Abstract 3256: Entinostat enhances the efficacy of checkpoint inhibition in breast-to-lung metastases and is associated with alterations in the phenotype and function of myeloid cell populations

Abstract Single agent immune checkpoint inhibitors (ICIs) are ineffective against metastatic breast cancer. One mechanism of intrinsic resistance is the presence of immunosuppressive myeloid cells and dysfunctional dendritic cells in the metastatic tumor microenvironment (TME) that impede the activation of anti-tumor T cells. A comparison of single-cell RNA sequencing analyses between breast-to-lung metastases and published data from breast tumors in the syngeneic NeuN mouse model of HER2+ breast cancer revealed that despite the presence of additional immune cell populations in the lungs, myeloid cells represented the predominant immune cell population in both TMEs and demonstrated similar changes in frequency and gene expression following treatment with the histone deacetylase inhibitor, entinostat. Our work highlights for the first time an increase in the frequency of conventional dendritic cells (cDC) with entinostat treatment in breast-to-lung metastases. Gene expression analyses of breast-to-lung metastases showed that entinostat-treated myeloid derived suppressor cells (MDSCs) and tumor associated macrophages (TAMs) downregulated genes associated with immunosuppression such as S100a8, Cd84, Trem2, and Mmp9. In addition, TAMs and cDCs upregulated the expression of genes associated with antigen presentation such as H2-K1, H2-Ab1, B2m, and Cd74 and pro-inflammatory markers such as Ccl3. NeuN mice were treated with the combination of entinostat and the immune checkpoint inhibitors, anti-PD-1 and anti-CTLA-4, and breast-to-lung metastases were analyzed via flow cytometry, demonstrating an increase in the frequency of cDC1s and CD8+ T cells expressing granzyme B. Furthermore, the treatment combination of entinostat + anti-PD-1 + anti-CTLA-4 significantly increased survival in an experimental model of lung metastasis using a newly characterized model derived from the original NT2.5 HER2+ cell line, NT2.5LM. These results confirm those of our breast expansion cohort from the phase I clinical trial NCI-9844 that demonstrate a 30% overall response rate in 20 heavily pretreated patients with metastatic disease treated with entinostat + Nivolumab (anti-PD-1) + Ipilimumab (anti-CTLA-4). We highlight for the first time significant alterations within multiple myeloid derived cell types, cDCs, TAMs and MDSCs, from within a native metastatic niche, that are likely driving an entinostat mediated improved response to ICI treatment. Citation Format: Edgar Gonzalez, Adam L. MacLean, Evanthia T. Roussos Torres, Aaron Baugh, Jesse Kreger, Sofi Castanon, Valerie Narumi, Elizabeth M. Jaffee. Entinostat enhances the efficacy of checkpoint inhibition in breast-to-lung metastases and is associated with alterations in the phenotype and function of myeloid cell populations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3256.