Safety and Efficacy of MT-4666

Inclusion Criteria: * Probable Alzheimer's disease consistent with National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and Alzheimer's Disease and Related Disorders Association (ADRDA) criteria * MMSE score of 10 to 24 inclusive at screening and at Day 1, and CDR-SB score of ≥ 2 at screening * Modified Hachinski Ischemic Score (mHIS) ≤ 4 at screening * Caregiver available; caregiver sees subject at least four days (at least 12 hours) each week * Subject living at home; if living at senior residential setting, or an institutional setting, subject with caregiver indicated above is available Exclusion Criteria: * Inability to perform cognitive tests (ADAS-cog-13 and MMSE) at screening and at Day 1 * Diagnosis of any other disease which may cause dementia * MRI or CT scan within 6 months before screening, with findings inconsistent with the diagnosis of Probable AD * Diagnosis of major depressive disorder as defined by Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) within last five years * History of or current diagnosis of any psychosis * History of myocardial infarction or unstable angina within six months before screening * History of cerebrovascular disorder within 18 months before screening