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BACKGROUND:Dermatomyositis is a chronic autoimmune disease with distinctive cutaneous eruptions and muscle weakness, and the pathophysiology is characterised by type I interferon (IFN) dysregulation. This study aims to assess the efficacy, safety, and target engagement of dazukibart, a potent, selective, humanised IgG1 neutralising monoclonal antibody directed against IFNβ, in adults with moderate-to-severe dermatomyositis. METHODS:This multicentre, double-blind, randomised, placebo-controlled, phase 2 trial was conducted at 25 university-based hospitals and outpatient sites in Germany, Hungary, Poland, Spain, and the USA. Adults aged 18-80 years with skin-predominant dermatomyositis were enrolled during stages 1, 2, and 2a, and had to have a Cutaneous Dermatomyositis Disease Area and Severity Index-Activity (CDASI-A) score of 14 or more and at least one unsuccessful systemic treatment with standard of care; whereas those with muscle-predominant dermatomyositis were enrolled in stage 3 and had to have active moderate muscle involvement. Patients were randomly assigned using an interactive response technology system to dazukibart 600 mg or placebo in stage 1; dazukibart 600 mg, dazukibart 150 mg, or placebo in stage 2; dazukibart 600 mg then placebo, dazukibart 150 mg then placebo, placebo then dazukibart 600 mg, or placebo then dazukibart 150 mg in stage 2a; and dazukibart 600 mg then placebo or placebo then dazukibart 600 mg in stage 3. For stage 2a and stage 3, treatments were switched at week 12. Patients, investigators, outcome assessors, and funders were masked to the treatment assignment. Dazukibart and placebo were administered intravenously on day 1 every 4 weeks, up to and including week 8 (stages 1 and 2, and stages 2a and 3 for patients starting dazukibart), or on week 12 every 4 weeks, up to and including week 20 (stages 2a and 3 for patients who started placebo and switched to dazukibart). The primary outcome for the skin-predominant cohorts was the change from baseline in CDASI-A score at week 12 assessed in the full analysis set (FAS; stage 1) and the pooled skin FAS (stages 1, 2, and 2a), and safety in the muscle-predominant cohort. This study is registered with ClinicalTrials.gov, NCT03181893. FINDINGS/RESULTS:Between Jan 23, 2018, and Feb 23, 2022, 125 adults were assessed and 50 were excluded. 75 patients were randomly assigned and treated (15 to dazukibart 150 mg, 37 to dazukibart 600 mg, and 23 to placebo). Most patients were female (53 [93%] of 57 in the skin-predominant cohort vs 13 [72%] of 18 in the muscle-predominant cohort and four [7%] vs five [28%] were male). In the FAS in stage 1 at week 12, the mean change from baseline in CDASI-A for dazukibart 600 mg was -18·8 (90% CI -21·8 to -15·8; placebo-adjusted difference -14·8 [-20·3 to -9·4]; p<0·0001). In the pooled skin FAS at week 12, the mean change from baseline in CDASI-A for the dazukibart 600 mg group was -19·2 (-21·5 to -16·8; placebo-adjusted difference -16·3 [-20·4 to -12·1]; p<0·0001), whereas the dazukibart 150 mg group was -16·6 (-19·8 to -13·4; placebo-adjusted difference -13·7 [-18·3 to -9·0]; p<0·0001). Treatment-emergent adverse events occurred in 12 (80%) of 15 patients in the dazukibart 150 mg group versus 30 (81%) of 37 in the dazukibart 600 mg group versus 18 (78%) of 23 in the placebo group, with the most common being infections and infestations (two [13%] vs 12 [32%] vs seven [30%]). Four (11%) patients in the dazukibart 150 mg group and one (4%) in the placebo group reported serious adverse events. One patient in stage 3 received dazukibart 600 mg then placebo and died during follow-up due to haemophagocytic lymphohistiocytosis and macrophage activation syndrome. INTERPRETATION/CONCLUSIONS:Dazukibart resulted in a pronounced reduction in disease activity and was generally well tolerated, supporting IFNβ inhibition as a highly promising therapeutic strategy in adults with dermatomyositis. FUNDING/BACKGROUND:Pfizer.