Antiestrogenic Effects Of The Natural Fetal Estrogen Estetrol (E-4) In Women With Estrogen-Receptor-Positive Early Breast Cancer.

280 Background: Estetrol (E4) is a fetal estrogen which exerts estrogenic effects on vaginal epithelium, hot flushes and bone, but has estrogen-antagonistic effects on breast cancer cell lines in vitro and in the rat DMBA model. Therefore E4 may be suitable for Hormone Replacement Therapy (HRT) in women with breast cancer including women treated with aromatase inhibitors. Methods: We have investigated the effect of 14 days pre-operative treatment with 20 mg E4 per day on proliferation, apoptosis, ER-receptors and sex steroid levels in a prospective, randomised, double-blind, placebo-controlled, neo-adjuvant study in 15 pre- and 15 postmenopausal women with estrogen-receptor positive early breast cancer. Results: Estetrol induced a significant increase of SHBG, a significant decrease of FSH in postmenopausal women and no increase of gonadotrophins in premenopausal women. Estetrol had no effect on Ki67 expression and on apoptosis-related Bax and Bcl-2, but the apoptosis index in tumor tissue increased significantly. Systemic IGF-1 levels decreased significantly. Surprisingly the intratumoral epithelial ER-alpha expression decreased significantly, whereas the ER-beta expression showed a trend to increase. Conclusions: This data show that E4 has estrogenic endocrine effects. The data support the hypothesis that E4, may be suitable and safe for HRT in women with spontaneous or induced menopausal symptoms, since apoptosis increases, IGF-1 decreases and no unfavourable effects are observed on Ki67, Bax and Bcl-2. The decrease of ER-alpha and the increase of ER-beta suggest a mechanism of action, explaining why the natural fetal estrogen E4 has estrogen-antagonistic effects on breast cancer tissue.