Background: In preclinical studies, combining M9241 (a novel immunocytokine containing interleukin (IL)-12 heterodimers) with avelumab (anti-programmed death ligand 1 antibody) resulted in additive or synergistic antitumor effects. We report dose-escalation and dose-expansion results from the phase Ib JAVELIN IL-12 trial investigating M9241 plus avelumab., Methods: In the dose-escalation part of JAVELIN IL-12 (NCT02994953), eligible patients had locally advanced or metastatic solid tumors; in the dose-expansion part, eligible patients had locally advanced or metastatic urothelial carcinoma (UC) that had progressed with first-line therapy. Patients received M9241 at 4, 8, 12, or 16.8 µg/kg every 4 weeks (Q4W) plus avelumab 10 mg/kg every 2 weeks (Q2W, dose levels (DLs) 1-4) or M9241 16.8 µg/kg Q4W plus avelumab 800 mg once a week for 12 weeks followed by Q2W (DL5/dose expansion). Primary endpoints for the dose-escalation part were adverse events (AEs) and dose-limiting toxicities (DLTs), and those for the dose-expansion part were confirmed best overall response (BOR) per investigator (Response Evaluation Criteria in Solid Tumors V.1.1) and safety. The dose-expansion part followed a two-stage design; 16 patients were enrolled and treated in stage 1 (single-arm part). A futility analysis based on BOR was planned to determine whether stage 2 (randomized controlled part) would be initiated., Results: At data cut-off, 36 patients had received M9241 plus avelumab in the dose-escalation part. All DLs were well tolerated; one DLT occurred at DL3 (grade 3 autoimmune hepatitis). The maximum-tolerated dose was not reached, and DL5 was declared the recommended phase II dose, considering an observed drug-drug interaction at DL4. Two patients with advanced bladder cancer (DL2 and DL4) had prolonged complete responses. In the dose-expansion part, no objective responses were recorded in the 16 patients with advanced UC; the study failed to meet the criterion (≥3 confirmed objective responses) to initiate stage 2. Any-grade treatment-related AEs occurred in 15 patients (93.8%), including grade ≥3 in 8 (50.0%); no treatment-related deaths occurred. Exposures for avelumab and M9241 concentrations were within expected ranges., Conclusions: M9241 plus avelumab was well tolerated at all DLs, including the dose-expansion part, with no new safety signals. However, the dose-expansion part did not meet the predefined efficacy criterion to proceed to stage 2., Competing Interests: Competing interests: JSt received institutional research funding from Bavarian Nordic, ImmunityBio, Merck, PDS Biotechnology, and Precigen. J-LD received honoraria from Bristol Myers Squibb, Janssen Oncology, and MSD; served as a consultant or advisor for Bristol Myers Squibb, Janssen Oncology, and Sanofi; and was a member of a speakers’ bureau for Astellas Pharma. MS served as a consultant or advisor for AbbVie, Adaptimmune, Agenus, Alligator Bioscience, Almac Diagnostics, Anaeropharma, Array BioPharma, AstraZeneca/MedImmune, BioNTech, Boehringer Ingelheim, Boston Pharmaceuticals, Bristol Myers Squibb, Chugai/Roche, Dragonfly Therapeutics, Genentech/Roche, Genmab, Genocea Biosciences, GI Innovation, Immunocore, Innate Pharma, Lilly, Modulate Pharma, Molecular Partners, Nektar, Newlink Genetics, Numab, Pieris Pharmaceuticals, Pierre Fabre, Seattle Genetics, SERVIER, Symphogen, Torque, Verastem, and Zelluna; owns stocks in Actym Therapeutics, Adaptive Biotechnologies, Amphivena, EvolveImmune Therapeutics, Intensity Therapeutics, Nextcure, and Torque; and had other relationships with CEC Oncology, Dava Oncology, Haymarket Media, and Physican’s Education Resource. AR received institutional research funding from Ipsen, Merck, and Pfizer; served as a consultant or advisor for AstraZeneca, Bristol Myers Squibb, Eisai, Ipsen, Merck, MSD, Novartis, Pfizer, and Roche; and received reimbursement for travel and accommodation expenses from AstraZeneca, Bristol Myers Squibb, Ipsen, Merck, MSD, Novartis, Pfizer, and Roche. MMar served as a consultant or advisor for Bristol Myers Squibb, Janssen Cilag, Ipsen, Merck, MSD, and Pfizer. RKP received research funding from Exelixis, Janssen Oncology, and Pharmacyclics; served as a consultant or advisor for AstraZeneca, Bayer, Blue Earth Diagnostics, Bristol Myers Squibb, Dendreon, Genomic Health, Jounce Therapeutics, Merck, Pfizer, and Sanofi; was a member of a speakers’ bureau for AstraZeneca, Dendreon, Genentech/Roche, Genomic Health, MSD, and Sanofi; and received reimbursement for travel and accommodation expenses from Genentech/Roche. TSG received research funding from Ferring and served as a consultant or advisor for Exelixis, Pfizer, and Merck. MMai received honoraria from Alfasigma, Amgen, AstraZeneca, Bristol Myers Squibb, Merck, MSD, Pierre Fabre, and Roche; served as a consultant or advisor for Alfasigma, AstraZeneca, Bristol Myers Squibb, Merck, MSD, Pierre Fabre, and Roche; owned patents, royalties, and other intellectual properties for ‘DNA hypomethylating agents for cancer therapy’; received reimbursement for travel and accommodation expenses from Alfasigma, Amgen, AstraZeneca, Bristol Myers Squibb, Merck, MSD, Pierre Fabre, and Roche; and owned stock in Theravance. LD had no competing interests. JSc received institutional research funding from Bavarian Nordic, ImmunityBio, Incyte, Merck, NextCure, PDS Biotechnology, and Precigen. RND received institutional research funding from Bavarian Nordic, ImmunityBio, Incyte, Merck, NextCure, PDS Biotechnology, and Precigen. Y-TT received institutional research funding from Bavarian Nordic, ImmunityBio, Incyte, Merck, NextCure, PDS Biotechnology, and Precigen. XW and YV are employed by EMD Serono Research & Development Institute, Inc., Billerica, MA, USA, an affiliate of Merck KGaA. FB, JSe, and AS are employed by Merck. SC was employed by Merck at the time of study. JLG received institutional research funding from Bavarian Nordic, ImmunityBio, Incyte, Janssen Oncology, and Merck., (© 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.)