A Study to Learn About the Safety of BIIB115 Injections and How BIIB115 is Processed in the Bodies of Healthy Adult Male Volunteers and of Pediatric Participants With Spinal Muscular Atrophy Who Previously Took Onasemnogene Abeparvovec

This study is looking for two main groups of people. For the first part of the study, they need healthy men, aged between 18 and 55, who are generally in good health.

For the second part, they are looking for children with SMA who are between 6 months and 12 years old. These children must have a confirmed genetic diagnosis of SMA and weigh at least 7 kg. It's also important that they have previously received a treatment called onasemnogene abeparvovec at least 180 days before potentially starting BIIB115, and doctors believe their SMA could still improve.

You would not be able to join if you have any health problems that would make a spinal injection risky, have certain serious medical conditions, or have had a serious infection recently. The doctors will check your medical history very carefully to make sure it's safe for you to take part.

If you decide to take part, the study will involve several visits to a clinic and some phone calls. For healthy adult men in Part A, you'll have up to 6 clinic visits and 4 phone calls over 13 months. You'll receive a single injection into your spinal canal on Day 1. You won't know if you're getting the actual medicine or a dummy injection. After this injection, you'll stay in the clinic for 24 hours so doctors can monitor you closely.

For children with SMA in Part B, the study lasts 24 months, with up to 14 clinic visits and 6 phone calls. You'll receive two injections of BIIB115 over this period, and everyone will know you're getting the active medicine. Again, there will be a 24-hour stay in the clinic after each injection. After completing Part B, some children might have the option to join a long-term follow-up called the LTE, which involves five more doses of BIIB115 and lasts for another 60 months, with additional clinic visits and phone calls. Please note that the 24-hour clinic stay after injections is not typically required in the LTE.