Background: Sarcoma cells are most immunogenic earlier in the disease course and prior to treatment when the immune system can recognize and destroy them. Hypothesis: Immune checkpoint inhibitors would be most effective when given as first line therapy. Methods: This is an IRB-approved dose-seeking Phase 1/2 protocol using defined doses of I (1 mg/kg i.v. q 12 weeks), N (3 mg/kg i.v. q 2 weeks) and escalating doses of T (1.0, 1.2, 1.5 mg/m2 i.v. q 3 weeks), employing the “Cohort of Three” design, followed by a Phase 2 using the MTD of trabectedin. Results: Nine subjects were treated in Phase 1 of the study, and 31 subjects in Phase 2. Safety analysis: at Dose 1: Grade 3 treatment-related adverse events (TRAEs) included fatigue (n = 1), increased TSH (n = 1). At Dose 2, Grade 4 TRAEs included thrombocytopenia with bleeding, DLT (n = 1), increased CK (n = 1); Grade 3 TRAEs included anemia (n = 1), myalgia (n = 1), increased TSH (n = 1), decreased TSH (n = 1), increased AST (n = 1). Efficacy analysis (evaluable patients): At Dose 1: Disease Control Rate (DCR = CR, PR, SD) was 67%, median PFS, 18 weeks; median OS, 50 weeks; At Dose 2: PR (n-1), DCR 60%, median PFS, 24 weeks; median OS, > 46 weeks. At Phase 2, MTD Dose 2 (PUPs): Safety analysis (n = 31): Grade 3 TRAEs included fatigue (n = 2), increased ALT (n = 6), increased AST (n = 4), hypernatremia (n = 1), hyponatremia (n = 1), dehydration (n = 1), rash (n = 1), port cellulitis (n = 1), psoriasis exacerbation (n = 1), increased TSH (n = 1), decreased hemoglobin (n = 2), neutropenia (n = 1). Efficacy analysis (N = 23 evaluable): PR (n-5; 2 UPS, 1 synovial sarcoma, 1 liposarcoma, 1 leiomyosarcoma,), BORR 22%, DCR 96%. Median PFS and OS not yet reached. After 4 treatment cycles, one resected tumor showed 80% necrosis and a greater number (30%) of CD8+ killer T cells, in the TME compared to archived pre-treatment tumor. Conclusions: These data suggest that the SAINT protocol (1) is safe with manageable adverse events, with no additive toxicity, and (2) may control tumor growth. Phase 2 of the study is on-going. Clinical trial information: NCT03138161