Background: Immunosuppressive factors such as regulatory T cells (Tregs) and myeloid-derived suppressive cells (MDSCs) limit the efficacy of immunotherapies. Histone deacetylase (HDAC) inhibitors have been shown to have immunomodulatory effects. We have previously reported that the class I selective HDAC inhibitor entinostat has synergistic antitumor effects in combination with high dose interleukin-2 (IL-2) in a renal cell carcinoma model by down-regulating Foxp3 expression and function of Tregs. Thus, we have conducted a Phase I/II clinical study with entinostat and high dose IL-2 in patients (pts) with metastatic clear cell renal cell carcinoma. Methods: The primary objectives were to evaluate the safety, tolerability and efficacy of this combination strategy. The main eligibility criteria were clear cell histology, no prior treatments, and being fit to receive high dose IL-2. The phase I portion consisted of two dose levels of entinostat (3 and 5 mg, PO every 14 days) and a fixed standard dose of IL-2 (600,000 units/kg every 8 hrs). To test our hypothesis, the fixed sample size at the Phase II dose level is 36 with a type I/II error of 10%. If 11 or more of the pts have a response, the hypothesis that the response rate is ≤ 20% is rejected. Results: Dose levels 1 and 2 were completed without DLTs and 5 mg was the Phase II recommended dose for entinostat. The most common transient grade 3/4 toxicities were hypophosphatemia (16%). lymphopenia (15%), and hypocalcemia (7%) We have enrolled 47 pts (44 at dose level 2), and 38 have completed one cycle (84 days) treatment. Four pts were not evaluable. 13 pts have achieved objective response (34%; 10 PR, 3 CR). The overall median PFS is 16.1 months (6.2, 27.8) and the median PFS of responders is 28.5 months (12,NR). To date the median OS is 65.3 months (52.6, 65.3). Decreased Tregs have been observed following treatment and lower numbers have been associated with response. Conclusions: The results from this clinical trial suggest that entinostat may increase the therapeutic effect of high dose IL-2 by modulating immunosuppressive cells. Clinical trial information: NCT01038778