The role of thrombolytic drugs in the management of myocardial infarction. Comparative clinical trials

Expression of a novel chimeric-truncated tPA in Pichia pastoris with improved biochemical properties

Thrombolytic therapy by plasminogen activators (PAs) has been a main goal in the treatment of acute myocardial infarction. Despite improved outcomes of currently available thrombolytic therapies, all these agents have different drawbacks that may result in less than optimal outcomes. In order to make tissue plasminogen activator (tPA) more potent, while being more resistant to plasminogen activator inhibitor-1 (PAI-1) and having a higher affinity to fibrin, a new chimeric-truncated form of tPA (CT tPA) was designed and expressed in Pichia pastoris. This novel variant consists of a finger domain of Desmoteplase, an epidermal growth factor (EGF) domain, a kringle 1 (K1) domain, a kringle 2 (K2) domain, in which the lysine binding site (LBS) was deleted, and a protease domain, where the four amino acids lysine 296, arginine 298, arginine 299, and arginine 304 were substituted by aspartic acid. The chimera CT tPA showed 14-fold increase in its activity in the presence of fibrin compared to the absence of fibrin. Furthermore, CT tPA showed about 10-fold more potency than commercially available full-length tPA (Actylase(®)) and provided 1.2-fold greater affinity to fibrin. A residual activity of only 68 % was observed after incubation of Actylase(®) with PAI-1, however, 91 % activity remained for CT tPA. These promising findings suggest that the novel CT tPA variant might be an acceptable PA with superior characteristics and properties.

Optimization of the Expression of Reteplase in Escherichia coli TOP10 Using Arabinose Promoter

Background:

Reteplase is a mutant version of t-PA (tissue plasminogen activator) with prolonged half-life. In the present study, E. coli Top 10 bacteria were utilized in the production of reteplase, which is the nonglycosylated active domain of t-PA. Reteplase gene was ligated into pBAD/gIII plasmid which, allows secretion of this protein in periplasmic space. It would allow the correct formation of disulfide bonds in protein structure.

Objectives:

This study aimed at expression of reteplase in optimum condition. In this study, the reteplase gene was cloned and expressed in Escherichia coli top 10 as a suitable host cell and its expression was optimized.

Materials and Methods:

The recombinant plasmid, pET15b/reteplase was digested by NcoI and BamHI restriction enzymes; while pBAD/gIIIA vector was digested by NcoI and BglII. Then the insert and vector were ligated and used for transformation of E. coli Top10 cells by heat shock method. Overnight culture of transformed bacteria was induced by L-arabinose in various concentrations (0.2, 0.02, 0.002, and 0.0002%) and at various temperatures.

Results:

The obtained recombinant plasmid was sequenced to confirm the presence and correct framing of reteplase gene regarding the expression of reteplase. Maximum production of this enzyme was obtained under the following condition: 0.0002% L-arabinose at 37°C for 2 hours incubation. The purified protein was detected on SDS-PAGE (sodium dodecyl sulfate Polyacrylamide gel electrophoresis) as a 66 kDa band. The concentration of t-PA standard was 1 unit which is equal to 12 µg/mL. The enzymatic activity of samples was measured as 0.8 units compared to the standards.

Conclusions:

Reteplase was expressed in E. coli Top 10 after activation of pBAD/gIIIA promoter region by arabinose and optimized.

A fed-batch based cultivation mode in Escherichia coli results in improved specific activity of a novel chimeric-truncated form of tissue plasminogen activator

AIMS

A novel chimeric-truncated form of tissue-type plasminogen activator (t-PA) with improved fibrin affinity and resistance to PAI was successfully produced in CHO expression system during our previous studies. Considering advantages of prokaryotic expression systems, the aim in this study was to produce the novel protein in Escherichia coli (BL21) strain and compare the protein potency in batch and fed-batch processes.

METHODS AND RESULTS

The expression cassette for the novel t-PA was prepared in pET-28a(+). The E. coli expression procedure was compared in traditional batch and newly developed fed batch, EnBase(®) Flo system. The protein was purified in soluble format, and potency results were identified using Chromolize t-PA Assay Kit. The fed-batch fermentation mode, coupled with a Ni-NTA affinity purification procedure under native condition, resulted in higher amounts of soluble protein, and about a 30% of improvement in the specific activity of the resulted recombinant protein (46.66 IU mg(-1) ) compared to traditional batch mode (35.8 IU mg(-1) ).

CONCLUSIONS

Considering the undeniable advantages of expression in the prokaryotic expression systems such as E. coli for recombinant protein production, applying alternative methods of cultivation is a promising approach. In this study, fed-batch cultivation methods showed the potential to replace miss-folded formats of protein with proper folded, soluble form with improved potency.

SIGNIFICANCE AND IMPACT OF THE STUDY

Escherichia coli expression of recombinant proteins still counts for nearly 40% of marketed biopharmaceuticals. The major drawback of this system is the lack of appropriate post-translational modifications, which may cause potency loss/decline. Therefore, applying alternative methods of cultivation as investigated here is a promising approach to overcome potency decrease problem in this protein production system.

Expression of a novel chimeric truncated t-PA in CHO cells based on in silico experiments

Tissue plasminogen activator (t-PA) is one of the fibrin-specific serine proteases that play a crucial role in the fibrinolytic system. The rapid clearance of the drug from the circulation, caused by its active uptake in the liver, has lead to complicated clinical applications. Different forms of plasminogen activators have been developed to treat thrombotic disease. Deletion of the first three domains of t-PA by gene manipulation techniques has shown a significant increase in its plasma half life. In order to compensate the disadvantage of higher bleeding risk, a novel chimeric truncated form of t-PA with 394 amino acids and more fibrin affinity compared to the truncated form was designed to be expressed in Chinese Hamster Ovarian (CHO) cells. The recombinant chimeric plasminogen activator consists of kringle 2 and serine protease (K2S) domains of t-PA, namely GHRP-SYQ-K2S. The level of expression was found to be 752 IU/ml with 566,917 IU/mg specific activity, based on amidolytic activity. The fibrin binding of this novel chimeric truncated t-PA was 86% of the full length t-PA at a fibrinogen concentration of 0.2 mg/ml. This could be a promising approach with more desirable pharmacodynamic properties compared to existing commercial forms.

Reteplase infusion in the treatment of acute lower extremity occlusions

PURPOSE

Reteplase, a recombinant tissue plasminogen activator, has been used in the treatment of acute coronary occlusion. Experience is accumulating regarding its use in the treatment of peripheral vascular disease. Increased experience with this thrombolytic agent may show that it can be an appropriate alternative agent to currently employed plasminogen activators.

MATERIALS AND METHODS

Over the course of six months, reteplase was used to treat 18 patients with peripheral arterial occlusions, confined to the lower extremities. The agent was infused via "pulse-spray" catheters. Fifteen patients were started on 0.5 U/h. Concurrent heparin therapy was used at 500 U/h in 16 of 18 patients.

RESULTS

The average total dose of reteplase infused was 13.3 units. It ranged from 6.2 to 41.5 units. The average duration of infusion was 26.9 hours with a range of 12-44 hours. Clinical success, defined as restoration of sufficient patency to reestablish limb function and resolution of pain in the symptomatic extremity was 89% (16 out of 18). Hemorrhagic complications occurred in five patients and were major in one patient (5%). No patient experienced intracranial hemorrhage.

CONCLUSION

Increased experience with the use of reteplase in the treatment of acute lower extremity occlusions is accumulating. The safety and efficacy profile in our group appears satisfactory and compares to other studies. However, definite conclusions must await the results of controlled comparisons of different reteplase infusion regimens and controlled comparison to other thrombolytic agents.

Comparison of double bolus urokinase versus front-loaded alteplase regimen for acute myocardial infarction. Thrombolysis in Myocardial Infarction in Korea (TIMIKO) study group

This study was performed to compare the double bolus urokinase regimen with the front-loaded alteplase regimen for acute myocardial infarction. Double bolus urokinase is an easy, safe, and effective thrombolytic regimen with comparable results to standard front-loaded alteplase in acute myocardial infarction.

Can we improve on front-loaded alteplase (r-TPA)?

The method of administration of alteplase has evolved since its introduction to clinical practice in the late 1980s. The initial dosage regimen of a graded administration of 100 mg was replaced by the front-loaded weight adjusted regimen, the efficacy of which was demonstrated in the GUSTO 1 trial. Double bolus administration was shown to achieve superior TIMI 3 patency of the infarct related artery in a small angiographic study, but the COBALT trial failed to show equivalence and indeed showed a slightly higher mortality and incidence of stroke, so cannot be recommended. Reteplase, a deletion mutant of alteplase, also showed superior efficacy in achieving coronary patency but no clinical superiority in outcomes in the 15,000 patient GUSTO 3 trial. The case of administration of reteplase, however, has some attraction as an alternative to alteplase. Trials of newer agents based on further modifications of alteplase are ongoing, but at present the front-loaded alteplase regimen remains the standard for clinical practice.

Fibrinogen breakdown, long-lasting systemic fibrinolysis, and procoagulant activation during alteplase double-bolus regimen in acute myocardial infarction

Recent clinical studies comparing accelerated versus bolus administration of alteplase tissue plasminogen activator (t-PA) suggest similar thrombolytic efficacy, but reveal higher bleeding complications among older patients during the double-bolus regimen. The objective of the present study was to characterize the hemostatic profile of t-PA administered as double-bolus doses of 50 mg, at intervals of 30 minutes. Among 50 patients with acute myocardial infarction treated by double-bolus t-PA thrombolysis, coagulation and fibrinolysis parameters, as well as t-PA levels, were monitored. Monitored t-PA levels peaked at 5 and 35 minutes and were detectable within the therapeutic range even after 90 minutes. Marked systemic fibrinolytic activation was indicated by 75% depletion of both plasminogen and fibrinogen, as well as by 19-fold and 300-fold increases of fibrin degradation and fibrinogen degradation products. Plasminogen-activator inhibitor activity was completely suppressed. Pronounced procoagulant activation was reflected by a 3.4-fold increase of both factor XIIa and prothrombin fragment 1+2, and by a threefold increase of thrombin-antithrombin complex. Independent of t-PA weight dosage, fibrinolytic activation was more pronounced among older patients (> or = 63 years). We conclude that t-PA after bolus administration has a long half-life. Double-bolus regimen leads to a long-lasting systemic fibrinolytic state, which is even more remarkable among older patients--a fact that may explain the higher bleeding complications reported for this age group.

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

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