Study of M5049 in DM and PM Participants (NEPTUNIA)

Inclusion Criteria:

• Diagnosis of probable or definite DM or PM as per 2017 ACR/EULAR classificationcriteria, with positive autoantibody status. Anti-synthetase syndrome (ASyS)participants that meet classification criteria are allowed

• Active disease on standard of care (SoC), must meet 1 of the criteria within 6months prior to Screening: Pathological evidence of active myositis in musclebiopsy; Evidence of active myositis by Electromyography (EMG); Magnetic resonanceimaging (MRI) with evidence of active myositis; or any muscle enzyme greater than orequal to (>=) 4 × upper limit of normal (ULN) at time of Screening; Active PM/DMskin rash as per cutaneous dermatomyositis area and severity index-A (CDASI-A) >= 7at time of Screening

• Minimum disease severity defined by: moderate to severe myopathy with manual muscletesting-8 (MMT-8) >= 80 and less than or equal to (<=) 142 AND at least 2 of thefollowing core set measures (CSM) abnormalities: Patient Global Activity (PtGA) >= 2centimeters (cm); Physician Global Activity (PGA) derived from myositis diseaseactivity assessment tool (MDAAT) >= 2 cm; Extramuscular Activity Assessment derivedfrom MDAAT >2 cm; At least 1 muscle enzyme > 1.5 times ULN; health assessmentquestionnaire-disability index (HAQ-DI) >= 0.25

• Stable doses of oral corticosteroids (CS) and/or maximum of 1 non-corticosteroidimmunosuppressive/immunomodulatory medications (methotrexate, 6 mercaptopurine,sulfasalazine, mycophenolate mofetil or sodium, azathioprine, leflunomide,cyclosporine, oral tacrolimus) for DM or PM

• Participants have a body mass index (BMI) lower or Equal to 40.0 kilograms persquare meter (kg/m^2)

• Other protocol defined inclusion criteria could apply

Exclusion Criteria:

• Primary diagnosis of inclusion body myositis (IBM), malignancy-associated myositis (defined as diagnosis of myositis within 3 years of cancer), immune mediatednecrotizing myopathy (IMNM) with a biopsy characterized as necrotizing biopsy orIMNM with positive anti-signal recognition particle antibody (SRP) or anti 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) auto antibodies.Participants with anti-transcription intermediary factor 1 (TIF1) gamma antibody ornewly diagnosed (within 1 year) anti MDAT5 antibody should have had adequatescreening for cancer within 12 months of Day 1. Adequate screening of cancer isdefined as up-to-date age and gender appropriate screening as per nationalguidelines

• Primary diagnosis of juvenile DM, or adult participants previously diagnosed withjuvenile DM

• Any other active concurrent connective tissue disease associated with inflammatorymyopathy in the Investigator's opinion. Eligibility of participants with diagnosisof concurrent connective tissue disease(s) will be reviewed and approved by anidiopathic inflammatory myopathies (IIM) expert committee

• Severe interstitial lung disease defined as supplemental oxygen required at rest, orforced vital capacity (FVC) of <60 percent (%) predicted. Participants within 1 yearof PM/DM diagnosis and anti-MDA5 antibody, should have been evaluated forinterstitial lung disease (ILD) with high resolution computed tomography (HRCT)Chest

• Any uncontrolled disease (for example [e.g.], severe respiratory, cardiovascular,gastrointestinal, neurological, psychiatric, hematological, metabolic [includingthyroiditis with increased/decreased thyroid stimulating hormone (TSH)], renal [Estimated glomerular filtration rate < 40 milliliter per minute/1.73 m^2 ascalculated by the Modification of Diet in Renal Disease equation by the centrallaboratory], hepatic, endocrine/reproductive organ disease) other than DM/PM, thatin the Investigator's or Sponsor/designee's opinion constitutes an inappropriaterisk or contraindication for participation in the study or that could interfere withthe study objectives, conduct, or evaluation

• Other protocol defined exclusion criteria could apply