The primary purpose of the study is to assess whether SAM-531, an investigational drug, is safe and well tolerated, compared with placebo (a medically inactive substance), in subjects with mild to moderate Alzheimer's disease.

Inclusion Criteria

1. Signed and dated written informed consent obtained from the subject or the subject's legally authorized representative (LAR) or next of kin (if applicable), in accordance with the local regulations. The subject's caregiver must also consent to participate in the study.
2. Diagnosis of probable Alzheimer's Disease according to National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria.
3. Men and postmenopausal or surgically sterile women aged from 50 to 90 inclusive. Postmenopausal women must have had 12 months of spontaneous amenorrhea. Surgically sterile women are defined as having a hysterectomy, bilateral ovariectomy [oophorectomy], or bilateral tubal ligation. Men who are sexually active will need to agree to use a form of contraception that is satisfactory as per the investigator.
4. Able to participate in all scheduled evaluations with a high probability of completing all required procedures and neuropsychological tests.

Exclusion Criteria

1. Significant neurological disease other than Alzheimer's disease, which may affect cognition (eg, epilepsy, Parkinson disease).
2. Current diagnosis of a major depressive disorder or other major psychiatric symptom according to the criteria of Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition Text Version (DSM-IV-TR).
3. Current clinically significant systemic illness, which is likely to deteriorate or affect the subject's safety, influence cognitive assessment or ability to complete the study.
4. Any clinically important deviation from normal limits in physical and neurological examination, vital signs, on electrocardiogram (ECG) or clinical laboratory test results that could compromise the study or be detrimental to the subject.