Background: Immunosuppressive regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) limit the efficacy of immunotherapies. We have previously reported that the HDAC inhibitor entinostat has synergistic antitumor effect in combination with immunotherapies in preclinical models by inhibiting Tregs and MDSCs function. The combination of atezolizumab (PD-L1 inhibitor) and bevacizumab (VEGF inhibitor) is active in renal cell carcinoma (RCC). Thus, we have conducted a Phase I study with entinostat, atezolizumab and bevacizumab in patients (pts) with metastatic RCC. Methods: The primary objective was to evaluate safety and tolerability. The phase I portion included 3 dose levels of entinostat (1 mg, 3 mg or 5 mg, PO weekly) and fixed, standard doses of atezolizumab (1200 mg IV every 21 days) and bevacizumab (15 mg/Kg IV every 21 days). Pts with any histological type and prior therapies were included. Results: Dose levels were completed with up to 1 DLT/dose level. 5 mg was the Phase II recommended dose for entinostat. DLTs included hypertension, encephalopathy, hyponatremia and pruritus. The most common resolved grade 3/4 toxicities were hypophosphatemia (33%), hypertension (17%), and pneumonitis (11%). We have enrolled 18 pts (17 evaluable for ORR by RECIST). 5 pts continue on treatment. 3 pts discontinued treatment because of adverse events, 9 pts for disease progression, and one pt for physician decision. Good risk and intermediate risk pts were 61% and 39%, respectively. Overall ORR was 47.1% (95% CI 23.0-72.2) and median PFS was 7.6 months (95% CI 1.6-16.3). In pts with no prior therapies (12) the ORR was 58.3% (95% CI 27.7-84.8) and median PFS was 13.4 months (95% CI 1.5-28.9). One additional PR was observed by ir-RC but was not confirmed within the data cut-off date of 11/11/19. In pts with prior immune checkpoint inhibitors (ICIs) (5) ORR was 20% (95% CI 0.5-71.6) and median PFS was 7.6 months (95% CI 1.3-NR). Preliminary data show a statistically significant lower % of circulating monocytic MDSC (HLADR⁻1CD11b⁺CD33^highCD14⁺CD15⁻) and exhausted T cells (CD45⁺CD3⁺CD8⁺TIM3⁺) following treatment in pts (4) with objective responses as compared to pts (4) with progressive disease. Conclusions: The results from this phase I suggest that the combination of entinostat, atezolizumab and bevacizumab is relatively well tolerated and is active in renal cell carcinoma patients, in both ICIs naïve and resistant disease. A phase II portion of this study is currently accruing patients. Clinical trial information: NCT03024437.