The maintenance of normal vascular tone in mammals involves a homeostatic balance between factors which promote vasodilation and factors which promote vasoconstriction. Thromboxane A2 (TxA2), for example, is a powerful vasoconstrictor (1) and also a potent mediator of platelet aggregation (14). Prostaglandins such as prostaglandin E2 and prostacyclin, on the other hand, have vasodilatory and anti-platelet aggregation effects.
Many diseases involve a perturbation of this normal homeostatic balance. For example, diabetes mellitus (5-7), hypertension (8-13), thrombosis (14-16) and septic shock (17-19) are associated with an imbalance of the thromboxane: prostacyclin or thromboxane: prostaglandin E2 ratios in favour of thromboxane. There is therefore considerable interest in finding ways of selectively controlling thromboxane formation, so as to block the vasoconstrictor and aggregatory component of the prostaglandin system (thromboxane) without affecting the vasodilator and anti-aggregatory prostaglandins.
Prostaglandins, including thromboxane, are formed from a common precursor, PGH2. This precursor is formed through the action of cyclooxygenase (COX) and is transformed by specific enzymes into each of the prostaglandins and thromboxane. Platelets convert PGH2 selectively into thromboxane A2 through the action of the PGH2 metabolizing enzyme, thromboxane A2 synthase. In other tissues and cells, PGH2 is converted into other prostaglandin types, e.g. prostaglandin E2 and I2. Since thromboxane A2 (TxA2) has a very short half life in the body (30 sec), stable analogs of TxA2 which mimic its actions have been explored; two such analogs are U44609 (20, 21) and U46619 (22). These analogs cause actions similar to TxA2, i.e. they cause platelet shape change and aggregation, as well as contraction of smooth muscle. These TxA2 mimics have also been used to develop inhibitors of TxA2 action as they work through the activation of the thromboxane receptors called TP receptors.
Several thromboxane synthase inhibitors (TSI) have been reported, some based on imidazole (23). In general, the IC50's of the imidazole-based inhibitors were in the range 10−4-10−7 m (24). Undesirable pressor effects that could not be resolved from their anti-thrombotic actions have been noted upon administration of these drugs, making the substituted imidazoles unsuitable for drug development.
Non-steroidal anti-inflammatory drugs such as aspirin have also been used to inhibit thromboxane synthesis. Aspirin, however, has frequent side effects, including gastric ulcers and Reye's syndrome. Attempts have been made to avoid these problems by developing COX-2 inhibitors such as CELEBREX® (Celecoxib) and VIOXX® (Rofecoxib) (42, 43). These drugs do not, however affect thromboxane formation in platelets where COX-1 is present and therefore are not of assitance in combatting thrombosis.
The hepoxilins are biologically active metabolites of arachidonic acid formed through the 12(S)-lipoxygenase pathway and hence are structurally unlike the prostaglandin endoperoxides (25-27). Four natural hepoxilins have been identified, the A-type hepoxilins consisting of two epimers having a hydroxyl group at carbon 8 (8(S, R)-hydroxy-11(S), 12(S)-epoxy-eicosa-5Z, 9E, 14Z-trienoic acid) and the B-type, two epimers having a hydroxyl group at carbon 10 (10(S,R)-hydroxy-11(S), 12(S)-epoxy-eicosa-5Z, 8Z,14Z-trienoic acid). Pharmacological studies have provided evidence that these compounds raise intracellular calcium by activating calcium stores in human neutrophils (28, 29). A hepoxilin-specific receptor responsible for this action has been suggested (30). The hepoxilins activate potassium channels in platelets (31) and in the Aplysia brain (32, 33). In platelets, the hepoxilins are formed endogenously in response of the cell to hypertonic volume expansion, and function to normalize cell volume (31).
Neither the hepoxilins nor the hepoxilin analogs described herein have previously been reported to affect thromboxane formation and action.