Abstract 4216: Advancing cancer immunotherapy through the development of a novel selective HDAC6 inhibitors

Abstract Immune cells can significantly influence the therapeutic outcomes of anti-cancer therapies. Particularly, tumor-associated macrophages (TAMs) mainly exhibit an anti-inflammatory M2-like phenotype and are well recognized for their pro-tumoral activity, which supports tumor growth and suppresses anti-tumor immune responses. Nonetheless, TAMs exhibit remarkable plasticity, allowing for their reprogramming toward an anti-tumoral M1-like phenotype through targeted epigenetic modulations. Our group has previously reported several highly selective histone deacetylase 6 inhibitors (HDAC6is) that promote the polarization of TAMs toward the M1-like phenotype while suppressing the M2-like pro-tumoral phenotype. Despite this promising discovery, the current HDAC6is face several drawbacks, including efficacy at micromolar concentrations, which results in poor isoform selectivity and potential off-target effects. To address these limitations, we developed an advanced screening pipeline for HDAC6is on macrophage functions, and the antitumor benefits associated with their treatment, alone or in combination with anti-PD1 in the immunocompetent SM1 murine melanoma model. Based on an in silico screening of 980 HDAC6is derivatives and a macrophage-based screening described previously, we identified two candidates, SM-05-947 and SM-06-09, that showed superior properties compared to their parent molecules. The screening included the determination of cytotoxicity, specificity of HDAC inhibition, and the modulation of macrophage phenotype. Using bone marrow-derived macrophages, the treatment-associated changes in macrophage functions were characterized. Following dose determination, we studied their antitumor effects on SM1 melanoma tumors when administered as a single agent or combined with anti-PD1 antibodies. The tumor microenvironment was further characterized by immunotyping using flow cytometry, CyToF, and sc-RNA sequencing. The functional assays provided further evidence of increased phagocytosis of tumor cells and improved antigen cross-presentation with HDAC6 inhibition. Specifically, the presence of SIINFEKL peptide in the context of MHC class I in macrophages treated with HDAC6is showed a significant increase in antigen presentation by M1 macrophages. Administration of SM-06-09 and SM-05-947 as a single agent significantly reduced tumor growth when compared to the vehicle group. The two combination groups of HDAC6is and anti-PD1 antibodies further significantly improved these antitumor effects, as observed by flow cytometry, CyToF, and sc-RNA sequencing analysis. In conclusion, these findings provide valuable insights and pave the way toward the development of next-generation HDAC6is with maximum efficiency, minimal toxicity, and enhanced anticancer potential of TAMs. Citation Format: Nithya Gajendran, Manasa Suresh, David Quiceno, Sonia Sebaoui, Xintang Li, Marie Durr, Sebastian J. Marquez Rodriguez, Mario A. Noboa, Sruthi Mohan, Barbora Havlinova, Julia Kudlacova, Annie Heuer, Satish Kumar Reddy Noonepalle, Cyril Barinka, Duncan J. Wardrop, Alejandro Villagra. Advancing cancer immunotherapy through the development of a novel selective HDAC6 inhibitors [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 4216.