A Study to Evaluate the Safety and Tolerability of SAGE-718 in Participants With Huntington's Disease

The primary purpose of the study is to evaluate the safety and tolerability of SAGE-718 softgel lipid capsule in participants with Huntington's Disease (HD)

Inclusion Criteria:For all participants:Completed 718-CIH-201 (NCT05107128) or 718-CIH-202 (NCT05358821) studies or meet eligibility criteria for the de novo cohort.Additional inclusion criteria for 718-CIH-201/202 completers who enroll after a gap of >7 days since completion of the parent study (Cohort 2)No significant decline in functional status since last visit in 718-CIH-201 or 718-CIH-202, in the opinion of the investigator.Additional inclusion criteria for the de novo cohort (Cohort 3)Be at least 25 years old, but not older than 65 years of age at Screening.Genetically confirmed disease with cytosine-adenine-guanine (CAG) expansion ≥40No features of juvenile HDCAG-Age-Product (CAP) score ≥90, as calculated using the CAP formula: AGE × (CAG - 30) / 6.49.UHDRS-Total Functional Capacity (TFC)=13 or Montreal Cognitive Assessment (MoCA) >25 score at Screening.Exclusion Criteria:For all participantsHave a diagnosis of an ongoing neurodegenerative condition other than HD, including but not limited to, Alzheimer's Disease, vascular dementia, dementia with Lewy bodies, or Parkinson's Disease.Additional exclusion criteria for the de novo cohort (Cohort 3):Have participated in a previous clinical study of SAGE-718, have participated in a previous gene therapy study, or have participated in any other drug, biologic, or device trial within 30 days or 5 half-lives (whichever is longer), unless the patient participated solely in the placebo arm of the study. Additionally, participants who have received treatment with antisense oligonucleotides or an mRNA splicing modifier will be excluded.Additional exclusion criteria for 718-CIH-201/202 completers (Cohorts 1 and 2):Have ongoing serious adverse events from the parent study.Have ongoing, unresolved AE(s), which in the opinion of the investigator or sponsor, is likely to interfere with study conduct or compliance.