Abstract 2148: HDAC6 inhibitors modulate macrophage functional activity in the post-radiotherapy tumor microenvironment

Abstract Introduction: In recent decades, radiation therapy (RT) has been a significant therapeutic tool for treating multiple cancer types. Primarily, radiation-induced immunogenic tumor cell death re-modulates the tumor microenvironment (TME) with the potential to enhance the efficacy of immunotherapies. Tumor-associated macrophages (TAMs) play a critical role in the anti-tumor innate immune response. However, post-RT, the phenotype of TAMs can easily convert from the pro-inflammatory M1-like to the immunosuppressive and tumor-promoting M2-like due to their plasticity. M2-like macrophages cause relapse and metastasis of the tumor. Therefore, reprogramming TAMs is critical. Our lab has reported that targeting Histone Deacetylase 6 (HDAC6) in macrophages can disrupt their phenotype shifting from M1-like to M2-like in TME. Methods: We demonstrated the suppressive effect of HDAC6 inhibitors (HDAC6i) on M2-like macrophage phenotype markers by flow cytometry and qRT-PCR. Furthermore, in-vitro assays were performed, including trans-well migration assays, phagocytosis assays, and in-vivo syngeneic tumor models against different tumor cell types, including SM1 murine melanoma, 4T1 breast cancer, and TRAMP-C2 prostate cancer. Results: The trans-well-based migration assay indicated an increment in M2-like macrophage migration to irradiated cancer cells compared to M1-like macrophages. This increment was suppressed when culturing M2 macrophages with HDAC6i. Furthermore, the co-culture assay between macrophages and cancer cells indicated that M1 macrophages showed a higher phagocytosis rate when co-culture with irradiated cells and HDAC6i treatment increased tumor cell phagocytosis. Meanwhile, an in-vivo study based on the 4T1 and SM1 tumor models model suggested that RT combined with HDAC6i treatment has better efficacy than the monotherapy treatment arms due to increased M1/M2 macrophage ratio. These results demonstrate that targeting macrophages with HDAC6is could enhance the overall therapeutic outcome of radiation therapy. Citation Format: Xintang Li, Satish Noonepalle, Karen Tan, Sonia Sebaoui, Manasa Suresh, Nithya Gajendran, Marie Durr, David Quiceno-Torres, Bryan Weselman, Vidhi Sharma, Abishek Sehgal, Scott Grindrod, Anatoly Dritschilo, Alejandro Villagra. HDAC6 inhibitors modulate macrophage functional activity in the post-radiotherapy tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2148.