To evaluate the safety, efficacy, and tolerability of 3 doses of K201 (oral) administered for up to 28 days in subjects with recent DC cardioversion to sinus rhythm from sustained symptomatic atrial fibrillation (AF duration >3 days and <6 months).

Inclusion Criteria:

• Symptomatic AF (sustained >3 days and <6 months) and clinically indicated for cardioversion;
• Adequate anticoagulant therapy for cardioversion in accordance with standard practice as recommended by ACC/AHA/ESC guidelines or with local clinical practice;
• Hemodynamically stable (90 mmHg < systolic blood pressure < 190 mmHg)at screening and on Day 1;

Exclusion Criteria:

• Known prolonged QT syndrome or QTc interval of >0.500 sec at screening; familial long QT syndrome; previous Torsade de Pointes; ventricular fibrillation; or sustained ventricular tachycardia (VT);
• QRS >0.130 sec;
• Previous episodes of second- or third-degree atrioventricular block;
• Unsuccessful DC cardioversion attempt within 3 months; prior ablation for AF;
• Persistent bradycardia with ventricular rate below 50 beats/min, sick-sinus syndrome or pacemaker (including CRT, AICD);
• Myocardial infarction (MI), cardiac surgery, angioplasty, unstable angina or acute coronary syndrome within 30 days prior to entry into the study;
• NYHA Class III or Class IV heart failure (HF) at screening or admission, or hospitalized for HF in previous 6 months;
• Known concurrent temporary secondary causes of AF;
• Received a Class I or Class III antiarrhythmic agent (including sotalol) within 5 half-lives of randomization or amiodarone or dronedarone within 4 weeks;
• Received treatment with other drugs known to prolong the QT interval within 5 half-lives.