Abstract 6577: Chemoproteomic evaluation of PROTACs targeting estrogen receptor alpha for ER+ breast cancer

Abstract In our investigation of estrogen receptor (ER) targeted protein degraders for advanced ER+ breast cancer, we assessed three proteolysis targeting chimeras (PROTACs), ARV471, ERD308, and ERD3111. Global proteomic analysis unveiled off-target protein degradation of progesterone receptor (PR) and phosphodiesterase 6D (PDE6D) by ER targeted PROTACs, but not selective ER degraders (SERDs). PR and PDE6D have previously been implicated as targets in cancer treatment. Protein degradation is rescued in CRBN- or VHL-knockout cells. Co-treating ER+ breast cancer cells with ligands for PR or PDE6D outcompeted PROTAC-mediated degradation and stabilized the respective protein, demonstrating the PROTACs are targeting the PR and PDE6D ligand-binding domains. Competition assays in ER-negative cell lines confirmed PR and PDE6D protein degradation is ER-independent. These findings underscore the importance of understanding off-target effects for developing more selective and effective therapeutics in ER+ breast cancer. Citation Format: Zachary Sandusky, Myles Brown. Chemoproteomic evaluation of PROTACs targeting estrogen receptor alpha for ER+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6577.