Background: Generalised myasthenia gravis is a chronic, unpredictable, and debilitating rare disease, often accompanied by high treatment burden and with an unmet need for more efficacious and well tolerated treatments. Zilucoplan is a subcutaneous, self-administered macrocyclic peptide complement C5 inhibitor. We aimed to assess safety, efficacy, and tolerability of zilucoplan in patients with acetylcholine receptor autoantibody (AChR)-positive generalised myasthenia gravis., Methods: RAISE was a randomised, double-blind, placebo-controlled, phase 3 trial that was done at 75 sites in Europe, Japan, and North America. We enrolled patients (aged 18-74 years) with AChR-positive generalised myasthenia gravis (Myasthenia Gravis Foundation of America disease class II-IV), a myasthenia gravis activities of daily living (MG-ADL) score of least 6, and a quantitative myasthenia gravis score of at least 12. Participants were randomly assigned (1:1) to receive subcutaneous zilucoplan 0·3 mg/kg once daily by self-injection, or matched placebo, for 12 weeks. The primary efficacy endpoint was change from baseline to week 12 in MG-ADL score in the modified intention-to-treat population (all randomly assigned patients who received at least one dose of study drug and had at least one post-dosing MG-ADL score). Safety was mainly assessed by the incidence of treatment-emergent adverse events (TEAEs) in all patients who had received at least one dose of zilucoplan or placebo. This trial is registered at ClinicalTrials.gov, NCT04115293. An open-label extension study is ongoing (NCT04225871)., Findings: Between Sept 17, 2019, and Sept 10, 2021, 239 patients were screened for the study, of whom 174 (73%) were eligible. 86 (49%) patients were randomly assigned to zilucoplan 0·3 mg/kg and 88 (51%) were assigned to placebo. Patients assigned to zilucoplan showed a greater reduction in MG-ADL score from baseline to week 12, compared with those assigned to placebo (least squares mean change -4·39 [95% CI -5·28 to -3·50] vs -2·30 [-3·17 to -1·43]; least squares mean difference -2·09 [-3·24 to -0·95]; p=0·0004). TEAEs occurred in 66 (77%) patients in the zilucoplan group and in 62 (70%) patients in the placebo group. The most common TEAE was injection-site bruising (n=14 [16%] in the zilucoplan group and n=8 [9%] in the placebo group). Incidences of serious TEAEs and serious infections were similar in both groups. One patient died in each group; neither death (COVID-19 [zilucoplan] and cerebral haemorrhage [placebo]) was considered related to the study drug., Interpretation: Zilucoplan treatment showed rapid and clinically meaningful improvements in myasthenia gravis-specific efficacy outcomes, had a favourable safety profile, and was well tolerated, with no major safety findings. Zilucoplan is a new potential treatment option for a broad population of patients with AChR-positive generalised myasthenia gravis. The long-term safety and efficacy of zilucoplan is being assessed in an ongoing open-label extension study., Funding: UCB Pharma., Competing Interests: Declaration of interests JFH has received research support (paid to his institution) from Alexion Pharmaceuticals, argenx BVBA, Cartesian Therapeutics, the US Centers for Disease Control and Prevention, the Myasthenia Gravis Foundation of America, the Muscular Dystrophy Association, the US National Institutes of Health (including the National Institute of Neurological Disorders and Stroke and the National Institute of Arthritis and Musculoskeletal and Skin Diseases), PCORI, Ra Pharmaceuticals (now UCB Biosciences), and Takeda Pharmaceuticals; honoraria from Alexion Pharmaceuticals, argenx BVBA, F Hoffman-LaRoche, Immunovant, Ra Pharmaceuticals (now UCB Biosciences), Regeneron Pharmaceuticals, Sanofi US, and Viela Bio (now Horizon Therapeutics); and non-financial support from Alexion Pharmaceuticals, argenx BVBA, Ra Pharmaceuticals (now UCB Biosciences), and Toleranzia. SB has served as a paid consultant for Sanofi Genzyme, Merck, Alexion, UCB, Biogen, and Bristol Myers Squibb. AG has served as a paid consultant for Medtronic, Atlantic Research Group, Calico, Apellis, Anexon, ALS Pharmaceuticals, QurAlis, Orion, Sanofi Genzyme, Ionis, Wave Life Therapies, Anelixis, Roche, Cytokinetics, Mitsubishi Tanabe Pharma, Amylyx, Alexion, UCB, Ra Pharmaceuticals (now UCB Biosciences), Biogen, Eli Lilly, and Amicus Therapeutics. CH has received funding for consultancy on scientific or educational advisory boards for UCB Pharma, argenx, Lupin, Roche and Biogen. HJK is a consultant for Roche, Cabaletta Bio, Lincoln Therapeutics, Takeda, and UCB Pharmaceuticals; and is chief executive officer and chief marketing office of ARC Biotechnology, based on US Patent 8,961,98. He is principal investigator of the Rare Disease Network for Myasthenia Gravis National Institute of Neurological Disorders & Stroke, and Targeted Therapy for Myasthenia Gravis. AM received funding for travel, meeting attendance, or advisory board participation from CSL Behring and UCB. RM received funding for travel, meeting attendance, or advisory board participation from Alexion, argenx, BioMarin, Catalyst, Sanofi, Regeneron, and UCB. MM has served as a paid consultant for argenx and Alexion Pharmaceuticals, and has received speaker honoraria from Asahi Kasei Medical, argenx and Alexion Pharmaceuticals. KU has served as a paid consultant for UCB Pharma, argenx, Janssen Pharma, Viela Bio, Chugai Pharma, Hanall BioPharma, and Mitsubishi Tanabe Pharma; and has received speaker honoraria from argenx, Alexion Pharmaceuticals, and the Japan Blood Products Organization. TV is the USF Site Principal Investigator for myasthenia gravis clinical trials sponsored by Alexion/AstraZeneca, argenx, Ra/UCB, Horizon/Viela Bio, Janssen/Momenta, Sanofi, Regeneron, and Cartesian Therapeutics; and has received speaking or consulting honoraria from Alexion, argenx, and UCB. MDW has received honoraria for serving on scientific advisory boards for Alexion, UCB-Ra, argenx, Biogen, Mitsubishi Tanabe Pharma, and Amylyx; consulting honoraria from Cytokinetics and CSL Behring; and speaker honoraria from Soleo Health. He also serves as a special government employee for the US Food and Drug Administration. MZ has served as a paid consultant for Signant Health. MZ was an employee of Medical University of Warsaw, Warsaw, Poland, during the conduct of the study, but is currently employed by MTZ Clinical Research powered by Pratia. BB, Melissa Brock, GdlB, PWD, and MV are employees and shareholders of UCB Pharma. RL was a Veramed consultant for UCB Pharma during the conduct of the study. Veramed are a full-service provider for UCB Pharma. RL is employed by ONO Pharma UK. MIL is funded by the UK National Health Service (Myasthenia and Related Disorders Service and National Specialised Commissioning Group for Neuromyelitis Optica, UK) and by the University of Oxford, Oxford, UK. She has been awarded research grants from the UK association for patients with myasthenia, Myaware, and the University of Oxford. She has received speaker honoraria or travel grants from Biogen Idec, Novartis, argenx, UCB, and the Guthy-Jackson Charitable Foundation. MIL serves on scientific or educational advisory boards for UCB Pharma, argenx, and Viela/Horizon. All other authors declare no competing interests., (Copyright © 2023 Elsevier Ltd. All rights reserved.)