Decreased Thrombin Generation Research Articles

Platelet C3G: a key player in vesicle exocytosis, spreading and clot retraction

C3G is a Rap1 GEF that plays a pivotal role in platelet-mediated processes such as angiogenesis, tumor growth, and metastasis by modulating the platelet secretome. Here, we explore the mechanisms through which C3G governs platelet secretion. For this, we utilized animal models featuring either overexpression or deletion of C3G in platelets, as well as PC12 cell clones expressing C3G mutants. We found that C3G specifically regulates α-granule secretion via PKCδ, but it does not affect δ-granules or lysosomes. C3G activated RalA through a GEF-dependent mechanism, facilitating vesicle docking, while interfering with the formation of the trans-SNARE complex, thereby restricting vesicle fusion. Furthermore, C3G promotes the formation of lamellipodia during platelet spreading on specific substrates by enhancing actin polymerization via Src and Rac1-Arp2/3 pathways, but not Rap1. Consequently, C3G deletion in platelets favored kiss-and-run exocytosis. C3G also controlled granule secretion in PC12 cells, including pore formation. Additionally, C3G-deficient platelets exhibited reduced phosphatidylserine exposure, resulting in decreased thrombin generation, which along with defective actin polymerization and spreading, led to impaired clot retraction. In summary, platelet C3G plays a dual role by facilitating platelet spreading and clot retraction through the promotion of outside-in signaling while concurrently downregulating α-granule secretion by restricting granule fusion.

Decreased Thrombin Generation is Associated with Increased Thrombin Generation Biomarkers and Blood Cellular Indices in Pulmonary Embolism.

Pulmonary embolism (PE) is a heterogenous condition with variable clinical presentations. Thrombin generation potential (TGP) and biomarkers, and blood cellular indices can reflect the underlying pathophysiology and risk stratification of PE. This case-control study analyzed TGP in 209 PE patients from Loyola University, Pulmonary Embolism Response Team program compared to normal human plasma (NHP) controls. The present study evaluates TGP and biomarkers, and cellular indices in relation to PE severity, according to the European Society of Cardiology (ESC) guidelines. Statistical analysis including median with interquartile range (IQR), 2-tailed Wilcoxon Mann-Whitney test, Chi-square test, and Spearman Correlational analysis were performed. There were 209 patients with PE, with an almost equal distribution between sex, and a median age of 63 years. Significant downregulation in peak thrombin and endogenous thrombin potential (ETP), as well as upregulation in lag time, were observed in PE patients versus controls. Biomarker analysis revealed pronounced elevations, with D-dimer demonstrating the most significant increase. Blood cellular indices also rose in PE patients, correlating with disease severity. PE severity was associated with higher TGP and biomarker levels. Mortality rates differed significantly across risk categories and were highest in patients with elevated cellular indices. TGP and biomarkers are intricately linked to PE severity and can aid in risk stratification. Elevated cellular indices are associated with increased mortality, highlighting their potential as prognostic markers. These findings could enhance the precision of PE management strategies.

Low molecular weight heparin decreases pro-coagulant activity in clinical MSC products

Background aimsMesenchymal stromal cells (MSCs) are multipotent adult cells that can be isolated from tissues including bone marrow [MSC(BM)], adipose [MSC(AT)] and umbilical cord [MSC(CT)]. Previous studies have linked expression of tissue factor (TF) on MSC surfaces to a procoagulant effect. Venous thromboembolism (VTE), immediate blood-mediated inflammatory reaction (IBMIR) and microvascular thrombosis remain a risk with intravascular MSC therapy. We examined the effect of low molecular weight heparin (LMWH) on clinical-grade MSCs using calibrated automated thrombography (CAT). MethodsClinical grade MSC(BM)s, MSC(AT)s and MSC(CT)s harvested at passage 4 were added to normal pooled plasma (NPP) to a final concentration of either 400 000 or 50 000 cells/mL. LMWH was added to plasma in increments of 0.1 U/mL. Thrombin generation (TG) was measured using CAT. Flow cytometry was conducted on the cells to measure MSC phenotype and TF load. ResultsPresence of MSCs decreased lag time and increased peak TG. All cell lines demonstrated a dose response to LMWH, with MSC(AT) demonstrating the least thrombogenicity and most sensitivity to LMWH. TG was significantly reduced in all cell lines at doses of 0.2 U/mL LMWH and higher. DiscussionAll MSC types and concentrations had a decrease in peak thrombin and TG with increasing amounts of LMWH. While this in vitro study cannot determine optimal dosing, it suggests that LMWH can be effectively used to lower the risk of VTE associated with intravascular administration of MSCs. Future in vivo work can be done to determine optimal dosing and effect on IBMIR and VTE.

The Anticoagulant Systems Cooperate Synergistically on Phospholipid Vesicle and Endothelial Cell Membranes, but Not on Platelets

Introduction: Thrombin, the enzyme which converts fibrinogen into a fibrin clot, is produced by the prothrombinase complex, composed of factor Xa (FXa) and factor Va (FVa). Down-regulation of this process is critical, as excess thrombin can lead to life threatening thrombotic events. FXa and FVa are inhibited by the anticoagulants tissue factor pathway inhibitor alpha (TFPIα) and activated protein C (APC), respectively, and their common cofactor protein S (PS). TFPIα, APC, and PS function synergistically on the surface of phospholipid vesicles (PLs). We have also recently reported a shear-dependent interaction between PS and von Willebrand factor (VWF), which partially blocks its anticoagulant function. Here, we investigated the function of the TFPIα/APC/PS anticoagulant system on physiologic membrane surfaces (platelets and endothelial cells, or ECs), and the effect of VWF on prothrombinase inhibition in these different membrane environments. Methods: Thrombin generation was measured in plasma and platelet-rich plasma (PRP) by calibrated automated thrombography, purified prothrombinase activity was assayed using a chromogenic substrate, and FVa cleavage was monitored by immunoblotting. Experiments were performed with synthetic PLs, washed platelets, or cultured EA.hy926 cells, an EC line. Experiments involving VWF were performed in the presence or absence of shear. Results: In plasma TG, as we have previously published, PS anticoagulant activity was greatly enhanced by addition of thrombomodulin (TM), to promote protein C (PC) activation. TM decreased thrombin generation in a PS-dependent manner. The effect of TM and PS was enhanced by the addition of TFPIα, particularly at low PS concentrations. Conversely, inhibitory antibodies against TFPI and PC had an additive effect on plasma TG which was mimicked by inhibiting PS alone. Similar results were obtained in plasma supplemented with ECs (which provide TM) or in PRP. However, in PRP, the antibodies reversed TG beyond what was seen with tissue factor-initiation alone. In purified protein systems, performed using either PLs or ECs, physiologic TFPIα concentrations had no detectable effect on thrombin activation by prothrombinase, unless APC was present in the system. The greatest inhibitory activity was observed when TFPIα, APC, and PS were all present, and was similar on ECs and PLs. Similarly, inhibition of FXa by TFPIα was found to render FVa susceptible to proteolysis by APC on either PLs or ECs. However, prothrombinase assembled on activated platelets was protected from inhibition, even in the presence of TFPIα, APC, and PS, as measured either by thrombin activation or FVa cleavage. Finally, we assessed the functional consequences of the PS/VWF interaction. In purified systems with PLs, sheared VWF dose-dependently reversed FXa inhibition by PS/TFPIα, measured using a chromogenic FXa substrate. Conversely, APC cofactor function, as measured by FVa cleavage, was unaffected on either PLs or activated platelets. Consistent with this, thrombin activation by prothrombinase was unaffected by sheared VWF on these surfaces. Interestingly, sheared VWF promoted prothrombinase function on ECs, increasing the rate of thrombin activation beyond that observed in the absence of the anticoagulants. Conclusions: We propose a model of prothrombinase inhibition through combined targeting of both FXa and FVa, and that this mechanism enables down-regulation of thrombin activation outside of a platelet clot. On PL or EC membranes in the plasma environment, APC, TFPIα, and PS cooperate synergistically to inhibit prothrombinase activity, preventing excessive clot propagation. Conversely, platelets protect prothrombinase from inhibition, supporting a procoagulant environment within the clot. The anticoagulant system appears to function similarly on PLs and on ECs, except that VWF has greater effect on ECs, suggesting that the protection mediated by platelets is due to a platelet component not present in ECs. We hypothesize that these regulatory mechanisms serve to localize procoagulant activity within the platelet plug and suppress it on ECs downstream of a clot.

BIO15: Inhibition of Leukocytes Decreases Thrombin Generation in a In Vitro ECMO model

Background: Despite systemic anti-coagulation, blood exposure to extracorporeal membrane oxygenation circuits (ECMO) generates excess thrombin, often resulting in thrombus formation, device failure, and life-threatening thromboembolism. Activated leukocytes and platelets promote ECMO thrombosis under controlled static and flow conditions. Previous studies document that phosphodiesterase type 4 inhibitor (PDE4) can selectively inhibit the pro-thrombotic function of leukocytes. Our hypothesis is that roflumilast (PDE4 inhibitor) decreases thrombin generation in an in vitro ECMO model using human blood. Methods: Simulated neonatal roller pump with a KIDS D100 oxygenator circulated human whole blood at a nominal flow rate (0.5 L/min) for six hours or until clot formation caused device failure. Samples were collected hourly from the ECMO model that received heparin alone or heparin plus roflumilast, a PDE4 inhibitor. Expression of pro-thrombotic proteins on leukocytes and extracellular vesicles (EV) was measured with high-resolution flow cytometry. A calibrated automated thrombogram without a trigger measured differences in time to thrombin and peak thrombin generation. Results: The addition of roflumilast to heparin sustained patency in 75% of tested circuits (3/4) after six hours, compared to 28% of circuits treated with heparin alone (2/7). Roflumilast decreased extracellular leukocytes expressing tissue factor. Roflumilast plus heparin prolonged time to thrombin generation at 13.9 minutes, compared to heparin alone at 6.8 minutes at the 3-hour time point (p<0.04). The addition of roflumilast significantly reduced peak thrombin generation compared to heparin alone (69 vs. 207 nm, p<0.02). Conclusions: Inhibition of leukocytes may be an effective adjunct to systematic anti-coagulation in extracorporeal life support.

Impact of Transfused Citrate on Pathophysiology in Massive Transfusion.

A limited library of curated articles was created using search terms including "citrate intoxication," "citrate massive transfusion," "citrate pharmacokinetics," "hypocalcemia of trauma," "citrate phosphate dextrose," and "hypocalcemia in massive transfusion." Review articles, as well as prospective and retrospective studies were selected based on their relevance for inclusion in this review. Given the limited number of relevant studies, studies were reviewed and included if they were written in English. This is not a systematic review nor a meta-analysis. As this is not a meta-analysis, new statistical analyses were not performed. Relevant data were summarized in the body of the text. The physiologic effects of citrate independent of hypocalcemia are poorly understood. While a healthy individual can rapidly clear the citrate in a unit of blood (either through the citric acid cycle or direct excretion in urine), the physiology of hemorrhagic shock can lead to decreased clearance and prolonged circulation of citrate. The so-called "Diamond of Death" of bleeding-coagulopathy, acidemia, hypothermia, and hypocalcemia-has a dynamic interaction with citrate that can lead to a death spiral. Hypothermia and acidemia both decrease citrate clearance while circulating citrate decreases thrombin generation and platelet function, leading to ionized hypocalcemia, coagulopathy, and need for further transfusion resulting in a new citrate load. Whole blood transfusion typically requires lower volumes of transfused product than component therapy alone, resulting in a lower citrate burden. Efforts should be made to limit the amount of citrate infused into a patient in hemorrhagic shock while simultaneously addressing the induced hypocalcemia.

IVIg treatment increases thrombin activation of platelets and thrombin generation in paediatric patients with immune thrombocytopenia.

Clinical manifestations and laboratory parameters of haemostasis were investigated in 23 children with newly diagnosed immune thrombocytopenia (ITP) before and after intravenous immunoglobulin (IVIg) treatment. ITP patients with platelet counts of less than 20 × 109 /L and mild bleeding symptoms, graded by a standardized bleeding score (BS), were compared with healthy children with normal platelet counts and children with chemotherapy-related thrombocytopenia. Markers of platelet activation and platelet apoptosis in the absence and presence of platelet activators were analysed by flow cytometry; thrombin generation in plasma was determined. ITP patients at diagnosis presented with increased proportions of platelets expressing CD62P and CD63 and activated caspases, and with decreased thrombin generation. Thrombin-induced activation of platelets was reduced in ITP compared with controls, while increased proportions of platelets with activated caspases were observed. Children with a higher BS had lower proportions of CD62P-expressing platelets compared with those with a lower BS. IVIg treatment increased the number of reticulated platelets, the platelet count to more than 20 × 109 /L and improved bleeding in all patients. Decreased thrombin-induced platelet activation, as well as thrombin generation, were ameliorated. Our results indicate that IVIg treatment helps to counteract diminished platelet function and coagulation in children with newly diagnosed ITP.

Inter-α-inhibitor heavy chain H4 and sepsis-related coagulation disturbances: Another link between innate immunity and coagulation

BackgroundThe protease inhibitor inter-α-inhibitor heavy chain H4 (ITIH4) has been described as an acute-phase reactant and could potentially aid in sepsis monitoring and prognostication. ObjectivesTo investigate ITIH4 plasma levels in sepsis patients compared with healthy controls and to examine the association between ITIH4 and acute-phase response markers, blood coagulation, and organ dysfunction in sepsis. MethodsWe performed a post hoc study to a prospective cohort study. Patients with septic shock (n = 39) were enrolled upon intensive care unit admission. ITIH4 was analyzed using an in-house immunoassay. Standard coagulation parameters, thrombin generation, fibrin formation and lysis, C-reactive protein, organ dysfunction markers, Sequential Organ Failure Assessment score, and disseminated intravascular coagulation (DIC) score were registered. ITIH4 levels were also investigated in a murine Escherichia coli sepsis model. ResultsITIH4 did not display acute-phase behavior as mean ITIH4 levels were not increased in patients with septic shock or in E. coli-infected mice. However, ITIH4 exhibited large interindividual variation in patients with septic shock compared with healthy controls. Low ITIH4 was associated with sepsis-related coagulopathy, including a high DIC score (mean ITIH4: DIC, 203 μg/mL vs non-DIC, 267 μg/mL, P = .01), low antithrombin (r = 0.70, P < .0001) and decreased thrombin generation (mean ITIH4: first peak thrombin tertile, 210 μg/mL vs third peak thrombin tertile, 303 μg/mL, P = .01). ITIH4 showed moderate correlation with arterial blood lactate (ρ = -0.50, P < .001) but only weak correlations with C-reactive protein, alanine transaminase, bilirubin, and Sequential Organ Failure Assessment score (all, ρ < 0.26, P > .05). ConclusionITIH4 is associated with sepsis-related coagulopathy but is not an acute-phase reactant during septic shock.

In patients with hemophilia, a decreased thrombin generation profile is associated with a severe bleeding phenotype

BackgroundHeterogeneity in clinical bleeding phenotype has been observed in hemophilia patients with similar FVIII or FIX activity levels. Thrombin generation and plasmin generation, as a global hemostasis assay, may contribute to a better prediction of which patients are at an increased risk of bleeding. ObjectivesThe objective of this study was to describe the association between clinical bleeding phenotype and thrombin generation and plasmin generation profiles in patients with hemophilia. MethodsThe Nijmegen Hemostasis Assay, which simultaneously measures thrombin and plasmin generation, was performed in plasma samples of patients with hemophilia participating in the sixth Hemophilia in the Netherlands study (HiN6). Patients receiving prophylaxis underwent a washout period. A severe clinical bleeding phenotype was defined as a self-reported annual bleeding rate of ≥5, a self-reported annual joint bleeding rate of ≥3, or the use of secondary/tertiary prophylaxis. ResultsIn total, 446 patients, with a median age of 44 years, were included in this substudy. Thrombin generation and plasmin generation parameters differed between patients with hemophilia and healthy individuals. The median thrombin peak height was 1.0 nM, 25.9 nM, 47.1 nM, and 143.9 nM in patients with severe, moderate, and mild hemophilia and healthy individuals, respectively. A severe bleeding phenotype was observed in patients with a thrombin peak height of <49% and a thrombin potential of <72% compared to healthy individuals, and was independent of the hemophilia severity. The median thrombin peak height was 0.70% in patients with a severe clinical bleeding phenotype and 30.3% in patients with a mild clinical bleeding phenotype. The median thrombin potentials for these patients were 0.06% and 59.3%, respectively. ConclusionA decreased thrombin generation profile is associated with a severe clinical bleeding phenotype in patients with hemophilia. Thrombin generation in combination with bleeding severity may be a better tool to personalize prophylactic replacement therapy irrespective of hemophilia severity.

Thrombin Generation Decrease After LMWH Administration in an Antithrombin-Deficient Pregnant Woman With a Homozygous HBS II Mutation.

The cases of antithrombin (AT)-deficient pregnant women with a homozygous HBS II mutation are relatively rare and are accompanied by an increased thrombophilic risk, which is manifested by increased thrombin generation (TG). It is very difficult to ensure their prophylactic treatment during pregnancy. We aimed to determine the utility of the thrombin generation assay (TGA) and anti-factor Xa (anti-FXa) test to monitor the effects of a prophylactic dose of low-molecular-weight heparin (LMWH) in a 28-year-old woman with homozygous AT deficiency caused by mutation c.391C > T#, (p.Leu131Phe†) in the SERPINC1 gene and to compare the findings with those from a group of pregnant and non-pregnant women also treated with LMWH. TG monitoring was chosen due to severe AT deficiency that was manifested by low levels of anti-FXa activity when monitoring the efficacy of LMWH treatment. A significant decrease in TG was detected in all monitored groups (P < .05). There were no thrombotic complications during the whole pregnancy of the woman with AT deficiency. Consistent monitoring of TG with LMWH anticoagulant therapy administration during pregnancy together with AT administration before and after delivery may improve the overall condition of pregnant women and the quality of their care.

Antithrombin-Specific RNA Aptamers As a Potential Therapy to Restore Hemostatic Balance in Hemophilia Patients

Hemophilia A and B are inherited blood bleeding disorders due to insufficient blood coagulation factors. Individuals with hemophilia A have a deficiency in factor VIII, while individuals with hemophilia B have a deficiency in factor IXa. Both of these factors are important in the intrinsic pathway of blood coagulation. In the absence of these factors, hemophilia patients are prone to bleeding. Thrombin, the primary factor in the blood coagulation pathway, promotes blood clotting and prevents bleeding. Hemophilia patients develop insufficient thrombin, resulting in an imbalance between the procoagulant and anticoagulant systems. Thus, generating more thrombin activity in these patients is an ideal therapeutic approach. We propose to increase thrombin's activity by inhibiting the primary inhibitor of thrombin and the natural anticoagulant protein, antithrombin (AT), using RNA molecules (aptamers) that bind to their target protein with high affinity and specificity. Several aptamers are evaluated in clinical trials as therapies for various blood diseases. We hypothesized that AT-specific aptamers could inhibit AT activity by preventing AT from forming a high molecular weight complex with thrombin, resulting in an overall increase in thrombin activity and restoring hemostatic equilibrium. The main goal is to design novel therapeutic molecules that promote thrombus formation in hemophilia patients. Using Systematic evolution of ligands by exponential enrichment (SELEX), we developed a library of RNA aptamers molecules that bind to AT with high to moderate affinity. We assessed the individual aptamers' ability to inhibit AT and increase thrombin activity using various in vitro methods. Three individual aptamers inhibited AT activity by preventing AT from forming a complex with thrombin in the presence and absence of heparin, resulting in a significant increase in thrombin activity. We also showed a decrease in thrombin generation using an ex vivo thrombin generation assay in the presence of aptamers. Our data illustrate that AT-specific RNA aptamers can inhibit AT activity and increase thrombin activity. Thus, these molecules could potentially be used to treat patients with both hemophilia A and B. Additional studies are being conducted to assess their ability to increase thrombin activity in a pre-clinical model.

Elevated Thrombin Generation Biomarkers and Their Relevance with Decreased Thrombin Generation Potential in Patients with Pulmonary Embolism

Background: Pulmonary embolism (PE) is a catastrophic complication in patients with venous thrombosis and is the third leading cardiovascular cause of death after myocardial infraction and stroke. Inflammation, hemostatic dysregulation, and generation of thrombotic mediators contribute to the pathogenesis of PE. Persistent thrombin generation contributes to the overall pathology of PE. The purpose of this study is to investigate endogenous thrombin potential (ETP) in PE patients and demonstrate its relationship with thrombin generation biomarkers, such as prothrombin fragment 1+2 (F1+2), thrombin anti-thrombin complex (TAT) and D-Dimer. Methods: Citrated blood samples from 201 patients with confirmed PE were collected under an IRB approved protocol. Fifty normal human plasma (NHP) samples were used as a reference. Thrombin generation studies were carried out by using fluorogenic substrate method (calibrated automated thrombogram, Diagnostica Stago; Paris, France). Thrombin generation parameters such as peak thrombin, lag time and endogenous thrombin potential (ETP) were compiled. Plasma samples were analyzed for such biomarkers as prothrombin fragment 1+2 (F1+2), thrombin anti-thrombin complex (TAT), and D-Dimer by using commercially available Sandwich ELISA methods. Results were compiled as mean ± SD and applicable statistical analysis was performed by using GraphPad Prism software. Result: PE patients exhibited a wide variation in thrombin generation parameters. ETP in PE patients decreased to 549.2±508.4 nM*min compared to NHP 701.8±214.6 nM*min, thrombin levels in PE patients also decreased to 83.7±80.1 nM compared to NHP 158.5±31.5 nM, while lag time increased 5.4±12.7 mins to 2.6±0.6 mins compared to NHP. Biomarker analysis showed elevated levels of F1+2 (972.3±1508.0 vs 294.2±105.0 pmol/L, 3.3-fold), TAT (34.9±54.3 vs 2.3±1.1ug/L, 15.3-fold), and D-Dimer (6720.0±4810.0 vs 182.6±205.8 ug/ml, 36.8-fold) compared to NHP. Endogenous thrombin potential levels demonstrate moderate and positive correlations with F1+2 (r = 0.22) while weak correlations were noted with TAT (r = 0.14) and D-Dimer (r = 0.05). F1+2 possess strong correlation with TAT (r = 0.55) and moderate for D-Dimer (r = 0.21). TAT also showed strong correlation with D-Dimer (r = 0.45). Thrombin generation parameters and biomarker levels based on PE severity has been shown in figure 1. Marked increase in thrombin generation biomarker levels were noted with increase PE severity. However, for the thrombin generation parameters, there is no differences were noted between the low grade and sub-massive PE, while the ETP and peak thrombin levels decreased, and lag time increased for the massive PE. Conclusion: These studies suggest that persistent thrombin generation in PE patients results in the consumption of prothrombin complex. However, such biomarkers of thrombin generation as F1+2, TAT and D-Dimer are formed, leading to higher circulating levels. Simultaneous measurement of thrombin generation biomarkers and thrombin generation parameters may be useful to determine the pathogenesis of thrombotic complications and optimization of treatment options. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Changes in fibrinolytic activity and coagulation factors after epicardial left atrial appendage closure in patients with atrial fibrillation.

The left atrial appendage (LAA) is known to be the primary source of thrombus formation in atrial fibrillation (AF). We investigate whether epicardial LAA occlusion (LAAO) from the cardiovascular system has an effect on coagulation and prothrombotic status in AF. Twenty-two patients with nonvalvular AF, who were not currently receiving oral anticoagulation (OAC) therapy, participated in a single-center prospective study. We measured fibrinogen and plasminogen levels along with plasma fibrin clot permeability, clot lysis time (CLT) and endogenous thrombin potential (ETP) before the LAAO procedure, at discharge and 1 month afterward. One month after the LAAO procedure, plasma fibrin clot permeability improved by 39.3% as measured by clots prepared from peripheral blood (P=0.019) and also after adjustment for fibrinogen (P=0.027). Higher plasma fibrin clot permeability was associated with improved clot susceptibility to lysis (r=-0.67, P=0.013). CLT was reduced by 10.3% (P=0.0020), plasminogen activator inhibitor-1 antigen levels were reduced by 52% (P=0.023) and plasminogen activity was increased by 8.9% (P=0.0077). A trend toward decreased thrombin generation, reflected by a decreased ETP and peak thrombin generated was also observed 1 month after LAAO procedure (P=0.072 and P=0.087, respectively). No differences were observed in tissue-type plasminogen activator and thrombin-activatable fibrinolysis inhibitor plasma levels (both P>0.05). Obtained results seem to confirm that LAA plays a key role in thrombogenesis. Elimination of LAA from the circulatory system may improve fibrin clot permeability and susceptibility to fibrinolysis in peripheral blood.

Anticoagulation Monitoring with Activated Partial ThromboPlastin Time and Anti-Xa Activity in Intensive Care Unit Patients: Interest of Thrombin Generation Assay.

Current guidelines recommend monitoring the anticoagulant effect of unfractionated heparin (UFH) by measuring anti-Xa activity rather than activated partial thromboplastin time (aPTT) in intensive care unit (ICU) patients. The primary objective of this study was to evaluate the correlation of aPTT, anti-Xa activity, and thrombin generation in UFH-treated ICU patients. A prospective observational pilot study was conducted in adult surgical ICU patients treated with UFH. aPTT and anti-Xa activity were monitored daily. The therapeutic target was aPTT between 50 s and 84 s, and/or anti-Xa between 0.3 and 0.7 U/mL. Correlation among aPTT, anti-Xa activity, and thrombin generation was determined by measuring endogenous thrombin potential (ETP), with the inflammatory response evaluated. C-reactive protein (CRP) was used as a marker of inflammatory response. The plasma of 107 samples from 30 ICU patients was analyzed. The correlation between aPTT and anti-Xa activity was 0.66, CI95% [0.54;0.76] (p < 0.0001). Although thrombin generation, aPTT, and anti-Xa were correlated with inflammatory responses, the correlation was higher with thrombin generation and anti-Xa activity compared to aPTT. When aPTT was in a therapeutic range, a low thrombin generation was observed but was 50% inhibited when anti-Xa was in a therapeutic range. Coagulation testing with aPTT, anti-Xa correlated with thrombin generation. A 50% decrease in thrombin generation was observed when anti-Xa was within a therapeutic range. Further work is needed to evaluate coagulation biomarker responses and clinical outcomes in specific ICU populations.

Statins in prevention of thromboembolic events: from seminal studies to recent advances.

Statins, 3‑hydroxy‑methylglutaryl coenzyme A reductase inhibitors, reduce the rate of cardiovascular events in primary and secondary prevention, and represent a cornerstone in the management of atherosclerotic vascular disease. Statins were also shown to be effective in lowering the risk of venous thromboembolism (VTE) in its primary and secondary prevention, but their use in VTE prophylaxis is still not approved and recommended in current guidelines. Several antithrombotic mechanisms of these cholesterol‑lowering agents, largely independent of the magnitude of low‑ density lipoprotein cholesterol reduction, were demonstrated in a broad spectrum of experimental in vitro and in vivo models. However, these studies yielded inconsistent results, such as substantial differences related to the use of specific statins, their dose or final concentration, and even the clinical setting (patients with cardiovascular disease or VTE vs healthy subjects). Anticoagulant properties of statins, reported for the first time 25 years ago, involve downregulation of tissue factor expression with a subsequent decreased thrombin generation and inhibition of thrombin‑mediated reactions, including factor V and factor XIII activation, and enhanced endothelial thrombomodulin expression resulting in increased protein C activation. Enhanced fibrinolysis was also reported partly as a result of reduced activity of fibrinolysis inhibitors such as plasminogen activator inhibitor‑1 (PAI‑1) and thrombin‑activatable fibrinolysis inhibitor. This review summarizes the findings of the studies from the 1990s until the most recent reports to update our knowledge on the impact of statins on blood coagulation and its potential clinical relevance.

Factor XII deficiency evaluated by thrombin generation assay

IntroductionCoagulation factor XII (FXII) plays a role in thrombin generation, fibrinolysis, inflammation, angiogenesis, chemotaxis and diapedesis. FXII deficiency is not associated with bleeding risk unlike other coagulation factors. Materials/methodsWe investigated thrombin generation assay (TGA) profile modification in FXII deficiency and the correlation with TGA and deficiency severity. TGA was performed in platelet poor plasma (PPP) with tissue factor (1 pmol/L) and phospholipid (4 µmol/L) standardized concentration. Thrombin generation profiles were compared in 54 patients with FXII deficiency, 25 healthy controls and 23 patients with hemophilia A (factor VIII (FVIII) deficiency. Patients with FXII deficiency were classified in three groups based on FXII activity (30–50%, 10–29%, <10%). FVIII deficiency was included as a bleeding control group. ResultsAs expected, we found a correlation between FXII deficiency and activated partial thromboplastin time (aPTT). A decrease of thrombin generation was observed in healthy controls and all FXII deficiency groups. A decrease of endogenous thrombin potential (ETP), peak and velocity was observed in patients with FVIII deficiency compared to FXII deficiency. A decrease of thrombin generation was noted in patients with FXII deficiency and bleeding history compared to patients with FXII deficiency and thrombosis history. ConclusionIn this study, thrombin generation profiles were not sensitive to FXII deficiency. TGA could distinguish bleeding and thrombotic tendency in FXII deficiency. Our results should therefore be considered as exploratory and deserve confirmation.

A pilot study evaluating the Calibrated Automated Thrombogram assay and application of plasma-thromboelastography for detection of hemostatic aberrations in horses with gastrointestinal disease

BackgroundCritically ill horses, such as horses with gastrointestinal (GI) disease, often suffer from hemostatic aberrations. Global hemostatic tests examining the initiation of coagulation, clot strength and fibrinolysis, such as the Calibrated Automated Thrombogram (CAT) and plasma-thromboelastography (TEG) have not been evaluated in horses. This study aimed to evaluate CAT and apply plasma-TEG in horses.Test performance of CAT was evaluated on equine platelet poor plasma with intra- and inter-assay variability (CV) and a heparin dilution curve. To examine clinical performance of both tests, group comparisons were assessed comparing healthy horses, horses with mild and severe GI disease with both CAT and plasma-TEG.ResultsFor CAT, intra- and inter-assay CVs were established for lag-time (1.7, 4.7%), endogenous thrombin potential (1.6, 4.6%), peak (2.6, 3.9%) and time to peak (ttPeak) (1.9, 3.4%). Increasing heparin concentrations led to the expected decrease in thrombin generation. In the group comparison analysis, CAT showed significant higher peak (p = 0.04) and ttPeak (p = 0.008) in the severe GI disease group compared to horses with mild GI disease and healthy horses, respectively. Plasma-TEG showed an increased angle (p = 0.032), maximum amplitude (p = 0.017) and shear elastic force (G) (p = 0.017) in the severe GI disease group compared to healthy horses.ConclusionsCAT performed well in horses. Both CAT and plasma-TEG identified hemostatic aberrations in horses with severe GI disease compared to healthy horses. Further studies including more horses, are needed to fully appreciate the use of CAT and plasma-TEG in this species.

Resistance to activated protein C and impaired TFPI activity in women with previous hormone-induced venous thromboembolism

IntroductionHormonal contraception is a well-known risk factor for venous thromboembolism (VTE). APC resistance and impaired functions of protein S and TFPI are thought to play an important role in the pathogenesis of hormone-related VTE. It is unknown, whether women, who develop VTE during hormonal contraception possess a vulnerability in these pathways, making them susceptible to thrombosis. Materials and methodsPlasma samples were obtained from 57 premenopausal women in average 15.3 years after hormone-associated VTE and from 31 healthy controls. Thrombin generation at high tissue factor (TF) in the absence and in the presence of activated protein C (APC) and at low TF without and with inhibiting anti-protein S- and anti-TFPI-antibodies was measured via calibrated automated thrombography. ResultsWomen with previous hormone-related thrombosis had higher thrombin generation at low TF, higher APC resistance, protein S- and TFPI ratios, differences: 219.9 nM IIa.min (95%CI:90.4 to 349.3); 1.88 (95%CI:0.71 to 3.05); 0.13 (95%CI:0.01 to 0.26) and 0.19 (95%CI:0.08 to 0.30), respectively. Thrombin generation at high TF without APC did not differ between the groups. Smoking decreased thrombin generation at low TF by −222.6 nM IIa.min (95%CI: −381.1 to −64.1), the APC sensitivity ratio by −2.20 (95%CI: −3.63 to −0.77) and the TFPI ratio by −0.16 (95%CI: −0.29 to −0.03), but did not influence thrombin generation at high TF. DiscussionWe demonstrated impairment of the protein S/TFPI system and increased APC resistance in women with previous hormone-induced VTE. Smoking decreased thrombin generation at assay conditions, dependent on the function of the TFPI system.

Regulation of Factor VIII By Activated Protein C: Assessment In Vitro and In Vivo

Factor VIII (FVIII) plays an essential role in the activation of factor X (FX), acting as a factor IXa (FIXa) cofactor in the intrinsic Xase complex. Activated protein C (APC) is a serine protease that inactivates FV/Va. Resistance of FV/FVa to APC due to the Leiden mutation is the most common inherited thrombophilia. By cleaving FVIII/FVIIIa at Arg336 and Arg 562, APC can also inactivate FVIII; however, the role of this reaction, especially in vivo, is not well defined. Further, thrombophilic mutations at either of these sites have yet to be identified, calling into question the importance of this reaction. Understanding the role of APC in FVIII regulation is important as there are several novel therapeutics in development for hemophilia A (HA) that either bypass FVIII regulatory pathways or target inhibition of APC. Additionally, while data using a high-specific activity factor IX variant has been successful in a hemophilia B gene transfer trial, there is no such described hyperactive FVIII protein. In principle, manipulation of APC cleavage sites may result in a more procoagulant FVIII protein with enhanced hemostastic function that may serve as a novel therapeutic for HA in the setting of a recombinant protein treatment or gene therapy.We sought to further understand the role of APC regulation of FVIII and to evaluate the in vivo hemostatic effect of an APC-resistant FVIII as a potential treatment strategy for HA. We conducted site-directed mutagenesis to introduce missense arginine to glutamine mutations at FVIII APC cleavage sites (R336Q and R562Q) on a B-domain deleted FVIII (FVIII-WT) backbone. The resulting recombinant, purified FVIII variant protein (FVIII-QQ) was then compared to FVIII-WT. Following incubation with APC, SDS-PAGE analysis of FVIII-QQ demonstrated no apparent cleavage by APC. In contrast, FVIII-WT demonstrated expected cleavage at R336 and R562, thereby confirming FVIII-QQ is resistant to APC cleavage. Surprisingly, one-stage clotting assay determination of FVIII-QQ specific activity was approximately 30% higher than FVIII-WT; this was unexpected given the absence of thrombomodulin or APC. We speculate this may be related to the ability of FXa to cleave FVIIIa at Arg336 and Arg562 inactivating FVIIIa that is likely disrupted in the FVIII-QQ variant (Eaton D et al. Biochemistry 1986 and Plantier JL et al. J Thromb Haemost 2010). To test the role of APC regulation of FVIII, we performed plasma-based in vitro thrombin generation assays in the presence of APC. Human FVIII deficient plasma was reconstituted to 1 nM (normal plasma FVIII concentration) with FVIII-QQ or FVIII-WT wherein increasing concentrations of APC were then added and thrombin generation was measured. Consistent with the known role of APC being an anticoagulant, the amount of thrombin generation decreased with increasing APC concentrations. However, the amount of APC necessary to inhibit 50% peak thrombin generation with FVIII-WT was 5.2 nM ± 0.3 nM versus FVIII-QQ was 11.3 nM ± 0.8 nM, which again demonstrated FVIII-QQ is resistant to APC inhibition. We suspect the observed decrease in thrombin generation with increasing APC concentrations is due to the role of APC in FV/FVa inhibition. Overall, our data support a role for APC inactivation of FVIIIa in this in vitro system. Interestingly, FVIII-QQ reliably demonstrated an approximate 2-fold increase in peak thrombin in the thrombin generation assay. This is being explored, but we again speculate this may be related to the effects of FXa on FVIII-WT at R336 and R562.We next evaluated the in vivo hemostatic efficacy of FVIII-WT vs. FVIII-QQ in HA mice. HA mice were infused with FVIII-WT or FVIII-QQ protein prior to tail clip assay. Comparison of both proteins at a dose of 3 µg/kg demonstrated greater blood loss in HA mice treated with FVIII-WT vs. FVIII-QQ (p=0.006). Further, blood loss in HA mice treated with FVIII-WT was greater than wild type mice (p=0.007); in contrast, there was no difference between wild type and HA mice treated with 3 µg/kg of FVIII-QQ (p=0.26).Our data demonstrate FVIII-QQ has a higher specific activity, requires 2-fold greater APC concentrations to inhibit thrombin generation in vitro and an approximate 2-fold greater in vivo hemostastic efficacy. Our results support that APC has physiologic significance in FVIII/FVIIIa inactivation and provide rationale for the study of APC in FVIII/FVIIIa regulation to develop potential novel HA therapeutics. DisclosuresDavidson:Spark Therapeutics: Consultancy. Camire:Spark Therapeutics: Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Patents & Royalties, Research Funding; Novo Nordisk: Research Funding; Bayer: Consultancy, Research Funding. George:Pfizer: Consultancy; Spark Therapeutics: Other: Principal Investigator of Ongoing Phase I/II Gene Therapy Trials for Hemophilia A and B.

Evaluation of Coagulation and Inflammatory Markers in Pediatric Patients on Extracorporeal Membrane Oxygenation (ECMO)

Background: Bleeding and thrombosis remain the primary complications related to the use of extracorporeal membrane oxygenation (ECMO). To date, no single test or parameter has been identified to accurately predict the risk of these hemostatic complications. Thrombin generation may be the key marker for both thrombosis and bleeding, and may be influenced by inflammation. The aims of this study were to evaluate if novel laboratory tests including thrombin generation assay, neutrophil extracellular traps (NETs), microparticles (MPs), and red blood cell (RBC) membrane fragility/adhesion could better predict patients at risk for bleeding and/or thrombosis compared to routine laboratory tests. We hypothesized that ECMO related thrombosis is associated with increased thrombin generation and ECMO related bleeding is associated with decreased thrombin generation. Methods: An IRB approved prospective pilot study of patients requiring ECMO was conducted at Children's Hospital of Michigan. Patients were enrolled within 24 hours of ECMO cannulation after informed consent. Patients aged 0-17 years placed on venoarterial (VA) or venovenous (VV) ECMO were included. Patient demographics, baseline laboratory, clinical data, and novel assays were collected. Presented is the preliminary data of the first 16 patients enrolled. SPSS was used for data analysis. Results: Of the 16 patients, there were 9 males and 7 females with the majority being cannulated for primary respiratory etiologies. Median age was 8.5 months with 6 patients being neonates (38%). Twelve patients (75%) survived the ECMO run, and 11 patients (69%) survived to hospital discharge. Laboratory data was grouped based on "no event" (N=94), "bleeding event" (N=13) and "clotting event" (N=21) days. There were no differences in the number of RBC, fresh frozen plasma, or cryoprecipitate transfusions between events. Platelet transfusions were used at higher volumes in bleeding events, p=0.011. Platelet counts did not differ between groups, whereas fibrinogen levels were significantly lower on days of bleeding events, p≤0.001 (Figure 1A). There were no differences seen in the activated partial thromboplastin time (aPTT) or activated clotting time (ACT) which are routinely used for monitoring anticoagulation. However, the prothrombin time (PT) was significantly prolonged in bleeding events with a median of 16 seconds (p=0.001), which may be related to the lower fibrinogen levels (Figure 1B). Thrombin generation was assessed using the maximum rate of thrombin generation (MRTG) value from the thromboelastograph (TEG) generated velocity curve (Figure 1D). Bleeding events had a significantly lower median MRTG of 7.93 mm/min compared to 12.36 mm/min in the no event group and 12.14 mm/min in the clotting event group (p=0.004). The thrombin generation assay results are in process. NETs were analyzed using an ELISA to detect citrullinated histone-3 (CitH3) levels (ng/mL). There were no significant differences seen in our population, however there is a trend toward increased levels in thrombotic events (Figure 1E). Both absolute MPs and platelet MPs were significantly increased in clotting events compared to bleeding events, p=0.003 and p≤0.001 respectively (Figure 1C). In our population, there was no evidence that hemolysis played a role in thrombosis as assessed by flow based RBC mechanical fragility testing. Of note, flow based RBC adhesion showed a non-significant trend toward increased adhesion to P-selectin. This finding requires further investigation because P-selectin is a primary site for adhesion on endothelial cells and plays a crucial role in coagulation (Figure 1F). Conclusions: Bleeding events on ECMO are associated with increased mortality. Our findings suggest that lower fibrinogen levels, prolonged PT and decreased thrombin generation (as assessed by the TEG) are associated with an increased risk for major bleeding. Increased levels of NETs, MPs and RBC adhesion have a trend towards increased thrombotic events. Thus, although preliminary, our results suggest that novel laboratory tests were effective in identifying bleeding events, and the trends seen in thrombotic events may allow for the use of new medications to reduce clotting in ECMO. The results illustrate the challenges in monitoring ECMO anticoagulation and additional study of novel tests are needed to identify and avoid clinical complications. Disclosures Tarasev: Functional Fluidics:Current Employment, Current equity holder in private company.Hines:Functional Fluidics:Current equity holder in private company.Chitlur:Biovertiv:Honoraria;Pfizer:Honoraria;Novo Nordisk:Consultancy, Honoraria;Takeda:Honoraria;Agios Pharmaceuticals:Research Funding.