Potent inhibitory effect of a new antiestrogen (RU 16117) on the growth of 7,12-dimethylbenz[a]anthracene-induced rat mammary tumors.

At the daily dose of 24 mug for a period of 4 weeks, RU 16117 (11alpha-methoxyethinyl estradiol), a new antiestrogen, led to 65% reduction of the number of already established dimethylbenz[a]anthracene (DMBA)-induced mammary tumors in female Sprague-Dawley rats. Not only the tumor number but also the tumor size was reduced by RU 16117 in a manner similar to that seen after ovariectomy. The absence of an inhibitory effect of doses of 0.1 to 12.5 mug 17beta-estradiol (E2) per day, a dose-range which covers the low estrogenic activity of the RU 16117 doses used, suggested that the inhibitory effect of RU 16117 was not due to its estrogenic activity. Decreased levels of receptors for E2, progesterone, and prolactin were found in the tumors remaining after ovariectomy; treatment with the dose of RU 16117 sufficient to inhibit tumor growth (24 mug) had a similar inhibitory effect on the levels of E2 and prolactin receptors. These data suggested that a reduction of hormone receptor levels in the tumor tissue could be a mechanism by which RU 16117 acts as a potent inhibitor of the growth of DMBA-induced mammary carcinoma.