Effect of Selective Factor Xa Inhibition on Arterial Thrombus Formation Triggered by Tissue Factor/Factor VIIa or Collagen in an Ex Vivo Model of Shear-Dependent Human Thrombogenesis

Tick anticoagulant peptide (TAP) is a potent and selective inhibitor of factor Xa. TAP has shown good antithrombotic efficacy in experimental animal models of disseminated intravascular coagulation and venous and arterial thrombogenesis. In the present study we evaluated the effect of recombinant TAP (rTAP) on acute thrombus formation in human nonanticoagulated blood triggered either by tissue factor (TF) or by collagen at arterial shear conditions. The main goal was to establish the role of factor Xa in thrombus formation by use of an optimal inhibitory concentration of rTAP. Blood was drawn directly from an antecubital vein by a pump over the respective thrombogenic surfaces, which were positioned in a parallel-plate perfusion chamber. rTAP was mixed homogeneously into the flowing blood by a heparin-coated device positioned proximal to the perfusion chamber. The passage of blood through this device caused minor activation of coagulation but little activation of platelets. Fibrinopeptide A and β-thromboglobulin levels after 5 minutes of blood perfusion were, on average, 14 ng/mL and 45 IU/mL, respectively. rTAP at a plasma concentration of 0.90 μmol/L completely inhibited TF/factor VIIa–dependent thrombus formation at wall shear rates of 650 and 2600 s−1 . These shear conditions are comparable to those in medium-sized arteries and in moderately stenosed small arteries, respectively. In contrast to the TF-coated surface, rTAP was less efficient in reducing collagen-induced thrombus formation. While a significant reduction of 53% was observed at 650 s−1 , thrombus formation at 2600 s−1 was not affected by rTAP. Thus, rTAP is an efficient inhibitor of thrombin-driven human thrombus formation on the TF-rich surface but less efficient when thrombus formation is elicited by type III collagen. The lack of antithrombotic effect on collagen type III at 2600 s−1 corroborates earlier findings, showing that collagen-induced thrombus formation in blood from patients with severe factor VIII deficiency is not affected at this blood flow condition and thus is not dependent on the prothrombotic effects of thrombin.