Thrombin generation testing is becoming an increasingly important tool to measure the variables of the clotting system. Under normal conditions the curve is bell-shaped, yet after addition of direct, reversible factor Xa (FXa) inhibitors, the shape of the curve changes. Direct FXa inhibitors cause a strong inhibition (in vitro and ex vivo) on the peak, whereas the ETP is affected to a smaller extent, due to a protraction of the thrombin generation process. This leads to a long plateau or a ‘double peak’.
To investigate the mechanism behind the protraction of the thrombin generation curve.
Thrombin generation (TG) was determined by using Calibrated Automated Thrombinography (CAT) in platelet poor plasma (PPP), platelet rich plasma and whole blood. The effect of direct FXa inhibitors was investigated with both a parenteral (otamixaban) and an oral agent (rivaroxaban). The two inhibitors were tested at different concentrations, including the plasma concentrations that would be found after therapeutic dosing (otamixaban: 100, 250 and 400 nM and rivaroxaban: 200, 400 and 800 nM). Thrombin generation was performed in control plasma, factor VII (FVII), FVIII, FIX, FXI and TFPI (tissue factor pathway inhibitor) deficient plasmas, activated with tissue factor (TF), FIXa or kaolin.
Thrombin generation curves in PPP in the presence of direct FXa inhibitors display a protracted shape with a strong inhibition of the peak. When the intrinsic pathway is bypassed (e.g. in FVIII or FIX deficient plasma), the first part of the peak is no longer present. When the extrinsic pathway is evaded (e.g. by activating with FIXa), the second part of the peak disappears. This leads us to believe that when a direct FXa inhibitor is present, the first part of thrombin generation is due to the intrinsic tenase complex and the second part can be attributed to the extrinsic tenase (prothrombinase). When TFPI deficient plasma is activated with TF, the curve returns to its normal bell-shape. So, TFPI is an important contributor to the second peak. When TG was activated with kaolin, we could not distinguish an effect. In platelet rich plasma and whole blood, a stronger effect on peak height than on ETP is found, however the distinct shape that can be discerned in platelet poor plasma is not seen.
Addition of direct FXa inhibitors to PPP results in a protracted TG curve, which could also be described as a double peak. The first peak can be attributed to the actions of the intrinsic pathway (FVIII and FIX), the second part of the peak is due to mechanisms in the extrinsic pathway. The protraction of the peak can also be restored by activating TFPI deficient plasma with TF, so this also contributes to the effect of the extrinsic pathway. The inhibition of the negative feedback on the TF-FVIIa complex by the TFPI-FXa complex will prevent the shutting off of the extrinsic pathway.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.