A new class of small molecule estrogen receptor-alpha antagonists that overcome anti-estrogen resistance

// Yongxian Ma 1 , Anju Preet 1 , York Tomita 1,4 , Eliseu De Oliveira 1,4 , Li Zhang 1,4 ,Yumi Ueda 1 , Robert Clarke 1 , Milton Brown 1,4 and Eliot M. Rosen 1,2,3 1 Department of Oncology, Georgetown University School of Medicine, Washington, DC, USA 2 Department of Biochemistry and Molecular and Cellular Biology, Georgetown University School of Medicine, Washington, DC, USA 3 Department of Radiation Medicine, Georgetown University School of Medicine, Washington, DC, USA 4 Department of Center for Drug Discovery Georgetown University School of Medicine, Washington, DC, USA Correspondence to: Eliot M. Rosen, email: // Keywords : BRCA1, estrogen receptor (ER-α), antagonist, agonist, Tamoxifen Received : June 24, 2015 Accepted : October 01, 2015 Published : November 13, 2015 Abstract Previous studies indicate that BRCA1 protein binds to estrogen receptor-alpha (ER) and inhibits its activity. Here, we found that BRCA1 over-expression not only inhibits ER activity in anti-estrogen-resistant LCC9 cells but also partially restores their sensitivity to Tamoxifen. To simulate the mechanism of BRCA1 inhibition of ER in the setting of Tamoxifen resistance, we created a three-dimensional model of a BRCA1-binding cavity within the ER/Tamoxifen complex; and we screened a pharmacophore database to identify small molecules that could fit into this cavity. Among the top 40 “hits”, six exhibited potent ER inhibitory activity in anti-estrogen-sensitive MCF-7 cells and four of the six exhibited similar activity (IC 50 ≤ 1.0 μM) in LCC9 cells. We validated the model by mutation analysis. Two representative compounds (4631-P/1 and 35466-L/1) inhibited ER-dependent cell proliferation in Tamoxifen-resistant cells (LCC9 and LCC2) and partially restored sensitivity to Tamoxifen. The compounds also disrupted the association of BRCA1 with ER. In electrophoretic mobility shift assays, the compounds caused dissociation of ER from a model estrogen response element. Finally, a modified form of compound 35446 (hydrochloride salt) inhibited growth of LCC9 tumor xenografts at non-toxic concentrations. These results identify a novel group of small molecules that can overcome Tamoxifen resistance.