Factor XII sits atop one branch of the clotting cascade but has not been pursued for treating and preventing thrombosis because it was seen as a minor player in blood coagulation. Now, a group from the Karolinska Institute and CSL Ltd. has shown that inhibiting the factor can prevent clotting without increasing the risk of bleeding,1 making it one of the only anticoagulants able to accomplish such a feat.

A group from Isis Pharmaceuticals Inc. and McMaster University obtained similar results with factor XII,2 but the biotech maintains that blocking a downstream target, factor XI, is a better approach.

Whether blocking factor XII or factor XI will be a better strategy will probably be determined in the clinic (see "Targeting the intrinsic pathway of coagulation and the kallikrein-kinin system").

Passing over factors

The main reason factor XII was passed over as an anticoagulant stemmed from observational studies of patients who had a deficiency of the enzyme. Those patients had normal hemostatic capacity-the formation of fibrin-based clots-and did not have an increased risk of bleeding from either an injury or a spontaneous event.3

As a result, said Thomas Renné, almost everyone assumed that factor XII was not important for forming clots. Renné, a professor of clinical chemistry and coagulation research at the Karolinska Institute and Karolinska University Hospital as well as a professor of clinical chemistry at the University Medical Center Hamburg-Eppendorf, suspected otherwise.

When Renné was a scientist at the University of Wuerzburg in 2005, he and his colleagues showed that factor XII-deficient mice were protected from thrombosis, which hinted that the enzyme could be a target for antithrombotic therapy.4

That work caught the interest of CSL, which set about exploring whether the outcomes in knockout mice could be replicated by pharmacological inhibition of factor XII.

Con Panousis, director of antibody technologies at CSL, said that the company spent several years testing small protein inhibitors of factor XII, but the compounds had off-target effects or were immunogenic.

Now, CSL and Karolinska have developed an antibody against factor XII
and have provided the clearest picture to date of what happens when the target is blocked in animals.

The team screened a human Fab-based phage antibody library for compounds that could specifically inhibit the proteolytic activity of factor XIIa, which is the active form of factor XII. The group isolated an antibody, 3F7, which dose-dependently inhibited factor XIIa activity in vitro with an IC50 of 13 nM.1

In a mouse model of thrombosis, 3F7 dose-dependently decreased occlusion rates and decreased time to occlusion compared with control antibody or saline. In the same mice, 3F7 neither increased blood loss nor prolonged bleeding.

In a rabbit model of cardiopulmonary bypass, 3F7 provided thromboprotection comparable to that of heparin. However, the antibody did not prolong incision-provoked bleeding.

That finding suggests that 3F7 might prevent thrombosis in acute indications like bypass surgery without increasing the risk of bleeding.

Results were published in Science Translational Medicine. Scientists from the Swedish University of Agricultural Sciences also contributed to the study.

The take-home message, the authors wrote, is that "FXIIa-driven fibrin formation is essential for pathological thrombus formation and propagation but has no function for fibrin formation during 'normal' hemostasis at a site of injury."

In a related paper, a team from Isis Pharmaceuticals and McMaster University led by Jeffrey Weitz also identified factor XII as an anticoagulation target. The group showed that factor XII-directed antisense oligonucleotides (ASOs) protected against catheter-induced thrombosis in rabbits.2

Similar results were seen with antisense inhibitors of factor XI, which sits downstream of factor XII (see "Factor XIIa pathways") and had previously been shown by Isis to prevent thrombus formation without causing an increase in bleeding in mouse models of stroke.

Weitz is chair of thrombosis and a professor of medicine at McMaster University and executive director of the Thrombosis and Atherosclerosis Research Institute.

Results were published in Blood.

Factor factory

Renné said that the data on factor XII may change the classical concept of coagulation, which holds that coagulation and bleeding are inextricably bound. Indeed, marketed anticoagulants including warfarin and antiplatelet therapies are used to prevent thrombosis, but they also result in dose-dependent increases in the risk of major bleeding.

Weitz agreed but added that "factor XI may be a better target because thrombin can feed back and activate factor XI, thereby bypassing factor XIIa inhibitors."

He did say that inhibitors of either target could have utility in a broad spectrum of indications.

"We believe that in addition to central venous catheters, other blood-contacting medical devices, such as vascular grafts, mechanical heart valves and left ventricular assist devices, also trigger clotting. Therefore, the ASOs could be used for long-term prevention of clotting for a wide variety of medical devices."

Isis' ISIS-FXIRx, a factor XI-targeted ASO, is in Phase II testing to prevent coagulation without increasing the risk of bleeding in patients undergoing knee replacement surgery. Results are expected this year.

At least one other company thinks that it has found a way to provide anticoagulation without increasing bleeding risk. XO1 Ltd.'s ichorcumab, a mAb against an allosteric site on thrombin (factor IIa; F2), is in preclinical development.5

CSL holds patents and patent applications covering 3F7, an optimized version of the mAb and FXII inhibition in prevention of thrombi formation. The IP is not currently available for licensing for antithrombotic indications. CSL said that it is focused on inhibiting factor XII to treat hereditary angioedema (HAE) and has an antibody against the target in preclinical development for that indication.

Panousis said that factor XII is also involved in bradykinin-induced edema, which is dysregulated in HAE. CSL markets Berinert, an injectable complement 1 (C1) esterase inhibitor derived from human plasma, to treat HAE.

Donner, A. SciBX 7(8); doi:10.1038/scibx.2014.217 Published online Feb. 27, 2014


1.   Larrson, M. et al. Sci. Transl. Med.; published online Feb. 5, 2014; doi:10.1126/scitranslmed.3006804 Contact: Thomas Renné, Karolinska Institute, Stockholm, Sweden e-mail: thomas@renne.net

2.   Yau, J.W. et al. Blood; published online Feb. 5, 2014; doi:10.1182/blood-2013-12-540872 Contact: Jeffrey I. Weitz, Thrombosis and Atherosclerosis Research Institute, Hamilton, Ontario, Canada e-mail: weitzj@taari.ca

3.   Renné, T. et al. Blood 120, 4296-4303 (2012)

4.   Renné, T. et al. J. Exp. Med. 202, 271-281 (2005)

5.   Hansen, S. BioCentury 21(26), A26; July 1, 2013


CSL Ltd. (ASX:CSL), Melbourne, Victoria, Australia

Isis Pharmaceuticals Inc. (NASDAQ:ISIS), Carlsbad, Calif.

Karolinska Institute, Stockholm, Sweden

Karolinska University Hospital, Stockholm, Sweden

McMaster University, Hamilton, Ontario, Canada

Swedish University of Agricultural Sciences, Uppsala, Sweden

Thrombosis and Atherosclerosis Research Institute, Hamilton, Ontario, Canada

University Medical Center Hamburg-Eppendorf, Hamburg, Germany

University of Wuerzburg, Wuerzburg, Germany

XO1 Ltd., Cambridge, U.K.