DRUG METABOLISM

Presented By:-Dr. Roohana Hasan

JR-1

Introduction Drug is any substance or product that

is used or is intended to be used to modify or explore physiological systems or pathological states for the benefit of the recipient.

Biotransformation means chemical alteration of the drug in the body.

The metabolites formed are less lipid soluble.

What does biotransformation do?It converts lipid soluble compounds to lipid insoluble so that they are not reabsorbed.

Role of biotransformation:› defensive mechanism› increases polarity of drug molecules,

restricts penetration through cellular membrane

reduces distribution promotes elimination

Sites of Metabolism

Liver

GIT

Lungs

Kidney

Plasma

Skin

Nasal Mucosa

Others

Consequences of Biotransformation

Inactive metabolite from an active drug

Active metabolite from an active drug

Active metabolite from an inactive drug

Chemical Pathways Phase 1 reactions1. Mainly microsomal - oxidation reduction hydrolysis

2. Few non microsomal-

3. Metabolite may be active or inactive.

Phase 2 reactions

1. Synthetic/Conjugation reactions

2. Can be catalysed by microsomal, mitochondrial or cytoplasmic enzymes.

3. Metabolite – mostly inactive

Endogenous radical

Structure of Cytochrome

Location of CYPs in the Cell

CYP- Oxidoreductase Complex

RH+02+NADPH+H+ ----------------► R0H+H20+NADP+

Drug Metabolising Enzymes

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OTHER36%

CYP2D62%

CYP2E17%

CYP 2C17%

CYP 1A212%

CYP 3A4-526%

CYP 1A214%

CYP 2C914%

CYP 2C1911%

CYP2D623%

CYP2E15%CYP 3A4-5

33%

RELATIVE HEPATIC CONTENT OF CYP ENZYMES

% DRUGS METABOLIZED BY CYP ENZYMES

Reactions involved in Drug Metabolism

Oxidation- This reaction involves addition of oxygen /negatively charged radical or removal of hydrogen /positively charged radical.

Oxidations are the most important drug metabolizing reactions.

Various oxidation reactions are: hydroxylation;oxygenation at C,N or S atoms; N or 0-dealkylation, oxidative deamination, etc.

Oxidative reactions are mostly carried out by a group of monooxygenases in the liver which in the final step involve a cytochrome P-450 haemoprotein, NADPH, cytochrome P-450 reductase and molecular 02.

More than 100 cytochrome P- 450 isoenzymes differing in their affinity for various substrates (drugs) have been identified.

Eg Imipramine, diazepam, codiene, phenytoin, barbiturates, paracetamol.

Reduction- A chemical reaction in which hydrogen s added to, or oxygen is removed from a compound.

Alcohols, aldehydes, quinones are reduced.

Drugs -chloralhydrate, chloramphenicol, halothane, warfarin.

Hydrolysis -This is cleavage of drug molecule by taking up a molecule of water.

Ester + H20 Esterase Acid + Alcohol

Hydrolysis occurs in liver, intestines, plasma and other tissues.

Examples are choline esters, procaine, lidocaine, procainamide, aspirin, c

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Flavin containing Monooxygenases (Ziegler’s enzyme)

Super family of phase I enzymes expressed at high levels in the liver bound to ER.

Six families of FMOs, FMO3 – most abundant in the liver. Metabolize nicotine, H2 receptor blocker, antipsychotics

[clozapine], antiemetics [itopride]. Genetic deficiency Fish odor syndrome due to lack of

metabolism of TMAO [ trimethylamine N oxide ] TMA. Minor contributors to drug metabolism - produce benign

metobolites. Not involved in drug-drug interactions.

Eg : Itopride metabolized by FMO3 : Cisapride metabolized by CYP3A4

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Conjugation Enzymes Phase 2 conjugation enzymes are synthetic in

nature – result in formation of metabolite with increase in molecular mass.

Terminate biological activity of the drug. Characteristic feature – dependency on the

catalytic reaction for cofactors such as UDP-GA, & PAPS,for UGT & SULT, which react with available functional groups on the substrates.

All reactions are carried out in cytosol of the cell, exception of glucuronidation.

Catalytic rates of phase 2 reaction are significantly faster than rates of CYP’s.

So rate of elimination depends on Phase 1.

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Glucuronidation

Most important phase 2 reaction catalyzed by UDP-Glucuronosyltransferases (UGTs).

UGT2 – Greater specificity for glucuronidation of steroids. UGT1A1 – Glucuronidation of bilirubin. Crigler Najjars syndrome type 1 & type 2 Most common genetic polymorphism – Gilberts

syndrome (10% popuplation) (mutation in UGT1A1 gene).

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Sulfation

SULT located in cytosol, metabolise various substrates.

13 SULT isoforms identified – role in human homeostasis.

SULT1B1 catalysis of cholesterol .SULT1A3 selective for catecholamineSULT1E1 estrogens are sulfatedSULT2A1 DHEASULT1 Sulfation of phenolic

molecules. Eg:- acetaminophen and minoxidil. SULT1A1 Most abundant in human tissue.

Glutathione conjugation The glutathione-S-transferases (GSTs)

catalyze the transfer of glutathione to reactive electrophiles, a function that serves to protect cellular macromolecules from interacting with electrophiles that contain electrophilic heteroatoms (-O, -N, and -S) and in turn protects the cellular environment from damage.

Glutathione exists in the cell as oxidized (GSSG) or reduced (GSH) forms, and the ratio of GSH:GSSG is critical in maintaining a cellular environment in the reduced state.

A severe reduction in GSH content can predispose cells to oxidative damage.

Over 20 human GSTs have been identified and divided into two subfamilies: the cytosolic and the microsomal forms.

The major differences in function between the microsomal and cytosolic GSTs reside in the selection of substrates for conjugation;

The cytosolic forms have more importance in the metabolism of drugs and xenobiotics,

Whereas the microsomal GSTs are important in the endogenous metabolism of leukotrienes and prostaglandins.

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N-acetylation› The cytosolic N-acetyltransferases (NATs) are

responsible for the metabolism of drugs and environmental agents that contain an aromatic amine or hydrazine group.

› NATs are among the most polymorphic of all the human xenobiotic drug-metabolizing enzymes.

› There are two functional NAT genes in humans, NAT1 and NAT2.

› The therapeutic relevance of NAT polymorphisms is in avoiding drug-induced toxicities.

NAT1 is ubiquitously expressed among most human tissues, whereas NAT2 is found predominantly in liver and the GI tract.

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Methylation Xenobiotics undergo O-, N-,S- methylation. N-methyltransferase are COMT, POMT, TPMT. The common theme among the MTs is the

generation of a methylated product, substrate specificity is high and distinguishes the individual enzymes.

TPMT – catalysis the S- methylation of aromatic and cyclic sulfhydryl compounds.

Genetic deficiency of TPMT – severe toxicities of thio purine drugs.(Azathioprine , 6-mercaptopurine )

Enzyme Induction Drugs on repeated administration stimulate

the growth of smooth endoplasmic reticulum.

This induction usually reversible, leads to increased microsomal enzyme activity.

Therefore, metabolism is increased and pharmacological response in decreased.

Mostly in liver, but can also occur in intestine, lung, placenta, kidney.

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1. It reduces efficacy and potency of drugs metabolized by these enzymes.

2. It reduces plasma half-life and duration of action of drugs.3. It enhances drug tolerance.4. It increases drug toxicity by enhancing concentration of

metabolite, if metabolite is toxic.5. It increases chances of drug interactions.6. Can be used for therapeutic benefits.

Clinical importance of enzyme induction

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Induction of Metabolism

CYP 450 enzymes and their inducers

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CYP3A4 Induction By Atorvastatin

It takes 3-10 days to induce and 1-3 weeks to return to normal levels after stoppage of inducer

Atorvastatin

Enzyme Inhibition One drug may inhibit the metabolism of another

drug with resultant increase in the circulating levels of the slowly metabolised drug and prolongation of its pharmacological effects.

Enzyme inhibition can be of hepatic microsomal MFOs or of enzymes having specific functions. Eg:xanthine oxidase, monoamine oxidase.

Rapid and usually reversible process.

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INHIBITORS ENZYME INHIBITED Drugs affected

Cimetidine Hepatic microsomal mixed function oxidase

Phenytoin, Warfarin, Anti Depressants, Theophyline, Diazepam, Quinidine,

Testosterone

Sodium valpaorate

Hepatic microsomal mixed function oxidase

Phenytoin, phenobarbital, primidon

Erythromycin Hepatic microsomal mixed function oxidase

Theophyline, Warfarin, carbamazepine, cyclosporin

Disulfiran,tolbutamide,

metranidazole

Aldehyde dehydrogenase Alcohol,Warfarin

Carbidopa L-aromatic aminoacid decorboxylase

L-dopa

Clinical significance Adverse consequences1. Unexpected nausea, vomiting and

tremors may occur when Theophylline is given with erythromycin.

2. Enhanced bleeding tendency when Dicumerol is given with cimetidine.

3. Severe respiratory depression may occur when Morphine is given with MAOs.

Therapeutically beneficial consequences

1. Increased accessibility of L-dopa in brain when given along with carbidopa.

2. Aversion to alcohol after prior administration of disulfiram.

3. Reversal of skeletal muscle paralysis due to tubocurarine by neostigmine.

Factors affecting Drug Metabolism

Age

Sex

Species

Race

Genetic variations

Nutrition and diet

Role of Xenobiotic Metabolism in the Safe and Effective Use of Drugs

Metabolism normally results in the inactivation of their therapeutic effectiveness and facilitates their elimination.

The extent of metabolism can determine the efficacy and toxicity of a drug by controlling its biological t1/2.

If a drug is metabolized too quickly, it rapidly loses its therapeutic efficacy.

This can occur if specific enzymes involved in metabolism are naturally overly active or are induced by dietary or environmental factors.

If a drug is metabolized too slowly, the drug can accumulate in the bloodstream; as a consequence, the pharmacokinetic parameter AUC (area under the plasma concentration-time curve) is elevated and the plasma clearance of the drug is decreased.

This increase in AUC can lead to overstimulation or excessive inhibition of some target receptors or undesired binding to other cellular macromolecules.

While environmental factors can alter the steady-state levels of specific enzymes or inhibit their catalytic potential.

The phenotypic changes in drug metabolism are also observed clinically in groups of individuals that are genetically predisposed to adverse drug reactions because of pharmacogenetic differences in the expression of xenobiotic-metabolizing enzymes.

Before a new drug application (NDA) is filed with the Food and Drug Administration, the route of metabolism and the enzymes involved in the metabolism must be known.

As a result, it is now routine practice in the pharmaceutical industry to establish which enzymes are involved in metabolism of a drug candidate and to identify the metabolites and determine their potential toxicity.

Thank you..!!