Appraisal of various thrombolytic agents in the treatment of acute myocardial infarction

Thrombolytic Therapy: A Comprehensive Review of its Use in Clinical Medicine—Part I

In the first part of this comprehensive review of thrombolytic therapy in clinical medicine, we begin with a brief history of fibrinolysis, followed by a review of the components of die endogenous fibrinolytic system and the currently available plasminogen activators. An in-depth examination of thrombolysis in treatment of acute myocardial infarction follows, Including recommendations for management based on available clinical trial data. New developments in thrombolytic therapy are also discussed.

Development and application of modern devices in vascular surgery including comment on modern antithrombotic and lytic drug therapy

Since modern vascular surgery started with the discovery of heparin and the development of modern vascular grafts including autogenous saphenous vein, the speciality has been technologically driven. At the outset, vascular surgery depended almost entirely on the development of specialized clamps, instruments, and tools to permit decisive attack on the problems of occlusion. No less important was the development of insight into the basis of atherosclerosis, the discovery that atherogenesis with thrombosis is electrochemically identical to corrosion in pipes and therefore subject to chemical forces, which have not yet been delineated, as well as mechanical forces which have been delineated and permit one to attack the atheroma directly. Thus, classic replumbing techniques including bypass and endarterectomy have long been a part of the fundamental firmament of vascular surgery. Most recently, modern techniques in cleaning out blood vessels, removing occlusive processes, and modern thrombus-dissolving enzymes have all come to the forefront. Frequently multiple techniques are used simultaneously. This article is an attempt to summarize various aspects of this activity and describe several of the patents which have been seminal in the ultimate application of the techniques in both experimental animals and man. This should be the first of a series of efforts to summarize this area. The process of change has never been more kinetic than now.

Time delay to thrombolytic therapy--a Sri Lankan perspective

Reducing the time delay in initiating thrombolytic therapy in acute myocardial infarction is critical in maximising the functional and survival benefit. We analysed 120 consecutive admissions for thrombolytic therapy to the Coronary Care Unit. The total delay was divided into prehospital, in-hospital and Coronary Care Unit stages, and the median delays were found to be 130, 70, and 15 minutes, respectively. The delay was significantly longer in patients who sought prehospital medical advice, and when the diagnosis was not made at the emergency treatment unit. Educating at-risk groups and modifying the admission system may help to minimise these delays, while the establishment of an emergency ambulance service with well-trained crew would also improve the prognosis after acute myocardial infarction in Sri Lanka.

Direct interactions of plasminogen activators with human aortic and pulmonary artery endothelial cells in vitro: implications for thrombolytic therapy

Direct interactions of plasminogen activators with arterial endothelial cells are important in the pathogenesis of vascular complications associated with thrombolytic therapy. We investigated the direct effects of various plasminogen activators on human aortic and pulmonary artery endothelial cell functions in vitro. The effects of plasminogen activators on endothelial cells were not caused by generation of plasmin, as shown by the absence of plasminogen and alpha(2)-plasmin inhibitor-plasmin complex both before and after addition of plasminogen activators to endothelial cells. High concentrations of plasminogen activators increased the permeability of aortic endothelial cells to albumin. Alteplase (50 x 10(3) IU/ml), a recombinant tissue-type plasminogen activator (t-PA), increased prostaglandin I(2) (PGI(2)) production by aortic endothelial cells from 175.5 +/- 13.8 to 870.8 +/- 131.0 pg/mg cellular protein during a 2-h incubation; other plasminogen activators increased PGI(2) production to a lesser extent. Alteplase (100 x 10(3) IU/ml) also increased PGI(2) production from 152.0 +/- 16.2 to 1,080 +/- 95.1 pg/mg cellular protein in human pulmonary artery endothelial cells. High concentrations of urokinases decreased the amount of endothelin-1 in the medium of aortic or pulmonary artery endothelial cells by as much as 93%; part of this decrease was attributable to degradation of endothelin-l by urokinases. Other plasminogen activators either had no effect on or slightly increased the production of endothelin-1. These changes in the function of human arterial endothelial cells induced by plasminogen activators may affect regional vascular tone, endothelial permeability, and platelet aggregability, all of which are important in the efficacy of thrombolysis and in the pathogenesis of such vascular complications as rethrombosis and hemorrhage.

Thrombolytic effects of recombinant fibrolase or APSAC in a canine model of carotid artery thrombosis

BACKGROUND

Thrombolytic agents used clinically rely on the activation of plasminogen to plasmin. Plasmin possesses multiple actions including increasing thrombin activity and activation of platelets. Thus, after successful thrombolytic therapy, arterial hyperactivity and reocclusion may be the result of a predominant plasmin-induced thrombogenic action at the site of the residual thrombus. Fibrolase, a direct-acting fibrinolytic enzyme from southern copperhead snake venom, induces rapid clot lysis in vitro. Fibrolase does not rely on plasminogen activation or any other bloodborne components for activity and is not inhibited by any of the rapidly acting serine proteinase inhibitors in blood.

METHODS AND RESULTS

We investigated the efficacy of fibrolase to lyse an occlusive thrombus formed in the carotid artery of the anesthetized dog. Electrolytic injury was initiated in both the right and left carotid arteries. Thirty minutes after both arteries were occluded, each vessel was infused with either fibrolase (4 mg/kg over 5 minutes) or physiological saline (over 5 minutes). In two separate groups of dogs, anisoylated plasminogen streptokinase activator complex (APSAC) (0.1 U/kg) was infused into the occluded vessel. In the artery infused with fibrolase, five of five dogs exhibited patency within 6 +/- 1 minutes of the infusion (P < .05 versus vehicle-treated artery; Fisher's exact test). In the contralateral carotid artery that received vehicle, the occlusion was maintained throughout the experimental protocol. APSAC alone lysed the thrombus in each vessel within 17 +/- 3 minutes. Five minutes after the end of fibrolase administration and in one of the groups administered APSAC, a glycoprotein (GP)IIb/IIIa antibody, 7E3 (0.8 mg/kg IV), was administered to prevent reocclusion of the patent artery. After 7E3 administration, the vessel treated with fibrolase remained patent in four of five dogs, and six of six APSAC-treated vessels were patent for the remainder of the observation period (2 hours).

CONCLUSIONS

These studies demonstrate that local administration of fibrolase lyses a carotid arterial thrombus rapidly without excessive hemorrhage or hemodynamic compromise. The enzyme in combination with antiplatelet therapy (7E3) offers a unique mechanism for clot dissolution and may prove useful as a clinically efficacious alternative to presently used thrombolytic agents or may act in a synergistic manner with plasminogen activators.

The severity of residual coronary stenosis immediately after thrombolytic therapy does not influence the size of later left ventricular asynergic area

To determine whether the severity of residual coronary artery stenosis immediately after thrombolytic therapy influences the size of later left ventricular (LV) asynergic area, we reviewed coronary angiograms (CAGs) and left ventriculograms (LVGs) of 31 patients with acute myocardial infarction (AMI). All patients received intracoronary urokinase therapy within 6 h after onset of AMI due to total occlusion of the proximal left anterior descending coronary artery (LAD). A dose of 960,000 IU urokinase was infused into the ostium of the left coronary artery over 40 min. Patients in whom antegrade blood flow without delayed distal filling was restored received rigorous anticoagulation. The patients were divided into three groups according to the severity of the coronary lesion immediately after urokinase therapy: 9 patients with complete occlusion in Group 1, 15 with > 90% stenosis in Group 2, and 7 with < 90% stenosis in Group 3. There were no significant differences in the baseline clinical characteristics among the patients in the three groups. The LADs in Group 1 were also totally occluded 1 month after urokinase therapy, the treated vessels in both Groups 2 and 3 were still patent, and patients in Group 2 showed a further reduction in residual stenosis. When LV asynergic area, regional wall motion, and global ejection fraction (EF) were compared among the three groups, no significant differences were demonstrated. In comparison with the data immediately after urokinase therapy, all parameters 1 month after therapy were significantly improved in both Groups 2 and 3.(ABSTRACT TRUNCATED AT 250 WORDS)

Pro-urokinase and prekallikrein are both associated with platelets. Implications for the intrinsic pathway of fibrinolysis and for therapeutic thrombolysis

The contact-dependent intrinsic pathway of fibrinolysis involving factor XII, prekallikrein (PK) and pro-urokinase (pro-UK) remains poorly understood. Casein autography of washed, intact platelets revealed both PK and pro-UK. Accordingly, platelets may mediate physiological thrombolysis by this pathway since factor XIIa activates PK and kallikrein activates pro-UK. Acid washing dissociated PK but not pro-UK from platelets. Exogenous pro-UK was specifically incorporated by platelets from the ambient fluid and similarly could not be dissociated from intact platelets. Therefore, platelets may also mediate an effect from therapeutically administered pro-UK by prolonging its half-life.

Thrombolytic, antiplatelet, and antithrombotic agents

The relative efficacy and safety of individual thrombolytic agents, administered alone and with antiplatelet and antithrombotic drugs, in the treatment of acute myocardial infarction are presented. The clinical benefits and risks of treatment choices are discussed in relation to the mechanisms of the formation and prevention of thrombus and thrombolysis. It is concluded that streptokinase, tissue plasminogen activator (t-PA), and anisoylated plasminogen-streptokinase activator complex (APSAC) significantly reduce mortality and improve left ventricular function equally, despite differences in the rate at which they achieve vascular patency, their durations of action, and the extent to which their use is associated with adverse events. The questions of how best to minimize reocclusion/reinfarction, bleeding, and stroke are discussed, with particular focus on the beneficial use of aspirin and the unresolved issue of how best to use heparin.

Experiences in intravenous urokinase treatment of 100 acute myocardial infarction patients

From 1980 to 1990 we treated 100 cases of AMI with i.v. urokinase (UK). According to the way of management and the dosage administered all these cases were divided into three groups: first stage of small dosage, second stage of trial big dosage, and third stage of comprehensive dosage. 36 patients of the first stage were treated with small dosage, 1-20,000 U b.i.d. for 1 week. 75% of the UK-treated and only 17% of the control group obtained relief of pain. Decrease of elevated ST reaching base line was 50 vs 8%, and FDP increased in 94%. 22 patients of the second stage were undergoing trial of big dosage. They were subdivided into larger dosage (more than 800,000 U) and smaller dosage (less than 300,000 U) groups. From the larger dosage group, 2 patients showed definite sign of recanalization, but unexpectedly 2 patients died of cardiac rupture. Since the recanalization rate of larger dosage group was 42.9%, but no case showed sign of recanalization in smaller dosage group, we are of the opinion that the dose of 800,000 U is rational for patients with symptoms' onset less than 3 h. Cardiac rupture was thought to be mostly due to reperfusion injury. Thus we designed the third stage of comprehensive dosage of UK. In this stage we used different dosage of UK and different ways of administration in 52 patients, based on the different symptoms' onset, so as to bring the effect of UK in full play. The aim of using UK is chiefly fibrinolysis as well as improvement of blood viscosity.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible mechanism of vascular reocclusion after initially successful thrombolysis with recombinant tissue-type plasminogen activator

We studied the effects of urokinase (UK), pro-urokinase (pro-UK), and recombinant tissue-type plasminogen activator (rt-PA) on platelet aggregation and the production of thromboxane A2 (TXA2) in vitro. Both UK and pro-UK inhibited the platelet aggregation induced by adenosine 5'-diphosphate (ADP), collagen, or thrombin in a concentration-dependent manner. In contrast, although a low dose of rt-PA (5 to 10 x 10(4) U/ml) blunted platelet aggregability, a high dose (40 to 60 x 10(4) U/ml) led to platelet hyperaggregation. UK and pro-UK markedly inhibited TXA2 synthesis during ADP-induced platelet aggregation. Despite the significant reduction of TXA2 synthesis by 10 x 10(4) U/ml rt-PA, a concentration of 60 x 10(4) U/ml rt-PA had no effect on synthesis. These results indicate that UK and pro-UK each inhibit platelet function, but a high concentration of rt-PA enhances platelet aggregability. This finding may at least in part contribute to the high incidence of reocclusion after initially successful thrombolysis with rt-PA.

Influence of thrombolytic agents on human platelet function

In this study, we have evaluated the effects of four different thrombolytic agents, including Streptokinase from Hoechst and from Kabivitrum, Urokinase from Abbott and tissue plasminogen activator (t-PA) from Genetech, on platelet-rich plasma clots and platelet aggregation. At concentrations lower than 50 ugs/ml, t-PA had no inhibitory effect on clot retraction or platelet aggregation induced by weak or potent agonist. At a higher concentration (greater than 100 ugs/ml), t-PA specifically antagonized the action of thrombin on clot formation and platelet aggregation. Streptokinase (Kabivitrum) potentiated the action of weak agonists on platelet aggregation, but the same agent from Hoechst had no negative or positive influence. None of the drugs tested had an adverse effect on platelet function at suggested therapeutic levels. None of the thrombolytic agents were capable of dissociating preformed clots made from platelet-rich plasma. However, all of them caused lysis of whole blood clots. Also, prior incubation of plasma alone or platelet-rich plasma with any of the agents prevented subsequent clot formation. The studies demonstrate that thrombolytic drugs at therapeutic concentrations do not affect platelet function adversely. They have a potent effect on whole blood clots, but not on clots from platelet-rich plasma. Therefore, platelets may play a critical role in determining the degree of reperfusion and the frequency of reocclusion following treatment with thrombolytic agents in vivo.

Submaximal exercise thallium-201 SPECT for assessment of interventional therapy in patients with acute myocardial infarction

Submaximal thallium-201 stress testing has been shown to provide important diagnostic and prognostic information in patients with acute myocardial infarction. The purpose of this investigation was to evaluate the diagnostic value of early submaximal stress testing and thallium-201 single photon emission computed tomography (SPECT) after interventional therapy. Scintigraphic results from 56 patients with infarctions, who underwent acute thrombolytic therapy, angioplasty, or both, were compared with late (6 weeks) functional outcome as assessed by radionuclide ventriculography and with results of discharge coronary angiography. A linear correlation was found between the extent of thallium-201 SPECT perfusion defect and late ventricular function (r = 0.74, p less than 0.01). Forty-two percent of patients with large SPECT perfusion defects had normal left ventricular ejection fractions, suggesting an overestimation of infarct size by early imaging. Sensitivity and specificity of thallium-201 SPECT for detection of coronary artery stenosis in noninfarct territories was 57% and 46%, respectively, indicating limited diagnostic definition of extent of underlying coronary artery disease. Results of follow-up coronary angiography showed a significant relationship between the size of the initial perfusion defect and early restenosis or reocclusion of the infarct artery. Thus the extent of early thallium-201 perfusion defects correlates with late functional outcome but appears to overestimate the degree of injury. Submaximal thallium-201 stress testing allows only limited characterization of underlying coronary artery disease. Early assessment of infarct size may identify a patient population at high risk for reocclusion of the infarct artery.

Sudden appearance of coronary thrombus observed by angiography--a case report

A sudden coronary thrombus formation was documented by chance during cardiac catheterization in a patient with postinfarction angina. The thrombus was successfully treated with intravenous urokinase and heparin infusions, and thereafter, coronary angioplasty was performed without any complication.

Immunological properties of the fibrinolytic enzyme (fibrolase) from southern copperhead (Agkistrodon contortrix contortrix) venom and its purification by immunoaffinity chromatography

An antibody to the fibrinolytic enzyme in southern copperhead venom was produced by immunizing rabbits with chromatographically purified enzyme. The antibody was purified from rabbit blood by ammonium sulfate fractionation and protein-A affinity chromatography. The purified antibody reacted only with the fibrinolytic enzyme in southern copperhead venom as demonstrated by immunodiffusion and immunoelectrophoresis. Western immunoblotting revealed that several snake venoms, including Agkistrodon piscivorus conanti, Crotalus atrox, Crotalus basiliscus basiliscus, and Bothrops asper, cross-reacted with the antibody to varying degrees. However, Deinagkistrodon acutus showed no cross-reaction. Immobilized antibody has been used, in combination with molecular sieve chromatography, to purify the fibrinolytic enzyme from southern copperhead venom. In this two-step purification procedure, the enzyme was purified in good yield within two days. The specific activity of the enzyme purified by the immunoaffinity chromatography procedure is comparable with that of enzyme purified by a four-step chromatographic procedure. The mol. wt of the purified enzyme is approximately 23,000-24,000 as determined by SDS-PAGE. Interestingly, the enzyme purified by this two-step immunoaffinity chromatography procedure possesses virtually no hemorrhagic activity.

Temporal effects of thrombolytic agents on platelet function in vivo and their modulation by prostaglandins

To examine the temporal effects of plasmin generated in vivo on platelet function, we infused tissue-type plasminogen activator (t-PA) in rabbits over 3 hours and measured ex vivo platelet aggregation. We noted an initial increase in the aggregation response to ADP occurring 30 minutes after the start of infusion. This enhanced response was short-lived and by 180 minutes was reduced, compared with pretreatment levels. Baseline aggregation response was restored by 240 minutes. This pattern of aggregation response to t-PA infusion was also seen with thrombin as the agonist. Coinfusion of either prostaglandin I2 or prostaglandin E1 abolished the initial hyperaggregable phase induced by t-PA; the hypoaggregable phase occurred earlier (after 60 minutes) and persisted throughout the 1-hour recovery period. Similarly, streptokinase infused for 1 hour also increased platelet aggregation at early times and then reduced aggregation responses after the first hour. Plasma plasmin activity increased as expected with t-PA infusion alone, peaking at 30 minutes and returning to baseline by the first hour. Interestingly, prostaglandin E1 blunted the rise in plasma plasmin activity. This same dose of prostaglandin E1 or prostaglandin I2 used alone did not appreciably alter platelet function at any time during the experiment. Our data show that therapeutic doses of t-PA or streptokinase produce a biphasic effect on platelet aggregation response in the rabbit. Coinfusion of either of the antiplatelet agents, prostaglandin E1 or prostaglandin I2, abolishes the hyperaggregable phase and prolongs the inhibitory effects on platelet aggregation produced by t-PA. These data suggest that the effects of thrombolytic agents on platelet function are complex and can be modulated by antiplatelet prostaglandins.

Time delays in the diagnosis and treatment of acute myocardial infarction: a tale of eight cities. Report from the Pre-hospital Study Group and the Cincinnati Heart Project

To establish the magnitude of prehospital and hospital delays in initiating thrombolytic therapy for acute myocardial infarction, the time from telephone 911 emergency medical system (EMS) activation to treatment and its components were analyzed from eight separate ongoing trials. This included estimates of ambulance response time, prehospital evaluation and treatment time, and time from admission to the hospital to initiation of thrombolytic therapy. The average time from EMS activation to patient arrival at the hospital was prospectively determined to be 46.1 +/- 8.2 minutes in 3715 patients from eight centers. The time from admission to the hospital to initiation of thrombolytic therapy was retrospectively determined to be 83.8 +/- 55.0 minutes in a separate group of 730 patients from six centers. Both the prehospital and hospital time delays were much longer than those perceived by paramedics and emergency department directors. Shorter hospital time delays were observed in patients in whom a prehospital ECG was obtained as part of a protocol-driven prehospital diagnostic strategy and a diagnosis of acute infarction made before arrival at the hospital (36.3 +/- 11.3 minutes in 13 patients). These results show that the magnitude of time required to evaluate, transport, and initiate thrombolytic therapy will preclude initiation of treatment to most patients within the first hour of symptoms. Implementation of a protocol-driven prehospital diagnostic strategy may be associated with a reduction in time to thrombolytic therapy.

Management of the patient following coronary thrombolysis

Myocardial salvage can be maximized by the early institution of thrombolytic therapy and aspirin. Certain patients may benefit from the administration of intravenous heparin, beta blockers, or nitroglycerin. The routine use of percutaneous transluminal coronary angioplasty (PTCA) or calcium-channel blockers does not appear to be warranted. Recurrent myocardial ischemia should be vigorously treated with medical therapy and there may be value in cardiac catheterization, followed by PTCA or bypass surgery, depending upon the extent of myocardium at risk and the underlying coronary anatomy. Long-term morbidity and mortality may be reduced by instituting aspirin and beta blockers as well as by modifying risk factors. There is no evidence for the long-term benefit from any calcium-channel blocker. Oral anticoagulation may be warranted in those patients with a mural thrombus, congestive heart failure, or atrial fibrillation. ACE inhibitors may be of value in the presence of left ventricular dysfunction and certainly in the presence of symptomatic congestive heart failure. Antiarrhythmic therapy is generally indicated only for symptomatic or life-threatening arrhythmias. Residual myocardial ischemia should be sought by exercise testing, and those patients with poor exercise tolerance generally warrant cardiac catheterization in consideration for revascularization.

Fatal streptokinase-induced intracerebral haemorrhage in cerebral amyloid angiopathy

A fatal intracerebral haemorrhage (ICH) associated with streptokinase (SK) treatment of an acute myocardial infarction is described. Autopsy examination showed a lobar ICH and severe cerebral amyloid angiopathy (CAA). The close temporal relationship between SK administration and intracranial haemorrhage, the absence of pretreatment risk factors for ICH, and the presence of CAA suggests that these are related phenomena. Accordingly: 1. There may be a synergistic relationship between CAA and intracranial haemorrhage induced by fibrinolytic agents; 2. Thrombolytic agents may induce more frequent than expected intracranial haemorrhage in conditions associated with a high incidence of CAA, notably old age and Alzheimer's disease; 3. A regional defect in haemostasis other than vessel fragility may contribute to the intracranial haemorrhagic predisposition of CAA; 4. Autopsy examination of cases of ICH is an essential part of the audit of clinical trials of fibrinolytic agents.

Thrombolytic therapy in acute myocardial infarction

Recombinant tissue-type plasminogen activator (rt-PA), streptokinase (SK), and anisoylated plasminogen-streptokinase activator complex (APSAC) have salutary effects on mortality when administered to patients with evolving acute myocardial infarction (MI). Studies suggest that intravenous rt-PA is more effective in reperfusing occluded infarct-related arteries than SK, and the results of ongoing studies directly comparing the influence of SK and rt-PA on mortality are awaited. The clinical role of agents such as APSAC, urokinase, and pro-urokinase, used alone or in combination, remains to be determined. It is evident that a variety of thrombolytic agents will be effective, and variables such as ease of administration, pharmacokinetics, fibrin specificity, effects on blood viscosity, and incidence of adverse effects need to be assessed to determine which agents are the most suitable for clinical use. There is an increased risk of bleeding at vascular puncture sites with all thrombolytic agents. Current indications for thrombolytic therapy include ischemic chest pain of at least 30 min duration that is unrelieved by nitroglycerin and is associated with ST-segment elevations of at least 0.1 mV in two contiguous electrocardiographic leads. Such therapy is usually reserved for patients less than 75 years old who are not at increased risk for bleeding and whose chest pain began less than 4-6 prior to treatment. Trials are under way to determine whether patients with shorter pain duration, transient ST-segment changes (ie, unstable angina patients), chest pain associated with ST-segment depressions or T-wave inversions (ie, non-Q-wave infarction patients), or patients whose pain began more than 4 to 6 h earlier will benefit from early thrombolytic therapy. Other factors such as patient age, the likelihood of the diagnosis of MI, and the estimated risk of bleeding should also be considered. The findings of available major randomized trials indicate that early invasive procedures are generally unnecessary and that meticulous care must be exercised in the selection and management of patients subjected to thrombolytic therapy.

New developments in thrombolytic therapy

Thrombotic complications of cardiovascular disease are a main cause of death and disability and, consequently, thrombolysis could favorably influence the outcome of such life-threatening diseases as myocardial infarction, cerebrovascular thrombosis and venous thromboembolism. Thrombolytic agents are plasminogen activators that convert plasminogen, the inactive proenzyme of the fibrinolytic system in blood, to the proteolytic enzyme plasmin. Plasmin dissolves the fibrin of a blood clot, but may also degrade normal components of the hemostatic system and predispose to bleeding. Currently, five thrombolytic agents are either approved for clinical use or under clinical investigation in patients with acute myocardial infarction. These include streptokinase, urokinase, recombinant tissue-type plasminogen activator (rt-PA), anisoylated plasminogen streptokinase activator complex (APSAC) and single chain urokinase-type plasminogen activator (scu-PA, prourokinase). The first generation thrombolytic agents, streptokinase (and probably also urokinase), are only moderately efficacious and their administration is associated with extensive systemic fibrinogen breakdown. In comparative studies performed in patients with acute myocardial infarction, recombinant tissue-type plasminogen activator (rt-PA) is a more effective and fibrin-specific thrombolytic agent than streptokinase. The acylated plasminogen streptokinase activator complex (APSAC) has a profile of thrombolytic efficacy and fibrin-specificity that is similar or somewhat better than that of streptokinase, but has the advantage that it can be administered by bolus injection. Single chain urokinase-type plasminogen activator is more fibrin-specific than urokinase. Comparative data on the efficacy and safety of this agent are limited as it is in the early stage of clinical investigation. Reduction of infarct size, preservation of ventricular function and/or reduction in mortality has been observed with streptokinase, rt-PA and APSAC. Therefore, thrombolytic therapy will probably become routine therapy for early acute myocardial infarction. In patients with acute myocardial infarction, intravenous streptokinase recanalizes 40-45 percent of occluded coronary arteries and reduces mortality by 25 percent; it costs approximately $200 for a therapeutic dose of 1,500,000 units. Recombinant tissue-type plasminogen activator (rt-PA) is more potent for coronary arterial thrombolysis, producing both more rapid and more frequent (65-70 percent) reperfusion, but it costs over $1,000 for a therapeutic dose of 100 mg. Side effects (mainly bleeding) and the incidence of reocclusion associated with the use of streptokinase and rt-PA are not markedly different. Whether the higher efficacy of rt-PA will translate into a comparably larger reduction of mortality remains to be determined in large comparative clinical trials. Both agents are available for clinical use.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of low-dose tissue plasminogen activator and streptokinase on platelet aggregation

Despite increasing success with low-dose intra-arterial thrombolysis, early rethrombosis still occurs. Platelet aggregation is thought to play a major part in this process. We have therefore investigated the effects of recombinant tissue plasminogen activator (rt-PA) and streptokinase on platelet function at doses currently used for peripheral arterial thrombolysis. Platelet-rich plasma was stirred at 37 degrees C, with either streptokinase (100, 300 or 1000 units ml-1) or rt-PA (10 (T10), 30 (T30) and 100 (T100) mg l-1), with immediate addition of an agonist for platelet aggregation (thrombin, collagen, adenosine diphosphate (ADP) or adrenaline) at a predetermined threshold dose. Significant inhibition of collagen-induced and adrenaline-induced platelet aggregation was produced with rt-PA at all doses used (P less than 0.05). With adrenaline as the agonist, T100 produced disaggregation to a mean (s.d.) level of 26 per cent. Thrombin-stimulated platelet aggregation was significantly reduced by T100 (P less than 0.001) and T30 (P less than 0.01) only, disaggregation being dose-dependent and complete with T100. Using ADP as the agonist, T100 produced a significant reduction in maximum platelet aggregation (P less than 0.01), and disaggregation was achieved to a mean (s.d.) level of 48(13) per cent. Streptokinase did not produce any significant changes in any parameter of aggregation.

After coronary thrombolysis and reperfusion, what next?

Thrombolytic therapy for the removal of intravascular thrombi was introduced when streptokinase was first given to humans 40 years ago, the same year the American College of Cardiology was founded. Streptokinase was first administered to patients with acute myocardial infarction in 1959. Today, thrombolytic therapy has been established to offer significant benefits to patients with acute myocardial infarction provided they are brought to medical attention early enough after the onset of symptoms. The two major agents, streptokinase and recombinant tissue-type plasminogen activator (rt-PA), have been shown to result in reperfusion of infarct-related arteries, to salvage ischemic myocardium, to improve myocardial performance and to reduce mortality. In spite of these impressive gains, this novel therapy has shortcomings. The interval from the start of thrombolytic treatment to coronary reperfusion varies significantly from patient to patient and may, at times, be too long to produce a real benefit in terms of salvage of ischemic myocardium. The rate of reocclusion lies somewhere between 10% and 20% and appears not to be influenced by concomitant heparin anticoagulation. The rate of bleeding complications even with the "fibrin-specific" rt-PA is higher than anticipated and may range from 10% to 30%. As a consequence, intensive efforts are being directed at the development of improved thrombolytic agents and for adjunctive therapy evaluating better anticoagulants than heparin and better antiplatelet agents than aspirin. This review is a status report summarizing where we are in thrombolytic therapy in acute myocardial infarction, where we need to improve treatment results and what is being done mainly at the preclinical level to bring about such improvements.

A new thrombolytic regimen for acute myocardial infarction using combination half dose tissue-type plasminogen activator with full dose streptokinase: a pilot study. KAMIT Study Group

Because a previous study utilizing a combination of recombinant tissue-type plasminogen activator (rt-PA) and urokinase demonstrated reduced reocclusion rates compared with rates obtained with rt-PA alone, this study was conducted to determine whether the combination of rt-PA and streptokinase might achieve similar results at reduced cost. Forty patients with acute myocardial infarction were treated with a 1 h infusion of rt-PA (50 mg) and streptokinase (1.5 million U) administered within 6 h (mean 3.6 +/- 1.2) of symptom onset. Emergency coronary arteriography revealed patency of the infarct-related artery in 30 (75%) of 40 patients. With the addition of coronary angioplasty in those who had unsuccessful thrombolytic reperfusion, the early patency rate was increased to 98%. In-hospital mortality rate (2.5%) and the incidence of significant bleeding requiring transfusion (15%) were low. Angiographically documented reocclusion of the infarct vessel occurred in 3 (8%) of 37 patients by day 7. Regional wall motion of the infarct zone improved by 0.9 +/- 0.9 SD/chord (p less than 0.0005), and ejection fraction increased 3.6 +/- 8% units (p less than 0.05) between immediate and day 7 studies. In contrast to the price of full dose rt-PA ($2,300) or rt-PA with urokinase ($3,500), the cost of this regimen was $1,230. This pilot study demonstrates that at half the cost, a combination of half dose rt-PA with full dose streptokinase offers high infarct vessel patency, recovery of ventricular function, a low rate of reocclusion and few bleeding complications.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics, thrombolytic efficacy and hemorrhagic risk of different streptokinase regimens in heparin-treated acute myocardial infarction

The systemic activator activity of 4 streptokinase (SK) regimens (250,000 IU intracoronary, group A; 500,000 IU, group B; 1.5 X 10(6) IU, group C; and 30 U anisoylated plasminogen streptokinase activator complex (APSAC) intravenously, group D) was tested with the fibrin plate technique. One hour after initiation of treatment, the activator activity was highest after APSAC (3.6 +/- 0.9 U), slightly but not significantly less after SK 1.5 X 10(6) IU (3.0 +/- 0.7), and significantly less after SK 500,000 IU (1.6 +/- 0.5) and 250,000 IU (0.6 +/- 0.5), p less than 0.001. After SK, activator activity half-lives were 184 minutes (group B) and 169 minutes (group C), and after APSAC 188 minutes (group D). These were all in agreement with greater than 12 hour duration of changes in other markers of systemic fibrinolysis (euglobulin lysis time) and substrates depletion (fibrinogen, plasminogen, alpha 2 antiplasmin). In extended pilot clinical groups given identical thrombolytic regimens during full anticoagulation with heparin, angiographic coronary patency was found in 83% (35 of 42) after intracoronary SK (group 1), in 73 and 75%, respectively, after 500,000 IU (31 of 43) and 1.5 X 10(6) IU (30 of 40) (group 2 and 3, difference not significant) and 80% (8 of 10) after the 30-U bolus of APSAC (group 4). The overall hemorrhagic risk was 24%, equally distributed among the 4 regimens and mostly (91%) related to catheters. The incidence of bleeding unrelated to vessel puncture was 4%; no deaths occurred. It is concluded that APSAC is the most fibrinolytic regimen but its potential thrombolytic superiority over SK remains to be demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

Newer agents for coronary thrombolysis. Perspectives from clinical studies

Myocardial infarction (MI) is a major cause of morbidity and mortality in western countries and the formation of intracoronary thrombi is recognized as a critical determinant of this ischaemic event. Since streptokinase and urokinase cause in vitro lysis of clots, it was though that these drugs were also effective in vivo in dissolving coronary thrombi. Clinical studies supported this concept. However, the beneficial effects of these drugs were, to some extent, offset by their inherent adverse reactions. Therefore new thrombolytic agents were developed, and for three of them (APSAC, tPA and proUK) there are enough clinical studies to allow for a comparison with 'old' agents. The data show that none of the new agents is safer or better than old agents with respect to easy handling, incidence of reperfusion of occluded coronary arteries, frequency of reocclusions, thrombus specificity, and bleeding complications. Thus, several directions are currently pursued to develop newer thrombolytic drugs with risk/benefit ratios better than those of 'old' agents. In this respect, it has been shown recently that the combination of aspirin with streptokinase is significantly better than streptokinase alone as far as mortality and incidence of rethrombosis is concerned. These data suggest that thrombolytic approaches safer and better than those currently available are possible and indicate that some of such new strategies are already available to enter the 'thrombolytic era' of acute MI.

Antithrombotic therapy after myocardial reperfusion in acute myocardial infarction

The problem of post-thrombolytic reocclusion can be approached in several ways. 1) Better thrombolytic agents with longer duration of effects and more powerful properties aimed at enhanced clot lysis and anticoagulation are under study. 2) The combination of high dose heparin and low dose aspirin is proposed for all patients with an acute myocardial infarction treated with thrombolytic agents. 3) Peptide inhibitors of thrombin and monoclonal antibodies against platelet glycoprotein receptors and adhesive macromolecules are potentially effective inhibitors of platelet aggregation and thrombus formation during or after thrombolytic therapy.

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.

Unresolved clinical pharmacologic questions in thrombolytic therapy for acute myocardial infarction

Thrombolytic therapy has now become established as a useful therapeutic measure for the immediate treatment of an acute evolving transmural infarction. Nevertheless, several important and fundamental aspects of a pharmacologic nature remain to be resolved. Prominent among these is whether or not fibrin specificity of a thrombolytic agent provides important benefits, and whether heparin therapy as commonly employed to prevent rethrombosis has been effective. Review of the available data raises serious questions as to the validity of current views and the appropriateness of prevailing trends.