Abstract 2352: Epigenetic modulation— unlocking the potential of checkpoint inhibition in advanced breast cancer

Immune checkpoint inhibition (ICI) has revolutionized treatment in immunogenic cancers by enabling infiltration of T cells into the tumor microenvironment (TME) and promoting cytotoxic signaling pathways. Tumors with complex immunosuppressive TME’s such as breast cancer present unique therapeutic obstacles as response rates to ICI remain low. Such tumors often recruit myeloid-derived suppressor cells (MDSCs) whose functioning prohibit both T cell activation and infiltration. Our current work aims to uncover the efficacy of ICI in advanced HER2 positive (HER2+) disease and to enhance response rates to these promising therapies by altering the metastatic TME epigenetically. Using a HER-2/neu transgenic mouse model, we syngeneically tumor challenge the NT2.5LM metastatic cell line to evaluate survival outcomes and metastatic burden upon treatment with combinations of the HDAC inhibitor entinostat (ENT), and the checkpoint inhibitors anti-PD-1 and anti-CTLA-4. We show that in the HER2+ mouse model of advanced disease, combining ENT + ICIs improves survival, and ENT + anti-CTLA-4 most significantly improves survival and decreases metastatic burden among responders. By investigating immune changes in sites of metastases, we show that treatment with ENT + ICIs significantly increases infiltration and proliferation of CD8+ T cells, increasing effector T cell infiltration, cytokine production, and markers of activation in cytotoxic T cells in the lung. Flow cytometry, ex vivo co-culture assays, western blots, and other functional assays performed on MDSCs and TIL elucidate further mechanisms behind response. We have found that the metastatic sites of animals treated with ENT + ICIs have significantly decreased infiltration of granulocytic-MDSCs and increased infiltration of monocytic-MDSCs, leading to the apparent cytotoxic anti-tumor response. In mouse models of early stage disease, ENT + ICI therapy alters MDSC infiltration and function in primary tumors, allowing for a more robust adaptive immune response. A significant anti-tumor effect is also seen in the metastatic state, though the function of MDSCs is not consistently altered, suggesting a key mechanism of ENT synergy with ICI to incite immune response and survival benefit that remains to be elucidated. In summary, addition of ICIs to ENT is beneficial in models of advanced disease by altering the recruitment of suppressive cells into the metastatic microenvironment, changing the dynamic interaction of T cells and tumor cells causing a robust anti-tumor response. These novel findings provide insight into how these combination therapies may function in patients with advanced stages of HER2+ breast cancer and suggest that responses are linked to stage of disease and likely follow different mechanisms of action within the different tumor microenvironments. Citation Format: Christine Rafie, Kayla Cruz, Skylar Woolman, Todd Armstrong, Elizabeth Jaffee, Evanthia Roussos Torres. Epigenetic modulation— unlocking the potential of checkpoint inhibition in advanced breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2352.