OBJECTIVES: We assessed pharmacodynamic (PD) response for the reduced prasugrel 5-mg maintenance dose in very elderly (≥75y; VE) patients. BACKGROUND: In TRITON-TIMI 38, prasugrel 10-mg reduced ischemic events versus clopidogrel 75-mg, but increased bleeding in VE patients. METHODS: We examined PD and active-metabolite pharmacokinetics with prasugrel 5-mg and 10-mg and clopidogrel 75-mg in a three-period (12 days each), blinded, cross-over study in VE (n=73, mean 79±3y) or non-elderly (≥45-<65y, NE) (n=82, 56±5y) stable coronary artery disease (CAD) patients on background aspirin. Assays included light transmission aggregometry (LTA), VerifyNow(®) P2Y12 (VN-P2Y12), and VASP. The primary comparison was non-inferiority of maximum platelet aggregation (MPA) comparing the median for prasugrel 5-mg in VE versus the 75th percentile for prasugrel 10-mg in NE, using a prespecified one-sided 97.5% confidence interval for the difference <15%. RESULTS: Prasugrel 5-mg in VE met the primary pharmacodynamic non-inferiority criterion versus prasugrel 10-mg in NE. For prasugrel 5-mg, MPA was significantly lower (mean±SD, 57±14%) than clopidogrel (63±14%) (p<0.001) in VE, but higher than prasugrel 10-mg in NE (46±12%) (p<0.001). PD response by LTA, VN-P2Y12, and VASP during all treatments appeared similar between age cohorts. Prasugrel 5-mg resulted in fewer VE poor responders versus clopidogrel. Rates of mild bleeding were higher with prasugrel 10-mg, but similar for prasugrel 5-mg versus clopidogrel 75-mg. CONCLUSIONS: In aspirin-treated stable CAD patients, prasugrel 5-mg in VE attenuated platelet inhibition while meeting prespecified non-inferiority criterion versus prasugrel 10-mg in NE, with significantly better PD response and fewer poor responders compared to clopidogrel 75-mg in VE.