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

Participants will be included in two groups: ambulatory participants with fractures and non-ambulatory participants with or without a history of fractures (Group 1) and ambulatory participants who are fracture-naïve (Group 2) at baseline. The study will assess the potential of satralizumab to improve bone fragility and to increase muscle function. A weight-tier-based dose of satralizumab will be given by subcutaneous (SC) injection every 4 weeks (Q4W).

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