Effect of STS-557 (17 alpha-cyanomethyl 17 beta-hydroxy-estra-4, 9(10)-dien-3-one) on blood hormone levels, the testis, accessory sex organs and fertility of rats.

Treatment with STS-557 (17 alpha-cyanomethyl 17 beta-hydroxy-estra-4,9(10)-dien-3-one; 10 mg kg-1 daily s.c.) for 4 weeks induced atrophy of the seminiferous tubules in adult rats with a reduction in tubule diameter and in the number of round spermatids at stage VII. Elongated spermatids were not detected. Leydig cells were atrophied from the second week of treatment with a concomitant decrease in blood levels of testosterone. The blood levels of FSH and LH were reduced from the third week of treatment. The weight of the reproductive organs was reduced after STS-557 treatment. The treatment induced sterility in 50% of rats after 2 weeks of treatment but after 4 weeks none of the treated males mated. Normal fertility and normal levels of testosterone and FSH were restored after 6 weeks and LH after 4 weeks of withdrawal of treatment. All other parameters studied recovered to pretreatment levels 6 weeks after withdrawal of treatment. STS-557 could act on the pituitary-gonadal axis (reducing gonadotrophin secretion) as well as directly affecting the Leydig cells. The consequent reduction in the blood levels of testosterone in combination with reduced gonadotrophins was presumably responsible for the suppression of spermatogenesis.