5 mg twice a day (BID) for 26 weeks. Each 5 mg dose will be taken as 4 tablets: one tablet of 5 mg and 3 tablets of matching placebo. The 5mg dose will not be titrated. Study drug will be provided in weekly blister cards.

Matching Placebo twice a day (BID) for 26 weeks. Each placebo dose will be taken as 4 tablets of matching Placebo. The placebo dose will not be titrated. Matching placebo will be provided in weekly blister cards.

Adverse event (AE) was any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship.

Adverse event (AE) was any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. An 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, life-threatening experience (immediate risk of dying), persistent or significant disability or incapacity, congenital anomaly.

Following parameters were analyzed for laboratory examination: hematology (hemoglobin, hematocrit, red blood cell count, platelet count, white blood cell count, total neutrophils, eosinophils, monocytes, basophils, lymphocytes);coagulation (PT international ratio); liver function (total bilirubin, direct bilirubin, indirect bilirubin, aspartate aminotransferase, alanine aminotransferase, gamma GT, LDH, alkaline phosphatase, total protein, albumin); renal function (blood urea nitrogen, creatinine, uric acid); electrolytes (calcium, sodium, potassium, chloride, total bicarbonate, magnesium, phosphate); clinical chemistry (glucose, glycosylated, hemoglobin, human chorionic gonadotropin, creatine kinase); urinalysis (decimal logarithm of reciprocal of hydrogen ion activity [pH], urine specific gravity, glucose, protein, blood, ketones, nitrite).

Following parameters were analyzed for laboratory examination: hematology (hemoglobin, hematocrit, red blood cell count, platelet count, white blood cell count, total neutrophils, eosinophils, monocytes, basophils, lymphocytes);coagulation (PT international ratio); liver function (total bilirubin, direct bilirubin, indirect bilirubin, aspartate aminotransferase, alanine aminotransferase, gamma GT, LDH, alkaline phosphatase, total protein, albumin); renal function (blood urea nitrogen, creatinine, uric acid); electrolytes (calcium, sodium, potassium, chloride, total bicarbonate, magnesium, phosphate); clinical chemistry (glucose, glycosylated, hemoglobin, human chorionic gonadotropin, creatine kinase); urinalysis (decimal logarithm of reciprocal of hydrogen ion activity [pH], urine specific gravity, glucose, protein, blood, ketones, nitrite).

Absolute values were analyzed for supine systolic blood pressure (SBP), standing SBP, supine diastolic blood pressure (DBP), standing DBP, supine pulse rate, and standing pulse rate. Number of participants with vital signs data meeting the following criteria was reported: Criterion A: supine SBP less than (<) 90 millimeter of mercury(mmHg); Criterion B: standing SBP < 90 mmHg; Criterion C: supine DBP <50 mmHg; Criterion D: standing DBP <50 mmHg; Criterion E: supine pulse rate < 40 beats per minute(BPM); Criterion F: supine pulse rate greater than (>)120 BPM; Criterion G: standing pulse rate < 40 beats per minute(BPM); Criterion H: standing pulse rate >120 BPM;

The number of participants with vital signs data of maximum increase from baseline meeting the following criteria was reported: Criterion A: maximum increase from baseline in supine SBP greater than or equal to (>=) 30 mmHg; Criterion B: maximum increase from baseline in standing SBP >= 30 mmHg; Criterion C: maximum increase from baseline in supine DBP >=20 mmHg; Criterion D: maximum increase from baseline in standing DBP >=20 mmHg

The number of participants with vital signs data of maximum decrease from baseline meeting the following criteria was reported: Criterion A: maximum decrease from baseline in supine SBP >= 30 mmHg; Criterion B: maximum decrease from baseline in standing SBP >= 30 mmHg; Criterion C: maximum decrease from baseline in supine DBP >=20 mmHg; Criterion D: maximum decrease from baseline in standing DBP >=20 mmHg

The number of participants with ECG absolute values meeting the following criteria was reported: Criterion A: maximum PR interval (time from the beginning of P wave to the start of QRS complex, corresponding to the end of atrial depolarization and onset of ventricular depolarization) >=300 msec; Criterion B: maximum QRS complex(time from Q wave to the end of S wave, corresponding to ventricle depolarization) >=140 msec; Criterion C: maximum QTcF interval (time from the beginning of Q wave to the end of T wave corresponding to electrical systole, corrected for heart rate using Fridericia's formula) 450-<480 msec; Criterion D: maximum QTcF interval 480-<500 msec; Criterion E: maximum QTcF interval (Fridericia's correction) >=500 msec

Number of participants with ECG meeting the following criteria was reported: Criterion A: maximum PR interval increase from baseline percentage change (PctChg)>= 25/50%; Criterion B: maximum QRS complex increase from baseline PctChg >=50%; Criterion C: maximum QTcF interval (Fridericia's correction) increase from baseline 30<=change<60 msec; Criterion D: maximum QTcF interval (Fridericia's correction) increase from baseline change >=60 msec.

Adverse event (AE) was any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. AE was assessed according to severity; mild (not causing any significant problem, dose adjustment not required), moderate (caused problem that does not interfere significantly with usual activities or the clinical status, dose adjustment needed due to adverse event) and severe (caused problem that interferes significantly with usual activities or the clinical status, study drug stopped due to adverse event). EPS were reported AEs of dystonia and akathisia.

The UHDRS was a clinical rating scale which has been developed by the Huntington Disease Study Group (HSG) to provide a uniform assessment of the clinical features and course of HD. The components of the full UHDRS assess motor function, cognition, behavior and functional abilities. The Total Maximum Chorea (TMC) was a subset of the TMS assessment. It was composed of the scoring of 7 chorea assessments (face, orobuccolingual, trunk, right and left upper extremities, right and left lower extremities). Each assessment was 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. n is the number of evaluable subjects in each visit.

The C-SSRS captured the occurrence, severity, and frequency of suicide-related thoughts and behaviors during the assessment period. C-SSRS assessed whether participant experienced following: completed suicide; suicide attempt; preparatory acts towards imminent suicidal behavior; suicidal ideation; self-injurious behavior, no suicidal intent. The results presented are the number of participants with completed suicide or non-fatal suicide events or behaviors. Worsening of suicidal ideation was an increase in severity of suicidal ideation from baseline.

CGI-I: 7-point clinician rated scale ranging from 1 (very much improved) to 7 (very much worse). Clinician responded to a question: "Compared to your subject's condition at the beginning of treatment, how much has your subject changed?". Improvement was compared to baseline and was defined as a score of 1 (very much improved), 2 (much improved), or 3 (minimally improved) on the scale. Higher score = more affected. n is the number of evaluable participants in each visit.

The 20 mg BID dose of PF-02545920 was titrated as follows: 5 mg BID for 7 days, 10 mg BID for 7 days, 15 mg BID for 7 days, then 20 mg BID for the remainder of the treatment phase. Participants took 4 tablets packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. The blister packs contained 3 placebo tablets and one 5 mg PF-02545920 tablet for Days 1-7, 2 placebo tablets and two 5 mg PF-02545920 tablets for Days 8-14, 1 placebo tablet and three 5 mg PF-02545920 tablets for Days 15-21, and four 5 mg PF-02545920 tablets from Day 22 through Day 182. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets packaged in blister packs (3 placebo tablets and one 5 mg PF-02545920) twice a day approximately every 12 hours from Baseline Day 1 (V2) to Week 26 (V9), at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets packaged in blister packs (3 placebo tablets and one 5 mg PF-02545920) twice a day approximately every 12 hours from Baseline Day 1 (V2) to Week 26 (V9), at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets packaged in blister packs (3 placebo tablets and one 5 mg PF-02545920) twice a day approximately every 12 hours from Baseline Day 1 (V2) to Week 26 (V9), at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets packaged in blister packs (3 placebo tablets and one 5 mg PF-02545920) twice a day approximately every 12 hours from Baseline Day 1 (V2) to Week 26 (V9), at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets packaged in blister packs (3 placebo tablets and one 5 mg PF-02545920) twice a day approximately every 12 hours from Baseline Day 1 (V2) to Week 26 (V9), at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets packaged in blister packs (3 placebo tablets and one 5 mg PF-02545920) twice a day approximately every 12 hours from Baseline Day 1 (V2) to Week 26 (V9), at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets packaged in blister packs (3 placebo tablets and one 5 mg PF-02545920) twice a day approximately every 12 hours from Baseline Day 1 (V2) to Week 26 (V9), at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.

Participants took 4 tablets of placebo packaged in blister packs twice a day approximately every 12 hours, at approximately the same time of day throughout the study. Study medication was swallowed whole, and was not manipulated or chewed prior to swallowing.