Three different patterns of calibrated automated thrombogram obtained with six different anticoagulants

Effect of PCC on Thrombin Generation among Patients on Factor Xa Inhibitors with Major Bleeding or Needing Urgent Surgery (GAUGE): Design and Rationale

Background  Direct factor Xa inhibitors (FXaIs) account for most oral anticoagulant use and FXaI-associated bleeding events are common. Clinicians have variable national and regional access to specific FXaI reversal agents such as andexanet alfa. Many centers have adopted the use of prothrombin complex concentrates (PCCs) as hemostatic therapy for FXaI-associated major bleeding events. PCC does not impact circulating FXaI levels and its mechanism of action to achieve hemostasis in FXaI-associated bleeding is uncertain. While PCC increases quantitative thrombin generation assay (TGA) parameters, it does not correct FXaI-altered thrombin generation kinetics, nor does it normalize thrombin generation. Clinical data supporting the use of PCC are based on cohort studies reporting clinical hemostatic efficacy, which is difficult to measure. The benefits of PCC for FXaI-associated bleeding beyond supportive care are uncertain. Objective  GAUGE is a prospective observational study designed to measure the effects of four-factor PCC administration (Octaplex) on TGA parameters among patients with FXaI-associated bleeding or needing urgent surgery. Methods  Laboratory outcomes will include the mean paired change in TGA parameters from pre- to post-PCC administration and the proportion of participants whose post-PCC TGA values fall within a defined reference range. Clinical outcomes will include hemostatic efficacy, thromboembolic complications, and all-cause death at 30 days post-PCC. Conclusion  Development of a viable and universally accessible FXaI bleed management strategy is crucial. GAUGE will provide in vivo data on the effects of PCC among patients with FXaI-associated bleeding.

DOAC-associated bleeding, hemostatic strategies, and thrombin generation assays - a review of the literature

Direct oral anticoagulants (DOACs) account for most oral anticoagulant use. DOAC-associated bleeding events are commonly encountered in clinical practice and are associated with substantial morbidity and mortality. Both specific reversal agents and nonspecific hemostatic therapies, such as prothrombin complex concentrates, are used in the management of DOAC-associated bleeding. Measuring hemostatic efficacy and demonstrating a clinical impact from these therapies among studies of bleeding patients is challenging. Thrombin generation assays provide information on the total hemostatic potential of plasma, and have emerged as a promising modality to both measure the impact of DOACs on coagulation and to evaluate the effects of hemostatic therapies among patients with DOAC-associated bleeding. The mechanisms by which nonspecific hemostatic agents impact coagulation and thrombin generation in the context of DOAC therapy are unclear. As a result, we undertook a review of the literature using a systematic search strategy with the goal of summarizing the effects of DOACs on thrombin generation and the effects of both specific reversal agents and nonspecific hemostatic therapies on DOAC-altered thrombin generation parameters. We sought to identify clinical studies focusing on whether altered thrombin generation is associated with clinical bleeding and whether correction of altered thrombin generation parameters predicts improvements in clinical hemostasis. Lastly, we sought to outline future directions for the application of thrombin generation assays toward anticoagulation therapies and the question of anticoagulation reversal.

Clinical use of thrombin generation assays

Determining patient's coagulation profile, i.e. detecting a bleeding tendency or the opposite, a thrombotic risk, is crucial for clinicians in many situations. Routine coagulation assays and even more specialized tests may not allow a relevant characterization of the hemostatic balance. In contrast, thrombin generation assay (TGA) is a global assay allowing the dynamic continuous and simultaneous recording of the combined effects of both thrombin generation and thrombin inactivation. TGA thus reflects the result of procoagulant and anticoagulant activities in blood and plasma. Because of this unique feature, TGA has been widely used in a wide array of settings from both research, clinical and pharmaceutical perspectives. This includes diagnosis, prognosis, prophylaxis, and treatment of inherited and acquired bleeding and thrombotic disorders. In addition, TGA has been shown to provide relevant information for the diagnosis of coagulopathies induced by infectious diseases, comprising also disturbance of the coagulation system in COVID-19, or for the assessment of early recurrence in breast cancer. This review article aims to document most clinical applications of TGA.

The Comparison of Therapeutic Efficacy Between Dabigatran Versus Warfarin in Patients With Nonvalvular Atrial Fibrillation

Background

Novel oral anticoagulants and warfarin are widely used for stroke prevention in patients with atrial fibrillation. The anticoagulation status of patients receiving warfarin or rivaroxaban has been studied. In this study, we aimed to evaluate the effect of dabigatran and warfarin on preventing thrombin generation (TG).

Methods

This retrospective study enrolled 237 nonvalvular atrial fibrillation (NVAF) subjects treated with 110 mg dabigatran etexilate twice daily and 224 NVAF patients received adjusted-dose warfarin (international normalized ratio [INR] of 2 to 3)). Coagulation assays, prothrombin fragment 1  +  2 (F₁₊₂), calibrated automated thrombogram, and thrombin–antithrombin complex (TAT) were detected at the steady state.

Results

Activated partial thromboplastin time (APTT), antithrombin III activity, fibrinogen, and lag time showed no difference between the two groups. Compared to the dabigatran group, prothrombin time and INR values were higher in the warfarin group (all P < .001). Thrombin time, endogenous thrombin potential, peak TG (C_max), F₁₊₂, and TAT were lower in the warfarin group. The inhibition of TG was still stronger in the warfarin group when the patients were divided into subgroups.

Conclusion

Conventional coagulation assays are suboptimal for assessing the coagulation status of dabigatran. TG could be used as supplementary assays to evaluate the anticoagulation effect of oral anticoagulants. Our results suggest that warfarin may inhibit TG more aggressively than dabigatran in patients regardless of age and kidney function.

Reference values for thrombin dynamics in platelet rich plasma

Thrombin generation (TG) is a better determinant of the overall function of the hemostatic system than routinely used clotting time-based assays and can be studied more in detail by thrombin dynamics analysis. Platelet poor plasma is often used to measure TG, however, measuring the contribution of the platelets is also important as patients with a low platelet count or with dysfunctional platelets have an increased risk of developing bleeding. In this study, platelet rich plasma (PRP) was collected from 117 healthy individuals. PRP was measured undiluted and diluted to a varying platelet concentration of 10*10⁹/L to 400*10⁹/L. Prothrombin conversion and thrombin inactivation were calculated from the data obtained by the TG parameters and coagulation factor levels (antithrombin, α₂Macroglobulin (α₂M) and fibrinogen). Reference ranges of TG and thrombin dynamics in PRP of 117 healthy individuals were established. Peak, velocity index and the maximum rate of prothrombin conversion increased linearly with platelet count, but endogenous thrombin potential reached a maximum at 150*10⁹/L as seen in a subset population (n = 20). More extensive analysis revealed that a platelet count below 50*10⁹/L did not affect TG parameters (except for the ETP). Correlation analysis indicated that the platelet count mainly affected the rate of prothrombin conversion. Inhibition of thrombin by antithrombin and α₂M increased with increasing TG, but the ratio of inhibition by antithrombin or α₂M remained the same independently of the total thrombin formed. In conclusion, TG and thrombin dynamics were assessed in PRP of healthy donors to provide reference values for future TG studies in PRP. Increasing the platelet count mainly affected the rate of prothrombin conversion and TG, rather than the total amount of thrombin formed.

Differential Assessment of Factor Xa Activity and Global Blood Coagulability Utilizing Novel Dielectric Coagulometry

An easy-to-use assessment for activated factor X (FXa) is lacking despite its pivotal role in the coagulation. Dielectric blood coagulometry (DBCM) was recently invented as a novel assessment tool for determining the whole blood coagulability by measuring the temporal change in the permittivity of blood. We previously reported that it could evaluate the global blood coagulability. This study aimed to apply the DBCM for assessing FXa activity and its inhibition by anticoagulants. We performed the DBCM analysis along with measurement of the FXa activity by a fluorometric assay in samples from healthy subjects, and identified a new index named maximum acceleration time (MAT) that had a correlation to the FXa activity. Next the DBCM analysis was performed using blood samples mixed with anticoagulants (unfractionated heparin, dalteparin, and edoxaban). Blood samples with three anticoagulants had different profiles of the temporal change in the permittivity, reflecting their different selectivity for FXa. We compared the MAT with the anti-FXa activity assay, and found that the prolongation of MAT was similarly correlated with the anti-FXa activity regardless of the type of anticoagulants. In conclusion, the DBCM has the possibility for evaluating the innate FXa activity and effect of anticoagulants focusing on their FXa inhibition.

Use of direct oral anticoagulants in antiphospholipid syndrome

The direct oral anticoagulants (DOACs) are therapeutic alternatives to warfarin and other vitamin K antagonists (VKAs), and constitute the standard of care for many indications. VKAs constitute the conventional therapy for the treatment and secondary thromboprophylaxis of thrombotic antiphospholipid syndrome (APS), but are often problematic, owing to the variable sensitivity of thromboplastins to lupus anticoagulant. Thus, the International Normalized Ratio may not accurately reflect anticoagulation intensity, or be clinically effective. Definition of the current role of DOACs in the treatment of APS is based on limited clinical trial data and information from other sources, including manufacturers' data, case series or cohort studies, and expert consensus. The Rivaroxaban in Antiphospholipid Syndrome (RAPS) randomized controlled trial (RCT), which had a laboratory surrogate primary outcome measure, suggests that rivaroxaban has the potential to be an effective and convenient alternative to warfarin in thrombotic APS patients with a single venous thromboembolism event requiring standard-intensity anticoagulation. However, further studies, in particular to provide better long-term efficacy and safety data, are needed before it can be widely recommended. APS patients are clinically heterogeneous, with the risk of recurrent thrombosis and the intensity of anticoagulation being influenced by their clinical phenotype and risk profile. DOAC trials involving homogeneous thrombotic APS populations, with the antiphospholipid antibody status well defined, will help to optimize the appropriate treatment in APS patient subgroups. Ongoing and emerging DOAC RCTs should provide further information to guide the use of DOACs in APS patients. Optimal identification of APS patients is a key step in working towards improved therapeutic strategies in these individuals.

Anwendungen des endogenen Thrombinpotenzials

An overview is given on the method and the applications of thrombin generation assays. Thrombin generation assays provide a tool which can be used to trace the entire thrombin formation and inactivation of a plasma sample. Therefore they are principally different from traditional global assays like PT and aPTT and promise new opportunities for the assessment of bleeding or thrombotic risks.

Effects of argatroban, danaparoid, and fondaparinux on trombin generation in heparin-induced thrombocytopenia

There is no in vitro data on the comparison of the effects of danaparoid, argatroban and fondaparinux on thrombin generation in patients with heparin-induced thrombocytopenia. It was the study objective to compare the in vitro anticoagulant potential of argatroban, danaparoid and fondaparinux using a thrombin generation assay TGA on a mixture of control platelet-rich plasma (PRP) and HIT patient platelet-poor plasma (PPP). The plasma of seven patients with a clear HIT diagnosed at our institution was selected. Mixtures of donor PRP and patient PPP were incubated with unfractionated heparin 0.2 U.mL-1, argatroban at 600 ng.mL-1, argatroban at 400 ng.mL-1, danaparoid at 0.65 IU.mL-1 and fondaparinux at 1 [uni03BC]g.mL-1. Thrombin generation was assessed by calibrated thrombinography. The percentage of inhibition of the endogenous thrombin potential observed with argatroban at 600 ng.mL-1 was statistically significantly higher compared with those observed with fondaparinux (median: 53.6% vs. 3.9%; p = 0.031) but not compared with argatroban at 400 ng.mL-1 and danaparoid. The percentage of inhibition of the thrombin peak observed with argatroban at 600 ng.mL-1 was statistically significantly higher compared with those observed with danaparoid (median: 71.2 vs. 56.8; p = 0.031) and fondaparinux (mean: 71.2 vs. 30; p = 0.031) but not with argatroban at 400 ng.mL-1. In conclusion, the in vitro effect of argatroban and danaparoid on thrombin generation seems to corroborate the results of clinical studies of these drugs in the treatment of HIT in term of efficiency. Fondaparinux showed a very small effect on thrombin generation evaluated by calibrated thrombinography.

Platelet aggregation impacts thrombin generation assessed by calibrated automated thrombography

A calibrated automated thrombogram (CAT) is performed usually with human platelet-free plasma (PFP) but may be more relevant with platelet-rich plasma (PRP). In this case, platelets are not stimulated by subendothelial molecules like collagen. Our aim was to assess the consequence of strong (collagen) or weak (ADP) induction of platelet release and aggregation on thrombin generation. Platelet aggregation in PRP was triggered with 10 µg/mL collagen or 10 µM ADP using a lumi-aggregometer. Thrombin generation curves were monitored by CAT in different conditions: PRP, PRP with activated platelets (actPRP), aggregated PRP (agPRP), aggregated platelets resuspended in autologous PFP (resPRP), PFP and PFP obtained after aggregation (agPFP). We found a 3-fold shortening of the lag time and time to peak and a marked increase in velocity and thrombin peak without changes in endogenous thrombin potential (ETP) in agPRP with both agonists compared with PRP. The same holds true in agPFP but with a marked increase in ETP compared with PFP. Similar changes in the kinetics of thrombin generation were observed with actPRP-collagen and to a lesser extent in resPRP-collagen compared with PRP. By contrast, there were no modifications of the thrombin generation curves in actPRP-ADP. Alpha-2-macroglobin-thrombin complexes were unchanged in the different PRP conditions but were increased in PFP prepared from agPFP compared to control PFP. Platelet aggregation during activation by agonists other than thrombin did not increase thrombin generation but accelerated its kinetics mainly via platelet content release and platelet-derived extracellular vesicules formation. In diseases characterized by altered platelet granule content or release as well as altered platelet activation, a platelet aggregation step prior to CAT analysis may be clinically relevant to improve laboratory estimation of the bleeding/thrombotic balance.

Comparative Assessment of the Anticoagulant Activity of Rivaroxaban and Dabigatran in Patients With Nonvalvular Atrial Fibrillation: A Noninterventional Study

There is a shortage of data in everyday clinical practice about the anticoagulant effects caused by the new oral anticoagulants (NOAs). Our aim was to estimate the intensity of anticoagulant activity induced by rivaroxaban 20 mg qd and dabigatran 110 mg bid among patients with nonvalvular atrial fibrillation (NV-AF).We studied 20 patients with NV-AF treated with dabigatran, and 20 patients treated with rivaroxaban. We performed conventional coagulation tests, thrombin generation (TG) test, thromboelastometry (ROTEM), and epinephrine-induced light transmission aggregometry (LTA) in all 40 patients and 20 controls. Hemoclot Thrombin Inhibitors (HTI) and Factor Xa Direct Inhibitor (DiXaI) assay were used to measure dabigatran and rivaroxaban plasma levels, respectively.Measurements of all assays estimating anticoagulant activity across the 2 patient groups were similar, except for aPTT. Patients on dabigatran exhibited statistically significantly prolonged aPTT values (P < 0.001). In LTA, patients on dabigatran also showed decreased aggregation compared to those on rivaroxaban (P = 0.045). Regarding the TG test, there was no association between endogenous thrombin potential (ETP) and rivaroxaban plasma levels (P = 0.33) as opposed to dabigatran levels (P < 0.001), but significant correlations were observed between rivaroxaban plasma concentrations and kinetic parameters of TG assay (Tlag, P = 0.045; Tmax, P = 0.016; and Cmax, P = 0.003).Based on ROTEM and TG assays, the anticoagulant effects induced by the 2 drugs given in the specific dose regimens in real-world patients were comparable. Only platelet aggregation was found to be more affected by dabigatran as compared to rivaroxaban.

Laboratory measurements of the oral direct factor Xa inhibitor edoxaban: comparison of prothrombin time, activated partial thromboplastin time, and thrombin generation assay

OBJECTIVES

Edoxaban, an oral direct factor Xa inhibitor, does not require routine monitoring. However, assessment of the anticoagulant effects may be required in certain situations.

METHODS

We investigated the effects of edoxaban on prothrombin time (PT), activated partial thromboplastin time (aPTT), and thrombin generation using human platelet-poor plasma (PPP) or platelet-rich plasma (PRP).

RESULTS

Edoxaban concentration-dependently prolonged PT and aPTT. There was a considerable variation in the magnitude of PT prolongation among the reagents used. The variability in aPTT prolongation among the reagents was smaller than that of PT. Edoxaban concentration-dependently inhibited thrombin generation, with a more potent effect seen in PPP than in PRP. Thrombin generation assay was three times more sensitive to edoxaban than PT and aPTT.

CONCLUSIONS

PT had disadvantages of a large variability among different PT reagents. aPTT could be used as a conventional and convenient test with a smaller variation among reagents. Thrombin generation was the most sensitive assay.

Warfarin versus dabigatran etexilate: an assessment of efficacy and safety in patients with atrial fibrillation

INTRODUCTION

Oral anticoagulation is the mainstay for stroke and thromboembolic event prevention in patients with atrial fibrillation (AF). Given limitations of warfarin therapy, non-vitamin K oral anticoagulants have been developed including direct thrombin inhibitors (i.e., dabigatran etexilate). Dabigatran etexilate has been tested thoroughly in terms of efficacy and safety in clinical trials and studies, involving 'real-world' cohorts. In this review, currently available evidence in patients with non-valvular AF is discussed.

AREAS COVERED

The pharmacology, efficacy and safety, and current aspects of use of dabigatran etexilate in patients with non-valvular AF are reviewed in a comparative manner to warfarin both for chronic anticoagulation and in different clinical settings.

EXPERT OPINION

Dabigatran etexilate appeared to have several pharmacokinetic and pharmacodynamic advantages over warfarin, as well as a favorable efficacy and safety profile being at least noninferior and often superior to warfarin in patients with non-valvular AF. The latter was shown in the clinical trials, meta-analyses and studies with 'real-world' data. Currently ongoing trials will expand the body of evidence on warfarin and will aid decision making in currently controversial areas. Important limitations of dabigatran etexilate include contraindications for its use in patients with prosthetic heart valves and end-stage chronic kidney disease.

Measurement of dabigatran and rivaroxaban in primary prevention of venous thromboembolism in 106 patients, who have undergone major orthopedic surgery: an observational study

No routine coagulation laboratory test is recommended during rivaroxaban or dabigatran treatment. However measuring drug concentration and/or anticoagulant activity can be desirable in some special clinical settings, such as bleeding, thrombosis recurrence or emergency surgery. The effects of dabigatran etexilate and rivaroxaban on various coagulation assays have been previously studied in normal plasma spiked with increasing concentrations of the drug. In contrast, few data are available in routinely treated patients. In order to perform and to interpret the results of these tests, it is necessary to determine the usual responses of patient's plasma. We have used several coagulation tests in a prospective study including 106 patients receiving thromboprophylactic treatment with dabigatran 150 or 220 mg od and rivaroxaban 10 mg od for major orthopaedic surgery. The most common tests--prothrombin time (PT) and activated partial thromboplastin time (aPTT)--give results, which vary according to the reagent used. To overcome this limitation, we advocate the use of plasma calibrators, which decreases the inter-laboratory heterogeneity of results. Anti-Xa measurement and Hemoclot, a thrombin diluted clotting assay, are specific assays which have been proposed for rivaroxaban and dabigatran respectively. These tests, conventional PT, aPTT and thrombin generation (TG) have been performed. We demonstrated that measurements of both drugs can determine reliably the drug concentration in patients' plasmas. PT is more prolonged with rivaroxaban than with dabigatran. Interestingly, the pattern of TG was clearly different in relation to the difference in the mechanism of action of the two new anticoagulants. A significant inter-individual variability of response is detected. Rivaroxaban--mean Cmax 140 ng/mL (extremes 0-412) induces a greater increase of PT than dabigatran. aPTT is sensitive to dabigatran. Rivaroxaban concentrations were in good agreement with two other studies while unexplained lower than expected concentrations were found in dabigatran patients receiving 220 mg once a day [mean Cmax 60 ng/mL (extremes 0-320)]. An interference by pantoprazole, a drug which reduces dabigatran absorption, could explain the observed lower than expected results.

Measurement of dabigatran and rivaroxaban in primary prevention of venous thromboembolism in 106 patients, who have undergone major orthopedic surgery: an observational study

No routine coagulation laboratory test is recommended during rivaroxaban or dabigatran treatment. However measuring drug concentration and/or anticoagulant activity can be desirable in some special clinical settings, such as bleeding, thrombosis recurrence or emergency surgery. The effects of dabigatran etexilate and rivaroxaban on various coagulation assays have been previously studied in normal plasma spiked with increasing concentrations of the drug. In contrast, few data are available in routinely treated patients. In order to perform and to interpret the results of these tests, it is necessary to determine the usual responses of patient's plasma. We have used several coagulation tests in a prospective study including 106 patients receiving thromboprophylactic treatment with dabigatran 150 or 220 mg od and rivaroxaban 10 mg od for major orthopaedic surgery. The most common tests--prothrombin time (PT) and activated partial thromboplastin time (aPTT)--give results, which vary according to the reagent used. To overcome this limitation, we advocate the use of plasma calibrators, which decreases the inter-laboratory heterogeneity of results. Anti-Xa measurement and Hemoclot, a thrombin diluted clotting assay, are specific assays which have been proposed for rivaroxaban and dabigatran respectively. These tests, conventional PT, aPTT and thrombin generation (TG) have been performed. We demonstrated that measurements of both drugs can determine reliably the drug concentration in patients' plasmas. PT is more prolonged with rivaroxaban than with dabigatran. Interestingly, the pattern of TG was clearly different in relation to the difference in the mechanism of action of the two new anticoagulants. A significant inter-individual variability of response is detected. Rivaroxaban--mean Cmax 140 ng/mL (extremes 0-412) induces a greater increase of PT than dabigatran. aPTT is sensitive to dabigatran. Rivaroxaban concentrations were in good agreement with two other studies while unexplained lower than expected concentrations were found in dabigatran patients receiving 220 mg once a day [mean Cmax 60 ng/mL (extremes 0-320)]. An interference by pantoprazole, a drug which reduces dabigatran absorption, could explain the observed lower than expected results.

In vitro studies using a global hemostasis assay to examine the anticoagulation effects in plasma by the direct thrombin inhibitors: dabigatran and argatroban

This study aimed to assess whether a global hemostatic assay we developed can measure the anticoagulant effects of the direct thrombin inhibitors (DTIs)--dabigatran and argatroban. A normal plasma pool (NPP) spiked with one of the DTIs and five plasma samples from patients with coronary heart disease spiked with dabigatran were examined. Fibrin formation and fibrin degradation were initiated by adding recombinant tissue factor (together with washed-frozen-thawed platelets and CaCl(2)) and recombinant tissue plasminogen activator. Fibrin optical density (OD) was recorded, based on which coagulation activation profile (Cp) and fibrinolysis activation profile (Fp) were determined. Moreover, the sum of OD values registered over time (fibrin OD-sum) was calculated to reflect the capacity of fibrin formation under the general effect by Cp and Fp. The endogenous thrombin potential (ETP) and the standard clotting markers i.e., activated partial thromboplastin time (APTT) and prothrombin time expressed as International Normalized Ratio (INR) were also analyzed. Results demonstrated that APTT, INR and ETP could detect the effects of the DTIs except for INR in NPP containing dabigatran. In our global assay, the DTIs depressed the fibrin formation (shown as decreased fibrin OD-sum value) by leading to decrease of Cp and increase of Fp. Thus, our global assay which examines both fibrin formation and degradation seems more advantageous than the other methods mentioned above, as regards the possibility of being a laboratory tool to monitor the antithrombotic therapy with DTIs.

Thrombin generation assay: a new tool to predict and optimize clinical outcome in cardiovascular patients?

Antithrombotic therapy (including antiplatelet and anticoagulant drugs) is the cornerstone of the current medical treatment of patients with acute coronary syndromes (ACS). This therapy and particularly the new antiplatelet and anticoagulant drugs have significantly reduced the ischemic risk, but have increased bleeding complications. Recently, several studies have emphasized the negative prognostic impact on long-term mortality of these bleeding adverse events. Thus, new assays to estimate the bleeding risk and the efficacy of these antithrombotic drugs are clearly in demand. Regarding the anticoagulant drugs, new promising data have emerged about the thrombin generation assay (TGA). TGA measures the ability of plasma to generate thrombin. TGA may be used to check coagulation function, to value risk of thrombosis and to compare the efficacy of different anticoagulants employed in clinical management of patients with ACS. The TGA result is a curve which describes the variation of thrombin's amount during the activation of the coagulation cascade. All available anticoagulant drugs influence the principal parameters generated by TGA and so it is possible to evaluate the effects of the medical treatment. In this review we provide a brief description of the assay and we summarize the principals of previous studies by analyzing the relationship between anticoagulant drugs and TGA. Moreover, a brief summary of its ability to predict ischemic and bleeding risks has been provided.

Impact of dabigatran on a large panel of routine or specific coagulation assays. Laboratory recommendations for monitoring of dabigatran etexilate

Due to low bioavailability and high inter-individual variability, monitoring of dabigatran may be required in specific situations to prevent the risk of bleedings or thrombosis. The aim of the study was to determine which coagulation assay(s) could be used to assess the impact of dabigatran on secondary haemostasis. Dabigatran was spiked at concentrations ranging from 4.7 ng/ml to 943.0 ng/ml in pooled citrated human platelet-poor plasma. The following clotting assays were performed: prothrombin time (PT); activated partial thromboplastin time (aPTT); thrombin time (TT); ecarin clotting time (ECT); ecarin chromogenic assay (ECA); prothrombinase-induced clotting time (PiCT); activated clotting time (ACT); Hemoclot Thrombin Inhibitor (HTI) and thrombin generation assay (TGA). A concentration-dependent prolongation of PT, dPT, and aPTT was observed with aPTT being the more sensitive test. The results varied mostly due to the clotting reagent. HTI, ECT and TGA were the most sensitive tests but are not available 24 hours a day. In addition, HTI showed a linear correlation with a good reproducibility. Dabigatran induced a concentration-dependent delay and inhibition of tissue factor-induced TGA. Cut-offs related with higher risk of bleedings or thrombosis were defined for each reagent of aPTT and HTI. In conclusion, aPTT could be used for the monitoring of dabigatran and as screening test for the risk of overdose. However, because of its higher sensitivity, good reproducibility, excellent linear correlation at all doses, its simplicity of use, and possibilities of automation, HTI should be considered as the gold-standard.

Global haemostasis assays, from bench to bedside

Bleeding and thrombosis are the ultimate clinical outcomes of aberrations in the haemostatic process. Haemostasis prevents excessive blood loss due to the effort of various compartments like the vasculature, blood cells, coagulation and fibrinolysis. The complexity of all processes involved makes the diagnosis of aberrations difficult, cumbersome and expensive. A single assay to detect any factor disturbing this haemostatic balance with high sensitivity and specificity would be of great value, especially if the outcome of this assay correlates well with clinical outcome. Despite years of research, such an assay is not yet available; however, some interesting candidates are under development and combine the effects of various compartments. This review describes the development of global haemostasis assays and summarizes the current state of the art of these haemostasis assays covering thrombin and plasmin generation, turbidity and thromboelastography/thromboelastometry. Finally, we discuss the applicability of global assays in clinical practice and we provide a future perspective on the ongoing development of automation and miniaturisation as it is our belief that these developments will benefit the standardization of global haemostasis assays.

Thrombin activatable fibrinolysis inhibitor: at the nexus of fibrinolysis and inflammation

TAFI (thrombin activatable fibrinolysis inhibitor) is the precursor of a basic carboxypeptidase (TAFIa) with strong antifibrinolytic and anti-inflammatory activity. Compelling evidence indicates that thrombin, either alone or in complex with thrombomodulin, is the main physiological activator of TAFI. For this reason derangements of thrombin formation, whatever the cause, may influence the fibrinolytic process too. Experimental models of thrombosis suggest that TAFI may participate in thrombus development and persistence under certain circumstances. In several models of pharmacological thrombolysis, the administration of TAFI inhibitors along with the fibrinolytic agent leads to a marked improvement of thrombus lysis, underscoring the potential of TAFI inhibitors as adjuvants for thrombolytic therapy. The role of TAFI in inflammatory diseases is more complex as it may serve as a defense mechanism, exacerbate the disease, or have no influence, depending on the nature of the model and the role played by the mediators controlled by TAFIa. Finally, the numerous clinical studies in patients with thrombotic disease support the idea that increased levels of TAFI and/or the enhancement of TAFI activation may represent a new risk factor for venous and arterial thrombosis.

[A new deal with new anticoagulants?]

The anticoagulant market has been very active recently with the development of new compounds including injectable anti-Xa such as fondaparinux, already available, and idraparinux, already replaced by its new biotynilateed form, and new oral drugs which can be divided into anti-IIa with dabigatran already available, and anti-Xa, such as the recently marketed rivaroxaban and apixaban still in the development stage. Others are coming forward. The competition is strong and the place for each drug remains to be determined. This review discusses these new anticoagulants in terms of efficacy and tolerance based on data in the literature. These recent reports mainly concern prophylaxis for orthopedic surgery but also consider treatment of deep venous thrombosis. The results of studies in heart patients have raised much curiosity since they will be determinant in the future use of innovating compounds, which could replace current oral anticoagulants. This will be upcoming but not yet for tomorrow.

Dabigatran enhances clot susceptibility to fibrinolysis by mechanisms dependent on and independent of thrombin-activatable fibrinolysis inhibitor

BACKGROUND

Anticoagulants are expected to promote fibrinolysis by counteracting the antifibrinolytic effects of thrombin, which include thrombin-activatable fibrinolysis inhibitor (TAFI) activation and clot structure enhancement. However, the efficiency of anticoagulants may vary remarkably, and the ability of direct thrombin inhibitors to facilitate clot lysis remains controversial.

OBJECTIVE

To evaluate the profibrinolytic effect of dabigatran, a new, direct thrombin inhibitor, using different in vitro models.

METHODS AND RESULTS

The resistance of tissue factor-induced plasma clots to fibrinolysis by exogenous tissue-type plasminogen activator (t-PA) (turbidimetric method) was reduced by dabigatran in a concentration-dependent manner, with > or = 50% shortening of lysis time at clinically relevant concentrations (1-2 microm). A similar effect was observed in the presence of low (0.1 and 1 nm) but not high (10 nm) concentrations of thrombomodulin. Acceleration of clot lysis by dabigatran was associated with a reduction in TAFI activation and thrombin generation, and was largely, although not completely, negated by an inhibitor of activated TAFI, potato tuber carboxypeptidase inhibitor. The assessment of the viscoelastic properties of clots showed that those generated in the presence of dabigatran were more permeable, were less rigid, and consisted of thicker fibers. The impact of these physical changes on fibrinolysis was investigated using a model under flow conditions, which demonstrated that dabigatran made the clots markedly more susceptible to flowing t-PA, by a mechanism that was largely TAFI-independent.

CONCLUSIONS

Dabigatran, at clinically relevant concentrations, enhances the susceptibility of plasma clots to t-PA-induced lysis by reducing TAFI activation and by altering the clot structure. These mechanisms might contribute to the antithrombotic activity of the drug.

Differential effects of factor IIa inhibitors on the endogenous thrombin potential

Calibrated automated thrombin generation measurement in clotting plasma (endogenous thrombin potential, ETP) is being used increasingly to monitor the effects of anticoagulant drugs. Calibrated automated thrombography measures the concentration of thrombin in clotting plasma by monitoring the cleavage of a fluorogenic substrate (Z-Gly-Gly-Arg-7-amino-4-methylcoumarin) and comparing it with a constant known thrombin activity in a parallel nonclotting sample. This study compared the concentration-dependent effects of different factor IIa inhibitors on the ETP. In accordance with a theoretical prediction, the monovalent factor IIa inhibitors melagatran and argatroban reduced peak thrombin concentrations without a marked effect on the lag time of thrombin generation. However, both bivalent factor IIa inhibitors lepirudin and bivalirudin markedly prolonged the lag time of thrombin generation and thus behaved like "super" factor Xa inhibitors according to the theoretical model. These findings have important consequences for therapeutic drug monitoring using thrombin generation assays. For monitoring of hirudins, the lag time of thrombin generation seems to be a very sensitive parameter. In contrast, for monitoring of argatroban, the evaluation of the ETP seems to be useful.

The Influence of Lepirudin, Bivalirudin and Dabigatran on the Calibrated Automated Thrombogram (CAT)

In a two centre study (laboratories in Diagnostica Stago and Biomnis) we compared the in vitro effect on thrombin generation (TG) of Dabigatran and Bivalirudin (reversible direct anti-IIa inhibitors) with that of Lepirudin (an irreversible direct anti-IIa inhibitor) spiked into normal pool plasma. The effect of Lepirudin, Bivalirudin and Dabigatran were evaluated in both centres using the CAT (Diagnostica Stago, France) TG method in a concentration ranges up to 5, 20 and 1 μg/mL respectively. Testing was done in triplicate and repeated over 2 days. To reduce assay variability both centres used the same reagents lots and the same normal pool plasma (George King, USA). The range of each drug tested extended well above the therapeutic range concentrations normally found in patient plasma (0.5 to 1.0 μg/mL, 5 to 10 μg/mL and 0.1 to 0.3 μg/mL respectively for Lepirudin, Bivalirudin and Dabigatran). To see the effect of increasing activation forces, TG was performed at 3 different final concentrations of Tissue Factor (TF) - 1, 5 and 20 pM. All reagents were used as recommended by the manufacturer (Thrombinoscope, The Netherlands). A prolongation in the lag time (LT) is observed with all 3 drugs with all 3 concentrations of TF, but this is more marked for Lepirudin and Bivalirudin than it is for Dabigatran. In the therapeutic range Dabigatran (at 5pM TF) shows both an increase in LT and a decrease in peak thrombin and the ETP. At low concentration of Bivalirudin or Lepirudin, there is a paradoxical increase in peak height, which is even more pronounced at low TF concentration. At 1pM TF, this paradoxical peak increase is also observed with Dabigatran.

Results obtained in both laboratories are similar and complement our previous results and those reported elsewhere (1–4). The effect of Lepirudin and Bivalirudin on TG is different from that of Dabigatran. We also note that at lower TF concentration the anticoagulant effect on TG initiation is more intense but the test becomes less reproducible.

Clinical pharmacokinetics and pharmacodynamics of the oral direct thrombin inhibitor dabigatran etexilate

The direct thrombin inhibitor dabigatran etexilate is currently in phase III of development for the prophylaxis and treatment of thromboembolic disorders, with three trials completed in primary venous thromboembolism (VTE) prevention. Dabigatran etexilate is an orally administered prodrug, which is rapidly absorbed and converted to the active form, dabigatran. Dabigatran has been shown to specifically and reversibly inhibit thrombin, the key enzyme in the coagulation cascade. Studies in healthy volunteers and in patients undergoing orthopaedic surgery have indicated that dabigatran has a predictable pharmacokinetic/pharmacodynamic profile, allowing for a fixed-dose regimen. Peak plasma concentrations of dabigatran are reached approximately 2 hours after oral administration in healthy volunteers, with no unexpected accumulation of drug concentrations upon multiple dosing. Excretion is predominantly via the renal route as unchanged drug. Dabigatran is not metabolized by cytochrome P450 isoenzymes. The small differences in dabigatran pharmacokinetics associated with age and gender are attributed to variations in renal function. Additional studies have shown that the pharmacokinetic/pharmacodynamic profile of dabigatran is consistent across a range of patient populations, with no effect of moderate hepatic impairment being observed. Drug-drug interactions are not observed with concomitant administration of atorvastatin, diclofenac or digoxin. The pharmacodynamic profile of dabigatran demonstrates effective anticoagulation combined with a low risk of bleeding. Further phase III studies are ongoing, including acute VTE treatment and stroke prevention in atrial fibrillation; the results obtained so far show that dabigatran etexilate is well tolerated and effective in the treatment and prevention of thromboembolic events.