Abstract 2366: HDAC inhibition improves immune checkpoint inhibitor efficacy in renal cell carcinoma

Background: Immune checkpoint inhibitors have shown clinical benefit in solid tumors, including renal cell carcinoma (RCC); however, the rate of clinical response remains modest and improved therapeutic approaches need to be tested. Growing evidence suggests that epigenetic modifying agents may have an immunomodulatory effect that improves the efficacy of immune checkpoint inhibitors. Our group has previously demonstrated that entinostat, a histone deacetylase (HDAC) inhibitor, decreases the function of regulatory T cells (Treg) and myeloid derived suppressor cells (MDSC), synergizing with PD-1 blockade. Here we assessed the combination of PD-1 blockade with pan-HDAC inhibition in a RCC model. Methods: To test the efficacy of combined PD-1 inhibition, mDX-400 (10 and 20 mg/kg I.P) (Merck & Co, Inc) with pan-HDAC inhibition, vorinostat (100 and 150 mg/kg I.P) (Merck & Co, Inc), we utilized a syngeneic mouse model of metastatic RCC following orthotopic implantation of RENCA cells in immunocompetent mice. Antitumor activity was assessed by measuring bioluminescence, end point tumor weights, and survival times. Immune profiling of tumor infiltrating lymphocytes (TILs) was performed by flow cytometry, immunohistochemistry, and immunofluorescence. Peripheral blood mononuclear cells (PBMC) were assessed for differential chromatin accessibility by Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq). Results: Significant reductions in tumor weight and lung metastases were observed in mice treated with the combination of vorinostat and mDX-400. Combination therapy significantly increased the survival of the mice (median survival = 60) compared to treatment with mDX-400 alone (median survival=42 days). Immune landscape profiling demonstrated an increase in natural killer cell infiltration (P=0.048) and decrease of exhausted T cells (P=0.049, CD8+ PD1+) in the combination group. Furthermore, decreased immunosuppressive Treg (CD4+ FOXP4+) and MDSC (CD11b+ Gr1+) populations were identified in the combination group. Analysis of the mouse PBMC ATAC-seq data in the combination and mDX400 alone conditions demonstrated numerous regions of differential chromatin accessibility. Pathway analysis of genes associated with increased accessibility in the combination treatment identified enrichment of cell cycle and immune activation pathways. Conclusions: Our results demonstrate that pan-HDAC inhibition augments the antitumor effect of immune checkpoint inhibitors, prolonging survival in our preclinical mouse model. This antitumor effect was achieved by changing the immune landscape in TILs and was associated with higher chromatin accessibility near genes involved in cell cycle progression and immune cell activation. Taken together, our results support the clinical testing of pan-HDAC inhibitors in combination with anti-PD-1. Citation Format: Justin Budka, Nur Damayanti, Roberto Pili. HDAC inhibition improves immune checkpoint inhibitor efficacy in renal cell carcinoma [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 2366.