This study consists of 2 parts, Part A and Part B. Part A is a single center, randomized, double-blind, placebo-controlled cohort designed to evaluate the safety, tolerability, and pharmacokinetics (PK) of NMRA-323511 among healthy elderly.

Part B is a multicenter, randomized, double-blinded, placebo-controlled, parallel-group cohort to evaluate the safety, tolerability, and efficacy of NMRA-323511 among adults with Agitation Associated with Dementia due to Alzheimer's Disease.

Part A consists of a Screening Period (up to 28 days), a 10-day Treatment Period, and a 10- day Follow-up clinic visit after last dose of study treatment.

Part B consists of a Screening Period (up to 28 days), an 8-week Treatment Period, and a 10-day Follow-up clinic visit after last dose of study treatment.

Inclusion Criteria:

Part A

• Healthy participants
• Age 65 to 80 years
• Body mass index (BMI) ≥18.0 and ≤32.0 kg/m^2 at the screening and check-in visit

Part B

• Participants aged 55 to 90 years
• Diagnosis of probable AD or Alzheimer's clinical syndrome according to the National Institute of Aging-Alzheimer's Association criteria at least 12 months prior to screening
• Agitation meets the International Psychogeriatric Association (IPA) consensus definition
• Mini-Mental State Examination (MMSE) score = 5 - 24 (mild to severe dementia) at screening

Exclusion Criteria:

Part A

• Participant is actively suicidal
• Is mentally or legally incapacitated or has significant emotional problems at the time of the screening visit
• Diagnosis of epilepsy taking anticonvulsants for seizure control, history of seizures

Part B

• Dementia or memory impairment due to a reason other than AD
• Clinically significant neurologic disorder other than AD
• Have any clinically significant and uncontrolled medical condition

Note: Other protocol defined inclusion/exclusion criteria may apply