• Change from baseline in the Severe Impairment Battery [ Time Frame: 26 weeks ] [ Designated as safety issue: No ]
• Change from Baseline in ADCS-Activities of Daily Living (severe) [ Time Frame: 26 weeks ] [ Designated as safety issue: No ]

• Change from baseline in the Neuropsychiatric Inventory [ Time Frame: 26 weeks ] [ Designated as safety issue: No ]
• Change from baseline in the Clinician's Global Impression of Change (plus caregiver input) [ Time Frame: 26 weeks ] [ Designated as safety issue: No ]
• Change from baseline in Mini-mental State Exam [ Time Frame: 26 weeks ] [ Designated as safety issue: No ]
• Change from baseline Resource Utilization in Dementia [ Time Frame: 26 weeks ] [ Designated as safety issue: No ]
• Change from baseline in EuroQoL - 5 Domain assessment [ Time Frame: 26 weeks ] [ Designated as safety issue: No ]
• Safety and Tolerability including adverse events and summary of clinically important changes in clinical safety laboratory assessments, electrocardiograms, and vital signs [ Time Frame: 26 weeks ] [ Designated as safety issue: Yes ]

No Dimebon clinical data exist yet in patients with disease that has advanced to the moderate-to-severe stage. Therefore, this study evaluates the safety and efficacy of Dimebon in patients with moderate-to-severe AD who are receiving existing background therapy with memantine.

• Drug: Dimebon 20 mg po TID
  Dimebon 10 mg po TID for 1 week followed by Dimebon 20 mg TID for 25 weeks
  Other Name: PF-01913539
• Drug: Placebo po TID
  Placebo (matched to Dimebon) po for 26 weeks

• Dimebon: Experimental
  Intervention: Drug: Dimebon 20 mg po TID
• Placebo: Placebo Comparator
  Intervention: Drug: Placebo po TID

Inclusion Criteria:

• Are men and women ≥ 50 years of age with a diagnosis of Alzheimers disease.
• Have a Mini-Mental State Exam between 5 and 14 inclusive.
• Have been taking the medication memantine (ie., Namenda) for at least six months prior to this study.
• Must have a caregiver who assists the patient at least five days per week for at least three hours per day, who can accompany patient to study visits, and who has an intimate knowledge of the patient's health states and personal care.

Exclusion Criteria:

• Have taken medicines for Alzheimers disease other than memantine (e.g., donepezil, rivastigmine, galantamine, tacrine) within 2 months prior to this study.
• Dementia other than Alzheimers disease.
• Any medical condition or reason that interferes with the ability of the patient to participate in or complete the trial or places the patient at undue risk, as judged by the study doctor.