Background: Novel checkpoint inhibitors are a promising treatment for advanced melanoma, as only a fraction of patients have durable responses to current FDA-approved immunotherapy. Lymphocyte activation gene 3 (LAG3) is an inhibitory checkpoint receptor on CD4+ and CD8+ T cells, where engagement results in suppression of T cell activation and proliferation. LAG3 and PD1 are co-expressed on T cells during T cell receptor signaling and are down-regulated after antigen clearance. Persistent stimulation leads to prolonged LAG3 and PD1 expression and to T cell exhaustion, a possible mechanism of resistance to immunotherapy. Both LAG3 and PD1 are expressed on tumor-infiltrating T cells in melanoma. Murine tumors treated with both anti-LAG3 and anti-PD1 have demonstrated increased tumor regression than tumors in mice treated with either single agent. Further, a phase I trial has demonstrated safety of combined anti-LAG3 monoclonal antibody, relatlimab and anti-PD1 monoclonal antibody, nivolumab. Methods: This phase II, single-center clinical trial is designed to enroll treatment naive patients with unresectable or metastatic melanoma to ultimately receive combined relatlimab and nivolumab after a lead-in arm where patients are randomized to receive relatlimab, nivolumab, or the combination for the first 4 week cycle. For the lead-in phase, patients will have baseline and post-treatment blood and tumor sampling. Disease assessment by imaging will occur after the lead-in phase at 4 weeks. After completion of the lead-in phase, all patients proceed to combination therapy with disease assessment at 12-week intervals. The primary endpoint for the lead-in phase is to evaluate changes in immune cell populations in peripheral blood and tumor with the single agents and combination treatment. The primary endpoint for the combination phase is best overall anti-tumor response. Secondary clinical endpoints include progression-free survival, overall survival, duration of response and toxicity. Exploratory endpoints are to determine the mechanistic effects of anti-LAG3 and anti-PD1 on the blood and tumor microenvironment, cytokine signatures, and correlation of these with clinical response. The study is currently accruing with enrollment of 9 out of 42 patients. Clinical trial information: NCT03743766