This study will evaluate the safety and efficacy of empasiprubart compared with placebo in adult participants with dermatomyositis (DM).

The study duration will be approximately 92 weeks for all participants. After the screening period, eligible participants will be randomized in a 2:1 ratio to receive either empasiprubart or placebo, respectively, during the treatment period (duration of 25 weeks). At the end of the treatment period, all the participants will enter a safety follow-up period (duration of 65 weeks).

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