Model for Hormonal Emergency Contraception (HEC) in cycling and mated guinea pigs - Studies with the Progesterone Receptor Modulators (PRM) Ulipristal Acetate (UPA/CDB2914) and EC317

A guinea pig model for new HEC methods is proposed. Two targets for HEC (Hormonal Emergency Contraception), ovulation and conception (post-mating study), were investigated using adjusted PRM treatments: (a) Ovulation inhibition study: Injections on cycle days 10-17, study of ovarian histology on day 18; (b) post-mating study: Injections on cycle days 1 and 2; rate of pregnant females was recorded at autopsy on day 18. P plasma levels permitted assessment of effects on ovulation in non-conceiving animals. Results: (a) All controls had recently ovulated. Statistically significant anti-ovulatory effects (p < 0.05, Fisher's Exact Test) were seen at 10 mg UPA (ulipristal acetate, CDB2914) and >= 0.3 mg EC317; 100% inhibition was found for EC317 at 10, 3, and 1 mg/day. No dosage of UPA was 100% effective. (b) In post-mating studies, 16 of 30 controls were pregnant. Both PFtMs (progesterone receptor modulator) exerted inhibitory effects on conception, none on imminent ovulation; 1 of 10 animals had living conceptuses after 10 mg UPA, none following 10 and 1 mg EC317/day, respectively. At pairwise comparison with controls, 10 mg was the lowest effective dosage for UPA (p < 0.05), and 1 mg for EC317 (p < 0.01). P plasma levels: Significantly lower P (p < 0.05) in subsequently pregnant vs non-pregnant controls was found on cycle day 3 or 4; this difference disappeared on day 8 or 9. This stage thus appears vulnerable to hormonal constellations and possibly PRM effects. HEC model: Effects on ovulation and conception were seen at the same dose levels of both PRM. Superior and more consistent effects of EC317 vs UPA (factor >= 10) suggest higher efficacy using EC317 for HEC.