Clinical Application of Cardiovascular Pharmacogenetics · Clinical Application of Cardiovascular Pharmacogenetics Deepak Voora, MD, Geoffrey S. Ginsburg, MD, PHD Durham, North Carolina

Journal of the American College of Cardiology Vol. 60, No. 1, 2012© 2012 by the American College of Cardiology Foundation ISSN 0735-1097/$36.00

STATE-OF-THE-ART PAPER

Clinical Application ofCardiovascular PharmacogeneticsDeepak Voora, MD, Geoffrey S. Ginsburg, MD, PHD

Durham, North Carolina

Pharmacogenetics primarily uses genetic variation to identify subgroups of patients who may respond differentlyto a certain medication. Since its first description, the field of pharmacogenetics has expanded to study a broadrange of cardiovascular drugs and has become a mainstream research discipline. Three principle classes ofpharmacogenetic markers have emerged: 1) pharmacokinetic; 2) pharmacodynamic; and 3) underlying diseasemechanism. In the realm of cardiovascular pharmacogenetics, significant advances have identified markers ineach class for a variety of therapeutics, some with a potential for improving patient outcomes. While ongoingclinical trials will determine if routine use of pharmacogenetic testing may be beneficial, the data today supportpharmacogenetic testing for certain variants on an individualized, case-by-case basis. Our primary goal is to re-view the association data for the major pharmacogenetic variants associated with commonly used cardiovascu-lar medications: antiplatelet agents, warfarin, statins, beta-blockers, diuretics, and antiarrhythmic drugs. In addi-tion, we highlight which variants and in which contexts pharmacogenetic testing can be implemented bypracticing clinicians. The pace of genetic discovery has outstripped the generation of the evidence justifying itsclinical adoption. Until the evidentiary gaps are filled, however, clinicians may choose to target therapeutics toindividual patients whose genetic background indicates that they stand to benefit the most from pharmaco-genetic testing. (J Am Coll Cardiol 2012;60:9–20) © 2012 by the American College of Cardiology Foundation

Published by Elsevier Inc. http://dx.doi.org/10.1016/j.jacc.2012.01.067

“The right dose of the right drug to the right person” isone of the goals of pharmacogenomics and personalizedmedicine. The need for pharmacogenomics in clinical prac-tice is underscored, for example, by the improved ischemicoutcomes with newer platelet P2RY12 receptor inhibitorswhich also have higher risk of adverse events compared toclopidogrel (1,2). Therefore, there is a critical need to targettherapeutics to individual patients who stand to benefit themost and suffer the least.

Principles of Pharmacogenetics

A prerequisite for pharmacogenetics is heterogeneity in drugresponse. The definitions of drug response are varied and caninclude surrogate measurements measured in the laboratory(e.g., international normalized ratio [INR] for warfarin) orclinical endpoints (e.g., stent thrombosis for clopidogrel).

A genetic basis for drug response is suggested whenresponses are similar within family members (and thereforeare heritable) or significantly different in across ethnicbackgrounds. Underlying genetic variation can be deter-mined either using a targeted approach where known

From the Duke Institute for Genome Science & Policy and Center for PersonalizedMedicine, Duke University, Durham, North Carolina. Dr. Voora has reported he hasno relationships relevant to the contents of this paper to disclose. Dr. Ginsburg is ascientific advisor to CardioDx.

Manuscript received May 12, 2011; revised manuscript received January 5, 2012,accepted January 18, 2012.

variants in candidate genes are hypothesized to influencedrug response or genome-wide association—an “unbiased”screen for common variants across the entire genome. Raregenetic variants missed by genome-wide association studies(GWAS) can be identified through sequencing candidategenes, exomes, or the entire genome in the case of raredrug-induced adverse events.

Three broad classes of genetic variants influence drugresponse: 1) pharmacokinetic; 2) pharmacodynamic; and3) those associated with the underlying disease mechanism(Table 1, Fig. 1). This review will discuss specific drugs, andin each case, we will apply these pharmacogenetic principlesfollowed by potential clinical implications.

Statins

Statins (3-hydroxy-3methylglutaryl-coenzyme A reduc-tase [HMGCR] inhibitors) can elicit 4 types (at least) of“responses”: 1) low-density lipoprotein cholesterol (LDLc)lowering; 2) protection from cardiovascular events; 3) mus-culoskeletal side effects; and 4) statin adherence.LDLc lowering. The majority of patients respond with30% to 50% LDLc reduction; however, there is wide (10%to 70%) variation (3). Two loci appear to affect LDLclowering: HMGCR and APOE (Table 2).

HMGCR. Carriers of a minor haplotype (defined by 3single nucleotide polymorphisms [SNPs]: rs17244841,
rs17238540, and rs3846662) of HMGCR experience a 5%

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10 Voora and Ginsburg JACC Vol. 60, No. 1, 2012Clinical Cardiovascular Pharmacogenetics July 3, 2012:9–20

to 20% reduced LDLc loweringwith pravastatin (4) or simvasta-tin (5) due to an alternativelyspliced HMGCR transcript thatproduces a version of HMGCRthat is less sensitive to simvasta-tin inhibition (6). This haplotypewas not identified in a GWASwith atorvastatin (7), and there-fore, whether this associationholds for other statins is unclear.

APOE. Two variants, rs7412and rs429358, define 3 haplotypes,namely, �2, �3, and �4, that areassociated with lipid, cognitive,and thrombotic traits. In general,the magnitude of LDLc loweringis greatest for carriers of �2, fol-lowed by �3, and �4 haplotypes,respectively. The influence ofAPOE haplotypes appears to beconsistent across variety of statin

ypes (4,7,8) and doses (8), and results in mild (�15%)ttenuations of LDLc lowering.dditional loci. Information on ABCB1 and ABCG2 cane found in the Online Appendix.eduction in cardiovascular events. Statins prevent car-iovascular disease by lowering LDLc and potential pleio-ropic effects. There are few genetic predictors of theseleiotropic benefits, but the rs20455 polymorphism ininesin-like protein 6 (KIF6, an underlying disease gene)ay be a candidate. Carriers of the risk allele may receive a

reater benefit from statin therapy compared to noncarriersespite equal LDLc, triglycerides, or C-reactive proteineduction (9,10). However, subsequent genome-wide meta-nalysis of large randomized placebo-controlled statin trialsound no evidence for association with coronary arteryisease (11) or any differential treatment benefit with statinherapy (12,13).tatin-induced musculoskeletal side effects. Statins havewell-defined safety profile, but come with a small risk ofusculoskeletal side effects (14). Genetic variants in the

epatic transporter, SLCO1B1, influence the risk of adverse

Abbreviationsand Acronyms

ACS � acute coronarysyndrome(s)

BP � blood pressure

CYP � cytochrome P450

GWAS � genome-wideassociation study

HR � heart rate

INR � internationalnormalized ratio

LDLc � low-densitylipoprotein cholesterol

LVEF � left ventricularejection fraction

MI � myocardial infarction

PCI � percutaneouscoronary intervention

RF � reduced function

SNP � single nucleotidepolymorphism

Sources of Pharmacogenetic VariationTable 1 Sources of Pharmacogenetic Variation

Category Description

Pharmacokinetic Variability in concentration of drug at site ofdrug effect

Pharmacodynamic Variability in drug ability to influence its targe

Underlying disease mechanisms Variability in disease being treated

vents (Table 2); it does not appear that cytochrome P450CYP) SNPs influence statin-induced side effects.

LCO1B1. The solute carrier organic anion transporterfamily, member 1B1 gene (SLCO1B1, also referred to asSLC21A6, OATP-C, or OATP1B1) harbors many geneticvariants, and each variant is numerically and sequentiallylabeled beginning with the unmutated copy of the gene, *1(referred to as “star 1”) followed by *2, *3, *4, and so forth.The *5 variant (rs4149056, Val174Ala) interferes with thelocalization of this transporter to the hepatocyte plasmamembrane (15) and leads to higher plasma statin concen-trations (16–18). In candidate gene and GWAS, carriers of*5 are at 4- to 5-fold increased risk of severe, creatine kinase(CK)- positive simvastatin-induced myopathy and 2- to3-fold increased risk of CK- negative myopathy (19,20).

In trials of randomly assigned statins as well as inobservational studies, the risk for myopathy with *5 dependson the statin type: the risk is greatest for simvastatin �atorvastatin � pravastatin, rosuvastatin, or fluvastatin (20–23).These effects parallel the influence of the *5 allele on theclearance of these statins (16–18,24) and thus appear to bestatin-specific.Adherence to statin therapy. Often, statin therapy ishampered by nonadherence. Although the genetics of ad-herence to statins has not been studied in any great depth,2 studies (a clinical trial and observational cohort) observedthat carriers of the SLCO1B1*5 allele have a higher rate ofstatin nonadherence (20,25).Clinical implications for statin pharmacogenetics. It isunlikely that genetic testing for statin efficacy will enterclinical care since the magnitude of associations is small(�10% to 15% differences in LDLc lowering), andphysicians can reasonably forecast the magnitude ofLDLc lowering based on statin type, dose, and baselineLDLc.

In contrast, statin-induced side effects and nonadherenceare less predictable. While the current level of evidencesurrounding SLCO1B1*5 may not support prospectivegenotyping at this time, the test is currently offered onconsumer-directed whole genome genotyping platforms(e.g., 23andMe, deCODEME, and so on). A potential strat-egy for prospective SLCO1B1*5 testing might offer carrierseither pravastatin or rosuvastatin or fluvastatin as first-line

Types of Genes Examples

Drug-metabolizing enzymesDrug transporters

Warfarin: CYP2C9Clopidogrel: CYP2C19Simvastatin: SLCO1B1Metoprolol: CYP2D6

Transmembrane receptorsIntracellular enzymes

Clopidogrel: P2RY12Simvastatin: HMGCRMetoprolol: ADBR1

Often downstream or independent of drug target Hydrochlorothiazide: ADD1

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11JACC Vol. 60, No. 1, 2012 Voora and GinsburgJuly 3, 2012:9–20 Clinical Cardiovascular Pharmacogenetics

agents, which seem to depend the least on SLCO1B1 for theirclearance.

Thienopyridines

Thienopyridines are effective in patients with acute coronarysyndrome (ACS) and percutaneous coronary intervention(PCI); however, some patients remain at risk for death,myocardial infarction (MI), and stent thrombosis. Plateletfunction in response to clopidogrel is variable (26), heritable(27), and reduced inhibition predicts future events (28);therefore, clopidogrel is a prime candidate for pharmacoge-netics. Clopidogrel pharmacogenetics centers around 2 loci,CYP2C19 (pharmacokinetic) and ABCB1 (pharmacoki-

etic); other loci, CYP2C9 (pharmacokinetic), PON1pharmacokinetic), and P2RY12 (pharmacodynamic) maylso influence response (Table 3).latelet function in response to clopidogrel. CYP2C19.lopidogrel is an inactive prodrug activated by several

nzymes, including CYP2C19, to an active metabolite thatnhibits the platelet adenosine diphosphate receptor,2RY12 (29). CYP2C19 *2 (rs4244285) is the most com-on reduced-function (RF) variant; additional, rare variantsimic the *2 allele: *3 [rs4986893], *4 [rs28399504], *5

rs56337013]) (30). An individual person’s genotype cane characterized in 3 ways: 1) presence of at least 1 RFllele (carrier vs. noncarrier); 2) the number of RF allelesi.e., 0, 1, 2); or 3) the predicted, total CYP2C19 enzymaticctivity (Table 4). Carriers of *2—also referred to as

Figure 1 Categories of Major Cardiovascular Pharmacogenetic

Genetic variants that influence the response to cardiovascular agents fall into 3 b3) underlying disease mechanism.

ntermediate metabolizers or poor metabolizers—produce

ower active metabolite and have attenuated platelet inhibi-ion (31). In general, there is a gene-dose effect wherencreasing number of RF alleles (i.e., 0, 1, 2, or extensive

etabolizers, intermediate metabolizers, and poor metabo-izers) predicts a decreasing amount of platelet inhibition27,31,32). Carriers of another variant *17 (rs3758581,ltrametabolizers) exhibit increased CYP2C19 activity, pro-uce more active metabolite, and improved platelet inhibi-ion, in most reports (33,34).

Higher loading and maintenance doses (e.g., 1,200 mgnd 150 mg/day) appear, in part, to overcome the effectsf the *2 allele (30,32,35,36), although not completely37), and can require up to 300 mg/day (38). Ticlopidine39), prasugrel (31,40,41), and ticagrelor (42) all produceniform platelet inhibition in *2 carriers and noncarriers.

BCB1. The adenosine triphosphate-binding cassette, sub-family B (MDR/TAP), member 1 (ABCB1) gene encodesan apical membrane protein in enterocytes and hepatocytesand serves to reduce bioavailability. Clopidogrel appears to behandled by this transporter, and 3 SNPs—C1236T (rs1128503),G2677T (rs2032582), and C3435T (rs1045642)—capture thecommon genetic variation at this locus. Persons who carry aT allele at each SNPs (i.e., the T-T-T haplotype) producedreduced active metabolite (43) in an initial report. Despitethis initial observation, the link to reduced platelet inhibi-tion has been difficult to establish (27,30), although may bepresent for persons who carry 2 copies of the T-T-T

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induced platelet inhibition is not affected by T-T-T haplo-type (44).Additional loci. Using GWAS, investigators have unsuc-cessfully searched for additional variants for clopidogrel(27). An additional discussion of genetic variation atABCB1, P2RY12, CYP2C9, and PON1 can be found in theOnline Appendix.Clinical response to clopidogrel. CYP2C19. In parallelwith the platelet function data, the CYP2C19*2 allele isassociated with a graded risk of death, MI, or stroke.Carriers of 1 allele (intermediate metabolizers) have a�1.5-fold increased risk, and carriers of 2 alleles (poormetabolizers) experience a �1.8-fold increase. This patternalso extends to stent thrombosis as well with a �2.6- and�4-fold increased risk in those with 1 and 2 *2 alleles,respectively (32,45–49). Therefore, the CYP2C19 geneticssociations with platelet function are mirrored in thelinical response to clopidogrel in the setting of PCI. Thesebservations formed the foundation for updating the clopi-ogrel label by the Food and Drug Administration tonclude pharmacogenetic information. Similarly, the gain ofunction variant, CYP2C19*17, is associated with increasedisk of bleeding (50), and protection from ischemic events51) CYP2C19*2 carriers treated with prasugrel or ticagreloro not show a heightened risk of cardiovascular death, MI,troke, or stent thrombosis (40,52).

BCB1. Carriers of 2 copies of the ABCB1 T-T-T haplo-type treated with clopidogrel are at an increased risk forsubsequent death, MI, or stroke compared to persons whocarried none, thus mirroring the platelet function data(44,45,52). Genetic variation at the ABCB1 locus does notappear to influence prasugrel- or ticagrelor-treated patients(40,52).Clinical implications. The current state of evidence reflectsa strong and consistent association with the CYP2C19*2 alleleand an increased risk for cardiovascular outcomes in ACS/PCI patients treated with clopidogrel, although not othersettings (51). Alternative antiplatelet agents (i.e., prasugreland ticagrelor) mitigate the adverse risk of CYP2C19*2-associated adverse events. Although CYP2C19 genotyping iscommercially available, whether genotype-guided therapywill improve outcomes or reduce costs is unknown. Con-sensus statements (53) currently do not recommend routinetesting. However, there is sufficient evidence to supportphysicians who may choose to pursue CYP2C19*2 testing inselected patients: 1) for diagnosis in patients with compli-cations of clopidogrel therapy such as stent thrombosis incompliant clopidogrel users; or 2) for the choice of dualantiplatelet therapy in the ACS/PCI setting where thephysician believes that additional information regardingthe risk/benefit profile for clopidogrel will influence thechoice of drug therapy (54). Outside of these scenarios,the ACS/PCI setting, or situations where there are ampledata to guide drug choice (e.g., ST-segment elevation MI
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transient ischemic attack/stroke), there is minimal ratio-nale to support CYP2C19 testing.

Based on the available data, it is reasonable that if aerson is found to be a poor metabolizer (Table 4), then anlternative to clopidogrel, such as prasugrel or ticagrelor,hould be considered. This seems preferable over increasedlopidogrel doses, which have not shown benefit overtandard dose clopidogrel (57–59). The greatest uncertaintys in the intermediate metabolizer group (i.e., those with 1oss of function allele) and is where integration of otherlinical risk factors such as diabetes, body mass index, cost,nd other bleeding and thrombosis risk factors need to beonsidered in determining the therapy with the optimalisk:benefit profile.

spirin

spirin irreversibly inhibits prostaglandin G/H synthase 1PTGS1, or COX-1) and the conversion of arachidonic acido thromboxane. With adequate dosing and compliance,spirin is capable of completely inhibiting COX-1 in �99%f persons; thus, true “aspirin resistance” is rare (60).owever, alternate agonists such as adenosine diphosphate

nd collagen can produce robust aggregation in the face ofomplete COX-1 inhibition (61), a response that demon-trates wide interindividual variability (62), heritability63,64), and at high levels, association with future cardio-ascular events (65,66), therefore making aspirin a candidateor pharmacogenetic discovery. Because aspirin uniformly

Main Genetic Associations With the Response to ClopidogrelTable 3 Main Genetic Associations With the Response to Clop

Gene Variant(s)

Risk Allele/Haplotype Frequencies

Drug RCaucasians Africans Asians

CYP2C19 *2 (rs4244285) 16% 14% 27% Drug concrecurreGWAS

CYP2C19 *17 (rs3758581) 5% 10% 6% Drug concbleedin

ABCB1 T-T-T haplotype defined byT allele at C1236T(rs1128503), G2677T(rs2032582), andC3435T (rs1045642)

71% 12% 65% Drug concrecurre

MI � myocardial infarction; DMET � drug metabolizing enzyme and transporter panel; other abbr

CYP2C19 Diplotype Classification*Table 4 CYP2C19 Diplotype Classification*

Alleles Classification

*1 and *1 Extensive metabolizer

*2/*3/*4/*5/*6 and *1/*17 Intermediate metabolizer

*2/*3/*4/*5/*6 and *2/*3/*4/*5/*6 Poor metabolizer

*1/*17 or *17/*17 Ultrametabolizer

a*Adapted from Ingelman-Sundberg et al. (147) and Scott et al. (148).

nhibits its target, COX-1, there are no pharmacokinetic orharmacodynamic genetic considerations. Instead, investi-ators have focused on platelet function loci (underlyingisease) and LPA (underlying disease).Variants in several platelet genes, namely, PEAR1,

ITGB3, VAV3, GPVI, F2R, and GP1BA, are associated withlatelet function in response to aspirin (67–70). One withoderate evidence is rs5918 in ITGB3 where carriers of the

isk allele have heightened platelet function on aspirin71–73). Recent GWAS of platelet function in response tospirin have identified additional genetic loci that have yeto be replicated (74). Finally, the most robust associationomes from a large study that found carriers of the minorllele for an intronic SNP, rs12041331, in PEAR1 haveigher PEAR1 platelet protein content and heightenedlatelet function in response to aspirin (75).The link to an increased risk of clinical events in aspirin

sers has not been established despite several large studies76,77), although the PEAR1 variant rs12041331 has notet been tested. An uncommon variant in LPA (rs3798220)eems to modify the protective effects of aspirin: carriersad a more than 2-fold reduction in the risk for cardio-ascular disease with aspirin, whereas noncarriers (�95%f Caucasians) had none in a large placebo-controlledlinical trial (78).linical implications. Because of the lack of consistent orreliminary associations with the variants described in thereceding text, there is currently no role for genetic testingor aspirin.

arfarin

hree loci contribute to the heterogeneity in response:YP2C9 (pharmacokinetic), VKORC1 (pharmacodynamic),nd CYP4F2 (pharmacodynamic) (Table 5).

arfarin dose requirements. CYP2C9. CYP2C9 is re-ponsible for S-warfarin (the more active enantiomer) clear-

el

se/Type of StudyEffect of VariantWith Risk Allele Ref. #

ion, platelet function,stent thrombosis/, CG

2Active metabolite concentration2Inhibition of platelet function1Risk of recurrent MI, stroke,

stent thrombosisGraded risk with 0, 1, or 2 alleles

31,32,45–47,52

ion, platelet function, 1Active metabolite concentration1Inhibition of platelet function1Risk bleeding

32,33,50

ion, platelet function,stroke, death/CG

2Active metabolite concentration2No change in inhibition of

platelet function1Risk of recurrent MI, stroke,

death (in carriers of 2 T-T-Thaplotypes)

27,30,43,45,48,49,52

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*2 (rs1799853) and *3 (rs1057910). The CYP2C9*2 allelereduces warfarin clearance by 30% and the *3 allele by90% (79), which translate into 19% and 33% reductionsper allele in warfarin dose requirements compared tononcarriers (80).

VKORC1. Vitamin K epoxide reductase complex, subunit 1(VKORC1) is warfarin’s target (81,82), and resequencingthis gene identified 2 haplotypes (A and B defined by thealleles at 5 SNPS: rs7196161, rs9923231, rs9934438,rs8050894, and rs2359612) that influence VKORC1 genexpression and predict warfarin dose requirements (83). Ofhese 5 SNPs, an A allele at rs9923231 (also referred to as

1639 G�A and the 3673 SNP) in the promoter region isesponsible for reducing: VKORC1 gene expression (84).

Therefore, carriers of an A allele at rs9923231 (or the Ahaplotype) require a 30% per allele reduction in warfarindose requirements (85).

CYP4F2. Several GWAS have not only confirmed theobservations above but also have identified a novel associa-tion between rs2108622 in CYP4F2 and reduced hepatic

YP4F2, higher levels of hepatic vitamin K, and higherarfarin dose requirements (86–90).dditional loci. Additional information regarding CALU

nd GGCX can be found in the Online Appendix.linical response to warfarin. RETROSPECTIVE STUDIES.

Standard dosing algorithms (i.e., a 5 mg or 10 mg loadingdose followed by titration based on the INR) lead to anincreased risk of an out-of-range INR (�4.0) or a delay inthe time-to-therapeutic range INR in carriers of theCYP2C9*2 or CYP2C9*3 alleles (91,92), the A haplotype or

allele of the �1639 SNP in VKORC1 (93), or the T allelein CYP4F2 (94). Importantly, only variants in CYP2C9 havebeen linked to the risk of bleeding (91,92).

PROSPECTIVE STUDIES OF TAILORED WARFARIN THERAPY.

Small randomized pilot studies, parallel cohort studies, andsingle arm studies with or without historical controls haveprospectively explored genetically guided warfarin therapy.

Main Genetic Associations With the Response to WarfarinTable 5 Main Genetic Associations With the Response to Warf

Gene Variant(s)

Risk Allele Frequencies

Caucasians Africans Asians

CYP2C9 *2 (rs1799853)*3 (rs1057910)

*2: 10%*3: 6%

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VKORC1 �1639 (rs9923231)* 60% 98% 2% Wa

CYP4F2 rs2108622 23% 6% 25% Wa

*Carriers of the risk, A, allele are also carriers of the A haplotype.INR � international normalized ratio; other abbreviations as in Tables 2 and 3.

Whereas some studies suggest a benefit over standard i

therapy (95–97), others failed to show any significantadvantage (98) except for smaller and fewer dosing changesand INR measurements (99). Definitive clinical trials areongoing to assess the benefit of incorporating genetics intowarfarin therapy.Clinical implications. There are strong genetic associa-tions surrounding CYP2C9, VKORC1, and CYP4F2 variantshat influence the response to warfarin. Commercial testingnd algorithms that assist in the interpretation of genotypesre available. Therefore, there is evidence to justify and toolso enable genotype-guided warfarin therapy. Until largecale trials demonstrate a benefit for routine testing, physi-ians may choose to pursue testing in selected patients whohey feel may benefit by: 1) diagnosing those with compli-ations from warfarin therapy (e.g., hemorrhage); 2) pre-icting dose for those at high risk of bleeding (e.g.,triple-therapy” with aspirin, clopidogrel, and warfarin); or) weighing the costs of newer anticoagulants againstarfarin.

iuretics

lood pressure–lowering response. The adducin 1 (alpha)ene (ADD1, underlying disease) is implicated in animalnd human sodium handling (100), and persons with aly460Trp substitution are more salt sensitive and have

nhanced blood pressure (BP) lowering with thiazide di-retics (100).linical outcomes. In an observational study, patientsith Trp460 who were treated with thiazide diuretics

xperienced a greater protection from MI and stroke thanatients with Gly460 (101). However, this difference in therotective effects of diuretics could not be replicated ineveral larger studies (102–104).linical implications. Given the inconsistent associations

nd lack of a commercially available test, the use of thisenetic variant in guiding treatment options in hypertension

g Response/ype of Study

Effect of Variant inCarriers of Risk Allele Ref. #

centration, warfarinequirements,range INR values,rhage/GWAS, CG

2Clearance of S-warfarin2Dose requirements1Risk of out-of-range INR values1Time to stable, therapeutic INR1Risk of hemorrhage

79,80,88,89,91–93

dose requirements,range INR values/e (human/animal),, CG

2Dose requirements1Risk of out-of-range INR values1Time to stable, therapeutic INR

81–83,88,89

dose requirements/, DMET

1Dose requirements 87–89

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Beta-Blockers

Beta-adrenergic receptor antagonists (or beta-blockers) area diverse class of agents that primarily antagonize the beta-1adrenergic receptor, encoded by ADBR1. Accordingly,changes in heart rate (HR), BP, and left ventricular ejectionfraction (LVEF) are considered the surrogate responses inplace of clinical responses such as protection from MI,death, or heart failure. Variation in CYP2D6 (pharmacoki-netic) and ADRB1, ADRB2, and GRK5 (all pharmacody-namic) have received the most attention (Table 6).Heart rate and blood pressure reduction. CYP2D6. Beta-blockers are substrates for CYP, and metoprolol is exten-sively metabolized by hepatic CYP2D6. The most commonRF variant CYP2D6*4 (rs3892097) results in the absence ofCYP2D6 activity. Carriers have a higher systemic exposureto metoprolol (105), which translates into a greater reduc-tion in HR and BP (106,107). Beta-blockers such asatenolol and carvedilol (108) do not require CYP2D6 forheir metabolism.

DRB1. Ethnic differences in the dose response for pro-pranolol (109) motivated pharmacodynamic genetic inves-tigations of beta-blockers. Two variants in ADBR1, theSer49Gly (rs1801252) and Arg389Gly (rs1801253), lead toimpaired down-regulation (110) and higher signal transduc-tion, respectively (111). Therefore, carriers of either varianthave enhanced, beta-1-receptor activity and more beta-blocker sensitivity. Healthy volunteers and patients withhypertension who carry 2 Arg389 variants have a greaterHR (112) or BP (113) reduction mainly with metoprolol,lthough not with all beta-blockers (114).

In patients with systolic heart failure treated with eitheretoprolol or carvedilol (115), but not bucindolol (116),

arriers of 2 copies of the ADBR1 Arg389 variant hadignificantly greater improvements in LVEF compared tohe Gly389 carriers.

DRB2. Genetic variation in the beta-2 adrenergic receptor(ADBR2) centers around 2 variants: Arg16Gly (rs1042713) and

lu27Gln (rs1042714). Receptors with Gly16 versus Arg16ave enhanced down-regulation of ADBR2. In contrast,eceptors with Glu27 appear to be resistant to down-egulation (117). Extension to variation in BP or HRowering in response to beta-blocker therapy has not beenemonstrated (114).

RK5. Downstream of the beta-1-adrenergic receptor areG-protein coupled receptor kinases responsible for desensi-tization of the beta-1-adrenergic receptor. A Glu41Leugenetic variant in 1 of these kinases, G protein-coupledreceptor kinase 5 (GRK5), is more prevalent in AfricanAmericans. The Leu41 more effectively uncoupledisoproterenol-stimulated responses than GRK5-Q41, thusproducing a pharmacological-like “beta-blockade” in mice(118), although with no differences in atenolol-induced HR
reduction in humans (119). M
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Clinical benefit in patients with cardiovascular disease.ADRB1. In heart failure patients, when compared to pla-cebo, ADRB1 Arg389 homozygotes had a greater reductionin the time to first hospitalization or death when treatedwith bucindolol (116) but not carvedilol or metoprolol(120–125) compared to Gly389 carriers. Whether thesedifferences are due to drug specific effects (i.e., bucindolol vs.metoprolol) or the play of chance has not been adequatelytested. In patients with chronic coronary artery diseaserandomly assigned to verapamil or atenolol, carriers of atleast 1 copy of the Ser49/ Arg389 haplotype had a 9- versus2-fold worsened prognosis when assigned to verapamilversus atenolol (126) despite equivalent BP and HR controlin both groups.

ADRB2. One study examined the influence of geneticvariation at ADRB2 and its influence on ACS patients andfound those homozygous for both the Arg16 and Glu27alleles had a 20% rate of subsequent death versus 6% ofthose homozygous for both Gly16 and Gln27 (125). Thistrend has been observed in some studies of patients withheart failure (124), although not all (122,123,126).

GRK5. Extension of the associations of the Glu41Leuvariant to clinical outcomes of patients was demonstrated in1 study where Leu41 carriers exhibited improvement in sur-vival compared to Glu41 carriers (118), although not inanother, larger study (121).Adverse events. Metoprolol-induced adverse events (e.g.,bradycardia) are associated with CYP2D6*4 in 2 studies,each numbering �1,000 treated patients (127,128), al-though not in smaller studies (129,130).Clinical implications. In general, carriers of the Arg389variant have: 1) enhanced reduction in HR and BP; 2) largerimprovements in LVEF; and 3) longer survival whentreated with chronic beta-blocker therapy compared topersons with the Gly389 variant. Although it is unlikely thatbeta-blocker therapy will ever be withheld for carriers of theGly389 variant, a potential application of these findingswould be to consider advanced heart failure therapies (e.g.,left ventricular assist devices, biventricular pacing, or trans-plantation) at an earlier stage in patients with the Gly389variant.

Because certain beta-blockers such as atenolol and carve-dilol are minimally handled by CYP2D6 (131), these may bereasonable alternates for carriers of CYP2D6*4 withmetoprolol-induced bradycardia. Commercial testing forCYP2D6*4 is available (e.g., Labcorp).

Antiarrhythmic Drugs

Digoxin and calcium-channel blockers. Many antiar-rhythmics are known ABCB1 (described in preceding text)substrates including verapamil, diltiazem, and digoxin. Ge-netic variation in ABCB1 is inconsistently associated withaltered pharmacokinetic profiles (132–134), and there are
no reports of different clinical outcomes.
Procainamide. Procainamide is rapidly converted by acet-ylation into N-acetylprocainamide (NAPA, an active me-tabolite) by hepatic acetyltransferases (NAT2, primarily).Variability in hepatic acetylation capacity has long beenobserved (135), and “slow acetylators” produce less NAPA.Common genetic variants that decrease NAT2 activity are*5/rs1801280, *6/rs1799930, *7/rs1799931, and *14/rs1801279 (136,137). Although efficacy does not appear tobe related to variation in its pharmacokinetics, the inductionof drug-induced lupus-associated autoantibodies (138,139),but not symptomatic, drug-induced lupus (138,139) hasbeen linked to the slow acetylator status.Propafenone. The hepatic hydroxylation by CYP2D6 ofpropafenone into 5-hydroxypropafenone demonstrates wideinterindividual variability (140). Carriers of CYP2D6*4 (areduced function allele) have reduced propafenone clearancecompared to noncarriers (140–142).Antiarrhythmic efficacy. At low doses of propafenone,CYP2D6*4 carriers have greater reduction in exercise- orisoproterenol-induced HR compared to noncarriers (143),although not with higher doses or without the “stress” ofexercise/isoproterenol (140,142,143). Further, CYP2D6*4carriers have enhanced suppression of atrial and ventriculararrhythmias compared to noncarriers in some studies(141,144), although not in all (140,142) Therefore, there areinsufficient data to make any conclusions regardingCYP2D6 genotype and antiarrhythmic efficacy.Toxicity. Central nervous system side effects withpropafenone, as well as excessive beta-blockade, are correlatedwith a higher concentration of systemic propafenone and,accordingly, the CYP2D6 genotype (140).Clinical implications. Antiarrhythmic drug pharmacoge-netics represents a mixed collection of associations that donot sufficiently translate to consistent associations of mean-ingful clinical outcomes. Therefore, there is currently norole for pharmacogenetic testing in the clinical use of thesemedications.

Future Directions

The data to date support many common variants that alterthe pharmacologic properties of cardiovascular agents.Newer medications such as prasugrel, ticagrelor, and apxi-ban as well as older medications such as enalapril, spirono-lactone, and angiotensin-receptor blockers that have variableclinical effects are also candidates for a pharmacogenetic approach.Novel systems approaches may elucidate additional deter-minants of drug response. Broad “omics” approaches haveseveral advantages over traditional genetics-based research,for example, 1) responding to the environment, and 2)elucidating how the activity of entire pathways, instead ofindividual genes, influence drug response.

The field of pharmacogenomics would benefit from thedevelopment of thresholds of evidence for testing andcoverage by insurance. With the expanding number and
complexity of “-omic” markers of drug response is an equally


profound evidentiary gap between association studies andthose that demonstrate clinical utility. It may be unrealisticto wait (or to require) a prospective, events-driven random-ized controlled trial of each drug-marker combination. Evenwhen such trials are conducted, the results may be madeobsolete by new observations and/or approval of new drugs.Therefore, genetic substudies of clinical trials and registriesmay become the highest levels of evidence for the potentialbenefits of using pharmacogenetic testing. Such standardsare common in many fields of medicine where there is oftena lack of randomized clinical trial data. Observationalstudies and comparative effectiveness research will lay thefoundation for broad-based recommendations and transla-tion of pharmacogenetic observations into a clinical para-digm of personalized medicine to improve health outcomes.

Reprint requests and correspondence: Dr. Deepak Voora or Dr.Geoffrey S. Ginsburg, Institute for Genome Sciences & Policy,Center for Genomic Medicine, 101 Science Drive, Duke Univer-sity, DUMC Box 3382, Durham, North Carolina 27708. E-mail:[email protected] or [email protected].

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Key Words: clopidogrel y pharmacogenetics y polymorphism y statinsy warfarin.

APPENDIX

For additional loci, see the online version of this article.