Background: IMCgp100 is a bispecific biologic capable of redirecting T cells against the melanocyte-associated antigen gp100. In the first-in-human (FIH) clinical trial,preliminary efficacy of IMCgp100 in advanced uveal melanoma (UM) was observed; however, cases of severe T cell-mediated toxicities with the first 2 doses limited dosing to 50 mcg QW. This phase 1 study was designed to implement intra-patient escalation to mitigate toxicity and maximize exposure of IMCgp100. Methods: Using a 3+3 design, HLA-A2+ pts with metastatic UM were treated with low weekly (QW) dosing of IMCgp100 iv at Cycle 1, Day 1 (C1D1) (20 mcg) and C1D8 (30 mcg), followed by the escalated cohort dose administered at C1D15 and beyond. Results: Nineteen metastatic UM pts with a median of 3 prior lines of therapy (range 0 - 8) were treated across 4 target dose cohorts (60, 70, 80, and 75 mcg). Fifteen of 19 pts remain on treatment at the data cutoff (25Nov16). Enrollment completed 15Sep16. DLT was observed in 1 of 6 pts in the 70-mcg cohort (LFT elevation), and 2 of 4 pts treated at 80 mcg QW (LFT and bilirubin elevation). The 80 mcg dose level was not tolerated and a 75 mcg cohort was enrolled. Six pts were treated at 75 mcg without DLT; this dose was identified as the MTD and RP2D. All 3 DLT resolved without corticosteroids and all pts resumed IMCgp100. Frequent related AE included pruritus (84%), pyrexia (84%), fatigue (74%), hypotension (74%) and peripheral edema (63%). Gr 3/4 drug-related AE include AST increased (15%), erythema (15%) and hypotension (15%). Preliminary efficacy data (RECIST v1.1) from 17 evaluable pts (2 pts had insufficient follow-up) showed no objective responses; 12 pts had a BOR of SD (63%) including 4 pts (24%) with a ≥10% reduction in target lesions. With median follow-up of 1.8 mo, the disease control rate (16 wks) was 32%. Conclusions: The intra-patient escalation regimen of IMCgp100 results in a 50% increase in dose above the FIH Phase 1, acceptable safety profile, and preliminary efficacy in UM pts. Considering the dismal prognosis of metastatic UM, the PFS observed in this study is encouraging. A Phase 2 expansion cohort and separate pivotal trial of IMCgp100 in advanced UM are ongoing. Clinical trial information: NCT02570308