A Study to Evaluate the Efficacy, Safety, Tolerability, and Pharmacokinetics of HSY244 in Participants With Atrial Fibrillation

Key Inclusion Criteria:

Hemodynamically stable men and women between 18 and 80 years of age with a clinical indication for direct current cardioversion of AF
Current episode of AF has been ongoing for >= 6 hours and =< 60 days
Successful initiation and achievement of therapeutic levels of national guideline and institution-specific anticoagulation therapy as appropriate for the duration of the AF episode and risk for the participant
Completion of national guideline and institution-specific imaging evaluation for left atrial thrombi as appropriate for the duration of AF episode and risk for the participant
Participants must weigh at least 60 kg to participate in the study, and must have a body mass index (BMI) within the range of 18 - 45 kg/m2. BMI = Body weight (kg) / [Height (m)2]
Written informed consent must be obtained before any assessment is performed and only participants able to provide written informed consent themselves will be included in this study

Key Exclusion Criteria:

Use of any class I or III anti-arrhythmic medication, or other prohibited medications, within 5 half-lives before randomization, including use of amiodarone within 3 months before randomization
History or current diagnosis of electrocardiogram (ECG) abnormalities or cardiac rhythm disorders indicating significant risk of safety for participant as determined by the Investigator's interpretation of the ECG findings
Attempted or unsuccessful cardioversion within 2 weeks prior to randomization
Presence of known severe mitral regurgitation and/or known severely dilated left atrium
Pre-existing or tachycardia-induced moderate to severe cardiac dysfunction (New York Heart Association class III or IV)
History within the preceding 3 months prior to randomization of either a: myocardial infarction, unstable angina, cardiac surgery, or a percutaneous coronary intervention
History of confirmed stroke or transient ischemic attack (TIA)
History or current diagnosis of any seizure disorder or major neurological disorder or major psychiatric disorder
Sexually active males, including those post-vasectomy, must use a condom during sexual intercourse for at least 96 hours after dosing to prevent delivery of the drug via seminal fluid and should not father a child during this period
Women must be either of non-childbearing potential or child-bearing potential using highly effective non-hormonal contraception

Other protocol-defined inclusion/exclusion may apply