Thrombin generation for monitoring hemostatic therapy in hemophilia A: A narrative review

Utility of Global Hemostatic Assays in Patients with Bleeding Disorders of Unknown Cause

Bleeding disorder of unknown cause (BDUC) is a diagnosis of exclusion after exhaustive evaluation of plasmatic coagulation and platelet function. This review explores the utility of global hemostatic assays as confirmatory tests and in elucidating the pathophysiology of BDUC. Unlike traditional hemostatic tests that focus on coagulation factors, global assays are conducted both in plasma and also whole blood. These assays provide a more comprehensive understanding of the cell-based model of coagulation, aid in the identification of plasmatic factor abnormalities that may reduce hemostatic capacity, and allow for the assessment of impaired platelet-endothelial interactions under shear stress, as well as hyperfibrinolytic states. While clinical tests such as skin bleeding time and global assays such as PFA-100 exhibit limited diagnostic capacity, the role of viscoelastic testing in identifying hemostatic dysfunction in patients with BDUC remains unclear. Thrombin generation assays have shown variable results in BDUC patients; some studies demonstrate differences compared with healthy controls or reference values, whereas others question its clinical utility. Fibrinolysis assessment in vitro remains challenging, with studies employing euglobulin clot lysis time, plasma clot lysis time, and fluorogenic plasmin generation yielding inconclusive or conflicting results. Notably, recent studies suggest that microfluidic analysis unveils shear-dependent platelet function defects in BDUC patients, undetected by conventional platelet function assays. Overall, global assays might be helpful for exploring underlying hemostatic impairments, when conventional hemostatic laboratory tests yield no results. However, due to limited data and/or discrepant results, further research is needed to evaluate the utility of global assays as screening tools.

Predictive parameters for spontaneous joint bleeding during emicizumab prophylaxis

ABSTRACT

Emicizumab is approved for prophylaxis of patients with hemophilia A (HA). Despite its efficacy in reducing bleeding, some patients on emicizumab still experience hemarthrosis, but no tool is yet available to identify those at a higher risk of spontaneous joint bleeding. This study aimed to evaluate whether laboratory measurements (global coagulation assays and emicizumab concentration) and/or arthropathy scores can distinguish patients at higher risk of spontaneous joint bleeding while on emicizumab prophylaxis. A thrombin generation assay was performed upon the addition of tissue factor and synthetic phospholipids. Nonactivated thromboelastography was performed on citrated whole blood. Emicizumab concentrations were measured using a modified 1-stage factor VIII assay. The degree of hemophilic arthropathy was assessed using the Hemophilia Joint Health Score and Hemophilia Early Arthropathy Detection with Ultrasound (HEAD-US) score. A Cox proportional hazards model was used to evaluate the association between variables and bleeding. The predictive power of these variables was investigated using receiver operating characteristic (ROC) analysis. Forty patients with severe HA, with or without inhibitors, on emicizumab prophylaxis were enrolled in an observational cohort study. Ten of 40 developed spontaneous joint bleeding. None of the laboratory parameters were able to distinguish patients with a higher risk of spontaneous joint bleeding. ROC analysis showed that during emicizumab prophylaxis, only the presence of synovitis and a higher HEAD-US score were associated with spontaneous joint bleeding (area under the curve, 0.84). A greater degree of arthropathy and the presence of synovitis could help predict the risk of spontaneous joint bleeding in patients with HA on emicizumab prophylaxis.

Two cases of Factor XI deficiency: Use of Thrombin Generation Assays (TGA) to detect a non-bleeding phenotype

Factor XI deficiency is a rare disorder of hemostasis. Previously also known as "hemophilia C", this defect has been regarded as a risk factor for bleeding. However, it has been known for long that bleeding tendency and severity of bleeding are not related to the residual factor XI activity in symptomatic patients. Moreover, a large proportion of patients with even severe factor XI deficiency are clinically unremarkable and do not show any signs of abnormal bleeding. Here, we present two cases of factor XI deficiency with a non-bleeding phenotype. Adequate diagnostic work-up and evaluation of the bleeding risk are reported and discussed with focus on thrombin generation assays (TGA) for the prediction of bleeding in affected patients. This is of high relevance in affected patients, particularly in the context of surgery.

Pharmacokinetic-Pharmacodynamic Modelling in Hemophilia A: Relating Thrombin and Plasmin Generation to Factor VIII Activity After Administration of a VWF/FVIII Concentrate

BACKGROUND

Hemophilia A patients are treated with factor (F) VIII prophylactically to prevent bleeding. In general, dosage and frequency are based on pharmacokinetic measurements. Ideally, an alternative dose adjustment can be based on the hemostatic potential, measured with a thrombin generation assay (TGA), like the Nijmegen hemostasis assay.

OBJECTIVE

The objective of this study was to investigate the predicted performance of a previously developed pharmacokinetic-pharmacodynamic model for FVIII replacement therapy, relating FVIII dose and FVIII activity levels with thrombin and plasmin generation parameters.

METHODS

Pharmacokinetic and pharmacodynamic measurements were obtained from 29 severe hemophilia A patients treated with pdVWF/FVIII concentrate (Haemate P®). The predictive performance of the previously developed pharmacokinetic-pharmacodynamic model was evaluated using nonlinear mixed-effects modeling (NONMEM). When predictions of FVIII activity or TGA parameters were inadequate [median prediction error (MPE) > 20%], a new model was developed.

RESULTS

The original pharmacokinetic model underestimated clearance and was refined based on a two-compartment model. The pharmacodynamic model displays no bias in the observed normalized thrombin peak height and normalized thrombin potential (MPE of 6.83% and 7.46%). After re-estimating pharmacodynamic parameters, EC50 and Emax values were relatively comparable between the original model and this group. Prediction of normalized plasmin peak height was inaccurate (MPE 58.9%).

CONCLUSION

Our predictive performance displayed adequate thrombin pharmacodynamic predictions of the original model, but a new pharmacokinetic model was required. The pharmacodynamic model is not factor specific and applicable to multiple factor concentrates. A prospective study is needed to validate the impact of the FVIII dosing pharmacodynamic model on bleeding reduction in patients.

[Basics for the Use of Andexanet]

BACKGROUND

 For life-threatening or uncontrollable bleeding in association with the thrombin inhibitor dabigatran, the monoclonal antibody fragment idarucizumab is available, and for bleeding in association with the direct factor Xa inhibitors rivaroxaban or apixaban, the modified recombinant FXa protein andexanet is available for reversal. These antidotes represent emergency drugs that are typically used only after performing guideline-compliant multimodal measures.

METHODS

 An interdisciplinary group of experienced experts in the fields of angiology, hematology, internal medicine, clinical pharmacology, laboratory medicine, transfusion medicine, anesthesiology, intensive care, and hemostaseology developed recommendations relevant to daily clinical practice based on the current scientific evidence.

RESULTS

 Reversal of oral anticoagulants should be considered for severe bleeding in the following situations: (1) life-threatening bleeding or refractory hemorrhagic shock, (2) intracerebral bleeding, or (3) endoscopically unstoppable gastrointestinal bleeding. After successful hemostasis, anticoagulation (e.g., direct oral anticoagulant, vitamin K antagonist, and heparin) should be resumed promptly, taking into account individual bleeding and thromboembolic risk.

DISCUSSION

 This article aims to facilitate the management of patients with andexanet by all medical disciplines involved, thereby ensuring optimal care of patients during bleeding episodes.

Single-Particle Plasmon Sensor to Monitor Proteolytic Activity in Real Time

We have established a label-free plasmonic platform that monitors proteolytic activity in real time. The sensor consists of a random array of gold nanorods that are functionalized with a design peptide that is specifically cleaved by thrombin, resulting in a blueshift of the longitudinal plasmon. By monitoring the plasmon of many individual nanorods, we determined thrombin's proteolytic activity in real time and inferred relevant kinetic parameters. Furthermore, a comparison to a kinetic model revealed that the plasmon shift is dictated by a competition between peptide cleavage and thrombin binding, which have opposing effects on the measured plasmon shift. The dynamic range of the sensor is greater than two orders of magnitude, and it is capable of detecting physiologically relevant levels of active thrombin down to 3 nM in buffered conditions. We expect these plasmon-mediated label-free sensors to open the window to a range of applications stretching from the diagnostic and characterization of bleeding disorders to fundamental proteolytic and pharmacological studies.

Hemophilia A: Emicizumab monitoring and impact on coagulation testing

Hemophilia A is an X-linked recessive bleeding disorder characterized by absent or ineffective coagulation factor VIII, a condition that could result in a severe and potentially life-threatening bleed. Although the current standard of care involves prophylactic replacement therapy of factor VIII, the development of neutralizing anti-factor VIII alloantibody inhibitors often complicates such therapeutic treatment. Emicizumab (Hemlibra®), a novel recombinant therapeutic agent for patients with hemophilia A, is a humanized asymmetric bispecific IgG4 monoclonal antibody designed to mimic activated factor VIII by bridging factor IXa and factor X thus effecting hemostasis. Importantly, this drug eliminates the need for factor VIII and complications associated with inhibitor generation. Emicizumab has been approved for use in several countries including the United States and Japan for prophylaxis of bleeding episodes in hemophilia A with and without FVIII inhibitors. Therapy is also approved in the European Union for routine prophylaxis of bleeds in hemophilia A with inhibitors or severe hemophilia A without inhibitors. Unfortunately, emicizumab therapy presents unique challenges for routine and specialty coagulation tests currently used to monitor hemophilia A. In this review, hemophilia A is presented, the biochemistry of factor VIII is discussed, and the impact of the therapeutic agent emicizumab is highlighted.

From a bispecific monoclonal antibody to gene therapy: A new era in the treatment of hemophilia A

The treatment of hemophilia A has progressed amazingly in recent years. Emicizumab, a bispecific-humanized monoclonal antibody, is able to improve coagulation by bridging activated factor IX and factor X. Emicizumab is administered subcutaneously and much less often compared to factor VIII products. It has low immunogenicity, does not require dose adjustment, and can be administered regardless of the presence of factor VIII inhibitors. Thrombin generation assays but not factor VIII activity are indicated to guide and monitor the treatment. Emicizumab has enabled the conversion of patients with severe forms into patients with milder forms of hemophilia A. It has reduced the number of bleeding episodes compared to both on-demand and prophylactic substitution therapy and has an excellent safety profile. Gene therapy can elevate factor VIII plasma levels for many years after a single treatment course, could offer long-term protection from bleeding episodes, and minimize or eliminate the need for substitutive treatment with factor VIII concentrates. Gene therapy can provoke an immune response, manifested by an increase in common liver enzymes, that require immunotherapy. Long term monitoring is necessary to identify possible adverse effects. Future objectives are: the development of an ideal viral vector, the possibility of its re-administration, the use of gene therapy in hemophiliac children, and determining whether it can be successfully used to induce immune tolerance to factor VIII ceteri paribus. The future will determine the place of each type of treatment and group of patients for which it is indicated.

Progress, and prospects in the therapeutic armamentarium of persons with congenital hemophilia. Defining the place for liver-directed gene therapy

In persons with congenital severe hemophilia A (HA) living in high-income countries, twice weekly intravenous infusions of extended half-life (EHL) factor VIII (FVIII) products, or weekly/biweekly/monthly subcutaneous injections of emicizumab are the gold standard home treatments to grant days without hurdles and limitations. Once weekly/twice monthly infusions of EHL Factor IX (FIX) products achieve the same target in severe hemophilia B (HB). Gene therapy, which is likely to be licensed for clinical use within 1-2 years, embodies a shift beyond these standards. At an individual patient level, a single functional gene transfer leads to a > 10-yr almost full correction of the hemostatic defect in HB and to a sustained (3-6-yrs) expression of FVIII sufficient to discontinue exogenous clotting factor administrations. At the doses employed, the limited liver toxicity of systemically infused recombinant adeno-associated virus (rAAV) vectors is documented by long-term (12-15 yrs) follow-ups, and pre-existing high-titer neutralizing antibodies to the AAV5 vector are no longer an exclusion criterion for effective transgene expression with this vector. A safe durable treatment that converts a challenging illness to a phenotypically curable disease, allows persons to feel virtually free from the fears and the obligations of hemophilia for years/decades. Along with patient organizations and health care professionals, communicating to government authorities and reimbursement agencies the liberating potential of this substantial innovation, and disseminating across the Centers updated information on benefits and risks of this strategy, will align expectations of different stakeholders and establish the notion of a potentially lifelong cure of hemophilia.

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 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.