Inhibitory mechanism of human platelet aggregation by nafamostat mesilate

We found that nafamostat mesilate (NM) inhibits platelet aggregation induced by all agonists tested, including ADP, collagen, arachidonic acid, thromboxane A analog, A23187, phorbol 12-myrisate 13-acetate (PMA), NaF and thrombin. The IC50 values were in the range of 9.3-17.8 mu M. NM inhibited agonists-induced aspirin-treated platelet aggregation at >10 mu M, suggesting that the action site lies beyond thromboxane (TXA)2 formation. However, NM inhibited thrombin (0.5 IU/ml)-induced TXB2 formation (IC50 = 1.9 +/- 0.6 mu M, mean +/- SD). Intracellular Ca2+ mobilization was also inhibited only when platelets were challenged by thrombin, but the effect was found at NM concentrations >50 mu M. This finding suggests that NM reduces the responses to thrombin by inhibiting its proteolytic activity on the platelet thrombin receptor (PAR1). NM did not affect the intracellular cAMP concentration or A-kinase activity. Agonists-induced surface expression of activated glycoprotein (GP)IIb-IIIa was inhibited by 10 mu M NM and was completely inhibited by 50 mu M NM. Since this inhibitory effect was parallel to the inhibition of platelet aggregation, the main inhibitory mechanism of NM against platelet aggregation seemed to be the suppression of activated GPIIb-IIIa expression, which makes it able to bind fibrinogen.