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

* 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. Main Inclusion (Continued): * The subject meets the Neurological and Communicative Disorders and Stroke/Alzheimer's Disease 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 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 Alzheimer's disease 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.

* The subject has taken galantamine or memantine within 60 days prior to Screening Visit 1. * The subject has received excluded concomitant medications. * 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.