Project Summary The median survival for metastatic melanoma is approximately 8–16 months, and few therapies offer a significant improvement in overall survival. However, immunotherapeutic strategies that abrogate immunologic checkpoints or improve immunosurveillance have shown promise, especially in melanoma. We have found that both genetic abrogation and pharmacologic inhibition of HDAC6 leads to a decreased infiltration of pro-tumoral tumor-associated macrophages and downregulation of immunosuppressive mediators. These effects translated into a pronounced delay of in vivo melanoma tumor growth, which is, at least in part, dependent on intact immunity, as evidenced by the restoration of tumor growth after CD4+ and CD8+ depletion. Our findings demonstrate a significant immunoregulatory role of HDAC6 in melanoma, providing a rationale for the use of selective HDAC6is to improve antitumor immunity. We are most interested in identifying HDAC6is that are best able to reduce the pro-tumoral phenotype of tumor-associated macrophages and decrease the expression of immunosuppressive surface molecules such as PD-L1 and PD-L2 while showing little cytotoxicity on their own. Our goal is thus to design, synthesize, and test new HDAC6i both in vitro and in vivo for use in the treatment of melanoma and other malignancies. The aims to be accomplished under this grant are as follows: 1. Using molecular modeling, design, and synthesize ~50 new HDAC6is per year, for the first three years and test these for HDAC isozyme selectivity; the last two years of the grant will focus on compound scale-up, preclinical translational studies, and advanced ADMET testing. 2. Evaluate compounds that have IC50 values of <50nM and at least 400-fold HDAC6 selectivity using in vitro melanoma models to measure acetylated tubulin, pSTAT3, PD-L1 levels, and cytotoxicity. For compounds found to decrease levels of pSTAT3 and PD-L1, while increasing acetylated tubulin, conduct early-stage ADMET studies (2 – 3/year) using a CRO, and if needed, modify the compounds using principles of medicinal chemistry and molecular modeling based methods to increase the compound’s drug-like character. 3. Lastly, test the best compounds for their ability to delay tumor growth in syngeneic murine melanoma models, both as stand-alone and in combination with PD-1 blocking antibodies. Additionally, we will evaluate our best candidates using humanized PDX models and as a cell therapy pre-treating macrophages to avoid their switch to M2-like protumoral phenotype. Those candidates showing the best in vivo efficacy will be sent for advanced ADMET, including chronic toxicity studies, cardiac activity, and pulmonary activity in rats. Our proposed work is significant as it helps to fill this critical knowledge gap and thus moves epigenetic- based combination immunotherapies to a new level. The concept to be validated is novel as it significantly deviates from the classical cytotoxic role of HDAC inhibitors as anti...