CS-747 (Prasugrel, LY640315) is a thienopyridine antiplatelet prodrug that is metabolized to the thiol-containing active metabolite R-138727,which binds to and irreversibly inhibits the platelet P2Y12ADP receptor. R-138727 is composed of 4 stereo-isomers, (R, S)-, (R, R)-, (S, S)-, and (S, R)-isomers (the first letter for the configuration of a chiral center at the sulfur-bearing position and the second for that at the benzylic position). In the present study, we determined the stereoselectivity of P2Y12 antagonist effects by assessing the antagonism of the [3H]-2-MeS-ADP that binds to human P2Y12 receptors expressed in Chinese hamster ovary cells as an affinity assay, and by the inhibition of ADP-induced aggregation of washed human platelets as a functional assay. R-138727 and its 2 components, R-99224, a mixture of (R, S)- and (S, R)-isomers and R-100364, a mixture of (R, R)- and (S, S)-isomers, inhibited [3H]-2-MeS-ADP binding and platelet aggregation. The rank order of potency of these compounds were identical in both assays: R-99224>R-138727>> R-100364. Inhibition of ADP-induced platelet aggregation by R-138727 and R-99224 was concentration- and time-related. In experiments using the 4 single stereo-isomers, all isomers inhibited ADP-induced platelet aggregation, but the (R, S)-isomer was found to be the most potent, followed by the (R, R)-isomer. These in vitro studies indicate that R- 138727 is an effective antagonist of P2Y12 and potent inhibitor of ADP-induced platelet aggregation, and that these antiplatelet activities of R-138727 are largely dependent on its (R, S)-isomer. This suggests that the (R)-configuration of the reactive thiol group of the active metabolite of CS-747 is critical for P2Y12 and platelet inhibitory activities.
The thienopyridine antiplatelet prodrug clopidogrel bisulfate (C) is widely used for atherothrombotic diseases. However, several reports have described a limitation of C, namely, poor platelet inhibition in many patients following oral dosing. Prasugrel (hydrochloride), currently under investigation in a Phase 3 trial in acute coronary syndrome patients undergoing percutaneous coronary intervention, is a novel thienopyridine antiplatelet prodrug. In recent clinical studies, prasugrel has shown more consistent platelet inhibition than C. To date, several preclinical studies have demonstrated that the prasugrel base formulation has more potent antiplatelet and antithrombotic effects than C. The effects of prasugrel hydrochloride (P), which has a higher solubility than the base formulation, have not been reported. The present studies examined the effects of P on platelet aggregation, arterial thrombosis, and bleeding time in rats. Single oral administration of P (0.3 to 3 mg/kg) resulted in inhibition of platelet aggregation (IPA) induced by ADP in a dose- and time-related manner with an ED50 value of 1.1 mg/kg at 4 hr postdose. After dosing of 3 and 10 mg/kg P, statistically significant IPA was observed at 30 and 15 min, and the time required to achieve 50% IPA was approximately 50 and 23 min, respectively (Figure). These results indicate that P has potent antiplatelet effects with an extremely fast onset of action. Multiple P dosing also resulted in potent IPA, with an ED50 value of 0.45 mg/kg/day (p.o.) at 4 hr after the last dose. Our previous report indicated an ED50 value of C at 4 hr after single oral dosing of 16 mg/kg (Br. J. Pharmacol. 129, 1439, 2000), demonstrating about 10-times greater potency of P compared to C. Potent dose-related antithrombotic effects of P were further confirmed in an arterial thrombosis model in rats. P (0.1–1 mg/kg, p.o.) inhibited thrombus formation in the arterial thrombosis induced by extravascular application of FeCl3 solution. All these results taken together indicate that prasugrel hydrochloride is a novel, potent and fast acting thienopyridine prodrug, potentially providing superior efficacy in the treatment of atherothrombotic disorders. Figure Figure
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