Background: Uveal melanoma (UM) is a rare disease but 50% of patients will eventually develop metastatic disease, for which not effective therapy is available. Liver-directed therapies, immunotherapy, targeted therapy and chemotherapy have limited activity [1]. Lymphocyte activation gene 3(LAG-3) is an immune checkpoint receptor associated with decreased T-cell effector function and tumor escape. Preclinical models have shown that dual inhibition of LAG-3 and PD-1 blockade generates synergistic anti-tumor activity [2]. Recent preclinical data indicates that uveal melanoma CD8+ T cells express the checkpoint receptor LAG3 to a greater extent than PD1 or CTLA4 [3,4]. Therefore, LAG3 is a potential candidate for checkpoint inhibitor immunotherapy in UM. Relatlimab is a human LAG-3-specific antibody isolated from immunized transgenic mice which binds to a defined epitope on LAG-3 with high affinity and specificity and potently blocks the interaction of LAG-3 with its ligand, MHC Class II. Methods: This is an open-label, single arm, single site investigator-initiated phase II study, NCT04552223. Based on Simon two-stage minimax design, 13 patients will be enrolled in Stage 1. If at least one response is noted, the study will proceed to Stage 2 and enroll an additional 14 patients. The null hypothesis will be rejected if 4 or more responses are observed among 27 patients. This design achieves 5% type I error and 80% power when the true ORR is 20%. The trial opened to accrual in December 2020. As of February 15, 2021 four patients had been enrolled the first stage of accrual. Main eligibility criteria include patients with biopsy proven metastatic uveal melanoma, previously untreated with PD-1, CTLA-4 and/or LAG-3 blocking antibodies and in good performance status. Enrolled patients are treated in the outpatient setting. Nivolumab 480 mg is mixed in the same bag with relatlimab 160 mg and administered intravenously over 60 minutes every 4 weeks until disease progression or intolerable toxicity for up to 24 months. The primary endpoint is best objective response rate (ORR). Secondary endpoints include disease control rate (DCR), progression-free survival (PFS), overall survival (OS), median duration of response (mDOR), and adverse events (AEs). Correlative studies will evaluate pre- and post-treatment characteristics of T cells in the tumor microenvironment and blood. Clinical trial information: NCT04552223