Pharmacogenomic Testing for CYP2C19 Variants among Stroke Patients Treated with Clopidogrel: Opportunity for the Clinical Laboratory?

BACKGROUND: Clopidogrel is a widely used antiplatelet agent used to prevent adverse events for patients suffering from acute coronary syndromes and ischemic stroke. As a prodrug, clopidogrel must be converted to the active form through the enzyme cytochrome (CYP) P450 2C19 (among other enzymes). Individuals carrying a loss of function (LOF) allele (i.e., *2 and/or *3) have reduced pharmacologic efficacy. Ticagrelor is an alternative antiplatelet medication that is not a prodrug. METHODS: We reviewed the Clopidogrel in High-Risk Patients with Acute Nondisabling Cerebrovascular Events (CHANCE2) Trial demonstrating the inferiority of clopidogrel dual therapy with aspirin vs ticagrelor dual therapy to prevent adverse events among patients suffering from a mild stroke among Chinese patients who carried a CYP2C19 LOF. We also summarized the pharmacogenomic testing policies within Chinese clinical laboratories after publication of this trial, and tabulated the CYP2C19 LOF allele frequencies among ancestries, as a criteria for justifying the expense required for establishing pharmacogenomic testing services for other populations. RESULTS: The CHANCE2 trial showed that stroke patients carrying a CYP2C19 LOF allele(s) had a reduction of 1.6% for recurrent stroke for those treated with ticagrelor vs clopidogrel. The LOF allele frequency was highest among Pacific Island and Western and Central Asian (e.g., Han Chinese) patients and lowest among European, Latin, and Hispanic Latino patients. CONCLUSIONS: Pharmacogenomic testing for CYP2C19 variants is more economically justified for laboratories that serve a population enriched with CYP2C19 LOF alleles, than populations exhibiting a lower allele frequency. Within a clinical laboratory offering testing, restricting testing to certain populations is not ethical.