The primary purpose of this study (CLN-504) is to determine the safety and develop a dosing regimen of an investigational anticoagulant drug (ATI-5923) in patients with atrial fibrillation.

ATI-5923 is a structural analog of warfarin that is being developed as an oral anticoagulant. It has a similar pharmacological profile to warfarin but was designed to provide more uniform and stable anticoagulation. ATI-5923 is a selective, non-competitive inhibitor of vitamin K epoxide reductase. ATI-5923 is metabolized by esterase to a single inactive metabolite and thus is expected to have less variable metabolism, drug-drug interactions and reduced instances of under/over coagulation, making the response safer and more predictable. The primary purpose of this study is to develop a dosing regimen and INR monitoring schedule that can be applied in an outpatient setting to safely and conveniently anticoagulate patients with atrial fibrillation to a therapeutic INR. Patients with atrial fibrillation (AF) were chosen for the study population of this trial because they require anticoagulation if they have risk factors for stroke.

Inclusion Criteria:

• Documented atrial fibrillation
• Candidate for anticoagulation or currently receiving warfarin at screening
• Males or females greater than 18 years of age
• Males or females with a CHADS2 score of 0 to 2

Exclusion Criteria:

• Contraindications to anticoagulation e.g., active bleeding
• Taking other anticoagulant or antiplatelet agents other than low-dose aspirin
• History of myocardial infarction, acute coronary syndrome, or coronary revascularization procedure within the past 3 months prior to Day 1
• History of laboratory values suggestive of anemia (i.e., Hb <10 gm/dL)
• Women of childbearing potential