Antiplatelet activity of J78 (2-chloro-3-[2′-bromo, 4′-fluoro-phenyl]-amino-8-hydroxy-1,4-naphthoquinone),an antithrombotic agent, is mediated by thromboxane (TX) A2 receptor blockade with TXA2 synthase inhibition and suppression of cytosolic Ca2+ mobilization

We previously reported that J78 (2-chloro-3-[2'-bromo,4'-fluoro-phenyl]-amino-8-hydroxy-1,4-naphthoquinone), a newly synthesized 1,4-naphthoquinone derivative, exhibited a potent antithrombotic effect, which might be due to antiplatelet rather than anticoagulation activity. In the present study, possible antiplatelet mechanism of J78 was investigated. J78 concentration-dependently inhibited rabbit platelet aggregation induced by collagen (10 mug/ml), thrombin (0.05 U/ml), arachidonic acid (100 muM), and U46619 (9,11-dideoxy-9,11-methanoepoxy-prostaglandin F-2; 1 muM), a thromboxane (TX) A(2) mimic, with IC50 values of 0.32+/-0.01, 0.44+/-0.02, 0.50+/-0.04, and 0.36+/-0.02 M, respectively. J78 also produced a shift to the right of the concentration-response curve of U46619, indicating an antagonistic effect on the TXA(2) receptor. J78 concentration-dependently inhibited collagen-induced arachidonic acid liberation. In addition, J78 potently suppressed TXA(2) formation by platelets that were exposed to arachidonic acid in a concentration-dependent manner but had no effect on the production of PGD(2), indicating an inhibitory effect on TXA(2) synthase. This was supported by a TXA(2) synthase activity assay that J78 concentration-dependently inhibited TXB2 formation converted from PGH(2). Furthermore, J78 was also able to inhibit the [Ca2+](i) mobilization induced by collagen or thrombin at such a concentration that completely inhibited platelet aggregation. Taken together, these results suggest that the antiplatelet activity of J78 may be mediated by TXA(2) receptor blockade with TXA 2 synthase inhibition and suppression of cytosolic Ca2+ mobilization.