An adverse event (AE) was any untoward medical occurrence in a clinical investigation participant administered a product or medical device, regardless of its causal relationship with study treatment. Serious adverse event (SAE) was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; was life-threatening (immediate risk of death); persistent or significant disability/incapacity; congenital anomaly. Treatment-emergent AEs are events between first dose of study treatment and up to 28 calendar days after the last administration of study treatment that were absent before treatment or that worsened after treatment. AEs included both serious and non-serious AEs.

Safety analysis population included all participants who entered the extension study with at least 1 dose of study medication.

The laboratory test included: hematology (hemoglobin, hematocrit, red blood cell count, platelet count, white blood cell count, absolute total neutrophils, eosinophils, monocytes, basophils, and lymphocytes), chemistry (blood urea nitrogen/urea, creatinine, glucose, glycosylated hemoglobin [diabetics only], calcium, phosphorus, magnesium, creatine kinase, sodium, potassium, chloride, total carbon dioxide, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, total bilirubin, alkaline phosphatase, uric acid, albumin, total protein), urinalysis (color, appearance, specific gravity, pH, qualitative glucose, qualitative protein, qualitative blood, ketones, nitrites, leukocyte esterase, and microscopy), and other tests (follicle stimulating hormone, urine drug screen, urine pregnancy [human chorionic gonadotropin, hCG], serum beta hCG). Abnormality was determined by the investigator.

Safety analysis population included all participants who entered the extension study with at least 1 dose of study medication.

Number of participants with vital signs data meeting the following criteria is presented: (1) supine systolic blood pressure (SBP) <90 millimeters of mercury (mmHg); (2) standing SBP <90 mmHg; (3) supine diastolic blood pressure (DBP) <50 mmHg; (4) standing DBP <50 mmHg; (5) supine pulse rate <40 beats per minute (bpm); (6) supine pulse rate >120 bpm; (7) standing pulse rate <40 bpm; (8) standing pulse rate >140 bpm; (9) maximum increase from baseline in supine SBP >= 30 mmHg; (10) maximum increase from baseline in standing SBP >= 30 mmHg; (11) maximum increase from baseline in supine DBP >= 20 mmHg; (12) maximum increase from baseline in standing DBP >= 20 mmHg; (13) maximum decrease from baseline in supine SBP >=30 mmHg; (14) maximum decrease from baseline in standing SBP >=30 mmHg; (15) maximum decrease from baseline in supine DBP >=20 mmHg; (16) maximum decrease from baseline in standing DBP >=20 mmHg.

The analysis population included all participants who entered the extension study with at least 1 dose of study medication, and with available data for at least 1 category.

Maximum absolute values and increases from baseline were summarized for PR interval (interval between the start of the ECG P wave and the start of the QRS complex corresponding to the time between the onset of the atrial depolarization and onset of ventricular depolarization), QRS complex (time from Q wave to the end of the S wave corresponding to ventricular depolarization), and QTcF interval (time from ECG Q wave to the end of T wave corresponding to electrical systole, corrected for heart rate using Fridericia's formula). Number of participants with ECG data meeting the following criteria is presented: (1) PR interval >=300 msec; (2) QRS complex >=140 msec; (3) QTcF interval: 450 to <480 msec; (4) QTcF interval: 480 to <500 msec; (5) QTcF interval >=500 msec; (6) PR interval increase from baseline >=25/50 percent; (7) QRS complex increase from baseline >=50 percent; (8) QTcF interval increase from baseline: 30 to <60 msec; (9) QTcF interval increase from baseline >=60 msec.

The analysis population included all participants who entered the extension study with at least 1 dose of study medication, and with available data for at least 1 category.

Number of participants with white blood cell (WBC) count and absolute neutrophil count (ANC) meeting the following criteria is presented: (1) WBC count <0.6 *the lower limit of normal (LLN); (2) WBC count >1.5 times the upper limit of normal (ULN); (3) ANC <0.8*LLN; and (4) ANC >1.2*ULN.

The analysis population included all participants who entered the extension study with at least 1 dose of study medication, and with available data for at least 1 category.

The laboratory test included: hematology (hemoglobin, hematocrit, red blood cell count, platelet count, white blood cell count, absolute total neutrophils, eosinophils, monocytes, basophils, and lymphocytes), chemistry (blood urea nitrogen/urea, creatinine, glucose, glycosylated hemoglobin [diabetics only], calcium, phosphorus, magnesium, creatine kinase, sodium, potassium, chloride, total carbon dioxide, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, total bilirubin, alkaline phosphatase, uric acid, albumin, total protein), urinalysis (color, appearance, specific gravity, pH, qualitative glucose, qualitative protein, qualitative blood, ketones, nitrites, leukocyte esterase, and microscopy), and other tests (follicle stimulating hormone, urine drug screen, urine pregnancy [human chorionic gonadotropin, hCG], serum beta hCG). Abnormality was determined by the investigator.

Safety analysis population included all participants who entered the extension study with at least 1 dose of study medication.

Adverse events related to extrapyramidal symptoms included dystonia, akathisia, tardive dyskinesia). Severity was assessed by the investigator. Mild means the AE didn't interfere with the participant's usual function. Moderate means the AE interfered to some extent the participant's usual function. Severe means the AE interfered significantly with the participant's usual function.

Safety analysis population included all participants who entered the extension study with at least 1 dose of study medication.

Columbia Suicide Severity Rating Scale (C-SSRS) was an interview-based rating scale to systematically assess suicidal ideation and suicidal behavior, and was used in this study. C-SSRS responses were mapped onto the Columbia Classification Algorithm of Suicide Assessment (C-CASA). Number of participants with any of the following behaviors occurring since last visit was summarized: completed suicide; suicide attempt; preparatory acts towards suicide; suicidal ideation; self-injurious behavior (no suicidal intent).

The analysis population included all participants who entered the extension study with at least 1 dose of study medication, and with available data for at least 1 time point.

The UHDRS is a clinical rating scale developed to provide a uniform assessment of the clinical features and course of Huntington's disease (HD). The components of the full UHDRS assess motor function, cognition, behavior and functional abilities. Total Motor Score (TMS) assesses motor features of HD with standardized ratings of oculomotor function, dysarthria, chorea, dystonia, gait, and postural ability. The total motor impairment scores was the sum of all the individual 31 motor sub-items (each rated from 0 to 4), with higher scores indicating more severe motor impairment than lower scores. The range of TMS is 0-124.

Full analysis set (FAS) included all participants who had an open-label extension study baseline efficacy evaluation and completed at least Week 2 visit with a valid UHDRS TMS score, and took >=1 dose of open-label study medication.

The UHDRS is a clinical rating scale developed to provide a uniform assessment of the clinical features and course of Huntington's disease (HD). The components of the full UHDRS assess motor function, cognition, behavior and functional abilities. Total Maximum Chorea (TMC) is a subset of the TMS assessment, and composed of the scoring of 7 chorea assessments (face, orobuccolingual, trunk, right and left upper extremities, right and left lower extremities). Each assessment is rated from 0 to 4 (absent to prolonged). TMC is obtained by adding up each of the separate scores, leading to max score of 28. The minimum score is 0. The higher the score, the worse the symptoms.

Full analysis set (FAS) included all participants who had an open-label extension study baseline efficacy evaluation and completed at least Week 2 visit with a valid UHDRS TMS score, and took >=1 dose of open-label study medication.

The Clinical Global Impression of Improvement (CGI-I) is a global measure of improvement or change based on the clinician's assessment of all available clinical data obtained from interviewing the participant. The CGI-I consists of a single 7-point rating of total improvement or change from baseline severity, regardless of whether or not the change is due entirely to drug treatment. Raters select 1 response based on the following question, "Compared to your participant's condition at the beginning of treatment, how much has he/she changed?" Scores are: 1: Very much improved; 2: Much improved; 3: Minimally improved; 4: No change; 5: Minimally worse; 6: Much worse; or 7: Very much worse.

Full analysis set (FAS) included all participants who had an open-label extension study baseline efficacy evaluation and completed at least Week 2 visit with a valid UHDRS TMS score, and took >=1 dose of open-label study medication.