Pharmacogenetics 2008 for Med Students

8/8/2019 Pharmacogenetics 2008 for Med Students

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Pharmacogenomics:Pharmacogenomics:

designer drugsdesigner drugs

Paula Gregory, Ph.D.Paula Gregory, Ph.D.

Dept. of GeneticsDept. of Genetics


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Ifit were notfor the great variability amongIfit were notfor the great variability among

individuals, medicine might as well be aindividuals, medicine might as well be a

science and not an art. Sir William Oslerscience and not an art. Sir William Osler


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Did you know?Did you know? Over 2 million people are hospitalized eachOver 2 million people are hospitalized each

year for adverse reactions to prescriptionyear for adverse reactions to prescription

drugs.drugs.

Every year more than 100,000 people dieEvery year more than 100,000 people die

from those reactions, making it the sixthfrom those reactions, making it the sixth

leading cause of death!leading cause of death!


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PharmacogenomicsPharmacogenomics

Will allow:Will allow:

individualized medication useindividualized medication usebased on geneticallybased on genetically

determined variation in effectsdetermined variation in effects

and side effectsand side effects

use of medications otherwiseuse of medications otherwiserejected because of side effectsrejected because of side effects

new medications for specificnew medications for specific

genotypic disease subtypesgenotypic disease subtypes


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How Pharmacogenomics works

How Pharmacogenomics works

Identifying gene sequenceIdentifying gene sequence

variations (SNPs) that affectvariations (SNPs) that affect

drug responsedrug response

Identifying diseaseIdentifying disease

susceptibility genes whichsusceptibility genes which

represent potential newrepresent potential new

drug targetsdrug targets

Identifying gene sequenceIdentifying gene sequence

variations that can causevariations that can cause

adverse drug reactionsadverse drug reactions


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Challenges to drug designChallenges to drug design


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Drug responses are genetic!Drug responses are genetic!

Drug metabolism/response can beDrug metabolism/response can be

monogenicmonogenic

alteration of the key metabolizing enzyme canalteration of the key metabolizing enzyme can

alter drugs effectalter drugs effect

Drug responses are polymorphicDrug responses are polymorphic

Drugs trigger downstream events that can varyDrugs trigger downstream events that can vary

among patientsamong patients


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Drug response curveDrug response curve


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Variation in drug response can beVariation in drug response can be

hereditaryhereditary

Variations in absorption ratesVariations in absorption rates

Variations in drug metabolismVariations in drug metabolism

Variations in drugVariations in drug

inactivation/eliminationinactivation/elimination Variation in target receptorsVariation in target receptors


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How genes alter drug responses:How genes alter drug responses:

Hereditary metabolic disorders thatHereditary metabolic disorders that

alter drug metabolismalter drug metabolism

Genetic variants of drugGenetic variants of drug

metabolizing enzymesmetabolizing enzymes

Genetic variants of drug receptorsGenetic variants of drug receptors


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Examples of genetic variationExamples of genetic variation

in drug metabolismin drug metabolism

Variants of plasma pseudocholinesteraseVariants of plasma pseudocholinesterase

experience prolonged apnea when treated withexperience prolonged apnea when treated with

the muscle relaxant succinylcholinthe muscle relaxant succinylcholin

Variants of AcetyltransferaseVariants of Acetyltransferase

Slow metabolizers experience toxic neuritis,Slow metabolizers experience toxic neuritis,

lupus and bladder cancer when treated withlupus and bladder cancer when treated withantianti--arrhythmic drug (procainamide)arrhythmic drug (procainamide)

Rapid metabolizers can experience colonRapid metabolizers can experience colon

cancercancer


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Hereditary metabolic disordersHereditary metabolic disorders

that alter drug metabolismthat alter drug metabolism

GG--66--PD deficiencyPD deficiency -- can cause hemolysiscan cause hemolysis

when exposed to antiwhen exposed to anti--malaria drugsmalaria drugs

FactorV Leiden alleleFactorV Leiden allele -- increases risk ofincreases risk of

venous thrombosis (dont use oralvenous thrombosis (dont use oral

contraceptives in these women)contraceptives in these women)

LeschLesch--Nyhan SyndromeNyhan Syndrome -- chemo and goutchemo and gout

treatments are ineffective (the drugs are nottreatments are ineffective (the drugs are not

metabolized to their active form)metabolized to their active form)


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Genetic Variation & PolymorphismGenetic Variation & Polymorphism

influence metabolisminfluence metabolism

Alcohol dehydrogenaseAlcohol dehydrogenase -- three loci; variantthree loci; variant

of ADH2 much more common in Japaneseof ADH2 much more common in Japanese

(90%) than Europeans (15%)(90%) than Europeans (15%)

Lactose activityLactose activity two major alleles; lowtwo major alleles; low

activity allele more common in Africansactivity allele more common in Africans

and Asiansand Asians


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Drug MetabolismDrug Metabolism

Gene XGene X

Enzyme XEnzyme X

Drug ADrug A Substance ASubstance A


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Variants of drug metabolism

Variants of drug metabolism

Variant ofGene XVariant ofGene X

Enzyme X Enzyme X



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Genetic variants that decrease levels ofGenetic variants that decrease levels of

drug metabolizing enzymesdrug metabolizing enzymes


Enzyme XEnzyme X



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Genetic variants that result in overGenetic variants that result in over

production of metabolizing enzymeproduction of metabolizing enzyme

Variant of Gene XVariant of Gene X

Enzyme XEnzyme X

Drug A

Drug A Substance ASubstance A


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Genetic Variants in DrugGenetic Variants in Drug

ReceptorsReceptors

Gene XGene X

Receptor XReceptor X

Drug ADrug A Cascade ofCascade of

cellular eventscellular events


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Genetic variants in drug receptorsGenetic variants in drug receptors


Aberrant Receptor XAberrant Receptor X

Drug ADrug A Cascade ofCascade of

cellular eventscellular events


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Over (or under) expression ofOver (or under) expression of

receptors can alter drug responsereceptors can alter drug response


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Mutant receptors may not bind the drug;Mutant receptors may not bind the drug;

therefore, altering drug responsetherefore, altering drug response

(may look like drug resistance)(may look like drug resistance)


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Genetic variation in drugGenetic variation in drug

receptorsreceptors

SteroidSteroid--induced glaucoma (dexamethasone)induced glaucoma (dexamethasone)

Malignant hyperthermia (general anesthesia)Malignant hyperthermia (general anesthesia) Anticoagulant resistance (Coumarin,Anticoagulant resistance (Coumarin,

warfarin) requires 7warfarin) requires 7--20X more drug to get a20X more drug to get a

responseresponse

Retinoic Acid suppression of leukemiaRetinoic Acid suppression of leukemia

Vasopressin resistanceVasopressin resistance


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Warfarin = coumadinWarfarin = coumadin

Warfarin inhibitsWarfarin inhibits

vitamin K reductase,vitamin K reductase,

which is the enzymewhich is the enzymeresponsible for recyclingresponsible for recycling

oxidated vitamin Koxidated vitamin K

back into the system.back into the system.

For this reason, drugs inFor this reason, drugs inthis class are alsothis class are also

referred to as vitamin Kreferred to as vitamin K

antagonists.antagonists.


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WarfarinWarfarin

Discovered 60 years ago and oneDiscovered 60 years ago and one

of the most widely prescribed drugs in the worldof the most widely prescribed drugs in the world Intended to prevent and treat thromboembolismsIntended to prevent and treat thromboembolisms

Afib, recurrent stroke, DVT, pulmonary embolism, heartAfib, recurrent stroke, DVT, pulmonary embolism, heartvalve prosthesisvalve prosthesis

MultiMulti--source anticoagulantsource anticoagulant 1, 2, 2.5, 3, 4, 5, 6, 7.5 and 10 mg tablet strengths1, 2, 2.5, 3, 4, 5, 6, 7.5 and 10 mg tablet strengths

Significant increase in Rxs over past 10 yearsSignificant increase in Rxs over past 10 yearsespecially in the elderlyespecially in the elderly


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Trends inWarfarin Use: 1.5Trends inWarfarin Use: 1.5--foldfold

Increase (45%) in Last 6 YearsIncrease (45%) in Last 6 Years

Prescriptions Dispensed in the U.S. for

Warfarin Tablets and Vials

10

15

20

25

30

1998 1999 2000 2001 2002 2003 2004 YTD

9/2005Year

Dis

pensedR

(millions)

Source: IMS Health National Prescription AuditPlusTM Data Extracted 11/2005


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Safety ofW

arfarinSafety ofW

arfarinMajor risk is bleeding: frequent

and severe

1.2 7 major bleeding episodesper 100 patients

Responsible for 1 in 10 hospital

admissions

Relative risk of fatal extracranial

bleeds 0 - 4.8%

Schulman, N Engl J Med 349:675-683, 2003

Pirmohamed, British Med J 329:15-19, 2004

DaSilva, Seminars Vasc Surg 15:256-267, 2002

Eikelboom, Med J Australia 180:549-551, 2004


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PurplePurple--toe syndrome & skin necrosistoe syndrome & skin necrosis::rare adverse reactions to warfarinrare adverse reactions to warfarin


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DosingDosing ofW

arfarin isC

omplex

ofW

arfarin isC

omplex

Narrow therapeutic indexNarrow therapeutic index

Small separation between doseSmall separation between dose--response curvesresponse curves

for preventing emboli and excess coagulationfor preventing emboli and excess coagulation Nonlinear doseNonlinear dose--response (INR)response (INR)

Small changes in dose may cause large changesSmall changes in dose may cause large changesin INR with a time lagin INR with a time lag

Wide range (50x) of doses (2Wide range (50x) of doses (2--112 mg/week)112 mg/week)to achieve target INR of 2to achieve target INR of 2--33

Patient intrinsic and extrinsic factorsPatient intrinsic and extrinsic factors


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DNA testing forDNA testing for

Warfarin sensitivity

Warfarin sensitivityThe FDA Clinical PharmacologyThe FDA Clinical Pharmacology

Subcommittee of the Advisory CommitteeSubcommittee of the Advisory Committee

for Pharmaceutical Sciences hasfor Pharmaceutical Sciences hasrecommended testing for variations in therecommended testing for variations in the

CYP2C9 and VKORC1 in patientsCYP2C9 and VKORC1 in patients

requiring warfarin therapy. The drug labelrequiring warfarin therapy. The drug label

will reflect this recommendation soon.will reflect this recommendation soon.

Article on this testArticle on this test


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Warfarin MetabolismWarfarin Metabolism

Two polymorphic genes, CYP2C9 andTwo polymorphic genes, CYP2C9 andVKORC1, affect warfarin metabolism andVKORC1, affect warfarin metabolism andresponse. Allelic frequencies of these two genesresponse. Allelic frequencies of these two genesare usually associated with ethnicity. Here areare usually associated with ethnicity. Here arethe concerns with prescribing warfarin tothe concerns with prescribing warfarin topatients with CYP2C9 or VKORC1patients with CYP2C9 or VKORC1polymorphisms:polymorphisms:

Overdose can result in bleeding which can beO

verdose can result in bleeding which can befatal.fatal.

Under dose can result in thrombosis which canUnder dose can result in thrombosis which canbe fatalbe fatal


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VKORC1 VariantsVKORC1 Variants

VKORC1 polymorphisms may explain up toVKORC1 polymorphisms may explain up to

25% of patient variability in response to25% of patient variability in response to

warfarin. Patients with VKORC1warfarin. Patients with VKORC1

polymorphisms are at risk for exaggeratedpolymorphisms are at risk for exaggerated

anticoagulant response.anticoagulant response.


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CYP2C9VariantsCYP2C9Variants

CYP2C9 variants take more time to achieveCYP2C9 variants take more time to achieve

stable dosing, and are associated withstable dosing, and are associated with

increased risk of bleeding events. Lowincreased risk of bleeding events. Low

CYP2C9 activity results in higher plasmaCYP2C9 activity results in higher plasma

levels of warfarin so the patient is at risk forlevels of warfarin so the patient is at risk for

bleedingbleeding


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Warfarin Sensitivity

Warfarin Sensitivity

TheWarfarin Sensitivity DNA TestTheWarfarin Sensitivity DNA Testdetermines the presence of specificdetermines the presence of specific

variations in the CYP2C9 and VKORC1variations in the CYP2C9 and VKORC1genes that confer sensitivity to warfaringenes that confer sensitivity to warfarin

and thus significantly reduce theand thus significantly reduce therequired maintenance dose. CYP2C9 isrequired maintenance dose. CYP2C9 is

involved in warfarin metabolism andinvolved in warfarin metabolism andVKORC1 influences warfarin'sVKORC1 influences warfarin's

anticoagulation effect through vitamin K.anticoagulation effect through vitamin K.


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Mechanistic Basis of DosingMechanistic Basis of Dosing

ProblemProblem

Large interindividual variability related to SLarge interindividual variability related to S--warfarinwarfarin

metabolism by CYP2C9 (genetics)metabolism by CYP2C9 (genetics)

*1 (wild type), *2 and *3 (variant alleles)*1 (wild type), *2 and *3 (variant alleles)

GenotypeGenotype

(N = 188)(N = 188)

PrevalencePrevalence % Enzyme% Enzyme

ActivityActivity

S/RS/R

WarfarinWarfarin

(mg/L)(mg/L)

WeeklyWeekly

DosesDoses

(mg)(mg)

Clearance/LClearance/L

BWBW

(ml/min/kg(ml/min/kg))

2C9 *1/*12C9 *1/*1 63%63% 100%100% 0.450.45

(0.11)(0.11)

34.134.1

(19.5)(19.5)

0.065 (0.025)0.065 (0.025)

2C9 *1/*X2C9 *1/*X 31%31% 5050--70%70% 0.690.69

(0.28)(0.28)

19.019.0

(10.8)(10.8)

0.041 (0.021)0.041 (0.021)

2C9 *X/*X2C9 *X/*X 6%6% 10%10% 1.431.43

(0.63)(0.63)

11.511.5

(7.2)(7.2)

0.020 (0.011)0.020 (0.011)

Herman et al, The Pharmacogenomics J 4:1-10. 2005


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Dosing Adjustments Based onDosing Adjustments Based on

GenotypeGenotype--Specific SSpecific S--WarfarinWarfarin

ClearanceClearance

0%

20%

40%

60%

80%

100%

PD

RRecommendedDose,

%

Wild Type *1/*2 *1/*3 *2/*2 *3/*3

Equivalent Warfarin Doses in Common Genotypes

Stefanovic and Samardzija, Clin Chem & Lab Med, 42(1) 2004


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Iressa: cancer miracle drugIressa: cancer miracle drug

Epidermal growth factorEpidermal growth factor

receptorreceptor--tyrosine kinase inhibitortyrosine kinase inhibitor

Used as a single treatment agent forUsed as a single treatment agent for

nonnon--small cell lung cancersmall cell lung cancer

Seems to work best on patientsSeems to work best on patients

with lung cancer who never smokedwith lung cancer who never smoked

It works by blocking tyrosine kinases found on the surface of normal &It works by blocking tyrosine kinases found on the surface of normal &

cancer cellscancer cells

Works on metastatic tumors as well.Works on metastatic tumors as well.


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Genetic variants in drugGenetic variants in drug

metabolismmetabolism

Fluorouracil (5Fluorouracil (5--FU) ToxicityFU) Toxicity

55--FU is a very common chemo agent for solid tumorsFU is a very common chemo agent for solid tumors

dihydropyrimidine dehydrogenase deficiency (DPD)dihydropyrimidine dehydrogenase deficiency (DPD)

increases the half life of 5increases the half life of 5--FU from 13 min to 160FU from 13 min to 160

min (prolonged exposure to the drug)min (prolonged exposure to the drug)

Incidence of this variant is 1Incidence of this variant is 1--3% of cancer patients3% of cancer patients The prolonged exposure to all tissues results inThe prolonged exposure to all tissues results in

toxicity, primarily BM and GI (neuro occassionally)toxicity, primarily BM and GI (neuro occassionally)


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Thiopurine methyltransferase null variantsThiopurine methyltransferase null variants

incidence of about 1 in 300incidence of about 1 in 300

Pts. Cannot metabolize chemo drugs used toPts. Cannot metabolize chemo drugs used totreat leukemia(6treat leukemia(6--mercaptopurine, 6mercaptopurine, 6--thioguaninethioguanine

& azathioprine) into their inactive methylated& azathioprine) into their inactive methylated

formsforms

Pts. Can be treated with 10Pts. Can be treated with 10--15 times less chemo15 times less chemo

than commonly prescribedthan commonly prescribed

Genotyping or functional enzyme assay is nowGenotyping or functional enzyme assay is now

the STANDARD PRACTICE in cancer centersthe STANDARD PRACTICE in cancer centers


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Slow acetylator phenotype results inSlow acetylator phenotype results in

peripheral neuropathy in pts treated with antiperipheral neuropathy in pts treated with anti--

TB drug (isoniazid)TB drug (isoniazid)

Causes slow clearance of the drug andCauses slow clearance of the drug and

associated toxicityassociated toxicity

This phenotype is found in 40This phenotype is found in 40 --60% of60% of

Caucasians, 80% of Middle Eastern pop butCaucasians, 80% of Middle Eastern pop but

only 20% of Japanese pop.only 20% of Japanese pop.


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Variants ofCytochrome P450Variants ofCytochrome P450

Cytochrome P450 enzymes are responsibleCytochrome P450 enzymes are responsible

for the MAJOR portion of drug metabolismfor the MAJOR portion of drug metabolism

CYP2D6 null alleles result in poorCYP2D6 null alleles result in poor

metabolizers and adverse reactions to cardiometabolizers and adverse reactions to cardio

and psyc medicationsand psyc medications

CYP2C9 mutations result in 5X decline inCYP2C9 mutations result in 5X decline inmetabolism of drugs (ibuprofen,metabolism of drugs (ibuprofen,

naproxen,etc)naproxen,etc)


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Genetic variations in the rate of drugGenetic variations in the rate of drug

metabolism can be specific for certainmetabolism can be specific for certain

ethnic groupsethnic groups


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Gleevec:Gleevec:

pharmacogenetics in action!pharmacogenetics in action! Used to treat CMLUsed to treat CML

4,500 middle4,500 middle--aged Americans diagnosed withaged Americans diagnosed with

CML each yearCML each year

Designed to block the activity of the BCRDesigned to block the activity of the BCR--

ABL proteinABL protein

BCRBCR--ABL is a fusion protein produced by theABL is a fusion protein produced by the

9;22 translocation associated with this9;22 translocation associated with this

leukemialeukemia


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SNPs = Single NucleotideSNPs = Single Nucleotide

PolymorphismsPolymorphisms

Occur throughout the genomeOccur throughout the genome

Occur about every 1,000 basesOccur about every 1,000 bases

May be linked toMay be linked to

polymorphisms in drug responspolymorphisms in drug respons

Under intense study byUnder intense study by

pharmaceutical companies.pharmaceutical companies.


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SNPs are throughout the genomeSNPs are throughout the genome


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SNPsSNPs

Characterization of SNPsCharacterization of SNPs

may help in identifyingmay help in identifying

subsets of individuals atsubsets of individuals atrisk for specificrisk for specific

diseasesdiseases

SNPs may predict drugSNPs may predict drugresponses/adverseresponses/adverse

reactionsreactions


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Gene Expression Analysis usingGene Expression Analysis using

MicroarraysMicroarrays

T i t P fili A h fT i t P fili A h f


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Copyright restrictions may apply.

Bumol, T. F. et al. JAMA 2001;285:551-555.

Transcript Profiling Approach forTranscript Profiling Approach for

Understanding Novel DrugUnderstanding Novel Drug

MechanismsMechanisms


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PharmacogenomicsPharmacogenomics

therapy with theright drug at the

right dose in the

right patient

Pharmacogenetics 2008 for Med Students

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