STS 557, a new orally active progestin with antiprogestational and contragestational properties in rabbits.

STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4.9(10)-diene-3-one) is a potent progestin. This compound was 10 times as potent as the standard levonorgestrel as judged by the classic Clauberg-McPhail Assay. The antiprogestational response is significant when STS 557 treatment starts 2 days before progesterone administration. It does not disturb the priming activity of estradiol. This compound alone is adequate for endometrial priming. Early pregnancy is maintained in ovariectomized does. Administration on day 1 of pregnancy reduces the number of implantations. In comparison with chlormadinone acetate and d-norgestrel, only STS 557 treatment causes complete inhibition of egg fertilization.