Novel pro-hemostatic and anti-thrombotic therapies

Prinicipal investigator: Prof Joost Meijers PhD

Blood coagulation is an important host defense mechanism to prevent bleeding after injury. However, this system is also involved in obstruction of vessels with a clot: thrombosis. In our research, we investigate the blood coagulation mechanism to better understand the events that lead to bleeding or thrombosis, and to identify (novel) agents that can be used for pro-hemostatic or anti-thrombotic therapy.

Coagulation occurs when the plasma protease activated factor VII comes into contact, and subsequently forms a complex, with tissue factor (TF). The TF/factor VIIa-complex can activate factor X and activated factor X can convert prothrombin into thrombin. Thrombin, a key contributor in coagulation, in turn converts fibrinogen into fibrin. In addition to this direct factor Xa generation, the TF/factor VIIa-complex can also indirectly activate factor X. The indirect route of factor X activation goes via the activation of factor IX. Factor IXa in the presence of cofactor factor VIIIa can activate factor X, thereby forming an amplification loop. For this sequence of events, TF must come into contact with blood, for instance upon injury or inflammation. Additionally, a TF independent pathway has evolved in vertebrates. Coagulation factor XII can be activated on charged surfaces (for instance polyphosphate, RNA) by a process called contact activation. Following autoactivation, factor XIIa can activate factor XI, eventually leading to the formation of thrombin, via factor IX, with its cofactor factor VIIIa, and factor X as described above. Deficiency in one of the factors involved in this pathway results in a variety of bleeding disorders. For example Hemophilia A (factor VIII) or Hemophilia B (factor IX) result in severe and often spontaneous bleeding. In contrast to factor VIII or factor IX deficiencies congenital factor XI deficiency, also known as Hemophilia C, typically causes only mild and injury-induced bleeding, where factor XII deficient patients do not have a bleeding tendency at all. These observations suggest that contact activation is not essential for normal haemostasis in vivo.
Two other proteins are important to mention regarding contact activation, namely high molecular weight kininogen (HK) and prekallikrein (PK). Patients deficient in either protein do not exhibit a bleeding phenotype, despite a prolonged aPTT clotting time. HK forms a noncovalent complex with factor XI, which is necessary for the binding of factor XI to negatively charged surfaces and for its activation to factor XIa by factor XIIa; HK serves as a nonenzymatic cofactor in this reaction. Prekallikrein also circulates in complex with HK and is the precursor of kallikrein (Kal), a serine protease that can liberate kinins, but can also cleave factor XII to generate additional factor XIIa.

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Fig.1 The contact system is a direct link between inflammation and coagulation. Activation of factor XII induces factor XI activation and this will ultimately lead to thrombin formation. Factor XII can be activated via negatively charged substances like polyphosphates, RNA and neutrophil extracellular traps (NETs). Activated factor XII can also cleave plasma prekallikrein (PK) into kallikrein (Kal), which on its turn releases several vasoactive substances like bradykinin. At the same time, plasma kallikrein produces additional factor XIIa. Prekallikrein and factor XI are both bound to high molecular weight kininogen (HK) in plasma. Indicated alongside of the coagulation factors are the various types of inhibitor (antisense oligonucleotides, antibodies, small molecule inhibitors and naturally occurring inhibitors).

In our research, we have identified the components of the intrinsic pathway and especially the contact system as optimal targets for antithrombotic therapy. With the use of antibodies and antisense technology, we could show effective anticoagulation without the bleeding risk that accompanies currently used anticoagulants. These studies are currently expanded to determine if this strategy is also effective and safe in thrombosis on atherosclerotic vessels.

Relevant publications

Last edited on: 18 February 2016