9,11-Epoxy-9-homo-14-thiaprost-5-enoic acid derivatives: potent thromboxane A2 antagonists

A novel bicyclic prostaglandin analogue, (1S)-[1 alpha,2 alpha(Z),3 alpha,4 alpha]-7-[3-[(hexylthio)methyl]-7- oxabicyclo [2.2.1]hept-2-yl]-5-heptenoic acid ((-)-10), and its cogeners were found to be potent antagonists at the TxA2 receptor. Compound (-)-10 was the only stereoisomer out of eight possible structures that was active. Thioether (-)-10 was 30-40-fold more potent than another TxA2 antagonist, BM 13.177, in inhibiting arachidonic acid (AA) induced aggregation of human platelet-rich plasma. Compound (-)-10 was effective (I50 = 0.5 +/- 0.4 microM) in inhibiting 9,11-azo-PGH2-induced (0.1 microgram/mL) contraction of guinea pig tracheal spirals. The bronchoconstriction in anesthetized guinea pigs induced by AA was also effectively antagonized by (-)-10 (1 mg/kg, iv); however, in this assay (-)-10 exhibited some direct agonist activity. Radioligand binding studies in washed (human) platelets revealed that (-)-10 is one of the most potent ligands for the PGH2/TxA2 receptor yet described (Kd = 1.6 +/- 0.4 nM).