Abstract BACKGROUND Melanoma brain metastases (MBM) is a major cause of morbidity and mortality. The way melanoma malignant cells interact with the microenvironment in distal sites including the brain, plays a major role in tumorigenesis, metastasis, and tumor survival. Data supports the tumor microenvironment (TME) in MBM is different at the molecular/cellular level than the TME in other metastatic sites. Objectives: (1) to determine the safety and feasibility of presurgical administration of Nivolumab (Nivo) in combination with Ipilimumab (Ipi) or Opdualag (Nivo + Relatlimab [Rela]); and (2) to investigate the TME and estimate the immune cell population difference between the different treatment groups. METHODS We are carrying a randomized open label pilot study with adult patients (n=24) with MBM with surgically resectable brain tumors, who have not received treatment with immune-checkpoint inhibitors for six months or with BRAF/MEK inhibitors for 1 month, and on ≤3 mg/day dexamethasone or equivalent/day. Patients will be treated with either Nivo+Ipi, Opdualag, or no treatment pre-surgery (NCT05704933). Following treatment, all subjects will undergo surgical resection of their brain tumors and systemic disease biopsy, if clinically indicated. Post-surgery, all subjects will receive stereotactic radiosurgery (SRS) or fractionated SRS (fSRS), followed by Nivo+Ipi x4 dosages and adjuvant Nivo for 1 year or until disease progression and/or intolerable adverse events. All subjects will be on levetiracetam 500 mg twice a day or equivalent medication throughout the duration of the study. A lumbar puncture for CSF collection will be performed if clinically indicated and CSF and blood collected at different time points. Collected tissue will be send for sc RNA-seq, TCR/BCR seq. A one-way ANOVA will be used to compare the cellular population differences between treatment arms. CNS clinical response will be defined using RANO-BM and i-RANO. Trial is open and actively recruiting.