Abstract PO3-24-10: Losartan enhances the radiosensitivity of triple-negative breast cancer by reshaping the tumor immune microenvironment

Abstract Triple-negative breast cancer (TNBC) represents the most aggressive subtype of breast cancer, and the therapeutic options available for TNBC are limited. In addition to surgery and chemotherapy, radiotherapy (RT) is a primary treatment modality for TNBC. However, radio-resistance poses significant challenges to TNBC treatment. Prior research has shown that losartan, an angiotensin receptor blocker, can improve vessel perfusion and drug delivery, thereby enhancing the efficacy of chemotherapy or radiotherapy in cancer. However, in our study, we found that losartan could enhance the radiosensitivity of TNBC in immunocompetent mice model, but not in immunodeficient mice model and TNBC cells. It suggested that losartan might enhance the radiosensitivity of TNBC in an immune system dependent way. Previous studies had noted that tumor immune microenvironment (TIME) played an important role in radio-resistance. Therefore, we focused on the TIME in our further study. For immune cells, it had reported that the recruitment and activation of immunosuppressive cells such as M2 phenotype tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs), could lead to the establishment of and immunosuppressive TIME and decreased the radiosensitivity. In this study, we found losartan facilitated the polarization of TAMs towards the M1 phenotype, and impeded the immunosuppressive function of MDSCs after RT in 4T1 TNBC mice model. Additionally, losartan also caused a significant increase of CD8+ T lymphocytes in the TIME, and also markedly enhanced the activity of infiltrated T cell. When the CD8+ T cells were eliminated by anti-CD8α antibody, the radiosensitization effect of losartan was significantly decreased. Therefore, T-cell mediated tumor suppression is essential for the radiosensitization caused by losartan in TNBC. For immune factors, we found that TNBC exhibited an increase in the expression of programmed death-ligand (PD-L1) and indoleamine-2,3-dioxygenase (IDO) upon exposure to irradiation, and the combination of losartan resulted in a significant reduction in their expression in vitro and in vivo. At the same time, the 4T1 TNBC transplantation tumors were significantly inhibited by losartan combined with PD-1 inhibitor after RT, which indicated that losartan might amplify the efficacy of radio-immunotherapy. Collectively, these findings suggest that losartan could reshape the TIME of TNBC through regulating immune cells (TAMs, MDCSs and CD8+ T cells) and immune factors (PD-L1 and IDO), which result in the enhancement of radiosensitivity of TNBC. In our investigation, losartan was innovatively found to increase the radiosensitivity of TNBC by reshaping the TIME rather than on the tumor stroma. As one of the most commonly prescribed drugs for hypertension, the safety and affordability of losartan have been extensively confirmed. In the future, losartan combined with radiotherapy might be a promising strategy for TNBC treatment. Citation Format: Cuiwei Liu, Xu Wang, Zihan Xia, Yuxi Ma, Yanxia Zhao. Losartan enhances the radiosensitivity of triple-negative breast cancer by reshaping the tumor immune microenvironment abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-24-10.