Epelsiban (GSK 557296) is a potent, selective and orally bioavailable oxytocin receptor antagonist, with a pKi of 9.9 for human oxytocin receptor.
Epelsiban is a potent oxytocin receptor, with a pKi of 9.9 for human oxytocin receptor, >31000-fold selectivity over all three human vasopressin receptors hV1aR (pKi,
Epelsiban shows an IC50 of 192 nM for oxytocin receptor in rats. Epelsiban has low levels of intrinsic clearance against the microsomes of rat, dog, and cynomolgus monkey, good bioavailability (55%), but is negative in the genotoxicity screens with a satisfactory oral safety profile in female rats[1].
[1]. Borthwick AD, et al. Pyridyl-2,5-diketopiperazines as potent, selective, and orally bioavailable oxytocin antagonists: synthesis, pharmacokinetics, and in vivo potency. J Med Chem. 2012 Jan 26;55(2):783-96.