Microplasmin: a novel thrombolytic that improves behavioral outcome after embolic strokes in rabbits

Plasmin Inhibitor in Health and Diabetes: Role of the Protein as a Therapeutic Target

The vascular obstructive thrombus is composed of a mesh of fibrin fibers with blood cells trapped in these networks. Enhanced fibrin clot formation and/or suppression of fibrinolysis are associated with an increased risk of vascular occlusive events. Inhibitors of coagulation factors and activators of plasminogen have been clinically used to limit fibrin network formation and enhance lysis. While these agents are effective at reducing vascular occlusion, they carry a significant risk of bleeding complications. Fibrin clot lysis, essential for normal hemostasis, is controlled by several factors including the incorporation of antifibrinolytic proteins into the clot. Plasmin inhibitor (PI), a key antifibrinolytic protein, is cross-linked into fibrin networks with higher concentrations of PI documented in fibrin clots and plasma from high vascular risk individuals. This review is focused on exploring PI as a target for the prevention and treatment of vascular occlusive disease. We first discuss the relationship between the PI structure and antifibrinolytic activity, followed by describing the function of the protein in normal physiology and its role in pathological vascular thrombosis. Subsequently, we describe in detail the potential use of PI as a therapeutic target, including the array of methods employed for the modulation of protein activity. Effective and safe inhibition of PI may prove to be an alternative and specific way to reduce vascular thrombotic events while keeping bleeding risk to a minimum. Key Points Plasmin inhibitor (PI) is a key protein that inhibits fibrinolysis and stabilizes the fibrin network.This review is focused on discussing mechanistic pathways for PI action, role of the molecule in disease states, and potential use as a therapeutic target.

Fibrinolytic nanocages dissolve clots in the tumor microenvironment, improving the distribution and therapeutic efficacy of anticancer drugs

Fibrin, one of the components of the extracellular matrix (ECM), acts as a transport barrier within the core of tumors by constricting the blood vessels and forming clots, leading to poor intratumoral distribution of anticancer drugs. Our group previously developed a microplasmin-based thrombolytic ferritin nanocage that efficiently targets and dissolves clots without causing systemic fibrinolysis or disrupting hemostatic clots. We hypothesized that the thrombolytic nanocage-mediated degradation of fibrin clots in the tumor ECM can lead to enhanced intratumoral drug delivery, especially for nanosized anticancer drugs. Fibrin clot deposition worsens after surgery and chemotherapy, further hindering drug delivery. Moreover, the risk of venous thromboembolism (VTE) also increases. Here, we used thrombolytic nanocages with multivalent clot-targeting peptides and fibrin degradation enzymes, such as microplasmin, to dissolve fibrin in the tumor microenvironment and named them fibrinolytic nanocages (FNCs). These FNCs target tumor clots specifically and effectively. FNCs efficiently dissolve fibrin clots inside of the tumor vessels, suggesting that they can mitigate the risk of VTE in cancer patients. Coadministration of FNC and doxorubicin led to improved chemotherapeutic activity in a syngeneic mouse melanoma model. Furthermore, the FNCs increased the distribution of Doxil/doxorubicin nanoparticles within mouse tumors. These results suggest that fibrinolytic cotherapy might help improve the therapeutic efficacy of anticancer nanomedicines. Thus, microplasmin-based fibrinolytic nanocages are promising candidates for this strategy due to their hemostatic safety and ability to home in on the tumor.

Preclinical animal studies in ischemic stroke: Challenges and some solutions

Despite the impressive efficacies demonstrated in preclinical research, hundreds of potentially neuroprotective drugs have failed to provide effective neuroprotection for ischemic stroke in human clinical trials. Lack of a powerful animal model for human ischemic stroke could be a major reason for the failure to develop successful neuroprotective drugs for ischemic stroke. This review recapitulates the available cerebral ischemia animal models, provides an anatomical comparison of the circle of Willis of each species, and describes the functional assessment tests used in these ischemic stroke models. The distinct differences between human ischemic stroke and experimental stroke in available animal models is explored. Innovative animal models more closely resembling human strokes, better techniques in functional outcome assessment and better experimental designs generating clearer and stronger evidence may help realise the development of truly neuroprotective drugs that will benefit human ischemic stroke patients. This may involve use of newer molecules or revisiting earlier studies with new experimental designs. Translation of any resultant successes may then be tested in human clinical trials with greater confidence and optimism.

Development and Testing of Thrombolytics in Stroke

Despite recent advances in recanalization therapy, mechanical thrombectomy will never be a treatment for every ischemic stroke because access to mechanical thrombectomy is still limited in many countries. Moreover, many ischemic strokes are caused by occlusion of cerebral arteries that cannot be reached by intra-arterial catheters. Reperfusion using thrombolytic agents will therefore remain an important therapy for hyperacute ischemic stroke. However, thrombolytic drugs have shown limited efficacy and notable hemorrhagic complication rates, leaving room for improvement. A comprehensive understanding of basic and clinical research pipelines as well as the current status of thrombolytic therapy will help facilitate the development of new thrombolytics. Compared with alteplase, an ideal thrombolytic agent is expected to provide faster reperfusion in more patients; prevent re-occlusions; have higher fibrin specificity for selective activation of clot-bound plasminogen to decrease bleeding complications; be retained in the blood for a longer time to minimize dosage and allow administration as a single bolus; be more resistant to inhibitors; and be less antigenic for repetitive usage. Here, we review the currently available thrombolytics, strategies for the development of new clot-dissolving substances, and the assessment of thrombolytic efficacies in vitro and in vivo.

Alpha2-Antiplasmin: The Devil You Don't Know in Cerebrovascular and Cardiovascular Disease

Alpha2-antiplasmin (α2AP), the fast-reacting, serine protease inhibitor (serpin) of plasmin, was originally thought to play a key role in protection against uncontrolled, plasmin-mediated proteolysis of coagulation factors and other molecules. However, studies of humans and mice with genetic deficiency of α2AP have expanded our understanding of this serpin, particularly in disease states. Epidemiology studies have shown an association between high α2AP levels and increased risk or poor outcome in cardiovascular diseases. Mechanistic studies in disease models indicate that α2AP stops the body's own fibrinolytic system from dissolving pathologic thrombi that cause venous thrombosis, pulmonary embolism, arterial thrombosis, and ischemic stroke. In addition, α2AP fosters the development of microvascular thrombosis and enhances matrix metalloproteinase-9 expression. Through these mechanisms and others, α2AP contributes to brain injury, hemorrhage and swelling in experimental ischemic stroke. Recent studies also show that α2AP is required for the development of stasis thrombosis by inhibiting the early activation of effective fibrinolysis. In this review, we will discuss the key role played by α2AP in controlling thrombosis and fibrinolysis and, we will consider its potential value as a therapeutic target in cardiovascular diseases and ischemic stroke.

Site-specific PEGylation of micro-plasmin for improved thrombolytic therapy through engineering enhanced resistance against serpin mediated inhibition

The relatively rapid inhibition of microplasmin by α₂-AP leads to short functional half-life of the molecule in vivo, causing inefficient clot dissolution, even after site-specific, local catheter-based delivery. Here, we describe a PEGylation approach for improving the therapeutic potential via improving the survival of microplasmin in presence of its cognate inhibitor, α₂-AP, wherein a series of strategically designed cysteine analogs of micro-plasminogen were prepared and expressed in E. coli, and further modified by covalent grafting in vitro with PEG groups of different molecular sizes so as to select single or double PEG chains that increase the molecular weight and hydrodynamic radii of the conjugates, but with a minimal discernible effect on intrinsic plasmin activity and structural framework, as explored by amidolytic activity and CD-spectroscopy, respectively. Interestingly, some of the purified PEG-coupled proteins after conversion to their corresponding proteolytically active forms were found to exhibit significantly reduced inhibition rates (up to 2-fold) by α₂-AP relative to that observed with wild-type microplasmin. These results indicate an interesting, and not often observed, effect of PEG groups through reduced/altered dynamics between protease and inhibitor, likely through a steric hindrance mechanism. Thus, the present study successfully identifies single- and double-site PEGylated muteins of microplasmin with significantly enhanced functional half-life through enhanced resistance to inactivation by its in vivo plasma inhibitor. Such an increased survival of bioactivity in situ, holds unmistakable potential for therapeutic exploitation, especially in ischemic strokes where a direct, catheter-based deposition within the cranium has been shown to be promising, but is currently limited by the very short in vivo bioactive half-life of the fibrin dissolving agent/s.

Large animals in neurointerventional research: A systematic review on models, techniques and their application in endovascular procedures for stroke, aneurysms and vascular malformations

Neuroendovascular procedures have led to breakthroughs in the treatment of ischemic stroke, intracranial aneurysms, and intracranial arteriovenous malformations. Due to these substantial successes, there is continuous development of novel and refined therapeutic approaches. Large animal models feature various conceptual advantages in translational research, which makes them appealing for the development of novel endovascular treatments. However, the availability and role of large animal models have not been systematically described so far. Based on comprehensive research in two databases, this systematic review describes current large animal models in neuroendovascular research including their primary use. It may therefore serve as a compact compendium for researchers entering the field or looking for opportunities to refine study concepts. It also describes particular applications for ischemic stroke and aneurysm therapy, as well as for the treatment of arteriovenous malformations. It focuses on most promising study designs and readout parameters, as well as on important pitfalls in endovascular translational research including ways to circumvent them.

Selective middle cerebral artery occlusion in the rabbit: Technique and characterization with pathologic findings and multimodal MRI

BACKGROUND

A reliable animal model of ischemic stroke is vital for pre-clinical evaluation of stroke therapies. We describe a reproducible middle cerebral artery (MCA) embolic occlusion in the French Lop rabbit characterized with multimodal MRI and histopathologic tissue analysis.

NEW METHOD

Fluoroscopic-guided microcatheter placement was performed in five consecutive subjects with angiographic confirmation of MCA occlusion with autologous clot. Multimodal MRI was obtained prior to occlusion and up to six hours post after which repeat angiography confirmed sustained occlusion. The brain was harvested for histopathologic examination.

RESULTS

Angiography confirmed successful MCA catheterization and durable (>6 h) MCA occlusion in all animals. There was increase of ADC volume over time and variable final core volume presumably related to individual variation in collateral flow. FLAIR hyperintensity indicative of cytotoxic edema and parenchymal contrast enhancement reflective of blood brain barrier disruption was observed over time. Tissue staining of the ischemic brain showed edema and structural alterations consistent with infarction.

COMPARISON WITH EXISTING METHODS

This study describes a technique of selective catheterization and embolic occlusion of the MCA in the rabbit with MRI characterization of evolution of ischemia in the model.

CONCLUSIONS

We demonstrate the feasibility of a rabbit model of embolic MCA occlusion with angiographic documentation. Serial MR imaging demonstrated changes comparable to those observed in human ischemic stroke, confirmed histopathologically.

Novel Thrombolytic Drug Based on Thrombin Cleavable Microplasminogen Coupled to a Single-Chain Antibody Specific for Activated GPIIb/IIIa

Background

Thrombolytic therapy for acute thrombosis is limited by life‐threatening side effects such as major bleeding and neurotoxicity. New treatment options with enhanced fibrinolytic potential are therefore required. Here, we report the development of a new thrombolytic molecule that exploits key features of thrombosis. We designed a recombinant microplasminogen modified to be activated by the prothrombotic serine‐protease thrombin (HtPlg), fused to an activation‐specific anti–glycoprotein IIb/IIIa single‐chain antibody (SCE5), thereby hijacking the coagulation system to initiate thrombolysis.

Methods and Results

The resulting fusion protein named SCE5‐HtPlg shows in vitro targeting towards the highly abundant activated form of the fibrinogen receptor glycoprotein IIb/IIIa expressed on activated human platelets. Following thrombin formation, SCE5‐HtPlg is activated to contain active microplasmin. We evaluate the effectiveness of our targeted thrombolytic construct in two models of thromboembolic disease. Administration of SCE5‐HtPlg (4 μg/g body weight) resulted in effective thrombolysis 20 minutes after injection in a ferric chloride–induced model of mesenteric thrombosis (48±3% versus 92±5% for saline control, P<0.01) and also reduced emboli formation in a model of pulmonary embolism (P<0.01 versus saline). Furthermore, at these effective therapeutic doses, the SCE5‐HtPlg did not prolong bleeding time compared with saline (P=0.99).

Conclusions

Our novel fusion molecule is a potent and effective treatment for thrombosis that enables in vivo thrombolysis without bleeding time prolongation. The activation of this construct by thrombin generated within the clot itself rather than by a plasminogen activator, which needs to be delivered systemically, provides a novel targeted approach to improve thrombolysis.

Dose-finding study of phototherapy on stroke outcome in a rabbit model of ischemic stroke

GOAL

While transcranial laser therapy (TLT) has been shown to improve clinical outcome in a preclinical model of ischemic stroke, optimal timing and dosing has yet to be tested adequately. The purpose of this study was to assess clinical stroke outcome in the Rabbit Small Clot Embolic Model (RSCEM) with dose escalating TLT.

METHODS

We utilized the rabbit small clot embolic stroke model (RSCEM) using dose-escalating regimens. Behavioral analysis was conducted at 24h post-embolization, allowing for the determination of the effective stroke dose (ES50) or clot amount (mg) that produces neurological deficits in 50% of a group of rabbits. Using the RSCEM, a treatment is considered beneficial if it significantly increases the ES50 compared with the control group.

FINDINGS

A significant behavioral benefit was seen at triple TLT of 111mW treatment of 2min at 2h post-embolization (6.47±1.06, n=17; p=0.03), compared with the previously used regimen (3.09±0.51, n=15).

CONCLUSION

TLT results in significant behavioral improvement when administered 2h post-embolization. Studies are warranted to evaluate this therapy in combination with thrombolysis.

Improving the translation of animal ischemic stroke studies to humans

Despite testing more than 1,026 therapeutic strategies in models of ischemic stroke and 114 therapies in human ischemic stroke, only one agent tissue plasminogen activator has successfully been translated to clinical practice as a treatment for acute stroke. Though disappointing, this immense body of work has led to a rethinking of animal stroke models and how to better translate therapies to patients with ischemic stroke. Several recommendations have been made, including the STAIR recommendations and statements of RIGOR from the NIH/NINDS. In this commentary we discuss additional aspects that may be important to improve the translational success of ischemic stroke therapies. These include use of tissue plasminogen activator in animal studies; modeling ischemic stroke heterogeneity in terms of infarct size and cause of human stroke; addressing the confounding effect of anesthesia; use of comparable therapeutic dosage between humans and animals based on biological effect; modeling the human immune system; and developing outcome measures in animals comparable to those used in human stroke trials. With additional study and improved animal modeling of factors involved in human ischemic stroke, we are optimistic that new stroke therapies will be developed.

The combination of astragalus membranaceus and ligustrazine ameliorates micro-haemorrhage by maintaining blood-brain barrier integrity in cerebrally ischaemic rats

ETHNOPHARMACOLOGICAL RELEVANCE

Haemorrhagic transformation is an asymptomatic event that frequently occurs after following ischaemic stroke, particularly when pharmaceutical thrombolysis is used. However, the mechanism responsible for haemorrhagic transformation remains unknown, and therapeutics have not been identified. In this study, we administered a combination of astragalus membranaceus and ligustrazine to rats with cerebral ischaemia that had undergone thrombolysis. We analysed the effect of this combination on the attenuation of haemorrhagic transformation and the maintenance of blood-brain barrier integrity.

METHODS

A rat model of focal cerebral ischaemia was induced with autologous blood clot injections. Thrombolysis was performed via the intravenous injection of rt-PA. Astragalus membranaceus, ligustrazine or a combination of Astragalus membranaceus and ligustrazine was administered immediately after the clot injection. The cerebral infarct area, neurological deficits, blood-brain barrier integrity, and cerebral haemorrhage status were determined after 3, 6 and 24h of ischaemia. The ultrastructure of the blood-brain barrier was examined with a transmission electron microscope. The expression of tight junction proteins, including claudin-1, claudin-5, occludin, and zonula occludens-1, and matrix metallopeptidase-9 activation was further evaluated in terms of their roles in the protective effects of the combination drug on the integrity of the blood-brain barrier.

RESULTS

Ischaemia-induced Evans blue leakage and cerebral haemorrhage were markedly reduced in the combination drug-treated rats compared to the rats treated with either astragalus membranaceus or ligustrazine alone (p<0.05). The disruption of the ultrastructure of the blood-brain barrier and the neurological deficits were ameliorated by the combination treatment (p<0.05). The reductions in the expression of laudin-1, claudin-5, occludin, and ZO-1 were smaller in the rats that received the combination treatment. In addition, MMP-9 activity was suppressed in the combination-treated rats compared to the controls (p<0.05).

CONCLUSIONS

Treatment with a combination of astragalus membranaceus and ligustrazine alleviated ischaemia-induced micro-haemorrhage transformation by maintaining the integrity of the blood-brain barrier.

rLOAD: does sex mediate the effect of acute antiplatelet loading on stroke outcome

BACKGROUND

Biologic sex can influence response to pharmacologic therapy. The purpose of this proof-of-concept study was to evaluate the medicating effects of estrogen in the efficacy of acute antiplatelet loading therapy on stroke outcome in the rabbit small clot embolic model.

METHODS

Female and male (20/group) New Zealand White rabbits were embolized to produce embolic stroke by injecting small blood clots into the middle cerebral artery via an internal carotid artery catheter. Two hours after embolization, rabbits were treated with standard dose antiplatelet loading (aspirin 10 mg/kg plus clopidogrel 10 mg/kg). Primary outcome measures were platelet inhibition, behavioral outcome P 50 (the weight of microclots (mg) that produces neurologic dysfunction in 50% of a group of animals), and effect of endogenous estrogen on outcome.

RESULTS

For the first time in a non-rodent model of stroke, it was found that higher endogenous estrogen levels resulted in significantly better behavioral outcome in female subjects (r s -0.70, p < 0.011). Platelet inhibition in response to collagen, arachidonic acid, and adenosine diphosphate (ADP) was not significantly different in females with higher vs. lower estrogen levels.

CONCLUSIONS

Behavioral outcomes are improved with females with higher endogenous estrogen levels treated with standard dose antiplatelet loading. This is the first non-rodent study to demonstrate that higher endogenous estrogen levels in female rabbits appear to be neuroprotective in ischemic stroke. This research supports the further study of the effect of endogenous estrogen levels on outcome with standard dose antiplatelet loading in stroke patients not eligible for revascularization therapies.

Hydrogen-rich saline attenuates neuronal ischemia--reperfusion injury by protecting mitochondrial function in rats

BACKGROUND

Hydrogen, a popular antioxidant gas, can selectively reduce cytotoxic oxygen radicals and has been found to protect against ischemia-reperfusion (I/R) injury of multiple organs. Acute neuronal death during I/R has been attributed to loss of mitochondrial permeability transition coupled with mitochondrial dysfunction. This study was designed to investigate the potential therapeutic effect of hydrogen-rich saline on neuronal mitochondrial injury from global cerebral I/R in rats.

MATERIALS AND METHODS

We used a four-vessel occlusion model of global cerebral ischemia and reperfusion, with Sprague-Dawley rats. The rats were divided randomly into six groups (n = 90): sham (group S), I/R (group I/R), normal saline (group NS), atractyloside (group A), hydrogen-rich saline (group H), and hydrogen-rich saline + atractyloside (group HA). In groups H and HA, intraperitoneal hydrogen-rich saline (5 mL/kg) was injected immediately after reperfusion, whereas the equal volume of NS was injected in the other four groups. In groups A and HA, atractyloside (15 μL) was intracerebroventricularly injected 10 min before reperfusion, whereas groups NS and H received equal NS. The mitochondrial permeability transition pore opening and mitochondrial membrane potential were measured by spectrophotometry. Cytochrome c protein expression in the mitochondria and cytoplasm was detected by western blot. The hippocampus mitochondria ultrastructure was examined with transmission electron microscope. The histologic damage in hippocampus was assessed by hematoxylin and eosin staining.

RESULTS

Hydrogen-rich saline treatment significantly improved the amount of surviving cells (P < 0.05). Furthermore, hydrogen-rich saline not only reduced tissue damage, the degree of mitochondrial swelling, and the loss of mitochondrial membrane potential but also preserved the mitochondrial cytochrome c content (P < 0.05).

CONCLUSIONS

Our study showed that hydrogen-rich saline was able to attenuate neuronal I/R injury, probably by protecting mitochondrial function in rats.

Alteplase for acute ischemic stroke

Alteplase, an intravenously administered form of recombinant tissue plasminogen activator (rt-PA), remains the only US FDA-approved thrombolytic treatment for acute ischemic stroke within 3 h of symptom onset. Patients treated with intravenous rt-PA are at least 30% more likely to have minimal or no disability at 3 months compared with placebo. Despite an increased risk of symptomatic intracranial hemorrhage, rt-PA does not increase mortality. The benefit achieved with rt-PA is cost effective and sustained 1 year after treatment. Despite its clear benefit, rt-PA remains underutilized. Although the future of acute ischemic stroke treatment will most likely involve a multi-faceted treatment approach, the primary objective remains to establish recanalization of the involved vessel. For patients with acute ischemic stroke within the first 3 h of symptom onset, rt-PA remains the first step in accomplishing this goal.

Extracellular proteolytic pathophysiology in the neurovascular unit after stroke

The NINDS Stroke Progress Review Group recommended a shift in emphasis from a purely neurocentric view of cell death towards a more integrative approach whereby responses in all brain cells and matrix are considered. The neurovascular unit (fundamentally comprising endothelium, astrocyte, and neuron) provides a conceptual framework where cell-cell and cell-matrix signaling underlies the overall tissue response to stroke and its treatments. Here, we briefly review recent data on extracellular proteolytic dysfunction in the neurovascular unit after a stroke. The breakdown of neurovascular matrix initiates blood-brain barrier disruption with edema and/or hemorrhage. Endothelial dysfunction amplifies inflammatory responses. Perturbation of cell-matrix homeostasis triggers multiple cell death pathways. Interactions between the major classes of extracellular proteases from the plasminogen and matrix metalloprotease families may underlie processes responsible for some of the hemorrhagic complications of thrombolytic stroke therapy. Targeting the proteolytic imbalance within the neurovascular unit may provide new approaches for improving the safety and efficacy of thrombolytic reperfusion therapy for stroke.

Microplasmin (ocriplasmin) in pediatric vitreoretinal surgery: update and review

PURPOSE

To discuss the potential role of microplasmin (ocriplasmin) as a surgical adjunct to vitrectomy in pediatric vitreoretinopathies.

METHODS

Literature review of the laboratory and clinical evidence to date for the use of both autologous plasmin enzyme as an adjunct to vitrectomy and more recently recombinant microplasmin (ocriplasmin) as monotherapy for focal vitreomacular traction in adults.

RESULTS

Autologous plasmin enzyme is currently being used as a surgical adjunct to vitrectomy, with supporting Levels 2 and 3 published evidence in a range of pediatric vitreoretinopathies including Stage 5 retinopathy of prematurity and congenital X-linked retinoschisis. The availability of autologous plasmin enzyme is limited. In recent Phase 3 clinical trials, intravitreal ocriplasmin versus sham injection resulted in resolution of focal vitreomacular traction in 27% versus 10% (P < 0.001, n = 652).

CONCLUSION

Ocriplasmin may potentially be used as a surgical adjunct to vitrectomy in place of autologous plasmin enzyme. A Phase 2, randomized, placebo-controlled surgical trial is under way to assess this.

Endovascular thrombectomy following acute ischemic stroke: a single-center case series and critical review of the literature

Acute ischemic stroke (AIS) due to thrombo-embolic occlusion in the cerebral vasculature is a major cause of morbidity and mortality in the United States and throughout the world. Although the prognosis is poor for many patients with AIS, a variety of strategies and devices are now available for achieving recanalization in patients with this disease. Here, we review the treatment options for cerebrovascular thromboembolic occlusion with a focus on the evolution of strategies and devices that are utilized for achieving endovascular clot extraction. In order to demonstrate the progression of this treatment strategy over the past decade, we will also present a single-center case series of AIS patients treated with endovascular thrombectomy.

Vitreomacular adhesion and neovascular age-related macular degeneration

We explore the hypothesis that vitreomacular adhesion (VMA) and vitreomacular traction (VMT) play a role in the pathogenesis and clinical course of neovascular ("wet") age-related macular degeneration (AMD). Several biological theories are offered to explain this possible association, including direct tractional force, altered vitreous oxygenation, altered diffusion coefficients of intravitreal molecules, and alterations in the pharmacokinetics of intravitreal drugs. Release of VMT may improve the clinical course of neovascular AMD, and a few case series suggest that vitrectomy can lead to both a functional and anatomic improvement. A large, randomized, controlled clinical trial is underway, investigating pharmacologic release of VMA in eyes with neovascular AMD.

Safety evaluation of a recombinant plasmin derivative lacking kringles 2-5 and rt-PA in a rat model of transient ischemic stroke

BACKGROUND

Tissue type plasminogen activator is the only approved thrombolytic agent for the treatment of ischemic stroke. However, it carries the disadvantage of a 10-fold increase in symptomatic and asymptomatic intracranial hemorrhage. A safer thrombolytic agent may improve patient prognosis and increase patient participation in thrombolytic treatment. A novel direct-acting thrombolytic agent, Δ(K2-K5) plasmin, promising an improved safety profile was examined for safety in the snare ligature model of stroke in the rat.

METHODS

Male spontaneously hypertensive rats were subjected to 6 hours middle cerebral artery occlusion followed by 18 hours reflow. Beginning 1 minute before reflow, they were dosed with saline, vehicle, Δ(K2-K5) plasmin (0.15, 0.5, 1.5, and 5 mg/kg) or recombinant tissue-type plasminogen activator (10 and 30 mg/kg) by local intra-arterial infusion lasting 10 to 60 minutes. The rats were assessed for bleeding score, infarct volume, modified Bederson score and general behavioral score. In a parallel study, temporal progression of infarct volume was determined. In an in vitro study, whole blood clots from humans, canines and rats were exposed to Δ(K2-K5). Clot lysis was monitored by absorbance at 280 nm.

RESULTS

The main focus of this study was intracranial hemorrhage safety. Δ(K2-K5) plasmin treatment at the highest dose caused no more intracranial hemorrhage than the lowest dose of recombinant tissue type plasminogen activator, but showed at least a 5-fold superior safety margin. Secondary results include: temporal infarct volume progression shows that the greatest expansion of infarct volume occurs within 2-3 hours of middle cerebral artery occlusion in the spontaneously hypertensive rat. A spike in infarct volume was observed at 6 hours ischemia with reflow. Δ(K2-K5) plasmin tended to reduce infarct volume and improve behavior compared to controls. In vitro data suggests that Δ(K2-K5) plasmin is equally effective at lysing clots from humans, canines and rats.

CONCLUSIONS

The superior intracranial hemorrhage safety profile of the direct-acting thrombolytic Δ(K2-K5) plasmin compared with recombinant tissue type plasminogen activator makes this agent a good candidate for clinical evaluation in the treatment of acute ischemic stroke.

Potentiation of aspirin-induced cerebroprotection by minocycline: a therapeutic approach to attenuate exacerbation of transient focal cerebral ischaemia

Cerebrovascular disease is a major cause of mortality and disability in adults. Diabetes mellitus increases the risk of cerebral ischaemia and is associated with worse clinical outcome following an event. Upregulation of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9) in diabetes appears to play a role in vascular complications of diabetes. We hypothesised that inhibition of MMP-2 and MMP-9 by minocycline can be potentiated by aspirin through inhibition of cyclooxygenase-2 and tissue plasminogen activator, resulting in amelioration of clinical cerebral ischaemia in diabetes. In the present study, cerebral ischaemia/reperfusion injury was induced in streptozotocin diabetic rats by 1 h middle cerebral artery occlusion and 24 h reperfusion. Infarct volume, cerebral oedema, neurological severity score and blood-brain barrier disruption were significantly increased in diabetic animals compared with the normoglycemic control group. The combination of aspirin and minocycline treatment significantly improved these parameters in diabetic animals. Moreover, this therapy was associated with significantly lower mortality and reduction in MMP-2 and MMP-9 levels. Our data indicate that combination of aspirin and minocycline therapy protects from the consequences of cerebral ischaemia in animal models of diabetes and is associated with inhibition of MMP-2 and MMP-9. Therefore, this combination therapy may represent a novel strategy to reduce the neurological complications of cerebral ischaemia in diabetes.

Characterization of a stabilized form of microplasmin for the induction of posterior vitreous detachment

PURPOSE

To investigate the stability and safety of a diluted acidified form of microplasmin and its ability to induce a posterior vitreous detachment (PVD) following intravitreal injection in post-mortem porcine eyes.

METHODS

Microplasmin diluted in normal saline (NS) and balanced salt solution (BSS+) was assayed for residual activity by hydrolysis of the chromogenic substrate Glu-Phe-Lys-pNA. Residual activity on vitreous was determined by injecting aliquots of microplasmin reconstituted in balanced salt solution (BSS+) or normal saline (NS) kept at room temperature (RT) for up to 1 hr, then injected in aliquots of porcine vitreous and incubated for 2 hr at 37°C. The breakdown products were submitted to SDS Page electrophoresis and compared to determine the level of enzymatic activity. Pig eyes were incubated with graded concentrations of microplasmin 0.625, 1.25, or 2.50 mg/mL reconstituted in BBS+ or NS. Morphologic alterations and the ability to induce a PVD was assessed by light and electron microscopy.

RESULTS

Microplasmin's enzymatic activity in an in vitro assay in BSS+ was 70% of its baseline value after 30 min, and about 50% after 60 min at RT. The corresponding effect on degradation of vitreous gel was 60 and 40% baseline at 30 and 60 min. There was no loss of activity in the microplasmin diluted in normal saline over this time period. Dilution of acidified microplasmin in normal saline did not lead to structural changes within the retina. A dose dependent PVD was observed in eyes treated with microplasmin diluted in NS.

CONCLUSIONS

Acidified (stabilized) microplasmin has the same intraocular activity profile as microplasmin at a neutral pH. Better retention of activity at room temperature makes it a better candidate for use in clinical practice.

Impacts of methylxanthines and adenosine receptors on neurodegeneration: human and experimental studies

Neurodegenerative disorders are some of the most feared illnesses in modern society, with no effective treatments to slow or halt this neurodegeneration. Several decades after the earliest attempt to treat Parkinson's disease using caffeine, tremendous amounts of information regarding the potential beneficial effect of caffeine as well as adenosine drugs on major neurodegenerative disorders have accumulated. In the first part of this review, we provide general background on the adenosine receptor signaling systems by which caffeine and methylxanthine modulate brain activity and their role in relationship to the development and treatment of neurodegenerative disorders. The demonstration of close interaction between adenosine receptor and other G protein coupled receptors and accessory proteins might offer distinct pharmacological properties from adenosine receptor monomers. This is followed by an outline of the major mechanism underlying neuroprotection against neurodegeneration offered by caffeine and adenosine receptor agents. In the second part, we discuss the current understanding of caffeine/methylxantheine and its major target adenosine receptors in development of individual neurodegenerative disorders, including stroke, traumatic brain injury Alzheimer's disease, Parkinson's disease, Huntington's disease and multiple sclerosis. The exciting findings to date include the specific in vivo functions of adenosine receptors revealed by genetic mouse models, the demonstration of a broad spectrum of neuroprotection by chronic treatment of caffeine and adenosine receptor ligands in animal models of neurodegenerative disorders, the encouraging development of several A(2A) receptor selective antagonists which are now in advanced clinical phase III trials for Parkinson's disease. Importantly, increasing body of the human and experimental studies reveals encouraging evidence that regular human consumption of caffeine in fact may have several beneficial effects on neurodegenerative disorders, from motor stimulation to cognitive enhancement to potential neuroprotection. Thus, with regard to neurodegenerative disorders, these potential benefits of methylxanthines, caffeine in particular, strongly argue against the common practice by clinicians to discourage regular human consumption of caffeine in aging populations.

Annexin A2: a tissue plasminogen activator amplifier for thrombolytic stroke therapy

Hemorrhagic transformation, incomplete reperfusion, neurotoxicity, and the short treatment time window comprise major challenges for thrombolytic therapy. Improving tissue plasminogen activator therapy has become one of the highest priorities in the stroke field. Recent efforts have been aimed at identifying new strategies that might enhance the thrombolytic efficacy of tissue plasminogen activator at the same time as reducing its associated complications related to hemorrhage and neurotoxicity. We believe that the combination of low-dose tissue plasminogen activator with recombinant annexin A2 (a tissue plasminogen activator and plasminogen coreceptor) might constitute a promising approach. Our pilot study using a focal embolic stroke model in rats supports this hypothesis.

Intra-arterial versus intra-venous thrombolysis within and after the first 3 hours of stroke onset

The NINDS trial demonstrated for the first time the effectiveness of intravenous thrombolysis in improving outcome after acute ischemic stroke. The absolute benefit of this intervention was 11-13% greater chance of being normal or near normal (MRS ≤ 1) at 3 months. However, if patients with severe stroke were considered (NIHSS ≥ 20), the absolute benefit dropped to 5-6%, indicating that IV thrombolysis may not be as effective for large vessel occlusion. This observation was further supported by TCD studies that clearly demonstrated that large artery occlusions had a recanalization rate of 13-18% with IV rt-PA. Intra-arterial thrombolysis achieves recanalization rates of 60-70%. Since tissue viability is clearly important, it is time to stop defining rigid time windows and if there is a large penumbra (20-50%) and the occlusion is in a large artery, there exists a logic and a growing evidence to consider either bridge therapy or direct intra-arterial therapy.

Stroke location and brain function in an embolic rabbit stroke model

PURPOSE

Current rabbit stroke models often depend on symptoms as endpoints for embolization and produce wide variation in location, size, and severity of strokes. In a further refinement of an angiographic embolic stroke model, localized infarctions were correlated to neurologic deficits with the goal to create a rabbit model for long-term studies of therapies after stroke.

MATERIALS AND METHODS

New Zealand White rabbits (4-5 kg; N = 71) had selective internal carotid artery (ICA) angiography and a single clot was injected. At 24 hours, neurologic assessment score (NAS) was measured on an 11-point scale (0, normal; 10, dead). Brains were removed and stained to identify stroke areas. All animals with single strokes (n = 31) were analyzed by specific brain structure involvement, and NAS values were correlated.

RESULTS

Stroke incidence differed by location, with cortex, subcortical, and basal ganglia regions highest. The middle cerebral artery (MCA), at 52%, and anterior cerebral artery (ACA), at 29%, were most commonly involved, with the largest stroke volumes in the ACA distribution. Brainstem and cerebellum strokes had disproportionately severe neurologic deficits, scoring 2.25 +/- 1.0 on the NAS, which represented a significant (P < .02) difference versus cortex (0.5 +/- 0.2), subcortical (1.3 +/- 0.4), and basal ganglia (0.5 +/- 0.3), all in the frontal or parietal regions.

CONCLUSIONS

MCA and ACA distributions included 81% of strokes. These sites were relatively silent (potentially allowing longer-term survival studies) whereas others in the posterior circulation produced disproportionately severe symptoms. Symptoms were not reliable indicators of stroke occurrence, and other endpoints such as imaging may be required. These are important steps toward refinement of the rabbit stroke model.

Current endovascular treatment of acute stroke and future aspects

Acute ischemic stroke remains a condition of high morbidity and mortality. Until now, the only established therapy has been intravenous (IV) tissue-type plasminogen activator (tPA). Only 3-10% of patients with acute ischemic stroke receive this treatment. On the basis of data from part 3 of the European Collaborative Acute Stroke Study (ECASS III), the time window for beneficial treatment of ischemic stroke with IV tPA has been extended from 3 to 4.5h after the onset of stroke symptoms. Beyond that window of opportunity, and additionally to IV treatment, interventional stroke therapy has assumed an important role for the treatment of acute ischemic stroke. Currently, new promising pharmacological and mechanical treatment options are being established as routine procedures to achieve a further improved outcome for stroke patients.

Does plasmin have anticoagulant activity?

The coagulation and fibrinolytic pathways regulate hemostasis and thrombosis, and an imbalance in these pathways may result in pathologic hemophilia or thrombosis. The plasminogen system is the primary proteolytic pathway for fibrinolysis, but also has important proteolytic functions in cell migration, extracellular matrix degradation, metalloproteinase activation, and hormone processing. Several studies have demonstrated plasmin cleavage and inactivation of several coagulation factors, suggesting plasmin may be not only be the primary fibrinolytic enzyme, but may have anticoagulant properties as well. The objective of this review is to examine both in vitro and in vivo evidence for plasmin inactivation of coagulation, and to consider whether plasmin may act as a physiological regulator of coagulation. While several studies have demonstrated strong evidence for plasmin cleavage and inactivation of coagulation factors FV, FVIII, FIX, and FX in vitro, in vivo evidence is lacking for a physiologic role for plasmin as an anticoagulant. However, inactivation of coagulation factors by plasmin may be useful as a localized anticoagulant therapy or as a combined thrombolytic and anticoagulant therapy.

Direct fibrinolytic agents: biochemical attributes, preclinical foundation and clinical potential

Direct fibrinolytics are proteolytic enzymes that degrade fibrin without requiring an intermediate step of plasminogen activation. This review summarizes the current information available for five such agents, namely, plasmin (the prototypical form), three derivatives of plasmin (mini-plasmin, micro-plasmin, and delta-plasmin), and alfimeprase, a recombinant variant of a snake venom alpha-fibrinogenase, fibrolase. Biochemical attributes of molecular size, fibrin binding and inhibitor neutralization are compared. Preclinical investigations that assess the potential for thrombolytic efficacy in vitro and in animal models of vascular occlusion and for hemostatic safety in animal models of bleeding are detailed. Clinical potential has been assessed in patients with peripheral arterial and graft occlusion, acute ischemic stroke, and access catheter and hemodialysis shunt occlusions. The direct fibrinolytic agents have impressive biochemical and preclinical foundations for ultimate clinical application. However, clinical trial results for micro-plasmin and alfimeprase have not measured up to their anticipated benefit. Plasmin has thus far shown encouraging hemostatic safety, but efficacy data await completion of clinical trials. Whether direct fibrinolytics will provide clinical superiority in major thrombotic disorders over currently utilized indirect fibrinolytics such as tissue plasminogen activator remains to be determined.

Randomized, Placebo-Controlled, Dose-Ranging Clinical Trial of Intravenous Microplasmin in Patients With Acute Ischemic Stroke

Background and Purpose— Microplasmin is a recombinant truncated form of human plasmin. It has demonstrated efficacy in experimental animal models of stroke and tolerability in healthy volunteers. We tested the tolerability of microplasmin in patients with acute ischemic stroke.

Methods— In a multicenter, double-blind, randomized, placebo-controlled Phase II trial, 40 patients with ischemic stroke were treated with either placebo or active drug between 3 and 12 hours after symptom onset in a dose-finding design. Ten patients received placebo, 6 patients received a total dose of 2 mg/kg, 12 patients received a total dose of 3 mg/kg, and 12 patients received a total dose of 4 mg/kg. We studied the pharmacodynamics of microplasmin and its effect on the clinical and hemodynamic parameters of the patients. MRI was used as a surrogate marker and matrix metalloproteinases serum concentrations were used as markers of neurovascular integrity. The study was underpowered to detect clinical efficacy.

Results— Microplasmin induced reversible effects on markers of systemic thrombolysis and neutralized α 2 -antiplasmin by up to 80%. It was well tolerated with one of 30 treated patients developing a fatal symptomatic intracerebral hemorrhage. No significant effect on reperfusion rate or on clinical outcome was observed. Matrix metalloproteinase-2 levels were reduced in microplasmin-treated patients.

Conclusions— Microplasmin was well tolerated and achieved neutralization of α 2 -antiplasmin. Further studies are warranted to determine whether microplasmin is an effective therapeutic agent for ischemic stroke.

Effect of internal carotid artery reperfusion in combination with Tenecteplase on clinical scores and hemorrhage in a rabbit embolic stroke model

In the present study, we used a modification of the rabbit small clot embolic stroke model (RSCEM), a multiple infarct ischemia model to achieve reperfusion (REP) through the internal carotid artery (ICA) following small clot embolization. We determined if increasing regional cortical blood flow (RCBF) following an embolic stroke is beneficial to neurological outcome. We compared this to cerebral reperfusion induced by the administration of the thrombolytic Tenecteplase (TNK, 1.5 mg/kg, IV bolus) in the presence or absence of REP. In this study, we also measured the incidence of ICH following REP and thrombolytic treatment. Following embolization, RCBF was reduced to 48-55% of baseline. When REP was induced by removal of a CCA ligature, RCBF initially increased to 185% of baseline. REP (P(50)=1.18+/-0.43 mg) had no effect on embolization-induced behavior measured 24 h following embolization compared to control (P(50)=1.01+/-0.48 mg). However, TNK treatment (2-hours post-embolization) in the absence or presence of REP (initiated 2 h following embolization) significantly (p<0.05) increased the group P(50) to 2.92+/-0.55 mg and 2.42+/-0.40 mg, respectively. In addition, ICH was increased in the REP (42%, p<0.05) and REP-TNK (35%, p>0.05) group compared to either the control group (5.5%) or TNK group (10%). This study show that reperfusion of ICA can increase RCBF following embolization, but this is not associated with improved neurological outcome measured using quantal analysis. However, TNK administration significantly increased behavioral outcome when given 2 h following embolization; an increase that is not affected by combining TNK with REP.

Endovascular approaches to acute stroke, part 1: Drugs, devices, and data

Despite years of research and pioneering clinical work, stroke remains a massive public health concern. Since 1996, we have lived in the era of US Food and Drug Administration-approved intravenous (i.v.) recombinant tissue plasminogen activator (rtPA). This treatment, despite its promise, continues to exhibit its limitations. Endovascular therapy has several theoretic advantages over i.v. rtPA, including site specificity, longer treatment windows, and higher recanalization rates. In this article, we will review the various pharmacologic strategies for acute stroke treatment, providing both a historic context and the state of the art. The drugs will be classified on the basis of their theoretic rationale for therapy. Next, we will review the various devices and strategies for mechanical revascularization with an aim toward comprehensiveness. These range from wire disruption of thrombus to preclinical trials for novel mechanical solutions. This first installment of this 2-part series will end with an analysis of retrograde reperfusion techniques.

Effects of microplasmin on recovery in a rat embolic stroke model

OBJECTIVES

The purpose of the present study was to examine the effects of microplasmin on behavioral performance and infarct volume after middle cerebral artery occlusion (MCAO) in rats. Some experiments support that microplasmin may have neuroprotective and thrombolytic properties.

METHODS

Eighty rats underwent surgery and were embolized in the right carotid territory with a fibrin-rich embolus and randomly assigned into three groups: 5 mg/kg microplasmin, 10 mg/kg microplasmin or saline (control). Groups treated with microplasmin received 50% bolus injection 10 minutes after embolization and 50% continuous infusion during the following hour. Animals from all groups were trained to obtain high baseline scores in Montoya's staircase test before embolization and were retested during 7-14 days after surgery.

RESULTS

When pre-maturely dead animals were excluded, no differences were observed among groups regarding infarct volumes. Furthermore, mortality was significantly lower in Group 1 than in Group 2 (p<0.05) and when performances were evaluated 7-14 days after surgery, Group 1 was significantly better than Group 2 concerning fine motor performance (p<0.05) and also achieved more normal bodyweight (p<0.05).

DISCUSSION

Among surviving animals, 5 mg/kg microplasmin treatment had no effect compared to saline-treated control animals; 5 mg/kg microplasmin reduced mortality and improved both behavioral rehabilitation and bodyweight compared to 10 mg/kg microplasmin treatment, while saline-treated animals did not differ from animals treated with 10 mg/kg microplasmin. Overall, these results indicate a potential beneficial effect of 5 mg/kg microplasmin treatment, while 10 mg/kg may worsen outcomes.

Neuroprotection in cerebral ischemia: emphasis on the SAINT trial

Acute ischemic stroke (AIS) is a significant cause of death and disability in the United States. It has been 10 years since tissue plasminogen activator became the first medication approved by the US Food and Drug Administration for treatment for AIS. However, this treatment simply reopens arteries. The identification of deleterious cellular reactions that occur secondary to cerebral ischemia has led investigators to search for neuroprotection strategies to complement reperfusion. More than 100 human trials, including a handful of phase III trials, had failed to produce an efficacious neuroprotective agent. In 2006, the first positive trial of neuroprotection was published: the SAINT I (Stroke-Acute Ischemic NXY Treatment) study. In February 2008, the SAINT II study was published, indicating that NXY-059 was not effective for AIS treatment.

Laser Therapy in Acute Stroke Treatment

Recent development of near infrared light therapy (NILT) as an acute stroke treatment is promising. In various preclinical animal stroke models, NILT has been shown to be effective in improving long-term stroke outcome. More importantly, NILT has a long postischemic therapeutic window that has not been previously observed in other treatment modalities. The preliminary efficacy and safety of NILT in acute stroke patients were demonstrated in the recently published phase II ***NeuroThera Effectiveness and Safety Trial (NEST-1). If confirmed by the NEST-II trial, ***NILT will revolutionize acute stroke management as ***ut has a long time window (possible 24 hr) for therapy. Moreover, understanding the mechanisms of action of NILT will provide a new therapeutic target for future drug or device development.

Tissue-type plasminogen activator as a therapeutic target in stroke

BACKGROUND

Ischemic stroke is a leading cause of morbidity and mortality worldwide and recombinant human tissue-type plasminogen activator (tPA) is the prominent therapeutic among very few therapeutics used in its treatment. Due to complications attributed to the drug, most notably transformation of ischemia to hemorrhage, tPA is only used in a small number of ischemic stroke cases, albeit significantly more often in specialized stroke centers.

OBJECTIVE

What are the mechanisms of tPA action and side effects in ischemic stroke, and can the knowledge about these mechanisms aid in making tPA a more efficacious and safe therapeutic or in developing alternative therapeutics?

METHODS

tPA use and alternative/combination therapies in acute ischemic stroke treatment are summarized. The review focuses on literature concerning tPA neurotoxicity and its implications for further development of tPA as a stroke therapeutic.

RESULTS/CONCLUSION

Exogenously administered recombinant tPA and endogenous tPA have both turned into promising therapeutic targets for the stroke patient.