The progesterone receptor (PR) is an important regulator of endometrial function. Blockade of PR function has been recognized as the potential basis for preventing gynecological conditions such as endometriosis and uterine fibroids. In this study, we examine the in vitro and in vivo properties of a nonsteroidal PR antagonist, 2-[4-(4-cyano-phenoxy)-3,5-dicyclopropyl-1H-pyrazol-1-yl]-N-methylacetamide (PF-02367982) in comparison with the nonselective steroidal antagonist RU-486 (mifepristone). PF-02367982 was found to be a potent PR antagonist with far greater selectivity over the glucocorticoid receptor than RU-486. Both PF-02367982 and RU-486 blocked progesterone-induced arborization of the rabbit endometrium in a dose-dependent manner at unbound drug exposures that were commensurate with their potencies as PR antagonists in vitro. Translation of this pharmacology to a clinically relevant system was required to bridge the pharmacological gap between nonmenstruating rabbits and humans. Thus, the pharmacokinetic (PK) and pharmacodynamic (PD) data from the rabbit were combined to predict pharmacological effects on the naturally cycling cynomolgus macaque endometrium. PF-02367982 blocked the effect of progesterone on the cynomolgus macaque endometrium to the same degree as RU-486 and at exposures predicted by the rabbit PK-PD model. With such an efficacious and superior selectivity profile to the nonselective RU-486, PF-02367982 may have significant therapeutic value in the treatment of gynecological conditions such as endometriosis.The discovery of synthetic progesterone receptor (PR) modulators has underscored the important role that progesterone plays in reproductive physiology, including the menstrual cycle, endometrial growth, breast development and maintenance of pregnancy (Chabbert-Buffet et al., 2005;Spitz, 2006). RU-38486 (RU-486, mifepristone) is the founding member of steroidal PR antagonists and is approved clinically for pregnancy termination and emergency contraception. Studies with RU-486 and other PR antagonists, such as onapristone, ZK-137316 (Zelinski-Wooten et al., 1998), and ZK-230211, have revealed that these agents, as well as directly antagonizing progesterone function, can block ovarian function and also arrest the effect of estrogen on the endometrium in women and nonhuman primates (NHP) (Wolf et al., 1989;Baird et al., 2003;Brenner et al., 2005; ChabbertBuffet et al., 2005;Slayden et al., 2006).The potential usefulness of RU-486 and other PR antagonists in the treatment of pathological conditions such as endometriosis, uterine fibroids, and cancer is supported by a number of preclinical observations and clinical findings (Grow et al., 1996;Kettel et al., 1996;Baird et al., 2003;Fiscella et al., 2006). Although RU-486 has potent antiprogestagenic activity, it is also an equipotent antagonist at the glucocorticoid receptor (GR) (Heikinheimo et al., 1987). As a consequence, RU-486 suppresses the inhibition of corticotropin secretion induced by corticosteroids, limiting its wider usefu...