Abstract 2730: Breast pulmonary metastases and associated myeloid derived suppressor cells are resistant to the effects of entinostat with checkpoint inhibitor in a murine tumor model
Authors: Jang Julie K. , Rafie Christine , Castanon Sofi , Christmas Brian J. , Cruz Kayla A. , Woolman Skylar , Roussos Torres Evanthia T. 
Published: July 2021

Abstract
Introduction: Immune checkpoint inhibitors (ICIs) are an effective strategy to engage the adaptive arm of the immune system in several solid cancers. While approved for metastatic triple negative breast cancer and breast cancer with microsatellite instability or mismatch repair deficiencies, ICIs are still under investigation in HER2+ breast cancer. In preclinical studies, the effects of ICIs are dampened by immunosuppressive cells, such as myeloid derived suppressor cells (MDSC), in the breast tumor microenvironment (TME). Previous studies in mice showed that addition of entinostat, a histone deacetylase inhibitor, to ICIs improved survival and reduced immunosuppression in an early HER2+ breast tumor model. Here, we evaluated the addition of entinostat to ICIs in a mouse model of metastatic HER2+ breast cancer. We hypothesize that the different immune cell composition within the TMEs of breast and lung metastases will affect mechanisms of response to this treatment combination.
Methods: mmTV-NeuN transgenic mice (NeuN) were challenged with syngeneic NT2.5-LM cells that spontaneously metastasize to the lungs. Mice were treated with different combinations of entinostat, anti-CTLA-4, anti-PD-1, and anti-HER2 for 3 weeks for analysis of survival and metastatic tumor burden. For analyses of tumor-infiltrating immune cells in pulmonary metastases, flow cytometry of dissociated lungs was done 6 weeks after tumor injection.
Results: Unlike the previously published model of early-stage HER2+ breast cancer, combinations of entinostat and ICIs did not improve survival in NT2.5-LM bearing mice. Anti-HER2 therapy was the only agent to improve survival, and its effect was hindered by the addition of entinostat and ICIs. The numbers of pulmonary metatastases among treatment groups were not significantly different. Although the combination of entinostat and ICIs increased the percentage of cytotoxic CD8 T cells in the lungs, it also increased the percentage of the more suppressive monocytic-MDSCs and decreased the percentage of less suppressive granulocytic-MDSCs. Whereas entinostat decreased suppressive activity of MDSCs from primary tumors, entinostat did not significantly change functional markers of MDSCs in the lung.
Conclusion: Entinostat and ICIs did not reduce breast metastases in the lung, nor did their combination improve survival. Previous studies showed that entinostat and ICIs reduced MDSC suppressive function within the TME. Reduced suppression was not observed in lung metastases. Investigations are underway to define mechanisms responsible for the differential effect of entinostat on MDSC phenotype in primary tumors but not in the metastatic niche.
Citation Format: Julie K. Jang, Christine Rafie, Sofi Castanon, Brian J. Christmas, Kayla A. Cruz, Skylar Woolman, Evanthia T. Roussos Torres. Breast pulmonary metastases and associated myeloid derived suppressor cells are resistant to the effects of entinostat with checkpoint inhibitor in a murine tumor model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2730.