Evaluation of STS-557 as an oral contraceptive in male rat.

To examine the nature and site of post-testicular antifertility action of STS-557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4,9-dien-3-one), male rats were given the steroid orally daily for 60 days. In doses of 1 and 5 mg per animal per day it had no effect on fertility at the end of 3 weeks of treatment. When the treatment was extended for 60 days, spermatogenic arrest and loss of libido were evident in animals treated with 5 mg dose; animals receiving 1 mg dose of steroid showed no decrease of spermatogenesis or sexual activity and their fertility remained unaffected. In 35-day-old growing rats the steroid produced inconsistent effects on spermatogenesis after a 15-day treatment period at 1 and 5 mg doses. Both in adult and in growing rats the steroid caused a significant reduction in the weights and secretory function of the epididymis and other accessory sex organs; a dose-dependent response was seen in all the sex organs. Evaluation in castrated rat model revealed that STS-557 is a weak anti-androgen. Although this steroid is a potent inhibitor of spermatogenesis, its inhibitory effect on Leydig cell function is a contraindication for its use as a male oral contraceptive.