Evaluate the Efficacy and Safety of ABT-126 in Subjects With Mild to Moderate Alzheimer's Disease on Stable Doses of Acetylcholinesterase Inhibitors

Inclusion Criteria:

• The subject and caregiver must voluntarily sign and date an informed consent. If thesubject does not have the capacity to provide informed consent, full informed consentmust be obtained from the subject's representative and assent must be obtained fromthe subject.

• The subject is a male or female between the ages of 55 and 90 years, inclusive, atScreening Visit 1. Main Inclusion (Continued):

• The subject meets the Neurological and Communicative Disorders and Stroke/Alzheimer'sDisease and Related Disorders Association (NINCDS/ADRDA) criteria for probable AD.

• The subject must be receiving a stable dose of an AChEI (donepezil or rivastigmine)for at least 90 days prior to Screening Visit 1.

• The subject has a Mini-Mental Status Examination (MMSE) total score of 12 to 24,inclusive, at Screening Visit 1.

• The subject has a Cornell Scale for Depression in Dementia (CSDD) score ≤ 10 atScreening Visit 1.

• The subject has a Modified Hachinski Ischemic Scale (MHIS) score of ≤ 4 at ScreeningVisit 1.

• With the exception of a diagnosis of mild-to-moderate Alzheimer's disease and thepresence of stable medical conditions, the subject is in general good health, basedupon the results of medical history, physical examination, vital signs, laboratoryprofile, and a 12-lead electrocardiogram (ECG).

• The subject has an identified, reliable caregiver who will provide support and ensurecompliance with the study medication and procedures, and provide accurate informationabout the subject's status during the study.

Exclusion Criteria:

• The subject has taken galantamine or memantine within 60 days prior to Screening Visit

• The subject has received excluded concomitant medications.

• The subject has clinically significant abnormal laboratory values at Screening Visit 1as determined by the investigator.

• The subject has a history of any significant neurologic disease other than Alzheimer'sdisease 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 headtrauma followed by persistent neurologic deficits or known structural brainabnormalities.

• In the opinion of the investigator, the subject has any clinically significantuncontrolled medical or psychiatric illness.