Pimavanserin is an antipsychotic may have some beneficial effects on core autism symptoms and co-morbid conditions such as irritability, anxiety, sleep disorders, mood instability due to epilepsy, etc.). These potential benefits stem from pimavanserin's impact on the serotonin system in the body. This study will compare pimavanserin to placebo in the treatment of irritability, core autism symptoms, and co-morbid conditions in children ages 6-17 years old.

In this research study a study drug named XEN1101 is being tested for the treatment of seizures. The main purpose of this study is to determine if XEN1101 can reduce the number of seizures and if it is safe to use. XEN1101 has been tested in other studies in epilepsy patients and it was considered to be well-tolerated. It had side effects that are similar to other anti-seizure medications commonly used to help patients with epilepsy.

Pimavanserin is an antipsychotic may have some beneficial effects on core autism symptoms and co-morbid conditions such as irritability, anxiety, sleep disorders, mood instability due to epilepsy, etc.). These potential benefits stem from pimavanserin's impact on the serotonin system in the body. This study will investigate pimavanserin in the treatment of irritability, core autism symptoms, and co-morbid conditions in children ages 6-17 years old.

The main purpose of this study is to determine if XEN1101 can reduce the seizure frequency and if it is safe to use. Subjects who successfully completed and did not terminate early from one of the antecedent studies (X-TOLE2, X-TOLE3, or X-ACKT) are eligible to participate in X-TOLE4. Following enrollment into X-TOLE4, subjects will undergo a treatment period of up to 3 years, during which there will be a visit at 2-, 4-, and 13-weeks post-entry, with subsequent visits occurring at 13-week intervals during the first year, and then at 26-week intervals (with a telephone call in between) until dosing is completed. All subjects will be initially assigned to receive 25 mg QD of XEN1101. Study drug is to be taken with the evening meal. Subjects will be expected to keep a daily seizure eDiary with a minimum of 80% compliance for the duration of the extension study (reporting on ≥80% of days between visits).
Upon completion of dosing at the end of the treatment period, there will be an 8-week follow-up period.