Key Inclusion Criteria:

   - Participants must have completed study NCT03626012 through the first follow-up clinic
   visit that follows the final dosing visit without missing more than 1 dose of study
   treatment.

   - Participants taking concomitant riluzole at study entry must be on a stable dose for
   ≥30 days prior to the first dose of study treatment (Day 1). Participants taking
   concomitant riluzole must be willing to continue with the same dose regimen throughout
   the study, unless the Investigator determines that riluzole should be discontinued for
   medical reasons, in which case it may not be restarted during the study.

   - Participants taking concomitant edaravone at study entry must be on a stable dose for
   ≥60 days prior to the first dose of study treatment (Day 1). Participants taking
   concomitant edaravone must be willing to continue with the same dose regimen
   throughout the study, unless the Investigator determines that edaravone should be
   discontinued for medical reasons, in which case it may not be restarted during the
   study. Edaravone may not be administered on dosing days of this study.

Key Exclusion Criteria:

   - History of drug abuse or alcoholism ≤6 months before study enrollment that would limit
   participation in the study, as determined by the Investigator.

   - Presence of an implanted shunt for the drainage of CSF or an implanted central nervous
   system (CNS) catheter.

   - History of or positive test result at Screening for human immunodeficiency virus. The
   requirement for testing at Screening may be omitted if it is not permitted by local
   regulations.

   - Treatment with another investigational drug (including investigational drugs for ALS
   through compassionate use programs) or biological agent within 1 month of Screening or
   5 half-lives of study agent, whichever is longer.

Note: Other protocol-specific inclusion/exclusion criteria may apply.