BIOPHARMACEUTICS ABSORPTION

SYSTEMIC ABSORPTION IS DEPENDENT ON:•The anatomy and physiology of the drug absorption site•The physicochemical properties of the drug•The nature of the drug product/formulation factors

Physiological Factors Affecting Oral Absorption

Objective:To understand the physiological factors which affect the oral

absorption of drug products Physiological Factors:A. Membrane physiology:

– Considering the structure of membranes – Transport processes

B. Gasstrointestinal physiology:– Characteristics of gastrointestinal physiology – Gastric motility and emptying – Influence of food – Other factors

Diagram Scheme of ADME processes

THERAPY

TOXIC

DISINTEGRATION DISSOLUTION ABSORPTION

DRUG IN SYST.CIRCULATION:FREE ⇄ BIND DISTRIBUTION

METABOLISM

TISSUE

RECEPTOR

ELIMINATION

PHARMACOLOGIAL EFFECT

PHARMACEUTICAL PHASE PHARMACOKINETIC PHASE

PHARMACODYNAMIC PHASE

Excretion

Protein Binding

• The ultimate goal of drug absorption is to have the drug reach the site of action in a concentration which produces a pharmacological effect. No matter how the drug is given (other than I.V.) it must pass through a number of biological membranes before it reaches the site of action.

A. Membrane physiology1. Membrane structure• In 1900 Overton performed some simple but

classic experiments related to membrane structure. By measuring the permeability of various types of compounds across the membranes of a frog muscle he found that lipid molecules could readily cross this membrane, larger lipid insoluble molecules couldn't and small polar compounds could slowly cross the membrane.

Diagram XI-2, the Davson-Danielli Model

Model of the plasma membrane including proteins and carbohydrates as well as lipids. Integral proteins are embedded in the lipid bilayer; peripheral proteins are merely associated with the membrane surface. Carbohydrate attached to proteins: glycoproteins, to lipid: glycolipids. These proteins provide a pathway for the selective transfer of certain polar molecules and charged ions through the lipid barrier Fluid Mosaic Model by Singer and Nicholson

• These results suggest that the biologic membrane is mainly lipid in nature but contains small aqueous channels or pores. Other experiments involving surface tension measurements have suggested that there is also a layer of protein on the membrane. These results and others have been incorporated into a general model for the biological membrane. This is the Davson-Danielli model.

• The membrane then acts as a lipid barrier with small holes throughout.

Nature of the drug transport in the body

• Transcellular Transport• Paracellular Transport• Intestinal Epithelial cell Transport

Diagram XI-3, Simplified Model of Membrane

This is the general structure. Membranes in different parts of the body have somewhat different characteristics which influence drug action and distribution. In particular, pore size and pore distribution is not uniform between different parts of the body.

Transcellular

Paracellular

Examples of some membrane types.• Blood-brain barrier. The membranes between

the blood and brain have effectively no pores. This will prevent many polar materials (often toxic materials) from entering the brain. However, smaller lipid materials or lipid soluble materials, such as diethyl ether, halothane, can easily enter the brain. These compounds are used as general anesthetics.

• Renal tubules. In the kidney there are a number of regions important for drug elimination. In the tubules drugs may be reabsorbed. However, because the membranes are relatively non-porous, only lipid compounds or non-ionized species (dependent of pH and pKa) are reabsorbed

• Blood capillaries and renal glomerular membranes. These membranes are quite porous allowing non-polar and polar molecules (up to a fairly large size, just below that of albumin, M.Wt 69,000) to pass through. This is especially useful in the kidney since it allows excretion of polar (drug and waste compounds) substances.

• Placenta Membranes• Testis Membranes

Transcellular Transport/pathway• Passive Diffusion• Carrier Mediated/Facilitated Transport or

Diffusion : Active transport, Facilitated transport, Carrier-Mediated Intestinal transport

• Vesicular Transport. Endocytosis/Exocytosis : pinocytosis, phagocytosis, receptor-mediated endocytosis, transcytosis

• Pore (Convective) Transport• Ion pair transport

Transport across the membranes (Transcellular Transport)

a)Passive Diffusion with a Concentration Gradient

• CGI

CP

h

• Most drugs cross biologic membranes by passive diffusion. Diffusion occurs when the drug concentration on one side of the membrane is higher than that on the other side.

• Drug diffuses across the membrane in an attempt to equalize the drug concentration on both sides of the membrane.

• If the drug partitions into the lipid membrane, a concentration gradient can be established.

Equation 1: Rate of DiffusionFick’s law of Diffusion

• The rate of transport of drug across the membrane can be described by Fick's first law of diffusion:

• Rate of diffusion = ( )PGI CCh

DAKdtdQ −=

dQ/dt = rate of diffusion; D = diffusion coefficient; K = lipid water patition coefficient of drug; A = surface area of membrane; h = membrane thickness; CGI-CP = difference between the concentration of drug in GIT and in the plasma

b)Carrier Mediated /Facilitated Transport or Diffusion1.Active transport. The body has a number of specialized

mechanisms for transporting particular compounds; for example, glucose and amino acids. Sometimes drugs can participate in this process; e.g. 5-fluorouracil. Active transport requires a carrier molecule and a form of energy

• Energy consuming process, ATP hydrolysis or transmembraneous sodium gradient and/or electrical potential

• Carrier/transporter mediated, the process can be saturated • against a concentration gradient across cell membrane• competitive inhibition is possible : metabolic inhibitors or

substrate analogues• Temperature dependence• Important role in the intestinal, renal and biliary excretion

of many drugs

• P-glycoprotein is one of the transmembrane protein acts as a carrier-mediated intestinal transporter identified in the intestine

• Pgp appears to reduce apparent intestinal epithelial cell permeability from lumen to blood for various lipophilic or cytotoxic drugs

Four types of membrane transport proteins couple the energy-releasing hydrolysis of ATP with the energy-requiring transport of substances against their concentration gradient

P-class Pumps• P-class pumps are composed of two different polypeptides,

α and β, and become phosphorylated as part of the transport cycle. The sequence around the phosphorylated residue, located in the larger α subunits, is homologous among different pumps

• In P-class pumps, phosphorylation of the α subunits and a change in conformational states are essential for coupled transport of H+, Na+, K+, or Ca2+ ions

• The P-class Na+/K+ ATPase pumps three Na+ ions out of and two K+ ions into the cell per ATP hydrolyzed. A homolog, the Ca2+ ATPase, pumps two Ca2+ ions out of the cell or, in muscle, into the sarcoplasmic reticulum per ATP hydrolyzed. The combined action of these pumps in animal creates an intracellular ion milieu oh high K+.ow Na+ very different from the extracellular fluid milieu of high Na+, high Ca2+, and low K+

F-class and V-class pumps

• F-class and V-class pumps do not form phosphoprotein intermediates. Their structures are similar and contain similar proteins but none of their subunits are related to those of P-class pumps

• In the multisubunit V-and F-class ATPases, which pumps protons exclusively, a phosphorylated protein is not an intermediate in transport

• A V-class H+ pump in animal lysosomal and endosomal membranes and plant vacuola membranes is responsible for maintaining a lower pH inside the organelles than in the surrounding cytosol

ABC superfamily ( ATP Binding Catalytic domains or Cassete)

• All members of the large ABC superfamily of proteins contain four domains: two transmembrane (T) domains and two cytosolic ATP-binding (A) domains that couple ATP hydrolysis to solute movement. These core domains are present as separate subunits in some ABC proteins, but are fused into a single polypeptide in other ABS proteins

• All members of the large and diverse ABC superfamily of transport proteins contain 4 core domains: two transmembrane domains, which form a pathway for solute movement and determine substrate specificity, and two cytosolic ATP-binding domains

• The ABC superfamily includes bacterial amino acid and sugar permeases; the mammalian MDR1 protein, which export a wide array of drug from cells; CTFR protein, a Cl channel that is defective in cystic fibrosis

Carrier Mediated Transport Process

2.Facilitated Diffusion • A drug carrier is required but no energy is

necessary. e.g. vitamin B12 transport • saturable if not enough carrier, subjected to inhibition

by competitive inhibitor• no transport against a concentration gradient, downhill

but faster • Requires concentration gradient for its driving force, as

does passive diffusion• Much faster rate than would be anticipated based on the

molecular size and polarity of the molecule• Minor role in drug absorption

3.Carrier-Mediated Intestinal Transport

• Various carrier-mediated systems (transporters) are present at the intestinal brush border and basolateral membrane for the absorption of specific ions and nutrients essential for the body. Many drug are absorbed by these carriers because of the structural similarity to natural substrates (see Table)

• Other carrier-mediated intestinal transporter: amino acid transporter, oligopeptide transporter, phosphate transporter, bile acid transporter, glucose transporter, monocarboxylic acid transporter

• Many oral cephalosporins are absorbed through the amino acid transporter. Cefazolin, a parenteral-only cephalosporin, cannot be absorbed through this mechanism, is not available orally

Intestine Transporters and Examples of Drug TransportedTransporter Examples

Amino acid Gabapentin MethyldopaL-dopa

p-cycloserineBaclofen

Oligopeptide CefadroxylCefiximeCephalexinLisinopril

CephradineCeftibutenCaptoprilThrombin inhibitor

Phosphate Fostomycin Foscarnet

Bile acid S3744

Glucose P-nitrophenyl-β-D-glucopyranoside

P-glycoprotein efflux EtoposideCyclosporin A

Vinblastine

Monocarboxylic acid Salicylic acidPravastatin

Benzoic acid

Figure 13-1 from your book p.374

• Summary of intestinal epithelial transporters. Transporters shown by square and oval shapes demonstrate active and facilitated transported, respectively

• Name of cloned transporters are shown with square or oval shapes

• Active transporters: arrows in same direction represent symport of substance and the driving force

• Arrows going in the reverse direction mean the antiport

Efflux of drugs from the intestine

• Counter transport efflux proteins that expel specific drugs back into the lumen of GIT after they have been absorbed

• Example: P-glycoproteins• Requires energy• Against a concentration gradient• Competitively inhibited by structural analogues

or metabolism inhibitors• Saturable process

Transport across Cell Membranes   Active Transport by ATP-Powered Pumps

Figure 15-17. Possible mechanisms of action of the MDR1 protein. (a) The flippase model proposes that a lipid-soluble molecule first dissolves in the cytosolic-facing leaflet of the plasma membrane ( 1 ) and then diffuses in the membrane until binding to a site on the MDR1 protein that is within the bilayer ( 2 ). Powered by ATP hydrolysis, the substrate molecule flips into the exoplasmic leaflet ( 3 ), from which it can move directly into the aqueous phase on the outside of the cell ( 4 ). (b) According to the pump model, MDR1 has a single multisubstrate binding site and transports molecules by a mechanism similar to that of other ATP-powered pumps. [Adapted from G. Ferro-Luzzi Ames and H. Legar, 1992, FASEB J. 6:2660; N. Nelson, 1992, Curr. Opin. Cell Biol. 4:654; C. F. Higgins and M. M. Gottesman, 1992, Trends Biochem. Sci. 17:18; and C. F. Higgins, 1995, Cell 82:693.]

Physiological Role of P-Glycoprotein• P-glycoprotein is found in high levels at apical

surface of cells typically associated with transport of: biliary canalicular membrane, brush border of renal proximal tubules, apical surface of intestinal mucosal cells, endothelial cells of brain and testis

• It has been proposed that the normal physiological role of P-glycoprotein is one of detoxification through active secretion of xenobiotics

Role of P-glycoprotein in cancer• Approximately 50% of human cancers express P-

glycoprotein at levels sufficient to confer MDR• Cancers which acquire expression of P-

glycoprotein following treatment of the patient include leukemias, myeloma, lymphomas, breast, ovarian cancer; preliminary results with P-gp inhibitors suggest improved response to chemotherapy in some of these patients

• Cancers which express P-gp at time of diagnosis include colon, kidney, pancreas, liver; these do not respond to P-gp inhibitors alone and have other mechanisms of resistance

P-Glycoprotein and Multi Drug Resistance

• Multi Drug Resistance (MDR) is the phenomenon whereby cancer cells develop resistance to cytotoxic drugs

• MDR is a result of over expression of P-glyco-protein: - MDR1 in human;

- mdr1 and pgp1 in rodents• P-glycoprotein utilizes ATP hydrolysis to pump

cytotoxic drugs out of cells

P-glycoprotein as a transmembrane drug efflux pump

• The Multi Drug Resistance gene MDR1, which encodes the cell-surface molecule P-glycoprotein (PGP) can confer resistance to a wide variety of drugs. PGP transport drugs out of the cell, which is a process that requires the presence of two ATP binding domains.These domains are a defining characteristic of this family of ATP Binding Cassete (ABC) transporters.

• The exact mechanism of drug efflux is not well understood, but might involve either direct transport out of the cytoplasm or redistribution of the drug as it transverses the plasma membrane. Some cytotoxic drugs that are known substrates for PGP include etoposide, daunomycin, taxol, vinblastine and doxorubicin. PGP is modified by sugar moieties on the external surface of the protein

c) Vesicular Transport/Endocytosis&Exocytosis:The processes of moving specific macro-molecules

into and out of cells, respectively Pinocytosis&Phagocytosis:• Engulfment particles or dissolved materials by the

cell• For example Vitamin A, D, E, and K, Sabin polio

vaccine and various large proteins.• Receptor-mediated endocytosis• Transcytosis d)Pore (Convective) Transporte) Ion pair transport• For example quaternary ammonium compounds

Pore (Convective) Transport

• Very small molecules such as urea, water and sugars, are able to cross cell membranes rapidly, as if the membrane contained channels or pores. The model of drug permeation through aqueous pores is used to explain renal excretion of drugs and uptake of drugs into the liver

• A certain type of protein called a transport protein may form an open channel across the lipid membrane of the cell. Small molecules including drugs move through the channel by diffusion more rapidly than at other parts of the membrane

Ion Pair Transport• Strong electrolytes drugs are highly ionized or charged

molecules, such as quaternary nitrogen compounds with extreme pKa values – maintain their charge at all physiologic pH values and penetrate membranes poorly

• When the ionized drug is linked up with an oppositely charged ion, an ion pair is formed with neutral charge – diffuses more easily across the membrane : propranolol (a basic drug) paired with oleic acid; quinine paired with hexylsalicylate; complexation of Amphotericin B and DSPG (DiSteroylPhosphatidylGlycerol)

• Ion pairing may transiently alter distribution, reduce high plasma free drug concentration, and reduce renal toxicity

Paracellular Transport/pathway• Water and small hydrophylic molecules pass through

numerous aqueous pores• Transport of material across aqueous pores between the

cells• The cells are joined together via closely fitting tight

junctions on their apical side• Generally, absorptive epithelia tend to be leakier than other

epithelia, decreases in importance down the length of the GIT (decreases in number and size of pores)

• Important for the transport of ions, sugars, amino acids and peptides at concentration above the capacity of their carriers

• Small, hydrophilic and charged drugs• Molecular weight cut-off: 200 Da• Convective (solvent drag) and diffusive component

The Mechanism of Paracellular Transport

• Filtration• Bulk flow

B. Gastrointestinal physiology

• Look at the GI Tract file• Conclusion of factors affecting GIT absorption

rate:1. Coefficient Partition between lipid-water2. Local blood flow3. Intestine surface area4. Gastric emptying time5. Gastrointestinal motility6. Intestinal motility7. Food8. Formulation factors

First-pass metabolism• Drugs may be absorbed well, but still fail to

reach the systemic circulation• All blood from the gut (except mouth and

lower rectal) passes through the portal system to the liver

• Many drugs are extracted and metabolized on their first-pass – may inactivate the drugs

• The alternative route: bucally (glyceryl trinitrate)

Influence of dietary components on the gastrointestinal metabolism and transport of

drugs• Ingestion of meal ~ physiologic changes

(gastric pH, gastric emptying, hepatic blood flow, etc) that significantly alter the rate and extent of drug absorption

• Components of food ~ alter drug absorption through alteration in drug solubility. Nutritional status ~ variability in the pharmacokinetic of certain drugs

• Grape fruit juice can increase the BA of certain drugs, by reducing presystemic intestinal metabolism, led to renewed interest in food-drug interactions

• Effects of grapefruit flavonoid, naringin, and furanocoumarin, 6’-7’-dihydrocybergamottin, on the activity of CYP3A4. The possibility of grape fruit juice might affect drug absorption via interaction with intestinal P-glycoprotein (P-gp) is being explored

• The use of herbal extracts, phytopharmaceutical raise: cause changes in pharmacokinetics of conventional drugs?

Absorpsi non Oral• Nasal Drug Delivery• Inhalation Drug Delivery• Topical and Transdermal Drug DeliveryNASAL• Nasal mucose ≅ sublingual mucose : good

absorption• Systemic - richly supplied with blood vessel:

α-adrenergic for infants• Local Decongestant : rhinitis• Diabetes incipidus : Desmopressin• Carcinoma prostat : Gonadi Liberin analog

( oligopeptide, damage in GIT)

Inhalation Drug Delivery• Local/systemic• Surface area 70 m2• Bronchodilator• Small particle droplet size

Topical Delivery• Usually : local, • now with Transdermal Drug Delivery:

systemic – patch. • Advantages: continuous release of drug

over a period of time, low presystemic clearance, good patient compliance

• Scopolamin, Nitroglycerin, Estradiol, HRT• Sometimes : local iritation

Skin Absorption• Transepidermal• Absorption barrier: non vascular stratum corneum

(less water content: ± 10%)• Absorption barrier : reservoir• Lipophylic compound with small hydrophylicity :

increase per cutan absorption• Hydrophobic compound : fat, oil, showed low per

cutan absorption because stratum corneum has less lipid

• Skin penetration for lipid-insoluble drug happened through hair folicle, sweat glands, sebaceae glands

Factors enhancing Skin Penetration

• Increasing skin temperature• Using hyperemic stimuli : DMSO• Increasing water content/hydration by compound

like urea• Irritated tissue• Damaging stratum corneum mechanically,

chemically, heat, burnt and wound• Absorption rate depends on age, infants good

absorption through skin because has not yet developed. Be careful to give corticosteroid cream