Background: Histone deacetylase (HDAC) inhibitors have single agent activity in various types of lymphoma. They have been shown to restore antigen-specific immune recognition in cancer cells and to downregulate PD-1 expression in circulating T lymphocytes. In preclinical studies, the combination of HDAC inhibitors and anti-PD-1 antibodies acts synergistically against various tumor models in mice. Accordingly, we investigated the safety and efficacy of the novel combination of the HDAC inhibitor ENT and the PD-1-blocking antibody PEM in patients with R/R HL. Methods: Patients with R/R HL received ENT 5-7 mg orally once weekly and PEM 200 mg intravenously once every three weeks. The primary objective is overall response rate (ORR) and 12-month progression-free survival (PFS). Multiplexed serum cytokine analysis of 20 pro-inflammatory cytokines and chemokines was performed on sera from peripheral blood samples collected at baseline and at 21 days on treatment. Results: At data cutoff on 2/5/20, 14 patients with HL have been enrolled. Out of 13 evaluable patients, 12 responded (92% ORR), including 3 who progressed on prior anti-PD-1 therapy. With a median duration of follow-up of 176 days (21-632), 9 patients are currently receiving treatment on study, 2 discontinued due to toxicity, 1 for progression, and 2 for consolidation with transplant or radiation. After 21 days on treatment, there was a decrease in median serum levels of eotaxin (-39%, p = 0.002), eotaxin-3 (-56%, p = 0.04), MDC (-78%, p = 0.025), MIP1a (-60%, p = 0.025), and TARC (-98%, p < 0.001) and a 3-fold increase in median levels of IFNγ (p = 0.032). There was an association between extent of tumor reduction and greater decrease in the cytokines eotaxin-3 (-62%, p = 0.064), MDC (-90%, p = 0.064), and MIP1a (-85%, p = 0.064), which trended towards statistical significance. Out of 22 total patients enrolled in this study (including 8 patients with follicular lymphoma), 62% had grade ≥3 adverse events (AE), which were predominantly hematologic, including neutropenia (48%), thrombocytopenia (19%), and anemia (10%). Immune-related AEs included 3 cases of hypothyroidism, 2 cases of hepatitis and 1 case of pneumonitis. Four patients who experienced serious AEs due to pericarditis (n = 2), hemophagocytic lymphohistiocytosis, and bullous dermatitis were taken off study. Conclusions: Early results from this ongoing phase II clinical trial suggest that the combination of PEM and ENT is safe with encouraging responses in HL. Clinical trial information: NCT03179930.