Comparative properties of two clinical preparations of recombinant human tissue-type plasminogen activator in patients with acute myocardial infarction

Comparison of pharmacokinetic properties of alteplase and tenecteplase. The future of thrombolysis in acute ischemic stroke

INTRODUCTION

The drug most frequently used for thrombolysis in cases of acute ischemic stroke (AIS) is alteplase. However, there is moderate-to-high-quality evidence that tenecteplase has similar or higher efficacy and safety. With improved pharmacokinetic properties over alteplase, tenecteplase could be a significant advantage in treating AIS.

AREAS COVERED

After conducting an extensive search on Scopus and PubMed, this manuscript reviews and compares the pharmacokinetic properties of alteplase and tenecteplase. Additionally, it provides information on pharmacodynamics, clinical efficacy, safety, tolerability, and drug-drug interactions.

EXPERT OPINION

The pharmacokinetic profile of alteplase and tenecteplase is derived from studies in patients with acute myocardial infarction. Thanks to its pharmacokinetic properties, tenecteplase is the drug closest to being the ideal fibrinolytic for AIS. Its longer half-life enables a single-bolus administration, which is particularly useful in emergencies. Tenecteplase has proven to have a good efficacy and safety profile in randomized clinical trials. Although we are awaiting the results of the ongoing phase 3 randomized clinical trials, we believe that tenecteplase has the potential to revolutionize the treatment of AIS through thrombolysis.

Early administration of therapeutic anticoagulation following intravenous thrombolysis for acute cardiogenic embolic stroke caused by left ventricular thrombus: case report and topic review

Cardiogenic cerebral embolism represents 20% of all acute ischemic strokes (AISs) with one-third of these being caused by left ventricular thrombus (LVT). LVT is not a contraindication for treatment with intravenous recombinant tissue plasminogen activator (IV rtPA) for AIS. However, the subsequent treatment of a potentially unstable LVT is contraindicated for 24 h following the use of IV rtPA according to current guidelines. We present a 66-year-old man with AIS treated with IV rtPA. Echocardiogram shortly after treatment demonstrated both a large apical and septal thrombus in the left ventricle and at 12 h post IV rtPA infusion, therapeutic anticoagulation with heparin was started without complication. In practice, the action of IV rtPA outlasts its apparent half-life because of thrombin-binding and the prolonged effects and longer half-life of its product, plasmin; however, the pharmacokinetics do not warrant prolonged avoidance of therapeutic anticoagulation when clinically indicated. Our case demonstrates that anticoagulation for potentially unstable LVT can be safely initiated at 12 h following IV rtPA treatment for AIS.

Thrombolytic therapy for ischemic stroke--a review. Part I--Intravenous thrombolysis

OBJECTIVE

Thrombolytic therapy for acute ischemic stroke was implemented into clinical routine 4 yrs ago. Unfortunately, at present <2% of eligible patients receive thrombolytic therapy. We present an overview of all hitherto completed trials of intravenous thrombolytic therapy for carotid artery stroke including recommendations for therapy and diagnostic procedures and their impact on patient selection and meta-analyses.

DATA SOURCES

We performed an extensive literature search not only to identify the larger and well-known randomized trials but also to identify smaller pilot studies and case series. Trials included in this review, among others, are the National Institute of Neurologic Disorders and Stroke (NINDS) study, European Cooperative Acute Stroke Study I and II, and Alteplase Thrombolysis for Acute Noninterventional Therapy in Ischemic Stroke (ATLANTIS) A and B and two large meta-analyses, including the Cochrane Library report.

CONCLUSION

Intravenous thrombolytic therapy with recombinant tissue plasminogen activator has demonstrated a significant benefit and has proven to be safe for patients who can be treated within 3-6 hrs after symptom onset. This benefit is at the cost of an increased rate of symptomatic intracranial hemorrhage without a significant effect on overall mortality. In general, the benefit of thrombolysis decreases and the risks increase with progressing time after symptom onset. Presently, thrombolytic therapy is still underutilized because of problems with clinical and time criteria, and lack of public and professional education to regard stroke as a treatable emergency. If applied more widely, thrombolytic therapy may result in profound cost savings in health care and reduction of long-term disability of stroke patients.

Achieving Optimal Reperfusion without Adjunctive Antithrombotic Therapy: Novel Thrombolytic Dosing Strategies

There is firm evidence that reperfusion therapy, to be effective must establish and maintain coronary arterial blood flow at a level sufficient to allow myocardial perfusion. However, current thrombolytic regimens have clear limitations, including a relatively low capacity to achieve TIMI Grade 3 blood flow and an unacceptable incidence of coronary reocclusion. Although it has been assumed that the key to achieving optimal reperfusion lies with adjunctive antithrombotic therapy, it may be that novel thrombolytics and dosing strategies can address the problem adequately. This possibility is attractive and requires careful consideration.

[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.

Human platelet activation by thrombolytic agents: effects of tissue-type plasminogen activator and urokinase on platelet surface P-selectin expression

The mechanisms that underlie reocclusion during thrombolytic therapy have not yet been clarified. The purpose of this study was to investigate the activating effects of tissue-type plasminogen activator and urokinase and the inhibitory effects of acetylsalicylic acid by measuring platelet surface P-selectin as a marker of platelet activation. After addition of urokinase (final concentration 192 U/ml, 1920 U/ml, or 19,200 U/ml) or tissue-type plasminogen activator (final concentration 120 U/ml, 1200 U/ml, or 12,000 U/ml) to platelet-rich plasma from 12 healthy persons, platelet surface P-selectin expression was measured by means of flow cytometry with an anti-CD62 monoclonal antibody. The presence of urokinase and tissue-type plasminogen activator increased platelet surface P-selectin expression in a concentration-dependent manner. In the next step, either 160 mg/day (n = 6) or 660 mg/day (n = 6) acetylsalicylic acid was administered to the 12 healthy persons, and venous blood samples were collected after 7 days of treatment. Platelet surface P-selectin expression was measured with the method used earlier and after addition of tissue-type plasminogen activator or urokinase. Although the effect of acetylsalicylic acid at 160 mg/day on P-selectin expression was minimal, a dose of 660 mg/day suppressed platelet P-selectin expression and inhibited the platelet activating effects of tissue-type plasminogen activator and urokinase in a statistically significant way. Platelets were activated by tissue-type plasminogen activator or urokinase, and this platelet activation was suppressed with administration of acetylsalicylic acid at 660 mg/day.

Effects of ramipril on plasma fibrinolytic balance in patients with acute anterior myocardial infarction. HEART Study Investigators

BACKGROUND

The long-term administration of ACE inhibitors to selected patients with left ventricular dysfunction appears to reduce the incidence of recurrent myocardial infarction (MI) and unstable angina pectoris. The mechanisms responsible for the reduction in ischemic events are unknown, but likely candidates include effects on the atherosclerotic process, thrombosis, and/or vascular tone.

METHODS AND RESULTS

The effects of ACE inhibitor therapy with ramipril on plasma fibrinolytic variables were assessed in 120 subjects participating in the Healing and Early Afterload Reduction Therapy (HEART) study, a double-blind, placebo-controlled trial of acute anterior MI patients who were randomly assigned within 24 hours of the onset of symptoms to receive low-dose ramipril (0.625 mg daily), full-dose ramipril (1.25 mg titrated to 10 mg/d), or placebo for 14 days. Plasma levels of plasminogen activator inhibitor-1 (PAI-1) activity and PAI-1 antigen and tissue plasminogen activator (TPA) antigen were measured before randomization and on day 14. Clinical characteristics of the three study groups were similar, as were the prerandomization plasma levels of PAI-1 antigen, PAI-1 activity, and TPA antigen. Compared with the placebo group, PAI-1 antigen levels were 44% lower (P=.004) at day 14 in the ramipril-treated patients, and PAI-1 activity levels were 22% lower (P=.02). In contrast, plasma TPA levels were not significantly different between the placebo-treated and ramipril-treated groups.

CONCLUSIONS

Treatment with ramipril has a significant impact on plasma fibrinolytic variables during the recovery phase after acute MI. The renin-angiotensin system appears to play an important role in the regulation of vascular fibrinolysis, and interruption of this regulatory pathway may contribute to the clinical benefits of ACE inhibitors.

Comparative studies of thrombolysis with single-chain and two-chain recombinant tissue-type plasminogen activators in canine coronary thrombosis

Tissue-type plasminogen activator (t-PA) can occur in two different forms, single- and two-chain t-PA. Such a difference in structure of the t-PA molecule may induce different biological functions. We compared the thrombolytic efficacy of single-chain t-PA with that of two-chain t-PA and urokinase in a canine thrombosis model. The thrombolytic properties of single- and two-chain recombinant t-PA and urokinase with equivalent activity in vitro after intravenous infusion were examined in 24 anesthetized, open-chest dogs with intracoronary thrombi. Repeated coronary angiography showed that the time to reperfusion was 29.0 +/- 4.8, 32.5 +/- 6.3, and 39.2 +/- 6.8 min, respectively. One dog in the single-chain t-PA group and all dogs in the vehicle group showed no recanalization. Plasma t-PA antigen reached similar plateau levels within 15. min in both the single- and two-chain t-PA groups. The infusion of single- and two-chain t-PA was not associated with systemic fibrinolytic activation, except for a decrease in alpha 2-plasmin inhibitor in single-chain t-PA. However, the infusion of urokinase was associated with significant depletion of fibrinogen. These findings suggest that single-chain t-PA has potent thrombolytic effect without systemic fibrinolytic activity similar to that of two-chain t-PA in a canine coronary artery thrombosis model.

alpha-Fucose-mediated binding and degradation of tissue-type plasminogen activator by HepG2 cells

The glycoprotein tissue-type plasminogen activator (t-PA) is subject to hepatic clearance in humans. Here, the interaction of t-PA with a well-differentiated hepatoma cell line (HepG2) was examined. Suspended HepG2 cells bound 125I-t-PA in a specific, saturable, and reversible fashion through a Ca(2+)-dependent, active site-independent mechanism. Binding isotherms indicated a high affinity system with a single class of saturable binding sites (Kd 39 nM; maximum binding capacity 493,000 sites per cell). Bound t-PA was rapidly degraded at 37 degrees C in a manner inhibited by lysosomotropic agents or metabolic inhibitors. Pretreatment of t-PA with monoclonal antibodies against the EGF/fibronectin finger domain, but not kringle 2 or kringle 1, reduced total binding by 86%. Binding of 125I-t-PA to HepG2 cells was inhibited by monosaccharides fucose and galactose and by the neoglycoprotein fucosyl-albumin. Enzymatic removal of alpha-fucose residues, but not alpha-galactose, high mannose, or complex oligosaccharide from 125I-t-PA, reduced specific binding by 60 +/- 5%. Binding was also inhibited by high, but not low, molecular weight urokinase, which contains an EGF-based threonine-linked alpha-fucose homologous to that of t-PA. These data suggest that EGF-associated O-linked alpha-fucose may mediate t-PA binding and degradation by HepG2 cells. This mechanism may be relevant to other proteins with analogous structures.

Acceleration of recombinant tissue-type plasminogen activator-induced reperfusion and prevention of reocclusion by recombinant antistasin, a selective factor Xa inhibitor, in a canine model of femoral arterial thrombosis

Antistasin is a 119-amino acid protein initially isolated from salivary glands of the Mexican leech, Haementeria officinalis, that exhibits potent anticoagulant properties resulting from selective inhibition of blood coagulation factor Xa. The comparative antithrombotic efficacies of recombinant antistasin (rATS), standard heparin (Hep), and aspirin (ASA) administered adjunctly with recombinant tissue-type plasminogen activator (tPA) on thrombolytic reperfusion and reocclusion were determined in a canine model of femoral arterial thrombosis. An occlusive thrombus was formed by insertion of a thrombogenic copper coil into the femoral artery, and blood flow velocity was monitored directly and continuously by Doppler flowmetry. Sixty minutes after occlusion, dogs received an intravenous infusion of either saline (vehicle) or rATS (0.31, 1.25, or 2.5 micrograms/kg/min), intravenous boluses of Hep (100 units/kg + 50 units/kg/hr or 200 units/kg + 150 units/kg/hr), or a single intravenous bolus of ASA (2.0 mg/kg), followed 45 minutes later by tPA (0.8 mg/kg i.v. over 90 minutes). The saline and rATS infusions were discontinued 60 minutes after termination of tPA, and the last Hep boluses were given 105 minutes after termination of tPA. All dogs achieved reperfusion. The time to reperfusion in the ASA group was similar to that in the vehicle group (50 +/- 9 versus 50 +/- 6 minutes, respectively). Reperfusion times were slightly decreased by the low and high doses of Hep (34 +/- 6 and 31 +/- 4 minutes, respectively) and the rATS doses of 0.31 and 1.25 micrograms/kg/min (37 +/- 4 and 36 +/- 5 minutes, respectively). However, the time to reperfusion was dramatically reduced with the 2.5 micrograms/kg/min rATS dose (15 +/- 3 minutes, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of tissue-type plasminogen activator during acute myocardial infarction in men. Effect of a prostacyclin analogue

BACKGROUND

Coronary reocclusion complicates the thrombolytic therapy of acute myocardial infarction despite the routine use of aspirin. This is consistent with experimental studies demonstrating that multiple agonists, in addition to thromboxane A2, mediate the platelet activation underlying reocclusion. Consequently, a more potent antiplatelet therapy with a broader spectrum of activity than aspirin may be required in this setting. Prostacyclin and its more stable analogue, iloprost, inhibit platelet aggregation to all known agonists and exert an additional effect over aspirin alone. Experiments in animal models have demonstrated, however, that iloprost increases the clearance of tissue-type plasminogen activator (t-PA) and impairs thrombolysis in vivo. This study examines whether a similar interaction occurs in humans.

METHODS AND RESULTS

Twelve patients with acute myocardial infarction received t-PA intravenously, 60 mg in the first hour and a maintenance infusion of 13.3 mg/hr for 3 hours. Patients were assigned in a double-blind fashion to iloprost (2 ng/kg/min) or placebo following the initial 90 minutes of the maintenance infusion of t-PA. Iloprost decreased mean arterial blood pressure (-10 +/- 2.9 mm Hg, p less than 0.05) but did not alter heart rate. Steady-state plasma iloprost concentration was 591 +/- 64 pmol/l. At this concentration, iloprost markedly inhibited platelet aggregation in vitro, particularly in the presence of aspirin. Steady-state clearance of t-PA was unchanged by iloprost (454 +/- 65 versus 443 +/- 136 ml/min in controls, p = NS). Furthermore, neither elimination kinetics nor plasma protein binding of t-PA was altered by iloprost.

CONCLUSIONS

At plasma levels that exert a potent antiplatelet effect, iloprost did not alter the pharmacokinetics of t-PA in men. Prostacyclin analogues may prove useful as an adjunct to plasminogen activators, particularly in patients at high risk for thrombotic reocclusion.

Evaluation of thrombolytic and systemic effects of the novel recombinant plasminogen activator BM 06.022 compared with alteplase, anistreplase, streptokinase and urokinase in a canine model of coronary artery thrombosis

The thrombolytic and systemic effects of BM 06.022 were evaluated and compared with those of alteplase, anistreplase, streptokinase and urokinase in a canine model of coronary artery thrombosis. BM 06.022 consists of the kringle-2 and protease domains of human tissue plasminogen activator (t-PA) and is unglycosylated because of its expression in Escherichia coli cells. Thrombus formation in anesthetized open chest dogs was induced by electrical injury to the intimal surface of the left circumflex coronary artery at a high level site of obstruction. In heparinized dogs, none of six vehicle-treated animals exhibited reperfusion. Reperfusion was achieved in four of six dogs at 18.3 +/- 6 min after intravenous bolus injection of 140 kU/kg (0.24 mg/kg) of BM 06.022, whereas four of six dogs exhibited reperfusion later (p less than 0.05) at 76.5 +/- 16.1 min during infusion of 1.33 mg/kg of alteplase (0.13 mg/kg as initial bolus injection, followed by 0.66 mg/kg over 1 h and 0.53 mg/kg over 2 h). Significantly later (p less than 0.05) reperfusion than that achieved with BM 06.022 was achieved in five of six dogs at 57.8 +/- 12.1 min after intravenous injection of 0.4 U/kg of anistreplase. Streptokinase (21,000 IU/kg over 60 min) and urokinase (20,000 IU/kg as an intravenous bolus injection, followed by 20,000 IU/kg over 89 min) each induced reperfusion in three of six dogs but at 67 +/- 12 and 84.3 +/- 17.1 min (p less than 0.05 vs. BM 06.022), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

A pilot study of the efficacy and safety of bolus administration of alteplase in acute myocardial infarction

OBJECTIVE

To examine the efficacy, safety, and the pharmacokinetic profile of a bolus dose administration regimen of alteplase in the treatment of acute myocardial infarction.

DESIGN

An open pilot study.

SETTING

District general hospital.

PATIENTS

33 suitable consecutive patients presenting within six hours of the onset of symptoms who satisfied the electrocardiographic criteria for acute myocardial infarction.

INTERVENTIONS

Two intravenous boluses of 35 mg alteplase, 30 minutes apart.

MAIN OUTCOME MEASURES

Angiographic coronary patency at 90 minutes and 24 hours. Plasma alteplase concentration-time profile and pharmacokinetic analysis.

RESULTS

Coronary patency at 90 minutes: 26 of 30 arteries (87%, 95% confidence interval (CI) 74-99%). Coronary patency at 24 hours: 24 of 29 arteries (83%, CI 69-97%). Mean (SD) plasma tissue plasminogen activator (t-PA) concentration reached 4434.8 (2117.8) and 4233.3 (2217.5) ng/ml within 10 minutes of each bolus and fell to 425.8 (288.3) ng/ml between boluses. The estimated peak concentrations at two minutes after boluses were 12,389 (8580) ng/ml and 10,811 (6802) ng/ml. The derived pharmacokinetic variables were volume of distribution 3.11 (1.89) 1, clearance 21.3 (9.3) 1/h, half life 5.9 (1.7) minutes.

CONCLUSIONS

This simple administration regimen achieved brief, high concentrations of plasma t-PA that were well tolerated. The regimen was associated with a high coronary patency rate at 90 minutes that was well maintained at 24 hours.

Induction of sustained patency after clot-selective coronary thrombolysis with Hybrid-B, a genetically engineered plasminogen activator with a prolonged biological half-life

BACKGROUND

Despite the utility of tissue-type plasminogen activator (t-PA) in eliciting coronary thrombolysis clinically, early reocclusion remains a problem, occurring despite anticoagulation in 5-30% of patients with initially successful recanalization. This study evaluated the utility of Hybrid-B, a molecular variant of t-PA with a prolonged half-life in the circulation, in eliciting coronary thrombolysis and maintaining patency in the presence of a continuing thrombogenic stimulus.

METHODS AND RESULTS

In intact, anesthetized dogs, either 18 mg Hybrid-B over 30 minutes (n = 15) or 50 mg t-PA (Activase) over 60 minutes (n = 8) was administered starting 60 minutes after left anterior descending coronary artery occlusion was induced with a thrombogenic copper coil. Time to lysis averaged 54 +/- 26 (means +/- SD) minutes and 64 +/- 34 minutes with Hybrid-B and t-PA, respectively (p = NS). When Hybrid-B was administered as a bolus (20 mg over 1 minute) to induce a high initial concentration in blood, time to lysis was shortened markedly and averaged 15 +/- 5 minutes. Dogs given Hybrid-B by either infusion or bolus exhibited prolonged time to reocclusion (337 +/- 192 minutes compared with 192 +/- 125 minutes in dogs given t-PA, p less than 0.03), reflecting maintenance of a subthrombolytic but persistently active concentration of activator in blood. Despite the persistence of Hybrid-B in blood, concentrations of fibrinogen and alpha 2-antiplasmin were not depleted markedly and remained at 77 +/- 25 and 56 +/- 24%, respectively, of control values.

CONCLUSIONS

Thus, Hybrid-B, a novel variant of t-PA with unique pharmacokinetic properties, elicits prompt, sustained, and clot-selective coronary thrombolysis.

Soluble fibrin degradation products potentiate tissue plasminogen activator-induced fibrinogen proteolysis

Despite its affinity for fibrin, tissue plasminogen activator (t-PA) administration causes systemic fibrinogenolysis. To investigate the mechanism, t-PA was incubated with plasma in the presence or absence of a fibrin clot, and the extent of fibrinogenolysis was determined by measuring B beta 1-42. In the presence of fibrin, there is a 21-fold increase in B beta 1-42 levels. The potentiation of fibrinogenolysis in the presence of fibrin is mediated by soluble fibrin degradation products because (a) the extent of t-PA induced fibrinogenolysis and clot lysis are directly related, (b) once clot lysis has been initiated, fibrinogenolysis continues even after the clot is removed, and (c) lysates of cross-linked fibrin clots potentiate t-PA-mediated fibrinogenolysis. Fibrin degradation products stimulate fibrinogenolysis by binding t-PA and plasminogen because approximately 70% of the labeled material in the clot lysates binds to both t-PA- and plasminogen-Sepharose, and only the bound fractions have potentiating activity. The binding site for t-PA and plasminogen is on the E domain because characterization of the potentiating fragments using gel filtration followed by PAGE and immunoblotting indicates that the major species is (DD)E complex, whereas minor components include high-molecular weight derivatives containing the (DD)E complex and fragment E. In contrast, D-dimer is the predominant species found in the fractions that do not bind to the adsorbants, and it has no potentiating activity. Thus, soluble products of t-PA-induced lysis of cross-linked fibrin potentiate t-PA-mediated fibrinogenolysis by providing a surface for t-PA and plasminogen binding thereby promoting plasmin generation. The occurrence of this phenomenon after therapeutic thrombolysis may explain the limited clot selectivity of t-PA.

New horizons in the treatment of coronary artery thrombosis

The application of recombinant DNA methodology to clinical medicine offers the clinician a new generation of more potent and specific therapies. Recombinant methods offer great promise in the treatment of coronary artery thrombosis. This review focuses on the characterization of 1) molecules that activate plasminogen locally (in the vicinity of a thrombus) rather than systemically, and 2) molecules that offer new approaches to the inhibition of platelet activation and thrombin activity. We first describe the methods used to uncover these molecules and their characterization at the molecular level. The ways in which this knowledge can lead to the development of agents tailored to clinical needs are then explored.

Prediction of coronary artery reperfusion after tissue plasminogen activator infusion

We investigated whether clinical and laboratory variables can predict perfusion status after t-PA administration, by using the data from 138 patients who received t-PA during the Thrombolysis in Myocardial Infarction (TIMI) I study. All clinical and laboratory variables that were collected at baseline or during perfusion for TIMI I were evaluated by the current study. Via stepwise discriminant analysis, 7 variables were closely associated with perfusion status at 90 minutes (listed in the order of their discriminant effect): baseline grade of stenosis in the infarct-related coronary artery, whether nausea was present during the infusion, baseline aspartate aminotransferase (SGOT) concentration, whether arrhythmias were present during the infusion, baseline fibrinogen concentration, baseline partial thromboplastin time, and baseline diastolic blood pressure. Baseline severity of stenosis and the likelihood of there being reperfusion were inversely related. Eighty-four percent of patients with adequate perfusion after 90 minutes of t-PA infusion were classified correctly, but only 50% of those without perfusion at 90 minutes were classified correctly. In addition, since 70% of the TIMI I patients, on average, did achieve perfusion, the use of these 7 variables added little predictive information. Our findings suggest that 1) there is as yet no practical way to predict reperfusion after t-PA therapy and 2) the severity of coronary stenoses, if known ahead of time, should be considered when selecting patients for thrombolytic therapy.

An overview of thrombolytic agents

The use of thrombolytic therapy has increased considerably in the past five years, particularly in patients with acute myocardial infarction. The agents that have been used in humans thus far include streptokinase and urokinase, as well as tissue-type plasminogen activator and, most recently, single-chain urokinase-type plasminogen activator or pro-urokinase. Each of these agents works by very different mechanisms to activate plasminogen and, as a result, to lyse fibrin clots. This article reviews the mechanisms by which pathophysiologic thrombi develop, the pharmacologic agents available to lyse thrombi, and the mechanisms of action of these agents.

Tissue-type plasminogen activator: intracoronary applications

Intracoronary tissue-type plasminogen activator (t-PA) was employed successfully before, after, or in place of coronary artery angioplasty in four patients referred for emergency cardiac catheterization during evolving myocardial infarction. The potential roles of intracoronary thrombolysis, dose considerations for intracoronary t-PA, factors influencing the choice of plasminogen activator, and safety issues are discussed.

Correlation between template bleeding times and spontaneous bleeding during treatment of acute myocardial infarction with recombinant tissue-type plasminogen activator

The purpose of this study was to correlate bleeding complications during and after treatment with recombinant tissue-type plasminogen activator (rt-PA) with serial template bleeding time measurements, with ADP-induced platelet aggregation, with clinical characteristics, and with hemostatic parameters. Fifty-two of 55 consecutive patients with acute myocardial infarction and template bleeding times (Ivy method) of less than 9.5 minutes were treated with rt-PA in a total dose of 55-212 mg (mean, 109 mg) over 90 to 360 minutes (median, 240 minutes) combined with heparin. The mean bleeding time was significantly prolonged at 90 minutes (from 5.0 +/- 1.9 to 8.2 +/- 4.3 minutes, p less than 0.0001) but returned toward baseline after 4 hours (from a median of 8.0 to 7.0 minutes, p less than 0.05). Thirteen patients (25%) suffered relatively minor but spontaneous bleeding that did not correlate with age, hypertension, smoking, partial thromboplastin time, platelet count, ADP-induced platelet aggregation, steady-state rt-PA level, or extent of fibrinogen degradation. In multivariate analysis, only the 90-minute bleeding time correlated with spontaneous bleeding (p = 0.01). Prolongation of the 90-minute bleeding time to greater than or equal to 9 minutes, which occurred in 21 patients, correlated with spontaneous bleeding with a sensitivity of 69% (95% confidence interval, 39-90%) and a specificity of 69% (95% confidence interval, 52-83%). Retrospective analysis revealed that in 14 patients taking aspirin, the bleeding time at 90 minutes was significantly more prolonged (p less than 0.05) and spontaneous bleeding significantly more frequent (p less than 0.01) than in patients not taking aspirin.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-bolus injection of recombinant tissue-type plasminogen activator in acute myocardial infarction

Recombinant tissue-type plasminogen activator (rt-PA) has hitherto been administered in acute myocardial infarction as an intravenous infusion with an initial bolus of about 10% of the total dose, both due to its short half-life and to avoid possible early reocclusions. A single-bolus dose would simplify the therapeutic regimen. Therefore, 20 patients with symptom duration of 125 +/- 58 minutes were given a single bolus of 50 mg of rt-PA over 2 minutes. Coronary angiography 60 minutes after the rt-PA bolus revealed a patent infarct-related artery in 15 of 20 patients (patency rate 75%, 95% confidence limits 51 to 91%). In the remaining patients, reperfusion was achieved by coronary angioplasty and intracoronary fibrinolysis; in 2 patients coronary artery bypass grafting was necessary. Control angiograms at 24 hours showed reocclusions in 4 of 18 patients. One woman died due to an intracranial hemorrhage 48 hours after the rt-PA bolus injection. Circulating fibrinogen decreased from 2.7 +/- 0.5 to 1.5 +/- 0.9 g/liter after 2 to 4 hours and reached the initial value within 24 hours. Pharmacokinetic parameters were obtained in 7 patients by measuring rt-PA antigen levels in multiple plasma samples. Mean peak rt-PA concentration was 9.8 +/- 3.6 micrograms/ml, total plasma clearance 476 +/- 148 ml/min and dominant half-life 4.8 +/- 1.0 minutes. Thus, rt-PA administered as a 50-mg single bolus appears to provide similar patency rates and shows similar kinetics in comparison with the conventional infusion regimen. Assessment of the incidence of bleeding complications requires further studies.

A canine model of coronary artery thrombosis with superimposed high grade stenosis for the investigation of rethrombosis after thrombolysis

A canine model was developed to investigate coronary artery thrombolysis and reocclusion in the setting of endothelial cell damage and fixed stenosis, which simulate anatomic features occurring in patients with acute myocardial infarction. In open chest dogs, endothelial cell damage was produced in the left anterior descending coronary artery by external compression with blunt forceps, greater than 90% stenosis was obtained by an external constrictor and thrombosis was induced by instillation of thrombin and fresh blood in an isolated arterial segment. In the absence of stenosis, intravenous infusion of 750,000 U of streptokinase over 1 h caused reperfusion in five of six dogs in 34 +/- 25 min (mean +/- SD). Urokinase, 600,000 U intravenously over 30 min followed by 600,000 U over 30 min by the intracoronary route, induced reperfusion in three of four dogs in 65 +/- 23 min. Recombinant two chain tissue-type plasminogen activator (rt-PA) (G11021), infused intravenously at a rate of 15 micrograms/kg per min for 30 min or until reflow, induced reperfusion in all 12 dogs in 28 +/- 13 min. In the absence of coronary artery stenosis, spontaneous reocclusion did not occur within 2 h after the end of the infusion. In the presence of the coronary artery constrictor, which reduced the blood flow to 40 +/- 10% of baseline, streptokinase, urokinase and rt-PA caused coronary thrombolysis to proceed at comparable or only slightly slower rates. Cyclical reocclusion during or after the end of infusion of these thrombolytic agents, caused by platelet-rich thrombus, was almost invariably observed, generally within 30 min after the onset of reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of thrombolytic therapy in surgical practice

The ability of streptokinase and urokinase to lyse intravascular fibrin-based clots is firmly established. However, there is a lack of enthusiasm for these agents because of serious haemorrhagic complications and a lack of controlled randomized studies indicating their efficacy. Thrombolytic therapy is suitable in only 15 per cent of patients with acute deep venous thrombosis. It restores the venous circulation to normal in up to 95 per cent of these patients if therapy is instituted within 5 days of the onset of symptoms. These patients have significantly fewer symptoms on follow-up than patients treated with heparin although the ability of thrombolytic therapy to preserve venous valvular function and to prevent the post-phlebitic syndrome is now in question. Thrombolytic therapy is as effective as heparin in preventing pulmonary embolism and may be superior in its treatment. Pulmonary haemodynamics are rapidly improved, diffusion capacity is restored and, although the evidence is inconclusive, long-term pulmonary hypertension may be prevented. Although the mortality rate is not decreased, controlled studies show that thrombolytic therapy may be beneficial in massive pulmonary embolism with clinical shock. Thrombolytic therapy is indicated for acute arterial and acute bypass graft occlusion when the surgical alternative is associated with a higher morbidity and mortality. Partial thrombolysis is achieved in up to 90 per cent of cases and the need for further therapeutic intervention is eliminated in one-third of the patients treated. New thrombolytic agents with greater specificity and potentially greater efficacy and fewer complications are being developed. Tissue plasminogen activator has been successfully used. Prourokinase, fibrin-seeking urokinase and acetylated streptokinase-plasminogen complex may expand the role of thrombolytic therapy in surgical practice.

Thrombolytic therapy in acute myocardial infarction: Part II--rt-PA

Thrombolysis with pharmacologic agents is a valuable modality for treatment of acute myocardial infarction. The results of several clinical studies indicate that early recanalization can be elicited with intravenous agents and that it is associated with substantial reductions of infarct size, improvement of ventricular function, and reduction in mortality. The recently introduced fibrin-selective agent, rt-PA, appears to represent a significant pharmacologic advance. Its use intravenously elicits high recanalization rates without marked derangements of the coagulation system, reflecting its relative fibrin selectivity. The efficacy of thrombolysis with any drug given by any route, unfortunately, is not 100 per cent. Bleeding remains an important risk. The optimal approach to management of residual stenosis after thrombolysis has not yet been delineated. Currently available information appears to justify the following conclusions: 1. Transmural myocardial infarction usually is caused by an acute obstructing coronary thrombus superimposed on a chronic atherosclerotic lesion. Myocardial necrosis following interruption of blood flow generally is complete within several hours. 2. The thrombus can be lysed and blood flow can be restored with intravenous agents that activate plasminogen. Intravenous rt-PA, a relatively fibrin-specific agent, elicits recanalization in 70 to 75 per cent of infarct-related arteries. 3. Recombinant t-PA evokes only modest depletion of fibrinogen (16 to 36 per cent reduction from baseline). 4. Early reperfusion preserves myocardium and ventricular function and reduces mortality. 5. The extent of benefit after pharmacologic reperfusion is correlated strongly with the brevity of myocardial ischemia prior to initiation of therapy. The greatest benefit is realized in patients treated within the first few hours of onset of acute myocardial infarction. 6. The incidence and optimal means of prevention of reocclusion and reinfarction following successful pharmacologic reperfusion are not yet entirely clear. Mechanical recanalization with PTCA in conjunction with thrombolysis is promising, but its routine immediate use on an emergency basis does not appear to be beneficial. Vigorous educational efforts are needed to heighten the awareness of prospective patients and all members of the health care team to the value of prompt diagnosis of incipient or evolving infarction so that prompt implementation of thrombolysis in appropriate candidates can be facilitated.

Plasminogen Activator Italian Multicenter Study (PAIMS): comparison of intravenous recombinant single-chain human tissue-type plasminogen activator (rt-PA) with intravenous streptokinase in acute myocardial infarction

A single chain preparation of recombinant tissue-type plasminogen activator (rt-PA) was compared with intravenous streptokinase to determine coronary thrombolytic efficacy in patients with acute myocardial infarction less than 3 h old. Eighty-six patients were randomly selected to receive the intravenous cumulative dose of 100 mg rt-PA and 85 patients to receive 1.5 million units streptokinase. At 240 min after initiation of the thrombolytic therapy noninvasive signs of sustained reperfusion occurred in 79% of patients in both groups (p = NS). Patency of the infarct-related vessel at follow-up angiography was observed in 81% of patients in the rt-PA group and 74% of patients in the streptokinase group (p = NS). At hospital discharge, compared with admission, echocardiographically determined left ventricular ejection fraction increased from 52 +/- 11% to 56 +/- 10% (p less than 0.01) in the rt-PA group; changes in the streptokinase group (50 +/- 9% to 51 +/- 11%) were not significant. A nadir value of less than 1 g/liter fibrinogen plasma level occurred in 6 patients (7%) receiving rt-PA versus 74 patients (87%) receiving streptokinase (p less than 0.0001). Plasma levels of fibrin(ogen) degradation products were more than doubled in the streptokinase group (p less than 0.01). One patient in the streptokinase group developed a fatal intracranial hemorrhage; five others showed a decline in hemoglobin greater than or equal to 5 g/dl. Other clinical events in the streptokinase-treated group included allergic reactions (four patients) and intrahospital reinfarction (two patients). None of these events occurred in the rt-PA group.(ABSTRACT TRUNCATED AT 250 WORDS)

Bolus dose response characteristics of single chain urokinase plasminogen activator and tissue plasminogen activator in a dog model of arterial thrombosis

Tissue plasminogen activator (t-PA) and single chain urokinase-plasminogen activator (scu-PA) are relatively "fibrin-specific" thrombolytic drugs with short plasma half lives of 6-8 minutes. Most treatment regimens with these agents utilize a bolus injection followed by continuous drug infusion, usually combined with anticoagulant therapy. The purpose of this study was to establish the dose-response characteristics for scu-PA and t-PA, when given as a single intravenous bolus injection, in a dog model of arterial thrombosis. Eight groups of 6 dogs each were given one of the following doses of scu-PA (mg/kg): 0.20, 0.50, 1.00, 2.00; or t-PA: 0.05, 0.10, 0.20; or an equivalent amount of saline (control group). All doses were given as a single bolus injection 60 minutes after formation of a totally occlusive femoral artery thrombus. Thrombolysis was measured by monitoring the continuous decrement of 125I activity from a radiolabelled thrombus. Ninety minutes after drug injection, all scu-PA treated dogs showed greater thrombolysis (30%, 45%, 56%, and 67%, respectively) than the control group (15%, p less than 0.01). The 0.10 and 0.20 mg/kg t-PA treated dogs showed greater thrombolysis (35% and 49%, respectively) than the control group (15%, p less than 0.01). Both scu-PA and t-PA caused a partial and dose-dependent decrease in alpha 2-antiplasmin activity but scu-PA caused a greater depletion (72% vs. 18%, respectively, p less than 0.05) at 60 minutes after the highest dose of drug administration. Both drugs showed a longer than expected thrombolytic effect based upon the known half lives. Neither drug caused significant changes in the prothrombin time, activated partial thromboplastin time, thrombin time, hematocrit, platelet count, or fibrin degradation product concentration. Single bolus injections of scu-PA and t-PA produce safe and effective thrombolysis in this dog model of arterial thrombosis.

New strategies in the development of thrombolytic agents

Recombinant DNA technology has allowed large-scale production of the physiological, fibrin-specific, plasminogen activators tissue-type plasminogen activator (t-PA) and single-chain urokinase-type plasminogen activator (scu-PA). The results of clinical trials with these agents, mainly for the treatment of acute myocardial infarction, have revealed a limited fibrin specificity at the large therapeutic doses required for efficient thrombolysis. Mutants and variants of t-PA and scu-PA have given important information on structure-function relationships in these proteins and have resulted in rt-PA variants with significantly prolonged half-lives in vivo. Construction of chimaeric plasminogen activators containing various portions of t-PA and scu-PA has produced functionally active enzymes, however with a lower fibrin-affinity than wild-type t-PA. The promise of antibody targeting and the use of synergistic combinations of thrombolytic agents remains to be further investigated. We anticipate that eventually these research lines will yield artificial plasminogen activators with improved efficacy, risk/benefit and cost/benefit ratios.