This is a 15-month study with two phases. During the first 12-month phase of this study, patients will be randomly assigned to receive either active study drug or placebo (approximately half of all patients will be on active study drug, the other half on placebo). The second phase is a 3-month randomized withdrawal period. For this phase approximately 10% of the patients will remain on the active drug.

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.

Patients on stable doses of marketed cholinesterase inhibitors approved for the treatment of AD (except for tacrine) for at least 3 months are eligible for study entry. Use of NSAIDs (except aspirin 81 mg per day) is not allowed during the study.

Inclusion Criteria:

• Patient is male or female and at least 50 years of age or older
• Female patients must be postmenopausal or surgically sterilized
• Clinical diagnosis of AD
• Patient must be in good health except for AD diagnosis
• Patient must have informant/caregiver who can monitor and assist patient during the study

Exclusion Criteria:

• Patient is living in a nursing home or skilled nursing facility
• Women will be ineligible if not postmenopausal or surgically sterilized
• Site physician or Sponsor may exclude patients with serious medical problems such as cancer, cardiovascular disease, major neurological or psychiatric illness, thyroid disease, or major head trauma