A randomized placebo-controlled study to evaluate the efficacy of PRX-03140 in subjects with Alzheimer's Disease receiving a stable dose of donepezil. The study consists of a 26-week double-blind treatment period.

• Men or Women with a clinical diagnosis of Probable AD
• MMSE score 12 to 22 inclusive
• Age >50 and <90 years
• Received at least 4 months of a stable dose of donepezil 10mg QD for AD
• Brain CT or MRI scan Consistent with a primary diagnosis of AD within 24 months
• Neurological examination without focal deficits (excluding changes attributable to peripheral nervous system disease, trauma or congenital birth deficits)
• No history or evidence of any other CNS disorder that could be interpreted as a cause of dementia
• No diagnosis of vascular dementia
• No history of significant psychiatric illness such as schizophrenia or bipolar affective disorder. Subjects with major depressive disorder on a stable dose of an antidepressant for >6 months may be eligible
• No evidence of the following: current vitamin B12 deficiency, positive syphilis serology, positive HIV test, or abnormalities in thyroid function
• No cognitive rehabilitation within 6 months of the study
• Subject has a regular caregiver willing to attend all study visits
• Signed informed consent by the subject (and legal guardian, if applicable)
• No history of drug or alcohol abuse
• No clinically significant laboratory abnormalities or medical history
• No investigational drug within 30 days of Randomization
• Cannot receive memantine within 2 months of enrollment
• No other prescription acetylcholinesterase inhibitors for AD (other than donepezil) within 4 months of enrollment
• No clinically significant ECG abnormalities or cardiac history
• No history of uncontrolled seizure disorder within 12 months of enrollment
• Cannot be taking MAO inhibitors, bupropion,fluoxetine, paroxetine, quinidine
• No history of malignancy within 3 years of randomization
• Women cannot be pregnant or breastfeeding