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

Prevention and treatment of atherothrombosis: Potential impact of nanotechnology

Complications with atherosclerosis can often lead to fatal clot formation and blood vessel occlusion - also known as atherothrombosis. A key component to the development of atherosclerosis and atherothrombosis is the endothelium and its ability to regulate the balance between prothrombotic and antithrombotic activities. Endothelial surface glycocalyx has a critical role in maintenance of vascular integrity. The endothelial glycocalyx, nitric oxide, prostacyclins, heparan sulfate, thrombomodulin, and tissue factor pathway inhibitor all prevent thrombosis, while P-selectin, among many other factors, favors thrombosis. However, endothelial dysfunction gives rise to the acceleration of thrombotic development and eventually the requirement of antithrombotic therapy. Most FDA-approved anticoagulant and antiplatelet therapies today carry a side effect profile of major bleed. Within the past five years, several preclinical studies using different endothelial targets and nanotechnology as a drug delivery method have emerged to target the endothelium and to enhance current antithrombosis without increasing bleed risk. While clinical studies are required, this review illustrates the proof-of-concept of nanotechnology in promoting a greater safety and efficacy profile through multiple in vitro and in vivo studies.

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.

Use of Exogenous Enzymes in Human Therapy: Approved Drugs and Potential Applications

The development of safe and efficacious enzyme-based human therapies has increased greatly in the last decades, thanks to remarkable advances in the understanding of the molecular mechanisms responsible for different diseases, and the characterization of the catalytic activity of relevant exogenous enzymes that may play a remedial effect in the treatment of such pathologies. Several enzyme-based biotherapeutics have been approved by FDA (the U.S. Food and Drug Administration) and EMA (the European Medicines Agency) and many are undergoing clinical trials. Apart from enzyme replacement therapy in human genetic diseases, which is not discussed in this review, approved enzymes for human therapy find applications in several fields, from cancer therapy to thrombolysis and the treatment, e.g., of clotting disorders, cystic fibrosis, lactose intolerance and collagen-based disorders. The majority of therapeutic enzymes are of microbial origin, the most convenient source due to fast, simple and cost-effective production and manipulation. The use of microbial recombinant enzymes has broadened prospects for human therapy but some hurdles such as high immunogenicity, protein instability, short half-life and low substrate affinity, still need to be tackled. Alternative sources of enzymes, with reduced side effects and improved activity, as well as genetic modification of the enzymes and novel delivery systems are constantly searched. Chemical modification strategies, targeted- and/or nanocarrier-mediated delivery, directed evolution and site-specific mutagenesis, fusion proteins generated by genetic manipulation are the most explored tools to reduce toxicity and improve bioavailability and cellular targeting. This review provides a description of exogenous enzymes that are presently employed for the therapeutic management of human diseases with their current FDA/EMA-approved status, along with those already experimented at the clinical level and potential promising candidates.

Phase I trial on robot assisted retinal vein cannulation with ocriplasmin infusion for central retinal vein occlusion

PURPOSE

To evaluate the safety and feasibility of robot-assisted retinal vein cannulation with Ocriplasmin infusion for central retinal vein occlusion.

METHODS

Prospective phase I trial including four patients suffering from central retinal vein occlusion (CRVO). Diagnosis was confirmed by preoperative fluo-angiography and followed by a standard three-port pars plana vitrectomy. Afterwards, a custom-built microneedle was inserted into a branch retinal vein with robotic assistance and infusion of Ocriplasmin started. Primary outcomes were the occurrence of intra-operative complications and success of cannulation. Secondary outcomes were change in visual acuity, central macular thickness (CMT) and venous filling times (VFT) during fluo-angiography two weeks after the intervention.

RESULTS

Cannulation with infusion of ocriplasmin was successful in all four eyes with a mean total infusion time of 355 ± 204 seconds (range 120-600 seconds). Best corrected visual acuity (BCVA) remained counting fingers (CF) in case 3 and 4, increased in case 1 from CF to 0.9LogMAR and decreased in case 2 from 0.4 to 1.3 LogMAR. CMT and VFT both showed a trend towards significant decrease comparing preoperative measurements with two weeks postintervention (1061 ± 541 μm versus 477 ± 376 μm, p = 0.068) and 24 ll 4 seconds versus 15 ± 1 seconds, p = 0.068, respectively). In one eye a needle tip broke and could be removed with an endoforceps. There were no other intervention-related complications.

CONCLUSION

Robot-assisted retinal vein cannulation is feasible and safe. Local intravenous infusion with Ocriplasmin led to an improved retinal circulation.

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.

Selection of mutant µplasmin for amyloid-β cleavage in vivo

One of the main culprits of Alzheimer's disease (AD) is the formation of toxic amyloid-β (Aβ) peptide polymers and the aggregation of Aβ to form plaques in the brain. We have developed techniques to purify the catalytic domain of plasmin, micro-plasmin (µPlm), which can be used for an Aβ-clearance based AD therapy. However, in serum, µPlm is irreversibly inhibited by its principal inhibitor α2-antiplasmin (α2-AP). In this study, we engineered and selected mutant forms of µPlm that are both catalytically active and insensitive to α2-AP inhibition. We identified surface residues of μPlm that might interact and bind α2-AP, and used an alanine-scanning mutagenesis method to select residues having higher activity but lower α2-AP inhibition. Then we employed saturation mutagenesis for further optimize both properties. Modeled complex structure of µPlm/α2-AP shows that F587 is a critical contact residue, which can be used as a starting position for further investigation.

Generation and characterization of tissue-type plasminogen activator transgenic rats

To address a species difference in the responsiveness to human recombinant tissue-type plasminogen activator (rt-PA) between rats and humans, tPA transgenic (Tg) rats were generated and characterized. In the rats, transcriptional regulation of tPA was designed under the control of the endogenous tPA promoter. There were no significant differences in hematological parameters between the tPA Tg and non Tg rats. Plasma tPA concentration was significantly increased and serum free PAI-1 was significantly decreased in the tPA Tg rats. Significant overexpression of tPA mRNA in five major organs was also confirmed in the tPA Tg rats. In contrast, the extent of tPA mRNA induction by pathophysiological stimuli (focal cerebral ischemia) was comparable in the two strains. Earlier increase in the plasma D-Dimer level was observed in the tPA Tg rats in a model of thromboembolism compared with the non Tg rats. On the other hand, there was no statistically significant prolongation of bleeding time in a rat model of bleeding between the two strains. rt-PA showed dose-related blood flow restoration in a rat model of thromboembolic stroke in the tPA Tg rats from a dose (1 mg/kg, i.v.) similar to clinical doses for human stroke patients. In conclusion, tPA Tg rats, in which tPA is overexpressed and endogenous fibrinolytic activity is enhanced without hemostatic abnormality, were generated. tPA Tg rats would be beneficial for the pharmacological and the toxicological evaluation of rt-PA and other various fibrinolytic enhancers.

Robot-assisted retinal vein cannulation in an in vivo porcine retinal vein occlusion model

PURPOSE

To evaluate the feasibility of robot-assisted retinal vein cannulation for retinal vein occlusion.

METHODS

Prospective experimental study performed in in vivo porcine eyes. A standard three port pars plana vitrectomy was followed by laser-induced branch retinal vein occlusion. Consequently, a retinal vein cannulation with the help of a surgical robot and a microneedle was performed. Complete success was defined as a stable intravenous position of the needle tip confirmed by blood washout for at least 3 min. Secondary outcomes were the occurrence of intra-operative complications and technical failures.

RESULTS

Cannulation was successful in 15 of 18 eyes with a complete success rate (duration of infusion of more than 3 min) of 73% after exclusion of two eyes from analysis due to failure in establishing a blood clot. There were no technical failures regarding the robotic device. The intravessel injections of ocriplasmin in two of two eyes led to a clot dissolution. In a subset of five eyes, a second cannulation attempt at the border of the optic disc resulted in a stable intravessel position and infusion during 362 (±138) seconds.

CONCLUSION

Robot-assisted retinal vein cannulation with prolonged infusion time is technically feasible. Human experiments are required to analyse the clinical benefit of this new therapy.

The evolution of recombinant thrombolytics: Current status and future directions

Cardiovascular disorders are on the rise worldwide due to alcohol abuse, obesity, hypertension, raised blood lipids, diabetes and age-related risks. The use of classical antiplatelet and anticoagulant therapies combined with surgical intervention helped to clear blood clots during the inceptive years. However, the discovery of streptokinase and urokinase ushered the way of using these enzymes as thrombolytic agents to degrade the fibrin network with an issue of systemic hemorrhage. The development of second generation plasminogen activators like anistreplase and tissue plasminogen activator partially controlled this problem. The third generation molecules, majorly t-PA variants, showed desirable properties of improved stability, safety and efficacy with enhanced fibrin specificity. Plasmin variants are produced as direct fibrinolytic agents as a futuristic approach with targeted delivery of these drugs using liposome technlogy. The novel molecules from microbial, plant and animal origin present the future of direct thrombolytics due to their safety and ease of administration.

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.

Characteristic adverse events and their incidence among patients participating in acute ischemic stroke trials

BACKGROUND AND PURPOSE

Adverse events (AE) in trial populations present a major burden to researchers and patients, yet most events are unrelated to investigational treatment. We aimed to develop a coherent list of expected AEs, whose incidence can be predicted by patient characteristics that will inform future trials and perhaps general poststroke care.

METHODS

We analyzed raw AE data from patients participating in acute ischemic stroke trials. We identified events that occurred with a lower 99% confidence bound greater than nil. Among these, we applied receiver operating characteristic principles to select the fewest types of events that together represented the greatest number of reports. Using ordinal logistic regression, we modeled the incidence of these events as a function of patient age, sex, baseline National Institutes of Health Stroke Scale, and multimorbidity status, defining P<0.05 as statistically significant.

RESULTS

We analyzed 5775 placebo-treated patients, reporting 21 217 AEs. Among 756 types of AEs, 132 accounted for 82.7%, of which 80% began within 10 days after stroke. Right hemisphere (odds ratio [OR], 1.67), increasing baseline National Institutes of Health Stroke Scale (OR, 1.11), multimorbidity status (OR, 1.09 per disease), patient age (OR, 1.01 per year), height (OR, 1.01 per centimeter), diastolic blood pressure (OR, 0.99 per mm Hg), and smoking (OR, 0.82) were independently associated with developing more AEs but together explained only 13% of the variation.

CONCLUSIONS

A list of 132 expected AEs after acute ischemic stroke may be used to simplify interpretation and reporting of complications. AEs can be modestly predicted by patient characteristics, facilitating stratification of patients by risk for poststroke complications.

Thrombolysis for acute ischaemic stroke

BACKGROUND

Most strokes are due to blockage of an artery in the brain by a blood clot. Prompt treatment with thrombolytic drugs can restore blood flow before major brain damage has occurred and improve recovery after stroke in some people. Thrombolytic drugs, however, can also cause serious bleeding in the brain, which can be fatal. One drug, recombinant tissue plasminogen activator (rt-PA), is licensed for use in selected patients within 4.5 hours of stroke in Europe and within three hours in the USA. There is an upper age limit of 80 years in some countries, and a limitation to mainly non-severe stroke in others. Forty per cent more data are available since this review was last updated in 2009.

OBJECTIVES

To determine whether, and in what circumstances, thrombolytic therapy might be an effective and safe treatment for acute ischaemic stroke.

SEARCH METHODS

We searched the Cochrane Stroke Group Trials Register (last searched November 2013), MEDLINE (1966 to November 2013) and EMBASE (1980 to November 2013). We also handsearched conference proceedings and journals, searched reference lists and contacted pharmaceutical companies and trialists.

SELECTION CRITERIA

Randomised trials of any thrombolytic agent compared with control in people with definite ischaemic stroke.

DATA COLLECTION AND ANALYSIS

Two review authors applied the inclusion criteria, extracted data and assessed trial quality. We verified the extracted data with investigators of all major trials, obtaining additional unpublished data if available.

MAIN RESULTS

We included 27 trials, involving 10,187 participants, testing urokinase, streptokinase, rt-PA, recombinant pro-urokinase or desmoteplase. Four trials used intra-arterial administration, while the rest used the intravenous route. Most data come from trials that started treatment up to six hours after stroke. About 44% of the trials (about 70% of the participants) were testing intravenous rt-PA. In earlier studies very few of the participants (0.5%) were aged over 80 years; in this update, 16% of participants are over 80 years of age due to the inclusion of IST-3 (53% of participants in this trial were aged over 80 years). Trials published more recently utilised computerised randomisation, so there are less likely to be baseline imbalances than in previous versions of the review. More than 50% of trials fulfilled criteria for high-grade concealment; there were few losses to follow-up for the main outcomes.Thrombolytic therapy, mostly administered up to six hours after ischaemic stroke, significantly reduced the proportion of participants who were dead or dependent (modified Rankin 3 to 6) at three to six months after stroke (odds ratio (OR) 0.85, 95% confidence interval (CI) 0.78 to 0.93). Thrombolytic therapy increased the risk of symptomatic intracranial haemorrhage (OR 3.75, 95% CI 3.11 to 4.51), early death (OR 1.69, 95% CI 1.44 to 1.98; 13 trials, 7458 participants) and death by three to six months after stroke (OR 1.18, 95% CI 1.06 to 1.30). Early death after thrombolysis was mostly attributable to intracranial haemorrhage. Treatment within three hours of stroke was more effective in reducing death or dependency (OR 0.66, 95% CI 0.56 to 0.79) without any increase in death (OR 0.99, 95% CI 0.82 to 1.21; 11 trials, 2187 participants). There was heterogeneity between the trials. Contemporaneous antithrombotic drugs increased the risk of death. Trials testing rt-PA showed a significant reduction in death or dependency with treatment up to six hours (OR 0.84, 95% CI 0.77 to 0.93, P = 0.0006; 8 trials, 6729 participants) with significant heterogeneity; treatment within three hours was more beneficial (OR 0.65, 95% CI 0.54 to 0.80, P < 0.0001; 6 trials, 1779 participants) without heterogeneity. Participants aged over 80 years benefited equally to those aged under 80 years, particularly if treated within three hours of stroke.

AUTHORS' CONCLUSIONS

Thrombolytic therapy given up to six hours after stroke reduces the proportion of dead or dependent people. Those treated within the first three hours derive substantially more benefit than with later treatment. This overall benefit was apparent despite an increase in symptomatic intracranial haemorrhage, deaths at seven to 10 days, and deaths at final follow-up (except for trials testing rt-PA, which had no effect on death at final follow-up). Further trials are needed to identify the latest time window, whether people with mild stroke benefit from thrombolysis, to find ways of reducing symptomatic intracranial haemorrhage and deaths, and to identify the environment in which thrombolysis may best be given in routine practice.

Thrombolytic effects of the snake venom disintegrin saxatilin determined by novel assessment methods: a FeCl3-induced thrombosis model in mice

Saxatilin, a novel disintegrin purified and cloned from the venom of the Korean snake Gloydius saxatilis, strongly inhibits activation and aggregation of platelets. Glycoprotein (GP) IIb/IIIa receptor antagonists can resolve thrombus, so saxatilin might also have thrombolytic effects. We investigated the thrombolytic effects of saxatilin in mice using a ferric chloride-induced carotid arterial thrombosis model. Thrombotic occlusion and thrombus resolution were evaluated quantitatively by measuring blood flow in the carotid artery with an ultrasonic flow meter and calculating the degree of flow restoration on a minute-by-minute basis; results were confirmed by histological examination. Saxatilin dissolved thrombi in a dose-dependent manner. Saxatilin at 5 mg/kg restored blood flow to baseline levels. As saxatilin dose increased, time to recanalization decreased. A bolus injection of 10% of a complete dose with continuous infusion of the remaining dose for 60 minutes resulted in effective recanalization without reocclusion. The thrombolytic effect of saxatilin was also demonstrated in vitro using platelet aggregometry by administering saxatilin in preformed thrombi. Bleeding complications were observed in 2 of 71 mice that received saxatilin. Fibrin/fibrinogen zymography and platelet aggregometry studies indicated that saxatilin does not have fibrinolytic activity, but exerted its action on platelets. Integrin-binding assays showed that saxatilin inhibited multiple integrins, specifically α2bβ3 (GP IIb/IIIa), α5β1, αvβ3, αvβ1, and αvβ5, which act on platelet adhesion/aggregation. Saxatilin inhibited multiple integrins by acting on platelets, and was safe and effective in resolving thrombi in mice.

Thrombolysis (different doses, routes of administration and agents) for acute ischaemic stroke

BACKGROUND

Stroke is a leading cause of death and disability world wide. Thrombolysis with recombinant tissue plasminogen activator (rt-PA) is licensed for treatment of acute ischaemic stroke in the early hours after symptom onset. It has been shown in randomised controlled trials (RCTs) and the 2009 Cochrane review of thrombolysis for acute ischaemic stroke to reduce dependency but at the increased risk of intracranial haemorrhage. Methods to reduce the risk of haemorrhage while retaining or enhancing the benefit could increase the use of thrombolytic treatment. While most available information comes from RCTs of intravenous rt-PA at 0.9 mg/kg, it is possible that other doses, drugs and other routes of administration might increase benefit and reduce the hazard.

OBJECTIVES

To assess the risks and benefits of different thrombolytic agents, doses and routes of administration for the treatment of acute ischaemic stroke.

SEARCH METHODS

We searched the Cochrane Stroke Group Trials Register (May 2012), MEDLINE (1966 to May 2012) and EMBASE (1980 to May 2012). We handsearched journals and conference proceedings, searched ongoing trials registers and contacted pharmaceutical companies and researchers.

SELECTION CRITERIA

Unconfounded randomised and quasi-randomised trials of different doses of a thrombolytic agent, or different agents, or the same agent given by different routes, in people with confirmed acute ischaemic stroke.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial eligibility and quality, and extracted the data using a structured proforma. We cross-checked and resolved discrepancies by discussion to reach consensus. We obtained translations and additional information from study authors where required.

MAIN RESULTS

We included 20 trials involving 2527 patients. Concealment of allocation was poorly described. Different doses (of tissue plasminogen activator, urokinase, desmoteplase or tenecteplase) were compared in 13 trials (N = 1433 patients). Different agents (tissue plasminogen activator versus urokinase, tissue-cultured urokinase versus conventional urokinase, tenecteplase versus tissue plasminogen activator) were compared in five trials (N = 875 patients). Five trials (N = 485) compared different routes of administration. As some trials compared different agents and different doses, some patients contributed to two analyses. There was an approximately three-fold increase in fatal intracranial haemorrhages in patients allocated to higher than to lower doses of the same thrombolytic drug (odds ratio (OR) 2.71, 95% confidence interval (CI) 1.22 to 6.04). There was no difference in the number of patients who were dead or dependent at the end of follow-up between those allocated higher or lower doses of thrombolytic drug (OR 0.86, 95% CI 0.62 to 1.19). Higher versus lower doses of desmoteplase were associated with more deaths at the end of follow-up (OR 3.21, 95% CI 1.23 to 8.39). There was no evidence of any benefit for intra-arterial over intravenous treatment.

AUTHORS' CONCLUSIONS

These limited data suggest that higher doses of thrombolytic agents may lead to higher rates of bleeding. However, the evidence is inadequate to conclude whether lower doses of thrombolytic agents are more effective than higher doses, or whether one agent is better than another, or which route of administration is the best, for acute ischaemic stroke. At present, intravenous rt-PA at 0.9mg/kg as licensed in many countries appears to represent best practice and other drugs, doses or routes of administration should only be used in randomised controlled trials.

A fibrin-specific thrombolytic nanomedicine approach to acute ischemic stroke

AIM

To develop a fibrin-specific urokinase nanomedicine thrombolytic agent.

MATERIALS & METHODS

In vitro fibrin-clot dissolution studies were utilized to develop and characterize simultaneous coupling and loading of anti-fibrin monoclonal antibody and urokinase onto perfluorocarbon nanoparticle (NP) surface. In vivo pharmacokinetics and fibrin-specific targeting of the nanolytic agent was studied in dogs.

RESULTS

Simultaneous coupling of up to 40 anti-fibrin antibodies and 400 urokinase enzymes per perfluorocarbon NP produced an effective targeted nanolytic agent with no significant surface protein-protein interference. Fibrin clot dissolution was not improved by increasing homing capacity from 10 to 40 antibodies/NP, but increasing enzymatic payload from 100 to 400/NP resulted in maximized lytic effect. Fluorescent microscopy showed that rhodamine-labeled urokinase nanoparticles densely decorated the intraluminal thrombus in canine clots in vivo analogous to the fibrin pattern, while an irrelevant-targeted agent had negligible binding.

CONCLUSION

This agent offers a vascularly constrained, simple to administer, low-dose nanomedicine approach that may present an attractive alternative for treating acute stroke victims.

Thrombolysis with plasmin: implications for stroke treatment

Plasmin is a direct-acting thrombolytic agent with a striking hemostatic safety advantage over plasminogen activators in animal models of thrombolysis and bleeding. In contradistinction to plasminogen activators, which risk bleeding at any effective thrombolytic dose, plasmin is tolerated without bleeding at several-fold higher amounts than those needed for thrombolysis. Plasmin has been safe in a current trial in patients with peripheral arterial or graft occlusion, and efforts are now directed toward therapy of stroke caused by cerebral artery occlusion. A rabbit (4 kg body weight) model of 2-hour, thrombin-induced middle cerebral artery occlusion using angiographic documentation of vascular patency and recanalization was used to perform a dose-ranging study of plasmin, delivered by catheter over a median duration of 10 minutes. Plasmin induced early recanalization in all animals (3 per group) within 10 minutes after discontinuation of 3, 2, or 1 mg of agent infusion. Control saline infusion failed to induce recanalization in 3 of 3 subjects. Plasmin rapidly induces middle cerebral artery recanalization, as determined in an angiogram-based animal model of arterial occlusion. Based on these data and other information, a phase I/IIa clinical trial of plasmin in human middle cerebral artery ischemic stroke has been initiated.

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.