DRUG METABOLISMDRUG METABOLISMF. Keyhanfar Ph.D,

D.I.C.

METABOLISM (BIOTRANSFORMATION)------------------------------------

Acetaminophen, NSAIDs inhibit cyclo-oxygenase

Amitriptylline & Cimetidine

Phenytoin

Diazepam -

Dopa & Carbidopa

Succinycholine

Isoniazid

TerminologyTerminology Metabolism: (Gk: metabole: change)Metabolism: (Gk: metabole: change) Metabolite: metabolism produce metaboliteMetabolite: metabolism produce metabolite Xenobiotics: xeno (Gk: foreign)… bioticsXenobiotics: xeno (Gk: foreign)… biotics Inhibition Inhibition (Latin inhibitus, ): to prohibit from (Latin inhibitus, ): to prohibit from

doing somethingdoing something Induction (Induction (Latin inducere): to induce: to call Latin inducere): to induce: to call

forth or bring about by influence or forth or bring about by influence or stimulationstimulation

Cytochrome P450s (CYPs)Cytochrome P450s (CYPs)

METABOLISM (BIOTRANSFORMATION)------------------------------------

The processes by which foreign molecules (Xenobiotics) are

chemically altered by a living organism.

METABOLISM (BIOTRANSFORMATION)

• The body recognizes these molecules and, if it is unable to eliminate

them unchanged, is able to increase their water

solubility by a variety of enzymic reactions

Result------------

• Water soluble metabolites

• Increased Excertion

• Reduced Biological Half-life

• Minimum Toxicity

Phase I Phase IIDRUG METABOLITE CONJUGATE

Expose or introduce a Conjugate the functional functional group that groups exposed or introduced can be conjugated by during Phase I biotransformation Phase II enzymes

Small in water solubility Large in water solubility

• Termination of Pharmacological activity or introduce toxicity

• The rate and extent to which a drug is metabolized determines the dose of the drug and the duration of the effect of the drug

Rate limiting/Affected by genetic and

environmental factors

Active/Inactive/Toxic/Mutagenic/Carcinogen

OH OGlucuronide

Two-phase biotransformation

Phase I (functionalization) reactions: Oxidation, Reduction, and hydrolytic reactions

(makes the drug more polar, but not necessarily inactive)

Phase II (conjugation) reactions: Conjugation to polar groups: glucuronidation,

sulfation, acetylation (most of these result in drug inactivation)

•Ultimate effect is to facilitate elimination

BIM

M11

8

• Phase I Reactions– Oxidation– Reduction– Hydrolytic cleavage– Alkylation (Methylation)– Dealkylation– Ring cyclization– N-carboxylation– Dimerization– Transamidation– Isomerization– Decarboxylation

Cytochromes P-450 (CYP)

Flavin Monooxygenase (FMO)

Monoamine Oxidase (MAO)

Aldehyde dehydrogenase

Alchohol dehydrogenase

Various amidases/esterases

BIM

M11

8

Drug Metabolism - Phase II

• Conjugation reactions– Glucuronidation by UDP-Glucuronosyltransferase:

(on -OH, -COOH, -NH2, -SH groups)

– Sulfation by Sulfotransferase:

(on -NH2, -SO2NH2, -OH groups)

– Acetylation by acetyltransferase:

(on -NH2, -SO2NH2, -OH groups)

– Amino acid conjugation

(on -COOH groups)– Glutathione conjugation by Glutathione-S-transferase:

(to epoxides or organic halides)– Fatty acid conjugation

(on -OH groups)– Condensation reactions

An Example of Drug An Example of Drug MetabolismMetabolism

Non – specificNon – specificHydrolytic enzymeHydrolytic enzyme

Complex Metabolism

Metabolism

Hepatic microsomal enzymes (oxidation, conjugation)

Extrahepatic microsomal enzymesExtrahepatic microsomal enzymes (oxidation, conjugation)(oxidation, conjugation)

Hepatic non-microsomal enzymesHepatic non-microsomal enzymes (acetylation, sulfation,GSH, (acetylation, sulfation,GSH, alcohol/aldehyde dehydrogenase,alcohol/aldehyde dehydrogenase,hydrolysis, ox/red)hydrolysis, ox/red)

Important CYP Isoforms (12 )

• CYP3A4 & 3A5 & 3A7

• CYP2D6

• CYP1A1 &1A2

• CYP2A6

• CYP2B6

• CYP2C8 & 2C9 & 2C19

• CYP2E1

Important CYP Isoforms (12 )

• CYP3A4 VERAPAMIL (CA CHANNEL BLOCER)

• CYP2D6 VERAPAMIL

• CYP1A1 &1A2

• CYP2A6

• CYP2B6• CYP2C8 & 2C9 VERAPAMIL

• CYP2C19

• CYP2E1

Important CYP Isoforms• CYP3A4

Midazolam, triazolam, cyclosporine, erythromycin, Ca channel blockers THE MOST PREDOMINANT IN HUMAN – AMOUNT IN THE LIVER & VARIETY OF DRUG BEING SUBSTRATES

50% of all CYP-mediated drug oxidations

also involved in the greatest number of drug-drug interactions.

active site is large , accept substrates up to M.wt = 1200

drugs bind in different regions of the enzyme active sites

CYP3A4Two drugs are metabolized by this enzyme.

Will coadministration result in a

drug–drug interation ?

drugs bind in different regions of the enzyme active sites

Two drugs can occupy the active site simultaneously both being available for metabolism by the enzyme.

absent interations

CYP3A5

Amino acid sequence is similar to that of CYP3A4,

It is not present in all individuals.

Patients expressing both CYP3A4 & 3A5

Shows increased metabolism of

CYP3A substrate

CYP3A7Expressed only in the fetus

Disappears following birth and replaced by CYP3A4 & 3A5

Different enzyme expression patterns and thus different drug metabolism capabilities throughout

the various stages of life.

CYP2D6

Tricyclic antidepressants, codeine, dextromethorphan, antipsychotics

antiarrhythmics etc

Its relative abundance in the liver is low.

Exhibit genetic polymorphism

MetabolismMetabolism

• Amitriptylline is metabolized by CYP1A2

• Cimetidine inhibits CYP1A2

• Coadministration results in elevated Amitriptylline levels

Cimetidine, Ritonavir, amiodarone, diltiazem, ketoconazole

Inhibit CYP3A4

Cimetidine, Fluoxetine, amiodarone

Inhibit CYP2D6

Cimetidine, Ketoconazole, Omeprazole

Inhibit CYP2C19

Potent InhibitorsPotent Inhibitors• Fluoroquinolones (ie: ciprofloxacin)

• H2Blockers (ie: most notably cimetidine)

• Imidazoles (ie fluconazole)

• INH

• Ritonavir

Mnemonic: “cip, cim, con, INH, and rit”

Barbiturates, Carbamazepine, Phenytoin, pioglitazone, glucocorticoids, …

Induce CYP3A4 & 3A5

Phenobarbital, dexamethasone

Induce CYP2A6 & 2B6 & 2C9

Smoking , Omeprazole

Induce CYP1A1 &1A2

Potent InducersPotent InducersNeuroleptics:

• Carbamazepine

• Phenobarbital

• Phenytoin

AND:

• Rifampin

Mnemonic: “carb, barb, pheny, and rif”

Hydrolysis• Esters & Amides Hydrolyzed by :

ESTERASES & AMIDASES

Found in :

Cytosol of cells in tissues

Plasma & microsomes

Metabolic Overloading

• Chemical being metabolised by an alternate pathway, not necessarily comparable

to metabolic stress in human

exposure situations

• The chemical may be unmetabolised

and accumulate

or stored in the body or be

excreted unchanged.

First order kinetics

A constant fraction of drug is eliminated per unit of time.

When drug concentration is high, rate of disappearanceis high.

Zero order kinetics

Rate of elimination is constant.

Rate of elimination is independent of drug concentration.

Constant amount eliminated per unit of time.

Example: Alcohol

Aspirin, Ethanol

Phenytoin

Metabolism rate is constant

Phase II Metabolism (conjugation)

GlutathioneGlucuronidation

SulfationGlycine

Acetylation Methylation

Coenzymes needed: various

Sulfation may produce active metabolite

N

N

NH2

O

H2N N

N

N

NHO

H2N N

S

O

HO

O

Minoxidil Minoxidil-sulfate

Paracetamol Overdose

• Most common drug taken in overdose

• Few symptoms or early signs• As little as 12g can be fatal• Hepatic and renal toxin

–Centrolobular necrosis• More toxic if liver enzymes induced

or reduced ability to conjugate toxin

Acetaminophen and p-Aminophenols

Acetanilide, 1886(accidental discovery ofantipyretic activity; high toxicity)

Phenacetin or acetophenetidin, 1887 (nephrotoxic, methemoglobinemia)

Acetaminophen, 1893

Recognized as active metabolite of acetanilide and phenacetin in 1948 (Brodie &Axelrod); popular in US since 1955

70-90%75-80%

HNCOCH 3

OH

HN

COCH 3

OC 2H5

NH2

OC 2H5

HN

COCH 3 NH2

Acetominophen Metabolism

~60% ~35%

CYP2E1*CYP1A2CYP3A4

NAPQIN-acetyl-p-benzoquinone imine

*induced by ethanol, isoniazid

Protein adducts,Oxidative stressToxicity

HN

COCH 3

OH HN

COCH 3

OSO 3H

HN

COCH 3

OO CO 2H

OH

OHHO

N

O

COCH 3

Paracetamol Metabolism

Management• General measures

• <8 hours– Take level after four hours

– Start N-aceylcysteine– Patients are usually declared fit for discharge from medical care on completion

of its administration.

– Patients should be advised to return to hospital if vomiting or abdominal pain develop or recur

N-acetylcysteine

• Supplies glutathione

• Dosage for NAC infusion - ADULT– (1) 150mg/kg IV infusion in 200ml 5% dextrose over 15 minutes,

then– (2) 50mg/kg IV infusion in 500ml 5% dextrose over 4 hours, then– (3) 100mg/kg IV infusion in 1000ml 5% dextrose over 16 hours

• Side-effects– Flushing, hypotension, wheezing, anaphylactoid reaction

• Alternative is methionine PO (<12 hours)

•Acetaminophen overdose results in more calls to poison control centers in the United States than overdose with any other pharmacologic substance.

•The American Liver Foundation reports that 35% of cases of severe liver failure are caused by acetaminophen poisoning which may require organ transplantation.

•N-acetyl cysteine is an effective antidote, especially if administered within 10 h of ingestion [NEJM 319:1557-1562, 1988]

•Addition of N-acetyl cysteine to acetaminophen tablets proposed to prevent liver toxicity. [British Medical Journal, Vol. 323, Sept. 15, 2001]

Acetaminophen Toxicity

Metabolism

Factors affecting drug metabolism

• Drug metabolism can be affected by:

– First pass effect

– Hepatic blood flow

– Liver disease

– Drugs which alter liver enzymes

Main site of drug metabolism = LIVER

The phenomenon of

“first pass effect”

or

“first pass metabolism”

and its clinical relevance

Some drugs are ineffective when given orally – examples: nitroglycerine, nor-adrenaline, insulin

Drug Admin: Formulation

• First Pass EffectBlood from the gastrointestinal tract passes through the liver before entering any other

organs. During this first pass through the liver, a fraction of the drug (in some cases nearly all) can

be metabolized to an inactive or less active derivative. The inactivation of some drugs is so

great that the agents are useless when given orally.

(e.g.. lidocaine)

Factors affecting drug metabolism

• Genetic factors– e.g acetylation status

• Other drugs– hepatic enzyme inducers– hepatic enzyme inhibitors

• Age– Impaired hepatic enzyme activity

• Elderly• Children < 6 months (especially premature babies)

Factors affecting biotransformation

• age (reduced in aged patients & children)

• sex (women slower ethanol metabilizers)

• species (phenylbutazone 3h rabbit, 6h horse, 8h monkey, 18h mouse, 36h man); biotransformation route can change

• clinical or physiological condition

• other drug administration (induction (not CYP2D6 ) or inhibition)

• food (charcoal grill ++CYP1A)(grapefruit juice --CYP3A)

• first-pass (pre-systemic) metabolism

Factors Influencing Activity and Level of CYP Enzymes

Nutrition 1A1;1A2;2E1; 3A3; 3A4,5

Smoking 1A1;1A2

Alcohol 2E1

Drugs 1A1,1A2; 2A6; 2B6; 2C; 2D6; 3A3, 3A4,5

Environment 1A1,1A2; 2A6; 1B; 2E1; 3A3, 3A4,5

Genetic Polymorphism

1A; 2A6; 2C9,19; 2D6; 2E1

Red indicates enzymes important in drug metabolism

EnantiomerDrug Shape

– Lock and Key phenomenon for the drug and its receptor site

– More than half of all useful drugs are chiral molecules and exist as enantiomeric pairs

Ex: Carvedilol has a single chiral center and two enantiomers

S(-) isomer – potent Beta receptor blockerR(+) isomer – 100-fold weaker at Beta

receptor– One drug enantiomer is often more susceptible than the

other to drug-metabolizing enzymes• One enantiomer could have a longer or shorter

duration of action than the other

Racemic mixtures

Drug Shape

– As of now, most clinical studies of drugs have only tested racemic mixtures (two enantiomer

pairs)

– 55% of drugs are only available as racemic mixtures where a patient is receiving 50% inactive

or actively toxic drug

Pharmacogenetics• For pharmacokinetics,

change may occur in– drug transporters

–drug metabolizing enzymes

» more active enzymes

» Inactive enzymes

» semi-active enzymes

Succinylcholine

● Used during anesthesia to induce muscle Used during anesthesia to induce muscle paralysisparalysis

● Paralysis usually lasts minutes, but in some Paralysis usually lasts minutes, but in some individuals, it may last up to one hourindividuals, it may last up to one hour

● Due to altered kinetics of Due to altered kinetics of pseudocholinesterase pseudocholinesterase

Isoniazid

● Used in the treatment of tuberculosisUsed in the treatment of tuberculosis● Observed variation in the amount of unchanged Observed variation in the amount of unchanged

isoniazid in the urineisoniazid in the urine● Differences were due to an individuals ability to Differences were due to an individuals ability to

convert isoniazid to acetylisoniazid.convert isoniazid to acetylisoniazid.● Caused by mutations in the Caused by mutations in the N-acetyltransferase-2 N-acetyltransferase-2

enzyme (NAT2) on chromosome 8enzyme (NAT2) on chromosome 8● Some individuals develop isoniazid toxicity Some individuals develop isoniazid toxicity

manifested as peripheral neuropathymanifested as peripheral neuropathy