INHIBITION OF PLATELET ACTIVATION BY THE NOVEL THROMBOXANE RECEPTOR ANTAGONIST BM 13.505

Abstract

The effects of BM 13.505 (4-[2-(4-Chlorobenzenesulfonylami-no)ethyl]-benzene acetic acid = BM) on human washed platelets and platelet-rich plasma (PRP) were studied in vitro and after oral application in 10 male volunteers ex vivo/in vitro. BM inhibited the shape change, aggregation and (1H)serotonin release when the platelets were activated by agents that stimulate via the thromboxane Az/prostaglandin H2 (TXA2/PGH2) receptor. Such agonists were collagen, methyl mercury chloride (methyl-Hg), arachidonic acid and the PGH2 analogue U 46,619. BM was 9 times more potent an inhibitor than sulotroban (= BM 13.177). The slope of the Schild plot for U 46,619-induced shape change was close to unity, which is consistent with competitive antagonism. The pA2 -value in PRP was 6.5. BM did not inhibit the primary platelet activation induced by ADP, PAF or serotonin in aspirin-treated platelets, indicating that the inhibition by BM was specific for the platelet TXA2/PGH2 receptor. BM also suppressed platelet activation by PGH2 which accumulated when the platelets were stimulated by collagen, methyl-Hg or arachidonic acid in the presence of the thromboxane synthase inhibitor dazoxiben. At concentrations beyond about 600 times the apparent KD (200 μM in PRP and 10 μM in washed platelets), BM induced a transient shape change. This effect was inhibited by sulotroban and might indicate a slight intrinsic activity of BM. BM 10 μM exerted no effect on the formation of 14C-labelled TXB2, PGE2, PGD2, PGF2α , HHT and 12-HETE from 14C-labelled arachidonic in washed platelet suspensions, irrespective of whether exogenous or endogenous (by methyl-Hg mobilized) arachidonic acid was metabolized. Volunteers who received 7 oral doses of 400 mg in 12 hour intervals reached peak plasma concentrations between 9.5 and 45 pM 1 hour after dosage. The bleeding time was prolonged by 90 %. Platelet activation by collagen, methyl-Hg and U 46,619 was inhibited for at least 9 hours after a single dose. No objective and subjective side effects were observed in any of the subjects. Thus, BM 13.505 is a specific, we11-tolerated and long-acting TXA2/PGH2 receptor antagonist. (Supported by the DFG, Grant Pa-263).