Discovery of Selective Estrogen Receptor Covalent Antagonists for the Treatment of ERα^WT and ERα^MUT Breast Cancer.

Abstract

Mutations in estrogen receptor alpha (ERα) that confer resistance to existing classes of endocrine therapies are detected in up to 30% of patients who have relapsed during endocrine treatments. Because a significant proportion of therapy-resistant breast cancer metastases continue to be dependent on ERα signaling, there remains a critical need to develop the next generation of ERα antagonists that can overcome aberrant ERα activity. Through our drug-discovery efforts, we identified H3B-5942, which covalently inactivates both wild-type and mutant ERα by targeting Cys530 and enforcing a unique antagonist conformation. H3B-5942 belongs to a class of ERα antagonists referred to as selective estrogen receptor covalent antagonists (SERCA). <i>In vitro</i> comparisons of H3B-5942 with standard-of-care (SoC) and experimental agents confirmed increased antagonist activity across a panel of ERα^WT and ERα^MUT cell lines. <i>In vivo</i>, H3B-5942 demonstrated significant single-agent antitumor activity in xenograft models representing ERα^WT and ERα^Y537S breast cancer that was superior to fulvestrant. Lastly, H3B-5942 potency can be further improved in combination with CDK4/6 or mTOR inhibitors in both ERα^WT and ERα^MUT cell lines and/or tumor models. In summary, H3B-5942 belongs to a class of orally available ERα covalent antagonists with an improved profile over SoCs.<b>Significance:</b> Nearly 30% of endocrine therapy-resistant breast cancer metastases harbor constitutively activating mutations in ERα. SERCA H3B-5942 engages C530 of both ERα^WT and ERα^MUT, promotes a unique antagonist conformation, and demonstrates improved <i>in vitro</i> and <i>in vivo</i> activity over SoC agents. Importantly, single-agent efficacy can be further enhanced by combining with CDK4/6 or mTOR inhibitors. <i>Cancer Discov; 8(9); 1176-93. ©2018 AACR.</i><i>This article is highlighted in the In This Issue feature, p. 1047</i>.