INTRODUCTION TO

BIOTRANSFORMATION OF DRUG

(EG METABOLISM OF PHENYTOIN AND

CODEINE )

PRESENTED BY

ADAM SHAHUL HAMEED

M.TECH COMPUTATIONAL BIOLOGY

INTRODUCTION

Biotransformation means chemical alteration of the drug in the

body.

Biotransformation is a major mechanism for drug elimination.

Enzymatic processes in liver and other tissues that

modify the chemical structure of drugs render them more

water-soluble, increase their elimination, decrease their

half-life.

Biotransformed metabolites are chemically different from the

parent molecule

Metabolism makes the compound less lipid soluble and more

polar and thus hydrophilic.

TYPES OF METABOLIC TRANSFORMATION

There are two phases in the metabolism of drugs:-

Phase 1 reaction. (Non synthetic phase).

This involves a change in drug molecule. It involves oxidation,

reduction or hydrolysis. This may result in activation, change

or inactivation of drug.

Phase II reaction. (Synthetic phase)

It involves formation of conjugates with drug or its metabolites

formed in phase 1 reaction. The conjugate is formed with an

endogenous substance such as carbohydrates and amino

acids.

OXIDATION

Loss of electrons M = M+ + e-

Gain of oxygen R + O = RO

Hydroxylation; Oxygenation at C,N and S atoms;N or O

dealkylation, oxidative deamination

CYTOCHROME P-450 MONOOXYGENASE

(MIXED FUNCTION OXIDASE)

A large number of families (at least 18 in mammals) of

cytochrome P-450 (abbreviated “CYP”) enzymes exists

Each member of which catalyzes the biotransformation of a

unique spectrum of drugs. some overlap in the substrate

specificities.

This enzyme system is the one most frequently involved in

phase I reactions.

CYTOCHROME P-450 MONOOXYGENASE

(MIXED FUNCTION OXIDASE)

Heme protein

Terminal oxidase of the mixed-function oxidase (MFO)

electron-transfer system

Located in the smooth endoplasmic reticulum of all

major organs and tissues

Uses NADPH as a source of reducing equivalents

THE P450 GENE SUPERFAMILY

Format of nomenclature:

CYPFamily/Subfamily/Gene

The cytochrome P-450 families are referred to using an

arabic numeral, e.g., CYP1, CYP2, etc.

Each family has a number of subfamilies denoted by an

upper case letter, e.g., CYP2A, CYP2B, etc.

The individual enzymes within each subfamily are denoted

by another arabic numeral, e.g., CYP3A1, CYP3A2, etc.

Localization

The primary location of cytochrome P-450 is the liver,

Liver is the major organ of biotransformation

Kidney, Lungs, testes, skin intestines are the secondary organ of

biotransformation.

Other tissues, including:

the adrenals

ovaries and testis

tissues involved in steroidogenesis and steroid metabolism.

The enzyme's subcellular location is the endoplasmic reticulum.

Mechanism of reaction

In the overall reaction:

the drug is oxidized

oxygen is reduced to water.

Reducing equivalents are provided by nicotinamide

adenine dinucleotide phosphate (NADPH), and

generation of this cofactor is coupled to cytochrome

Most of the reactions of cytochromeP450 involve

the addition of a hydroxyl group to a compound

which may be represented as

REDUCTION

Gain of electrons M+ + e- = M

Loss of oxygen RO = R + O

Gain of hydrogen R + H = RH

REDUCTION

Nitro to amino group

Chromium VI to Chromium III

NO NOHNO2 NHH H

Cr6+ + 3 e- Cr3+

CYCLIZATION AND DECYCLIZATION

Cyclization

This is formation of ring structure from a straight

chain compound, e.g. proguanil.

De cyclization

This is opening up of ring structure of the cyclic

drug molecule, e.g. barbiturates, phenytoin.

SYNTHETIC REACTIONS (PHASE 2 METABOLISM)

These involve conjugation of the drug or its phase I

metabolite with an endogenous substrate, generally derived

from carbohydrate or amino acid, to form a polar highly

ionized organic acid, which is easily excreted in urine or bile

Conjugation reactions have high energy requirement

For conjugation to take place, a compound should have an

appropriate group or centre eg – COOH, -OH, -NH, or –SH.

Conjugated metabolites are in variably less lipid soluble than their

parent compound

GLUCURONIDE CONJUGATION

This is the most important synthetic reaction carried out by a group

of UDP-glucuronosyl transferases (UGTs).

Compounds with a hydroxyl or carboxylic acid group are easily

conjugated with glucuronic acid which is derived from glucose.

Examples are chloramphenicol, aspum, paracetamol, lorazepam,

morphine, metronidazole.

Not only drugs but endogenous substrates like bilirubin, steroidal

hormones and thyroxine utilize this pathway.

Acetylation

Compounds having amino or hydrazine residues are

conjugated with the help of acetyl coenzyme-A, e.g.

sulfonamides, isoniazid, PAS, hydralazine, clonazepam,

procainamide.

Methylation

The amines and phenols can be methylated;

methionine and cysteine acting as methyl donors, e.g.

adrenaline, histamine, nicotinic acid, methyldopa,

captopril

SULFATE AND GLYCINE CONJUGATION

Sulfate conjugation

The phenolic compounds and steroids are

sulfated by sulfotransferases (SULTs).

e.g. chloramphenicol, methyldopa, adrenal and sex

steroids.

Glycine conjugation

Salicylates and other drugs having

carboxylic acid group are conjugated with glycine, but this

is not a major pathway of metabolism.

Glutathione conjugation:

Forming a mercapturate is normally a minor pathway. However, it

serves to inactivate highly reactive quinone or epoxide intermediates

formed during metabolism of certain drugs, e.g. paracetamol.

When large amount of such intermediates are formed (in poisoning

or after enzyme induction), glutathione supply falls short-toxic

adducts are formed with tissue constituents to tissue damage.

Ribonucleoside/nucleotide synthesis:

This pathway is important for the activation of many purine and

pyrimidine antimetabolites used in cancer chemotherapy.

MICROSOMAL ENZYMES

These are located on smooth endoplasmic reticulum

(a system or microtubules inside the cell), primarily in liver,

also in kidney, intestinal mucosa and lungs.

The monooxygenases, cytochrome P 450, glucuronyl

transferase, etc. are microsomal enzymes.

They catalyse most of the oxidations, reductions, hydrolysis

and glucuronide conjugation

Microsomal enzymes are inducible by drugs, diet and other

agencies.

NONMICROSOMAL ENZYMES

These are present in the cytoplasm and mitochondria of

hepatic cells as well as in other tissues including plasma

The flavoprotein oxidases, esterases, amidases and

conjugases are nonmicrosomal enzymes.

Reactions catalysed are:

Some oxidations and reductions, many hydrolytic

reactions and all conjugations except-glucuronidation.

1. Cytochrome P-450

2. Alcohol

dehydrogenase

3. Deaminases

4. Esterases

5. Amidases

6. Epoxide hydratases

1. Glucuronyl transferase

(glucuronide conjugation)

2. Sulfotransferase (sulfate

conjugation)

3. Transacylases (amino acid

conjugation)

4. Acetylases

5. Ethylases

6. Methylases

7. Glutathione transferase.

Enzymes catalyzing phase I

biotransformation

Enzymes catalyzing phase II

biotransformation

HOFMANN ELIMINATION

This refers to inactivation of the drug in the body fluids

by spontaneous molecular rearrangement without the

agency of any enzyme, e.g. atracurium.

RESULTS OF BIOTRANSFORMATION

Production of metabolites that are more polar than the

parent drug

Usually terminates the pharmacologic action of the

parent drug

After phase I reactions, similar or different

pharmacologic activity, or toxicologic activity.

METABOLISM OF DRUG LEADS TO

Inactivation of drug: The active drug is converted in to inactive

metabolites & excreted. Ex-lidocaine, ibuprofen. Also conversion of

phenytoin to p-hydroxy phenytoin.

Active metabolites from equally active metabolites: The drug is

converted in to similar active metabolites. Ex- conversion of codeine in to

morphine, having similar activity.

Active metabolites from inactive drug (prodrug): Some drugs given in

the form of inactive form which are made active by metabolism.

Ex- enalapril which activated in the form of enalaprilat, which is prodrug.

METABOLISM OF DRUG LEADS TO

Conversion in to toxic substances: Xenobiotics metabolizing

enzymes are responsible for elimination of drug but may convert it in to

toxic metabolites.

This occurs when the enzyme convert the drug in to unstable

intermediate which has affinity towards cellular component & causing

toxic effect.

They have carcinogenic activity, when the electron deficient atom is

formed which reacted with DNA & RNA of the cell and causing

mutation of gene. Other ex- conversion of paracetamol to toxic

metabolite causing hepatic toxicity.

PHENYTOIN

EPILEPSY:

It is a Chronic medical condition produced by sudden

changes in the electrical function of the brain.

METABOLISM OF PHENYTOIN

Pharmacokinetics of phenytoin

Well absorbed when given orally, however, it is also available as iv. (foremergency)

80-90% protein bound

Induces liver enzymes (Very Important)

Metabolized by the liver to inactive metabolite

Metabolism shows saturation kinetics and hence t ½ increases as the doseincreased

Excreted in urine as glucuronide conjugate

Plasma t ½ approx. 20 hours

Therapeutic plasma concentration 10-20 µg/ml (narrow)

Dose 300-400 mg/day

CODEINE

Codeine or 3methylmorphine (a naturally occurring

methylated morphine) is an opiate used for its

analgesic, antidiarrheal, antihypertensive, anxiolytic,

antidepressant, sedative and hypnotic properties.

Codeine is used to treat mild to moderate pain and to

relieve cough.

METABOLISM OF CODEINE

METABOLISM OF CODEINE

REFERENCE

1.Essential of medical Pharmacology by K.D Tripathi and

Jaypee page no 23 to 30

2.Biochemistry by U.Sathya narayana and U.Chakrapani.

3.Modern Pharmacology with Clinical applications by

Lipinncott (Sixth Edition )

4.Pharmacy Tutor www.pharmatutor.com

5.Wikipedia Biotransformation of Drug.

My Sincere Thanks to SHRIKANTH (Comp Bio )