A Study to Evaluate the Effect of SAGE-718 on Cognitive Function in Participants With Huntington's Disease (HD)

The UHDRS is a multi-domain clinical rating scale for assessment of functional capacity in HD. Independence Scale, Part V of the UHDRS, is a single-item measure to assess a participant's ability to function independently in daily activities across all stages of HD. Total score ranges from 10 to 100, with higher scores indicating better functioning. Negative change from baseline indicates worsening in functional ability.

The Trail Making test is a speeded graphomotor test of visual attention and task switching. Part B includes a set-switching component that requires participants to connect a series of alternating numbers and letters in order from lowest to highest (i.e., 1-A-2-B-3-C) in the shortest time possible. The time limit to complete the task is 240 seconds, at which point the test is stopped and scored using the maximum allowable time if not yet completed. Greater time indicates greater impairment. Negative change from baseline indicates less impairment.

The OTS is a computerized test of executive function to assess problem-solving and planning ability. In this task, participants are shown a set of colored balls and must determine the minimum number of moves required to match a target arrangement by moving one ball at a time into one of two available locations. The goal is to perform the task using the smallest number of moves possible. The participant selects the correct number of moves from the options shown on the screen. The time to correct response is measured. Longer times indicate greater impairment. Negative change from baseline indicates an improvement in performance.

The PTAP is a test of motor timing in which participants synchronize their finger taps with auditory pacing tones during an initial paced phase. This is followed by a self-paced phase, where the tones are removed, and the participant continues tapping until a final auditory cue signals the end of the test. The primary outcome is paced tapping consistency, calculated as the inverse of the standard deviation of the intertap intervals per millisecond (1/msec), with no range specified. Higher values indicate greater timing accuracy.

Hi-DEF Scale: self-reported measure to capture difficulties experienced in daily life due to HD across 4 areas of functioning: At home, At work, Driving, \& Relationships. Full scale comprises 40 items which ask participants to rate their functioning difficulty using 5-point Likert scale from 1 (No Difficulty) to 5 (Cannot do this anymore) on first 3 domains (home, work \& driving), \& 4-point scale from 1 (Never) to 4 (Always)for Relationships. Hi-DEF total score ranges from 0-154, higher score=greater difficulty. For Home subdomain, before scores can be calculated, data from item responses should be rescored so as: lowest item-level score=0 \& highest item-level score= 4. Hence, Home subdomain has 15 items scored on 5-point Likert scale, 0 indicating no difficulty \& 4 indicating cannot be done anymore, where total raw score of 0-60 are transformed to 0-100. Higher score=greater difficulty. Negative change from baseline=improvement in daily functioning.

The CGI-S scale is a validated instrument developed by the National Institute of Mental Health specifically for use in clinical studies. Cognitive Status Subdomain is one of clinical global impression severity scales domains, for which clinicians generate ratings of the severity of the participant's condition over the past 7 days (including the day of the clinic visit). The responses are scored on a scale ranging from 0 (normal - no symptoms present) to 6 (severely disabled; helpless; complete assistance needed). Higher scores indicate greater disease symptom severity. Negative change from baseline indicates an improvement in the participant's condition.

An adverse event (AE) is any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product that does not necessarily have a causal relationship with this treatment. A TEAE is defined as an AE with onset after the start of the IP, or any worsening of a pre-existing medical condition/AE with onset after the start of the IP and throughout the study.