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

Key Inclusion Criteria: * At screening, written informed consent were required to be obtained before any assessment was performed and only participants able to provide written informed consent themselves were included in this study. * Hemodynamically stable men and women (either of non-child-bearing potential or child bearing potential with highly effective contraception) between 18 and 80 years of age (inclusive) at screening with a clinical indication for direct current cardioversion of AF. * At screening, current episode of AF had 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. * At screening, participants were required to weigh at least 60 kg to participate in the study and were required to have a body mass index (BMI) within the range of 18 - 45 kg/m\^2. BMI = Body weight (kg) / \[Height (m)\]\^2 * At screening, vital signs (systolic blood pressure and pulse rate) were assessed in the sitting position. Sitting vital signs were required to be within the following ranges (exclusive): * systolic blood pressure between 100-160 mmHg and diastolic blood pressure 60-100 mmHg * pulse rate (ventricular rate) between 60-120 bpm. Key Exclusion Criteria: * Women of child-bearing potential, defined as all women physiologically capable of becoming pregnant, unless they were using highly effective methods of contraception during dosing and for 4 days after stopping of investigational drug. * Sexually active males unwilling to use a condom during intercourse while taking investigational drug and for 96 hours after study drug administration. A condom was required for all sexually active male participants to prevent them from fathering a child AND to prevent delivery of the investigational drug via seminal fluid to their partner. In addition, male participants could not donate sperm for the time period specified above. * Use of any anti-arrhythmic class I or III drug (including Ranolazine \[Ranexa\]) within 5 half lives before randomization; including use of amiodarone within 3 months before randomization. * At screening, history of current diagnosis of ECG abnormalities or cardiac rhythm disorders as determined by the Investigator's interpretation of the ECG findings indicating a significant risk for participating in the study, such as: * History of Torsades de Pointes (TdP), any other polymorphic ventricular tachycardia, sustained monomorphic ventricular tachycardia, long QT syndrome, or Brugada syndrome. * Wolfe-Parkinson-White (WPW) syndrome * In the absence of a complete bundle branch block a resting QTcF \>460 msec for men and \>470 msec for women (mean of ≥ 5 consecutive QT intervals) * In the presence of a complete bundle branch block, a prolonged QTcF or JTc that, in the opinion of the investigator, may pose a risk to patient safety * Third-degree (complete) heart block, or second-degree Mobitz type II heart block * 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 and IV). * History within the preceding 3 months prior to randomization of: myocardial infarction, unstable angina, cardiac surgery, or a percutaneous coronary intervention. * History of a confirmed stroke or transient ischemic attack (TIA). * History or current diagnosis of any seizure disorder, epilepsy, significant head trauma, or other disorders increasing the risk for seizures. * History or current diagnosis of a major neurologic or psychiatric disorder that, in the opinion of the investigator, poses a risk to patient safety to participate.