Abstract 948: Preventing estrogen receptor alpha-positive breast cancer outgrowth with the use of hormone replacement therapy

Abstract

Abstract Menopause occurs in all women, usually between the ages of 45 and 55. As a result of menopause, women often experience undesirable vasomotor symptoms, which can be alleviated by hormone replacement therapy (HRT). The Women's Health Initiative (WHI) trial concluded that PremPro, a HRT formulation that combines conjugated equine estrogens (CE) with medroxyprogesterone acetate, increases the risk of breast cancer. Over the years, the number of women taking HRT has dramatically decreased due to the perceived risk based largely on the results of the WHI trial. Follow-up sudies suggest that breast cancer cases from PremPro treatment were primarily due to the outgrowth of occult tumors, not the formation of new disease. Duavee, a more recent form of HRT that combines CE and bazedoxifene (BZA), a selective estrogen receptor modulator (SERM) and degrader (SERD), has been approved by the FDA for treatment of moderate to severe hot flashes and to reduce the risk of osteoporosis. More importantly, this CE+BZA mixture not only relieves symptoms associated with menopause, but it also does not stimulate the breast or uterus. Several preclinical studies suggest that CE+BZA might be protective in the breast, however the mechanism of action of this new combination therapy is not known. Our goal, therefore, is to elucidate the underlying molecular mechanisms by which CE+BZA differentially affects estrogen receptor alpha (ERα) action in the mammary gland, using transcriptome and whole genome occupancy analysis in breast cancer cell lines. RNA-Seq and ChIP-Seq studies suggest that CE+BZA decreases pathways related to S-phase entry, cell cycle regulation and BRCA1. We are also studying the effects of CE+BZA on early mammary cancer progression in the estrogen-sensitive polyoma middle T antigen (PyMT) transgenic mouse model and have observed that treatment with CE+BZA delayed the onset of tumors and decreased their rate of growth. Mice treated with CE+BZA also survived longer. In addition, CE+BZA was able to decrease the rate of tumor growth in an ERα-positive patient-derived xenograft (PDX) mouse model. An improved understanding of the molecular mechanisms of CE+BZA action in hormone sensitive breast cancer cell and animal models should have important implications for women considering HRT. Citation Format: Anna G. Dembo, Emily S. Aledort, Geoffrey L. Greene. Preventing estrogen receptor alpha-positive breast cancer outgrowth with the use of hormone replacement therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 948.