Background: The majority of unresectable melanoma patients eventually progress on first-line checkpoint blockade therapy. Duvelisib is a potent phosphoinositide 3-kinase (PI3K) δ and γ isoform inhibitor approved for relapsed/refractory CLL. PI3Kγ inhibition has been shown to restore anti-PD1 activity in preclinical tumor models through conversion of tumor associated macrophages (TAMs) from an immunosuppressive to a pro-inflammatory state. PI3Kδ inactivation has been associated with impaired T regulatory cell (Treg) function and reduced intratumoral Treg numbers. We report study design and results of a phase I study to evaluate the safety and efficacy of duvelisib and nivolumab in anti-PD1 refractory patients. Methods: Utilizing a modified 3+3 design with escalating doses of duvelisib (15 mg daily, 25 mg daily, and 25 mg BID), patients with unresectable melanoma were treated with nivolumab and duvelisib until unacceptable toxicity or disease progression. Primary endpoints for the phase I portion were maximum tolerated dose (MTD) and recommended phase II dose (RP2D) of duvelisib. Secondary endpoints include early and late toxicities and anti-tumor activity based on RECIST v1.1. Exploratory endpoints were to evaluate dose-dependent changes in the immune cell populations of peripheral blood and tumor tissue with duvelisib and nivolumab. Results: Ten patients have been enrolled (7 at dose level I, 3 at dose level II) with mean age 64. Two patients had BRAF V600E mutation and were previously treated with BRAF/MEK inhibitors. No dose limiting toxicities (DLT) were observed. Grade 3 or higher adverse events (AEs) attributable to study treatment occurred in 4 patients (40%), including G3 neutropenia (10%), G3 diarrhea (10%), G3 vomiting (10%) and G4 hepatitis (20%). Both instances of hepatitis resolved rapidly with 1-2mg/kg of prednisone. In one patient, hepatitis and nausea/vomiting led to discontinuation of treatment. Out of 6 patients who had follow-up imaging, 4 exhibited progressive disease, one stable disease, and one had partial response of 11 months duration. Given the published data on the regulation of macrophages and Tregs by PI3Kδγ inhibitors, we are utilizing multispectral imaging to measure the spatial interactions of key immune cells within the tumor microenvironment (T cells, B cells, and macrophages) in all matched pre-/post-treatment paraffin-embedded tissues, for which results will be presented. Conclusions: The combination of duvelisib and nivolumab was well-tolerated in patients with advanced unresectable melanoma refractory to ICI without DLTs to date at 15 and 25mg daily dosages. Significant anti-tumor activity was noted in one patient. Enrollment at dose level II is ongoing. Clinical trial information: NCT04688658.