Study of STA-1 as an Add-on Treatment to Donepezil

Inclusion Criteria: * Male or female patient aged ≥ 50 years; * Probable Alzheimer's disease diagnosed by the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria; * MMSE score between 10-26, inclusive; * Patient has been treated with donepezil 10 mg per day for at least 3 months prior to screening; * Patient able to participate in all study evaluations and ingest oral medication as indicated; * Patient has a responsible caregiver who will accompany the patient to all clinic visits during the study; * Patient and the responsible caregiver have provided written informed consent before undergoing any study procedures. Exclusion Criteria: * Brain image (computed tomography (CT) scan or Magnetic Resonance Imaging (MRI) done within past 12 months prior to the study) and laboratory tests to exclude secondary dementia or non-Alzheimer's dementia; * Patient with significant clinically central nervous system illness other than AD (e.g. Parkinson's disease, Human Immunodeficiency Virus (HIV) induced dementia, Hachinski Ischaemic Score (HIS) \>4) or dementia complicated by other organic disease or delirium; * Patient with a severe or uncontrolled Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) Axis I disorder other than Alzheimer's disease, including amnestic disorders, schizophrenia or schizoaffective disorder, bipolar disorder, current major depressive episode, psychosis, panic, or post-traumatic stress disorder; * Patient suffering from cerebral disturbances following a stroke or a cerebral trauma (if the event occurred within the last 6 months); * Patient with a history of hypersensitivity to study drugs; * Patient who has a history or evidence of a medical condition that would expose them to an undue risk of a significant adverse event or interfere with assessments of safety or efficacy during the course of the trial, including but not limited to hepatic, renal, respiratory, cardiovascular, endocrine (e.g., Addison's Disease), immune, neurologic, or hematologic disease as determined by the clinical judgment of the investigator; * Participation in any research study within the last 30 days; * Patient with significant alcohol or drug abuse as judged by the investigator.