Factor XI and factor XII as targets for new anticoagulants

Thrombosis Research 141S2 (2016) S40–S45

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Thrombosis Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r . c o m / l o c a t e / t h r o m r e s

Factor XI and factor XII as targets for new anticoagulants Jeffrey I. Weitz* Departments of Medicine and Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Canada; and Thrombosis and Atherosclerosis Research Institute, Hamilton, Canada

K E Y W O R D S

A B S T R A C T

anticoagulants contact system intrinsic pathway factor XII factor XI thrombosis

 lthough the non-vitamin antagonist oral anticoagulants produce less intracranial bleeding than warfaA rin, serious bleeding still occurs. Therefore, the search for safer anticoagulants continues. Factor XII and factor XI have emerged as promising targets whose inhibition has the potential to prevent thrombosis with little or no disruption of hemostasis. Thus, thrombosis is attenuated in mice deficient in factor XII or factor XI and patients with congenital factor XII deficiency do not bleed and those with factor XI deficiency rarely have spontaneous bleeding. Strategies targeting factor XII and XI include antisense oligonucleotides to decrease their synthesis, inhibitory antibodies or aptamers, and small molecule inhibitors. These strategies attenuate thrombosis in various animal models and factor XI knockdown with an antisense oligonucleotide in patients undergoing knee replacement surgery reduced postoperative venous thromboembolism to a greater extent than enoxaparin without increasing bleeding. Therefore, current efforts are focused on evaluating the efficacy and safety of factor XII and factor XI directed anticoagulant strategies. © 2016 Elsevier Ltd. All rights reserved.

Introduction

epidemiological evidence that patients with congenital deficiency of factor XI are at lower risk for VTE and ischemic stroke [5,6], the observation that mice deficient in factor XI or XII have attenuated thrombosis at sites of arterial or venous injury [7,8], the recent identification of naturally-occurring polyphosphates as potential physiological activators of the contact system [9-12], and the fact that patients with congenital factor XII deficiency do not bleed [3,4], and those with congenital factor XI deficiency have only a mild bleeding diathesis [13]. Therefore, if factor XII and XI are important in thrombosis, they are promising targets for development of safer anticoagulants because they have little or no role in hemostasis. Focusing on factors XI and XII, this review (a) describes the contact activation pathway, (b) identifies the roles of factors XI and XII in thrombosis, (c) lists the factor XI and factor XII directed anticoagulant strategies under development, (d) highlights the relative advantages and limitations of factor XI versus factor XII as targets for new anticoagulants, (e) describes the clinical data with these agents, and (f) summarizes the opportunities and challenges for factor XI or factor XII directed anticoagulant strategies for various indications.

The holy grail of anticoagulation is to attenuate thrombosis without affecting hemostasis. Although the non-vitamin K antagonist oral anticoagulants (NOACs) come closer to this goal than vitamin K antagonists such as warfarin, we still have a long way to go. Thus, the NOACs have been shown to be at least as effective as warfarin for stroke prevention in atrial fibrillation and for treatment of venous thromboembolism (VTE), but produce less serious bleeding, particularly intracranial bleeding [1,2]. Nonetheless, serious bleeding still occurs with the NOACs. Thus, the annual rate of major bleeding with the NOACs in patients with AF is 2% to 3%, while the annual rate of intracranial bleeding is 0.3% to 0.5% [1]. Therefore, there remains a need for safer anticoagulants for long-term indications. Development of safer anticoagulants depends on identification of targets beyond thrombin and factor Xa, the enzymes inhibited by dabigatran and the oral factor Xa inhibitors, respectively. Recent interest has focused on components of the contact pathway, particularly factors XI and XII, as potential targets [3,4]. Interest in these upstream clotting factors stems from

Contact Activation Pathway   * Correspondence to: Thrombosis & Atherosclerosis Research Institute, 237 Barton Street East, Hamilton, Ontario, L8L 2X2, Canada.

E-mail address: [email protected] (Jeffrey I. Weitz).

Exposure of blood to negatively charged substances or artificial surfaces triggers thrombin generation and fibrin formation



Jeffrey I. Weitz / Thrombosis Research 141S2 (2016) S40–S45

via a series of reactions known as contact activation (Figure 1). The process starts with reciprocal activation of factor XII and prekallikrein (PK); reactions that are enhanced in the presence of high molecular weight kininogen (HK) [14,15]. Factor XIa activates factor IX and the resultant factor IXa binds to factor VIIIa on the activated platelet surface to form intrinsic tenase, which then activates factor X and triggers thrombin generation and fibrin formation. Patients with congenital deficiency of factor XII, PK or HK have evidence of impaired thrombin generation and fibrin formation in surface-dependent tests of coagulation, such as the activated partial thromboplastin time (aPTT). Despite a prolonged aPTT, such patients do not bleed, indicating that these contact proteins are not required for hemostasis [3,4]. Factor XIIa propagates coagulation by activating factor XI, whereas kallikrein cleaves HK to yield bradykinin; a pro-inflammatory peptide [14,15]. In contrast, patients with congenital factor XI deficiency have a mild bleeding diathesis [13]. Thus, spontaneous bleeding is rare, but hemorrhage can occur with trauma or surgery. These observations suggest that factor XI contributes to hemostasis in a contact pathway-independent manner. Supporting this concept is the

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evidence that factor XI can be back activated by thrombin in a reaction that is enhanced by inorganic polyphosphates, DNA and RNA [10,16,17]. The lack of bleeding in patients with factor XII deficiency and the mild bleeding diathesis in those with factor XI deficiency prompted the interest in these proteins as targets for safer anticoagulants [3,4]. However, inhibiting these factors only makes sense if factor XII and XI are important drivers of thrombosis. Role of Factors XII and XI in Thrombosis Epidemiological evidence supports a role for factor XI in thrombosis. Thus, patients with congenital factor XI deficiency appear to be protected from VTE and ischemic stroke [5,6]. Furthermore, subjects with higher levels of factor XI are at greater risk for VTE and ischemic stroke than those with lower levels [18,19], and the levels of factor XI correlate with stroke risk in women taking oral contraceptives [20]. Therefore, factor XI appears to be important in the pathogenesis of VTE and ischemic stroke. The role of factor XI in myocardial infarction is less clear. Although factor XI levels correlated with the risk of myocardial

Figure 1. Contact activation and thrombin generation. Factor (F) XII and prekallikrein (PK) are activated on anionic surfaces to FXIIa and kallikrein (K), respectively. These reactions are enhanced by high molecular weight kininogen (HK). Kallikrein can activate additional FXII and can cleave HK to liberate bradykinin, which triggers inflammation. FXIIa propagates coagulation by activating FXI. The resultant FXIa then activates FIX. FIXa binds to FVIIIa on the activated platelet surface to form intrinsic tenase, which is an efficient activator of FX. FX can also be activated by the tissue factor/FVIIa complex, so called extrinsic tenase, which forms on the surface of tissue factor expressing cells or microparticles. Extrinsic tenase can also activate FIX to generate additional FXa. FXa binds to factor Va on the activated platelet surface to form prothrombinase, which converts prothrombin to thrombin. Thrombin triggers clotting by converting fibrinogen to fibrin and by serving as a potent platelet agonist. Thrombin feeds back to activate FXI, a reaction enhanced by polyphosphates, DNA and RNA. Thrombin also feeds back to activate FVIII and FV, the cofactors for intrinsic tenase and prothrombinase, respectively.

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infarction in some studies [21], this finding was not confirmed in others [22]. The divergent results may reflect differences in study design. Alternatively, it is possible that the contribution of factor XI to thrombosis in the coronary circulation differs from that in other vascular beds. Additional studies are needed to distinguish between these two possibilities. The epidemiological evidence for a role of factor XII in thrombosis is less convincing. Thus, patients with hereditary angioedema secondary to impaired regulation of factor XIIa and kallikrein are not prone to thrombosis and John Hageman, the first person identified with factor XII deficiency, died of a pulmonary embolism as a complication of immobilization after pelvic fracture. Likewise, patients with congenital factor XII deficiency do not appear to be at lower risk for VTE or to be protected from ischemic stroke, nor is there an association between factor XII levels and the risk of VTE [19,23,24]. Therefore, at least on the basis of currently available epidemiological data, the evidence for a role in thrombosis in humans is stronger for factor XI than for factor XII. In mouse models, factor XI and XII appear to be equally important drivers of thrombosis, whereas in non-human primate models, factor XI is the more important contributor. Thus, mice deficient in factor XII or factor XI exhibit equally attenuated thrombosis at sites of arterial or venous injury [7,8]. Thrombi formed in the vessels of such mice are unstable under flow conditions, and are prone to fragmentation. Therefore, these findings support the hypotheses that thrombosis in these models is driven by contact activation and that factor XII and factor XI are important for thrombus stabilization. The results in non-human primates are different. Thus, in a baboon arterio-venous shunt model, antibodies against factor XI attenuated platelet and fibrin deposition more than those directed against factor XII [25,26]. Supporting the predominant role of factor XI in this model, factor XI knockdown with an antisense oligonucleotide (ASO) reduced thrombosis in a concentration-dependent manner. Attenuation of thrombosis was observed with as little as a 50% reduction in factor XI levels, and was maximal with reductions over 80% [27]. Therefore, factor XI appears to be a more important driver of thrombosis than factor XII in non-human primate models. It is unclear whether the same is true in human and studies are needed to compare the efficacy of factor XI and XII directed anticoagulant strategies. Strategies to Target Factor XII or Factor XI Strategies to target factor XII and factor XI include (a) factor XII or factor XI directed ASOs that reduce hepatic synthesis of the clotting proteins [27-29], (b) monoclonal antibodies that bind to factor XII or factor XI and block their activation or their activity [27-30], (c) factor XII directed aptamers [31], and (d) inhibitors that block the active site of factor XIa [32-34]; allosteric modulators of factor XI have also been described (Table1) [35,36]. Aptamers against factor XI and active site directed inhibitors of factor XIIa have not been described. Each strategy has a different mechanism of action and unique characteristics (Table 2). Thus, antibodies against factor XII or factor XI [27-30], the factor XII directed aptamers [31] and factor XI directed active site inhibitors [32-34] have a rapid onset of action, whereas ASOs have a delayed onset of action because it takes at least 2 to 3 weeks of treatment to lower factor XII or factor XI levels into the therapeutic range [27-29]. The slow onset of action of ASOs limits their utility for initial treatment of thrombosis or for immediate thromboprophylaxis. Instead, they will be most useful for chronic therapy. In contrast, with a rapid onset of action, antibodies, aptamers, and active site inhibitors could be used for acute or chronic treatment. The offset of action also varies with the different strategies. Active site inhibitors and aptamers have a rapid offset of action, whereas the offset of action of ASOs and antibodies is slow.

Table 1 Mode of Action of Factor XII or Factor XI Directed Anticoagulants. Target Agent

Factor XII

Factor XI

Antisense oligonucelotides [27,29,39,42,43]

Reduce hepatic synthesis of factor XII

Reduce hepatic synthesis of factor XI

Antibodies [25,26,30,40]

Bind factor XII and block its activation

Bind factors XI and block its activation and its capacity to activate factor IX. Bind factor XIa and block its activity

Small molecule inhibitors [32-34]

Not reported

Bind to the active site of factor XIa and block its activity

Allosteric inhibitors [35,36]

Not reported

Bind to charged residues on factor XI and modulate factor XIa activity

Aptamer [31]

Binds to factor XII and blocks autoactivation and factor XIIa activity

Not reported

The prolonged half-life of factor XI directed antibodies or ASOs could be problematic if there is bleeding with trauma or surgery. Although replacement with factor XI would provide temporary reversal with ASO therapy, factor XIa directed antibodies would not be reversed with factor XI replacement and factor VIIa may be required. Therefore, each strategy has its strengths and weaknesses. Factor XII or Factor XI: Which is the Better Target? The advantage of factor XII as a target is safety; because factor XII has no role in hemostasis, strategies targeting it will not induce bleeding. In contrast, strategies targeting factor XI may be associated with bleeding, particularly mucosal bleeding, such as bleeding from the oropharynx with dental procedures or excessive menstrual bleeding because these are the complications that can occur in patients with severe factor XI deficiency [13]. A potential limitation of factor XII as a target is that its role in thrombosis is less certain than that of factor XI based on epidemiological data [18-24]. In addition, targeting factor XII may be of little or no benefit in situations where thrombosis is initiated by tissue factor because it is likely that thrombin generated via extrinsic tenase will back activate factor XI, thereby bypassing factor XII [37]. Therefore, despite the potential for mild bleeding, factor XI may be a better target than factor XII for most indications. An exception may be clotting on blood-contacting medical devices or extracorporeal circuits because thrombosis on artificial surfaces is triggered by activation of factor XII [38]. Consequently, factor XII may be a better target than factor XI in this setting, particularly because inhibition of factor XIIa will not only prevent clotting, but will also attenuate inflammation by Table 2 Comparison of the Characteristics of the Various Factor XI Directed Anticoagulant Therapies. Characteristic

ASO [27,29,39,42,43]

Antibodies [25,30]

Small Molecules [32-36]

Mechanism

Reduces hepatic synthesis of factor XI

Bind factor XI and blocks its activation or bind factor XIa and block its activity

Reversible active site inhibition

Delivery

Subcutaneous

Intravenous or subcutaneous

Oral

Onset of action

Slow

Rapid

Rapid

Offset of action

Slow

Slow

Rapid

Indications

Chronic

Acute or chronic

Acute or chronic



Jeffrey I. Weitz / Thrombosis Research 141S2 (2016) S40–S45

blocking bradykinin generation [14,15]. Several lines of investigation support these concepts. Consistent with the importance of factor XII as a driver of clotting on medical devices, catheters coated with corn trypsin inhibitor, a potent and specific inhibitor of factor XIIa, remained patent significantly longer than uncoated catheters or catheters coated with albumin when inserted in the jugular veins of rabbits [39]. Furthermore, corn trypsin inhibitor coating endowed catheters with anticoagulant properties compared with uncoated catheters or those coated with albumin [39]. Likewise, knockdown of factor XII with an ASO prolonged the time to occlusion of such catheters by over 2-fold, whereas factor VII knockdown had little effect [28]. Furthermore, an antibody directed against factor XIIa was as effective as heparin at preventing clotting in an extracorporeal membrane oxygenation circuit in rabbits, but produced less bleeding [40]. Therefore, these studies suggest that contact activation of factor XII is the initiator of clotting on artificial surfaces. Although factor XII may initiate clotting on artificial surfaces, factor XI also appears to be important. Thus, factor XI knockdown was as effective as factor XII knockdown at preventing catheter occlusion in rabbits [28]. Furthermore, even though factor XII depletion reduced thrombin generation induced by components of mechanical heart valves to background levels, factor XI depletion abolished it [41]. Therefore, strategies targeting factor XI may be as or more effective than those targeting factor XII for prevention of clotting on artificial surfaces. Human studies are needed to determine the contribution of factors XII and XI to thrombosis in various settings. Unfortunately, only limited data are available at this time. Studies in Humans The only strategy to be tested in humans so far has been the factor XI directed ASO (ISIS-416858), which is given subcutaneously. In a phase I study in healthy volunteers, the ASO reduced factor XI antigen and activity levels in a concentration-dependent manner [42]. In this study, the ASO was administered subcutaneously at doses of 50 to 300 mg. Volunteers received three injections the first week and weekly injections thereafter. As was seen in animals, the maximum decrease in factor XI was observed after 3 weeks of ASO administration. Aside from minor irritation at injection sites, no bleeding or other adverse events were noted. Recovery of factor XI levels was delayed for several weeks after stopping the ASO injections [42]. The phase I results prompted a proof-of-concept phase II study with the ASO in 300 patients undergoing elective knee arthroplasty [43]. In this open-label, parallel group study, patients were randomized to receive the ASO at doses of 200 or 300 mg starting 35 days prior to surgery, or to enoxaparin at a dose of 40 mg once daily starting after surgery and continued for at least 10 days. All patients underwent bilateral venography at day 10±2 after surgery [43]. The primary efficacy outcome was VTE, which included the composite of asymptomatic deep vein thrombosis detected by venography, objectively documented symptomatic deep vein thrombosis or pulmonary embolism and VTE-related mortality, while the principal safety outcome was bleeding, which was defined as major or clinically relevant non-major. All the venograms and the suspected efficacy and safety outcomes were adjudicated by an independent committee that was blinded with respect to treatment allocation. At the time of surgery, mean factor XI levels were reduced to 38±0.01% and 28±0.01% of baseline in the groups receiving the 200 and 300 mg doses of ASO, respectively, compared with a reduction to 93±0.02% of the baseline value in the enoxaparin group [43]. The primary efficacy outcome occurred in 36 of 134 patients (27%) and in 3 of 71 patients (4%) who received the 200 and 300 mg doses of the ASO, respectively, as compared with 21 of 69 patients (30%) who received enoxaparin. The 200 mg ASO regimen was non-inferior and the 300 mg ASO regimen was

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superior to enoxaparin (P<0.001). The rates of major or clinically relevant non-major bleeding were 3% in both ASO groups and 8% in the enoxaparin groups. Therefore, the results of this study suggest that lowering factor XI levels reduces the risk of postoperative VTE to a greater extent than enoxaparin without increasing the risk of bleeding. The incidence of VTE was higher in patients with factor XI levels greater than 0.2 U/mL at the time of surgery than it was in those with lower factor XI levels (25.2% and 4.8%, respectively) [43], raising the possibility that reducing factor XI to levels below 0.2 U/mL may provide the greatest efficacy. The few thrombi that formed in patients treated with the ASO were smaller than those in the enoxaparin group [43]. This finding is consistent with the concept that factor XI is important for thrombus stabilization and growth [8]. Furthermore, despite the fact that at the time of surgery the levels of factor XI were less than 20% those of normal in many of the patients in the ASO treated groups, rates of bleeding were not increased. Therefore, this study highlights the safety of factor XI knockdown and reveals the promise of factor XI directed strategies for thromboprophylaxis in patients at high risk for thrombosis and bleeding. The potential clinical indications for these therapies need to be chosen with care to maximize potential benefit. Potential Indications for Factor XII or Factor XI Directed Therapies The promising results with the factor XI directed ASO in patients undergoing knee arthroplasty not only highlight the potential of this approach, but change our thinking about the pathogenesis of venous thrombosis after surgery. There is no question that thrombin generation is increased after major orthopedic surgery, likely as a result of tissue factor exposure at the surgical site. There are two potential explanations for the reduced risk of VTE with factor XI knockdown that are not mutually exclusive. First, tissue factor-induced thrombin generation may amplify coagulation by feedback activation of factor XI; a process attenuated with factor XI knockdown. Second, platelet activation and tissue damage at the surgical site may trigger the release of polyphosphates from activated platelets and DNA and RNA from activated or damaged cells. These polyanions could activate factor XII, thereby initiating the contact system and inducing factor XI activation. Knockdown of factor XI would prevent propagation of coagulation by either of these pathways, whereas strategies that target factor XII would only block contact activation. Where do we go from here? As outlined in Table 3, additional studies are needed to confirm the results of factor XI directed strategies for VTE prevention and other indications. Starting with knee arthroplasty, a head-to-head comparison of factor XI and factor XII directed strategies, perhaps using ASOs, would not only give valuable insight into the relative contribution of these two factors to postoperative VTE, but would also help to identify the optimal target. Because of their slow onset of action, ASOs are best suited for chronic indications. These might include secondary prevention in patients with unprovoked VTE and stroke prevention in AF patients at high risk for bleeding, such as those with end stage renal failure who are on hemodialysis. Unprovoked VTE is a potential indication because these patients have a risk of recurrent thrombosis of about 10% at one year and 30% at five years if anticoagulant therapy is stopped [44]. For this reason, many of them are maintained on indefinite anticoagulant therapy, which carries a risk of bleeding even with the NOACs [2]. Factor XII or factor XI directed strategies may be safer, and adherence may be better with once or twice monthly injections of ASOs or antibodies than with oral medications that must be taken one or twice daily (Table 3). These possibilities need to be tested. Stroke prevention in hemodialysis patients with atrial fibrillation represents an unmet medical need because the NOACs are

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Jeffrey I. Weitz / Thrombosis Research 141S2 (2016) S40–S45

contraindicated in such patients and because there is uncertainty as to whether the harms of warfarin outweigh its benefits in this setting (Table 3). Factor XI is likely to be a better target than factor XII for this indication because factor XI inhibition will prevent thrombus stabilization and growth regardless of whether the stimulus for clotting in the left atrial appendage is thrombin generation driven by tissue factor or factor XII activation by polyphosphates. Inhibition of factor XI may also attenuate clotting on the hemodialysis membrane, thereby obviating the need for heparin and further lowering the risk of bleeding. Capitalizing on the fact that thrombosis on artificial surfaces is driven by contact activation [38], factor XII or XI directed therapies may be safer than heparin for prevention of clotting on extracorporeal membrane oxygenation circuits, and safer than warfarin for prevention of thromboembolic events in patients with left ventricular assist devices. Dabigatran failed against warfarin in patients with mechanical heart valves [45]; a finding that prompted black box warnings against the use of NOACs in such patients. Factor XI directed strategies may be very effective in this setting because factor XI depletion abolished mechanical valve induced thrombin generation in vitro [41]. Factor XI directed strategies may also provide a safer platform than current anticoagulants in patients requiring dual or triple therapy (Table 3). Thus, when added to dual antiplatelet therapy with aspirin and clopidogrel in stabilized acute coronary syndrome patients, rivaroxaban reduced the risk of recurrent ischemic events and stent thrombosis [46]. These beneficial effects came at a cost of increased bleeding, including intracranial bleeding. The efficacy of rivaroxaban in this setting suggests that thrombin contributes to both recurrent ischemia and stent thrombosis [47]. Factor XI directed strategies are likely to be safer than rivaroxaban and should not only block contact activation on stents but should also prevent factor XI-mediated thrombus stabilization and growth. An unanswered question is whether inhibition of factor XII or factor XI will be sufficient for treatment of established venous or arterial thrombosis when used as sole therapy. Studies are needed to address this question. Thrombus associated factor Xa and clot-bound thrombin are likely to be important drivers of thrombus growth at sites of venous or arterial injury and neutrophil extracellular traps (NETs) may amplify this process by promoting contact activation [11,12]. Factor XI inhibition has the potential to block thrombus growth if thrombin-mediated activation of factor XI and contact activation by NETs are responsible for thrombus expansion. Therefore, considerably more work Table 3 Potential Clinical Indications and Rationale for Factor XII or Factor XI Directed Anticoagulants. Indication

Rationale

Elective knee arthroplasty

Provides proof-of-principle and permits head-to-head comparison of factor XII and factor XI directed therapies

Secondary prevention of venous thromboembolism

May be safer than current therapies and once or twice monthly injections with antibodies or ASO may be more convenient than daily oral therapy

Stroke prevention in atrial fibrillation patients with end stage renal disease on dialysis

Unmet medical need because of high risk of stroke, myocardial infarction and bleeding; lack of clear benefit of warfarin and NOACs contraindicated; factor XII or XI directed therapies may have a superior benefit-risk profile compared with aspirin or warfarin and may enable dialysis without heparin

Extracorporeal membrane oxygenation, left ventricular assist devices or mechanical heart valves

May be better than heparin at preventing clotting on extracorporeal membrane oxygenation circuits and may be safer than warfarin in patients with left ventricular assist devices or mechanical heart valves

is needed to identify the optimal indications for factor XII or XI directed anticoagulant strategies. Conclusions and Future Directions Factor XI and factor XII are promising targets for new anticoagulants that are likely to be safer than those that inhibit downstream enzymes such as factor Xa or thrombin. With the availability of ASOs, antibodies and small molecules, the requisite armamentarium is available to determine whether factor XI or factor XII is the best target and to compare the efficacy and safety of these new strategies with current standards of care for prevention or treatment of thrombosis. Selection of indications should focus on those where there are unmet medical needs, particularly those where current therapies are limited in both efficacy and safety. The clinical potential of factor XII and XI directed anticoagulant strategies should become clear over the next few years. Acknowledgements Dr. Weitz holds the Canada Research Chair (tier I) in Thrombosis and the Heart and Stroke Foundation J. Fraser Mustard Chari in Cardiovascular Research. Conflicts of interest Dr Weitz has served as a consultant and has received honoraria from Ionis Pharmaceuticals, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Pfizer, Merck and Daiichi Sankyo. References [1] Ruff CT, Giugliano RP, Braunwald E, Hoffman EB, Deenadayalu N, Ezekowitz MD, Camm AJ, Weitz JI, Lewis BS, Parkhomenko A, Yamashita T, Antman EM. Comparison of the efficacy and safety of new oral anticoagulants with warfarin in patients with atrial fibrillation: a meta-analysis of randomised trials. Lancet 383 (2014) 955–62. [2] van der Hulle T, Kooiman J, den Exter PL, Dekkers OM, Klok FA, Huisman MV. Effectiveness and safety of novel oral anticoagulants as compared with vitamin K antagonists in the treatment of acute symptomatic venous thromboembolism: a systematic review and meta-analysis. J. Thromb. Haemost. 12 (2014) 320–28. [3] Müller F, Gailani D, Renné T. Factor XI and XII as antithrombotic targets. Curr. Opin. Hematol. 18 (2011) 349–55. [4] Gailani D, Bane E, Gruber A. Factor XI and contact activation as targets for antithombotic therapy. J. Thromb. Haemost. 13 (2015) 1383–95. [5] Salomon O, Steinberg DM, Zucker M, Varon D, Zivelin A, Seligsohn U. Patients with severe factor XI deficiency have a reduced incidence of deep-vein thrombosis. Thromb. Haemost. 105 (2011) 269–73. [6] Salomon O, Steinberg DM, Koren-Morag N, Tanne D, Seligsohn U. Reduced incidence of ischemic stroke in patients with severe factor XI deficiency. Blood 111 (2008) 4113–17. [7] Renné T, Pozgajová M, Grüner S, Schuh K, Pauer HU, Burfeind P, Gailani D, Nieswandt B. Defective thrombus formation in mice lacking factor XII. J. Exp. Med. 202 (2005) 271–81. [8] Renné T, Oschatz C, Seifert S, Müller F, Antovic J, Karlman M, Benz PM. Factor XI deficiency in animal models. J. Thromb. Haemost. 7 (2009) 79–83. [9] Smith SA, Morrissey JH. Polyphosphate; a new player in the field of hemostasis. Curr. Opin Hematol. 21 (2014) 388–94. [10] Kannemeier C, Shibamiya A, Nakazawa F, Trusheim H, Ruppert C, Markart P, Song Y, Tzima E, Kennerknecht E, Niepmann M, von Bruehl M-L, Sedding D, Massberg S, Günther A, Engelmann B, Klaus T. Preissner Extracellular RNA constitutes a natural procoagulant cofactor in blood coagulation. Proc. Natl. Acad. Sci. USA 104 (2007) 6388–93. [11] Fuchs TA, Brill A, Duerschmied D, Schatzberg D, Monestier M, Myers DD Jr, Wrobleski SK, Wakefield TW, Hartwig JH, Wagner DD. Extracellular DNA traps promote thrombosis. Proc. Natl. Acad. Sci. USA. 107 (2010) 15880–85. [12] Gould TJ, Vu TT, Swystun LL, Dwivedi DJ, Mai SH, Weitz JI, Liaw PC. Neutrophil extracellular traps promote thrombin generation through platelet-dependent and platelet-independent mechanisms. Arterioscler. Thromb. Vasc. Biol. 34 (2014) 1977–84. [13] Seligsohn U. Factor XI deficiency in humans. J. Thromb. Hemost. 7 (2009) 84–87. [14] Renne T. The procoagulant and proinflammatory plasma contact system. Semin. Immunopathol. 34 (2012) 31–41. [15] Schmaier AH. Physiologic activities of the contact activation system. Thromb. Res. 133 (2014) S41–44.



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