Mechanism of Inhibitory Action of Cyclooxygenase-2 Inhibitors, Nimesulide and NS-398 in Human Platelets

Cyclooxygenase (COX) enzymes convert arachidonic acid to prostaglandin H2 (PG H2), which is further metabolized by other enzymes to various PGs, prostacyclins and thromboxanes. COX-1 is expressed constitutively whereas COX-2 is induced in cells exposed to proinflammatory agents including cytokines, mitogens and endotoxin. Human platelets predominantly express COX-1. Nimesulide (CAS 51803-78-2) has been shown to exert marked anti-inflammatory effects in several in vivo models of inflammation. Since nimesulide is considered to be a selective inhibitor of COX-2, it has not been studied in detail in relation to its mechanistic effects on human platelets, which expresses cyclooxygenase-1 (COX-1). This study was conducted to investigate the effects of nimesulide on human platelet aggregation. We found that nimesulide (1-100 μM) inhibited platelet aggregation induced by adrenaline (20 μM). It also inhibited thromboxane A2 (TXA2) formation by platelets at a low concentration (IC50; 1 μM). However, much lower concentrations of nimesulide (0.01-0.1 μM) potentiated the aggregatory response to subthreshold concentrations of adrenaline (0.22 μM) whereas NS-398, a selective inhibitor of COX-2 however, had no such effect. The aggregatory effect of nimesulide on platelets was blocked by Ca2+-channel blockers, verapamil and diltiazem (IC50; 7 and 46 μM, respectively) and a nitric oxide donor, S-Nitroso-N-acetylpenicillamine (SNAP) (IC50, 2 μM) but not by genistein (up to 10 μM).These results are indicative of the concentration-dependent dual effects of nimesulide on human platelet aggregation