Efficacy and Safety of MK-7622 as Adjunct Therapy in Participants With Alzheimer's Disease (MK-7622-012)

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 participant well * On a stable and effective daily dose of AChEI (either donepezil, rivastigmine, or galantamine), for at least two months before Screening, and willing to remain on the same dose for the duration of the trial. Effective doses are considered to be: donepezil, 10 mg total daily dose administered orally; rivastigmine, 9.5 or 13.3 mg/24 hours administered by transdermal patch or 6-12 mg total daily dose administered orally; galantamine, 16-24 mg total daily dose administered orally * 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 * Have a reliable and competent trial partner who must have a close relationship with the subject Exclusion Criteria: * History of clinically significant 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 * 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 * Does not have a magnetic resonance imaging (MRI) scan obtained within 12 months of Screening and is unwilling or not eligible to undergo an MRI scan at Screening * 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 participant. 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 * 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 which has been treated with potentially curative therapy with no evidence of recurrence for ≥3 year post-therapy * Clinically significant vitamin B12 deficiency, or increased thyroid stimulating hormone (TSH) in the six months before Screening * Major surgery within 3 months of Screening