Inherited factor XI deficiency confers no protection against acute myocardial infarction

Factor XI Inhibitors: perspectives in primary and secondary prevention of ischemic stroke

Stroke is one of the most common causes of mortality and disability worldwide. Antithrombotic therapy represents the mainstay in primary and secondary prevention, both in cardioembolic and non-cardioembolic stroke. Particularly, direct oral anticoagulants play a crucial role in atrial fibrillation, the most common cause of cardioembolic stroke, whereas single or dual antiplatelet therapy is preferred in non-cardioembolic stroke. However, the limitations related to the residual risk of cardioembolic or cerebrovascular events, and the risk of major bleeding, still represent unmet medical needs. To overcome them, new drugs inhibiting Factor XI (FXI) and Factor XII have been proposed, with a selective inhibition of contact pathway of coagulation, delineating a new anticoagulant approach. This review provides a summary of the currently available evidence and future perspectives on FXI inhibitors, that can represent an additional therapeutic option in the primary and secondary prevention of cardioembolic and non-cardioembolic ischemic stroke, also in challenging therapeutic contexts.

Factor XI deficiency: phenotypic age-related considerations and clinical approach towards bleeding risk assessment

ABSTRACT

Factor XI (FXI) deficiency is a rare bleeding disorder that presents complex challenges in patient assessment and bleeding risk management. Despite generally causing mild to moderate bleeding symptoms, clinical manifestations can vary, and bleeding tendency does not always correlate with FXI plasma levels or genotype. Our manuscript delves into the age-related nuances of FXI deficiency across an individual's lifespan. We emphasize issues faced by specific groups, including neonates and females of reproductive age experiencing abnormal uterine bleeding and postpartum hemorrhage. Older patients present unique challenges and concerns related to the management of bleeding as well as thrombotic complications. The current assortment of diagnostic laboratory assays shows limited success in predicting bleeding risk in the perisurgical setting of patients with FXI deficiency. This review explores the intricate interplay between individual bleeding profiles, surgical sites, and FXI activity levels. We also evaluate the accuracy of existing laboratory assays in predicting bleeding and discuss the potential role of investigational global assays in perioperative assessment. Furthermore, we outline our suggested diagnostic approach to refine treatment strategies and decision making. Available treatment options are presented, including antifibrinolytics, replacement products, and recombinant activated FVII. Finally, we discuss promising nonreplacement therapies for the treatment of rare bleeding disorders that can potentially address the challenges faced when managing FXI deficiency-related bleeding complications.

Development of new anticoagulant in 2023: Prime time for anti-factor XI and XIa inhibitors

Thrombosis remains one of the leading causes of death in the world. The history of anticoagulation has evolved considerably from non-specific drugs (i.e., heparins and vitamin K antagonists, VKA) to agents that directly target specific coagulation factors (i.e., argatroban, fondaparinux and direct oral anticoagulants, DOAC). Since the last decade, DOAC are widely used in clinical practice because of their ease to use with favorable pharmacological profile and not requiring monitoring, particularly for venous thromboembolism treatment and prevention and stroke prevention in atrial fibrillation. However, despite having a better safety profile than VKA, their bleeding risk is not negligible. Therefore, research is underway to develop new anticoagulant therapies with a better safety profile. One of these news approaches to reduce the risk of bleeding is to target the coagulation in the intrinsic pathway, in particular the contact activation, with the ultimate goal of preventing thrombosis without impairing hemostasis. Based on epidemiological data with patients with inherited factor XI (FXI) deficiency and preclinical studies, FXI emerged as the most promising candidate target separating hemostasis from thrombosis. This review summaries the role of FXI and FXIa in hemostasis, provides evidence of initial success with FXI pathway inhibitors in clinical trials (such as IONIS-FXI_Rx, fesomersen, osocimab, abelacimab, milvexian, asundexian or xisomab 3G3) and highlights the opportunities and challenges for this next generation of anticoagulants.

Discovery of Milvexian, a High-Affinity, Orally Bioavailable Inhibitor of Factor XIa in Clinical Studies for Antithrombotic Therapy

Factor XIa (FXIa) is an enzyme in the coagulation cascade thought to amplify thrombin generation but has a limited role in hemostasis. From preclinical models and human genetics, an inhibitor of FXIa has the potential to be an antithrombotic agent with superior efficacy and safety. Reversible and irreversible inhibitors of FXIa have demonstrated excellent antithrombotic efficacy without increased bleeding time in animal models (Weitz, J. I., Chan, N. C. Arterioscler. Thromb. Vasc. Biol. 2019, 39 (1), 7-12). Herein, we report the discovery of a novel series of macrocyclic FXIa inhibitors containing a pyrazole P2' moiety. Optimization of the series for (pharmacokinetic) PK properties, free fraction, and solubility resulted in the identification of milvexian (BMS-986177/JNJ-70033093, 17, FXIa Ki = 0.11 nM) as a clinical candidate for the prevention and treatment of thromboembolic disorders, suitable for oral administration.

Rare Defects: Looking at the Dark Face of the Thrombosis

Venous thromboembolism (VTE) constitutes a serious and potentially fatal disease, often complicated by pulmonary embolism and is associated with inherited or acquired factors risk. A series of risk factors are known to predispose to venous thrombosis, and these include mutations in the genes that encode anticoagulant proteins as antithrombin, protein C and protein S, and variants in genes that encode instead pro-coagulant factors as factor V (FV Leiden) and factor II (FII G20210A). However, the molecular causes responsible for thrombotic events in some individuals with evident inherited thrombosis remain unknown. An improved knowledge of risk factors, as well as a clear understanding of their role in the pathophysiology of VTE, are crucial to achieve a better identification of patients at higher risk. Moreover, the identification of genes with rare variants but a large effect size may pave the way for studies addressing new antithrombotic agents in order to improve the management of VTE patients. Over the past 20 years, qualitative or quantitative genetic risk factors such as inhibitor proteins of the hemostasis and of the fibrinolytic system, including fibrinogen, thrombomodulin, plasminogen activator inhibitor-1, and elevated concentrations of factors II, FV, VIII, IX, XI, have been associated with thrombotic events, often with conflicting results. The aim of this review is to evaluate available data in literature on these genetic variations to give a contribution to our understanding of the complex molecular mechanisms involved in physiologic and pathophysiologic clot formation and their role in clinical practice.

The rebirth of the contact pathway: a new therapeutic target

PURPOSE OF REVIEW

Anticoagulation with vitamin-K antagonists or direct oral anticoagulants is associated with a significant risk of bleeding. There is a major effort underway to develop antithrombotic drugs that have a smaller impact on hemostasis. The plasma contact proteins factor XI (FXI) and factor XII (FXII) have drawn considerable interest because they contribute to thrombosis but have limited roles in hemostasis. Here, we discuss results of preclinical and clinical trials supporting the hypothesis that the contact system contributes to thromboembolic disease.

RECENT FINDINGS

Numerous compounds targeting FXI or FXII have shown antithrombotic properties in preclinical studies. In phase 2 studies, drugs-targeting FXI or its protease form FXIa compared favorably with standard care for venous thrombosis prophylaxis in patients undergoing knee replacement. While less work has been done with FXII inhibitors, they may be particularly useful for limiting thrombosis in situations where blood comes into contact with artificial surfaces of medical devices.

SUMMARY

Inhibitors of contact activation, and particularly of FXI, are showing promise for prevention of thromboembolic disease. Larger studies are required to establish their efficacy, and to establish that they are safer than current therapy from a bleeding standpoint.

Osocimab: A Novel Agent in Preventing Venous Thromboembolism

The nature of orthopedic surgery, and specifically total knee arthroplasty, lends itself to the development of venous thromboembolism given endothelial injury from the surgical procedure, promotion of an acute hypercoagulable state, and the prolonged period of immobilization after surgery promoting stasis; all factors of Virchow's triad. Current guidelines recommend the direct acting oral anticoagulants, enoxaparin, fondaparinux, and warfarin as options for venous thromboembolism prevention. However, these agents may still be prone to unacceptable bleeding risk, given they mostly target the extrinsic pathway of the clotting cascade, and have other characteristics which can be problematic for use. Investigators have determined patients with factor XI deficiency seem to be protected for thrombotic risk and seem to be devoid of bleeding sequelae. This has led to the development of osocimab, a fully humanized monoclonal G1 antibody designed specifically to functionally neutralize factor XIa. Phase 1 clinical trials have demonstrated an agent with a long half-life (∼30 days) with minimal requirement of renal elimination and hepatic metabolism. Phase 2 trials have identified that an optimal dose range, 0.6-1.2 mg/kg, as a 1-time dose preoperatively or postoperatively is effective in preventing thrombotic complications with minimal bleeding risk compared with standard of care for elective total knee arthroplasty patients. Future clinical development will be able to clearly outline the role this agent will play in the future.

Turning Up to Eleven: Factor XI Inhibitors as Novel Agents to Maximize Safety and Maintain Efficacy in Thromboembolic Disease

Within the past decade nonvitamin K oral anticoagulants have emerged as the standard of care for the prevention and treatment of thromboembolic disorders, however safety of anticoagulants remain a concern for many patients and providers. There exists new interest in factor XI inhibition as novel therapeutic target based on observations of lower thrombotic rates and without significant bleed risk in individuals with inherited factor XI deficiency. Several classes of factor XI inhibitors including antisense oligonucleotides, monoclonal antibodies, and small molecule inhibitors have undergone preclinical studies and clinical trials in humans. Both osocimab and IONIS-FXI have been evaluated in patients undergoing orthopedic surgery and demonstrated superiority to enoxaparin without increasing major bleeding. Future studies with both these agents are ongoing, as well as the continued development of other inhibitors of factor XI. Early data regarding factor XI inhibition is encouraging as a potent anticoagulant and may offer a safer alternative compared to therapeutic currently available in contemporary practice for thromboembolic disease.

Clinical and laboratory diagnosis of rare coagulation disorders (RCDs)

Rare coagulation disorders (RCDs) are a group of diseases due to coagulation factors deficiency leading to life-long bleeding diathesis. The diagnosis of RCDs is challenging due to the limited knowledge of these disorders and the large heterogeneity of their bleeding patterns. The clinical symptoms of RCDs are extremely diverse in terms of bleeding type, site, severity, age at onset, and duration. The strength of the association between clotting factor activity level in plasma and clinical symptoms is also variable within each RCD. The clinical evaluation of RCDs starts with a detailed collection of clinical history and has been facilitated by bleeding assessment tools, however their effectiveness in diagnosing RCDs requires further investigation. The following laboratory diagnosis of RCDs involves coagulation screening tests, including activated partial thromboplastin time, prothrombin time, and thrombin time. After ruling out the presence of an inhibitor by mixing studies, in case of abnormal results, the specific deficiency is identified by performing one-stage clotting assays using the specific factor-depleted plasmas as substrate. In fibrinogen and FXIII deficiencies coagulation screening tests are not informative, therefore additional tests are needed. Global assays have been developed and are thought to aid in patient management, however, they are not well standardized yet. In addition to outlining the principles of clinical and laboratory diagnosis, this review explores molecular basis of RCDs and laboratory techniques for genetic analysis, and discusses the importance and effectiveness of quality control programs to ensure standardized laboratory results.

Factor XII Activation Promotes Platelet Consumption in the Presence of Bacterial-Type Long-Chain Polyphosphate In Vitro and In Vivo

Objective- Terminal complications of bacterial sepsis include development of disseminated intravascular consumptive coagulopathy. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway in flowing whole blood promotes thrombin generation and platelet activation and consumption distal to thrombus formation ex vivo and in vivo. Here, we sought to determine whether presence of long-chain polyP or bacteria in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner. Approach and Results- Long-chain polyP promoted platelet P-selectin expression, microaggregate formation, and platelet consumption in flowing whole blood in a contact activation pathway-dependent manner. Moreover, long-chain polyP promoted local fibrin formation on collagen under shear flow in a FXI-dependent manner. Distal to the site of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the blood flow in a FXI- and FXII-dependent manner. In a murine model, long-chain polyP promoted platelet deposition and fibrin generation in lungs in a FXII-dependent manner. In a nonhuman primate model of bacterial sepsis, pre-treatment of animals with an antibody blocking FXI activation by FXIIa reduced lethal dose₁₀₀ Staphylococcus aureus-induced platelet and fibrinogen consumption. Conclusions- This study demonstrates that bacterial-type long-chain polyP promotes platelet activation in a FXII-dependent manner in flowing blood, which may contribute to sepsis-associated thrombotic processes, consumptive coagulopathy, and thrombocytopenia.

Factor XI contributes to myocardial ischemia-reperfusion injury in mice

Inhibiting contact activation of factor XI during reperfusion of acute myocardial ischemia reduces infarct size in mice. Factor XII/XI contact axis inhibition may improve the outcome of coronary artery recanalization in acute myocardial infarction.

Management of rare coagulation disorders in 2018

Rare bleeding disorders (RBDs) comprise inherited deficiencies of factors I (fibrinogen), II (prothrombin), V, VII, X, XI, and XIII as well as combined factor V + VIII and vitamin K-dependent factors. They represent 3-5% of all congenital bleeding disorders and are usually transmitted as autosomal recessive traits. These disorders often manifest during childhood and have varied clinical presentations from mucocutaneous bleeding to life-threatening symptoms such as central nervous system and gastrointestinal bleeding. Bleeding manifestations generally vary within the same RBD and may also vary from 1 RBD to the other. Laboratory diagnosis is based on coagulation screening tests and specific factor assays, with molecular techniques providing diagnostic accuracy and enabling prenatal counseling. The approach to treatment of bleeding episodes and invasive procedures needs to be individualized and depends on the severity, frequency and procedure-related risk of bleeding. The first line of treatment of RBDs is replacement of the deficient factor, using specific plasma-derived or recombinant products and using fresh frozen plasma or cryoprecipitate when specific products are not available or in resource-limited countries. Prophylaxis may be considered in individuals with recurrent serious bleeding and especially after life-threatening bleeding episodes. Novel no-replacement strategies promoting hemostasis by through different mechanisms need to be studied in RBDs as alternative therapeutic options.

Relation of coagulation factor XI with incident coronary heart disease and stroke: the Cardiovascular Health Study

: The role of coagulation factor XI (FXI) in the cause of arterial thrombotic events remains uncertain. We examined the association of FXI with incident coronary heart disease (CHD), ischemic stroke, and hemorrhagic stroke. Data were from 3394 adults (mean age: 74.5 years) enrolled in the Cardiovascular Health Study who had FXI antigen from plasma samples drawn in 1992-1993 and were followed for cardiovascular events until 30 June 2013. Approximately 63% of participants were women and 17% were black. FXI levels were higher in blacks and women, showed positive associations with high-density lipoprotein and total cholesterol, BMI and diabetes, and negative associations with age and alcohol intake. During median follow-up of 13 years, we identified 1232 incident CHD, 473 ischemic stroke, and 84 hemorrhagic stroke events. In multivariable Cox models adjusted for traditional cardiovascular disease risk factors, the hazard ratio per one SD (32.2 mg/dl) increment of FXI was 1.02 (95% confidence interval: 0.96-1.08) for CHD; 0.94 (0.85-1.04) for ischemic stroke, and 0.85 (0.65-1.10) for hemorrhagic stroke. In this prospective cohort of elderly adults, there was no statistically significant association of higher FXI levels with incident CHD and stroke.

Plasma contact factors as therapeutic targets

Direct oral anticoagulants (DOACs) are small molecule inhibitors of the coagulation proteases thrombin and factor Xa that demonstrate comparable efficacy to warfarin for several common indications, while causing less serious bleeding. However, because their targets are required for the normal host-response to bleeding (hemostasis), DOACs are associated with therapy-induced bleeding that limits their use in certain patient populations and clinical situations. The plasma contact factors (factor XII, factor XI, and prekallikrein) initiate blood coagulation in the activated partial thromboplastin time assay. While serving limited roles in hemostasis, pre-clinical and epidemiologic data indicate that these proteins contribute to pathologic coagulation. It is anticipated that drugs targeting the contact factors will reduce risk of thrombosis with minimal impact on hemostasis. Here, we discuss the biochemistry of contact activation, the contributions of contact factors in thrombosis, and novel antithrombotic agents targeting contact factors that are undergoing pre-clinical and early clinical testing.

Anticoagulation beyond direct thrombin and factor Xa inhibitors: indications for targeting the intrinsic pathway?

Antithrombotic drugs like vitamin K antagonists and heparin have been the gold standard for the treatment and prevention of thromboembolic disease for many years. Unfortunately, there are several disadvantages of these antithrombotic drugs: they are accompanied by serious bleeding problems, it is necessary to monitor the therapeutic window, and there are various interactions with food and other drugs. This has led to the development of new oral anticoagulants, specifically inhibiting either thrombin or factor Xa. In terms of effectiveness, these drugs are comparable to the currently available anticoagulants; however, they are still associated with issues such as bleeding, reversal of the drug and complicated laboratory monitoring. Vitamin K antagonists, heparin, direct thrombin and factor Xa inhibitors have in common that they target key proteins of the haemostatic system. In an attempt to overcome these difficulties we investigated whether the intrinsic coagulation factors (VIII, IX, XI, XII, prekallikrein and high-molecular-weight kininogen) are superior targets for anticoagulation. We analysed epidemiological data concerning thrombosis and bleeding in patients deficient in one of the intrinsic pathway proteins. Furthermore, we discuss several thrombotic models in intrinsic coagulation factor-deficient animals. The combined results suggest that intrinsic coagulation factors could be suitable targets for anticoagulant drugs.

Inhibition of Factors XI and XII for Prevention of Thrombosis Induced by Artificial Surfaces

Exposure of blood to a variety of artificial surface induces contact activation, a process that contributes to the host innate response to foreign substances. On the foreign surface, the contact factors, factor XII (FXII), and plasma prekallikrein undergo reciprocal conversion to their fully active protease forms (FXIIa and α-kallikrein, respectively) by a process supported by the cofactor high-molecular-weight kininogen. Contact activation can trigger blood coagulation by conversion of factor XI (FXI) to the protease FXIa. There is interest in developing therapeutic inhibitors to FXIa and FXIIa because these activated factors can contribute to thrombosis in certain situations. Drugs targeting these proteases may be particularly effective in thrombosis triggered by exposure of blood to the surfaces of implantable medical devices. Here, we review clinical data supporting roles for FXII and FXI in thrombosis induced by medical devices, and preclinical data suggesting that therapeutic targeting of these proteins may limit surface-induced thrombosis.

Associations of coagulation factors IX and XI levels with incident coronary heart disease and ischemic stroke: the REGARDS study

Essentials Coagulation factors (F) IX and XI have been implicated in cardiovascular disease (CVD) risk. We studied associations of FIX and FXI with incident coronary heart disease (CHD) and stroke. Higher FIX antigen was associated with incident CHD risk in blacks but not whites. Higher levels of FIX antigen may be a CHD risk factor among blacks.

SUMMARY

Background Recent studies have suggested the importance of coagulation factor IX and FXI in cardiovascular disease (CVD) risk. Objectives To determine whether basal levels of FIX or FXI antigen were associated with the risk of incident coronary heart disease (CHD) or ischemic stroke. Patients/Methods The REasons for Geographic And Racial Differences in Stroke (REGARDS) study recruited 30 239 participants across the contiguous USA between 2003 and 2007. In a case-cohort study within REGARDS, FIX and FXI antigen were measured in participants with incident CHD (n = 609), in participants with incident ischemic stroke (n = 538), and in a cohort random sample (n = 1038). Hazard ratios (HRs) for CHD and ischemic stroke risk were estimated with Cox models per standard deviation higher FIX or FXI level, adjusted for CVD risk factors. Results In models adjusting for CHD risk factors, higher FIX levels were associated with incident CHD risk (HR 1.19; 95% confidence interval [CI] 1.01-1.40) and the relationship of higher FXI levels was slightly weaker (HR 1.15; 95% CI 0.97-1.36). When stratified by race, the HR of FIX was higher in blacks (HR 1.39; 95% CI 1.10-1.75) than in whites (HR 1.06; 95% CI 0.86-1.31). After adjustment for stroke risk factors, there was no longer an association of FIX levels with ischemic stroke, whereas the association of FXI levels with ischemic stroke was slightly attenuated. Conclusions Higher FIX antigen levels were associated with incident CHD in blacks but not in whites. FIX levels may increase CHD risk among blacks.

The Intrinsic Pathway of Coagulation as a Target for Antithrombotic Therapy

Plasma coagulation in the activated partial thromboplastin time assay is initiated by sequential activation of coagulation factors XII, XI, and IX. While this series of proteolytic reactions is not an accurate model for hemostasis in vivo, there is mounting evidence that factor XI and factor XII contribute to thrombosis, and that inhibiting them can produce an antithrombotic effect with a small effect on hemostasis. This article discusses the contributions of components of the intrinsic pathway to thrombosis in animal models and humans, and results of early clinical trials of drugs targeting factors IX, XI, and XII.

Cross Talk Pathways Between Coagulation and Inflammation

Anatomic pathology studies performed over 150 years ago revealed that excessive activation of coagulation occurs in the setting of inflammation. However, it has taken over a century since these seminal observations were made to delineate the molecular mechanisms by which these systems interact and the extent to which they participate in the pathogenesis of multiple diseases. There is, in fact, extensive cross talk between coagulation and inflammation, whereby activation of one system may amplify activation of the other, a situation that, if unopposed, may result in tissue damage or even multiorgan failure. Characterizing the common triggers and pathways are key for the strategic design of effective therapeutic interventions. In this review, we highlight some of the key molecular interactions, some of which are already showing promise as therapeutic targets for inflammatory and thrombotic disorders.

Factor XI and factor XII as targets for new anticoagulants

Although the non-vitamin antagonist oral anticoagulants produce less intracranial bleeding than warfarin, 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.

Factor XI deficiency is associated with lower risk for cardiovascular and venous thromboembolism events

Factor XI deficiency is associated with reduced risk of cardiovascular events. Factor XI deficiency is associated with reduced risk of VTE.

Factors XI and XII as Targets for New Anticoagulants

Compared with vitamin K antagonists, the direct oral anticoagulants (DOACs) are simpler to administer and are associated with less intracranial bleeding. Nonetheless, even with the DOACs, bleeding still occurs and many patients with atrial fibrillation fail to receive anticoagulant thromboprophylaxis because of the fear of bleeding. Therefore, there is an urgent need for safer anticoagulants. Recent investigations into the biochemistry of hemostasis and thrombosis have identified new targets for development of novel anticoagulants. Using data from complementary sources, including epidemiological studies and investigations in various animal models, the contact pathway has emerged as a potential mediator of thrombosis that plays a minor part in hemostasis. Consequently, factor (F) XII of the contact system and FXI in the intrinsic pathway have been identified as potentially safer targets of anticoagulation than thrombin or FXa. However, further studies are needed to identify which is the better target for the appropriate indication. This review highlights the evidence for focusing on FXI and FXII and examines the novel approaches directed at these new targets. These emerging strategies should address current unmet medical needs and provide new avenues by which to improve anticoagulant therapy by reducing the risk of bleeding.

Emerging anticoagulant strategies

Despite the introduction of direct oral anticoagulants (DOACs), the search for more effective and safer antithrombotic strategies continues. Better understanding of the pathogenesis of thrombosis has fostered 2 new approaches to achieving this goal. First, evidence that thrombin may be as important as platelets to thrombosis at sites of arterial injury and that platelets contribute to venous thrombosis has prompted trials comparing anticoagulants with aspirin for secondary prevention in arterial thrombosis and aspirin with anticoagulants for primary and secondary prevention of venous thrombosis. These studies will help identify novel treatment strategies. Second, emerging data that naturally occurring polyphosphates activate the contact system and that this system is critical for thrombus stabilization and growth have identified factor XII (FXII) and FXI as targets for new anticoagulants that may be even safer than the DOACs. Studies are needed to determine whether FXI or FXII is the better target and to compare the efficacy and safety of these new strategies with current standards of care for the prevention or treatment of thrombosis. Focusing on these advances, this article outlines how treatment strategies for thrombosis are evolving and describes the rationale and approaches to targeting FXII and FXI. These emerging anticoagulant strategies should address unmet needs and reduce the systemic underuse of anticoagulation because of the fear of bleeding.

Thrombotic Events in Asymptomatic FXII Deficiency versus Symptomatic FXI Deficiency: Surprising Observations

OBJECTIVE

To evaluate the impact of an asymptomatic congenital clotting defect (FXII deficiency) versus that of a similar but symptomatic defect (FXI deficiency) on protection from thrombosis.

PATIENTS AND METHODS

All patients with FXII or FXI deficiency and thrombosis were gathered from a time-unlimited PubMed search that was carried out twice and from personal records. Combined defects were excluded. The defect had to be proven by the demonstration of a suited hereditary pattern and by a specific clotting assay. Only patients with a factor activity level of <30% of normal were selected.

RESULTS

Twenty-eight patients with an FXII deficiency presented with arterial thrombosis, mainly myocardial infarction, and 29 showed venous thrombosis; for FXI deficiency, these figures were 43 and 10, respectively. The ratio of arterial and venous thrombosis was 0.96 and 4.3, respectively, for FXII and FXI deficiency.

CONCLUSIONS

Factor FXII deficiency supplies no protection from arterial or venous thrombosis. FXI deficiency shows no protection from arterial thrombosis but appears to guarantee protection from venous thrombosis. A symptomatic, namely bleeding, condition (FXI deficiency) provides protection from venous thrombosis whereas an asymptomatic one (FXII deficiency) does not.

Factor XI as a Therapeutic Target

Factor XIa is a plasma serine protease that contributes to thrombin generation primarily through proteolytic activation of factor IX. Traditionally considered part of the intrinsic pathway of coagulation, several lines of evidence now suggest that factor XIa serves as an interface between the vitamin-K-dependent thrombin generation mechanism and the proinflammatory kallikrein-kinin system, allowing the 2 systems to influence each other. Work with animal models and results from epidemiological surveys of human populations support a role for factor XIa in thromboembolic disease. These data and the clinical observation that deficiency of factor XI, the zymogen of factor XIa, produces a relatively mild bleeding disorder suggest that drugs targeting factor XI or XIa could produce an antithrombotic effect while leaving hemostasis largely intact. Results of a recent trial comparing antisense-induced factor XI reduction to standard-dose low molecular-weight heparin as prophylaxis for venous thrombosis during knee replacement are encouraging in this regard. Here, we discuss recent findings on the biochemistry, physiology, and pathology of factor XI as they relate to thromboembolic disease.

Congenital factor XI and factor VII deficiencies assure an apparent opposite protection against arterial or venous thrombosis: An intriguing observation

OBJECTIVE

To investigate the prevalence and type of thrombotic events reported in patients with congenital factor XI (FXI) or factor VII (FVII) deficiency.

PATIENTS AND METHODS

Data on all patients with congenital FXI or FVII deficiency and a thrombotic event were gathered by means of a time unlimited PubMed search carried out in June 2014 and in February 2015. Appropriate keywords including the medical subject headings were used in both instances. Side tables were also consulted and cross-checking of the references was carried out to avoid omissions. The thrombosis event had to be proven by objective methods.

RESULTS

Forty-three patients with FXI deficiency had arterial thrombosis and only eight had venous thrombosis. On the contrary, only five patients with FVII deficiency had arterial thrombosis whereas 31 patients had venous thrombosis. The arterial/venous ratios were 5.37 and 0.17 for FXI or FVII, respectively.

CONCLUSIONS

Arterial thrombosis is frequent in FXI deficiency whereas venous thrombosis is rare. The reverse is true for FVII deficiency. The significance of these findings is discussed especially in view of the recent use of synthetic anti-FXI compounds in the prophylaxis of post-orthopedic surgery of venous thrombosis complications.

Factor XI Deficiency Protects Against Atherogenesis in Apolipoprotein E/Factor XI Double Knockout Mice

OBJECTIVE

Atherosclerosis and atherothrombosis are still major causes of mortality in the Western world, even after the widespread use of cholesterol-lowering medications. Recently, an association between local thrombin generation and atherosclerotic burden has been reported. Here, we studied the role of factor XI (FXI) deficiency in the process of atherosclerosis in mice.

APPROACH AND RESULTS

Apolipoprotein E/FXI double knockout mice, created for the first time in our laboratory. There was no difference in cholesterol levels or lipoprotein profiles between apolipoprotein E knockout and double knockout mice. Nevertheless, in 24-week-old double knockout mice, the atherosclerotic lesion area in the aortic sinus was reduced by 32% (P=0.004) in comparison with apolipoprotein E knockout mice. In 42-week-old double knockout mice, FXI deficiency inhibited atherosclerosis progression significantly in the aortic sinus (25% reduction, P=0.024) and in the aortic arch (49% reduction, P=0.028), with a prominent reduction of macrophage infiltration in the atherosclerotic lesions.

CONCLUSIONS

FXI deprivation was shown to slow down atherogenesis in mice. The results suggest that the development of atherosclerosis can be prevented by targeting FXI.

Coagulation factor XII genetic variation, ex vivo thrombin generation, and stroke risk in the elderly: results from the Cardiovascular Health Study

BACKGROUND

The relationships of thrombin generation (TG) with cardiovascular disease risk are underevaluated in population-based cohorts.

OBJECTIVES

To evaluate the relationships of TG influenced by the contact and tissue factor coagulation pathways ex vivo with common single-nucleotide polymorphisms (SNPs) and incident cardiovascular disease and stroke.

PATIENTS/METHODS

We measured peak TG (pTG) in baseline plasma samples of Cardiovascular Health Study participants (n = 5411), both with and without inhibitory anti-factor XIa antibody (pTG/FXIa(-) ). We evaluated their associations with ~ 50 000 SNPs by using the IBCv2 genotyping array, and with incident cardiovascular disease and stroke events over a median follow-up of 13.2 years.

RESULTS

The minor allele for an SNP in the FXII gene (F12), rs1801020, was associated with lower pTG in European-Americans (β = - 34.2 ± 3.5 nm; P = 3.3 × 10(-22) ; minor allele frequency [MAF] = 0.23) and African-Americans (β = - 31.1 ± 7.9 nm; P = 9.0 × 10(-5) ; MAF = 0.42). Lower FXIa-independent pTG (pTG/FXIa(-) ) was associated with the F12 rs1801020 minor allele, and higher pTG/FXIa(-) was associated with the ABO SNP rs657152 minor allele (β = 16.3 nm; P = 4.3 × 10(-9) ; MAF = 0.37). The risk factor-adjusted ischemic stroke hazard ratios were 1.09 (95% confidence interval CI 1.01-1.17; P = 0.03) for pTG, 1.06 (95% CI 0.98-1.15; P = 0.17) for pTG/FXIa(-) , and 1.11 (95% CI 1.02-1.21; P = 0.02) for FXIa-dependent pTG (pTG/FXIa(+) ), per one standard deviation increment (n = 834 ischemic strokes). In a multicohort candidate gene analysis, rs1801020 was not associated with incident ischemic stroke (β = - 0.02; standard error = 0.08; P = 0.81).

CONCLUSIONS

These results support the importance of contact activation pathway-dependent TG as a risk factor for ischemic stroke, and indicate the importance of F12 SNPs for TG ex vivo and in vivo.

Factor XI and contact activation as targets for antithrombotic therapy

The most commonly used anticoagulants produce therapeutic antithrombotic effects either by inhibiting thrombin or factor Xa (FXa) or by lowering the plasma levels of the precursors of these key enzymes, prothrombin and FX. These drugs do not distinguish between thrombin generation contributing to thrombosis from thrombin generation required for hemostasis. Thus, anticoagulants increase bleeding risk, and many patients who would benefit from therapy go untreated because of comorbidities that place them at unacceptable risk for hemorrhage. Studies in animals demonstrate that components of the plasma contact activation system contribute to experimentally induced thrombosis, despite playing little or no role in hemostasis. Attention has focused on FXII, the zymogen of a protease (FXIIa) that initiates contact activation when blood is exposed to foreign surfaces, and FXI, the zymogen of the protease FXIa, which links contact activation to the thrombin generation mechanism. In the case of FXI, epidemiologic data indicate this protein contributes to stroke and venous thromboembolism, and perhaps myocardial infarction, in humans. A phase 2 trial showing that reduction of FXI may be more effective than low molecular weight heparin at preventing venous thrombosis during knee replacement surgery provides proof of concept for the premise that an antithrombotic effect can be uncoupled from an anticoagulant effect in humans by targeting components of contact activation. Here, we review data on the role of FXI and FXII in thrombosis and results of preclinical and human trials for therapies targeting these proteins.

Epidemiologic and clinical data linking factors XI and XII to thrombosis

Currently available evidence supports the contention that elevated levels of factor XI (fXI) are associated with a greater risk of venous thromboembolism and ischemic stroke, but, less convincingly, with myocardial infarction. Conversely, reduced plasma levels of fXI seem to offer some protection from venous thromboembolism and stroke, but not myocardial infarction. Factor XI-deficient patients are at risk for certain types of bleeding, particularly posttraumatic hemorrhage on mucosal surfaces where there is a high endogenous fibrinolytic activity. In contrast, the situation with fXII in human thrombosis remains enigmatic. Deficiency of fXII is clearly not associated with any bleeding risk, but neither does it seem to be protective against thrombosis. The longstanding debate as to whether partial fXII deficiency represents a risk factor for thrombosis remains unresolved, with seemingly conflicting results depending on study design, type of assay used, and analyte evaluated. The possibility that elevated fXII levels represent a risk factor for thrombosis is not borne out in the literature.

Myocardial Infarctions and Other Acute Coronary Syndromes in Rare Congenital Bleeding Disorders

Objective:

To investigate the occurrence of myocardial infarction or other acute coronary syndromes in rare congenital bleeding disorders.

Patients:

All patients with factor I (FI), factor II (FII), factor V (FV), factor VII (FVII), factor X (FX), factor XI (FXI), or factor XIII (FXIII) deficiency or abnormality reported to have presented a myocardial infarction or another acute coronary syndrome were investigated. The condition had to be demonstrated by objective means, including a coronary/angiography. Cases of stable angina were excluded.

Results:

A total of 53 patients (4 had FI, 2 had FV, 2 had FVII, 36 had FXI, 1 had FXIII deficiency, and 8 patients had platelet disorders) met the inclusion criteria . No patient with FII or FX deficiency and acute coronary disease met the inclusion criteria. In the majority of patients, common risk factors were present, namely hypertension, hypercholesterolemia, smoking, and diabetes. Replacement therapy was involved in 5 cases.

Conclusion:

The congenital hypocoagulability present in these patients was unable to allow a protection from acute coronary diseases. The significance of the findings is discussed.

Antigen levels of coagulation factor XII, coagulation factor XI and prekallikrein, and the risk of myocardial infarction and ischemic stroke in young women

BACKGROUND

High levels of activated protein–inhibitor complexes of the intrinsic coagulation proteins are associated with ischemic stroke (IS) but not with myocardial infarction (MI). This study was aimed at determining whether the antigen levels of coagulation factors(factor XII, FXII, and FXI and prekallikrein (PK)are associated with MI and IS, and whether this association is independent of levels of activated protein–inhibitor complexes.

PATIENTS AND METHODS

The RATIO study included young women (< 50 years) with MI (N = 205)and IS (N = 175), and 638 healthy controls. Antigen levels of FXII, FXI and PK were measured and expressed as percentages of of those in pooled normal plasmas. Odds ratios (ORs) and corresponding 99% confidence intervals (CIs) were calculated for high levels (i.e. ≥ 90th percentile of controls) as measures of rate ratios.

RESULTS

After adjustment for potential confounders, high levels of FXII antigen were not associated with MI risk or IS risk(OR(MI) 1.18, 99% CI 0.51–2.74; ORIS 1.03, 9% CI 0.41–2.55). High levels of FXI antigen were slightly associated with an increase in MI risk (OR(MI) 1.55, 9% CI 0.74–3.21), whereas there was a substantial association with IS risk (ORIS 2.65, 9% CI 1.27–5.56). PK antigen was slightly associated with MI risk but not with IS risk(ORMI 1.54, 9% CI 0.67–3.52; ORIS 0.90, 9% CI 0.35–2.33). All associations remained similar after adjustment for levels of protein–inhibitor complexes.

CONCLUSION

Increased levels of FXI antigen were associated with an increase in IS risk, whereas they showed only a marginal association with MI risk. FXII antigen and PK antigen levels were not substantially associated with MI risk and IS risk.

Genetic variants of coagulation factor XI show association with ischemic stroke up to 70 years of age

BACKGROUND AND PURPOSE

Coagulation factor XI (FXI) has an important role in the propagation and stabilization of a thrombus upon vessel injury. High FXI levels have been implicated in thrombotic diseases including ischemic stroke. The aim of our study was to investigate whether FXI gene (F11) variants are associated with ischemic stroke.

METHODS

The discovery sample, the Sahlgrenska Academy Study on Ischemic Stroke (SAHLSIS), included 844 patients with ischemic stroke and 668 controls, all aged 18-70 years. Replication was performed in the Lund Stroke Register (LSR) and Malmö Diet and Cancer study (MDC), together including 1213 patients and 788 controls up to 70 years of age, and in total 3145 patients and 1793 controls (18-102 years). Seven F11 single-nucleotide polymorphisms (SNPs) were selected using a tagging approach.

RESULTS

The SNPs rs3733403, rs925451, and rs1593 showed independent associations with overall ischemic stroke in SAHLSIS, ORs of 0.74 (95% CI 0.59-0.94), 1.24 (95% CI 1.06-1.46), and 0.70 (95% CI 0.55-0.90), respectively. The association for rs925451 was replicated in the LSR and MDC sample in a pre-specified analysis of subjects aged 70 years or younger, OR of 1.16 (95% CI 1.00-1.34), whereas no SNP was replicated when all ages were included. In line with this, one F11 haplotype was associated with overall ischemic stroke in the discovery sample and in the replication sample ≤70 years.

CONCLUSIONS

We found significant associations between F11 variation and overall ischemic stroke up to 70 years of age. These findings motivate further studies on the role of F11 in ischemic stroke, especially in younger individuals.