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

Inclusion Criteria:

1. Meet the following criteria for mild cognitive impairment (MCI) or mild dementia dueto Alzheimer's Disease (AD) at Screening:

2. A memory complaint reported by the participant or their study partner

3. A Clinical Dementia Rating (CDR) score of 0.5 to 1.0 (inclusive) with a memorybox score ≥ 0.5

4. Essentially preserved activities of daily living, in the opinion of theinvestigator

5. Brain Magnetic Resonance Imaging (MRI) report, obtained within the 2 yearspreceding the Baseline Period, that is consistent with the diagnosis of AD withno clinically significant findings of non-AD pathology that could account forthe observed cognitive impairment

6. Have a score of 15 to 25 (inclusive) on the Montreal Cognitive Assessment (MoCA)with years of education adjustment at Screening.

Exclusion Criteria:

1. Have participated in a previous clinical study of SAGE-718, have participated in aprevious 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 antisenseoligonucleotides (ASO) will be excluded

2. Have a condition that precludes undergoing an MRI, in accordance with standardoperating procedures at the imaging facility (eg, ferromagnetic metal in the body,claustrophobia), in a participant requiring MRI during Screening

3. Have any medical or neurological condition (other than AD) that might becontributing to the participant's cognitive impairment or history of cognitivedecline

4. Have a history, presence, and/or current evidence of

5. Brain surgery, deep brain stimulation, or any history of hospitalization due toa brain injury

6. Possible or probable cerebral amyloid angiopathy, according to the BostonCriteria

7. Treatment with an anti-amyloid therapy (including biologics) without subsequentMRI demonstrating the absence of amyloid-related imaging abnormalities

8. Seizures or epilepsy, with the exception of childhood febrile seizures

9. Participants has a history of suicidal behavior within 2 years or answers "YES" toQuestions 3, 4, or 5 on the Columbia-Suicide Severity Rating Scale (C-SSRS) atScreening or at Day 1 or is currently at risk of suicide in the opinion of theinvestigator

10. Have any of the following medical conditions:

11. Any clinically significant finding on 12-lead electrocardiogram (ECG) duringScreening in the opinion of the investigator

12. Any clinically significant supine vital signs (heart rate, systolic anddiastolic blood pressure) during Screening (note: vital sign measurements maybe repeated once)

13. Have a history, presence, and/or current evidence of serologic positive results forhuman immunodeficiency virus (HIV)-1 or HIV-2, or hepatitis B or C

14. Have a positive pregnancy test, or be lactating, or intend to breastfeed during thestudy

15. Is known to be allergic to any of SAGE-718 excipients, including soy lecithin