Drug Metabolism / Biotransformation

Dr Kiew LV

Objectives

• Define drug elimination and describe the two processes of elimination, i.e.metabolism and excretion

• List the major phase I and phase II reactions and give examples of enzymes (esp.CYP450 enzymes) involved in metabolism.

• Describe the effects of drug metabolism (esp. in liver) and the clinical relevance ofdrug metabolism.

General info ondrug metab.

General Rx steps+ exceptions

Enzyme info +Rx examples

MFO & Cyp450

En. Induction /inhibition

Genetics

Drug Elimination

• All processes which reducethe effective drug concentrationin the body fluids

• Two main mechanisms:

- Metabolism / biotransformation(in liver + other tissues)

- Excretion (in kidney & GIT)

Drug biotransformation / metabolism

• Conversion of drugs into another chemical species by body enzymes

• Purpose:– To inactivate drug (detoxification / remove drug’s therapeutic effects)

• Location: Mainly in liver cells (smooth endoplasmic reticulum, sER)

• General direction for conversion:

inactivate the drug and make the drug more polar / hydrophilic (for renalexcretion) via Phase I & Phase II metabolisms

Inactive** polardrug metaboliteDrug

Very polar inactive drugmetabolite conjugate

Phase I Rx Phase II Rx

Excreted if the metabolite is sufficiently polar Excreted

Inactive** polardrug metaboliteDrug

Very polar inactive drugmetabolite conjugate

Phase I Rx Phase II Rx

Excreted if the metabolite is sufficiently polar Excreted

H OH

Phase I Rx: + polar group

Phase II Rx:+ bigger polargroup (conjugate) bigger polar

group

excretion

excretion

Water soluble metabolite

Drug

** Although phase 1 reaction intends to deactivate drugs, the reaction may also:

– converts a pharmacologically inactive compound (a prodrug) topharmacologically active drug.e.g. levodopa → dopamine

enalapril → enalaprilatzidovudine → AZT-triphosphate

– turn an active drug into active / reactive / toxic metabolitese.g. diazepam → nordiazepam → oxazepam (more active)

paracetamol → N-acetyl-p-benzoquinoneimine (NAPQI)(reactive/toxic)

Inactive** polardrug metaboliteDrug

Very polar inactive drugmetabolite conjugate

Phase I Rx Phase II Rx

Excreted if the metabolite is sufficiently polar Excreted

Phase 1 Phase 2

Objective To add / unmask a polar functionalgroup to drug (e.g. –OH, –NH2, –SH, –COOH)

serves as the point to attack inphase II reaction

Drugs are either inactivated / activated

To add a conjugate to the polar functional groups of thephase I drug metabolite

phase II metabolites (much more polar & often inactive*)

* except for a few, e.g. morphine 6-glucuronide

Location sER sER sER sER Cytosol Cytosol Cytosol

Main Rx Oxidation reduction hydrolysis Glucuronidation(most common)

Sulphation Acetylation Glutathione(GSH)conjugation

Mainenzymes

Mixed-functionoxygenase(MFO)system

CytochromeP450reductase(part of MFO)

Esterase &Amidase

UDP-glucuronosyl -transferases(UGT)

Sulfo -transferases

N-acetyl -transferases

Bile acid-CoA: amino acidN-acyl –transferases

GlutathioneS -transferases

* cytoplasmic alcohol dehydrogenase: oxidize alcohol

Drug biotransformation / metabolism

Oxidation: add oxygen

Alkyl alcohol

Reduction: Add hydrogen / remove oxygen

-N=N- or -NO2 -NH2

azo nitro amines

Hydrolysis:breakdown molecule, + water

Ester Alcohol Acid

Glucuronidation: add glucuronic acid

Sulphation: add sulphate

Acetylation: add acetyl group

Glutathione conjugation: add glutathione (GSH)

e.g. drug metabolism: Paracetamol metabolism

P2 P2

P2

P1

N-acetyl-p-benzoquinoneimine(NAPQI):- Toxic paracetamol metabolite

- may accumulate in the liverwhen GSH is depleted fulminant liver failure

- GSH will deplete in cases of:- Paracetamol overdose- Fasting

- Antidote: acetylcysteine

* Around 5% paracetamol willtake this path

• Enzymes of the smooth ER (sER) of liver cells = microsomal enzymes

Reason: when isolated from liver cells for in vitro study, sER reform itself into vesiclescalled microsomes

• Main microsomal enzymes in the liver :enzymes of the mixed-function oxygenase (MFO) system

• MFO system : 4 components, catalyze drug oxidation

– Cytochrome P450– Cytochrome P450 reductase– NADPH as a proton carrier Drug-H + O2 + 2H+ + 2e– → Drug-OH + H2O– O2

• Other microsomal enzymes:catalyze reactions e.g. dehydrogenation, hydroxylation, epoxidation, oxygenation

Microsomal enzymes in the liver

Mixed-function oxygenase (MFO) systemCytochrome P450

- key enzymes in MFO system & phase 1drug metabolism

- important enzyme in pharmacogenetics /pharmacogenomics studies

- has many subtypes

Enzymes Substrates

CYP2E1 Acetaminophen

CYP2D6 dextromethorphan

CYP1A2 Acetaminophen,caffeine, polycyclichydrocarbons

CYP3A4(most abundant)

Acetaminophen,nifedipine, cyclosporine

CYP2C19 omeprazole

* Acetaminophen = paracetamol

Other locations for drug metabolism

Enzymes involved:MFO, monoamine oxidases (MAOs), Catechol-O-methyltransferase (COMT),

peroxidases, plasma cholinesterase etc.

Location Substrate / drug metabolized

GIT epithelial cells Tyramine, salbutamol, NA, adrenaline

Lung epithelial cells Prostaglandin E2

kidney epithelial cells Imipenam

Skin epithelial cells Betamethasone , Sulfamethoxazole

Plasma ACh, suxamethonium, procaine

Drugs /substance Target en. Effect on enzyme Influence on other drug’s activity

Phenobarbitone CYP450(CYP2B1)

Induction(

↑ warfarin metab.↓ warfarin effect

(pharmacokinetic drug tolerance)

Phenobarbitone CYP450(CYP3A4)

Induction ↑ paracetamol metab.

↑ N-acetylp-benzo-quinone imine (NAPQI)↑ toxicity

Phenytoin

Carbamazepine

Rifampicin

CYP450 Induction ↑ benzodiazepines & cyclosporin metab.

↓ benzodiazepines & cyclosporin effect

Polycyclichydrocarbon(chemicals incigarette smoke )

Cabbage

CYP450 Induction ↑ theophylline (bronchodilator) metab.↓ theophylline effect

(pharmacokinetic drug tolerance)

Metabolic enzyme induction vs. drug metabolism

Drugs /substances

Targetenzyme

Effect on enzyme Influence on other drug’s activity

Cimetidine

Sulphonamides

Ketoconazole

CYP450 Inhibition ↓ warfarin metab.↑ warfarin effect

Grapefruit juice

Alcohol(acute ingestion)

CYP450 Inhibition ↓ benzodiazepines & cyclosporin metab.

↑ benzodiazepines & cyclosporin effect

Metabolic enzyme induction / inhibition vs. drug metabolism

* Chronic ingestion ofalcohol induces CYP450

Genetic predisposition & drug metabolism

• Differences in genetic (inherited) makeup amongindividuals affect drug metabolism.

• Pharmacogenetics :The study on the genetic differences that cause the variation in patient drug response

• Variations in genetic makups of individual code different drug metabolizing enzyme subtypes that has different level ofactiveness

• Slow metabolizer: people with enzyme that process (metabolize) drugs slowly Accumulation of drug risk of toxicity

• Fast metabolizer: people with enzyme that process (metabolize) drugs quickly Fast elimination of drug risk of underdose

Pharmacogenomics : Study on the influence of genetic variation on drug response in patients bycorrelating gene expression or single-nucleotide polymorphisms with a drug's efficacy or toxicity.

Enzyme People e.g. drug effect

N-acetyltransferase

slow acetylators

fast acetylators

Isoniazid(treat tuberculosis)

Higher [plasma isoniazid],longer t1/2 in slow acetylators

Plasmapseudocholin-esterase

Low level enzyme

Normal level enzyme

Suxamethonium Prolonged muscle relaxationin individual with low levelenzyme (> 3hr vs. 6 min innormal)

cannot breathe on theirown as soon after surgery

need a ventilator for anextended time.

Cytochrome P450enzyme (CYP2D6)

Metabolizer type:

• Ultra-rapid• Extensive / fast• Intermediate• Poor

Codeine(changed intomorphine byenzyme)

Variable analgesic effects

Genetic predisposition affecting pharmacokinetics