Safety and Tolerability Study Evaluating a New Treatment for Subjects With Mild to Moderate Alzheimer's Disease

* 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 was randomized into Study M11-793 and completed dosing through Week 24 in that study. * With the exception of a diagnosis of mild to moderate AD 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 ECG. * If male, the subject is surgically sterile (vasectomy, is sexually inactive, or is using a barrier method of birth control (condom) with spermicidal foam/gel/film/cream/suppository for the duration of the study and for 30 days following the last dose of study drug. However, if the male subject's partner has been postmenopausal for at least two years or is surgically sterile, then use of a barrier method of birth control is not required. * 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 and caregiver must have sufficient visual, hearing and graphomotor skills to complete the study procedures.

* The subject experienced an adverse event or abnormal finding in physical examination, vital signs, laboratory profile and/or ECG measurements in Study M11-793 that indicates the subject could become medically unstable during the current study. * The subject is currently taking or is expected to be prescribed any excluded medications without the approval of Abbott medical monitor. * The subject was noncompliant with donepezil or rivastigmine during Study M11-793 or is expected to discontinue these medications prior to conclusion of this study. * The subject anticipates a move outside the geographic area of the investigative site or is planning extended travel inconsistent with the recommended visit intervals. * The subject is currently enrolled in, or plans to participate in, another experimental study during the course of this trial. * The subject developed any significant medical or psychiatric condition that, in the opinion of the investigator, renders the subject an unsuitable candidate to participate in this study. * For any other reason the investigator considers the subject to be an unsuitable candidate to receive ABT-126 or to participate in this study.