Drug Metabolism

Consequences of Drug Metabolism Enzyme (E) Substrates (s)

Drug (D)

MAJOR

Active

Influence of Substrate on Enzyme Activity E + S ↔ ES → P

Inactive

MINOR

Inactive Less Lipid Soluble more readily excreted Active

Toxic

Non-toxic

Non-toxic

Toxic

Active

Equal, less or more active

Effect of Enzyme Level on Activity Amount of substrate transformed

Dr. Robert G. Lamb Professor Pharmacology & Toxicology

Metabolites (DS)

E + S ↔ ES → P

4x

3x

2x

1x

Increasing enzyme concentration

Drug Metabolism

Time of reaction

Cellular Location of Drug Metabolizing Enzymes

Overall Metabolism Scheme

reduction

1

Hydrolysis of Aspirin O R

–

E

C – OR

H2N

O C CH3 COOH

E

OH COOH

O C

R - COOH + ROH acid alcohol

O

aspirin

Hydrolysis of Procaine

E = plasma esterase

C2H5 O

CH2

CH2

Short-acting local anesthetic

N C2H5

procaine (novocaine)

E = plasma esterase

O + HO C CH3

salicylic acid

Hydrolysis of Aspirin

acetic acid

H2N

O C

C2H5 O H

para-aminobenzoic acid (PABA)

+ HO

CH2

CH2

N C2H5

diethylaminoethanol

Procaine Hydrolysis Amide Hydrolysis

Lidocaine Hydrolysis Reduction of Chloral Hydrate

2

Overall Scheme of Oxidative Metabolism

P450-Dependent Drug Oxidation

NADP+

Non-specific system associated with ER

NADPH+

1- Substrate Binding

P 450 Reductase

Multiple forms of CYP-P450 [enzyme] 1A2[12%] induced by Smoking and Charcoal Cooking 2B6 [20%] induced by Phenobarbital (PB) and Rifampin 2E1 [6%] induced by Alcohol and Isoniazid 3A4 [28%] induced by PB, Phenytoin, Rifampin, etc.

Flavoprotein (reduced)

Flavoprotein (oxidized)

2- Substrate Reduction

2

e-

3- Substrate Oxygenation P -4 5 0 -F e 2 +

RH

3

NADPH Cytochrome P450 reductase [enzyme]

RH

P -4 5 0 -F e 3 +

O2 RH

P -4 5 0 -F e 2 + O2

H 2O

Substrates: Oxygen, NADPH and Drug

1

Regulation of Oxidative Metabolism 1. Level of CYP-P450 and Reductase Enzymes Higher in alcoholics and smokers (more drug) Higher with drug intake (PB etc.) [more drug] Lower in elderly, infants (less drug) 2. Level of substrates (drugs, oxygen and NADPH)

Examples of Oxidative Metabolism I N-Oxidation Primary amines Secondary amines

RNH 2

S

S

R2

Deamination

O

R2 OH

RCHCH 3

R

NH2

Desulfuratio n

Thioridazine, cimetidine, chlorpromazine

R1

C CH3

R

NH2

R1

Amphetamine, diazepam.

NH

S

Thiopental. C

N

OH

2-Acetylaminofluorene, acetaminophen.

R2

R1

Nicotine, methaqualone

R1 N

R2 N

O

R3

Role of Phase I and II Reactions Phase I reactions usually precede Phase II reactions. Phase I reactions produce chemically reactive sites. Phase II reactions occur at reactive sites.

O

R1 C

R2

CCH 3 + NH 3

R1

R2

Tertiary amines

Aniline chlorphentermine

RNHOH

R1

R3

Examples of Oxidative Metabolism II

4- Product Dissociation

R-OH (oxidized product)

R2

R1

4- Substrate Rearrangement

4 P -4 5 0 -F e 3 +

R-H (parent drug)

S-Oxidation

3- Substrate Reduction

e-

Phase II metabolites are usually inactive.

O

R2

3

Glucuronidation of Aspirin Acetylation of Sulfanilamide NH2

NH CO +

UDPG is UDP-glucuronic acid

SA = salicylic acid metabolite of aspir

E is glucuronosyl transferase

OH E COOH UDPG

CH3

CH3

N - AT

SO2NH2

SO2NH2 Sulfanilamide

OC6H9O6 COOH

HOOC

Acetic Acid

Acetylsulfanilamide

N-AT is N-Acyltransferase

ether glucuronide of SA 10%

SA

Methylation Reactions

Glycine Conjugation of Aspirin Metabolite (SA)

OH + H 2N COOH

CH2 COOH

N - AT

OH CONH

CH 2 COOH

HO HO

OH

CH3

CH CH2 NH

OMT SAM

epinephrine

SA (salicylic acid)

CH3

OH

CH3O HO

CH CH2 NH

metanephrine

salicyluric acid

glycine

75% major metabolite of Aspirin

HO HO

OH CH CH2 NH2

norepinephrine

NMT SAM

HO HO

OH

CH3

CH CH2

NH

epinephrine

O-, N-methyltransferase (OMT & NMT) S-Adenosylmethionine (SAM)

Acetaminophen Hepatotoxicity ACETAMINOPHEN HNCOCH3

HNCOCH3

PAPS

SULFATE

OH

45 - 50%

HNCOCH3

UDPGA

P-450 MIXED FUNCTION OXIDASE

GLUCURONIDE

45 - 50%

HO-N-COCH3 OXIDATIVE STRESS (•OH, O 2 •–)

4 - 5%

OH

POSTULATED TOXIC INTERMEDIATES

NCOCH3

HIGH DOSE (10-15g)

LOW DOSE (1-2g)

Key Factor

GLUTATHIONE 1+

HNCOCH3

Diseases: Hyperthyroidism

Conditions: smoking, alcoholism

HNCOCH3

O CELL MACROMOLECULES

GLUTATHIONE

MERCAPTURIC ACID

Enzyme Induction (slow) increases drug clearance

Drugs [many]: PB, Rifmpin, Phenytoin, etc.

NUCLEOPHILIC CELL MACROMOLECULES

OH

Factors Influencing Drug Metabolism I

Higher doses of drugs are required

OH

Alcoholic N-Acetylcysteine

Only one induction period then stable level CELL DEATH

4

Factors Influencing Drug Metabolism II Enzyme Inhibition (fast) reduces drug clearance Diseases:Hypothyroidism, Liver Disease Drugs (many): Chloramphenicaol, Cimetidine, Disulfiram, Ethanol (acute), etc. Conditions: Pregnancy, Aging, Newborn

Factors Influencing Drug Metabolism III Age: low metabolism in elderly and newborn start low and go slow with drug dose Nutrition: high metabolism with chronic intake of alcohol and charcoal cooked food and lower with high acute alcohol intake.

Factors Influencing Drug Metabolism IV Genetic Variations: Isoniazid [prophylaxis of tuberculosis] produces liver injury in slow acetylators. Succinylcholine [surgical muscle relaxant] produces prolonged respiratory depression (apnea) in patients with abnormal plasma cholinesterase which reduces the hydrolysis of succinylcholine.

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