BACKGROUND: Given burdensome side-effects and long latency for efficacy with conventional agents, there is a continued need for generalised myasthenia gravis treatments that are safe and provide consistently sustained, long-term disease control. Nipocalimab, a neonatal Fc receptor blocker, was associated with dose-dependent reductions in total IgG and anti-acetylcholine receptor (AChR) antibodies and clinically meaningful improvements in the Myasthenia Gravis Activities of Daily Living (MG-ADL) scale in patients with generalised myasthenia gravis in a phase 2 study. We aimed to assess the safety and efficacy of nipocalimab in a phase 3 study. METHODS: Vivacity-MG3 was a phase 3, randomised, double-blind, placebo-controlled, phase 3 study conducted at 81 outpatient centres with expertise in myasthenia gravis in 17 countries in Asia-Pacific, Europe, and North America. Adults (aged ≥18 years) with generalised myasthenia gravis inadequately controlled with standard-of-care therapy (MG-ADL score ≥6) were randomly assigned (1:1) to either nipocalimab (30 mg/kg loading dose then 15 mg/kg every 2 weeks for maintenance dosing) or placebo infusions every 2 weeks, added to standard-of-care therapy in both groups, for 24 weeks. Randomisation was stratified by antibody status, day 1 MG-ADL total score, and region. The sponsor, investigators, clinical raters, and participants were masked to treatment assignment. The primary endpoint was the difference between nipocalimab and placebo based on least-squares mean change from baseline in MG-ADL total score averaged over weeks 22, 23, and 24 in the intention-to-treat population of patients who were antibody-positive (for AChR, anti-muscle-specific tyrosine kinase [MuSK], or anti-low-density lipoprotein receptor-related protein 4 [LRP4]). Adverse events were assessed in patients who received at least one dose of study drug. This study is registered at ClinicalTrials.gov, NCT04951622; the double-blind phase is completed and an open-label extension phase is ongoing. FINDINGS: Between July 15, 2021, and Nov 17, 2023, 199 patients were enrolled, and 196 patients received study drug (98 in the nipocalimab group and 98 in the placebo group); of these, 153 (77 in the nipocalimab group and 76 in the placebo group) were antibody-positive. The least-squares mean change in MG-ADL score from baseline to weeks 22, 23, and 24 was -4·70 (SE 0·329) in the nipocalimab group versus -3·25 (0·335) in the placebo group (difference -1·45 [95% CI -2·38 to -0·52]; p=0·0024). The incidence of adverse events was similar between groups (82 [84%] of 98 in both the nipocalimab and placebo groups), including infections (42 [43%] of 98 in the nipocalimab group and placebo group) and headache (14 [14%] of 98 in the nipocalimab group and 17 [17%] of 98 in the placebo group). Serious adverse events were reported for nine (9%) of 98 patients in the nipocalimab group and 14 (14%) of 98 patients in the placebo group, three of which had a fatal outcome (nipocalimab: myasthenic crisis; placebo: cardiac arrest and myocardial infarction). INTERPRETATION: Results from the completed double-blind phase of Vivacity-MG3 support the role of nipocalimab, added to standard-of-care therapies, as a safe treatment for sustained disease control over 6 months for a broad population of patients with generalised myasthenia gravis who are antibody-positive. The ongoing open-label extension phase should provide longer term sustained safety and efficacy data with nipocalimab. FUNDING: Janssen Research & Development, LLC, a Johnson & Johnson company.

Centrum Medyczne NeuroProtect Warszawa Poland

Department of Medicine Hospital Universitari i Politècnic and IIS La Fe and University of Valencia Valencia Spain

Department of Medicine University of Toronto University Health Network Toronto ON Canada

Department of Neurology Ghent University Hospital Ghent Belgium

Department of Neurology Masaryk University and St Anne's Hospital Brno Czechia

Department of Neurology National Hospital Organisation Sendai Medical Centre Sendai Japan

Department of Neurology Neuroscience Clinical Research Center and Integrated Myasthenia Gravis Centre Charité Universitätsmedizin Corporate Member of Freie Universität Berlin and Humboldt Universität zu Berlin Berlin Germany

Department of Neurology School of Medicine Kyungpook National University Chilgok Hospital Daegu South Korea

Department of Neurology University of Kansas Medical Centre Kansas City KS USA

Department of Neurology University of South Florida Morsani College of Medicine Tampa FL USA

Department of Neurology Xiangya Hospital Central South University Hunan China

Department of Neurology Yale University School of Medicine New Haven CT USA

Global Medical Affairs Janssen Global Services a Johnson and Johnson Company Raritan NJ USA

Janssen Research and Development a Johnson and Johnson Company Titusville NJ USA

Neuroimmunology and Neuromuscular Diseases Unit and Apheresis and Immunotherapy Unit IRCCS Carlo Besta Neurological Institute Foundation Milan Italy

Silesian Neurology Medical Centre Katowice Poland

Statistics and Decision Sciences Janssen Research and Development a Johnson and Johnson Company Titusville NJ USA

The Neuromuscular Research Centre and Neuromuscular Clinic of Arizona Phoenix AZ USA

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