Background: AMG 228 is an agonistic human IgG1 monoclonal antibody that binds to GITR (CD357), a TNFSFR costimulatory molecule expressed by effector/regulatory T cells. Dose escalation of this open label, first in human, phase 1 study evaluated the safety, pharmacokinetics (PK), pharmacodynamics, and maximum tolerated dose (MTD) and recommended phase 2 dose of AMG 228 in pts with advanced solid tumors. Methods: Pts with refractory advanced colorectal cancer (n = 13), squamous cell carcinoma of head and neck (n = 10), non–small cell lung cancer (n = 2), urothelial transitional cell carcinoma (n = 4), and melanoma (n = 1) received AMG 228 IV Q3W. Dose escalation was in two stages: single-pt cohorts until AMG 228-related grade > 2 adverse events (AEs), efficacy, or 90 mg dose reached (4 cohorts: 3, 9, 30, and 90 mg), followed by rolling six design (n = 2 to 6) until MTD or highest planned dose of 1200 mg reached (5 cohorts: 180, 360, 600, 900, and 1200 mg). Primary endpoints included incidence of dose-limiting toxicities (DLTs) and AEs and PK. Additional endpoints were objective response (RECIST 1.1) and evidence of biological activity. Results: In total, 30 pts (median age 63 y) were treated (3, 9, 30, and 90 mg, n = 1; 180 mg, n = 6; 360 mg, n = 4; 600 mg, n = 6; 900 mg, n = 4; 1200 mg, n = 6). Twenty-seven (90%) pts had treatment-emergent AEs; the most common were hypophosphatemia (23%), fatigue (23%), anemia (23%), nausea (20%), and pyrexia (20%). No DLTs occurred; the MTD was not reached. AMG 228 exposure was dose-related, with PK profiles at low doses (3 to 90 mg) consistent with target mediated drug disposition; doses > 360 mg achieved serum levels needed for 95% receptor occupancy on activated PBMCs. No evidence of T cell activation was observed despite complete target coverage in both tumor and peripheral blood. Among 29 evaluable pts, none had an objective response. Conclusions: In this population of pts with advanced solid tumors, AMG 228 Q3W was tolerable up to the highest tested dose (1200 mg), showing favorable PK. However, no clinical or immunological activity was observed in this limited number of pts. Clinical trial information: NCT02437916