Inclusion Criteria:

1. Ability to consent in the jurisdiction in which the study is taking place and capable of giving signed informed consent.
2. A definite or probable clinical diagnosis of idiopathic inflammatory myopathy (IIM)
3. One of the following medical histories:

-- Diagnosis of dermatomyositis (DM) or juvenile dermatomyositis (JDM), (age of disease onset <18 years of age). The diagnosis date for juvenile dermatomyositis should not be >5 years from the screening date.

-- Diagnosis of polymyositis (PM) (including antisynthetase syndrome (ASyS))

-- Diagnosis of immune-mediated necrotizing myopathy (IMNM)

1. Diagnosed with active disease as defined by the presence of at least 1 of the following criteria:

-- Abnormal levels of at least 1 of the following enzymes: creatine kinase (CK), aldolase, lactate dehydrogenase, aspartate aminotransaminase (AST), alanine aminotransferase (ALT), based on central laboratory results

-- Electromyography demonstrating active disease within the past 3 months

-- Active dermatomyositis (DM) skin rash

-- Muscle biopsy indicative of active idiopathic inflammatory myopathy (IIM) in the past 3 months

-- Magnetic resonance imaging within the past 3 months indicative of active inflammation

1. Muscle weakness
2. Receiving a permitted background treatment for idiopathic inflammatory myopathy.
3. Contraceptive use by nonsterilized male participants and women of childbearing potential will be consistent with local regulations, where available, for individuals participating in clinical studies. Women of childbearing potential must have a negative serum pregnancy test during screening and a negative urine pregnancy test at baseline to establish the nonpregnant state before receiving investigational medicinal product (IMP).

The full list of inclusion/ exclusion criteria can be found in the protocol.