Haemostatic effects of recombinant coagulation factor VIIa

Recombinant coagulation factor VIIa (rFVIIa) has recently become available for treatment of patients with inhibitor-complicated haemophilia. It has been postulated that rFVIIa could become a universal haemostatic agent. Case reports and small studies confirm efficacy and safety of rFVIIa in a variety of haemostatic disorders including liver disease, platelet-related bleeding disorders, and in patients without prior coagulopathy who are bleeding excessively due to trauma or surgery. Although clinical evidence for efficacy and safety of rFVIIa in its various applications is rapidly accumulating, very little is know about the molecular mechanisms by which rFVIIa induces haemostasis. In this thesis, novel hypotheses on the mechanism of action of rFVIIa in different haemostatic disorders are described. In the major part of this thesis, the focus is on the involvement of thrombin activatable fibrinolysis inhibitor (TAFI)-mediated down-regulation of fibrinolysis in in vitro systems representing various haemostatic disorders, and the effect of rFVIIa. In chapter 2, it has been described that rFVIIa, next to its procoagulant properties, possesses antifibrinolyic properties in plasma from patients with severe haemophilia A. This antifibrinolytic effect was due to enhanced activation of TAFI. Chapter 3 describes the complex haemostatic disorder induced by liver disease, and in chapter 4 the role of TAFI-mediated downregulation of fibrinolysis in liver disease was investigated. In contrast to earlier reports, we did not find evidence of hyperfibrinolysis in liver disease, and also TAFI activation appeared normal, despite decreased plasma levels of TAFI antigen. In chapter 5, the effect of rFVIIa on coagulation and fibrinolysis in plasma from patients with liver disease are described. In contrast to the situation in haemophilia, rFVIIa did not display an antifibrinolytic effect in plasma from patients with liver disease. In contrast to the haemophilia situation, TAFI activation is already optimal in plasma from these patients. In chapter 6, the effect of anticoagulants targeting different steps in the coagulation cascade on coagulation and fibrinolysis were examined. It was found that anticoagulant drugs targeting factor Xa, but not those inhibiting thrombin or tissue factor accelerated fibrinolysis in vitro. In chapter 6, the effects of rFVIIa on coagulation and fibrinolysis of plasma anticoagulated with fondaparinux (a novel anticoagulant, specifically directed against factor Xa) were investigated. rFVIIa was able to completely reverse the effects of fondaparinux on coagulation, and was partially able to reverse the induction of hyperfibrinolysis by the drug. In chapter 8, the focus was on the effects of rFVIIa on platelet adhesion under flow conditions. More specifically, a model for the mechanism of action of rFVIIa in patients with Glanzmanns thrombasthenia, and patients treated with anti-aIIbB3 drugs is proposed based on results of in vitro perfusion studies. It was shown that tissue factor-indepdent thrombin generation via rFVIIa restored the defective platelet deposition induced by aIIbB3-deficiency. In chapter 9, the results presented in the previous chapters are discussed in a broader context. The advances in the understanding of the mechanism of action of rFVIIa in its different applications are discussed.