Studies on the mechanism of action of Win 49596: a steroidal androgen receptor antagonist.

Win 49596 is an orally active antiandrogen in the rat. This report describes a series of in vitro and in vivo studies which were performed to characterize the mechanism of action of this compound. In vitro competition and Lineweaver-Burk analyses indicate that Win 49596 binds competitively to the rat ventral prostate androgen receptor with a Ki of 2.2 +/- 0.4 microM. Similar to other androgen antagonists, the relative binding affinity (RBA) of Win 49596 was greater after 1 h of incubation with androgen receptor than after an 18 h incubation (RBA of 2.2 versus 0.05, respectively). Win 49596 did not bind to rat cytosolic uterine estrogen or progesterone receptors or thymus glucocorticoid receptors. Furthermore, Win 49596 did not inhibit rat ventral prostate 5 alpha-reductase or 3 alpha-oxidoreductase, rat adrenal 3 beta-hydroxysteroid dehydrogenase or human placental aromatase activity in vitro at concentrations as high as 10 microM. A series of in vivo studies demonstrated that Win 49596 inhibited the uptake of [3H]testosterone as well as testosterone-induced nuclear accumulation of androgen receptor in the rat ventral prostate. Collectively, these results support direct androgen receptor antagonism as the mechanism for the antiandrogenic effects of Win 49596.