Effects of the selective inhibition of platelet thromboxane synthesis on the platelet-subendothelium interaction.

1. Drugs that inhibit TxA(2) synthesis are used to reduce platelet aggregation. The aim of this study was to compare the effects of a cyclo-oxygenase (COX) inhibitor (acetylsalicylic acid, ASA), a thromboxane synthetase (TxS) inhibitor (dazoxiben) and a dual TxS inhibitor and TxA(2) receptor blocker (DT-TX 30) on platelet aggregation and the platelet-subendothelium interaction in flow conditions. 2. The techniques used in this in vitro study were platelet aggregometry in whole blood, and measurement of platelet thromboxane B(2) and prostaglandin E(2) production and leucocyte production of 6-keto-PGF(1alpha). The platelet-subendothelium interaction was evaluated in rabbit aorta subendothelium preparations exposed to flowing blood at a shear stress of 800 s(-1). Morphometric methods were used to calculate the percentage of subendothelium occupied by platelets. 3. The 50% inhibitory concentration (IC(50)) of DT-TX 30 in whole blood was in the range of 10(-7) micro M (induced with collagen or arachidonic acid) to 10(-5) micro M (induced with thrombin) or 10(-4) (induced with ADP). IC(50) values under all experimental conditions were lower with DT-TX 30 than with ASA. For thromboxane B(2) the IC(50) were: ASA 0.84+/-0.05 micro M, dazoxiben 765+/-54 micro M, DT-TX 30 8.54+/-0.60 micro M. Prostaglandin E(2) was inhibited only by ASA (IC(50) 1.21+/-0.08 micro M). Leucocyte 6-keto-PGF(1alpha) was inhibited by ASA (IC(50) 6.58+/-0.76 micro M) and increased by dazoxiben and DT-TX 30. The greatest reduction in percentage subendothelial surface occupied by platelets after blood perfusion was seen after treatment with DT-TX 30 in the range of concentrations that inhibited collagen-induced platelet aggregation (control group: 31.20+/-3.8%, DT-TX 30 at 0.1 micro M: 10.71+/-0.55%, at 1.0 micro M: 6.53+/-0.44%, at 5.0 micro M; 1.48+/-0.07%). All three drugs reduced thrombus formation, although ASA (unlike dazoxiben or DT-TX 30) increased the percentage surface occupied by adhesions. 4. In conclusion, the effect of specific blockage of TxS together with blockage of membrane receptors for TxA(2) can surpass the effect of ASA in inhibiting the platelet-subendothelium interaction in flow conditions.