Factors Affecting Drug Metabolism - The University of ...

Factors Affecting

Drug Metabolism

H HH
O N NH2 O NN O

NN

Prof. Patrick Davis

Basic Med Chem Principles

PHR 143M Fall-08

Factor Affecting Drug Metabolism
The Big Picture

Factor Affecting Drug Metabolism
The Simple View

• Consider: Route(s) of metabolism (Phase-1
& Phase 2) depend on enzymes.

• What happens if metabolism decreased (i.e.,
if enzyme levels or activity go down)?

• What happens if metabolism increased (i.e.,
if enzyme levels or activity go up)?

• Generally changes are quantitative rather
than qualitative.

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Factor Affecting Drug Metabolism

Drug Enz A Metabolite A
Inactive
Enz B
Enz C Metabolite B
Enz D Active

Metabolite C
Toxic

Metabolite D
Undetectable

Factor Affecting Drug Metabolism

Drug Enz A Metabolite A
Inactive
Enz B
Enz C Metabolite B
Enz D Active

Metabolite C
Toxic

Metabolite D
Undetectable

Factor Affecting Drug Metabolism

Drug Enz A Metabolite A
Inactive
Enz B
Enz C Metabolite B
Enz D Active

Metabolite C
Toxic

Metabolite D
Detectable

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Cytochrome P-450 Multiplicity

• Devlin, “Textbook of Biochemistry With Clinical
Correlation”, 5th Ed. (2002); Chap 11, The
Cytochromes P-450’s.

• Lewis, “Guide to Cytochrome P-450: Structure
and Function (2001).

• Foye, “Principles of Medicinal Chemistry” 6th Ed.
(2007); Chap 10, Drug Metabolism.

• D.R. Nelson’s P-450 site::

http://drnelson.utmem.edu/CytochromeP450.html

Cytochrome P-450 Multiplicity

• Superfamily: Current estimates >1000 genes.
• Some of these “pseudogenes”.
• Humans: Current estimates:

~57 distinct P-450’s in 17 “families”.
• Initially simple P-450’s cholesterol & FA’s

form membranes) Millions of years; evolved
Endogenous -> Endogenous + Exogenous

Cytochrome P-450 Multiplicity

Fe Heme Center

P-450
Enzyme
Active
Site

Substrate Binding Site

3

Cytochrome P-450 Multiplicity

Cytochrome P-450

Superfamily Nomenclature

CYP2D6

Cyt P-450
Family (>40% homology)
Sub-Family (>55% homology)
Individual Form

Human P-450 Multiplicity

CYP1 CYP2 CYP3 CYP4 CYP11 CYP17 CYP19 CYP21

1A1 2A6 3A3 4A9 11A1 21A2

1A2 2A7 3A4 4A11 11B1

2B6 3A5 4B1 11B2

2C8 3A7 4F2

2C9 4F3

2C10

2C18

2C19

2D6

2E1

From Foye

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Human P-450 Multiplicity

CYP1 CYP2 CYP3 CYP4 CYP11 CYP17 CYP19 CYP21

Drugs Endobiotics
Xenobiotics

PAH’s Fatty Acids
Aryl Amines Arach Acid

Drugs Endogenous Steroids, Bile Acids
Steroids

Drugs

Human P-450 Multiplicity

CYP1 CYP2 CYP3 CYP4 CYP11 CYP17 CYP19 CYP21

1A1 2A6 3A3 4A9 11A1 21A2

1A2 2A7 3A4 4A11 11B1

2B6 3A5 4B1 11B2

2C8 3A7 4F2

2C9 4F3

2C10

2C18 2D6 = See Table 8.9 (Foye)
2C19 2E1 = See Table 8.10 (Foye)

2D6 Ethanol, Acetaminophen
2E1 3A4/3A5 = See Table 8.12 (Foye)

Factor Affecting Drug Metabolism

• Age • Drug Dose

• Disease • Enzyme Induction

• Species Differences • Enzyme Inhibition

• Gender • Diet

• Pregnancy • Heredity/Genetics
• Environmental =>Pharmacogenetics
=>Pharmacogenomics

5

Factor Affecting Drug Metabolism

• Age: Very Young

– Not fully ‘metabolically competent.’
– Chloramphenicol toxicity in newborns

(Gray Baby Syndrome) due to poor
glucuronidation (virtually no Phase-2
enzymes).
– Fetus: CYP3A Sub-family only
(poor metabolism overall).
– FDA questions re fetus/infants….

Factor Affecting Drug Metabolism

• Age: Elderly

– Diminished metabolism and excretion.
– Diminished enzyme induction.
– Multiple drugs: average 10 drugs/patient

(health care facilities)
– Dose using escalation strategy.
– Beer’s List

Factor Affecting Drug Metabolism

• Disease

– Hepatitis, hepatic cancer, nephritis, etc.
– General decrease with acute or chronic

liver disease (assess).
– Is drug cleared by liver??

Overdose danger!

6

Factor Affecting Drug Metabolism

• Species Differences

– Inter- and intraspecies variation.
– Cats form sulfates (lack UDP-GT) but

Pigs form glucuronides (lack of
sulfotransferase enzymes).
– Animal models for predicting metabolism.

• Humans have ONE CYP2D isoform
(CYP2D6).

• Rats have SIX CYP2D isoforms.

Factor Affecting Drug Metabolism

• Species Differences

Rabbits COOH
Guinea Pigs
O
Humans

NH2 NH2

Rats

HO

Factor Affecting Drug Metabolism

• Gender

– Humans few examples
(contraceptives?).

– Differences probably hormonally based
(menstrual cycle can affect PKin).

– N-Demethylation of erythromycin F>M.
– Propranolol oxidation M>F.
– Significance not well understood.

7

Factor Affecting Drug Metabolism

• Pregnancy

– Pregnancy = Concern for fetus (Age)
– Placenta high in CYP1A family if smoker.

Consequences to fetus or neonate:
teratogenicity, carcinogenicity, hepatotoxicity
– Can have profound induction in pregnancy.
e.g., may have to increase anticonvulsants.

Factor Affecting Drug Metabolism

• Environmental Factors
• Example: Cigarette smoke

– Cigarette Smoke -> PAH’s
– PAH’s -> Induce CYP1A2
– CYP1A2 metabolizes PAH to carcinogens.
– Carcinogens -> lung & colon cancer.
– Difficult to correlate (carcinogenesis complex

and takes a long time)

Factor Affecting Drug Metabolism

• Drug Dose

– Classic: Acetaminophen
– Try graphing Dose/Toxicity curve!

UDP-GT Acetaminophen
Sulfo- Glucuronide

Acetaminophen Transferase Acetaminophen
Sulfate
CYP2E1
p-quinoneimine
Toxic

8

Factor Affecting Drug Metabolism

• Enzyme Induction => More Enzyme!
• “Adaptive” process based on exposure.
• Transcriptional (classic derepression):

“inducer” + receptor protein =>
binds “repressor” upstream of regulatory
gene resulting in derepression (expression).

• Post-Transcriptional: Stabilize mRNA.

Factor Affecting Drug Metabolism

• Enzyme Induction: Example

Oral CYP3A4

Contraceptive Inactive, Excreted

Steroids Induction

Rifampin

Consequences??

Factor Affecting Drug Metabolism

• Enzyme Induction: Example

Acetaminophen CYP2E1 p-Quinone Imine

(TOXIC!)

Induction

Ethanol

Consequences??

9

Factor Affecting Drug Metabolism

• Enzyme Induction: Example

Warfarin CYP3A1

CYP2B2 OH-Warfarins

(inactive)

Induction

Phenobarbital

Consequences?? Consequences??
=> Levetiracetam (Keppra®)

Factor Affecting Drug Metabolism

• Enz Induction: Drug/Herbal Interaction

CYP3A4 Metabolites

Cyclosporin (inactive)

(Immunosuppressive)

Factor Affecting Drug Metabolism

• Enz Induction: Drug/Herbal Interaction

CYP3A4

Cyclosporin Metabolites

(Immunosuppressive) Induction (inactive)

St. John’s Wort (hyperforin)

Consequences: Liver transplant rejection!

[Transplantation, 71:239 (2001)]
Huge number of interactions coming to light!

10

Factor Affecting Drug Metabolism

• Enzyme Induction: Additional Points

– Especially P-450 Isozymes:
– 3A4 Inducible (See Table 8-11)

(large number of drugs affected; Table 8-12).

– 2D6 Not Inducible.
– 2E1 Inducible

(large number of drugs and solvents affected;
Table 8-10).

Factor Affecting Drug Metabolism

• Enzyme Inhibition: Example

Warfarin CYP3A1

CYP2B2 OH-Warfarins

(inactive)

Inhibition

Chloramphenicol (AB)
Miconazole (AF)

Consequences??

Factor Affecting Drug Metabolism

• Enzyme Inhibition: Example

Terfenadine CYP3A4 Active
(Seldane®)
Antihistamine

Inhibition

Erythromycin

Ketoconazole

Many drugs

Consequences?? => Arrhythmias

Consequences?? => Fexofenadine

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Factor Affecting Drug Metabolism

• Enzyme Inhibition: Example

Many CYP2D6 Inactive
Drugs
Metabolites

Inhibition

Quinidine

Consequences??

Factor Affecting Drug Metabolism

• Enzyme Inhibition: Example

Many Many Inactive
P-450’s

Drugs Metabolites

S Inhibition
S

HN N NH Cimetidine O NH

NC CH3 O2N CH3
NH CH3 N
N H
CH3

Consequences?? => Ranitidine

Factor Affecting Drug Metabolism

• Enzyme Inhibition: Additional Points

– Virtually any enzyme involved in
metabolism.

– 3A4 Inhibition (See Table 8-11)

(large number of drugs affected; Table 8-12).

– 2D6 Inhibition (See Table 8-11)

(large number of drugs affected; Table 8-9).

12

Factor Affecting Drug Metabolism

• Diet: Example Grapefruit Juice

Terfenadine CYP3A4 Active
(Seldane®)
Antihistamine

Inhibition

Grapefruit Juice

(Bioflavinoids, e.g. naringin)

Consequences??

Critically assess Austin Statesman Article

Factor Affecting Drug Metabolism

• Heredity/Genetics

– Genetic Polymorphism = demonstrably
different forms or levels of enzyme(s) in
distinct population

– Defined by >1% incidence in population.
– e.g. lower levels or non-functional P450’s

-> “poor metabolizers” -> toxicity.
– e.g. higher P450 levels.

-> “fast metabolizers” -> therapeutic failure.
– Inter-individual or interracial differences.

Pharmacogenomics

“Influence of DNA-sequence variation
on drug response”

• A. Var. in drug metabolizing enzymes
• B. Var. in drug transport (abs, dist, excr)
• C. Var. in drug targets (receptors, enzymes)

=> adverse effects
=> therapeutic failures
=> drug interactions

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Pharmacogenomics

Drug results are ‘polygenic’:

• A. Enzymes = 
• B. Transport = 
• C. Receptors = 

Pharmacogenomics

Drug results are ‘polygenic’:

• A. Enzymes =
• B. Transport =
• C. Receptors =

• D. Other Factors (environment, induction,
inhibition, foods) => Complex

Factor Affecting Drug Metabolism

• A Clear Example: CYP2D6!

– Genetic Polymorphism: 80 different alleles*

– SNP’s = Single Nucleotide Polymorphisms.

– Also gene duplication (1-13 copies)
– => Many possibilities

• Normal enzyme (fully functional), normal amts.
• Normal enzyme in diminished amounts.
• Poorly active or inactive enzyme.
• Massive amounts of enzyme.

* http://www.imm.ki.se/cypalleles

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Factor Affecting Drug Metabolism

• A Clear Example: CYP2D6!

– CYP2D6 normal enzyme
– CYP2D6*4 = defective splicing => inactive
– CYP2D6*2xn = gene duplicate => 2X active
– CYP2D6*5 = gene deletion => no enzyme
– CYP2D6*17 = 3 bases => dec substrate affin

• Note nomenclature; it’s important!

http://www.imm.ki.se/cypalleles

Factor Affecting Drug Metabolism

• A Clear Example: CYP2D6!

– => Four 2D6 phenotypic sub-populations

• Poor metabolizers (PM)
• Intermediate metabolizers (IM)
• Extensive metabolizers (EM)
• Ultrarapid metabolizers (UM)

http://www.imm.ki.se/cypalleles

Factor Affecting Drug Metabolism

• Heredity/Genetics: Example CYP2D6

– Marker Probe: Debrisoquine NH

N NH2

NH
N NH2

OH

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Factor Affecting Drug Metabolism

• A Clear Example: CYP2D6!
– Inter-racial Differences

• 5-10% Caucasians (European) => PM’s
• 1-2% SE-Asian (descent) => PM’s

– => Potential consequences?

• PM’s toxicity
• UM’s failure
• Prodrugs? (e.g. consider codeine to morphine

bioconversion for each group)

Factor Affecting Drug Metabolism

• A Clear Example: CYP2D6!

– Racial (geographic) differences in gene duplication.
– 104-fold variation in rates.
– Arose 3-5,000 years ago (alkaloid metabolism)

Factor Affecting Drug Metabolism

• Azathioprine (Antileukemia Drug)

CH3

N

N SH CH3
S S

NO2 N N TPMT N N
N N

N N N N N N
H H H

• Autosomal codominant inheritance [TPMTH/TPMTH]

• If homozygous recessive (1% Caucasians
[TPMTL/TPMTL]) no activity.
=> low TPMT => fatal myelosuppression.

16

Factor Affecting Drug Metabolism

• Azathioprine (Antileukemia Drug)

Factor Affecting Drug Metabolism

• Azathioprine (Antileukemia Drug)

CH3
N

N SH CH3
S S

NO2 N N TPMT N N
N N

N N N N N N
H H H

• RBC test for TPMT deficiency (first PGenomic)

• Serious liability issues.

Factor Affecting Drug Metabolism

• Heredity/Genetics: Example INH

– N-Acetyltransferase NAT-2): Isoniazid

H HH
O N NH2 O NN O

NN

– "Slow Acetylators" (50% Caucasians and African-
Americans; only 5% Asian-Amer)

– "Fast Acetylators" (Eskimos & Asian-Amer)
– Applies to INH, phenelzine, procainamide.

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Factor Affecting Drug Metabolism

• Heredity/Genetics: Example INH

– N-Acetyltransferase: Isoniazid

Factor Affecting Drug Metabolism

• Heredity/Genetics: Excellent Pharmacogenetics &
Pharmacogenomics Reviews on Website
– “Pharmacogenomics: Translating Functional
Genomics into Rational Therapeutics”, W.E. Evans

and M.V. Relling, Science, 286:487 (1999).

– “Inheritance and Drug Response”, R. Weinshilboum,

New Eng. J. Medicine, 348:529 (2003).

– “Pharmacogenomics - Drug Disposition, Drug
Targets, and Side Effects”, W.E. Evans, H.L. McLeod,

New Eng. J. Medicine, 348:538 (2003).
– “Cytochromes P450, Drugs and Disease”, F.P. Guengerich,

Molecular Interventions, 3:194 (2003).

Factor Affecting Drug Metabolism

•

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Factor Affecting Drug Metabolism

Age Drug Dose

Disease Enzyme Induction

Species Differences Enzyme Inhibition

Gender Diet

Pregnancy Heredity/Genetics

Environmental =>Pharmacogenetics
=>Pharmacogenomics

• May look complex, but it's usually COMMON

SENSE -and-

PREDICTABLE!

Factors Affecting

Drug Metabolism

H HH
O N NH2 O NN O

NN

Prof. Patrick Davis

Basic Med Chem Principles

PHR 143M Fall-08

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