Inhibition of male fertility by STS 557

To ascertain the effects of STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4, 9-dien-3-one) on the reproductive and endocrine functions, male rabbits were given the substance orally daily over 8 weeks. In doses of 10 and 20 mg STS 557 per animal per day fertility inhibition was accompanied by a decrease of spermatogenesis and sexual activity. At the chosen dose of 5 mg per day, STS 557 caused a decrease of sperm motility, semen fructose content and sterility of the bucks. On the other hand, libido, semen volume, sperm number, sialic acid content in semen, serum LH and testosterone remained unaffected. Concomitant injection of testosterone did not reverse the suppressive effect of STS 557 on fertility. Motility of human sperm was lowered markedly by STS 557 in vitro. The human sperm penetration and pronuclear formation were significantly reduced in the vitro fertilization assay with zona-free hamster eggs. Findings are discussed in view of the development of male contraceptives.