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Background: Whether genetic polymorphisms (GP) impact residual platelet aggregation (RPA) following prasugrel is unclear, especially during maintenance phase. We assessed the influence of CYP2C19 GP carriers on RPA in the prospective observational cohort study. Methods and Results: All post-stent patients (n = 206) received prasugrel 60 mg loading and either 5 or 10 mg daily maintenance with aspirin100 mg. RPA levels by light transmission aggregometry (LTA), multiplate electrode aggregometry (MEA), and VerifyNow (P2Y12 reaction units, PRU) with CYP2C19 GP were measured simultaneously. Demographics and clinical characteristics were not useful for predicting response after prasugrel. GP carriers exhibited higher RPA (PRU: p = 0.001, LTA: p = 0.001, MEA: p = 0.023) than noncarriers. CYP2C19 carriers had higher RPA for 5 mg (n = 35; LTA: p = 0.043, MEA: p = 0.023) and reached significance for 10 mg (n = 27; LTA: p = 0.001, PRU: p = 0.001) prasugrel. When divided into extensive, intermediate, and poor metabolizers, all exhibited statistical differences among the 3 groups (LTA: 14.9 ± 12.3%, 22.6 ± 14.9%, 22.9 ± 15.6%, p = 0.002; PRU: 104.1 ± 70.8%, 141.8 ± 78.0%, 151.0 ± 84.8%, p = 0.003; MEA: 19.7 ± 8.9%, 24.4 ± 12.2%, 28.1 ± 14.7%, p = 0.002). Conclusion: CYP2C19 GP impacts RPA during maintenance phase prasugrel in Korean outpatients. This effect is consistent for both of the approved prasugrel doses potentially affecting long-term outcomes including bleeding risks. However, the clinical utility of these findings is still uncertain, and requires more evidence from larger randomized trials beyond East Asians.