Background: Novel combination therapies to overcome anti-PD-1 resistance are required. Enadenotucirev, a tumor-selective blood stable adenoviral vector, has demonstrated a manageable safety profile and ability to increase tumor immune-cell infiltration in phase I studies in solid tumors., Methods: We conducted a phase I multicenter study of intravenous enadenotucirev plus nivolumab in patients with advanced/metastatic epithelial cancer not responding to standard therapy. Co-primary objectives were safety/tolerability and maximum tolerated dose and/or maximum feasible dose (MTD/MFD) of enadenotucirev plus nivolumab. Additional endpoints included response rate, cytokine responses, and anti-tumor immune responses., Results: Overall, 51 heavily pre-treated patients were treated, 45/51 (88%) of whom had colorectal cancer (35/35 patients with information available were microsatellite instability-low/microsatellite stable) and 6/51 (12%) had squamous cell carcinoma of the head and neck. The MTD/MFD of enadenotucirev plus nivolumab was not reached, with the highest dose level tested (1×10 12 vp day 1; 6×10 12 vp days 3 and 5) shown to be tolerable. Overall, 31/51 (61%) patients experienced a grade 3-4 treatment-emergent adverse event (TEAE), most frequently anemia (12%), infusion-related reaction (8%), hyponatremia (6%), and large intestinal obstruction (6%). Seven (14%) patients experienced serious TEAEs related to enadenotucirev; the only serious TEAE related to enadenotucirev occurring in >1 patient was infusion-related reaction (n=2). Among the 47 patients included in efficacy analyses, median progression-free survival was 1.6 months, objective response rate was 2% (one partial response for 10 months), and 45% of patients achieved stable disease. Median overall survival was 16.0 months; 69% of patients were alive at 12 months. Persistent increases in Th1 and related cytokines (IFNγ, IL-12p70, IL-17A) were seen from ~day 15 in two patients, one of whom had a partial response. Among the 14 patients with matching pre-tumor and post-tumor biopsies, 12 had an increase in intra-tumoral CD8 + T-cell infiltration and 7 had increased markers of CD8 T-cell cytolytic activity., Conclusions: Intravenously dosed enadenotucirev plus nivolumab demonstrated manageable tolerability, an encouraging overall survival and induced immune cell infiltration and activation in patients with advanced/metastatic epithelial cancer. Studies of next-generation variants of enadenotucirev (T-SIGn vectors) designed to further re-program the tumor microenvironment by expressing immune-enhancer transgenes are ongoing., Trial Registration Number: NCT02636036., Competing Interests: Competing interests: MF: Honoraria (advisory, speakers bureau): Amgen, Inc. Consulting: AstraZeneca, Bayer Corporation, Bristol Myers Squibb, Incyte Corporation, Mirati Therapeutics, Inc., Akamis Bio Ltd, Taiho Oncology. Advisory: Bayer Corporation, Roche/Genentech, Xenthera. Advisory board: Mirati Therapeutics Inc., Nouscom. Editorial board: Mirati Therapeutics Inc. Grants to institutions: Bristol Myers Squibb (study-related drugs provided to Institution), Genentech, Verastem. WH: None. DM: Speakers Bureau: Caris Life Sciences and Guardant Health. Steering Committee: Janssen. HB: Consultancy: Myovant, Corea Therapeutics, Novocure, Coherus BioSciences. Speakers Bureau: Guardant360. Research funds: Blue Earth, Novocure, Spirita Oncology. Tumor board: CARIS. Steering Committee/Medical Advisory Board: Novocure, Virogin Biotech, Idera, and JS InnoPharm. JB: Advisory board member: Insmed, Bayer, Mirati, Ipsen, QED, Oxford Biotherapeutics. Data and Safety Monitoring Board: Novocure, Pancreatic Cancer Action Network, Karyopharm. Research funding to institution: I-Mab, Dragonfly, Astellas, Atreca, AbbVie, Pfizer, Karyopharm, Boston Biomedical, Akamis Bio Ltd, EMD Serono, BMS; Grants (to Institution): Novartis, Abbvie, Bayer, Lilly, Incyte, EMD Serono, Dragonfly, I-Mab, Incyte, Pfizer, BMS, Transcenta, Totus, Tyra, 23 and me, Sumitomo Dainippon Pharma; Advisory board: Bayer, Mirati, Insmed, Oxford Biotherapeutics, Biosapien, EMD Serono, Ipsen, Merck, Merus; Data safety monitoring board: AstraZeneca, Novocure; Scientific and medical advisory board: Pancreatic Cancer Action Network, Debbie’s Dream Foundation; Nominating committee: ASCO. LR: Akamis Bio Ltd: Research Funding to Institution. TL, DK, CC, JC, MPa, LP, MPo: employees and stock options, Akamis Bio Ltd., (© Author(s) (or their employer(s)) 2023. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)