Comparative effects of tissue plasminogen activator, streptokinase, and staphylokinase on cerebral ischemic infarction and pulmonary clot lysis in hamster models

Combined Treatment With 2-(2-Benzofu-Ranyl)-2-Imidazoline and Recombinant Tissue Plasminogen Activator Protects Blood-Brain Barrier Integrity in a Rat Model of Embolic Middle Cerebral Artery Occlusion

Recombinant tissue plasminogen activator (rt-PA) is used to treat acute ischemic stroke but is only effective if administered within 4.5 h after stroke onset. Delayed rt-PA treatment causes blood-brain barrier (BBB) disruption and hemorrhagic transformation. The compound 2-(-2-benzofuranyl)-2-imidazoline (2-BFI), a newly discovered antagonist of high-affinity postsynaptic N-methyl-D-aspartate (NMDA) receptors, has been shown to have neuroprotective effects in ischemia. Here, we investigated whether combining 2-BFI and rt-PA can ameliorate BBB disruption and prolong the therapeutic window in a rat model of embolic middle cerebral artery occlusion (eMCAO). Ischemia was induced in male Sprague Dawley rats by eMCAO, after which they were treated with 2-BFI (3 mg/kg) at 0.5 h in combination with rt-PA (10 mg/kg) at 6 or 8 h. Control rats were treated with saline or 2-BFI or rt-PA. Combined therapy with 2-BFI and rt-PA (6 h) reduced the infarct volume, denatured cell index, BBB permeability, and brain edema. This was associated with increased expression of aquaporin 4 (AQP4) and tight junction proteins (occludin and ZO-1) and downregulation of intercellular adhesion molecule 1 (ICAM-1) and matrix metalloproteinases 2 and 9 (MMP2 and MMP9). We conclude that 2-BFI protects the BBB from damage caused by delayed rt-PA treatment in ischemia. 2-BFI may therefore extend the therapeutic window up to 6 h after stroke onset in rats and may be a promising therapeutic strategy for humans. However, mechanisms to explain the effects oberved in the present study are not yet elucidated.

Integration of VEK-30 peptide enhances fibrinolytic properties of staphylokinase

Staphylokinase (SAK), a 136 amino acid bacterial protein with profibrinolytic properties, has emerged as an important thrombolytic agent because of its fibrin specificity and reduced inhibition by α-2 antiplasmin. In an attempt to enhance the clot dissolution ability of SAK, a 30 amino acid peptide (VEK-30) derived from a plasminogen (Pg) binding protein (PAM), was fused at the C-terminal end of SAK with a RGD (Arg-Gly-Asp) linker. The chimeric protein, SAKVEK, was expressed in E. coli and purified as a soluble protein. Pg activation by equimolar complexes of SAKVEK and SAK with plasmin revealed that the fusion of VEK-30 peptide has significantly enhanced the catalytic activity of SAK. The kinetic constant, k_cat /K_m , of SAKVEK for the substrate Pg appeared 2.7 times higher than that of SAK and the time required for the fibrin and platelet rich clot lysis was shortened by 30% and 50%, respectively. The binary activator complex of SAKVEK with plasmin gets inhibited by α2- antiplasmin but remains protected in the presence of fibrin, very similar to SAK. Thus, the present study suggests that SAKVEK is more potent and effective as a thrombolytic agent due to its higher catalytic activity for Pg activation in a fibrin-specific manner and its ability to clear platelet-rich plasma clot faster than SAK.

Impact of Cholesterol on Ischemic Stroke in Different Human-Like Hamster Models: A New Animal Model for Ischemic Stroke Study

Rationale: While high low-density lipoprotein cholesterol (LDL-C) and low high-density lipoprotein cholesterol (HDL-C) levels are positively associated with cardiovascular events, it is still unclear whether familial hypercholesterolemia (FH) and Tangier’s disease (TD), caused by mutations in LDLR and ABCA1, respectively, influence ischemic stroke (IS) in humans. Objective: We sought to establish an easier, more effective, and time-saving method to induce IS, then studied the precise effects of different types of lipoproteins on IS. Methods and Results: A new technique termed contralateral middle cerebral artery occlusion (c-MCAO) was introduced to human-like hamster models to induce IS. Compared to traditional distal MCAO (d-MCAO) induced by electrocoagulation, c-MCAO resulted in a more severe IS with larger infarct sizes and more blood–brain barrier (BBB) disruption after 24 h. It was shown that c-MCAO markedly elicited an increase in brain infarct volume and BBB leakage in both homozygous LDLR (LDLR^–/–) and ABCA1 knockout (ABCA1^–/–) hamsters, but not in heterozygous LDLR knockout (LDLR^+/–) hamsters when compared to wild-type (WT) controls. Conclusions: Using human-like genetically engineered hamsters, our findings demonstrated that both high LDL-C level caused by homozygous LDLR deficiency and severe low HDL-C level caused by deleting ABCA1 were risk factors of IS. As such, we believe the development of this novel IS hamster model is suitable for future ischemic/reperfusion studies.

Protective effects of 2-(2-benzonfuranyl)-2-imidazoline combined with tissue plasminogen activator after embolic stroke in rats

Stroke is the third leading cause of death and disability in developing countries. The effective therapy for acute ischemic stroke is thrombolysis with recombinant tissue plasminogen activator (rt-PA) within 4.5 h of stroke onset. An effective post-ischemic neuroprotectant would extend the advantages of rt-PA, and protect against complications of thrombolysis. We previously reported that 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a newly discovered ligand for high-affinity type 2 imidazoline receptor (I2R), provides neuroprotection against ischemic stroke in rats. Here we investigated the protective effects of 2-BFI in combination with delayed intravenous rt-PA after stroke induced by embolic middle cerebral artery occlusion (eMCAO) in rats. Infarct size was determined using 2,3,5-triphenyltrazolium chloride staining, while neurological deficit was assessed based on neurological score. Numbers of apoptotic cells in vivo were estimated using TUNEL stain, and expression of the pro-apoptotic protein BAX and anti-apoptotic protein BCL-2 were quantified by Western blotting. The results showed that 2-BFI (3 mg/kg) administered at 0.5 h after embolic MCAO combined with rt-PA (10 mg/kg) administered at 6 h reduced brain infarct size, mitigated neurological deficit, decreased the number of TUNEL-positive cells, down-regulated BAX expression, and up-regulated BCL-2 expression. These findings suggest that 2-BFI may extend the therapeutic window of rt-PA to 6 h after embolic stroke onset in rats.

Impact of Bradykinin Generation During Thrombolysis in Ischemic Stroke

Ischemic stroke is one of the leading causes of death and disability worldwide. Current medical management in the acute phase is based on the activation of the fibrinolytic cascade by intravenous injection of a plasminogen activator (such as tissue-type plasminogen activator, tPA) that promotes restauration of the cerebral blood flow and improves stroke outcome. Unfortunately, the use of tPA is associated with deleterious effects such as hemorrhagic transformation, symptomatic brain edema, and angioedema, which limit the efficacy of this therapeutic strategy. Preclinical and clinical evidence suggests that intravenous thrombolysis generates large amounts of bradykinin, a peptide with potent pro-inflammatory, and pro-edematous effects. This tPA-triggered generation of bradykinin could participate in the deleterious effects of thrombolysis and is a potential target to improve neurological outcome in tPA-treated patients. The present review aims at summarizing current evidence linking thrombolysis, bradykinin generation, and neurovascular damage.

Increase in blood-brain barrier permeability does not directly induce neuronal death but may accelerate ischemic neuronal damage

It is observed that the increase in blood-brain barrier (BBB) permeability (BBBP) is associated with ischemic stroke and thought to trigger neuronal damage and deteriorate ischemic infarction, even though there is no experimental proof. Here, we investigated the effect of BBBP increase on brain damage, using a combination of photochemically-induced thrombotic brain damage (PIT-BD) model, a focal brain ischemic model, and transient bilateral carotid artery occlusion model (CAO, a whole brain ischemic model), in mice. In PIT-BD, BBBP increased in the region surrounding the ischemic damage from 4 h till 24 h with a peak at 8 h. On day 4, the damaged did not expand to the region with BBBP increase in mice with PIT-BD alone or with 30 min CAO at 1 h before PIT-BD, but expanded in mice with 30 min CAO at 3.5 h after PIT-BD. This expansion was paralleled with the increase in the number of apoptotic cells. These findings indicate that increase in BBBP does not cause direct neuronal death, but it facilitates ischemic neuronal loss, which was attributed, at least partially, to acceleration of apoptotic cell death.

Stroke, Vascular Dementia, and Alzheimer's Disease: Molecular Links

Stroke is a brain disease that occurs when blood flow stops, resulting in reduced oxygen supply to neurons. Stroke occurs at any time and at any age, but increases after the age of 55. It is the second leading cause of death and the third leading cause of disability-adjusted, life-years. The pathophysiology of ischemic stroke is complex and recent molecular, cellular, and animal models and postmortem brain studies have revealed that multiple cellular changes have been implicated, including oxidative stress/mitochondrial dysfunction, inflammatory responses, micro RNA alterations, and marked changes in brain proteins. These cellular changes provide new information for developing therapeutic strategies for ischemic stroke treatment. Research also revealed that stroke increases with a number of modifiable factors and most strokes can be prevented and/or controlled through pharmacological or surgical interventions and lifestyle changes. Ischemic stroke is the major risk factor for vascular dementia and Alzheimer's disease. This review summarizes the latest research findings on stroke, including causal factors and molecular links between stroke and vascular disease/Alzheimer's disease.

α2-Antiplasmin: New Insights and Opportunities for Ischemic Stroke

Thrombotic vascular occlusion is the leading cause of ischemic stroke. High blood levels of α₂-antiplasmin (a2AP), an ultrafast, covalent inhibitor of plasmin, have been linked in humans to increased risk of ischemic stroke and failure of tissue plasminogen activator (tPA) therapy. Consistent with these observations, a2AP neutralizes the therapeutic benefit of tPA therapy in experimental stroke. In addition, a2AP has deleterious, dose-related effects on ischemic brain injury in the absence of therapy. Experimental therapeutic inactivation of a2AP markedly reduces microvascular thrombosis, ischemic brain injury, brain swelling, brain hemorrhage, and death after thromboembolic stroke. These data provide new insights into the critical importance of a2AP in the pathogenesis of ischemic brain injury and suggest that transiently inactivating a2AP may have therapeutic value in ischemic stroke.

Tissue Plasminogen Activator Neurotoxicity is Neutralized by Recombinant ADAMTS 13

Tissue plasminogen activator (tPA) is an effective treatment for ischemic stroke, but its neurotoxicity is a significant problem. Here we tested the hypothesis that recombinant ADAMTS 13 (rADAMTS 13) would reduce tPA neurotoxicity in a mouse model of stroke. We show that treatment with rADAMTS 13 in combination with tPA significantly reduced infarct volume compared with mice treated with tPA alone 48 hours after stroke. The combination treatment significantly improved neurological deficits compared with mice treated with tPA or vehicle alone. These neuroprotective effects were associated with significant reductions in fibrin deposits in ischemic vessels and less severe cell death in ischemic brain. The effect of rADAMTS13 on tPA neurotoxicity was mimicked by the N-methyl-D-aspartate (NMDA) receptor antagonist M-801, and was abolished by injection of NMDA. Moreover, rADAMTS 13 prevents the neurotoxicity effect of tPA, by blocking its interaction with the NMDA receptor NR2B and the attendant phosphorylation of NR2B and activation of ERK1/2. Finally, the NR2B-specific NMDA receptor antagonist ifenprodil abolished tPA neurotoxicity and rADAMTS 13 treatment had no further beneficial effect. Our data suggest that the combination of rADAMTS 13 and tPA may provide a novel treatment of ischemic stroke by diminishing the neurotoxic effects of exogenous tPA.

Effects of edaravone on early outcomes in acute ischemic stroke patients treated with recombinant tissue plasminogen activator

BACKGROUND

We investigated whether edaravone could improve early outcomes in acute ischemic stroke patients treated with recombinant tissue plasminogen activator (rtPA).

METHODS

We conducted a retrospective cohort study using the Japanese Diagnosis Procedure Combination database. We identified patients admitted with a primary diagnosis of ischemic stroke from 1 July 2010 to 31 March 2012 and treated with rtPA on the same day of stroke onset or the following day. Thereafter, we selected those who received edaravone on the same day of rtPA administration (edaravone group), and those who received rtPA without edaravone (control group). The primary outcomes were modified Rankin Scale (mRS) scores at discharge. One-to-one propensity-score matching was performed between the edaravone and control groups. An ordinal logistic regression analysis for mRS scores at discharge was performed with adjustment for possible variables as well as clustering of patients within hospitals using a generalized estimating equation.

RESULTS

We identified 6336 eligible patients for inclusion in the edaravone group (n=5979; 94%) and the control group (n=357; 6%) as the total population. In 356 pairs of the propensity-matched population, the ordinal logistic regression analysis showed that edaravone was significantly associated with lower mRS scores of patients at discharge (adjusted odds ratio: 0.74; 95% confidence interval: 0.57-0.96).

CONCLUSIONS

Edaravone may improve early outcomes in acute ischemic stroke patients treated with rtPA.

The role of TPA I/D and PAI-1 4G/5G polymorphisms in multiple sclerosis

Background. Previous studies have shown impaired fibrinolysis in multiple sclerosis (MS) and implicated extracellular proteolytic enzymes as important factors in demyelinating neuroinflammatory disorders. Tissue-type plasminogen activator (t-PA) and its inhibitor (PAI-1) are key molecules in both fibrinolysis and extracellular proteolysis. In the present study, an association of the TPA Alu I/D and PAI-1 4G/5G polymorphisms with MS was analyzed within the Genomic Network for Multiple Sclerosis (GENoMS). Methods. The GENoMS includes four populations (Croatian, Slovenian, Serbian, and Bosnian and Herzegovinian) sharing the same geographic location and a similar ethnic background. A total of 885 patients and 656 ethnically matched healthy blood donors with no history of MS in their families were genotyped using PCR-RFLP. Results. TPA DD homozygosity was protective (OR = 0.79, 95% CI 0.63–0.99, P = 0.037) and PAI 5G5G was a risk factor for MS (OR = 1.30, 95% CI 1.01–1.66, P = 0.038). A significant effect of the genotype/carrier combination was detected in 5G5G/I carriers (OR = 1.39 95% CI 1.06–1.82, P = 0.017). Conclusions. We found a significantly harmful effect of the combination of the PAI-1 5G/5G genotype and TPA I allele on MS susceptibility, which indicates the importance of gene-gene interactions in complex diseases such as MS.

An activated protein C analog with reduced anticoagulant activity extends the therapeutic window of tissue plasminogen activator for ischemic stroke in rodents

BACKGROUND AND PURPOSE

Tissue plasminogen activator (tPA) is the only approved therapy for acute ischemic stroke. However, tPA has a brief therapeutic window. Its side effects include intracerebral bleeding and neurotoxicity. Therefore, a combination therapy with tPA and agents that can extend the therapeutic window of tPA and/or counteract its side effects are warranted. Here, we studied whether 3K3A-APC, a neuroprotective analog of activated protein C with reduced anticoagulant activity, can enhance the therapeutic effects of tPA in models of ischemic stroke in rodents.

METHODS

Human recombinant tPA (10 mg/kg), alone or in combination with human recombinant 3K3A-APC (2 mg/kg), was administered intravenously 4 hours after proximal or distal transient middle cerebral artery occlusion in mice and embolic stroke in rats. The 3K3A-APC was additionally administered for 3 to 4 consecutive days after stroke. The neuropathological and neurological analyses were performed at 1 to 7 days after stroke.

RESULTS

In all models, tPA alone had no effects on the infarct volume or behavior (ie, neurological score, foot-fault, forelimb asymmetry, adhesive removal) compared with vehicle. The tPA and 3K3A-APC combination therapy reduced the infarct volume 24 hours and 7 days after proximal or distal transient middle cerebral artery occlusion in mice and 7 days after embolic stroke in rats by 65%, 63%, and 52%, respectively, significantly (P<0.05) improved behavior and eliminated tPA-induced intracerebral microhemorrhages.

CONCLUSIONS

The 3K3A-APC extends the therapeutic window of tPA for ischemic stroke in rodents. Therefore, this combination therapy also should be considered for treating stroke in humans.

Protein S blocks the extrinsic apoptotic cascade in tissue plasminogen activator/N-methyl D-aspartate-treated neurons via Tyro3-Akt-FKHRL1 signaling pathway

Background

Thrombolytic therapy with tissue plasminogen activator (tPA) benefits patients with acute ischemic stroke. However, tPA increases the risk for intracerebral bleeding and enhances post-ischemic neuronal injury if administered 3-4 hours after stroke. Therefore, combination therapies with tPA and neuroprotective agents have been considered to increase tPA's therapeutic window and reduce toxicity. The anticoagulant factor protein S (PS) protects neurons from hypoxic/ischemic injury. PS also inhibits N-methyl-D-aspartate (NMDA) excitotoxicity by phosphorylating Bad and Mdm2 which blocks the downstream steps in the intrinsic apoptotic cascade. To test whether PS can protect neurons from tPA toxicity we studied its effects on tPA/NMDA combined injury which in contrast to NMDA alone kills neurons by activating the extrinsic apoptotic pathway. Neither Bad nor Mdm2 which are PS's targets and control the intrinsic apoptotic pathway can influence the extrinsic cascade. Thus, based on published data one cannot predict whether PS can protect neurons from tPA/NMDA injury by blocking the extrinsic pathway. Neurons express all three TAM (Tyro3, Axl, Mer) receptors that can potentially interact with PS. Therefore, we studied whether PS can activate TAM receptors during a tPA/NMDA insult.

Results

We show that PS protects neurons from tPA/NMDA-induced apoptosis by suppressing Fas-ligand (FasL) production and FasL-dependent caspase-8 activation within the extrinsic apoptotic pathway. By transducing neurons with adenoviral vectors expressing the kinase-deficient Akt mutant Akt^K179A and a triple FKHRL1 Akt phosphorylation site mutant (FKHRL1-TM), we show that Akt activation and Akt-mediated phosphorylation of FKHRL1, a member of the Forkhead family of transcription factors, are critical for FasL down-regulation and caspase-8 inhibition. Using cultured neurons from Tyro3, Axl and Mer mutants, we show that Tyro3, but not Axl and Mer, mediates phosphorylation of FHKRL1 that is required for PS-mediated neuronal protection after tPA/NMDA-induced injury.

Conclusions

PS blocks the extrinsic apoptotic cascade through a novel mechanism mediated by Tyro3-dependent FKHRL1 phosphorylation which inhibits FasL-dependent caspase-8 activation and can control tPA-induced neurotoxicity associated with pathologic activation of NMDA receptors. The present findings should encourage future studies in animal stroke models to determine whether PS can increase the therapeutic window of tPA by reducing its post-ischemic neuronal toxicity.

VEGF-B inhibits apoptosis via VEGFR-1-mediated suppression of the expression of BH3-only protein genes in mice and rats

Despite its early discovery and high sequence homology to the other VEGF family members, the biological functions of VEGF-B remain poorly understood. We revealed here a novel function for VEGF-B as a potent inhibitor of apoptosis. Using gene expression profiling of mouse primary aortic smooth muscle cells, and confirming the results by real-time PCR using mouse and rat cell lines, we showed that VEGF-B inhibited the expression of genes encoding the proapoptotic BH3-only proteins and other apoptosis- and cell death-related proteins, including p53 and members of the caspase family, via activation of VEGFR-1. Consistent with this, VEGF-B treatment rescued neurons from apoptosis in the retina and brain in mouse models of ocular neurodegenerative disorders and stroke, respectively. Interestingly, VEGF-B treatment at the dose effective for neuronal survival did not cause retinal neovascularization, suggesting that VEGF-B is the first member of the VEGF family that has a potent antiapoptotic effect while lacking a general angiogenic activity. These findings indicate that VEGF-B may potentially offer a new therapeutic option for the treatment of neurodegenerative diseases.

Enhanced poly(ADP-ribose) polymerase-1 activation contributes to recombinant tissue plasminogen activator-induced aggravation of ischemic brain injury in vivo

Recombinant tissue plasminogen activator (rt-PA) treatment improves functional outcome after acute ischemic stroke, inducing reperfusion by its thrombolytic activity. Conversely, there is evidence that rt-PA can mediate neuronal damage after ischemic brain injury in vivo. In addition to other mechanisms, enhancement of N-methyl-D-aspartate (NMDA) receptor signalling has been proposed to underlie rt-PA-mediated neurotoxicity. However, the role of poly(ADP-ribose) polymerase-1 (PARP-1) activation, which mediates postischemic excitotoxic cell death, in rt-PA-mediated aggravation of ischemic brain injury has not been established and was therefore addressed in this study. After permanent focal cerebral ischemia, intravenous rt-PA application significantly increased early postischemic PARP-1 activation within ischemic hemispheres and infarct volumes compared with control mice without affecting cerebral blood flow. Rt-PA induced increase in PARP-1 activation, and infarct volumes could be blocked by the PARP inhibitor 3-aminobenzamide. Moreover, the rt-PA-induced increase in PARP-1 activation was also prevented by the NMDA antagonist MK-801. In summary, we demonstrate that rt-PA treatment enhances postischemic PARP-1 activation, which contributes to rt-PA induced aggravation of ischemic brain injury in vivo. Furthermore, we provide evidence that NMDA receptor activation is required for rt-PA-mediated effects on postischemic PARP-1 activation.

Neutralizing the neurotoxic effects of exogenous and endogenous tPA

The clinical use of tissue-type plasminogen activator (tPA) in the treatment of stroke is profoundly constrained by its serious side effects. We report that the deleterious effects of tPA on cerebral edema and intracranial bleeding are separable from its fibrinolytic activity and can be neutralized. A hexapeptide (EEIIMD) corresponding to amino acids 350-355 of plasminogen activator inhibitor type 1 (PAI-1) abolished the tPA-induced increase in infarct size and intracranial bleeding in both mechanical and embolic models of stroke in rats, and reduced brain edema and neuronal loss after traumatic brain injury in pigs. These experiments suggest mechanisms to reduce the neurotoxic effects of tPA without compromising its fibrinolytic activity, through the use of selective antagonists and new tPA formulations.

1,026 experimental treatments in acute stroke

OBJECTIVE

Preclinical evaluation of neuroprotectants fostered high expectations of clinical efficacy. When not matched, the question arises whether experiments are poor indicators of clinical outcome or whether the best drugs were not taken forward to clinical trial. Therefore, we endeavored to contrast experimental efficacy and scope of testing of drugs used clinically and those tested only experimentally.

METHODS

We identified neuroprotectants and reports of experimental efficacy via a systematic search. Controlled in vivo and in vitro experiments using functional or histological end points were selected for analysis. Relationships between outcome, drug mechanism, scope of testing, and clinical trial status were assessed statistically.

RESULTS

There was no evidence that drugs used clinically (114 drugs) were more effective experimentally than those tested only in animal models (912 drugs), for example, improvement in focal models averaged 31.3 +/- 16.7% versus 24.4 +/- 32.9%, p > 0.05, respectively. Scope of testing using Stroke Therapy Academic Industry Roundtable (STAIR) criteria was highly variable, and no relationship was found between mechanism and efficacy.

INTERPRETATION

The results question whether the most efficacious drugs are being selected for stroke clinical trials. This may partially explain the slow progress in developing treatments. Greater rigor in the conduct, reporting, and analysis of animal data will improve the transition of scientific advances from bench to bedside.

Effect of thrombolysis on myocardial injury: recombinant tissue plasminogen activator vs. alfimeprase

Plasmin-dependent thrombolytic agents are potentially prothrombotic and proinflammatory. Alfimeprase, a zinc-containing metalloproteinase, degrades fibrin directly and achieves thrombolysis independent of plasmin formation. This study examines the hypothesis that thrombolysis in the absence of plasmin generation results in improved myocardial salvage on reperfusion. The thrombolytic effects of recombinant tissue plasminogen activator [rt-PA; 0.022 mg/kg, 1/10 of which was administered as a loading dose; the rest (9/10) was infused over 60 min by intracoronary (ic) administration] or alfimeprase (0.5 mg/kg over 1 min ic) were evaluated in a canine model of arterial thrombosis involving electrolytic injury of the left circumflex (LCX) coronary artery. Both agents induced thrombolysis, with onset of reperfusion being more rapid after alfimeprase compared with rt-PA (1.5 ± 0.6 vs. 10.1 ± 2.1 min). In the absence of adjunctive therapy, time to reocclusion after alfimeprase was 3.2 ± 0.5 min compared with 77.5 ± 31.9 min with rt-PA. The glycoprotein IIb/IIIa platelet receptor antagonist CRL-42796 prolonged reperfusion time after thrombolysis with alfimeprase or rt-PA. The effect of each lytic agent on myocardial infarct size was examined in a separate group of dogs subjected to 60 min of LCX coronary artery ligation and 4 h of reperfusion. Myocardial infarct size, expressed as percentage of the risk region, was larger (32.16 ± 3.95%) after rt-PA compared with alfimeprase (19.85 ± 3.61%) or that of the saline control group (18.46 ± 3.34%). rt-PA in contrast to alfimeprase, a direct-acting fibrinolytic agent, is associated with an increase in myocyte reperfusion injury.

Rt-PA causes a significant increase in endogenous u-PA during experimental focal cerebral ischemia

The aim of this study was to investigate the effects of different doses of exogenous recombinant human tissue plasminogen activator (rt-PA) on the endogenous cerebral plasminogen-plasmin system in focal ischemia in rats. Ischemia was induced using the suture model. Each group of rats (n = 6) received either treatment (0.9, 9 or 18 mg rt-PA/kg body weight) or saline (control group) at the end of ischemia; a sham-operated group was added. The activity of the plasminogen activators was measured by casein-dependent plasminogen zymography. In the cortex urokinase (u-PA) rose from sham (no ischemia), 91 +/- 7% to ischemia, 176 +/- 10% (P < 0.005). Increasing rt-PA doses led to further significant (P < 0.001) cortical u-PA activation which was maximal at 18 mg: 249 +/- 13%. An extreme increase in the u-PA activity was observed in the basal ganglia to 1019 +/- 22% (P < 0.001). This increase was further aggravated by higher rt-PA doses (18 mg, 1236 +/- 15%; P < 0.001). The t-PA level did not change I3R24 during (3 h ischemia followed by reperfusion for 24 h); however, during low and moderate doses of rt-PA, endogenous t-PA was reduced. In conclusion, while ischemia leads to a significant increase in u-PA, mainly in the basal ganglia, t-PA is not altered. Increasing doses of rt-PA lead to a further elevation of u-PA. Thus, u-PA seems to play a major role in the endogenous plasminogen activator system following focal cerebral ischemia.

The neurotoxicity of tissue plasminogen activator?

Tissue plasminogen activator (tPA), a fibrin specific activator for the conversion of plasminogen to plasmin, stimulates thrombolysis and rescues ischemic brain by restoring blood flow. However, emerging data suggests that under some conditions, both tPA and plasmin, which are broad spectrum protease enzymes, are potentially neurotoxic if they reach the extracellular space. Animal models suggest that in severe ischemia with injury to the blood brain barrier (BBB) there is injury attributed to the protease effects of this exogenous tPA. Besides clot lysis per se, tPA may have pleiotropic actions in the brain, including direct vasoactivity, cleaveage of the N-methyl-D-aspartate (NMDA) NR1 subunit, amplification of intracellular Ca++ conductance, and activation of other extracellular proteases from the matrix metalloproteinase (MMP) family, e.g. MMP-9. These effects may increase excitotoxicity, further damage the BBB, and worsen edema and cerebral hemorrhage. If tPA is effective and reverses ischemia promptly, the BBB remains intact and exogenous tPA remains within the vascular space. If tPA is ineffective and ischemia is prolonged, there is the risk that exogenous tPA will injure both the neurovascular unit and the brain. Methods of neuroprotection, which prevent tPA toxicity or additional mechanical means to open cerebral vessels, are now needed.

Comparative effects of microplasmin and tissue-type plasminogen activator (tPA) on cerebral hemorrhage in a middle cerebral artery occlusion model in mice

BACKGROUND

Thrombolytic therapy of ischemic stroke with tissue-type plasminogen activator (tPA) improves clinical outcome which may, however, be partially offset by significant intracerebral bleeding (ICB).

OBJECTS

The comparative effects of microplasmin (microPli) and tPA on ICB were evaluated in a thrombotic middle cerebral artery (MCA) occlusion model in mice.

METHODS

A dose of microPli (5 mg kg(-1)) or tPA (4 mg kg(-1)) which are comparably effective for reduction of brain damage, or a double dose (10 or 8 mg kg(-1), respectively) or the microPli excipient as a control were intravenously administered as a bolus at 30 min or 4 h after MCA occlusion. ICB was measured at 24 h by hemoglobin assay of exsanguinated brain extracts. Bleeding time was measured by tail cutting.

RESULTS

In controls given solvent at 4 h, ICB was on average 8.8 micro L, which was significantly increased with 10 mg kg(-1) microPli and with 4 and 8 mg kg(-1) tPA to 12-13 micro L (P < 0.05 each vs. controls, n = 7-9), whereas 5 mg kg(-1) microPli did not affect bleeding (8.5 micro L P = NS vs. controls, n = 7). When given at 30 min, neither microPli nor tPA altered ICB (6.3-6.8 micro L, mean; n = 7-9). tPA but not microPli increased bleeding time; from 2.4 min in controls to 5.9 min (median, P < 0.05 vs. controls) and 8.7 min (P < 0.01 vs. controls) with 4 and 8 mg kg(-1), respectively, and to 2.3 and 2.9 min with 5 and 10 mg kg(-1) microPli, respectively (n = 10).

CONCLUSIONS

microPli at a dose comparably effective as tPA for brain damage reduction induced significantly less ICB, and less bleeding time prolongation in mice with thrombotic MCA occlusion.

Can the time window for administration of thrombolytics in stroke be increased?

Level 1 evidence now shows that thrombolysis in cases of acute ischaemic stroke is effective if administered within 3 hours of stroke onset. This benefit has been shown to be time dependent and potentially extends beyond 3 hours, with evidence that potentially viable penumbral tissue may be present in a significant proportion of cases well beyond 3-6 hours and, in isolated cases, perhaps up to 48 hours. This exposes a "stroke recovery gap", the difference observed between the clinical response to thrombolytic therapy in a given population of patients presenting with ischaemic stroke and the potential clinical recovery if all of the penumbra were salvaged under ideal circumstances. The means of bridging this "stroke recovery gap" using thrombolysis must involve extending the therapeutic time window (i.e. the time between stroke onset and administration of thrombolytics). Approaches to do this include the use of: (i) improved patient selection with modern neuroimaging techniques, particularly magnetic resonance imaging using perfusion-weighted image/diffusion-weighted image mismatch; (ii) newer thrombolytic agents; (iii) lower doses of these agents; (iv) varied methods of administration of thrombolytic therapy including combined intravenous and intra-arterial approaches; and (v) adjunctive therapies such as neuroprotectants. Should these means of extending the time window for thrombolysis prove successful, a more widespread use of this form of acute stroke therapy will be possible.

Recombinant human microplasmin: production and potential therapeutic properties

The effect of recombinant human microplasmin was studied in ischemic stroke models in mice and in an extracorporeal loop thrombosis model in rabbits. Human microplasminogen ( micro Plg), which lacks the five 'kringle' domains of plasminogen was expressed with high yield in Pichia pastoris. It was purified, converted to microplasmin ( micro Pli) and equilibrated with 5 mmol L(-1) citrate, pH 3.1, yielding a stable preparation. In mice with middle cerebral artery (MCA) ligation, an intravenous (i.v.) bolus of 5.0 mg kg(-1) micro Pli reduced infarct size at 24 h from 27 (26-30) to 25 (21-28) mm3 (median and range, n= 16 each, P= 0.0001), whereas 4.0 mg kg(-1) rt-PA and 40 mg kg(-1) micro Plg had no effect. Infarct reduction was observed with administration at 4 h after occlusion. In mice with MCA, infarct size at 24 h was reduced from 20 (14-30) to 9.1 (3.1-25) mm3 with 5.0 mg kg(-1) micro Pli (n = 15 each, P < 0.002) and to 11 (5.2-27) mm3 with 4.0 mg kg(-1) rt-PA (n = 6; P= 0.02). Infarct reduction was still observed at 10 h after occlusion with micro Pli but not with t-PA. In rabbits with radiolabeled clots in an extracorporeal arteriovenous loop, local infusion of 2.5 mg kg(-1) micro Pli over 2 h, induced 51 +/- 15% lysis (mean +/- SD, n= 11) vs. a control value of 23 +/- 5.5%. micro Pli did not prolong template bleeding times, whereas equipotent doses of rt-PA were associated with extensive rebleeding. The potency of micro Pli in both models was similar to that of intact plasmin. These findings indicate that recombinant micro Pli may be useful for treatment of ischemic stroke and arterial thrombosis.

Localization of plasminogen in the extracellular matrix of hamster eggs: exogenous activation by streptokinase

The plasminogen activator (PA)/plasminogen/plasmin proteolytic system has begun to be taken into account in the fertilization process. In this study, we demonstrated the presence of plasminogen in the extracellular matrix (ECM) of hamster oocytes by indirect immunofluorescence and immunoperoxidase assays using human anti-plasminogen. Plasminogen appeared first on the zona pellucida (ZP) of ovarian oocytes and later on the plasma membrane (PM) of oviducal eggs. This would suggest that oviducal oocytes modulate the expression of plasminogen binding sites on the PM. Human plasminogen as well as that of other species, known to be activated by streptokinase (SK), is rapidly converted to a plasmin-SK complex. We demonstrated the rapid formation of a SK-plasminogen complex that yields plasmin in the blood plasma of hamsters. Both the in vivo and in vitro SK treatment of eggs from superovulated female hamsters caused a decreased in the ZP dissolution time (ZPdt), probably either due to the proteolytic effect of plasmin or due to the SK-Plasminogen. Extracellular proteolysis assays carried out on agar-casein plates confirmed the proteolytic activity of SK-incubated eggs; the controls, on the contrary, failed to display a halo. These studies show that (1) superovulated hamster eggs contain plasminogen in their ECM, (2) oviducal eggs exhibit plasminogen on their PMs, indicating the presence of their corresponding binding sites, (3) in hamsters, SK, a non-enzymatic exogenous protein would be capable of activating ECM plasminogen to plasmin, and (4) the complex SK-plasminogen and/or the plasmin are capable of changing the ZPdt with alpha-chymotrypsin.

ATTEMPTS: a heparin/protamine-based delivery system for enzyme drugs

A prodrug delivery system termed "Antibody Targeted, Triggered, Electrically Modified Prodrug-Type Strategy (ATTEMPTS)" has been developed to permit the antibody-directed administration of inactive enzyme drug including tissue-type plasminogen activator (tPA), and allow a subsequent triggered release of the active tPA at the target site. Cation-modified tPA (mtPA) was attached to a heparin-antifibrin complex via ionic interaction, and the active tPA can subsequently be released by the addition of protamine, a competitive heparin inhibitor. Anti-fibrin IgG was conjugated to heparin via an end-point attachment to form the heparin-antifibrin complex which provides the targeting efficiency of the final heparin/mtPA complex. Cation modification was performed by either chemical conjugation by linking (Arg)7Cys to tPA with N-succinimidy-3-(2-pyridyldithio) propionate or by recombinant DNA methods. Results show that the modification process did not significantly alter the specific activity of tPA with regard to plasminogen activation, fibrin-binding ability, and response toward fibrinogen. The complexes of both modified tPA-heparin did not yield any intrinsic catalytic activity owing to the blockage of the active site of tPA by the attached heparin. On the other hand, heparin-induced inhibition of modified tPA activity was reversed by adding protamine, which is similar to that of a prodrug delivery system. These results suggest that heparin/protamine-based enzyme delivery systems may be a useful tool to improve current enzyme therapeutic status, as well as thrombolytic therapy, by both regulating the release of active enzyme and aborting the associated systemic toxic effect. Currently, modification of enzyme drugs has been optimized by recombinant DNA technology assisted by computer simulation. In addition, the original strategy has been revised to obtain enhanced therapeutic efficacy.

Tissue-type plasminogen activator is involved in the process of neuronal death induced by oxygen-glucose deprivation in culture

Effect of tissue-type plasminogen activator (tPA) on oxygen-glucose deprivation (OGD) was studied in cultured cortical neurons prepared from tPA gene knockout (tPA-KO) and wild-type (Wt) mice. Three hours of OGD induced 45% and 23% of neuronal death in Wt and tPA-KO mice, respectively. Neuronal death in tPA-KO mice was increased to 42% by additional tPA. Six hours of OGD induced 80% and 40% of neuronal death in Wt and tPA-KO mice, respectively, whereas the addition of tPA increased to 62% in tPA-KO mice. These results suggest that tPA is directly involved in the process of neuronal death induced by ischemia-mimic stress without involving vascular or circulatory components.

Depletion of circulating alpha(2)-antiplasmin by intravenous plasmin or immunoneutralization reduces focal cerebral ischemic injury in the absence of arterial recanalization

In the absence of arterial recanalization, thrombolytic agents induce a dose-related extension of focal cerebral ischemic injury (FII) in experimental animals. However, FII is smaller in mice lacking alpha(2)-antiplasmin (alpha(2)-AP), the physiologic inhibitor of plasmin, suggesting its depletion might reduce FII in the absence of reperfusion. Therefore, the effect of human plasmin (Pli), human miniplasmin (mPli), and an Fab fragment neutralizing murine alpha(2)-AP (Fab-4H9) on FII after middle cerebral artery (MCA) ligation was studied in mice and in hamsters. In BALB/c mice, the median FII after 24 hours was 28 microL (range, 20-34) (n = 10) with saline and 23 microL (range, 17-26) (n = 9) with a single bolus of 0.07 mg Pli, given after MCA ligation (P =.010), which reduced alpha(2)-AP to 44% and fibrinogen from 0.75 to 0.44 g/L. FII was 20 microL (range, 13-26) (n = 6, P =.025) with 0.2 mg mPli and was 24 microL (range, 20-27) (n = 6, P =.020) with 1.7 mg Fab-4H9. Neuronal atrophy and reduction of laminin immunoreactivity were comparably observed in the infarct area after saline and Pli. In hamsters, a single bolus injection of 1 mg Pli, after MCA ligation, depleted alpha(2)-AP and fibrinogen and reduced FII at 24 hours from 20 microL (range, 9.9-38) (n = 6) to 7.0 microL (range, 0.44-31) (n = 7, P =.032). Thus, reduction of circulating alpha(2)-AP, with a single bolus of plasmin or of a neutralizing antibody fragment, significantly reduced FII after MCA ligation in mouse and hamster models, suggesting that, provided these observations can be extrapolated to human beings, transient depletion of circulating alpha(2)-AP might reduce ischemic stroke in the absence of reperfusion.

ATTEMPTS: a heparin/protamine-based prodrug approach for delivery of thrombolytic drugs

The aim of this study is to develop a heparin/protamine-based prodrug system for the controlled delivery of enzyme such as tissue-type plasminogen activator (tPA). This approach, termed antibody targeted, triggered, electrically modified prodrug-type strategy (ATTEMPTS), would permit antibody-directed administration of inactive tPA, and allow a subsequent triggered release of the active tPA at the target site. Cation-modified tPA (mtPA) was attached to a heparin--antifibrin complex via ionic interaction. The active tPA can be subsequently released by the addition of protamine, a competitive heparin inhibitor. Anti-fibrin IgG was conjugated to heparin via an end-point attachment to form the heparin--antifibrin--complex which provides the targeting efficiency of the final heparin--mtPA complex. Cation-modification was performed either by chemical conjugation by linking (Arg)(7)Cys to tPA with N-succinimidy-3-(2-pyridyldithio) propionate or by recombinant DNA method. Results show that the chemical modification process did not significantly alter specific activity of tPA with regard to plasminogen activation, fibrin-binding ability, and response toward fibrinogen. Expressed modified tPA (EmtPA) produced by recombinant DNA methods retained the same catalytic activity of the parent tPA, as well as a dynamic catalytic behavior depending upon the presence of heparin and protamine. Both types of modified tPA, especially the mtPA demonstrated a significantly higher affinity toward heparin or heparin--antifibrin complex than native tPA. In addition, the complexes of mtPA--heparin did not yield any intrinsic clot lysis activity owing to the blockage of the active site of tPA by attached heparin. On the other hand, heparin-induced inhibition of both mtPA and EmtPA activity was reversed by adding protamine, as confirmed by chromogenic and in vitro clot lysis assays. These results suggested that a heparin/protamine-based tPA delivery system may be a useful tool to improve current thrombolytic therapeutic status, by both precisely regulating the release of active tPA and aborting the associated bleeding risk. Alternatively, this ATTEMPTS approach could also be used to deliver enzyme drugs while diminishing their associated toxic effects.