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 arecurrently taking ranolazine or Class I antiarrhythmic drugs such as mexiletine) at 3or 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 QTsyndrome (LQT2)

• Known or suspected history of seizures or epilepsy

• History of heart failure defined as New York Heart Association (NYHA) Class IV and/orknown 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 filtrationrate (eGFR) < 30 mL/min/1.73m^2, using the 4 Variable Modification of Diet in RenalDisease (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 totalbilirubin > 1.5 x ULN

• An aborted cardiac arrest (ACA), implantable cardioverter-defibrillator (ICD)implantation, syncopal episode, or appropriate ICD therapy within 3 months prior toscreening

• Any other condition or circumstance that in the opinion of the investigator wouldpreclude compliance with the study protocol Note: Other protocol defined Inclusion/Exclusion criteria may apply.