Summary

CS-747 (Prasugrel, LY640315) is a thienopyridine antiplateletprodrug that is metabolized to the thiol-containing active metaboliteR-138727, which binds to and irreversibly inhibits the plateletP2Y₁₂ ADP receptor. R-138727 is composed of 4 stereoisomers,( R , S )-, ( R , R )-, ( S , S )-, and ( S , R )-isomers (the first letterfor the configuration of a chiral center at the sulfur-bearing positionand the second for that at the benzylic position).In the presentstudy,we determined the stereoselectivity of P2Y₁₂ antagonisteffects by assessing the antagonism of the [³ H]-2-MeS-ADPthat binds to human P2Y₁₂ receptors expressed in Chinesehamster ovary cells as an affinity assay, and by the inhibition ofADP-induced aggregation of washed human platelets as a functionalassay. R-138727 and its 2 components, R-99224, a mixtureof ( R , S )- and ( S , R )-isomers and R-100364, a mixture of ( R , R )-and ( S , S )-isomers, inhibited [³ H]-2-MeS-ADP binding and platelet aggregation. The rank order of potency of these compoundswere identical in both assays: R-99224>R-138727>>R-100364. Inhibition of ADP-induced platelet aggregation byR-138727 and R-99224 was concentration- and time-related. Inexperiments using the 4 single stereo-isomers, all isomers inhibitedADP-induced platelet aggregation, but the ( R , S )-isomerwas found to be the most potent, followed by the ( R , R )-isomer.These in vitro studies indicate that R-138727 is an effective antagonistof P2Y₁₂and potent inhibitor of ADP-induced platelet aggregation,and that these antiplatelet activities of R-138727 arelargely dependent on its ( R , S )-isomer.This suggests that the ( R )-configuration of the reactive thiol group of the active metaboliteof CS-747 is critical for P2Y₁₂ and platelet inhibitory activities.