Drug Disposition Overview - Pharmacology and at UCSD

BIOM/PHAR 255 Winter 2013 Halpert - Jan. 17, 2013 

Drug Metabolism and Disposition 

BIOM/PHAR 255 

James R. Halpert, Ph.D. 

Tuesday, January 17, 2013 

10:00 -12:00 

• Absorption 

– Transporters 

• Distribution 


• Metabolism 

– Enzymes 

• Excretion 


Routes of Elimination of the Top 200 Prescribed 

Drugs in 2002 (Wienkers and Heath, 2005) Overview 

1 of 22 

Drug Disposition 

• Importance and general principles of drug 

metabolism (Phase I and II) 

• Cytochromes P450 

• Factors affecting drug metabolism and adverse 

reactions 

• Conjugating enzymes


Importance and General Principles 

of Drug Metabolism 

Drug Therapy: The challenges 

1. Non-responders 

Only 30-60 % of patients respond to the 

common drugs. 

2. Serious adverse drug reactions (ADRs): 

5-7 % of all hospital admissions 

2 days prolonged hospital visit 

$2,500 per patient, $100 billion in USA per year 

100,000 deaths annually in US 

2 of 22 

Drug Metabolism in Clinical 

Practice 

• Goals 

– Understand factors that 

affect metabolism of a 

drug 

– Predict metabolism of 

the drug in an individual 

– Design rational therapy 

regimen that is safe and 

effective. 

• Need to know 

– Which enzymes 

(isoforms) metabolize 

the drug 

– How each enzyme is 

affected by genetic 

and environmental 

factors (including other 

drugs) 

– The relative 

therapeutic efficacies 

and toxicities of drug 

and metabolites 

Reasons for termination of drug candidates in 

development in 2000 

Attrition due to PK/bioavailability decreased from 40% to 8% between 1991 and 

2000 in large measure due to incorporation of these concepts in early drug 

discovery. Guengerich, F.P. (2007). Chem Res. 

Toxicol. 20:344-369.


Metabolism → Termination of drug action 

Bioinactivation 

Detoxification 

Elimination 

Metabolism → Bioactivation 

Active Metabolites 

Prodrugs 

Toxification 

Role of the liver in drug 

metabolism 

• The liver is the most important organ in the 

biotransformation of most xenobiotics (foreign 

compounds). 

– All compounds absorbed from the g.i. tract reach 

the liver via the portal vein before they reach 

other organs. 

– The liver is the largest organ in the body and 

contains the highest content of xenobioticmetabolizing 

enzymes. 

3 of 22 

Sites of drug 

biotransformation 

• Liver 

• Gastrointestinal tract 

• Kidney 

• Respiratory tissues 

– lung 

– nose 

• Brain, skin, heart 

Drug metabolism and elimination 

From Weinshilboum, R.(2003) NEJM 348 (6): 529-537


Conversion of a Hydrophobic Drug to a 

Water-Soluble Metabolite 

Phenytoin 

O 

H 

N 

NH 

O 

CYP 

HO 

4-Hydroxy 

phenytoin 

O 

Water solubility 

H 

N 

NH 

O 

UGT 

COOH 

O 

O 

HO OH 

General Pathways of Drug Metabolism 

REACTION ENZYME 

Hydrolysis 

Reduction 

Oxidation 

Esterase, peptidase, epoxide hydrolase 

Azo and nitro reduction 

Carbonyl, disulfide, sulfoxide, quinone 

Alchohol and aldehyde dehydrogenase 

Xanthine and diamine oxidase 

Monomine oxidase 

Prostaglandin H synthase 

Flavin-monooxygenases 

Cytochrome P450s 

Conjugation Glucuronide, sulfate, glutathione, acylation, 

methylation 

4-Hydroxy-phenytoin 

Β-D-glucuronide 

OH 

O 

H 

N 

Expose or 

Introduce. 

-OH 

-NH 2 

-SH 

-COOH 

NH 

Synthetic 

O 

4 of 22 

Phase I: 

Purpose of phase I and phase II 

metabolism 

Detoxify 

Enhance excretion 

RH --> ROH (more water 

soluble and prepares for 

phase II) 

Phase II = Conjugation: 

Enhance excretion 

Detoxify (e.g. GSH) 

Acts on parent drug 

Acts on phase I 

metabolite 

Cytochrome 

P450 

Cytochromes P450 

Excretion 

Xenobiotic 

Phase I 

Metabolism 

Excretion 

Phase II 

Metabolism 

Glucuronides 

Sulfates 

GSH-conjugates 

Other conjugates 

Excretion


General Features of Cytochromes P450 

• Heme protein 

• Located in the smooth endoplasmic reticulum (and 

sometimes mitochondria) of all major organs and 

tissues 

• Require NADPH cytochrome P450 reductase to 

deliver electrons 

• Many are inducible 

Class II (Mammalian) Xenobiotic Metabolism System 

more polar 

metabolite 

H 2O 

Cytochrome 

P450 

heme 

lipophilic substrate 

O 2 

Cytochrome 

P450 Reductase 

FMN 

FAD 

2 NADPH 

2 e - 

2 NADP + 

5 of 22 

Cytochromes P450 (CYP) 

Show peak at 450 nm in the 

P450 red-CO form 

Biochemical reaction: 

RH + O 2 + NADPH + H + ROH + H 2O + NADP + 

Heme pocket of cytochromes P450 

H H 

O 

Fe 3+ 

S 

The heme group in P450 is invariant. It is the size and shape of the 

binding pocket that distinguishes different P450 enzymes.


Types of P450 Reactions 

1. Hydroxylation of aliphatic or aromatic carbons 

2. Epoxidation of a double bond 

3. Heteroatom (S-, N-) oxygenation and N-hydroxyl 

4. Heteroatom (O-, S-, N-) dealkylation 

5. Oxidative group transfer 

6. Cleavage of esters 

7. Dehydrogenation 

P450 oxidation reactions 

Epoxidation (arene oxide formation) 

benzo[a]pyrene 

N-, O- and S-dealkylation 

O 

H3 C CH3 N 

CH3CH2 C C CH2CH CH3 methadone 

O 

benzo[a]pyrene-7,8-epoxide 

H3C H 

O N 

CH3 CH2 C C CH2CH CH3 + 

H C 

H 

O 

6 of 22 

P450 reactions: oxidation at 

C 

aliphatic hydroxylation 

H 3 C 

CH 3CH 2 

O 

CH2 CH2 CH2 CH3 pentobarbital 

aromatic hydroxylation 

CH 3CH 2 

O 

phenobarbital 

N 

NH ONa 

O 

NH ONa 

O 

N 

HO 

H 3C 

CH 3 CH 2 

P450 nomenclature 

Based on sequence identity: 

O 

CH 

OH 

CH 3 CH 2 

CH 2 

N 

O 

CH2 CH3 NH ONa 

O 

NH ONa 

MELSVLLLLALLTGLLLLMARGHPKAYGHLPPGPRP… 

MEFSLLLLLAFLAGLLLLLFRGHPKAHGRLPPGPSP… 

Family: ≥ 40% sequence identity. 

for example: CYP1, CYP2 

Subfamily: 40% - 55% sequence identity. 

for example: CYP2A, CYP2B 

Within subfamily: ≥ 55% sequence identity. 

for example: CYP2B1, CYP2B4 

O 

N


Major Human Liver Drug 

Metabolizing Cytochromes P450 

P450 Substrate Inducer Inhibitor 

1A2 Caffeine PAH Furafylline 

2A6 Coumarin Phenytoin Tranylcypromine 

2B6 Bupropion Phenobarbital Ticlopidine 

2C8 Taxol Rifampicin Trimethoprim 

2C9 Tolbutamide Secobarbital Sulfaphenazole 

2C19 S-Mephenytoin Rifampicin Benzylnirvanol 

2D6 Debrisoquine Quinidine 

2E1 Chlorzoxazone Ethanol Diethyldithiocarbamate 

3A4 Midazolam Rifampicin Troleandomycin 

3A5 Midazolam Glucocorticoids Ketoconazole 

Cytochrome P450 1A2 

Planar aromatic/heterocyclic amines and amides 

Substrates Inhibitor 

Caffeine 

Phenacetin 

Furafylline (MBI) 

MBI = mechanismbased 

inactivator 

7 of 22 

Characteristics of Human P450 Substrates 

•1A2: Planar aromatic/heterocyclic amines and amides 

• 2A6: Small polar compounds with a ketone or nitroso group 

•2B6: Non-planar (often V-shaped), lipophilic molecules 

•2C8: Large organic anions 

•2C9: Weakly acidic compounds 

•2C19:Neutral or basic compounds 

•2D6: Nitrogenous bases with site of metabolism 4-7 Å 

from basic nitrogen 

•2E1 Generally neutral compounds of low molecular weight 

•3A4 Structurally diverse compounds of relatively high 

molecular weight 


Small polar compounds with a ketone or nitroso group 

Substrates Inhibitors 

Coumarin 

(R)-(+)Menthofuran (MBI) 

Tranylcypromine


Cytochrome P450 2B6 

Substrates Inhibitor 

(S)-Mephenytoin 

Buproprion 

Non-planar (often V-shaped), lipophilic molecules 

Cytochrome P450 2C9 

Ticlopidine (MBI) 


Tolbutamide 

Diclofenac 

Weakly acidic compounds 

Tienilic acid (MBI) 

Sulfaphenazole 

8 of 22 


Large organic anions 

Substrate Inhibitor 

Taxol (paclitaxel) Trimethoprim 


Neutral or basic compounds 

Substrate Inhibitor 

(S)-Mephenytoin 

(+)-N-3-Benzyl-nirvanol


Cytochrome P450 2D6 

Nitrogenous bases with site of metabolism 4- 

7 Å from basic nitrogen 


Debrisoquine 

Dextromorphan Hydrobromide 

Testosterone 


Quinidine 

Paroxetine (QI) 

QI = quasi-irreversible 

Structurally diverse compounds of relatively high molecular weight 


Midazolam 

Ketoconazole 

9 of 22 

Cytochrome P450 2E1 

Generally neutral compounds of low molecular weight 


Chlorzoxazone 

N-Nitrosodimethylamine 


4-Methylpyrazole 

Diethyldithiocarbamate 


Erythromycin Troleandomycin (QI)


Cytochromes P450 (Enzyme 

Structures) 

Six Protein Regions that Contribute to 

Substrate Selectivity: SRS 

Substrate Recognition Sites 

(SRS) 

Gotoh (1992) J. Biol. Chem. 267, 

83-90. 

SRS1 

SRS2 

SRS3 

SRS4 

SRS5 

SRS6 

heme 

2B4 

PDB 1SUO 

10 of 22 

General Structure of Mammalian 

Cytochromes P450 

Structural 

components 

alpha helix 

beta sheet 

heme 

protoporphyrin 

IX prosthetic 

group 

2B4 

PDB: 1PO5 

Major structural features that 

determine specificity of P450 enzymes 

• Length and positioning of α−helices and βsheets 

determines size and shape of binding 

pocket 

• Identity of active site residues determines 

binding orientation of substrates


Comparison between the active site cavity volumes for human P450 1A2 (A) (PDB: 2HI4), 

P450 2A6 (B) (PDB: 1Z10 (6)), and P450 3A4 (C) (PDB: 1TQN (14)). 

Sansen S et al. J. Biol. Chem. 2007;282:14348-14355 

©2007 by American Society for Biochemistry and Molecular Biology 

Comparison of P450 Inhibition and Induction 

Inhibition Induction 

Mechanism Direct effect on Synthesis of new 

existing enzyme protein 

Onset Rapid Slow 

Prior exposure Not needed Needed 

11 of 22 

Factors Affecting Drug Metabolism and 

Adverse Drug Reactions 

Age 

Diet 

Disease 

Drug Dose 

Enzyme Induction 

Enzyme Inhibition 

Gender 

Genetics 

Pregnancy 

Metabolic Basis of Drug Interactions 

Perpetrator Victim Consequence 

P450 Inhibitor Drug Increased effect/ADR 

P450 Inhibitor Pro-Drug Decreased effect 

P450 Inducer Drug Decreased effect 

P450 Inducer Pro-Drug Increased effect/ADR


Mechanisms of P450 Inhibition 

•Alternate substrate inhibition 

•Binding of nitrogen heterocycles to heme 

iron (ketoconazole, itraconazole) 

•Binding of a metabolite to the reduced 

heme iron (troleandomycin, paroxetine) 

•Irreversible modification of the protein or 

heme moiety by a reactive metabolite 

(tienilic acid, ticlopidine, chloramphenicol) 

Factors affecting drug metabolism 

Enzyme induction: examples 

12 of 22 

Effect of fluovoxamine on caffeine 

pharmacokinetics (>10-fold increase in 

t 1/2 and AUC) 

Nuclear Receptors That Induce Drug Metabolism 

RECEPTOR LIGANDS 

• Aryl hydrocarbon receptor (AhR) 

• Constitutive androstane receptor 

(CAR) 

• Pregnane X receptor (PXR) 

• Farnesoid X receptor (FXR) 

• Vitamin D receptor 

• Peroxisome proliferator activated 

receptor (PPAR) 

• Retinoic acid receptor (RAR) 

• Retinoid X receptor (RXR) 

• Omeprazole, Flavonoids 

• Phenobarbital 

• Rifampin 

• Bile Acids 

• Vitamin D 

• Fibrates 

• all-trans-Retinoic acid 

• 9-cis-Retinoic acid


Model for PXR in Xenobiotic Regulation 

Xenobiotic Pregnane X 

PXR RXR 

CYP3A gene 

“Other drug metabolism genes” 

Xenobiotic 

CYP3A 

Common Perpetrator Drugs 

HO-Xenobiotic 

Steroid 

HO-Steroid 

•Strong inducers of CYP3A4 (rifampicin, dexamethasone, 

phenobarbital, carbamazepine) 

•Strong inhibitors of CYP3A4 (ritonavir, ketoconazole) 

•Strong inhibitors of CYP2D6 (quinidine, fluoxetine, 

paroxetine) 

•Inhibitors of multiple P450 enzymes (fluvoxamine) 

Common Victim Drugs 

•Drugs with a narrow therapeutic index (warfarin, 

phenytoin, digoxin) 

•Substrates of CYP3A with low oral bioavailability 

(triazolam) 

13 of 22 

Why are clinically significant drug 

interactions relatively uncommon? 

•The interaction, while statistically significant, is 

too small to produce a clinically important 

change in the dynamics of the drug. 

•The therapeutic index of the victim drug is 

large enough that even a substantial change in 

plasma levels does not alter therapeutics or 

toxicity. 

•Kinetics and response to the victim drug are so 

variable that they overshadow changes in 

plasma levels due to the interaction. 

CYP dependent metabolism of drugs (80 % of 

all phase I metabolism of drugs) 

Beta blockers 

Antidepressants 

Antipsychotics 

Dextromethorphan 

Codeine 

Debrisoquine 

CYP2D6* CYP2C19* 

CYP2E1 

CYP1A2 

CYP2B6* 

CYP3A4/5/7 

Cyclosporin 

Taxol 

Tamoxifen 

Tacrolimus 

Amprenavir 

Amiodarone 

Cerivastatin 

Erythromycin 

Methadone 

Quinine 

CYP2C9* 

Tolbutamide 

Warfarin 

Phenytoin 

NSAID 

Diazepam 

Citalopram 

Anti ulcer drugs 

Clozapine 

Ropivacaine 

Efavirenz 

Cyclophosphamide 

40 % of the phase I 

metabolism is carried out 

by polymorphic P450s 

(enzymes in Italics)


Consequences of genetically impaired 

metabolism (similar to chemical inhibition) 

•When drug is inactivated by 

metabolism: Higher blood levels 

= increased therapeutic effect 

and risk for side effects. 

•When drug is activated by 

metabolism: Higher blood levels 

of unchanged prodrug and lower 

levels of active metabolite = 

decreased therapeutic effect. 

Single dose 

Poor metabolizer 

(PM) 

Time 

Mult iple dosage 

PM, t 1 / 2 = 2 4 h 

Cm ean = 2 8 m / L 

EM, t1/ 2 = 6 h 

C mean = 7 .5 

mg/ L 

Time 

Extensive 

m e t aboliz e r 

(EM) 

Molecular Mechanisms that Alter CYP Function 

Log (plasma conc.) 

TiPS, 20: 342, 1999 

14 of 22 

Gene Dosage Effect (similar to induction) 

Nortriptyline 

antidepressant 

CYP2D6 genes 

Major P450 Alleles 

10-hydroxynortriptyline 

TiPS, 20: 342, 1999 

TiPS, 20: 342, 1999


Most Clinically Relevant CYP 

Polymorphisms (2C9, 2C19. 2D6) 

Ingelman-Sundberg, M. (2004) TIPS 25: 193-200 

Metabolic Activation of Ticlopidine 

and Clopidogrel 

Metabolic activation of ticlopidine, 1a, and clopidogrel, 1b, into their pharmacologically active thiol 

derivatives 3a (5) and 3b (6). Metabolite 3b was identified after trapping with acrylonitrile leading to 

3′b (6). 

Published in: Patrick M. Dansette; Julie Libraire; Gildas Bertho; Daniel Mansuy; Chem. Res. Toxicol. 2009, 22, 369-373. 

DOI: 10.1021/tx8004828 

Copyright © 2009 American Chemical Society 

15 of 22 

S-mephenytoin 

hexobarbital 

R-mephobarbital 

phenytoin 

diazepam 

citalopram 

omeprazole 

lansoprazole 

pantoprazole 

CYP2C19 Substrates 

R-warfarin (8-OH) 

propranolol (in part) 

imipramine 

clomipramine 

amitryptylline 

proguanil 

teniposide 

nilutamide 

indomethacin 

moclobemide 

Human Cytochromes P450 Involved in the Oxidation 

of Clopidogrel 

CYP2C19 contributes substantially to both oxidation steps required to 

form the active metabolite of clopidogrel. Kazui, M. et al., Drug Metab 

Dispos. 38:92-9. 2010.


Relationship between CYP2C19 Genetic Classification and 

Pharmacokinetic and Pharmacodynamic Responses after the 

Relationship between CYP2C19 Genetic Classification and Pharmacokinetic and 

Pharmacodynamic Responses after the Administration of Loading and Maintenance Doses of 

Administration of Loading and Maintenance Doses of 

Clopidogrel in Healthy Subjects 

Clopidogrel in Healthy Subjects 

Mega JL et al. N Engl J Med 2009;360:354-362 

Carriers of a reduced-function CYP2C19 allele had lower levels of active metabolite formation, 

diminished platelet inhibition, and higher risk of major adverse cardiovascular effects. 

Role of Genetics in Codeine Metabolism 

1. Codeine to norcodeine and 

codeine 6G accounts for 80% 

of the metabolism 

2. Conversion of codeine to 

morphine by CYP2D6 accounts 

for 10% of the metabolism 

3. Fast CYP2D6 metabolizers will 

accumulate excess morphine 

4. Morphine and morphine 6G 

have opioid activity 

Gashe Y. et al. N. Engl. J. Med 2005 

16 of 22 

Conversion of an active drug to an 

active metabolite 

CYP2D6 

Codeine 

CYP3A4 

Drug metabolizing systems 

Evans, W.E. (1999). Science 286:487-491.


Conjugating Enzymes 

A comparison of the major conjugation 

pathways 

Reaction Cosubstrate Functional Groups Capacity 

Glucuronidation UDP-glucuronic acid -OH, -COOH, -NH 2, 

-SH, -NR 2, -R 3C-H 

Sulfation 3´-phosphoadeno sine-5´phosphosulfate 

(PAPS) 

-OH, -NH 2 

High 

Acetylation Acetyl-CoA -OH, -NH 2 Variable 

Glutathione 

conjugation 

Amino acid 

conjugation 

Low 

Glutathione Electrophiles Low 

Amino acids -COOH Medium 

Methylation S-adenosylmethionine -OH, -NH 2 

17 of 22 

Properties of Phase II Enzymes 

• Catalyze biosynthetic reactions 

• Utilize a high-energy co-factor 

• Generally react with nucleophilic groups (OH 

and NH2 most common) 

• Generally lead to increase in water solubility 

Notable exceptions: 

• Glutathione reacts with electrophiles 

• Acetylation leads to decreased water solubility 

Cofactors Utilized in Phase 2 Reactions 

UDP-glucuronic acid 


N 

H 

COOH 

H 

OH 

O 

H 

H 

OH 

O UDP 

H OH 

Phosphoadenosine- 

Phosphosulfate (PAPS) 

N 

H 2 

N 

N 

N 

O- 

CH2O P 

OH 

O 

P 

O 

O SO 3 - 

C 

H 3 

N 

H 2 

N 

N 

C 

N 

H 2 

CH 2 CH 2 CO NH CH 

H 2 C 

SH 

Acetyl-CoA 

CH 2 NH CH 2 COOH 

N 

OH OH 

N 

CH2O P O P O 

O 

O O 

CH3 OH 

CH2 CH 

H3C CO NH CH2 CH2 S 

O 

C 

OH O PO3H2 S-adenosyl-methionine 

N 

NH 2 

N 

N 

N 

CH2 OH HO 

O 

S + 

CH 3 

CH 2 CH 2 CH 2 COOH 

CH 3


Properties of UGTs 

• Normally detoxifying, but can be involved in bioactivation 

• Glucuronides excreted in bile or urine 

– Smaller glucuronides excreted in urine 

– Larger glucuronides excreted in bile (undergo enterohepatic 

circulation) 

• Genetic defect in UGT1A1 leads to hyperbilirubinemia 

diseases 

• Glucuronidation can be impaired when cofactor is 

depleted (e.g fasting) 

18 of 22 

OH 

OCH 2 CHCH 2 NHCH(CH 3 ) 2 

Propanolol 

(Alkylhydroxy-glucuronide) 

N 

CH 2 CH 2 CH 2 N 

Imipramine 

(Quaternary-glucuronide) 

Examples: UGT Substrates 

(CH 3 ) 2 

HO 

O 

Morphine 

OH 

H 2 N 

N 

N 

CH3 N-Hydroxy-PhIP 

(N-glucuronide) 

N 

Benzidine 

(Arylamine-glucuronide) 

A single gene encodes multiple UGTs with differing 

substrate preferences 

(Ritter et al, 1992) 

Each isoform of UGT1 results from differential splicing of 

exon1s (N-terminal 287 AA) to common exons 2-5 (245 AA). 

1A12 

1A10 1A8 1A6 1A4 1A2 2 3 4 5 

1A11 1A9 1A7 1A5 1A3 1A1 

pseudogene 

H 

OH 

NH 2 

UGT1A


Sulfotransferases 

(sulfate ester formation) 

The reaction 

PAPS + R-OH 

sulfotransferases 

-----------------> R-OSO - 

3 + ADP 

UGT1A1 

ONLY 

Synthesis of PAPS 

SO 2- 

4 + ATP 

ATP sulfurylase 

-----------------> Adenosine 5'-phosphosulfate (APS) + PPi 

APS kinase 

APS + ATP -----------------> 3'-phosophoadenosine- 5'-phosphosulfate 

(PAPS) + ATP 

NHCOCH 3 

NHCOCH 3 

+ 3'-phosphoadenosine- 

5'-phosphosulfate 

(PAPS) 

sulfotransferase 

+ PAP 

OH 

- 

OSO 3 

acetaminophen acetaminophen sulfate 

19 of 22 

Reduced levels of conjugated bilirubin 

Sulfotransferases and sulfonation 

reactions 

• PAPS concentrations in cells are relatively low: highaffinity, 

low-capacity system 

– e.g. acetaminophen is mainly sulfated at low 

concentrations, mainly glucuronidated at high 

concentrations 

• Nucleophilic attack of substrate on electrophilic sulfur of 

PAPS 

• Substrates include 

– -OH groups: alcohols, phenols, catechols 

–NH 2 groups and N-oxides


Human cytosolic sulfotransferases 

SULT Substrates Remarks 

1A1 

1A2 

simple phenols, Minoxidil, 

acetaminophen, N-OHPhIP 

N-OH-AAF, simple phenols 

Most abundant 1A form in liver 

Only found in humans. Protein may not be 

expressed. 

1A3 Dopamine, other catecholamines Low in liver, high in intestine, placenta 

1A4 

1B1 

Catecholamines 

Thyroid hormones Liver, intestine, leukocytes 

1C2 N-OH-AAF, simple phenols 

1C4 N-OH-AAF, simple phenols Highest in fetal liver, kidney 

Highest in fetal liver, kidney 

1E1 estrogens Liver, intestine, secretory endometrium 

Inhibited by PCB metabolites 

2A1 Steroids, DHEA, hydroxymethyl 

PAHs 

Liver, intestine, steroidogenic organs 

2B1 Steroids, cholesterol 

2 variants with alternative exons 1 

4A1 ? 

2 variants. High in brain 

N-Acetyl Transferase 

• Significant route of biotransformation 

• Two enzymes in humans 

NAT1-most tissues, abundant in bladder 

NAT2-primarily in liver 

Aromatic - R-NH 2 

Hydrazines R-NH-NH 2 

CoA-S-COCH 3 

Aromatic - R-NH-COCH 3 

Hydrazines R-NH-NH 2-COCH 3 

NAT2 – approximately 50% slow acetylators in US 

Leads to increased side effects of isoniazid, 

sulfonamides, procainamide, and hydralazine. 

20 of 22 

N-acetyltransferases 

CH 3 

N 

COO - 

CO NH NH 2 

H 2N S NH 2 

sulfanilamide 

O 

CoA-S-acetyltransferase 

+ CoASH CH3 C 

O 

O 

N-acetyltransferase 

H 3 C 

O 

C 

CO NH NH C 

N 

O 

O 

S CoA 

CH 3 

N S NH 2 

H 

Less water soluble 

Glutathione conjugation 

• Enzymatic and nonenzymatic reactions 

• Crucial role in cellular protection from electrophilic, oxidizing xenobiotics 

and their metabolites 

e.g. epoxides 

• Glutathione S-transferases catalyze both substitution and addition reactions 

on electrophilic substrates 

• [GSH] is usually very high in cells-10 mM range 

Substitution 

Addition 

Cl 

NO 2 

+ 

γ-Glu-Cys-Gly 

glutathione (GSH) 

NO2 2,4-dinitro-1-chlorobenzene 

O 

CH2 CH2 + 

γ-Glu-Cys-Gly 

glutathione (GSH) 


S-transferase 


S-transferase 

NO 2 

O 

S glutathione 

NO2 OH 

CH CH2 SG


Properties of Glutathione S-Transferases 

• Cytosolic GSTs: Homo- and heterodimers of at least 15 subunits 

– Alpha family: hGSTA1-5 (main forms in liver and kidney) 

– Pi family: hGSTP1a,1b (allelic variants: lung, gut, tumors) 

– Mu family: hGSTM1-5 (muscle, brain) 

– Theta family: hGSTT1-2 

– Single members of sigma and zeta families in humans 

• Microsomal GST - membrane associated proteins in eicosanoid and 

glutathione metabolism (MAPEG) - 

• Mitochondrial GST - hGSTK1 (soluble) 

• Widespread tissue distribution 

• Some GSTs have noncatalytic binding sites for hydrophobic ligands: 

involved in intracellular transport 

• Differential regulation of GSTs 

– Chemicals 

– Sex differences, development 

– Increased expression in tumors - drug resistance 

Routes of Elimination of the Top 200 Prescribed 

Drugs in 2002 (Wienkers and Heath, 2005) 

Other Conjugation Pathways 

Amino acid conjugation 

COOH 

+ ATP 

CoASH 

CO S CoA 

N-acetyltransferase 

+ NH 2 -CH 2 -COOH 

benzoic acid benzoyl-CoA glycine hippuric acid 

Methylation 

+ S-adenosylmethionine 

methyltransferase 

N H 

(CH2 ) 2 N 

CH3 desmethylimipramine 

N CH3 (CH2 ) 2 N 

CH3 imipramine 

Ingelman-Sundberg, M., J Int Med 250: 186-200, 2001, 

21 of 22 

Interindividual differences in drug action 

CO NHCH 2 COOH 

+ 

+ CoASH 

S-adenosylhomocysteine


Clinically significant drug interactions due to 

inhibition of Phase II enzymes are relatively rare 

because: 

• N-acetylation and sulfation represent the primary 

clearance mechanism for few drugs. 

• UGTs are the primary clearance mechanism for

Drug Disposition Overview - Pharmacology and at UCSD

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