An Open-label Study of XEN1101 in Epilepsy

This is an Open Label Extension study of the following Phase 3 clinical studies: XPF-010-301 (X-TOLE2), XPF-010-302 (X-TOLE3), and XPF-010-303 (X-ACKT). This study will evaluate the long-term safety, tolerability, PK, and efficacy of XEN1101 in subjects with FOS or PGTCS for the treatment of seizures for up to 6 years. 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 6 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.

Subjects will be initially assigned to XEN1101 as follows:

* 25 mg QD for subjects aged ≥18 years * For subjects aged ≥12 and \<18 years

* 15 mg QD for those

* who weigh \<45 kg at the start of the X-TOLE4 study * who weighed \<45 kg during the X-ACKT study * who weighed ≥45 kg at randomization in the X-ACKT study and had dose reduction(s) for intolerability * 25 mg QD for all others

Subjects will be instructed to orally take XEN1101 once daily (QD) with an 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.