Efficacy and Safety Study for Subjects With Mild-to-Moderate Alzheimer's Disease

* The subject is a male or female between the ages of 55 and 90 years, inclusive, at Screening Visit 1. * The subject meets the NINCDS/ADRDA criteria for probable AD. * The subject has a Mini-Mental Status Examination (MMSE) total score of 10 to 24, inclusive, at Screening Visit 1. * The subject has a Modified Hachinski Ischemic Scale (MHIS) score of less than or equal to 4 at Screening Visit 1. * If female, subject must be postmenopausal for at least two years or surgically sterile * The subject has an identified, reliable, caregiver.

* 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 * The subject has a history of any significant neurologic disease other than AD. * In the opinion of the investigator, the subject has any clinically significant uncontrolled medical or psychiatric illness. * The subject has reported history of discontinuation of donepezil due to lack of efficacy. * The subject has a history of intolerance or adverse reaction to donepezil that led to discontinuation. * The subject has a known hypersensitivity to donepezil, piperidine derivatives, or any of the excipients in either donepezil hydrochloride or ABT-126 formulations. * Patient uses non-prescribed drugs of abuse or has a history of drug or alcohol abuse/dependence. * Subjects with AST and ALT values at Screening Visit 1 that are greater than or equal to 1.5-fold the upper limit of normal (ULN).