Abstract A13: Entinostat enhances antitumor immune responses and cooperates with PD-1 blockade in bladder cancer mouse models

Bladder cancer harbors a high frequency of somatic mutations that occur randomly across the genome. A fraction of these mutations can be expressed and processed into peptides called neoantigens. When presented on the surface of tumor cells by the major histocompatibility complexes (MHC), these neoantigens can invoke an adaptive immune response. In general, a high neoantigen burden is associated with a significant increase in overall survival. However, tumor cells can avoid the detection of immune cells by silencing the expression of the neoantigens. The post-translational acetylation of histone tails is catalyzed by histone acetyltransferases (HATs) and is counterbalanced by histone deacetylases (HDACs), which mediate their removal. Histone acetylation is associated with open chromatin and active transcription. HDAC inhibition would, therefore, promote histone acetylation and restore the expression of suppressed genes. Many of the frequently mutated genes in bladder cancer are involved in chromatin remodeling, such as KMT2D, KDM6A, and KMT2C, suggesting the contribution of epigenetic dysregulation to disease progression. Therefore, therapy that targets epigenetic processes such as HDAC inhibition is a rational treatment approach for bladder cancer. Here, we evaluated the antitumor efficacy and mechanisms of entinostat, a selective HDAC1 and HDAC3 inhibitor, in a preclinical murine model of high-grade muscle-invasive bladder cancer, BBN963. In this study, BBN963 cells were implanted subcutaneously into immunocompetent C57BL/6 (B6) mice or immunodeficient NOD/scid/gamma (NSG) mice. Entinostat exhibited robust in vivo activity in B6 mice implanted with BBN963 cells with a number of complete responses. However, the effect was much weaker in BBN963 cells grown in NSG mice. RNAseq analysis demonstrated increased immune gene signature expression in entinostat treated tumors from B6 (but not NSG) mice. Flow cytometry verified the increased numbers of CD8+ effector memory T cells. Most interestingly, the expressions of predicted neoantigens were almost completely lost in the entinostat-treated tumors in B6 mice but were unchanged in tumors from NSG mice. Additionally, the in vivo antitumor effect of entinostat in the BBN963 cells lacking MHC class 1 was significantly reduced. Finally, the combination of anti-PD1 therapy and entinostat robustly out-performed each agent alone in inhibiting BBN963 tumor growth with 67% incident of complete response. These data support the hypothesis that entinostat mediates antitumor effect through enhanced neoantigen expression and antigen-driven immune response. Citation Format: Andrew S. Truong, Mi Zhou, Takanobu Utsumi, Bhavani Krishnan, Kyle G. Stewart, Ryoichi Saito, Ujjawal Manocha, Sean T. Bailey, William Y. Kim. Entinostat enhances antitumor immune responses and cooperates with PD-1 blockade in bladder cancer mouse models [abstract]. In: Proceedings of the AACR Special Conference on Bladder Cancer: Transforming the Field; 2019 May 18-21; Denver, CO. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(15_Suppl):Abstract nr A13.