Extension Study of Pimavanserin in Irritability Associated With Autism Spectrum Disorder

INCLUSION CRITERIA: * Has completed the treatment period of study ACP-103-069 * Informed consent prior to the conduct of any study procedures * Continues to be both clinically stable and not at imminent risk of suicide or injury to self, others, or property * Continues to be medically stable at enrollment * For female patients only: unable to become pregnant or agree to use a highly effective non-hormonal method of contraception. Females of childbearing potential must have a negative pregnancy test EXCLUSION CRITERIA: * Patient or parent/legally accepted representative is judged by the Investigator to be inappropriate for the study * Requires treatment with a medication prohibited by the protocol, including concomitant psychotropic drugs targeting irritability, including those used off-label (clonidine, guanfacine, and propranolol; lithium, valproate), medications that prolong the QT interval; and strong cytochrome P450 (CYP) 3A4 enzyme (CYP3A4) inhibitors and inducers * At a significant risk of suicide, or is a danger to self or others * At risk of significant violent behavior to the extent that participation would pose an undue risk to other patients, caregivers, or others * Positive urine drug test * Serious and/or unstable psychiatric, neurologic, cardiovascular, respiratory, gastrointestinal, renal, hepatic, hematologic, or other medical disorder, including cancer or malignancies * Any change in medical or treatment status that may increase the risk associated with taking pimavanserin, would interfere with safety assessments, or would confound the interpretation of study results * Clinically significant abnormal ECG of protocol-defined cardiac conduction abnormalities * Weight \<15 kg * Additional inclusion/exclusion criteria apply. Subjects will be evaluated at screening to ensure that all criteria for study participation are met.