Thrombin generation and bleeding in haemophilia A

Exploring the effects of Factor Xa inhibitors on thrombin generation in people with haemophilia

Optimal management of cardiovascular disease (CVD) in people with haemophilia (PWH) is a growing issue, given the continuing improvement in life expectancy among PWH. The evolving treatment paradigms targeting higher trough levels and the advent of non-factor replacement therapies (NFRT) means much of the 'protection' PWH were thought to have against CVD may be lost. There is a paucity of evidence regarding the safety of using anticoagulants in PWH. We designed a study assessing the thrombin generation (TG) of PWH of different severities and treatments, compared to non-haemophilia patients receiving a Factor Xa (FXa) inhibitor (apixaban or rivaroxaban), healthy controls, and assessing TG parameters of adding FXa inhibitor to the plasma of PWH receiving emicizumab prophylaxis. In total, 40 patients were included. TG was initiated with 5pM tissue factor (TF) using the calibrated automated thrombinoscope. Compared to those with mild haemophilia, patients receiving a FXa inhibitor had higher endogenous thrombin potential (ETP) (1278.42 vs 1831.36) and velocity index (40.71 vs 112.56), but both had a similar peak height (154.0 vs 262.63) and time to peak (both 5.83). People with severe haemophilia receiving emicizumab had significantly improved TG parameters compared to those not receiving emicizumab - ETP 1678.11 vs 809.96 and peak height 233.8 vs 92.05; however, when FXa inhibitor was added their TG parameters deteriorated to the severe haemophilia range (ETP 1179.60 and peak height 103.05). TG may provide additional useful information regarding the use of anticoagulants in PWH.

Relationship between plasma tissue Factor Pathway Inhibitor (TFPI) levels, thrombin generation and clinical risk of bleeding in patients with severe haemophilia A or B

INTRODUCTION

Bleeding severity in severe haemophilic patients, with low thrombin generation (TG) capacity, can vary widely between patients, possibly reflecting differences in tissue factor pathway inhibitor (TFPI) level.

AIM

To compare free TFPI (fTFPI) levels in patients with severe haemophilia A (sHA) and severe haemophilia B (sHB) and to investigate in these patients as a whole the relationships between bleeding and TG potential, between TG potential and fTFPI level and between fTFPI level and bleeding tendency.

METHODS

Data on bleeding episodes retrospectively recorded during follow-up visits over 5-10 years were collected and used to calculate the annualised joint bleeding rate (AJBR). fTFPI levels and basal TG parameters were determined in platelet-poor plasma (PPP) and platelet-rich plasma (PRP) using calibrated automated tomography (CAT).

RESULTS

Mean fTFPI levels did not differ significantly between sHA (n = 34) and sHB (n = 19) patients. Mean values of endogenous thrombin potential (ETP) and thrombin peak (peak) in PPP and PRP were two-fold higher when fTFPI levels < 9.4 versus > 14.3 ng/mL. In patients treated on demand, ETP and peak in PRP were doubled when AJBR was≤ 4.9 $ \le 4.9$ , AJBR being halved in patients with a low fTFPI level (9.4 ng/mL). In patients on factor prophylaxis, no association was found between TG parameters and either fTFPI level or AJBR.

CONCLUSION

In patients treated on demand, bleeding tendency was influenced by fTFPI levels, which in turn affected basal TG potential. In patients on prophylaxis, bleeding tendency is probably determined primarily by the intensity of this treatment.

Comprehensive Analysis of the Role of Fibrinogen and Thrombin in Clot Formation and Structure for Plasma and Purified Fibrinogen

Altered properties of fibrin clots have been associated with bleeding and thrombotic disorders, including hemophilia or trauma and heart attack or stroke. Clotting factors, such as thrombin and tissue factor, or blood plasma proteins, such as fibrinogen, play critical roles in fibrin network polymerization. The concentrations and combinations of these proteins affect the structure and stability of clots, which can lead to downstream complications. The present work includes clots made from plasma and purified fibrinogen and shows how varying fibrinogen and activation factor concentrations affect the fibrin properties under both conditions. We used a combination of scanning electron microscopy, confocal microscopy, and turbidimetry to analyze clot/fiber structure and polymerization. We quantified the structural and polymerization features and found similar trends with increasing/decreasing fibrinogen and thrombin concentrations for both purified fibrinogen and plasma clots. Using our compiled results, we were able to generate multiple linear regressions that predict structural and polymerization features using various fibrinogen and clotting agent concentrations. This study provides an analysis of structural and polymerization features of clots made with purified fibrinogen or plasma at various fibrinogen and clotting agent concentrations. Our results could be utilized to aid in interpreting results, designing future experiments, or developing relevant mathematical models.

Antithrombin lowering in hemophilia: a closer look at fitusiran

Thrombin is a key enzyme in the maintenance of normal hemostatic function and is the central product of an interconnected set of simultaneously occurring cellular and proteolytic events. Antithrombin (AT) is a natural anticoagulant that downregulates different components of the clotting process, particularly thrombin generation. In good health, well-regulated hemostasis is the result of a balance between procoagulant and anticoagulant elements. Cumulative understanding of the regulation of thrombin generation and its central role in hemostasis and bleeding disorders has led to the clinical development of therapeutic strategies that aim to rebalance hemostasis in individuals with hemophilia and other coagulation factor deficiencies to improve bleeding phenotype. The aim of this review is to discuss the rationale for AT lowering in individuals with hemophilia, with a focus on fitusiran, its mechanism of action, and its potential as a prophylactic therapy for individuals with hemophilia A or B, with or without inhibitors. Fitusiran is an investigational small, interfering RNA therapeutic that targets and lowers AT. It is currently in phase III clinical trials and results have shown its potential to increase thrombin generation, leading to enhanced hemostasis and improved quality of life while reducing the overall treatment burden.

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

Background

Heterogeneity 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.

Objectives

The objective of this study was to describe the association between clinical bleeding phenotype and thrombin generation and plasmin generation profiles in patients with hemophilia.

Methods

The 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.

Results

In 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.

Conclusion

A 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 and implications for hemophilia therapies: A narrative review

Thrombin plays an essential role in achieving and maintaining effective hemostasis and stable clot formation. In people with hemophilia, deficiency of procoagulant factor (F)VIII or FIX results in insufficient thrombin generation, leading to reduced clot stability and various bleeding manifestations. A correlation has been found between the bleeding phenotype of people with hemophilia and the extent of thrombin generation, with individuals with increased thrombin generation being protected from bleeding and those with lower thrombin generation having increased bleeding tendency. The amount, location, and timing of thrombin generation have been found to affect the formation and stability of the resulting clot. The goal of all therapies for hemophilia is to enhance the generation of thrombin with the aim of restoring effective hemostasis and preventing or controlling bleeding; current treatment approaches rely on either replacing or mimicking the missing procoagulant (ie, FVIII or FIX) or rebalancing hemostasis through lowering natural anticoagulants, such as antithrombin. Global coagulation assays, such as the thrombin generation assay, may help guide the overall management of hemostasis by measuring and monitoring the hemostatic potential of patients and, thus, assessing the efficacy of treatment in people with hemophilia. Nevertheless, standardization of the thrombin generation assay is needed before it can be adopted in routine clinical practice.

Only Subclinical Alterations in the Haemostatic System of People with Diabetes after COVID-19 Vaccination

People with diabetes have an increased risk of experiencing adverse COVID-19 outcomes. COVID-19 vaccination is, therefore, highly recommended. However, people with diabetes have an inherently elevated risk of thrombotic events and the impact of the vaccination on the coagulation system in this patient population remains to be elucidated. The aim of this study was to investigate the impact of COVID-19 vaccination on the haemostatic system in people with type 1 or type 2 diabetes. We evaluated the effects of COVID-19 vaccination (BioNTech Pfizer, Moderna, AstraZeneca) on standard coagulation parameters, whole blood coagulation (Thrombelastometry), platelet function (impedance aggregation), and thrombin generation (calibrated automated thrombography) in people with type 1 diabetes mellitus (n = 41) and type 2 diabetes mellitus (n = 37). Blood sampling points were prior to vaccination and two weeks after the respective vaccination. Thrombelastometry measurements indicated moderately increased clot formation post-vaccination in people with type 1, as well as with type 2, diabetes: "Clot formation times" were significantly shorter, and both "maximum clot firmness" and "alpha angles" were significantly higher, as compared to the respective pre-vaccination values. Therefore, TEM parameters were not altered after vaccination in patients receiving ASA. Moreover, platelet aggregation was enhanced in people with type 1 diabetes, and plasma levels of D-Dimer were increased in people with type 2 diabetes, following COVID-19 vaccination. All other standard coagulation parameters, as well as thrombin generation, were not affected by the vaccination. The coagulation responses of people with diabetes to COVID-19 vaccination were only subclinical and comparable to those observed in healthy individuals. Our findings suggest that people with diabetes do not face an increased activation of the coagulation post-vaccination.

Thrombin generation assays to personalize treatment in bleeding and thrombotic diseases

Treatment of bleeding and thrombotic disorders is highly standardized and based on evidence-based medicine guidelines. These evidence-based treatment schemes are well accepted but may lead to either insufficient treatment or over-dosing, because the individuals' hemostatic properties are not taken into account. This can potentially introduce bleeding or thrombotic complications in individual patients. With the incorporation of pharmacokinetic (PK) and pharmacodynamic (PK-PD) parameters, based on global assays such as thrombin generation assays (TGAs), a more personalized approach can be applied to treat either bleeding or thrombotic disorders. In this review, we will discuss the recent literature about the technical aspects of TGAs and the relation to diagnosis and management of bleeding and thrombotic disorders. In patients with bleeding disorders, such as hemophilia A or factor VII deficiency, TGAs can be used to identify patients with a more severe bleeding phenotype and also in the management with non-replacement therapy and/or bypassing therapy. These assays have also a role in patients with venous thrombo-embolism, but the usage of TGAs in patients with arterial thrombosis is less clear. However, there is a potential role for TGAs in the monitoring of (long-term) antithrombotic therapy, for example with the use of direct oral anticoagulants. Finally this review will discuss controversies, limitations and knowledge gaps in relation to the introduction of TGAs to personalize medicine in daily medical practice.

Assessing the individual roles of FII, FV, and FX activity in the thrombin generation process

Thrombin generation (TG) is known as a physiological approach to assess the hemostatic function. Although it correlates well with thrombosis and bleeding, in the current setup it is not sensitive to the effects of fluctuations in single coagulation factors. We optimized the calibrated automated thrombinography (CAT) method to quantify FII, FV and FX activity within the coagulation system. The CAT assay was fine-tuned for the assessment of FII, FV and FX by diluting the samples in FII-, FV-, or FX-deficient plasma, respectively, and measuring TG. Plasma FII levels correlated linearly with the ETP up to a plasma concentration of 100% FII. FV and FX levels correlated linearly with the peak height up to a plasma level of 2.5% FV and 10% FX, respectively. Sensitized CAT protocols were designed by adding a fixed volume of a pre-diluted patient sample to FII, FV, and FX deficient plasma in TG experiments. This approach makes the TG measurement dependent on the activity of the respective coagulation factor. The ETP or peak height were quantified as readouts for the coagulation factor activity. The intra- and inter-assay variation coefficients varied from 5.0 to 8.6%, and from 3.5 to 5.9%, respectively. Reference values were determined in 120 healthy subjects and the assays were clinically validated in 60 patients undergoing coronary artery bypass grafting (CABG). The sensitized CAT assays revealed that the contribution of FII, FV, and FX to the TG process was reduced after CABG surgery, leading to reduced prothrombin conversion and subsequently, lower TG.

Use of population PK/PD approach to model the thrombin generation assay: assessment in haemophilia A plasma samples spiked by a TFPI antibody

The thrombin generation (TG) assay is a well-established tool to capture the clotting potential of any healthy or haemophiliac subject. It measures ex vivo the kinetics of thrombin activation throughout the coagulation. Clinical studies allowed to create two databases gathering the coagulation factor levels and the thrombin generation profile of 40 healthy and 40 haemophiliac A (HA) subjects. Besides, portions of all HA samples were spiked with increasing levels of a TFPI antibody (considered as a possible therapeutic target) and corresponding TG profiles were determined. The non-linear mixed-effect (NLME) modelling aims at describing and explaining the experimentally observed important variability of the TG curves between subjects and the individual effects of spiking with a TFPI antibody. The models consist of an empirical description of the TG kinetics, accounting for an additive residual error and between-subject variability on its parameters. Factor VIII and TFPI were found to significantly explain and reduce the variability of the TG of haemophilia A samples. Besides, the model is shown to correctly reproduce the variability in the response to the ex vivo spiking with the TFPI antibody, by combining the empirical description of TG to a simple Hill equation that accounts for the binding between TFPI and different doses of its antibody. Such models can be useful for clinical practice, with the analysis and comparison of the distributions of TG profiles in healthy and haemophilia populations; and also for research, with the analysis of the effect of TFPI and its neutralization on individual TG profiles.

In Silico Hemostasis Modeling and Prediction

Computational physiology, i.e., reproduction of physiological (and, by extension, pathophysiological) processes in silico, could be considered one of the major goals in computational biology. One might use computers to simulate molecular interactions, enzyme kinetics, gene expression, or whole networks of biochemical reactions, but it is (patho)physiological meaning that is usually the meaningful goal of the research even when a single enzyme is its subject. Although exponential rise in the use of computational and mathematical models in the field of hemostasis and thrombosis began in the 1980s (first for blood coagulation, then for platelet adhesion, and finally for platelet signal transduction), the majority of their successful applications are still focused on simulating the elements of the hemostatic system rather than the total (patho)physiological response in situ. Here we discuss the state of the art, the state of the progress toward the efficient "virtual thrombus formation," and what one can already get from the existing models.

Factor VIII prophylaxis effects outweigh other hemostasis contributors in predicting severe haemophilia A joint outcomes

INTRODUCTION

The Joint Outcome Study (JOS) demonstrated that previously untreated children with severe haemophilia A treated with prophylactic factor VIII (FVIII) concentrate had superior joint outcomes at age 6 years compared to those children treated episodically for bleeding. However, variation in joint outcome within each treatment arm was not well explained.

AIM

In this study, we sought to better understand variation in joint outcomes at age 6 years in participants of the JOS.

METHODS

We evaluated the influence of FVIII half-life, treatment adherence, constitutional coagulant and anticoagulant proteins, and global assays on joint outcomes (number of joint bleeds, total number of bleeds, total MRI score and joint physical exam score). Logistic regression was used to evaluate the association of variables with joint failure status on MRI, defined as presence of subchondral cyst, surface erosion or joint-space narrowing. Each parameter was also correlated with each joint outcome using Spearman correlations.

RESULTS

Prophylaxis treatment arm and FVIII trough were each found to reduce risk of joint failure on univariate logistic regression analysis. When controlling for treatment arm, FVIII trough was no longer significant, likely because of the high level of covariation between these variables. We found no consistent correlation between any laboratory assay performed and any joint outcome parameter measured.

CONCLUSION

In the JOS, the effect of prescribed prophylactic FVIII infusions on joint outcome overshadowed the contribution of treatment adherence, FVIII half-life, global assays of coagulation and constitutional coagulation proteins. (ClinicalTrials.gov number, NCT00207597).

Burden of mild haemophilia A: Systematic literature review

Introduction

Although the clinical manifestations of severe haemophilia A (HA) are well studied, the challenges, if any, of living with mild HA are not clearly delineated to date.

Aim

To assess available evidence of clinical risks and societal/economic impacts of disease in adult patients with mild HA using a systematic literature review.

Methods

Prespecified study selection criteria were applied in a comprehensive literature search. Included studies varied in design and reported outcomes of interest for adults (≥13 years of age) with mild HA.

Results

Seventeen studies with a total of 3213 patients met eligibility criteria (published or presented in English, 1966‐2017). Most studies were observational, and the outcomes reported were too sparse and dissimilar to support a formal meta‐analysis. Mean annual bleeding rates ranged from 0.44 to 4.5 episodes per patient per year. Quality of life (QoL; SF‐36 General Health) was impacted compared to healthy controls. Health care costs and productivity were seldom assessed and no robust comparisons to healthy controls were available.

Conclusion

Quantifying outcomes for adult patients with mild HA remains challenging, with estimates of key QoL and cost data often based on small data sets and without comparison to population norms. Therefore, the clinical impact of mild haemophilia may be under‐represented and unmet needs may remain unaddressed. As paradigm‐changing therapies for HA emerge, stronger knowledge of mild HA can guide the development of care options that minimize burden and enhance the QoL for this segment of the haemophilia community, and for the haemophilia community in totality.

The central role of thrombin in bleeding disorders

Maintaining normal hemostasis relies on a regulated system of procoagulant and anticoagulant pathways, and disruption of these processes leads to the loss of hemostatic control, with the potential for excessive bleeding or thrombosis. Evaluation of bleeding disorders has conventionally been achieved by laboratory assays that measure the activity of individual coagulation factors. While such assays have proven effective for detecting abnormalities of the coagulation system and aiding diagnosis, inherent limitations prevent them from capturing a complete picture of hemostatic function. An improved understanding of thrombin activity and its central role in hemostasis and bleeding disorders has led to the clinical development of global assays that are more physiologically relevant than traditional assays; furthermore, these global assays are able to monitor responses to therapy. In this review, we provide an overview of the role of thrombin in hemostasis, and describe the clinical benefits of thrombin monitoring in patients with bleeding disorders. Moreover, we discuss recent advances in thrombin-targeting therapeutic strategies that aim to correct thrombin deficiency and prevent bleeding in patients with hemophilia and other rare bleeding disorders.

Looking for a partner: ceruloplasmin in protein-protein interactions

Ceruloplasmin (CP) is a mammalian blood plasma ferroxidase. More than 95% of the copper found in plasma is carried by this protein, which is a member of the multicopper oxidase family. Proteins from this group are able to oxidize substrates through the transfer of four electrons to oxygen. The essential role of CP in iron metabolism in humans is particularly evident in the case of loss-of-function mutations in the CP gene resulting in a neurodegenerative syndrome known as aceruloplasminaemia. However, the functions of CP are not limited to the oxidation of ferrous iron to ferric iron, which allows loading of the ferric iron into transferrin and prevents the deleterious reactions of Fenton chemistry. In recent years, a number of novel CP functions have been reported, and many of these functions depend on the ability of CP to form stable complexes with a number of proteins.

Importance of post-translational modifications on the function of key haemostatic proteins

Post-translational modifications (PTMs) such as glycosylation and phosphorylation play an important role on the function of haemostatic proteins and are critical in the setting of disease. Such secondary level changes to haemostatic proteins have wide ranging effects on their ability to interact with other proteins. This review aimed to summarize the knowledge of the common PTMs associated with haemostatic proteins and the implications of such modifications on protein function. Haemostatic proteins that represent the main focus for studies specific to PTMs are von Willebrand factor, tissue factor, factor VIII, antithrombin and fibrinogen. These proteins are susceptible to PTMs by glycosylation, phosphorylation, sulphation, citrullination and nitration, respectively, with a significant impact on their function. During synthesis, vWF must undergo extensive PTMs, with N-linked glycosylation being the most common. Increased phosphorylation of tissue factor results in increased affinity for platelets to the vessel endothelium. Citrullination of antithrombin leads to an increased anticoagulant function of this protein and therefore an anticoagulant state that inhibits clot formation. On the contrary, nitration of fibrinogen has been shown to result in a prothrombotic state, whilst sulphation is required for the normal function of Factor VIII. From this review, it is evident that PTMs of haemostatic proteins as a change in protein structure at a secondary level greatly influences the behaviour of the protein at a tertiary level.

The utility of thromboelastography in inherited and acquired bleeding disorders

Thromboelastography (TEG) was first described by Hartert in 1948, and was designed to monitor viscoelastic clot strength in whole blood in real time. The current TEG method and Rotational Thromboelastometry (ROTEM) were subsequently developed from the original principles. Both of the modern methods provide data by measuring changes in the viscoelastic strength of a small sample of clotting blood in response to a constant rotational force. The important advantage of these techniques is to visually observe and quantify blood coagulation including the propagation, stabilization and dissolution phases of clot formation under low shear conditions. Analysis of the results provides detailed kinetic data on fibrin generation, clot strength and fibrinolysis. These TEG/ROTEM analyses therefore enable evaluation of global clotting function and the monitoring of haemostatic treatment in various clinical situations, not only in patients with genetic bleeding disorders, such as haemophilia, but also in patients undergoing cardiac surgery, liver transplantation or suffering from traumatic injury. Some evidence suggests that haemostatic management using TEG/ROTEM leads to a reduction in total transfusions of whole blood or clotting factors. Wider clinical application of this technology seems likely.

Thrombin generation and bleeding in haemophilia inhibitor patients during immune tolerance induction

BACKGROUND

Inhibitor formation complicates haemophilia treatment and requires immune tolerance induction to rid inhibitors over 5 BU. In the prospective, randomized International Immune Tolerance Study, immune tolerance induction was equally effective with high-dose (HD) (200 IU kg^-1 day^-1 ) and low-dose (LD) (50 IU kg^-1 3× per week) factor VIII, but haemorrhages were twofold higher in the LD arm. This finding was unexpected as inhibitors neutralize FVIII activity. We hypothesized that the thrombin generation assay (TGA), a global measure of clot formation, might predict bleeding better than FVIII levels.

METHODS

We evaluated TGA using relipidated tissue factor (TF) on 83 thawed, recalcified corn trypsin inhibitor/citrate plasma samples from 31 subjects (17 HD, 14 LD) who participated on the ITI study, and who had sufficient sample available and appropriate informed consent.

RESULTS

There were no significant differences in peak thrombin, estimated thrombin potential, maximum rate or lag time between HD and LD arms; between pre-, during and post-ITI time points, or after FVIII spiking. In 19 subjects (12 HD, 7 LD) with anti-FVIII<1.0 BU, the prevalence of non-neutralizing antibody (NNA) and neutralizing antibody (NA) was 89.5% (17/19), and the latter strongly correlated with anti-VIII titer, r = 0.73 [95% CI: 0.55, 0.88].

CONCLUSION

In haemophilia inhibitor patients, thrombin generation is present, but does not predict bleeding risk. Following tolerance induction, NNA remains detectable in the majority.

Modeling thrombin generation: plasma composition based approach

Thrombin has multiple functions in blood coagulation and its regulation is central to maintaining the balance between hemorrhage and thrombosis. Empirical and computational methods that capture thrombin generation can provide advancements to current clinical screening of the hemostatic balance at the level of the individual. In any individual, procoagulant and anticoagulant factor levels together act to generate a unique coagulation phenotype (net balance) that is reflective of the sum of its developmental, environmental, genetic, nutritional and pharmacological influences. Defining such thrombin phenotypes may provide a means to track disease progression pre-crisis. In this review we briefly describe thrombin function, methods for assessing thrombin dynamics as a phenotypic marker, computationally derived thrombin phenotypes versus determined clinical phenotypes, the boundaries of normal range thrombin generation using plasma composition based approaches and the feasibility of these approaches for predicting risk.

The effect of corn trypsin inhibitor and inhibiting antibodies for FXIa and FXIIa on coagulation of plasma and whole blood

BACKGROUND

Corn trypsin inhibitor (CTI), an inhibitor of FXIIa, is used to prevent plasma coagulation by contact activation, to specifically investigate tissue factor (TF)-initiated coagulation.

OBJECTIVE

In the present work the specificity of CTI for factor (F) XIIa is questioned.

METHODS AND RESULTS

In the commercial available plasma coagulation assays CTI was found to double activated partial thromboplastin time (APTT) at a plasma concentration of 7.3 ± 1.5 μm CTI (assay concentration 2.4 μm). No effect was found on the prothrombin time (PT) when high TF concentrations were used. Also, with specific antibodies for FXIIa and for FXIa only APTT was found to be extended but not PT. With specific enzyme assays using chromogenic substrates CTI was shown to be a strong inhibitor of FXIIa and a competitive inhibitor of FXIa with Ki  = 8.1 ± 0.3 μm, without effect on the coagulation factors FVIIa, FIXa, FXa and thrombin. In thrombin generation and coagulation (free oscillation rheometry, FOR) assays, initiated with low TF concentrations, no effect of CTI (plasma concentrations of 4.4 and 13.6 μm CTI, 25 resp. 100 mg L(-1) in blood) was found with ≥ 1 pm TF. At ≤ 0.1 pm TF in the FOR whole blood assay the coagulation time (CT) concentration dependently increased while the plasma CT became longer than the observation time.

CONCLUSION

To avoid inhibition of FXIa and the thrombin feedback loop we recommend that for coagulation assays the concentration of CTI in blood should be below 20 mg L(-1) (1.6 μm) and in plasma below 3 μm.

HIV replication alters the composition of extrinsic pathway coagulation factors and increases thrombin generation

Background

HIV infection leads to activation of coagulation, which may increase the risk for atherosclerosis and venous thromboembolic disease. We hypothesized that HIV replication increases coagulation potentially through alterations in extrinsic pathway factors.

Methods and Results

Extrinsic pathway factors were measured among a subset of HIV participants from the Strategies for Management of Anti‐Retroviral Therapy (SMART) trial. Thrombin generation was estimated using validated computational modeling based on factor composition. We characterized the effect of antiretroviral therapy (ART) treatment versus the untreated state (HIV replication) via 3 separate analyses: (1) a cross‐sectional comparison of those on and off ART (n=717); (2) a randomized comparison of deferring versus starting ART (n=217); and (3) a randomized comparison of stopping versus continuing ART (n=500). Compared with viral suppression, HIV replication consistently showed short‐term increases in some procoagulants (eg, 15% to 23% higher FVIII; P<0.001) and decreases in key anticoagulants (eg, 5% to 9% lower antithrombin [AT] and 6% to 10% lower protein C; P<0.01). The net effect of HIV replication was to increase coagulation potential (eg, 24% to 48% greater thrombin generation from computational models; P<0.01 for all). The pattern of changes from HIV replication was reversed with ART treatment and consistent across all 3 independent comparisons.

Conclusions

HIV replication leads to complex changes in extrinsic pathway factors, with the net effect of increasing coagulation potential to a degree that may be clinically relevant. The key influence of changes in FVIII and AT suggests that HIV‐related coagulation abnormalities may involve changes in hepatocyte function in the context of systemic inflammation.

Clinical Trial Registration

URL: ClinicalTrials.gov. Unique identifier: NCT00027352.

Progressive improvement in wound healing with increased therapy in haemophilia B mice

Previous work has shown that normalized haemostasis only at the time of an injury is not sufficient to promote optimal wound healing in haemophilia B (HB) mice. However, the duration of treatment required for optimal healing has not been established. The goal of these studies was to determine the effect of different durations of replacement or bypassing therapy [factor IX(FIX) or factor VIIa (FVIIa)] on wound healing parameters in a mouse model of HB. A dermal wound was placed on the back of HB mice. Animals were either untreated or pretreated and then subsequently treated for 3 days, 5 days, or 7 days with FIX or FVIIa. Wound area, time to wound healing, haematoma formation and iron deposition were measured. All treated animals showed shortened time to healing relative to untreated animals. Haematoma formation was prevented by treatment and bleeding into the wounds, measured by iron scores, was reduced by treatment. In addition, there was a progressive improvement in healing with 7 days of treatment more effective than 5 days which was more effective than 3 days. Replacement therapy with FIX had slightly shorter healing times than bypassing therapy with FVIIa. HB mice treated with FIX had slightly smaller wound area than untreated animals; by contrast, FVIIa-treated animals had much smaller wound areas that were close to the wound areas seen in wild-type animals. The data suggest that sustained therapy is required for normal wound healing.

Models for thrombin generation and risk of disease

Computational models can offer an integrated view of blood clotting dynamics and may ultimately be instructive regarding an individual's risk of bleeding or clotting. Appropriately, developed and validated models could allow clinicians to simulate the outcomes of therapeutics and estimate risk of disease. Computational models that describe the dynamics of thrombin generation have been developed and have been used in combination with empirical studies to understand thrombin dynamics on a mechanistic basis. The translation of an individual's specific coagulation factor composition data using these models into an integrated assessment of hemostatic status may provide a route for advancing the long-term goal of individualized medicine. This review details the integrated approaches to understanding: (i) What is normal thrombin generation in individuals? (ii) What is the effect of normal range plasma composition variation on thrombin generation in pathologic states? (iii) Can disease progression or anticoagulation be followed by understanding the boundaries of normal thrombin generation defined by plasma composition? (iv) What are the controversies and limitations of current computational approaches? Progress in these areas can bring us closer to developing models that can be used to aid in identifying hemostatic risk.

Investigation of the phenotype heterogeneity in severe hemophilia A using thromboelastography, thrombin generation, and thrombodynamics

BACKGROUND

Hemophilia A (HA) patients with similar factor VIII levels can demonstrate varying bleeding tendencies. In particular, 10-15% of all severe HA patients (FVIII:C<1IUdL(-1)) do not require regular replacement therapy. Modern global coagulation assays can help to detect and study this "mild" bleeding phenotype. Here, we investigated the coagulation status of different bleeding phenotypes using various types of global coagulation assays.

MATERIALS AND METHODS

Ten HA patients with severe phenotype and eleven patients with mild phenotypes were included in the study. For each patient, thromboelastography (TE), thrombodynamics (TD), and kaolin- or tissue factor-induced thrombin generation (TG) were measured. TG in platelet-rich plasma (PRP) was investigated using our original modification when the thrombin generation curve showed two peaks, previously shown to depend on platelet activity. We also utilized TG and TD with the addition of thrombomodulin.

RESULTS

The second peak amplitude and ETP of PRP TG were the only parameters that were significantly higher in mild bleeders (peak 41.6 ± 3.5 nM, ETP 1966 ± 169 nM*min) than in patients with severe bleeding (peak 28.3 ± 3.3 nM, ETP 1359 ± 130 nM*min).

CONCLUSIONS

Our results suggest that severe and mild HA phenotypes could be distiguished by TG assay in PRP suggesting that difference in platelet activity can be involved in the phenotype formation. According to our previous results we can suppose that the mechanism of the phenotypic heterogeneity is linked with TG mediated by PS-expressing platelets.

Thrombin generation and whole blood viscoelastic assays in the management of hemophilia: current state of art and future perspectives

Hemophilia is a bleeding disorder that afflicts about 1 in 5000 males. Treatment relies upon replacement of the deficient factor, and response to treatment both in clinical research and practice is based upon subjective parameters such as pain and joint mobility. Existing laboratory assays quantify the amount of factor in plasma, which is useful diagnostically and prognostically. However, these assays are limited in their ability to fully evaluate the patient's clot-forming capability. Newer assays, known as global assays, provide a far more detailed view of thrombin generation and clot formation and have been studied in hemophilia for about 10 years. They have the potential to offer a more objective measure of both the hemophilic phenotype as well as the response to treatment. In particular, in patients who develop inhibitors to deficient clotting factors and in whom bypassing agents are required for hemostasis, these assays offer the opportunity to determine the laboratory response to these interventions where traditional coagulation assays cannot. In this article we review the existing literature and discuss several controversial issues surrounding the assays. Last, a vision of future clinical uses of these assays is briefly described.

Potentiation of thrombin generation in hemophilia A plasma by coagulation factor VIII and characterization of antibody-specific inhibition

Development of inhibitory antibodies to coagulation factor VIII (fVIII) is the primary obstacle to the treatment of hemophilia A in the developed world. This adverse reaction occurs in 20-30% of persons with severe hemophilia A treated with fVIII-replacement products and is characterized by the development of a humoral and neutralizing immune response to fVIII. Patients with inhibitory anti-fVIII antibodies are treated with bypassing agents including recombinant factor VIIa (rfVIIa). However, some patients display poor hemostatic response to bypass therapy and improved treatment options are needed. Recently, we demonstrated that fVIII inhibitors display widely variable kinetics of inhibition that correlate with their respective target epitopes. Thus, it was hypothesized that for antibodies that display slow rates of inhibition, supplementation of rfVIIa with fVIII would result in improved thrombin generation and be predictive of clinical responses to this novel treatment regimen. In order to test this hypothesis, 10 murine monoclonal antibodies (MAbs) with non-overlapping epitopes spanning fVIII, differential inhibition titers, and inhibition kinetics were studied using a thrombin generation assay. Of the 3 MAbs with high inhibitory titers, only the one with fast and complete (classically defined as "type I") kinetics displayed significant inhibition of thrombin generation with no improvement upon supplementation of rfVIIa with fVIII. The other two MAbs that displayed incomplete (classically defined as "type II") inhibition did not suppress the potentiation of thrombin generation by fVIII. All antibodies that did not completely inhibit fVIII activity demonstrated potentiation of thrombin generation by the addition of fVIII as compared to rfVIIa alone. In conclusion, fVIII alone or in combination with rfVIIa corrects the thrombin generation defect produced by the majority of anti-fVIII MAbs better than single agent rfVIIa. Therefore, combined fVIII/rfVIIa therapy may provide better hemostatic control than current therapy in some patients with anti-fVIII inhibitors.

Low molecular weight heparin inhibits plasma thrombin generation via direct targeting of factor IXa: contribution of the serpin-independent mechanism

BACKGROUND

Although heparin possesses multiple mechanisms of action, enhanced factor Xa inhibition by antithrombin is accepted as the predominant therapeutic mechanism. The contribution of FIXa inhibition to heparin activity in human plasma remains incompletely defined.

OBJECTIVES

To determine the relevance of FIXa as a therapeutic target for heparins, particularly serpin-independent inhibition of intrinsic tenase (FIXa-FVIIIa) activity.

PATIENTS/METHODS

Thrombin generation was detected by fluorogenic substrate cleavage. The inhibitory potencies (EC(50) s) of low molecular weight heparin (LMWH), super-sulfated LMWH (ssLMWH), fondaparinux and unfractionated heparin (UFH) were determined by plotting concentration vs. relative velocity index (ratio ± heparin). Inhibition was compared under FIX-dependent and FIX-independent conditions (0.2 or 4 pm tissue factor [TF], respectively) in normal plasma, and in mock-depleted or antithrombin/FIX-depleted plasma supplemented with recombinant FIX.

RESULTS

UFH and fondaparinux demonstrated similar potency under FIX-dependent and FIX-independent conditions, whereas LMWH (2.9-fold) and ssLMWH (5.1-fold) demonstrated increased potency with limiting TF. UFH (62-fold) and fondaparinux (42-fold) demonstrated markedly increased EC(50) values in antithrombin-depleted plasma, whereas LMWH (9.4-fold) and ssLMWH (two-fold) were less affected, with an EC(50) within the therapeutic range for LMWH. The molecular target for LMWH/ssLMWH was confirmed by supplementing FIX/antithrombin-depleted plasma with 90 nm recombinant FIX possessing mutations in the heparin-binding exosite. Mutated FIX demonstrated resistance to inhibition of thrombin generation by LMWH and ssLMWH that paralleled the effect of these mutations on intrinsic tenase inhibition.

CONCLUSIONS

Therapeutic LMWH concentrations inhibit plasma thrombin generation via antithrombin-independent interaction with the FIXa heparin-binding exosite.

Activated protein C inhibitor for correction of thrombin generation in hemophilia A blood and plasma1

BACKGROUND

Replacement therapy for hemophilic patient treatment is costly, because of the high price of pharmacologic products, and is not affordable for the majority of patients in developing countries.

OBJECTIVE

To generate and evaluate low molecular weight agents that could be useful for hemophilia treatment.

METHODS

Potential agents were generated by synthesizing specific inhibitors [6-(Lys-Lys-Thr-[homo]Arg)amino-2-(Lys[carbobenzoxy]-Lys[carbobenzoxy]-O-benzyl)naphthalenesulfonamide] (PNASN-1)] for activated protein C (APC) and tested in plasma and fresh blood from hemophilia A patients.

RESULTS

In the activated partial thromboplastin time-based APC resistance assay, PNASN-1 partially neutralized the effect of APC. In calibrated automated thrombography, PNASN-1 neutralized the effect of APC on thrombin generation in normal and congenital factor VIII-deficient plasma (FVIII:C < 1%). The addition of PNASN-1 to tissue factor-triggered (5 pm) contact pathway-inhibited fresh blood from 15 hemophilia A patients with various degrees of FVIII deficiency (FVIII:C < 1-51%) increased the maximum level of thrombin generated from 78 to 162 nm, which approached that observed in blood from a healthy individual (201 nm). PNASN-1 also caused a 47% increase in clot weight in hemophilia A blood.

CONCLUSIONS

Specific APC inhibitors compensate to a significant extent for FVIII deficiency, and could be used for hemophilia treatment.

Rationale for a randomized controlled trial comparing two prophylaxis regimens in adults with severe hemophilia A: the Hemophilia Adult Prophylaxis Trial

A major goal of comprehensive hemophilia care is to prevent occurrence of bleeds by prophylaxis or regular preventive factor, one or more times weekly. Although prophylaxis is effective in reducing bleeding and joint damage in children, whether it is necessary to continue into adulthood is not known. The purpose of this article is to describe a Phase III randomized controlled trial to evaluate prophylaxis comparing two dose regimens in adults with severe hemophilia A. I hypothesize that adults with mature cartilage and joints are less susceptible to joint bleeds and joint damage, and that once-weekly recombinant factor VIII prophylaxis, with up to two rescue doses per week, is as effective as thrice-weekly prophylaxis in reducing bleeding frequency, but less costly and more acceptable, with higher quality of life. The ultimate goal of this project is to determine whether once-weekly prophylaxis is any worse than thrice-weekly prophylaxis in reducing joint bleeding frequency, while potentially utilizing less factor, at lower cost, leading to a better quality of life. This is an innovative concept, as it challenges the current paradigm of thrice-weekly prophylaxis in adults, which is based on dosing in children. Furthermore, this trial will assess interdose thrombin generation, a novel tissue factor-based assay of hemostasis, to determine if individualized thrombin generation can predict more individualized prophylaxis dosing, which would be practice changing.

Thrombin activatable fibrinolysis inhibitor activation and bleeding in haemophilia A

Individuals with haemophilia A exhibit bleeding tendencies that are not always predicted by their factor (F)VIII level. It has been suggested that bleeding in haemophilia is due not only to defective prothrombin activation but also aberrant fibrinolysis. Thrombin activatable fibrinolysis inhibitor (TAFI) activation was measured in tissue factor (TF)-initiated blood coagulation in blood samples of 28 haemophiliacs and five controls. Reactions were quenched over time with FPRck and citrate and assayed for TAFIa and thrombin-antithrombin (TAT). The TAFIa potential (TP), TAFI activation rate and the TAFIa level at 20 min (TAFIa(20 min)) was extracted from the TAFI activation progress curve. In general, the time course of TAFI activation follows thrombin generation regardless of FVIII activity and as expected the rate of TAFI activation and TP decreases as FVIII decreases. The magnitude of TP was similar among the control subjects and subjects with <11% FVIII. In severe subjects with <1% FVIII at the time of blood collection, the TAFIa(20 min) was inversely and significantly correlated with haemarthrosis (-0.77, P = 0.03) and total bleeds (-0.75, P = 0.03). In all cases, TAFIa(20 min) was more strongly correlated with bleeding than TAT levels at 20 min. Overall, this study shows that TAFI activation in whole blood can be quantified and related to the clinical bleeding phenotype. Measuring TAFIa along with thrombin generation can potentially be useful to evaluate the differential bleeding phenotype in haemophilia A.

The influence of prophylactic factor VIII in severe haemophilia A

Haemophilia A individuals displaying a similar genetic defect have heterogeneous clinical phenotypes. Our objective was to evaluate the underlying effect of exogenous factor (f)VIII on tissue factor (Tf)-initiated blood coagulation in severe haemophilia utilizing both empirical and computational models. We investigated twenty-five clinically severe haemophilia A patients. All individuals were on fVIII prophylaxis and had not received fVIII from 0.25 to 4 days prior to phlebotomy. Coagulation was initiated by the addition of Tf to contact-pathway inhibited whole blood ± an anti-fVIII antibody. Aliquots were quenched over 20 min and analyzed for thrombin generation and fibrin formation. Coagulation factor levels were obtained and used to computationally predict thrombin generation with fVIII set to either zero or its value at the time of the draw. As a result of prophylactic fVIII, at the time of the blood draw, the individuals had fVIII levels that ranged from <1% to 22%. Thrombin generation (maximum level and rate) in both empirical and computational systems increased as the level of fVIII increased. FXIII activation rates also increased as the fVIII level increased. Upon suppression of fVIII, thrombin generation became comparable in both systems. Plasma composition analysis showed a negative correlation between bleeding history and computational thrombin generation in the absence of fVIII. Residual prophylactic fVIII directly causes an increase in thrombin generation and fibrin cross-linking in individuals with clinically severe haemophilia A. The combination of each individual's coagulation factors (outside of fVIII) determine each individual's baseline thrombin potential and may affect bleeding risk.

Factor XIII combined with recombinant factor VIIa: a new means of treating severe hemophilia A

BACKGROUND

Abnormal thrombin generation is considered the key defect in hemophilia. Conventional treatment seeks to correct this using coagulation factor replacement or bypassing agents, for example recombinant factor VIIa (rFVIIa). Previous studies demonstrate abnormal FXIII activation in patients with hemophilia. FXIII activation is essential for formation of structurally normal, stable clots.

OBJECTIVES

The present study challenges the hypothesis that in hemophilia the use of plasma-derived FXIII (pdFXIII) in combination with rFVIIa will produce a greater improvement in clot stability than promotion of thrombin generation alone.

METHODS

Fourteen individuals with severe hemophila A were enrolled. Whole blood was spiked ex vivo with buffer, rFVIIa (2 μg mL(-1)) or rFVIIa (2 μg mL(-1)) plus pdFXIII (10 μg mL(-1)). Whole blood thromboelastometry assessed clot stability, after activation with tissue factor (TF) (0.15 pm) plus tissue-type plasminogen activator (tPa) (2 nm). The primary outcome measure of clot stability was area under the elasticity curve (AUEC).

RESULTS

The combination of pdFXIII and rFVIIa significantly improved clot stability as measured by AUEC (P < 0.05) compared with rFVIIa alone.

CONCLUSION

The use of pdFXIII resulted in superior clot stability compared with solely enhancing thrombin generation and we suggest that increasing thrombin generation alone fails to fully correct dysregulation of clot-stabilizing mechanisms associated with bleeding disorders. Hemorrhage control in hemophilia may be improved using clot stabilizing drugs. FXIII shows potential as a novel agent.

Challenges of multiple mutations in individual patients with haemophilia

Multiple mutations in the same gene within a haemophilia family are being increasingly reported and raise many issues with regard to the specificity of the mutations in causing the disease. In a proportion of families with multiple mutations, discordant phenotypic severity is often observed among the affected members. Understanding whether these mutations influence additively or non-additively the structure, stability and function of the protein will help in a better clinical evaluation of these patients. In case of haemophilia A, out of 2740 entries, ten are double mutants. Among the 2891 patient entries in the Haemophilia B mutation database, there are 34 double mutants and one triple mutant. The major challenge in patients with multiple mutations lies in genetic diagnosis and counselling especially in developing countries wherein the entire gene is not being sequenced and the screening is stopped as soon as the mutation is identified. As of now, the presence of multiple mutations stresses the importance of additional DNA testing in patients with known mutations who have unusual phenotypes or additional, unexplained clinical problems, until more cost-effective techniques for screening the entire gene are identified.

Empirical and theoretical phenotypic discrimination

We have developed an integrated approach that combines empirical and computational methodologies to define an individual's thrombin phenotype. We have evaluated the process of thrombin generation in healthy individuals and individuals with defined pathologies in order to develop general criteria relevant to assess an individual's propensity for hemorrhage or thrombosis. Three complementary hypotheses have emerged from our work: (i) compensation by the ensemble of other coagulation proteins in individuals with specific factor deficiencies can 'normalize' an individual's thrombin generation process and represents a rationale for their unexpected phenotype; (ii) individuals with clinically unremarkable factor levels may present thrombin generation profiles typical of individuals with hemostatic complications; and (iii) in some hemostatic disorders a specific pattern of expression of a small ensemble of coagulation factors may be sufficient to explain the overall phenotype.