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Background and methods: Prasugrel is a novel orally active thienopyridine prodrug with potent and long-lasting antiplatelet effects. Platelet inhibition reflects inhibition of P2Y12 receptors by its active metabolite (AM). Previous studies have shown that the antiplatelet potency of prasugrel is at least 10 times higher than that of clopidogrel in rats and humans, but the mechanism of its higher potency has not yet been fully elucidated.Results: Oral administration of prasugrel to rats resulted in dose-related and time-related inhibition of ex vivo platelet aggregation, and its effect was about 10 times more potent than that of clopidogrel. The plasma concentration of prasugrel AM was higher than that of clopidogrel AM despite tenfold higher doses of clopidogrel, indicating more efficient in vivo production of prasugrel AM than of clopidogrel AM. In rat platelets, prasugrel AM inhibited in vitro platelet aggregation induced by adenosine 5′-diphosphate (ADP) (10 μm) with an IC50 value of 1.8 μm. Clopidogrel AM similarly inhibited platelet aggregation with an IC50 value of 2.4 μm. Similar results were also observed for ADP-induced (10 μm) decreases in prostaglandin E1-stimulated rat platelet cAMP levels. These results indicate that both AMs have similar in vitro antiplatelet activities.Conclusions: The greater in vivo antiplatelet potency of prasugrel as compared to clopidogrel reflects more efficient in vivo generation of its AM, which demonstrates similar in vitro activity to clopidogrel AM.