Purpose: In this first-in-human phase I study (NCT02132754), we explored MK-4166 [humanized IgG1 agonist mAb targeting glucocorticoid-induced TNF receptor (GITR)] with and without pembrolizumab in advanced solid tumors. Patients and Methods: MK-4166 was tested alone (0.0015. 900 mg i.v. every 3 weeks for four doses) or with pembrolizumab (200 mg i.v. every 3 weeks for ≤35 doses) in patients with metastatic solid tumors (dose escalation/confirmation) and advanced melanoma (expansion). Primary objectives were to evaluate the safety and tolerability and establish the MTD of MK-4166. Exploratory endpoints were objective response rate (ORR) and T cell-inflamed gene expression profile (GEP) analysis using RNA from baseline tumor samples. Results: A total of 113 patients were enrolled [monotherapy, n = 48; combination therapy, n = 65 (20 in the expansion)]. Fortysix patients (40.7%) had grade ≥3 adverse events, 9 (8.0%) of which were treatment related. No treatment-related deaths were observed. One dose-limiting toxicity event with monotherapy (bladder perforation in patient with neobladder) was considered related to study drug. MTD was not reached. MK-4166 pharmacodynamics showed decreased GITR availability on circulating T cells with increasing doses. One objective response (ORR, 2.2%) was achieved with combination therapy in the dose escalation/confirmation (n=45). In the expansion, 8 of 13 patients with immune checkpoint inhibitor (ICI)-naive melanoma achieved a response (ORR, 62%; 95% confidence interval, 32.86; 5 complete responses and 3 partial responses). None of the ICI-pretreated patients (n=7) responded. Highresponse rates were observed in ICI-naive patients irrespective of GEP status. Conclusions: MK-4166 900 mg i.v. every 3 weeks as monotherapy and with pembrolizumab was tolerable. Responses were observed with combination therapy, mostly in patients with ICI-naïve melanoma.