Habu coagulotoxicity: Clinical implications of the functional diversification of Protobothrops snake venoms upon blood clotting factors.

Venom can affect any part of the body reachable via the bloodstream. Toxins which specifically act upon the coagulation cascade do so either by anticoagulant or procoagulant mechanisms. Here we investigated the coagulotoxic effects of six species within the medically important pit viper genus Protobothrops (Habu) from the Chinese mainland and Japanese islands, a genus known to produce hemorrhagic shock in envenomed patients. Differential coagulotoxicity was revealed: P. jerdonii and P. mangshanensis produced an overall net anticoagulant effect through the pseudo-procoagulant clotting of fibrinogen; P. flavoviridis and P. tokarensis exhibit a strong anticoagulant activity through the destructive cleavage of fibrinogen; and while P. elegans and P. mucrosquamatus both cleaved the A-alpha and B-beta chains of fibrinogen they did not exhibit strong anticoagulant activity. These variations in coagulant properties were congruent with phylogeny, with the closest relatives exhibiting similar venom effects in their action upon fibrinogen. Ancestral state reconstruction indicated that anticoagulation mediated by pseudo-procoagulant cleavage of fibrinogen is the basal state, while anticoagulation produced by destructive cleavage of fibrinogen is the derived state within this genus. This is the first in depth study of its kind highlighting extreme enzymatic variability, functional diversification and clotting diversification within one genus surrounding one target site, governed by variability in co-factor dependency. The documentation that the same net overall function, anticoagulation, is mediated by differential underlying mechanics suggests limited antivenom cross-reactivity, although this must be tested in future work. These results add to the body of knowledge necessary to inform clinical management of the envenomed patient.

Roles of Coagulation Proteases and PARs (Protease-Activated Receptors) in Mouse Models of Inflammatory Diseases.

Activation of the blood coagulation cascade leads to fibrin deposition and platelet activation that are required for hemostasis. However, aberrant activation of coagulation can lead to thrombosis. Thrombi can cause tissue ischemia, and fibrin degradation products and activated platelets can enhance inflammation. In addition, coagulation proteases activate cells by cleavage of PARs (protease-activated receptors), including PAR1 and PAR2. Direct oral anticoagulants have recently been developed to specifically inhibit the coagulation proteases FXa (factor Xa) and thrombin. Administration of these inhibitors to wild-type mice can be used to determine the roles of FXa and thrombin in different inflammatory diseases. These results can be compared with the phenotypes of mice with deficiencies of either Par1 (F2r) or Par2 (F2rl1). However, inhibition of coagulation proteases will have effects beyond reducing PAR signaling, and a deficiency of PARs will abolish signaling from all proteases that activate these receptors. We will summarize studies that examine the roles of coagulation proteases, particularly FXa and thrombin, and PARs in different mouse models of inflammatory disease. Targeting FXa and thrombin or PARs may reduce inflammatory diseases in humans.

Thrombin and factor Xa link the coagulation system with liver fibrosis.

BACKGROUND: Thrombin activates hepatic stellate cells via protease-activated receptor-1. The role of Factor Xa (FXa) in hepatic fibrosis has not been elucidated. We aimed to evaluate the impact of FXa and thrombin in vitro on stellate cells and their respective inhibition in vivo using a rodent model of hepatic fibrosis. METHODS: HSC-LX2 cells were incubated with FXa and/or thrombin in cell culture, stained for αSMA and relative gene expression and gel contraction calculated. C57BL/6 J mice were administered thioacetamide (TAA) for 8 weeks with Rivaroxaban (n = 15) or Dabigatran (n = 15). Control animals received TAA alone (n = 15). Fibrosis was scored and quantified using digital image analysis and hepatic tissue hydroxyproline estimated. RESULTS: Stellate cells treated with FXa and thrombin demonstrated upregulation of procollagen, TGF-beta, αSMA and significant cell contraction (43.48%+/- 4.12) compared to culturing with FXa or thrombin alone (26.90%+/- 8.90, p = 0.02; 13.1%+/- 9.84, p < 0.001). Mean fibrosis score, percentage area of fibrosis and hepatic hydroxyproline content (2.46 vs 4.08, p = 0.008; 2.02% vs 3.76%, p = 0.012; 276.0 vs 651.3, p = 0.0001) were significantly reduced in mice treated with the FXa inhibitor compared to control mice. FXa inhibition was significantly more effective than thrombin inhibition in reducing percentage area of fibrosis and hepatic hydroxyproline content (2.02% vs 3.70%,p = 0.031; 276.0 vs 413.1,p = 0.001). CONCLUSIONS: FXa promotes stellate cell contractility and activation. Early inhibition of coagulation using a FXa inhibitor significantly reduces TAA induced murine liver fibrosis and may be a viable treatment for liver fibrosis in patients.

A direct oral factor Xa inhibitor edoxaban ameliorates neointimal hyperplasia following vascular injury and thrombosis in apolipoprotein E-deficient mice.

Vascular injury activates the coagulation cascade. Some studies report that coagulation factor Xa and thrombin are implicated in proliferation of vascular smooth muscle cells and neointimal hyperplasia after vascular injury. The aim of this study was to determine the effect of an oral direct factor Xa inhibitor, edoxaban, on neointimal hyperplasia following the carotid artery injury in apolipoprotein E (ApoE)-deficient mice. Vascular injury was induced by the application of 10% ferric chloride to the carotid artery for 3 min in ApoE-deficient mice. After vascular injury, all animals were fed with high-cholesterol chow for 6 weeks. Edoxaban at 15 mg/kg was orally administered to the mice 1 h before (n = 10) or 1 h after (n = 9) ferric chloride injury, and thereafter 10 mg/kg edoxaban was orally administered b.i.d. for 6 weeks. Thrombus formation and neointimal hyperplasia were evaluated. Treatment with 15 mg/kg edoxaban before vascular injury almost completely inhibited thrombus formation, and following chronic administration of edoxaban significantly suppressed neointimal hyperplasia. In the mice treated with edoxaban after vascular injury, there was wide interindividual variability. In some mice (four out of nine) the neointimal hyperplasia was inhibited like in edoxaban-pretreated mice, but there was no statistical difference compared with control. This study demonstrated that inhibition of the coagulation and thrombosis by edoxaban ameliorated neointimal hyperplasia caused by vascular injury and high-cholesterol diets in ApoE-deficient mice. This suggests that factor Xa has a crucial role in the formation of neointima following vascular injury.The abstract should be followed by 3-4 bullet points that highlight major findings. The final bullet point should emphasize future directions for research.

[Roles of coagulation pathway and factor Xa in chronic kidney disease (CKD)].

Considering that fibrin deposition is observed in glomerulonephritis as well as in diabetic nephropathy, we performed studies to clarify the roles of the coagulation pathway and the active type of coagulation factor X (factor Xa) in the development of chronic kidney disease (CKD) using animal models. Factor Xa activates various cell types through protease-activated receptor 2 (PAR2). Several in vitro studies have demonstrated that PAR2 can mediate factor Xa signaling, but not thrombin signaling. Coagulation processes proceed together with the extracellular matrix (ECM) accumulation through factor V expression in rat Thy-1 nephritis. DX-9065a, a factor Xa inhibitor, suppresses this type of glomerulonephritis. The factor Xa inhibitor danaparoid ameliorated proteinuria, cellular proliferation, and fibrin deposition in lipopolysaccharide (LPS)-triggered activation of High IgA (HIGA) strain of ddY mice. Another factor Xa inhibitor, fondaparinux, suppressed urinary protein, glomerular hypertrophy, and connective tissue growth factor (CTGF), and ECM protein deposition together with angiogenesis in diabetic db/db mice. Finally, in the model of peritoneal fibrosis, fondaparinux treatment decreased the thickness of submesothelial fibrotic tissue and angiogenesis. In consideration of the results to potential human therapy, factor Xa regulation may be promising for the treatment of the aggravation in glomerulonephritis and of the early phase of diabetic nephropathy. In the near future, novel factor Xa inhibitors with the characteristics of oral administration and biliary elimination may appear in the clinical use for treatment of cardiovascular diseases.

A role for coagulation factor Xa in experimental pulmonary arterial hypertension.

AIMS: Anticoagulation with warfarin is recommended for the treatment of patients with pulmonary arterial hypertension (PAH). However, the therapeutic benefit of anticoagulation has not yet been demonstrated experimentally or clinically. Here, rivaroxaban, an oral, direct factor Xa (FXa) inhibitor, was compared with warfarin and enoxaparin in the prevention of right ventricular (RV) dysfunction and hypertrophy in the monocrotaline (MCT) model of pulmonary hypertension. METHODS AND RESULTS: Sprague-Dawley rats (n = 10 per group) were randomized to receive rivaroxaban, warfarin, enoxaparin, or placebo before receiving a subcutaneous injection of MCT 60 mg/kg or saline. Rivaroxaban and enoxaparin were administered for 28 days starting 4 h before MCT injection; warfarin was given for 35 days initiated 7 days before MCT injection. RV haemodynamics and hypertrophy were assessed 28 days after MCT administration. Rivaroxaban dose-dependently reduced systolic and end-diastolic RV pressure increase and RV hypertrophy. Warfarin reduced RV pressure increase only. Enoxaparin had no effect on either parameter. Severe bleeding occurred in four and five rats treated with warfarin and enoxaparin, respectively, whereas no overt bleeding was observed in rats treated with rivaroxaban. CONCLUSION: Selective, direct inhibition of FXa by rivaroxaban effectively prevented RV dysfunction and hypertrophy in MCT-injected rats, indicating a role for coagulation factors in experimental pulmonary hypertension. Clinical investigation of the impact of early and continued administration of a specific FXa inhibitor such as rivaroxaban on the course of PAH should be considered.

Role of protease-activated receptor-2 in idiopathic pulmonary fibrosis.

RATIONALE: Activation of the coagulation cascade has been demonstrated in pulmonary fibrosis. In addition to its procoagulant function, various coagulation proteases exhibit cellular effects that may also contribute to fibrotic processes in the lung. OBJECTIVE: To investigate the importance of protease-activated receptor (PAR)-2 and its activators, coagulation factor VIIa (FVIIa)/tissue factor (TF), in the development of idiopathic pulmonary fibrosis (IPF). METHODS: Expression and localization of PAR-2 and its activators were examined in IPF lung tissue. The ability of PAR-2 to mediate various cellular processes was studied in vitro. MEASUREMENTS AND MAIN RESULTS: Expression of PAR-2 was strongly elevated in IPF lungs and was attributable to alveolar type II cells and fibroblasts/myofibroblasts. Transforming growth factor-ß(1), a key profibrotic cytokine, considerably enhanced PAR-2 expression in human lung fibroblasts. FVIIa stimulated proliferation of human lung fibroblasts and extracellular matrix production in a PAR-2-dependent manner, but did not initiate differentiation of fibroblasts into myofibroblasts. PAR-2/FVIIa-driven mitogenic activities were mediated via the p44/42 mitogen-activated protein kinase pathway and were independent of factor Xa and thrombin production. Proproliferative properties of FVIIa were markedly potentiated in the presence of TF and abrogated by TF antisense oligonucleotides. Hyperplastic alveolar type II cells overlying fibroblastic foci were found to be the source of FVII in IPF lungs. Moreover, TF colocalized with PAR-2 on fibroblasts/myofibroblasts in IPF lungs. CONCLUSIONS: The PAR-2/TF/FVIIa axis may contribute to the development of pulmonary fibrosis; thus, interference with this pathway confers novel therapeutic potential for the treatment of IPF.

Coagulation-induced shedding of platelet glycoprotein VI mediated by factor Xa.

This study evaluated shedding of the platelet collagen receptor, glycoprotein VI (GPVI) in human plasma. Collagen or other ligands induce metalloproteinase-mediated GPVI ectodomain shedding, generating approximately 55-kDa soluble GPVI (sGPVI) and approximately 10-kDa platelet-associated fragments. In the absence of GPVI ligands, coagulation of platelet-rich plasma from healthy persons induced GPVI shedding, independent of added tissue factor, but inhibitable by metalloproteinase inhibitor, GM6001. Factor Xa (FXa) common to intrinsic and tissue factor-mediated coagulation pathways was critical for sGPVI release because (1) shedding was strongly blocked by the FXa-selective inhibitor rivaroxaban but not FIIa (thrombin) inhibitors dabigatran or hirudin; (2) Russell viper venom that directly activates FX generated sGPVI, with complete inhibition by enoxaparin (inhibits FXa and FIIa) but not hirudin; (3) impaired GPVI shedding during coagulation of washed platelets resuspended in FX-depleted plasma was restored by adding purified FX; and (4) purified FXa induced GM6001-inhibitable GPVI shedding from washed platelets. In 29 patients with disseminated intravascular coagulation, mean plasma sGPVI was 53.9 ng/mL (95% confidence interval, 39.9-72.8 ng/mL) compared with 12.5 ng/mL (95% confidence interval, 9.0-17.3 ng/mL) in thrombocytopenic controls (n = 36, P < .0001), and 14.6 ng/mL (95% confidence interval, 7.9-27.1 ng/mL) in healthy subjects (n = 25, P = .002). In conclusion, coagulation-induced GPVI shedding via FXa down-regulates GPVI under procoagulant conditions. FXa inhibitors have an unexpected role in preventing GPVI down-regulation.

Contributions of extravascular and intravascular cells to fibrin network formation, structure, and stability.

Fibrin is essential for hemostasis; however, abnormal fibrin formation is hypothesized to increase thrombotic risk. We previously showed that in situ thrombin generation on a cell's surface modulates the 3-dimensional structure and stability of the fibrin network. Currently, we compared the abilities of extravascular and intravascular cells to support fibrin formation, structure, and stability. Extravascular cells (fibroblasts, smooth muscle) supported formation of dense fibrin networks that resisted fibrinolysis, whereas unstimulated intravascular (endothelial) cells produced coarse networks that were susceptible to fibrinolysis. All 3 cell types produced a fibrin structural gradient, with a denser network near, versus distal to, the cell surface. Although fibrin structure depended on cellular procoagulant activity, it did not reflect interactions between integrins and fibrin. These findings contrasted with those on platelets, which influenced fibrin structure via interactions between beta3 integrins and fibrin. Inflammatory cytokines that induced prothrombotic activity on endothelial cells caused the production of abnormally dense fibrin networks that resisted fibrinolysis. Blocking tissue factor activity significantly reduced the density and stability of fibrin networks produced by cytokine-stimulated endothelial cells. Together, these findings indicate fibrin structure and stability reflect the procoagulant phenotype of the endogenous cells, and suggest abnormal fibrin structure is a novel link between inflammation and thrombosis.

Synthesis and biological evaluation of the metabolites of 2-(1-{3-[(6-chloronaphthalen-2-yl)sulfonyl]propanoyl}piperidin-4-yl)-5-methyl-1,2-dihydro-3H-imidazo[1,5-c]imidazol-3-one.

We have recently discovered imidazo[1,5-c]imidazol-3-one derivative 1 as a potent, selective, and orally bioavailable factor Xa (FXa) inhibitor. In this study, we have synthesized metabolites of 1 and evaluated their biological activities. As a result, we identified the active metabolites S-5 and 6 with a potent FXa inhibitory activity comparable to 1 and a favorable pharmacokinetic profile in monkeys.

The effect of a low molecular weight heparin on coagulation parameters in healthy cats.

The low molecular weight heparin (LMWH), dalteparin sodium, was administered subcutaneously (100 IU/kg) to 8 healthy cats twice daily for 13 doses. Anti-activated factor X (anti-Xa) activity was measured prior to administration (time 0), and 4, 6, 8, and 12 h after the 1st dose, 4 h after administration of the 3rd dose, and at 4, 6, 8, and 12 h after the last dose. Four cats developed measurable anti-Xa activity 4 h following a single dose, returning to baseline by 6 h. Anti-Xa activity was not detected at any time point in 4 cats. Prothrombin time (PT), activated partial thromboplastin time (APTT), and antithrombin (AT) concentrations were unaffected by LMWH administration. Dalteparin, at 100 IU/kg SC, did not achieve anti-Xa activity in 4 out of 8 cats and failed to maintain anti-Xa activity beyond 4 h in the other 4 healthy cats.

Factor Xa: at the crossroads between coagulation and signaling in physiology and disease.

Activated factor Xa (FXa) is traditionally known as an important player in the coagulation cascade responsible for thrombin generation. Long considered a passive bystander, it is now evident that FXa exerts direct effects on a wide variety of cell types via activation of its two main receptors, protease-activated receptor-1 (PAR-1) and PAR-2. Recent findings suggest that PAR-2 plays a crucial role in fibro-proliferative diseases such as fibrosis, tissue remodeling and cancer and point towards FXa as the important mediator coordinating the interface between coagulation and disease progression. Here, we provide an overview of the FXa signaling pathways that mediate its effects in pathophysiology and explore the potential therapeutic implications of targeting FXa; in terms of arresting disease progression, the modulation of FXa activity might be more important than the modulation of FVIIa or thrombin.

Persistent factor VIII-dependent factor X activation on endothelial cells is independent of von Willebrand factor.

Endothelial cells are able to support the activation of coagulation factor X by activated factor IX in the presence of its cofactor, factor VIII. We have previously reported that this reaction is persistent on endothelial cells, but transient on activated platelets and phospholipid vesicles when activated factor X (Xa) is used as activator of factor VIII. Aim of the present study was to explore the influence of von Willebrand factor and that of the factor VIII activator, either factor Xa or thrombin, on the decay of factor X activation on the endothelial cell surface. Kinetics of factor X activation on human umbilical vein endothelial cells was compared with that on phospholipid vesicles employing purified coagulation factors from plasma as well as recombinant factor VIII variants. Employing factor Xa as factor VIII activator, rate constants for decay of membrane-bound factor X activation were consistently low on endothelial cells (0.02 min) as compared with phospholipid vesicles (0.2 min). Activation of factor VIII by thrombin resulted in two-fold increased decay rates. In the presence of excess of von Willebrand factor over factor VIII, decay rates were not significantly changed. Factor VIII variants with and without a Tyr to Phe substitution, which abolishes high-affinity binding to von Willebrand factor, displayed the same factor X activation decay kinetics. Although previous studies have shown that von Willebrand factor modulates factor VIII activation and stabilisation, this apparently does not affect the progression of factor X activation at the endothelium.

Coagulation signalling following tissue injury: focus on the role of factor Xa.

The primary function of the coagulation cascade is to promote haemostasis and limit blood loss in response to tissue injury. However, it is now recognized that the physiological functions of the coagulation cascade extend beyond blood coagulation and that this cascade plays a pivotal role in influencing inflammatory and tissue repair responses via the activation of their signalling responses, the proteinase-activated receptors (PARs). Consequently, uncontrolled coagulation activity and PAR signalling contributes to the pathophysiology of several conditions, including thrombosis, arthritis, cancer, kidney disease, and acute and chronic lung injury. Much of the work thus far has focused on the role of thrombin-mediated signalling in the pathophysiology of these conditions. However, recent evidence suggests that coagulation proteinases upstream of thrombin, including factor Xa (FXa), may also signal via PARs and thus induce cellular effects independent of thrombin generation. These studies have highlighted a novel and important role for FXa signalling in influencing proinflammatory and pro-fibrotic effects following tissue injury. This article will provide an overview of FXa as a central proteinase of the coagulation cascade and will review more recent evidence that FXa signalling may contribute to inflammation and tissue remodelling. The novel opportunities that this may present for therapeutic intervention will also be highlighted.

The haemostatic role of tissue factor pathway inhibitor.

Under normal conditions the blood circulates freely within the confines of the vascular system, carrying oxygen, nutrients, and hormonal information around the body and removing metabolic waste. If blood gains access to extravascular sites, or the vasculature becomes pathologically challenged, hemostasis may be activated. This process is finely regulated by positive and negative feedback loops that modulate fibrin clot formation. Blood coagulation revolves around the activation and assembly of the components of the prothrombinase complex, which converts the inactive zymogen, prothrombin, into its active form, thrombin. This serine protease catalyzes the conversion of fibrinogen to fibrin, the structural scaffold that stabilizes platelet aggregates at sites of vascular injury. The extent of the hemostatic response is controlled by the action of inhibitory pathways, which ensure that thrombin activity and the spread of the hemostatic plug is limited to the site of vessel damage. This review article focuses on the major physiological regulator of tissue factor-induced coagulation, tissue factor pathway inhibitor, its expression, anticoagulant function, and its role in normal hemostasis.

Lipid-bound factor Xa regulates tissue factor activity.

The activation of coagulation factor X by tissue factor (TF) and coagulation factor VIIa (VIIa) on a phospholipid surface is thought to be the key step in the initiation of blood coagulation. In this reaction, the product, fXa, is transiently and reversibly bound to the TF-VIIa enzyme complex. This in effect leads to a probabilistic inhibition of subsequent fX activations; a new fX substrate molecule cannot be activated until the old fXa molecule leaves. In this study, we demonstrate that benzamidine and soybean trypsin inhibitor-conjugated Sepharose beads, which bind fXa and sequester it away from the reaction, serve to enhance fX activation by the TF-VIIa complex. Thus, removal of fXa from the reactive zone, by either flow, fXa sequestration, or binding to distant lipid surfaces, can serve to enhance the levels of TF-VIIa activity. Using resonance energy transfer, we found the dissociation constants of fX and fXa for 100 nm diameter phospholipid vesicles to be on the order of 30-60 nM, consistent with previous measurements employing planar lipid surfaces. On the basis of the measurements of binding of fXa to phospholipid surfaces, we demonstrate that the rates of fX activation by the TF-VIIa complex under a variety of experimental conditions depend inversely on the amount of product (fXa) bound to the TF-phospholipid surface. These data support an inhibitory role for the reaction product, fXa, and indicate that models previously employed in understanding this initial coagulation reaction must now be re-evaluated to account for both the product occupancy of the phospholipid surface and the binding of the product to the enzyme. Moreover, the inhibitory properties of fXa can be described on the basis of the estimated surface density of fXa molecules on the TF-phospholipid surface.

Coagulation factor Xa modulates airway remodeling in a murine model of asthma.

RATIONALE: Previous studies have demonstrated that dysregulated coagulation and fibrinolysis contribute to the pathogenesis of asthma. OBJECTIVE: The role of procoagulant factor X in a murine model of ovalbumin (OVA)-induced asthma was investigated. METHODS: Biochemical, cellular, and physiologic in vivo and in vitro approaches were used to determine effects of factor X on the asthmatic response in mice. MEASUREMENTS AND MAIN RESULTS: Factor X transcript levels and factor Xa activity were increased in lungs of asthmatic mice challenged with OVA, compared with controls treated with phosphate-buffered saline. Factor X was highly expressed in bronchoalveolar lavage fluid macrophages from asthmatic mice. Treatment of mice with the factor Xa inhibitor fondaparinux during the last 4 wk of OVA challenge resulted in the attenuation of airway hyperresponsiveness but did not alter infiltration of inflammatory cells into the lung. There was a significant decrease in the thickness of the mucosal layer and in lung collagen deposition in fondaparinux-treated mice. In vitro investigations using human mucus-producing NCI-H292 cells indicated that exogenous factor Xa enhanced mucin production in a dose-dependent manner. Levels of amphiregulin, a protein that induces mucin production, were also increased in cells stimulated by factor Xa. CONCLUSIONS: The results of this study introduce a novel participant in the asthmatic response and indicate that factor Xa functions in airway remodeling in asthma by stimulating mucin production, through regulation of amphiregulin expression and collagen deposition.

Factor Xa inhibition by immobilized recombinant tissue factor pathway inhibitor.

The recombinant form of the endogenous anticoagulant, tissue factor pathway inhibitor (rTFPI), is a potent inhibitor of Factor Xa (FXa) and the tissue factor-factor VIIa (TF:VIIa) complex. Surface-immobilized rTFPI reduces the thrombogenicity and intimal hyperplasia associated with synthetic vascular grafts in animal models and specifically reduces fibrin deposition on collagen-impregnated Dacron grafts from native blood in an in vitro flow model. The FXa inhibitory capacity of rTFPI in the bulk phase has been demonstrated in static systems and immobilized rTFPI reduces fibrin deposition in whole blood in vitro and animal studies; however, the specific mode of this anticoagulation has not been studied. Therefore, a comparison was made between the FXa binding capacity of two forms of immobilized rTFPI, i.e., passively adsorbed and covalently bound. The rTFPI-coated surfaces were evaluated using a parallel-plate flow reactor and comparing the amount of FXa exiting the flow chamber after exposure to an rTFPI-coated versus an uncoated plate. The results demonstrate that adsorbed rTFPI exhibits increased binding capacity (1.5-3.6 times) the expected stoichiometry via interactions with the C-terminus, whereas covalently-bound rTFPI interacts with FXa in a 1:1 stoichiometry. Thus, the results imply that specific FXa inhibition is a key component of the anticoagulant effect of rTFPI-coated surfaces and that passive adsorption of rTFPI to glass surfaces produces a more effective coating than covalent binding of rTFPI.