Dear Sir, Once associated with the thrombus, blood coagulation factor Xa (FXa) still retains its enzymatic activity, leading to the generation of thrombin and fibrin (1–3). The persistence of clot-bound FXa and thrombin activity, which is resistant to inactivation by antithrombin III (ATIII)-dependent mechanisms, could promote sustained thrombus growth and rethrombosis after successful thrombolysis (1, 4).

Recently, Sinha et al. reported that basic small-molecule direct factor Xa (FXa) inhibitors (ie compounds that exist in positive ionised state at physiological pH) were more efficacious than neutral inhibitors in blocking the prothrombinase-bound FXa activity even though these two classes of inhibitors exhibited equivalent nanomolar Ki against free FXa (5). Accordingly, this study raises the possibility that neutral direct FXa inhibitors, which are potentially potent against free FXa activity, may be relatively weak against clot-bound FXa activity. We therefore investigated the effects of apixaban, a neutral direct FXa inhibitor (6–8) on human clot-bound and free FXa activity. Apixaban is a promising oral direct FXa inhibitor in late-stage clinical development for the prevention and treatment of venous thromboembolism, stroke prevention in atrial fibrillation and secondary prevention in acute coronary syndrome, and has shown promising safety and efficacious results for the prevention and treatment of venous thrombosis in phase 2 clinical trials (9, 10). Reagents used in this study included: S-2765 from Chromogenix AB (distributed by DiaPharma Group, West Chester, OH, USA); Tris-buffered saline (TBS: 0.05 M TRIS-HCl/0.1 M