Background: Inebilizumab, an anti-CD19 B-cell-depleting antibody, demonstrated safety and efficacy in neuromyelitis optica spectrum disorder in the randomised controlled period of the N-MOmentum trial. Here, end-of-study data, including the randomised controlled period and open-label extension period, are reported., Methods: In the double-blind, randomised, placebo-controlled, phase 2/3 N-MOmentum trial, adults aged 18 years and older with an neuromyelitis optica spectrum disorder diagnosis, Expanded Disability Status Scale score of 8·0 or less, and history of either at least one acute inflammatory attack requiring rescue therapy in the past year or two attacks requiring rescue therapy in the past 2 years, were recruited from 81 outpatient specialty clinics or hospitals in 24 countries. Eligible participants were randomly assigned (3:1), using a central interactive voice system or interactive web response system, and a permuted block randomisation scheme (block size of 4), to receive intravenous inebilizumab (300 mg) or identical placebo on days 1 and 15 of the randomised period, which lasted up to 197 days. Participants and all study staff were masked to treatment assignment. The primary endpoint of the randomised period of the trial was time to onset of adjudicated neuromyelitis optica spectrum disorder attack on or before day 197. Participants in the randomised controlled period who had an adjudicated attack, completed 197 days in the study, or were in the randomised controlled period when enrolment stopped, could voluntarily enter the open-label period. In the open-label period, participants either initiated inebilizumab if assigned placebo (receiving 300 mg on days 1 and 15 of the open-label period) or continued treatment if assigned inebilizumab (receiving 300 mg on day 1 and placebo on day 15, to maintain B-cell depletion and masking of the randomised controlled period). All participants subsequently received inebilizumab 300 mg every 6 months for a minimum of 2 years. The end-of-study analysis endpoints were time to adjudicated attack and annualised attack rate (assessed in all participants who received inebilizumab at any point during the randomised controlled period or open-label period [any inebilizumab population] and the aquaporin-4 [AQP4]-IgG seropositive subgroup [any inebilizumab-AQP4-IgG seropositive population]) and safety outcomes (in all participants who were exposed to inebilizumab, analysed as-treated). This study is registered with ClinicalTrials.gov, NCT02200770, and is now complete., Findings: Between Jan 6, 2015, and Sept 24, 2018, 467 individuals were screened, 231 were randomly assigned, and 230 received at least one dose of inebilizumab (n=174) or placebo (n=56). Between May 19, 2015, and Nov 8, 2018, 165 (95%) of 174 participants in the inebilizumab group and 51 (91%) of 56 in the placebo group entered the open-label period (mean age 42·9 years [SD 12·4], 197 [91%] of 216 were female, 19 [9%] were male, 115 [53%] were White, 45 [21%] were Asian, 19 [9%] were American Indian or Alaskan Native, and 19 [9%] were Black or African American). As of data cutoff for this end of study analysis (Dec 18, 2020; median exposure 1178 days [IQR 856-1538], total exposure of 730 person-years) 225 participants formed the any inebilizumab population, and 208 (92%) participants were AQP4-IgG seropositive. Overall, 63 adjudicated neuromyelitis optica spectrum disorder attacks occurred in 47 (21%) of 225 treated participants (60 attacks occurred in 44 [21%] of 208 in the AQP4-IgG seropositive subgroup); 40 (63%) of 63 attacks occurred in 34 (15%) of 225 treated participants during the first year of treatment. Of individuals who had an adjudicated attack while receiving inebilizumab, 36 (77%) of 47 were subsequently attack-free at the end of 4 years. Annualised attack rates decreased year-on-year, with end-of-study adjusted annualised attack rates being similar in the any inebilizumab-AQP4-IgG seropositive subgroup (0·097 [95% CI 0·070-0·14]) and any inebilizumab populations (0·092 [0·067-0·13]). Overall, 208 (92%) of 225 participants who received any inebilizumab had at least one treatment-emergent adverse event, the most frequent of which were urinary tract infection (59 [26%]), nasopharyngitis (47 [21%]), and arthralgia (39 [17%]). Infection rates did not increase over 4 years. Three (1%) of 225 participants in the any inebilizumab population died during the open-label period (one each due to a CNS event of unknown cause and pneumonia, respiratory insufficiency resulting from an neuromyelitis optica spectrum disorder attack and viral pneumonia related to COVID-19), all of which were deemed to be unrelated to treatment., Interpretation: Data from the end-of-study analysis of the N-MOmentum trial showed continued and sustained clinical benefits of long-term inebilizumab treatment in individuals with neuromyelitis optica spectrum disorder, which supports the role of inebilizumab as a CD19+ B-cell-depleting therapy in neuromyelitis optica spectrum disorder., Funding: MedImmune and Viela Bio/Horizon Therapeutics, now part of Amgen., Competing Interests: Declaration of interests BACC reports personal fees for consulting from Alexion, Atara, Autobahn, Avotres, Biogen, Boston Pharma, EMD Serono, Gossamer Bio, Hexal/Sandoz, Horizon Therapeutics, Neuron23, Novartis, Sanofi, Siemens, TG Therapeutics, and Therini Bio, and has received research support from Genentech. HJK has received a grant from the National Research Foundation of Korea and research support from AprilBio, Eisai, and UCB; consultancy or speaker fees from Alexion, Altos Biologics, AstraZeneca, Biogen, Daewoong Pharmaceutical, Eisai, GC Pharma, Handok Pharmaceuticals, Kaigene, Kolon Life Science, MDimune, Merck Serono, Mitsubishi Tanabe Pharma, Roche, and Sanofi Genzyme; serves on a steering committee for MedImmune/Viela Bio; and is a co-editor for the Multiple Sclerosis Journal and an associate editor for the Journal of Clinical Neurology. BGW receives payments for serving as chair of attack adjudication committees for clinical trials in neuromyelitis optica spectrum disorder for Alexion, MedImmune, UCB Bioscience, and Viela Bio/Horizon Therapeutics; has consulted with CANbridge Pharmaceuticals, Chugai, Genentech, Horizon Therapeutics, Mitsubishi Tanabe Pharma, and Roche Pharmaceuticals; has received payments for speaking for Genentech, Horizon Therapeutics, and Roche; and has a patent for NMO-IgG for diagnosis of neuromyelitis optica, with royalties paid by Hospices Civils de Lyon, MVZ Labor PD Dr Volkmann und Kollegen GbR, RSR Limited, and the University of Oxford. SJP reports grants, personal fees, and non-financial support from Alexion; grants from Autoimmune Encephalitis Alliance and Grifols; grants, personal fees, and non-financial support from MedImmune and Viela Bio/Horizon Therapeutics; consulting support from Astellas, Genentech, and Sage Therapeutics; personal fees for consulting services from for F Hoffman-La Roche AG, Genentech, and UCB; and has a patent #9,891,219 (Application#12-573942) “Methods for Treating Neuromyelitis Optica (NMO) by Administration of Eculizumab to an individual that is Aquaporin-4 (AQP4)-IgG Autoantibody positive”. DMW reports personal fees from Alexion, AstraZeneca, Bristol Myers Squibb, Genentech, Horizon Therapeutics, Imcyse, Merck, Novartis, and Reistone Biopharma; serves on a clinical trial adjudication committee for MedImmune, Viela Bio/Horizon Therapeutics, and UCB Pharma; and serves on a data and safety monitoring board for Alexion and Abscuro. KF serves on scientific advisory boards for Alexion, Bayer Schering, Biogen, Chugai, MedImmune, Merck Serono, Mitsubishi Tanabe Pharma, Nihon Pharmaceutical, Novartis, Ono Pharmaceutical, and Viela Bio/Horizon Therapeutics; has received funding for travel and speaker fees from Asahi Kasei Medical, Astellas, Bayer Schering, Biogen, Daiichi Sankyo, Dainippon Sumitomo, Eisai, Mitsubishi Tanabe Pharma, Nihon Pharmaceutical, Novartis, and Takeda; and research support from Asahi Kasei Medical, Bayer Schering, Biogen, Chemo-Sero-Therapeutic Research Institute, Chugai, Genzyme Japan, the Ministry of Education, Culture, Sports, Science, and Technology of Japan, the Ministry of Health, Welfare, and Labor of Japan, Mitsubishi Tanabe Pharma, Nihon Pharmaceutical, Ono Pharmaceutical, Teijin, and Teva. FP has received research support, speaker fees, and travel reimbursement from Bayer, Biogen, Merck Serono, Novartis, Sanofi Genzyme, and Teva; is supported by the German Competence Network for Multiple Sclerosis and the German Research Council (DFG Exc 257); has received travel reimbursement from the Guthy–Jackson Charitable Foundation; and serves on the steering committee of the OCTiMS study sponsored by Novartis. GRC has received personal fees for participation on data and safety monitoring boards from AI Therapeutics, AMO Pharma, Applied Therapeutics, AstraZeneca, Avexis Pharmaceuticals, BioLineRx, Brainstorm Cell Therapeutics, Bristol Myers Squibb/Celgene, CSL Behring, Galmed Pharmaceuticals, Green Valley Pharma, Horizon Therapeutics, Immunic Therapeutics, Karuna Therapeutics, Mapi Pharma, Merck, Mitsubishi Tanabe Pharma, the National Heart, Lung, and Blood Institute (US; Protocol Review Committee), Novartis, OPKO Biologics, Prothena Biosciences, Reata Pharmaceuticals, Regeneron, Sanofi-Aventis, Teva Pharmaceuticals, the University of Texas Southwestern, the University of Pennsylvania, and Visioneering Technologies; personal fees for consulting or advisory board participation from Alexion, Antisense Therapeutics, Biogen, Clinical Trial Solutions, Entelexo Biotherapeutics, Genentech, Genzyme, GW Pharmaceuticals, Immunic Therapeutics, Immunosis, Klein Buendel, Merck/Serono, Novartis, Osmotica Pharmaceuticals, Perception Neurosciences, Protalix BioTherapeutics, Recursion/CereXis, Regeneron, Roche, and SAB Biotherapeutics. GRC is employed by the University of Alabama at Birmingham and is President of Pythagoras, a private consulting company based in Birmingham, AL, USA. RM serves on scientific advisory boards for MedImmune and Viela Bio/Horizon Therapeutics; and has received funding for travel and fees from Alexion, Biogen, Merck Serono, Novartis, Roche, Sanofi Genzyme, Teva, and Viela Bio/Horizon Therapeutics. AJG reports grants from the Conrad N Hilton Foundation and the Tom Sherak MS Hope Foundation; other financial relationships for activities as expert witness, associate editor, advisory board or steering committee participation, and endpoint adjudication with Bionure, Inception Sciences, JAMA Neurology, MedImmune/Viela Bio, Mylan, Synthon, and Trims Pharma; and personal fees from Pipeline Therapeutics. OA reports grants from the German Ministry of Education and Research (known as BMBF) and the German Research Foundation (known as DFG); grants and personal fees from Bayer HealthCare, Biogen, Genzyme, Novartis, Teva, and Viela Bio/Horizon Therapeutics; and personal fees from Alexion, Almirall, Horizon Therapeutics, Merck Serono, Roche, and Zambon. H-PH has received fees for consulting, speaking, and serving on steering committees from Bayer HealthCare, Biogen, Celgene Receptos, CSL Behring, GeNeuro, Genzyme, MedDay, MedImmune, Merck Serono, Novartis, Roche, Sanofi, TG Therapeutics, and Viela Bio with approval by the Rector of Heinrich Heine University Düsseldorf. DS, WR, MS, and DC hold stock or are employees of Viela Bio/Horizon Therapeutics, or both. EK was an employee of Viela Bio/Horizon Therapeutics during the preparation of this manuscript. JLB reports payment for study design or consultation from MedImmune/Viela Bio; personal fees from AbbVie, Alexion, Antigenomycs, BeiGene, Chugai, Clene Nanomedicine, EMD Serono, Genentech, Genzyme, Horizon Therapeutics, Mitsubishi Tanabe Pharma, Novartis, Reistone Biopharma, Roche, and TG Therapeutics; grants and personal fees from Novartis; grants from Alexion, the Guthy–Jackson Charitable Foundation, Mallinckrodt, and the National Institutes of Health; and has a patent for aquaporumab issued (US10654916B2: Compositions and methods for the treatment of neuromyelitis optica)., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)