To evaluate the pharmacodynamic efficacy of de-escalating P2Y12 inhibition from prasugrel to clopidogrel based on cytochrome P450 (CYP) 2C19 genotyping, we genotyped 50 Korean patients with AMI who underwent percutaneous coronary intervention (PCI) for CYP2C19 *2,*3, or *17 using real-time PCR. They were discharged on prasugrel 10 mg daily. A control group of 48 AMI patients who underwent PCI and were discharged on clopidogrel but did not undergo genotyping was identified retrospectively. Based on genotyping results available at 3 weeks, 12 patients found to have 2 copies of either CYP2C19 *2 or *3 loss of function alleles continued prasugrel while the remaining 38 patients switched to clopidogrel 75 mg daily. The rate of patients within the therapeutic window (TW) of on-treatment platelet reactivity (OPR), 85<P2Y12 reactivity unit (PRU) ≤275, was compared in the genotype-directed cohort before (3 weeks) and after genotype-directed antiplatelet treatment (5 weeks), as well as with the control cohort at 5 weeks. In the genotype-directed group, there was an increase in the proportion of patients within the TW after genotype-directed antiplatelet treatment (48–76%, p=0.007), primarily driven by a decrease of patients with PRU <85 (52–16%, p <0.001). The proportion of patients within the TW was similar between the genotype-guided and control groups (76% vs. 72.9% p=0.726). In conclusion, individualized antiplatelet regimens based on CYP2C19 genotyping may improve likelihood of achieving a TW of OPR compared to fixed dose of prasugrel 10 mg during maintenance periods of AMI in East Asians.
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