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

• Mini Mental Status Exam [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Questionnaire which provides a quantitative measure of cognition
• Alzheimer's Disease Cooperative Study - Activities of Daily Living (ADCS-ADL) [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Caregiver-based assessment of activities of daily living
• DEMentia Quality of Life (DEMQOL) [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Health-related quality of life measurement tool
• Clinician Interview-Based Impression of Change - plus (CIBIC-plus) [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Measures a global impression of change in severity of dementia
• Neuropsychiatry Inventory (NPI) [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Assesses the presence of psychopathology in subjects with Alzheimer's disease and other dementias
• Partner-Patient Questionnaire for Shared Activities (PPQSA) [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Measures the extent to which mood and mental state interferes with the patient-partner relationship
• Resource Use in Dementia (RUD-Lite) [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Brief measurement tool for resource utilization
• EuroQol-5D Questionnaires [ Time Frame: Measurements up through 24 weeks ] [ Designated as safety issue: No ]
  Assesses patient's mobility, self-care, usual activity, pain/discomfort and anxiety/depression
• Wechsler Memory Scale-III (WMS-III) Working Memory Index [ Time Frame: Measurements up through 18 weeks ] [ Designated as safety issue: No ]
  Assesses working memory

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 420 adults with mild to moderate Alzheimer's disease who are taking stable doses of acetylcholinesterase inhibitors. Subjects will be randomized to one of two ABT-126 dose arms or placebo for a 24-week treatment period.

• Drug: ABT-126
  See Arm Description
• Drug: placebo
  See Arm Description

• Experimental: ABT-126 Low Dose
  low dose
  Intervention: Drug: ABT-126
• Experimental: ABT-126 High Dose
  high dose
  Intervention: Drug: ABT-126
• Placebo Comparator: sugar pill
  Intervention: Drug: placebo

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. 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.

Exclusion Criteria:

• 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.