Relapse Prevention Study of Pimavanserin in Dementia-related Psychosis

Inclusion Criteria: 1. Meets criteria for All-cause Dementia according to NIA-AA guidelines 2. Meets clinical criteria for one of the following disorders: Dementia associated with Parkinson's disease, Dementia with Lewy bodies, Possible or probable Alzheimer's disease, Frontotemporal degeneration spectrum disorders, Vascular dementia 3. Has an MMSE score ≥6 and ≤24 4. Has had psychotic symptoms for at least 2 months 5. Must be on a stable does of cholinesterase inhibitor or memantine, if applicable 6. If the subject is female, she must not be pregnant or breastfeeding. She must also be of non-childbearing potential or must agree to use a clinically acceptable method of contraception for the duration of the study Exclusion Criteria: 1. Has psychotic symptoms that are primarily attributable to a condition other than dementia 2. Has had a recent major depressive episode 3. Has experienced suicidal ideation or behavior within 3 months prior to study enrollment 4. Has evidence of a non-neurologic medical comorbidity or medication use that could substantially impair cognition 5. Has a history of ischemic stroke within the last 12 months or any evidence of hemorrhagic stroke 6. Has a known history of cerebral amyloid angiopathy (CAA), epilepsy, CNS neoplasm, or unexplained syncope 7. Has any of the following: greater than New York Heart Association (NYHA) Class 2 congestive heart failure, Grade 2 or greater angina pectoris, sustained ventricular tachycardia, ventricular fibrillation, torsade de pointes, syncope due to an arrhythmia, an implantable cardiac defibrillator 8. Had a myocardial infarction within the last 6 months 9. Has a known personal or family history or symptoms of long QT syndrome 10. Has a significant unstable medical condition that could interfere with subject's ability to complete the study or comply with study procedures 11. Requires treatment with a medication or other substance that is prohibited by the protocol Additional inclusion/exclusion criteria apply. Subjects will be evaluated at screening to ensure that all criteria for study participation are met.