Effect of Eleclazine on Shortening of the QT Interval, Safety, and Tolerability in Adults With Long QT Syndrome Type 3

Key Inclusion Criteria: * Individuals with an established diagnosis of LQT3 (by genotype testing) * Mean (of triplicate) QTc interval ≥ 480 msec (or ≥ 460 msec, for individuals who are currently taking ranolazine or Class I antiarrhythmic drugs such as mexiletine) at 3 or more time points, determined by standard 12-lead ECG, at screening Key Exclusion Criteria: * Known mutations associated with type 1 long QT syndrome (LQT1) or type 2 long QT syndrome (LQT2) * Known or suspected history of seizures or epilepsy * History of heart failure defined as New York Heart Association (NYHA) Class IV and/or known left ventricular ejection fraction (EF) ≤ 45% * Body mass index (BMI) ≥ 40 kg/m\^2 at screening * Severe renal impairment at screening (defined as an estimated glomerular filtration rate (eGFR) \< 30 mL/min/1.73m\^2, using the 4 Variable Modification of Diet in Renal Disease (MDRD) equation, as determined by the study center) * Abnormal liver function tests at screening, defined as alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \> 2 x upper limit of normal (ULN), or total bilirubin \> 1.5 x ULN * An aborted cardiac arrest (ACA), implantable cardioverter-defibrillator (ICD) implantation, syncopal episode, or appropriate ICD therapy within 3 months prior to screening * Any other condition or circumstance that in the opinion of the investigator would preclude compliance with the study protocol Note: Other protocol defined Inclusion/Exclusion criteria may ap…