Alexander Disease| ION373

Inclusion Criteria:

• Clinical phenotype and brain imaging consistent with a diagnosis of Alexander disease
• Documented genetic mutation in the GFAP gene
• Aged ≥ 2 to 65 years old at the time of informed consent
• Able and willing to meet all study requirements, including travel to Study Center, procedures, measurements and visits
• Patients < 18 years old at Screening must have a trial partner (parent, caregiver or other)

Exclusion Criteria:

• Clinically significant abnormalities in medical history or physical examination
• Any clinically significant laboratory abnormalities that would render a patient unsuitable for inclusion
• Any contraindication or unwillingness to undergo MRI
• Treatment with another investigational drug, biological agent, or device within 1 month of Screening, or 5 half-lives of investigational agent, whichever is longer; concurrent participation in any other clinical study (including observational and non-interventional studies)
• Previous treatment with an oligonucleotide (including small interfering ribonucleic acid [siRNA]) within 4 months of Screening if single dose received, or within 12 months of Screening if multiple doses received. This exclusion does not apply to vaccines (both messenger ribonucleic acid [mRNA] and viral vector vaccines).
• History of gene therapy or cell transplantation or any other experimental brain surgery [ROW]
• Obstructive hydrocephalus
• Presence of a functional ventriculoperitoneal shunt for the drainage of cerebrospinal fluid (CSF) or an implanted central nervous system (CNS) catheter
• Known brain or spinal disease that would interfere with the lumbar puncture (LP) process, CSF circulation or safety assessment.
• Hospitalization for any major medical or surgical procedure involving general anesthesia within 12 weeks prior to Screening or planned during the study
• Have any other conditions, which, in the opinion of the Investigator would make the patient unsuitable for inclusion, or could interfere with the patient participating in or completing the study