A Study to Evaluate Safety and Efficacy of Empasiprubart in Adults With Dermatomyositis

Inclusion Criteria: * Is at least 18 years of age and the local legal age of consent for clinical studies when signing the Informed Consent Form * Is capable of providing signed informed consent and complying with protocol requirements * Agrees to use contraceptive measures consistent with local regulations and women of child-bearing potential must have a negative serum pregnancy test at screening and a negative urine pregnancy test before receiving the study drug * Has a clinical diagnosis of dermatomyositis or juvenile dermatomyositis. The diagnosis date for juvenile dermatomyositis should be ≤5 years before screening * Has active muscle disease associated with classic dermatomyositis or juvenile dermatomyositis at screening and before the first study drug adminisitration and at least 1 of the following: elevated levels of creatine kinase, aldolase, lactate dehydrogenase, aspartate aminotransaminase or alanine aminotransferase at screening; or electromyography ≤18 weeks before the first study drug administration; or an MRI depicting active muscle inflammation ≤18 weeks before the first study drug administration; or muscle biopsy demonstrating signs of active inflammation ≤18 weeks before the first study drug administration * Has at least mild skin disease at screening * Complies with the permitted background dermatomyositis treatment requirements at screening * Has had immunization with the first meningococcal, pneumococcal, and the single Haemophilus influenza type B vaccine ≥14 days before the first study drug administration Exclusion Criteria: * Known autoimmune disease or any medical condition that would interfere with an accurate assessment of clinical symptoms of dermatomyositis or puts the participant at undue risk * Naïve to standard dermatomyositis treatment according to local recommendations * History of malignancy unless considered cured by adequate treatment with no evidence of recurrence for ≥3 years before the first study drug administration. Adequately treated participants with the following cancers can be included at any time: Basal cell or squamous cell skin cancer; Carcinoma in situ of the cervix; Carcinoma in situ of the breast; Incidental histological findings of prostate cancer * Clinically significant active infection that is not sufficiently resolved before the first study drug administration in the investigator's opinion * Positive serum test at screening for active infection with any of the following: Hepatitis B virus, Hepatitis C virus, HIV * Clinically significant disease, recent major surgery, or intention to have major surgery during the study; or any other medical condition that, in the investigator's opinion, would confound the results of the study or put the participant at undue risk * Current participation in another interventional clinical study * Known hypersensitivity to the study drug or any of its excipients * History (within 12 months before screening) of or current alcohol, drug, or medication abuse, as assessed by the investigator * Pregnant or lactating state or intending to become pregnant during the study * Previous participation in an empasiprubart clinical study with at least 1 dose of study drug received * Known complement component deficiency as assessed by the investigator * Change in dermatomyositis physical therapy or exercise program from ≤4 weeks before screening * Inflammatory or non-inflammatory myopathies other than dermatomyositis, such as drug-induced or endocrine-induced myositis, infective myositis, polymyositis, immune-mediated necrotizing myopathy, inclusion body myositis, overlap myositis, metabolic myopathies, or muscle dystrophies * Paraneoplastic dermatomyositis secondary to malignancy * Glucocorticoid-induced myopathy * Severe muscle damage * Extensive or severe calcinosis * Interstitial lung disease with at least 1 of the following: forced vital capacity (FVC) ≤60%; supplemental oxygen therapy; rapidly progressing uncontrolled interstitial lung disease; moderate or severe interstitial lung disease