The objective of this study is to evaluate the safety and efficacy as assessed by the Alzheimers Disease Assessment Scale-cognitive subscale 13-item (ADAS-cog-13) of two doses of MT-4666 or placebo administered daily for 24 weeks to subjects with mild to moderate Alzheimer's disease.

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