Less Carcinogenic Chlorinated Estrogens Applicable to Hormone Replacement Therapy

Human estrogens prescribed for hormone replacement therapy (HRT) are known to be potent carcinogens. To find safer estrogens, several chlorinated estrogens were synthesized and their carcinogenic potential were determined. A pellet containing either 2-chloro-17 beta-estradiol (2-ClE2) or 4-chloro-17 beta-estradiol (4-ClE2) was implanted subcutaneously for 52 weeks into August Copenhagen Irish (ACI) rats, a preferred animal model for human breast cancer. 17 beta-Estradiol (E-2) frequently induced mammary tumors while both 2-ClE2 and 4-ClE2 did not. Their 17 alpha-ethinyl forms, thought to be orally active estrogens, were also synthesized. Neither 2-chloro-17 alpha-ethinylestradiol (2-ClEE2) nor 4-chloro-17 alpha-ethinylestradiol (4-ClEE2) induced tumors. The less carcinogenic effects were supported by histological examination of mammary glands of ACI rats treated with the chlorinated estrogens. A chlorine atom positioned at the 2- or 4-position of E-2 may prevent the metabolic activation, resulting in reducing the carcinogenicity. 2-ClE2 and 4-ClE2 administered subcutaneously and 2-ClEE2 and 4-ClEE2 given orally to ovariectomized rats all showed uterotrophic potency, albeit slightly weaker than that of E-2. Our results indicate that less carcinogenic chlorinated estrogens retaining estrogenic potential could be safer alternatives to the carcinogenic estrogens now in use for HRT.