Effect of warfarin treatment on thrombin activatable fibrinolysis inhibitor (TAFI) activation and TAFI-mediated inhibition of fibrinolysis

Effect of anticoagulants on fibrin clot structure: A comparison between vitamin K antagonists and factor Xa inhibitors

BACKGROUND

Abnormal clot structure has been identified in patients with thrombotic disorders. Anticoagulant therapy offers clear benefits for thrombosis prevention and treatment by reducing blood clot formation and size; nevertheless, there are limited data on the effects of different anticoagulants, where clotting is initiated with different triggers, on clot structure.

OBJECTIVES

Our aim was to investigate the effects of vitamin K antagonists and factor Xa inhibitors on clot structure.

METHODS

Clots from pooled plasma spiked with rivaroxaban, apixaban, or enoxaparin, as well as plasma from patients on warfarin, were compared to plasma without anticoagulation. The kinetic profile of polymerizing clots was obtained by turbidity, fiber density was determined by confocal microscopy, clot pore size was investigated by permeation, and fiber size was analyzed using scanning electron microscopy. Clotting agonist was either tissue factor or thrombin.

RESULTS

Following clotting with tissue factor, all anticoagulated clots had a significantly increased lag time, with the exception of enoxaparin. Rivaroxaban additionally led to significantly less dense and more permeable clots, with thicker fibers. In contrast, turbidity analysis following initiation with thrombin showed few effects of anticoagulation, with only enoxaparin leading to a prolonged lag time. Enoxaparin clots made with thrombin were less dense and more permeable.

CONCLUSION

Our results show that anticoagulants modulate clot structure particularly when induced by tissue factor, most likely due to reduction of thrombin generation. We propose that the effects of different anticoagulants could be assessed with a global clot structure measurement such as permeation or turbidity, providing information on clot phenotype.

Thrombin activatable fibrinolysis inhibitor pathway alterations correlate with bleeding phenotype in patients with severe hemophilia A

BACKGROUND

Patients with severe hemophilia A display varied bleeding phenotypes despite similar factor VIII (FVIII) activity levels.

OBJECTIVE

We investigated different thrombin activatable fibrinolysis inhibitor (TAFI)-related variables in patients with severe hemophilia A and their possible correlation with bleeding tendency.

PATIENTS/METHODS

Sixty-one patients with severe hemophilia A (FVIII:C <1%], treated on demand, were included. Patients were categorized as mild, moderate, and severe bleeders according to number of bleeds per year (≤2, 3-24, ≥25, respectively). Thirty healthy males served as controls. Clot lysis time was assessed by turbidimetric assay, TAFI activation by two-stage functional assay, and response to TAFIa as the prolongation of fibrinolysis time upon addition of purified TAFIa. Circulating levels of activated TAFI (TAFIa/ai) were measured by specific enzyme-linked immunosorbent assay.

RESULTS

As compared to controls, hemophilic patients displayed shorter lysis time, less TAFIa generation, and reduced response to TAFIa, but similar TAFIa/ai levels. Clot lysis time was similar in mild, moderate, and severe bleeders, whereas TAFIa generation and response to TAFIa decreased with the increase in bleeding tendency; moreover, circulating TAFIa/ai levels were highest in severe bleeders. Patients with markedly impaired TAFIa generation or TAFIa response (below median) displayed 3-fold to 4-fold higher bleeding rate and factor consumption than patients whose TAFI-related values approached the control ones.

CONCLUSION

The TAFI pathway impairment correlates with bleeding phenotype in severe hemophilia and may represent a promising tool to stratify the bleeding risk.

Stability of warfarin sodium flavoured preservative-free oral liquid formulations

Objectives

This study was designed to assess the physicochemical stability of warfarin sodium liquid formulations stored at room temperature and under refrigeration.

Methods

Formulations of warfarin sodium 1mg/mL were prepared from either powder or tablets dispersed in mixture containing deionised water, glycerin, sodium phosphate and strawberry alcohol-free concentrate flavour and stored in amber glass bottles at 25°C and 4°C. Stability indicating high-performance liquid chromatography assay procedure was used to analyse the concentration of the drug at the time of preparation and on days 7, 14, 21, 30, 45, 60 and 90. Changes in colour, taste and pH, as well as visual evidence of precipitation, were assessed for physicochemical instability.

Results

Between day 0 and day 90, the pH of the powder and tablet formulations stored at 25°C decreased by -1.40 units and -1.16 units, respectively. There were minor changes in pH for both formulations stored at 4°C. The formulation from powder stored at 25°C and 4°C maintained at least 90% of the initial warfarin concentration for 30 days and 45 days, respectively. The formulation from tablets stored at 25°C and 4°C maintained at least 90% of initial warfarin concentration for 21 days and 30 days, respectively.

Conclusion

Warfarin sodium in flavoured preservative-free formulation from powder is recommended and stable in amber glass bottles for at least 30 days stored at 25°C and 45 days stored at 4°C.

Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) inhibition stimulates the fibrinolytic rate in different in vitro models

Essentials AZD9684 is a potent inhibitor of carboxypeptidase U (CPU, TAFIa, CPB2). The effect of AZD9684 on fibrinolysis was investigated in four in vitro systems. The CPU system also attenuates fibrinolysis in more advanced hemostatic systems. The size of the observed effect on fibrinolysis is dependent on the exact experimental conditions.

SUMMARY

Background Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) is a basic carboxypeptidase that attenuates fibrinolysis. This characteristic has raised interest in the scientific community and pharmaceutical industry for the development of inhibitors as profibrinolytic agents. Objectives Little is known about the contribution of CPU to clot resistance in more advanced hemostatic models, which include blood cells and shear stress. The aim of this study was to evaluate the effects of the CPU system in in vitro systems for fibrinolysis with different grades of complexity. Methods The contribution of the CPU system was evaluated in the following systems: (i) plasma clot lysis; (ii) rotational thromboelastometry (ROTEM) in whole blood; (iii) front lysis with confocal microscopy in platelet-free and platelet-rich plasma; and (iv) a microfluidic system with whole blood under arterial shear stress. Experiments were carried out in the presence or absence of AZD9684, a specific CPU inhibitor. Results During plasma clot lysis, addition of AZD9684 resulted in 33% faster lysis. In ROTEM, the lysis onset time was decreased by 38%. For both clot lysis and ROTEM, an AZD9684 dose-dependent response was observed. CPU inhibition in front lysis experiments resulted in 47% and 50% faster lysis for platelet-free plasma and platelet-rich plasma, respectively. Finally, a tendency for faster lysis was observed only in the microfluidic system when AZD9684 was added. Conclusions Overall, these experiments provide novel evidence that the CPU system can also modulate fibrinolysis in more advanced hemostatic systems. The extent of the effects appears to be dependent upon the exact experimental conditions.

Reduced fibrinolytic resistance in patients with factor XI deficiency. Evidence of a thrombin-independent impairment of the thrombin-activatable fibrinolysis inhibitor pathway

Essentials Plasma of factor XI-deficient patients (FXI-dp) displays enhanced fibrinolysis. We investigated the role of thrombin activatable fibrinolysis inhibitor (TAFI) in 18 FXI-dp. FXI-dp generated less activated TAFI (TAFIa) on clotting challenge and were resistant to TAFIa. TAFI activation and TAFIa resistance correlated with bleeding score and bleeding phenotype.

SUMMARY

Background Factor XI (FXI) deficiency, a rare disorder with unpredictable bleeding, has been associated with reduced fibrinolytic resistance as a result of abnormal fibrin density. Objective We investigated the involvement of thrombin-activatable fibrinolysis inhibitor (TAFI) in the increased lysability of FXI-deficient (FXI-def) clots and the role of thrombin. Patients/Methods Eighteen patients with FXI deficiency (1-58%) and 17 matched controls were investigated for fibrinolytic resistance to t-PA, thrombin generation, TAFI activation and response to TAFIa. Results When clotting was induced by 0.5 pm tissue factor (TF), FXI-def plasmas displayed less thrombin and TAFIa generation and shorter lysis time than controls. A 100-fold higher TF concentration (to bypass FXI) abolished the difference in thrombin generation but not in lysis time between patients and controls. Normalization of FXI levels by a FXI concentrate increased thrombin generation but had no effect on the lysis time of FXI-def plasma. Moreover, when clots were induced by purified thrombin and high concentrations of FXa inhibitor, FXI-def plasma still generated less TAFIa and displayed a shorter lysis time than controls. Finally, upon TAFIa addition, the lysis time of FXI-def plasma was prolonged significantly less than that of control plasma, suggesting a TAFIa resistance. TAFIa generation and TAFIa resistance were correlated with the bleeding score, displaying a considerable capacity to discriminate between patients with and without bleeding. Conclusions TAFI pathway impairment, largely caused by a hitherto unknown TAFIa resistance, appears to be one main cause of decreased fibrinolytic resistance in FXI deficiency and might be clinically useful for assessing the bleeding risk of FXI-def patients.

Warfarin-related recurrent knee haemarthrosis treated with arterial embolisation and intra-articular injection of tranexamic acid

Haemarthrosis is an uncommon complication of anticoagulation therapy. Tranexamic acid (TXA) has a high clinical value for the treatment of bleeding due to fibrinolysis. We describe a case of a 61-year-old woman with a mechanical heart valve who presented with warfarin-related recurrent haemarthrosis of her right knee, which recurred after transarterial embolisation. Intra-articular injection of TXA led to a cessation of haemarthrosis without any adverse event for 1 year. Intra-articular injection of TXA may be an effective treatment for warfarin-related haemarthrosis.