A Study of Nipocalimab in Participants With Active Idiopathic Inflammatory Myopathies

Inclusion Criteria:

• Disease classification criteria: Participant meets the diagnostic criteria ofprobable or definite idiopathic inflammatory myopathies (IIM) based on 2017 TheEuropean League Against Rheumatism (EULAR)/American College of Rheumatology (ACR)classification criteria for adult IIM at least 6 weeks prior to first administrationof the study intervention

• If a participant is on regular or as needed treatment with low potency topicalglucocorticoids (GC) that are allowed in the study or topical tacrolimus (TAC) totreat skin lesions, the dose and frequency should be stable for greater than orequal to (>=) 4 weeks prior to first administration of the study intervention aswell as maintained at the same dose until Week 52 of the study

• Antibody positivity criteria: Any 1 of the myositis-specific antibodies (MSAs)positive: dermatomyositis (DM): anti-Mi-2 (Mi-2/nucleosome remodeling anddeacetylase [NuRD] complex), anti-transcription intermediary factor 1-Gamma (TIF1-Gamma), anti- nuclear matrix protein 2 (NXP-2), anti-small ubiquitin-likemodifier-1 activating enzyme; anti- antimelanoma differentiation-associated gene 5 (MDA-5) antibodies. Or immune-mediated necrotizing myopathy (IMNM): anti- signalrecognition particle (SRP) and anti- 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibodies. Or anti-synthetase syndrome (ASyS): anti- histidyl- ribonucleicacid [tRNA] synthetase (Jo-1), anti- threonyl-tRNA synthetase (PL7), anti-alanyl-tRNA synthetase (PL12), anti- isoleucyl-tRNA synthetase (OJ), and anti-glycyl-tRNA synthetase (EJ) antibodies. If all MSAs are negative or more than 1 MSAis positive within different subtypes (defined by the central laboratory) atscreening, the tests should be repeated during the screening period. If the sameresults are observed at retesting, the participant should not be enrolled in thestudy

Exclusion Criteria:

• Has a juvenile myositis diagnosis and now >=18 years old

• Has cancer-associated myositis defined as cancer diagnosis within 3 years ofmyositis diagnosis except for cervical carcinoma in situ and non-melanoma skincancer (squamous cell carcinoma, basal cell carcinoma of the skin)

• Has comorbidities (example, asthma, chronic obstructive pulmonary disease [COPD])which have required 3 or more courses of oral GC within 1 year prior to screening

• Has a history of primary immunodeficiency or secondary immunodeficiency not relatedto the treatment of the participants IIM

• Has experienced myocardial infarction (MI), unstable ischemic heart disease, orstroke within 12 weeks of screening