Context

The antiplatelet effect of clopidogrel varies among users as measured by ex vivo platelet function studies and clinical outcomes. Studies have identified common reduced-function variants in the cytochrome P450 enzyme gene, CYP2C19, that are associated with clopidogrel responsiveness. The fully functional form of the gene, designated CYP2C19*1, metabolises clopidogrel from its native prodrug state to its active antiplatelet metabolite more efficiently than CYP2C19*2, found commonly in Caucasians, African Americans and Asians, and CYP2C9*3, common only in Asians. Individuals carrying one of these reduced-function alleles are intermediate metabolisers, whereas those carrying two reduced-function alleles are poor metabolisers. In patients on clopidogrel, these variants have been associated with poorer clinical outcomes in many studies, although not all have shown an effect, particularly in patients with one reduced-function allele.