The purpose of this study is to determine if Dimebon is safe and effective for the treatment of cognitive impairment in Huntington disease.

This study is a multicenter Phase 3, randomized, double-blind, placebo-controlled safety and efficacy study of Dimebon treatment in subjects with Huntington disease (HD). The study will evaluate Dimebon 20 mg three times daily (TID) administered orally (PO) for six months (26 weeks) compared with matching placebo TID for the primary safety and efficacy analyses. Safety and tolerability will be assessed by recording of adverse events and by monitoring of vital signs, physical examinations, safety laboratory evaluations, and 12-lead electrocardiogram(ECG)assessments.

Inclusion Criteria:

• Have clinical features of HD and a CAG polyglutamate repeat expansion ≥ 36

• Have cognitive impairment as noted by the following:

  1. A Screening MMSE AND a baseline (pre-dose) MMSE score between 10 and 26 (inclusive); and
  2. A subjective assessment of cognitive impairment with decline from pre-HD levels by the Investigator after interviewing the subject and caregiver;

• Are willing and able to give informed consent

• Aged 30 years or older

• Have a caregiver who assists/spends time with the subject at least five days per week for at least three hours per day and has intimate knowledge of the subject's cognitive, functional, and emotional states, and of the subject's personal care.

Exclusion Criteria:

• Had onset of symptoms prior to age 18
• Have any major medical illness or unstable medical condition within 180 days of screening that may interfere with the subject's ability to comply with study procedures and abide by study restrictions, or with the ability to interpret safety data