A Study to Evaluate the Safety and Efficacy of Zilganersen (ION373) in Patients With Alexander Disease (AxD)

This is a Phase 1-3, multi-center, double-blind, placebo-controlled, multiple-ascending dose (MAD) study in approximately 73 patients with AxD. Participants will be randomized in a 2:1 ratio to receive zilganersen (ION373) or matching placebo during 5 treatment periods: 1) a 60-week double-blind treatment period; 2) a 60-week open-label treatment period; 3) a 120-week open-label, Long-Term Extension (LTE) period; 4) an open-label Extended Long-Term Extension Treatment Period (X-LTE) of up to 240 weeks; 5) a 28-week post-treatment follow-up period. Multiple dose cohorts will be evaluated in the study. Cohorts will be enrolled sequentially. The initial participants in each dose cohort must be at least 8 years of age at the time of screening. The study will include an optional open-label sub-study in participants \<2 years of age at some sites. Treatment extension period was added to provide continued access to open label zilganersen for patients completing the main study and sub-study until the drug may be commercially available in the patient's country, or until the Sponsor discontinues the zilganersen development program, whichever occurs earlier.