The drug being tested in this study is called soticlestat (TAK-935). Soticlestat administered long-term in pediatric and adult participants who participated in either of the antecedent Phase 3 clinical studies, TAK-935-3001 [NCT04940624] (participants with DS) or TAK-935-3002 [NCT04938427] (participants with LGS) will be assessed for additional safety and tolerability data along with efficacy analysis.~The study will enroll approximately 376 participants.~All participants will receive soticlestat based on their weight in the 2-week Titration Period. Following the Titration Period, participants will continue to receive the same dose in the Maintenance Period. At the end of maintenance period, the dose will be down-tapered (unless already at the lowest dose) and then stopped. Participants not tolerating minimum dose of 100 mg twice a day (BID) will be discontinued from the study.~This multi-center trial will be conducted worldwide. The overall time to participate in the study will be approximately 4 years, or until the study is stopped at the discretion of the sponsor, or the product is approved for marketing. Participants who discontinue study drug treatment before the completion of the study, will continue to be followed per protocol and maintain a daily seizure diary until the final follow-up phone call
The main aim of the study is to learn if soticlestat, when given as an add-on therapy, reduces the number of seizures in children and adults with Dravet Syndrome (DS) or Lennox-Gastaut Syndrome (LGS).~Participants will receive their standard anti-seizure therapy, plus tablets of soticlestat. There will be scheduled visits and follow-up phone calls throughout the study.