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Background: Activation of OX40, a costimulatory protein in the tumor necrosis factor receptor super family, enhances T effector cell activation and inhibits T regulatory (Treg) cell-mediated suppression. Preclinical data showed that combination of an OX40 agonist with checkpoint blockade (anti-PD-1 or anti-CTLA-4) enhanced antitumor activity vs checkpoint blockade alone. BMS-986178 is a fully human IgG1 agonist monoclonal antibody that binds with high affinity to OX40 and was well tolerated ± NIVO (anti-PD-1; Olszanski AJ, et al. J Immunother Cancer 2017;5(suppl2) [abstract O17]). The relationship between receptor modulation and the activity of agonist OX40 agents in the clinic is not clear. Here we present findings on the regulation of OX40 receptor expression and T-cell activation by BMS-986178 ± NIVO or IPI (anti–CTLA-4) from the phase 1/2a study in patients with advanced solid tumors (NCT02737475). Methods: Patients with ≥ 1 prior therapy were treated in this open-label, dose-escalation and -expansion study. During escalation, patients received BMS-986178 monotherapy 20-320 mg IV Q2W, BMS-986178 20-320 mg + NIVO 240 mg IV Q2W, or BMS-986178 20-320 mg + IPI 1 mg/kg IV Q3W. Pharmacokinetics (PK) and systemic pharmacodynamics (PD) were evaluated, including analysis of OX40 receptor occupancy (RO), CD4+ T cell- and Treg-cell surface OX40 expression, soluble OX40 (sOX40) levels, cytokines, and proliferation of effector memory T cells in circulation. Results: BMS-986178 PK was linear from 20 to 320 mg when administered as monotherapy (n = 20), + NIVO (n = 38), or + IPI (n = 32). Moreover, BMS-986178 ± NIVO or IPI stimulated the production of IFN-γ and increased proliferating (Ki-67+) effector memory T cells. Concomitant downregulation of OX40 expression on the cell surface of CD4+ T cells and Tregs was observed as OX40 RO approached saturation at BMS-986178 doses of ≥ 40 mg. Furthermore, BMS-986178 doses of 20-160 mg ± NIVO or IPI increased sOX40 levels, while BMS-986178 doses of ≥ 160 mg ± NIVO or IPI led to sOX40 plateau. Conclusions: Clinical PK/PD findings showed efficient BMS-986178 target engagement and confirmed preclinical observations that BMS-986178 can modulate RO and OX40/sOX40 expression. Furthermore, peripheral T-cell activation was observed in patients treated with BMS-986178 ± NIVO or IPI. Coupled with our preclinical observations, these findings highlight a complex dose-response relationship between BMS-986178 receptor saturation, receptor modulation, and induction of soluble receptor. These data support further clinical investigation of a broader dose range and optimal schedules for BMS-986178 ± NIVO or IPI. Citation Format: Rui Wang, Yan Feng, Ed Hilt, Xiling Yuan, Chan Gao, Xiao Shao, Yongliang Sun, Michael D'silva, Katherine Yang, Becky Penhallow, Goce Bogdanoski, Rajesh Anand, Irene Pak, Danielle Greenawalt, Anke Klippel, Nataly Manjarrez-Orduno, Robert Neely, Michael Quigley, Michael Hedrick, Praveen Aanur, Z Cao. From bench to bedside: Exploring OX40 receptor modulation in a phase 1/2a study of the OX40 costimulatory agonist BMS-986178 ± nivolumab (NIVO) or ipilimumab (IPI) in patients with advanced solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-127.