The long-term extension study was terminated because the parent study NBI-921352-FOS2021 (NCT05159908) failed to meet its primary endpoint.

This Phase 2, prospective, interventional, active extension study was designed to evaluate the long-term safety and tolerability of NBI-921352 as adjunctive therapy in adult participants with focal onset seizures who completed 11 weeks of treatment in randomized, double-blind, placebo-controlled Study NBI-921352-FOS2021. Eligible participants may enroll directly following the completion of the Week 11 study visit of Study NBI-921352-FOS2021 or after a gap following completion of that study.

Key Inclusion Criteria:

• Provided informed consent.
• Completed 11 weeks of treatment in Study NBI-921352-FOS2021.
• Stable treatment with at least 1 but not more than 4 antiseizure medicines.

Key Exclusion Criteria:

• Have developed any other disorder for which the treatment takes priority over the treatment of focal onset seizure or is likely to interfere with study treatment or impair treatment compliance.

For participants who enroll directly from Study NBI-921352-FOS2021, the study will include a Blinded Dose Conversion Period (1 week) before proceeding to the Open-Label Treatment Period.