A Study to Learn About the Safety of BIIB147 and Its Effects in Adults With Amyotrophic Lateral Sclerosis (ALS)

The primary objectives of this study are to evaluate the safety and tolerability of BIIB147 during the MAD Period and the long-term safety and tolerability of BIIB147 during the Open Label Extension (OLE) and the Integrated MAD and OLE Periods. The secondary objectives of MAD Period are to evaluate the pharmacokinetics (PK) profile of total active BIIB147 related species in serum and cerebrospinal fluid (CSF), and to evaluate the impact of BIIB147 on neurodegeneration. The secondary objective for the OLE is to evaluate the long-term PK profile of total active BIIB147 related species in serum and CSF. The secondary objectives of the Integrated MAD Period and OLE are to evaluate the long-term impact of BIIB147 on neurodegeneration, on clinical outcomes of disease progression and survival for participants with earlier initiation of BIIB147 (i.e., start of MAD), compared with delayed initiation of BIIB147 (i.e., at start of the OLE), and long-term PK profile of total active BIIB147 related species in serum and CSF.