Comparative study of antiplatelet drugs in vitro: distinct effects of cAMP-elevating drugs and GPIIb/IIIa antagonists on thrombin-induced platelet responses.

Among various categories of antiplatelet drugs, cAMP-elevating agents and GP IIb/IIIa antagonists have been reported to inhibit platelet aggregation stimulated by a wide variety of platelet agonists. To clarify the qualitative difference between these two agents, their effects on various platelet responses in washed platelets evoked by thrombin (0.05 U/mL) were compared in vitro. Two types of cAMP-elevating drugs, cilostazol (a phosphodiesterase III inhibitor) and prostaglandin E1 (an adenylate cyclase activator), both inhibited platelet aggregation, thromboxane A2 formation, and platelet factor 4 release in a concentration-dependent manner. In addition, both agents suppressed intracellular Ca++ elevation induced by thrombin. However, two classes of GP IIb/IIIa antagonists, abciximab (Fab fragment of antibody) and tirofiban (a synthetic compound), showed no inhibitory effects against thromboxane A2 formation and platelet factor 4 release, although these drugs inhibited platelet aggregation. Essentially the same results were obtained in platelet-rich plasma stimulated with high concentration (100 microM) of thrombin receptor activating peptide. In contrast to these different profiles on thromboxane A2 formation and release reaction, both cAMP-elevating agents and GP IIb/IIIa antagonists potently suppressed procoagulant activity in thrombin-stimulated platelets. These results suggest that the development of platelet procoagulant activity induced by thrombin is exclusively dependent on platelet aggregation or aggregation-dependent processes. These observations also indicate that cAMP-elevating agents possess wider inhibitory effects on platelet responses evoked by strong agonists than GP IIb/IIIa antagonists.