Purpose: The First-in-Human Phase 1/2 ICONIC trial evaluated an investigational ICOS agonist, vopratelimab, alone and in combination with nivolumab in patients with advanced solid tumors. Experimental Design: In Phase 1, patients were treated with escalating doses of intravenous vopratelimab alone or with nivolumab. Primary objectives were safety, tolerability, maximum tolerated dose and recommended Phase 2 dose (RP2D). Phase 2 enriched for ICOS+ tumors; patients were treated with vopratelimab at the monotherapy RP2D alone or with nivolumab. Pharmacokinetics, pharmacodynamics and predictive biomarkers of response to vopratelimab were assessed. Results: ICONIC enrolled 201 patients. Vopratelimab alone and with nivolumab was well tolerated; Phase 1 established 0.3 mg/kg q3w as the vopratelimab RP2D. Vopratelimab resulted in modest objective response rates of 1.4% and with nivolumab of 2.3%. The prospective selection for ICOS+ tumors did not enrich for responses. A vopratelimab-specific peripheral blood pharmacodynamic biomarker, ICOS-hi CD4 T-cells, was identified in a subset of patients who demonstrated greater clinical benefit versus those with no emergence of these cells [overall survival (OS), p=0.0025]. A potential genomic predictive biomarker of ICOS-hi CD4 T-cell emergence was identified which demonstrated improvement in clinical outcomes, including OS (p=0.0062). Conclusions: Vopratelimab demonstrated a favorable safety profile alone and in combination with nivolumab. Efficacy was observed only in a subset of patients with a vopratelimab-specific pharmacodynamic biomarker. A potential predictive biomarker of response was identified, which is being prospectively evaluated in a randomized Phase 2 non-small cell lung cancer trial.