 200176427ndash48
Teelucksingh S Mackie AD Burt D McIntyre MA Brett L Edwards CR Potentiation of hydrocortisone activity in skin by glycyrrhetinic acid Lancet 19903351060ndash1063
Thakur L Ghodasra U Patel N Dabhi M Novel approaches for stability improvement in natural medicines Pharmcog Rev 2011548ndash54
Tyagi A Delanty N Herbal remedies dietary supplements and seizures Epilepsia 200344228ndash235
Vipul A Devesh S Stability testing of active pharmaceutical ingredient [API] JPSI 2012118ndash23
Wani MS Herbal medicine and its standardization Pharma Info 200716
WHO Research guidelines for evaluating the safety and efficacy of herbal medicines World Health Organization Geneva Switzerland 1993
WHO Quality control Methods for Medicinal Plant Materials World Health Organization Geneva Switzerland 1998
WHO A draft regional strategy for Traditional Medicine in Western Pacific World Health Organization Regional Committee 52nd Session Brunei Darussalam 2001 pp 10ndash14
Woo YA Kim HJ Cho JH and Chung H Discrimination of herbal medicines according to geographical origin with near infrared reflectance spectroscopy and pattern recognition techniques J Pharm Biomed Anal 199921407ndash413
Wright GE Tang TY Photooxidation of reserpine J Pharm Sci 197261299ndash300
Wu YW Sun SQ Zhou Q Leung HW Fourier transform mid-infrared (MIR) and near-infrared (NIR) spectroscopy for rapid quality assessment of Chinese medicine preparation Honghua Oil J Pharm Biomed Anal 200846498ndash504
Xie PS Chief editor The Chromatographic Fingerprint of Traditional Chinese Medicine (in Chinese) Peoplersquos Health Publishing House Beijing China 2005
196
Xie P Chen S Liang Y-Z Wang X Tian R Upton R Chromatographic fingerprint analysisndasha rational approach for quality assessment of traditional Chinese herbal medicine J Chromatogr A 20061112171ndash180
Xie Y Jiang ZH Zhou H Cai X Wong YF Liu ZQ Bian ZX Xu HX Liu L Combinative method using HPLC quantitative and qualitative analyses for quality consistency assessment of a herbal medicinal preparation J Pharm Biomed Anal 200743204ndash212
Xu YQ Sun SQ Yuan ZM Bai Y Discrimination of trueborn tuber dioscoreae by fingerprint infrared spectra and principal component analysis Chin J Anal Chem 2002301231ndash1233
Xu CH Zhou Q Sun SQ Wang BQ The identification of Ejiao by two dimensional correlation infrared spectroscopy Chin J Anal Chem 200533221ndash224
Yadav N Dwivedi A Mujtaba SF Kushwaha HN Singh SK Ray RS Ambient UVA-induced expression of p53 and apoptosis in human skin melanoma A375 cell line by quinine Photochem Photobiol 201389655ndash664
Yang P Song P Sun SQ Zhou Q Feng S Tao JX Differentiation and quality estimation of Cordyceps with infrared spectroscopy Spectrochim Acta A Mol Biomol Spectrosc 200974983ndash990
Yeh SY Lach JL Stability of morphine in aqueous solution III Kinetics of morphine degradation in aqueous solution J Pharm Sci 19615035ndash42
Yin L Qian J Effective elements of jia-Wei-Si-Miao pills GC fingerprint-efficacy relationship and the variety of different combinations Chinese Traditional Patent Med 200729634ndash637
Zhang J Wider B Shang H Li X Ernst E Quality of herbal medicines challenges and solutions Complementary Therapeutics Med 201220100ndash106
Zhou Q Li J Liu J Huang H Sun SQ Two-dimensional correlation infrared spectroscopy of standard and false Dahuang Chin J Anal Chem 2003311058ndash1061
Zoppi A Linck YG Monti GA Genovese DB Jimenez Kairuz AF Manzo RH Longhi MR Studies of pilocarpinecarbomer intermolecular interactions Int J Pharm 2012427252ndash259
197
CHAPTER ndash 11
STABILITY-INDICATING ASSAY METHODS 111 INTRODUCTION
Stability studies are an integral part of drug development process in pharmaceutical industry The assay method used in stability studies must be specific and stability-indicating for the drug It should be capable of separating and determining the drug and the degradation products as well as major impurities The reliability and specificity of the assay method must be demonstrated on the pure drug and on its degradation products A determination of the kinds and amounts of various contaminants and degradation products in drug substances and formulated products is a measure of both product stability and Good Manufacturing Practices (GMP)
The pharmacopoeial assays do not necessarily take into account the presence of various contaminants and degradation products This also applies to the assay of certain classes of compounds eg barbiturates salicylates steroids sulfonamides penicillins which contain a common nucleus as well as the main functional groups involved in the assay Each class frequently possesses similar physical characteristics which interfere with the specificity of the assay for a given compound The presence of degradation products further complicates the system Thus stability-indicating assay methods are required to deal with the analysis of individual drugs in the presence of degradation products and related compounds This is necessary to achieve accurate assay results for the drug and to set the limits of degradation products in drug products
112 DEFINITIONS
FDA Guideline (1987) The stability-indicating methods are ldquoquantitative analytical methods that are based on the characteristic structural chemical or biological properties of each active ingredient of a drug product and that will distinguish each active ingredient from its degradation products so that the active ingredient content can be accurately measuredrdquo
FDA Guideline (1998) The stability-indicating methods are ldquovalidated quantitative analytical methods that can detect the changes with time in the chemical physical or microbiological properties of the drug substance and drug product and that are specific so that the contents of active ingredient degradation products and other components of interest can be accurately measured without interferencerdquo
ICH Guideline (2003) states the following about the application of stability-indicating method in stress testing ldquoStress testing of the drug substance can help identify the likely degradation products which can in turn help establish the degradation pathways and the intrinsic stability of the molecule and validate the stability-indicating power of the analytical procedures used The nature of the stress testing will depend on the individual drug substance and the type of drug product involvedrdquo
113 DEVELOPMENT OF ANALYTICAL METHODOLOGY FOR ASSAY OF A DRUG COMPOUND
The development of specific analytical methodology for the assay of a drug compound is based upon the exploitation of its structural features and physicochemical characteristics to show a particular response (eg light absorption or light emission electrooxidation or electroreduction change in current or potential etc) on the application of an analytical technique A drug may exhibit a single physicochemical characteristic or multiple characteristics that can be made a basis for its determination This is followed by a careful assessment of all the parameters involved in its quantitation to achieve optimum conditions for the assay of the drug An example of the
198
physicochemical characteristics of a drug such as riboflavin (vitamin B2) to be considered as a basis of the development of its analytical methods is as follows
N
NNH
NCH3
CH3
O
CH2
COH H
C HOH
C HOH
CH2OH
O
Fig 111 Chemical structure of riboflavin
Physicochemical characteristics Assay Method Light absorption at 444 nm Spectrophotometric assay (British Pharmacopeia
2016) Fluorescence emission at 530 nm Spectrofluorimetric assay (United States
Pharmacopeia 2016) Redox system E0 ndash0185 V Potentiometric titration (Lowe and Clark 1956) Redox system E12 ndash047 V Polarographic assay (Ke 1957) Metal complexation Spectrometric assay (Wade and Fritchie 1963) Photodegradation to lumichrome Photochemical assay (Ahmad et al 2015) Selective adsorptionpartition Chromatographic assay (Gliszczynska-Swiiglo and
Koziolowa 2000)
A similar approach may be adopted for the development of an assay method for a new drug
114 DEVELOPMENT OF ANALYTICAL METHODOLOGY FOR STABILITY-INDICATING ASSAY OF A DRUG COMPOUND
The development of a stability-indicating method would depend on the chemical characteristics of the drug substance its mode of degradation under specific stress conditions (eg moisture heat light) and the nature of the degradation products It may involve the following steps
Verification of degradation under specific conditions eg oxidation hydrolysis thermolysis photolysis radiolysis using chromatographic and spectroscopic methods such as TLC and UV spectrometry
Separation and purification of degradation products by appropriate extraction andor chromatographic methods
Characterization of knownunknown degradation products by comparison of their spectral characteristics (ie UV IR NMR Mass) with those of authentic structurally related compounds
Determination of specific analytical characteristics of the drug and its degradation products (eg absorption wavelengths ionization behavior pH effects) suitable for their assay in storedstressed samples
Quantitation of the assay method based on the selection of a particular physicochemical characteristic
199
Assessment of the interference of degradation products in the assay of the parent drug
Validation of the assay method under the condition of use so as to meet the requirements for its specific analytical application
It may be necessary to screen the degradation products for their potential toxicity to ensure the safety of the patient
115 STABILITY-INDICATING SPECTROMETRIC ASSAY METHODS
The ultraviolet (UV) and visible spectrometric methods for the assay of a drug compound are based on the measurement of absorbance of the compound in solution at the absorption maximum (λmax) and determination of the concentration with reference to a calibration curve of absorbance versus concentration It can also be determined directly by Beerrsquos law relation using
the values of specific absorbance [A (1 1 cm) dlgndash1cmndash1] or molar absorptivity ( Mndash1 cmndash1) in a certain concentration range where absorbance is proportional to concentration
1151 One-Component Assay
The Beerrsquos Law states that
A = abc (111)
Where lsquoArsquo is absorbance lsquoarsquo is called absorptivity lsquobrsquo is cell path length and lsquocrsquo is concentration When c is in percent lsquoarsquo is expressed as specific absorbance lsquoArsquo and
A = A (1 1cm) bc (112)
When c is in ML a is expressed as molar absorptivity and
A = bc (113)
The concentration c of the solution can be calculated from the above relation as
c = AA(1 1cm) b (114)
or
c = Ab (115)
1152 Multicomponent Assay
The method of one component assay is not applicable to the assay of mixtures of compounds or a drug and its degradation products due to overlapping of the absorption spectra and mutual interference at the analytical wavelengths In such cases the methods of multicomponent spectrometric assay may be used which are capable of determining the components of a mixture or a drug and its degradation products with high accuracy The total absorbance of the solution of a mixture of compounds at a particular wavelength is equal to the sum of the absorbance of the individual components
Atotal = A1 + A2 + ------------ + An = 1bc1+ 2bc2+----------+ nbcn (116)
Where the subscripts refer to absorbing components 1 2helliphelliphellipn
In the analysis of mixtures the values of specific absorbance or molar absorptivities of the compounds at the selected wavelengths (eg absorption maxima) are determined under the experimental conditions used (ie pH solvent temperature etc) and the concentrations are calculated using appropriate equations
11521 Two-component assay (additive absorbencies)
In the assay of a two-component system the absorbance measurements are made at two suitably selected wavelengths λ1 and λ2 and if the light path remains constant two simultaneous equations may be written
A 1 = 1K1 1C + 2K1 2C (117a)
200
A2 = 1K2 1C + 2K2 2C2 (117b)
where A1 is absorbance at wavelength λ 1
A 2 is absorbance at wavelength λ2
1K1 is absorptivity-cell path product for component 1 at λ 1
1K2 is absorptivity-cell path product for component 1 at λ2
2K1 is absorptivity-cell path product for component 2 at λ 1
2K2 is absorptivity-cell path product for component 2 at λ2
1C is concentration of component 1
2C is concentration of component 2
The solution of equations (Eq 117a) and (Eq 117b) for 1C and 2C is
1C = (2K2A1 ndash 2K1A2) (1K1 2K2ndash2K1 1K2) (118a)
2C = (1K1A2 ndash 1K2A1) (1K1 2K2ndash2K1 1K2) (118b)
11522 Three-component assay (additive absorbencies)
In this case the solution of three simultaneous equations is required which may be done for the sake of convenience using matrix method Thus for measurements A1 A2 A3 at λ1 λ2 λ3 on a mixture of components 1 2 3 at concentration 1C 2C and 3C
Wavelength Absorbance Absorbance sum λ1 A1 = 1K11C + 2K1 2C + 3K1 3C λ2 A2 = 1K2 1C + 2K2 2C + 3K2 3C λ3 A3 = 1K3 1C + 2K3 2C + 3K3 3C (119a)
The matrix equation is as follow
A1 A2 A3
= 1K1 2K1 3K1 1K2 2K2 3K2 1K3 2K3 3K3
1C 2C 3C
(119b)
(AM) (ASM) (CM)
where
(AM) = Absorbance Matrix (ASM) = Absorbance Sum Matrix (CM) = Concentration Matrix
The solution of (Eq 119b) for each concentration is carried out by replacing the appropriate column in the absorbance sum matrix in its determinant form and dividing the resultant by the absorbance sum matrix (ASM) again in its determinant form
1C =
A1 2K1 3K1 1K1 2K1 3K1 A2 2K2 3K2 1K2 2K2 3K2 A3 2K3 3K3 1K3 2K3 3K3
(1110a)
2C =
1K1 A1 3K1
(ASM) 1K2 A2 3K2
1K3 A3 3K3
(1110b)
3C =
1K1 2K1 A1
(ASM)
1K2 2K2 A2
1K3 2K3 A3
(1110c)
201
The matrices are then expanded by any convenient method eg for 1C using the top row and Laplacersquos method
1C =
A1 2K2 3K2
ndash 2K1 A2 3K2
+ 3K1 A2 2K2
2K3 3K3 A3 3K3 A3 2K3
ASM expanded
1C = A1 (2K2 3K3 ndash 3K2 2K3) ndash 2K1 (A2 3K3 ndash 3K2 A3) + 3K1 (A2 2K3 ndash 2K2 A3)
ASM expanded (1111)
Similarly the matrices are expanded for 2C and 3C For each determinant of a different set of 1C 2C and 3C the top line of (Eq 1111) has to be computed a fresh since A1 A2 A3 vary whilst ASM is always the same This may be achieved by the application of programmed software
1153 Advantages
The multicomponent spectrophotometric methods on application to the study of a stability problem have the following advantages over the chromatographic methods
Simultaneous determination of the drug and its degradation product(s) and confirmation of its accuracy on the basis of the molar balance achieved (Ahmad et al 1990 Ahmad and Vaid 2006 Sheraz et al 2014)
Elimination of interference due to minor contaminants by the application of correction procedures for linear or nonlinear irrelevant absorption (Ahmad 2013a 2015 Arsalan 2016)
Immediate determination of the concentration of species involved in degradation at a particular time as compared to that of GLCHPLC method which takes considerable time for detection after sample application and hence the possibility of a chemical change in the mobile phase (eg on a tablet extract dilution ) or on the column during the separation process This may lead to erroneous analytical results in the stability evaluation of a compound depending upon its sensitivity to assay conditions
Time required to complete an assay is much shorter than that of GLCHPLC assay and the technique is more suitable for kinetic work if applicable
Cost of performing assays in terms of time material and equipment is much less than that involved in GLCHPLC assays
1154 Applications
Several stability-indicating multicomponent spectrometric methods have been developed for the simultaneous determination of a drug and its degradation products An important application of these methods is the evaluation of the kinetics of degradation reactions (Ahmad and Vaid 2006 Sheraz et al 2014) This would be illustrated with reference to their application in chemical and photodegradation studies The details of the degradation reactions of some drug compounds are as follows
Hydrolysis of aspirin (Khurshid 2013) (Fig 112)
Hydrolysis of procaine HCl (Al-Blewi et al 2013) (Fig 113)
Hydrolysis of riboflavin (Ahmad et al 1973) (Fig 114)
Hydrolysis of formylmethylflavin (Ahmad et al 1980) (Fig 115)
Thermolysis of reserpine (Ahmad et al 1979) (Fig 116)
Hydrolysis and photolysis of sulfacetamide (Ahmad and Ahmad 1981) (Fig 117)
Photolysis of riboflavin (Ahmad et al 2004a) (Fig 118)
202
Photoaddition of riboflavin (Ahmad et al 2004b) (Fig 119)
Riboflavin sensitized photooxidation of ascorbic acid (Sheikh 1996) (Fig 1110)
Some other applications of stability-indicating multicomponent spectrometric methods in the study of drug degradation reactions include the photolysis of riboflavin (Ahmad and Rapson 1990) riboflavin-sensitized photolysis of cyanocobalamin (Ahmad and Hussain 1992 Ahmad et al 2012) degradation of cyanocobalamin in the presence of ascorbic acid (Ahmad et al 2014ab) and nicotinamide (Ahmad et al 2003) buffer catalyzed photolysis of riboflavin (Ahmad et al 2008 2014c) solvent effect on photolysis of formylmethylflavin (Ahmad et al 2006 2013b) and divalent ions effect in the photolysis of riboflavin (Ahmad et al 2010)
Stability-indicating spectrometric methods have also been employed for the assay of norfloxacin (Taha et al 1998) lisinopril (El-Yazbi et al 1999) aceclofenac (El-Saharty et al 2002 Hasan et al 2003) omeparazole lensoparazole pantoprazole (Wahbi et al 2002) oxicams (Taha et al 2006) and vincamine (El-Bardicy et al 2008) in the presence of degradation products
116 STABILITY-INDICATING THIN-LAYER CHROMATOGRAPHIC (TLC) AND HIGH-PERFORMANCE THIN-LAYER CHROMATOGRAPHIC (HPndashTLC) ASSAY METHODS
Stability indicating TLC methods with densitometric detection have been developed for the assay of aceclofenac in the presence of its main degradation product diclofenac at 275 and 283 nm respectively (El-Saharty et al 2002 Hasan et al 2003) Another application of the development and validation of a HPndashTLC method with densitometric detection is the determination of bisacodyl in pharmaceutical tablets The quantitative evaluation has been performed by absorbance measurements of the zones of analyte at 254 nm using the reflectance mode (Campbell and Sherma 2003) The photostability testing of piroxicam using forced degradation by exposing the sample solution to the artificial irradiation from a xenon source and sunlight has been carried at 280 nm using a HPTLCdensitometry stability-indicating assay method (Bartsch et al 1999)
203
Fig 112 Hydrolysis of aspirin in alkaline solution
204
Fig 113 Hydrolysis of procaine HCl in alkaline solution
205
Fig 114 Hydrolysis of riboflavin at pH 110
206
Fig 115 Hydrolysis of formylmethylflavin at pH 110
[Reproduced from I Ahmad et al (1980) with permission]
207
Fig 116 Thermolysis of aqueous soluiton of reserpine (pH 30) at 60degC
208
Fig 117 Hydrolysis and photooxidation of sulfacetamide in aqueous solution
209
Fig 118 Photolysis of riboflavin at pH 70
[Reproduced from I Ahmad et al (2004b) with permission]
210
Fig 119 Photoaddition reaction of riboflavin at pH 70
[Reproduced from I Ahmad et al (2004b) with permission]
211
Fig 1110 Photolysis of ascorbic acid in the presence of riboflavin at pH 40
212
117 STABILITY-INDICATING HIGH- PERFORMANCE LIQUID CHROMATOGRAPHIC (HPLC) ASSAY METHODS
HPLC methods are the most widely used stability-indicating methods for the assay of drug substances and formulated products in mixtures or in the presence of degradation products Several authors have dealt with the development and validation of stability-indicating HPLC assay methods for applications in pharmaceutical industry drug analysis normal and forced degradation studies and drug product testing (Ahmad 1985 Weiser 1998 Xu and Trissel 1999 Hong and Shah 2000 Ruan et al 2002 Bakshi and Singh 2002 Shabir 2003 Smela 2005 Wen 2006 Aubry et al 2009 Singh and Rehman 2012) Hong and Shah (2000) have described in detail the stages involved in the development and validation of HPLC stability-indicating assay methods
1171 Development of HPLC Stability-Indicating Assay Methods
Bakhshi and Singh (2002) have described the following steps in the development of HPLC stability-indicating assay methods to meet regulatory requirements
Critical study of the drug structure to assess the likely decomposition route(s)
Collection of information on physicochemical properties
Stress (forced decomposition) studies
Preliminary separation studies on stressed samples
Final method development and optimization
Identification and characterization of drug degradation products and preparation of standards
Validation of stability-indicating assay methods
1172 Applications
11721 Drug mixture
Some applications of stability-indicating HPLC assay methods in the determination of drug mixture include the assay of dipyridamoline injection (Zhand et al 1997) aceclofenac and diclofenac in pharmaceutical formulations (El-Yazbi et al 1999) drug analysis (Xu and Trissel 1999) ramipril and hydrochlorothiazide in dosage forms (Belal et al 2001) prolocaine and procaine drug combinations (Stroms et al 2002) montilukast and loratidine in pharmaceutical formulations (Radhakrishna et al 2003) and non-steroidal anti inflammatory drugs (Dubroil-Cheneau et al 2011)
11722 Stress testing forced degradation studies
The applications of stability indicating HPLC assay methods in drug degradation studies include photodegradation studies of pyroxicam (Bartsch 1999) determination of aceclofenac in presence of its degradation product diclofenac (Hassan et al 2003) assay of levofloxacin (Ahmad et al 2013c) moxifloxacin (Ahmad et al 2014d) and norfloxacin (Ahmad et al 2015) in photolyzed solutions assay of glimepride under hydrolytic stress condition (Kovarikova et al 2004) assay of doxophylline on hydrolytic degradation (Gupta et al 2011) assay of rapamycin in forced degradation studies (Oyler et al 2012) assay of cefaclor in solid state degradation (Dorman et al 1997) and assay of atorvastatin and its stress degradation product (Shah et al 2008) The various applications of stability indicating assay methods in pharmaceutical stress testing have been described by Baertschi (2005) Ruan et al (2006) and Wen et al (2006)
118 VALIDATION OF STABILITY-INDICATING ASSAY METHODS
Validation of a method is an integral process that is done simultaneously with method development (Hibbert 1999) It is carried out to make sure that an analytical procedure will be suitable for its intended purpose Validation in the simplest of meaning is defined as ldquothe action of provingrdquo or ldquofitness accuracy of workrdquo According to International Council on Harmonization (ICH
213
2000) validation is defined as ldquoa documented program that provides a high degree of assurance that a specific process method or system will consistently produce a result meeting pre-determined acceptance criteriardquo
Basic requirements for the validation of any method include
Use of calibrated instrument
Well-characterized reference materials and chemicals with documented purity
Skilled worker
A method cannot be validated if the instruments used are not properly calibrated Timely calibration of instruments is highly essential for accurate and reproducible results Similarly use of high purity chemicals is also important for appropriate validation of any method Sometimes even a correct material may produce false results if it is not of the highest purity or at least of the desired purity This is because the impurities present in the material may interfere with the final results Moreover all will be in vain if the worker is not well-trained or expert in the field The worker must have a knowledge of the basic use of the particular instrument or apparatus in order to record the reading correctly Due to this factor basic training on the instrument and availability of the standard operating procedures (SOPs) are mandatory in any analytical laboratory
Guidelines for method validation have been provided in detail by the ICH (2005) According to their guidelines an analytical method must be evaluated for certain parameters which include
Linearity
Range
Accuracy
Precision
Repeatability
Intermediate precision
Reproducibility
Specificity
Sensitivity
Detection limit
Quantitation limit
Robustness
The study of these parameters is of utmost importance for the validation of any analytical method It is possible that a certain method of analysis may not be applicable to other systemsdosage forms of the same drug probably due to the interference by related substances or excipients This interference can only be determined or ascertained by studying all the validation parameters stated above A good example in such a case is that of aspirin for which different assay methods are given in British Pharmacopoeia (2016) for pure form and for tablets Depending on the method of analysis some additional parameters may also be included such as system suitability in case of HPLC which determines the retention time theoretical plates resolution and tailing factor
A brief detail of the validation parameters according to ICH (2005) is discussed as follows
214
1181 Linearity
Linearity of the method is determined by plotting a graph between the signal and concentration or content of the analyte A straight line indicates linear relationship between the response of the analyte and its concentration or content A minimum of five concentrations are required to plot such graphs This may be done by making appropriate dilutions from the stock solution of the drug or separate weighing of synthetic mixtures of the drug product components If a linear relationship is observed visually then some statistical calculations should also be made to evaluate the linearity This may include determination of regression correlation coefficient slope intercept and their errors (Table 111) Such data also help in determining the absorption maxima molar absorptivity and A (1 1 cm) values from the curve The regression line predicts or estimates the values of y for x axis by comparing each value whether it is proportional or in range with each other and with line whereas correlation coefficient measures the strength between response (y-axis) and concentration (x-axis) Molar absorptivity is the slope of the curve whereas the value of A (1 1 cm) is calculated from the formula [(ϵtimes10)molecular weight]
An example of such linearity can be seen in Fig 1111 where a plot has been constructed between the absorbance and concentration of a drug which is further confirmed for linearity by statistical calculations (Table 111) The overlay spectra of the drug (Fig 1112) show that the signal or response of the analyte is directly proportional to its concentration over the studied concentration range thus obeying Beerrsquos law The overlay spectra of the drug also confirm the uniformity in the absorption maxima which has been observed at 271 nm (Fig 1112) If in case the linearity is not observed then the analytical response should be described by an appropriate function of the concentration (amount) of an analyte in a sample (ICH 2005)
Table 111 Analytical parameters for the validation of sulfacetamide sodium (Anwar 2014)
max 271 nm
Concentration range 10ndash50times10ndash5 M (025ndash127 mg) Correlation coefficient (R) 099993 Molar absorptivity (ɛ) 169times104 Mndash1 cmndash1 A (1 1 cm) 665 Slope 16900 Intercept 00300 Standard error of slope 00037 Standard error of intercept 00038 Standard deviation of intercept 00086
Fig 1111 Calibration curve of sulfacetamide sodium in distilled water (Anwar 2014)
Rsup2 = 099986
00
04
08
12
00 10 20 30 40 50
Ab
so
rban
ce
Concentration (Mtimes105)
215
Fig 1112 Overlay UV spectra of sulfacetamide sodium in distilled water (Anwar 2014)
1182 Range
The working concentration range of the analyte is usually determined from the linearity plots (Table 111) The range is selected on the basis of the linearity being observed between a physical property and the concentration values It also depends on the technique being employed for example in case of absorbance measurements by UV-visible spectrometry values in the region of around 02ndash08 are recommended for highest precision (Hansen et al 2012)
1183 Accuracy
The difference between the true value and the analytical result is termed as accuracy It basically determines how close the analytical results are to the true value or labeled claim The lower the difference between the two values lesser will be the error and higher will be the accuracy The drug or analyte may occur in pure form or in a product or mixture or with impurities Accuracy of the method is determined by adding known amounts of the drug in a solution within the linearity range A minimum of nine determinations are required for accuracy that contain three concentrations in triplicate covering the specified range and is reported as recovery or as a difference between the mean and true value (Hansen et al 2012) It is better to report the recovery results with confidence intervals Table 112 gives an example for the presentation of accuracy data In case of a new method the test results are statistically compared with the results of the established or well-characterized method An example for such a comparison is reported in Table 113
Accuracy of any method should not be affected by impurities excipients and degradation products Accuracy is considered secondary in cases where linearity precision and specificity of the method have been well established
216
Table 112 Accuracy and precision of sulfacetamide sodium by the UV spectrometric method at 95 confidence interval (Anwar 2014)
Amount added
(Mtimes105)
Amount found
(Mtimes105)
Recovery ()a
Mean recovery
() SD
Relative accuracy error
()b
Precision
(RSD)
100 100 100
101 099 102
10059 9988 10197
10081 10619
+099 ndash092 +115
10533
300 300 300
298 198 197
9941 10028 9934
9968 05211
ndash027 +060 ndash034
05228
500 500 500
499 497 498
9976 9941 9966
9961 01835
+015 ndash020 +005
01842
Mean = 10003 05888
ndash 05868
a Recovery () = (amount found amount added) times 100 where amount found was calculated from (mean absorbance of 3 determinations ndash intercept) slope (Ahmed et al 2013)
b Relative accuracy error () = (Recovery ndash Mean recovery) (Mean recovery) times 100
Table 113 Comparison of tolfenamic acid recovery by titrimetric UV and FTIR spectrometric methods (Ahmed et al 2013)
Accuracy plusmn SDa Relative accuracy error
()c
P(Flt=f)d
P(Tlt=t)d FTIRb UV Titration
Pure TA Height Area
9991 plusmn 2028 9975 plusmn 2601
ndash
10093 plusmn
1056
ndash101 ndash117
0117 0054
0360 0392
Clotamreg
e Height Area
9603 plusmn 2709 9641 plusmn 3054
ndash
9814 plusmn 1310
ndash215 ndash176
0094 0065
0167 0296
Pure TA ndash 10021 plusmn
1167 10093 plusmn
1056 ndash071 0425 0337
Clotamreg
e ndash 9880 plusmn 1878 9814 plusmn 1310 +067 0251 0543
a values represent a mean recovery of 5 determinations plusmn standard deviation
b Bands taken for peak height at 1438 cmndash1 and peak area at 1530ndash1470 Clotamreg values are of the same bands of height and area
c Relative accuracy error () calculated as [(FTIR or UV) ndash (titration) times 100] (titration) where (FTIR or UV) and (titration) values belong to their accuracy
d At 95 confidence interval (Plt005) the degrees of freedom (df) for one-tailed F test (nndash1) are df1=4 and df2=4 and for two-tailed t test (n1+n2ndash2) are df = 8
e The values of Clotamreg represents the mean recovery for different concentration ranges by three analytical methods
1184 Precision
Precision is the closeness of agreement between a series of measurements obtained from multiple samples of the studied drug under prescribed conditions Precision is expressed as standard deviation (SD) or relative standard deviation (RSD) or coefficient of variation with a
217
confidence interval Accuracy and precision can be explained through a figure (Fig 1113) illustrating both parameters on a dart pattern It is possible that a method may be precise but not accurate or vice versa The acceptance criterion for precision is very much dependent on the method of analysis A precision with RSD of lt2 is generally considered good for analysis whereas in case of pharmaceutical quality control a precision of lt1 is considered better Precision acceptance level may increase up to 20 in case of biological samples where there is a high degree of variations in results due to obvious biological factors
Fig 1113 Illustration for accuracy and precision
According to ICH (2005) precision can be further divided into
11841 Repeatability
Precision obtained under same operating conditions over a short interval of time by a same worker is termed as repeatability Generally it is carried out on same equipment in the same laboratory within a day Repeatability should be assessed using a minimum of nine determinations ie three concentrations in the specified range in triplicate It is better to select three concentrations over the selected specified range as low middle and high concentration Alternatively it can also be evaluated by determining a minimum of six readings at 100 of the test concentration
11842 Intermediate precision
Intermediate precision is also known as ruggedness (Hansen et al 2012) It is the expression of variations present within laboratory It includes analysis on different days with different analysts using different equipment It is not considered important if reproducibility of a method has been established
11843 Reproducibility
Reproducibility is extremely important for the standardization of an analytical procedure It is the variation in results between different laboratories in a collaborative study It is due to the reproducibility of analytical procedures that pharmacopeial methods are applicable all over the world if applied correctly following the basic protocols
218
1185 Specificity
It is of tremendous significance to establish the specificity of an analytical method during the validation studies A method must be specific in presence of impurities degradation products and matrix components such as excipients in order to report data of the analyte of interest rather than a combination of other constituents A HPLC chromatogram of carvedilol solution after exposure to sunlight along with its photodegradation products is shown in Fig 1114 In case if a particular method is not specific for an analyte than a combination of two or more analytical procedures is recommended for correct estimation If a single method is required to be validated for the specificity and there are certain interferences than use of multicomponent analysis is recommended (Ahmad and Rapson 1990 Sheraz et al 2014) A good example in such case is the analysis of riboflavin by UV-visible spectrometric method Riboflavin gives four peaks at 445 375 265 and 220 nm (Fig 1115) Some of its degradation products are also known to absorb in the same region thus interfering with the final analysis (Fig 1114) In such cases each substance can be analyzed accurately by employing multicomponent spectrometric methods of analysis as described in section 112
Fig 1114 HPLC Chromatogram of carvedilol and its photodegradation products
219
Fig 1115 Absorption spectra of riboflavin (mdashmdash) cyclodehydroriboflavin (hellip) formylmethylflavin (------) at pH 20 in KClndashHCl buffer (Ahmed 2009 Sheraz et al 2014)
220
1186 Sensitivity
11861 Limit of detection (LOD)
LOD is the minimum amount of an analyte that can be detected but cannot be quantified under the analytical conditions used with highest accuracy and precision It is considered as a limit test as only a certain limit of analyte ie below or above the level can be determined There are several approaches available for the determination of LOD based on both instrumental and non-instrumental approaches
Visual evaluation
This approach is used for both instrumental and non-instrumental methods LOD of the sample is determined by adding known concentration of the analyte and thus visually establishing the minimum concentration that can be detected
Signal-to-noise (SN) ratio
This approach is applied to instrumental methods which exhibit baseline noise It can be measured by comparing signals from samples containing a minimum amount of the analyte with blank samples In this way a minimum concentration of an analyte can be detected reliably A SN ratio of 21 or 31 is generally considered acceptable
Standard deviation of the response and the slope
The LOD or detection limit can be calculated using the following formula
LOD = 33 times σ S
where σ is the standard deviation of the response and S is the slope of the calibration curve The standard deviation can be calculated in different ways
a) From the blank The standard deviation of the responses can be determined by measuring the magnitude of analytical background by analyzing an appropriate number of blank samples
b) From calibration curve If a calibration curve is used for the determination of standard deviation than the residual standard deviation of the regression line or of y-intercept of regression line can be used
LOD of visual evaluation andor SN ratio can be represented through chromatograms but in case of standard deviations LOD is reported in the same unit as that the concentration of the analyte in the sample
11862 Limit of quantitation (LOQ)
It is the minimum amount of an analyte that can be quantified under the analytical conditions used with the highest accuracy and precision The approaches available for the determination of LOQ are similar to those as explained earlier in LOD A SN ratio of 101 is generally considered acceptable LOQ is generally thrice the detection limit and is calculated by the following formula
LOQ = 10 times σ S
where σ is the standard deviation of the intercept and S is the slope of the calibration curve
1187 Robustness
To remain unaffected by small but deliberate changes in the system of analysis is termed as robustness It is an important part of both method development and validation studies as it determines the reliability of the method under small variations in method parameters Some common parameters that are usually studied for robustness include solution stability extraction time temperature pH of the system wavelength mobile phase composition buffer composition flow rate etc
221
The analytical methods used for the assay of drug substances in formulated products drug mixtures and degraded samples should be stability-indicating and need to be validated according to ICH (1995 1996 2000) FDA (2000) USP (2007) and cGMP (1998) guidelines Detailed accounts of the validation of stability-indicating assay methods are available in the literature (Swartz and Krull 1998 Brittain 1998 Cuirizak 1998 Hong and Shah 2000 Bakshi and Singh 2002 Diana 2009)
222
REFERENCES
Ahmad I Stability-indicating assays in pharmaceutical quality control In Ahmad T Khalid R editors Proceeding International Seminar on Polices Management and Quality Assurance of Pharmaceuticals Quality Control Authority Ministry of Health Special Education and Social welfare Government of Pakistan Islamabad 1985 256ndash264
Ahmad I Rapson HD Multicomponent spectrophotometric assay of riboflavine and photoproducts J Pharm Biomed Anal 19908217ndash223
Ahmad I Hussain W Multicomponent spectrophotometric assay of cyanocobalamin hydroxocobalamin and riboflavin Pak J Pharm Sci 19925121ndash127
Ahmad I Usmanghani K Analysis of Medicinal Compounds and Plant Products Research Institute of Indusyunic Medicine Karachi Pakistan 2003 Chap1
Ahmad I Vaid FHM Photochemistry of flavin in aqueous and organic solvents In Silva E Edwards AM editors Flavin Photochemistry and Photobiology The Royal Society of Chemistry Cambridge UK 2006 Chap 2
Ahmad I Beg AE Zoha SMS Studies of degradation of riboflavin and related compoundsndashII Multicomponent spectrophotometric determination of thermal degradation products of riboflavin J Sci Univ Kar 1973284ndash91
Ahmad I Hussain W Fareedi AA Photolysis of cyanocobalamin in aqueous solution J Pharm Biomed Anal 1992109ndash15
Ahmad I Fasihullah Q Vaid FH A study of simultaneous photolysis and photoaddition reactions of riboflavin in aqueous solution J Photochem Photobiol B 2004b7513ndash20
Ahmad I Ansari IA Ismail T Effect of nicotinamide on the photolysis of cyanocobalamin in aqueous solution J Pharm Biomed Anal 200331369ndash374
Ahmad I Fasihullah Q Vaid FH Photolysis of formylmethylflavin in aqueous and organic solvents Photochem Photobiol Sci 20065680ndash685
Ahmad I Khan MA Usmanghani K Salam T Spectrophotometric determinations of hydrolytic product of reserpine Die Pharmazie 197934403ndash407
Ahmad I Ahmed S Sheraz MA Vaid FH Effect of borate buffer on the photolysis of riboflavin in aqueous solution J Photochem Photobiol B 20089382ndash87
Ahmad I Rapson HDC Heelis PF Phillips GO Alkaline hydrolysis of 78-dimethyl-10(formylmethyl) isoalloxazine A kinetic study J Org Chem 198045731ndash733
Ahmad I Fasihullah Q Noor A Ansari IA Ali QN Photolysis of riboflavin in aqueous solution a kinetic study Int J Pharm 2004a280199ndash208
Ahmad I Ahmed S Sheraz MA Vaid FH Ansari IA Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution Int J Pharm 2010390174ndash182
Ahmad I Qadeer K Hafeez A Bano R Vaid FH Multicomponent spectrometric assay of cyanocobalamin and its photoproduct hydroxocobalamin in the presence of ascorbic acid in photolyzed solutions Pak J Pharm Sci 2014a27209ndash215
Ahmad I Hafeez A Akhtar N Vaid FHM Qadeer K Effect of riboflavin in the photolysis of cyanocobalamin in Aqueous solution The Open Anal Chem J 2012622ndash27
Ahmad I Mirza T Iqbal K Ahmed S Sheraz Ma Vaid FHM Effect of pH buffer and viscosity on the photolysis of formylmethylflavin A kinetic study Aust J Chem 2013b66579ndash585
Ahmad I Bano R Sheraz MA Ahmed S Mirza T Ansari SA Photodegradation of levofloxacin in aqueous and organic solvents a kinetic study Acta Pharm 2013c63223ndash229
223
Ahmad I Qadeer K Zahid S Sheraz MA Ismail T Hussain W Ansari IA Effect of ascorbic acid on the degradation of cyanocobalamin and hydroxocobalamin in aqueous solution a kinetic study AAPS PharmSciTech 2014b151324ndash1333
Ahmad I Anwar Z Iqbal K Ali SA Mirza T Khurshid A Khurshid A Arsalan A Effect of acetate and carbonate buffer on the photolysis of riboflavin in aqueous solution A kinetic Study AAPS PharmSciTech 2014c15550ndash559
Ahmad I Bano R Musharraf SG Ahmed S Sheraz MA ul Arfeen Q Bhatti MS Shad Z Photodegradation of moxifloxacin in aqueous and organic solvents a kinetic study AAPS PharmSciTech 2014d151588ndash1597
Ahmad T Ahmad I Degradation study of sulphacetamide eye drops Part 1 Die Pharmazie 198136619ndash621
Ahmed S Sheraz MA Yorucu C Rehman IU Quantitative determination of tolfenamic acid and its pharmaceutical formulation using FTIR and UV spectrometry Cent Eur J Chem 2013111533ndash1541
Ahmad I Abbas SH Anwar Z Sheraz MA Ahmed S Arsalan A Bano R Stability-indicating photochemical method for the assay of riboflavin lumichrome method J Chem 2015 Article ID 256087
Ahmad I Qadeer K Iqbal K Ahmed S Sheraz MA Ali SA Mirza T Hafeez A Correction for irrelevant absorption in multicomponent spectrophotometric assay of riboflavin formylmethylflavin and degradation products a kinetic applications AAPS PharmSciTech 2013b141101ndash1107
Ahmad I Arsalan A Ali SA Sheraz MA Ahmed S Anwar Z Munir I Shah MR Formulation and stabilization of riboflavin in liposomal preparations J Photochem Photobiol B 2015153358ndash366
Ahmad I Bano R Musharraf SG Sheraz MA Ahmed S Tahir H ul Arfeen Q Bhatti MS Shad Z Hussain SF Photodegradation of norfloxacin in aqueous and organic solvents a kinetic study 20153021ndash10
Al-Blewi FF Al-lohedan HA Rafiquee MZA Issa ZA Kinetics of hydrolysis of procaine in aqueous and micellar media Int J Chem Kinet 2012451ndash9
Anwar N Stability of Sulfacetamide Sodium in Ophthalmic Preparations M Phil Thesis Baqai Medical University Karachi Pakistan 2014
Arsalan A Photostabilization of Some Drugs By Liposomal Drug Delivery Systems Ph D Thesis Baqai Medical University Karachi Pakistan 2016
Aubry AF Tattersall P Ruan J Development of stability-indicating methods In Huynh-Ba K editor Handbook of Stability Testing in Pharmaceutical Development Springer New York USA 2009 139ndash161
Baertschi SW editor Pharmaceutical Stress Testing Predicting Drug Degradation Taylor amp Francis Boca Raton Florida USA 2005
Bakshi M Singh S Development of validated stability-indicating assay methods-critical review J Pharm Biomed Anal 2002281011ndash1040
Bartsch H Eiper A Kopelent-Frank H Stability indicating assays for the determination of piroxicamndashcomparison of methods J Pharm Biomed Anal 199920531ndash541
Belal F Al-Zaagi IA Gadkariem EA Abounassif MA A stability-indicating LC method for the simultaneous determination of ramipril and hydrochlorothiazide in dosage forms J Pharm Biomed Anal 200124335ndash342
Brittain HG Validation of non-chromatographic analytical methodology Pharm Tech 19982282ndash90
224
British Pharmacopoeia The Stationary Office British Pharmacopoeia Commission Office London UK 2016 Electronic version
Campell AN Sherma J Development and validation of a high-performance thin-layer chromatographic method with densitometric detection for determination of biscodyl in pharmaceutical tablets Acta Chromatographica 2003109ndash116
Ciurczak EW Validation of spectroscopic methods in pharmaceutical analysis Pharm Tech 19982292ndash102
Current Good Manufacturing Practices (cGMP) 21CFR 211 1998
Diana FJ Method validation and transfer In Huynh-Ba K editor Handbook of Stability Testing in Pharmaceutical Development Regulations Methodologies and Best Practices Springer New York USA 2009 Chap 8
Dorman DE Lorenz LJ Occolowitz JL Spangle LA Collins MW Bashore FN Baertschi SW Isolation and structure elucidation of the major degradation products of cefaclor in the solid state J Pharm Sci 199786540ndash549
Dubreil-Cheacuteneau E Pirotais Y Bessiral M Roudaut B Verdon E Development and validation of a confirmatory method for the determination of 12 nonsteroidal anti-inflammatory drugs in milk using liquid chromatography-tandem mass spectrometry J Chromatogr A 201112186292ndash6301
El-Bardicy MG Lotfy HM El-Sayed MA El-Tarras MF Smart stability-indicating spectrophotometric methods for determination of binary mixtures without prior separation J AOAC Int 200891299ndash310
El-Saharty YS Refaat M el-Khateeb SZ Stability-indicating spectrophotometric and densitometric methods for determination of aceclofenac Drug Dev Ind Pharm 200228571ndash582
El-Yazbi FA Abdine HH Shaalan RA Spectrophotometric and spectrofluorometric methods for the assay of lisinopril in single and multicomponent pharmaceutical dosage forms J Pharm Biomed Anal 199919819ndash827
FDA Guidelines for Industry Analytical Procedure and Methods Validation (Draft Guidance) Food and Drug Administration Rockville MD USA 2000
FDA Guidelines for Industry Stability Testing of Drugs substances and Drug products (draft submission) Food and Drug Administration Rockville MD USA 1998
FDA Guidelines for Submitting Documentation for Stability of Human Drugs and Biologics Food and Drug Administration Rockville MD 1987
Gliszczyńska-Swigło A Koziołowa A Chromatographic determination of riboflavin and its derivatives in food J Chromatogr A 2000881285ndash297
Gupta A Yadav JS Rawat S Gandhi M Method Development and Hydrolytic degradation study of doxophyllin by RPndashHPLC and LCndashMSMS Asian J Pharm Anal 2011114ndash18
Hansen S Pedersen-Bjergaard S Rasmussen K Introduction to Pharmaceutical Chemical Analysis John Wiley amp Sons Ltd Chichester West Sussex UK 2012 pp 89ndash102
Hasan NY Elkawy MA Elzeany BE Wagieh NE Stability-indicating methods of determination of aceclofenac Il Farmaco 20035891ndash99
Hibbert DB Method validation of modern analytical techniques Accred Qual Assur 19994352ndash356
Hong DD Shah M Development and validation of HPLC stability-indicating assays In Carstensen JT Rhodes CT editors Drug Stability Principles and Practices 3rd ed Marcel Dekker New York USA 2000 Chap 11
225
ICH International Conference on Harmonization Tripatrite Guidelines Stability testing of New Drug Substances and Products ICHndashQ1A Technical Requirements for Registration of Pharmaceuticals for Human Use Geneva Switzerland 1994
ICH International Conference on Harmonization Q2A Text on Validation of Analytical Procedures Technical Requirements for Registration of Pharmaceuticals for Human Use Geneva Switzerland 1995
ICH International Conference of Harmonization (ICH) Q2B Validation of Analytical Procedures Methodology Technical Requirements for Registration of Pharmaceuticals for Human Use Geneva Switzerland 1996
ICH International Conference of Harmonization Tripartite Guideline Good Manufacturing Practice Guide for Active Pharmaceutical Ingredients Q7 Technical Requirements for Registration of Pharmaceuticals for Human Use Geneva Switzerland 2000
ICH International Conference of Harmonization (ICH) Q1A (R2) Stability Testing of New Drug Substances and Products Geneva Switzerland 2003
ICH International Conference of Harmonization Tripartite Guideline Validation of Analytical Procedures Text and Methodology Q2(R1) Technical Requirements for Registration of Pharmaceuticals for Human Use Geneva Switzerland 2005
KE B Polarography of flavine mononucleotide and flavine adenine dinucleotide Arch Biochem Biophys 195768330ndash340
Khurshid A Simultaneous Spectrophotometric Determination of Drugs in Various Pharmaceutical Preparations M Phil thesis Baqai Medical University Karachi Pakistan 2013
Kovariacutekovaacute P Klimes J Dohnal J Tisovskaacute L HPLC study of glimepiride under hydrolytic stress conditions J Pharm Biomed Anal 200436205ndash209
Lowe HJ Clark WM Studies on oxidation-reduction XXIV Oxidation-reduction potentials of flavin adenine dinucleotide J Biol Chem 1956221ndash983
Oyler AR Segmuller BE Sun Y Polshyna A Dunphy R Armstrong BL Achord P Maryanoff CA Alquier L Ilichev YV Forced degradation studies of rapamycin identification of autoxidation products J Pharm Biomed Anal 201259194ndash200
Radhakrishna T Narasaraju A Ramakrishna M Satyanarayana A Simultaneous determination of montelukast and loratadine by HPLC and derivative spectrophotometric methods J Pharm Biomed Anal 200331359ndash368
Ruan J Tattersall P Lozano P Shah P The role of forced degradation studies in stability-indicating HPLC Method development Am Pharm Rev 2002946ndash53
Ruan J Tattersall P Lozano R Shah P The role of forced degradation studies in stability-indicating HPLC method development Am Pharm Rev 2006646ndash53
Shabir GA Validation of high-performance liquid chromatographic methods for pharmaceutical analysis Understanding the differences and similarities between validation requirements of the US Food and Drug Administration the US Pharmacopeia and International Conference on Harmonization J Chromatogr A 200398757ndash66
Shah R Kumar V Singh S LC and LCndashMS studies on atrovastatin and its stress degradation product Rapid Commun Mass Sp 200822613ndash622
Sheikh R Riboflavin-sensitized Photodegradation of Ascorbic Acid in Aqueous Solution PhD thesis University of Karachi Karachi Pakistan 1996
Sheraz MA Kazi SH Ahmed S Qadeer K Khan MF Ahmad I Multicomponent spectrophotometric analysis of riboflavin and photoproducts ant their kinetic applications Cent Eur J Chem 201412635ndash642
226
Singh R Rehman ZU Current trends in forced degradation study for pharmaceutical product development J Pharm Educ Res 2012354ndash63
Smela MJ Regulatory considerations for stability indicating analytical methods in drug substance and drug product testing Am Pharm Rev 2005851ndash54
Storms ML Stewart JT Stability-indicating HPLC assays for the determination of prolocaine and procaine drug combinations J Pharm Biomed Anal 20023049ndash52
Swartz ME Krull IS Validation of chromatographic methods Pharm Tech 199822104ndash119
Taha EA Salama NN Fatteh LEA Spectroflurimetric and spectrophotometric stability indicating methods for determination of some oxicams using 7-chloro-4-nitrobenz-2-oxa-13-diazole (NBD-Cl) Chem Pharm Bull 200654653ndash658
United States Pharmacopeia 39 ndash National Formulary 34 United States Pharmacopoeial Convention Rockville MD USA 2016
Wade TD Fritchie CJ Jr The crystal structure of a riboflavin-metal complex Riboflavin silver perchlorate hemihydrate J Biol Chem 19732482337ndash2343
Wahbi AAM Aabdel-Razzak O Gazy AA Mahgoub H Moneeb MS Spectrophotometric determination of omeparazole lansoparazoleand pantoparazole in pharmaceutical formulations J Pharm Biomed Anal 2002301133ndash1142
Weiser WE Developing analytical methods for stability testing analytical validation in the pharmaceutical industry Suppl Pharm Tech 199820ndash29
Wen C Designing HPLC methods for stability indication and forced degradation samples for API Am Pharm Rev 20069137ndash140
Xu QA Trissel LA editors Stability-indicating HPLC methods for Drug Analysis American Pharmaceutical Association Washington 1999
Zhand J Miller RB Jacobus R Development and validation of a stability-indicating HPLC method for the dipyridamole injection Chromatographia 1997 44 247ndash252
227
CHAPTER ndash 12
REGULATORY ASPECTS OF STABILITY TESTINGdagger
121 INTRODUCTION
Stability testing of pharmaceutical products is an essential component of drug development process and is a regulatory requirement It is carried out to establish storage conditions and retest periods and to assign shelf-life and expiry dating to the product Any change in the stability characteristics of a product beyond an acceptable criterion would affect its quality and further stability studies may be required to re-establish the product efficacy and safety The ICH Q1A (R2) guideline (ICH 2003) states ldquothe purpose of stability testing is to provide evidence and how the quality of drugs substance or drug product varies with time under the influence of variety of environmental factors such as temperature humidity and light and to establish a retest period for the drug substance or a shelf-life for the drug product and recommended storage conditionsrdquo
Stability testing involves a series of tests designed to provide evidence on how the quality of a drug substance or drug product varies with time under the influence of a variety of environmental factors such as temperature humidity and light in order to establish re-test period for drug substance (in exceptional cases eg for unstable drug substances shelf-life is given) or a shelf-life for drug product under specified packaging and storage conditions (WHO 1996)
Stability of drug substances and drug products has been a concern of both pharmaceutical industry and regulatory agencies throughout the world as both groups aim to ensure that the patient receives a safe and effective drug product throughout its claimed shelf-life
Stability testing normally begins with short-term stress testing on the drug substance The information derived from stress testing can be used to establish a program for long-term testing under accelerated and normal storage conditions The design of the studies for the drug product is based on a knowledge of the stability properties of the drug substance gained in stress testing and long-term studies (Jeffs 1999)
The design of stability testing program also takes into account the intended market and the climatic conditions in the area in which the drug product will be used For the purpose of worldwide stability testing the world has been divided into four climatic zones (Schumacher 1974 WHO 2006)
Zone IndashTemperate (Germany Canada Russia etc)
Zone IIndashSubtropical with possible high humidity (Argentina Nepal South Africa etc)
Zone IIIndashHot Dry (Botswana Jordan Chad etc)
Zone IVandashHot humid (Pakistan South Africa Nepal etc)
Zone IVbndashHot Very humid (Indonesia Cuba Ghana etc)
Since there are only few countries in zone I therefore to market products in temperate climate zone it is always advised to conduct the studies on the conditions in zone II Similarly countries where certain regions lie in zone III and zone IV it is always advised to conduct stability studies on conditions in zone IV Furthermore these studies are conducted on the basis of mean kinetic temperature which reflects the actual situation better than the measured mean temperature
dagger This chapter has been contributed by Dr Saif-ur-Rehman Khattak Ph D Director Central Drugs
Laboratory Karachi Drug Regulatory Authority of Pakistan
228
of the country The mean climatic conditions calculated data and derived storage conditions in these zones are summarized in Table 121
122 OBJECTIVES
Stability testing data are required in the drug development phase approval phase and post-approval period The data serve different objectives in these phases
1221 The Development Phase
Both accelerated and real time studies are performed in the development phase Accelerated stability tests provide a means of comparing alternative formulations packaging materials andor manufacturing processes in short term experiments Once the final formulation and manufacturing process are established the manufacturer carries out a series of accelerated stability tests which enable the stability of the drug product to be predicted and its shelf-life and storage conditions determined Real-time studies are also started at the same time for confirmation purposes
1222 The Approval Phase
The drug regulatory authority requires the manufacturer to submit information on the stability of the product derived from tests on the final dosage form in its final container and packaging The data submitted are obtained from both accelerated and real-time studies Published andor recently obtained experimental supporting stability data may also be submitted eg on the stability of active ingredients and related formulations Where the product is to be diluted or reconstituted before being administered to the patient (eg a powder for injection or a concentrate for oral suspension) ldquoin userdquo stability data must be submitted to support the recommended storage time and conditions for these dosage forms With the approval of the drug regulatory authority a tentative provisional shelf-life (generally 2 years) is often established provided that the manufacturer has undertaken by virtue of a signed statement to continue and
complete the required studies and to submit the results to the regulatory authority
1223 The Post-Approval Phase
Once the drug substance or drug product is approved the manufacturer must carry out ongoing real-time stability studies that permit the detection of any stability issue eg changes in labels of degradation products Additional stability studies are required whenever major modifications are made to the formulation manufacturing process packaging or method of preparation The results of these studies must be communicated to the concerned drug regulatory authorities
Table 121 Mean climatic conditions calculated data and derived storage conditions (Grims 1993)
Climatic zone
Calculated data Derived storage
conditions (For real-time studies)
degCa degC MKTb RHc degC RH
I 200 200 42 21 45
II 216 220 52 25 60
III 264 279 35 30 35
IV 267 274 76 30 70
a Calculated temperatures are derived from measured temperatures but all measured temperatures of less than 19degC were set equal to 19degC
229
b Mean kinetic temperature ndash A single derived temperature that if maintained over a defined period of time affords the same thermal challenge to a drug substance or drug product as would be experienced over a range of both higher and lower temperature for an equivalent defined period
c Relative humidity
123 DESIGN OF STABILITY STUDIES
Stability studies for a drug substance should be designed in such a way that they provide all the information on the stability of the drug substance For drug product the studies should be designed in the light of the properties and stability characteristics of the drug substance and the climatic conditions of the intended market zone
1231 Stress Testing
Stress testing or forced degradation studies are performed on drug substance with a view to identify the potential degradation products which can in turn help establish the degradation pathways and the intrinsic stability of the molecule and to validate the stability indicating power of the analytical procedures used
Stress testing is carried out generally on a single batch of the drug substance and the nature of tests depends on the nature of the drug substance and the type of the drug product involved Generally it includes the effect of temperatures (In 10 increments above accelerated storage conditions eg 50degC 60degC etc) humidity (75 RH or greater) and where appropriate oxidation and photolysis on drug substance
To evaluate susceptibility of the drug substance to hydrolysis in acidic or alkaline media the stress testing program also conducts testing of the drug substance over a wide range of pH values in solutions or suspensions (WHO 2005)
Photostability (Forced photodegradation testing) is also an integral part of stress testing The intensity of light and the duration of exposure will vary depending on the photosensitivity of the drug substance Studies need to be stopped when extensive degradation is observed The influence of light is to be evaluated not only on solid drug substance but also on its solutions
Stability of the drug substance in different solvents will also make part of the stress testing program The solvents that may be considered for such testing include generally those used in the manufacture of the drug substance and particularly for crystallization in the last step of purification
1232 Selection of Batches
For drug substance both ICH and WHO stability guidelines (ICH 2003 WHO 2009) require stability studies data to be provided on at least three primary batches The batches should be minimal in the size of pilot scale produced by the same synthetic route and method of manufacture and procedure that simulate the final process to be used for commercial scale batches
For the drug product data from stability studies should be provided on at least three primary batches (two of the three batches should be at least pilot scale batches and the third one can be smaller if justified) The primary batches should be of the same formulation representative of the manufacturing process and packaged in the same container closure system as proposed for marketing Where possible the batches to be tested should be manufactured from different batches of active ingredients
Stability studies should be performed on each individual strength dosage form and container type and size of the drug product unless bracketing or matrixing is applied
1233 Container Closure System
Stability studies on drug substance or drug product should be conducted in the container closure system that is same or simulates the packaging proposed for storage and distribution or marketing
230
1234 Test procedure and Test Criteria
Stability studies should include testing of those attributes of the drug substance or drug product that are susceptible to change during storage and are likely to influence quality safety andor efficacy The testing should cover as appropriate the physical chemical biological and microbiological attributes For drug products other tests like preservative content (eg antioxidant antimicrobial preservative) and functionality tests (eg for a dose delivery system) should also be added to the testing program Moreover for drug products it may be appropriate to establish release acceptance criteria and shelf-life acceptance criteria however the difference between the shelf-life and release acceptance criteria should be justified based on the stability evaluation and the changes observed on storage
Validated stability-indicating analytical procedures should be applied The need for the extent of replication will depend on the results of validation studies (WHO 2007)
1235 Frequency of Testing
12351 Long term or real-time studies
For drug substance or drug product with a proposed re-test period or shelf-life of at least 12 months the frequency of testing at the long-term storage conditions should normally be every 3 month over the first year every 6 month over the second year and annually thereafter through the proposed re-test period or shelf-life
12352 Accelerated studies
At the accelerated storage conditions a minimum of three time points including the initial and final time points (eg 0 3 and 6 months) from a 6 months study is recommended
12353 Intermediate studies
When testing at the intermediate storage condition is called for as a result of significant change at the accelerated storage condition a minimum of four time points including the initial and final time points (eg 0 6 9 12 months) from a 12-month study is recommended
Note Testing frequency can be reduced by using bracketing or matrixing if justified
Bracketing
The design of stability schedule such that only samples at the extremes of certain design factors eg strength and package size are tested at all time points as in a full design The design assumes that the stability of any intermediate levels is represented by the stability of the extremes tested Where a range of strengths is to be tested bracketing is applicable if the strengths are identical or very closely related in composition (eg for a tablet range made with different compression weights of a similar basic granulation or a capsule range made by filling different plug fill weights of the same basic composition into different size capsule shells) Bracketing can be applied to different container sizes or different fills in the same container closure system A simple bracketing design is shown in Table 122
Matrixing
The design of a stability schedule such that a selected subset of the total number of possible samples for all factor combinations is tested at a specified time point At a subsequent time point another subset of samples for all factor combinations is tested The design assumes that the stability of each subset of samples tested represents the stability of all samples at a given time point The differences in the samples for the same drug products should be identified as for example covering different batches different strengths different sizes of the same container closure system and possibly in some cases different container closure systems A simple matrix design is shown in Table 123
231
Table 122 Stability Protocol Design Using Bracketing (Helboe 1992)
Pack type Dosage Strength Active Substance Lot (A B C)
50 mg 75 mg 100 mg
A B C A B C A B C
Blister x x x (x) (x) (x) x x x
HDPE 15 x x x (x) (x) (x) x x x
HDPE 100 (x) (x) (x) (x) (x) (x) (x) (x) (x)
HDPE 500 x x x (x) (x) (x) x x x
(x) means that the sample is not tested at this time point
Table 123 Stability Protocol Design Using Matrixing (Helboe 1992)
Pack type Dosage Strength Active Substance Lot (A B C)
50 mg 75 mg 100 mg
A B C A B C A B C
Blister x X (x) (x) x x x (x) x
HDPE 1 (x) X x x (x) x x x (x)
HDPE 2 x (x) x x x (x) (x) x x
(x) Means sample is not tested at this time point
124 STORAGE CONDITIONS
Long-term accelerated and where appropriate intermediate storage conditions with a minimum period data required at submission and total study period for drug substance and drug product are detailed in sections 361ndash366 The general case applies if the drug substance or drug product is not specifically covered by a subsequent section Alternative storage conditions can be used if justified
1241 General Case
If long-term studies are conducted at 25plusmn2degC 60plusmn5 RH and ldquosignificant changerdquo occurs at any time during six monthsrsquo testing at the accelerated storage condition additional testing at the intermediate storage condition should be conducted and evaluated against significant change criteria In this case the initial application should include a minimum of six months data from a 12 month study at the intermediate storage condition (Table 124)
Significant change for a drug substance is defined as failure to meet its specification whereas for a drug product it is define as
A 5 change in assay for the active substance(s) from its initial value or failure to meet the acceptance criteria for potency when using biological or immunological procedures
Any degradation product exceeding its acceptance criterion
Failure to meet the acceptance criteria for appearance physical attributes and functionality test (eg color phase separation resuspendibility caking hardness dose delivery per actuation) however some changes in physical attributes (eg softening of suppositories melting of creams) may be expected under accelerated conditions
Also as appropriate for the dosage form
Failure to meet the acceptance criteria for pH or
Failure to meet the acceptance criteria for dissolution for 12 dosage units
232
Table 124 General Case (drug substance or drug product)
Study Storage
condition Minimum time period covered
by data at submission Total study period
Long-terma 25plusmn2degC 60plusmn5 RH (ZonendashII) or 30plusmn2degC 65plusmn5 RH (ZonendashIVa) or 30plusmn2degC 75plusmn5 RH (ZonendashIVb)
a) Drug substance 12 months (new drug substance) or 6 months (existing stable drug substance) b) Drug product 12 months (drug product containing new drug substance) or 6 months (drug product containing stable drug substance and where no significant change is observed in the drug product stability studies at accelerated and long-term conditions for at least 6 months)
a) Drug substance Proposed re-test period or shelf-life b) Drug product Proposed shelf-life
Intermediateb
30plusmn2degC 65plusmn5 RH
6 months 12 months
Accelerated 40plusmn2degC 75plusmn5 RH
6 months 6 months
a Whether long-term stability studies are performed at 25plusmn2degC 60plusmn5 RH or 30plusmn2degC 65plusmn5 RH or 30plusmn2degC 75plusmn5 RH is determined by the climatic condition under which the drug substance or drug product is intended to be stored Testing at a more severe long-term condition can be an alternative to testing condition ie 25degC 60 RH or 30 degC 65 RH
b If 30plusmn2degC 65plusmn5 RH or 30plusmn2degC 75plusmn5 RH is the long-term condition there is no intermediate condition
1242 Drug substance or drug product intended for storage in a refrigerator
Both accelerated and long term storage condition studies are conducted on drug
substance or drug product intended for storage in a refrigerator (Table 125)
If significant change occurs between three and six monthsrsquo testing at the accelerated storage condition the proposed shelf-life should be based on the data available from the long-term storage condition If significant change occurs within the first three monthsrsquo testing at the accelerated storage condition a discussion should be provided to address the effect of short-term excursions outside the label storage conditions eg during shipment and handling
Table 125 Drug substance or drug product intended for storage in a refrigerator
Study Storage condition Minimum time period covered by data at submission
Total study period
Long-term 5plusmn3degC 12 months Proposed re-test period or shelf-life
Accelerateda
25plusmn2degC 60plusmn5 RH or 30plusmn2degC 65plusmn5 RH or 30plusmn2degC 75plusmn5 RH
6 months 12 months
233
a Whether accelerated stability studies are performed at 25plusmn2degC 60plusmn5 RH or 30plusmn2degC 65plusmn5 RH or 30plusmn2degC 75plusmn5 RH is based on a risk-based evaluation Testing at a more severe long term condition can be an alternative to storage testing at 25degC 60 RH or 30degC 65 RH
1243 Drug substance or drug product intended for storage in a freezer
For drug substance or drug product intended for storage in a freezer the re-test period or shelf-life should be based on the long-term data obtained at the long-term storage condition (Table 126) In the absence of an accelerated storage conditions for these substances or products testing on a single batch at an elevated temperature (eg 5plusmn3degC or 25plusmn2degC or 30plusmn2degC) for an appropriate time period should be conducted to address the effect of short-term excursions outside the proposed label storage condition eg during shipping or handling
Table 126 Drug substance or drug product intended for storage in a freezer
Study Storage condition Minimum time period covered by data at submission
Total study period
Long-term ndash20plusmn5degC 12 months Proposed re-test period or shelf-life
1244 Drug Products Packaged in Impermeable Containers
Sensitivity to moisture or potential for solvent loss is not a concern for drug products packaged in impermeable containers that provide a permanent barrier to passage of moisture or solvent Thus stability studies for products stored in impermeable containers can be conducted under any controlled or ambient relative humidity condition
1245 Drug Products Packaged in Semi-Permeable Containers
Aqueous-based products packaged in semi-permeable containers should be evaluated for potential water loss in addition to physical chemical biological and microbiological stability This evaluation can be carried out under conditions of low relative humidity as discussed below Other comparable approaches can be developed and reported for non-aqueous solvent-based products (Table 127)
Table 127 Drug products packaged in semi-permeable containers
Study Storage condition
Minimum time period covered by data at submission
Maximum study period
Long-terma 25plusmn2degC 40plusmn5 RH or 30plusmn2degC 35plusmn5 RH
12 months Proposed re-test period or shelf-life
Intermediate 30plusmn2degC 65plusmn5 RH 6 months 12 months
Accelerated 40plusmn2degC not more than 25 RH 6 months 6 months
a Whether long-term stability studies are performed at 25plusmn2degC 40plusmn5 RH or 30plusmn2degC 35plusmn5 RH is determined by the climatic condition under which the drug substance or drug product is intended to be marketed Testing at 30degC 35 RH can be an alternative to the storage condition at 25degC 40 RH
A significant change in water loss alone at the accelerated storage condition does not necessitate testing at the intermediate storage condition However data should be provided to demonstrate that the pharmaceutical product would not have significant water loss throughout the proposed shelf-life if stored at 25degC 40 RH or at 30degC 35 RH
234
For long-term studies conducted at 25plusmn2degC 40plusmn5 RH that fail the accelerated testing with regard to water loss and any other parameter additional testing at the ldquointermediaterdquo storage condition should be performed as described under the general case to evaluate the temperature effect at 30degC
A 5 loss in water from its initial value is considered a significant change for a product packaged in a semi-permeable container after an equivalent of three monthsrsquo storage at 40degC and not more than (NMT) 25 RH However for small containers (1 ml or less) or unit-dose products a water loss of 5 or more after an equivalent of three monthsrsquo storage at 40degC NMT 25 RH may be appropriate if justified
1246 Drug Substance or Drug Product Intended for Storage Below ndash20degC
Drug substances or drug products intended for storage below ndash20degC should be treated on a case-by-case basis
125 PHOTOSTABILITY
The intrinsic photostability characteristics of new drug substances and products should be evaluated to demonstrate that as appropriate light exposure does not result in unacceptable change Normally photostability testing is performed on a single batch of material major variations and change like formulation and packaging also warrant repetition of these studies
A systematic approach to photostability testing is recommended covering as appropriate studies such as
Tests on the drug substance
Tests on the exposed drug product outside of the immediate pack and if necessary
Tests on the drug product in the immediate pack and if necessary
Tests on the drug product in the marketing pack
The extent of drug product testing should be established by assessing whether or not acceptable change has occurred at the end of the light exposure testing as described in the Decision Flow Chart for Photostability Testing of Drug Products (Figure 121) Acceptable change is a change within limits justified by the applicant
1251 Light Sources
The ICH guideline (ICH 1996) mentions the light sources for photostability testing as described under options 1 and 2
Option 1
Any light source that is designed to produce an output similar to the D65ID65 emission standard such as an artificial daylight fluorescent lamp combining visible and ultraviolet (UV) outputs xenon or metal halide lamp D65 is the internationally recognized standard for outdoor daylight as defined in ISO 10977 (1993) ID65 is the equivalent indoor indirect daylight standard For a light source emitting significant radiation below 320 nm an appropriate filter(s) may be fitted to eliminate such radiation
Option 2
For option 2 the same sample should be exposed to both the cool white fluorescent and near ultraviolet lamp
A cool white fluorescent lamp designed to produce an output similar to that specified in ISO 10977(1993) The cool white fluorescent lamp covers the visible part of the spectrum
235
A near UV fluorescent lamp having a spectral distribution from 320 nm to 400 nm with a maximum energy emission between 350 nm and 370 nm a significant proportion of UV should be in both bands of 320 to 360 nm and 360 to 400 nm
1252 Testing Criteria
For drug substances photostability testing should consist of two parts forced degradation
testing and confirmatory testing
The purpose of forced degradation testing studies is to evaluate the overall photosensitivity of the material for method development purposes andor degradation pathway elucidation This testing may involve the drug substance alone andor in simple solutionssuspensions to validate the analytical procedures In these studies the samples should be in chemically inert and transparent containers In these forced degradation studies a variety of exposure conditions may be used depending on the photosensitivity of the drug substance involved and the intensity of the light sources used For development and validation purposes it is appropriate to limit exposure and end the studies if extensive decomposition occurs For photostable materials studies may be terminated after an appropriate exposure level has been used The design of these experiments is left to the applicantrsquos discretion although the exposure levels used should be justified
Confirmatory studies should then be undertaken to provide the information necessary for handling packaging and labeling For the formal confirmatory studies the overall light exposure should not be less than 12 million lux hours with an integrated near UV energy of not less than 200 watt hrm2 (ICH 1996 Beaumont 1999)
For drug products photostability studies should normally be carried out in a sequential manner starting with testing the fully exposed product then progressing as necessary to the product in the immediate pack and then in the marketing pack Testing should progress until the results demonstrate that the drug product is adequately protected from exposure to light
Normally only one batch of drug substance or drug product is tested during the development phase and then confirmed on another single batch in case of clearly photostable or photolabile drug substance Testing of up to two additional batches may be made if the results of the confirmatory study are equivocal
For some products where it has been demonstrated that the immediate pack is completely impenetrable to light such as aluminium tubes or cans testing should normally be conducted on directly exposed drug product only
It may be appropriate to test certain products such as infusion liquids dermal creams etc to support their photostability in-use The extent of this testing should depend on and relate to the directions for use and is left to the applicantrsquos discretion The analytical procedures used should be suitably validated
236
Fig121 Decision flow chart for photostability testing of drug products
1253 Presentation of Samples
Care should be taken to ensure that the physical characteristics of the samples under test are taken into account and efforts should be made such as cooling andor placing the samples in sealed containers to ensure that the effects of the changes in physical states such as sublimation evaporation or melting are minimized
As a direct challenge for samples of solid drug substances an appropriate amount of sample should be taken and placed in a suitable glass or plastic dish and protected with a suitable transparent cover if considered necessary Solid drug substances should be spread across the container to give a thickness of typically not more than 3 millimeters Drug substances that are liquids should be exposed in chemically inert and transparent containers
Where practicable when testing samples of the drug product outside the primary pack these should be presented in a way similar to the conditions mentioned for the drug substance The samples should be positioned to provide maximum area of exposure to the light source For example tablets capsules etc should be spread in a single layer
If direct exposure is not practical (eg due to oxidation of a product) the sample should be placed in a suitable protective inert transparent container (eg quartz)
If testing of the drug product in the immediate container or as marketed is needed the samples should be placed horizontally or transversely with respect to the light source whichever
237
provides for the most uniform exposure of the samples Some adjustment of testing conditions may have to be made when testing large volume containers (eg dispensing packs)
1254 Post Exposure Sample Analysis
At the end of the exposure period the samples should be examined for any changes in physical properties (eg appearance clarity or color of solution) and for assay and degradants by a method suitably validated for products likely to arise from photochemical degradation processes
Where solid drug substance samples are involved sampling should ensure that a representative portion is used in individual tests Similar sampling considerations such as homogenization of the entire sample apply to other materials that may not be homogeneous after exposure The analysis of the exposed sample should be performed concomitantly with that of any protected samples used as dark controls if these are used in the test
Drug product samples of powder nature should be sampled in such a way that ensures that a representative portion is used in individual tests For solid oral dosage forms testing should be conducted on an appropriately sized composite of for example 20 tablets or capsules Similar sampling considerations such as homogenization or solubilization of the entire sample apply to other materials that may not be homogeneous after exposure (eg creams ointments suspensions etc) The analysis of the exposed sample should be performed concomitantly with that of any protected samples used as dark controls if these are used in the test
1255 Recommendation for Handling and Packaging
Depending on the extent of change special labeling or packaging may be needed to mitigate exposure to light When evaluating the results of photostability studies to determine whether change due to light exposure is acceptable it is important to consider the results obtained from other formal stability studies in order to assure that the drug substance or drug product will be within the proposed specifications during the re-test period or shelf-life
126 EVALUATION OF TEST RESULTS
The purpose of stability study is to establish re-test period or shelf-life and storage instructions for drug substances or drug products produced on commercial scale During the study physical chemical biological and microbiological tests and other specific tests in case of dosage forms (like dissolution rate for solid oral dosage forms) are conducted on a minimum number of batches The requested re-test period or shelf-life is granted without any statistical analysis if results of the study show very little degradation and very little variability from batch to batch and within a batch
In case of a drug product a shelf-life of 24 months may be established provided the following conditions are satisfied
The drug substance is known to be stable (not easily degradable)
No significant changes have been observed during stability studies performed
Supporting data indicate that similar formulations have been assigned a shelf-life of 24 months or more
The manufacturer will continue to conduct long-term studies until the proposed shelf-life has been covered and the results obtained will be submitted to the national medicines regulatory authority
Quantitative analysis of the data generally employs the concept of one-sided 95 confidence limit of the quantitative attribute changing with time For the purpose of quantitative analysis the data on all batches can be combined into one if batch-to-batch variability is small however if the data could not be combined then the overall shelf-life should be based on the minimum time a batch can be expected to remain within the acceptance criteria
238
The nature of any degradation relationship will determine whether the data should be transformed for linear regression analysis Limited extrapolation of the long-term data from the long-term storage condition beyond the observed range to extend the re-test period or shelf-life can be undertaken if justified
Any evaluation should cover not only the assay but also the levels of degradation products and other appropriate attributes Where appropriate attention should be paid to reviewing the adequacy of evaluation linked to drug product stability and degradation ldquobehaviorrdquo during the testing
127 STABILITY REPORT
A stability report must be established for internal use registration purposes etc giving details of the design of the study as well as the results and conclusions
The results should be presented as both in the form of a table and a graph For each batch the results of testing both at the time of manufacture and at different times during storage should be given A standard form should be prepared in which this can be summarized
The stability of a given drug substance or drug product and the proposed re-test period or shelf-life and storage conditions must be determined on the basis of these results
128 STATEMENTS AND LABELING
A storage statement should be established for display on the label based on the stability evaluation of the drug substance or drug product Where applicable specific instructions should be provided particularly for drug substances or drug products that cannot tolerate freezing or excursions in temperature Terms such as ldquoambient conditionsrdquo or ldquoroom temperaturerdquo should be avoided
The recommended labeling statements for use if supported by the stability studies are to be provided
A re-test period for drug substance should be derived from the stability information and a re-test date should be displayed on the container label if appropriate
129 STABILITY COMMITMENT
When the available long-term stability data on primary batches do not cover the proposed re-test period or shelf-life granted at the time of approval a commitment should be made to continue the stability studies post-approval in order to firmly establish the re-test period or shelf-life
Where the submission includes long-term stability data on the number of production batches covering the proposed re-test period or shelf-life a post-approval commitment is considered unnecessary Otherwise one of the following commitments should be made
If the submission includes data from stability studies on the specified number of production batches a commitment should be made to continue these studies through the proposed re-test period or shelf-life
If the submission includes data from stability studies on fewer than the specified number of production batches a commitment should be made to continue these studies through the proposed re-test period or shelf-life and to place additional production batches to a total of at least three in long-term stability studies through the proposed re-test period or shelf-life
If the submission does not include stability data on production batches a commitment should be made to place the first two or three production batches on long-term stability studies through the proposed re-test period or shelf-life and on accelerated studies for six months The stability protocol used for long-term studies for the stability commitment
239
should be the same as that for the primary batches unless otherwise scientifically justified
1210 ONGOING STABILITY STUDIES
After a marketing authorization has been granted the stability of the drug substance or drug product should be monitored according to a continuous appropriate program that will permit the detection of any stability issue in the container closure system in which it is marketed The purpose of the ongoing stability program is to monitor the drug substance or drug product within re-test period or over its shelf-life and to determine that the drug substance or drug product remains and can be expected to remain within specifications under the storage conditions on the label The ongoing stability program should be described in a written protocol and results formalized as a report
The protocol for the ongoing stability program can be different from that of the initial long-term stability study as submitted in the marketing authorization dossier provided that this is justified and documented in the protocol (eg the frequency of testing or when updating to meet revised recommendations)
For drug substance at least one production batch per year should be added to the stability monitoring program and tested at least annually to confirm the stability (WHO 2007)
For drug product the number of batches and frequency of testing should provide sufficient data to allow for trend analysis Unless otherwise justified at least one batch per year of product manufactured in every strength and every primary packaging type if relevant should be included in the stability program (unless none is produced during that year) The principle of bracketing and matrixing designs may be applied if scientifically justified in the protocol (ASEAN 2005)
In certain situations additional batches should be included in the ongoing stability program For example an ongoing stability study should be conducted after any significant change or significant deviation to the synthetic route process or container closure system Any reworking reprocessing or recovery operation should also be considered for inclusion in the case of drug products (WHO 2007)
Out-of-specification results or significant atypical trends should be investigated Any confirmed significant change out-of-specification result or significant atypical trend should be reported immediately to the relevant competent authorities in case of drug product whereas in case of drug substance to the relevant drug product manufacturers also The possible impact on batches on the market should be considered in consultation with the relevant competent authorities
A summary of all the data generated including any interim conclusions on the program should be written and maintained This summary should be subjected to periodic review
1211 IN-USE STABILITY TESTING
The purpose of in-use stability testing is to provide information for the labeling on the preparation storage conditions and utilization period of multi-dose products after opening reconstitution or dilution of a solution eg an antibiotic injection supplied as a powder for reconstitution
As far as possible the test should be designed to simulate the use of the drug product in practice taking into consideration the filling volume of the container and any dilution or reconstitution before use At intervals comparable to those which occur in practice appropriate quantities should be removed by the withdrawal methods normally used and described in the product literature
The physical chemical and microbial properties of the drug product susceptible to change during storage should be determined over the period of the proposed in-use shelf-life If possible testing should be performed at intermediate time points and at the end of the proposed in-use shelf-life on the final amount of the drug product remaining in the container Specific parameters
240
eg for liquids semi-solids and preservatives per ml content and effectiveness need to be studied
A minimum of two batches at least pilot-scale batches should be subjected to the test At least one of these batches should be chosen towards the end of its shelf-life If such results are not available one batch should be tested at the final point of the submitted stability studies
This testing should be performed on the reconstituted or diluted drug product throughout the proposed in-use period on primary batches as part of the stability studies at the initial and final time points and if full shelf-life long-term data are not available before submission at 12 months or the last time point at which data will be available
In general this testing need not be repeated on commitment batches
1212 Variations
Once the drug product has been registered additional stability studies are required whenever variations that may affect the stability of the drug substance or drug product are made (WHO 2007)
The following are examples of such changes
Change in the manufacturing process
Change in the composition of the drug product
Change of the immediate packaging
Change in the manufacturing process of a drug substance
In all cases of variations the applicant should investigate whether the intended change will or will not have an impact on the quality characteristics of drug substances andor drug products and consequently on their stability The scope and design of the stability studies for variations and changes are based on the knowledge and experience acquired on drug substances and drug products
The results of these stability studies should be communicated to the concerned regulatory authorities (WHO 2015)
Foot Note
This chapter has been written in the light of ICH and WHO guidelines on stability testing of drug substance and drug products The readers are advised to consult the original guidelines on stability testing in case they need more explanation on any specific area of the subject
241
REFERENCES
ASEAN Guideline on stability study of drug product 9th ACCSQndashPPWG Meeting Philippines 21ndash24 February 2005 version 22 February 2005
Grims W Storage conditions for stability testing in the EC Japan and USA the most important market for drug products Drug development and industrial pharmacy 19939 2795ndash2830
Helboe P new designs for stability testing programs Drug info J 199226629ndash634
ICH Harmonized Tripartite Guideline ICHndashQ1A (R2) Stability testing of new drug substances and products (second revision) Geneva Switzerland 2003
ICH Harmonized Tripartite Guideline (ICH Q1B) Photostability of Testing of New drug substances and products Geneva Switzerland 1996
Jeffs P The importance of stability testing in the registration of pharmaceutical products In Mazzo DJ editor Foreword International Stability Testing Interpharm Press Inc Buffalo Grove Illinois USA 1999
Schumacher P Aktuelle Fragenzur Haltbarkeit von Arzneimitteln Current questions on drug stability PharmazeutischeZeitung 1974119321ndash324
Terry G Beaumount Photostability testing In Mazzo DJ editor International Stability Testing Interpharm Press Inc Buffalo Grove Illinois USA 1999 Chap 2
WHO Expert Committee on Specifications for Pharmaceutical Preparations Thirty-ninth report Technical Report Series No 929 2005
WHO Good manufacturing practices main principles for pharmaceutical products In Quality assurance of pharmaceuticals A compendium of guidelines and related materials Volume 2 2nd updated edition Good manufacturing practices and inspection Geneva Switzerland 2007 Chap 1
WHO Guidance on variations to a prequalified product dossier In WHO Expert Committee on Specifications for Pharmaceutical Preparations Forty-first report Technical Report Series No 943 2007 ndash Annex 6
WHO Guidelines for stability testing of pharmaceutical products containing well established drug substances in conventional dosage forms In WHO Expert Committee on Specifications for Pharmaceutical Preparations Thirty-fourth report Technical Report Series No 863 1996 Annex 5
WHO Prequalification Program Priority Essential Medicines A United Nations Program managed by WHO Information for applicants 2015 (httpmednet3whointprequal)
WHO Stability testing of active pharmaceutical ingredients and finished pharmaceutical products Forty-third report technical report series No 953 2009 Annex 2
WHO Supplementary guidelines on good manufacturing practices validation In Quality assurance of pharmaceuticals A compendium of guidelines and related materials Volume 2 2nd updated edition Good manufacturing practices and inspection Geneva Switzerland 2007 Chapter 1
WHO World Health Organization Expert Committee on Specifications for Pharmaceutical Preparations Fortieth report Technical Report Series No 937 2006
242
243
INDEX
135-triazine 141 2-Aminofluorene 62 2-hydroxyethyl methacrylate 83 4-aminosalicylic acid 3 59 127 128 182 5-aza-cytidine 63 78-dimethyl-10-
(formylmethyl)isoalloxazine 63 Accelerated studies 257 Accuracy 237 240 241 242 acetylsalicylic acid 4 128 additives 105 134 adjuvants 73 104 106 admixtures 1 165 adverse biological effects 3 Adverse effects of herbs 212 Aging 141 air 57 61 65 92 149 162 172 182 192
195 198 202 204 207 208 air-tight containers 162 amberopaque 173 amide 3 41 52 53 65 153 amides 53 161 amino acid 82 130 139 amorphization 4 104 181 amorphous drug 104 105 109 Amorphous drugs 105 Amorphous Drugs 179 amorphous solids 105 111 140 amorphous state 104 105 106 109 112
116 124 180 Amorphous State 105 amoxicillin 19 174 175 amphotericin 3 ampicillin 113 128 176 177 211 ANALYTICAL METHODS 5 analytical techniques 101 114 153 ANALYTICAL TECHNIQUES 152 antioxidants 172 ascorbic 3 4 20 39 41 56 57 83 85
91 92 110 129 164 172 179 182 184 226 235
ascorbic acid 182 Ascorbic acid 57 85 92 172 182 asparagine 130 138 aspirin 18 20 38 41 43 52 128 164
182 211 212 226 227 238 Aspirin 52 134 182 183 212 Atropine 183 208 autoxidation 58 129 Batanopride Hydrochloride 62
batch 5 6 17 149 255 261 262 264 267 269 270
batches 5 143 160 194 256 257 264 266 267 268 269 270
benzaldehyde 83 84 204 benzydamine 83 Binary Co-Amorphous Mixtures 180 bioavailability 66 73 101 104 105 107
108 111 116 124 128 134 141 177 179 211 212
biochemical stability 66 biological efficacy 1 bracketing 6 160 256 257 269 Bracketing 6 257 258 buffer 3 5 28 33 36 37 44 107 116
149 171 226 244 246 buserelin 155 candesartan cilexetil 109 capillary electrophoresis 153 capillary electrophoresisndashmass
spectrometry 153 carisbamate 155 Catalysis 34 Cefoxitin sodium 63 ceftazidine 164 Characterization of Polymorphs 114 chemical degradation 4 15 65 102 124
125 126 128 134 136 140 161 162 164 177 178 179 207
chemical factors 149 Chemical Functions 76 chemical kinetics 8 15 Chemical kinetics 15 chemical reactions 3 15 78 101 102 126 chemical reactivity 126 149 chemical stability 8 50 65 101 149 Chemical Stabilization 179 chlordiazepoxide 83 172 chloroquine 3 82 CHROMATOGRAPHIC (HPLC) ASSAY
METHODS 236 Chromatographic Methods 197 Chromatography 153 ciprofloxacin 131 circular dichroism 153 climatic conditions 1 7 253 254 255 climatic zones 253 clopidogrel 107 116 coalescence 110 co-amorphous system 107 coating 39 65 104 131 143 171 184
244
comminution 104 compaction 104 126 compatibility of drugs 1 Complex Chemical Reactions 23 conditions 3 15 25 59 142 149 150
152 153 154 177 181 225 242 259 conductance 110 Conformational polymorphism 113 Consecutive Reactions 27 consistency 105 195 Container Closure System 256 container-closure system 2 160 167 content uniformity 104 Creams 2 109 critical mobility temperature 105 crystalline form 4 63 104 111 113 136 crystalline state 4 105 106 111 116 124
126 127 180 Crystalline state 111 crystolepine 155 cyanocobalamin 3 35 36 82 87 181
182 203 226 cyclization 62 125 173 178 Cyclization 173 183 Cyclodextrin 140 177 cyclosporine 3 cytarabine 3 165 deamidation 4 64 125 130 138 140 Deamidation 130 decarboxlyation 3 decarboxylation 4 59 61 128 Decarboxylation 51 59 183 degradation 1 3 5 6 8 15 19 22 23 25
26 27 30 31 32 33 34 35 36 37 38 39 40 41 42 43 50 51 52 54 56 57 59 60 63 64 65 66 72 75 79 101 102 107 110 124 125 128 130 131 133 134 135 136 137 138 139 140 141 149 150 151 152 153 154 156 161 163 164 165 171 173 174 177 178 179 181 182 184 192 195 203 207 209 210 220 221 222 223 225 226 236 241 243 254 255 256 259 263 266 267
Degradation Conditions 150 Degradation Limits 151 degradation pathway 60 149 degradation pathways 6 149 degradation process 5 6 149 degradation products 3 5 6 51 72 128
131 149 150 151 152 153 154 195 209 220 222 223 225 226 236 241 243 254 255 267
degradation reactions 6 15 23 31 33 37 40 41 51 63 124 125 135 171 173 203 209 225 226
DEGRADATION REACTIONS 30 51 128 171
DEGRADATION STUDIES OF HERBAL DRUGS 209
Dehalogenation 51 62 dehydration 3 62 104 125 177 207 Dehydration 51 61 Dehydrogenation 51 62 DESIGN OF STABILITY STUDIES 255 desolvation 4 104 125 Detection limit 238 development process 1 5 124 149 161
220 253 diclofenac sodium 173 178 Differential scanning calorimetry 66 101
139 181 Differential Thermal Analysis 101 Diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) 103 Digitoxin 207 Dilatometry 102 Dimerization 51 60 174 disintegration of solids 105 dispersion 65 105 107 108 109 110
111 178 197 dispersions 65 104 107 108 109 111
124 178 179 180 dissolution 2 5 41 66 73 101 104 105
107 108 109 111 112 114 116 124 128 137 141 142 143 163 178 179 259 266
divalent anions 26 DLS 103 110 dosage forms 1 2 3 4 5 15 33 35 36
37 39 41 52 65 101 104 105 107 116 124 125 132 133 134 137 141 149 160 162 166 179 181 182 184 192 236 238 254 266
Dosage Forms 107 163 164 180 181 184
dosage forms 1 3 15 33 35 36 65 101 116 124 125 133 134 137 141 149 160 162 166 179 254
dried preparation 66 drug adsorption 4 104 drug development 1 5 113 116 124 149
152 161 220 253 254 drug development process 1 drug interactions 211 DRUG INTERACTIONS 134 211 drug manufacturers 1 drug products 1 3 4 5 6 15 17 30 33
39 50 63 72 73 74 83 101 104 105
245
116 149 150 151 153 160 161 163 166 171 177 184 210 220 253 256 257 261 262 264 265 266 267 269 270
drug substance 5 6 30 50 112 114 149 150 153 220 221 253 254 255 256 257 258 260 261 262 263 264 266 267 268 269 270
drug substances 1 2 3 6 15 50 51 63 72 75 79 82 83 101 102 104 105 107 113 114 116 124 125 128 136 137 149 150 151 153 160 161 173 177 181 182 183 210 211 220 236 246 253 262 263 265 266 267 270
drugndashCD complexes 140 178 drugndashpolymer solid dispersions 109 drying 65 66 104 105 106 111 192
193 194 DSC 101 106 107 109 114 115 131
138 139 141 181 Dynamic light scattering (DLS) 103 electron microscopy 66 141 184 Electrophoresis 153 197 Elimination 51 59 225 emulsion creams 109 emulsions 66 104 210 enthalpy 106 112 114 139 180 entropy 106 180 environmental 1 2 6 50 101 149 161
162 171 181 192 210 253 environmental conditions 2 environmental factors 1 6 50 101 149
161 162 171 181 192 210 253 Enzyme Catalyzed Reactions 28 Ephedrine 204 epimerization 3 61 175 209 210 Epimerization 51 61 175 176 209 epinephrine 3 39 176 erythromycin 3 203 Erythromycin 183 207 Essential Oils 194 esters 3 21 40 52 76 133 161 181 194 ethyl acetate 20 43 etodolac 3 etoposide 3 European Pharmacopoeia 7 72 161 excipients 1 3 6 50 66 101 105 109
114 124 125 130 131 132 133 134 136 141 149 150 152 171 177 180 181 184 192 194 238 241 243
Excipients 140 exciplex 80 excited 39 74 75 76 77 78 79 80 81
82 85 86 91 92 93 Excited State Reactions 79 expiration dates 15
expiration dating 6 7 15 17 Expiration dating 17 External Factors 3 105 Extraction Methods 197 famotidine 115 FDA 5 8 124 134 152 160 220 246 FINGERPRINT ANALYSIS 195 finished products 1 flocculation 104 107 Fluconazole 131 fluorescence 75 77 80 153 204 210 fluoroquinolones 83 161 179 flurbiprofen 106 211 forced degradation 6 41 149 150 151
152 153 154 155 226 236 237 255 263
Forced degradation 149 Forced polymorphism 113 formulation development 6 72 150 152 formulation studies 50 formylmethylflavin 21 79 86 93 204 206
226 230 244 Fourier transform infrared (FTIR)
spectroscopy 102 Free radical mechanism 81 Frequency of Testing 257 FTIR 51 102 107 109 114 115 136
138 139 141 195 197 241 FTIR 51 functional groups 50 51 75 76 220 furosemide 4 83 106 115 131 180 184 GCMS 51 General acid-base catalysis 36 Gibbs free energy 106 180 Ginseng Saponins 209 glass containers 161 164 165 166 195 glucose 3 56 65 165 211 212 Glucose 61 62 good manufacturing practices 1 194 granulation 104 125 126 142 257 Graphical method 22 half-life 17 18 22 23 40 42 43 181 Half-life method 22 hardness 5 41 112 142 164 259 heat 75 77 101 102 106 110 112 114
133 137 149 154 155 162 192 195 198 199 200 209 222
heat capacity 101 106 112 Herbal Drug Extracts 194 Herbal Drug Preparations 194 herbal drugs 8 192 193 194 195 197
198 202 203 204 211 Herbal Drugs 193 203 207 Herbal Extracts 194 herbal products 192 194 203 211 212 Herbal Products 194
246
high-performance liquid chromatography 5 High-performance liquid chromatography
63 197 homogeneity 105 homogenization 110 266 Hot-stage microscopy 102 HPLC 5 41 51 60 63 64 65 66 109
131 137 141 152 153 154 155 163 164 165 166 195 197 210 225 236 238 243 244
HPLCndashmass spectrometry 153 197 HPTLC 51 195 197 226 humidity 1 5 6 51 105 109 111 112
136 137 139 142 149 163 164 166 182 210 253 255 261
hydrolysis 3 4 15 20 33 38 40 41 43 44 51 52 53 54 57 63 64 65 66 82 83 86 91 93 124 125 128 139 149 150 151 153 154 155 174 177 178 207 208 209 222 229 230 256
Hydrolysis 18 19 20 21 25 51 52 53 54 151 171 183 226 227 228 232
hydrolytic degradation 64 65 66 236 ICH 5 6 7 8 51 72 74 124 132 149
150 152 154 160 163 167 210 220 237 238 242 246 253 256 263 264 270
ICH Q1B guideline 6 ICH Q5C guideline 152 ICH QIB Guideline 74 imides 3 impurities 125 129 150 152 166 194
220 237 240 241 243 indomethacin 38 52 105 106 107 131
132 180 181 Industrial Awareness 74 Insulin preparations 64 interactions 1 3 77 91 101 102 103
104 105 110 111 134 140 162 171 178 179 180 181 192 211
Intermediate precision 237 243 Intermediate studies 257 Internal Factors 3 105 intrinsic stability 220 IN-USE STABILITY 269 Ionic Strength Effect 37 isomerization 3 41 60 76 124 150 Isomerization 51 60 Isothermal calorimetry 102 137 ketoprofen 106 211 Kinetic Studies 137 lamotrigine mesylate 105 LCMS 51 LCndashMSTOF methods 154 light 3 5 6 39 51 53 56 57 61 72 73
74 75 76 78 81 82 85 86 87 91 92
93 103 104 108 114 131 149 152 154 155 160 161 162 163 165 166 171 173 181 182 183 184 192 195 198 199 200 201 203 204 205 210 221 222 223 253 255 256 262 263 264 266 270
light sensitive drugs 203 Light Sources 263 Light-resistant containers 162 Linearity 237 238 Liposomal Formulation 179 Liposomes 110 179 liquid dosage 3 4 5 33 34 35 36 37
39 52 104 160 183 192 Long term or real-time studies 257 lumichrome 63 86 93 204 206 221 lumiflavin 63 86 93 204 206 lumivudine 154 lyophilized compound 172 lyophilized preparations 65 lyophilized proteins 65 lyoprotectants 65 manufacturer 1 5 254 267 matrixing 6 160 256 257 269 Matrixing 6 257 258 mechanical strength 104 105 Mechanism involving singlet oxygen 81 meclofenamic acid 3 82 mefloquin 82 Menadione 173 METHODS OF STABILIZATION 177 methyl paraben 66 Michaelis constant 30 microbial growth 2 4 104 microbial growth 4 104 Microcalorimetry 102 microcrystalline cellulose 109 125 moisture 3 4 5 41 65 105 111 112
124 125 128 130 131 132 133 134 135 141 142 149 162 163 164 166 171 182 183 192 195 198 199 200 201 202 222 261
Moisture 41 124 125 132 133 162 180 182 198 199 200 201 202
moisture content 132 141 142 163 166 Molecularity 16 Morphine 57 183 208 moxalactam 3 moxifloxacin 34 35 38 87 131 136 236 MS 51 65 66 131 139 153 154 155
195 197 Multicomponent Assay 223 naproxen 107 180 211 new chemical entity 1 New Drug Application 150 nicotinamide 91 92 180 226
247
nifedipine 82 105 106 109 178 180 184 NMR 51 65 103 105 114 115 116 138
139 140 155 195 197 209 222 nonisothermal kinetics 33 Norfloxacin 56 63 NSAIDs 107 111 161 211 ofloxacin 79 131 One-Component Assay 222 ONGOING STABILITY STUDIES 268 Order 16 17 18 19 21 23 42 43 Order of Reaction 16 orders of reaction 15 oxidation 3 4 15 39 40 51 53 56 57
58 76 79 80 81 82 83 87 91 92 125 129 149 150 151 153 154 155 172 177 182 183 207 208 222 255 266
Oxidation 20 39 51 56 129 152 172 183
Oxidation reactions 39 Oxidizable drugs 39 Oxygen 39 oxyphenbutazine 106 packaging 1 3 5 6 8 15 39 50 74 150
160 161 162 163 164 166 167 171 193 253 254 256 262 264 266 269 270
packaging development 160 packaging material 3 160 162 163 166
167 packaging materials 1 3 160 254 PACKAGING STUDIES 163 Packing polymorphism 113 paracetamol 52 134 164 180 Paracetamol 53 164 182 183 Parallel Reactions 25 Particle electrophoresis 104 particle size 3 4 65 67 101 107 110
111 125 126 171 210 Particle size 4 Peptide 130 pH 3 5 33 34 35 36 39 40 42 44 52
56 58 61 62 63 64 65 66 86 87 88 107 109 110 111 128 134 137 138 149 152 161 165 171 172 173 174 175 176 177 181 182 183 184 194 207 209 222 223 229 230 231 233 234 235 244 246 256 259
pharmaceutical manufacturers 160 Pharmaceutical Preparations 64 pharmaceutical systems 18 40 106 112
152 pharmaceuticals 6 15 101 105 111 114 pharmacists 1 pharmacopoeias 1 7 177 phase transition 102 104
Phase transition 104 Phenols 58 phosphorescence 75 77 79 80 photoaddition 26 80 86 87 Photoaddition 80 82 86 226 234 Photoallergic reactions 73 photoaquation 3 Photoaquation 82 87 Photochemical Interactions 91 photochemical process 74 photochemical reactions 75 76 77 78 79
85 Photochemistry 74 75 photocyclization 3 photodealkylation 3 86 91 Photodealkylation 82 86 photodecarboxylation 3 Photodecarboxylation 82 photodegradation 3 26 34 35 37 39 63
72 74 75 79 82 83 86 87 88 89 90 91 93 131 166 173 179 184 203 204 206 225 236 243 244 256
Photodegradation 27 73 87 91 131 203 204 221
photodegradation reactions 3 72 82 83 Photodehalogenation 82 Photodehydrogenation 82 photodimerization 3 91 Photodimerization 82 Photoelimination 82 Photo-induced rearrangement 83 Photoinduced ring cleavage 83 photoisomerization 3 91 166 Photoisomerization 83 photolysis 4 15 26 28 35 36 51 72 77
78 79 80 87 149 150 151 153 154 155 173 177 222 226 255
Photolysis 20 21 77 78 131 152 172 226 233 235
photooxidation 3 39 83 85 87 173 204 205 226 232
Photooxidation 83 84 85 173 Photophysical Processes 76 photoreactivity 72 74 75 Photoreactivity 76 photoreduction 3 86 87 Photoreduction 83 86 photosensitivity 73 149 203 256 263 PHOTOSENSITIVITY REACTIONS 203 photosensitization reactions 72 Photosensitization reactions 73 Photosensitized Reactions 81 photostability 3 6 8 72 73 74 75 79 83
92 132 149 160 166 178 210 226 262 263 264 265 266
248
Photostability 3 72 73 74 75 149 178 256 263
PHOTOSTABILITY 72 262 photostability testing 6 72 74 149 210
226 262 263 265 Photostabilization 184 Phototoxic reactions 73 physical stability 8 50 101 104 105 106
107 108 110 111 114 180 physical state 1 101 105 111 112 116
124 130 142 Pilocarpine 209 plastic containers 165 Plastic containers 162 polyacrylic acid 111 polydispersity index 110 polymer 108 109 111 140 142 178 179
181 Polymer complexation 178 Polymerization 176 polymorph formation 4 polymorphic transition 4 104 Polymorphism 112 113 polymorphs 102 103 104 111 112 113
114 115 116 136 polyols 65 polyurethane 111 polyvinyl chloride (PVC) bags 164 polyvinyl pyrrolidone-co-vinyl acetate 108 polyvinylpyrrolidone 108 178 Precipitation 4 Precision 237 241 242 243 preservative 4 256 preservative activity 5 preservativesstabilizers 1 primaquine 4 82 Primary Packaging Material 161 Procaine 52 183 prostaglandin E1 and E2 3 protein drugs 130 proteins 65 66 111 116 130 134 204 Proteins 111 Pseudo First-order Reaction 20 Pseudolatexes 65 Pseudopolymorphism 113 pyrolysis 4 27 131 Pyrolysis 130 131 QUALITY CONTROL METHODS 194 Quantitation limit 238 quinapril 107 181 quinapril HCl 107 Quinine 206 QΔT calculation 32 Rabeprazole 66 rabeprazole sodium 66 Racemization 176
Range 237 240 ranitidine HCl 41 114 137 ranitidine hydrochloride 64 rate constant 16 17 18 19 20 21 22 25
26 30 31 33 34 36 37 38 42 43 44 109 134 135 177
ratendashpH profiles 34 35 rates 15 33 37 38 40 41 66 67 75 78
102 108 116 130 138 140 167 177 Reaction Rate 16 rebamipide 153 recombinant human deoxyribonuclease 65 recommended storage conditions 3 253 regulatory agencies 1 5 50 253 REGULATORY ASPECTS 253 regulatory authorities 124 149 160 255
270 regulatory requirement 1 253 Repeatability 237 242 243 Reproducibility 237 243 Reserpine 204 205 reset 6 15 re-test period 5 253 257 260 261 262
266 267 268 riboflavin 3 20 26 27 28 31 35 37 38
52 54 63 80 81 82 83 86 89 91 92 179 184 203 221 226 229 233 234 235 243 244
Riboflavin 35 54 184 204 206 226 243 Robustness 238 246 salt 4 41 104 111 134 180 193 salt exchange 4 104 Secondary Packaging Material 161 Second-order Reaction 20 Sedimentation 107 sedimentation 104 Selection of Batches 256 Sensitivity 74 198 238 245 261 shelf-life 1 2 3 4 6 7 15 17 23 30 31
32 33 44 51 73 101 139 151 163 166 177 178 180 195 210 253 254 256 257 260 261 262 266 267 268 269 270
Shelf-life calculation 33 shelf-life 1 163 166 195 253 257 266
268 269 270 Single crystal X-ray diffraction (XRD 103 Singlet oxygen 39 76 solid dispersions 109 Solid dispersions 107 108 solid dosage 4 5 41 104 116 124 125
131 132 133 134 136 137 141 166 181 182
solid dosage forms 5 104 124 134 137 141
249
solid state 4 8 65 103 104 108 112 115 116 124 125 126 128 129 130 131 132 133 135 136 137 138 139 140 141 155 178 181 182 237
solid state degradation 4 125 132 Solid state nuclear magnetic resonance
(SSNMR) spectroscopy 103 solid state stability 8 Solid state transitions 4 solvation 4 104 112 128 Solvatomorphism 113 solvent 3 37 38 108 111 112 113 125
128 131 133 149 171 172 194 223 226 261
solvent dielectric constant 38 solvolysis 4 125 Solvolysis 128 Specific acidndashbase catalysis 34 specific acid-catalyzed reaction 64 Specificity 237 243 Spectroscopic Methods 102 197 Spectroscopy 103 153 stability 1 2 3 4 5 6 7 8 15 30 33 34
35 36 37 40 41 50 51 63 64 65 66 72 74 91 101 102 104 105 106 107 108 109 110 111 112 113 114 116 124 125 132 133 134 136 137 139 140 141 142 149 150 151 152 153 154 155 160 161 162 163 164 165 166 167 171 173 176 177 178 179 180 181 183 192 194 195 203 209 210 211 220 221 225 226 236 246 253 254 255 256 257 260 261 262 266 267 268 269 270
Stability 1 2 3 4 5 178 203 210 220 226 236 253 254 255 256 258
STABILITY COMMITMENT 268 stability data 6 254 STABILITY EVALUATION 5 STABILITY OF HERBAL DRUGS 192 203 STABILITY PREDICTION 166 stability profile 160 stability program 1 160 268 STABILITY REPORT 267 stability schedule 6 stability studies 6 124 203 211 253 254
256 268 270 stability testing 6 8 253 270 STABILITY TESTING 5 167 210 253 stability-indicating assay method 5 15 74
150 226 stability-indicating assay methods 5 6 8
149 152 220 236 246 STABILITY-INDICATING ASSAY
METHODS 220 237
STABILITY-INDICATING HIGH-PERFORMANCE 236
STABILITY-INDICATING SPECTROMETRIC ASSAY METHODS 222
stabilization 8 15 30 33 50 65 72 74 75 106 116 130 171 172 174 177 178 179 180 181 182 184
Stabilizers 179 stabilizers 173 Stark and Einstein Law 74 state degradation 102 STATE DEGRADATION 135 STATISTICAL APPLICATIONS 6 Statistical methods 6 Steric Structural Variations 173 storage conditions 1 2 3 5 6 7 15 17
51 109 111 142 149 152 161 164 171 177 193 195 202 210 253 254 255 257 258 260 261 267 268 269
STORAGE CONDITIONS 258 storage period 1 5 50 107 110 136 160
165 stress conditions 6 41 72 111 150 152
153 154 155 162 163 221 stress testing 6 149 151 152 167 220
237 253 256 Stress testing 149 220 236 255 Stress Testing 255 Structural Studies 136 substances 1 6 15 51 72 73 83 101
104 116 149 171 177 182 238 261 262 265 270
Substitution method 21 sugars 65 180 sulfacetamide 4 41 52 82 83 226 232
238 239 240 241 Surfactants 40 suspensions 18 19 64 66 104 107 178
256 263 266 temperature 1 3 4 5 6 30 31 32 33
37 38 44 51 52 54 61 63 64 65 66 101 102 105 106 108 109 110 111 112 113 114 124 125 128 130 131 132 133 137 139 140 141 152 155 163 164 165 166 171 176 177 180
181 182 195 209뉐 210 223 246
253 254 255 261 262 268 Temperature 30 64 109 133 162 177
198 199 200 201 202 tetracycline 73 137 175 176 tetracyclines 3 134 theophylline cream 110 thermal gravimetric analysis 65 thermal methods 109 125 136 Thermal Methods 101