Pharmacogenetics and Clopidogrel Response in Patients Undergoing Percutaneous Coronary Interventions

Clopidogrel is one of the most widely prescribed drugs in the United States, but it is associated with a great deal of interpatient variability in response (e.g., clopidogrel resistance).1 It is a prodrug that requires activation by cytochrome P450 enzymes to its active form, with only ~15% of the dose being bioactivated. This active metabolite binds irreversibly to platelet P2Y12 (ADP) receptors to inhibit aggregation (Figure 1). Cytochrome P450 2C19 (CYP2C19) is the most important enzyme in this process. The CYP2C19 gene has several common variants. The fully functional and most prevalent version of the gene is designated CYP2C19*1, and the two most common gene variants that encode enzymes with reduced activity are designated CYP2C19*2 and CYP2C19*3, with the *3 allele present primarily in Asians.2 The frequency of these alleles in various racial and ethnic groups is shown in Table 1. In addition to the reduced function alleles, there is a common gain-of-function allele designated as CYP2C19*17. Figure 1 Clopidogrel activation and platelet aggregation pathways. (a) Activation pathway for clopidogrel in liver.28 (b) Mechanism of clopidogrel’s antiplatelet action.29 Figure copyright PharmGKB; reprinted with permission of PharmGKB and Stanford University. ... Table 1 Frequencies of CYP2C19 *2 and*3 minor alleles and phenotype prevalence in various ethnic groups Numerous high-profile studies have been published in the past 2 years that used both candidate gene and genome-wide approaches to assess variability in clopidogrel response. Most, but not all, of the studies have found a significant association between CYP2C19 reduced-function variants and clopidogrel treatment outcomes, as discussed further below. This association with outcomes was first noted in 2006 among healthy volunteers, in ex vivo platelet aggregation studies;3 this was followed, in 2009, by three candidate-gene studies showing that CYP2C19 is also associated with cardiovascular outcomes among clopidogrel- treated patients.4–6 Using a genome-wide association study, our group found CYP2C19*2 to be a major determinant of clopidogrel response as measured by ex vivo platelet reactivity in healthy Caucasian subjects.7 This association occurred in a gene–dose-dependent manner, whereby *1/*1 individuals (extensive metabolizers) had the greatest reduction in platelet aggregation in response to clopidogrel, *1/*2 individuals (intermediate metabolizers) had intermediate reduction in platelet aggregation in response to clopidogrel, and homozygous variant *2/*2 individuals (PMs) had the greatest residual platelet reactivity. The *2 variant accounted for ~12% of the total variation in platelet aggregation in response to clopidogrel. Importantly, in this genome-wide study, no other common variants with a similar effect size were discovered. Furthermore, in a separate population undergoing cardiac catheterization, carriers of the *2 allele had a 3.4-fold (95% confidence interval (CI), 1.36–8.46, P = 0.004) higher rate of occurrence of cardiovascular events while on clopidogrel therapy as compared with noncarriers.7 The majority of the candidate gene–related studies evaluating the association between CYP2C19*2 genotype and response to clopidogrel (as measured by platelet aggregation and clinical outcomes) have noted similar findings.4,5,8–11 Two meta- analyses12,13 found that, among patients on clopidogrel therapy, those carrying the CYP2C19*2 allele had a greater risk of cardiovascular events as compared with those with the *1/*1 genotype. In 9,685 patients from nine studies, 91.3% of whom underwent PCI, carriers of one CYP2C19*2 allele and two CYP2C19*2 alleles had significantly increased rates of cardiovascular events as compared with noncarriers (hazard ratio (HR) 1.55, 95% CI 1.10–2.17, P = 0.01 and HR 1.76, 95% CI 1.24–2.5, P = 0.002, respectively).12 The effect of CYP2C19*2 on stent thrombosis in clopidogrel-treated patients was even greater (HR 2.67, 95% CI 1.69–4.22, P = 0.0001 and HR 3.97, 95% CI 1.75–9.02, P = 0.001 in those with one and two copies of CYP2C19*2, respectively, relative to noncarriers).12 The findings in these studies indicate that the increase in risk for carriers of the CYP2C19*2 variants begins very early and remains throughout follow-up (1–100 months; median 12 months). These findings have led to three US Food and Drug Administration–mandated updates of clopidogrel’s label, the most recent being in March 2010. The new boxed warning states that tests are available to determine a patient’s CYP2C19 genotype and that physicians should consider alternative treatment or treatment strategies in patients identified as PMs. However, a growing number of publications have been reporting findings that conflict with the earlier reports. Recent large retrospective analyses of the CURE (Clopidogrel in Unstable Angina to Prevent Recurrent Events)14 and PLATO (Platelet Inhibition and Patient Outcomes)15 trials have not found an association between CYP2C19 genotype and cardiovascular outcomes. Of note, the populations in these studies differed from many of the other reported populations in that they were not necessarily undergoing PCI (with only 66% having a planned invasive strategy in PLATO and only 14.5% receiving stents in CURE), thereby suggesting that the major effect of CYP2C19*2 on clopidogrel response may be indication specific, which makes comparison with more recent studies difficult.14,15 This speculation is consistent with the observation that, among studies that have shown an association between CYP2C19*2 genotype and adverse outcomes with clopidogrel, the largest effects are seen on the outcome of stent thrombosis, with relative risks in the 3.0–6.0 range for individuals carrying a reduced-function allele as compared with those with the *1/*1 genotype.4,5,8,11 For cardiovascular outcomes other than stent thrombosis, the relative risks are lower.4,5 Interestingly, the PLATO study did see an association between CYP2C19 genotype and 30-day outcome with clopidogrel despite the association not holding true at the end of the 12-month follow-up period.15 An additional inconsistency of the clopidogrel loading dose (300 mg vs. 600 mg) exists across the cumulative experience of these studies, including those that did and those that did not demonstrate a genotype–outcome association.14,15 Particularly because increased risk linked to the CYP2C19*2 allele begins early, whether a 600-mg loading dose can to some degree overcome this PM effect remains to be elucidated. Biologically consistent with reports of CYP2C19 being associated with clopidogrel treatment outcomes is the finding that proton pump inhibitors, which are CYP2C19 inhibitors, reduce the efficacy of clopidogrel. However, this issue remains unresolved, with some investigators observing the interaction and others not.16,17