Pharmacological concentrations of recombinant factor VIIa restore hemostasis independent of tissue factor in antibody‐induced hemophilia mice

Abstract

Summary Background Recombinant factor VIIa (rFVIIa) has been used widely for treating hemophilia patients with inhibitory autoantibodies against factor VIII or IX. Its mechanism of action is not entirely known. A majority of in vitro studies suggested that pharmacological concentrations of rFVIIa restore hemostasis in hemophilia in a phospholipid-dependent mechanism, independent of tissue factor (TF). However, a few studies suggested that a TF-dependent mechanism plays a primary role in rFVIIa correction of bleeding in hemophilia patients. Here, we investigated the potential contribution of TF in rFVIIa-induced hemostasis in hemophilia employing a model system of FVIII antibody-induced hemophilia in TF transgenic mice. Methods Mice expressing low levels of human TF (LTF mice), relatively high levels of human TF (HTF mice) or wild-type mice (WT mice) were administered with neutralizing anti-FVIII antibodies to induce hemophilia in these mice. The mice were then treated with varying concentrations of rFVIIa. rFVIIa-induced hemostasis was evaluated with the saphenous vein bleeding model. Results Administration of FVIII inhibitory antibodies induced the hemophilic bleeding phenotype in all three genotypes. rFVIIa administration rescued the bleeding phenotype in all three genotypes. No significant differences were observed in rFVIIa-induced correction in the bleeding of LTF and HTF mice administered with FVIII antibodies. Conclusions Our results provide strong evidence supporting that the hemostatic effect of pharmacological doses of rFVIIa stems from a TF-independent mechanism.

Limited factor VIIa surface localization requirement of the factor VIIa–induced overall thrombin generation in platelet‐rich hemophilia A plasma

“…Finally, FVIIa mode of action studies using TGA should be performed with caution; in sharp contrast to our TGA finding, the relatively poor factor X–activating ability of FVIIa in solution leaves little doubt in our minds that phospholipid association is utterly important for the pharmacologic effect of FVIIa, and neither does the documented effect of FVIIa on platelet adhesion and activation under flow conditions . Recent data strongly supporting a TF‐independent mode of action of FVIIa, neither competing with zymogen for TF nor requiring TF for its own effect, support the necessity of a direct interaction with procoagulant membrane surfaces …”

Factor VIIa interaction with EPCR modulates the hemostatic effect of rFVIIa in hemophilia therapy: mode of its action

“…FVIIa AI , which binds TF with very high affinity, 15 could effectively block TF-FVIIa complex formation and thus inhibits TF-dependent thrombus formation. Studies from our laboratory 13 and others 30 showed that the hemostatic effect of rFVIIa in hemophilia stems from the TF-independent mechanism. FVIIa binding to phospholipids exposed on the activated platelets and the platelet-bound FVIIa activation of FX are thought to be responsible for the hemostatic effect of rFVIIa in hemophilia patients.…”

“…13 Briefly, bleeding was induced by a sharp incision to the saphenous vein, and the bleeding from the cut was monitored for 30 min. After each hemostasis incident, the clot was disrupted gently to reinitiate a new bleeding episode.…”

“…9;10 Although a number of mechanisms have been proposed to explain the therapeutic effect of rFVIIa, either involving TF-dependent or platelet-dependent/TF-independent mechanisms, 9;11–13 the mode of rFVIIa action in treating hemophilia is not entirely clear. We postulated earlier that FVIIa binding to EPCR might augment the hemostatic effect of rFVIIa in therapeutic conditions by effectively competing with protein C for limited EPCR on the endothelium and thus down-regulating APC generation.…”

Concizumab promotes haemostasis via a tissue factor‐factor VIIa–dependent mechanism supporting prophylactic treatment of haemophilia: Results from a rabbit haemophilia bleeding model

“…2 Thus, inhibition of TFPI with concizumab extends FX activation within the initiation phase of coagulation and protects the prothrombinase complex at the platelet surface. 6,7 In the present study, the effect of concizumab and rFVIIa on blood loss in a cuticle bleeding model in haemophilic rabbits was compared when the compounds were administered at different time points after bleeding induction. 3,4 Concizumab exhibits cross-reactivity with rabbit TFPI and reduced bleeding dose-dependently in haemophilic rabbits after intravenous or subcutaneous administration prior to and up to 5 minutes after bleeding initiation.…”