Evaluation of a novel flow chamber system to assess clot formation in factor VIII-deficient mouse and anti-factor IXa-treated human blood

Quantification of the contribution of individual coagulation factors to haemostasis using a microchip flow chamber system and reconstituted blood from deficient plasma

BACKGROUND AND OBJECTIVES

Quantifying the contribution of individual coagulation factors to haemostasis may aid our understanding of the haemostatic function in patients with rare coagulation deficiencies (RCDs) and the exploration of suitable treatments.

MATERIALS AND METHODS

Reconstituted blood prepared from specific coagulation factor-deficient plasma (factor [F]II; prothrombin, FV, FVII, FVIII, FIX, FX, FXI or FXII) and red blood cell/platelet products were used to simulate the whole blood of patients with RCD. We prepared in vitro treatment models for patients with prothrombin deficiency using coagulation factor agents and fresh frozen plasma. Haemostatic function was measured using a microchip flow chamber system at 600 s-1.

RESULTS

The haemostatic function was low, especially in blood samples reconstituted with prothrombin- and FX-deficient plasma. In a plasma transfusion model of prothrombin deficiency, haemostatic function recovered after 10% replacement with normal plasma and reached a plateau at ≧60% replacement. A treatment model of prothrombin deficiency with prothrombin complex concentrates revealed dose-dependent therapeutic effects in the range of 0-50 IU/kg.

CONCLUSION

Microchip flow chamber system-based quantification of haemostatic function using reconstituted blood could predict haemostasis and therapeutic effects of treatments in patients with prothrombin deficiency.

In vitro effects of Gla-domainless factor Xa analog on procoagulant and fibrinolytic pathways in apixaban-treated plasma and whole blood

BACKGROUND

Andexanet alfa is a Gla-domainless FXa (GDXa) analog used as an antidote to FXa inhibitors. Despite its clinical use, laboratory monitoring for anti-Xa reversal and the effect of andexanet on fibrinolysis has not been explored. We used a GDXa with a serine-to-alanine mutation at position 195 (chymotrypsin numbering) to model the interaction between andexanet and apixaban.

METHODS

Six batches of pooled plasma, and whole blood from healthy volunteers were treated with increasing concentrations of apixaban with/without GDXa. Thrombin generation and plasmin generation (TG and PG) were tested in plasma, and whole blood thrombus formation was tested using thromboelastometry or a flow-chamber system. FXa was also tested in isolation for its ability to support plasmin activation with/without apixaban and GDXa.

RESULTS

Apixaban (250-800 nM) concentration-dependently decreased the velocity and peak of TG in plasma. Apixaban prolonged the onset of thrombus formation in thromboelastometry and flow-chamber tests. GDXa normalized apixaban-induced delays in TG and whole blood thrombus formation. However, GDXa minimally affected the low PG velocity and peak caused by apixaban at higher concentrations (500-800 nM). FXa promoted plasmin generation independent of fibrin that was inhibited by apixaban at supratherapeutic concentrations.

CONCLUSIONS

This study demonstrated the feasibility of assessing coagulation lag time recovery in plasma and whole blood following in vitro apixaban reversal using GDXa, a biosimilar to andexanet. In contrast, GDXa-induced changes in plasmin generation and fibrinolysis were limited in PG and tPA-added ROTEM assays, supporting the endogenous profibrinolytic activity of FXa and its inhibition at elevated apixaban concentrations.

The Use of Total Thrombus Formation Analysis System as a Tool to Assess Platelet Function in Bleeding and Thrombosis Risk-A Systematic Review

BACKGROUND

Today there are many devices that can be used to study blood clotting disorders by identifying abnormalities in blood platelets. The Total Thrombus Formation Analysis System is an automated microchip flow chamber system that is used for the quantitative analysis of clot formation under blood flow conditions. For several years, researchers have been using a tool to analyse various clinical situations of patients to identify the properties and biochemical processes occurring within platelets and their microenvironment.

METHODS

An investigation of recent published literature was conducted based on PRISMA. This review includes 52 science papers directly related to the use of the Total Clot Formation Analysis System in relation to bleeding, surgery, platelet function assessment, anticoagulation monitoring, von Willebrand factor and others.

CONCLUSION

Most available studies indicate that The Total Thrombus Formation Analysis System may be useful in diagnostic issues, with devices used to monitor therapy or as a significant tool for predicting bleeding events. However, T-TAS not that has the potential for diagnostic indications, but allows the direct observation of the flow and the interactions between blood cells, including the intensity and dynamics of clot formation. The device is expected to be of significant value for basic research to observe the interactions and changes within platelets and their microenvironment.

Point-of-Care Diagnostic Assays and Novel Preclinical Technologies for Hemostasis and Thrombosis

Hemostasis is a complex wound-healing process involving numerous mechanical and biochemical mechanisms and influenced by many factors including platelets, coagulation factors, and endothelial components. Slight alterations in these mechanisms can lead to either prothrombotic or bleeding consequences, and such hemostatic imbalances can lead to significant clinical consequences with resultant morbidity and mortality. An ideal hemostasis assay would not only address all the unique processes involved in clot formation and resolution but also take place under flow conditions to account for endothelial involvement. Global assays do exist; however, these assays are not flow based. Flow-based assays have been limited secondary to their large blood volume requirements and low throughput, limiting potential clinical applications. Microfluidic-based assays address the aforementioned limitations of both global and flow-based assays by utilizing standardized devices that require low blood volumes, offer reproducible analysis, and have functionality under a range of shear stresses and flow conditions. While still largely confined to the preclinical space, here we aim to discuss these novel technologies and potential clinical implications, particularly in comparison to the current, commercially available point-of-care assays.

The Total Thrombus Formation (T-TAS) platelet (PL) assay, a novel test that evaluates whole blood platelet thrombus formation under physiological conditions

Dual antiplatelet therapy (DAPT, aspirin, and a P2Y₁₂ inhibitor) reduces thrombotic events in patients with coronary artery disease (CAD). The T-TAS PL assay uses arterial shear flow over collagen surface, better mimicking in vivo conditions compared to conventional agonist-based platelet function assays, to evaluate platelet function. Here, the platelet function in patients taking DAPT is evaluated with the T-TAS PL assay. In 57 patients with CAD, taking DAPT ≥7 days (n = 22 for clopidogrel, n = 15 for prasugrel, n = 20 for ticagrelor), T-TAS PL assessments were performed in duplicate, and expressed as area under the flow pressure curve within a 10-minute period (AUC10). The duplicate measurements were strongly correlated (r = 0.90, p < .001), with an intra-assay coefficient of variation (CV) of 11,5%. For clopidogrel, the median AUC10 was 11.5 (IQR5.9-41.8), for prasugrel 28.8 (IQR10.3-37.6), and for ticagrelor 8.9 (IQR 6.4-10.9). All measurements were below the AUC10 cutoff of 260 measured in healthy volunteers, suggesting excellent discrimination of DAPT-treated and untreated persons. The new T-TAS PL assay demonstrated complete discrimination of platelet function in patients on DAPT based on an established cutoff. Ticagrelor showed lower levels of platelet function and a more uniform response compared to prasugrel and clopidogrel.

Poor glycaemic control contributes to a shift towards prothrombotic and antifibrinolytic state in pregnant women with type 1 diabetes mellitus

OJECTIVES

Thrombotic and antifibrinolytic influence of Diabetes mellitus type 1 (T1DM) on haemostasis have been well demonstrated. There has been no research assessing the influence of poor glycemic control on thrombus formation under flow conditions in vitro or in pregnant type 1 diabetic women to date.

PATIENTS/METHODS

This study compared singleton pregnant T1DM women (n = 21) and control pregnant subjects without any metabolic disease (n = 15). The T1DM group was divided into two subgroups of sufficient (SGC-DM; HbA1c ≤6,5%,n = 15) and poor glycaemic control (PGC-DM; HbA1c >6,5%,n = 6). Evaluation of the whole blood thrombogenicity we assessed using T-TAS® at a shear rate of 240 s-1 (Total-Thrombus Analysis System, Zacros, Japan).

RESULTS

Blood clot formation initiation time (T10) was significantly shortened in PGC-DM subgroup when compared to SGC-DM subgroup (p = 0,03). The area under the curve (AUC30) of blood clot time formation and the MPV (mean platelet volume) values were substantially higher in the PGC-DM subgroup in comparison to the SGC-DM group (p = 0,03). Negative correlations were noted between HbA1c and T10 values (p = 0,02) and between T10 and MPV values in the T1DM group (p = 0,04).

CONCLUSIONS

Poor glycaemic control in T1DM subjects triggers a shift towards a prothrombotic and antifibrinolytic state. This phenomenon can be detected using the novel system for quantitative assessment of the platelet thrombus formation process under flow conditions in vitro. The alteration of T-TAS values in PGC-DM subgroup proves that a poor glycemic control-related shift of the equilibrium toward thrombogenesis occurs in this group of patients. Our findings need a further elucidation in research on more massive data sets to be confirmed.

Evaluation of the Total Thrombus-Formation System (T-TAS): application to human and mouse blood analysis

The Total Thrombus-formation Analyser System (T-TAS) is a whole blood flow chamber system for the measurement of in vitro thrombus formation under variable shear stress conditions. Our current study sought to evaluate the potential utility of the T-TAS for the measurement of thrombus formation within human and mouse whole blood. T-TAS microchips (collagen, PL chip; collagen/tissue thromboplastin, AR chip) were used to analyze platelet (PL) or fibrin-rich thrombus formation, respectively. Blood samples from humans (healthy and patients with mild bleeding disorders) and wild-type (WT), mice were tested. Light transmission lumi-aggregometer (lumi-LTA) was performed in PRP using several concentrations of ADP, adrenaline, arachidonic acid, collagen, PAR-1 peptide and ristocetin. Thrombus growth (N = 22) increased with shear within PL (4:40 ± 1.11, 3:25 ± 0.43 and 3:12 ± 0.48 mins [1000, 1500 and 2000s-1]) and AR chips (3:55 ± 0.42 and 1:49 ± 0.19 [240s-1 and 600s-1]). The area under the curve (AUC) on the PL chip was also reduced at 1000s-1 compared to 1500/2000s-1 (260 ± 51.7, 317 ± 55.4 and 301 ± 66.2, respectively). In contrast, no differences in the AUC between 240s-1 and 600s-1 were observed in the AR chip (1593 ± 122 and 1591 ± 158). The intra-assay coefficient of variation (CV) (n = 10) in the PL chip (1000s-1) and AR chip (240s-1) were T1014.1%, T6016.7%, T10-6022.8% and AUC1024.4% or T10 9.03%, T808.64%, T10-8023.8% and AUC305.1%. AR chip thrombus formation was inhibited by rivaroxaban (1 µM), but not with ticagrelor (10 µM). In contrast, PL chip thrombus formation was totally inhibited by ticagrelor. T-TAS shows an overall agreement with lumi-LTA in 87% of patients (n = 30) with normal PL counts recruited into the genotyping and phenotyping of platelet (GAPP) study and suspected to have a PL function defect. The onset (T10) of thrombus formation in WT mice (N = 4) was shorter when compared to humans e.g. PL chip (1000s-1) T10 were 02:02 ± 00:23 and 03:30 ± 0:45, respectively). T-TAS measures in vitro thrombus formation and can be used for monitoring antithrombotic therapy, investigating patients with suspected PL function defects and monitoring PL function within mice.

Prohemostatic Activity of Factor X in Combination With Activated Factor VII in Dilutional Coagulopathy

BACKGROUND

Recombinant activated factor VII (rFVIIa) concentrate reduces allogeneic blood transfusions, but it may increase thromboembolic complications in complex cardiac surgery. The mixture of activated factor VII (FVIIa) and factor X (FX) (FVIIa/FX) (FVIIa:FX = 1:10) is a novel bypassing agent for hemophilia patients. We hypothesized that the combination of FX and FVIIa could improve thrombin generation (TG) in acquired multifactorial coagulation defects such as seen in cardiac surgery and conducted in vitro evaluation of FVIIa/FX in parallel with other coagulation factor concentrates using in vitro and in vivo diluted plasma samples.

METHODS

Plasma samples were collected from 9 healthy volunteers and 12 cardiac surgical patients. We measured TG (Thrombinoscope) using in vitro 50% dilution plasma and in vivo dilution plasma after cardiopulmonary bypass, in parallel with thromboelastometry (ROTEM) and standard coagulation assays. In vitro additions of FVIIa/FX (0.35, 0.7, and 1.4 μg/mL, based on the FVIIa level), rFVIIa (1.4, 2.8, and 6.4 μg/mL), prothrombin complex concentrate (0.3 international unit), and 20% plasma replacement were evaluated.

RESULTS

In diluted plasma, the addition of either FVIIa/FX or rFVIIa shortened the lag time and increased the peak TG, but the effect in lag time of FVIIa/FX at 0.35 μg/mL was more extensive than rFVIIa at 6.4 μg/mL. Prothrombin complex concentrate increased peak TG by increasing the prothrombin level but failed to shorten the lag time. No improvement in any of the TG variables was observed after 20% volume replacement with plasma. The addition of factor concentrates normalized prothrombin time/international normalized ratio but not with plasma replacement. In cardiac patients, similar patterns were observed on TG in post-cardiopulmonary bypass samples. FVIIa/FX shortened clotting time (CT) in a concentration-dependent manner on CT on thromboelastometry. Plasma replacement did not improve CT, but a combination of plasma and FVIIa/FX (0.35 μg/mL) more effectively shortened CT than FVIIa/FX alone.

CONCLUSIONS

The combination of FVIIa and FX improved TG more efficiently than rFVIIa alone or plasma in dilutional coagulopathy models. The required FVIIa dose in FVIIa/FX was considerably lower than those reported during bypassing therapy in hemophilia patients (1.4-2.8 μg/mL). The combination of plasma could restore coagulation more efficiently compared to FVIIa/FX alone. Lesser FVIIa requirement to exert procoagulant activity may be favorable in terms of reducing systemic thromboembolic complications.

In vitro studies show synergistic effects of a procoagulant bispecific antibody and bypassing agents

Essentials Patients with hemophilia A and inhibitors receiving emicizumab experience breakthrough bleeding. Safety concerns may exist when combining emicizumab with bypassing agents. Combined bypassing agent and bispecific antibody increased thrombin generation up to 17-fold. Thrombotic effects should be considered when combining emicizumab with plasma bypassing agent.

SUMMARY

Background Investigational non-factor products such as emicizumab offer a treatment option for patients with hemophilia and inhibitors. However, their mechanism of action raises questions regarding safety when they are combined with treatments for breakthrough bleeding. Objectives To evaluate in vitro thrombin generation (TG) and clot formation for combinations of activated prothrombin complex concentrate (aPCC), recombinant activated factor VII (rFVIIa), and a sequence-identical analog of emicizumab (SIA). Methods Therapeutic concentrations of SIA (20-600 nm) alone or with aPCC (0.05-1 U mL-1 ), isolated aPCC components or rFVIIa (0.88-5.25 μg mL-1 ) were tested for TG and compared with reference ranges for healthy donor plasma. Coagulation of FVIII-inhibited blood was determined with a widely established method, i.e. rotational thromboelastometry (ROTEM), and confirmed with the Total Thrombus-formation Analysis System. Results and conclusions SIA (600 nm) or aPCC (0.5 U mL-1 ) alone resulted in peak thrombin levels of 21.4 nm and 38.6 nm, respectively, both of which are lower than normal (83.7 ± 29.8 nm). SIA plus aPCC (0.5 U mL-1 ) increased the peak thrombin level 17-fold over SIA alone, exceeding the reference plasma value by 4.2-fold. This hypercoagulable effect occurred with 600 nmSIA combined with as little as 0.25 U mL-1 aPCC, confirmed by ROTEM. FIX was the main driver for enhanced TG. SIA plus rFVIIa (1.75 μg mL-1 ) induced a 1.8-fold increase in the peak thrombin level in platelet-rich plasma, but it did not reach the normal range. These in vitro experiments demonstrate excessive TG after administration of a combination of aPCC and SIA at clinically relevant doses. Careful judgement may be required when breakthrough bleeding is treated in patients receiving emicizumab.

Recombinant factor VIIa addition to haemophilic blood perfused over collagen/tissue factor can sufficiently bypass the factor IXa/VIIIa defect to rescue fibrin generation

INTRODUCTION

Factor VIII (FVIII) or factor IX (FIX)-deficient haemophilic patients display deficits in platelet and fibrin deposition under flow detectable in microfluidics. Compared to fibrin generation, decreased platelet deposition in haemophilic blood flow is more easily rescued with recombinant factor VIIa (rFVIIa), whereas rFVIIa requires FXIIa participation to generate fibrin when tissue factor (TF) is absent.

AIMS

Perfusion of haemophilic whole blood (WB) over collagen/TF surfaces was used to determine whether rFVIIa/TF was sufficient to bypass poor FIXa/FVIIIa function in blood from patients with haemophilia A and B.

METHODS

Whole blood treated with high-dose corn trypsin inhibitor (40 μg mL-1 ) from seven healthy donors and 10 patients was perfused over fibrillar collagen presenting low or high TF (TFlow or TFhigh ) at wall shear rate of 100 s-1 .

RESULTS

With WB from healthy controls, platelet deposition and fibrin accumulation increased as TF increased. Factor-deficient WB (1-3% of normal) displayed striking deficits in platelet deposition and fibrin formation at either TFlow or TFhigh . In contrast, mildly factor-deficient WB (14-32%) supported fibrin formation under flow on TFhigh /collagen. With either TFlow or TFhigh , exogenously added rFVIIa (20 nm) increased platelet deposition and fibrin accumulation in WB from factor-deficient patients (1-3% of normal) to levels commensurate with untreated healthy WB.

CONCLUSION

The absence of FIXa/FVIIIa in patients with severe haemophilia results in deficits in fibrin formation that cannot be rescued by wall-derived TF ex vivo. The effects of rFVIIa on platelet adhesion and rFVIIa/TF can act together to reinforce thrombin generation, platelet deposition and fibrin formation under flow.

Flow chamber and microfluidic approaches for measuring thrombus formation in genetic bleeding disorders

Platelet adhesion and aggregation, coagulation, fibrin formation, and fibrinolysis are regulated by the forces and flows imposed by blood at the site of a vascular injury. Flow chambers designed to observe these events are an indispensable part of doing hemostasis and thrombosis research, especially with human blood. Microfluidic methods have provided the flexibility to design flow chambers with complex geometries and features that more closely mimic the anatomy and physiology of blood vessels. Additionally, microfluidic systems with integrated optics and/or pressure sensors and on-board signal processing could transform what have been primarily research tools into clinical assays. Here, we describe a historical review of how flow-based approaches have informed biophysical mechanisms in genetic bleeding disorders, challenges and potential solutions for developing models of bleeding in vitro, and outstanding issues that need to be addressed prior to their use in clinical settings.

Application of microfluidic devices in studies of thrombosis and hemostasis

Due to the importance of fluid flow during thrombotic episodes, it is quite appropriate to study clotting and bleeding processes in devices that have well-defined fluid shear environments. Two common devices for applying these defined shear stresses include the cone-and-plate viscometer and parallel-plate flow chamber. While such tools have many salient features, they require large amounts of blood or other protein components. With growth in the area of microfluidics over the last two decades, it has become feasible to miniaturize such flow devices. Such miniaturization not only enables saving of precious samples but also increases the throughput of fluid shear devices, thus enabling the design of combinatorial experiments and making the technique more accessible to the larger scientific community. In addition to simple flows that are common in traditional flow apparatus, more complex geometries that mimic stenosed arteries and the human microvasculature can also be generated. The composition of the microfluidics cell substrate can also be varied for diverse basic science investigations, and clinical investigations that aim to assay either individual patient coagulopathy or response to anti-coagulation treatment. This review summarizes the current state of the art for such microfluidic devices and their applications in the field of thrombosis and hemostasis.

Role of red blood cells in the anemia-associated bleeding under high shear conditions

BACKGROUND

Red blood cells (RBCs) contribute to hemostasis under blood-flow, and anemia might contribute to a hemorrhagic diathesis. The majority of current laboratory techniques to assess hemostasis do not consider the effects of RBCs. An assay to determine the role of RBCs in hemostasis could be beneficial for clinical management.

OBJECTIVES

To investigate the influence of RBCs in hemostasis.

METHODS

Hemostasis was investigated using a novel microchip flow-chamber system (T-TAS^® ) in an anemic patient with von Willebrand disease. Subsequently, the effects of RBCs in total thrombus analysis system (T-TAS) were examined using reconstituted whole blood at various hematocrit levels.

RESULTS

In vivo: When the patient was anemic and demonstrated persisted hemorrhagic symptoms despite the maintained adequate von Willebrand factor ristocetin cofactor activity levels, thrombus formation determined by T-TAS was delayed. However, transfusions of RBCs resolved bleeding symptom and, accordingly, the thrombus formation in T-TAS improved. In vitro: Thrombus formation determined by T-TAS at 1000 s^-1 was dose-dependent on hematocrit (the time to reach 10 kPa (T₁₀ ): 10.0 ± 0, 9.5 ± 1.4, 6.7 ± 2.4, 2.8 ± 1.6 min at hematocrits of 0%, 12.5%, 25% and 50%, respectively). Markedly defective thrombus formation (T₁₀ >10 min) was confirmed at a hematocrit <25% at 2000 s^-1 .

CONCLUSION

Red blood cells play an essential role in hemostasis under high shear, and RBC transfusions could be effective for refractory bleeding in patients with anemia. T-TAS measurements appear to reflect the hemostatic consequences of diminished red cell numbers under blood-flow, and could provide a valuable means for monitoring patients.

Usefulness of the Total Thrombus-Formation Analysis System (T-TAS) in the diagnosis and characterization of von Willebrand disease

INTRODUCTION

The heterogeneity of von Willebrand disease (VWD) makes its diagnosis a difficult task.

METHODS

We report here on the usefulness of a microchip-based flow-chamber system, the total thrombus-formation analysis system (T-TAS), in the identification and characterization of VWD. Thirty VWD patients and 20 healthy subjects were studied with the T-TAS platelet (PL) and atherome (AR) microchips developed for the in vitro assessment of platelet thrombus formation and fibrin-rich platelet thrombus formation respectively.

RESULTS

Samples from severe type 1 VWD, characterized by von Willebrand factor (VWF) levels below 10 U dL^-1 , failed to occlude either the PL or the AR chip capillaries, while the occlusion times were normal in patients with mild type 1 VWD (VWF above 25 U dL^-1 ). PL and/or AR chip occlusion occurred, but took longer than normal, for samples from type Vicenza and type 1 VWD patients, whose VWF levels ranged between 10 and 25 U dL^-1 . No PL or AR chip capillary occlusion was seen for samples from patients with type 2A or 2B VWD featuring the absence of large VWF multimers, whereas no abnormalities emerged for type 2B patients with normal multimer patterns.

CONCLUSION

The T-TAS appears to be sensitive mainly to plasma VWF concentration and the presence of large multimers. Failure of the PL and AR chips to become occluded points to a lack of large VWF multimers, or type 1 VWD with VWF levels below 10 U dL^-1 . Although the T-TAS does not assure a precise VWD diagnosis, it does point us in the right direction, and thus seems a useful global preliminary test.

Assessment of platelet-derived thrombogenicity with the total thrombus-formation analysis system in coronary artery disease patients receiving antiplatelet therapy

BACKGROUND

Accurate evaluation of thrombogenicity helps to prevent thrombosis and excessive bleeding. The total thrombus-formation analysis system (T-TAS) was developed for quantitative analysis of platelet thrombus formation by the use of microchips with thrombogenic surfaces (collagen, platelet chip [PL-chip]; collagen plus tissue factor, atherome chip [AR-chip]). We examined the utility of the T-TAS in the assessment of the efficacy of antiplatelet therapy in patients with coronary artery disease (CAD).

METHODS AND RESULTS

In this cross-sectional study, 372 consecutive patients admitted to the cardiovascular department were divided into three groups: patients not receiving any antiplatelet therapy (control, n = 56), patients receiving aspirin only (n = 69), and patients receiving aspirin and clopidogrel (n = 149). Blood samples were used for the T-TAS to measure the platelet thrombus-formation area under the curve (AUC) at various shear rates (1500 s(-1) [PL18 -AUC10 ] and 2000 s(-1) [PL24 -AUC10 ] for the PL-chip; 300 s(-1) [AR10 -AUC30 ] for the AR-chip). The on-clopidogrel platelet aggregation was measured by the use of P2Y12 reaction units (PRUs) with the VerifyNow system. The mean PL24 -AUC10 levels were 358 ± 111 (± standard deviation) (95% confidence interval [CI] 328.9-387.1) in the control group, 256 ± 108 (95% CI 230.5-281.5) in the aspirin group, and 113 ± 91 (95% CI 98.4-127.6) in the aspirin/clopidogrel group. In the aspirin/clopidogrel group, the PL24 -AUC10 was higher in poor metabolizers (PMs) with cytochrome P450 2C19(CYP2C19) polymorphisms (152 ± 112, 95% CI 103.4-200.6) than in the non-PM group (87 ± 74, 95% CI 73.8-100.2).

CONCLUSIONS

Our findings suggest that the PL24 -AUC10 level measured by the T-TAS is a potentially suitable index for the assessment of antiplatelet therapy in CAD patients.

Total Thrombus-Formation Analysis System (T-TAS) Can Predict Periprocedural Bleeding Events in Patients Undergoing Catheter Ablation for Atrial Fibrillation

Background

Non–vitamin K antagonist oral anticoagulants are used to prevent thromboembolism in patients with atrial fibrillation. The T‐TAS “Total Thrombus‐formation Analysis System” (Fujimori Kogyo Co Ltd) was developed for quantitative analysis of thrombus formation using microchips with thrombogenic surfaces (collagen, platelet chip [PL] ; collagen plus tissue factor, atheroma chip [AR]). We evaluated the utility of T‐TAS in predicting periprocedural bleeding in atrial fibrillation patients undergoing catheter ablation (CA).

Methods and Results

After exclusion of 20 from 148 consecutive patients undergoing CA, the remaining 128 patients were divided into 2 treatment groups: the warfarin group (n=30) and the non–vitamin K antagonist oral anticoagulants group (n=98). Blood samples obtained on the day of CA (anticoagulant‐free point) and at 3 and 30 days after CA were used in T‐TAS to compute the thrombus formation area under the curve (AUC; AUC for the first 10 minutes for PL tested at flow rate of 24 μL/min [PL₂₄‐AUC₁₀]; AUC for the first 30 minutes for AR tested at flow rate of 10 μL/min [AR₁₀‐AUC₃₀]). AR₁₀‐AUC₃₀ and PL₂₄‐AUC₁₀ levels were similar in the 2 groups on the day of CA. Levels of AR₁₀‐AUC₃₀, but not PL₂₄‐AUC₁₀, were significantly lower in the 2 groups at days 3 and 30 after CA. Multiple logistic regression analyses identified the AR₁₀‐AUC₃₀ level on the day of CA as a significant predictor of periprocedural bleeding events (odds ratio 5.7; 95% CI 1.54–21.1; P=0.009). Receiver operating characteristic analysis showed that the AR₁₀‐AUC₃₀ level on the day of CA significantly predicted periprocedural bleeding events (AUC 0.859, 95% CI 0.766–0.951; P<0.001). The cutoff AR₁₀‐AUC₃₀ level was 1648 for identification of periprocedural bleeding events.

Conclusions

These results suggested that the AR₁₀‐AUC₃₀ level determined by T‐TAS is a potentially useful marker for prediction of bleeding events in atrial fibrillation patients undergoing CA.

Microfluidic technology as an emerging clinical tool to evaluate thrombosis and hemostasis

Assessment of platelet function and coagulation under flow conditions can augment traditional static assays used to evaluate patients with suspected hemostatic or thrombotic disorders. Among the available flow-based assays, microfluidic devices require the smallest blood volume and provide multiple output options. These assays are based on the presence of wall shear stress that mimics in vivo interactions between blood components and vessel walls. Microfluidic devices can generate essential information regarding homeostatic regulation of platelet activation and subsequent engagement of the coagulation cascade leading to fibrin deposition and clot formation. Emerging data suggest that microfluidic assays may also reveal consistent patterns of hemostatic or thrombotic pathology, and could aid in assessing and monitoring patient-specific effects of coagulation-modifying therapies.

Rate-limiting roles of the tenase complex of factors VIII and IX in platelet procoagulant activity and formation of platelet-fibrin thrombi under flow

The importance of factor Xa generation in thrombus formation has not been studied extensively so far. Here, we used mice deficient in either factor VIII or factor IX to determine the role of platelet-stimulated tenase activity in the formation of platelet-fibrin thrombi on collagen. With tissue factor present, deficiency in factor VIII or IX markedly suppressed thrombus growth, fibrin formation and platelet procoagulant activity (phosphatidylserine exposure). In either case, residual fibrin formation was eliminated in the absence of tissue factor. Effects of factor deficiencies were antagonized by supplementation of the missing coagulation factor. In wild-type thrombi generated under flow, phosphatidylserine-exposing platelets bound (activated) factor IX and factor X, whereas factor VIII preferentially co-localized at sites of von Willebrand factor binding. Furthermore, proteolytic activity of the generated activated factor X and thrombin was confined to the sites of phosphatidylserine exposure. With blood from a hemophilia A or B patient, the formation of platelet-fibrin thrombi was greatly delayed and reduced, even in the presence of high concentrations of tissue factor. A direct activated factor X inhibitor, rivaroxaban, added to human blood, suppressed both thrombin and fibrin formation. Together, these data point to a potent enforcement loop in thrombus formation due to factor X activation, subsequent thrombin and fibrin generation, causing activated factor X-mediated stimulation of platelet phosphatidylserine exposure. This implies that the factor VIII/factor IX-dependent stimulation of platelet procoagulant activity is a limiting factor for fibrin formation under flow conditions, even at high tissue factor concentrations.

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

BACKGROUND

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

OBJECTIVE

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

METHODS AND RESULTS

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

CONCLUSION

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

The use of microfluidics in hemostasis: clinical diagnostics and biomimetic models of vascular injury

PURPOSE OF REVIEW

This article reviews the application of microfluidic technologies in hemostasis. The emphasis is on promising developments in devices for clinical applications and novel approaches to modeling complex hemodynamics.

RECENT FINDINGS

Microfluidics combined with micropatterning of prothrombotic substrates provides devices for measuring platelet function and coagulation with low blood volumes (∼100 μl) over a wide range of shear stresses. This technology has been applied to the diagnosis of bleeding and thrombotic disorders, as well as to dosing and monitoring of anticoagulation and antiplatelet agents. Microfluidic devices that mimic vascular geometries such as bifurcations, stenosis, and complex interconnected networks yield complex flow fields that have revealed new mechanisms of platelet adhesion and aggregation. Applying techniques from tissue engineering by endothelializing these networks is beginning to close the gap between in-vitro and in-vivo models of vascular injury.

SUMMARY

Microfluidic technology enables researchers to create in-vitro models of vascular disease with unprecedented control of the biochemical and biophysical conditions. Two promising directions are flow-dependent clinical assays and biomimetic vascular networks. These approaches are particularly well suited for modeling the microvasculature. However, caution should be used when extrapolating results from microfluidic channels to the pathophysiology of thrombosis in large arteries and veins.

The effect of factor VIII deficiencies and replacement and bypass therapies on thrombus formation under venous flow conditions in microfluidic and computational models

Clinical evidence suggests that individuals with factor VIII (FVIII) deficiency (hemophilia A) are protected against venous thrombosis, but treatment with recombinant proteins can increase their risk for thrombosis. In this study we examined the dynamics of thrombus formation in individuals with hemophilia A and their response to replacement and bypass therapies under venous flow conditions. Fibrin and platelet accumulation were measured in microfluidic flow assays on a TF-rich surface at a shear rate of 100 s⁻¹. Thrombin generation was calculated with a computational spatial-temporal model of thrombus formation. Mild FVIII deficiencies (5–30% normal levels) could support fibrin fiber formation, while severe (<1%) and moderate (1–5%) deficiencies could not. Based on these experimental observations, computational calculations estimate an average thrombin concentration of ∼10 nM is necessary to support fibrin formation under flow. There was no difference in fibrin formation between severe and moderate deficiencies, but platelet aggregate size was significantly larger for moderate deficiencies. Computational calculations estimate that the local thrombin concentration in moderate deficiencies is high enough to induce platelet activation (>1 nM), but too low to support fibrin formation (<10 nM). In the absence of platelets, fibrin formation was not supported even at normal FVIII levels, suggesting platelet adhesion is necessary for fibrin formation. Individuals treated by replacement therapy, recombinant FVIII, showed normalized fibrin formation. Individuals treated with bypass therapy, recombinant FVIIa, had a reduced lag time in fibrin formation, as well as elevated fibrin accumulation compared to healthy controls. Treatment of rFVIIa, but not rFVIII, resulted in significant changes in fibrin dynamics that could lead to a prothrombotic state.

Studies of a microchip flow-chamber system to characterize whole blood thrombogenicity in healthy individuals

INTRODUCTION

A whole blood flow-chamber system, the Total Thrombus-formation Analysis System (T-TAS), was developed for quantitative analysis of platelet thrombus formation (PTF) using microchips with thrombogenic surfaces (collagen, PL chip; collagen plus tissue thromboplastin, AR chip) under shear stress conditions. We evaluated the usefulness of the T-TAS for assessing individual thrombogenicity compared with other platelet function tests.

MATERIALS AND METHODS

Blood samples from 31 healthy volunteers were applied to the T-TAS to measure PTF starting time (T10: time to reach 10 kPa), occlusion time (T60 for PL chip; T80 for AR chip), and area under the curve (AUC10, area under curve until 10 min for PL chip; AUC30, 30 min for AR chip) under various shear rates (1000, 1500, 2000s(-1) for PL chip; 300 s(-1) for AR chip). Platelet functions were also tested using platelet aggregometry, the PFA-100 (collagen and epinephrine [C/EPI], collagen and adenosine diphosphate [C/ADP]), and the VerifyNow P2Y12 assay.

RESULTS

Individual pressure waveforms, including PTF starting and ending points, varied among healthy subjects. In the PL chip, T10 and AUC10 showed a shear-dependent correlation with C/EPI or C/ADP. VerifyNow P2Y12 values were not significantly associated with the parameters of the T-TAS. Platelet counts were correlated with all AR measurements, and mostly with PL measurements.

CONCLUSION

The results of the T-TAS were associated with those of the PFA-100 in many respects, indicating that its characteristics are related to shear-induced PTF. The T-TAS showed few correlations with platelet aggregometry and the VerifyNow P2Y12 assay. The T-TAS may allow for the measurement of comprehensive parameters of individual thrombogenicity under whole blood flow conditions.

Biomechanics of haemostasis and thrombosis in health and disease: from the macro- to molecular scale

Although the processes of haemostasis and thrombosis have been studied extensively in the past several decades, much of the effort has been spent characterizing the biological and biochemical aspects of clotting. More recently, researchers have discovered that the function and physiology of blood cells and plasma proteins relevant in haematologic processes are mechanically, as well as biologically, regulated. This is not entirely surprising considering the extremely dynamic fluidic environment that these blood components exist in. Other cells in the body such as fibroblasts and endothelial cells have been found to biologically respond to their physical and mechanical environments, affecting aspects of cellular physiology as diverse as cytoskeletal architecture to gene expression to alterations of vital signalling pathways. In the circulation, blood cells and plasma proteins are constantly exposed to forces while they, in turn, also exert forces to regulate clot formation. These mechanical factors lead to biochemical and biomechanical changes on the macro- to molecular scale. Likewise, biochemical and biomechanical alterations in the microenvironment can ultimately impact the mechanical regulation of clot formation. The ways in which these factors all balance each other can be the difference between haemostasis and thrombosis. Here, we review how the biomechanics of blood cells intimately interact with the cellular and molecular biology to regulate haemostasis and thrombosis in the context of health and disease from the macro- to molecular scale. We will also show how these biomechanical forces in the context of haemostasis and thrombosis have been replicated or measured in vitro.