Endocrine and antiprostatic effects of raloxifene (LY156758) in the male rat

The benzothiophene anti-estrogen, raloxifene [LY156758; (6-hydroxy-2-(4-hydroxyphenyl) benzo(b)thien-3-yl)(4-(2-1-piperidinyl)ethoxy)phenyl methanone hydrochloride] has selective estrogen pharmacological antagonist activity in female rats. The present studies were done in the male rat to assess activity of raloxifene related to inhibition of prostatic growth and effects on the hypothalamic-pituitary-gonadal axis. Raloxifene did not compete for binding of the androgen, [3H]-methyltrienolone (R1881) in cytosolic extracts of ventral prostate. Similarly, the compound did not inhibit prostatic 5 alpha-reductase or testicular 17 alpha-hydroxy/C17,20-lyase activities. Raloxifene had no effect on the ventral prostatic uptake of [3H]-R1881 in vivo. Administration of estradiol to castrated male rats stimulated fourfold increases of in vitro ventral prostatic binding of [3H]-R1881. Raloxifene was devoid of agonist activity in castrated animals, because the compound had no stimulatory effect on prostatic androgen receptor binding activity. When raloxifene was coadministered with estradiol, the compound markedly antagonized the estrogen-induced increase of prostatic [3H]-R1881 binding, confirming its antiestrogenic properties in male rats. Serum prolactin was also elevated significantly (P < 0.05) with a single injection of raloxifene (20.0 mg/kg). In these same animals, serum FSH was significantly (P < 0.05) decreased by one dose (10.0 mg/kg) of the compound. Luteinizing hormone levels in castrated male rats were unaffected by raloxifene administration. Raloxifene treatment of castrated males significantly (P < 0.05) antagonized the stimulatory response of the ventral prostate (VP) to exogenous androgens in a dose-dependent manner. Raloxifene treatment of intact male rats for 14 and 28 days produced significant (P < 0.05) dose-dependent regression of the VP and seminal vesicles (SV). The VP regressive responses to raloxifene were associated with a decline in serum testosterone levels. Histological analysis of the VPs in raloxifene-treated rats was consistent with an androgen-deprived state. These findings support the contention that raloxifene is a pure estrogen antagonist and a physiological antagonist of androgen action in male rats. These pharmacological properties provide support for further structure-activity and mechanistic investigations with benzothiophenes in the medical management of prostatic neoplasia.