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 clinicalfindings and prior confirmatory genetic testing using a clinical diagnostic genetictest

• 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 priorhistory of fractures: a) Prior history of low-trauma fracture defined as: evidenceof at least one prevalent vertebral compression fracture of Genant Grade 1 or 2 (orradiographic signs of vertebral fractures [VF]) or history of at least onelow-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 ofnon-ambulatory status defined as participant- or caregiver-reported age ofcontinuous wheelchair use, approximated to the nearest month, and an North StarAmbulatory Assessment (NSAA) walk score of "0" and inability to perform the 10-MeterWalk/Run (10 MWR) at the baseline visit, with or without fractures

• Group 2 participants are required to meet the following criteria: - Befracture-naïve, defined as: no history of prior low-trauma fractures before thebaseline visit nor any radiological findings indicative of prevalent VF at thescreening visit - Be ambulatory defined as able to walk independently withoutassistive 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 plannedsurgery or procedure that would interfere with the conduct of the study for any timeduring 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 atscreening 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 baselinevisit or expects to receive a live or live attenuated virus vaccine during the study

• Has abnormal laboratory values considered clinically significant as defined by theprotocol

• Any medical condition that might interfere with the evaluation of LS BMD, such assevere scoliosis or spinal fusion

• Participant has previous or ongoing medical condition, medical history, physicalfindings or laboratory abnormalities that could affect safety, make it unlikely thattreatment and follow-up will be correctly completed or impair the assessment ofstudy results, in the opinion of the investigator

• Participant has an allergy or hypersensitivity to the study medication or to any ofits constituents. Other protocol defined inclusion and exclusion criteria may apply