Clopidogrel is the most important antiplatelet agent identified since aspirin. Its activity, however, is characterized by highly variable efficacy. This variability results from differences among individuals in their ability to metabolize the drug. These differences have clinical consequences, as evidenced by the observation that resistance to clopidogrel is associated with stent thrombosis in the setting of percutaneous coronary intervention (PCI) and myocardial infarction in coronary artery disease. Several of the enzymes responsible for the conversion of clopidogrel to its active metabolite have been identified. Cytochrome P450 isozymes are the most widely studied and function in the conversion of clopidogrel to 2-oxo-clopidogrel (Figure). Loss-of-function alleles of the cytochrome P450 2C19 gene are associated with increased stent thrombosis. A genome-wide association study linked CYP2C19 variants with differences in platelet response to clopidogrel. However, CYP2C19 variants account for only 12 percent of the heterogeneity in clopidogrel response. Bouman and colleagues from The Netherlands now show that paraoxonase-1, a second enzyme mediating clopidogrel metabolism, may contribute more significantly to response variability.
They used a metabolic profiling approach to identify enzymes responsible for clopidogrel metabolism. They broke the process down into two stages (Figure): cytochrome P450-mediated oxidation and esterase-mediated thiophene ring hydrolysis. Paraoxonase-1 was identified as the critical esterase in the second stage. In addition, it was noted that the Arg192 allozyme of paraoxonase-1 had a much better cleavage efficiency than the Gln192 allozyme.
The authors then conducted two clinical trials to evaluate the contribution of paraoxonase-1 gene (PON1) variants to variability in response to clopidogrel. In a case-cohort study in individuals who underwent PCI and received clopidogrel for six to 12 months, the authors found that individuals with the QQ192 PON1 variant (encoding the Gln192 allozyme) had a 12-fold increase in stent thrombosis compared with individuals with the RR192 PON1 variant (encoding the Arg192 allozyme). In addition, those with stent thrombosis demonstrated a lower plasma concentration of active metabolite and a higher concentration of 2-oxo-clopidogrel, indicating deficient paraoxonase-1 activity. Individuals with the QQ192 PON1 variant also showed lower inhibition of platelet aggregation following clopidogrel administration. The results of this trial were corroborated in a second clinical trial in which nearly 2,000 individuals with acute coronary syndromes receiving clopidogrel were followed prospectively. This trial demonstrated a 10-fold increased risk of stent thrombosis and a nearly five-fold increase in myocardial infarction in QQ192 versus RR192 homozygous individuals. In contrast, individuals with the QQ192 variant demonstrated a significantly lower risk of bleeding. The authors calculated that the PON1 Q192R polymorphism explained 72.5 percent of the variability in ADP-stimulated platelet aggregation after clopidogrel administration.
Clopidogral Metabolism Broken Down into Two Stages: Cytochrome P450-Mediated Oxidation and Esterase-Mediated Thiophene Ring Hydrolysis. Clopidogrel is metabolized via oxidation by cytochrome P450 isozymes followed by hydrolytic cleavage by paraoxonases, particularly PON1.
Clopidogral Metabolism Broken Down into Two Stages: Cytochrome P450-Mediated Oxidation and Esterase-Mediated Thiophene Ring Hydrolysis. Clopidogrel is metabolized via oxidation by cytochrome P450 isozymes followed by hydrolytic cleavage by paraoxonases, particularly PON1.
In Brief
The discovery of the PON1 Q192R polymorphism as a determinant of responsiveness to clopidogrel is an important advance in our understanding of this widely used drug. Yet some questions about this finding remain. The reasons why the PON1 gene region was not associated with clopidogrel response variability in a genome-wide association study of an Amish population is not entirely clear. Differences in populations or in methodology of platelet testing between the two studies might explain the discrepancy. Conversely, Bouman et al. did not demonstrate an impact of CYP2C19 genes, which has been previously associated with response variability, in their studies. These discrepancies notwithstanding, the impressive hazard ratio of thrombotic risk associated with the PON1 QQ192 variant in the setting of clopidogrel therapy makes a strong case for further evaluation of this gene variant. If follow-up studies are confirmatory, PON1 genotyping or PON1 activity measurements, perhaps in combination with assessment of CYP2C19, will help guide therapeutic decisions. Such studies will be particularly useful in identifying candidates for newer alternative P2Y12 receptor inhibitors, such as prasugrel or ticagrelor, that do not require similar metabolism.
