Stroke prevention in atrial fibrillation: understanding the new oral anticoagulants dabigatran, rivaroxaban, and apixaban

Abstract

Unlike vitamin K antagonists (VKAs), the new oral anticoagulants (NOACs)-direct thrombin inhibitor, dabigatran, and direct activated factor X inhibitors, rivaroxaban, and apixaban-do not require routine INR monitoring. Compared to VKAs, they possess relatively rapid onset of action and short halflives, but vary in relative degrees of renal excretion as well as interaction with p-glycoprotein membrane transporters and liver cytochrome P450 metabolic enzymes. Recent completed phase III trials comparing NOACs with VKAs for stroke prevention in atrial fibrillation (AF)-the RE-LY, ROCKET AF, and ARISTOTLE trials-demonstrated at least noninferior efficacy, largely driven by significant reductions in haemorrhagic stroke. Major and nonmajor clinically relevant bleeding rates were acceptable compared to VKAs. Of note, the NOACs caused significantly less intracranial haemorrhagic events compared to VKAs, the mechanisms of which are not completely clear. With convenient fixed-dose administration, the NOACs facilitate anticoagulant management in AF in the community, which has hitherto been grossly underutilised. Guidelines should evolve towards simplicity in anticipation of greater use of NOACs among primary care physicians. At the same time, the need for caution with their use in patients with severely impaired renal function should be emphasised.

Figure 1

Dabigatran etexilate: a p-gp substrate. Using energy from adenosine triphosphate, p-gp receptors in the intestinal wall actively transport molecules across the epithelial monolayer. Because of its low bioavailability, dabigatran etexilate, with its moderate affinity for the p-gp receptor, is sensitive to the actions of p-gp efflux at the intestinal wall. Once absorbed into the intestinal bloodstream, dabigatran etexilate is hydrolysed by plasma esterases to its active principle, dabigatran. The latter is no longer a substrate for p-gp efflux.