Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy

Whole Blood Based Multiparameter Assessment of Thrombus Formation in Standard Microfluidic Devices to Proxy In Vivo Haemostasis and Thrombosis

Microfluidic assays are versatile tests which, using only small amounts of blood, enable high throughput analyses of platelet function in several minutes. In combination with fluorescence microscopy, these flow tests allow real-time visualisation of platelet activation with the possibility of examining combinatorial effects of wall shear rate, coagulation and modulation by endothelial cells. In particular, the ability to use blood and blood cells from healthy subjects or patients makes this technology promising, both for research and (pre)clinical diagnostic purposes. In the present review, we describe how microfluidic devices are used to assess the roles of platelets in thrombosis and haemostasis. We place emphasis on technical aspects and on experimental designs that make the concept of "blood-vessel-component-on-a-chip" an attractive, rapidly developing technology for the study of the complex biological processes of blood coagulability in the presence of flow.

Electrical Analysis Of Normal And Diabetic Blood For Evaluation Of Aggregation And Coagulation Under Different Rheological Conditions

Introduction

Erythrocyte aggregation and blood coagulation are of great interest and are still under investigation by many researchers. Erythrocytes have a direct effect on hemorheological properties. Real-time in vitro studies on blood coagulation and aggregation provide a chance to understand their mechanisms in normal and pathological conditions. Additionally, this method offers control over the physical and chemical conditions during the study.

Objective

The present study introduced a simple in vitro technique to study blood aggregation and coagulation under controlled conditions.

Methods

The technique used in this study is based on the measurement of the electrical properties of blood. A simple flow chamber was made from two cylinders with a gap between them. The outer cylinder remains stationary, and the inner cylinder rotates about its axis. The inner cylinder velocity is controlled by a stepper motor. Blood samples are introduced in the gap between the two cylinders. Capacitance and impedance of blood samples were recorded by two electrodes attached to the outer cylinders and in direct contact with blood.

Results

Quantitative parameters were extracted from the capacitance and impedance time courses. These parameters were used to describe the aggregation and coagulation processes under different shear rates. Strong correlations between the aggregation index and shear rate were found for normal and diabetic blood samples. Additionally, strong negative correlations of coagulation time were found for normal and diabetic blood samples. In conclusion, the electrical analysis of blood reflects well the interactions between internal blood contents.

Conclusion

The parameters extracted from this technique can be used in the quantitative description of hemorheological processes under different physical conditions.

Thromboprophylaxis strategies for children with single-ventricle circulations (superior or total cavo-pulmonary connections) after stent implantation

OBJECTIVE

To define optimal thromboprophylaxis strategy after stent implantation in superior or total cavopulmonary connections.

BACKGROUND

Stent thrombosis is a rare complication of intravascular stenting, with a perceived higher risk in single-ventricle patients.

METHODS

All patients who underwent stent implantation within superior or total cavopulmonary connections (caval vein, innominate vein, Fontan, or branch pulmonary arteries) were included. Cohort was divided into aspirin therapy alone versus advanced anticoagulation, including warfarin, enoxaparin, heparin, or clopidogrel. Primary endpoint was in-stent or downstream thrombus, and secondary endpoints included bleeding complications.

RESULTS

A total of 58 patients with single-ventricle circulation underwent 72 stent implantations. Of them 14 stents (19%) were implanted post-superior cavopulmonary connection and 58 (81%) post-total cavopulmonary connection. Indications for stenting included vessel/conduit stenosis (67%), external compression (18%), and thrombotic occlusion (15%). Advanced anticoagulation was prescribed for 32 (44%) patients and aspirin for 40 (56%) patients. Median follow up was 1.1 (25th-75th percentile, 0.5-2.6) years. Echocardiograms were available in 71 patients (99%), and advanced imaging in 44 patients (61%). Thrombosis was present in two patients on advanced anticoagulation (6.3%) and none noted in patients on aspirin (p = 0.187). Both patients with in-stent thrombus underwent initial stenting due to occlusive left pulmonary artery thrombus acutely post-superior cavopulmonary connection. There were seven (22%) significant bleeding complications for advanced anticoagulation and none for aspirin (p < 0.001).

CONCLUSIONS

Antithrombotic strategy does not appear to affect rates of in-stent thrombus in single-ventricle circulations. Aspirin alone may be sufficient for most patients undergoing stent implantation, while pre-existing thrombus may warrant advanced anticoagulation.

Anti-GPVI Fab SAR264565 effectively blocks GPVI function in ex vivo human platelets under arterial shear in a perfusion chamber

INTRODUCTION

Glycoprotein VI (GPVI) is the major platelet receptor for collagen-mediated platelet adhesion and activation. SAR264565 is an anti-GPVI-Fab, binds to GPVI with high affinity, and blocks GPVI function in human platelets in vitro.

METHODS

The effect of SAR26456 on platelet responsiveness in the blood of 21 healthy male subjects was investigated using Sakariassen's ex vivo thrombogenesis perfusion chamber model on a collagen-coated surface under conditions mimicking arterial flow. Ex vivo effects of SAR264565 (10 and 100 μg/mL) were investigated before administration of aspirin or clopidogrel to study subjects (baseline), after aspirin (2× 300 mg) administration alone, and after combined aspirin (2× 300 mg)/clopidogrel (600 mg) administration. Additional ex vivo and in vitro platelet tests were also performed.

RESULTS

Addition of SAR264565 to the perfusion chamber dose-dependently reduced platelet and fibrin deposition, reaching statistical significance at 100 μg/mL (415 ± 67 compared to 137 ± 36 platelets/cm², [p < 0.01] and fibrin 0.095 ± 0.014 compared to 0.032 ± 0.008 μg/cm², [p < 0.001]). Aspirin administration caused an additive and dose-dependent reduction of SAR264565-induced platelet and fibrin deposition. Combined aspirin/clopidogrel administration did not lead to additional SAR264565-induced inhibition of platelet or fibrin deposition.

CONCLUSION

GPVI antagonism by the anti-GPVI-Fab fragment SAR264565 dose-dependently inhibits platelet adhesion and fibrin formation on a collagen surface under arterial shear. Additive inhibition is observed after prior aspirin administration with no further amplification on top of a combination of aspirin with clopidogrel. Ex vivo antiplatelet tests confirmed a selective inhibiting effect of SAR264565 on collagen-induced platelet activation.

Measurement of platelet aggregation functions using whole blood migration ratio in a microfluidic chip

Platelets play a major role in maintaining endothelial integrity and hemostasis. Of the various soluble agonists, ADP is an important in vivo stimulus for inducing platelet aggregation. In this study, a simple, rapid, and affordable method was designed for testing bleeding time (BT) and platelet aggregation with a two-channel microfluidic chip. Whole blood migration ratio (MR) from a microchip system was evaluated in comparison to the closure time (CT) from PFA-100 assays (Siemens, Germany) and CD62P expression on platelets. To induce platelet aggregation, a combination of collagen (1.84 mg/ml) and ADP (37.5 mg/ml) were used as agonists. After adding the agonists to samples, whole blood MR from the microchip system was measured. The outcome of the assessment depended on reaction time and agonist concentration. MR of whole blood from the microchip system was significantly correlated with CT from PFA-100 (r = 0.61, p <  0.05, n = 60). In addition, MR was negatively correlated with CD62P expression (r =-0.95, p <  0.05, n = 60). These results suggest that the measurement of MR using agonists is an easy, simple and efficient method for monitoring platelet aggregation in normal and ADP-receptors defective samples, along with the BT test. Thus, usage of the current microfluidic method could expand to diverse applications, including efficacy assessments in platelet therapy.

The impact of blood shear rate on arterial thrombus formation

The shear rate and corresponding shear stress have impacts on arterial thrombus formation. In particular, the effects of increasing concentration of platelets at the vessel wall and activation of platelets at this site increase the growth and stability of the thrombi which may result in a fatal narrowing of the arterial lumen. The efficacy of many antithrombotic agents is shear dependent as well. It is apparent that there is a need for a point-of-care device to rapidly monitor the risk for arterial thrombosis and to optimize antithrombotic therapy in vitro. The present review focuses on the essential role of shear rate on arterial thrombus formation in native human blood drawn directly from an antecubital vein.

Effects of Rivaroxaban on Platelet Activation and Platelet-Coagulation Pathway Interaction: In Vitro and In Vivo Studies

Introduction:

Activation of coagulation and platelets is closely linked, and arterial thrombosis involves coagulation activation as well as platelet activation and aggregation. In these studies, we investigated the possible synergistic effects of rivaroxaban in combination with antiplatelet agents on thrombin generation and platelet aggregation in vitro and on arterial thrombosis and hemostasis in rat models.

Materials and Methods:

Thrombin generation was measured by the Calibrated Automated Thrombogram method (0.5 pmol/L tissue factor) using human platelet-rich plasma (PRP) spiked with rivaroxaban (15, 30, or 60 ng/mL), ticagrelor (1.0 µg/mL), and acetylsalicylic acid (ASA; 100 µg/mL). Tissue factor-induced platelet aggregation was measured in PRP spiked with rivaroxaban (15 or 30 ng/mL), ticagrelor (1 or 3 µg/mL), or a combination of these. An arteriovenous (AV) shunt model in rats was used to determine the effects of rivaroxaban (0.01, 0.03, or 0.1 mg/kg), clopidogrel (1 mg/kg), ASA (3 mg/kg), and combinations on arterial thrombosis.

Results:

Rivaroxaban inhibited thrombin generation in a concentration-dependent manner and the effect was enhanced with ticagrelor and ticagrelor plus ASA. Rivaroxaban and ticagrelor also concentration-dependently inhibited tissue factor-induced platelet aggregation, and their combination increased the inhibition synergistically. In the AV shunt model, rivaroxaban dose-dependently reduced thrombus formation. Combining subefficacious or weakly efficacious doses of rivaroxaban with ASA or ASA plus clopidogrel increased the antithrombotic effect.

Conclusion:

These data indicate that the combination of rivaroxaban with single or dual antiplatelet agents works synergistically to reduce platelet activation, which may in turn lead to the delayed/reduced formation of coagulation complexes and vice versa, thereby enhancing antithrombotic potency.

Monomerization of C-reactive protein requires glycoprotein IIb-IIIa activation: pentraxins and platelet deposition

BACKGROUND

Pentraxins are inflammatory mediators linked to cardiovascular disease; however, their role in thrombosis remains to be fully elucidated.

AIMS

We investigated the role of pentraxins in thrombus formation on different vascular substrates under flow conditions.

METHODS

Native C-reactive protein (nCRP) and serum amyloid P (SAP) effects on thrombosis were evaluated under flow conditions on substrates placed in flat perfusion chambers. nCRP and dissociated monomeric CRP (mCRP) distributions were visualized by use of confocal microscopy. The effects of nCRP on vascular substrates were tested in the Badimon chamber.

RESULTS

mCRP, but not nCRP, induced a significant activation in platelet deposition, whereas SAP induced an activation only on fibrinogen-coated substrates. The effects of CRP on platelet deposition were significantly reduced by statin treatment. mCRP resulting from recirculation of blood containing nCRP over a thrombogenic vessel wall induced increased platelet deposition. Blocking glycoprotein IIb-IIIa prevented the effects of CRP dissociation and significantly reduced platelet deposition. Annexin V treatment did not block monomerization of CRP on activated platelets.

CONCLUSIONS

Under flow conditions, platelet deposited on all tested biological substrates support nCRP dissociation into mCRP. The effect is dependent on the thrombogenic potency of the substrate to trigger initial platelet deposition. Exposure of glycoprotein IIb-IIIa in the platelet surface supports nCRP dissociation. CRP monomerization was not dependent on the aminophospholipid exposed on the surface of activated platelets. The dissociated mCRP is trapped in the growing platelet aggregate and stimulates further platelet deposition. SAP increases platelet deposition only on fibrin monolayers. Therefore, pentraxins induce a platelet activation effect linking inflammation and thrombosis.

Assaying the efficacy of dual-antiplatelet therapy: use of a controlled-shear-rate microfluidic device with a well-defined collagen surface to track dynamic platelet adhesion

We report the development and demonstration of an assay that distinguishes the pharmacological effects of two widely used antiplatelet therapies, aspirin (COX-1 inhibitor) and clopidogrel (P2Y12 inhibitor). Whole blood is perfused through a low-volume microfluidic device in contact with a well-characterized (ellipsometry, atomic force microscopy) acid-soluble type I collagen surface. Whole human blood treated in vitro with a P2Y12 inhibitor 2-methylthioadenosine 5'-monophosphate triethylammonium salt (2-MeSAMP) extended the time to the start of platelet recruitment, i.e., platelet binding to the collagen surface. Treatment with 2-MeSAMP also slowed the rate of aggregate buildup, with an overall reduced average platelet aggregate area after 8 min of constant blood flow. A far smaller effect was observed for in vitro treatment with aspirin, for which the rate of change of surface coverage is indistinguishable from controls. In whole blood obtained from patients under treatment with dual-antiplatelet therapy (aspirin and clopidogrel), a significant extension of time to platelet recruitment was observed along with a slowed rate of aggregate buildup and an average aggregate size approximately half that of control measurements. Differentiation of the pharmacological effects of these two well-targeted antiplatelet pathways suggests a role for this assay in determining the antiplatelet effects of these and related new therapeutics in clinical settings.

Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Custom-made and commercial parallel-plate flow chambers are widely used for studies of platelet activation and thrombus formation in whole blood at defined shear rates. When used in a reproducible way, such flow chamber devices give valuable information on the thrombogenic potential of human, mouse, or rat blood. This article aims to provide a practical guide for the use of parallel-plate flow chambers in combination with routine microscopic imaging techniques. The following methodological aspects are addressed: preparation of surface coatings, calculation of blood flow and shear rate, control of pre-analytical variables, protocols for routine performing of flow chamber tests with non-coagulating or coagulating blood, and procedures for real-time and end-point analysis of thrombus formation. Frequently encountered experimental problems and artifacts are discussed, as well as possibilities for using flow chamber devices as a diagnostic tool to test antithrombotic medication.

Monitoring in vitro thrombus formation with novel microfluidic devices

Cardiovascular disease is a major cause of mortality globally and is subject to ongoing research to improve clinical treatment. It is established that activation of platelets and coagulation are central to thrombosis, yet at different extents in the arterial and venous system. In vitro perfusion chamber technology has contributed significant knowledge on the function of platelets in the thrombotic process under shear conditions. Recent efforts to downscale this technique with a variety of microfluidic devices has opened new possibilities to study this process under precisely controlled flow conditions. Such microfluidic devices possess the capability to execute platelet function tests more quickly than current assays, while using small blood samples. Gradually becoming available to the clinic now, they may provide a new means to manage the treatment of cardiovascular diseases, although accurate validation studies still are missing. This review highlights the progress that has been made in monitoring aspects of thrombus formation using microfluidic devices.

Development of a perfusion chamber assay to study in real time the kinetics of thrombosis and the antithrombotic characteristics of antiplatelet drugs

Background

Arterial thrombosis triggered by vascular injury is a balance between thrombus growth and thrombus fragmentation (dethrombosis). Unbalance towards thrombus growth can lead to vascular occlusion, downstream ischemia and tissue damage.

Here we describe the development of a simple methodology that allows for continuous real time monitoring and quantification of both processes during perfusion of human blood under arterial shear rate conditions. Using this methodology, we have studied the effects of antiplatelet agents targeting COX-1 (aspirin), P2Y₁₂ (2-MeSAMP, clopidogrel), GP IIb-IIIa (eptifibatide) and their combinations on the kinetics of thrombosis over time.

Results

Untreated samples of blood perfused over type III collagen at arterial rates of shear promoted the growth of stable thrombi. Modulation by eptifibatide affected thrombus growth, while that mediated by 2-MeSAMP and aspirin affected thrombus stability. Using this technique, we confirmed the primacy of continuous signaling by the ADP autocrine loop acting on P2Y₁₂ in the maintenance of thrombus stability. Analysis of the kinetics of thrombosis revealed that continuous and prolonged analysis of thrombosis is required to capture the role of platelet signaling pathways in their entirety. Furthermore, studies evaluating the thrombotic profiles of 20 healthy volunteers treated with aspirin, clopidogrel or their combination indicated that while three individuals did not benefits from either aspirin or clopidogrel treatments, all individuals displayed marked destabilization profiles when treated with the combination regimen.

Conclusions

These results show the utility of a simple perfusion chamber technology to assess in real time the activity of antiplatelet drugs and their combinations. It offers the opportunity to perform pharmacodynamic monitoring of arterial thrombosis in clinical trials and to investigate novel strategies directed at inhibiting thrombus stability in the management of cardiovascular disease.

Effective hydrodynamic shaping of sample streams in a microfluidic parallel-plate flow-assay device: matching whole blood dynamic viscosity

We report the development of an aqueous buffer system tailored to the fluidic and hemodynamic requirements of our recently reported microfluidic platelet dynamic assay device, which uses hydrodynamic focusing to "shape" a blood sample into a thin flowing layer adjacent to its protein-functionalized surface. By matching the dynamic viscosity of whole blood (3.13 ± 0.08 mPa·s, from healthy donors), the selected buffer minimizes interfacial fluid mixing and better controls shear rate within the device, permitting platelet/protein-surface interaction assays with as little as 50 μL of whole blood. Buffers containing the viscosity-enhancing components bovine serum albumin (BSA), gelofusine/glycine, or histopaque (Ficoll gradient solution) were found not to activate platelets when incubated with blood at concentrations up to 50%, as assessed by flow cytometry quantitation of P-selectin expression and αIIbβ (3) activation. In contrast, glycerol-based buffer activated platelets (two-fold increase in P-selectin levels) at concentrations as low as 10% by volume. BSA- and gelofusine/glycine-based buffers were problematic in preparation and use, and therefore, were not used beyond initial characterization. The histopaque solution selected as the best choice for flow studies stabilizes sample contact with the device's thrombogenic surface, does not activate platelets, and does not interfere with the action of agonists added to deliberately activate platelets.

Platelet adhesion from shear blood flow is controlled by near-wall rebounding collisions with erythrocytes

The efficacy of platelet adhesion in shear flow is known to be substantially modulated by the physical presence of red blood cells (RBCs). The mechanisms of this regulation remain obscure due to the complicated character of platelet interactions with RBCs and vascular walls. To investigate this problem, we have created a mathematical model that takes into account shear-induced transport of platelets across the flow, platelet expulsion by the RBCs from the near-wall layer of the flow onto the wall, and reversible capture of platelets by the wall and their firm adhesion to it. This model analysis allowed us to obtain, for the first time to our knowledge, an analytical determination of the platelet adhesion rate constant as a function of the wall shear rate, hematocrit, and average sizes of platelets and RBCs. This formula provided a quantitative description of the results of previous in vitro adhesion experiments in perfusion chambers. The results of the simulations suggest that under a wide range of shear rates and hematocrit values, the rate of platelet adhesion from the blood flow is mainly limited by the frequency of their near-wall rebounding collisions with RBCs. This finding reveals the mechanism by which erythrocytes physically control platelet hemostasis.

Effect of blood flow on near-the-wall mass transport of drugs and other bioactive agents: a simple formula to estimate boundary layer concentrations

Transport of bioactive agents through the blood is essential for cardiovascular regulatory processes and drug delivery. Bioactive agents and other solutes infused into the blood through the wall of a blood vessel or released into the blood from an area in the vessel wall spread downstream of the infusion/release region and form a thin boundary layer in which solute concentration is higher than in the rest of the blood. Bioactive agents distributed along the vessel wall affect endothelial cells and regulate biological processes, such as thrombus formation, atherogenesis, and vascular remodeling. To calculate the concentration of solutes in the boundary layer, researchers have generally used numerical simulations. However, to investigate the effect of blood flow, infusion rate, and vessel geometry on the concentration of different solutes, many simulations are needed, leading to a time-consuming effort. In this paper, a relatively simple formula to quantify concentrations in a tube downstream of an infusion/release region is presented. Given known blood-flow rates, tube radius, solute diffusivity, and the length of the infusion region, this formula can be used to quickly estimate solute concentrations when infusion rates are known or to estimate infusion rates when solute concentrations at a point downstream of the infusion region are known. The developed formula is based on boundary layer theory and physical principles. The formula is an approximate solution of the advection-diffusion equations in the boundary layer region when solute concentration is small (dilute solution), infusion rate is modeled as a mass flux, and there is no transport of solute through the wall or chemical reactions downstream of the infusion region. Wall concentrations calculated using the formula developed in this paper were compared to the results from finite element models. Agreement between the results was within 10%. The developed formula could be used in experimental procedures to evaluate drug efficacy, in the design of drug-eluting stents, and to calculate rates of release of bioactive substances at active surfaces using downstream concentration measurements. In addition to being simple and fast to use, the formula gives accurate quantifications of concentrations and infusion rates under steady-state and oscillatory flow conditions, and therefore can be used to estimate boundary layer concentrations under physiological conditions.

Threshold response of initiation of blood coagulation by tissue factor in patterned microfluidic capillaries is controlled by shear rate

OBJECTIVE

Blood flow is considered one of the important parameters that contribute to venous thrombosis. We quantitatively test the relationship between initiation of coagulation and shear rate and suggest a biophysical mechanism to understand this relationship.

METHODS AND RESULTS

Flowing human blood and plasma were exposed to cylindrical surfaces patterned with patches of tissue factor (TF) by using microfluidics. Initiation of coagulation of normal pooled plasma depended on shear rate, not volumetric flow rate or flow velocity, and coagulation initiated only at shear rates below a critical value. Initiation of coagulation of platelet-rich plasma and whole blood showed similar behavior. At constant shear rate, coagulation of plasma also showed a threshold response to the size of a patch of TF, consistent with our previous work in the absence of flow.

CONCLUSIONS

Initiation of coagulation of flowing blood displays a threshold response to shear rate and to the size of a surface patch of TF. Combined with the results of others, these results set the range of shear rates that limit initiation of coagulation by small surface areas of TF and by shear activation of platelets. This range fits the relatively narrow range of physiological shear rates described by Murray's law.

Biomarkers and Bioassays for Cardiovascular Diseases: Present and Future

Stratification of cardiac patients arriving at the emergency department is now being made according to the levels of acute cardiac biomarkers (i.e. cardiac troponin (cTn) or creatine kinase myocardial band (CK-MB)). Ongoing efforts are undertaken in an attempt to identify and validate additional cardiac biomarkers, for example, interleukin-6, soluble CD40L, and C-reactive protein, in order to further risk stratify patients with acute coronary syndrome. Several studies have also now shown an association of platelet transcriptome and genomic single nucleotide polymorphisms with myocardial infarction by using advanced genomic tools. A number of markers, such as myeloid-related protein 14 (MRP-14), cyclooxygenase-1 (COX-1), 5-lipoxygenase activating protein (FLAP), leukotriene A4 hydrolase (LTA4H) and myocyte enhancing factor 2A (MEF2A), have been linked to acute coronary syndromes, including myocardial infarction. In the future, these novel markers may pave the way toward personalized disease-prevention programs based on a person's genomic, thrombotic and cardiovascular profiles. Current and future biomarkers and bioassays for identifying at-risk patients will be discussed in this review.

Validation of the human tissue factor/FVIIa complex as an antithrombotic target and the discovery of a synthetic peptide

This review focuses on the validation of the principal initiator of human coagulation, the tissue factor (TF)/coagulation factor (F)VIIa complex, as an antithrombotic target, as well as on the discovery of a cyclic pentapeptide (PN7051), which dose-dependently inhibits TF/FVIIa-induced coagulation and thrombus formation. Target validation and studies of antithrombotic efficacy were performed with a human thrombosis model employing non-anticoagulated blood from severe homozygous FVII-deficient patients and healthy individuals at blood-flow conditions mimicking those in healthy and diseased vessels. Additional validation included an anti-TF monoclonal antibody, recombinant TF pathway inhibitor, recombinant inactivated-active site FVIIa and all-trans retinoic acid. Structural and biological characterization of PN7051 and other peptides from the same FVII domain indicate that PN7051 interferes with an essential interaction between the epidermal growth factor domain-2-like and the catalytic domains of FVIIa. A peptidomimetics approach is suggested to further improve the antithrombotic potency of PN7051.

Mechanical Stress Activates Platelets at a Subhemolysis Level: An In Vitro Study

A feasibility study is performed to quantify sheep platelets (PLTs) and to identify the relationship between PLT count and hemolysis as a consequence of mechanical stress. Six adult, healthy Dorset sheep have been used for in vitro blood sampling test procedures in a hemoresistometer device (HRM). In each experiment, blood of the same animal was exposed to six different shear rates. Free hemoglobin levels and PLT count for each shear rate were detected. In all animals (A-F), hemolysis increased significantly between the shear rates of 2325 and 3100/s (P < 0.05) and the mean PLT count dropped immediately (contact, low shear) 40% in the beginning, between the shear rates of 0 and 775/s (P < 0.05). PLT count increased slightly as soon as hemolysis started. At higher shear rates, hemolysis increased and PLTs reduced further. Precise counting of PLTs indicates that PLTs are consumed dramatically at very low shear (by contact) and further by applied mechanical stress when hemolysis is obvious. A repetition of these tests with human blood could indicate species differences.

Blood flow devices in medical research and clinical testing in humans: are we approaching personalized medicine?

This review focuses on studies of blood flow devices employed in man to unravel the mechanisms of bleeding and thrombotic disorders, and on the characterization of novel experimental antithrombotic entities and drug candidates in biopharmaceutical research and development. Clinical studies with drug candidates and new therapeutic strategies have also been performed, and the predictability of these experimental approaches to clinical situations is excellent. Based on the solid validation of these flow devices, miniature flow devices employing nonanticoagulated blood drawn directly from an antecubital vein should be developed for diagnostic purposes. It is anticipated that such a diagnostic flow device could develop into a personalized medicine approach.

Predilution versus postdilution during continuous venovenous hemofiltration: a comparison of circuit thrombogenesis

During continuous venovenous hemofiltration, predilution can prolong circuit survival time, but the underlying mechanism has not been elucidated. The aim of the present study was to compare predilution with postdilution, with respect to circuit thrombogenesis. Eight critically ill patients were treated with both predilutional and postdilutional continuous venovenous hemofiltration in a crossover fashion. A filtration flow of 60 ml/min was used in both modes. We chose blood flows of 140 and 200 ml/min during predilution and postdilution, respectively, to keep the total flow through the hemofilter constant. Extracorporeal circuit pressures were measured hourly, and samples of blood and ultrafiltrate were collected at five different time points. Thrombin-antithrombin complexes and prothrombin fragments F1 + 2 were measured by ELISA, and platelet activation was assessed by flow cytometry. No signs of thrombin generation or platelet activation were found during either mode. During postdilution, baseline platelet count and maximal prefilter pressure had a linear relation, whereas both parameters were inversely related with circuit survival time. In summary, predilution and postdilution did not differ with respect to extracorporeal circuit thrombogenesis. During postdilution, baseline platelet count and maximal prefilter pressure were inversely related with circuit survival time.

CHO cells expressing the high affinity alpha(IIb)beta3 T562N integrin demonstrate enhanced adhesion under shear

BACKGROUND

Alpha(IIb)beta3-mediated platelet adhesive interactions in the vasculature, which are dependent on the functional state of this receptor, may be sensitive to shear forces.

OBJECTIVES

To evaluate the influence of the alpha(IIb)beta3 affinity state on cell attachment under flow, we compared Chinese hamster ovary cells expressing the low affinity alpha(IIb)beta3 wild-type (wt) receptor to those expressing the high affinity alpha(IIb)beta3 T562N receptor.

MATERIALS AND METHODS

We designed a real-time videomicroscopy adhesion assay for von Willebrand factor (VWF) or fibrinogen under flow conditions.

RESULTS

At 50 s(-1), alpha(IIb)beta3 T562N supported higher cell adhesion to fibrinogen (63.3 +/- 2.9 cells/field) than alpha(IIb)beta3 wt (38.7 +/- 2.4 cells/field, P < 0.0001). At 100 s(-1), alpha(IIb)beta3 T562N mediated cell adhesion (40.5 +/- 3.8 cells/field), while alpha(IIb)beta3 wt did not (5.3 +/- 1.4 cells/field, P < 0.001), allowing to discriminate the efficiency of each receptor. Similar findings were observed for adhesion to VWF. Complete inhibition of cell adhesion to fibrinogen was achieved with 800 microM fibrinogen gamma-chain dodecapeptide [HHLGGAKQAGDV (H12)], while Arg-Gly-Asp-Ser (RGDS) peptide (10-1000 microM) induced a dose-dependent cell detachment. These results suggest that the H12 motif allows initial attachment, in contrast to the RGDS site, which strengthens the stability of adhesion. Interestingly, compared with wt, a 10-fold lower concentration of RGDS was required to reach a similar reduction of cell adhesion mediated by alpha(IIb)beta3 T562N.

CONCLUSIONS

Our data show that alpha(IIb)beta3 activation is associated with a stabilization of integrin binding to fibrinogen or VWF under shear.

Resistance to antiplatelet drugs: current status and future research

Platelet reactivity and activation are important factors during the development of atherothrombotic processes and subsequent ischaemic complications. Pharmacological agents that suppress platelet function are proved to be the most efficient in the prevention and treatment of thrombotic complications. As the activation of platelets during thrombus generation involves many complex and redundant pathways, simultaneous use of different antiplatelet drugs that are directed against different targets have been effective in reducing adverse clinical events. The main antiplatelet drugs are aspirin (which inhibits thromboxane synthesis), thienopyridines (which block P2Y12 receptors) and glycoprotein IIb/IIIa antagonists (which block glycoprotein IIb/IIIa receptors). In recent years, resistance or nonresponsiveness to antiplatelet therapy has been reported and, more importantly, are linked to the occurrence of adverse cardiovascular events. New treatment strategies to overcome nonresponsiveness are being sought. A focus on the development of simple, reproducible and user friendly point-of-care methods to determine aspirin/clopidogrel responsiveness should be undertaken to assist clinicians in tailoring antiplatelet therapy to the individual patient.

Prothrombotic responses to exercise are little influenced by clopidogrel treatment

AIMS

Adenosine diphosphate (ADP) is involved in shear-induced platelet activation, which may be important for platelet responses to stress. We therefore tested the hypothesis that ADP receptor antagonism by clopidogrel treatment would attenuate exercise-induced platelet activation.

METHODS AND RESULTS

Fifteen healthy volunteers performed exhaustive exercise without and with clopidogrel pretreatment (75 mg/day; 7 days) in a randomised crossover study. Filtragometry readings (reflecting platelet aggregability in vivo) and 11-dehydro-thromboxane B(2) (TxM) in plasma were determined before and after exercise. Platelet and leukocyte activity, platelet-platelet (PPA), and platelet-leukocyte aggregates (PLAs) in vivo and their responsiveness to agonist stimulation in vitro were assessed by flow cytometry. Clopidogrel treatment inhibited ADP-induced platelet P-selectin expression by 72% (54-85%). Exercise increased platelet aggregation (filtragometry and PPAs), elevated plasma TxM, increased single platelet P-selectin expression, elevated circulating PLAs, and enhanced ADP and thrombin-stimulated P-selectin expression. Clopidogrel prolonged filtragometry readings and attenuated agonist stimulated P-selectin expression at rest, but did not influence TxM in plasma or urine or attenuate platelet or leukocyte responses to exercise. Clopidogrel treatment did not influence plasma CD40L (ligand) at rest or after exercise.

CONCLUSION

Clopidogrel treatment attenuates platelet activity in vivo at rest, but exercise counteracts the platelet stabilizing effects of clopidogrel. The hypothesis that ADP is involved in stress-induced platelet activation was not supported.

Platelet-derived NO slows thrombus growth on a collagen type III surface

Nitric oxide (NO) is a free radical that plays an important role in modulating platelet adhesion and aggregation. Platelets are a source of vascular NO, but since erythrocytes avidly scavenge NO, the functional significance of platelet-derived NO is not clear. Our purpose was to determine if NO from platelets affects platelet thrombus formation in the presence of anticoagulated whole blood in an in vitro parallel plate flow system. We studied platelet adhesion and aggregation on a collagen type III surface in the presence of physiologically relevant fluid mechanical shear stress. We found that certain receptor mediated agonists (insulin and isoproterenol) caused a concentration dependent reduction in thrombus formation at a shear rate of 1000 s-1. This effect was mediated by NO since it was abolished in the presence of the NO inhibitor L-nitro-arginine-methyl-ester (L-NAME). As expected, at venous levels of shear rate (100 s-1) neither of the agonists had any effect on thrombus formation since platelet adhesion does not depend on activation at these low levels of shear. Interestingly, at a shear rate of 2000 s-1 the addition of L-NAME caused an increase in platelet coverage suggesting that shear, by itself, induces NO production by platelets. This is the first demonstration of shear stress causing platelets to produce an inhibitor of platelet activation. These results demonstrate that the development of a platelet thrombus is regulated in a complex way and that platelets produce functionally significant amounts of NO even in the presence of whole blood.

Interactions between aspirin and COX-2 inhibitors or NSAIDs in a rat thrombosis model

Recent in vitro studies, clinical trials and epidemiological studies have suggested possible interactions between aspirin and other cyclo-oxygenase (COX) inhibitors, such as ibuprofen of the COX-2 inhibitors celecoxib and rofecoxib. The objective of this study was to test the effects of aspirin (1, 2.5 and 5 mg/kg), and ibuprofen (4 and 15 mg/kg), diclofenac (2.5 mg/kg), flurbiprofen (2 mg/kg), celecoxib (7.5 mg/kg), and rofecoxib (1 mg/kg), alone or combined on a rat model of arterial thrombosis. Drugs were given orally daily for 7 days, before insertion of an arterio-venous shunt thrombosis system, left in place for 15 min. Main parameter was thrombus weight. Five to 12 rats were used per experiment, and 35 controls overall. Aspirin inhibited thrombus formation in a dose-dependent manner. All NSAIDS given alone also inhibited thrombus formation to approximately the same level as aspirin 1 mg/kg/day. Ibuprofen, celecoxib and rofecoxib inhibited the effects of aspirin, but not diclofenac or flurbiprofen. The interactions with aspirin do not seem to affect all NSAIDs to equal levels. The clinical impact of this needs to be confirmed in adequately powered clinical trials or pharmaco-epidemiological studies.

Purinergic P2Y12 receptor blockade inhibits shear-induced platelet phosphatidylinositol 3-kinase activation

Pathologically elevated shear stress triggers aspirin-insensitive platelet thrombosis. Signaling mechanisms involved in shear-induced platelet thrombosis are not well understood. To investigate these, we examined the hypothesis that functionally important platelet phosphatidylinositol 3-kinase (PI3-K) activity is stimulated by an in vitro shear stress of 120 dynes/cm(2) (shear rate of 6,000 sec(-1)). Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) production was examined in washed human platelets subjected to pathological shear stress in a cone-plate viscometer. PIP(3) production peaks 30 s after shear begins and is initiated by von Willebrand factor (VWF) binding to the glycoprotein (Gp) Ib-IX-V complex. Inhibiting PI3-K with wortmannin or 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) results in the inhibition of shear-induced platelet aggregation. In resting platelets, class IA PI3-K associates with the tyrosine kinase Syk. Within 30 s of beginning shear, PI3-K-associated Syk becomes tyrosine phosphorylated. Inhibiting Syk activation with piceatannol results in the inhibition of PIP(3) production and aggregation. Selective blockade of the P2Y(12) receptor results in the inhibition of Syk phosphorylation, PIP(3) production, and aggregation. These results indicate that shear-induced VWF binding to platelet GpIb-IX-V stimulates functionally important PI3-K activity. PI3-K activation is signaled by rapid feedback amplification that involves P2Y(12) receptor-mediated activation of Syk.

Effects of the selective inhibition of platelet thromboxane synthesis on the platelet-subendothelium interaction

1. Drugs that inhibit TxA(2) synthesis are used to reduce platelet aggregation. The aim of this study was to compare the effects of a cyclo-oxygenase (COX) inhibitor (acetylsalicylic acid, ASA), a thromboxane synthetase (TxS) inhibitor (dazoxiben) and a dual TxS inhibitor and TxA(2) receptor blocker (DT-TX 30) on platelet aggregation and the platelet-subendothelium interaction in flow conditions. 2. The techniques used in this in vitro study were platelet aggregometry in whole blood, and measurement of platelet thromboxane B(2) and prostaglandin E(2) production and leucocyte production of 6-keto-PGF(1alpha). The platelet-subendothelium interaction was evaluated in rabbit aorta subendothelium preparations exposed to flowing blood at a shear stress of 800 s(-1). Morphometric methods were used to calculate the percentage of subendothelium occupied by platelets. 3. The 50% inhibitory concentration (IC(50)) of DT-TX 30 in whole blood was in the range of 10(-7) micro M (induced with collagen or arachidonic acid) to 10(-5) micro M (induced with thrombin) or 10(-4) (induced with ADP). IC(50) values under all experimental conditions were lower with DT-TX 30 than with ASA. For thromboxane B(2) the IC(50) were: ASA 0.84+/-0.05 micro M, dazoxiben 765+/-54 micro M, DT-TX 30 8.54+/-0.60 micro M. Prostaglandin E(2) was inhibited only by ASA (IC(50) 1.21+/-0.08 micro M). Leucocyte 6-keto-PGF(1alpha) was inhibited by ASA (IC(50) 6.58+/-0.76 micro M) and increased by dazoxiben and DT-TX 30. The greatest reduction in percentage subendothelial surface occupied by platelets after blood perfusion was seen after treatment with DT-TX 30 in the range of concentrations that inhibited collagen-induced platelet aggregation (control group: 31.20+/-3.8%, DT-TX 30 at 0.1 micro M: 10.71+/-0.55%, at 1.0 micro M: 6.53+/-0.44%, at 5.0 micro M; 1.48+/-0.07%). All three drugs reduced thrombus formation, although ASA (unlike dazoxiben or DT-TX 30) increased the percentage surface occupied by adhesions. 4. In conclusion, the effect of specific blockage of TxS together with blockage of membrane receptors for TxA(2) can surpass the effect of ASA in inhibiting the platelet-subendothelium interaction in flow conditions.