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 apply.