Studies of the Coagulation of the Blood: Part I. Some Physico-Chemical Aspects of Coagulation

Antithrombotic potential of the contact activation pathway

PURPOSE OF REVIEW

This report examines the mechanism(s) by which each protein of the contact activation system - factor XII (FXII), high-molecular-weight kininogen, and prekallikrein - influences thrombosis risk.

RECENT FINDINGS

FXII generates thrombin through contact activation via interaction with artificial surfaces as on medical instruments such as indwelling catheters, mechanical valves, stents, and ventricular assist devices. Inhibition of FXIIa-mediated contact activation prevents thrombosis under contact activation circumstances without affecting hemostasis. Current studies suggest that high-molecular-weight kininogen deficiency parallels that of FXII and inhibits contact activation. Prekallikrein inhibition contributes to thrombosis prevention by contact activation inhibition in the nylon monofilament model of transient middle cerebral artery occlusion. However, in arterial thrombosis models where reactive oxygen species are generated, prekallikrein deficiency results in downregulation of vessel wall tissue factor generation with reduced thrombin generation. Exploiting this latter prekallikrein pathway for thrombosis risk reduction provides a general, overall reduced tissue factor, antithrombotic pathway without risk for bleeding.

SUMMARY

These investigations indicate that the proteins of the contact activation and kallikrein/kinin systems influence thrombosis risk by several mechanisms and understanding of these pathway provides insight into several novel targets to prevent thrombosis without increase in bleeding risk.

The contact activation and kallikrein/kinin systems: pathophysiologic and physiologic activities

The contact activation system (CAS) and kallikrein/kinin system (KKS) are older recognized biochemical pathways that include several proteins that skirt the fringes of the blood coagulation, fibrinolytic, complement and renin-angiotensin fields. These proteins initially were proposed as part of the hemostatic pathways because their deficiencies are associated with prolonged clinical assays. However, the absence of bleeding states with deficiencies of factor XII (FXII), prekallikrein (PK) and high-molecular-weight kininogen indicates that the CAS and KKS do not contribute to hemostasis. Since the discovery of the Hageman factor 60 years ago much has been learned about the biochemistry, cell biology and animal physiology of these proteins. The CAS is a pathophysiologic surface defense mechanism against foreign proteins, organisms and artificial materials. The KKS is an inflammatory response mechanism. Targeting their activation through FXIIa or plasma kallikrein inhibition when blood interacts with the artificial surfaces of modern interventional medicine or in acute attacks of hereditary angioedema restores vascular homeostasis. FXII/FXIIa and products that arise with PK deficiency also offer novel ways to reduce arterial and venous thrombosis without an effect on hemostasis. In summary, there is revived interest in the CAS and KKS due to better understanding of their activities. The new appreciation of these systems will lead to several new therapies for a variety of medical disorders.