A Study to Learn About the Safety and Effects of Salanersen (BIIB115) When Given to Babies With Spinal Muscular Atrophy (SMA) Who Were Previously Treated With Onasemnogene Abeparvovec

Exclusion Criteria

• •Any unresolved post-OA laboratory abnormalities defined as follows:
• •* Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) must be less than 2 × Upper Limit of Normal (ULN) while not receiving corticosteroids within 30 days prior to dosing with salanersen or sham procedure (repeat testing may be performed if necessary).
• •* Evidence of thrombocytopenia, indicated by the platelet count being lower than the normal range for the laboratory.
• •* Evidence of elevated troponin-I levels, identified as elevated post-OA, and has not returned to the normal range.
• •Confirmed demonstration of corrected QT interval, using Fridericia's correction method, of \> 450 milliseconds (ms).
• •Other than OA, any prior treatment with an approved SMA disease modifying therapy (e.g. nusinersen and/or risdiplam), a myostatin inhibitor therapy, or an investigational drug given for the treatment of SMA.
• •Steroid treatment administered for the purpose of treating complications following OA within 14 days prior to dosing with salanersen or sham procedure on Day 1.