Benzothiophenone Derivatives Targeting Mutant Forms of Estrogen Receptor-α in Hormone-Resistant Breast Cancers

Estrogen receptor- positive (ER+) breast cancers represent 75% of all invasive breast cancer cases, while de novo or acquired resistance to ER-directed therapy is also on the rise. Numerous factors contribute to this phenomenon including the recently-reported ESR1 gene mutations such as Y537S, which amplifies co-activator interactions with ER and promotes constitutive activation of ER function. Herein, we propose that direct targeting of the activation function-2 (AF2) site on ER represents a promising alternative therapeutic strategy to overcome mutation-driven resistance in breast cancer. A systematic computer-guided drug discovery approach was employed to develop a potent ER inhibitor that was extensively evaluated by a series of experiments to confirm its AF2-specific activity. We demonstrate that the developed small-molecule inhibitor effectively prevents ER-coactivator interactions and exhibits a strong anti-proliferative effect against tamoxifen-resistant cells, as well as downregulates ER-dependent genes and effectively diminishes the receptor binding to chromatin. Notably, the identified lead compound successfully inhibits known constitutively-active, resistance-associated mutant forms of ER observed in clinical settings. Overall, this study reports the development of a novel class of ER AF2 inhibitors, which have the potential to effectively inhibit ER activity by a unique mechanism and to circumvent the issue of mutation-driven resistance in breast cancer.