Study Comparing 3 Dosage Levels Of SAM-531 In Outpatients With Mild To Moderate Alzheimer Disease

The study will evaluate the efficacy and safety of an investigational drug called SAM-531 at three dosage levels. Subjects will receive either one of the 3 dosage levels of SAM-531, donepezil or placebo for the first 24 weeks of the study (period I). Subjects who receive placebo for period I will be assigned to receive the highest dose of SAM-531 SAM-531 for the remaining 28 weeks of the study, while subjects who received one of the three SAM-531 dosage levels or donepezil in period I will continue with the same study drug (period II).