Drug metabolism

Refers to enzyme-mediated biotransformations (detoxication) that alter the pharmacological activity of both endogenous and exogenous compounds.

DrugMetabolic enzymes

Metabolite

Drug metabolism

• Drugs undergo a variety of chemical changes in the animal organism by enzymes of the liver, intestine, kidney, lung, plasma and other tissues.

• Many enzymes take place in such biotransformations; oxidase, hydrolase, lipase, synthetase, dehydrogenase, …etc

Blood

CNS

Muscles

Heart

Metabolism may result in:

– Pharmacologically inactive drug (detoxification).

– Pharmacologically active drug (bioactivation…prodrug approach).

– Change the pharmacological activity (toxic effect).

The importance of studying drug metabolism:

– Understanding the pharmacological and toxicological activity of drugs.

– The importance of shortening the drug’s duration of action.

– The complications of drug-drug interactions mainly depends on the induction or inhibition of metabolic enzymes

Drug metabolism• Can be divided into two distinct categories:

– Phase-I: Reactions which introduce or unmask hydrophilic

groups in the drug structure (functionalisations).

– Phase-II: Reactions which conjugate the drug or its phase-I

metabolite with a hydrophilic, endogenous species (conjugation reactions).

Phase-I reactions

• Aliphatic hydroxylation.• Oxidation:

– Oxidative Dealkylation.– Oxidative deamination.– N and S oxidation.– Alcohol/aldehyde dehydrogenase.

• Reduction.• Hydrolysis.

Phase-II metabolism

• Involves the following conjugation reactions that are catalyzed by transferase enzymes:

– Glucuronidation.– Sulfation.– Amino acid conjugation.– Methylation.– Acetylation.

Phase-I reactions

• Two general types of enzyme systems take part in these reactions:

– Microsomal Mixed Function Oxidases (MFOs)• Flavoprotein, NADPH-monooxygenase• Cytochrome P450

– Non-cytochrome oxidizing enzymes.• Xanthine oxidase• Alcohol/aldehyde dehydrogenase

General features of Cytochrome P-450

• A large number of families (at least 18 in mammals) of cytochrome P-450 (abbreviated “CYP”) enzymes exists as well as many subfamilies.

• each member catalyzes the biotransformation of a unique group of drugs

• some overlap in the substrate specificities.

CYP 450Families and subfamilies

Foye's principles of medicinal chemistry

CYP Biotransformations

• Chemically diverse small molecules are converted, generally to more polar compounds

• Reactions include:

– Aliphatic hydroxylation, aromatic hydroxylation– Dealkylation (N-,O-, S-)– N-oxidation, S-oxidation– Deamination– Dehalogenation

Oxidative Phase-I involving cytochrome P-450 enzymes:

• Aliphatic hydroxylation:– Mainly occur on the ultimate (ω) or penultimate

(ω-1) carbon atom in the structure.– Also it occurs at an activated carbon atom, that is

next to sp , sp2 carbons:

RR

R

O

R R

Oxidative Phase-I involving cytochrome P-450 enzymes:

• Aliphatic hydroxylation:– Mainly occur on the ultimate (ω) or penultimate

(ω-1) carbon atom in the structure.– Also it occurs at an activated carbon atom, that is

next to sp , sp2 carbons:

RR

R

O

R R

Oxidative Phase-I involving cytochrome P-450 enzymes:

• Aromatic hydroxylation:

HN

HNO

O

C6H6

Phenytoin

HN

HNO

O

C6H6 OH

Phase-II conjugate

• Aromatic epoxidation:

Benzo[a]pyrene

O NH

N

N

N

N

O

Covalently bound Deoxyguanosine adduct(systemic toxicity)

O

HO

OH

Deoxyribose

NH

N

N

N

N

O

Deoxyribose

DNA

Oxidative Phase-I involving cytochrome P-450 enzymes:

• Alkene epoxidation:

N

O NH2

Carbamazipine

N

O NH2

O

N

O NH2

HO OH

HN

NH

O

O O

Secobarbital

HN

NH

O

O O

OH

HO

Secodiol

Glutathione conjugation

• For electrophilic drugs and metabolites:

G-SHO

R R SG

HH H HOH

G-SH O OH

SG

HS

NH

O COOH

HNH O

COOHNH2

Glutathione (tripeptide: glutamate+cysteine+glycine)

Detoxication by glutathione adduct formation

OH

O

O

N

O GSH

OH

O

O

N

GS

HO

[O]O

Arene oxide

OHSG

Glutathione conjugation

• Toxicity of aromatic compounds came from the formation of arene oxide during the metabolism that will be attacked by endogenous nucleophile such as proteins, DNA or RNA.

Glutathione conjugation

O

OH

H

NIH

Shift

OH

H2O

Or epoxide hydrolase

OHOH

GSHOH

SG

DNA, RNA, proteins

OHM

• O-dealkylation:

O

N

O

OH

CH3

O

N

HO

OH

Oxidative demethylation

Codeine Morphine

• N-dealkylation:

HN R

OH

NH2R

O

H

N-dealkylation

Cl

NN

C6H6

Cl

NNH

C6H6

N-t-butylnorchlorcyclizine norchlorcyclizine

Br

N

Br

NH2

Br

O

OH

Brompheniramine

Oxidative Phase-I involving cytochrome P-450 enzymes:

• Oxidative deamination:

Oxidative Phase-I involving cytochrome P-450 enzymes:

• N-oxidation:– Mostly for primary and secondary amines as well

as aromatic amines:– This gives N-oxide that will be rapidly converted to

hydroxylamines.

R NH2 R NH

OHR NO2

NH2HN

OHN

O

proteins and nucleic acids

NO2

NOH

Protein

Oxidize Fe+2 in hemoglobin to Fe+3

(methemoglobin or ferrhemoglobin)this form is no longer capable to transport oxygen (methemoglobinemia toxicity)

NO

HNO

2-acetylaminofluorene

NOHO

NO

-O3SO

Sulfate

conjugation

NO

NO

Nu

|Nitrenium ion

NO

Nu

Hydrolytic phase-I metabolism

• By non-specific esterase and amidase enzymes that present in plasma, gut, liver and kidney.

• It has a beneficial role in most of prodrugs that after hydrolysis inside the body release the active form of the drug.

Ester vs. Amide bond

Ester bond is relatively weaker than amide bond, it will be rapidly hydrolyzed by esterase enzyme

R O

O

R NH

O

The reactivity of ester and amide bond depend on how much the carbonyl carbon is electropositive

Nitrogen atom is less electronegative than oxygen, so it will be waeker electron withdrawing atom

therefore, the crabonyl carbon attached to oxygen atom will be more electropositive, and more reactive toward nucleophilic attack of water molecule during hydrolysis.

Nucleophilic attack of hydroxide anion on ester and amide

R O

O

R NH

O

OH OH

Example

ProcaineShort acting local anesthetic

ProcainamideLong acting antiarrhythmic

T1/2 = 2.5-4.5 hrT1/2

= 40-84 second

Hydrolytic phase-I metabolism

• Examples of prodrugs activated by hydrolytic enzymes:

– Dipivefrine: is a di-tertbutylcarboxy ester of adrenaline…. More lipophilic… better penetration through the corneal membrane….then will be hydrolyzed to give the active form (adrenaline)

Why Dipifevrine has been prepared?

– Adrenaline is a polar drug….difficult access into the ocular cavity.

– Adrenaline has a generalized adrenergic effect…. Many side effects such as increase blood pressure, heart rate and bronchodilation.

– Dipifevrine is more lipophilic, better penetration… localized effect.

HO

HO

HN

OH

Adrenaline

Other phase-I metabolic enzymes

• Alcohol dehydrogenase and aldehyde dehydrogenase:

R

OH

R

O

H R

O

OH

Alcohol

dehydrogenase

Aldehyde

dehydrogenase

R

O

RNo oxidation

Other phase-I metabolism• Heterocyclic ring oxidation:

• S-dealkylation:

O

NH

Phenmetrazine

O

NH

O

N

N N

HN

S

6-methylmercapropurine

N

N N

HN

SH

6-mercapropurine

Other phase-I metabolism• Sulfoxidation: by flavin monooxygenase

N

S

N

Chlorpromazine

N

S

N

O

N

S

NH2

O

N

SO

OHO

Deaminated metabolite Dealkylated metabolite

Other phase-I metabolism• Azoreduction:

NN

COOHOH

SHNN

O

O

Azulfidine

NH2

SHNN

O

O

H2N

COOHOH

P-aminosalicylic acidSulfapyridine

Antibacterial action Anti-inflammatory action

General notes regarding phase-I metabolism

• Hydrolysis normally catalyzed by carboxylesterases:– Cholinesterase…. Hydrolyzes choline-like esters (such as

succinylcholine), procaine and acetylsalicylic acid.

– Arylcarboxyesterase.– Liver carboxyesterase

O

O

O

O

N

N

Syccinylcholine

OH

O

OH

O

NHO

N

COOC2H5

HN

COO

HN

COOCH3

HN

COOZwitter ionicPolarEasily excreted

General notes regarding phase-I metabolism

• Esters that are sterically hindered are hydrolyzed more slowly and may be appeared unchanged in urine:

OH

O

O

N

Atropine

50% excreted unchanged in urine

OH

O

O

N

Atropine

General notes regarding phase-I metabolism

• Amides are more stable to hydrolysis than esters….large fraction of amide containing drugs are normally excreted unchanged.

H2N

NH

ON

Procainamide

60% excreted unchanged in urine

H2N

O

ON

Procaine

HN

ON

Lidocaine

Procaine has a short duration of anesthesia

lidocaine has a long duration of anesthesia

CH3

CH3

HN

ON

Lidocaine

Nucleophilic attack of HO-

Aromatic hydroxylation

• The least substituted aromatic ring will be favorably oxidized, especially at the least hindered carbon atom

• The activated ring will be better oxidized (the ring bearing an electron donating group)

• The least substituted aromatic ring will be favorably oxidized, especially at the least hindered carbon atom

NN

Cl

MeclizineAntiemetic agent

NN

Cl

OHMajor metabolite

• The activated ring will be better oxidized (the ring bearing an electron donating group)

HN

HN

N

Cl

Cl

ClonidineNo aromatic hydroxylation