Epithioandrostanol(10275-S), an estrogen antagonist.

2ƒ¿,3ƒ¿-Epithio-5ƒ¿-androstan-17ƒÀ-ol (10275-S) has been known to possess a potent and long-acting anti-estrogenic activity in rodents. In this experiment, 1.25mg 10275-S injected once subcutaneously into the ovariectomized and estradiol-treated mice caused a complete block of vaginal cornification for 24.3 days in spite of the successive treatment with estrogen. Pre-treatment with 10275-S in ovariectomized mice either 4 or 24 hr before tritiated estradiol injection caused significant inhibition of accumulation of radioactivity in the uterus. Pre-incubation of uterine cytosol from immature rat with 10275-S competitively suppressed the formation of estradiol-macromolecule complex maximally around the 8S region, and 50% suppression was obtained at 4.4•~10-7M, about 70-fold over the estradiol. Under the same conditions, 10-5M progesterone or testosterone gave no effect on estradiol binding at the 8S region. The relation of these in vitro results to the in vivo activities of this steroid was discussed.