Phase 2 Study of the Safety, Tolerability and Pilot Efficacy of Oral Factor Xa Inhibitor Betrixaban Compared to Warfarin

Inclusion Criteria: * Male or female, age ≥18 years. * If the patient is a woman, she must be without reproductive potential (i.e., postmenopausal for ≥2 years or after hysterectomy). * AF at the time of enrollment (randomization) or documented within the last year by Holter, ECG, rhythm strip, pacemaker or other intracardiac recording, resulting in an indication for anticoagulation with warfarin, acenocumerol, phenprocoumon, or other Vitamin K antagonist in the opinion of the treating physician. * One or more of the following risk factor(s) for stroke: 1. Age 75 years or older. 2. Prior stroke, TIA or systemic (i.e., central nervous system) embolus at least 30 days remote from the time of screening. 3. Symptomatic congestive heart failure within 3 months echocardiography, radionuclide study or contrast angiography. 4. Hypertension requiring pharmacological treatment. 5. Diabetes. 6. Age of 55 years or older and previous coronary artery disease or known peripheral artery disease. Exclusion Criteria: * Body weight less than 40 kg (88 lbs). * Need for either hemodialysis or peritoneal dialysis (or likely to require it within one year). * AF due to reversible causes (e.g., thyrotoxicosis, pericarditis, cardiac surgery, pulmonary embolism). * Mechanical prosthetic valve (bioprosthetic valve is allowed) or valvular disease likely to be operated on within one year. * History (including family history) or symptoms of a congenital or acquired bleeding disorder or vascular malformation; or a history of intracranial, retroperitoneal, or intraocular bleeding within the last 6 months; or is felt to be at high risk for bleeding for other reasons including from significant liver disease. This also includes gastrointestinal bleeding within 90 days before randomization or endoscopically verified ulcer disease within 30 days of screening. * Conditions other than AF that require chronic anticoagulation (e.g. prosthetic mechanical heart valve). * Persistent, uncontrolled hypertension (SBP \>160 mm Hg on repeated measurements). * Active infective endocarditis. * Scheduled major surgery. * Planned pulmonary vein ablation or surgical procedure for cure of AF or flutter. * Recent ischemic stroke, systemic embolic event or acute coronary syndrome within 30 days. * Severe co-morbid condition with life expectancy of ≤1 year. * Previous known history of genetic coagulopathy (e.g., Factor V Leiden, Protein C Deficiency, Protein S Deficiency, Antiphospholipid Syndrome, etc.). * Evidence at Screening of: 1. Platelet count \<100,000/mm3. 2. Serum alanine aminotransferase (ALT) or aspirate aminotransferase (AST) \>2 times upper limit of normal (ULN). 3. A history (including family history) of "Long QT Syndrome". * Aspirin \>162 mg daily. * Use of verapamil (pending the availability of a drug interaction study with betrixaban). * Active alcohol or drug abuse, or psychosocial reasons that make study participation impractical. * Use of an investigational drug or device within the past 30 days. * Inability to comply with INR monitoring or other protocol-related activities. * Unable to give written informed consent.