1. To compare the pharmacokinetics of Dimebon in subjects with mild and moderate hepatic impairment to subjects with normal hepatic function.
2. To assess the safety and tolerability of Dimebon in subjects with hepatic impairment and subjects with normal hepatic function.
3. To explore the pharmacokinetics of Dimebon in subjects with severely-impaired hepatic function.

Inclusion Criteria:

• Healthy (healthy is defined as the absence of clinically-relevant abnormalities identified by a detailed medical history, full physical examination, 12-lead ECG and clinical laboratory tests).
• Free of any medical or surgical conditions that might significantly interfere with gastrointestinal absorption, distribution, metabolism, or excretion of Dimebon.
• Demographically comparable to subjects with mild and moderate hepatic impairment.
• Subjects with hepatic impairment: Screening medical history, physical examination, vital signs, 12-lead ECG, and clinical laboratory tests performed within 28 days before the first dose of study medication, abnormal findings that are related to the subject's underlying condition are acceptable.
• Satisfy the criteria for Class A, B, or C of the modified Child-Pugh classification [Mild (Child-Pugh Scores 5-6 points), moderate (Child-Pugh Scores 7-9 points), and severe (Child-Pugh Scores >9 and <12 points)] within 14 days before the first dose of study medication.
• A diagnosis of hepatic impairment due to cirrhosis, not secondary to other diseases, which is confirmed and documented by medical history, physical examination, liver biopsy or hepatic ultrasound, CT scan or MRI.

Exclusion Criteria:

• Subjects presenting with any of the following will not be included in the trial:CYP2D6 PM genotype, as identified by screening genotyping.
• A known sensitivity to Dimebon.
• Exposure within the previous three months to a drug known to have a negative effect on skeletal muscle or reproductive organs.
• History of febrile illness within 5 days prior to the first dose.
• Any condition possibly affecting drug absorption (e.g., gastrectomy, active peptic ulcer within last 3 months).