Towards the Identification of Novel Breast Cancer Inhibitors Specific for GPER-Expressing Cells

Together with ERα/β, GPER (G protein-coupled estrogen receptor) mediates important pathophysiological signaling pathways induced by estrogens and is currently regarded as a promising target for ER-negative and triple-negative breast cancer. Only a few selective GPER modulators have been reported to date and their use in cancer cell lines has often led to contradictory results. Here we report the application of virtual screening and cell-based studies for the identification of new chemical scaffolds with a specific antiproliferative effect against GPER-expressing breast cancer cell lines. Out of the four different scaffolds identified, the pyrrolo[1,2a]quinoxaline 14c was found as the most promising compound able to induce i) antiproliferative activity in GPER expressing cell lines (MCF7 and SKBR3) similar to G15, ii) no effect on cells that do not express the GPER receptor (HEK293), iii) reduction in Cyclin D1 expression, and iv) sustained induction of the cell cycle negative regulators p53 and p21. Breast cancer represents the most common cancer diagnosis in women and the most common cause of cancer-related mortality among females worldwide, with current predictions and statistics suggesting that both incidence and mortality rate are on the rise. Hormone receptor (HR)-positive breast cancer (HR+) is the most common form of breast cancer (85% of all breast cancers) and has a good prognosis thanks to endocrine therapy (e.g. Tamoxifen) and administration of aromatase inhibitors (e.g. Letrozole). HRhuman epidermal growth factor receptor 2 positive (HER2+), and triple negative (TN) breast cancers (HER2-, ERα-, ERβ-) have a lower incidence but, while HER2+ tumors can be treated with targeted therapies (e.g. Trastuzumab), TN tumors have the worst prognosis, higher chance of recurrence and no targeted treatments. Estrogen hormones play a key role not only in the regulation of multiple physiological processes but also in promoting the proliferation of the neoplastic breast epithelium by acting on soluble nuclear estrogen receptors (ERα and ERβ). While Tamoxifen is a widely-used selective estrogen receptor modulator (SERM) and displays anti-estrogenic (and therefore anticancer) effects in the breast, it also has pro-estrogenic activity in the endometrium, enhancing the risk of developing endometrial cancer. However, the therapeutic benefit of Tamoxifen in the treatment of breast cancer has been considered to outweigh the risk of developing endometrial cancer in premenopausal women. These side-effects seems to be due to the active metabolite of Tamoxifen (4hydroxytamoxifen, 4OHT) which acts as an agonist for a third estrogen receptor involved in the rapid non-genomic response to estrogens: the G-protein estrogen receptor (GPER or GPR30). GPER is now emerging as a new potential target for the treatment of several human diseases, such as cardio-vascular, renal and proteinuria diseases and malignancy. In the field of anticancer drug-discovery, several evidences collected in the last decade point toward GPER as a key modulator in cancer cell proliferation, migration and invasion. GPER has particular significance in TN breast cancer, even if conflicting results have been reported in the literature. Since the de-orphanization of GPER, researchers have sought modulators to assess its function. Several steroid-based modulators have been described (1,2, and 9, see Figure 1). Tamoxifen (3) and its metabolite (4) as well as flavonoids (5), biaryl methanes (6,7) and biaryl olefins (8) can apparently function as steroid mimetics. The compounds most selective for affecting only GPER-expressing cells are compounds 10-12, named G1, G15, and G36 (Figure 1). Figure 1. Representative structures of GPER modulators 10.1002/cmdc.201700145 ChemMedChem This article is protected by copyright. All rights reserved.