Abstract PS17-16: Sar439859, a novel selective estrogen receptor degrader (serd), demonstrates effective and broad antitumor activity in wild-type and mutant er-positive breast cancer models

Primary treatment for estrogen receptor-positive (ER+) breast cancer is endocrine therapy. However, substantial evidence indicates a continued role for ER signaling in tumor progression. Selective estrogen receptor degraders (SERDs), such as fulvestrant, induce effective ER signaling inhibition, although clinical studies with fulvestrant report insufficient blockade of ER signaling, possibly due to suboptimal pharmaceutical properties. Furthermore, activating mutations in the ER have emerged as a resistance mechanism to current endocrine therapies. New oral SERDs with improved drug properties are under clinical investigation, but the biological profile that could translate to improved therapeutic benefit remains unclear. Here we describe the discovery of SAR439859, a novel, orally bioavailable SERD with potent antagonist and degradation activities against both wild-type and mutant Y537S ER. Driven by its fluoropropyl pyrrolidinyl side chain, SAR439859 has demonstrated broader and superior ER antagonist and degrader activities across a large panel of ER+ cells, compared with other SERDs characterized by a cinnamic acid side chain, including improved inhibition of ER signaling and tumor cell growth. Similarly, in vivo treatment with SAR439859 demonstrated significant tumor regression in ER+ breast cancer models including MCF7-ESR1 wild type and mutant-Y537S mouse tumors, and HCI013, a patient-derived tamoxifen-resistant xenograft tumor. These findings indicate that SAR439859 may provide therapeutic benefit to patients with ER+ breast cancer, including those have resistance to endocrine therapy with both wild-type and mutant ER. This presentation will highlight the global profile and key features of new SERD and its translation to patient. Citation Format: Maysoun Shomali, Fangxian Sun, Mikhail Levit, Jack Pollard, Joon Sang Lee, Laurent Schio, Youssef El-Ahmad, Chris Soria, Patrick Cohen, Laurent Debussche, Monsif Bouaboula. Sar439859, a novel selective estrogen receptor degrader (serd), demonstrates effective and broad antitumor activity in wild-type and mutant er-positive breast cancer models [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS17-16.