Thrombi Formed in a Chandler Loop Mimic Human Arterial Thrombi in Structure and PAI-1 Content and Distribution

Flow affects the structural and mechanical properties of the fibrin network in plasma clots

The fibrin network is one of the main components of thrombi. Altered fibrin network properties are known to influence the development and progression of thrombotic disorders, at least partly through effects on the mechanical stability of fibrin. Most studies investigating the role of fibrin in thrombus properties prepare clots under static conditions, missing the influence of blood flow which is present in vivo. In this study, plasma clots in the presence and absence of flow were prepared inside a Chandler loop. Recitrated plasma from healthy donors were spun at 0 and 30 RPM. The clot structure was characterized using scanning electron microscopy and confocal microscopy and correlated with the stiffness measured by unconfined compression testing. We quantified fibrin fiber density, pore size, and fiber thickness and bulk stiffness at low and high strain values. Clots formed under flow had thinner fibrin fibers, smaller pores, and a denser fibrin network with higher stiffness values compared to clots formed in absence of flow. Our findings indicate that fluid flow is an essential factor to consider when developing physiologically relevant in vitro thrombus models used in researching thrombectomy outcomes or risk of embolization.

Effect of Chandler loop shear and tubing size on thrombus architecture

Thrombosis can lead to a wide variety of life-threatening circumstances. As current thrombolytic drug screening models often poorly predict drug profiles, leading to failure of thrombolytic therapy or clinical translation, more representative clot substrates are necessary for drug evaluation. Utilizing a Chandler loop device to form clot analogs at high shear has gained popularity in stroke societies. However, shear-dependent clot microstructure has not been fully addressed and low shear conditions are often overlooked. We herein characterized the impact of wall shear rate (126 to 951 s^-1) on clot properties in the Chandler loop. Different revolutions (20-60) per minute and tubing sizes (3.2 to 7.9 mm) were employed to create different sized clots to mimic various thrombosis applications. Increased shear resulted in decreased RBC counts (76.9 ± 4.3% to 17.6 ± 0.9%) and increased fibrin (10 to 60%) based on clot histology. Increased fibrin sheet morphology and platelet aggregates were observed at higher shear under scanning electron microscope. These results show the significant impact of shear and tubing size on resulting clot properties and demonstrate the capability of forming a variety of reproducible in-vivo-like clot analogs in the Chandler loop device controlling for simple parameters to tune clot characteristics.

Real-time tracking of fibrinolysis under constant wall shear and various pulsatile flows in an in-vitro thrombolysis model

A great need exists for the development of a more representative in-vitro model to efficiently screen novel thrombolytic therapies. We herein report the design, validation, and characterization of a highly reproducible, physiological scale, flowing clot lysis platform with real-time fibrinolysis monitoring to screen thrombolytic drugs utilizing a fluorescein isothiocyanate (FITC)-labeled clot analog. Using this Real-Time Fluorometric Flowing Fibrinolysis assay (RT-FluFF assay), a tPa-dependent degree of thrombolysis was observed both via clot mass loss as well as fluorometrically monitored release of FITC-labeled fibrin degradation products. Percent clot mass loss ranged from 33.6% to 85.9% with fluorescence release rates of 0.53 to 1.17 RFU/min in 40 and 1000 ng/mL tPa conditions, respectively. The platform is easily adapted to produce pulsatile flows. Hemodynamics of human main pulmonary artery were mimicked through matching dimensionless flow parameters calculated using clinical data. Increasing pressure amplitude range (4-40 mmHg) results in a 20% increase of fibrinolysis at 1000 ng/mL tPA. Increasing shear flow rate (205-913 s^-1) significantly increases fibrinolysis and mechanical digestion. These findings suggest pulsatile level affects thrombolytic drug activities and the proposed in-vitro clot model offers a versatile testing platform for thrombolytic drug screening.

A triple fusion tissue-type plasminogen activator (TriF-ΔtPA) enhanced thrombolysis in carotid embolism-induced stroke model

Recombinant tissue-type plasminogen activator (rtPA) is the first approved thrombolytic agent in acute ischemic stroke, but suffers from a short half-life and poor resistance to plasminogen activator inhibitor (PAI-1), limiting its clinical use. Thus, the development of novel thrombolytic agents with improved benefit/risk balance has always been of great significance. In this study, We identified a serine protease domain of tPA mutant (named ΔtPA^A146V) capable of escaping the inhibition by endogenous PAI-1 with 66-fold increased resistance compared to the wild type. Based on this mutant, we generated a triple fusion ΔtPA (TriF-ΔtPA), including albumin and fibrin binding peptide(FBP). The fusion with albumin effectively prolonged the plasma half-life of ΔtPA in mice to 144 minutes, which is much longer than ΔtPA and did not affect its thrombolytic activity. Furthermore, FBP rendered fibrin specificity of the fusion protein, giving a KD of ∼25 ± 0.9 μM. In a novel murine carotid embolism-induced stroke (CES) model, i.v. administration of TriF-ΔtPA promoted vascular recanalization, reduced infarct volume, and mitigated neurobehavioral deficits more significantly compared to ΔtPA-HSA or Alteplase, showing little bleeding risk. Together, this long-acting PAI-1-resistant thrombolytic agent holds great potential for clinical applications.

Location, location, location: Fibrin, cells, and fibrinolytic factors in thrombi

Thrombi are heterogenous in nature with composition and structure being dictated by the site of formation, initiating stimuli, shear stress, and cellular influences. Arterial thrombi are historically associated with high platelet content and more tightly packed fibrin, reflecting the shear stress in these vessels. In contrast, venous thrombi are generally erythrocyte and fibrin-rich with reduced platelet contribution. However, these conventional views on the composition of thrombi in divergent vascular beds have shifted in recent years, largely due to recent advances in thromboectomy and high-resolution imaging. Interestingly, the distribution of fibrinolytic proteins within thrombi is directly influenced by the cellular composition and vascular bed. This in turn influences the susceptibility of thrombi to proteolytic degradation. Our current knowledge of thrombus composition and its impact on resistance to thrombolytic therapy and success of thrombectomy is advancing, but nonetheless in its infancy. We require a deeper understanding of thrombus architecture and the downstream influence on fibrinolytic susceptibility. Ultimately, this will aid in a stratified and targeted approach to tailored antithrombotic strategies in patients with various thromboembolic diseases.

“Going with the flow” in modeling fibrinolysis

The formation of thrombi is shaped by intravascular shear stress, influencing both fibrin architecture and the cellular composition which has downstream implications in terms of stability against mechanical and fibrinolytic forces. There have been many advancements in the development of models that incorporate flow rates akin to those found in vivo. Both thrombus formation and breakdown are simultaneous processes, the balance of which dictates the size, persistence and resolution of thrombi. Therefore, there is a requirement to have models which mimic the physiological shear experienced within the vasculature which in turn influences the fibrinolytic degradation of the thrombus. Here, we discuss various assays for fibrinolysis and importantly the development of novel models that incorporate physiological shear rates. These models are essential tools to untangle the molecular and cellular processes which govern fibrinolysis and can recreate the conditions within normal and diseased vessels to determine how these processes become perturbed in a pathophysiological setting. They also have utility to assess novel drug targets and antithrombotic drugs that influence thrombus stability.

Comparison of Large Animal Models for Acute Ischemic Stroke: Which Model to Use?

Translation of acute ischemic stroke research to the clinical setting remains limited over the last few decades with only one drug, recombinant tissue-type plasminogen activator, successfully completing the path from experimental study to clinical practice. To improve the selection of experimental treatments before testing in clinical studies, the use of large gyrencephalic animal models of acute ischemic stroke has been recommended. Currently, these models include, among others, dogs, swine, sheep, and nonhuman primates that closely emulate aspects of the human setting of brain ischemia and reperfusion. Species-specific characteristics, such as the cerebrovascular architecture or pathophysiology of thrombotic/ischemic processes, significantly influence the suitability of a model to address specific research questions. In this article, we review key characteristics of the main large animal models used in translational studies of acute ischemic stroke, regarding (1) anatomy and physiology of the cerebral vasculature, including brain morphology, coagulation characteristics, and immune function; (2) ischemic stroke modeling, including vessel occlusion approaches, reproducibility of infarct size, procedural complications, and functional outcome assessment; and (3) implementation aspects, including ethics, logistics, and costs. This review specifically aims to facilitate the selection of the appropriate large animal model for studies on acute ischemic stroke, based on specific research questions and large animal model characteristics.

Experimental Evaluation of the Effectiveness of Aspiration-Based Techniques to Treat Different Types of Acute Thromboembolic Occlusions in the Femoropopliteal Vascular System Using an In Vitro Flow Model

PURPOSE

In this in vitro study, the effectiveness and safety of four aspiration-based techniques for thrombectomy are evaluated for three types of thrombi in a flow model simulating the femoropopliteal segment.

MATERIAL AND METHODS

Red, white, and mixed thrombi were produced in a standardized manner and used to simulate occlusion of a superficial femoral artery using a pulsatile flow model. Four techniques were compared: aspiration alone, aspiration + stent retriever, exposing thrombus to laser by an excimer laser system and a laser catheter + aspiration, and aspiration + mechanical fragmentation by a separator. Rate of first-pass recanalization, embolic events, and number of embolized fragments > 1 mm were compared.

RESULTS

Aspiration alone, stent retriever, laser, and separator differed in rates of first-pass recanalization (53.3%; 86.6%; 20%; and 100%) and embolic events (40%; 93.3%; 73.3%; and 60%). Number of embolized fragments was lowest with aspiration and higher with separator, laser, and stent retriever. Rates of first-pass-recanalization (75%; 75%; and 45%) and embolic events (65%; 60%; and 75%) differed for red, white, and mixed thrombi. The mixed thrombus caused the highest number of embolized fragments, which was particularly high using the stent retriever.

CONCLUSION

Additional use of mechanical techniques significantly enhances the effectiveness of thrombectomy but simultaneously provokes more embolism. Laser seems to negatively alter the structure of a thrombus and thus diminishes the effectiveness, while provoking embolism. All techniques had lowest effectiveness, but highest embolism with the mixed thrombus. This was particularly striking when a stent retriever was used with the mixed thrombus.

Role of Shear Stress and tPA Concentration in the Fibrinolytic Potential of Thrombi

The resolution of arterial thrombi is critically dependent on the endogenous fibrinolytic system. Using well-established and complementary whole blood models, we investigated the endogenous fibrinolytic potential of the tissue-type plasminogen activator (tPA) and the intra-thrombus distribution of fibrinolytic proteins, formed ex vivo under shear. tPA was present at physiologically relevant concentrations and fibrinolysis was monitored using an FITC-labelled fibrinogen tracer. Thrombi were formed from anticoagulated blood using a Chandler Loop and from non-anticoagulated blood perfused over specially-prepared porcine aorta strips under low (212 s⁻¹) and high shear (1690 s⁻¹) conditions in a Badimon Chamber. Plasminogen, tPA and plasminogen activator inhibitor-1 (PAI-1) concentrations were measured by ELISA. The tPA–PAI-1 complex was abundant in Chandler model thrombi serum. In contrast, free tPA was evident in the head of thrombi and correlated with fibrinolytic activity. Badimon thrombi formed under high shear conditions were more resistant to fibrinolysis than those formed at low shear. Plasminogen and tPA concentrations were elevated in thrombi formed at low shear, while PAI-1 concentrations were augmented at high shear rates. In conclusion, tPA primarily localises to the thrombus head in a free and active form. Thrombi formed at high shear incorporate less tPA and plasminogen and increased PAI-1, thereby enhancing resistance to degradation.

Simultaneous R2* and quantitative susceptibility mapping measurement enables differentiation of thrombus hematocrit and age: an in vitro study at 3 T

BACKGROUND

The efficacy of acute ischemic stroke treatment is affected by thrombus composition and age, yet no diagnostic method capable of quantitative thrombus characterization currently exists. This in vitro study evaluates the use of R_2^* , quantitative susceptibility mapping (QSM), and proton density fat fraction (FF) maps derived from a single gradient echo (GRE) MRI acquisition for characterizing clot of various hematocrit, as well as added calcified and lipidic components, throughout aging.

METHODS

Two thrombus phantoms containing porcine clots (10-60% hematocrit, one with added calcium or lard) were scanned serially throughout 6 days of aging. Three-dimensional multi-echo GRE imaging was used to generate R_2^* , QSM, and FF maps, from which mean values for all clots at every time point were obtained. Receiver operating characteristic analysis was used to derive thresholds differentiating acute from chronic clot, and measured R_2^* and QSM were tested for their ability to estimate clot hematocrit.

RESULTS

R_2^* and QSM varied minimally over the first 6 hours of aging (acute), and QSM was found to linearly relate to clot hematocrit. Beyond 6 hours (chronic), R_2^* and QSM increased considerably over time and hematocrit could be estimated from the R_2^* /QSM ratio. R_2^* and QSM thresholds of 22 s^-1 and 0.165 ppm differentiated acute from chronic clots with a sensitivity/specificity of 100%/100% and 85%/92%, respectively. QSM and FF maps definitively distinguished calcium and lipid, respectively, from clots of any hematocrit and age.

CONCLUSIONS

R_2^* , QSM, and FF from a single multi-echo GRE scan discriminated hematocrit and age, and distinguished calcification and lipid withinin vitro clot.

Control of fibrinolytic drug injection via real-time ultrasonic monitoring of blood coagulation

In the present study, we investigated the capabilities of a novel ultrasonic approach for real-time control of fibrinolysis under flow conditions. Ultrasonic monitoring was performed in a specially designed experimental in vitro system. Fibrinolytic agents were automatically injected at ultrasonically determined stages of the blood clotting. The following clots dissolution in the system was investigated by means of ultrasonic monitoring. It was shown, that clots resistance to fibrinolysis significantly increases during the first 5 minutes since the formation of primary micro-clots. The efficiency of clot lysis strongly depends on the concentration of the fibrinolytic agent as well as the delay of its injection moment. The ultrasonic method was able to detect the coagulation at early stages, when timely pharmacological intervention can still prevent the formation of macroscopic clots in the experimental system. This result serves as evidence that ultrasonic methods may provide new opportunities for real-time monitoring and the early pharmacological correction of thrombotic complications in clinical practice.

Intra-arterial pulse wave analysis during thrombectomy for the assessment of collateral status - A feasibility study

Purpose

Knowledge of the collateralization of an occluded vessel is important for the risk-benefit analysis of difficult revascularization maneuvers during mechanical thrombectomy. If the territory behind a clot is well perfused, one could desist from performing a risky thrombectomy maneuver. The arterial pulse pressure curve may serve as an indicator for the collateralization of an occluded target vessel. We investigated the feasibility of arterial pulse measurements with a standard microcatheter.

Methods

We measured the intra-arterial blood pressure proximal and distal to the clot in 40 thrombectomy maneuvers in a porcine stroke model. We used a microcatheter (Trevo Pro 18, Stryker, Kalamazoo, CA, USA), a pressure transducer (MEMSCAP SP844), an AdInstruments Powerlab 16/35 workstation, and LabChart 8 Software (AdInstruments, Dunedin, New Zealand).

Results

Median arterial blood pressure proximal and distal to the clot was 96.0 mmHg (IQR, 23.8 mmHg) and 47.5 mmHg (IQR, 43.5 mmHg), respectively (p < .001). The median difference between systolic maximum and diastolic minimum proximal and distal to the clot decreased significantly from 1.8 mmHg (IQR, 3.6 mmHg) to 0.0 mmHg (IQR, 0.5 mmHg) (p < .001). There was loss of the curve in 26 of 40 cases and loss of pressure in 23 of 40 cases (p = .008). There was no significant correlation between vessel diameter and either loss of the pulse pressure curve (p = .20) or overall pressure loss (p = .31).

Conclusion

It is possible to measure the pulse pressure proximal and distal to the clot with a standard microcatheter used during mechanical thrombectomy.

Recent advances in mouse models for systemic sclerosis

SSc is a complex rheumatoid disease characterized by autoimmunity, fibrosis and vasculopathy. Mouse models provide powerful research tools for exploring the pathogenesis of the human diseases. Each mouse model can represent a specific way leading to the development of disease. Moreover, mouse models can be used to investigate the role of candidate molecule in the pathogenesis of disease. So far, more than twenty mouse models for SSc have been established and provide new insights in the understanding of the pathogenesis of SSc. In this review, we provide an overview on recent advances in the field of experimental SSc. We introduce novel mouse models generated in the recent years and discuss their relevance to the SSc pathogenesis. Moreover, we summarize and discuss recent findings in the pathogenesis of classical SSc mouse models.

Measuring fibrinolysis: from research to routine diagnostic assays

Development and standardization of fibrinolysis methods have progressed more slowly than coagulation testing and routine high-throughput screening tests for fibrinolysis are still lacking. In laboratory research, a variety of approaches are available and are applied to understand the regulation of fibrinolysis and its contribution to the hemostatic balance. Fibrinolysis in normal blood is slow to develop. For practical purposes plasminogen activators can be added to clotting plasma, or euglobulin prepared to reduce endogenous inhibitors, but results are complicated by these manipulations. Observational studies to identify a 'fibrinolysis deficit' have concluded that excess fibrinolysis inhibitors, plasminogen activator inhibitor 1 (PAI-1) or thrombin-activatable fibrinolysis inhibitor (TAFI), zymogen or active enzyme, may be associated with an increased risk of thrombosis. However, results are not always consistent and problems of adequate standardization are evident with these inhibitors and also for measurement of fibrin degradation products (D-dimer). Few methods are available to investigate fibrinolysis under flow, or in whole blood, but viscoelastic methods (VMs) such as ROTEM and TEG do permit the contribution of cells, and importantly platelets, to be explored. VMs are used to diagnose clinical hyperfibrinolysis, which is associated with high mortality. There is a debate on the usefulness of VMs as a point-of-care test method, particularly in trauma. Despite the difficulties of many fibrinolysis methods, research on the fibrinolysis system, taking in wider interactions with hemostasis proteins, is progressing so that in future we may have more complete models and better diagnostic methods and therapeutics.

Necessary Catheter Diameters for Mechanical Thrombectomy with ADAPT

BACKGROUND AND PURPOSE

Large-bore catheters allow mechanical thrombectomy in ischemic stroke by engaging and retrieving clots without additional devices (direct aspiration first-pass technique [ADAPT]). The purpose of this study was to establish a model for minimal catheter diameters needed for ADAPT.

MATERIALS AND METHODS

We established a theoretic model for the calculation of minimal catheter diameters needed for ADAPT. We then verified its validity in 28 ADAPT maneuvers in a porcine in vivo model. To account for different mechanical thrombectomy techniques, we factored in ADAPT with/without a hypothetic 0.021-inch microcatheter or 0.014-inch microwire inside the lumen of the aspiration catheter and aspiration with a 60-mL syringe versus an aspiration pump.

RESULTS

According to our calculations, catheters with an inner diameter of >0.040 inch and >0.064 inch, respectively, are needed to be effective in the middle cerebral artery (2.5-mm diameter) or in the internal carotid artery (4 mm) in an average patient. There was a significant correlation between predicted and actual thrombectomy results (P = .010). Our theoretic model had a positive and negative predictive value of 78% and 79%, respectively. Sensitivity and specificity were 88% and 64%, respectively.

CONCLUSIONS

Our theoretic model allows estimating the minimal catheter diameters needed for successful mechanical thrombectomy with ADAPT, as demonstrated by the good agreement with our animal experiments. Our model will be helpful to interventionalists in avoiding selecting catheters that are likely too small to be effective.

Inertial Cavitation Ultrasound with Microbubbles Improves Reperfusion Efficacy When Combined with Tissue Plasminogen Activator in an In Vitro Model of Microvascular Obstruction

We have previously reported that long-tone-burst, high-mechanical-index ultrasound (US) and microbubble (MB) therapy can restore perfusion in both in vitro and in vivo models of microvascular obstruction (MVO). Addition of MBs to US has been found to potentiate the efficacy of thrombolytics on large venous thrombi; however, the optimal US parameters for achieving microvascular reperfusion of MVO caused by microthrombi, when combined with tissue plasminogen activator (tPA), are unknown. We sought to elucidate the specific effects of US, with and without tPA, for effective reperfusion of MVO in an in vitro model using both venous and arterial microthrombi. Venous- and arterial-type microthrombi were infused onto a mesh with 40-μm pores to simulate MVO. Pulsed US (1 MHz) was delivered with inertial cavitation (IC) (1.0 MPa, 1000 cycles, 0.33 Hz) and stable cavitation (SC) US (0.23 MPa, 20% duty cycle, 0.33 Hz) regimes while MB suspension (2 × 106 MBs/mL) was infused. The efficacy of sonoreperfusion with these parameters was tested with and without tPA. Sonoreperfusion efficacy was significantly greater for IC + tPA compared with tPA alone, IC, SC and SC + tPA, suggesting lytic synergism between tPA and US for both venous- and arterial-type microthrombi. In contrast to our previous in vitro studies using 1.5 MPa at 5000 US cycles without tPA, the IC regime employed herein used 90% less US energy. These findings suggest an IC regime can be used with tPA synergistically to achieve a high degree of fibrinolysis for both thrombus types.

Effect of Thrombus Composition and Viscosity on Sonoreperfusion Efficacy in a Model of Micro-Vascular Obstruction

Distal embolization of micro-thrombi during stenting for myocardial infarction causes micro-vascular obstruction (MVO). We have previously shown that sonoreperfusion (SRP), a microbubble (MB)-mediated ultrasound (US) therapy, resolves MVO from venous micro-thrombi in vitro in saline. However, blood is more viscous than saline, and arterial thrombi that embolize during stenting are mechanically distinct from venous clot. Therefore, we tested the hypothesis that MVO created with arterial micro-thrombi are more resistant to SRP therapy compared with venous micro-thrombi, and higher viscosity further increases the US requirement for effective SRP in an in vitro model of MVO. Lipid MBs suspended in plasma with adjusted viscosity (1.1 cP or 4.0 cP) were passed through tubing bearing a mesh with 40-μm pores to simulate a micro-vascular cross-section; upstream pressure reflected thrombus burden. To simulate MVO, the mesh was occluded with either arterial or venous micro-thrombi to increase upstream pressure to 40 mmHg ± 5 mmHg. Therapeutic long-tone-burst US was delivered to the occluded area for 20 min. MB activity was recorded with a passive cavitation detector. MVO caused by arterial micro-thrombi at either blood or plasma viscosity resulted in less effective SRP therapy compared to venous thrombi. Higher viscosity further reduced the effectiveness of SRP therapy. The passive cavitation detector showed a decrease in inertial cavitation when viscosity was increased, while stable cavitation was affected in a more complex manner. Overall, these data suggest that arterial thrombi may require higher acoustic pressure US than venous thrombi to achieve similar SRP efficacy; increased viscosity decreases SRP efficacy; and both inertial and stable cavitation are implicated in observed SRP efficacy.

Antithrombogenic Properties of Amphiphilic Block Copolymer Coatings: Evaluation of Hemocompatibility Using Whole Blood

Antithrombogenicity is one of the most critical properties required for materials used in biomedical devices, particularly in devices that contact blood. The antithrombogenicity of surfaces coated with amphiphilic block copolymers composed of hydrophobic poly(2-methoxyethyl acrylate) (M) and hydrophilic poly(N,N-dimethylacrylamide) (D) segments was investigated using plasma protein and whole blood with regard to protein adsorption, thrombus formation, platelet activation, and clotting kinetics. Three types of block copolymers and a random copolymer were synthesized using one-pot reversible addition-fragmentation chain-transfer (RAFT) polymerization under conditions of high yield and high molecular weight. Triblock and 4-arm block copolymers with MDM and (MD)₄ architecture, respectively, showed good adhesion to both organic and inorganic substrates, including polyvinyl chloride (PVC) tubes, and the resulting coated surfaces showed superior protein repellency and hemocompatibility compared to the diblock or random copolymer coatings and noncoated control. In a Chandler-loop method with whole blood, PVC tubes coated with MDM and (MD)₄ showed improved thromboresistance and adsorption resistance to blood-derived proteins. This high hemocompatibility was also confirmed with human whole blood by thrombelastography (suppression of blood-clotting behavior in both intrinsic and extrinsic coagulation pathways) and platelet function analyses (significant reductions in the aggregation activity of platelets under two types of stimulation). The antithrombogenicity has been discussed based on the structural analyses of the MDM-coated surface. The results of this study will enable the development of more effective biomedical and analytical devices with excellent antithrombogenic characteristics by using a simple and environmentally friendly approach.

Plasminogen associates with phosphatidylserine-exposing platelets and contributes to thrombus lysis under flow

The interaction of plasminogen with platelets and their localization during thrombus formation and fibrinolysis under flow are not defined. Using a novel model of whole blood thrombi, formed under flow, we examine dose-dependent fibrinolysis using fluorescence microscopy. Fibrinolysis was dependent upon flow and the balance between fibrin formation and plasminogen activation, with tissue plasminogen activator-mediated lysis being more efficient than urokinase plasminogen activator-mediated lysis. Fluorescently labeled plasminogen radiates from platelet aggregates at the base of thrombi, primarily in association with fibrin. Hirudin attenuates, but does not abolish plasminogen binding, denoting the importance of fibrin. Flow cytometry revealed that stimulation of platelets with thrombin/convulxin significantly increased the plasminogen signal associated with phosphatidylserine (PS)-exposing platelets. Binding was attenuated by tirofiban and Gly-Pro-Arg-Pro amide, confirming a role for fibrin in amplifying plasminogen binding to PS-exposing platelets. Confocal microscopy revealed direct binding of plasminogen and fibrinogen to different platelet subpopulations. Binding of plasminogen and fibrinogen co-localized with PAC-1 in the center of spread platelets. In contrast, PS-exposing platelets were PAC-1 negative, and bound plasminogen and fibrinogen in a protruding "cap." These data show that different subpopulations of platelets harbor plasminogen by diverse mechanisms and provide an essential scaffold for the accumulation of fibrinolytic proteins that mediate fibrinolysis under flow.

Improvement of stent retriever design and efficacy of mechanical thrombectomy in a flow model

PURPOSE

In vitro experiments were performed to evaluate the efficacy of mechanical intracranial thrombectomy comparing the newly developed Aperio stent retriever and standard devices for stroke treatment.

METHODS

The Aperio (A), with an increased working length of 4 cm and a special cell design for capturing and withholding clots, was compared to three benchmark devices: the Solitaire retrievable stent (B), the Merci X6 (C), and the Merci L5 retriever (D). In a vascular glass model with pulsatile flow, reminiscent of the M1 segment of the middle cerebral artery, we repeatedly induced occlusion by generating thrombi via a modified Chandler loop system. The numbers of recanalization attempts, peripheral embolizations, and recanalizations at the site of occlusion were recorded during 10 retrieval experiments with each device.

RESULTS

Eleven devices were able to remove the blood clots from the occluded branch. In 34 of 40 experiments, restoration of flow was obtained in 1-3 attempts. The main differences between the study devices were observed in terms of clot withholding and fragmentation during retrieval. Although there was only one fragmentation recorded for device A, disengagement of the whole clot or peripheral embolization of fragments occurred more frequently (5-7 times) with devices B, C, and D.

CONCLUSION

In a vascular model, the design of device A was best at capturing and withholding thrombi during retrieval. Further study will be necessary to see whether this holds true in clinical applications.

The role of thrombin activatable fibrinolysis inhibitor and factor XI in platelet-mediated fibrinolysis resistance: a thromboelastographic study in whole blood

BACKGROUND

The resistance of platelet-rich thrombi to fibrinolysis is generally attributed to clot retraction and platelet PAI-1 release. The role of TAFI in platelet-mediated resistance to lysis is unclear.

OBJECTIVE

We investigated the contribution of TAFI to the antifibrinolytic effect of platelets in whole blood by thromboelastography.

METHODS

Platelet-poor (PP-WB, < 40 × 10(3) μL(-1) ) and platelet-rich (PR-WB, > 400 × 10(3) μL(-1) ) blood samples were obtained from normal human blood (N-WB, 150-220 × 10(3) μL(-1) ). Clot lysis time was measured by thromboelastography in recalcified blood supplemented with t-PA (100 ng mL(-1) ) and tissue factor (1:1000 Recombiplastin).

RESULTS

t-PA-induced lysis time increased in parallel with platelet concentration (up to 3-fold). Neutralization of TAFI, but not of PAI-1, shortened the lysis time by ∼ 50% in PR-WB and by < 10% in PP-WB. Accordingly, prothrombin F1+2 and TAFIa accumulation was greater in PR-WB than in PP-WB. A similar TAFI-dependent inhibition of fibrinolysis was observed when clot retraction was prevented by cytochalasin D or abciximab, or when platelet membranes were tested. Moreover, in blood with an intact contact system, platelet-mediated fibrinolysis resistance was attenuated by an anti-FXI but not by an anti F-XII antibody. Finally, platelets made the clots resistant to the profibrinolytic effect of heparin concentrations displaying a strong anticoagulant activity.

CONCLUSIONS

Our data indicate that TAFI activation is one major mechanism whereby platelets make clots resistant to fibrinolysis and underscore the importance of TAFI inhibitors as new antithrombotic agents.

Secretion and antifibrinolytic function of thrombin-activatable fibrinolysis inhibitor from human platelets

BACKGROUND

The thrombin-activatable fibrinolysis inhibitor (TAFI) is a zymogen first characterized in human plasma that is activated through proteolytic cleavage by thrombin, thrombin in complex with thrombomodulin, or plasmin. Active TAFI attenuates fibrinolysis by removing C-terminal lysine residues from partially degraded fibrin, thereby inhibiting a potent positive feedback loop in the fibrinolytic cascade. The existence of a separate pool of TAFI within platelets has been described.

OBJECTIVES AND METHODS

We aimed to confirm the presence of TAFI in the medium of washed, thrombin-stimulated platelets and to evaluate the characteristics of platelet TAFI by western blot analysis and with a quantitative assay for activated TAFI. We also assessed the ability of platelet TAFI to inhibit fibrinolysis in vitro, using a platelet-rich thrombus lysis assay.

RESULTS

Our data are consistent with the presence of TAFI in the α-granules of resting platelets. In contrast to previous reports, platelet TAFI is very similar in electrophoretic mobility to plasma-derived TAFI. We also show, for the first time, that platelet-derived TAFI is capable of attenuating platelet-rich thrombus lysis in vitro independently of plasma TAFI. Moreover, we demonstrate additive effects on thrombolysis of platelet-derived TAFI and TAFI present in plasma.

CONCLUSIONS

Taken together, these observations indicate that the secretion of platelet-derived TAFI can augment the concentrations of TAFI already present in plasma to enhance attenuation of the fibrinolytic cascade. This could be significant in regions of vascular damage or pathologic thrombosis, where activated platelets are known to accumulate.

Model thrombi formed under flow reveal the role of factor XIII-mediated cross-linking in resistance to fibrinolysis

BACKGROUND

Activated factor XIII (FXIIIa), a transglutaminase, introduces fibrin-fibrin and fibrin-inhibitor cross-links, resulting in more mechanically stable clots. The impact of cross-linking on resistance to fibrinolysis has proved challenging to evaluate quantitatively.

METHODS

We used a whole blood model thrombus system to characterize the role of cross-linking in resistance to fibrinolytic degradation. Model thrombi, which mimic arterial thrombi formed in vivo, were prepared with incorporated fluorescently labeled fibrinogen, in order to allow quantification of fibrinolysis as released fluorescence units per minute.

RESULTS

A site-specific inhibitor of transglutaminases, added to blood from normal donors, yielded model thrombi that lysed more easily, either spontaneously or by plasminogen activators. This was observed both in the cell/platelet-rich head and fibrin-rich tail. Model thrombi from an FXIII-deficient patient lysed more quickly than normal thrombi; replacement therapy with FXIII concentrate normalized lysis. In vitro addition of purified FXIII to the patient's preprophylaxis blood, but not to normal control blood, resulted in more stable thrombi, indicating no further efficacy of supraphysiologic FXIII. However, addition of tissue transglutaminase, which is synthesized by endothelial cells, generated thrombi that were more resistant to fibrinolysis; this may stabilize mural thrombi in vivo.

CONCLUSIONS

Model thrombi formed under flow, even those prepared as plasma 'thrombi', reveal the effect of FXIII on fibrinolysis. Although very low levels of FXIII are known to produce mechanical clot stability, and to achieve γ-dimerization, they appear to be suboptimal in conferring full resistance to fibrinolysis.

In vitro methods of evaluating antithrombotics and thrombolytics

Platelets play a crucial role in primary hemostasis by forming hemostatic plugs at sites of vascular injury. There is abundant evidence that platelets also play a pivotal role in the pathogenesis of arterial thrombotic disorders, including unstable angina (UA), myocardial infarction (MI), and stroke. The underlying pathophysiological mechanism of these processes has been recognized as the disruption or erosion of a vulnerable atherosclerotic plaque, leading to local platelet adhesion and subsequent formation of partially or completely occlusive platelet thrombi. A variety of methods have been used to assess platelet aggregation, blood coagulation, and the ex vivo and/or in vitro efficacy of platelet antagonists, anticoagulants, and thrombolytics.

Bispecific targeting of thrombin activatable fibrinolysis inhibitor and plasminogen activator inhibitor-1 by a heterodimer diabody

BACKGROUND AND OBJECTIVES

Thrombin activatable fibrinolysis inhibitor (TAFI) and plasminogen activator inhibitor-1 (PAI-1) play important roles in fibrinolysis. Both reduce plasmin generation, but they exert their antifibrinolytic effects via different mechanisms. This study reports the cloning and characterization of a heterodimer diabody that inhibits TAFI and PAI-1 simultaneously.

METHODS AND RESULTS

The diabody was derived from two inhibiting monoclonal antibodies, i.e. MA-33H1F7, an anti-PAI-1 antibody that induces non-inhibitory substrate behavior of PAI-1, and MA-T12D11, an anti-TAFI antibody that inhibits activation of TAFI by the thrombin-thrombomodulin complex. A single-chain variable fragment (scFv) was derived from MA-T12D11 that displayed slightly reduced binding and inhibitory properties as compared to MA-T12D11. Characterization of the diabody revealed a similar affinity for TAFI and PAI-1 as that of the parental antibodies. Furthermore, the inhibitory properties of MA-33H1F7 and MA-T12D11 were fully preserved in the diabody format. In platelet-free plasma (PFP) clots, addition of the diabody had a stronger effect in shortening lysis times than either MA-T12D11 or MA-33H1F7. A similar reduction in clot lysis time was observed in platelet-rich plasma (PRP) clots. The same effect on clot lysis times in PFP and PRP was also achieved by the combined addition of MA-T12D11 and MA-33H1F7. The lysis rate of human model thrombi, made from whole blood, was approximately doubled after addition of the diabody. Moreover, this effect was significantly better than after the combined addition of the individual antibodies.

CONCLUSIONS

These observations demonstrate that simultaneous inhibition of TAFI and PAI-1 results in faster lysis of the formed thrombus.

The antithrombotic effect of dextran-40 in man is due to enhanced fibrinolysis in vivo

BACKGROUND

Dextran-40 is effective in reducing postoperative Doppler-detectable embolization in patients undergoing carotid endarterectomy (CEA). Dextrans are thought to have antithrombotic and antiplatelet effects. The mode of action is unclear. In rats, dextran blocks uptake of tissue plasminogen activator (tPA) by mannose-binding receptors. Because this would have the effect of enhancing endogenous fibrinolysis, we explored this effect of dextran-40 on fibrinolysis in man.

METHODS

Twenty patients undergoing endovascular stenting for abdominal aortic aneurysm were randomized to receive 100 mL of 10% dextran-40 or saline, over 1 hour, during their operation in addition to heparin. Blood samples were taken preoperatively, intraoperatively (immediately after operative procedure), and 24 hours postoperatively. Thrombi were formed in a Chandler loop and used to assess endogenous fibrinolysis over 24 hours, measured as the fall in thrombus weight, and the release of fluorescently labelled fibrinogen from the thrombus. Plasma samples were analyzed for markers of fibrinolysis; plasmin-antiplasmin (PAP), PAI-1, and t-PA, and for functional von Willebrand factor (vWF). Platelet response to thrombin and other agonists was measured by flow cytometry.

RESULTS

Thrombi formed ex vivo from the intraoperative blood samples from the dextran-treated patients exhibited significantly greater fibrinolysis vs preoperative samples, seen both as a significantly greater percentage reduction in thrombus weight (from 34.7% to 70.6% reduction) and as an 175% increase in the release of fluorescence (P < .05). Fibrinolysis returned to baseline levels the next day. No change was seen in the saline-treated group. Plasma levels of PAP and PAI-1 increased significantly postoperatively in the dextran-treated group vs the saline group (P < .05). The postoperative level of functional VWF was significantly lower in the dextran-treated group vs controls. A specific reduction occurred in the platelet response to thrombin, but not to other agonists, in the intraoperative samples from the dextran-treated group (11.1% vs 37.1%; P = .022), which was not seen in the controls.

CONCLUSIONS

These data are consistent with a rise in plasmin due to dextran blockade of tPA uptake in vivo, leading to enhanced fibrinolysis, cleavage of vWF and of the platelet protease-activated receptor-1 (PAR-1) thrombin receptor. This suggests that dextran exerts a combined therapeutic effect, enhancing endogenous fibrinolysis, whilst also reducing platelet adhesion to vWF and platelet activation by thrombin. The proven antithrombotic efficacy of low-dose dextran in carotid surgery may be applicable to wider therapeutic use.

The use of the Chandler loop to examine the interaction potential of NXY-059 on the thrombolytic properties of rtPA on human thrombi in vitro

BACKGROUND AND PURPOSE

Recombinant tissue-type plasminogen activator (rtPA) is the only globally approved treatment for acute ischaemic stroke. Other potential treatments might be administered with rtPA, making it important to discover whether compounds interfere with rtPA-induced lysis. We evaluated methods for examining the effect of the neuroprotectant NXY-059 on the lytic property of rtPA.

EXPERIMENTAL APPROACH

Plasma clot formation and lysis in the presence of rtPA and NXY-059 was measured as the change in plasma turbidity. The effect of NXY-059 on rtPA-induced lysis was similarly assessed on preformed clots. Lysis of the thrombus formed in a Chandler loop measured release of fluorescent-tagged fibrinogen that had been incorporated during thrombus formation. Thrombi were exposed to both rtPA and NXY-059 throughout lysis in the presence of 80% autologous plasma and the release of label during lysis was measured.

KEY RESULTS

Data interpretation is limited in the clot lysis experiments because either the rtPA was present during clot formation or the drug was added to a clot formed in static conditions. In contrast, thrombi were formed in dynamic flow conditions in the Chandler loop and the time course of lysis in plasma was examined. rtPA increased thrombolysis and the antifibrinolytic trans-4-(aminomethyl) cyclohexane carboxylic acid (AMCA) inhibited lysis. Lysis induced by rtPA was unaltered by NXY-059.

CONCLUSIONS AND IMPLICATIONS

The Chandler loop method provides a reliable technique for examining the effect of compounds on rtPA-induced lysis in vitro and demonstrated that NXY-059 does not alter rtPA-induced lysis at clinically relevant concentrations of either drug.

Enhanced Detection of Thromboemboli With the Use of Targeted Microbubbles

Background and Purpose— Targeted ultrasound contrast agents have recently been developed to adhere selectively to specific pathogenic materials such as plaque or thrombus. Administration of such microbubbles has potential to aid transcranial Doppler ultrasound (TCD) detection of emboli and to act as markers for distinguishing one embolic material from another. The purpose of this study was to investigate whether TCD detection of circulating thrombus emboli would be enhanced by the addition of targeted microbubbles.

Methods— Binding of microbubbles to the surface of the thrombus was confirmed by scanning electron microscopy. Targeted and control bubbles were then introduced to thrombus and tissue-mimicking material circulated under pulsatile-flow conditions in an in vitro flow rig. Embolic signal intensities before and after introduction of the bubbles were measured by TCD.

Results— Targeted microbubbles enhanced TCD signal intensities from thrombus emboli by up to 13 dB. The bubbles were capable of binding to moving thrombus when injected into the flow circuit in low concentrations (≈36 bubbles per 100 mL) and were retained on the thrombus under pulsatile-flow conditions. Signal intensities from similarly sized pieces of tissue-mimicking material were not enhanced by injection of targeted bubbles.

Conclusions— Injection of appropriately targeted microbubbles significantly enhances TCD detection of circulating thrombus emboli in vitro.

Fibrinolytic properties of lysine-derivatized polyethylene in contact with flowing whole blood (Chandler loop model)

This article reports on the concept of a fibrinolytic surface based on the preferential adsorption of endogenous plasminogen from blood. Data are presented indicating that such a surface, when pretreated with tissue-type plasminogen activator (tPA), is able to dissolve nascent thrombus generated in contact with flowing whole blood. Polyethylene (PE) surfaces were modified by attaching a lysine-containing polymer using photochemical methods as reported previously (McClung et al., J Biomed Mater Res 2000;49:409-414). The lysine residues were bound chemically to the polymer via the alpha-amino groups leaving the epsilon-amino groups free (epsilon-Lys surface). Control surfaces were (a) unmodified PE, (b) PE modified with the coating polymer containing no lysine, and (c) PE modified with the polymer containing lysine bound via the epsilon-amino group. The materials in tubing form were evaluated in contact with nonanticoagulated flowing human whole blood in a modified Chandler Loop experiment. They were first treated with tPA to allow activation of adsorbed plasminogen to plasmin. It was found that thrombus formation was initiated within 15-25 min (depending on donor blood) on all surfaces, as indicated by the formation of platelet aggregates. On the controls (including the lysine-containing material in which the epsilon-amino group was used in the binding reaction) thrombogenesis continued till the tubing was occluded and blood flow ceased. On the epsilon-Lys surface, thrombogenesis was interrupted at various stages depending on the donor blood; in all cases any thrombus generated was dissolved within minutes. It was shown that thrombolysis was due to the fibrinolytic action of plasmin generated at the surface and not to plasmin formed by traces of tPA released into the blood. This work provides further evidence of the efficacy of this approach to the development of a fibrinolytic surface.

TAFIa, PAI-1 and alpha-antiplasmin: complementary roles in regulating lysis of thrombi and plasma clots

PAI-1 and alpha(2)-antiplasmin (alpha(2)AP) are the principal direct inhibitors of fibrinolytic proteases. Thrombin activatable fibrinolysis inhibitor (TAFI), a plasma procarboxypeptidase activated by thrombin-thrombomodulin to form TAFIa, also regulates fibrinolysis by modulating fibrin. In this study, the relative contributions of PAI-1, alpha(2)AP and TAFIa to inhibition of lysis were assessed. In platelet-poor plasma clots, alpha(2)AP, TAFIa and PAI-1 all inhibited lysis, as shown by the addition of neutralizing antibodies to alpha(2)AP and PAI-1 +/- CPI, a potato carboxypeptidase inhibitor. alpha(2)AP played the largest role in regulating plasma clot lysis, but neutralization of inhibitors in combinations was more effective in shortening lysis times, with a maximal effect when all three inhibitors were neutralized. In platelet-rich clots, a larger contribution of PAI-1 was evident. Tissue plasminogen activator induced lysis of model thrombi, made from whole blood, was approximately doubled on incorporation of CPI, illustrating a substantial contribution of TAFIa to inhibition of thrombus lysis. Similar increases in thrombus lysis were observed on inclusion of neutralizing antibodies to PAI-1 and alpha(2)AP, with alpha(2)AP playing the dominant role. Maximal thrombus lysis occurred upon neutralization of all three inhibitors. These observations suggest that, despite the differences in concentrations and activities of inhibitors, and the different modes of action, the roles of the three are complementary in both plasma clot lysis and thrombus lysis.

Mechanism of inactivation of plasminogen activator inhibitor-1 by a small molecule inhibitor

The inactivation of plasminogen activator inhibitor-1 (PAI-1) by the small molecule PAI-1 inhibitor PAI-039 (tiplaxtinin) has been investigated using enzymatic analysis, direct binding studies, site-directed mutagenesis, and molecular modeling studies. Previously PAI-039 has been shown to exhibit in vivo activity in various animal models, but the mechanism of inhibition is unknown. PAI-039 bound specifically to the active conformation of PAI-1 and exhibited reversible inactivation of PAI-1 in vitro. SDS-PAGE indicated that PAI-039 inactivated PAI-1 predominantly through induction of PAI-1 substrate behavior. Preincubation of PAI-1 with vitronectin, but not bovine serum albumin, blocked PAI-039 activity while analysis of the reciprocal experiment demonstrated that preincubation of PAI-1 with PAI-039 blocked the binding of PAI-1 to vitronectin. Together, these data suggest that the site of interaction of the drug on PAI-1 is inaccessible when PAI-1 is bound to vitronectin and may overlap with the PAI-1 vitronectin binding domain. This was confirmed by site-directed mutagenesis and molecular modeling studies, which suggest that the binding epitope for PAI-039 is localized adjacent to the previously identified interaction site for vitronectin. Thus, these studies provide a detailed characterization of the mechanism of inhibition of PAI-1 by PAI-039 against free, but not vitronectin-bound PAI-1, suggesting for the first time a novel pool of PAI-1 exists that is vulnerable to inhibition by inactivators that bind at the vitronectin binding site.

Polymorphonuclear leukocytes from patients with severe sepsis have lost the ability to degrade fibrin via u-PA

Fibrin persistence in the vasculature is an important complication of sepsis that can often lead to mortality. We have previously established that polymorphonuclear leukocytes (PMN) from healthy individuals have the capacity to degrade fibrin via urokinase-type plasminogen activator (u-PA). We have also demonstrated an increase in u-PA antigen in the plasma of patients suffering from septic shock. In this study, we investigate the hypothesis that PMN from patients with sepsis have lost their fibrinolytic ability and that this might contribute to the persistence of fibrin deposits. We show here that PMN from these patients do not express any u-PA activity, despite retaining some u-PA antigen. Additionally, thrombi prepared from the whole blood of the patients exhibit reduced endogenous lysis compared with those from healthy individuals. These data indicate that loss of fibrinolytic activity from PMN may be a contributing factor in fibrin persistence in the microvasculature in sepsis.

Differential effects of the iodinated contrast agents Ioxaglate, Iohexol and Iodixanol on thrombus formation and fibrinolysis

INTRODUCTION

Contrast media (CM) possess both pro-thrombotic and anticoagulant properties. Here we investigate the effect of three classes of CM; Iohexol, Iodixanol and Ioxaglate, on thrombus formation and fibrinolysis in vitro and evaluated the contribution of platelets to this process.

MATERIALS AND METHODS

Non-anticoagulated blood was mixed with CM or saline (50% or 20% (v/v)) for 1 min then citrated. Thrombi were produced in vitro under flow in a Chandler loop. Thrombus structure was visualized by immunohistochemistry using FITC-fibrin/ogen antibodies, and propidium iodide to identify nucleated blood cells. To measure fibrino/thrombolysis release of FITC-labeled fibrinogen, added prior to thrombus formation, was measured over 24 h. Platelet degranulation was analyzed by whole blood flow-cytometry.

RESULTS

No thrombi formed from blood incubated with Ioxaglate. Thrombi formed with Iohexol or Iodixanol weighed > OR = 10x more than saline controls (116+/-52 and 230+/-128 mg vs. 11+/-3 mg, respectively; p<0.0005), and were more resistant to thrombolysis as evidenced by the release of FITC over 24 h (19.1+/-8.9 and 31.9+/-17.2 U vs. 65.1+/-19.1 U, respectively; p<0.02). Thrombi formed with Iodixanol and Iohexol had larger, more diffuse platelet-rich head areas and tail regions composed of a more open fibrinogen/fibrin meshwork enclosing denser RBC rich areas compared to controls. Iodixanol and Ioxaglate did not increase platelet degranulation, but Iohexol caused a significant increase compared to the saline control (percentage of platelets expressing P-selectin 68.9+/-23.43% vs. 4.93+/-1.44%, respectively; p=0.001).

CONCLUSION

Thus thrombi formed with Iodixanol or Iohexol will be larger and more resistant to thrombolysis, whereas Ioxaglate may reduce the risk of thrombus formation.

Thrombus lysis by uPA, scuPA and tPA is regulated by plasma TAFI

The carboxypeptidase, TAFIa or CPU, is known to prolong plasma clot lysis by tissue plasminogen activator (tPA) and to have a role in thrombus stability in vivo. This current study examined lysis by urokinase (uPA) and single chain urokinase (scuPA) in addition to tPA. Further, we investigated the role of TAFIa in a model thrombus system, in which thrombi are formed under conditions of flow. We show that human thrombi, formed in vivo, and model thrombi both contain TAFI. No effect of thrombus TAFIa was observed in thrombus lysis assays, except when thrombi were bathed in plasma, in which case addition of potato tuber carboxypeptidase inhibitor (CPI) resulted in doubling of the rate of lysis. TAFIa inhibited lysis of model thrombi and plasma clots by uPA, scuPA in addition to lysis by tPA. The effect of TAFIa was more evident at high concentrations of plasminogen activator such as those used in thrombolytic therapy. Addition of plasminogen increased lysis and, in its presence, the enhancement by CPI was smaller. Thus the action of TAFIa could be partially overcome by plasminogen, whether lysis was by tPA, uPA or scuPA. This is consistent with TAFIa exerting its effect primarily through modifying the binding of plasminogen to fibrin and to a lesser extent through modification of the binding of tPA to fibrin.

Engineering design of optimal strategies for blood clot dissolution

Blood clots form under hemodynamic conditions and can obstruct flow during angina, acute myocardial infarction, stroke, deep vein thrombosis, pulmonary embolism, peripheral thrombosis, or dialysis access graft thrombosis. Therapies to remove these clots through enzymatic and/or mechanical approaches require consideration of the biochemistry and structure of blood clots in conjunction with local transport phenomena. Because blood clots are porous objects exposed to local hemodynamic forces, pressure-driven interstitial permeation often controls drug penetration and the overall lysis rate of an occlusive thrombus. Reaction engineering and transport phenomena provide a framework to relate dosage of a given agent to potential outcomes. The design and testing of thrombolytic agents and the design of therapies must account for (a) the binding, catalytic, and systemic clearance properties of the therapeutic enzyme; (b) the dose and delivery regimen; (c) the biochemical and structural aspects of the thrombotic occlusion; (d) the prevailing hemodynamics and anatomical location of the thrombus; and (e) therapeutic constraints and risks of side effects. These principles also impact the design and analysis of local delivery devices.

Polymorphonuclear leucocytes mediate endogenous thrombus lysis via a u-PA-dependent mechanism

Many human thrombi lyse spontaneously without the administration of lytic drugs and cause no clinical symptoms. The mechanisms by which this occurs are incompletely understood. We found that model thrombi prepared from whole human blood in a Chandler loop also exhibited significant spontaneous lysis. Lysis was inhibited by chemical protease inhibitors, consistent with proteolysis resulting primarily from serine proteases, with a small contribution from matrix metalloproteinases. Whole blood was fractionated into platelet-rich plasma and cell populations. Significant spontaneous lysis was observed in platelet-rich thrombi enriched with polymorphonuclear leucocytes (PMNs), whereas mononuclear cells (MCs) and erythrocytes did not contribute to lysis. Incorporation of antibodies to urokinase (u-PA) and its receptor u-PAR neutralized a large proportion of the activity. Incubation of plasma with PMNs generated free u-PA activity, which was also detectable in model thrombi and in vivo human thrombi. Purified neutrophils, free of eosinophils, generated activity identical to PMNs. Smaller contributions to lysis by tissue-type plasminogen activator (t-PA), elastase and cathepsin G were also identified. These findings suggest a major role for circulating PMNs in endogenous thrombus lysis.

Effective lysis of model thrombi by a t-PA mutant (A473S) that is resistant to alpha2-antiplasmin

This study used two mutants of tissue-type plasminogen activator (t-PA) with resistance to inhibitors of fibrinolysis to define the contribution of plasminogen activator inhibitor (PAI)-1 and alpha2-antiplasmin (alpha2-AP) to the control of fibrin lysis. Wild-type t-PA was compared with KHRR296-299AAAA, which is resistant to PAI-1, and with A473S, which is resistant to alpha2-AP. We examined these forms of t-PA in model systems that are physiologically relevant. Neutralization of alpha2-AP was essential for lysis of plasma clots, irrespective of their platelet content, by either wild-type t-PA or KHRR296-299AAAA. In marked contrast, A473S lysed plasma clots without neutralization of alpha2-AP. Model thrombi, with structures similar to in vivo thrombi, were lysed slowly by wild-type t-PA; the rate and extent of lysis were enhanced by the addition of antibodies to alpha2-AP or PAI-1. A473S was more effective than wild-type t-PA without the addition of antibodies by virtue of its resistance to alpha2-AP. This resistance was remarkable, in that no complex formed between A473S t-PA and alpha2-AP, even after extended incubation, when 50% of wild-type t-PA could be converted to complex. Comparison of A473S and KHRR296-299AAAA mutants showed their similar effectiveness in lysis of platelet-rich model thrombi. Thus, PAI-1 and alpha2-AP contribute approximately equally to the inhibition of thrombus lysis. This study underlines the functional significance of alpha2-AP as a direct inhibitor of t-PA and further explains the basis of the accepted role of alpha2-AP as a regulator of fibrin persistence and thrombus resistance to lysis.

Plasminogen activator inhibitor 2 and urokinase-type plasminogen activator in plasma and leucocytes in patients with severe sepsis

Proteins influencing plasminogen activation to plasmin, namely plasminogen activators tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA) and their principal inhibitors, plasminogen activator inhibitor 1 (PAI-1) and PAI-2, were measured in the plasma, the polymorph and mononuclear cell fractions taken from patients with major sepsis who were entering a general intensive care unit. The purpose of this study was to elucidate the factors favouring the persistence of fibrin in the microvasculature and thus contributing to multiple organ failure. Levels of u-PA antigen in plasma rose in sepsis and u-PA activity, not detectable in normal plasma, appeared. Levels of u-PA antigen in the cell fractions fell concomitantly. t-PA antigen in plasma and in the mononuclear cell fraction rose in sepsis, but t-PA activity was not detectable. Plasma PAI-1 antigen levels were strikingly raised in sepsis, presumably accounting for the complete neutralization of t-PA activity. PAI-2 antigen, not normally detected in plasma, appeared in the plasma of some patients, whereas it disappeared from the cellular fractions. Appearance of PAI-2 in plasma was associated with non-survival of the patient. The observations indicate that all the agents involved in plasminogen activation are released into the plasma in major sepsis. The levels of PAI-1 reached were quantitatively sufficient to suppress all activity of the released t-PA, but the inhibitors did not prevent expression of u-PA activity in the circulation. Circulating active u-PA and PAI-2 in the plasma of patients with severe sepsis may represent material originating from leucocytes. Leucocyte release of these agents within fibrin deposits may influence the persistence of fibrin and thus the development of multiple organ failure.

Fibrinolysis and thrombosis

The fibrinolytic system generates plasmin, which dissolves fibrin in haemostatic plugs and in thrombi. It is often regarded simply as a secondary phenomenon responsive to the generation of thrombi but it is, rather, in dynamic balance with fibrin formation, such that abnormalities in either can lead to thrombosis. This chapter summarizes the fibrinolytic system and its regulation. It considers the components of the system in blood, both in plasma and in circulating cells, with emphasis on protease-inhibitor balance. It goes on to discuss local fibrinolytic potential in thrombi, both venous and arterial, and in the diseased vessel wall, presenting evidence that increased local inhibition of fibrinolysis by PAI-1, PAI-2 and alpha2-antiplasmin is intimately involved in thrombus stability and in the generation of fibrin-rich vessel wall lesions. Finally, it reviews the evidence that defective plasma fibrinolysis has a causal role in venous and arterial thrombosis.

A reconstituted dilute blood clot lysis assay for the medium throughput screening of thrombolytic compounds

Antithrombotic and clotting factors have long been targets for drug discovery, necessitating the development of blood assays to determine the efficacy of lead compounds prior to animal testing. We have developed a reconstituted blood clot lysis assay which eliminates the need for on-site donors. The assay uses whole blood stored at 4 degrees C obtained from a local blood bank, diluted 1:10 in phosphate buffer. This blood was supplemented with 125I-labeled fibrinogen and the release of radioactive fibrinopeptides from formed clots was measured. The whole blood used in this assay, which had been stored at 4 degrees C for several days, no longer formed solid or retracting clots. Thus, platelets 5-7 days ex vivo (165 x 10(6) platelets) were added to the whole blood in the presence of thrombin (0.80 IU/ml) to form clots. Solid clots formed within 2 min of thrombin addition and began retracting shortly thereafter. In the absence of any thrombolytic agent, clots fully retracted within 2.5 h and remained stable. Thrombin-stimulated clot formation was completely inhibited by heparin. Clots could be lysed in a dose-dependent fashion in the presence of tissue-type plasminogen activator. Clot lysis could be completely inhibited in a dose-dependent fashion with plasminogen activator inhibitor type 1. To demonstrate the utility of this assay as a screen for thrombolytic agents, a 14-amino-acid PAI-1-inhibitory peptide relieved the PAI-1 effect on tPA in a dose-dependent fashion. These data describe an assay for the screening of potential pro-fibrinolytic agents that target PAI-1 inhibition in a human plasma-based system that is versatile, cost-effective, and physiologically relevant and does not rely on the availability of on-site blood donors.