This study will evaluate safety and efficacy of PF-05212377 in subjects with mild-to-moderate Alzheimer's Disease with existing neuropsychiatric symptoms on a stable dose of Donepezil. The 4-week run-in will minimize placebo effect. The 12-week treatment period is considered the minimum length necessary to reliably evaluate the effect PF-05212377 on cognition and and neuropsychiatric symptoms in this population. The 2-week washout will allow to monitor re-emergence of neuropsychiatric and cognitive symptoms.