Large inter-individual variation of the pharmacodynamic effect of anticoagulant drugs on thrombin generation

Manufacture and Initial Characterisation of RAPIDTM Biodynamic Haematogel, an Autologous Platelet and Leukocyte-Rich Plasma Gel for Diabetic Foot Ulcers

This observational study reports the process for the manufacture of RAPIDTM Biodynamic Haematogel and explores the properties of the platelet and leukocyte-rich plasma gels formed. Gels were manufactured from 60 mL of human blood using the protocol of Biotherapy Services. Platelet and leukocyte content, time-to-gel, gel weight and the temporal profile of liquid exudation from the gels were measured, along with the content of growth factors VEGF and PDGF in the releasate. The effect of the releasate on human keratinocyte (HaCat) cell proliferation was also determined. The platelet and leukocyte concentrations in donor blood were 1.60-8.10 × 108 and 1.00 × 106-2.00 × 107 cells/mL, which were concentrated 2.67- and 1.12-fold, respectively, during processing. Structurally weak gels were formed which exuded a clear liquid releasate (77.4% w/w of gel weight over 60 min) that contained 278 pg/mL VEGF and 1319 pg/mL PDGF. The releasate produced concentration-dependent proliferation of HaCat cells: 5-15% releasate produced a 2.7-8.9-fold increase in growth over 48 h. In conclusion, we have described the point-of-care manufacturing protocol and characterised the gel properties of RAPIDTM Biodynamic Haematogel. This is an essential first step towards identifying, understanding and controlling critical processing parameters that impact on this medicinal product's quality.

Development and Validation of a Nomogram for Predicting Heparin Resistance in Neonates and Young Infants Undergoing Cardiac Surgery: A Retrospective Study

BACKGROUND

Heparin resistance (HR) is a common finding in pediatric cardiac surgery and generally refers to decreased sensitivity to heparin. Antithrombin (AT) deficiency is considered the primary mechanism of HR; however, the etiology of HR may be multifactorial. Early identification of HR might help optimize heparin anticoagulation management. This study aimed to develop a predictive nomogram for HR in neonates and young infants undergoing cardiac surgery.

METHODS

From January 2020 to August 2022, a total of 296 pediatric patients 1 to 180 days of age were included in this retrospective study. The patients were randomly divided into development and validation cohorts in a 7:3 ratio. Univariable logistic regression and the Least Absolute Shrinkage and Selection Operator (LASSO) regularization were used for variable selection. A multivariable logistic regression was performed to identify predictors and establish a nomogram to predict HR risk. Discrimination, calibration, and clinical usefulness were assessed in the development and validation cohorts.

RESULTS

After the multistep variable selection, AT activity, platelet count, and fibrinogen were predictors for HR in neonates and young infants. The prediction model constructed using these 3 factors achieved an area under the receiver operating characteristic curve (ROC-AUC) of 0.874 and 0.873 in the development and validation cohorts. The Hosmer-Lemeshow test did not find evidence of a lack of fit ( P = .768). The calibration curve of the nomogram was close to the ideal diagonal line. Furthermore, the model performed well in neonate and infant subgroups.

CONCLUSIONS

A nomogram based on preoperative variables was developed to predict the HR risk in neonates and young infants undergoing cardiac surgery. This provides clinicians with a simple tool for the early prediction of HR, which may help optimize heparin anticoagulation strategies in this vulnerable patient population.

Bioimpedance Analysis as Early Predictor for Clot Formation Inside a Blood-Perfused Test Chamber: Proof of Concept Using an In Vitro Test-Circuit

Clot formation inside a membrane oxygenator (MO) due to blood-to-foreign surface interaction represents a frequent complication during extracorporeal membrane oxygenation. Since current standard monitoring methods of coagulation status inside the MO fail to detect clot formation at an early stage, reliable sensors for early clot detection are in demand to reduce associated complications and adverse events. Bioimpedance analysis offers a monitoring concept by integrating sensor fibers into the MO. Herein, the feasibility of clot detection via bioimpedance analysis is evaluated. A custom-made test chamber with integrated titanium fibers acting as sensors was perfused with heparinized human whole blood in an in vitro test circuit until clot formation occurred. The clot detection capability of bioimpedance analysis was directly compared to the pressure difference across the test chamber (ΔP-TC), analogous to the measurement across MOs (ΔP-MO), the clinical gold standard for clot detection. We found that bioimpedance measurement increased significantly 8 min prior to a significant increase in ΔP-TC, indicating fulminant clot formation. Experiments without clot formation resulted in a lack of increase in bioimpedance or ΔP-TC. This study shows that clot detection via bioimpedance analysis under flow conditions in a blood-perfused test chamber is generally feasible, thus paving the way for further investigation.

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.

Titrated versus conventional anticoagulation management for thrombin generation in cardiac surgery: a randomized controlled trial

PURPOSE

Optimal heparin titration during cardiopulmonary bypass (CPB) may reduce coagulation system activation and preserve hemostatic function post-CPB. Our objective was to assess if the Heparin Management System (HMS) Plus improves heparin titration, thereby leading to higher thrombin generation post-CPB compared with activated clotting time (ACT)-guided management.

METHODS

We conducted a randomized controlled trial of 100 patients undergoing cardiac surgery with CPB at a single center. A total of 50 patients were randomized to conventional ACT-guided management, and 50 to the HMS Plus system. The primary outcome was change in thrombin generation post-CPB compared with baseline, as assessed by calibrated automated thrombography. Secondary outcomes included intraoperative blood loss, chest drain output up to 72 hr, and transfusions. In an exploratory analysis, we compared the quintile of patients with the highest average heparin concentration on CPB (≥ 4.0 mg⋅kg-1) with the rest of the cohort.

RESULTS

A total of 100 patients were included in an intent-to-treat analysis. We observed no difference in post-CPB thrombin generation or secondary outcomes. However, patients in the HMS Plus group had higher average heparin concentrations while on CPB than patients in the conventional management group did (mean difference, -0.21; 95% confidence interval, -0.42 to -0.01). The quintile of patients with the highest average heparin concentration (4.0 mg⋅kg-1) had higher thrombin generation post-CPB than the rest of the cohort did.

CONCLUSIONS

The HMS Plus system did not show significant benefits in thrombin generation, bleeding outcomes, or transfusion in patients undergoing cardiac surgery with CPB. Higher average heparin concentrations on CPB were associated with higher post-CPB thrombin generation.

STUDY REGISTRATION

www.

CLINICALTRIALS

gov (NCT03347201); first submitted 12 October 2017.

Anticoagulated patients exhibit intact endogenous thrombin potential using ST Genesia unlike the Calibrated Automated Thrombogram

Background

The thrombin generation (TG) assay is a feasible but labor‐intensive method for detecting global coagulation. It enables comprehensive assessment of anticoagulation, while drug‐specific assays assess only exposure. Traditionally, the Calibrated Automated Thrombogram (CAT) has been used, however the ST Genesia (Diagnostica Stago) allows automated evaluation.

Objective

We aimed to observe coagulation using the ST Genesia and compare the data with those of CAT in anticoagulated patients.

Patients and methods

In total, 43 frozen‐thawed samples were studied using DrugScreen to assess direct oral anticoagulants (DOACs), warfarin, and low‐molecular‐weight heparin. Twenty samples (nine rivaroxaban, five apixaban, three warfarin, and three heparin) were also compared using CAT (5 pM tissue factor).

Results

TG reduction in DrugScreen depended on the specific drug and modestly correlated with DOAC levels (lag time R² = 0.36; peak R² = 0.50). The best correlation was observed with peak thrombin and rivaroxaban‐specified anti–activated factor X (anti‐Xa) activity (R² = 0.60). When comparing ST Genesia with CAT, only the results for apixaban concorded (R² = 0.97). Unlike CAT, ST Genesia yielded a normal endogenous thrombin potential (ETP) in 77% (24/31) activated factor X inhibitor cases, and it failed to give readouts at international normalized ratio (INR) ≥4.5 and at anti‐Xa ≥1.0 IU/mL.

Conclusion

The ST Genesia data did not correlate with CAT, but it was independently associated with INR, anti‐Xa, and DOAC concentrations. The lag time and peak responses were similar; the major differences were that ST Genesia showed no ETP effect of DOACs and failed to give readout at high INR or anti‐Xa activity.

A pilot study of procoagulant platelet extracellular vesicles and P-selectin increase during induction treatment in acute lymphoblastic leukaemia paediatric patients: two new biomarkers of thrombogenic risk?

In paediatric acute lymphoblastic leukaemia (ALL), focus has shifted towards preventing treatment-related complications, including venous thromboembolism, the cause of significant mortality and morbidity. To better understand thrombogenic mechanisms during induction treatment, we studied the number, origin and procoagulant activity of extracellular vesicles (EVs) and P-selectin level throughout the induction course in 24 paediatric patients. EVs were mainly of platelet origin. We observed a significant increase in EV number, in platelet EV number and P-selectin level throughout the induction course. There was a correlation between higher EV and platelet EV number, P-selectin level, higher platelet count and leucocyte count. We also observed a correlation between higher EV procoagulant activity and higher platelet count and leucocyte count and higher P-selectin level. Older age and T phenotype were associated with a higher EV procoagulant activity. Platelet EV generation may play a role in thrombogenic complications in ALL patients and could serve as a biomarker to identify patients with a high risk of thrombosis. As a marker of platelet activation, P-selectin may be another relevant marker with the advantage of being easier to analyse in clinical practice.

Inhibition of thrombin generation 12 hours after intake of direct oral anticoagulants

BACKGROUND

The residual antithrombotic activity 12 hours after intake of direct oral anticoagulants (DOACs) is of clinical relevance in the setting of bleeding or urgent surgery.

OBJECTIVE

To evaluate the effects of DOACs on thrombin generation 12 hours after DOAC intake in comparison to baseline and a healthy control group.

METHODS

Eighty patients were recruited, 20 patients for each approved DOAC: apixaban, edoxaban, rivaroxaban, and dabigatran. The patients were either to be put on anticoagulation for the first time or had stopped taking oral anticoagulation for at least 48 hours. Blood plasma was sampled before (baseline) and 12 hours after starting DOAC for quantification of drug levels and thrombin generation assayed using an automated system (ST Genesia). Sixty-one blood donors served as control group.

RESULTS

The factor Xa inhibitors significantly increased lag time (137%-219%) and reduced thrombin peak (47%-76%) and velocity index (17%-44%) after 12 hours compared to baseline. Dabigatran showed prolongation of lag time to 133% and time to peak to 119%. All patients had residual antithrombotic activity, with reduced thrombin generation parameters 12 hours after DOAC intake compared to baseline and to the healthy control group. This effect remained significant in patients with low residual DOAC plasma levels <50 ng/mL.

CONCLUSION

Thrombin generation remains reduced 12 hours after DOAC intake. While thrombin peak is particularly modified by factor Xa inhibitors, all DOACs prolong the lag time and time to thrombin peak. In the setting of bleeding or urgent surgery, the automated thrombin generation assay may assist in decision making and antidote administration.

Heparins: A Shift of Paradigm

Heparins inhibit the thrombin forming capacity of plasma, i. e., the endogenous thrombin potential (ETP), by their anti-thrombin (aIIa) activity, the anti-factor Xa (aXa) activity is of minimal importance. This holds for both unfractionated heparin (UFH) and low molecular weight heparin (LMWH) at aXa/aIIa ratios < 25. Clinical experience and epidemiological evidence show a direct relationship between the ETP and the risk of thrombosis and bleeding. Consequently, the therapeutic potency of a heparin is determined by its aIIa activity, i.e., the concentration of a domain in which 12 sugar flank the high affinity antithrombin-binding pentasaccharide (HA5) at one side. The response of individual plasmas to a fixed dose of any heparin is highly variable. This suggests that individualization of heparin dosage, on basis of the ETP, might reduce bleeding or re-thrombosis. There exist simple laboratory methods for both the ETP and the concentration of the active domain. These methods can be used both for unequivocally characterization of a heparin preparation and for controlling heparin therapy and allow arbitrary units relative to a standard to be abandoned. These tests are as robust as any hematological routine test but not yet routinely available, which severely encumbers progress in the field.

Rivaroxaban pharmacodynamics in healthy volunteers evaluated with thrombin generation and the active protein C system: Modeling and assessing interindividual variability

BACKGROUND

Rivaroxaban is a direct factor Xa inhibitor with substantial inter-individual pharmacokinetic (PK) variability. Pharmacodynamic (PD) variability, especially assessed with thrombin generation (TG), has been less documented.

OBJECTIVES

(i) To assess TG parameter time profiles in healthy volunteers, with TG being studied under different conditions and (ii) to model the relationship between rivaroxaban concentrations and TG parameters and subsequently estimate interindividual variability.

METHODS

Sixty healthy male volunteers (DRIVING-NCT01627665) received a single 40-mg rivaroxaban dose. Blood sampling was performed at baseline and 10 predefined time points over 24 h. The TG was investigated with the fully automated ST-Genesia system (Stago), using two tissue-factor (TF) concentrations, in the absence (-), or presence (+) of thrombomodulin (TM) for the lowest one. The PD models were built to characterize the relationships between plasma rivaroxaban concentrations and endogenous thrombin potential (ETP) or peak height induced by the lowest TF concentration.

RESULTS

Thrombin generation parameter time profiles with the lowest TF concentration showed a good sensitivity to rivaroxaban, especially +TM (active protein C negative feedback). The relationship between rivaroxaban concentrations and TG parameters was modeled with a sigmoidal relation. Mean rivaroxaban concentrations halving the baseline value of ETP and peak height (-TM) (C50 ) were of 284 and 33.2 ng/mL, respectively: +TM, C50 declined to 19.4 and 13.8 ng/mL, reflecting a powerful inhibitory effect. The estimated C50 population coefficients of variation were of 12.2% (-TM) and 31.3% (+TM) with the peak height models, 34.8% (+TM) with the ETP model.

CONCLUSIONS

This low-rivaroxaban to moderate-rivaroxaban PD variability in healthy volunteers contrasts with the substantial PK variability and deserves to be studied in different patient settings.

Assessment of the analytical performances and sample stability on ST Genesia system using the STG-DrugScreen application

BACKGROUND

Thrombin generation testing has been used to provide information on the coagulation phenotype of patients. The most used technique is the calibrated automated thrombogram (CAT) but it suffers from a lack of standardization, preventing its implementation in routine. The ST Genesia is a new analyzer designed to assess thrombin generation based on the same principle as the CAT. Unlike the CAT system, the ST Genesia is a benchtop, fully automated analyzer, able to perform the analyses individually and not by batch, with strict control of variables such as temperature and volumes, ensuring, theoretically, maximal reproducibility.

OBJECTIVES

This study aimed at assessing the performance of the STG-DrugScreen application on the ST Genesia analyzer. We also aimed at exploring stability of plasma samples after freezing and defining a reference normal range.

RESULTS

Results demonstrated the excellent interexperiment precision of the ST Genesia and confirmed that the use of a reference plasma helps reducing the inter-experiments variability. Stability revealed that plasma samples are stable for at least 11 months at -70°C or lower, except for those containing low molecular weight heparins which have to be tested within 6 months. Freezing had no effect on the majority of thrombin generation parameters except on time to peak.

CONCLUSIONS

Our results suggest an easy implementation of thrombin generation with the use of ST Genesia in the routine laboratory. This will facilitate the design of multicentric studies and enable the establishment of reliable and evidence-based thresholds, which may improve the management of patients treated with anticoagulants.

Periprocedural management of anticoagulation for atrial fibrillation catheter ablation in direct oral anticoagulant-treated patients

BACKGROUND

Guidelines recommend performing atrial fibrillation (AF) catheter ablation without interruption of a direct oral anticoagulants (DOACs) and to administer unfractionated heparin (UFH) for an activated clotting time (ACT) ≥300 seconds, by analogy with vitamin K antagonist (VKA). Nevertheless, pharmacological differences between DOACs and VKA, especially regarding ACT sensitivity and UFH response, prevent extrapolation from VKA to DOACs.

HYPOTHESIS

The level of anticoagulation at the time of the procedure in uninterrupted DOAC-treated patients is unpredictable and would complicate intraprocedural UFH administration and monitoring.

METHODS

This prospective study included interrupted DOAC-treated patients requiring AF ablation. Preprocedural DOAC concentration ([DOAC]), intraprocedural UFH administration, and ACT values were recorded. A cohort of DOAC-treated patients requiring flutter catheter ablation was considered to illustrate [DOAC] without DOAC interruption.

RESULTS

Forty-eight patients underwent AF and 14 patients underwent flutter ablation, respectively. In uninterrupted DOAC-treated patients, [DOAC] ranged from ≤30 to 466 ng/mL. When DOAC were interrupted, from 54 to 218 hours, [DOAC] were minimal (maximum: 36 ng/mL), preventing DOAC-ACT interference. Anyway, ACT values were poorly correlated with UFH doses (R ²  = 0.2256).

CONCLUSIONS

Our data showed that uninterrupted DOAC therapy resulted in an unpredictable and highly variable initial level of anticoagulation before catheter ablation. Moreover, even with DOAC interruption preventing interference between DOAC, UFH, and ACT, intraprocedural UFH monitoring was complex. Altogether, our exploratory results call into question the appropriateness of transposing UFH dose protocols, as well as the relevance of ACT monitoring in uninterrupted DOAC-treated patients.

Thrombodynamics, a new global coagulation test: Measurement of heparin efficiency

The actual coagulation status may be reliably measured using only highly sensitive global functional tests; however, they are not numerous and all of them have disadvantages. Thrombodynamics (TD), a novel global coagulation test, is sensitive to hypo- and hypercoagulable states. The main properties of this test were investigated, and its capabilities for hemostasis analysis were verified through pharmacodynamic monitoring of the most widely used anticoagulants, heparins. The anticoagulant effects in the plasma of donors (n = 20) and patients after hip replacement (n = 20) spiked with unfractionated heparin or enoxaparin were measured in vitro to eliminate the influence of pharmacokinetic factors. Sensitivity for heparins was compared for activated partial thromboplastin time, thrombin generation tests and TD. TD was shown to reliably characterize the pharmacodynamics of any heparin in the entire range of its prophylactic and therapeutic concentrations. Inter-individual variability for the anticoagulant action of heparins was also calculated using the TD data. This variability did not differ between the investigated groups and did not exceed 12% and 20% for the stationary clot growth rate in the presence of unfractionated heparin and enoxaparin, respectively. That finding was in accordance with the values determined earlier using the thrombin generation test. The study results showed that TD has advantages over the other global methods of coagulation analysis. These advantages are good standardization, high reproducibility, independence of the parameter values from patient age and gender, and a narrower parameter distribution in a normal population. These results indicate that TD is a promising universal assessment method that improves the quality of hemostasis analysis because it more reliably detects deviations from the parameters' reference values.

Prediction of bleeding risk in patients taking vitamin K antagonists using thrombin generation testing

Until recently, vitamin K antagonists (VKAs) were the mainstay of oral anticoagulant treatment with bleeding as the most prevalent adverse effect. One to four percent of patients experience major bleeding episodes, while clinically relevant bleeding occurs in up to 20%. At this moment no laboratory assays are available to identify patients at risk for bleeding. With this study we aimed to investigate whether thrombin generation tests might identify a bleeding risk in patients taking VKAs. This prospective cohort study included 129 patients taking VKAs for more than three months. Calibrated automated thrombinography (CAT) was performed in whole blood, platelet rich and platelet poor plasma. Hematocrit, hemoglobin concentrations and the International Normalized Ratio (INR) were defined and coagulation factor levels were measured. Forty clinically relevant bleeding episodes were registered in 26 patients during follow-up. No differences were found in plasma CAT parameters or INR values. Bleeding was not associated with age, sex, hematocrit, hemoglobin levels or coagulation factor levels. In whole blood a significantly lower endogenous thrombin potential (ETP) and peak were found in patients with bleeding (median ETP: 182.5 versus 256.2 nM.min, p = 0.002; peak: 23.9 versus 39.1 nM, p = 0.029). Additionally, the area under the receiver operating curve (AUC ROC) was significantly associated with bleeding (ETP: 0.700, p = 0.002; peak: 0.642, p = 0.029). HAS-BLED scores were also significantly higher in bleeding patients (3 versus 2, p = 0.003), with an AUC ROC 0.682 (p = 0.004). In conclusion, bleeding in patients taking VKAs is associated with a decreased whole blood ETP and peak as well as with an increased HAS-BLED score.

A century of heparin: past, present and future

Heparin was discovered around 1922 by Howell (Baltimore) and was further developed by the teams of Best (Toronto) and Jorpes (Stockholm). Kakkar (London) propagated its routine use for the prevention of postoperative thrombosis from 1971 onwards. The discovery of low molecular weight heparins (1976, Johnson, London) and their development in the subsequent years led to the present arsenal of clinically useful drugs. In 1976, three teams independently found that a specific structure in heparin binds tightly to antithrombin. This enabled the teams of Lindahl (Stockholm) and Casu (Milan) to determine the pentasaccharide structure responsible for this binding and Petitou, from the Choay team (Paris), to synthesize it (1983). It was found (Olson and others) that heparin facilitates the interaction between antithrombin and a clotting enzyme by allosteric changes in the antithrombin (important for factor Xa) and by facilitating the approach of the enzyme to antithrombin via its "sliding" along the heparin molecule (important for thrombin). Antithrombin action therefore requires a minimum length of seven sugar units next to the pentasaccharide whereas anti-factor Xa action does not. The effect of heparin is almost entirely due to anti-thrombin action (B≐guin), so anti-factor Xa activity does not reflect the concentration of anticoagulant heparin. The anticoagulant effect is poorly reflected by the activated partial thromboplastin time. Because present clinical use is based on the latter tests, it is not generally known that the individual response to heparin shows an extremely wide variation. Individualization of heparin dosage is likely to improve clinical results.

Unfractionated and Low-Molecular-Weight Heparin and the Phosphodiesterase Inhibitors, IBMX and Cilostazol, Block Ex Vivo Equid Herpesvirus Type-1-Induced Platelet Activation

Equid herpes virus type-1 (EHV-1) is a major pathogen of horses, causing abortion storms and outbreaks of herpes virus myeloencephalopathy. These clinical syndromes are partly attributed to ischemic injury from thrombosis in placental and spinal vessels. The mechanism of thrombosis in affected horses is unknown. We have previously shown that EHV-1 activates platelets through virus-associated tissue factor-initiated thrombin generation. Activated platelets participate in thrombus formation by providing a surface to localize coagulation factor complexes that amplify and propagate thrombin generation. We hypothesized that coagulation inhibitors that suppress thrombin generation (heparins) or platelet inhibitors that impede post-receptor thrombin signaling [phosphodiesterase (PDE) antagonists] would inhibit EHV-1-induced platelet activation ex vivo. We exposed platelet-rich plasma (PRP) collected from healthy horses to the RacL11 abortigenic and Ab4 neuropathogenic strains of EHV-1 at 1 plaque-forming unit/cell in the presence or absence of unfractionated heparin (UFH), low-molecular-weight heparin (LMWH) or the PDE inhibitors, 3-isobutyl-1methylxanthine (IBMX), and cilostazol. We assessed platelet activation status in flow cytometric assays by measuring P-selectin expression. We found that all of the inhibitors blocked EHV-1- and thrombin-induced platelet activation in a dose-dependent manner. Platelet activation in PRP was maximally inhibited at concentrations of 0.05 U/mL UFH and 2.5 μg/mL LMWH. These concentrations represented 0.1–0.2 U/mL anti-factor Xa activity measured in chromogenic assays. Both IBMX and cilostazol showed maximal inhibition of platelet activation at the highest tested concentration of 50 μM, but inhibition was lower than that seen with UFH and LMWH. Our results indicate that heparin anticoagulants and strong non-selective (IBMX) or isoenzyme-3 selective (cilostazol) PDE antagonists inhibit ex vivo EHV-1-induced platelet activation. These drugs have potential as adjunctive therapy to reduce the serious complications associated with EHV-1-induced thrombosis. Treatment trials are warranted to determine whether these drugs yield clinical benefit when administered to horses infected with EHV-1.

The modification of the thrombin generation test for the clinical assessment of dabigatran etexilate efficiency

A new oral anticoagulant, dabigatran etexilate (DE, a prodrug of direct thrombin inhibitor (DTI) dabigatran), has been used clinically to prevent thrombosis. The assessment of dabigatran efficiency is necessary in some clinical cases, such as renal insufficiency, risk of bleeding, and drug interactions. However, a specific thrombin generation test (TGT) that is one of the most informative and sensitive to anticoagulant therapy (calibrated automated thrombinography (САТ)) shows a paradoxical increase of test parameters, such as endogenous thrombin potential (ETP) and peak thrombin, in patients receiving DE. The paradoxical behaviour of ETP and peak thrombin in these patients in the presence of DTIs is mostly caused by a decrease in the activity of thrombin in the α₂-macroglobulin-thrombin complex that is used as a calibrator in CAT. For a correct estimation of the TGT parameters in patient’s plasma containing DTIs we proposed to use our previously described alternative calibration method that is based on the measurement of the fluorescence signal of a well-known concentration of the reaction product (7-amino-4-methylcoumarin). In this study, the validity of such approach was demonstrated in an ex vivo study in patients with knee replacement and two special patients with multiple myeloma, who received DE for thrombosis prophylaxis.

Short-Term Heparin Kinetics during Catheter Ablation of Atrial Fibrillation

BACKGROUND

Percutaneous catheter ablation of atrial fibrillation (CA-AF) is a treatment option for symptomatic drug-refractory atrial fibrillation (AF). CA-AF carries a risk for thromboembolic complications that has been minimized by the use of intraprocedural intravenous unfractionated heparin (UFH). The optimal administration of UFH as well as its kinetics are not well established and need to be precisely determined.

METHODS AND RESULTS

A total 102 of consecutive patients suffering from symptomatic drug-refractory AF underwent CA-AF. The mean age was 61 ± 10 years old. After transseptal puncture of the fossa ovalis, weight-adjusted UFH bolus (100 U/kg) was infused. A significant increase in activated clotting time (ACT) was observed from an average value of 100 ± 27 seconds at baseline, to 355 ± 94 seconds at 10 min (T10), to 375 ± 90 seconds at 20 min (T20). Twenty-four patients failed to reach the targeted ACT value of ≥300 seconds at T10 and more than half of these remained with subtherapeutic ACT values at T20. This subset of patients showed similar clinical characteristics and amount of UFH but were more frequently prescribed preprocedural vitamin K1 than the rest of the study population.

CONCLUSIONS

In a typical intervention setting, UFH displays unexpected slow anticoagulation kinetics in a significant proportion of procedures up to 20 minutes after infusion. These findings support the infusion of UFH before transseptal puncture or any left-sided catheterization with early ACT measurements to identify patients with delayed kinetics. They are in line with recent guidelines to perform CA-AF under therapeutic anticoagulation.

Rivaroxaban and warfarin achieve effective anticoagulation, as assessed by inhibition of TG and in-vivo markers of coagulation activation, in patients with venous thromboembolism

INTRODUCTION

Rivaroxaban is non-inferior to warfarin for the treatment of venous thromboembolism, with regard to clinical efficacy and safety. The ex-vivo effects of warfarin versus therapeutic dose rivaroxaban on in-vivo markers of coagulation activation and thrombin generation remain undefined. The aim of this study was to compare the effects of warfarin and therapeutic dose rivaroxaban on ex-vivo thrombin generation (TG), and the in-vivo markers of coagulation activation, prothrombin fragment 1.2 (F1.2), thrombin-antithrombin complex (TAT), and D-dimer.

METHODS

Eighty-five patients with venous thromboembolism were studied, 45 on warfarin, target INR 2.5 and 40 on rivaroxaban 20mg once daily.

RESULTS

Anticoagulation was in therapeutic range in 71% (32/45) warfarin and 65% (26/40) rivaroxaban treated patients. 8 patients on warfarin and 9 patients on rivaroxaban had subtherapeutic INR and rivaroxaban levels respectively. Both rivaroxaban and warfarin reduced endogenous thrombin potential (ETP) and peak thrombin, and prolonged lag time and time to peak, compared to normal controls (p<0.0001). The lag time and time to peak TG were longer, and peak thrombin was lower in patients receiving rivaroxaban (p<0.0001) compared with warfarin, although warfarin-treated patients had lower ETP (p=0.0008). In-vivo coagulation activation markers were within the normal ranges in all rivaroxaban-treated patients (including those with levels considered to be subtherapeutic) and in 37/45 warfarin-treated patients who had an INR≥2.0. The warfarin-treated patients with subtherapeutic INRs exhibited slightly raised F1.2 and/or TAT.

CONCLUSION

In conclusion, both rivaroxaban and warfarin provided effective anticoagulation, as assessed by inhibition of TG and makers of in-vivo coagulation activation.

Global Thrombosis Test (GTT) can detect major determinants of haemostasis including platelet reactivity, endogenous fibrinolytic and thrombin generating potential

BACKGROUND

Detection of both thrombosis and bleeding risk are essential in clinical cardiology. Thrombin generated by activated platelets and from the extrinsic coagulation pathway is the major determinant of thrombogenesis and hemostasis. Although novel oral anticoagulants further increase the bleeding risk of antiplatelet drugs, platelet function tests do not reliably predict hemorrhagic complications. It seems that in addition to platelet aggregation, true assessment of bleeding risks requires the measurement of both platelet and plasma derived thrombin activity.

OBJECTIVE

To adapt a novel, near-patient test for the assessment of both antithrombotic and anticoagulant effects of oral thrombin inhibitors.

METHODS

The point-of-care Global Thrombosis Test (GTT), which measures platelet reactivity to shear-activation in native blood, was used. Thrombin, generated from activated platelets (procoagulant activity) plays a pivotal role in GTT measurement. In order to assess endogenous thrombin potential, in a separate blood sample thrombin generation was induced by microparticles formed during hypotonic hemolysis. Thus two blood samples were tested to measure simultaneously platelet reactivity (occlusion time, OT) and hemolysis (microparticles)-induced endogenous thrombin potential (OT-H).

RESULTS

In healthy subjects (n=32), OT measured in native blood was reduced in hemolysed blood (100% vs. 43 ± 4%; OT vs. OT-H respectively). Shortening of OT in hemolysed blood (OT-H) was dose-dependently inhibited by the in vitro added thrombin inhibitor argatroban. In patients receiving dabigatran (n=27), OT and, to a lesser extent, OT-H was prolonged, compared to healthy volunteers. Intra-assay variation of OT-H was low (4.5%), but interindividual variation was great, both in healthy subjects (61%) and in patients on dabigatran (65%). Thrombin inhibitors argatroban, heparin (in vitro) and dabigatran (in vivo) all prolonged both OT and OT-H. There was no correlation between the measured OT and OT-H data.

CONCLUSIONS

Microparticles shed from erythrocytes during hypotonic lysis of native blood considerably shortened OT. In a direct proportion to the applied concentrations, various thrombin inhibitors prolonged both OT (antithrombotic effect) and to a lesser extent, OT-H (anticoagulant effect). Further large studies are required to evaluate the usefulness of this technique in a clinical setting, in assessing the anticoagulant and antithrombotic effects of medication and relating GTT results with observed thrombotic and bleeding events.

Differential in vitro inhibition of thrombin generation by anticoagulant drugs in plasma from patients with cirrhosis

Background

Treatment and prevention of thrombotic complications is frequently required in patients with cirrhosis. However anticoagulant therapy is often withheld from these patients, because of the perceived bleeding diathesis. As a result of the limited clinical experience, the anticoagulant of choice for the various indications is still not known.

Objectives

We evaluated the in vitro effect of clinically approved anticoagulant drugs in plasma from patients with cirrhosis.

Patients/Methods

Thirty patients with cirrhosis and thirty healthy controls were studied. Thrombin generation assays were performed before and after addition of unfractionated heparin, low molecular weight heparin, fondaparinux, dabigatran, and rivaroxaban, to estimate anticoagulant potencies of these drugs.

Results

Addition of dabigatran led to a much more pronounced reduction in endogenous thrombin potential in patients compared to controls (72.6% reduction in patients vs. 12.8% reduction in controls, P<0.0001). The enhanced effect of dabigatran was proportional to the severity of disease. In contrast, only a slightly increased anticoagulant response to heparin and low molecular weight heparin and even a reduced response to fondaparinux and rivaroxaban was observed in plasma from cirrhotic patients as compared to control plasma.

Conclusions

The anticoagulant potency of clinically approved drugs differs substantially between patients with cirrhosis and healthy individuals. Whereas dabigatran and, to a lesser extent, heparin and low molecular weight heparin are more potent in plasma from patients with cirrhosis, fondaparinux and rivaroxaban showed a decreased anticoagulant effect. These results may imply that in addition to dose adjustments based on altered pharmacokinetics, drug-specific dose adjustments based on altered anticoagulant potency may be required in patients with cirrhosis.

New oral anticoagulants: are coagulation units still required?

Chronic antithrombotic therapy involves the use of anticoagulants, antiplatelets given either as monotherapy or in combination for the prevention of thrombotic complications. The most feared and sometimes fatal complication with this therapy is bleeding. It should be considered a "golden rule" that a drug or combination of drugs that maximizes efficiency (decreased thromboembolic risk) will probably be less safe (increased risk of bleeding), and this holds true either for single therapy or during combined therapy. The chances of bleeding indicated by risk tables can be useful but show only a snapshot, and the biological, social, environmental, and drug changes and therapeutic adherence also determine changes in the risk of thrombosis and bleeding. Bleeding is an eventuality that occurs in places of "locus minoris resistentiae," and the results of careful phase 3 studies thus cannot be completely predictive of outcomes when a medication is introduced on the pharmaceutical market. With the use of warfarin, the International Normalized Ratio (INR) that has been established to indicate adequately balanced therapy is between 2.0 and 3.0. With the new oral anticoagulants, the pharmaceutical companies emphasize that it is not necessary to monitor anticoagulant effects. In studies with different doses of new oral anticoagulants, however, incidence of clinically significant bleeding complications have been directly related to the doses. Therefore, therapeutic excesses can condition bleeding risk and therapeutic limitation can increase thrombotic risk, especially when short-acting drugs such as the new oral anticoagulants are used. Hence, it is imperative to establish an appropriate method for monitoring new oral anticoagulants, setting levels of safety and effectiveness through periodic dosage and monitoring of their anticoagulant effects. Therefore, we still recommend the use of anticoagulation units for monitoring during treatment with the new oral anticoagulants.