This is an efficacy and safety study evaluating a 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 410 adults with mild to moderate Alzheimer's disease. Subjects will be randomized to one of 5 treatment groups (ABT-126, donepezil or placebo) for a 24-week treatment period.

Inclusion Criteria:

• The subject and caregiver must voluntarily sign and date an informed consent. If the subject does not have the capacity to provide informed consent, full informed consent must be obtained from the subject's representative and assent must be obtained from the subject.
• The subject is a male or female between the ages of 55 and 90 years, inclusive, at Screening Visit 1.
• The subject meets the National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS/ADRDA) criteria for probable Alzheimer's disease.
• The subject has a Mini-Mental Status Examination (MMSE) total score of 10 to 24, inclusive, at Screening Visit 1.
• The subject has a Cornell Scale for Depression in Dementia (CSDD) score ≤ 10 at Screening Visit 1.
• The subject has a Modified Hachinski Ischemic Scale (MHIS) score of ≤ 4 at Screening Visit 1.
• With the exception of a diagnosis of mild to moderate AD and the presence of stable medical conditions, the subject is in general good health, based upon the results of medical history, physical examination, vital signs, laboratory profile, and a 12-lead electrocardiogram (ECG).
• The subject has an identified, reliable caregiver who will provide support and ensure compliance with the study medication and procedures, and provide accurate information about the subject's status during the study.

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, or is participating in cognitive therapy for the treatment of Alzheimer's disease or dementia.
• The subject has clinically significant abnormal laboratory values at Screening Visit 1 as determined by the investigator.
• The subject has a history of any significant neurologic disease other than Alzheimer's disease including Parkinson's disease, multi-infarct or vascular dementia, Huntington's disease, normal pressure hydrocephalus, progressive supranuclear palsy, multiple sclerosis, any seizures, mental retardation or a history of significant head trauma followed by persistent neurologic deficits or known structural brain abnormalities.
• In the opinion of the investigator, the subject has any clinically significant uncontrolled medical or psychiatric illness.
• The subject has a known hypersensitivity or intolerance to donepezil that lead to discontinuation or a known reported history of donepezil treatment failure.