Background: Immunotherapy (IMT) with checkpoint blocking antibodies has led to progress in metastatic melanoma with 3 FDA-approved drugs, including the combination of ipilimumab (IPI), a CTLA-4 antibody, and nivolumab (NIVO), a PD-1 antibody. Although radiotherapy (RT) is primarily used as local palliative therapy in metastatic melanoma, it also possibly affects systemic antitumor immunity. Preclinical data suggest RT alters the tumor microenvironment and renders tumor cells more susceptible to immunologically-mediated disease regression. These preclinical immunologic effects of RT have been shown to vary by RT dose and fractionation. We are now conducting the first clinical trial in patients to evaluate the triple combination of IPI + NIVO + RT using 2 different dose/fractionation schemes of RT. Methods: This ongoing Phase 1, open-label, multicenter study (NCT02659540) evaluates safety, efficacy, and immunologic effects of IPI + NIVO + RT in 18 patients with unresectable stage IV melanoma. Patients must have 1 melanoma metastasis that can be safely irradiated for palliative purposes and at least 1 measurable lesion that will not be irradiated. Patients receive concurrent IPI (3 mg/kg) and NIVO (1 mg/kg) every 3 weeks (Q3W) x 4, followed by NIVO monotherapy (240 mg Q2W), with RT initiated between the first and second doses of IPI + NIVO. In Cohort A, the irradiated metastasis receives a conventionally fractionated low dose of 30 Gy in 10 fractions of 3 Gy each over 2 weeks. If ≤7 of 9 patients (78%) in Cohort A have Grade 3/4 drug- or radiation-related adverse events, safety is deemed acceptable and Cohort B enrollment opens. In Cohort B, the irradiated metastasis receives a hypofractionated high dose of 27 Gy in 3 fractions of 9 Gy each over 2 weeks. The primary endpoint is safety. Secondary endpoints are objective response rate and disease control rate by RECIST and immune-related RECIST measured at Weeks 12 and 18, duration of response, progression-free survival, and overall survival. Exploratory endpoints include correlative studies of immunological effects. Enrollment opened on 05 Aug 2016. As of 31 Dec 2016, 4 patients are enrolled in Cohort A; enrollment is ongoing. Clinical trial information: NCT02659540