E7766 is a novel stimulator of interferon genes (STING) agonist, capable of potent activation of immune cells and generating strong antitumor response in preclinical murine tumor models. Here we present the safety, efficacy, and biomarker results of the first-in-human phase I/Ib study of intratumoral E7766 in patients with advanced solid tumors. Eligible patients with relapsing/refractory cancers (n=24) were enrolled in dose-escalating cohorts to receive intratumoral injections of E7766 from 75 to 1000 µg. The most frequent treatment-related treatment-emergent adverse events were chills (50.0%
85.7%), fever (40.0%
85.7%), and fatigue (30.0%
35.7%) in patients who received non-visceral and visceral injections, respectively. Eight patients (33.3%) achieved stable disease as their best response per modified Response Evaluation Criteria In Solid Tumors version 1.1 with variability between injected and non-injected lesions. Plasma levels of IFN-α, IFN-β, IFN-γ, TNF-α, IL-6, IP-10, MCP1, and MIP1b transiently increased in all evaluable patients within 10 hours postinjection, then dropped to baseline levels. Levels of blood and tumor gene expression increased in most interferon-related and STING genes tested. Further increases in programmed death ligand 1 and cluster of differentiation 8 expression at both the RNA and protein levels were also observed in some patients across dose levels. In total, E7766 generated on-target pharmacodynamic effects in patients with solid tumors. Further exploration in a homogeneous patient population is necessary to assess efficacy.