A Study Evaluating Soticlestat in Participants With Dravet Syndrome or Lennox-Gastaut Syndrome Who Have Been Exposed to Fenfluramine

The purpose of this study is to check how soticlestat impacts symptoms of Dravet syndrome \[DS\] and Lennox-Gastaut syndrome \[LGS\] in participants who have been exposed to fenfluramine.

The drug being tested in this study is called soticlestat. Soticlestat is being tested to treat people who have DS or LGS and have been exposed to fenfluramine. This study will assess the efficacy and safety of soticlestat in addition to standard care in the treatment of DS or LGS.

The study will enroll approximately 45 patients. This study comprises a screening period of up to 6 weeks, a 4-week titration period, a 48-week maintenance period, a taper period of up to 1 week and a follow-up safety visit. Participants will be enrolled to receive soticlestat along with the standard of care:

• Soticlestat 100-300 milligrams (mg)

Participants will receive oral administration of soticlestat Dose 1 (days 1 to 7), Dose 2 (days 8 to 14), and Dose 3 (Days 15 to 28) with a minimum dose of 100 mg to a maximum dose of 300 mg depending on participant's body weight in the titration period followed by maintenance period up to end of treatment (up to approximately 52 weeks). Percent change from baseline in convulsive in participants with DS and major motor drop (MMD) in participants with LGS seizure frequency per 28 days during the initial 12 weeks of the maintenance period will be assessed.

This multi-center trial will be conducted in the United Kingdom and Europe. The overall time to participate in this study is approximately 60 weeks. Participants will make multiple visits to the clinic and will be followed up for safety by visiting the clinic or by telephone approximately 2 weeks after the last dose of the study drug.

Eligibility Criteria:
Inclusion Criteria:

1. The participant has been exposed to fenfluramine (currently on or used previously).
2. The participant has a clinical diagnosis of LGS and a history of, on average, ≥12 MMD seizures in the last 90 days immediately before screening based on historical information, and the participant has ≥4 MMD seizures during a minimum of 4 weeks of seizure data collection during the prospective baseline period.
3. The participant is currently taking 0 to 5 antiseizure treatments (eg. antiseizure medications \[ASMs\], vagus nerve stimulation \[VNS\], ketogenic diet) at stable doses.

Exclusion Criteria:

1. The participant is currently enrolled in a clinical study involving an investigational product or treatment device (ie, not approved in that country, other than soticlestat), or concurrently enrolled in any other type of medical research judged not to be scientifically or medically compatible with this study. Note: Compatibility will be determined on the basis of consultation with the sponsor/designee.
2. The participant has a known hypersensitivity to any component of the soticlestat formulation.
3. Participants aged ≥6 years who have positive answers on item numbers 4 or 5 on the Columbia-Suicide Severity Rating Scale (C-SSRS) before dosing are excluded. This scale will only be administered to participants aged ≥6 years at the time of enrollment or participants who turn 6 after enrollment.