This is a Phase 2b open-label extension study to evaluate the long-term safety and efficacy of SPN-817.

This is a Phase 2b, multicenter, open-label extension, one-year, safety, tolerability, and efficacy study in adults who previously completed an applicable double-blind SPN-817 clinical study. This study will include a double-blind Dose Titration/Bridging Period of 8-10 weeks in which SPN-817 will be titrated to the participant's maximum tolerated dose based on response. Following the Dose Titration/Bridging Period, participants will enter an Open Label Extension (OLE) Period of 42-44 weeks. When participants finish the OLE Period, they will initiate a Tapering Period (up to 4 weeks) followed by an End-of-Tapering Period video contact (VC) after the last dose of study drug. The duration of study treatment before starting the 4-week Tapering Period will be one year (52 weeks).

Inclusion Criteria:

1. Completed antecedent SPN-817 double-blind study
2. Taking a stable dosage regimen (maintained during the antecedent study) of at least one antiseizure medication (ASM) and no more than 4 ASMs

Exclusion Criteria:

1. Has current nonepileptic events that could be confused by the participant and/or study staff as epileptic seizures
2. Has any suicidal behavior or suicidal ideation related to Item 4 (active suicidal ideation with some intent to act without specific plan) or Item 5 (active suicidal ideation with specific plan and intent) based on the Columbia-Suicide Severity Rating Scale (C-SSRS) assessments in the antecedent study and at Visit 1 or more than one lifetime suicide attempt.