U-51605, a stable analog of the endoperoxide prostaglandin H2 (PGH2), functions as an inhibitor with greater selectivity towards prostacyclin (PGI) synthase over thromboxane (TX) synthase. It also acts as a partial agonist at TP receptors. Studies show that at a concentration of 2.8 µM, U-51605 effectively inhibits PGI synthase in human foreskin fibroblasts, while a concentration of 5.6 µM is required to inhibit human platelet TX synthase. Furthermore, U-51605, at up to 1 µM, decreases the release of prostacyclin in SHR aorta triggered by the calcium ionophore A-23187 without impacting TXA2 production, and notably enhances the release of PGE2 and PGF2α.