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

Key Inclusion Criteria: * Signed Informed Consent Form 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 Informed Consent Form * 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: 1. 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 VF) or history of at least one low-trauma long-bone fracture (upper or lower extremity) OR 2. 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, or hepatitis B 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 kg * Evidence of a severe vertebral fracture (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 vaccination during the first 3 months after Baseline. * 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