The Regulation of the Factor VII-Dependent Coagulation Pathway: Rationale for the Effectiveness of Recombinant Factor VIIa in Refractory Bleeding Disorders

Abstract

We have explored the molecular basis of the clinical therapeutic effect of factor VIIa in hemophilia A using empirical reconstituted in vitro thrombin generation models. Tissue factor acts as a receptor and activator of preexistent but virtually inactive two-chain plasma factor VIIa. However, most of the factor VII circulates as a single-chain inactive zymogen (10 nM) and a trace (approximately 10-100 pM) circulates in the active two-chain form. Empirical reconstitution (purified factors VIIa, X, IX, VIII, V, prothrombin, and relipidated tissue factor) showed that plasma concentrations of factor VII (10 nM) prolong the initiation phase of thrombin generation significantly at low concentrations of tissue factor and 100 pM factor VIIa. Thus, we show for the first time that the zymogen factor VII may have a very significant inhibitory action on thrombin generation at physiologic ratios of factor VII to factor VIIa. The inhibition kinetics of factor Xa generation by low concentrations of tissue factor indicate that factor VII inhibits the reaction by competition for the initial binding of factor VIIa to tissue factor. Physiological concentrations of factor VII also inhibit the maximal rate of thrombin generation by 100 pM factor VIIa in the absence of factor VIII. Increasing the concentration of factor VIIa to 2 nM in this hemophilia A model overcame the inhibition of thrombin generation by 10 nM factor VII. Increasing the concentration up to 10 nM factor VIIa in the absence of factor VIII completely normalized the thrombin generation profile to that observed in the presence of factor VIII and 10 nM factor VII/100 pM factor VIIa. The levels of factor VIIa that overcome the inhibitory effect of factor VII and that normalize thrombin generation in our model are consistent with the observed plasma levels of factor VIIa needed to manage hemophilia A. Our data strongly indicate that the therapeutic mechanism of factor VIIa in the medical treatment of hemophiliacs with inhibitors is in large part based on overcoming the inhibitory effect of factor VII on thrombin generation.

Use of Recombinant Factor VIIa in Patients With Warfarin-Associated Intracranial Hemorrhage

“…It acts by directly binding to tissue factor to activate thrombin generation, bypassing much of the cascade of coagulation factors (14). There may also be an independent effect on activated platelet function (15).…”

The use of recombinant factor VIIa in the treatment of bleeding disorders

“…By contrast, Mann and colleagues [23][24][25][26] hold the view that the mechanism of action of factor VIIa is dominated by the tissue factor-mediated activity with only a minimal, if any, contribution from TF-independent fVIIa activity. In studies using TF incorporated into lipid vesicles, van't Veer et al observed that fVIIa overcomes the inhibitory effect of zymogen fVII on thrombin generation.…”

“…In studies using TF incorporated into lipid vesicles, van't Veer et al observed that fVIIa overcomes the inhibitory effect of zymogen fVII on thrombin generation. 23,24 While the influence of rfVIIa is dependent on TF, Butenas et al observed that phospholipids or platelets substantially increase the hemostatic potential of rfVIIa. 25,26 Lisman and De Groot have recently analyzed the alternative mechanisms of action of rfVIIa.…”

The glycoprotein Ib-IX-V complex contributes to tissue factor–independent thrombin generation by recombinant factor VIIa on the activated platelet surface

“…4,5 However, different independent experiments have shown that the effect of rFVIIa can proceed via TF-dependent 3,6 as well as TF-independent 7-9 pathways, and it has been postulated that both mechanisms are operative in vivo. 10 Although the mechanisms through which rFVIIa exerts its activity have been studied extensively, there are still some unresolved questions.…”