Study to Examine the Safety and Efficacy of Pimavanserin for the Treatment of Agitation and Aggression in Alzheimer's Disease (SERENE)

Inclusion Criteria: 1. Male or female, 50 years of age or older 2. Can understand the nature of the trial and protocol requirements and provide signed informed consent * from patient, if deemed competent to provide consent * from an appropriate person (e.g. patient's Legally Authorized Representative (LAR) with the patient's assent) if patient is deemed not competent to provide informed consent. 3. Has a diagnosis of probable AD according to the National Institute on Aging-Alzheimer's Association (NIA-AA) guidelines 4. Meets criteria for agitation according to the International Psychogeriatric Association (IPA) guidelines 5. Lives at home or in an assisted living or care facility (but has the capacity to visit the clinic as an outpatient). Subjects must have been at their current location for at least 3 weeks prior to Screening and plan to remain at the same location for the duration of the trial. 6. Has a designated study partner/caregiver who is in contact with the patient at least 3 times a week on 3 separate days 7. Female patients must be of non-childbearing potential or must agree to use an acceptable method of contraception or abstinence , for at least 1 month prior to randomization, during the study, and 1 month following completion of the study 8. The patient and caregiver are willing and able to participate in all schedule evaluations and complete all required tests Exclusion Criteria: 1. The agitation/aggression is attributable to concomitant medications, environmental conditions, substance abuse, or active medical or psychiatric condition 2. Patient is receiving skilled nursing care for any medical condition other than dementia 3. Treatment with an antipsychotic medication within 2 weeks of Baseline visit or 5 half lives, whichever is longer 4. Patient or study partner/caregiver has a medical condition (e.g., hearing, vision impairments) that would impair the ability to perform the study assessments. 5. Has had a myocardial infarction within the last six months 6. Has a history or symptoms of long QT syndrome 7. Has a history of a significant psychotic disorder before or during the diagnosis of probable Alzheimer's disease (including, but not limited to schizophrenia or bipolar disorder) 8. Patient is bedridden or has any significant medical condition that is unstable and would place the patient at undue risk from study drug or study procedures 9. Has a sensitivity to pimavanserin or its excipients 10\. Has previously participated in a clinical study with pimavanserin 11\. Has a Global Clinician Assessment of Suicidality (GCAS) score of 3 or 4 based on Investigator's assessment of behavior within the last 3 months at Screening or since last visit at Baseline