Abstract 6047: Discovery and characterization of PVTX-321, an estrogen receptor heterobifunctional degrader

Abstract The estrogen receptor α (ERα) is a clinically validated target for the treatment of estrogen receptor-positive and human epidermal growth factor receptor 2-negative (ER+/HER2-) breast cancer. However, resistance to early line endocrine treatments due to mutations of the ESR1 gene encoding ERα is associated with poor prognosis and remains an area of unmet clinical need. ERα degradation using heterobifunctional degraders has emerged as a promising approach to tackle endocrine resistance through proteasome mediated degradation of the receptor. Here, we present the preclinical results for the development candidate, PVTX-321, an ERα heterobifunctional degrader. PVTX-321 is a rapid and potent ER heterobifunctional degrader as well as a full ERα antagonist. Both properties translate into potent antiproliferative activity across multiple ER+ breast cancer cell lines bearing either wild-type or clinically relevant ERα mutations (E380Q, Y537S and D538G). PVTX-321 demonstrates favorable oral bioavailability across different species and effectively induces dose-dependent degradation of ERα in vivo. Moreover, PVTX-321 achieves tumor regression as an oral agent at 10 mg/kg in MCF-7 mouse xenograft model. Furthermore, PVTX-321 possesses excellent in vitro and in vivo safety profiles and warrants further development for the treatment of ER+/HER2- breast cancer. Citation Format: Courtney G. Havens, Guozhang Xu, Cass Lowenstein, Debangshu Samanta, Brian Vidal, Elham Behshad, Rakesh Nagilla, Mike Russell, Peter Orth, Winston Wu, Larry Jolivette, Scott Priestley, Helai Mohammad, Zhihua Sui. Discovery and characterization of PVTX-321, an estrogen receptor heterobifunctional degrader 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 (6Suppl): Abstract nr 6047.