Satigrel (E5510, 4-cyano-5,5-bis(4-methoxyphenyl)-4-pentenoic acid) is a potent inhibitor of platelet aggregation. Like cyclooxygenase/prostaglandin H synthase (PGHS) inhibitors such as aspirin, which suppress platelet aggregation by inhibiting thromboxane A2 production, satigrel inhibits collagen- and arachidonic acid-induced aggregation of human platelets. In contrast to other PGHS inhibitors, satigrel, like cyclic nucleotide phosphodiesterase (PDE) inhibitors such as cilostazol, shows inhibitory activity against thrombin-induced platelet aggregation. To investigate the mechanism of the anti-platelet activity of satigrel, we examined the selectivity and potency of satigrel against PGHS isozyme activities and PDE isoform activities. Two isozymes of PGHS are known; constitutive enzyme (PGHS1) and inducible enzyme (PGHS2). Satigrel showed inhibitory activity against PGHS1 (IC50: 0.081 microM) and PGHS2 (IC50: 5.9 microM), suggesting the selective inhibition of PGHS1. Indomethacin, which is a selective inhibitor of PGHS1, showed similar selectivity against PGHS isozymes (IC50: 0.12 microM and 1.4 microM, respectively). These results support that satigrel suppresses thromboxane A2 production by inhibiting PGHS1. It is known that three isozymes of PDE exist in human platelets: Type V, which specifically hydrolyzes guanosine 3',5'-cyclic monophosphate (cGMP), Type III, which mainly hydrolyzes cAMP, and Type II, which hydrolyzes both cGMP and cAMP. We separated these three isozymes from human platelets and examined the inhibitory activity of satigrel against each enzyme. Of the three isozymes, the inhibitory activity of satigrel was the most potent against Type III PDE (IC50: 15.7 microM). The IC50 value for Type III corresponded with that for thrombin-induced platelet aggregation. Type V and Type II were also inhibited by satigrel (IC50: 39.8 and 62.4 microM, respectively). In human platelets, satigrel increased both cAMP and cGMP levels in a dose-dependent manner (100, 300 microM). In conclusion, satigrel inhibits collagen- and arachidonic acid-induced platelet aggregation through preventing thromboxane A2 synthesis by selective inhibition of the target enzyme, PGHS1, which exists in platelets. The anti-aggregating activity of satigrel against thrombin-induced aggregation may be due to elevation of the cyclic nucleotide levels through the inhibition of PDE isozymes.