1. Male or female adult individuals from ≥18 years of age (or alternative age for adults based on local regulations) on the date of signing the informed consent.
2. A definite or probable clinical diagnosis of idiopathic inflammatory myopathy (IIM) according to the 2017 American college of rheumatology (ACR)/European alliance of associations of rheumatology (EULAR) classification criteria.
3. Participants with overlap myositis (OM) must have IIM as the predominant disease.
4. Participants ≥18 years of age (or alternative age for adults based on local regulations) with juvenile dermatomyositis (JDM) can only be included if they were 15 years old or older at first onset of DM symptoms.
5. Disease activity defined by moderate to severe myopathy.
6. The participant must be on background therapy for IIM.
7. Woman (or women) of childbearing potential (WOCBP) must be ready and able to use highly effective methods of birth control for a period of 28 days before treatment initiation, throughout the trial, and for a period of at least 5 days after the last dose of investigational medicinal product (IMP). WOCBP taking oral contraceptives also have to use one barrier method.
You may not be if
1. Major surgery (major according to the investigator's assessment, e.g. hip replacement) performed within 6 weeks prior to randomisation or planned during the trial period.
2. Any documented active or suspected malignancy or history of malignancy within 5 years prior to screening, except appropriately treated basal cell carcinoma of the skin, or in situ squamous cell carcinoma of the skin, or in situ carcinoma of uterine cervix.
3. Participants who must or wish to continue the intake of restricted medications or any drug considered likely to interfere with the safe conduct of the trial.
4. Participants diagnosed with advanced interstitial lung disease (ILD).
