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STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4, 9-dien-3-one) is a interceptive agent in rabbits, mice and rats. In rats, it also shows post-implantational pregnancyterminating activity. In rabbits treated orally before mating or after ovulation for three consecutive days, it inhibited pregnancy largely at total doses of 0.08 mg/kg in the former and completely with 8.0 mg/kg in the latter case. A single subcutaneous dose of 40 mg/kg given on day 1 of pregnancy inhibited completely nidation in rats. In inhibition of pregnancy in rats could also be realized when the dose of 40 mg/kg was distributed on several days and given subcutaneously at daily doses of 5.0 mg/kg on days 1--8 of pregnancy, and even only daily doses of 2 mg STS 557/kg were needed in this respect, if animals which showed no living conceptuses were scored as "non-pregnant". STS 557 was effective in terminating pregnancy in rats, too, when given subcutaneously after implantation for 4 days at daily doses of 50 mg/kg, beginning on day 5 or on day 8 of pregnancy. The nidation inhibiting effects of STS 557 in rabbits treated pre-coitally, and in mice as well as in rats treated post-coitally were more marked than those of levonorgestrel. The interceptive and post-implantational inhibiting activity of STS 557 may be based on its antiprogestagenic properties. Luteolysis in the nidation phase can be excluded as shown by radioimmunoassay of progesterone in rats. The antigestagenic effects on the endometrium in rats were evident in the diamine oxidase assay. The acceleration of tubal egg transport, the morphological changes of the endometrium shown by scanning electron microscopy, and the effects on blastocyst transfer, all investigated in rats, suggest peripheral mechanisms of action.

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STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4, 9-dien-3-one) was tested for progestagenic activity in rabbits and for ovulation-inhibiting activities in rabbits and rats in comparison with levonorgestrel and, in some cases, with norethisterone acetate or chlormadinone acetate. In the immature rabbit, the endometrium transformation-inducing activity of STS 557 was about 5 times higher than levonorgestrel for both oral and subcutaneous administration. The ovulation-inhibiting effect of STS 557 was 3.5 times higher than that of levonorgestrel in the rabbit after oral administration, and about 7 times higher in the rat. After subcutaneous injection, however, the antiovulatory activity of STS 557 in rats was only about 4% of that of levonorgestrel when the ED 50 were compared.

Two synthetic estrogens, STS 456 and STS 593, with noteworthy post-coital antifertility properties in the rat were studied for their estrogenic, antiestrogenic, progestagenic, antiprogestagenic, antigonadotrophic, androgenic and antiandrogenic effects in laboratory animals. Beside their weak estrogenic activity which corresponds to their slight affinity to the uterine estrogen receptor in the rat, both steroids show remarkable antiestrogenic effects. No progestagenic, androgenic and antiandrogenic effects were found but STS 593 showed some antiprogestagenic activity. The antigonadotrophic efficacy of STS 456 was relatively high while STS 593 was scarcely active. The results are discussed with respect to possible application of these drugs for interceptive purposes.