Purpose: This phase I study aimed to define the recommended phase II dose (RP2D) of tebentafusp, a first-in-class T-cell receptor/anti-CD3 bispecific protein, using a three-week step-up dosing regimen, and to assess its safety, pharmacokinetics, pharmacodynamics, and preliminary clinical activity in patients with metastatic uveal melanoma (mUM)., Methods: In this open-label, international, phase I/II study, HLA-A*02 or HLA-A*02:01+ patients with mUM received tebentafusp 20 μg once in week 1 and 30 μg once in week 2. Dose escalation (starting at 54 μg) began at week 3 in a standard 3 + 3 design to define RP2D. Expansion-phase patients were treated at the RP2D (20-30-68 μg). Blood and tumor samples were collected for pharmacokinetics/pharmacodynamics assessment, and treatment efficacy was evaluated for all patients with baseline efficacy data as of December 2017., Results: Between March 2016 and December 2017, 42 eligible patients who failed a median of two previous treatments were enrolled: 19 in the dose escalation cohort and 23 in an initial dose expansion cohort. Of the dose levels investigated, 68 μg was identified as the RP2D. Most frequent treatment-emergent adverse events regardless of attribution were pyrexia (91%), rash (83%), pruritus (83%), nausea (74%), fatigue (71%), and chills (69%). Toxicity attenuated following the first three doses. The overall response rate was 11.9% (95% CI, 4.0 to 25.6). With a median follow-up of 32.4 months, median overall survival was 25.5 months (range, 0.89-31.1 months) and 1-year overall survival rate was 67%. Treatment was associated with increased tumor T-cell infiltration and transient increases in serum inflammatory mediators., Conclusion: Using a step-up dosing regimen of tebentafusp allowed a 36% increase in the RP2D compared with weekly fixed dosing, with a manageable side-effect profile and a signal of efficacy in mUM., Competing Interests: Richard D. CarvajalConsulting or Advisory Role: Merck, Aura Biosciences, Castle Biosciences, Immunocore, PureTech, Sorrento Therapeutics, Chimeron Bio, Rgenix, InxMed, Pierre Fabre, TriSalus Life Sciences, Iovance Biotherapeutics, Oncosec, Regeneron, Genzyme, Amgen (Inst), Astellas Pharma (Inst), AstraZeneca (Inst), Bristol Myers Squibb/Medarex (Inst), Corvus Pharmaceuticals (Inst), Ideya (Inst), Mirati Therapeutics (Inst), Novartis (Inst), Pfizer (Inst), Plexxikon (Inst), Roche/Genentech (Inst)Speakers' Bureau: Bristol Myers Squibb/MedarexResearch Funding: Amgen (Inst), Astellas Pharma (Inst), AstraZeneca (Inst), Bayer (Inst), Bellicum Pharmaceuticals (Inst), Bristol Myers Squibb (Inst), Corvus Pharmaceuticals (Inst), Lilly (Inst), Immunocore (Inst), Incyte (Inst), Macrogenics (Inst), Merck (Inst), Mirati Therapeutics (Inst), Novartis (Inst), Pfizer (Inst), Plexxikon (Inst), Roche/Genentech (Inst), Array BioPharma (Inst), IDEAYA Biosciences (Inst), Regeneron (Inst) Paul NathanConsulting or Advisory Role: AstraZeneca, Bristol Myers Squibb, MSD, Immunocore, Pfizer, Pierre Fabre, Novartis, GlaxoSmithKline, Ipsen, 4SC, MerckSpeakers' Bureau: Bristol Myers Squibb, Novartis, MSD, MerckTravel, Accommodations, Expenses: Bristol Myers Squibb, MSD Joseph J. SaccoHonoraria: Immunocore, Pierre Fabre, NovartisConsulting or Advisory Role: Immunocore (Inst), Delcath Systems, MSD, BMS, ImmunocoreResearch Funding: AstraZeneca (Inst), Immunocore (Inst), Replimune (Inst), BMS (Inst)Travel, Accommodations, Expenses: BMS, MSD Marlana OrloffConsulting or Advisory Role: Immunocore, TriSalus Life Sciences, IDEAYA BiosciencesSpeakers' Bureau: Bristol Myers SquibbResearch Funding: Bristol Myers Squibb (Inst), Immunocore (Inst), Delcath Systems (Inst), Plexxikon (Inst), IDEAYA Biosciences (Inst), Linnaeus Therapeutics (Inst) Leonel F. Hernandez-AyaConsulting or Advisory Role: Massive Bio, Bristol Myers Squibb, Castle BiosciencesSpeakers' Bureau: Sanofi/RegeneronResearch Funding: Bristol Myers Squibb (Inst), Regeneron (Inst), Immunocore (Inst), Merck (Inst), Polynoma (Inst), Corvus Pharmaceuticals (Inst), Roche/Genentech (Inst), Merck Serono (Inst), Amgen (Inst), MedImmune (Inst), Takeda (Inst), Moderna Therapeutics (Inst), Foghorn Therapeutics (Inst)Travel, Accommodations, Expenses: Sanofi/Regeneron, Bristol Myers Squibb, Castle Biosciences Jason J. LukeStock and Other Ownership Interests: Actym Therapeutics, Mavu Pharmaceutical, Pyxis, Alphamab, Tempest Therapeutics, Kanaph Therapeutics, Onc.AI, Arch OncologyConsulting or Advisory Role: Bristol Myers Squibb, Merck, EMD Serono, Novartis, 7 Hills Pharma, Janssen, Reflexion Medical, Tempest Therapeutics, Alphamab, Spring Bank, AbbVie, Astellas Pharma, Bayer, Incyte, Mersana, Partner Therapeutics, Synlogic, Eisai, Werewolf Therapeutics, Ribon Therapeutics, Checkmate Pharmaceuticals, CStone Pharmaceuticals, Nektar, Regeneron, Rubius Therapeutics, Tesaro, Xilio Therapeutics, Xencor, Alnylam, Crown Bioscience, Flame Biosciences, Genentech, Kadmon, KSQ Therapeutics, Immunocore, Inzen Therapeutics, Pfizer, Silicon Therapeutics, TRex BioResearch Funding: Merck (Inst), Bristol Myers Squibb (Inst), Incyte (Inst), Corvus Pharmaceuticals (Inst), AbbVie (Inst), Macrogenics (Inst), Xencor (Inst), Array BioPharma, Agios, Astellas Pharma, EMD Serono, Immatics, Kadmon, Moderna Therapeutics, Nektar, Spring bank, Trishula Therapeutics, KAHR Medical, Fstar, Genmab, Ikena Oncology, Numab, Replimune, Rubius Therapeutics, Synlogic, Takeda, Tizona Therapeutics Inc, Trishula Therapeutics, BioNTech, Scholar RockPatents, Royalties, Other Intellectual Property: Serial #15/612,657 (Cancer Immunotherapy), Serial #PCT/US18/36052 (Microbiome Biomarkers for Anti–PD-1/PD-L1 Responsiveness: Diagnostic, Prognostic and Therapeutic Uses Thereof)Travel, Accommodations, Expenses: Bristol Myers Squibb, Array BioPharma, EMD Serono, Janssen, Merck, Novartis, Reflexion Medical, Mersana, Pyxis, Xilio Therapeutics Marcus O. ButlerHonoraria: Roche, Merck, Bristol Myers Squibb, NovartisConsulting or Advisory Role: Merck, Bristol Myers Squibb, Novartis, Immunovaccine, Immunocore, Adaptimmune, EMD Serono, GlaxoSmithKline, Genzyme, Sanofi, LaRoche Posay, Sun Pharma, Instil Bio, Iovance Biotherapeutics, PfizerResearch Funding: Merck, Takara BioExpert Testimony: Merck Sarah StanhopeEmployment: ImmunocoreStock and Other Ownership Interests: Immunocore Laura CollinsEmployment: ImmunocoreStock and Other Ownership Interests: Immunocore Cheryl McAlpineEmployment: Immunocore, Adaptimmune Chris HollandEmployment: ImmunocoreStock and Other Ownership Interests: Immunocore, Amgen, MacroGenicsPatents, Royalties, Other Intellectual Property: Coinvolvement in a patent for the compositions and methods for treating diffuse large B-cell lymphoma Shaad E. AbdullahEmployment: ImmunocoreStock and Other Ownership Interests: ImmunocorePatents, Royalties, Other Intellectual Property: AU2019270277A1—Treatment of Cancer, WO2020225552A1—Combination of monalizumab, durvalumab, chemotherapy, and bevacizumab or cetuximab for the treatment of colorectal cancer Takami SatoConsulting or Advisory Role: Immunocore, Castle BiosciencesResearch Funding: Immunocore (Inst), Verastem (Inst)Travel, Accommodations, Expenses: Castle BiosciencesNo other potential conflicts of interest were reported.