THROMBOXANE A(2) SYNTHASE INHIBITION AND THROMBOXANE A(2) RECEPTOR BLOCKADE BY 4-CYANOPHENYL)AMINO]-3-CHLORO-1,4-NAPHTHALENEDIONE (NQ-Y15) IN RAT PLATELETS

The effects of -acetylphenyl)amino]-3-chloro-1,4-naphthalenedione (NQ- Y15), a synthetic 1,4-naphthoquinone derivative, on platelet activity and its mechanism oi action were investigated. NQ-Y15 caused a concent ration-dependent inhibition of the aggregation induced by thrombin, co llagen, arachidonic acid (AA), and A23187. The IC50 values of NQ-Y15 o n thrombin (0.1 U/mL)-, collagen (10 mu g/mL)-, AA (50 mu M)-, and A23 187 (2 mu M)-induced aggregation were 36.2 +/- 1.5, 6.7 +/- 0.7, 35.4 +/- 1.7, and 93.1 +/- 1.4 mu M, respectively. NQ-Y15 also inhibited th rombin-, collagen-, AA-, and A23187-stimulated serotonin secretion in a concentration-dependent manner. However, a high concentration (100 m u M) of Na-Y15 showed no significant inhibitory effect on ADP-induced primary aggregation, which is independent of thromboxane A(2) (TXA(2)) production in rat platelets. In fura-2-loaded platelets, the elevatio n of intracellular free calcium concentration stimulated by AA, thromb in, and 4-bromo-A23187 was inhibited by NQ-Y15 in a concentration depe ndent manner. The formation of TXA(2) caused by AA, thrombin, and coll agen was inhibited significantly by NQ-Y15. NQ-Y15 inhibited TXA(2) sy nthase in intact rat platelets, since this agent reduced the conversio n of prostaglandin (PG) H-2 to TXA(2). Similarly, NQ-Y15 selectively i nhibited the TXA(2) synthase activity in human platelet microsomes, wh ereas it had no effect on activity of phospholipase A(2), cyclooxygena se, and PGI(2) synthase in vitro. Na-Y15 inhibited platelet aggregatio n induced by the endoperoxide analogue U46619 in human platelets, indi cating TXA(2) receptor antagonism, possibly of a competitive nature. T hese results suggest that the antiplatelet effect of NQ-Y15 is due to a combination of TXA(2) synthase inhibition with TXA(2) receptor block ade, and that it may be useful as an antithrombotic agent. (C) 1997 El sevier Science Inc.