Evaluation of Efficacy and Safety of THRV-1268 in Long QT Syndrome Type 2 (LQTS 2)

Inclusion Criteria: * Male and female participants over 15 years of age at Screening (at least the first 5 participants enrolled in Part A must be 18 years of age or older). * LQTS 2 genotype: Demonstration of KCNH2 mutation by clinical laboratory test result that is autosomal dominant (heterozygous) and considered to be pathologic or likely pathologic can be included after approval from the sponsor medical monitor or qualified delegate. * QTcF interval \>480 ms and ≤600 ms based on Screening ECG. * Body weight of at least 45 kg with body mass index between 18.0 and 40.0 kg/m2, inclusively at Screening. * Male and female participants of childbearing potential must agree to use highly effective contraception throughout the duration of the study. * Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. * Participants age 15 to \<18 years capable of providing signed assent. Exclusion Criteria: * Within 2 months prior to Screening, Participant has a history of an aborted cardiac arrest, ICD implantation, or syncopal episode due to a ventricular arrhythmia. Participants can be enrolled after the 2-month period has lapsed. * History of structural or functional cardiovascular disease, myocardial infarction or stroke or transient ischemic attack (TIA), atrial fibrillation or atrial flutter, heart failure, bundle branch block, angina pectoris, or hemodynamically signi…