A Study to Assess the Efficacy and Safety of Satralizumab in Duchenne Muscular Dystrophy (DMD)

Key Inclusion Criteria: * Signed Informed Consent Form (ICF) and signed Assent Form when appropriate * Male at birth * A definitive diagnosis of DMD prior to screening based on documentation of clinical findings and prior confirmatory genetic testing using a clinical diagnostic genetic test * Age ≥ 8 and \< 18 years at the time of signing ICF * Group 1 participants are required to meet the following criteria: - Ambulatory (defined as able to walk independently without assistive devices) with a prior history of fractures: a) Prior history of low-trauma fracture defined as: evidence of at least one prevalent vertebral compression fracture of Genant Grade 1 or 2 (or radiographic signs of vertebral fractures \[VF\]) or history of at least one low-trauma long-bone fracture (upper or lower extremity) or b) Non-ambulatory, characterized as being non-ambulatory for a minimum of 6 months with onset of non-ambulatory status defined as participant- or caregiver-reported age of continuous wheelchair use, approximated to the nearest month, and an North Star Ambulatory Assessment (NSAA) walk score of "0" and inability to perform the 10-Meter Walk/Run (10 MWR) at the baseline visit, with or without fractures * Group 2 participants are required to meet the following criteria: - Be fracture-naïve, defined as: no history of prior low-trauma fractures before the baseline visit nor any radiological findings indicative of prevalent VF at the screening visit - Be ambulatory defined as able to walk independently without assistive devices - Age ≥ 8 to \< 12 years old at the time of screening * Daily oral corticosteroids Key Exclusion Criteria: * Major surgery (e.g., spinal surgery) within 3 months prior to baseline or planned surgery or procedure that would interfere with the conduct of the study for any time during this study * Presence of any clinically significant illness * Has serological evidence of current, chronic, or active human immunodeficiency virus (HIV), tuberculosis (TB), hepatitis C virus (HCV), or hepatitis B virus (HBV) infection * Has a symptomatic infection (e.g., upper respiratory tract infection, pneumonia, pyelonephritis, meningitis) within 4 weeks prior to baseline * Body weight at screening \< 20 or \> 100 kilograms (kg) * Evidence of a severe VF (defined as Grade 3), assessed by radiographic imaging at screening and quantified using the Genant semiquantitative method * Treatment with prohibited therapies as defined by the protocol * Has received a live or live attenuated virus vaccine within 6 weeks of the baseline visit or expects to receive a live or live attenuated virus vaccine during the study * Has abnormal laboratory values considered clinically significant as defined by the protocol * Any medical condition that might interfere with the evaluation of LS BMD, such as severe scoliosis or spinal fusion * Participant has previous or ongoing medical condition, medical history, physical findings or laboratory abnormalities that could affect safety, make it unlikely that treatment and follow-up will be correctly completed or impair the assessment of study results, in the opinion of the investigator * Participant has an allergy or hypersensitivity to the study medication or to any of its constituents. Other protocol defined inclusion and exclusion criteria may apply