Atrial fibrillation is already the most common clinically significant cardiac 
arrhythmia and a common cause of stroke. Vitamin K antagonists are very 
effective for the prevention of cardioembolic stroke but have numerous 
limitations that limit their uptake in eligible patients with AF and reduce 
their effectiveness in treated patients. Multiple new anticoagulants are under 
development as potential replacements for vitamin K antagonists. Most are small 
synthetic molecules that target factor IIa (e.g., dabigatran etexilate, 
AZD-0837) or factor Xa (e.g., rivaroxaban, apixaban, betrixaban, DU176b, 
idrabiotaparinux). These drugs have predictable pharmacokinetics that allow 
fixed dosing without laboratory monitoring, and are being compared with vitamin 
K antagonists or aspirin in phase III clinical trials [corrected]. A new vitamin 
K antagonist (ATI-5923) with improved pharmacological properties compared with 
warfarin is also being evaluated in a phase III trial. None of the new agents 
have as yet been approved for clinical use.