Recombinant human factor VIIa (rFVIIa) cleared principally by antithrombin following intravenous administration in hemophilia patients

Summary. Objective: The objective of the present study was to evaluate the pharmacokinetics and the clearance pathways of rFVIIa after intravenous administration to hemophilia patients. Methods: Ten severe hemophilia patients were included in the study; all patients were intravenously administered a clinically relevant dose of 90 μg kg^?1 (1.8 nmol kg^?1) rFVIIa. Blood samples were collected consecutively to describe the pharmacokinetics of rFVIIa. All samples were analyzed using three different assays: a clot assay to measure the activity (FVIIa:C), an enzyme immunoassay (EIA) to measure the antigen levels (FVII:Ag), and an EIA (FVIIa‐AT) to measure the FVIIa antithrombin III (AT) complex. Pharmacokinetic parameters were evaluated both by use of standard non‐compartmental methods and by use of mixed effects methods. A population pharmacokinetic model was used to simultaneously model all three datasets. The total body clearance of rFVIIa:C was estimated to be 38 mL h^?1 kg^?1. The rFVII‐AT complex formation was responsible for 65% of the total rFVIIa:C clearance. The initial and the terminal half‐life of rFVIIa:C was estimated to be 0.6 and 2.6 h, respectively. The formation of rFVII‐AT complex was able to explain the difference observed between the rFVIIa:C and the rFVII:Ag concentration. The non‐compartmental analysis resulted in almost identical parameters.     

The assembly of the factor X-activating complex on activated human platelets

Summary.  Platelet membranes provide procoagulant surfaces for the assembly and expression of the factor X-activating complex and promote the proteolytic activation and assembly of the prothrombinase complex resulting in normal hemostasis. Recent studies from our laboratory and others indicate that platelets possess specific, high-affinity, saturable, receptors for factors XI, XIa, IX, IXa, X, VIII, VIIIa, V, Va and Xa, prothrombin, and thrombin. Studies described in this review support the hypothesis that the factor X-activating complex on the platelet surface consists of three receptors (for the enzyme, factor IXa; the substrate, factor X; and the cofactor, factor VIIIa), the colocalization of which results in a 24 million-fold acceleration of the rate of factor X activation. Whether the procoagulant surface of platelets is defined exclusively by procoagulant phospholipids, or whether specific protein receptors exist for the coagulant factors and proteases, is currently unresolved. The interaction between coagulation proteins and platelets is critical to the maintenance of normal hemostasis and is pathogenetically important in human disease.     

Bio‐distribution of pharmacologically administered recombinant factor VIIa (rFVIIa)

Summary. Background: Recent clinical studies suggest that the prophylactic use of recombinant factor VIIa (rFVIIa) markedly reduces the number of bleeding episodes in hemophilic patients with inhibitors. Given the short biological half‐life of rFVIIa, it is unclear how rFVIIa could be effective in prophylactic treatment. Objectives: To examine the extravascular distribution of pharmacologically administered rFVIIa to obtain clues on how rFVIIa could work in prophylaxis. Methods: Recombinant mouse FVIIa tagged with AF488 fluorophore (AF488‐FVIIa) was administered into mice via the tail vein. At different time intervals following the administration, mice were exsanguinated and various tissues were collected. The tissue sections were processed for immunohistochemistry to evaluate distribution of rFVIIa. Results: rFVIIa, immediately following the administration, associated with the endothelium lining of large blood vessels. Within 1 h, rFVIIa bound to endothelial cells was transferred to the perivascular tissue surrounding the blood vessels and thereafter diffused throughout the tissue. In the liver, rFVIIa was localized to sinusoidal capillaries and accumulated in hepatocytes. In bone, rFVIIa was accumulated in the zone of calcified cartilage and some of it was retained there for a week. The common finding of the present study is that rFVIIa in extravascular spaces was mostly localized to regions that contain TF expressing cells. Conclusions: The present study demonstrates that pharmacologically administered rFVIIa readily associates with the vascular endothelium and subsequently enters into extravascular spaces where it is likely to bind to TF and is retained for extended time periods. This may explain the prolonged pharmacological effect of rFVIIa.     

Recombinant activated factor VII (rFVIIa) as salvage treatment for intractable hemorrhage

Background  

Recently, there has been an increased use of recombinant activated factor VII (rFVIIa) to promote hemostasis in various hemorrhagic conditions. The objective of this study was to determine the outcome of patients treated with rFVIIa who had intractable bleeding associated with cardiac surgery (CSP) or as a result of other causes (OBP).     

Recombinant factor VIIa enhances platelet adhesion and activation under flow conditions at normal and reduced platelet count.

BACKGROUND: Recombinant factor VIIa (rFVIIa), which was developed for treatment of inhibitor-complicated hemophilia, appears suitable as prohemostatic agent in other clinical disorders including patients with thrombocytopenia. It is generally accepted that rFVIIa functions by enhancement of thrombin generation at the site of injury. It is, however, unknown if and how this affects platelet adhesion and aggregation. OBJECTIVES: To determine the effect of rFVIIa-mediated thrombin generation on platelet adhesion and aggregation under flow conditions at normal and reduced platelet counts. METHODS: Washed platelets and red cells were combined to obtain plasma-free blood with different platelet counts. The reconstituted blood was perfused over a collagen- or fibrinogen-coated surface in the absence or presence of a thrombin generating system consisting of purified coagulation factors rFVIIa, factor (F)X and prothrombin. RESULTS: Addition of coagulation factors rFVIIa, FX and prothrombin to washed platelets and red cells enhanced platelet adhesion and aggregation to collagen and adhesion and spreading to fibrinogen at normal platelet count and at platelet numbers as low as 10 000 microL(-1). rFVIIa-mediated thrombin generation enhanced the activation state of platelets as measured by intracellular calcium fluxes, and enhanced the exposure of procoagulant phospholipids as measured by annexin A5 binding. CONCLUSIONS: Taken together, increased platelet adhesion and aggregation by rFVIIa-mediated thrombin formation may explain the therapeutic effects of rFVIIa in thrombocytopenic conditions and in patients with a normal platelet count by (i) enhancement of primary hemostasis and (ii) enhancement of procoagulant surface leading to elevated fibrin formation.     

Recombinant factor VIIa reverses the inhibitory effect of aspirin or aspirin plus clopidogrel on in vitro thrombin generation

BACKGROUND: Antiplatelet drugs constitute the therapy of choice for acute coronary syndromes, but bleeding can be a side-effect requiring treatment. Restoration of normal platelet activity is also mandatory before urgent surgery. This study investigated: (a) whether a regimen of aspirin or clopidogrel plus aspirin significantly inhibited platelet thrombin generation (TG); and (b) the reversal of this inhibition by recombinant activated factor VII (rFVIIa). METHODS AND RESULTS: TG was evaluated by the lag time, time to peak, peak of TG, and area under the curve after 35 min of assay (AUC(0 --> 35 min)). These measures were examined by the calibrated automated thrombography method in 22 healthy volunteers, 22 volunteers after a 100 mg day(-1) aspirin intake (200 mg first day) for 5-7 days, and 22 healthy volunteers after aspirin 100 mg day(-1) (200 mg first day) plus clopidogrel 75 mg day(-1) (300 mg first day) for 4-7 days. The TG parameters were measured under basal conditions and after platelet stimulation by sodium arachidonate (AA), adenosine 5'-diphosphate (ADP), collagen and rFVIIa in normal non-aspirinated as well as in vivo aspirinated platelet-rich plasma (PRP) or aspirin plus clopidogrel PRP. Lag time was shorter (P < 0.05), and peak of TG and AUC(0 --> 35 min) were significantly greater (P < 0.01 for both), in PRP activated with ADP, collagen, AA or FVIIa than in non-activated PRP from normal subjects. Both non-activated PRP and activated PRP prepared from platelets obtained from volunteers after aspirin intake showed significant prolongation of the time parameters but there was less effect on peak of TG and AUC(0 --> 35 min). For most parameters, aspirin plus clopidogrel administration showed to be more effective compared with the effect obtained by aspirin alone. When rFVIIa was added to ASA-PRP or ASA + Clop PRP, lag time (P < 0.001 for all) and time to peak (P < 0.001-0.017) were significantly shortened, indicating that rFVIIa reverses the inhibitory effect of these anti-aggregating agents. CONCLUSION: Platelets activated by AA, ADP, collagen or FVIIa triggered TG. This effect was inhibited by aspirin plus clopidogrel, suggesting an additional benefit of this drug combination for preventing thrombosis. rFVIIa reverses the inhibitory effect of aspirin or aspirin plus clopidogrel, and could be useful for bleeding complications or when acute surgery is needed during treatment with these antiplatelet drugs.     

von Willebrand factor (VWF)-dependent human platelet activation: porcine VWF utilizes different transmembrane signaling pathways than does thrombin to activate platelets, but both require protein phosphatase function

Summary. The interaction between von Willebrand factor (VWF) and glycoprotein (GP) Ib results in platelet agglutination and activation of many signaling intermediates. To determine if VWF‐dependent platelet activation requires the participation of pivotal transmembrane signaling pathways, we analyzed VWF‐dependent platelet activation profiles following inhibition of several transmembrane signaling intermediates. This was accomplished using porcine VWF, which has been shown to interact with human GPIb independently of shear stress or ristocetin. Platelet alpha (CD62) and lysozomal granule release (CD63), microparticle formation, and platelet agglutination/aggregation were evaluated. The ability of signaling inhibitors to prevent VWF‐dependent platelet activation was compared to their ability to inhibit thrombin‐dependent activation. The results demonstrate that VWF‐dependent platelet activation can occur independently of the activities of protein kinase C (PKC), wortmannin‐sensitive phosphatidylinositide 3‐kinase, and phospholipase C, as well as independently of elevations in the concentration of intracellular calcium. In sharp contrast, these transmembrane signaling intermediates are required for thrombin‐dependent platelet activation. In addition, thrombin‐dependent but not VWF‐dependent platelet activation was associated with elevations in the concentration of intracellular calcium under the conditions used. The family of signaling intermediates which appeared to be pivotal for both thrombin‐ and VWF‐dependent platelet activation were the protein tyrosine phosphatases and the serine/threonine phosphatases. It is concluded that thrombin‐dependent platelet activation relies on the activation of several transmembrane signaling pathways, whereas VWF‐dependent platelet activation is dependent upon the activity of protein phosphatases. Inhibition of these phosphatases in vivo may provide a novel therapeutic approach for treating VWF‐dependent platelet disorders such as thrombotic thrombocytopenic purpura or arterial thrombosis.     

Real-time quantitative PCR analysis of factor XI mRNA variants in human platelets.

Coagulation factor XI (FXI) plays an essential role in blood coagulation. A deficiency of FXI is an unusual hemorrhagic diathesis in that the bleeding tendency can be highly variable, ranging from severe deficiencies with no symptoms to mild and moderate deficiencies requiring multiple blood transfusions for hemorrhages. This variability in bleeding has been attributed to a number of factors including the presence of a novel form of FXI associated with platelets, which ameliorates the bleeding in some cases of FXI deficiency. However, the nature of this platelet FXI molecule is controversial. Hsu et al. (J Biol Chem 1998; 273: 13787-93) suggest that it is a product of normal FXI - but lacking exon V whilst Martincic et al. (Blood 1999; 94: 3397-404) were unable to detect this alternatively spliced variant using RT-PCR. In order to resolve this controversy, we have employed the highly sensitive technique of real-time quantitative RT-PCR using RNA isolated from FXI-deficient patients. Our results indicate that the platelets of both normal and FXI deficient individuals contain FXI mRNA that is identical to the mRNA found in liver. An exon V deleted splice variant was not detected. Thus the FXI message is not alternatively spliced in platelets and therefore would not be able to produce an unusual FXI protein.     

Safety and pharmacokinetics of subcutaneously administered recombinant activated factor VII (rFVIIa)

Summary. Background: Recombinant activated factor VIIa (rFVIIa) is used to treat bleeds in hemophilia patients with inhibitors. A subcutaneous formulation could potentially improve its half‐life and make it suitable for prophylactic treatment. Objectives: A study was conducted to determine the safety of subcutaneously administered rFVIIa in patients with hemophilia and the pharmacokinetic profile (including bioavailability). Patients/Methods: This was a multicenter, open‐label, cross‐over comparison of single doses of intravenous rFVIIa 90 μg kg^?1 and a new formulation of rFVIIa for subcutaneous injection at dose levels of 45, 90, 180, 270 and 360 μg kg^?1. Sixty subjects (12 per dose cohort) with hemophilia A or B were enrolled. Results: Subcutaneously administered rFVIIa showed lower mean peak plasma concentrations and prolonged FVII activity (C_max, 0.44–5.16 IU mL^?1 [across doses]; t_1/2, 12.4 h; t_max, 5.6 h) compared with intravenously administered rFVIIa (C_max, 51.7 IU mL^?1; t_1/2, 2.7 h; t_max, < 10 min). The absolute bioavailability of subcutaneous rFVIIa ranged from 21.1 to 30.1% across dose levels. Dose proportionality was observed within a 2‐fold dose increase but not across the full dose range. No thromboembolic events, drug‐related serious adverse events, severe injection‐site reactions or neutralizing antibodies were reported (primary endpoint). Mild and moderate injection‐site reactions were more frequent with subcutaneous than with intravenous injections. Conclusion: This phase I clinical trial did not identify safety concerns of prolonged exposure to rFVIIa administered subcutaneously in single doses to hemophilia patients.     

Human platelets express endothelial protein C receptor,which can be utilized to enhance localization of factor VIIa activity

Essentials

• Factor VIIa binds activated platelets to promote hemostasis in hemophilia patients with inhibitors.
• The interactions and sites responsible for platelet‐FVIIa binding are not fully understood.
• Endothelial cell protein C receptor (EPCR) is expressed on activated human platelets.
• EPCR binding enhances the efficacy of a FVIIa variant and could impact design of new therapeutics.

Summary

Background

High‐dose factor VIIa (FVIIa) is routinely used as an effective bypassing agent to treat hemophilia patients with inhibitory antibodies that compromise factor replacement. However, the mechanism by which FVIIa binds activated platelets to promote hemostasis is not fully understood. FVIIa‐DVQ is an analog of FVIIa with enhanced tissue factor (TF)‐independent activity and hemostatic efficacy relative to FVIIa. Our previous studies have shown that FVIIa‐DVQ exhibits greater platelet binding, thereby suggesting that features in addition to lipid composition contribute to platelet–FVIIa interactions.

Objectives

Endothelial cell protein C receptor (EPCR) also functions as a receptor for FVIIa on endothelial cells. We therefore hypothesized that an interaction with EPCR might play a role in platelet–FVIIa binding.

Methods/results

In the present study, we used flow cytometric analyses to show that platelet binding of both FVIIa and FVIIa‐DVQ is partially inhibited in the presence of excess protein C or an anti‐EPCR antibody. This decreased binding results in a corresponding decrease in the activity of both molecules in FXa and thrombin generation assays. Enhanced binding to EPCR was sufficient to account for the increased platelet binding of FVIIa‐DVQ compared with wild‐type FVIIa. As EPCR protein expression has not previously been shown in platelets, we confirmed the presence of EPCR in platelets using immunofluorescence, flow cytometry, immunoprecipitation, and mass spectrometry.

Conclusions

This work represents the first demonstration that human platelets express EPCR and suggests that modulation of EPCR binding could be utilized to enhance the hemostatic efficacy of rationally designed FVIIa analogs.

     

Preferential localization of recombinant factor VIIa to platelets activated with a combination of thrombin and a glycoprotein VI receptor agonist

BACKGROUND: Activation of platelets with a combination of collagen and thrombin generates a subpopulation of highly procoagulant 'coated' platelets characterized by high surface expression of fibrinogen and other procoagulant proteins. OBJECTIVES: To analyze the interaction of recombinant factor VIIa (rFVIIa) with coated platelets. METHODS AND RESULTS: rFVIIa localized to the coated platelets in flow cytometry experiments, while minimal rFVIIa was found on platelets activated with adenosine diphosphate, thrombin or via glycoprotein VI individually, and essentially no rFVIIa was found on non-stimulated platelets. Removal of the gamma-carboxyglutamic acid (Gla) domain of rFVIIa, and addition of EDTA, annexin V or excess prothrombin inhibited rFVIIa localization to the coated platelets, indicating that the interaction was mediated by the calcium-dependent conformation of the Gla domain and platelet exposure of negatively charged phospholipids. A reduced level of platelet fibrinogen exposure was observed at hemophilia A-like conditions in a model system of cell-based coagulation, indicating that coated platelet formation in hemophilia may be diminished. Addition of rFVIIa dose-dependently enhanced thrombin generation and partly restored platelet fibrinogen exposure. CONCLUSIONS: The data suggest that rFVIIa localized preferentially on platelets activated with dual agonists, thereby ensuring enhanced thrombin generation localized at the site of injury where both collagen and tissue factor are exposed, the latter ensuring the formation of thrombin necessary for coated platelet formation.     

Severe factor XI deficiency caused by a Gly555 to Glu mutation (factor XI-Glu555): a cross-reactive material positive variant defective in factor IX activation.

During normal hemostasis, the coagulation protease factor (F)XIa activates FIX. Hereditary deficiency of the FXIa precursor, FXI, is usually associated with reduced FXI protein in plasma, and circulating dysfunctional FXI variants are rare. We identified a patient with < 1% normal plasma FXI activity and normal levels of FXI antigen, who is homozygous for a FXI Gly555 to Glu substitution. Gly555 is two amino acids N-terminal to the protease active site serine residue, and is highly conserved among serine proteases. Recombinant FXI-Glu555 is activated normally by FXIIa and thrombin, and FXIa-Glu555 binds activated factor IX similarly to wild type FXIa (FXIa(WT)). When compared with FXIa(WT), FXIa-Glu555 activates factor IX at a greatly reduced rate ( approximately 400-fold), and is resistant to inhibition by antithrombin. Interestingly, FXIa(WT) and FXIa-Glu555 cleave the small tripeptide substrate S-2366 with comparable k(cat)s. Modeling indicates that the side chain of Glu555 significantly alters the electrostatic charge around the active site, and would sterically interfere with the interaction between the FXIa S2' site and the P2' residues on factor IX and antithrombin. FXI-Glu555 is the first reported example of a naturally occurring FXI variant with a significant defect in FIX activation.     

The role of recombinant factor VIIa (FVIIa) in fibrin structure in the absence of FVIII/FIX

Summary.  Patients with hemophilia have an impaired thrombin generation and therefore form loose fibrin hemostatic plugs that are easily dissolved by fibrinolysis. This prevents maintained hemostasis in these patients, resulting in a severe bleeding disorder. Recombinant (F)VIIa has been shown to enhance thrombin generation on already thrombin-activated platelets in the absence of FVIII and FIX. An efficacy rate of 80–90% has been found in hemophilia patients with inhibitors against FVIII or FIX both in association with major surgery and in the treatment of serious bleedings. In a model measuring fibrin clot permeability in a platelet-containing system described by Blombäck et al . (1994) this was demonstrated to be dependent on the concentration of FVIII and FIX. The addition of rFVIIa in concentrations of 1.9, 4.8 and 9.6 µg mL⁻¹ normalized fibrin clot permeability. The concentration of 1.9 µg mL⁻¹ of rFVIIa normalized clot permeability in this system and the higher concentrations of rFVIIa added only slightly to the effect. No further decrease in clot permeability was found when rFVIIa in a concentration of 1.9 µg mL⁻¹ was added to a sample with a normal concentration (100%) of FVIII or FIX. Higher concentrations of rFVIIa added to the plasma containing 100% of FVIII or FIX induced only a slight further decrease of fibrin permeability constant, arguing against any unwanted effect of extra rFVIIa on clot permeability in the case of a normal hemostasis. Furthermore, the fibrin network was studied with 3D microscopy and the loose network found in the absence of FVIII or FIX increased in density with increasing FVIII or FIX concentrations. The addition of rFVIIa to FVIII- or FIX-deficient systems altered the network structure, making the fibers thinner and more tightly packed.     

Glycogen synthase kinase‐3 negatively regulates tissue factor expression in monocytes interacting with activated platelets

Summary. Background: At the site of vascular injury, monocytes (MN) interacting with activated platelets (PLT) synthesize tissue factor (TF) and promote thrombus formation. Intracellular signals necessary for the expression of TF in MN, in the context of a developing thrombus, remain unknown. Objective: The study was designed to investigate the role of the glycogen synthase kinase 3 (GSK3, a serine‐threonine kinase) downstream insulin receptor pathway, in PLT‐induced TF expression in MN. Methods: To this purpose we used a well‐characterized in vitro model of human MN‐PLT interactions that allows detailed analysis of TF activity, TF protein and gene expression.Results: The results demonstrated that, in MN interacting with activated PLT: (i) TF activity, antigen and mRNA were low until 8–10 h and dramatically increased thereafter, up to 24 h; (ii) according to the kinetics of TF expression in MN, GSK3β undergoes phosphorylation on serine 9, a process associated with down‐regulation of enzyme activity; (iii) pharmacological blockade of GSK3 further increased TF expression and was accompanied by increased accumulation of NF‐kB, in the nucleus; (iv) blockade of phosphoinositide‐3 kinase (PI(3)K) by wortmannin inhibited PLT‐induced TF expression; and (v) according to the established role of the GSK3 downstream insulin receptor, insulin increased PLT‐induced TF expression in a PI(3)K‐dependent manner. Conclusion: GSK3 acts as a molecular brake on the signaling pathway, leading to TF expression in MN interacting with activated PLT. PI(3)K, through Akt‐dependent phosphorylation of GSK3, relieves this brake and allows TF gene expression. This study identifies a novel molecular link between thrombotic risk and metabolic disorders.     

von Willebrand factor stimulates thrombin-induced exposure of procoagulant phospholipids on the surface of fibrin-adherent platelets

Summary. Studies from our laboratory have demonstrated that von Willebrand factor (VWF) stimulates thrombin generation in platelet‐rich plasma. The precise role of VWF and fibrin in this reaction, however, remained to be clarified. In the present study we utilized thrombin‐free planar fibrin layers and washed platelets to examine the relationship between platelet–fibrin interaction and exposure of coagulation‐stimulating phosphatidylserine (PS) under conditions of low and high shear stress. Our study confirms that platelet adhesion to fibrin at a shear rate of 1000 s^?1 requires fibrin‐bound VWF. The cytosolic calcium concentration ([Ca²⁺]_i) of stationary platelets was not elevated and PS exposing platelets were virtually absent (2 ± 2%). However, thrombin activation resulted in a marked increase in the number of PS exposing platelets (up to 85 ± 14%) along with a transient elevation in [Ca²⁺]_i from 0.05 µmol L^?1 up to 1.1 ± 0.2 µmol L^?1. Platelet adhesion to fibrin at a shear rate of 50 s^?1 is mediated by thrombin but not by fibrin‐bound VWF. The [Ca²⁺]_i of these thrombin‐activated platelets was elevated (0.2 ± 0.1 µmol L^?1), but only a minority of the platelets (11 ± 8%) exposed PS. The essential role of VWF in this thrombin‐induced procoagulant response became apparent from low shear rate perfusion studies over fibrin that was incubated with VWF and botrocetin. After treatment with thrombin, the majority of the adherent platelets (57 ± 23%) exposed PS and had peak values of [Ca²⁺]_i of about 0.6 µmol L^?1. Taken together, these results demonstrate that thrombin‐induced exposure of PS and high calcium response on fibrin‐adherent platelets depends on shear‐ or botrocetin‐induced VWF–platelet interaction.     

Recombinant human factor VIIa and a factor VIIa-analogue reduces heparin and low molecular weight heparin (LMWH)-induced bleeding in rats

Summary.  Background:  Heparin and low molecular weight heparin (LMWH) are widely used for prevention and treatment of thromboemobolic events, but may occasionally cause uncontrollable bleeding. Heparin can readily be antagonized with protamine, but this is less effective against LMWH. Objectives:  To test the effects of rFVIIa or an analogue of rFVIIa, NN1731, on heparin- and LMWH-induced bleeding in rats. Methods:  Initially the doses of heparin and tinzaparin (a LMWH) were determined by dose-titration. Following pretreatment with heparin or tinzaparin in rats, tail-transection was performed, and the effect of rFVIIa and NN1731 on the bleeding was observed. Results:  rFVIIa (5, 10 and 20 mg kg⁻¹) reduced bleeding time and blood loss caused by heparin- and tinzaparin-induced bleeding, using doses of 200 IU kg⁻¹ ( n  = 8) and 500 IU kg⁻¹ ( n  = 9), respectively. Similarly, 10 mg kg⁻¹ NN1731 significantly reduced both heparin- and tinzaparin-induced bleeding to the normal level. Following severe anticoagulation with 1800 IU kg⁻¹ tinzaparin, 10 mg kg⁻¹ NN1731 reduced and normalized the bleeding, while the effect of 20 mg kg⁻¹ rFVIIa failed to reach statistical significance. These data are consistent with the hypothesis that rFVIIa/NN1731 are capable of generating sufficient thrombin locally on the surface of activated platelets to induce hemostasis in the presence of heparin/LMWH. Conclusions:  This study suggests that rFVIIa and NN1731 may have the potential to control bleedings caused by heparin or LMWH.     

Role of protease‐activated and ADP receptor subtypes in thrombin generation on human platelets

Summary. The activated platelet surface serves as an integral part of the prothrombinase complex upon activation by potent platelet agonists such as thrombin and collagen. We determined the receptor specificity through which thrombin was enhancing collagen‐induced thrombin generation. Whereas SFLLRN or AYPGKF alone produced minimal thrombin generation or phosphatidylserine exposure through protease activated receptor (PAR) stimulation, they caused a leftward shift in the collagen‐induced thrombin generation dose–response curve. Although SFLLRN or AYPGKF potentiated collagen‐induced thrombin generation, neither of them potentiated to the same extent as thrombin. However, SFLLRN and AYPGKF together potentiated collagen‐induced thrombin generation to the same extent as thrombin. We conclude that thrombin mediates its procoagulant activity through activation of both PAR1 and PAR4 receptors. Similarly, neither PAR1 nor PAR4 stimulation alone mimicked the annexin V‐binding response caused by thrombin stimulation. The combination of PAR activating peptides caused minimal increases in annexin V binding, but caused significant thrombin generation, suggesting that events other than phosphatidylserine exposure may play a role in platelet prothrombinase complex formation. We also investigated the ability of ADP to potentiate agonist‐induced thrombin generation. Whereas P2Y₁ antagonism did not affect collagen or thrombin‐induced thrombin generation, P2Y₁₂ antagonism did decrease both collagen‐ and thrombin‐induced thrombin generation, suggesting that ADP potentiates thrombin generation primarily through the P2Y₁₂ receptor. Collectively, these results suggest that stimulation of both the PAR1 and PAR4 receptors are necessary for thrombin‐induced procoagulant activity, and that the P2Y₁₂ receptor, but not the P2Y₁ receptor, is responsible for the potentiation of agonist‐induced platelet procoagulant activity.     

Recombinant nematode anticoagulant protein c2, an inhibitor of tissue factor/factor VIIa, attenuates coagulation and the interleukin-10 response in human endotoxemia

Summary.  The tissue factor–factor (F)VIIa complex (TF/FVIIa) is responsible for the initiation of blood coagulation under both physiological and pathological conditions. Recombinant nematode anticoagulant protein c2 (rNAPc2) is a potent inhibitor of TF/FVIIa, mechanistically distinct from tissue factor pathway inhibitor. The first aim of this study was to elucidate the pharmacokinetics and pharmacodynamics of a single intravenous (i.v.) dose of rNAPc2. The second aim was to study its effect on endotoxin-induced coagulation and inflammation. Initially, rNAPc2 was administered to healthy volunteers in three different doses. There were no safety concerns and the pharmacokinetics were consistent with previous studies, in which rNAPc2 was administered subcutaneously. rNAPc2 elicited a dose-dependent reduction of the endogenous thrombin potential and a selective prolongation of prothrombin time. Subsequently, the effect on endotoxin-induced coagulation and inflammation was studied. The administration of rNAPc2 completely blocked the endotoxin-induced thrombin generation, as measured by plasma prothrombin fragment F1+2. The endotoxin-induced effect on fibrinolytic parameters such as plasmin–antiplasmin complexes and plasminogen activator inhibitor type 1 was not affected by rNAPc2. The administration of rNAPc2 attenuated the endotoxin-induced rise in interleukin (IL)-10, without affecting the rise in other cytokines. In conclusion, rNAPc2 is a potent inhibitor of TF/FVIIa, which was well tolerated and could safely be used intravenously in this Phase I study in healthy male volunteers. A single i.v. dose rNAPc2 completely blocked endotoxin-induced thrombin generation without affecting the fibrinolytic response. In addition, rNAPc2 attenuated the endotoxin-induced rise in IL-10, without affecting the rises in other cytokines.     

Extracellular vesicles from human saliva promote hemostasis by delivering coagulant tissue factor to activated platelets

Essentials

• Human salivary extracellular vesicles (EVs) expose coagulant tissue factor (TF).
• Salivary EVs expose CD24, a ligand of P‐selectin.
• CD24 and coagulant TF co‐localize on salivary EVs.
• TF⁺/CD24⁺ salivary EVs bind to activated platelets and trigger coagulation.

Summary

Background

Extracellular vesicles (EVs) from human saliva expose coagulant tissue factor (TF). Whether such TF‐exposing EVs contribute to hemostasis, however, is unknown. Recently, in a mice model, tumor cell‐derived EVs were shown to deliver coagulant TF to activated platelets at a site of vascular injury via interaction between P‐selectin glycoprotein ligand‐1 (PSGL‐1) and P‐selectin.

Objectives

We hypothesized that salivary EVs may deliver coagulant TF to activated platelets via interaction with P‐selectin.

Methods

We investigated the presence of two ligands of P‐selectin on salivary EVs, PSGL‐1 and CD24.

Results

Salivary EVs expose CD24 but PSGL‐1 was not detected. Immune depletion of CD24‐exposing EVs completely abolished the TF‐dependent coagulant activity of cell‐free saliva, showing that coagulant TF and CD24 co‐localize on salivary EVs. In a whole blood perfusion model, salivary EVs accumulated at the surface of activated platelets and promoted fibrin generation, which was abolished by an inhibitory antibody against human CD24.

Conclusions

A subset of EVs in human saliva expose coagulant TF and CD24, a ligand of P‐selectin, suggesting that such EVs may facilitate hemostasis at a site of skin injury where the wound is licked in a reflex action.