This is a efficacy and safety study evaluating new treatment for subjects with mild to moderate Alzheimer's Disease.

This is a Phase 2 study designed to evaluate the efficacy and safety of ABT-126 in approximately 260 adults with mild-to-moderate Alzheimer's disease (AD). Subjects will be randomized to one of the four treatment groups (ABT-126, donepezil, or placebo) for a 12-week Treatment Period. Acronyms are listed in the secondary outcome section, below, you will find a list of the acronyms defined:

• MMSE - Mini Mental Status Exam
• QoL-AD - Quality of Life - Alzheimer's Disease
• CIBIC-plus - Clinician Interview-Based Impression of Change
• NPI - Neuropsychiatric Inventory
• CSDD - The Cornell Scale for depression in Dementia
• ADCS-ADL - Alzheimer's Disease Cooperative Study - Activities of Daily Living

Inclusion Criteria:

• The subject is a male or female between the ages of 55 and 90 years, inclusive, at Screening Visit 1.
• The subject meets the NINCDS/ADRDA criteria for probable AD.
• The subject has a Mini-Mental Status Examination (MMSE) total score of 10 to 24, inclusive, at Screening Visit 1.
• The subject has a Modified Hachinski Ischemic Scale (MHIS) score of less than or equal to 4 at Screening Visit 1.
• If female, subject must be postmenopausal for at least two years or surgically sterile
• The subject has an identified, reliable, caregiver.

Exclusion Criteria:

• The subject is currently taking or has taken a medication for the treatment of Alzheimer's disease or dementia within 60 days prior to Screening Visit 1
• The subject has a history of any significant neurologic disease other than AD.
• In the opinion of the investigator, the subject has any clinically significant uncontrolled medical or psychiatric illness.
• The subject has reported history of discontinuation of donepezil due to lack of efficacy.
• The subject has a history of intolerance or adverse reaction to donepezil that led to discontinuation.
• The subject has a known hypersensitivity to donepezil, piperidine derivatives, or any of the excipients in either donepezil hydrochloride or ABT-126 formulations.
• Patient uses non-prescribed drugs of abuse or has a history of drug or alcohol abuse/dependence.
• Subjects with AST and ALT values at Screening Visit 1 that are greater than or equal to 1.5-fold the upper limit of normal (ULN).