(E)-7-Phenyl-7-(3-pyridyl)-6-heptenoic acid (CV-4151) inhibited horse platelet microsomal thromboxane (TX) A2 synthetase with an IC50 of 2.6 X 10(-8) M, but even at a high concentration of 10(-4) M it had little effect on cyclooxygenase, PGI2 synthetase and 5-lipoxygenase in in vitro enzymatic assays. CV-4151 did not affect PGI2 release from rat and rabbit aortic tissues in in vitro (10(-4) M) and ex vivo (10 and 100 mg/kg, p.o.) experiments, whereas aspirin (10(-4) M or 10 and 100 mg/kg, p.o.) markedly inhibited PGI2 release in these preparations. When given orally to rats and dogs, CV-4151 markedly inhibited blood TXA2 synthetase activity: the ID50 values (mg/kg, 2 hr later) were 0.05 in rats and 0.17 in dogs. The inhibitory effects at an oral dose of 1 mg/kg lasted more than 24 hr in both species; the inhibition was 41% in rats and 32% in dogs 24 hr after the administration. When injected i.v. to rats and dogs, CV-4151 caused inhibitory effects on TXA2 synthetase equipotent to those observed with the oral administration. In both species, CV-4151 given orally increased concentration of serum immunoreactive 6-keto-PGF1 alpha concomitant with a decrease of serum TXB2-8 concentration. CV-4151 was equipotent to OKY-1580 (IC50: 2.3 X 10 M), a well documented TXA2 synthetase inhibitor, in an in vitro TXA2 synthetase assay. However, CV-4151, given orally or i.v. to rats and dogs, was much more potent and longer acting in inhibition of blood TXA2 production than OKY-1580. Dazoxiben was less potent than these compounds in vitro. In rats, serial oral administration of CV-4151 (10 mg/kg) once daily for 14 days produced a constant and marked reduction of serum TXB2 concentration with concomitant increase of serum immunoreactive 6-keto-PGF1 alpha concentration. No rebound phenomenon in inhibition of TXA2 synthetase was observed after the dosing was stopped. These findings indicate that CV-4151 is a potent and long acting selective inhibitor of TXA2 synthetase and may reorient the metabolism of PG endoperoxides to PGI2.