Reversibility of antigestagenic action of antiprogestin onapristone by exogenous progestagens during early pregnancy in guinea pig

Abstract Ability of progesterone, gestodene, promegestone and cyproterone acetate (CPA) to reverse antigestagenic action of onapristone in adult female guinea pigs was investigated. Onapristone (10 mg/kg, s.c.) administered on postconception days 8–11 caused resorption of implantations and vaginal bleeding in all animals. Simultaneous administration of progesterone, gestodene or promegestone on days 7–13 successfully reversed antigestagenic action of this antiprogestin, since most animals supplemented with these progestagens had viable implantations at autopsy on day 14. CPA was, however, ineffective and animals supplemented with it had only resorbed implantations and blood in uterus and vagina like that in onapristone per se treated animals. High plasma progesterone and low PGFM concentration were generally observed in all pregnant animals bearing viable implantations. PGFM (13, 14-dihydro-15-keto PGF 2α ) was significantly elevated by day 14 in onapristone-treated (Group II) and CPA-supplemented (Group X) animals. No discernible effect on pregnancy or post-implantation embryonic development was observed in animals treated per se with these progestagens.