A Dose-ranging, Randomized, Double-blind , Placebo-controlled Study of the Effect of RO5313534, Used as add-on Therapy to Donepezil, on Cognitive Function in Patients With Mild to Moderate Symptoms of Alzheimer's Disease

This 4 arm study will assess the efficacy and safety of RO5313534 (MEM3454) versus placebo added to donepezil, in patients with mild to moderate Alzheimer's disease. Following a screening period, patients will be randomized to one of 4 treatments (placebo, or RO5313534 1mg, 5mg or 15mg po daily) with background therapy of donepezil (5mg or 10mg).The anticipated time on study treatment is 6 months, and the target sample size is 100-500 individuals.