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., Competing Interests: Competing interests: JJL: DSMB: AbbVie, Agenus, Immutep, Evaxion; Scientific Advisory Board: (no stock) 7 Hills, Affivant, BioCytics, Bright Peak, Exo, Fstar, Inzen, RefleXion, Xilio (stock) Actym, Alphamab Oncology, Arch Oncology, Duke Street Bio, Kanaph, Mavu, NeoTx, Onc.AI, OncoNano, physIQ, Pyxis, Saros, STipe, Tempest; Consultancy with compensation: AbbVie, Agenus, Alnylam, AstraZeneca, Askgene, Atomwise, Bayer, Bristol Myers Squibb, Castle, Checkmate, Codiak, Crown, Cugene, Curadev, Day One, Eisai, EMD Serono, Endeavor, Flame, G1 Therapeutics, Genentech, Gilead, Glenmark, HotSpot, Kadmon, Ko Bio Labs, Krystal, KSQ, Janssen, Ikena, Inzen, Immatics, Immunocore, Incyte, Instil, IO Biotech, LegoChem, Macrogenics, Merck, Mersana, Nektar, Novartis, Partner, Pfizer, Pioneering Medicines, PsiOxus, Regeneron, Replimmune, Ribon, Roivant, Servier, STINGthera, Storm, Sumoitomo, Synlogic, Synthekine, Teva; Research Support: (all to institution for clinical trials unless noted) AbbVie, Astellas, AstraZeneca, Bristol Myers Squibb, Corvus, Day One, EMD Serono, Fstar, Genmab, Hot Spot, Ikena, Immatics, Incyte, Kadmon, KAHR, Macrogenics, Merck, Moderna, Nektar, Next Cure, Novartis, Numab, Palleon, Pfizer, Replimmune, Rubius, Servier, Scholar Rock, Synlogic, Takeda, Trishula, Tizona, Xencor; Patents: US-11638728 (Microbiome Biomarkers for Anti-PD-1/PD-L1 Responsiveness: Diagnostic, Prognostic and Therapeutic Uses Thereof). DJP: lecture fees from ViiV Healthcare, Bayer Healthcare, BMS, Roche, Eisai, Falk Foundation, Incyte, Boston Scientific, travel expenses from BMS and Bayer Healthcare; consulting fees for Mina Therapeutics, Elevar Therapeutics, Eisai, Roche, DaVolterra, Mursla, Starpharma, Exact Sciences, and Astra Zeneca; research funding (paid to institution) from MSD, GSK, and BMS. DJ: Consulting or advisory role for AstraZeneca, Eisai, Genentech, Lilly, MapKure, Novartis, Pfizer, PIC Therapeutics, Relay Therapeutics, Vibliome Therapeutics; research funding (paid to institution) from Amgen, Arvinas, AstraZeneca, Blueprint Medicines, Eisai, Genentech, Infinity Pharmaceuticals, InventisBio, Novartis, Pfizer, Ribon Therapeutics, Scorpion Therapeutics, Syros Pharmaceuticals, Takeda; stock and other ownership interests from PIC Therapeutics, Relay Therapeutics, Vibliome Therapeutics. PL: Participation on a Data Safety Monitoring Board or Advisory Board for: AbbVie, Takeda, Agenus, IQVIA, Glaxo-Smith Kline, Pfizer, QED Therapeutics, AstraZeneca, EMD Serono, Kyowa Kirin Pharmaceutical Development, Kineta, Inc., Zentalis Pharmaceuticals, Molecular Templates, ABL Bio, STCube Pharmaceuticals, I-Mab, Seagen, imCheck, Relay Therapeutics, Stemline, Compass BADX, Mekanistic, Mersana Therapeutics, BAKX Therapeutics, Scenic Biotech, Qualigen, NeuroTrials, Actuate Therapeutics, Atreca Development, Cullinan, Amgen, DrenBio, Quanta Therapeutics, Schrodinger, Boehrigner Ingelheim; Consultant for Sotio, IDMC, I-Mab, Roivant Sciences; Member of imCORE Alliance-Roche. PJH: consultant/advisory role for Fibrogen. AMD: nothing to disclose. RH: consulting or advisory role at Merck, Eisai, Bristol Myers Squibb, AstraZeneca, GSK, EMD Serono, Bayer, Coherus Biosciences, Boehringer Ingelheim, Genmab, Galera Therapeutics, Merus, and ALX Oncology; stock and other ownership interests from Tosk; research funding from Merck, Eisai, Bristol Myers Squibb, AstraZeneca, Genentech, Pfizer, Kura, EMD Serono, and Incyte; other from Nanobiotix, ISA Pharmaceuticals, Boehringer Ingelheim, and Hookipa Pharma. MdM: nothing to disclose. AC: grants and contracts paid to institutions from Genentech, Merck Serono, Roche, Beigene, Bayer, Servier, Lilly, Novartis, Takeda, Astellas, Fibrogen, Amcure, Sierra Oncology, Astra Zeneca, Medimmune, BMS, and MSD. Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Merck Serono, Roche, Angem, Bayer, Servier, and Foundation Medicine. Participation on a Data Safety Monitoring Board or Advisory Board for GSK, Merck Serono, Agenus, MSD, AbbVie, Roche, Beigene, Bayer, Servier, Lilly, Novartis, Takeda, and Astellas. WSK: grant/research support from Sanofi, Beigene, Boryong, Roche, Kyowa-Kirin, and Donga. AM: manuscript writing support from Eisai. Consulting and scientific advisory boards on STING for Eisai, Merck (MSD), Boehringer Ingelheim, and GSK. YZ: employee of Eisai. YR: Employee of Eisai. XY: employee of Eisai. SC: Speaking Engagement: Amgen, Astellas, AstraZeneca, Bristol Myers Squibb, Eisai, Genmab, Janssen, Merck KGaA, MSD, Novartis, Roche, Servier, Takeda. Principal investigator of clinical trials for: AbbVie, Amgen, Boehringer Ingelheim, Bolt Biotherapeutics, Centessa Pharmaceuticals, Cytovation, Eisai, GlaxoSmithKline, Imcheck Therapeutics, Immunocore, Molecular Partners Ag, MSD, Ose Immunotherapeutics, Pierre Fabre, Replimune, Roche, Sanofi Aventis, Seagen, Sotio A.S., Transgene. Advisory Board/Consulting: Alderaan Biotechnology, Amgen, AstraZeneca, Avacta, Beigene, BioNTech, Celanese, Domain Therapeutics, Ellipses Pharma, Genmab, Immunicom, Mariana Oncology, Nanobiotix, Nextcure, Oncovita, Pierre Fabre, Seagen, Takeda, Tatum Bioscience, Tollys, UltraHuman8; Travel and congress: Amgen, AstraZeneca, Boehringer Ingelheim, Bristol Myers Squibb, MSD, Ose Immunotherapeutics, Roche, Sotio., (© Author(s) (or their employer(s)) 2025. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ Group.)