Chapter 13. Drug Metabolism

Introduction: the process of drugs in the body includes absorption, distribution, metabolism and elimination. Drug metabolism is also named “drug biotransformation”

Drug Blood

Tissues

Kidney

Effects

Metabolism

Elimination

Absortionor

Injection

Important Terms• Biotransformation: Processes of drugs

or toxins in the body, which may change the physical, chemical or biological properties of the drugs or toxins.

• Bioavailability: F, the fraction of the dose that reaches the systemic circulation. F=1 for IV administration.

• Distribution: Movement of drug from the central compartment (tissues) to peripheral compartments (tissues) where the drug is present.

• Elimination: The processes that encompass the effective "removal" of drug from "the body" through excretion or metabolism.

• Half-Life: the length of time necessary to eliminate 50% of the remaining amount of drug present in the body.

Routes of Administration

• Oral• Injection: Intravenous, Subcutaneou

s, Intramuscular, Intraperitoneal• Transdermal (patch)• Mucous membranes of mouth or nos

e (includesnasal sprays)

• Inhalation• Rectal or vaginal

1. Biotransformation and the enzymes

The major site for drug biotransformation is the liver. The extrahepatic sites include: the lung, kidney, intestine, brain, skin, etc.

The major organelles for drug biotransformation is microsome, and others include cytosol and mitochondria.

The major enzymes for drug biotransformation are microsomal enzymes.

Drug 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)

Reactions in biotransformation

Include Phase 1 & Phase 2 Reactions.

Phase 1: involves metabolic oxygenation, reduction, or hydrolysis; result in changes in biological activity (increased or decreased)

Phase 2: conjugation—bound by polar molecules or modified by functional groups, in almost all cases results in detoxication.

1) The first phase reactions

A. Metabolic oxygenation

Microsomal enzymes catalyze hydroxylation, dealkylation, deamination, S-oxidation, N-oxidation and hydroxylation, dehalogenation, etc.

a) Hydroxylation Hydroxylations include aliphatic and aro

matic hydroxylation

E x a m p l e s : i b u p r o f e n , p e n t o b a r b i t a l

Aliphatic hydroxylation

R CH 2 CH 3 R CHCH 3

OH

i b u p r o f e n

CO 2 H CO 2 H

HO

p e n to b a r b ita l

HN

N

O

O

H

O

HN

N

O

O

H

O OH

E x a m p le s : a c e ta n ilid e , p h e n y to in , p ro p ra n o lo lE n d o g e n o u s s u b s tra te s : s te ro id h o rm o n e s (n o t a ro m a tic a m in o a c id s )

A ro m a tic H y d ro x y la t io n R

unstable arene epoxideintermediate

non-enzymatic

HYL1epoxide

hydrolase

R OH

OH

R OH

O DNA, Proteintoxic

reactions

O

R

R

OH

R

OH

or

phenyto in

A rene epoxide in term ediate produces m ultip le products

N

N

O

CYP2C8,9 N

N

O

OH

H

HYL1

phenytoin

3,4-dihydro-dihydroxyphentoin

N

N

O

HO

N

N

O

OH

N

N

O

HO OH

para-hydroxyphenytoin meta-hydroxyphenytoin

propranolol O

N

OH

H

ON

OH

H

OH

ON

OH

H

OH

b) Dealkylation

Dealkylations include N-, O- and S-dealkylation.

R-X-CH2-R’

[R-X-CH(OH)-R’]

R-XH + O=CH-R’

[O]

X = O, N, S

N-dealkylationDealkylation of secondary or tertiary amines

will produce primary amines and aldehydes. R N

CH2

CH2

R

CH2

OH

R NCH2

CH2

R

CH2

R NCH2

CH2

R

CH3 R NCH2

CH2

R

CH3+

O 2

-H+-1e-

HCHO+R N

CH2

CH2

R

H

O-dealkylation

Dealkylation of ethers or esters will produce phenols and aldehydes.

Codeine Morphine

S-dealkylation

S-dealkylation usually produces sulfhydryl group and aldehyde.

R-S-CH3 [R-S-CH2OH] R-SH + HCHO N

N

N

N

SCH3

N

N

N

N

SH

HCHO+

6-methylthiopurine 6-thiopurine

[O]

c) DeaminationDeamination may produce ketone and ammon

ia. RCHCH3

NH2

RC

NH2

CH3

OH RCCH3

O+ NH3

NH2 ONH3+

For example, deamination of amphetamine:

d) S-oxidation S-Oxidation

SR

R2

SR

R2

O

For example, S-oxidation of chlorpromazine: S

N Cl

N

S

N Cl

N

O

e) N-oxidation N-Oxidation

R NH2 R NHOH

R NR

RR N

+

RR

O_

N

N

Cl

N

N

Cl

O

For example, N-oxidation of chlorpheniramine

B. Microsomal oxidases and their action mechanisms

The enzymes that catalyze the above oxygenation of drugs are called “mixed- function oxidase” or “monooxygenase”. In the reactions, one oxygen is reduced into water and the other is integrated into the substrate molecule.

RH + O2 + NADPH + H+ ROH + NADP+ + H2O

Mixed-function oxidase contains cytochrome P450 (CYP) and NADPH as electron carrier and hydrogen provider.

The CYP family: Human CYPs – have several types and subtypes, named CYP1, 2, 3…; CYP1a, 1b, and so on. They are important in drug metabolism.

Human Liver CYPs

CYP enzyme

Level (%total)

Extent of variability

1A2 ~ 13 ~40-fold 1B1 <1 2A6 ~4 ~30 - 100-fold 2B6 <1 ~50-fold 2C ~18 25-100-fold 2D6 Up to 2.5 >1000-fold 2E1 Up to 7 ~20-fold 2F1 2J2 3A4 Up to 28 ~20-fold 4A, 4B

2E

S. Rendic & F.J. DiCarlo, Drug Metab Rev 29:413-80, 1997

Electron flow in microsomal drug oxidizing system

CO

h

CO

h

CO

hCYP-Fe+2

Drug

CO

CYP-Fe+2

Drug

CO

O2O2

e-

e-

2H+

H2O

2H+

H2O

DrugDrug

CYPR-Ase

NADPH

NADP+

CYPR-Ase

NADPH

NADP+

OHDrug OHDrug OHDrug

CYP Fe+3CYP Fe+3CYP Fe+3

PCPCDrug

CYP Fe+2

DrugCYP Fe+2

Drug

CYP Fe+2

Drug

O2

CYP Fe+2

Drug

O2

CYP Fe+3

OHDrug

CYP Fe+3

OHDrug OHDrug

C. Other oxidases a) Monoamine oxidaseThese enzymes exist in mitochondria. They catalyze oxidation of amines into al

dehyde and ammonia. For example, degradation of 5-hydroxytryptamine.

RCH2-NH2 RCH=NH RCHO + NH3

[O] H2O

b) Alcohol and aldehyde oxidases

R-CHOH R-CHO R-COOH

Alcohol dehydrogenase

Aldehyde dehydrogenase

D. Reductions

a) Aldehyde and ketone reductases: these enzymes catalyze reduction of ketones or aldehydes to alcohols.

For example:

CCl3CHO CCl3CH2OH

The coenzyme may be NADH or NADPH.

2H

Trichloroacetaldehyde Trichloroethanol

b) Reductases for Azo or nitro compounds These reductases mainly exist in hepatic

mitochondria with NADH or NADPH as coenzyme.

N N N N

H H

NH2

2H 2H2

NO2 NO NHOH NH2

2H 2H2H

Azo

Nitrobenzene

Aniline

E. Hydrolysis

Esters and amides may be hydrolyzed to produce acids and alcohol or amine.

H2N CO

OCH2CH2N(C2H5)2

H2N COOH HOCH2CH2N(C2H5)2+H2O

H2N CO

NHCH2CH2N(C2H5)2

H2O+ H2NCH2CH2N(C2H5)2H2N COOH

Ester(Procain)

Amide(Procainamide)

Para-aminobenzoic acid

2) The second phase reactions

The second phase reactions of drugs are also named “Conjugation Reactions” . These reactions include glucuronidation, sulfation, acetylation, methylation and amino acid binding.

Glucuronidation O

OH

OHOOH

CO2H

P O P O

O

HO

OH

O

CH2

O NNH

O

O

OOH

OHOH

CO2HO R

+ ROH

orR3N

UGT

UDP- -D-glucuronic acidO

OH

OHOH

CO2HN+ R

R

R

O-glucuronide

N+-glucuronide

SulfationPAPS is the phosphate donor.

R OHR O S OH

O

O H H

NH2

N

NN

N

OH

O

H HHO

O P

OH

O

O SOH

O

O

(PAPS, 3’-phosphoadenosine-5’-phosphosulfate)

Acetylation Ar NH2

R SH

R OH

R NH2

+

Ar NCH3

O

H

Acetyl transferase

CoA SO

R NO

CH3H

R OO

CH3

R SO

CH3

Acetylation may reduce the water solubility of the compounds.

P r o c a i n a m i d e

U n c h a n g e di n U r i n e , 5 9 %

3 %2 4 % F a s t1 7 % S l o w

U n c h a n g e di n U r i n e , 8 5 %

N A P A

0 . 3 %

1 %

H 2 N

O

N

H

N N

O

N

H

N

O

H H 2 N

O

N

H

N

H

NO

N

H

N

O

H H

Methylation

Methylation of phenols, amines and biologically active molecules may change their activity or toxicity. Generally, methylation reduces the hydrophilicity of the compound.

S-adenosylmethionine (SAM) is the donor of methyl group.

Methylation includes N- or O-methylation.

Methylation

RH R-CH3

HO

OH

CHOHCH2NH2

HO

OH

CHOHCH2NH CH3

HO

O

CHOHCH2NH CH3

CH3

-CH3 -CH3

Norepinephrine Epinephrine O-methylepinephrine(no activity)

SAM

2. Factors that affect drug metabolism

A. Inducers Inducers are those that promote drug meta

bolism in the body. Most inducers are lipophilic compounds and have no specificity in actions.

Examples: barbital, ether, amidopyrine, miltown (meprobamate), glucocorticoids, vit. C, etc. Repeated administration of these drugs may result in drug-resistance.

The mechanism by which inducers enhance drug metabolism in the body is believed to be the induction of the enzymes involved in the drug metabolism.

For example, phenobarbital stimulates proliferation of SER and increases production of some enzymes in the metabolisn of drugs, such as liver CYPs and UDP-glucuronate transferase, both of which enhance metabolism of many drugs in the liver (oxygenation and conjugation).

B. Inhibitors Inhibitors are those that inhibit drug metabo

lism in the body. Include competitive and non-competitive inhibitors.

a) A drug inhibits the metabolism of other drugs: such as chloramphenicol and isoniazid. They inhibit hepatic microsomal enzymes. Combined administration of these drugs and others such as barbitals may increase the toxicity of the latter.

b) Non-drug compounds inhibit the metabolism of drugs: such as pyrogallol ( 没食子酚 ). This compound inhibits o-methylation of epinephrine and thus enhances the activity of the hormone in body (it competes with epinephrine for methyltransferase).

C. Other factors

a) Species difference.

b) Sex, age, nutrition conditions have effects on drug metabolism.

c) Hepatic functions.

3. Significance of drug biotransformation

A. Effective removal of drug from the body through excretion or metabolism. For example, sulfation and glucuronidation increase secretion of the drug in urine.

B. Change of the biological activity or toxicity of drugs in the body. For example, trichloroacetaldehyde is first reduced into trichloroethanol and then conjugated by glucuronate to become a non-toxic compound.

C. Inactivation of bioactive molecules in the body. For example, some hormones are inactivated through biotransformation in the liver (epinephrine, steroid hormones).

D. Exploration of new drugs. Based on the mechanisms of biotransformation, it is possible to design new drugs with longer half-lives and fewer side-effects.

E. Explanation for the carcinogenic property of some drugs. For example, after biotransformation some “non-toxic” drugs may become toxic or carcinogenic.

N-acetylation may form nitrenium ion which is a potent carcinogenic agent

NH2 NH

OHNH

O

C OCH3

N+

NAT2CYP1A2

Reactive Nitrenium ion

Carcinogenic DNA Adduct

F. The mechanisms of biotransformation may be used to improve the efficacy of drugs. For example, those that are mainly metabolized in the liver may have less efficacy through oral administration than IV route.