Search

There are situations where monitoring direct oral anticoagulants (DOACs) would be useful, including bleedings and trauma. The thromboelastographic technique has proven useful in bleeding situations in trauma and heart surgery. The aim of this study was to examine the effect of DOACs on all currently commercially available conventional TEG®5000 assays as well as novel modified assay using Ecarin and human factor Xa (HFXa). Healthy male volunteers were given single dose of oral dabigatran 150 mg, rivaroxaban 20 mg, or apixaban 5 mg. Kaolin, RapidTEG, functional fibrinogen, PlateletMapping assay, and novel modified assays using Ecarin and HFXa were prepared. All TEG parameters were recorded. DOAC concentrations were correlated to the parameters with highest response to the DOAC effect. Sensitivity and negative predictive value of the parameter with highest response to DOAC concentration of 50 ng/mL was calculated. None of the conventional TEG assays demonstrated significant response to the effect on apixaban. Using Ecarin, reaction time R was strongly correlated with dabigatran concentrations. Using HFXa assay, R was strongly correlated with rivaroxaban and apixaban concentrations: r = 0.96, 0.84, and 0.86, respectively; p

Objective This article evaluates the efficacy of enoxaparin when targeting anti-factor Xa levels of 0.5 to 1 units per milliliter in the neonatal intensive care unit. Study Design This is a retrospective chart review of 45 neonates receiving enoxaparin for the treatment of venous thromboembolism. Enoxaparin dosing and corresponding anti-factor Xa levels were collected. Time to resolution of clot was confirmed by imaging and compared between clots in various locations. Results The median time to clot resolution was 76 days (interquartile range 40–91 days). Clot location, postnatal age, and sex at the clot onset were significantly associated with time to clot resolution in a multivariable Cox model (p-value: 0.03, 0.03, and Conclusion Based on our findings, 50% of all patients evaluated, regardless of thrombus location, achieved resolution within the first 76 days of therapy. Clots located in the extremities tended to resolve sooner, hence earlier reimaging should be considered.

SARS-CoV-2 infection (COVID-19) results in local and systemic activation of inflammation and coagulation. In this review article, we will discuss the potential role of coagulation factor Xa (FXa) in the pathophysiology of COVID-19. FXa, a serine protease, has been shown to play a role in the cleavage of SARS-CoV-1 spike protein (SP), with the inhibition of FXa resulting in the inhibition of viral infectivity. FX is known to be primarily produced in the liver, but it is also expressed by multiple cells types, including alveolar epithelium, cardiac myocytes, and macrophages. Considering that patients with preexisting conditions, including cardiopulmonary disease, are at an increased risk of severe COVID-19, we discuss the potential role of increased levels of FX in these patients, resulting in a potential increased propensity to have a higher infectious rate and viral load, increased activation of coagulation and inflammation, and development of fibrosis. With these observations in mind, we postulate as to the potential therapeutic role of FXa inhibitors as a prophylactic and therapeutic treatment for high-risk patients with COVID-19.

Without conjunctive administration of an anticoagulant, endothelial injury-induced thrombosis is resistant to thrombolysis and prone to re-thrombosis. We hypothesized that co-delivery of recombinant tissue plasminogen activator (rtPA) with annexin V–containing anticoagulants that specifically target the injured endothelium may passivate the thrombogenic elements of the vascular injury site and enhance rtPA-induced thrombolysis. In this study, the effects of conjunctive administration of Kinexins (Kunitz inhibitor–annexin V fusion proteins) with rtPA on thrombolysis were determined in vitro and in vivo. Thromboelastometry showed that both TAP-A (tick anticoagulant peptide–annexin V fusion protein; an inhibitor of factor Xa [FXa] and prothrombinase) and A-6L15 (annexin V-6L15 fusion protein; an inhibitor of tissue factor/FVIIa) exerted concentration-dependent (10–100 nM) effects on clot formation, with TAP-A being several folds more potent than A-6L15 in whole blood. Combination of TAP-A or A-6L15 with rtPA (1 μg/mL) led to decrease in lysis index, suggesting conjunctive enhancement of thrombolysis by combined use of rtPA with TAP-A or A-6L15. In a rat cremaster muscle preparation subjected to photochemical injury, conjunctive administration of rtPA and TAP-A significantly restored tissue perfusion to 56%, which is approximately two fold of that by rtPA or TAP-A alone. Near-infrared fluorescence images demonstrated local retention of a fluorescent A-6L15-S288 at the injury site, suggesting a targeting effect of the fusion protein. Pharmacokinetic analysis showed that 123I-labelled TAP-A and A-6L15 had initial distribution half-lives (T1/2α) of approximately 6 minutes and elimination half-lives (T1/2β) of approximately 2.3 hours. In conclusion, Kinexins were potentially useful adjunctive agents with rtPA thrombolytic therapy especially for thrombosis induced by endothelial injury.

Antithrombin (AT) activity tests are used for diagnosing hereditary AT deficiency, a main genetic determinant of thrombophilia. They are either based on inhibition of thrombin (FIIa) or activated factor X (FXa). FXa-based assays have been suggested to be preferable to FIIa-based assays due to their higher sensitivity for certain AT deficiency causing mutations. To assess the performance of these two methods in a real-world scenario, 745 consecutively collected samples from patients referred to our institute during a 3-month period for thrombophilia testing were analysed. In samples from patients not receiving direct-acting oral anticoagulants or heparins (n = 485), both methods showed good agreement (r = 0.874, Bland–Altman limits of agreement 6.57%, −15.76%). While similar results were obtained in patients receiving low-molecular-weight heparin (LMWH, n = 76, r = 0.891, 4.09%, −14.35%), the agreement was lower in patients receiving rivaroxaban (n = 86, r = 0.570, 5.97%, −49.43%) and apixaban (n = 72, r = 0.735, 3.77%, −42.45%). Direct FXa inhibitors but not LMWH increased FXa-based assay results in a dose-dependent manner, while the FIIa-based test was unaffected. Both assay types were equally successful in detecting hereditary AT deficiency in our study population, as samples from 9 out of 10 patients with AT deficiency causing mutations were detected by each method. These data suggest that FXa-based AT testing can be preferred over FIIa-based methods only in the absence of direct FXa inhibitors. In patients receiving direct FXa inhibitors, AT activity testing should be performed using FIIa-based assays.

SummaryEmicizumab, a humanised bispecific antibody recognising factors (F) IX/IXa and X/Xa, can accelerate FIXa-catalysed FX activation by bridging FIXa and FX in a manner similar to FVIIIa. However, details of the emicizumab–antigen interactions have not been reported so far. In this study, we first showed by surface plasmon resonance analysis that emicizumab bound FIX, FIXa, FX, and FXa with moderate affinities (K D = 1.58, 1.52, 1.85, and 0.978 μM, respectively). We next showed by immunoblotting analysis that emicizumab recognised the antigens’ epidermal growth factor (EGF)-like domains. We then performed K D-based simulation of equilibrium states in plasma for quantitatively predicting the ways that emicizumab would interact with the antigens. The simulation predicted that only a small part of plasma FIX, FX, and emicizumab would form antigen-bridging FIX–emicizumab–FX ternary complex, of which concentration would form a bell-shaped relationship with emicizumab concentration. The bell-shaped concentration dependency was reproduced by plasma thrombin generation assays, suggesting that the plasma concentration of the ternary complex would correlate with emicizumab’s cofactor activity. The simulation also predicted that at 10.0–100 μg/ml of emicizumab–levels shown in a previous study to be clinically effective–the majority of plasma FIX, FX, and emicizumab would exist as monomers. In conclusion, emicizumab binds FIX/FIXa and FX/FXa with micromolar affinities at their EGF-like domains. The K D-based simulation predicted that the antigen-bridging ternary complex formed in circulating plasma would correlate with emicizumab’s cofactor activity, and the majority of FIX and FX would be free and available for other coagulation reactions.Institution where the work was carried out: Research Division, Chugai Pharmaceutical Co., Ltd.Supplementary Material to this article is available online at www.thrombosis-online.com.

Direct oral anticoagulants (DOACs) are increasingly used for prevention and treatment of venous thromboembolism and for prevention of stroke in patients with nonvalvular atrial fibrillation. In phase III clinical trials that included more than 100,000 patients, the DOACs were at least as effective as vitamin K antagonists (VKAs) and were associated with less serious bleeding, particularly less intracranial bleeding. Real-world evidence supports these outcomes. Despite this, some physicians and patients are concerned about serious bleeding or emergencies unless specific reversal agents for the DOACs are available. However, in clinical trials performed without reversal agents, the outcome of major bleeds was similar or better in patients receiving DOACs than in those taking VKAs. Because of their short half-lives, supportive measures are sufficient to manage most bleeds in patients receiving DOACs. Anticoagulant reversal should only be considered with life-threatening bleeds, with bleeds that fail to respond to usual measures and in patients requiring urgent surgery. Idarucizumab is licensed for dabigatran reversal and andexanet alfa is likely to be soon licensed for reversal of rivaroxaban, apixaban, and edoxaban. To ensure responsible use of these agents, every hospital needs a bleeding management algorithm that identifies patients eligible for reversal and outlines appropriate dosing regimens.

SummaryInflammatory bowel disease (IBD)-associated thromboembolic event often lacks precise aetiology. The aim of this study was to investigate the contribution of phosphatidylserine (PS) exposure and neutrophil extracellular traps (NETs) towards the hypercoagulable state in IBD. We demonstrated that the levels of PS exposed MPs and the sources of MP-origin, platelets, erythrocytes, leukocytes and cultured endothelial cells (ECs) were higher in IBD groups than in healthy controls using flow cytometry and confocal microscopy. Wright-Giemsa and immunofluorescence staining demonstrated that the elevated NETs were released by activated IBD neutrophils or by control neutrophils treated with IBD sera obtained from patients with the active disease. MPs and MP-origin cells in IBD groups, especially in active stage, markedly shortened coagulation time and had increased levels of fibrin, thrombin and FXa production as assessed by coagulation function assays. Importantly, we found that on stimulated ECs, PS rich membranes provided binding sites for FXa and FVa, promoting fibrin formation while TNF blockage or IgG depletion attenuated this effect. Treatment of control neutrophils with TNF and isolated IgG from PR3-ANCA-positive active IBD patients also resulted in the release of NETs. Blockade of PS with lactadherin prolonged coagulation time, decreased fibrin formation to control levels, and inhibited the procoagulant enzymes production in the MPs and MP-origin cells. NET cleavage by DNase I partly decreased PCA in IBD or stimulated neutrophils. Our study reveals a previously unrecognised link between hypercoagulable state and PS exposure or NETs, and may further explain the epidemiological association of thrombosis within IBD patients.

It is generally accepted that the initial event in coagulation and intravascular thrombus formation is the exposure of the flowing blood to cell-surface protein, such as tissue factor (TF). Vascular injury and/or atherosclerotic plaque complication is responsible for this exposure, leading to clinical manifestations such as acute coronary syndromes. For many years, TF has been considered one of the major determinants of plaque-related thrombosis. However, the discoveries of different pools of TF that circulates in the blood as cell-associated TF, microparticles bound, and as soluble form have changed this dogma. Recent evidence suggests that an increased circulating TF activity may potentiate the initial thrombogenic stimulus related to vessel wall-associated TF, leading to the formation of larger and/or more stable thrombus and thus more severe clinical manifestations. Different pathological conditions, such as inflammatory status, enhance TF expression and activity. Conversely, TF upregulation may facilitate inflammation by formation of proinflammatory fibrin fragments and coagulation proteases generation, including FVIIa, FXa, and thrombin. Furthermore, the biology of TF has become more complex by the demonstration that, apart from its known effects on blood coagulation, it is involved in intracellular signaling, proliferation, angiogenesis, and tumor metastasis.

SummarySeveral new anticoagulants have entered the clinical arena or are under clinical development. These drugs include indirect (fondaparinux) and direct oral factor Xa inhibitors (rivaroxaban, apixaban, edoxaban, betrixaban), and the direct thrombin inhibitor dabigatran. Especially the oral direct FXa and FIIa inhibitors overcome many of the shortcomings of heparins and vitamin K antagonists (VKAs). They are administered orally at a fixed dose; regular monitoring is not necessary; interaction with other drugs or nutrition occur less than with VKAs and they are at least as effective as VKAs for most indications tested. They are associated with about 50 % less intracranial bleeding than VKAs. Nevertheless, they are still associated with bleeding complications. Bleeding can occur spontaneously or as a result of trauma or urgent surgery. In such situations rapid reversal of the anticoagulant effect is highly desirable. For unfractionated heparin protamine, and for VKAs prothrombin complex concentrates are available as specific antidotes. Under clinical development are: for the direct and indirect FXa inhibitors a modified recombinant FXa (andexanet alpha), which lacks enzymatic activity; and for dabigatran a Fab fragment of a monoclonal antibody (idarucizumab). In addition a small molecule (aripazine) has entered phase I clinical trials, which seems to inhibit nearly all anticoagulants but VKAs and argatroban. This review summarises the current options and strategies in development to antagonise anticoagulants with a focus on the status of the development of antidotes for the oral direct FXa and FIIa inhibitors.

SummaryFVIIIa is labile due to the dissociation of A2 subunit. Previously, we introduced hydrophobic mutations at select A1/A2/A3 subunit interfaces yielding more stable FVIII(a) variants. Separately we showed that altering the sequence flanking the primary FXa cleavage site in FVIIIa (Arg336) yielded reduced rates of proteolytic inactivation of FVIIIa. In this study we prepared the FXa-cleavage resistant mutant (336(P4-P3’)562) combined with mutations of Ala108Ile, Asp519Val/ Glu665Val or Ala108Ile/Asp519Val/Glu665Val and examined the effects of these combinations relative to FVIII thermal stability, rates of FVIIIa decay and proteolytic inactivation of FVIIIa by FXa. Thermal decay rates for 336(P4-P3’)562/Ala108Ile, 336(P4-P3’)562/Asp519Val/ Glu665Val, and 336(P4-P3’)562/Ala108Ile/Asp519Val/Glu665Val variants were reduced by ∼2– to 5-fold as compared with wild-type (WT) primarily reflecting the effects of the A domain interface mutations. FVIIIa decay rates for 336(P4-P3’)562/Asp519Val/Glu665Val and 336(P4-P3’)562/Ala108Ile/Asp519Val/Glu665Val variants were reduced by ∼25 fold, indicating greater stability than the control Asp519Val/Glu665Val variant (∼14-fold). Interestingly, 336(P4-P3’)562/Asp519Val/Glu665Val and 336(P4-P3’)562/Ala108Ile/ Asp519Val/Glu665Val variants showed reduced FXa-inactivation rates compared with the 336(P4-P3’)562 control (∼4-fold), suggesting A2 subunit destabilisation is a component of proteolytic inactivation. Thrombin generation assays using the combination variants were similar to the Asp519Val/Glu665Val control. These results indicate that combining multiple gain-of-function FVIII mutations yields FVIII variants with increased stability relative to a single type of mutation.

SummaryThrombin is essential for blood coagulation but functions also as a mediator of cellular signalling. Gene expression microarray experiments in human monocytes revealed thrombin-induced upregulation of a limited subset of genes, which are almost exclusively involved in inflammation and wound healing. Among these, the expression of F3 gene encoding for tissue factor (TF) was enhanced indicating that this physiological initiator of coagulation cascade may create a feed-forward loop to enhance blood coagulation. Activation of protease-activated receptor type 1 (PAR1) was shown to play a main role in promoting TF expression. Moreover, thrombin induced phosphorylation of ERK1/2, an event that is required for expression of thrombin-regulated genes. Thrombin also increased the expression of TF at the protein level in monocytes as evidenced by Western blot and immunostaining. Furthermore, FXa generation induced by thrombin-stimulated monocytes was abolished by a TF blocking antibody and therefore it is entirely attributable to the expression of tissue factor. This cellular activity of thrombin provides a new molecular link between coagulation, inflammation and wound healing.

SummaryWe previously reported that the incorporation of tissue factor (TF) into cell-derived microvesicles (MVs) is regulated by the phosphorylation of the cytoplasmic domain of TF. Since the cytoskeletal protein filamin-A is known to bind to the cytoplasmic domain of TF in a phosphorylation-dependent manner, the involvement of filamin-A in the incorporation of TF into MVs was examined. Endothelial cells were transfected to express TF, whereas MDA-MB-231 cells were used to examine endogenously expressed TF. MV release was induced by activating protease-activated receptor-2 (PAR2). Partial suppression of filamin-A expression using two different filamin-A siRNA sequences resulted in significant reductions in the incorporation of TF antigen into MVs as determined by TF-ELISA and western blot analysis, and was reflected in reduced thrombin-generation and FXa-generation capacities of these MVs. Deletion of the cytoplasmic domain of TF also resulted in reduced incorporation of TF into MVs, whereas the suppression of filamin-A expression had no additional effect on the incorporation of truncated TF into MVs. Partial suppression of filamin-A expression had no effect on the number and size distribution of the released MVs. However, >90% suppression of filamin-A expression resulted in increased MV release, possibly as a result of increased instability of the plasma membrane and underlying cytoskeleton. In conclusion, the presence of filamin-A appears to be essential for the incorporation of TF into MVs following PAR2 activation, but is not required for the process of MV formation and release following PAR2 activation.

Chromogenic assays for measurement of rivaroxaban from EDTA anticoagulated plasma samples

Because their anticoagulant effect is dose predictable, steady, and little influenced by diet and drugs, laboratory monitoring was deemed unnecessary in trials on venous and arterial thromboprophylaxis with the direct oral anticoagulant drugs (DOACs) rivaroxaban, apixaban, edoxaban, or dabigatran. However, there are special clinical settings in which measurement of their anticoagulant effect may be clinically desired. Because of lack of standardization between reagents employed and their global nature, the activated partial thromboplastin time (APTT) and the prothrombin time (PT) are not generally suitable for accurately assessing the anticoagulant effect of DOACs. The modified (diluted) PT reliably quantifies the anticoagulant effect of rivaroxaban or of apixaban. However, a higher intraindividual variability from one PT reagent to another is found when the data are compared with those obtained with standard PT. The HEMOCLOT (HYPHEN BioMed, France) assay is a modified (diluted) thrombin time (TT) provided with dabigatran calibrators. However, it is performed only in few specialized centers. Anti-factor Xa (anti-FXa) chromogenic assays employ routine automated coagulometers or manual spectrometers, and kits for anti-FIIa assays that measure dabigatran levels and kits for anti-Xa with rivaroxaban calibrators are commercially available. However, no correlation has been identified between any of these tests and clinical outcomes in patients taking any of the DOACs. Thus, currently, there are evidence gaps regarding the role of laboratory testing for surveillance and management of adverse events associated with DOACs. In view of this, in addition to standardization, validation of such assays is mandatory before they can be used to make clinical decisions.

SummaryAnticoagulant plasma protein S (PS) is essential for maintaining haemostatic balance. About 2.5% of PS is stored in platelets and released upon platelet stimulation. So far, little is known about the functionality and importance of platelet (plt)PS. A platelet-associated protease cleaves plasma-derived (pd)PS and pltPS in the “thrombin-sensitive region”, abolishing activated protein C (APC) cofactor activity. However, we showed that cleaved PS retains APC-independent anticoagulant activities (“PS-direct”). To investigate whether pltPS or pdPS exert PS-direct on platelets or platelet-shed microparticles, thrombin and factor (F)Xa generation on unstimulated or stimulated washed platelets and microparticles were measured. Western blotting revealed that pltPS and pdPS bound to washed, stimulated platelets and microparticles, and that pltPS had slower electrophoretic mobility than pdPS. Platelet stimulation in the presence of inhibitory anti-PS antibodies resulted in 2.6 ± 1.6-fold (p

SummaryGlobal coagulation assays display variable effects at different concentrations of rivaroxaban. The aim of this study is to quantify the ex vivo effects of low-dose rivaroxaban on thrombophilia screening assays and coagulation factor activities based on the administration time, and to show how to mask possible interferences. Plasma samples from 40 patients receiving rivaroxaban 10 mg daily were investigated to measure activities of clotting factor II, V, VII, VIII, IX, XI, XII and XIII; protein C- and protein S-levels; lupus anticoagulants; anticardiolipin IgG and IgM; D-dimer, heparin-platelet factor 4 (HPF4) antibodies and screening tests for von Willebrand disease (VWD). Two hours after rivaroxaban administration, the activities of clotting factors were significantly decreased to different extents, except for factor XIII. Dilution of plasma samples resulted in neutralisation of these interferences. The chromogenic protein C activity assay was not affected by rivaroxaban. Depending on the timing of tablet intake in relation to blood sampling protein S activity was measured falsely high when a clotting assay was used. False-positive results for lupus anticoagulants were observed depending on the assay system used and the administration time of rivaroxaban. ELISA-based assays such as anticardiolipin IgG and IgM, D-dimer, HPF4-antibodies and the turbidimetric assays for VWD were not affected by rivaroxaban. Specific haemostasis clotting tests should be performed directly prior to rivaroxaban intake. Assay optimisation in the presence of rivaroxaban can be achieved by plasma dilution. Immunologic assays are not influenced by rivaroxaban, while chromogenic assays can be used, when they do not depend on factor Xa.

SummaryFactor (F)Xa within the prothrombinase complex is protected from inhibition by unfractionated heparin (UFH), enoxaparin and fondaparinux. We have developed a covalent antithrombin-heparin complex (ATH) with enhanced anticoagulant activity. We have also demonstrated that ATH is superior at inhibiting coagulation factors when assembled on artificial surfaces. The objective of the present study is to determine the ability of ATH vs AT+UFH to inhibit FXa within the prothrombinase complex when the enzyme complex is assembled on the more native platelet system. Discontinuous inhibition assays were performed to determine final k 2-values for inhibition of FXa, FXa within the platelet-prothrombinase, or FXa within prothrombinase devoid of various components. Thrombin generation and plasma clotting was also assayed in the presence of resting/activated platelets ± inhibitors. Protection of FXa was not observed for ATH, whereas a moderate 60% protection was observed for AT+UFH. ATH inhibited platelet-prothrombinase ∼4-fold faster than AT+UFH. Relative to intact prothrombinase, rates for FXa inhibition by AT+UFH in prothrombinase complexes devoid of either prothrombin (II)/activated platelets/FVa were higher. However, inhibition by AT+UFH of prothrombinase devoid of FII yielded slightly lower rates compared to free FXa inhibition. Thrombin generation and plasma clotting was enhanced with activated platelets, while inhibition was better by ATH compared to AT+UFH, thus suggesting an overall enhanced anticoagulant activity of ATH against platelet-bound prothrombinase complexes.

SummaryAlthough factor (F) VIIIa is inactivated by activated protein C (APC) through cleavages in the FVIII heavy chain-derived A1 (Arg336) and A2 subunits (Arg562), the FVIII light chain (LC) contributes to catalysis by binding the enzyme. ELISA-based binding assays showed that FVIII and FVIII LC bound to immobilised active site-modified APC (DEGRAPC) (apparent K d ~270 nM and 1.0 μM, respectively). Fluid-phase binding studies using fluorescence indicated an estimated K d of ~590 nM for acrylodan-labelled LC binding to DEGR-APC. Furthermore, FVIII LC effectively competed with FVIIIa in blocking APC-catalysed cleavage at Arg336 (K i = 709 nM). A binding site previously identified near the C-terminal end of the A3 domain (residues 2007–2016) of FVIII LC was subjected to Ala-scanning mutagenesis. FXa generation assays and western and dot blotting were employed to assess the contribution of these residues to FVIIIa interactions with APC. Virtually all variants tested showed reductions in the rates of APC-catalysed inactivation of the cofactor and cleavage at the primary inactivation site (Arg336), with maximal reductions in inactivation rates (~3-fold relative to WT) and cleavage rates (~3 to ~9-fold relative to WT) observed for the Met2010Ala, Ser2011Ala, and Leu2013Ala variants. Titration of FVIIIa substrate concentration monitoring cleavage by a dot blot assay indicated that these variants also showed ~3-fold increases relative to WT while a double mutant (Met2010Ala/Ser2011Ala) showed a >4-fold increase in K m. These results show a contribution of a number of residues within the 2007–2016 sequence, and in particular residues Met2010, Ser2011, and Leu2013 to an APC-interactive site.

SummaryThe clinical severity in some patients with haemophilia A appears to be unrelated to the levels of factor (F)VIII activity (FVIII:C), but mechanisms are poorly understood. We have investigated a patient with a FVIII gene mutation at Arg1781 to His (R1781H) presenting with a mild phenotype despite FVIII:C of 0.9 IU/dl. Rotational thromboelastometry using the patient’s whole blood demonstrated that the clot time and clot firmness were comparable to those usually observed at FVIII:C 5–10 IU/dl. Thrombin and FXa assays using plasma samples also showed that the peak levels of thrombin formation and the initial rate of FXa generation were comparable to those observed at FVIII:C 5–10 IU/dl. The results suggested a significantly greater haemostatic potential in this individual than in those with severe phenotype. The addition of incremental amounts of FX to control plasma with FVIII:C 0.9 IU/dl in clot waveform analyses suggested that the enhanced functional tenase assembly might have been related to changes in association between FVIII and FX. To further investigate this mechanism, we prepared a stably expressed, recombinant, B-domainless FVIII R1781H mutant. Thrombin generation assays using mixtures of control plasma and FVIII revealed that the coagulation function observed with the R1781H mutant (0.9 IU/dl) was comparable to that seen with wild-type FVIII:C at ∼5 IU/dl. In addition, the R1781H mutant demonstrated an ∼1.9-fold decrease in K m for FX compared to wild type. These results indicated that relatively enhanced binding affinity of FVIII R1781H for FX appeared to moderate the severity of the haemophilia A phenotype.Note: An account of this work was presented, in part, at the 23rd Congress of the International Society of Thrombosis and Haemostasis, July 27, 2011, Kyoto, Japan.

SummaryThe variant serpin α1-PI M358R inhibits thrombin and other proteases such as activated protein C (APC) and factor XIa. We previously described recombinant proteins HAPI M358R (α1-PI M358R containing an N-terminal extension corresponding to residues 1–75 of heparin cofactor II) and HAPI RCL5 (HAPI M358R with F352-I356 and I360 substituted for the corresponding residues of antithrombin), with enhanced selectivity for thrombin over APC inhibition. We tested the hypotheses that these recombinant proteins would limit thrombosis in three mouse models, and that the HAPI chimeric proteins would be more effective than α1-PI M358R. Recombinant serpins were purified from Escherichia coli by nickel chelate and ion exchange affinity chromatography, and administered to mice intravenously. HAPI RCL5 reduced incorporation of radiolabelled fibrin(ogen) into thrombi in the ferric chloride-injured vena cava in a dose-dependent manner; HAPI M358R was less effective and α1-PI M358R was without effect. In a model of murine endotoxaemia, HAPI RCL5 was more effective than α1-PI M358R in reducing radiolabelled fibrin(ogen) deposition in heart and kidneys; immunohis-tochemistry of tissue sections showed lesser staining with anti-fibrin(ogen) antibodies with both treatments. In the ferric chloride-injured murine carotid artery, administration of both recombinant serpins was equally effective in lengthening the vessel’s time to occlusion. Our results show that the antithrombotic efficacy of the recombinant serpins correlates with their potency as thrombin inhibitors, since HAPI RCL5 inhibits thrombin, but not factors Xa, XIa, XIIa, or neutrophil elastase, more rapidly than α1-PI M358R.

SummaryRecent studies showed that an imbalance of prothrombotic and antithrombotic factors and impaired thrombolytic activity contribute to the thrombophilia of the nephrotic syndrome (NS). However, it is not clear whether blood cell injury and/or activation is involved in hypercoagulability in NS patients. Our objectives were to study the increase in microparticle (MP) release and phosphatidylserine (PS) exposure on the outer membrane of MP-origin cells in NS patients, and to evaluate their procoagulant activity (PCA). The subjects were patients with membranous nephropathy (MN), minimal change nephrotic syndrome (MCNS) and healthy controls. Analyses of MPs and PS exposure were performed using a flow cytometer. PCA was determined by clotting time and purified coagulation complex assays. We found that lactadherin+ MPs, which derived from red blood cells (RBC), platelet and endothelial cell, increased in NS patients. Moreover, PS exposure on RBCs and platelets in each NS group, especially in MN, are higher than that in controls. MP shedding and PS exposure of RBCs/platelets were highly procoagulant in NS patients. However, blockade of PS with lactadherin inhibited over 90% of PCA while an anti-tissue factor antibody had no significant inhibition effect. Our results demonstrate that the thrombophilic susceptibility of NS may be partly ascribed to MP release and PS exposure of RBCs, platelets and endothelial cells. Lactadherin is a sensitive probe for PS that has high anticoagulant activity.

SummaryPatent protection for enoxaparin has expired. Generic preparations are developed and approved for clinical use in different countries. However, there is still skepticism about the possibility of making an exact copy of the original drug due to the complex processes involved in generating low-molecular-weight heparins. We have undertaken a careful analysis of generic versions of enoxaparin available for clinical use in Brazil. Thirty-three batches of active ingredient and 70 of the final pharmaceutical product were obtained from six different suppliers. They were analysed for their chemical composition, molecular size distribution, in vitro anticoagulant activity and pharmacological effects on animal models of experimental thrombosis and bleeding. Clearly, the generic versions of enoxaparin available for clinical use in Brazil are similar to the original drug. Only three out of 33 batches of active ingredient from one supplier showed differences in molecular size distribution, resulting from a low percentage of tetrasaccharide or the presence of a minor component eluted as monosaccharide. Three out of 70 batches of the final pharmaceutical products contained lower amounts of the active ingredient than that declared by the suppliers. Our results suggest that the generic versions of enoxaparin are a viable therapeutic option, but their use requires strict regulations to ensure accurate standards.

SummaryRivaroxaban is a direct factor Xa inhibitor, which can be monitored by anti-factor Xa chromogenic assays. This ex vivo study evaluated different assays for accurate determination of rivaroxaban levels. Eighty plasma samples from patients receiving rivaroxaban (Xarelto®) 10 mg once daily and 20 plasma samples from healthy volunteers were investigated using one anti-factor Xa assay with the addition of exogenous antithrombin and two assays without the addition of antithrombin. Two different lyophilised rivaroxaban calibration sets were used for each assay (low concentration set: 0, 14.5, 59.6 and 97.1 ng/ml; high concentration set: 0, 48.3, 101.3, 194.2 and 433.3 ng/ml). Using a blinded study design, the rivaroxaban concentrations determined by the assays were compared with concentrations measured by HPLC-MS/MS. All assays showed a linear relationship between the rivaroxaban concentrations measured by HPLC-MS/MS and the optical density of the anti-FXa assays. However, the assay with the addition of exogenous anti-thrombin detected falsely high concentrations of rivaroxaban even in plasma samples from controls who had not taken rivaroxaban (intercept values using the high calibrator set and the low calibrator set: +26.49 ng/ml and +13.71 ng/ml, respectively). Plasma samples, initially determined by the high calibrator setting and containing rivaroxaban concentrations

SummaryPharmaceutical grade heparins from porcine intestine and bovine lung consist mainly of repeating tri-sulfated units, of the disaccharide →4-α-IdoA2S-1 →4-α-GlcNS6S-1 →. Heparin preparations from bovine intestine, in contrast, are more heterogeneous. Nuclear magnetic resonance (NMR) and disaccharide analysis after heparinase digestions show that heparin from bovine intestine contains α-glucosamine with significant substitutive variations: 64% are 6-O-sulfated and N-sulfated, as in porcine intestinal heparin while 36% are 6-desulfated. Desulfated α-iduronic acid units are contained in slightly lower proportions in bovine than in porcine heparin. NMR data also indicate N-, 3-and 6-trisulfated a-glucosamine (lower proportions) and α-GlcNS-1→4-α-GlcA and α-IdoA2S-1→4-α-GlcNAc (higher amounts) in bovine than in porcine heparin. Porcine and bovine heparins can be fractionated by anion exchange chromatography into three fractions containing different substitutions on the a-glucosamine units. Each individual fraction shows close disaccharide composition and anticoagulant activity, regardless of their origin (bovine or porcine intestine). However, these two heparins differ markedly in the proportions of the three fractions. Interestingly, fractions with the typical he-parin disaccharides of porcine intestine are present in bovine intestinal heparin. These fractions contain high in vitro anticoagulant activity, reduced antithrombotic effect and high bleeding tendency. These observations indicate that the prediction of haemostatic effects of heparin preparations cannot rely exclusively on structural analysis and anticoagulant assays in vitro. Minor structural components may account for variations on in vivo effects. In conclusion, we suggest that pharmaceutical grade bovine intestinal heparin, even after purification procedures, is not an equivalent drug to porcine intestinal heparin.

SummaryProtein Z-dependent protease inhibitor (ZPI) is a plasma inhibitor of factor (F)Xa and FXIa. In an earlier study, five mutations were identified within the ZPI gene of venous thrombosis patients and healthy controls. Two of these were nonsense mutations and three were missense mutations in important regions of the protein. Here we report that two of these latter three mutations, F145L and Q384R, impair the inhibitory function of ZPI in vitro. Recombinant wild-type and mutant proteins were prepared; stability in response to thermal challenge was similar. Inhibition of FXa in the presence of the cofactor protein Z was reduced 68-fold by the Q384R mutant; inhibition of FXIa by the F145L mutant was reduced two- to three-fold compared to the wild-type ZPI. An analysis of all five ZPI mutations was undertaken in a cohort of venous thrombosis patients (n=550) compared to healthy controls (n=600). Overall, there was a modest increase in incidence of these mutations in the thrombosis group (odds ratio 2.0, 1.05–3.7, p=0.044). However, in contrast to W324X (nonsense mutation), the Q384R missense mutation and R88X nonsense mutation were evenly distributed in patients and controls; F145L was rare. The final mutation (S143Y) was also rare and did not significantly alter ZPI function in laboratory studies. The F145L and particularly the Q384R mutation impaired the function of the coagulation inhibitor ZPI; however, there was no convincing association between these mutations and venous thrombosis risk. The functional role for ZPI in vivo has yet to be clarified.

SummaryEdoxaban is a novel, orally available, highly specific direct inhibitor of factor Xa and is currently being developed for the treatment and prevention of venous thromboembolism and prevention of stroke and systemic embolism in patients with non-valvular atrial fibrillation (NVAF). The objectives of the present analyses were to characterise edoxaban population pharmacokinetics (PPK) and identify potential intrinsic and extrinsic factors affecting variability in edoxaban exposure, determine if there are relationships between edoxaban pharmacokinetics or biomarkers and the risk of bleeding in patients with NVAF using an exposure-response model, and to use the PPK and exposure-response model to support dose selection for a phase III trial of edoxaban in patients with NVAF. PPK analysis of data from 1,281 edoxaban-dosed subjects with intrinsic factors such as renal impairment or NVAF and extrinsic factors such as concomitant medications revealed significant effects of renal impairment and concomitant strong P-glycoprotein (P-gp) inhibitors on the pharmacokinetics of edoxaban. Exposure-response analysis found that in patients with NVAF, the incidence of bleeding events increased significantly with increasing edoxaban exposure, with steady-state minimum concentration (Cmin,ss) showing the strongest association. Clinical trial simulations of bleeding incidence were used to select 30 mg and 60 mg once-daily edoxaban with 50% dose reductions for patients with moderate renal impairment or receiving concomitant strong P-gp inhibitors as the treatment regimens in the ENGAGE AF-TIMI 48 (NCT00781391) trial.The results of this study were previously presented at the 2009 International Society on Thrombosis and Haemostasis, July 2009, Boston, Massachusetts, USA.

SummaryEdoxaban is a new oral direct factor Xa inhibitor. The purpose of this study was to evaluate the efficacy and safety of different doses of edoxaban for the prevention of venous thromboembolism (VTE) in patients undergoing elective total hip replacement. A total of 903 patients were randomised to oral edoxaban 15, 30, 60 or 90 mg once daily or subcutaneous dalteparin once daily (initial dose 2,500 IU, subsequent doses 5,000 IU). Both drugs were begun 6–8 hours postoperatively and continued for 7–10 days, when bilateral venography was performed. The primary efficacy endpoint was the incidence of total VTE, which included proximal and/or distal deep-vein thrombosis (DVT) by venography or symptomatic, objectively confirmed DVT or pulmonary embolism during the treatment period. The primary safety outcome was the incidence of the composite of major and clinically relevant non-major bleeding. All venograms and bleeding events were reviewed by a central independent adjudication committee blinded as to treatment allocation. Of the 903 patients randomised, 776 were evaluable for the primary efficacy analysis. The incidences of VTE were 28.2%, 21.2%, 15.2%, and 10.6% in patients receiving edoxaban 15, 30, 60 and 90 mg, respectively, compared with 43.8% in the dalteparin group (p

SummaryApixaban is an oral, direct and highly selective factor Xa (FXa) inhibitor in late-stage clinical development. This study evaluated the in vitro effect of apixaban on human platelet aggregation induced by thrombin derived via the extrinsic pathway. Direct inhibitors of FXa (rivaroxaban), FVIIa (BMS-593214), thrombin (dabigatran, argatroban) and FXIa (BMS-262084) were included for comparison. Citrated human plateletsrich plasma (PRP) was treated with 50 μg/ml corn trypsin inhibitor (to block the contact factor pathway) and 3 mM H-Gly-Pro-Arg- Pro-OH-AcOH (to prevent fibrin polymerisation). Human tissue factor (TF) (Innovin®; dilution 1:1,000 to 1:1,500) plus 7.5 mM CaCl2 was added to PRP pre-incubated with vehicle or increasing concentrations of inhibitors. The TF-induced platelet aggregation was measured by optical aggregometry. TF produced 85 ± 3% aggregation of human platelets in the vehicle-treated group (n=10). Apixaban and other factor inhibitors, except the FXIa inhibitor, inhibited TF-induced platelet aggregation with IC50 (nM) values as follows: 4 ± 1 (apixaban), 8 ± 2 (rivaroxaban), 13 ± 1 (BMS-593214), 46 ± 1 (dabigatran) and 79 ± 1 (argatroban). BMS-262084 (IC50 = 2.8 nM vs. human FXIa) had no effect on TF-induced platelet aggregation at 10 μM. These inhibitors at 10 μM had no effect on platelet aggregation induced by ADP and collagen, as expected from their mechanism of action. This study demonstrates that inhibition of thrombin generation by blocking upstream proteases (FVIIa and FXa) in the blood coagulation cascade is as effective as direct thrombin inhibition in preventing TF-induced platelet aggregation. Under these experimental conditions, a FXIa inhibitor did not prevent TF-induced platelet aggregation.

SummaryPrevious studies investigating continuous unfractionated heparin (UFH) therapy report age-related differences in UFH response in children, as measured by APTT and anti-Xa assay. This study determined the age-related response following administration of a single UFH bolus of 75–100 IU/kg in children. Venous blood samples were collected from children (n=56) at 15, 30, 45 and 120 minutes post-UFH. Anti-Xa, anti-IIa, APTT, TCT and protamine titration were performed on all samples. Age-dependent differences in the effect and concentration of UFH were identified for the anti-Xa, anti-IIa and protamine titration as-says, respectively. In addition, a trend suggesting a proportional increase in anti-Xa and anti-IIa-mediated UFH effect with age was evident. Logistic regression demonstrated an increase in protamine titration of 0.6 IU/ml for every year of age in samples collected 15 minutes post-UFH. UFH-mediated anti-IIa activity was reduced compared to anti-Xa activity across childhood, with a two-fold increase in anti-Xa to anti-IIa ratio in infants less than one year of age compared to teenagers in the setting of high UFH concentrations. This study demonstrates that the previously reported age-dependent response to UFH occurs in the context of an age-dependent serum concentration of UFH. The trend toward increased UFH serum concentration and anticoagulant activity with age may be related to short-term differences in UFH binding to coagulant and competitive plasma proteins in vivo.

SummaryApixaban is an oral, direct factor Xa inhibitor under development for secondary prevention in acute coronary syndrome (ACS). Apixaban‘s effect on D-dimer and prothrombin fragment 1.2 (F1.2) (coagulation activity biomarkers ) was determined in a randomised, double-blinded, placebo-controlled, phase 2 study. Patients (n=1,715) with either ST-segment elevation or non-ST-segment elevation ACS received either placebo or apixaban 2.5 mg twice daily, 10 mg once daily, 10 mg twice daily or 20 mg once daily for six months. Samples were obtained at baseline (before study drug administration), week 3 and week 26. Apixaban plasma concentrations were measured directly by liquid chromatography/mass spectometry, and anti-Xa activity was determined using apixaban as a reference standard. D-dimer and F 1.2 were measured using ELISA-based methods. Most patients had elevated D-dimer and F1.2 levels at baseline. Both coagulation activity biomarkers decreased by week 3 in all treatment groups, but to a greater degree with apixaban than placebo (p

SummaryThrombin amplifies the blood coagulation via factor V (FV)-mediated positive feedback loop. We hypothesised that factor Xa (FXa) inhibitors would interfere more gradually with this feedback activation loop than thrombin inhibitors, thereby achieving a better balance between haemostasis and prevention of thrombosis. In this study, we compared the effects of TAK-442, a novel FXa inhibitor, versus ximelagatran, a thrombin inhibitor, on FV-mediated positive feedback, venous thrombosis and bleeding. In normal plasma, TAK-442 delayed the onset of tissue factor-induced thrombin generation and prolonged prothrombin time (PT) with more gradual concentration-response curve than melagatran, the active form of ximelagatran. The effect of melagatran on the onset of thrombin generation decreased in an FVa-concentration-dependent manner in FV-deficient plasma supplemented with FVa. Furthermore, in FV-deficient plasma, the PT-prolonging potency of melagatran was markedly increased with a change in its concentration-response curve from steep to gradual. In the rat venous thrombosis model, TAK-442 (10 mg/kg, p.o.) prevented thrombus formation by 55% with 1.2 times prolongation of PT; a similar effect was observed in ximelagatran-treated (3 mg/kg, p.o.) rats. TAK-442 at 100 mg/kg prolonged PT by only 2.1 times with no change in bleeding time (BT), whereas ximelagatran at 10 mg/kg prolonged PT by 3.9 times and significantly increased BT. These results suggest that the differential effects of the two agents on FV-mediated amplification of thrombin generation may underlie the observation of a wider therapeutic window for TAK-442 than for ximelagatran.

Increasing reports of bleeding and peri- or post-operative blood dyscrasias in Brazil were possibly associated with the use of heparin from bovine instead of porcine intestine. These two pharmaceutical grade heparins were analysed for potential differences. NMR analyses confirmed that porcine heparin is composed of mainly trisulfated disaccharides --4-alpha-IdoA2S-1--4-alpha-GlcNS6S-1--. Heparin from bovine intestine is also composed of highly 2-sulfated alpha-iduronic acid residues, but the sulfation of the alpha-glucosamine units vary significantly: approximately 50% are 6- and N -disulfated, as in porcine heparin, while approximately 36% are 6-desulfated and approximately 14% N -acetylated. These heparins differ significantly in their effects on coagulation, thrombosis and bleeding. Bovine heparin acts mostly through factor Xa. Compared to porcine heparin on a weight basis, bovine heparin exhibited approximately half of the anticoagulant and antithrombotic effects, but similar effect on bleeding. These two heparins also differ in their protamine neutralisation curves. The doses of heparin from bovine intestine required for effective antithrombotic protection and the production of adverse bleeding effects are closer than those for porcine heparin. This observation may explain the increasing bleeding observed among Brazilian patients. Our results suggest that these two types of heparin are not equivalent drugs.

SummaryThe direct thrombin inhibitor argatroban offers some significant advantages over unfractionated heparin (UFH) and is recommended as an alternative anticoagulant during percutaneous coronary interventions (PCI). The impact of argatroban on cardiac catheter thrombosis – a severe potential complication of PCI – has not been systematically studied yet. The aim of the present study was to test in vitro the hypothesis that argatroban is equivalent to the more established anticoagulants UFH and enoxaparin in preventing catheter thrombus formation. Blood pretreated with the anticoagulants of interest was continuously circulated through a guiding catheter by using a roller pump for a maximum experimental period of 60 minutes. In an alternate model, coagulation was mechanically induced by a magnetic stirrer. Coagulation parameters, overall thrombus weight and electron microscopic features (deposits of platelets and fibrin on the catheter surface) were quantified as endpoints. Argatroban (administered as bolus or continuous in-fusion), UFH (bolus), and enoxaparin (bolus) significantly reduced catheter thrombus formation compared to untreated controls. Here, neither overall thrombus weight nor platelet/fibrin deposition was different among the specific anticoagulants. Declining ACT (activated clotting time) levels – which were found in the argatroban bolus group – could be prevented by continuous infusion. In magnetic stirrer-induced coagulation, thrombus weight was lower following bolus treatment with UFH and enoxaparin compared to argatroban. These data suggest that the potential for argatroban in preventing catheter thrombosis is comparable to that of UFH and enoxaparin. However, the anticoagula-tory efficacy varied, depending on the model of coagulation activation, which demonstrates the necessity for specific testing.

Abstract 5185 Administration of various chemotherapeutic agents is associated with an increased risk of thrombotic events. Although vascular endothelium plays a predominant role in blood coagulation and fibrinolysis, the effect of cytotoxic drugs on the procoagulant activity (PCA) of endothelial cells has not been well evaluated. Our study aims to investigate the possibility that daunorubicin, a chemotherapeutic agent, exerts prothrombotic effect on endothelial cells. We tested the impact of daunorubicin on phosphatidylserine (PS) exposure, endothelial microparticles (EMPs) release and consequent PCA. Human umbilical vein endothelial cells (HUVECs) were treated with daunorubicin (0.1, 0.2, 0.5, 1, 2 μ M) for 24 h. PCA of HUVECs was measured using clotting time and purified coagulation complex assays. Counts and PCA of EMPs were evaluated by flow cytometry and clotting time assay, respectively. Lactadherin was used as a novel probe for detection of PS exposure and EMPs release. We found that daunorubicin dose-dependently increased the PS exposure and consequent PCA of HUVECs. Moreover, daunorubicin treatment also enhanced the release of EMPs which were highly procoagulant. This increment was especially significant at 0.2 μ M of daunorubicin or over. Blockade of PS with lactadherin inhibited over 90% of HUVECs and EMPs PCA. However, anti-TF antibody had no significant inhibition effect. Our results demonstrate that daunorubicin treatment enhanced PCA of HUVECs through PS exposure and shedding of procoagulant EMPs. Lactadherin acts as an efficient anticoagulant in this process. Disclosures: No relevant conflicts of interest to declare.

SummaryHeparins, either unfractionated or low-molecular-weight (UFH and LMWHs), and vitamin K antagonists (VKAs) are currently the anticoagulants of choice for the prevention of post-operative venous thromboembolism (VTE) and for the treatment of acute venous and arterial thromboembolism. While VKAs are widely used in the US, LMWHs are the standard of care in the EU. Although efficacious, these agents are associated with a number of drawbacks, such as the risk of heparin-induced thrombocytopenia, the need for frequent coagulation monitoring in the case of UFH and VKAs, and the parenteral mode of administration in the case of heparins, which can lead to problems associated with patient compliance. There is a need for new anticoagulants that overcome these limitations. Direct, small-molecule inhibitors of coagulation proteins targeting a single enzyme in the coagulation cascade – particularly thrombin or Factor Xa – have been developed in recent years. Two agents, the direct thrombin inhibitor dabigatran and the direct Factor Xa inhibitor rivaroxaban, have recently been approved in the EU and several other countries for the prevention of VTE after total hip or knee replacement surgery. Here we will review data that suggest that the antithrombin-independent mechanism of action of these agents, particularly that of direct Factor Xa inhibitors, leads to increased efficacy with similar safety profiles compared with the antithrombin-dependent heparins. Although the end of the heparins era is not to be expected, the new anticoagulants presented in this review potentially represent the future of anticoagulation.

SummaryAn algal sulfated galactan has high anticoagulant and antithrombotic activities. Its serpin-dependent anticoagulant action is due to promoting thrombin and factor (F)Xa inhibition by antithrombin and heparin cofactor II. Here, we evaluated the anticoagulant effect of the algal sulfated galactan using serpin-free plasma. In contrast to heparin, the sulfated galactan is still able to prolong coagulation time and delay thrombin and FXa generation in serpin-free plasma. We further investigated this effect using purified blood coagulation proteins, discovering that sulfated galactan inhibits the intrinsic tenase and prothrombinase complexes, which are critical for FXa and thrombin generation, respectively. We also investigated the mechanism by which sulfated galactan promotes FXa inhibition by antithrombin using specific recombinant mutants of the protease. We show that sulfated galactan interacts with the heparin-binding exosite of FXa and Arg-236 and Lys-240 of this site are critical residues for this interaction, as observed for heparin. Thus, sulfated galactan and heparin have similar high-affinity and specificity for interaction with FXa, though they have differences in their chemical structures. Similar to heparin, the ability of sulfated galactan to potentiate FXa inhibition by antithrombin is calcium-dependent. However, in contrast to heparin, this effect is not entirely dependent on the conformation of the γ-carboxyglutamic acid-rich domain of the protease. In conclusion, sulfated galactan and heparin have some similar effects on blood coagulation, but also differ significantly at the molecular level. This sulfated galactan opens new perspective for the development of antithrombotic drugs.

SummaryThe hypothesis that prothrombin (FII) protects coagulation factor Va (FVa) from proteolytic inactivation by activated protein C (APC) was tested using purified proteins. FII dose-dependently protected FVa from APC proteolysis under conditions where competition of proteins for binding to negatively-charged phospholipid surface was not relevant (i.e. either at high phospholipid vesicle concentrations or using soluble dicaproylphosphatidylserine at levels below its critical micellar concentration). Cleavages in FVa at both Arg506 and Arg306 by APC were inhibited by FII. FII did not alter the amidolytic activity of APC towards chromogenic oligopeptide substrates or inhibit FVIIIa inactivation by APC, implying that the FII-mediated protection of FVa from APC proteolysis was due to the ability of FII to inhibit protein-protein interactions between FVa and APC. FII also protected FVa from inactivation by Gla-domainless APC, ruling out a role for the APC Gla domain for these observations. To identify domains of FII responsible for the observed phenomenon, various forms or fragments of FII were employed. Biotin-PheProArg-CMK-inhibited meizothrombin and fII-fragment 1•2 protected FVa from proteolysis by APC. In contrast, no significant protection of FVa from APC cleavage was observed for Gladomainless-FII, prethrombin-1, prethrombin-2, FII fragment 1 or active site inhibited-thrombin (DEGR-thrombin). Overall, these data demonstrate that the Gla domain of FII linked to kringle 1 and 2 is necessary for the ability of FII to protect FVa from APC cleavage and support the general concept that assembly of the FII activation complex (FXa•FVa•FII•lipid surface) protects FVa from APC inactivation so that the procoagulant, thrombin generating pathway can act unhindered by APC. Only following FII activation and dissociation of the FII Gla domain fragments from the FII-ase complex, can APC inactivate FVa and down-regulate thrombin generation.

SummaryTissue factor (TF) is a transmembrane glycoprotein that initiates blood coagulation when complexed with activated factor VII (FVIIa).TF is constitutively expressed in a variety of tumor cells and has been implicated in cellular signaling, angiogenesis, and tumor progression. Formation of TF-FVIIa complex and generation of downstream coagulation proteases, including activated factor X (FXa) and thrombin, initiate signaling by activation of protease-activated receptors (PARs).We have previously shown thatTFFVIIa-Xa complex formation promotes phosphorylation of p44/42 mitogen-activated protein kinase and Akt/protein kinase B in human breast cancer cells. In the present study, we show that formation of TF-FVIIa-FXa complex induces phosphorylation of mammalian target of rapamycin (mTOR) and p70 S6 kinase in a human breast cancer cell line, Adr-MCF-7. Activation of the mTOR pathway, which is probably mediated by PAR1 and/or PAR2, was associated with enhanced cell migration, a key step in the metastatic cascade. Inhibition of this pathway with the specific mTOR inhibitor, rapamycin, markedly decreased cell migration induced by formation of TFFVIIa-FXa complex. These studies suggest thatTFFVIIa-mediated signaling modulates mTOR pathway activation, which regulates in part breast cancer cell migration. Targeting the TF-mediated cell signaling pathway might represent a novel strategy for the treatment of breast cancer.

SummarySulfated galactan from the red alga Botryocladia occidentalis has a potent anticoagulant activity,due to its ability to enhance thrombin and factor Xa inhibition by antithrombin and/or heparin cofactor II. It is less active than unfractionated heparin in arterial thrombosis,but in a venous thrombosis presents a dual effect,inhibiting thrombosis in low but not in high doses.This dual effect on venous thrombosis is a consequence of two actions, one that inhibits thrombin and factor Xa and one that induces factor XII activation. In order to dissociate these effects, we prepared derivatives of the sulfated galactan with low molecular weights. Two fractions that were similar in size to unfractionated heparin and low-molecular-weight heparin were obtained. As the molec- ular weight decreased, the ability to activate factor XII and to promote inhibition of coagulation proteases in the presence of antithrombin and heparin cofactor II diminished.At ∼5 kDa, the sulfated galactan fragment had no effect on factor XII activation, and showed the same effect as unfractionated heparin in a venous thrombosis model.The ∼5-kDa fragment is an antithrombotic with several advantages: i) It is as active as unfractionated heparin in venous thrombosis, but it has little activity in arterial thrombosis; ii) It inhibits venous thrombosis with very little anticoagulant effect; iii) It does not cause bleeding; and iv) It is not obtained from mammals.

SummaryHeparinization requires monitoring, but optimal methods for measuring the anticoagulant effects of heparin remain to be determined. We compared prothrombinase-induced clotting time (PiCT) and two chromogenic anti-factor Xa activity (anti-Xa) assays in monitoring high-dose heparinization during cardiopulmonary by-pass (CPB). Heparin effects were serially measured with PiCT and two anti-Xa assays in 100 patients. Antithrombin and protein C activities were measured preoperatively, and antithrombin activity was measured during CPB. Activation of coagulation was assessed with measurements of prothrombin fragment F1+2, soluble fibrin complexes, and D-dimer before, during, and after CPB. During CPB mean ranges of PiCT and of anti-Xa heparin levels measured with (anti-Xa A) and without (anti-Xa B) dextran sulfate and antithrombin supplementation were 5.0–5.2, 4.7–5.0, and 4.5–4.9 IU/ml, respectively. There was poor agreement between PiCT and anti-Xa and between the two anti-Xa assays (r=0.32–0.65 and broad limits of agreement). Patients with low preoperative antithrombin or protein C levels had lower PiCT (p=0.028 and p=0.01) and anti-Xa A (both p

During the last 15 years, transgenic mice have been generated that carry defective and/or mutant alleles of the natural anticoagulant pathways and display a spontaneous thrombotic phenotype. With the generation of these mouse lines, better opportunities became available for investigating both existing and novel risk factors for venous thrombosis. In addition, these models could serve as a tool for evaluating novel antithrombotic strategies. This review summarizes these mouse models and evaluates whether they have fulfilled the expectations.

Summary Tissue factor (TF) is a transmembrane glycoprotein that initiates blood coagulation when complexed with factorVIIa (FVIIa).TF is constitutively expressed ina variety of tumor cells and has been shown to playa role in cellular signaling and tumor progression. In this study, we investigated the effect of TF-FVIIa mediated signaling on apoptosis in human breast cancer cells. Apoptosis was induced by prolonged serum starvation and studied using the Adr-MCF-7 cell line, which has high endogenous TF expression. Treatment of the cells with the combination of FVIIa (10 nM) and FX (150 nM), reduced apoptosis by nearly 50% compared with untreated, control cells using an ELISA that detects histone-DNA fragments. In contrast, FVIIa (10 nM) alone did not significantly prevent apoptosis. Pretreatment of the Adr-MCF-7 cells with hirudin, a specific thrombin inhibitor, did not inhibit the anti-apoptotic effect of the combination of FVIIa and FX, whereas this effect could be abrogated by inhibition of phosphorylation of either p44/42 mitogen-activated protein kinase (MAPK) or protein kinaseB (PKB/Akt). In addition, treatment of theAdr-MCF-7 cells with the combination of FVIIa and FX led to a 30-50% increase in the level of the anti-apoptotic protein, survivin, compared with untreated cells usingWestern blot analysis. These results indicate that formation of TF-FVIIa-FXa complex prevents apoptosis in breast cancer cells by a thrombin-independent pathway. Moreover, the anti-apoptotic effect of this signaling pathway involves phosphorylation of both p44/42 MAPK and PKB/Akt and might be mediated in part by an increase in cell survivin levels.

SummaryFour direct thrombin inhibitors (DTIs), lepirudin, bivalirudin, argatroban, and melagatran, differ in their ability to prolong the prothrombin time (PT). Paradoxically, the DTI in clinical use with the lowest affinity for thrombin (argatroban) causes the greatest PT prolongation. We compared the effects of these DTIs on various clotting assays and on inhibition of human and bovine factor Xa (FXa). On a mole-for-mole basis, lepirudin was most able to prolong the PT, activated partial thromboplastin time (APTT), and thrombin clotting time (TCT), whereas argatroban had the least effect. At concentrations that doubled the APTT (argatroban, 1 μmol/l; melagatran, 0.5 μmol/l; bivalirudin, 0.25 μmol/l; lepirudin, 0.06 μmol/l), the rank order for PT prolongation was: argatroban > melagatran > bivalirudin > lepirudin. Although the Ki’s associated with inhibition of human FXa by melagatran (1.4 μmol/l) and argatroban (3.2 μmol/l) approach their therapeutic concentrations, inhibition of FXa did not appear to be a major contributor to PT prolongation, since argatroban also prolonged the PT of bovine plasma (despite a Ki for bovine FXa of 2,600 μmol/l). Only melagatran inhibited prothrombinase-bound FXa. We conclude that the differing effects of the DTIs on PT prolongation are primarily driven by their respective molar plasma concentrations required for clinical effect. DTIs with a relatively low affinity for thrombin require high plasma concentrations to double the APTT; these higher plasma concentrations, in turn, quench more of the thrombin generated in the PT, thereby more greatly prolonging the PT.

SummaryThe factors responsible for the removal of injected factor IX (fIX) from the blood of individuals with haemophilia B are only partly understood, and may include binding to endothelial or subendothelial sites, passive extravasation related to size or charge, or interactions requiring fIX activation. To investigate these issues, we have produced and characterised recombinant fIX proteins with amino acid changes: Δ155–177, an internal deletion which removes most of the activation peptide while retaining the activation cleavage sites; S365A, which inactivates the serine protease activity of fIXa; and K5A, previously shown to eliminate fIX binding of endothelial/subendothelial collagen IV. All proteins were expressed in stably transfected HEK 293 cells, purified by immunoaffinity chromatography, and compared to the wild type HEK 293-derived protein (fIX (WT)). Mutant fIX proteins K5A and Δ155–177 exhibited 72 and 202% of the specific activity of fIX (WT), respectively; S365A was without activity. Following intravenous injection in haemophilia B (fIX knockout) mice, recoveries did not differ for fIX (WT) and Δ155–177, but were higher for K5A and S365A. The terminal catabolic halflife of Δ155–177, alone among the mutants, was increased, by 45% versus fIX (WT). Nine hours post-injection, the observed areas under the clearance curve (AUCs) of Δ155–177 and K5, but not S365A, were elevated 2-fold. Δ155–177 was equally effective as fIX (WT) in reducing blood loss following tail vein transection in haemophilia B mice. Our results suggest that deletion of the multiple sites of fIX post-translational modification found within the activation peptide eliminated important fIX clearance motifs.

SummaryPseutarin C, a group C prothrombin activator from Pseudonaja textilis venom, is a large protein complex consisting of catalytic and nonenzymatic subunits, which are functionally similar to the mammalian FXa-FVa complex. Here, we present the complete cDNA sequence of the catalytic subunit of pseutarin C. The cDNA of the catalytic subunit encodes a protein of 449 amino acids, which includes a 22-residue signal peptide, 18-residue propeptide and a mature protein of 409 amino acids. The deduced amino acid sequence shows 74-83% identity to group D prothrombin activators from snake venom and ∼42% identity to mammalian FX and has identical domain structure. The precursor of the catalytic subunit of pseutarin C has several unique features. The activation peptide of the catalytic subunit of pseutarin C is significantly smaller (27 as compared to 52 residues in mammalian FX) and does not contain any glycosylation sites. Unlike coagulation FXa, Ser52 and Asn45 of the light and heavy chains are O- and N-glycosylated in pseutarin C catalytic subunit. There is a 12-residue insertion in pseutarin C catalytic subunit close to the region that is implicated in binding to FVa. This is the first sequence of the catalytic subunit of a group C prothrombin activator.

SummaryIn a preliminary study, we have demonstrated that tissue factor (TF) is immunohistochemically stained in monocytes from patients with disseminated intravascular coagulation (DIC) but not from healthy volunteers, and that leukocytes from DIC patients induce enhanced activated factor X (FXa) generation in the presence of a mixture of FVIIa, FX and Ca2+. Then, TF mRNA levels in leukocytes were measured to evaluate the role of TF in the pathophysiology of various diseases. TF mRNA levels in leukocytes were low in healthy volunteers but they were significantly increased in various diseases, especially in patients with infectious diseases, solid cancer, anemd hematopoietic tumors. TF mRNA levels in leukocytes were significantly higher in patients with high levels of C reactive protein (CRP) than in those with low CRP. TF mRNA levels were significantly higher in patients with DIC than in those without DIC. TF mRNA levels were well correlated with TF antigens in plasma and leukocytes. These findings suggest that the expression of TF mRNA in leukocytes is increased in various diseases and that this may play an important role in hypercoagulability or DIC.

SummaryRecent studies have shown that inhibitors develop against acidic regions of the FVIII molecule, which contain important functional sites. However, their mechanisms of inhibition are not well understood. In this study, two anti-human FVIII mouse monoclonal antibodies (MAbs), directed towards the exposed acidic regions of the FVIII molecule, were developed, characterised and their mechanisms of inhibition investigated. The two MAbs, F7B4 and F26F6, had inhibitory titres of 32 and 944 BU/mg respectively, had high affinities for the FVIII molecule (KD ~ nM range) and recognised sequences V357-F360 on the acidic a1 region and E724-L731 on the acidic a2 region of the FVIII heavy-chain (HC), respectively. F7B4 inhibited the rate of FXa generation by activated FVIII, whilst both antibodies inhibited FVIII activation by thrombin and blocked thrombin cleavage of FVIII. Furthermore, F7B4 and F26F6 inhibited FVIII binding to (a) phospholipids (IC50: 77 nM and 40 nM respectively), and (b) VWF (IC50: 93 nM and 267 nM respectively), despite both having HC specificity. Experiments with F(ab’)2 fragments confirmed the above findings. Taken together these data represent novel findings in that anti-acidic HC antibodies can inhibit FVIII function by a variety of mechanisms, in particular by interfering with the binding of FVIII to phospholipids & VWF.

A homogeneous set of low-molecular weight heparins, chemically modified to yield different degrees of sulfation, were investigated for their ability to interfere with the antithrombin (AT)-factor Xa (FXa) interaction process in the presence or absence of physiological concentrations of calcium ions. The heparin-AT dissociation constants were not appreciably affected by the presence of the metal ion, whereas the catalytic process was strongly dependent on Ca 2+ . Our data suggest that AT binding to heparin represents the main factor driving the FXa inhibition process. In addition, the presence of the metal ion is likely to mask favorable AT- heparin ionic contacts occurring with the highly sulfated material. These results help in assessing proper structure-activity relationships for glycosaminoglycans, a multitarget family of biologically active compounds.