Background: Entinostat is an oral, class I selective histone deacetylase inhibitor shown preclinically to enhance immune checkpoint inhibitor activity by regulation of immune suppressor cells in the tumor microenvironment. ENCORE 601 is designed to evaluate safety and efficacy of ENT in combination with PEMBRO. Phase 1b identified ENT 5 mg PO weekly and PEMBRO 200 mg IV every 3 weeks as the recommended Phase 2 dose for further investigation. Methods: Phase 2 of ENCORE 601 employs a Simon 2-stage design to assess activity across 3 cohorts: 1) NSCLC not previously treated with a PD-1/L1 blocking antibody, 2&3) NSCLC & melanoma previously progressing on or after a PD-1/L1 blocking antibody. Patients were enrolled irrespective of PD-L1 expression levels. The primary endpoint is ORR as assessed by irRECIST. Results of the first stage of Cohort 3 are reported. Criteria for advancing to Stage 2 are ≥2 responses out of 13 evaluable patients. Results: Stage 1 enrollment has been completed (n = 13). All patients received a prior PD-1 inhibitor; in addition, 7 patients received prior ipilimumab and 2 patients received a prior BRAF inhibitor. 9 patients are still on treatment; 4 patients have discontinued due to progression. To date, 1 confirmed and 1 unconfirmed objective response have been observed. The confirmed response (PR) was in a patient who was previously treated with ipilimumab/nivolumab for 6 months (best response of SD with subsequent progression on therapy). Treatment-related AEs occurred in 5 patients (most common being nausea and fatigue occurring in 2 patients); one patient experienced Gr3/4 events (fatigue and rash). Blood samples and pre-treatment biopsies were obtained in all patients along with paired post-treatment biopsies in 8 patients. Evaluation of PD-L1 expression, gene expression, myeloid and lymphoid compartments in blood and tumor samples is in progress. Of note, the patient with a confirmed PR converted from a PD-L1 negative, non-inflamed gene signature to PD-L1 positive, inflamed after 2 weeks of treatment. Conclusions: ENT combined with PEMBRO demonstrates acceptable safety and encouraging preliminary activity in melanoma patients refractory to checkpoint inhibitors. Clinical trial information: NCT02437136