STS 557 as an interceptive in rodents and baboons

Studies with mice, rats, guinea pigs and baboons were undertaken to define the interceptive action of the new progestin STS 557 (17 alpha -cyanomethyl-17 beta-hydroxy-estra-4.9(10)-diene-3-one) and to compare it with other progestins used in oral contraceptives. STS 557, norethindrone and norethindrone acetate reduced deciduoma formation as well as the number of implantations in mice and rats. Chlormadinone acetate and levonorgestrel when administered at the appropriate dose could not prevent early pregnancy or deciduoma formation. But in contrast to STS 557, levonorgestrel maintained early pregnancy in ovariectomized rats. On the other hand, STS 557 was ineffective as a postcoital agent in guinea pigs. When 0.4 mg STS 557 was orally administered to 37 female baboons 3 or 6 h after the mating period, only one pregnancy occurred in a total of 60 cycles investigated (controls: 11 pregnancies in 12 cycles investigated). The results are discussed in view of the development of an interceptive method based on STS 557.