ABSTRACT 1 Merkel cell carcinoma MCC is an often-lethal skin cancer with increasing incidence and few treatment 2 options. The dismal five-year survival rate of advanced MCC is less than 18% and the mortality rate is 3-times 3 higher than melanoma. Although immune checkpoint inhibitors (ICI) have become standard of care in advanced 4 MCC, 50% of all MCC patients are ineligible for ICIs and resistance develops in the majority of treated patients. 5 Importantly, there is no therapeutic alternative for these patients. Given this urgent clinical demand, our long- 6 term goal is to identify novel therapies which can be used to augment ICI efficacy and overcome ICI resistance, 7 or to serve as alternative treatments for MCC patients who are ineligible for ICIs. Utilizing MCC cell lines 8 established in our laboratory and high throughput dose-response drug screening of 430 clinically relevant 9 kinase inhibitors, we identified fimepinostat, the first-in-class PI3K/HDAC inhibitor, with potent anti-MCC 10 activities. The objective of this proposal is to determine therapeutic efficacy of fimepinostat alone and in 11 combination with ICIs, and resolve intrinsic and extrinsic mechanisms of anti-MCC activities. The central 12 hypothesis is that fimepinostat inhibits MCC growth and progression by targeting PI3K and HDAC epigenetic 13 pathways, and by sensitizing tumor cells to ICIs through augmentation of antitumor immunity in the MCC 14 microenvironment. We will test this hypothesis by utilizing systems-level approaches integrating cellular, 15 molecular and imaging analyses in single cell resolution to fully study the functional complexity of the MCC 16 ecosystem in our clinically relevant animal model representing the closest preclinical models of human MCC- 17 immune microenvironment. This singularly powerful biological platform will enable us to study direct effects of 18 fimepinostat on tumors and tumor-immune interactions as a monotherapy and in combination with 19 immunotherapy, as well as uncover new insights into TME-mediated resistance and response. The proposed 20 research is significant because successful completion of the project will lay foundation for a new treatment 21 paradigm for MCC, overcoming or circumventing current limitations in therapeutic options, and lending insights 22 into immune resistance will be applicable across human cancers and improve patient outcomes.