Estetrol, a pregnancy-specific human steroid, prevents and suppresses mammary tumor growth in a rat model.

Estetrol (E4) is a pregnancy-specific D-ring metabolite of estradiol (E2) and estriol (E3) produced by the human fetal liver and present in both male and female fetuses. In adults, female exposure is restricted to the gestational period. We report that E4, dose-dependently, prevents the growth of chemically induced (7,12-dimethylbenz(a)anthracene, DMBA) mammary tumors in female Sprague-Dawley rats and that E4 has the potential to reduce the number and size of pre-existing mammary tumors. We performed two prevention studies and one intervention study. In the prevention studies, we investigated the effect of oral doses of E4 over a dose range of 0.5-3.0 mg/kg. The intervention study used oral dose levels of 1, 3 and 10 mg/kg E4. The antiestrogen tamoxifen was used as reference compound in all three studies and ovariectomy and ethinylestradiol, at estrogenic doses pharmacologically equipotent to E4, acted as control treatments in the second prevention study and in the intervention study. Rats treated with DMBA develop estrogen-responsive breast tumors. This model has become the standard pharmacological model to investigate the effect of new compounds on breast tumors. When DMBA-induced rats were co-treated with E4 for 8 weeks, this resulted in a dose-dependent reduction in the number and size of tumors, an effect that appeared equally effective as tamoxifen treatment or ovariectomy and was not seen with ethinylestradiol. When E4 was administered to rats in which tumors had already developed, a significant decrease in the number and size of tumors was observed after 4 weeks. This decrease was dose-dependent, comparable to tamoxifen-treated animals, and at high dose levels E4 was as effective as ovariectomy.