Thromboxane A2/prostaglandin H2 receptor antagonists. A new therapeutic principle.

Neither low- nor very-low-dose Aspirin suppresses thromboxane A2 biosynthesis without inhibiting the formation of its functional antagonist prostacyclin. Although thromboxane synthase inhibitors selectively inhibit thromboxane A2 biosynthesis and increase prostacyclin formation in vivo, thromboxane synthase inhibitors can lead to an accumulation of prostaglandin H2 that substitutes for thromboxane A2 at their common receptors in platelets and smooth muscle. In contrast to those inhibitors of thromboxane A2 biosynthesis, thromboxane A2/prostaglandin H2 receptor antagonists do not affect the synthesis of prostacyclin and other prostaglandins but prevent thromboxane A2 and prostaglandin H2 from activating platelets and inducing vasoconstriction. The available thromboxane A2/prostaglandin H2 receptor antagonists are competitive antagonists. However, some of them, such as daltroban, S-145, GR 32.191, and Bay U 3405, produce a noncompetitive antagonism in human platelets due to their low dissociation rate. As a consequence, agonists equilibrate with the antagonist-occupied receptor pool so slowly that they fail to induce platelet activation. This property of some antagonists strongly increases their potency and the duration of their inhibitory effect. Therefore, a low dissociation rate is an important measure of the effectiveness of a thromboxane A2/prostaglandin H2 receptor antagonist in addition to its receptor affinity. Another approach in thromboxane A2 pharmacology is a combination of thromboxane synthase inhibition with thromboxane receptor antagonism in one drug, such as ridogrel. Such dual inhibitors present a very high inhibitory potential and prolong the skin bleeding time to a greater extent than Aspirin, thromboxane synthase inhibitors, or thromboxane A2/prostaglandin H2 receptor antagonists.