Clinical and experimental aspects of the anti-mammary carinogenic activity of estriol.

Intermittent implantation of 600--1,300 microgram estriol subcutaneously beginning 48 h before oral administration of 7,12-dimethylbenz(a)anthracene or procarbazine prevents development of 80--90% of carcinomas of the breast occurring during the natural life span of the intact female Sprague-Dawley rat. Some estriol precursors were less inhibitory of breast cancer development among 23 other estrogens and androgens, progestins and glucocorticoids tested. More frequent or lower estriol doses than 100--200 microgram/kg/24 h every 2 months were less inhibitory of breast carcinogenesis. No other types of neoplasms were reduced in incidence by estriol implants, which also reduced uterine weights by 20--25%. Intermittent substitution of estriol for estrone or estradiol in the nuclear receptor complexes of target cells probably accounts for these observations, which resemble the effect of castration in reducing breast cancer incidence. Human studies indicate excellent tolerance for oral estriol doses of 10--200 microgram/kg/24 h, which may correct subnormal estriol/estrone + estradiol urinary quotients associated with elevated risk of breast carcinogenesis in epidemiologic investigations.