Pharmaco Genetics

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PHARMACOGENETICSThe study of genetically controlled variations in drug response

I. Key Concepts and TermsMonogenic: due to allelic variation at a single gene

Polygenic: due to variations at two or more genes

Polymorphic: frequently occurring monogenic variants occurring at a frequency >1%

Normal DistributionFrequencyActivity

Polymorphic DistributionFrom Pratt WB,Taylor P. Fig 7-5b

GENETIC POLYMORPHISMSPharmacokineticPharmacodynamicTransportersPlasma protein bindingMetabolism

ReceptorsIon channelsEnzymesImmune molecules

From: Evans WE, Relling MV. Pharmacogenomics: Translating functional genomics into rational therapeutics. Science 286:487-491, 1999.

From: Evans WE, Relling MV. Pharmacogenomics: Translating functional genomics into rational therapeutics. Science 286:487-491, 1999.II. Genetic polymorphisms in drug metabolizing enzymes

A. Atypical Plasma Cholinesterasea rapid acting, rapid recovery muscle relaxant - 1951usual paralysis lasted 2 to 6 min in patientsoccasional pt exhibited paralysis lasting hrscause identified as an atypical plasma cholinesteraseHydrolysis by pseudocholinesterasecholinesuccinylmonocholine

Atypical plasma cholinesterase has 1/100 the affinity for succinylcholine as normal enzymeoccurs in 1:2500 individualstested clinically via the abilityof dibucaine to inhibit esterase hydrolysis of benzoylcholineAdapted from: Pharmac Ther 47:35-60, 1990.normal enzyme inhibited > 70%abnormal inhibited < 30%

Atypical plasma cholinesterase has 1/100 the affinity for succinylcholine as normal enzymeoccurs in 1:2500 individualstested clinically via the abilityof dibucaine to inhibit esterase hydrolysis of benzoylcholineFamily studies indicate variability in plasma cholinesterase activity consistent with 2 allelic, autosomal, codominant genesother variant forms exist as well

B. Glucose-6-phosphate dehydrogenase activityEffects >100 million worldwideR-NH2 CYP MPOPGH SynthaseR-NOHERYTHROCYTER-NOH O2HgbFe+2R-NOHgbFe+3Reactive OxygenNADHNAD+MetHgbReductaseNADPHor GSH(?)NADP+ or GSSG(?)HMP ShuntG-6-PDDependentSODCatalaseGSH PeroxidaseDetoxification SplenicSequestrationHemolytic AnemiaGSHSemi-mercaptalsulfinamideR-NH2

Drugs and Chemicals Unequivocally Demonstrated to Precipitate Hemolytic Anemia in Subjects with G6PD DeficiencyAcetanilideNitrofurantoinPrimaquineMethylene BlueSulfacetamideNalidixic AcidNaphthaleneSulfanilamideSulfapyridineSulfamethoxazole

INCIDENCE OF G6PD DEFICIENCY IN DIFFERENT ETHNIC POPULATIONSEthnic GroupIncidence(%)Ashkenazic Jews0.4Sephardic Jews Kurds53 Iraq24 Persia15 Cochin10 Yemen 5 North Africa

INCIDENCE OF G6PD DEFICIENCY IN DIFFERENT ETHNIC POPULATIONSEthnic GroupIncidence(%)Asiatics Chinese 2 Filipinos13 Indians-Parsees16 Javanese13 Micronesians

C. N-ACETYLTRANSFERASE ACTIVITYDistribution of plasma isoniazid concentration in 483 subjectsafter and oral dose. Reproduced from Evans DAP. Br Med J 2:485, 1960.

NAT1*4NAT2*4NAT2*5ANAT2*6ANAT2*7APABAPASSMXPADDSSMZAFModified from Grant DM. Pharmacogenetics 3:45-52, 1993

ETHNIC DIFFERENCES IN THE DISTRIBUTION OF ACETYLATOR PHENOTYPEPopulation% Slow % Hetero Fast% Homo FastSouth Indians 5935.65.4Caucasians 58.635.95.5Blacks 54.638.66.8Eskimos 10.543.8 45.7Japanese 1245.3 42.7Chinese 2249.8 28.2From: Kalo W. Clin Pharmacokinet 7:373-4000, 1982.

XENOBIOTICS SUBJECT TO POLYMORPHIC ACETYLATION IN MAN Hydrazines isoniazid hydralazine phenylzineacetylhydrazine hydrazine Arylamines dapsone procainamide sulfamethazine sulfapyridineaminoglutethimide Carcinogenic Arylamines benzidine-naphthylamine4-aminobiphenylDrugs metabolized to aminessulfasalazinenitrazepamclonazepamcaffeine

ADVERSE EFFECTS TO SULFASALAZINE IN PATIENTS WITH INFLAMMATORY BOWEL DISEASE

ADVERSE EFFECTS TO SULFASALAZINE IN PATIENTS WITH INFLAMMATORY BOWEL DISEASEData from: Das et al. N Engl J Med 289:491-495, 1973. Side Effect cyanosis hemolysistransient reticulocytosis Frequency of side effectSlow Acetylators Fast Acetylators915060

Relationship Between Onset of Lupus Syndrome in Fast and Slow Acetylators Receiving Procainamide. Data from: Woosley RL, et al. N Engl J Med 298:1157-1159, 1978.

Distribution of acetylator phenotype in control subjects and those experiencing a sulfonamide hypersensitivity reaction.Rieder et al. Clin Pharmacol Ther 49:13-17, 1991.

NAT1N-acetyl-SMXUDPGTSMX-glucuronideCYP2C9MPOPGH SYNTHASESMX hydroxylamineNitrosoDetoxCovalent binding tocellular macromolecules/cytotoxicityHypersensitivity/Adverse ReactionO-acetylationAcetoxy esterNAT1HydroxamicacidN,O-ATDetoxified metabolite

D. CYP2D6 ACTIVITY

DRUGS WHOSE METABOLISM CO-SEGREGATES WITH DEBRISOQUINEalprenololamitriptylinebufuralolclomipraminecodeinedesipramineencainideethylmorphineflecainidefluoxetineguanoxanimipraminemetoprololnortriptylineparoxetinephenforminpropafenonepropranolol

Plasma metoprolol concentrations in poor () and extensive () metabolizers of debrisoquine after 200 mg of metoprolol tartrate administered orally. Redrawn from Lennard MS, et al. NEJM 307:1558-1560, 1982.

Dose requirements for nortriptyline in patients with different CYP2D6Phenotypes. From: Meyer U. Lancet 356:1667, 2000.

O-demethylationmorphineN-demethylationnorcodeine6-glucuronidationcodeine-6-glucuronideM-6-GM-3-Gnormorphinenorcodeine-6-glucuronideCYP2D6

Effect of Quinidine on the Analgesic Response to Codeine in Extensive Metabolizers of CYP2D6 (Phenotyped with Dextromethorphan)Data from: Desmeules J, et al. Eur J Clin Pharmacol 41:23:26, 1991

What is the cause of hypermetabolizers?

Debrisoquine phenotype in subjects with different CYP2D6 genotypes(CYP2D6L)2 - gene duplication; CYP2D6A - single base deletionCYP2D6B - multiple point mutationsData from: Agundez JG et al. Clin Pharmacol Ther 57:265, 1995.

Genotype

# of Subjects

Metabolic Ratio

CYP2D6wt/(CYP2D6L)2

9

0.33

CYP2D6wt/CYP2D6wt

12

1.50

CYP2D6wt/CYP2D6(A or B)

9

2.14

CYP2D6B/CYP2D6B

6

48.84

From: Dalen P, et al. Clin Pharmacol Ther 63:444-452, 1998.

E. CYP2C9 ACTIVITYPrescribed Daily Warfarin Dose and CYP2C9 Genotype Warfarin Dose* Genotype5.63 (2.56)*1/*14.88 (2.57)*1/*23.32 (0.94)*1/*34.07 (1.48)*2/*22.34 (0.35)*2/*31.60 (0.81)*3/*3*Data presented as mean (SD) daily dose in mgFrom: Higashi MK, et al. Association between CYP2C9 genetic variants and anticoagulation-related outcomes during warfarin therapy. JAMA 287:1690-1698, 2002.

F. THIOPURINE METHYLTRANSFERASE (TPMT)

Distribution of Thiopurine Methyl-transferase Activity. Reproduced from: Weinshelboum RM, Sladek SL. Am J Hum Genet 32:651-662, 1980.

G. GENETIC POLYMORPHISMS, MATERNAL SMOKING AND LOW BIRTH WEIGHT (LBW)65% of all infant deaths occur among LBW infants, while LBW infants account for 7.6% of all live birthsReduction in birth wgt among smoking women

GenotypeWeight Reduction CYP1A1 AA252 g CYP1A1 Aa/aa520 g

GST1 AA/Aa285 g GST1 aa642 gData from: Wang X, et al. JAMA 287:195-2002, 2002.

From: Esteller M, et al. Inactivation of the DNA-repair gene MGMT and the clinical response of gliomas to alkylating agents. NEJM 243:1350-1354, 2000.Why are some gliomas resistant to nitrosourea alkylating agents?

Evidence suggests this may be the result of an epigenetic phenomenon one that does not involve a change in DNA sequence.

MGMT methylguanine-DNA methyltransferaseMethylation of the promoter region of MGMT may silence the gene

From: Esteller M, et al. Inactivation of the DNA-repair gene MGMT and the clinical response of gliomas to alkylating agents. NEJM 243:1350-1354, 2000.

Future Role of SNPs and PharmacogeneticsSNP - Single Nucleotide Polymorphisms. G G T A A C T G . G G C A A C T G ...AS of February 2001, 1.42 million SNPs had been identified in the human genome.

Patients with efficacyin clinical trialsPatients without efficacyin clinical trialsPredictive of efficacyPredictive of no efficacy

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Pharmaco Genetics