Full-length Tissue Factor Pathway Inhibitor Research Articles

Plasma thrombin generation in the presence of TIX-5 may contribute significantly to a prediction model for major bleeding in patients on VKA anticoagulant therapy

IntroductionOral anticoagulant therapy comes at the cost of a significant bleeding risk. However, it is hard to predict which patients are at risk of major bleeding. Previously we found associations of Calibrated Automated Thrombinography (CAT) parameters obtained in the presence of TIX-5 (an inhibitor of the FV activation by FXa), and plasma levels of TFPIα, γ’-fibrinogen and soluble thrombomodulin with major bleeding in the BLEEDS cohort, a cohort especially powered to find new biomarkers of major bleeding during VKA therapy. MethodsTo determine and compare the predictive capability for major bleeding in the BLEEDS cohort of the above biomarkers, also in a combined model with clinical risk factors, we performed Univariable Prentice-weighted Cox regression analyses and Bayesian variable selection. ResultsThe highest predictive value among the laboratory measures were found for thrombin generation lagtime in the presence of TIX-5 (TIX-5 lagtime per 25% increase, hazard ratio (HR) 1.11, 95%CI 1.04–1.18, p=0.001) and full-length tissue factor pathway inhibitor (TFPIα) (per 25% increase HR 1.12, 95%CI 1.03–1.21, p=0.008), which remained significant after correction for multiple testing, and independently associated with major bleeding after Bayesian variable selection. Only the addition of TIX-5 lagtime to the clinical risk factors improved prediction of major bleeding significantly (p<0.001). ConclusionWe established predictive value of the lagtime of thrombin generation measured in the presence of TIX-5 for the risk of a major bleeding of patients on VKA therapy.

The influences of selected clotting and fibrinolysis factors on survival of patients with kidney tumors - a prospective study.

Multiple studies suggest that cancer leads to activation of clotting and fibrinolysis pathways, elevating the risk of thromboembolic events. Kidney cancer is often complicated by clotting disorders. In this study, we hypothesized that preoperative clotting and fibrinolysis parameters are altered in healthy volunteers and kidney tumor patients. We also hypothesized that these differences may be associated with survival in patients who have undergone operations due to kidney tumors. In this study, 96 patients with kidney tumors and 30 healthy volunteers were recruited at a single university center. All patients were assessed for pre-operative serum concentrations of tissue factor (TF), tissue factor pathway inhibitor (TFPI, total TFPI, full-length TFPI, truncated TFPI), plasmin-antiplasmin complex (PAP), thrombin-antithrombin complex (TAT), von Willebrand factor (vWF), clotting factor XIII A1 (FXIIIA1), D-dimers, and fibrinogen. Additionally, standard peripheral blood morphology was evaluated. Malignant kidney tumors were diagnosed in 85 of 96 tumor patients. In patients with kidney tumors, there were statistically significantly higher concentrations of fibrinogen, D-dimers, TAT, PAF, TF, TFPI, vWF, FXIIIA1, and leukocyte counts compared to the control group. Statistically significant correlations were found between multiple parameters. This points to significant clotting system alterations. Cox stepwise hazard analysis showed that pre-operative fibrinogen and D-Dimer concentrations were significantly associated with survival. In patients with kidney tumors, multiple clotting and fibrinolysis parameters are significantly altered. Routine pre-operative measures should include determination of fibrinogen and D-dimer concentrations as these markers aid in prediction of survival probability.

Effect of blood loss during caesarean section on coagulation parameters

IntroductionVenous thrombosis is the leading cause of pregnancy-related maternal morbidity and mortality. The thrombosis risk is increased by caesarean section and blood loss, though underlying mechanisms of these prothrombotic changes remain unknown. Materials and methodsThis prospective study recruited 50 pregnant women at term undergoing elective caesarean section at University Hospital Magdeburg, Germany. Blood loss during surgery was correlated with the changes in total protein S, full length TFPI (TFPIfl), prothrombin, the endogenous thrombin potential (ETP) and resistance to activated protein C (APCsr) determined via calibrated automated thrombography. ResultsMean blood loss was 506 ml (95%CI: 456 to 557 ml). Total protein S was 0.63 (95%CI: 0.60 to 0.67) U/ml preoperatively, decreased by 14.8% after caesarean section and almost normalised five days later. TFPIfl was 0.47 (95%CI: 0.41 to 0.53) U/ml before, remained unchanged immediately after and increased by 11.5% five days after surgery. Prothormbin was 1.10 (95%CI: 1.03 to 1.16) U/ml preoperatively, reduced by 10.4% immediately after and increased again five days after caesarean section, exceeding the preoperative values by 4.4% (−0.7 to 9.6). The ETP decreased by 3.9%, whereas the APCsr increased by 37.0% immediately after caesarean section. The changes in total protein S, prothrombin, thrombin generation and APC resistance showed a trend to be more pronounced in the subgroups with higher blood loss. DiscussionModerate blood loss during caesarean section hardly reduces thrombin generation but aggravates pregnancy-induced APC resistance and combined deficiency of TFPI and protein S, which can account for the increased thrombosis risk in early puerperium.

Maintaining extraembryonic expression allows generation of mice with severe tissue factor pathway inhibitor deficiency

AbstractTissue factor pathway inhibitor (TFPI) is a serine protease with multiple anticoagulant activities. The Kunitz1 (K1) domain of TFPI binds the active site of factor VIIa and is required for inhibition of tissue factor (TF)/factor VIIa catalytic activity. Mice lacking TFPI K1 domain die in utero. TFPI is highly expressed on trophoblast cells of the placenta. We used genetic strategies to selectively ablate exon 4 encoding TFPI K1 domain in the embryo, while maintaining expression in trophoblast cells. This approach resulted in expected Mendelian frequency of TFPI K1 domain–deficient mice. Real-time polymerase chain reaction confirmed 95% to 99% genetic deletion and a similar reduction in transcript expression. Western blotting confirmed the presence of a truncated protein instead of full-length TFPI. Mice with severe TFPI K1 deficiency exhibited elevated thrombin-antithrombin (TAT) levels, frequent fibrin deposition in renal medulla, and increased susceptibility to TF-induced pulmonary embolism. They were fertile, and most lived normal life spans without any overt thrombotic events. Of 43 mice observed, 2 displayed extensive brain ischemia and infarction. We conclude that in contrast to complete absence of TFPI K1 domain, severe deficiency is compatible with in utero development, adult survival, and reproductive functions in mice. Inhibition of TFPI activity is being evaluated as a means of boosting thrombin generation in hemophilia patients. Our results show that in mice severe reduction of TFPI K1 activity is associated with a prothrombotic state without overt developmental outcomes. We note fibrin deposits in the kidney and rare cases of brain ischemia.

Tissue Factor Pathway Inhibitor Enhances Transendothelial Migration of Chronic Lymphocytic Leukemia Cells through Binding to Glypican-3

Chronic lymphocytic leukemia (CLL) is the most common leukemia in the western world. Cell trafficking and homing of CLL cells play a critical role in organ infiltration and contribute to the clinical course of CLL. Interaction between CLL cells and endothelial cells affects gene expression in CLL cells and further regulates cell trafficking. Endothelial cells are the main source of tissue factor (TF) pathway inhibitor (TFPI), which is the primary inhibitor of TF. However, the role of TFPI in leukemic cell trafficking has not been explored before. CXCL12 and its receptor CXCR4 play an important role in CLL cell trafficking and homing. To assess the effect of TFPI on CXCL12-mediated migration of leukemic cells, we investigated transendothelial migration (TEM) of B cells from CLL patients by using CXCL12 as a chemoattractant. We found that full-length recombinant TFPI (rTFPI) increased CXCL12-mediated migration of CLL cells dose-dependently by using MACS flow cytometry to count the migrated cells. Accordingly, Western blot showed the binding of rTFPI to the CLL cells after 24 h treatment. However, the chemokine receptor CXCR4 expression in CLL cells was not increased or even slightly decreased by rTFPI treatment, suggesting that TFPI enhances leukemic cell migration probably through interacting with other factor(s) than overexpressing CXCR4 in CLL cells. Surface expression and intracellular trafficking of TFPI is regulated by glycosylphosphatidylinositol (GPI)-anchored co-receptor(s). Glypican-3, a member of proteoglycan family, binds to the plasma membrane by a GPI anchor and its heparan sulfate (HS)-type glycosaminoglycan chains are considered to mediate the interaction with other cell membrane proteins. To determine whether TFPI interacts with glypican-3, immunofluorescence staining and co-immunoprecipitation were performed in K562 and HL60 cells. Immunofluorescence analysis revealed colocalization of TFPI and glypican-3 on the surface of K562 and HL60 cells. Co-immunoprecipitation confirmed the binding between TFPI and glypican-3 in K562 cells. Moreover, co-localized TFPI and glypican-3 were found in bone marrow biopsies of leukemia patients. We further employed an antibody against HS chains of glypican-3, which inhibits the activity of glypican-3. Western blot showed that rTFPI treatment failed to increase TFPI protein in CLL cells when glypican-3 inhibitor HS20 was applied. In addition, TEM assay showed that HS20 treatment abolished the effect of TFPI on cell migration, which suggests that inhibition of glypican-3 blocks the binding of TFPI to CLL cells and then abolishes the effect of TFPI on cell migration. It indicates that TFPI affects CLL cell migration through binding to glypican-3. Using a factor Xa generation assay, we found that HS20 suppressed TF activity on the surface of HL60 cells, suggesting that the inhibition of glypican-3 impaired the procoagulant activity of leukemic cells. In addition, Western blot and RT-PCR showed that HS20 downregulated TF mRNA and protein expression in CLL cells. Finally, we explored the role of Wnt/β-catenin signaling, which has been shown to regulate tumor progression and metastasis. We found a dose-dependent increase of activated β-catenin in the CLL cells after rTFPI treatment. Inhibition of Wnt/β-catenin signaling by a potent inhibitor IWP4 strongly impaired the migration of CLL cells induced by rTFPI. In this study, we show that TFPI enhances CLL cell migration through binding to glypican-3 and activating Wnt/β-catenin signaling pathway. Our findings suggest a novel molecular mechanism regulating the transmigration of leukemic cells. It may open a new window for therapeutic targeting in the treatment of organ infiltration of CLL patients. DisclosuresNo relevant conflicts of interest to declare.

Factor V has an anticoagulant cofactor activity that targets the early phase of coagulation

Tissue factor pathway inhibitor (TFPI), the main inhibitor of initiation of coagulation, exerts an important anticoagulant role through the factor Xa (FXa)-dependent inhibition of tissue factor/factor VIIa. Protein S is a TFPI cofactor, enhancing the efficiency of FXa inhibition. TFPI can also inhibit prothrombinase assembly by directly interacting with coagulation factor V (FV), which has been activated by FXa. Because full-length TFPI associates with FV in plasma, we hypothesized that FV may influence TFPI inhibitory function. Using pure component FXa inhibition assays, we found that although FV alone did not influence TFPI-mediated FXa inhibition, it further enhanced TFPI in the presence of protein S, resulting in an ∼8-fold reduction in Ki compared with TFPI alone. A FV variant (R709Q/R1018Q/R1545Q, FVΔIIa) that cannot be cleaved/activated by thrombin or FXa also enhanced TFPI-mediated inhibition of FXa ∼12-fold in the presence of protein S. In contrast, neither activated FV nor recombinant B-domain-deleted FV could enhance TFPI-mediated inhibition of FXa in the presence of protein S, suggesting a functional contribution of the B domain. Using TFPI and protein S variants, we show further that the enhancement of TFPI-mediated FXa inhibition by protein S and FV depends on a direct protein S/TFPI interaction and that the TFPI C-terminal tail is not essential for this enhancement. In FXa-catalyzed prothrombin activation assays, both FV and FVΔIIa (but not activated FV) enhanced TFPI function in the presence of protein S. These results demonstrate a new anticoagulant (cofactor) function of FV that targets the early phase of coagulation before prothrombinase assembly.

New functional assays to selectively quantify the activated protein C‐ and tissue factor pathway inhibitor‐cofactor activities of protein S in plasma

Background Protein S plays an important role in the down-regulation of coagulation as cofactor for activated protein C (APC) and tissue factor pathway inhibitor (TFPI). Aim To develop functional assays to quantify the APC- and TFPI-cofactor activities of protein S in plasma. Methods APC- and TFPI-cofactor activities of protein S in plasma were measured using calibrated automated thrombography in protein S-depleted plasma supplemented with a small amount of sample plasma either in the presence of anti-TFPI antibodies and APC (APC-cofactor activity) or at excess full-length TFPI without APC (TFPI-cofactor activity). Total and free protein S levels in plasma were measured by ELISAs. Results Average APC-cofactor activities of protein S were 113%, 108% and 89% in plasma from normal individuals (n = 15), FV Leiden heterozygotes (n = 14) and FV Leiden homozygotes (n = 7), respectively, whereas the average APC-cofactor activity of protein S in plasma from heterozygous protein S-deficient individuals (n = 21) was significantly lower (55%). Similar trends were observed for the TFPI-cofactor activity of protein S, with averages of 109%, 115% and 124% in plasma from individuals with normal protein S levels and different FV Leiden genotypes, and 64% in plasma from protein S-deficient patients. APC-cofactor activities of protein S correlated significantly with free and total protein S antigen levels, whereas TFPI-cofactor activities correlated less with protein S antigen levels. Conclusion We have developed functional protein S assays that measure both the APC- and TFPI-cofactor activities of protein S in plasma, which are hardly if at all affected by the FV Leiden mutation.

Platelet full length TFPI-α in healthy volunteers is not affected by sex or hormonal use.

BackgroundOnly 10% of plasma TFPIα (TFPI) exists in the full length form, the rest circulates as a C-terminally truncated form. However, blood platelets exclusively contain full length TFPI, which is released at the site of injury upon platelet activation, and which could play an important local regulatory role in thrombin generation and prevention of thrombosis.MethodsThe anticoagulant activities of full length and truncated TFPI were investigated using thrombin generation assays. Blood samples were obtained from 30 healthy volunteers (10 male subjects, 10 female subjects, and 10 females using oral contraceptives). Platelet TFPI was released in platelet rich plasma and in platelet isolates using convulxin or thrombin, and measured by free TFPI ELISA and thrombin generation assays.ResultsFull length TFPI and platelet TFPI were much more potent inhibitors of thrombin generation than truncated TFPI, which was virtually inactive. Although mean plasma TFPI antigen levels decreased from men (0.30 nM) to women (0.20 nM) to women using oral contraceptives (0.11 nM), no relevant differences were found in platelet TFPI among those subgroups.ConclusionsPlatelets release similar amounts of TFPI regardless of plasma TFPI concentrations and is unaffected by sex or oral contraceptive use. We speculate that platelet TFPI is important to prevent systemic coagulation and thrombosis and restrict thrombus formation to the site of the growing platelet plug. The stable contribution of platelet TFPI to the anticoagulant potential in plasma is likely to become particularly relevant under conditions of low plasma TFPI levels in combination of oral contraceptives use.

New Functional Assays to Selectively Quantify APC- and TFPI-Cofactor Activities of Protein S in Plasma

Background: Protein S is an important anticoagulant in the down-regulation of blood coagulation by acting as cofactor for two main regulators of coagulation. Protein S acts as a cofactor for activated protein C (APC) in the inactivation of the procoagulants FVa and FVIIIa and for TFPI in the formation of the TFPI-FXa encounter complex that subsequently inactivates TF-FVIIa. Several studies have indicated that protein S deficiency is associated with increased risks of venous thrombosis.Aims: Current protein S functional assays are influenced by plasma determinants such as FVLeiden. The aim of the study was to develop thrombin generation based assays that enable quantification of both the APC- and TFPI-cofactor activities of protein S in plasma whereas the outcome of the assays should not be affected by the presence of FVLeiden or other plasma variables.Methods: APC- and TFPI-cofactor activities of protein S in plasma were measured using calibrated automated thrombography (CAT) in protein S-depleted plasma supplemented with a small amount of sample plasma either in the presence of anti-TFPI antibodies and APC (APC-cofactor activity) or at excess full-length TFPI without APC (TFPI-cofactor activity). Total and free protein S levels in plasma were measured by ELISA's.Results: Average APC-cofactor activities of protein S were 113%, 122%, and 87% in plasma from normal individuals (n=10), FV Leiden heterozygotes (n=4), and FV Leiden homozygotes (n=2), respectively, while the average APC-cofactor activity of protein S in plasma from heterozygous protein S-deficient individuals (n=21) was significantly lower (55%). Similar trends were observed for the TFPI-cofactor activity of protein S, with averages of 108%, 112%, and 97% in plasma from individuals with normal protein S levels and different FV Leiden genotypes, and 64% in plasma from protein S-deficient patients. APC-cofactor activities of protein S correlated significantly with free and total protein S antigen levels, while TFPI-cofactor activities correlated less with protein S antigen levels.Summary/Conclusion: We have developed functional protein S assays that measure both the TFPI- and APC-cofactor activities of protein S in plasma and that are not affected by the FV Leiden mutation or other plasma influences. DisclosuresNo relevant conflicts of interest to declare.

Inhibition of tissue factor:factor VIIa–catalyzed factor IX and factor X activation by TFPI and TFPI constructs

TFPI is a Kunitz-type protease inhibitor that downregulates the extrinsic coagulation pathway by inhibiting factor Xa (FXa) and FVIIa. All three Kunitz domains (KD1, KD2, and KD3) and protein S are required for optimal inhibition of FXa and FVIIa. There is limited information on Kunitz domain requirements of the inhibition of TF:FVIIa-catalyzed FIX and FX activation by TFPI. To investigate the role of the Kunitz domains of TFPI and protein S in the inhibition of FX and FIX activation. Inhibition of TF:FVIIa-catalyzed FX and FIX activation by full-length TFPI (TFPIFL ) and TFPI constructs was quantified from progress curves of FXa and FIXa generation measured with chromogenic substrates. TFPIFL inhibited TF:FVIIa-catalyzed FIX activation with a Ki of 16.7 nmol L(-1) . Protein S reduced the Ki to 1.0 nmol L(-1) . TFPI1-150 and KD1-KD2 had 10-fold higher Ki values and were not stimulated by protein S. Single Kunitz domains were poor inhibitors of TF:FVIIa-catalyzed FIX activation (Ki >800 nm). FX activation was measured at limiting FVIIa and excess TF or vice versa. At both conditions, TFPIFL , TFPI1-150 , and KD1-KD2 showed similar inhibition of FX activation. However, at low phospholipid concentrations, TFPIFL was ~ 15-fold more active than TFPI1-150 or KD1-KD2. Apparently, excess phospholipids act as a kind of sink for TFPIFL , limiting its availability for TF:FVIIa inhibition. Preformed FXa:TFPIFL/1-150 complexes rapidly and stoichiometrically inhibited FIX and FX activation by TF:FVIIa, indicating that binary TFPI:FXa complex formation is the limiting step in TF:FVIIa inhibition. Protein S also enhanced inhibition of TF:FVIIa-catalyzed FX activation by TFPI.

Inhibitory effects of LDL-associated tissue factor pathway inhibitor

IntroductionTissue factor pathway inhibitor (TFPI) is present in plasma as full-length free TFPI (TFPIα) and as C-terminally degraded forms mainly associated with low-density lipoprotein particles (LDL-TFPI). Addition of TFPIα to plasma induces a prolongation of the clotting time when tested in a diluted prothrombin time (dPT) assay, whereas no prolongation is observed with LDL-TFPI or truncated recombinant TFPI (TFPI1–161). The aim was to further characterize kinetic properties of purified LDL-TFPI in thrombin generation and chromogenic activity assays. Materials and MethodsLDL-TFPI was purified from human plasma by sequential flotation ultracentrifugation. Thrombin generation was measured in human plasma or in FVIII-immunodepleted plasma using either 1 pM tissue factor and 4μM phospholipids or 0.5nM factor Xa (FXa) and 4μM phospholipids, respectively. ResultsTFPIα prolonged the lag-phase and decreased the thrombin peak in tissue factor-induced thrombin generation, whereas LDL-TFPI exclusively decreased the peak height of thrombin without effecting the lag phase. Steady-state and transient kinetics showed that LDL-TFPI was a more potent inhibitor of FXa than TFPIα and TFPI1–161, indicating that FXa inhibition was not rate determining for the lag phase, whereas it appeared to affect thrombin generation during the propagation phase. This was supported by FXa-induced thrombin generation showing that LDL-TFPI, compared with TFPIα, more actively decreased the peak height. ConclusionsOur results suggest that LDL-TFPI affects thrombin generation during the propagation phase, and is kinetically different from TFPI1–161. It may therefore play a more prominent physiological role in vivo than hereto anticipated from dPT measurements.

Design Of a Most Efficient Inhibitor Of TFPI By Molecular Fusion Of Two Inhibitory Peptides

TFPI is an important inhibitor of the extrinsic coagulation pathway. It efficiently inhibits TF-FVIIa and FXa by quaternary complex formation. Plasma contains various truncated forms of TFPI which are poor inhibitors, and full length (fl)TFPI (0.3 – 0.5 nM) which is the most active TFPI in plasma. flTFPI is released from platelets upon activation, and increases flTFPI concentrations locally up to 30-fold. Most intravascular TFPI (∼80%) is associated with endothelial cells. Both endothelial forms, TFPIa and TFPIb, are similarily effective inhibitors of FX activation on the endothelial cell surface. Inhibition of TFPI in hemophilia models with blocking antibodies, aptamers or peptide inhibitors improves hemostasis and may become an option to treat hemophilia. Recently, we presented peptide inhibitors of TFPI that enhance coagulation in hemophilia models. Two optimized peptides, JBT-A7 and JBT-B5, efficiently blocked inhibitory activity of TFPI and bound to distinct binding sites.We demonstrated the crystal structure of JBT-A7, a linear TFPI inhibitory peptide composed of 20 amino acids, bound to NtermK1 (TFPI 1-83). JBT-B5, a cyclic TFPI inhibitory peptide of 23 amino acids, co-crystallized with TFPI KD1-KD2 (TFPI 22-150). Overlaying the KD1 structure in the KD1-KD2/JBT-B5 and the NTermK1/JBT-A7 complex provided atomic details for linking the two peptide entities. Binding of peptides to TFPI and TFPI fragments was studied by BioCore. The TFPI inhibitory potential of the resulting fusion peptide was tested in model systems (FXa inhibition and TF-FVIIa catalyzed FX activation) and global hemostatic assays (TF-triggered thrombin generation) using hemophilia plasma. To model situations of increased TFPI concentration, both model and plasma assays were carried out at TFPI concentrations up to 10 nM, which is 40-50-fold higher than the physiological flTFPI plasma concentration. To characterize the inhibition of platelet TFPI, we used platelets isolated from blood samples and platelet rich plasma from different donors. Binding of a biotinylated fusion peptide on living HUVE cells was assessed by fluorescence activated cell sorting (FACS) and fluorescence microscopy. Inhibition of cell surface TFPI was analyzed on cultivated HUVECs stimulated with TNFa for TF expression. We monitored FXa generation by the TFPI-dependent cell surface FX activation complex by conversion of an FXa-specific fluorogenic substrate.The overlay of the crystal structures of KD1-KD2/JBT-B5 and the NTermKD1/JBT-A7 complexes revealed non-overlapping epitopes and close proximity of the termini of both peptides. The distance could be bridged by an approximately ten amino acid linker. A fusion peptide with a 10-serine-linker was synthesized and showed highly improved dissociation in Biacore experiments and most efficiently inhibited TFPI activity in the model assays. In contrast, single peptides only partially inhibit TFPI especially at high TFPI concentrations. In thrombin generation assays using hemophilia plasma, the fusion peptide showed a substantially higher ability than the single peptides to increase the thrombin peak even at elevated TFPI. The fusion peptide efficiently inhibited TFPI released from platelets and improved thrombin generation in TFPI deficient plasma reconstituted with platelets as the only source of TFPI released upon platelet activation. The fusion peptide was also shown to bind TFPI on the surface of living HUVECs. This is consistent with its binding epitopes on KD1 and KD2 which result in inhibition of cell surface TFPI in a cell based FX activation assay. Thus, we demonstrate that a molecular fusion peptide most efficiently inhibits all physiologic forms of TFPI.X-ray structures of binary and ternary peptide TFPI complexes provided atomic details for linking two single peptides to generate a fusion peptide that most efficiently blocks TFPI in plasma, released from platelets and associated with endothelial cells. It most efficiently neutralizes TFPI even at substantially elevated concentrations occurring at sites of platelet activation. Our observations support the notion that targeting TFPI with TFPI inhibitors is a promising novel strategy to mitigate the bleeding risk in hemophilia patients. Disclosures:Dockal:Baxter Innovations GmbH, Vienna, Austria: Employment. Hartmann:Baxter Innovations GmbH, Vienna, Austria: Employment. Polakowski:3B Pharmaceuticals, Berlin, Germany: Employment. Brandstetter:Baxter Innovations GmbH, Vienna, Austria: Research Funding. Kammlander:Baxter Innovations GmbH, Vienna, Austria: Employment. Panholzer:Baxter Innovations GmbH, Vienna, Austria: Employment. Redl:Baxter Innovations GmbH, Vienna, Austria: Employment. Osterkamp:3B Pharmaceuticals, Berlin, Germany: Employment. Rosing:Baxter Innovations GmbH, Vienna, Austria: Consultancy, Research Funding. Scheiflinger:Baxter Innovations GmbH, Vienna, Austria: Employment.

Aptamer BAX 499 mediates inhibition of tissue factor pathway inhibitor via interaction with multiple domains of the protein

Tissue factor pathway inhibitor (TFPI) is a multidomain protein that negatively regulates the coagulation cascade. TFPI inhibits the tissue factor (TF)-activated factorVII-activated FX (FXa) complex during TF-mediated coagulation initiation. The aptamer BAX499 binds specifically to TFPI and inhibits its function, mediating a procoagulant effect in both invitro and invivo models of hemophilia. This study sought to identify the regions of TFPI that are critical for BAX499 binding, and to determine how binding mediates aptamer inhibition of TFPI. Invitro biochemical methods were used to evaluate the BAX499 interaction with and inhibition of TFPI. Binding experiments indicated that the full-length TFPI protein is required for tight aptamer binding. Binding-competition experiments implicated the Kunitz1, Kunitz3 and C-terminal domains of TFPI in aptamer binding, a finding that is supported by hydrogen-deuterium exchange experiments, and indicated that aptamer and FXa can bind simultaneously to TFPI. In enzymatic assays, BAX499 inhibited TFPI in a manner that is distinct from domain-specific antibodies, and aptamer inhibitory activity is reduced in the presence of the TFPI cofactor proteinS. These studies demonstrate that BAX499 binds to TFPI via multiple domains of the protein in a manner that is distinct from other TFPI inhibitors, mediating a mechanism of inhibition that does not involve direct competition with FXa. With this unique inhibitory mechanism, BAX499 provides a useful tool for studying TFPI biology in health and disease.

Direct inhibition of factor VIIa by TFPI and TFPI constructs

Tissue factor pathway inhibitor (TFPI) is a multi-Kunitz domain protease inhibitor that down-regulates the extrinsic coagulation pathway by inhibiting FXa and FVIIa. To investigate the role of the three Kunitz domains (KDs) of TFPI in FVIIa inhibition using full-length TFPI (TFPIfl ) and truncated TFPI constructs. Inhibition of FVIIa with/without relipidated tissue factor (TF) or soluble TF (sTF) by TFPIfl /TFPI constructs was quantified with a FVIIa-specific chromogenic substrate. TFPIfl inhibited TF-FVIIa via a monophasic reaction, which is rather slow at low TFPIfl concentrations (t½ ≈5min at 2nm TFPI) and has a Ki of 4.6nm. In the presence of sTF and without TF, TFPIfl was a poor FVIIa inhibitor, with Ki values of 122nm and 1118nm, respectively. This indicates that phospholipids and TF significantly contribute to FVIIa inhibition by TFPIfl . TFPI constructs without the KD3-c-terminus (TFPI1-150 and KD1-KD2) were 7-10-fold less effective than TFPIfl in inhibiting TF-FVIIa and sTF-FVIIa, indicating that the KD3-C-terminus significantly contributes to direct inhibition of FVIIa by TFPI. Compared with KD1-KD2, KD1 was a poor TF-FVIIa inhibitor (Ki =434nm), which shows that the KD2 domain of TFPI also contributes to FVIIa inhibition. Protein S stimulated TF-FVIIa inhibition by TFPIfl (Ki =0.7nm). In the presence of FXa, a tight quaternary TF-FVIIa-TFPI-FXa complex is formed with TFPIfl , TFPI1-150 and KD1-KD2, with Ki values of <0.15nm, 0.5nm and 0.8nm, respectively, indicating the KD3-C-terminus is not a prerequisite for quaternary complex formation. Phospholipids and the Gla-domain of FXa are required for quaternary complex formation.

Plasmatic tissue factor pathway inhibitor is a major determinant of clotting in factor VIII inhibited plasma or blood

Tissue factor pathway inhibitor (TFPI) is a major inhibitor of coagulation. We therefore hypothesised that high plasmatic TFPI levels are associated with impaired ex vivo clotting in a model of acquired haemophilia. Blood samples were collected in a prospective clinical study from 30 healthy volunteers. Coagulation in normal or factor VIII (FVIII)-inhibited human blood or plasma was measured by the calibrated automated thrombogram (CAT) and rotational thromboelastometry (ROTEM). Both methods are global haemostatic assays that provide insight into the whole coagulation process. Monoclonal mouse antibodies raised against either the C-terminus or the Kunitz domain 2 of TFPI were used to determine full-length (fl-) and total TFPI by an enzyme-immunoassay. Clotting times and parameters of thrombin generation correlated with TFPI levels. Subjects with low fl-TFPI levels had significantly shorter clotting times and a higher endogenous thrombin potential (ETP) compared to those with high fl-TFPI levels (p≤0.005 for all). An even stronger effect was seen in FVIII-inhibited blood/plasma: ROTEM clotting time was 26% shorter (p=0.01) and the ETP assessed by CAT was >2-fold higher in subjects with low fl-TFPI levels (p≤0.0001). Plasmatic TFPI is a major determinant of coagulation in global haemostatic tests particularly when FVIII is missing. Thus, inhibition of TFPI might be a promising novel treatment approach, especially in haemophilia patients with FVIII inhibitors.

Structure-Activity Relationship of Pro- and Anticoagulant Effects of Fucus Vesiculosus Fucoidan

AbstractAbstract 3353Fucoidans are sulfated polysaccharides extracted from brown algae. Fucoidans have a wide variety of biological activities including pro- and anticoagulant activities, which occur at different concentration ranges. Therefore, fucoidans have also been described as non-anticoagulant sulfated polysaccharides (NASPs, Liu et al. Thromb Haemost 2006; 95:68). Fucoidans have complex structures due to their large molecular weight (Mw), wide Mw distribution, variable degree and pattern of sulfation, diverse monosaccharide composition, branching of the sugar chain and different monomer linkages. This structural complexity challenges identification of the components responsible for fucoidan activities.The aim of the presented study was to fractionate Fucus vesiculosus (F.v.) fucoidan by size and to de- and oversulfate it to obtain compounds of varying Mw or degree of sulfation (DS), respectively. The fucoidans were then to be analyzed for their pro-and anticoagulant activities and structure, and the effect of modified fucoidan on the target protein Tissue Factor Pathway Inhibitor (TFPI) investigated.Two approaches were applied to generate F.v. fucoidan with different structural properties to the original. Firstly, fucoidan was fractionated by size using ultrafiltration. The Mw of the resulting fractions ranged from 8–180 kD. NMR, elemental analysis and HPAEC showed that other structural features, such as sulfate content and monosaccharide composition, were similar to those of the original fucoidan. Thrombin generation (CAT) assays showed an EC50 for procoagulant activity of 0.3–3 μg/mL; aPTT increased by 50% at 4–25 μg/mL. Generally, higher Mw increased procoagulant activity. Below 15 kD, this activity was markedly reduced. The minimum active length of F.v. fucoidan was 70 carbohydrate units.De- and oversulfated F.v. fucoidans were used to investigate the impact of charge on pro- and anticoagulant activities. In the CAT assay, oversulfated fucoidans showed improved procoagulant activity with an EC50 of 0.09–0.12 μg/mL, while desulfated fucoidans demonstrated reduced procoagulant activity compared to the original. A DS of 0.5 was estimated to be the limit for procoagulant activity. Inhibition of TFPI by fucoidan was assessed with a dilute prothrombin time assay (dPT) with added full-length TFPI. TFPI-blocking activity was mainly dependent on the DS and less on the fucoidans’ Mw. Interestingly, oversulfation also stimulated an undesired activation of the contact pathway.The presented study determined the minimal structural requirements for procoagulant activity of fucoidan molecules and identified features causing undesired biological effects. Disclosures:No relevant conflicts of interest to declare.

Biological Explanation of Clinically Observed Elevation of TFPI Plasma Levels After Treatment with TFPI-Antagonistic Aptamer BAX 499

Abstract 1104BAX 499, an aptamer that binds with a high affinity to TFPI (KD ∼0.1 nM), blocks the anticoagulant action of TFPI in plasma and model systems. BAX 499 was recently tested in a phase 1 safety, tolerability and ex vivo efficacy study in hemophilia patients. The study was prematurely stopped due to an increased number of bleeding events. Baxter subsequently performed several analyses to investigate why clinical safety was inconsistent with preclinical observations.Analysis of plasma levels of full length TFPI (fl-TFPI) and BAX 499, of global hemostatic parameters via thrombin generation (CAT) and thromboelastography (ROTEM) demonstrated that BAX 499 induced a considerable increase in fl-TFPI plasma levels: over 25-fold at the highest dose (72 mg, SubQ). Pharmacodynamic assessment showed that high fl-TFPI concentrations substantially reduce thrombin generation even at excess BAX 499 plasma concentrations (∼1 μM). This strongly suggests that the BAX 499-induced rise in plasma fl-TFPI levels caused the increased bleeding.Assessment of biological mechanisms underlying the increase in fl-TFPI plasma levels showed that BAX 499 can potentially raise plasma levels by 1) increasing the synthesis of TFPI by endothelial cells, 2) affecting the distribution of TFPI which is bound to the endothelial surface, stored in platelets and circulated in plasma, 3) interfering with the metabolism of plasma TFPI, i.e. preventing the conversion of full length to truncated TFPI by proteases and 4) affecting TFPI clearance.BAX 499 treatment of Human Umbilical Vein Endothelial Cells (HUVECs) hardly affected TFPI mRNA production, indicating that BAX 499 has virtually no effect on TFPI synthesis at the transcriptional level.Treatment of HUVECs with BAX 499 stimulated release of TFPI to the cell culture supernatant in a dose dependent manner. FACS analysis showed that BAX 499 has a minimal influence on cell surface TFPI of endothelial cells but, like Heparin, is able to mobilize and release intracellularly-stored TFPI. TFPI released from HUVECs, however, is too low to explain the increased plasma fl-TFPI levels detected in clinical samples.Most TFPI circulating in blood is truncated (80–90%) and bound to lipoproteins. The Lys86-Gln90 region in TFPI is a hot spot for proteolytic cleavage by a wide variety of proteases such as elastase. Cleavage results in the simultaneous removal of TFPI Kunitz 1 domain and loss in activity. BAX 499 (1μM) caused a ∼4-fold decrease in elastase-catalyzed cleavage of TFPI, showing that BAX 499 can delay the proteolytic processing of full length TFPI.BAX 499 was tested in Biacore studies for the interference of fl- TFPI binding to low density lipoprotein receptor-related protein 1 (LRP1), involved in receptor-mediated endocytosis.BAX 499 (1 μM) strongly reduced TFPI binding to LRP1.Pharmacokinetic studies in mice showed that injected human fl-TFPI is rapidly cleared from their circulation and that administration of TFPI with a molar excess of a non-PEGylated variant of BAX 499 resulted in prolonged TFPI clearance. This effect was even more pronounced when TFPI was dosed with PEGylated BAX 499.In conclusion, the elevated TFPI plasma levels in hemophilia patients treated with aptamer BAX 499 can be explained as follows: BAX 499 releases intracellularly-stored TFPI, impacts its metabolism and prolongs the circulatory half life of fl-TFPI, most likely due to binding of BAX 499 to the Kunitz 3-C terminus domain of TFPI, a region required for fl-TFPI clearance. The net result of these effects is elevated plasma fl-TFPI, which even at a molar excess of BAX 499 retains anti-coagulant activity. Disclosures:No relevant conflicts of interest to declare.

Levels of heparin-releasable TFPI are increased in first-ever lacunar stroke patients

New insights in the pathophysiology of lacunar stroke (LS) suggest that it is caused by increased permeability of the blood-brain barrier due to endothelial activation. Because endothelial cells are the major production and storage site of tissue factor pathway inhibitor (TFPI), this protein can be used as marker of endothelial activation. In this observational study we measured the different pools of TFPI, as a marker of endothelial function, in first-ever lacunar stroke patients. We determined antigen levels of total and free full-length (FL) TFPI using ELISA in 149 patients and 42 controls. Heparin-releasable free FL TFPI was determined in a random subset of 17 patients and 15 controls. By brain MRI, we classified LS patients as having isolated lacunar infarct (ILA) or silent ischemic lesions (SILs). Plasma levels of total TFPI were highest in patients with SILs compared with those with ILA, but this association disappeared after correction for age and levels of low-density lipoprotein cholesterol. However, levels of heparin-releasable free FL TFPI were higher in patients than in controls. Although ambient plasma levels of total TFPI were not different in subtypes of LS, the increased levels of heparin-releasable TFPI in patients suggest a role of endothelial activation in the pathogenesis of LS.

The Kunitz 1 and Kunitz 3 domains of tissue factor pathway inhibitor are required for efficient inhibition of factor Xa

Tissue factor pathway inhibitor (TFPI) is a slow tight-binding inhibitor that inhibits factor (F)Xa in a biphasic fashion: a rapid formation of loose FXa·TFPI encounter complex is followed by slow rearrangement into a tight FXa·TFPI* complex in which the Kunitz-2 (K2) domain of TFPI binds and inhibits FXa. In the current study, full-length TFPI (TFPIfl) and various truncated TFPI constructs were used to assess the importance of TFPI domains other than K2 in the inhibition of FXa. In the absence of Ca2+ ions, FXa was more effectively inhibited by TFPIfl than Gla-domain less FXa. In turn, Ca2+ ions impaired FXa inhibition by TFPIfl but not by TFPI constructs that lack the C-terminus. This suggests that, in absence of Ca2+ ions, interactions between the C-terminus of TFPI and the Gla-domain of FXa promote FXa-inhibition. TFPIfl and K2K3 had similar efficiencies for encounter complex formation. However, K2K3 showed monophasic inhibition instead of biphasic inhibition, indicating absence of rearrangement into a tight complex. K1K2 and TFPI1-161 showed biphasic inhibition, but had less efficient encounter complex formation than TFPIfl. Finally, K2K3 was a 10-fold more efficient FXa- inhibitor than K2. These results indicate that K3-C-terminus enhances the formation of encounter complex and that K1 is required for isomerisation of the encounter- into tight complex. Since TFPIfl has a 10-fold higher Ki than K2K3-C-terminus, we propose that K1 is not only required for the transition of the loose to the tight FXa·TFPI* complex, but also inhibits FXa·TFPI encounter complex formation. This inhibitory activity is counteracted by K3 and C-terminus.

Strong Correlation of High Endogenous Plasma Levels of Tissue Factor Pathway Inhibitor (TFPI) with Poor Blood Clotting and Corrective Action of BAX 513 Under FVIII-Inhibited Conditions

Abstract 1201▪▪This icon denotes a clinically relevant abstractTFPI is the major physiological inhibitor of the extrinsic pathway of blood coagulation. Thus, blocking TFPI activity is a possible approach to treat bleeding disorders such as hemophilia A and B. BAX 513, a fucoidan derived from the brown seaweed L. japonica, and other non-anticoagulant sulfated polysaccharides (NASPs) improve coagulation in hemophilic blood and plasma (Liu et al. Thromb Haemost 2006;95:68). This procoagulant activity of NASPs can at least partially be attributed to the inhibition of TFPI. For this study, we predicted that endogenous TFPI levels are associated with decreased hemostatic capacity under hemophilic conditions. We correlated clotting parameters measured by global hemostatic assays with the total and full-length (FL) TFPI levels in human plasma samples in the absence or presence of BAX 513.Tissue factor-triggered coagulation of normal and FVIII-inhibited plasma or blood from 30 healthy subjects was monitored by calibrated automated thrombograph (CAT) assay and rotation thromboelastometry (ROTEM). The experiments were performed with or without factor VIII blockade with a goat anti-human Factor VIII antibody to simulate acquired hemophilia. The total and FL-TFPI plasma levels were quantified by ELISA. We observed longer clotting and clot formation times by ROTEM and decreased thrombin generation in the presence of high TFPI levels in normal plasma (p<0.05). These correlations became substantially stronger when FVIII was inhibited (reaching up to r2= 0.64; p<0.001). In the presence of BAX 513 (0.1–1.2 μg/mL), most plasma samples showed a pro-coagulant response with an at least 50% increase in thrombin peak and a decrease in time to peak. The BAX 513-mediated increase in thrombin generation correlated strongly with the individual FL-TFPI plasma level. The effect was even more apparent when FVIII was inhibited, where the extrinsic pathway is the only driver of thrombin formation.These data indicate that high endogenous TFPI levels critically compromise hemostasis in acquired factor VIII deficiency and that TFPI is a threshold factor for the intrinsic amplification of thrombin formation. As low TFPI levels are associated with increased blood clotting, TFPI inhibition is a rationale treatment approach in patients with hemophilia. Importantly, our findings validate the inhibition of TFPI activity as a mode of action for BAX 513 and related pro-coagulant compounds. Disclosures:Dockal:Baxter Innovations GmbH: Employment. Gorczyca:Medical University of Vienna: Employment. Knappe:Baxter Innovations GmbH: Employment. Palige:Baxter Innovations GmbH: Employment. Ehrlich:Baxter Innovations GmbH: Employment. Scheiflinger:Baxter Innovations GmbH: Employment.