A Clinical Trial for Participants With DEE to Assess Efficacy, Safety, Tolerability, and PK of Relutrigine

Inclusion Criteria: * Has a documented diagnosis of a developmental and epileptic encephalopathy. * Onset of seizures \<12 years old. * Has a weight \>7 kg at the time of signing consent/assent. Exclusion Criteria: * Has a history of left bundle branch block, arrhythmias, Brugada syndrome, congenital heart disease, familial short QT syndrome, or family history of sudden death or ventricular arrhythmias, including idiopathic ventricular fibrillation. * Had 2 or more episodes of convulsive status epilepticus requiring hospitalization and intubation in the 6 months prior to Screening. * Has an abnormal ECG reading, including a QT interval corrected for heart rate using Bazett's method (QTcB) \<350 and \>450 ms (males), or \<360 and \>460 ms (females) at Screening and/or on Day 1. * Any nerve stimulation must have been placed at least 3 months prior to Screening with at least 1 month of stable settings prior to Screening. * Has received any other experimental or investigational drug, device, or other therapy within 30 days or 5 half-lives (whichever is longer) prior to Screening, including any prior use of gene therapy. * Is currently pregnant or breastfeeding or is planning to become pregnant during the clinical trial or within 5 half-lives of the last study drug dose.