The purpose of this Phase 3 study is to evaluate the efficacy and safety of 3APS as an add-on therapy to most standard medication for Alzheimer's disease compared to placebo (inactive substance pill) in patients with mild to moderate Alzheimer's disease.

• Duration of treatment: 18 months
• 3 treatment arms:

Placebo and 2 different doses of active drug.

RECRUITMENT OF PARTICIPANTS IS PERFORMED ONLY BY STUDY SITES.

INCLUSION CRITERIA:

Participants must meet the following inclusion criteria to be eligible.

• Male or Female (age 50 years and older): Female must be of non-childbearing potential (i.e. surgically sterilized or at least 2 years post-menopausal).
• Diagnosis of probable Alzheimer's disease based on the National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA criteria).
• Severity of dementia of mild to moderate degree as assessed by the Mini Mental State Examination (MMSE) performed at the screening visit.
• Patient must be living in the community with a reliable caregiver. Participant living in an assisted living facility may be included if study medication intake is supervised and participant has a reliable caregiver.
• Potential participant must be treated with an acetylcholinesterase inhibitor (donepezil, galantamine or rivastigmine) and must be on stable dose for at least 4 months prior to the screening visit and during the entire study period.
• Participants must not have taken memantine for at least 4 months prior to the commencement of screening. The use of memantine is prohibited during the course of the study.
• Fluency (oral and written) in the language in which the standardized tests will be administered.
• Signed informed consent from potential participant or legal representative and caregiver.

EXCLUSION CRITERIA:

Patients will not be eligible to participate in the study if they meet any of the following criteria:

• Potential participant with any other cause of dementia.
• Life expectancy less than 2 years.
• Potential participant with a clinically significant and/or uncontrolled condition or other significant medical disease.
• Previous use of anti-amyloid or vaccine treatment for Alzheimer's disease.
• Use of an investigational drug within 30 days prior to the screening visit or during the entire study.
• Previous exposure to 3APS.
• Inability to swallow pills.