The purpose of this study is to assess the efficacy, safety, pharmacokinetics (PK) and pharmacodynamics (PD) of satralizumab, a humanized anti-interleukin-6 receptor (aIL-6R) monoclonal antibody, in ambulatory and non-ambulatory participants with DMD aged ≥ 8 to < 18 years old receiving corticosteroid therapy.

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

Male participants

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing