A Study to Evaluate the Effects of SAGE-718 in Participants With Mild Cognitive Impairment or Mild Dementia Due to Alzheimer's Disease (AD)

The primary purpose of this study is to evaluate the effect of SAGE-718 on cognitive performance in participants with Alzheimer's Disease.

Inclusion Criteria:Meet the following criteria for mild cognitive impairment (MCI) or mild dementia due to Alzheimer's Disease (AD) at Screening:A memory complaint reported by the participant or their study partnerA Clinical Dementia Rating (CDR) score of 0.5 to 1.0 (inclusive) with a memory box score ≥0.5Essentially preserved activities of daily living, in the opinion of the investigatorBrain Magnetic Resonance Imaging (MRI) report, obtained within the 2 years preceding the Baseline Period, that is consistent with the diagnosis of AD with no clinically significant findings of non-AD pathology that could account for the observed cognitive impairmentHave a score of 15 to 25 (inclusive) on the Montreal Cognitive Assessment (MoCA) at Screening.Exclusion Criteria:Have participated in a previous clinical study of SAGE-718, have participated in a previous gene therapy study, or have received study treatment in any other drug, biologic, or device trial within 30 days or 5 half-lives (whichever is longer), unless the participant participated solely in the placebo arm of the study. Additionally, participants who have received treatment with antisense oligonucleotides (ASO) will be excludedHave a condition that precludes undergoing an MRI, in accordance with standard operating procedures at the imaging facility (eg, ferromagnetic metal in the body, claustrophobia), in a participant requiring MRI during ScreeningHave any medical or neurological condition (other than AD) that might be contributing to the participant's cognitive impairment or history of cognitive declineHave a history, presence, and/or current evidence ofBrain surgery, deep brain stimulation, or any history of hospitalization due to a brain injuryPossible or probable cerebral amyloid angiopathy, according to the Boston CriteriaTreatment with an anti-amyloid therapy (including biologics) without subsequent MRI demonstrating the absence of amyloid-related imaging abnormalitiesSeizures or epilepsy, with the exception of childhood febrile seizuresBe at risk for suicidal ideation as per Columbia-Suicide Severity Rating Scale (C-SSRS) and/or in the opinion of the investigatorHave any of the following medical conditions:Any clinically significant finding on 12-lead electrocardiogram (ECG) during Screening in the opinion of the investigatorSupine vital signs outside of the following ranges during Screening (vital sign measurements may be repeated once if initial values exceed these ranges):i. Heart rate <50 or >100 bpm ii. Systolic blood pressure <100 or >160 mmHg iii. Diastolic blood pressure <60 or >100 mmHgHave a history, presence, and/or current evidence of serologic positive results for human immunodeficiency virus (HIV)-1 or HIV-2, or hepatitis B or CHave a positive pregnancy test, or be lactating, or intend to breastfeed during the studyIs known to be allergic to any of SAGE-718 excipients, including soy lecithin