Coronary thrombolysis with Desmodus salivary plasminogen activator in dogs. Fast and persistent recanalization by intravenous bolus administration

Identification, purification, and pharmacological activity analysis of Desmodus rotundus salivary plasminogen activator alpha1 (DSPAα1) expressed in transgenic rabbit mammary glands

Desmodus rotundus plasminogen activator alpha 1(DSPAα1) is a thrombolytic protein with advantages, such as a long half-life, high accuracy and specificity for thrombolysis, wide therapeutic window, and no neurotoxicity. To date, DSPAα1 has only been expressed in the Chinese hamster ovary, insect cells, transgenic tobacco plants, and Pichia pastoris. To the best of our knowledge, we are the first to report the expression of DSPAα1 in transgenic rabbit mammary glands, extract the product, and analyze its pharmacology activity. An efficient mammary gland-specific expression vector pCL25/DSPAα1 was transferred to prokaryotic zygotes in rabbits by microinjection to generate six DSPAα1 transgenic rabbits. The recombinant DSPAα1 (rDSPAα1) expression in transgenic rabbit milk was 1.19 ± 0.26 mg/mL. The rDSPAα1 purification protocol included pretreatment, ammonium sulfate precipitation, benzamidine affinity chromatography, cation exchange chromatography, and Cibacron blue affinity chromatography; approximately 98% purity was achieved using gel electrophoresis. According to sequencing results, the primary structure of rDSPAα1 was consistent with the theoretical design sequence, and its molecular weight was consistent with that of the natural protein. N-terminal sequencing results indicated rDSPAα1 to be a mature protein, as the goat signal peptide sequence of the expression vector was no longer detected. The fibrinolytic activity of rDSPAα1 was estimated to be 773,333 IU/mg. Fibrin-agarose plate assay and in vitro rat blood clot degradation assay showed that rDSPAα1 had strong thrombolytic activity. In conclusion, we report recombinant DSPAα1 with high thrombolytic activity expressed in transgenic rabbit mammary glands.

Clot Formation in the Presence of Acetylsalicylic Acid Leads to Increased Lysis Rates Regardless of the Chosen Thrombolysis Strategy

BACKGROUND

Patients with acute ischemic strokes frequently take an acetylsalicylic acid (ASA) premedication. We determined the impact of ASA on different thrombolysis strategies in vitro.

METHODS

For two clot types made from platelet-rich plasma (one with and one without ASA) lysis rates were measured by weight loss after 1 h for five different groups: in control group A clots were solely placed in plasma; in groups B and C clots were treated with rt-PA (60 kU/ml), and in groups D and E clots were treated with desmoteplase (DSPA; 2 µg/ml). Ultrasound (2 MHz, 0.179 W/cm2) was included in groups C and E. The fibrin mesh structures of the clots were investigated by electron microscopy.

RESULTS

For both clot types lysis rates increased significantly for all treatment strategies compared to their control group (each p < 0.001). The addition of ASA significantly increased the lysis rate in all 5 groups (each p < 0.001) and led to a ceiling effect concerning the treatment. A semiquantitative analysis of transmission electron micrographs revealed a decreased fibrin density for clots with ASA. For both clot types DSPA and ultrasound led to a significant dissolution of the fibrin mesh (both p = 0.029).

CONCLUSIONS

In vitro ASA pretreatment leads to significantly increased lysis rates due to a weaker fibrin mesh in platelet-rich plasma clots.

In Vitro Examination of the Thrombolytic Efficacy of Desmoteplase and Therapeutic Ultrasound Compared with rt-PA

The aim of the study described here was to evaluate the thrombolytic efficacy of combined treatment with the fibrin-selective plasminogen activator desmoteplase (DSPA) and therapeutic ultrasound (sonothrombolysis [STL]) compared with conventional rt-PA (recombinant tissue plasminogen activator) treatment in vitro. Lysis rates were determined by the weight loss of platelet-rich plasma (PRP) clots treated with rt-PA (60 kU/mL) or DSPA (2 μg/mL) combined with pulsed wave ultrasound (2 MHz, 0.179 W/cm(2)). To reveal the individual effects of medication and ultrasound, lysis rates were also determined for DSPA monotherapy and for combined treatment with rt-PA and ultrasound. Clots solely placed in plasma served as the control group. Lysis increased significantly with rt-PA (26.5 ± 7.8%) and DSPA (30.5 ± 6%) compared with the control group (18.2 ± 5.9%) (each p < 0.001). DSPA lysis was more effective than rt-PA lysis (without STL: p = 0.015, with STL: p = 0.01). Combined treatment with DSPA and 2-MHz STL significantly exceeded rt-PA lysis (32.8% vs. 26.5%, p < 0.001).

Hematologic profile of hematophagous Desmodus rotundus bats before and after experimental infection with rabies virus

INTRODUCTION

Hematophagous Desmodus rotundus bats play an important role in the rabies lifecycle. This study describes the hematological profile of these bats before and after experimental infection with rabies virus.

METHODS

Cells counts were performed in a Neubauer chamber.

RESULTS

The average values of erythrocytes and leucocytes counts in blood before experimental infections were 9.97 × 10(6)mm3 and 4.80 × 10(3)mm3, respectively. Neutrophils represented 69.9% of white blood cells and the lymphocytes represented 26.9%. Following the experimental infections, the average numbers of erythrocytes and leucocytes was 9.43 × 106mm3 and 3.98 × 10(3)mm3, respectively. Neutrophils represented 40% of white blood cells and the lymphocytes represented 59%.

CONCLUSIONS

The hematological profile given in this study can serve as reference values for D. rotundus bats.

Desmoteplase in the treatment of acute ischemic stroke

Desmoteplase, developed by Paion, Forest and Lundbeck, is a novel plasminogen activator that selectively activates fibrin-bound plasminogen and is currently being investigated for the treatment of acute ischemic stroke within the time window of 3-9 h after symptom onset. Desmoteplase is believed to offer pharmacologic advantages over currently approved treatment options. To date, two published Phase II perfusion imaging-based clinical trials have reported the safety and potential efficacy of desmoteplase in ischemic stroke. Results from a recently completed Phase III trial in Europe, Asia and the USA are awaited. This article reviews the available data on desmoteplase, including discussion of its favorable features and potential benefit beyond the 3-h time window in the treatment of ischemic stroke.

Newer Pharmaceutical Agents for STEMI Interventions

ST-elevation myocardial infarction (STEMI) causes 12.6% of deaths worldwide. Treatment strategies involve early revascularization by percutaneous coronary intervention and/or fibrinolytics, with adjunctive pharmacologic therapy. While antiplatelet therapy remains the cornerstone of pharmacologic management, newer antithrombotic therapies are showing benefit in the reduction of long-term thrombotic events following acute vessel occlusion. Future directions in adjunctive STEMI management include the use of hematopoietic stem cell therapy or growth factors to induce proliferation and differentiation of cardiac myocytes.

Desmoteplase: discovery, insights and opportunities for ischaemic stroke

Nature has provided a vast array of bioactive compounds that have been exploited for either diagnostic or therapeutic use. The field of thrombosis and haemostasis in particular has enjoyed much benefit from compounds derived from nature, notably from snakes and blood-feeding animals. Indeed, the likelihood that blood-feeding animals would harbour reagents with relevant pharmacology and with potential pharmaceutical benefit in haemostasis was not too far-fetched. Blood-feeding animals including leeches and ticks have evolved a means to keep blood from clotting or to at least maintain the liquid state, and some of these have been the subject of clinical development. A more recent example of this has been the saliva of the common vampire bat Desmodus rotundus, which has proven to harbour a veritable treasure trove of novel regulatory molecules. Among the bioactive compounds present is a fibrinolytic compound that was shown over 40 years ago to be a potent plasminogen activator. Studies of this vampire bat-derived plasminogen activator, more recently referred to as desmoteplase, revealed that this protease shared a number of structural and functional similarities to the human fibrinolytic protease, tissue-type plasminogen activator (t-PA) yet harboured critically important differences that have rendered this molecule attractive for clinical development for patients with ischaemic stroke.

Venomous mammals: a review

The occurrence of venom in mammals has long been considered of minor importance, but recent fossil discoveries and advances in experimental techniques have cast new light into this subject. Mammalian venoms form a heterogeneous group having different compositions and modes of action and are present in three classes of mammals, Insectivora, Monotremata, and Chiroptera. A fourth order, Primates, is proposed to have venomous representatives. In this review we highlight recent advances in the field while summarizing biochemical characteristics of these secretions and their effects upon humans and other animals. Historical aspects of venom discovery and evolutionary hypothesis regarding their origin are also discussed.

Desmoteplase

The high fibrin specificity of Desmodus rotundus salivary plasminogen activator alpha1 (desmoteplase) renders it a promising candidate for the treatment of acute ischemic stroke. In the DIAS (Desmoteplase in Acute Ischemic Stroke) and DEDAS (Dose Escalation study of Desmoteplase in Acute ischemic Stroke) Phase II studies, doses of 90 microg/kg and 125 microg/kg desmoteplase were reported to have acceptable safety profiles, leading to potentially superior reperfusion compared with placebo, with possible clinical efficacy for up to 9 h after the onset of symptoms in patients with a significant ischemic penumbra selected from magnetic resonance perfusion-diffusion weighted mismatches imaging. However, a Phase III clinical trial (DIAS-2) was unable to detect any benefit from desmoteplase when given 3 - 9 h after stroke onset. In this study with a modest sample size, certain methodological factors may have reduced its potential to detect a desmoteplase effect, as only 30% of these patients had a visible occlusion at presentation, with only small core and mismatched lesion volumes. Indeed, it is surprising that a study testing an occluded vessel 'reopener' was conducted in a cohort of stroke patients, the majority of whom was known not to have a detected vessel occlusion. It has also been claimed that the DIAS-2 patients selection using core/penumbra mismatch calculation may not have followed an appropriate mismatch threshold. However, the corrective value of changing the mismatch threshold remains unclear, because the relative mismatch volumes were in fact higher in the 'negative' DIAS-2 than in the 'positive' DIAS and DEDAS. Two Phase II randomized trials with tPA, Diffusion-weighted imaging Evaluation For Understanding Stroke Evolution (DEFUSE) and Echoplanar Imaging Thrombolytic Evaluation Trial (EPITHET) provided strong biological support for the relation between infarct growth, reperfusion and clinical outcome in the 3 - 6 h time window after onset of stroke using penumbral imaging. In this frame, why exactly desmoteplase should have specific advantages over tPA, is not clear. Taken together, these findings may also lead to the disappointing conclusion that vessel recanalization after 4.5 - 5 h from stroke onset may generally be inefficacious for tissue salvage. Nevertheless, other randomized Phase III clinical trials (DIAS-3 and DIAS-4) are currently under way with a planned sample size of 320 patients having vessel occlusion or high-grade stenosis on MRI or CT-angiography in the proximal cerebral arteries.

Novel thrombolytic drugs: will they make a difference in the treatment of ischaemic stroke?

Treatment of acute ischaemic stroke aims to recanalize the occluded artery, salvage the at-risk brain tissue and thus minimize neurological sequelae. Efforts a decade ago have led to the only currently approved medical treatment for acute ischaemic stroke, i.e. intravenous alteplase given within 3 hours of stroke onset. Recanalization occurs in only one-half of the patients receiving alteplase, and only approximately 5% of all ischaemic stroke patients in industrialized countries receive this treatment. Studies are currently being carried out to determine whether intravenous alteplase would be safe and effective for up to 4.5 hours after ischaemic stroke onset, and whether it should be followed by an intra-arterial approach. Two novel thrombolytic drugs being studied for acute ischaemic stroke are desmoteplase and tenecteplase. Although the first trials were promising, the most recent evidence suggests that desmoteplase is not superior to placebo, even in carefully selected patients, in the 3- to 9-hour time window after stroke onset. Tenecteplase has only been studied for acute ischaemic stroke in a single noncontrolled, dose-finding trial in the 3-hour time window after stroke onset, which suggested a similar efficacy to that demonstrated in the historical data from the alteplase trials. A trial to compare the safety and efficacy of tenecteplase versus alteplase is ongoing. Safer and more effective thrombolytic drugs for the treatment of ischaemic stroke are thus being sought. Such agents will be welcome, but they are not here yet. While waiting we are likely to see the emergence of additive therapies, including ultrasound insonation, neuroprotective/regenerative agents and invasive intra-arterial techniques. Novel thrombolytic drugs, or other novel therapies, possess great potential to make a difference in the future, but the most urgent priority now is in the organization of stroke treatment in such a way that more patients receive the currently available optimal treatments.

Production of Desmodus rotundus salivary plasminogen activator alpha1 (DSPAalpha1) in tobacco is hampered by proteolysis

The high fibrin specificity of Desmodus rotundus salivary plasminogen activator alpha1 (DSPAalpha1 or desmoteplase (INN)) makes it a promising candidate for the treatment of acute ischemic stroke. In the current study we explored the use of transgenic tobacco plants and BY-2 suspension cells as alternative production platforms for this drug. Four different N-terminal signal peptides, from plants and animals, were used to translocate the recombinant DSPAalpha1 protein to the endomembrane system. Intact recombinant DSPAalpha1 was produced in transgenic plants and BY-2 cells, although a certain degree of degradation was observed in immunoblotted extracts. The choice of signal peptide had no major influence on the degradation pattern or recombinant protein accumulation, which reached a maximum level of 38 microg/g leaf material. N-terminal sequencing of purified, His6-tagged DSPAalpha1 revealed only minor changes in the position of signal peptide cleavage compared to the same protein expressed in Chinese hamster ovary cells. However, correctly processed recombinant DSPAalpha1 was also detected. The enzymatic activity of the recombinant protein was confirmed using an in vitro assay with unpurified and purified samples, demonstrating that plants are suitable for the production of functional DSPAalpha1. In contrast to whole plant cell extracts, no recombinant DSPAalpha1 was detected in the culture supernatant of transgenic BY-2 cells. Further analysis showed that recombinant DSPAalpha1 is subject to proteolysis and that endogenous secreted BY-2 proteases are responsible for DSPAalpha1 degradation in the culture medium. The addition of a highly concentrated protease inhibitor mixture or 5 mM EDTA reduced DSPAalpha1 proteolysis, improving the accumulation of intact product in the culture medium. Strategies to improve the plant cell suspension system for the production of secreted recombinant proteins are discussed.

Pharmacologic reperfusion therapy for acute myocardial infarction

Acute myocardial infarction (MI) remains a significant problem in terms of morbidity, mortality and healthcare costs. Pharmacologic reperfusion therapies for MI are becoming increasingly complex. This review therefore places contemporary pharmacologic MI developments into perspective. An historical overview of pharmacologic reperfusion therapy for MI is provided, followed by an analysis of current limitations, treatment options, and present and likely future pharmacologic therapies. Adjunctive percutaneous and other treatments are also discussed, to clarify what is becoming a rapidly changing field.

A recombinant antibody-targeted plasminogen activator with high affinity for activated platelets increases thrombolytic potency in vitro and in vivo

To increase thrombolytic specificity of urokinase (uPA), we engineered a recombinant chimeric plasminogen activator SZ51Hu-scuPA, which consists of a humanized monoclonal antibody (SZ-51Hu) specifically against P-selectin on activated human platelet and a single-chain urokinase (scuPA). The cDNA, encoding scuPA amino acids 1-411, was inserted in 5' end to 3' end orientation immediately after the CH3 of SZ-51Hu heavy-chain sequence in the expression vector alphaLys30. The resulting construct alphaLys30-SZ51VH/Hu-scuPA was used to transfect into SP2/0 murine myeloma cell line, which was pretransfected with SZ51Hu light chain. The fusion protein SZ51Hu-scuPA was expressed at 5 mg/L in the supernatant of cell culture. The fusion protein purified by affinity chromatography had a molecular weight of 160 kDa with fibrinolytic activity of 39,000 IU/mg and its affinity to activated human platelet was 67% of the parent murine mAb SZ-51. The thrombolytic property of the fusion protein was first characterized in an in vitro system, which consists of a 125I-fibrin-labeled human plasma clot containing different concentrations of human platelets suspended in citrated human plasma. Fifty percent lysis was reached with SZ51Hu-scuPA in 1 hour at a concentration of 20 IU/mL or in 2 hours at a concentration of 10 IU/ mL, which was much faster than uPA at the same concentration. The maximal lysis of the clots by SZ51Hu-scuPA was 4.1 to 8.4 times more potent than that by uPA. The fusion protein was further characterized in the hamster pulmonary embolism model with clots prepared from fresh platelet-rich human plasma containing 125I-labeled fibrinogen. The thrombolytic activity of SZ51-scuPA was 3.9 times more potent than that of uPA at 2,000 IU/kg in this model. Almost no significant fibrinogen breakdown was observed either in vitro and in vivo.

Thrombolytic therapy: a review of its use in acute myocardial infarction

OBJECTIVE

To review the literature on the use of thrombolytic agents in the pharmacotherapeutic management of acute myocardial infarction (AMI).

DATA SOURCE

English-language clinical trials, reviews, and editorials derived from MEDLINE (January 1966-September 1997) and/or cross-referencing of selected articles.

STUDY SELECTION

Articles that were selected best represent the clinical trials researching the role for thrombolytics in the therapy of AMI to improve morbidity and mortality.

DATA SYNTHESIS

AMI is one of the leading causes of mortality in the US. Following supportive data that the most common cause of an AMI is an intracoronary thrombus, clinical investigation has demonstrated that intravenous thrombolytic agents improve survival rates in patients who experience an AMI. Several clinical trials have been conducted to determine whether one thrombolytic agent is superior to others with respect to improving mortality. At present, only the first Global Use of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries (GUSTO-I) trial has reported any statistically significant difference in mortality rate. In this trial, "front-loaded" alteplase induced a statistically significant (p < 0.001) 1% absolute reduction in 30-day and 1-year mortality compared with streptokinase. This has led to alteplase being the preferred thrombolytic at many US institutions. However, the results of GUSTO-I have been questioned by some on the basis of either study design or clinical significance.

CONCLUSIONS

Thrombolytic agents have secured a place in the treatment of AMI due to their well-proven reduction in mortality rates. In general, comparative trials have demonstrated minimal differences in efficacy among these agents. Probably just as important as choosing which thrombolytic agent to use is ensuring that a patient experiencing an AMI is administered thrombolytic therapy unless a contraindication to receive such therapy exists in the patient and/or the patient is a candidate to receive an emergent intracoronary procedure. Trials also indicate that the sooner thrombolytics can be administered, the greater the benefit to the patient.

[Optimization of thrombolytic treatment in acute myocardial infarct: the role of new fibrinoselective drugs and their combination with new antithrombotics]

Although reperfusion therapy is well recognized as the mainstay of treatment of acute myocardial infarction, mortality of myocardial infarction is still high, thrombolytic treatment remains underutilized and, usually, applied too late. Additionally, most of the patients do not experience optimal reperfusion because of the suboptimal flow rate in the infarct-related artery, abnormal microvascular flow, and reocclusion of the infarct-related artery. Strategies to enhance the results of reperfusion therapy include, expanding the population of potential candidates, earlier treatment, and newer methods to improve infarct-related artery flow rates. In this sense, new thrombolytic agents, and combination therapies with or without addition of more potent and specific new antithrombotic agents are being extensively investigated. Also, it is important to promote studies of ancillary treatments to reduce reperfusion injury, which may be one cause of decreased microvascular flow. Although aspirin and heparin have been the conventionally used agents for inhibiting thrombin and platelet function, newer agents such as hirudin or hirulog and inhibitors of the platelet glycoprotein IIb-IIIa receptors are becoming available, and their clinical application will increase in the future.

Affinity binding of a vampire bat plasminogen activator to SEC resins

DSPAalpha1 is a recombinant form of the vampire bat plasminogen activator which we have produced in mammalian cell culture. During the development of a recovery process for DSPAalpha1 we observed an unexpected binding interaction between this protein and several types of gel filtration chromatography resins. Under typical operating conditions using neutral pH buffers, we found that DSPA flows through the sizing resin and is fractionated, as expected, according to its molecular size. However, DSPA applied under certain acidic conditions (<pH 3) binds tightly to the Sephacryl series of resins. The protein is not released until solvent conditions are changed, specifically the pH is raised above 3. From the results presented we conclude that this unexpected interaction with the gel filtration media is not simply an ion exchange nor a hydrophobic interaction, but rather a more complex, mixed mode "affinity" like binding. Several structural features of the DSPA protein which may be involved in this unique binding have been examined, including binding after inactivation of its active site, chemical deglycosylation, chemical denaturation, or limited proteolysis. The "affinity" interactions persist despite these treatments and lead us to conclude that there may be a unique peptide sequence(s) within the protein which is responsible for the binding interaction to Sephacryl resins.

Thrombolytic therapy for cerebral infarction

Until recently, no clinically effective therapy for acute ischemic stroke has been available. Recent advances in the use of thrombolytic therapy for ischemic stroke appear promising in clinical care. As the use of thrombolytic therapy in acute stroke progress, emergency physicians (EPs) will become increasingly involved in its implementation. The EP must be cognizant of both prior and ongoing investigations in acute ischemic stroke therapy. To that end, this article reviews research in the field of thrombolytic therapy for acute ischemic stroke.

Application of high-performance liquid chromatograph-electrospray ionization mass spectrometry and matrix-assisted laser-desorption ionization time-of-flight mass spectrometry in combination with selective enzymatic modifications in the characterization of glycosylation patterns in single-chain plasminogen activator

The application of high-performance liquid chromatography (HPLC), electrospray ionization mass spectrometry (ESI-MS) and matrix-assisted laser-desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and selective enzymatic deglycosylation treatments is demonstrated in the analysis of glycosylation patterns in recombinant Desmodus salivary plasminogen activator, a heterogeneous glycoprotein. The sample was initially digested with a proteolytic enzyme (endoproteinase Lys-C) and then further treated with either PNGase F to remove N-linked carbohydrates or a combination of neuraminidase and O-glycosidase to remove sialic acid and O-linked carbohydrates. By comparison of the LC-ESI-MS peptide maps for the fully glycosylated and deglycosylated samples, it was possible to unambiguously identify the sites of N-linked glycosylation as well a number of N-linked glycopeptides. The O-link glycopeptides, which are present at low level ( < 1%), were not detected prior to the deglycosylation, nor could changes in peptide elution in the map following deglycosylation be correlated with potential O-linked glycosylation sites.

O-linked L-fucose is present in Desmodus rotundus salivary plasminogen activator

DSPAalpha1 (Desmodus rotundus salivary plasminogen activator), a plasminogen activator from the saliva of the vampire bat Desmodus rotundus, is an effective thrombolytic agent. An unusual type of posttranslational modification, in which L-fucose is O-glycosidically linked to threonine 61 in the epidermal growth factor domain was found for natural DSPAalpha1 and its recombinant form isolated from Chinese hamster ovary cells. In the present study a combination of carbohydrate and amino acid composition analysis, amino acid sequencing, and mass spectrometry revealed that the L-fucose is bound to residues 56-68 of DSPAalpha1. The amino acid sequence of this glycosylation site agreed with the suggested consensus sequence Cys-Xaa-Xaa-Gly-Gly-Ser/Thr-Cys described for other proteins. Anew strategy for the identification of the modified amino acid was established. Direct evidence for the occurrence of fucosyl-threonine was obtained by mass spectrometry after digestion of the glycopeptide with a mixture of peptidases. On the basis of these results, DSPAalpha1 is a suitable model for studying the influence of O-fucosylation on clearance rates, particularly in comparative studies with the identically fucosylated and structurally related tissue plasminogen activator.

Application of capillary electrophoresis, high-performance liquid chromatography, on-line electrospray mass spectrometry and matrix-assisted laser desorption ionization--time of flight mass spectrometry to the characterization of single-chain plasminogen activator

The analysis of recombinant Desmodus salivary plasminogen activator (DSPA alpha 1), a heterogeneous glycoprotein, is demonstrated through the use of high-performance liquid chromatography (HPLC), high-performance capillary electrophoresis (HPCE), liquid chromatography-electrospray mass spectrometry (LC--ES-MS), and matrix-assisted laser desorption ionization--time of flight mass spectrometry (MALDI--TOF-MS). The proteins is analyzed at three specific levels of detail: the intact protein, proteolytic digests of the protein, and fractions from the proteolytic digest. A method for "on-column" collection of HPLC fractions for subsequent transfer and analysis by HPCE and MALDI--TOF-MS is shown.

Limited fibrin specificity of tissue-type plasminogen activator and its potential link to bleeding

Plasminogen activators initiate the fibrinolytic process by converting plasminogen to plasmin. Though plasminogen activators are effective in the treatment of thrombotic disorders, bleeding complications are associated with their use. The development of plasminogen activators with greater fibrin specificity was expected to reduce the incidence of bleeding complications; however, this has not occurred. In our rabbit model (a) bleeding from standardized ear incisions induced by tissue-type plasminogen activator (t-PA) is attenuated when fibrinogenolysis is reduced by the coadministration of alpha 2-antiplasmin and (b) when used in doses that produce equivalent thrombolysis, vampire bat plasminogen activator (b-PA), an agent that is more fibrin specific than t-PA, causes less bleeding than t-PA. In addition, we have found that the (DD)E complex formed as a result of degradation of crosslinked fibrin is a potent stimulator of t-PA-induced plasminogen activation but has no effect on b-PA. Fragment X, a high-molecular-weight clottable fibrinogen degradation product, accumulates after treatment with t-PA but not with t-PA given with alpha 2-antiplasmin or with b-PA. These findings suggest that there is a link between plasminogen activator-induced fibrinogenolysis and bleeding, and that the composition of fibrin within hemostatic plugs may influence susceptibility to lysis. Whether these results mean that fibrin-specific plasminogen activators like b-PA will have a better risk-to-benefit profile in humans requires rigorous testing in well-designed clinical trials. However, at the very least, our findings suggest that the development of plasminogen activators that are more fibrin specific than t-PA is a worth-while exercise.

Structural features mediating fibrin selectivity of vampire bat plasminogen activators

The distinguishing characteristic of vampire bat (Desmodus rotundus) salivary plasminogen activators (DSPAs) is their strict requirement for fibrin as a cofactor. DSPAs consist of structural modules known from urokinase (u-PA) and tissue-type plasminogen activator (t-PA) such as finger (F), epidermal growth factor (E), kringle (K), and protease (P), combining to four genetically and biochemically distinct isoenzymes, exhibiting the formulas FEKP (DSPA alpha 1 and alpha 2) and EKP and KP (DSPA beta and DSPA gamma). Only DSPA alpha 1 and alpha 2 bind to fibrin. All DSPAs are single-chain molecules, displaying substantial amidolytic activity. In a plasminogen activation assay, all four DSPAs are almost inactive in the absence of fibrin but strongly stimulated by fibrin addition. The catalytic efficiency (kcat/Km) of DSPA alpha 1 increases 10(5)-fold, whereas the corresponding value of t-PA is only 550. The ratio of the bimolecular rate constants of plasminogen activation in the presence of fibrin versus fibrinogen (fibrin selectivity) of DSPA alpha 1, alpha 2, beta, gamma, and t-PA was found to be 13,000, 6500, 250, 90, and 72, respectively. Whereas all DSPAs are therefore more fibrin dependent and fibrin selective than t-PA, the extent depends on the respective presence of the various domains. The introduction of a plasmin-sensitive cleavage site in a position akin to the one in t-PA partially obliterates fibrin cofactor requirement. Fibrin dependence and fibrin selectivity of DSPAs are accordingly mediated by fibrin binding, which involves the F domain, as yet undefined determinants within the K and P domains, and by the absence of a plasmin-sensitive activation site. These findings transcend the current understanding of fibrin-mediated stimulation of plasminogen activation: in addition to fibrin binding, specific protein-protein interactions come into play, which stabilize the enzyme in its active conformation.

New thrombolytic agents and strategies

Despite their widespread use in patients with acute myocardial infarction, all currently available thrombolytic agents suffer from a number of significant limitations, including resistance to reperfusion, the occurrence of acute coronary reocclusion, and bleeding complications. Several lines of research towards improvement of thrombolytic therapy are being explored, including strategies to enhance the fibrinolytic potency of plasminogen activators and to improve conjunctive antiplatelet or antithrombotic agents. Mutants and variants of plasminogen activators, chimeric plasminogen activators, and conjugates of plasminogen activators with monoclonal antibodies have been constructed, and plasminogen activators from animal or bacterial origin have been evaluated. Some of these new thrombolytic agents have shown promise in animal models of venous or arterial thrombosis and in pilot studies in patients with acute myocardial infarction. Such molecules include mutants of tissue-type plasminogen activator (t-PA) with prolonged half-life and/or resistance to protease inhibitors and staphylokinase. Antiplatelet strategies include the use of platelet glycoprotein IIb/IIIa receptor blocking agents, of thromboxane synthase inhibitors and endoperoxide receptor antagonists. Antithrombotic strategies include the use of selective inhibitors of thrombin, tissue factor or factor Xa. The efficiency and safety of these new agents in man will have to be carefully evaluated.