This is a first-in-human (FIH) study to evaluate the safety, tolerability, and PK of BIIB115, and consists two parts.~Part A includes cohorts 1 through 4, where healthy volunteer (HV) participants will be assigned to receive a single dose of either BIIB115 or placebo. This part will have a 4-week Screening Period, a Treatment Period, and a 13-month Follow-up Period. The study duration for each participant will be approximately 14 months.~Part B includes cohorts 5 and 6, where pediatric participants with SMA will be assigned to receive 2 doses of BIIB115. This part will have a 4-week Screening Period, a Treatment Period, and a 24-month Follow-up Period. The study duration for each participant will be approximately 25 months.~The study will be conducted in approximately 20 sites globally.
The primary objective of the study is to evaluate the safety and tolerability of single ascending dose of BIIB115 administered via intrathecal (IT) bolus injection to healthy male participants in Part A and multiple ascending doses of BIIB115 administered via IT bolus injection to pediatric Spinal Muscular Atrophy (SMA) participants previously treated with onasemnogene abeparvovec in Part B. The secondary objective of the study is to evaluate the pharmacokinetics (PK) of single-dose of BIIB115 administered via IT bolus injection to healthy male participants in Part A and multiple ascending doses of BIIB115 administered via IT bolus injection to pediatric SMA participants who previously received onasemnogene abeparvovec in Part B.