Study closed for enrollment

Inclusion Criteria:

• Diagnosis of probable AD based on both a) the National Institute of Neurological and Communicative Diseases and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria and b) the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision (DSM-IV-TR) criteria for AD
• AD is of mild to moderate severity
• Clear history of cognitive and functional decline over at least one year that is either a) documented in medical records or b) documented by history from an informant who knows the subject well
• Able to read at a 6th grade level or equivalent, and must have a history of academic achievement and/or employment sufficient to exclude mental retardation
• If a participant is receiving an acetylcholinesterase inhibitor, memantine, medical food/supplement (e.g., vitamin E) and/or herbal medications for AD, the dose must have been stable for at least three months before Screening, and the participant must be willing to remain on the same dose for the duration of the trial. Participants may need to be on AD treatments in accordance with local requirements
• Participant must have a reliable and competent trial partner/caregiver who must have a close relationship with the subject

Inclusion Criteria for Extension Period (Part II):

• Tolerated study drug and completed the initial 78-week period of the trial (Part I)
• Participant must have a reliable and competent trial partner who must have a close relationship with the subject

Exclusion Criteria:

• History of stroke
• Evidence of a neurological disorder other than the disease being studied (i.e., probable AD)
• History of seizures or epilepsy within the last 5 years before Screening
• Evidence of a clinically relevant or unstable psychiatric disorder, excluding major depression in remission
• Participant is at imminent risk of self-harm or of harm to others
• History of alcoholism or drug dependency/abuse within the last 5 years before Screening
• Participant is unwilling or not eligible to undergo a magnetic resonance imaging (MRI) scan at Screening or does not have an MRI scan obtained within 12 months before screening and available for central review cannot be randomized. Exception: A head CT scan may be accepted on a case-by-case basis, as approved by the Sponsor (e.g., when MRI is contraindicated for the subject). The head CT must be suitable for central review.
• History of hepatitis or liver disease that has been active within the six months prior to Screening Visit
• Recent or ongoing, uncontrolled, clinically significant medical condition within 3 months of the Screening Visit (e.g., diabetes, hypertension, thyroid or endocrine disease, congestive heart failure, angina, cardiac or gastrointestinal disease, dialysis, or abnormal renal function) other than the condition being studied such that participation in the trial would pose a significant medical risk to the subject. Controlled co-morbid conditions are not exclusionary if stable within three months of the Screening Visit
• History or current evidence of long QT syndrome, corrected QT (QTc) interval ≥470 milliseconds (for male subjects) or ≥480 milliseconds (for female subjects), or torsades de pointes
  • Subjects with stable bundle branch block who exceed these limits for QTc interval are eligible for the trial if judged by the investigator not to be at increased risk for Torsades
• History of malignancy occurring within the five years before Screening, except for adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, or localized prostate carcinoma; or malignancy which has been treated with potentially curative therapy with no evidence of recurrence for ≥3 year post-therapy
• Clinically significant vitamin B12 or folate deficiency in the six months before Screening Visit
• Use of any investigational drugs within 30 days (or longer depending on drug) before Screening or participation in studies involving repeated cognitive testing within 30 days before Screening. Participation in an observational study, such as those involving annual cognitive assessments, may be allowed if approved by Sponsor
• History of a hypersensitivity reaction to more than three drugs
• Has tested positive for human immunodeficiency virus (HIV)
• Close family member (including the caregiver, the spouse or any children) who is among the personnel of the investigational or sponsor staff directly involved with this trial

Additional Exclusion Criteria for Safety Cohort:

• History of an ongoing medical condition that has been poorly controlled within 6 months of the Screening Visit (e.g., hypotension, diabetes, hypertension, cerebrovascular disease, thyroid disease, endocrine disturbance, congestive heart failure, cardiac or gastrointestinal disease, dialysis, or abnormal renal function) other than the condition being studied such that a subject's participation in the trial would pose a significant medical risk
• History of congestive heart failure (moderate or greater severity), myocardial infarction, heart surgery, syncope, bradycardia, or clinically significant hypotension within one year before Screening

Exclusion Criteria for Extension Period (Part II):

• Participant is at imminent risk of self-harm or of harm to others
• Has developed a recent or ongoing, uncontrolled, clinically significant medical condition other than AD
• Has history of or has developed during Part I evidence of long QT syndrome, QTc interval ≥470 milliseconds (for male subjects) or ≥480 milliseconds (for female subjects), or torsades de pointes
• Has developed a form of dementia that is not AD