This is a phase 2a, proof-of-concept, 26-week, double-blind, multicenter, randomized, parallel group, placebo-controlled study to compare the efficacy and safety of treatment with SUVN-502 to placebo treatment in subjects with moderate Alzheimer's disease receiving stable doses of donepezil HCl and memantine HCl.

This is a phase 2a, proof-of-concept, 26-week, double-blind, multicenter, randomized, parallel group, placebo-controlled study to compare the efficacy and safety of treatment with SUVN-502 to placebo treatment in subjects with moderate Alzheimer's disease receiving donepezil HCl (10 mg qd) and either memantine HCl (10 mg twice daily [bid]) or Namenda XR® (Extended Release, 28 mg qd) or the combination therapy, Namzaric™

The primary objective of the study is to evaluate the efficacy of a serotonin receptor subtype 6 (5-HT6) antagonist, SUVN-502, compared to placebo, as adjunct treatment in subjects with moderate Alzheimer's disease (Mini-Mental State Examination [MMSE] score of 12 to 20) currently treated with the acetylcholinesterase inhibitor, donepezil hydrochloride (HCl), and the N-methyl-D-aspartic acid (NMDA) antagonist, memantine HCl. Efficacy will be assessed by the 11-item Alzheimer's Disease Assessment Scale for Cognitive Behavior (ADAScog-11) after 26 weeks of treatment.

Inclusion Criteria:

• Has a diagnosis of probable Alzheimer's disease based on the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria at least 1 year prior to the screening visit.
• Has a score between 12 and 20 inclusive on the MMSE at the screening and baseline visits.
• Has a MRI or CT scan performed within 12 months prior to screening with findings consistent with the diagnosis of dementia due to Alzheimer's disease without any other clinically significant comorbid pathologies.
• Must be receiving treatment with stable doses of donepezil HCl and memantine HCl for at least 3 months prior to the screening visit
• Availability of an eligible and reliable caregiver
• Must be living in the community or an assisted living facility.
• Must be ambulatory or ambulatory aided (use of cane or walker).
• Is not pregnant or planning to become pregnant during the study.
• Subject (or subject's legally acceptable representative) and caregiver must sign an Informed Consent to participate in the study.

Exclusion Criteria:

• Has a diagnosis of dementia due to other than Alzheimer's Disease
• Is taking cholinesterase inhibitors other than donepezil HCl or taking doses of donepezil HCl other than 10 mg
• Is taking doses of memantine HCl other than 10 mg bid or Namenda XR® 28 mg qd.
• Has uncontrolled cardiac disease or hypertension.
• Has clinically significant renal or hepatic impairment.
• Has cancer or a malignant tumor, untreated thyroid disorder or has a history of seizure disorder
• Is treated or likely to require treatment during the study, with any medications prohibited by the study protocol.