This is a placebo-controlled, parallel-groups, 12 month, double-blind study, followed by a 3-month, double-blind, randomized withdrawal period to evaluate the efficacy and safety of an investigational drug (currently approved for other indications) to slow the progression of symptoms of AD. Patients with probable AD according to NINCDS-ADRDA criteria who have a Mini Mental State Examination (MMSE) score between 13 and 26 inclusive will be eligible for participation in the study.