Study to Evaluate the Safety, Tolerability, and Pharmacokinetics of ABT-957 in Subjects With Mild-to-Moderate Alzheimer's Disease on Stable Doses of Acetylcholinesterase Inhibitors

Inclusion Criteria: * Meets the National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association criteria for probable Alzheimer's disease (AD); * Has a Mini-Mental State Examination total score of 16 to 26; * Has a Modified Hachinski Ischemia Scale score of ≤ 4; * Is taking a stable dose of donepezil, galantamine or rivastigmine for at least 30 days; * Has had a computerized tomography or magnetic resonance imaging. The scan must not show evidence for an alternative etiology for dementia; * With the exception of a diagnosis of mild-to-moderate AD and the presence of stable medical conditions, is in general good health. Exclusion Criteria: * Positive screen for drugs of abuse, alcohol or cotinine; * Females must not have positive results for pregnancy; * Focal neurological signs on examination; * Has a clinically significant abnormal value, in serum chemistry, hematology or urinalysis; * History of any significant neurologic disease other than AD; * History of head trauma, motor vehicle accident, concussion.