Abstract Background: Resistance to endocrine therapy and CDK4/6 inhibitors (CDK4/6i) poses a major barrier to effective management of estrogen receptor-positive (ER+) breast cancer, especially tumors harboring activating ESR1 mutations such as Y537S. Previous work by our group has demonstrated that combining ER inhibition with CDK4/6i significantly enhances radiosensitivity in ER+ breast cancer models. Given this synergy, we hypothesized that targeted ER degradation using Vepdegestrant (ARV-471), a potent PROTAC-based ER degrader, would further augment radiosensitization in models resistant to CDK4/6 inhibition and endocrine therapy. Methods:We evaluated ARV-471 in ER+ breast cancer cell lines, including wild-type MCF-7, tamoxifen-resistant ESR1 Y537S-mutant MCF-7, and CDK4/6i-resistant variants, with and without radiation. Radiosensitization was quantified using clonogenic survival assays. Mechanistic effects on DNA double-strand break repair were assessed through non-homologous end joining (NHEJ) and homologous recombination (HR) reporter assays. Results: ARV-471 induced potent, dose-dependent (5 nM – 100 nM) ERα degradation across all cell lines, including tamoxifen-resistant and CDK4/6i-resistant variants. ARV-471 treatment (3 nM) significantly enhanced radiosensitivity (radiation enhancement ratio (rER) 1.23 – 1.35, p-value 0.05), even in the highly resistant ESR1-mutant and CDK4/6i-resistant models. Mechanistically, ARV-471 impaired DNA repair by reducing NHEJ efficiency by 30% and shifting DNA repair towards HR. Conclusion:ARV-471 effectively radiosensitizes ER+ breast cancer cells, including ERi and CD4/6i resistant models, through ER degradation and disruption of DNA repair mechanisms, overcoming critical resistance to endocrine therapy and CDK4/6 inhibitors. Building upon prior evidence of radiosensitization achieved by combined ER and CDK4/6i inhibition, these findings highlight ER degradation as a potentially effective therapeutic approach to enhance radiotherapy efficacy in resistant ER+ breast cancer, addressing an unmet clinical need, especially in women with metastatic, treatment-refractory breast cancer undergoing palliative RT. Citation Format: P. S. Rana, E. Hochmuth, R. Abou Zeidane, B. Hauk, A. Davis, M. Endraws, M. Tao, V. Mercer, C. Speers. Vepdegestrant Overcomes Endocrine and CDK4/6i Resistance to Enhance Radiosensitivity in ESR1 mutant ER+ Breast Cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-07-11.
Rana et al. (Tue,) studied this question.