Elevated levels of Factor XI are associated with cardiovascular disease in women

Targeting the Contact Pathway of Coagulation for the Prevention and Management of Medical Device-Associated Thrombosis

Hemorrhage remains a major complication of anticoagulants, with bleeding leading to serious and even life-threatening outcomes in rare settings. Currently available anticoagulants target either multiple coagulation factors or specifically coagulation factor (F) Xa or thrombin; however, inhibiting these pathways universally impairs hemostasis. Bleeding complications are especially salient in the medically complex population who benefit from medical devices. Extracorporeal devices-such as extracorporeal membrane oxygenation, hemodialysis, and cardiac bypass-require anticoagulation for optimal use. Nonetheless, bleeding complications are common, and with certain devices, highly morbid. Likewise, pharmacologic prophylaxis to prevent thrombosis is not commonly used with many medical devices like central venous catheters due to high rates of bleeding. The contact pathway members FXI, FXII, and prekallikrein serve as a nexus, connecting biomaterial surface-mediated thrombin generation and inflammation, and may represent safe, druggable targets to improve medical device hemocompatibility and thrombogenicity. Recent in vivo and clinical data suggest that selectively targeting the contact pathway of coagulation through the inhibition of FXI and FXII can reduce the incidence of medical device-associated thrombotic events, and potentially systemic inflammation, without impairing hemostasis. In the following review, we will outline the current in vivo and clinical data encompassing the mechanism of action of drugs targeting the contact pathway. This new class of inhibitors has the potential to herald a new era of effective and low-risk anticoagulation for the management of patients requiring the use of medical devices.

Targeting factor XI and factor XIa to prevent thrombosis

ABSTRACT

Direct oral anticoagulants (DOACs) that inhibit the coagulation proteases thrombin or factor Xa (FXa) have replaced warfarin and other vitamin K antagonists (VKAs) for most indications requiring long-term anticoagulation. In many clinical situations, DOACs are as effective as VKAs, cause less bleeding, and do not require laboratory monitoring. However, because DOACs target proteases that are required for hemostasis, their use increases the risk of serious bleeding. Concerns over therapy-related bleeding undoubtedly contribute to undertreatment of many patients who would benefit from anticoagulation therapy. There is considerable interest in the plasma zymogen factor XI (FXI) and its protease form factor XIa (FXIa) as drug targets for treating and preventing thrombosis. Laboratory and epidemiologic studies support the conclusion that FXI contributes to venous and arterial thrombosis. Based on 70 years of clinical observations of patients lacking FXI, it is anticipated that drugs targeting this protein will cause less severe bleeding than warfarin or DOACs. In phase 2 studies, drugs that inhibit FXI or FXIa prevent venous thromboembolism after total knee arthroplasty as well as, or better than, low molecular weight heparin. Patients with heart disease on FXI or FXIa inhibitors experienced less bleeding than patients taking DOACs. Based on these early results, phase 3 trials have been initiated that compare drugs targeting FXI and FXIa to standard treatments or placebo. Here, we review the contributions of FXI to normal and abnormal coagulation and discuss results from preclinical, nonclinical, and clinical studies of FXI and FXIa inhibitors.

Persistently High Levels of Coagulation Factor XI as a Risk Factor for Venous Thrombosis

Coagulation factor XI (FXI) promotes fibrin formation and inhibits fibrinolysis. Elevated plasma FXI levels, limited to a single measurement, are associated with a higher thrombotic risk. Our case-control study aimed to identify the effect of persistently increased plasma FXI levels on the risk of deep vein thrombosis (DVT). All patients evaluated between January 2016 and January 2018 for a first episode of proximal DVT of the lower extremity were considered for enrolment. Plasma FXI levels were measured at least 1 month after the discontinuation of anticoagulant treatment (T1). The patients with increased plasma FXI levels (>90th percentile of controls) were tested again 3 months later (T2). Among the 200 enrolled patients (M/F 114/86, age range 26-87 years), 47 patients had increased plasma FXI levels at T1 and16 patients had persistently increased plasma FXI levels at T2. The adjusted odds ratio for DVT was 2.4 (95% CI, 1.3 to 5.5, p < 0.001) for patients with increased FXI levels at T1 and 5.2 (95% CI, 2.3 to 13.2, p < 0.001) for patients with persistently high FXI levels at T2. Elevated FXI levels constitute a risk factor for deep vein thrombosis, and this risk nearly doubled in patients with persistently increased plasma FXI levels. Larger prospective studies are needed to confirm our findings.

An update on novel therapies for treating patients with arterial thrombosis

INTRODUCTION

Antithrombotic therapy field is undergoing rapid and significant changes during the past decade. In addition to new therapeutic strategies with existing targets, investigators are exploring the potential use of new targets to address unmet needs to treat patients with arterial diseases.

AREAS COVERED

We aim to provide an update on and a comprehensive review of the antithrombic agents that are being explored in patients with arterial diseases. We discuss latest developments with respect to upstream antiplatelet agents, and collagen and thrombin pathway inhibitors. We searched PubMed databases for English language articles using keywords: antiplatelet agents, thrombin pathway inhibitors, collagen receptors, arterial disease.

EXPERT OPINION

Despite implementation of potent P2Y12 inhibitors, there are numerous unmet needs in the treatment of arterial diseases including ceiling effect of currently available antiplatelet agents along with and an elevated risk of bleeding. The latter observations encouraged investigators to explore new targets that can attenuate the generation of platelet-fibrin clot formation and subsequent ischemic event occurrences with minimal effect on bleeding. These targets include collagen receptors on platelets and thrombin generation including FXa, FXIa, and FXIIa. In addition, investigators are studying novel antiplatelet agents/strategies to facilitate upstream therapy in high-risk patients.

Venous thromboembolism-related genetic determinant F11 rs4253417 is a potential prognostic factor in ischaemic stroke

Ischaemic stroke (IS) and venous thromboembolism (VTE) are two forms of thromboembolism that, although distinct, seem to share numerous risk factors. Concerning genetic risk factors, while many VTE genetic markers have been reported, inclusively by genome-wide association studies (GWAS), the identification and validation of genetic determinants underlying IS pathogenesis have been challenging. Considering that IS and VTE shared biological pathways and aetiological factors, the severity of IS might be also influenced by VTE-related genetic variants. Thus, the present study was designed to analyse the impact of six VTE GWAS-identified genetic variants on the clinical outcome of 363 acute IS patients. Results revealed that the single-nucleotide polymorphism (SNP) F11 rs4253417 was an independent predictor of the 5-year risk of death among patients with total anterior circulation infarct (TACI). Namely, the ones carrying the SNP C allele presented a fourfold increase in the 5-year risk of death compared to TT genotype carriers (CC/CT vs. TT; adjusted HR, 4.240; 95% CI, 1.260-14.270; P = 0.020). This SNP is known to be associated with coagulation factor XI (FXI) levels, thus with implications in haemostasis and inflammation. As such, F11 rs4253417 might be a promising prognostic biomarker among TACI patients to aid in clinical decision-making. However, additional investigation is required to confirm the study's results and dissect the underlying mechanisms.

Factor XIa Inhibitors as a Novel Anticoagulation Target: Recent Clinical Research Advances

BACKGROUND

While current clinically administered anticoagulant medications have demonstrated effectiveness, they have also precipitated significant risks: severe bleeding complications including, but not limited to, gastrointestinal hemorrhaging and intracranial and other life-threatening major bleedings. An ongoing effort is being made to identify the best targets for anticoagulant-targeted drugs. Coagulation factor XIa (FXIa) is emerging as an important target of current anticoagulant treatment.

OBJECTIVE

This review will summarize the development of anticoagulants and recent advances in clinical trials of experimental factor XI inhibitors from a clinical application perspective.

RESULTS

As of 1 January 2023, our search screening included 33 clinical trials. We summarized the research progress of FXIa inhibitors from seven clinical trials that evaluated their efficacy and safety. The results showed no statistically meaningful distinction in the primary efficacy between patients receiving FXIa inhibitors compared to controls (RR = 0.796; 95% CI: 0.606-1.046; I² = 68%). The outcomes did not indicate a statistical difference in the occurrence of any bleeding between patients receiving FXIa inhibitors compared to controls (RR = 0.717; 95% CI: 0.502-1.023; I² = 60%). A subgroup analysis found significant differences in severe bleeding and clinically relevant hemorrhaging in subjects receiving FXIa inhibitors compared to Enoxaparin (RR = 0.457; 95% CI: 0.256-0.816; I² = 0%).

CONCLUSIONS

Clinical trials to date have indicated that factor XIa is a potential anticoagulation target, and factor XIa inhibitors may play an important role in the development of anticoagulants.

Effects of rifampin on the pharmacokinetics and pharmacodynamics of milvexian, a potent, selective, oral small molecule factor XIa inhibitor

Milvexian (BMS-986177/JNJ-70033093) is a potent, oral small molecule that inhibits the active form of factor XI with high affinity and selectivity. This study assessed the single-dose pharmacokinetic and pharmacodynamic properties of milvexian co-administered with rifampin, an organic anion transport protein (OATP) inhibitor and potent cytochrome P450 (CYP) 3A and P-glycoprotein (P-gp) inducer. In this open-label, nonrandomized, single-sequence study, healthy participants (N = 16) received single doses of milvexian on Day 1 (100 mg), milvexian and rifampin (600 mg) on Day 4, rifampin on Days 5-11, milvexian and rifampin on Day 12, and rifampin on Days 13-14. Pharmacokinetic data were summarized using descriptive statistics. Administration of milvexian, alone or in combination with rifampin, was generally safe and well tolerated. Single-dose co-administration of rifampin and milvexian demonstrated no meaningful changes in milvexian exposure versus milvexian alone (C_max, 110%; AUC_[0-T], 102%; AUC_[INF], 101%). After multiple doses of rifampin and milvexian, peak and total milvexian exposure substantially decreased versus milvexian alone (C_max, 22%; AUC_[0-T], 15%; AUC_[INF], 15%). Results were consistent with preclinical data, indicating that milvexian is a substrate for CYP3A4/5 and P-gp but not OATP. The implications of these results on the need for dose adjustment of milvexian will be further elucidated following the completion of phase 2 and 3 trials.Trial registration The study was registered with ClinicalTrials.gov (NCT02959060; submitted 7/11/2016, first posted 8/11/2016).

Single-Dose Pharmacokinetics of Milvexian in Participants with Mild or Moderate Hepatic Impairment Compared with Healthy Participants

Background

Patients with hepatic impairment receiving antithrombotic agents metabolized primarily through the liver can be at risk for bleeding. Milvexian (BMS-986177/JNJ-70033093) is a small-molecule, active-site inhibitor of activated Factor XI (FXIa). Modulation of FXI may provide systemic anticoagulation without increased risk of clinically significant bleeding.

Objective

This open-label study evaluated the effects of mild or moderate hepatic impairment on the pharmacokinetics of milvexian to assess their impact on safety and dosing.

Methods

Single doses of milvexian 60 mg were administered to participants with mild hepatic impairment (n = 9), moderate hepatic impairment (n = 8), and normal hepatic function (n = 9). Healthy participants were matched to participants with hepatic impairment by body weight, age, and sex. Analysis of variance was performed on natural log-transformed milvexian exposure parameters, with hepatic function group as a fixed effect.

Results

Single doses of milvexian 60 mg were generally well tolerated, with no serious adverse events (AEs), bleeding AEs, or discontinuations due to AEs. Geometric mean ratios (90% confidence interval) for total milvexian maximum observed plasma concentration and area under the plasma concentration–time curve from time zero extrapolated to infinite time were 1.180 (0.735–1.895) and 1.168 (0.725–1.882), respectively, for mild hepatic impairment versus normal hepatic function and 1.140 (0.699–1.857) and 0.996 (0.609–1.628), respectively, for moderate hepatic impairment versus normal hepatic function. Across groups, milvexian exposure–related increases were observed for activated partial thromboplastin time.

Conclusion

Milvexian was well tolerated in participants with normal, mildly impaired, and moderately impaired hepatic function. Observed pharmacokinetic changes suggest it is unlikely that dose adjustments will be necessary in patients with mild or moderate hepatic impairment. Clinical Trial RegistrationClinicaltrials.gov identifier: NCT02982707.

First-in-human study of milvexian, an oral, direct, small molecule factor XIa inhibitor

Milvexian (BMS-986177/JNJ-70033093) is a small molecule, active-site inhibitor of factor XIa (FXIa) being developed to prevent and treat thrombotic events. The safety, tolerability, pharmacokinetics (PKs), and pharmacodynamics (PDs) of milvexian were assessed in a two-part, double-blind, placebo-controlled, sequential single ascending dose (SAD) and multiple ascending dose (MAD) study in healthy adults. Participants in SAD panels (6 panels of 8 participants; n = 48) were randomized (3:1) to receive milvexian (4, 20, 60, 200, 300, or 500 mg) or placebo. The 200- and 500-mg panels investigated the pharmacokinetic impact of a high-fat meal. Participants in MAD panels (7 panels of 8 participants; n = 56) were randomized (3:1) to receive milvexian (once- or twice-daily) or placebo for 14 days. All milvexian dosing regimens were safe and well-tolerated, with only mild treatment-emergent adverse events and no clinically significant bleeding events. In SAD panels, maximum milvexian plasma concentration occurred 3 h postdose in all fasted panels. The terminal half-life (T1/2 ) ranged from 8.3 to 13.8 h. In fasted panels from 20 to 200 mg, absorption was dose-proportional; results at higher doses (300 and 500 mg) were consistent with saturable absorption. Food increased milvexian bioavailability in a dose-dependent fashion. In MAD panels, steady-state milvexian plasma concentration was reached within 3 and 6 dosing days with once- and twice-daily dosing, respectively. Renal excretion was less than 20% in all panels. Prolongation of activated partial thromboplastin time was observed and was directly related to drug exposure. These results suggest that the safety, tolerability, PK, and PD properties of milvexian are suitable for further clinical development.

WPK5, a Novel Kunitz-Type Peptide from the Leech Whitmania pigra Inhibiting Factor XIa, and Its Loop-Replaced Mutant to Improve Potency

Kunitz-type proteins or peptides have been found in many blood-sucking animals, but the identity of them in leeches remained elusive. In the present study, five Kunitz-type peptides named WPK1-WPK5 were identified from the leech Whitmania pigra. Recombinant WPK1-WPK5 were expressed in Pichia pastoris GS115, and their inhibitory activity against Factor XIa (FXIa) was tested. WPK5 showed inhibitory activity against FXIa with an IC₅₀ value of 978.20 nM. To improve its potency, the loop replacement strategy was used. The loop 1 (TGPCRSNLER) and loop 2 (QYGGC) in WPK5 were replaced by loop 1 (TGPCRAMISR) and loop 2 (FYGGC) in PN2KPI, respectively, and the resulting peptide named WPK5-Mut showed an IC₅₀ value of 8.34 nM to FXIa, which is about 100-fold the potency of FXIa compared to that of WPK5. WPK5-Mut was further evaluated for its extensive bioactivity in vitro and in vivo. It dose-dependently prolonged APTT on both murine plasma and human plasma, and potently inhibited FeCl₃-induced carotid artery thrombosis in mice at a dose of 1.5 mg/kg. Additionally, WPK5-Mut did not show significant bleeding risk at a dose of 6 mg/kg. Together, these results showed that WPK5-Mut is a promising candidate for the development of an antithrombotic drug.

Factor XI(a) inhibitors for thrombosis: an updated patent review (2016-present)

Introduction: Anticoagulation without bleeding is an ideal goal in treating thrombosis, however, this goal has not been achieved. All current anticoagulants are associated with significant bleeding which limits their safe use. Genetic and pharmacological findings indicate that factor XIa is a key player in thrombosis, yet it is a relatively marginal one in hemostasis. Thus, factor XIa and its zymogen offer a unique opportunity to develop anticoagulants with low bleeding risk.Areas covered: A survey of patent literature has retrieved more than 50 patents on the discovery of novel therapeutics targeting factor XI(a) since 2016. Small molecules, monoclonal antibodies, oligonucleotides, and polypeptides have been developed to inhibit factor XI(a). Many inhibitors are in early development and few have been evaluated in clinical trials.Expert opinion: Factor XI(a) is being actively pursued as a drug target for the development of effective and safer anticoagulants. Although many patents claiming factor XI(a) inhibitors were filed prior to 2016, recent literature reveals a moderately declining trend. Nevertheless, more agents have entered different levels of clinical trials. These agents exploit diverse mechanistic strategies for inhibition. Although further development is warranted, reaching one or more of these agents to the clinic will transform the anticoagulation therapy.

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.

Recent advances in the discovery and development of factor XI/XIa inhibitors

Factor XIa (FXIa) is a serine protease homodimer that belongs to the intrinsic coagulation pathway. FXIa primarily catalyzes factor IX activation to factor IXa, which subsequently activates factor X to factor Xa in the common coagulation pathway. Growing evidence suggests that FXIa plays an important role in thrombosis with a relatively limited contribution to hemostasis. Therefore, inhibitors targeting factor XI (FXI)/FXIa system have emerged as a paradigm-shifting strategy so as to develop a new generation of anticoagulants to effectively prevent and/or treat thromboembolic diseases without the life-threatening risk of internal bleeding. Several inhibitors of FXI/FXIa proteins have been discovered or designed over the last decade including polypeptides, active site peptidomimetic inhibitors, allosteric inhibitors, antibodies, and aptamers. Antisense oligonucleotides (ASOs), which ultimately reduce the hepatic biosynthesis of FXI, have also been introduced. A phase II study, which included patients undergoing elective primary unilateral total knee arthroplasty, revealed that a specific FXI ASO effectively protects patients against venous thrombosis with a relatively limited risk of bleeding. Initial findings have also demonstrated the potential of FXI/FXIa inhibitors in sepsis, listeriosis, and arterial hypertension. This review highlights various chemical, biochemical, and pharmacological aspects of FXI/FXIa inhibitors with the goal of advancing their development toward clinical use.

Thrombus Formation and Propagation in the Onset of Cardiovascular Events

Ischemic cardiovascular disease is a major cause of morbidity and mortality worldwide and thrombus formation on disrupted atherosclerotic plaques is considered to trigger its onset. Although the activation of platelets and coagulation pathways has been investigated intensively, the mechanisms of thrombus formation on disrupted plaques have not been understood in detail. Platelets are thought to play a central role in the formation of arterial thrombus because of rapid flow conditions; however, thrombus that develops on disrupted plaques consistently includes large amounts of fibrin in addition to aggregated platelets. While, thrombus does not always become large enough to completely occlude the vascular lumen, indicating that the propagation of thrombus is also critical for the onset of cardiovascular events. Various factors, such as vascular wall thrombogenicity, altered blood flow and imbalanced blood hemostasis, modulate thrombus formation and propagation on disrupted plaques. Pathological findings derived from humans and experimental animal models of atherothrombosis have identified important factors that affect thrombus formation and propagation, namely platelets, extrinsic and intrinsic coagulation factors, proinflammatory factors, plaque hypoxia and blood flow alteration. These findings might provide insight into the mechanisms of thrombus formation and propagation on disrupted plaques that lead to the onset of cardiovascular events.

The relevance of coagulation in cardiovascular disease: what do the biomarkers tell us?

Several haemostatic factors have been associated with incident arterial cardiovascular disease in prospective studies and meta-analyses. Plasma fibrinogen shows a strong and consistent association with risk; however, this may reflect its inflammatory marker status, and causality remains to be proven. The common haemostatic gene polymorphisms for factor II, factor V and the von Willebrand factor: Factor VIII (non-O blood group) show significant associations with coronary heart disease (CHD) risk, consistent with potential causality. Increased D-dimer and t-PA antigen levels are associated with CHD risk, suggesting roles for coagulation activation and endothelial disturbance. There is little evidence for associations with CVD with other haemostatic factors.

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.

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.

Reaction-diffusion waves of blood coagulation

One of the main characteristics of blood coagulation is the speed of clot growth. In the current work we consider a mathematical model of the coagulation cascade and study existence, stability and speed of propagation of the reaction-diffusion waves of blood coagulation. We also develop a simplified one-equation model that reflects the main features of the thrombin wave propagation. For this equation we estimate the wave speed analytically. The resulting formulas provide a good approximation for the speed of wave propagation in a more complex model as well as for the experimental data.

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.

Allosteric inhibition of factor XIa. Sulfated non-saccharide glycosaminoglycan mimetics as promising anticoagulants

Recent development of sulfated non-saccharide glycosaminoglycan mimetics, especially sulfated pentagalloyl glucopyranoside (SPGG), as potent inhibitors of factor XIa (FXIa) (J. Med. Chem. 2013; 56:867-878 and J. Med. Chem. 2014; 57:4805-4818) has led to a strong possibility of developing a new line of factor XIa-based anticoagulants. In fact, SPGG represents the first synthetic, small molecule inhibitor that appears to bind in site remote from the active site. Considering that allosteric inhibition of FXIa is a new mechanism for developing a distinct line of anticoagulants, we have studied SPGG's interaction with FXIa with a goal of evaluating its pre-clinical relevance. Comparative inhibition studies with several glycosaminoglycans revealed the importance of SPGG's non-saccharide backbone. SPGG did not affect the activity of plasma kallikrein, activated protein C and factor XIIIa suggesting that SPGG-based anticoagulation is unlikely to affect other pathways connected with coagulation factors. SPGG's effect on APTT of citrated human plasma was also not dependent on antithrombin or heparin cofactor II. Interestingly, SPGG's anticoagulant potential was diminished by serum albumin as well as factor XI, while it could be reversed by protamine or polybrene, which implies possible avenues for developing antidote strategy. Studies with FXIa mutants indicated that SPGG engages Lys529, Arg530 and Arg532, but not Arg250, Lys252, Lys253 and Lys255. Finally, SPGG competes with unfractionated heparin, but not with polyphosphates and/or glycoprotein Ibα, for binding to FXIa. These studies enhance understanding on the first allosteric inhibitor of FXIa and highlight its value as a promising anticoagulant.

Sulfated pentagalloylglucoside is a potent, allosteric, and selective inhibitor of factor XIa

Inhibition of factor XIa (FXIa) is a novel paradigm for developing anticoagulants without major bleeding consequences. We present the discovery of sulfated pentagalloylglucoside (6) as a highly selective inhibitor of human FXIa. Biochemical screening of a focused library led to the identification of 6, a sulfated aromatic mimetic of heparin. Inhibitor 6 displayed a potency of 551 nM against FXIa, which was at least 200-fold more selective than other relevant enzymes. It also prevented activation of factor IX and prolonged human plasma and whole blood clotting. Inhibitor 6 reduced V(MAX) of FXIa hydrolysis of chromogenic substrate without affecting the K(M), suggesting an allosteric mechanism. Competitive studies showed that 6 bound in the heparin-binding site of FXIa. No allosteric small molecule has been discovered to date that exhibits equivalent potency against FXIa. Inhibitor 6 is expected to open up a major route to allosteric FXIa anticoagulants with clinical relevance.

P1 and P2' site mutations convert protease nexin-2 from a factor XIa inhibitor to a plasmin inhibitor

The kunitz protease inhibitor domain of PN2 (PN2KPI) is a potent and specific inhibitor (K(i) 0.5-2 nM) of factor XIa (FXIa) and inhibits cerebrovascular thrombosis in mice. To determine whether the antithrombotic properties of PN2KPI arise from its FXIa-inhibitory activity, we have now prepared mutant forms of PN2KPI. Mutations at the P1 (Arg(15)) site in combination with P2' (Met(17)) mutations profoundly affect inhibition of FXIa, plasmin, kallikrein, factor Xa and thrombin. The mutant proteins PN2KPI-R(15)K, -M(17)K, -R(15)K,M(17)K and -R(15)K,M(17)R lost inhibitory activity against FXIa (K(i) 34, 94, 3081 and 707 nM, respectively) and kallikrein (no inhibition) and gained inhibitory activity against plasmin (K(i) 108, 7, 8 and 8 nM, respectively). The intravenous administration of rPN2KPI into mice dramatically decreased thrombus formation in a murine model of FeCl(3)-induced carotid injury, whereas rPN2KPI-R(15)K,M(17)K failed to inhibit thrombus formation. Molecular modelling studies showed that fine structural variations explain the observed functional differences in FXIa and plasmin inhibition. PN2KPI has potent antithrombotic activity due to its specific FXIa anticoagulant activity, whereas PN2KPI-R(15)K,M(17)K and PN2KPI-R(15)K,M(17)R have potent antifibrinolytic (antiplasmin) activity without anticoagulant or antithrombotic activity.

The kunitz protease inhibitor domain of protease nexin-2 inhibits factor XIa and murine carotid artery and middle cerebral artery thrombosis

Coagulation factor XI (FXI) plays an important part in both venous and arterial thrombosis, rendering FXIa a potential target for the development of antithrombotic therapy. The kunitz protease inhibitor (KPI) domain of protease nexin-2 (PN2) is a potent, highly specific inhibitor of FXIa, suggesting its possible role in the inhibition of FXI-dependent thrombosis in vivo. Therefore, we examined the effect of PN2KPI on thrombosis in the murine carotid artery and the middle cerebral artery. Intravenous administration of PN2KPI prolonged the clotting time of both human and murine plasma, and PN2KPI inhibited FXIa activity in both human and murine plasma in vitro. The intravenous administration of PN2KPI into WT mice dramatically decreased the progress of FeCl(3)-induced thrombus formation in the carotid artery. After a similar initial rate of thrombus formation with and without PN2KPI treatment, the propagation of thrombus formation after 10 minutes and the amount of thrombus formed were significantly decreased in mice treated with PN2KPI injection compared with untreated mice. In the middle cerebral artery occlusion model, the volume and fraction of ischemic brain tissue were significantly decreased in PN2KPI-treated compared with untreated mice. Thus, inhibition of FXIa by PN2KPI is a promising approach to antithrombotic therapy.

Coagulation factor XI as a novel target for antithrombotic treatment

Coagulation factor (F)XI was first described as a member of the contact pathway of coagulation. However, the 'classic' theory of the extrinsic and intrinsic pathway has been revised and FXI was found to be activated by thrombin and to play a role in sustained thrombin generation and fibrinolysis inhibition. Recent studies have pointed to a disproportionate role of FXI in thrombosis and hemostasis. The observations that human congenital FXI deficiency is generally accompanied by mild and injury-related bleeding, and that experimental, provoked bleeding in animals is unaffected by FXI deficiency or FXI inhibition, suggest that the FXI amplification pathway is less important for normal hemostasis in vivo. In contrast, elevated plasma levels of FXI may contribute to human thromboembolic disease and the antithrombotic efficacy of FXI inhibition has been demonstrated in numerous animal models of arterial, venous and cerebral thrombosis. Whether severe FXI deficiency in humans protects against thromboembolic events remains unclear, although some evidence exists that the occurrence of ischemic stroke or venous thrombosis is low in severely FXI-deficient patients. Because of its distinctive function in thrombosis and hemostasis, FXI is an attractive target for the treatment and prevention of thromboembolism. A novel strategy for FXI inhibition is the use of antisense technology which has been studied in various thrombosis and bleeding animal models. The results are promising and support the concept that targeting FXI might serve as a new, effective and potentially safer alternative for the treatment of thromboembolic disease in humans.

Intrinsic clotting factors in dependency of age, sex, body mass index, and oral contraceptives: definition and risk of elevated clotting factor levels

Elevated clotting factors have been demonstrated to be a risk factor for venous thromboembolism (VTE). The aim of our study was to investigate the impact of age, sex, body mass index, and oral contraceptives on the clotting factor activities of factors VIII, IX, XI, and XII and their impact on the cutoff definition and risk of VTE associated with elevated clotting factors. Factor VIII, IX, XI, and XII activities were measured in 499 blood donors and 286 patients with VTE. Age and body mass index predicted significantly and independently the clotting factor activities of factors VIII, IX, and XI, whereas use of oral contraceptives predicted factor IX, XI, and XII levels. Percentiles of clotting factor activities, which are often used for the cutoff definition of elevated clotting factors, varied due to the effect of age, body mass index, and oral contraceptives. The adjusted odds ratios for VTE were 10.3 [95% confidence interval (CI) 5.1-20.7], 6.1 (95% CI 3.1-12.0), and 3.3 (95% CI 1.9-5.8) for elevated factors VIII, IX, and XI, respectively. Furthermore, our study demonstrates for the first time that elevated factor XII is associated with an increased risk of VTE (adjusted odds ratio 2.9, 95% CI 1.6-5.3).

Coagulation factors II, V, IX, X, XI, and XII, plasminogen, and alpha-2 antiplasmin and risk of coronary heart disease

AIM

To examine whether plasma levels of coagulation factors II, V, IX, X, XI, and XII, plasminogen, and alpha-2 antiplasmin are associated with coronary heart disease (CHD) in a prospective case-cohort study.

METHODS

This case-cohort sample consisted of 368 African-Americans or whites with incident CHD that occurred between 1990-92 and 1998, from the Atherosclerosis Risk in Communities (ARIC) study, and a cohort random sample of n=412. Hemostatic factors were measured in the case-cohort sample using plasma stored at -70 degrees C since 1990-92.

RESULTS

After adjustments for age, sex and race, coagulation factors IX and XI, and alpha-2 antiplasmin were associated positively with risk of CHD: The hazard ratio [95% confidence interval] for the highest vs lowest quartiles was 1.52 [1.01-2.27] for factor IX; 2.26 [1.47-3.48] for factor XI; and 1.64 [1.05-2.57] for alpha-2 antiplasmin. However, as these hemostatic factors were correlated with classical risk factors, their association with CHD was attenuated and no longer statistically significant after multivariable adjustments. No association was observed between CHD and factor II, V, X, or XII, or plasminogen.

CONCLUSIONS

Positive associations of factors IX and XI, and alpha-2 antiplasmin with incident CHD were not strong and accounted for by classical coronary risk factors.

Higher risk of ischaemic stroke associated with factor XI levels in dyslipidaemic patients

BACKGROUND

Ischaemic stroke (IS) is a complex disease that involves genetic and environmental factors. The role of factor XI (FXI) in arterial thrombosis is unclear. We have investigated the risk of IS related to FXI levels in a case-control study.

METHODS

We studied 445 individuals: 218 diagnosed with IS and 227 age-gender-ethnic matched control subjects. We measured factor VIIIc, fibrinogen and factor XIc levels. FXI < 144% was taken as the reference group in the statistical analysis.

RESULTS

There were 104 women and 114 men in the IS group and 108 women and 119 men in the control group. The crude odds ratio (OR) of IS in dyslipidaemic patients with high levels of FXI was 4.2 [95% confidence interval (CI): 1.2-14.8] compared with dyslipidaemic controls and low levels of FXI, but the OR in the non-dyslipidaemic with high levels of FXI subgroup was 1.2 (95% CI: 0.4-3.2). The adjusted OR of IS in dyslipidaemic patients with high levels of FXI was 6.4 (95% CI: 1.6-26.1) compared with dyslipidaemic controls, but the OR in non-dyslipidaemic subgroup with high levels of FXI was 0.6 (95% CI: 0.2-2.1).

CONCLUSION

We found almost a sixfold higher risk of IS in patients with dyslipidaemia and high levels of FXI. Further studies should elucidate the role of FXI in IS and therapeutic approaches should become apparent for patients with dyslipidaemia and high-FXI plasma levels.

Thrombophilia-hypofibrinolysis and atherothrombotic cardiovascular disease < or = age 45 years

Thrombophilia-hypofibrinolysis may play an important role in rare premature (< or = age 45 years) arterial occlusive events in atherothrombotic cardiovascular (ATCVD) disease, particularly in normolipidemic patients. Whether thrombophilia-hypofibrinolysis contributed to ATCVD < or = age 45 years was assessed in 78 men and 40 women with 230 ATCVD events (myocardial infarction (MI) [n = 60], coronary artery bypass graft [CABG, n = 33], angioplasty [n = 52], chronic angina [n = 41], ischemic stroke [n = 11], transient ischemic attack [TIA, n = 24], claudication [n = 9]). Cases were compared with healthy normal adult controls (44 men and 76 women). In men, the Factor V Leiden mutation was present in 6/63 (10%) cases versus 0/44 (0%) controls (P = 0.042), Factor VIII was high (>150%) in 16/60 (27%) cases versus 1/42 (2%) controls (P = 0.001), Factor XI was high (>150%) in 9/57 (16%) cases versus 0/42 (0%) controls (P = 0.009), and plasminogen activator inhibitor activity (PAI-Fx) was high (>21.1 U/mL) in 15/63 (24%) cases versus 3/43 (7%) controls (P = 0.023). In women, protein C was low (<73%) in 4/26 (15%) cases versus 0/74 (0%) controls (P = 0.004), and free protein S was low (<66%) in 5/27 (19%) cases versus 2/74 (3%) controls (P = 0.014). In women, Factor XI was high (>150%) in 3/27 (11%) cases versus 1/74 (1%) controls (P = 0.057), and the lupus anticoagulant was present in 9/32 (28%) cases versus 2/51 (4%) controls (P = 0.002). In patients with ATCVD < or = age 45 years, thrombophilias (Factor V Leiden, Factor VIII, Factor XI, protein C and S deficiency, lupus anticoagulant) and hypofibrinolysis (PAI-Fx, Lp[a]) may promote arterial thrombosis, which is synergistic with atherosclerotic endothelial injury.

Identification of the SELDI ProteinChip human serum retentate by microcapillary liquid chromatography-tandem mass spectrometry

Surface-enhanced laser desorption/ionization (SELDI) time-of-flight (TOF) mass spectrometry (MS) has been widely applied for conducting biomarker research with the goal of discovering patterns of proteins and/or peptides from biological samples that reflect disease status. Many diseases, ranging from cancers of the colon, breast, and prostate to Alzheimer's disease, have been studied through serum protein profiling using SELDI-based methods. Although the results from SELDI-based diagnostic studies have generated a great deal of excitement and skepticism alike, the basis of the molecular identities of the features that underpin the diagnostic potential of the mass spectra is still largely unexplored. A detailed investigation has been undertaken to identify the compliment of serum proteins that bind to the commonly used weak cation exchange (WCX-2) SELDI protein chip. Following incubation and washing of a standard serum sample on the WCX-2 sorbent, proteins were harvested, digested with trypsin, fractionated by strong cation exchange liquid chromatography (LC), and subsequently analyzed by microcapillary reversed-phase LC coupled online with an ion-trap mass spectrometer. This analysis resulted in the identification of 383 unique proteins in the WCX-2 serum retentate. Among the proteins identified, 50 (13%) are documented clinical biomarkers with 36 of these (72%) identified from multiple peptides.

Elevated factor XI activity levels are associated with an increased odds ratio for cerebrovascular events

High levels of factor XI have been implicated as a risk factor for deep venous thrombosis and possibly cardiovascular disease; however, the relationship between elevated factor XI activity and stroke has yet to be established. We retrospectively evaluated factor XI activity, factor XI antigen, and high-sensitivity C-reactive protein (hs-CRP) values in samples from 65 patients with stroke, 13 with transient ischemic attack (TIA), and 17 with venous thrombosis, younger than 55 years with normal prothrombin and partial thromboplastin times who underwent evaluation for a hypercoagulable state. Factor XI activity levels were more than normal in 22% of patients with stroke or TIA and 18% of patients with venous thrombosis, producing odds ratios of 5.3 and 4.1 for stroke or TIA and venous thrombosis, respectively. Factor XI activity levels correlate with factor XI antigen levels by Deming regression analysis (slope, 1.3; R = 0.667) and a lack of correlation of both with hs-CRP suggests that factor XI is not an acute phase reactant. Our findings suggest an association between elevated factor XI activity and stroke.

Factor XI contributes to thrombus propagation on injured neointima of the rabbit iliac artery

BACKGROUND

Thrombus formation through the activation of tissue factor (TF) and factor (F) XI is a critical event in the onset of cardiovascular disease. TF expressed in atherosclerotic plaques and circulating blood is an important determinant of thrombogenicity that contributes to fibrin-rich thrombus formation after plaque disruption. However, the contribution of FXI to thrombus formation on disrupted plaques remains unclear.

METHODS

A mouse monoclonal antibody against FXI and activated FXI (FXIa) (XI-5108) was generated by immunization with activated human FXI. Prothrombin time (PT), activated partial thromboplastin time (APTT), bleeding time, and ex vivo platelet aggregation in rabbits were measured before and after an intravenous bolus injection of XI-5108. We investigated the role of FXI upon arterial thrombus growth in the rabbit iliac artery in the presence of repeated balloon injury.

RESULTS

The XI-5108 antibody reacted to the light chain of human and rabbit FXI/FXIa, and inhibited FXIa-initiated FXa and FXIa generation. Fibrin-rich thrombi developed on the injured neointima that was obviously immunopositive for glycoprotein IIb-IIIa, fibrin, TF, and FXI. Intravenous administration of XI-5108 (3.0 mg kg(-1)) remarkably reduced thrombus growth, and the APTT was significantly prolonged. However, PT, bleeding time and platelet aggregation were not affected.

CONCLUSIONS

These results indicate that plasma FXI plays a potent role in thrombus growth on the injured neointima. Inhibition of plasma FXI activity might help to reduce thrombus growth on ruptured plaques without prolonging bleeding time.

Role of thrombotic and fibrinolytic factors in acute coronary syndromes

There is a growing body of literature concerning the contribution of hemostatic factors to the development of cardiovascular disease. The mechanisms of the coagulation/fibrinolytic system are complicated and one factor is intimately interrelated with another; thus the contribution of each factor cannot be clearly understood, unless hemostatic factors are considered in accordance with endothelial function and vessel morphology. Although there are many clinical studies about the correlation between hemostatic factors and cardiovascular risk, the results are inconsistent and conflicting at times. Fibrinogen and D-dimer are associated with atherosclerosis or coronary events across multiple studies, even after multivariate adjustment. But the hemostatic factors are intimately correlated, so it can be said that focusing on one to the exclusion of others is inappropriate. The clinical trials with statins or angiotensin converting enzyme inhibitors have shown favorable effects on the prognosis of cardiovascular disease. The study of hemostatic factors in relation to these drugs has provided insights into understanding how these drugs produce beneficial effects.

Beta 2-Glycoprotein I binds factor XI and inhibits its activation by thrombin and factor XIIa: loss of inhibition by clipped beta 2-glycoprotein I

Activation of factor XI (FXI) by thrombin in vivo plays a role in coagulation by providing an important positive feedback mechanism for additional thrombin generation. FXI is activated in vitro by thrombin, or FXIIa in the presence of dextran sulfate. In this report, we investigated the effect of beta(2)-glycoprotein I (beta(2)GPI) on the activation of FXI. beta(2)GPI bound FXI in vitro and inhibited its activation to FXIa by thrombin and FXIIa. The affinity of the interaction between beta(2)GPI and FXI was equivalent to the interaction between FXI and high molecular weight kininogen. Inhibition of FXI activation occurred with lower concentrations of beta(2)GPI than found in human plasma. Proteolytic clipping of beta(2)GPI by plasmin abolished its inhibition of FXI activation. The results suggest a mechanism of regulation whereby physiological concentrations of beta(2)GPI may attenuate thrombin generation in vivo by inhibition of FXI activation. Plasmin cleavage of beta(2)GPI provides a negative feedback that counteracts its inhibition of FXI activation.

The blood coagulation system as a molecular machine

The human blood coagulation system comprises a series of linked glycoproteins that upon activation induce the generation of downstream enzymes ultimately forming fibrin. This process is primarily important to arrest bleeding (hemostasis). Hemostasis is a typical example of a molecular machine, where the assembly of substrates, enzymes, protein cofactors and calcium ions on a phospholipid surface markedly accelerates the rate of coagulation. Excess, pathological, coagulation activity occurs in "thrombosis", the formation of an intravascular clot, which in the most dramatic form precipitates in the microvasculature as disseminated intravascular coagulation. Thrombosis occurs according to a biochemical machine model in the case of atherothrombosis on a ruptured atherosclerotic plaque, but may develop at a slower rate in venous thrombosis, illustrating that the coagulation machinery can act at different velocities. The separate coagulation enzymes are also important in other biological processes, including inflammation for which the rapid conversion of one coagulation factor by the other is not a prerequisite. The latter role of coagulation enzymes may be related to the old and probably maintained function of the coagulation machine in innate immunity.

Factor XI-dependence of surface- and tissue factor-initiated thrombus propagation in primates

Thrombin, generated through activation of factor XI (FXI) and/or tissue factor (TF)-factor VIIa, is essential for thrombosis and hemostasis. We investigated the role of FXI-dependent thrombus propagation under arterial flow conditions producing rapid thrombus growth that, after the initiation phase, could limit the availability of TF at the blood/thrombus interface. Thrombosis was initiated by knitted dacron or TF-presenting teflon grafts deployed into arteriovenous shunts in baboons treated with antihuman FXI antibody (aFXI). Although aFXI did not prevent thrombus initiation, it markedly reduced intraluminal thrombus growth on both surfaces. The antithrombotic effect of aFXI was comparable with that of heparin at doses that significantly prolonged the partial thromboplastin time (APTT), prothrombin time (PT), and bleeding time (BT). aFXI also prolonged the APTT, but the PT and BT were unaffected. Thus, antithrombotic targeting of FXI might inhibit thrombosis with relatively modest hemostatic impairment versus strategies targeting other coagulation factors.