Effects of TAK-442, a Direct FXa Inhibitor, and Fondaparinux on Platelet-Associated Prothrombinase in the Balloon Injured Artery of Rats

PCI patients routinely experience arterial injury during balloon angioplasty and stent deployment. At the injured arterial site, endothelial damage triggers platelet adhesion to the vascular wall and initiates platelet-associated prothrombinase complex (factor Xa [FXa]-factor Va-phospholipids-calcium) formation, resulting in the progression of thrombus formation. Recently, it was demonstrated that treatment with fondaparinux reduced the risk of death or recurrent myocardial infarction without an increase in the incidence of major bleeding compared to standard therapy in patients with STEMI. However, fondaparinux showed no superiority to placebo or heparin in patients undergoing PCI. On the other hand, small molecule direct FXa inhibitors, which are still in early stages of clinical development for the prevention of ACS, have been reported to strongly block reconstituted prothrombinase. Here, we compared the effects of TAK-442, a novel direct FXa inhibitor on platelet-associated prothrombinase activity, with fondaparinux. TAK- 442 and fondaparinux inhibited endogenous FXa activity in platelet-poor plasma from humans (IC₅₀: 53 nM, TAK-442; 11 nM, fondaparinux) and rats (IC₅₀: 32 nM, TAK-442; 19 nM, fondaparinux). When TAK-442 or fondaparinux were preincubated with free FXa, they each inhibited in vitro reconstituted prothrombinase activity, with IC₅₀ values of 28 nM and 3.9 nM, respectively. In the absence of free FXa preincubation, TAK-442 retained this inhibitory effect (IC₅₀, 51 nM); the inhibitory effect of fondaparinux, however, decreased dramatically (IC₅₀, 1700 nM). In a rat model of balloon injury, thrombin activity on the surface of injured vessels increased to 3.3-, 20- and 5.7-fold that of intact aorta at 5 min, 1 hr, and 24 hrs after the injury, respectively. At 1 hr after the injury, 300 nM TAK-442 significantly (p<0.025) inhibited platelet-associated thrombin generation on the surface of injured aorta by 99% (IC₅₀, 19 nM), whereas the same concentration of fondaparinux showed no significant (p=0.076) inhibition (IC₅₀, >300 nM). These results highlight a limitation of fondaparinux in inhibiting platelet-associated prothombinase activity that leads to thrombus formation compared with TAK-442, and suggest that TAK-442 may be more effective in preventing arterial thrombosis in patients undergoing percutaneous coronary interventions.