Thrombolytic therapy: 2001

Fibrinogen binding to activated platelets and its biomimetic thrombus-targeted thrombolytic strategies

Thrombosis is associated with various fatal arteriovenous syndromes including ischemic stroke, myocardial infarction, and pulmonary embolism. However, current clinical thrombolytic treatment strategies still have many problems in targeting and safety to meet the thrombolytic therapy needs. Understanding the molecular mechanism that underlies thrombosis is critical in developing effective thrombolytic strategies. It is well known that platelets play a central role in thrombosis and the binding of fibrinogen to activated platelets is a common pathway in the process of clot formation. Based on this, a concept of biomimetic thrombus-targeted thrombolytic strategy inspired from fibrinogen binding to activated platelets in thrombosis was proposed, which could selectively bind to activated platelets at a thrombus site, thus enabling targeted delivery and local release of thrombolytic agents for effective thrombolysis. In this review, we first summarized the main characteristics of platelets and fibrinogen, and then introduced the classical molecular mechanisms of thrombosis, including platelet adhesion, platelet activation and platelet aggregation through the interactions of activated platelets with fibrinogen. In addition, we highlighted the recent advances in biomimetic thrombus-targeted thrombolytic strategies which inspired from fibrinogen binding to activated platelets in thrombosis. The possible future directions and perspectives in this emerging area are briefly discussed.

Selective binding of cationic fibrinogen-mimicking chitosan nanoparticles to activated platelets and efficient drug release for antithrombotic therapy

Thrombosis is the main cause of catastrophic events including ischemic stroke, myocardial infarction and pulmonary embolism. Acetylsalicylic acid (ASA) therapy offers a desirable approach to antithrombosis through a reduction of platelet reactivity. However, major bleeding complications, severe off-target side effects, and resistance or nonresponse to ASA greatly attenuate its clinical outcomes. Herein, we report a cationic fibrinogen-mimicking nanoparticle, denoted as ASA-RGD-CS@TPP, to achieve activated-platelet-targeted delivery and efficient release of ASA for safer and more effective antithrombotic therapy. This biomimetic antithrombotic system was prepared by one-pot ionic gelation between cationic arginine-glycine-aspartic acid (RGD)-grafted chitosan (RGD-CS) and anionic tripolyphosphate (TPP). The platform exhibited selective binding to activated platelets, leading to efficient release of ASA and subsequent attenuation of platelet functions, including the remarkable inhibition of platelet aggregation through a potent blockage of cyclooxygenase-1 (COX-1). After intravenous administration, ASA-RGD-CS@TPP displayed significantly prolonged circulation time and successful prevention of thrombosis in a mouse model. ASA-RGD-CS@TPP was demonstrated to significantly enhance antithrombotic therapy while showing minimal coagulation and hemorrhagic risks and excellent biocompatibility in vivo as compared to free ASA. This platform provides a simple, safe, effective and targeted strategy for the development of antithrombotic nanomedicines.

An in vitro Model for Experimental Evaluation of Sonothrombolysis under Tissue-mimicking Material Conditions

Background

The mechanical properties of therapeutic ultrasound (US) have attracted scientific interest for thrombolysis enhancement in combination with thrombolytic agents and microbubbles (MBs). The aim of the study was to develop an in vitro model to observe how the effects of sonothrombolysis change in the case where a tissue-mimicking material (TMM) is placed in the path of the US beam before the clot.

Methods

Fully retracted blood clots were prepared and pulse sonicated for 1 h under various conditions. The system was in a state of real circulating flow with a branch of an open bypass and an occluded tube containing a blood clot, thus mimicking the case of ischemic stroke. The effectiveness of thrombolysis was quantified in milligrams of clots removed. An agar-based TMM was developed around the occluded tube.

Results

The clot breakdown in a TMM was found to be more pronounced than in water, presumably due to the retention of the acoustic field. A higher level of acoustic power was required to initiate clot lysis (>76 W acoustic power) using only focused US (FUS). The greatest thrombolysis enhancement was observed with the largest chosen pulse duration (PD) and the use of MBs (150 mg clot mass lysis). The synergistic effect of FUS in combination with MBs on the enzymatic fibrinolysis enhanced thrombolysis efficacy by 260% compared to thrombolysis induced using only FUS. A reduction in the degree of clot lysis was detected due to the attenuation factor of the intervening material (30 mg at 1 and 4 ms PD).

Conclusion

In vitro thrombolytic models including a TMM can provide a more realistic evaluation of new thrombolytic protocols. However, higher acoustic power should be considered to compensate for the attenuation factor. The rate of clot lysis is slow and the clinical use of this method will be challenging.

A Fibrinogen-Mimicking, Activated-Platelet-Sensitive Nanocoacervate Enhances Thrombus Targeting and Penetration of Tissue Plasminogen Activator for Effective Thrombolytic Therapy

The development of a fibrinolytic system with long circulation time, high thrombus targeting, efficient thrombus penetration, effective thrombolysis, and minimal hemorrhagic risk remains a major challenge. Herein, inspired by fibrinogen binding to activated platelets in thrombosis, this article reports a fibrinogen-mimicking, activated-platelet-sensitive nanocoacervate to enhance thrombus penetration of tissue plasminogen activator (tPA) for targeted thrombolytic therapy. This biomimetic nanothrombolytic system, denoted as RGD-Chi@tPA, is constructed by "one-pot" coacervation through electrostatic interactions between positively charged arginine-glycine-aspartic acid (RGD)-grafted chitosan (RGD-Chi) and negatively charged tPA. Flow cytometry and confocal laser scanning microscopy measurements show targeting of RGD-Chi@tPA to activated platelets. Controlled tPA release triggered by activated platelets at a thrombus site is demonstrated. Its targeted fibrinolytic and thrombolytic activities are measured in in vitro models. The pharmacokinetic profiles show that RGD-Chi@tPA can significantly prolong circulation time compared to free tPA. In a mouse tail thrombus model, RGD-Chi@tPA displays efficient thrombus targeting and penetration, enabling a complete vascular recanalization as confirmed by the fluorescence imaging, histochemical assay, and laser speckle contrast imager. Consequently, RGD-Chi@tPA induces a substantial enhancement in thrombolysis with minimal hemorrhagic risk compared to free tPA. This simple, effective, and safe platform holds great promise for the development of thrombolytic nanomedicines.

Multiphysics Modelling and Simulation of Thrombolysis via Activated Platelet-Targeted Nanomedicine

PURPOSE

This study establishes a multiphysics simulation platform for both conventional and targeted thrombolysis using tissue plasminogen activator (tPA). Based on our computational results, the effects of therapeutic parameters on the dynamics of thrombolysis and the risk of side effects are investigated.

METHODS

The model extends our previously developed one-dimensional(1D) mathematical models for fibrinolysis by incorporating targeted thrombolysis. It consists of two parts: (i) a coupled mathematical model of systemic pharmacokinetics (PK) and pharmacodynamics (PD) and local PD in a 1D occluded artery, and (ii) a mechanistic model for a targeted thrombolytic system via activated platelet-targeted tPA-loaded nanovesicles (tPA-NV), with model parameters derived from our in vitro experiments. A total of 16 therapeutic scenarios are simulated by varying the clot location and composition as well as the dosing regimen with free tPA or tPA-NV.

RESULTS

Our simulation results indicate that tPA-NV offers several advantages over free tPA for thrombolysis. It reduces systemic exposure of tPA, thereby minimising the risk of bleeding complications. Simulations with different tPA-NV doses reveal that tPA-NV at 10% of the recommended dose can be as effective as the standard regimen with the full recommended dose of free tPA, demonstrating the potential of our tPA-NV as a new thrombolytic strategy with a reduced tPA dose. Moreover, faster recanalisation can be achieved with tPA-NV, especially for platelet-rich(or fibrin-poor) clots.

CONCLUSIONS

Our simulation platform for thrombolysis with well-tuned model parameters can be used to evaluate and optimise treatment regimens of existing and new thrombolytic therapies via benefit/risk assessment under various therapeutic scenarios.

Fibrinogen-mimicking, multiarm nanovesicles for human thrombus-specific delivery of tissue plasminogen activator and targeted thrombolytic therapy

Clinical use of tissue plasminogen activator (tPA) in thrombolytic therapy is limited by its short circulation time and hemorrhagic side effects. Inspired by fibrinogen binding to activated platelets, we report a fibrinogen-mimicking, multiarm nanovesicle for thrombus-specific tPA delivery and targeted thrombolysis. This biomimetic system is based on the lipid nanovesicle coated with polyethylene glycol (PEG) terminally conjugated with a cyclic RGD (cRGD) peptide. Our experiments with human blood demonstrated its highly selective binding to activated platelets and efficient tPA release at a thrombus site under both static and physiological flow conditions. Its clot dissolution time in a microfluidic system was comparable to that of free tPA. Furthermore, we report a purpose-built computational model capable of simulating targeted thrombolysis of the tPA-loaded nanovesicle and with a potential in predicting the dynamics of thrombolysis in physiologically realistic scenarios. This combined experimental and computational work presents a promising platform for development of thrombolytic nanomedicines.

Intrapleural Tissue Plasminogen Activator for Traumatic Retained Hemothorax

Objective: To describe the efficacy, safety, dosing regimen, and administration technique of intrapleural alteplase for the treatment of retained hemothorax. Data Sources: A PubMed, EMBASE, and Google Scholar search (January 2000 to February 2019) was conducted with the search terms intrapleural, fibrinolytic, fibrinolysis, alteplase, tissue plasminogen activator, and hemothorax. Study Selection and Data Extraction: Articles were included if they described the use of intrapleural alteplase in adult patients with a retained hemothorax; single patient case reports and abstracts were excluded. Data Synthesis: A total of 6 retrospective reviews and 1 meta-analysis were identified for inclusion. A variety of dosing strategies have been defined for the administration of intrapleural alteplase ranging from 6 to 100 mg, volume of fluid from 50 to 120 mL of normal saline, and the number of total doses has ranged from 1 to 8 over the treatment course. A majority of studies showed a greater than 80% success rate and less than 7% bleeding rate. Relevance to Patient Care and Clinical Practice: Because of the paucity of data for use of alteplase in retained hemothorax and administration of a high-risk medication, this review provides dosing and administration recommendations based on reported safety and efficacy. Conclusion: Administration of intrapleural alteplase should be considered in patients with retained hemothorax as an alternative to surgical intervention. In contrast to intrapleural alteplase administration for other indications such as empyema, higher doses and volumes of alteplase are recommended for retained hemothorax.

An activated-platelet-sensitive nanocarrier enables targeted delivery of tissue plasminogen activator for effective thrombolytic therapy

It remains a major challenge to develop a selective and effective fibrinolytic system for thrombolysis with minimal undesirable side effects. Herein, we report a multifunctional liposomal system (164.6 ± 5.3 nm in diameter) which can address this challenge through targeted delivery and controlled release of tissue plasminogen activator (tPA) at the thrombus site. The tPA-loaded liposomes were PEGylated to improve their stability, and surface coated with a conformationally-constrained, cyclic arginine-glycine-aspartic acid (cRGD) to enable highly selective binding to activated platelets. The in vitro drug release profiles at 37 °C showed that over 90% of tPA was released through liposomal membrane destabilization involving membrane fusion upon incubation with activated platelets within 1 h, whereas passive release of the encapsulated tPA in pH 7.4 PBS buffer was 10% after 6 h. The release of tPA could be readily manipulated by changing the concentration of activated platelets. The presence of activated platelets enabled the tPA-loaded, cRGD-coated, PEGylated liposomes to induce efficient fibrin clot lysis in a fibrin-agar plate model and the encapsulated tPA retained 97.4 ± 1.7% of fibrinolytic activity as compared with that of native tPA. Furthermore, almost complete blood clot lysis was achieved in 75 min, showing considerably higher and quicker thrombolytic activity compared to the tPA-loaded liposomes without cRGD labelling. These results suggest that the nano-sized, activated-platelet-sensitive, multifunctional liposomes could facilitate selective delivery and effective release of tPA at the site of thrombus, thus achieving efficient clot dissolution whilst minimising undesirable side effects.

Effect of encapsulation on plasminogen activator delivery to the microcirculation and its implications for bleeding

BACKGROUND AND PURPOSE

It is known that encapsulation can alter the delivery of plasminogen activators by flow to accelerate fibrinolysis while other experimental studies suggest encapsulation may reduce the risk of hemorrhage with administration of the agent. The aim of this research is to resolve the effect of encapsulation on fibrinolysis and bleeding in the microcirculation.

METHODS

An established rabbit model of fibrinolytic hemorrhage was utilized to explore the potential of encapsulation to limit bleeding. Equal dosages of free or microencapsulated streptokinase (MESK) were infused to initiate thrombolysis of small vessel clots while tracking blood loss.

RESULTS

Compared to free streptokinase, significant improvements in bleeding were observed with MESK as demonstrated by (1) delayed onset of bleeding, (2) shortened duration, and (3) reduction in the volume of lost blood, consistent with less systemic fibrinogen degradation.

CONCLUSIONS

Findings demonstrate that encapsulation of streptokinase can inhibit clot lysis in small vessels. Combined with prior work on accelerated thrombolysis, results suggest a time-based regimen for avoiding bleeding complications during thrombolytic therapy with encapsulated agent.

Magnetically controlled release of recombinant tissue plasminogen activator from chitosan nanocomposites for targeted thrombolysis

Ionic cross-linking of water-soluble chitosan with sodium tripolyphosphate in the presence of recombinant tissue plasminogen activator (rtPA) and magnetite (Fe₃O₄) nanoparticles could produce rtPA-encapsulated magnetic chitosan nanoparticles (MCNPs-rtPA). MCNPs do not elicit cytotoxicity and hemolysis in vitro. MCNPs-rtPA showed a negligible release of the rtPA protein when stored in phosphate buffer for 28 days. In contrast, the burst release of rtPA from MCNPs-rtPA was found in the serum with 60% of the original activity released in 30 min. The drug release into the serum is also magnet-sensitive; the release could be turned down with a magnetic field when MCNPs-rtPA was pelleted and reversibly turned on after removing the magnetic field when MCNPs-rtPA was dispersed. An in vitro thrombolytic study by thromboelastometry indicated a controlled release of rtPA from MCNPs-rtPA. In a rat embolic model where a preformed blood clot lodged in the left iliac artery upstream of the pudic epigastric branch, MCNPs-rtPA (0.2 mg kg^-1 rtPA) was administered and guided magnetically to the clot, followed by mobile magnetic guidance for 60 min. Iliac blood flow increased immediately in response to the treatment, and reached a stable level ∼50 min after drug administration and the hind limb perfusion rate was restored from 53% to 75% of the basal level. Effective thrombolysis was therefore successfully demonstrated at an rtPA dose equivalent to 20% of the regular dose when the MCNPs-rtPA pellet was magnet-guided to the blood clot, followed by a triggered release of rtPA when switched to mobile magnetic guidance.

Biphasic release of protein from polyethylene glycol and polyethylene glycol/modified dextran microspheres

Dextrans show great promise for delivery of therapeutic agents. Dextran acetates (DAs) were synthesized with increasing degrees of substitution (DA1 < DA2 < DA3) by the reaction of the polysaccharide dextran (70 kDa) with acetic anhydride. A series of polyethylene glycol (PEG)/DA microspheres were prepared and tested with bovine serum albumin (BSA) functioning as a model protein. Particle size (0.74-0.85 μm) and encapsulation efficiency (56-70%) increased with the degree of substitution along with a slower release rate of protein from PEG/DA microspheres. Time to release 90% of protein rose from 31 to 118 min. Percentage of BSA released from PEG and PEG/DA3 microspheres with time (min) was modeled mathematically [Y(PEG) = 100(1 - e(-0.12t)); Y(PEG/DA3) = 100(1 - e(-0.024t))] to predict cumulative delivery from mixtures in vitro over a period of hours when constrained to a target level at 30 min. The system is examined for potential application in thrombolytic therapy.

Bioconjugation of recombinant tissue plasminogen activator to magnetic nanocarriers for targeted thrombolysis

Low-toxicity magnetic nanocarriers (MNCs) composed of a shell of poly [aniline-co-N-(1-one-butyric acid) aniline] over a Fe(3)O(4) magnetic nanoparticle core were developed to carry recombinant tissue plasminogen activator (rtPA) in MNC-rtPA for targeted thrombolysis. With an average diameter of 14.8 nm, the MNCs exerted superparamagnetic properties. Up to 276 μg of active rtPA was immobilized per mg of MNCs, and the stability of the immobilized rtPA was greatly improved during storage at 4°C and 25°C. In vitro thrombolysis testing with a tubing system demonstrated that magnet-guided MNC-rtPA showed significantly improved thrombolysis compared with free rtPA and reduced the clot lysis time from 39.2 ± 3.2 minutes to 10.8 ± 4.2 minutes. In addition, magnet-guided MNC-rtPA at 20% of the regular rtPA dose restored blood flow within 15-25 minutes of treatment in a rat embolism model without triggering hematological toxicity. In conclusion, this improved system is based on magnetic targeting accelerated thrombolysis and is potentially amenable to therapeutic applications in thromboembolic diseases.

The role of tissue plasminogen activator in the management of complex intra-abdominal abscesses in children

OBJECTIVE

The objective of this study is to assess the safety of fibrinolytic therapy using tissue plasminogen activator (tPA) in children with complex intra-abdominal abscesses.

SUMMARY BACKGROUND DATA

Intra-abdominal abscesses are common in children. Antibiotics and percutaneous drainage are the mainstays of treatment, but drainage may be less effective when the fluid is thick or septated. Fibrinolytic therapy using tPA is effective in a rat model of intra-abdominal abscesses, has recently been reported for the treatment of intra-abdominal abscesses in adults, and is commonly used in the treatment of empyema in children.

METHODS

This is a retrospective review of all patients over a 10-year period who had intra-abdominal collections managed with tPA abscess drainage.

RESULTS

Sixty-four children had a total of 66 drains placed and 92 doses of tPA. Appendicitis was the cause of the abscesses in 52 of 64 children. Mean length of stay pre-tPA was 11.7 ± 7.63 days, mean time from drain insertion to tPA was 4.3 ± 3.78 days, and mean time from tPA to discharge was 8.6 ± 8.85 days. Thirty patients underwent an operation before tPA administration. No patients experienced bleeding complications, anastomotic or appendiceal stump leak, or wound dehiscence after the administration of tPA, and no patients had abnormalities in coagulation studies related to tPA administration. One child died of sepsis.

CONCLUSIONS

Tissue plasminogen activator is safe for the management of thick or septated intra-abdominal abscesses in children. A prospective controlled study will be needed to evaluate the efficacy of this technique.

Molecular Modeling and Structure-Activity Relationship of Podophyllotoxin and Its Congeners

A quantitative structure-activity relationship (QSAR) model has been developed between cytotoxic activity and structural properties by considering a data set of 119 podophyllotoxin analogs based on 2D and 3D structural descriptors. A systematic stepwise searching approach of zero tests, a missing value test, a simple correlation test, a multicollinearity test, and a genetic algorithm method of variable selection was used to generate the model. A statistically significant model ( r train2 = 0.906; q cv2 = 0.893) was obtained with the molecular descriptors. The robustness of the QSAR model was characterized by the values of the internal leave-one-out cross-validated regression coefficient ( q cv2) for the training set and r test2 for the test set. The overall root mean square error (RMSE) between the experimental and predicted pIC50 value was 0.265 and r test2 = 0.824, revealing good predictability of the QSAR model. For an external data set of 16 podophyllotoxin analogs, the QSAR model was able to predict the tubulin polymerization inhibition and mechanistically cytotoxic activity with an RMSE value of 0.295 in comparison to experimental values. The QSAR model developed in this study shall aid further design of novel potent podophyllotoxin derivatives.

Accelerating thrombolysis with chitosan-coated plasminogen activators encapsulated in poly-(lactide-co-glycolide) (PLGA) nanoparticles

Accelerating thrombolysis using plasminogen activators (PAs) encapsulated liposomes or PEG microparticles by pressure-driven permeation have been demonstrated in vitro and in vivo in animal models. However, designing and delivering PA-encapsulated nanoparticles (NPs) to enhance thrombolysis by applying electrostatic forces or ligand-receptor interactions between the NPs and blood clots has not been proposed. Therefore, without a pressure-driving factor, tissue-plasminogen activator (t-PA) encapsulated in PLGA NPs with chitosan (CS) and CS-GRGD coating and their thrombolysis capabilities in a blood clot-occluded tube model were evaluated by determining clot lysis times and the masses of the digested clots. The characteristics and release profiles of t-PA-encapsulated PLGA, PLGA/CS and PLGA/CS-GRGD NPs are determined by FT-IR, a laser particle/zeta potential analyzer and HPLC. Additionally, the permeation capacities of the NPs into flat blood clots were examined. For example, the mean particle sizes and encapsulation efficacies of t-PA for the NPs are in the ranges 260-320 nm and 65.5-70.5%, respectively. The results reveal that the NPs for the shortest clot lysis time and the highest weight percentages of digested clot are PLGA/CS (20.7 +/- 0.7 min) and PLGA/CS-GRGD (25.7 +/- 1.3 wt%), respectively. Compared with t-PA solution, the NPs can significantly shorten clot lysis times in the following order: PLGA/CS NPs (38.8 +/- 1.5%) > PLGA/CS-GRGD NPs (16.3 +/- 1.0%) > PLGA NPs (7.7 +/- 1.2%). Compared with t-PA solution, the NPs significantly increase the weight of digested clots in the order, PLGA/CS-GRGD (40.9 +/- 1.5%) > PLGA/CS (27.8 +/- 1.2%) > PLGA (8.6 +/- 0.6%). The highest release rate of t-PA in the fast release phase and the highest permeability into intra-clots of PLGA/CS and PLGA/CS-GRGD NPs, respectively, correspond to the shortest clot lysis time and the largest increase in weight of the digested clots among the NP system. In conclusion, the NPs designed based on new concepts significantly accelerate thrombolysis in vitro in this model, and may be useful in clinical study.

Quantitative structure activity relationship studies of aryl heterocycle-based thrombin inhibitors

A quantitative structure activity relationship (QSAR) analysis has been performed on a data set of 42 aryl heterocycle-based thrombin inhibitors. Several types of descriptors including topological, spatial, thermodynamic, information content and E-state indices were used to derive a quantitative relationship between the anti thrombin activity and structural properties. Genetic algorithm based genetic function approximation method of variable selection was used to generate the model. Best model was developed when number of descriptors in the equation was set to five. Highly statistically significant model was obtained with atom type logP descriptors, logP and Shadow_YZ. The model is not only able to predict the activity of new compounds but also explained the important regions in the molecules in a quantitative manner.

Human complement receptor type 1-directed loading of tissue plasminogen activator on circulating erythrocytes for prophylactic fibrinolysis

Plasminogen activators (PAs) are not used for thromboprophylaxis due to rapid clearance, bleeding, and extravascular toxicity. We describe a novel strategy that overcomes these limitations. We conjugated tissue-type PA (tPA) to a monoclonal antibody (mAb) against complement receptor type 1 (CR1) expressed primarily on human RBCs. Anti-CR1/tPA conjugate, but not control conjugate (mIgG/tPA), bound to human RBCs (1.2 x 10(3) tPA molecules/cell at saturation), endowing them with fibrinolytic activity. In vitro, RBC-bound anti-CR1/tPA caused 90% clot lysis versus 20% by naive RBCs. In vivo, more than 40% of anti-CR1/(125)I-tPA remained within the circulation ( approximately 90% bound to RBCs) 3 hours after injection in transgenic mice expressing human CR1 (TgN-hCR1) versus less than 10% in wild-type (WT) mice, without RBC damage; approximately 90% of mIgG/(125)I-tPA was cleared from the circulation within 30 minutes in both WT and TgN-hCR1 mice. Anti-CR1/tPA accelerated lysis of pulmonary emboli and prevented stable occlusive carotid arterial thrombi from forming after injection in TgN-hCR1 mice, but not in WT mice, whereas soluble tPA and mIgG/tPA were ineffective. Anti-CR1/tPA caused 20-fold less rebleeding in TgN-hCR1 mice than the same dose of tPA. CR1-directed immunotargeting of PAs to circulating RBCs provides a safe and practical means to deliver fibrinolytics for thromboprophylaxis in settings characterized by a high imminent risk of thrombosis.

Discovery and Evaluation of Potent P1 Aryl Heterocycle-Based Thrombin Inhibitors

In an effort to discover potent, clinically useful thrombin inhibitors, a rapid analogue synthetic approach was used to explore the P(1) region. Various benzylamines were coupled to a pyridine/pyrazinone P(2)-P(3) template. One compound with an o-thiadiazole benzylic substitution was found to have a thrombin K(i) of 0.84 nM. A study of ortho-substituted five-membered-ring heterocycles was undertaken and subsequently demonstrated that the o-triazole and tetrazole rings were optimal. Combination of these potent P(1) aryl heterocycles with a variety of P(2)-P(3) groups produced a compound with an extraordinary thrombin inhibitory activity of 1.4 pM. It is hoped that this potency enhancement in P(1) will allow for more diversification in the P(2)-P(3) region to ultimately address additional pharmacological concerns.

Equivocal roles of tissue-type plasminogen activator in stroke-induced injury

Stroke represents a major health problem in the ever-ageing population of industrialized nations. Each year, over three million people in the USA alone suffer from this affliction. Stroke, which results from the obstruction of an intra- or extra-cerebral artery, induces irreversible neuronal damage. The clot-busting drug tissue-type plasminogen activator (tPA) is the only FDA-approved therapy for acute stroke. Although tPA has been successfully used to treat myocardial infarction due to clot formation, its use in the treatment of occlusive cerebrovascular diseases remains controversial. Indeed, tPA is clearly beneficial as a thrombolytic agent. However, increasing evidence suggests that tPA could have direct and deleterious effects on neurons and glial cells.

Safety of plasmin in the setting of concomitant aspirin and heparin administration in an animal model of bleeding

Plasmin is a direct thrombolytic which has been shown to have a strikingly favorable benefit to risk profile in comparison with plasminogen activators, notably tissue plasminogen activator (t-PA). As heparin is known to increase the risk of hemorrhage when co-administered with a plasminogen activator, we asked whether adjunct antithrombotic agents such as aspirin and heparin would affect the safety of plasmin. Three groups of rabbits were administered plasmin at a dose (4 mg kg-1) designed to induce significant decreases in antiplasmin, fibrinogen and factor (F)VIII, to about 25, 40 and 40%, respectively, of baseline values, but not cause prolongation of the ear puncture bleeding time. In a blinded and randomized trial, the results show that an intravenous aspirin bolus plus heparin administered as a bolus followed by a maintenance continuous infusion did not significantly prolong the bleeding time during plasmin infusion. These data indicate that in the rabbit, concomitant use of aspirin plus heparin does not affect the safety of a therapeutic dose of plasmin.

Benefit-risk assessment of treatments for heparin-induced thrombocytopenia

Patients with heparin-induced thrombocytopenia (HIT) are at high risk of thrombosis and should be treated with alternative anticoagulant therapy to reduce complications. The current treatment of choice is one of the approved direct thrombin inhibitors, argatroban or lepirudin. These drugs have been proven to be safe and effective in multicentre clinical trials where dosage regimens have been established for prophylaxis and treatment of thrombosis. Argatroban has also been tested and approved for use in invasive cardiology procedures in the HIT patient. Dosage regimens for other clinical uses, such as cardiac surgery, have not yet been established for either drug. The safety and effectiveness of the thrombin inhibitors is dependent on their use according to established guidelines. Other treatment options that may be effective for the patient with HIT include dextran, plasmapheresis, intravenous gammaglobulin and aspirin (acetylsalicylic acid). Although used historically, these options have not been tested in rigorous clinical trials. For life- and limb-threatening thrombosis, thrombolytic agents and/or surgery may provide benefit. Because the risk of bleeding is high from these procedures, they should be performed only by an experienced practitioner. Several studies have shown that patients with HIT requiring continued anticoagulation are best managed with a warfarin derivative initiated while under full anticoagulation with a thrombin inhibitor. There is a risk of skin necrosis and bleeding if guidelines for dose administration and monitoring of warfarin are not followed. Subsequent use of heparin or a low molecular weight heparin after resolution of the clinical episode of HIT can be hazardous, particularly within the first 3 months. If laboratory testing is negative, heparin may be cautiously reinstituted for short-term use (1-2 hours) with monitoring for platelet count decrease and thromboembolism. The pregnant patient with HIT requiring anticoagulation represents a particular challenge, where there is no drug of choice at present. Although today there are realistic treatment options for the patient with HIT, the morbidity and mortality associated with this disease have not been eliminated. Awareness and early treatment of HIT remain important components of the clinical care for patients exposed to heparins. Future therapeutic developments based on a better understanding of the pathophysiology of HIT may further improve clinical outcomes. Despite some limitations, the current treatment options for patients with HIT provide unparalleled benefit compared with the treatment options available only a few years ago.

Monitoring manufacturing process yields, purity and stability of structural variants of PEGylated staphylokinase mutant SY161 by quantitative reverse-phase chromatography

Staphylokinase variant SY161 is a recombinant mutant of the Staphylococcus aureus polypeptide staphylokinase (Sak), and is currently in human clinical trials as a thrombolytic agent. The 15 kDa single chain SY161 protein is expressed as a soluble cytoplasmic product in E. coli with a single cysteine inserted near the N-terminus. The protein as extracted from E. coli is a mixture of both monomeric and intermolecularly disulfide crosslinked species. To improve protein purification yields SY161 is sulfitolyzed during the early stages of production, preventing disulfide formation. The protein is later modified during manufacturing to incorporate a single 5 kDa polyethylene glycol group on the single sulfhydryl sidechain. We have developed and qualified a reverse-phase chromatographic method to quantitate SY161 during product manufacturing. We discuss the use of the assay during manufacturing development to monitor fermentation yields, the SY161 PEGylation reaction, and as an in-process manufacturing control assay. The assay has been applied as a product purity and identity release assay and is suitable for use in assessing product structural integrity during stability testing. The assay has a linear range of quantitation for SY161 from at least 0.15 to 16 micro g, and is-in addition capable of detecting and quantitating protein de-PEGylation events and host cell-derived protein contaminants.

Thrombolytic potency of acid-stabilized plasmin: superiority over tissue-type plasminogen activator in an in vitro model of catheter-assisted thrombolysis

Plasmin, the direct fibrinolytic enzyme, was compared with tissue plasminogen activator (t-PA) in an in vitro thrombolysis model. Plasmin has been prepared in a highly pure form from human plasma and has been stabilized against auto-degradation by low-pH formulation. This acidified formulation of plasmin has been designed to have a low buffering capacity so that it can be directly infused into clots in a stable and latently active form. This low-pH formulation has been shown to be equivalent to a neutral-pH formulation of plasmin in its extent of clot lysis. An in vitro model of catheter-assisted thrombolysis has been devised in which large (12 x 0.6 cm), retracted clots are treated with an intrathrombus thrombolytic agent via a multi-sideport catheter. Plasmin dissolves these plasminogen-deficient clots in a dose-dependent manner and is clearly superior to t-PA. In this model system, t-PA exhibits efficacy only when retracted clots are replenished with plasminogen.

Distinct dose-dependent effects of plasmin and TPA on coagulation and hemorrhage

All thrombolytic agents in current clinical usage are plasminogen activators. Although effective, plasminogen activators uniformly increase the risk of bleeding complications, especially intracranial hemorrhage, and no laboratory test is applicable to avoid such bleeding. We report results of a randomized, blinded, dose-ranging comparison of tissue-type plasminogen activator (TPA) with a direct-acting thrombolytic agent, plasmin, in an animal model of fibrinolytic hemorrhage. This study focuses on the role of plasma coagulation factors in hemostatic competence. Plasmin at 4-fold, 6-fold, and 8-fold the thrombolytic dose (1 mg/kg) induced a dose-dependent effect on coagulation, depleting antiplasmin activity completely, then degrading fibrinogen and factor VIII. However, even with complete consumption of antiplasmin and decreases in fibrinogen and factor VIII to 20% of initial activity, excessive bleeding did not occur. Bleeding occurred only at 8-fold the thrombolytic dose, on complete depletion of fibrinogen and factor VIII, manifest as prolonged primary bleeding, but with minimal effect on stable hemostatic sites. Although TPA had minimal effect on coagulation, hemostasis was disrupted in a dose-dependent manner, even at 25% of the thrombolytic dose (1 mg/kg), manifest as rebleeding from hemostatically stable ear puncture sites. Plasmin degrades plasma fibrinogen and factor VIII in a dose-dependent manner, but it does not disrupt hemostasis until clotting factors are completely depleted, at an 8-fold higher dose than is needed for thrombolysis. Plasmin has a 6-fold margin of safety, in contrast with TPA, which causes hemorrhage at thrombolytic dosages.

Towards safer thrombolytic therapy

Plasminogen activators (PA) are unique agents that are currently applied as thrombolytic therapy to achieve rapid vascular reperfusion. Regimens of PA plus anticoagulants and antiplatelet drugs have attained a high degree of sophistication and predictable rates of positive clinical outcomes for acute myocardial infarction (MI), ischemic stroke, pulmonary embolism (PE), deep vein thrombosis (DVT), and thrombosed catheters. Included in the repertoire are newly approved mutants of tissue plasminogen activator (TPA), which have biochemical advantages that allow for bolus administration. Yet, despite tremendous effort devoted to enormous trials to establish the clinical efficacy of these agents in acute MI, mortality results are not superior to those with native TPA or streptokinase (SK). Furthermore, all PAs have the potential for hemorrhagic complication, most critically intracranial hemorrhage (ICH), occurring in 0.9% of patients treated with native or mutant TPA. It is possible that a limit of clinical effectiveness has been reached, beyond which more potent PAs do not achieve greater benefit without a serious increase in risk of bleeding. A breakthrough is possible, however, if the risk of ICH could be avoided. One solution is the application of the direct-acting thrombolytic enzyme, plasmin. While intravenous plasmin is not effective when administered systemically, regional infusion to a thrombus induces local thrombolysis. Unlike the PAs, plasmin treatment should not cause hemorrhage from vascular trauma sites, as it is neutralized by antiplasmin in the blood. Animal studies are fully consistent with this approach, which offers potential for achieving a truly regional thrombolytic treatment.