Inhibition of platelet adhesion and aggregation by E4021, a type V phosphodiesterase inhibitor, in guinea pigs

E4021 (sodium 1-[6-chloro-4-(3, 4-methylenedioxybenzyl)-aminoquinazolin-2-yl]piperidine-4-carboxylate sesquihydrate) is a highly selective and potent inhibitor of type V phosphodiesterase(PDE5). The in vitro and in vivo effect of E4021 on platelet function was evaluated, using echistatin, a potent disintegrin, as a positive reference agent. E4021 inhibits aggregatory response to collagen in washed human platelets (IC 50 =5μM, vs. 0.14μM with echistatin). In the ex vivoplatelet aggregation assay using whole blood from treated guinea pigs, E4021 (9mg/kg i.v.) showed a moderate inhibition (43%) against collagen (0.125μg/ml), whereas echistatin (250μg/kg i.v.) exerted a 88% inhibition. The absence of endothelium-derived factors (NO) may account for the moderate in vitro and ex vivo antiplatelet activity of E4021. In an in vivo model of reversible platelet aggregation elicited by collagen (100μg/kg i.v.), both E4021 and echistatin attenuated the intrapulmonary platelet accumulation in guinea pigs (-36% and -44%, respectively). In addition, E4021 (9mg/kg i.v.) and echistatin (250μg/kg i.v.) caused a similar inhibition of platelet adhesion at sites of microfilament-induced vascular injury in guinea pigs (52% and 65%, respectively). The two agents in combination did not show additive effect, suggesting that E4021 inhibits platelet activation and impairs interactions of adhesion receptors with matrix proteins. E4021 caused a selective increase in cGMP concentrations in the platelets isolated from treated guinea pigs; cAMP was not affected. It is concluded that the antiplatelet activity of E4021 is mediated through cGMP mechanism by virtue of selective inhibition of PDE5 in the platelets.