2512 Background: Programmed death-1 (PD-1) is an inhibitory T-cell coreceptor that may lead to suppression of antitumor immunity. MK-3475 is a humanized monoclonal IgG4 antibody against PD-1. Preclinically, MK-3475 has shown antitumor activity in multiple tumor types. This first-in-human phase I trial explored safety, PK, PD, and antitumor activity of MK-3475. Methods: An open-label, dose escalation study was conducted in patients with advanced malignancy refractory to standard therapy. Cohorts of 3-6 patients were enrolled (3+3 design) at escalating IV doses of 1, 3, and 10 mg/kg. Following an initial dose and 28-day Cycle 1, patients were allowed to subsequently receive multiple doses given every 2 wks. Radiographic assessment was conducted every 8 wks using RECIST 1.1 guidelines. Results: Nine patients, 3 at each dose level, completed the dose-limiting toxicity (DLT) period (28 d). Patients had non–small cell lung cancer (NSCLC, n=3), rectal cancer (n=2), melanoma (MEL, n=2), sarcoma (n=1), or carcinoi...