Streptokinase-induced, antibody-mediated platelet aggregation: a potential cause of clot propagation in vivo

A rapid agglutination assay to detect anti-streptokinase antibodies

BACKGROUND

Streptokinase resistance may cause suboptimal thrombolytic therapy.

AIM

To develop a rapid latex-bead assay to detect streptokinase antibodies.

METHODS

Sera were obtained from 16 patients presenting with acute myocardial infarction (MI) before treatment with streptokinase and 1 and 6 months post treatment, and from 100 controls. Sera were assayed for anti-streptokinase antibodies using a functional streptokinase-neutralising assay.

RESULTS

Streptokinase-neutralising activity was low in controls (54 +/- 5U/ml) and patients prior to treatment (101 +/- 18), increasing to 2,110 +/- 823 and 1,017 +/- 169 at 1 and 6 months (mean +/- SEM). The latex assay had a sensitivity of 94% and a specificity of 93% for detecting individuals with > 350U/ml of streptokinase resistance, which is sufficient to neutralise the drug clinically.

CONCLUSIONS

Estimation of streptokinase resistance using an enzyme immunoassay and a latex bead assay correlated well with serum neutralising activity. This assay can rapidly identify patients who have a high level of streptokinase-neutralising activity.

Inhibition of Streptokinase-Induced, Antibody-Mediated Platelet Aggregation with Tirofiban After Exposure to Streptokinase or Streptococcal Infection

STUDY OBJECTIVE

To evaluate the effect of tirofiban (a glycoprotein IIb-IIIa inhibitor) in preventing streptokinase-induced, antibody-mediated platelet aggregation after administration of streptokinase or development of a streptococcal infection.

DESIGN

Prospective analysis.

SETTING

Research center of a Canadian hospital.

PARTICIPANTS

Forty-five healthy volunteers, 45 patients who had received streptokinase within the past 3 years, and 13 patients who had a severe streptococcal infection also within the past 3 years.

INTERVENTION

Blood samples were drawn to measure the extent of inhibition of streptokinase-induced, antibody-mediated platelet activation and aggregation by tirofiban.

MEASUREMENTS AND MAIN RESULTS

Platelet aggregation was measured by using a turbidimetric method. The extent of inhibition by tirofiban was measured by incubating tirofiban for 2 minutes before adding streptokinase 5000 U/ml. Also, tirofiban was added 2 minutes before adding adenosine 5'-diphosphate (ADP) 2 microM/L into the last tube as a comparison. Strepto-kinase-induced, antibody-mediated platelet aggregation was observed in 10 (22%) of the 45 patients treated with streptokinase, in 3 (23%) of the 13 patients with streptococcal infection, and in none of the 45 healthy volunteers. Tirofiban inhibited streptokinase-induced, antibody-mediated platelet aggregation by 89 +/- 14% (p<0.001). Similarly, ADP-induced platelet aggregation was inhibited by 92 +/- 6% (p<0.001) with tirofiban.

CONCLUSION

Streptokinase-induced, antibody-mediated platelet aggregation occurred in 13 (22%) of 58 patients who received streptokinase or were exposed to a streptococcal infection in the past 3 years. Such patients may not benefit from streptokinase therapy. In these patients, tirofiban significantly decreased the extent of antistreptokinase antibody-mediated platelet aggregation. Hence, patients undergoing streptokinase therapy may benefit from tirofiban as adjunctive therapy.

Antistreptokinase platelet-activating antibodies are common and heterogeneous

BACKGROUND

Platelet activation by antistreptokinase (SK) antibodies could impair the clinical effect of SK administration.

OBJECTIVE

To better describe anti-SK antibodies with particular emphasis on procoagulant activities as a result of platelet activation.

PATIENTS AND METHODS

Sera were collected from 146 patients with coronary artery disease: non-SK-treated, 95 from mainland France, 31 from French Polynesia; 20 patients from mainland in year 2 after SK treatment. Serum-induced SK-dependent platelet activation resulting in procoagulant activities was assessed with washed platelets from five donors representative of the known patterns of reactivities to IgG.

RESULTS

Concentrations (2-5252 microg mL(-1)) and fibrinolytic neutralization titres (< 10 to > 1280) were found in the expected wide range and correlated (rho = 0.66, P < 0.0001). Platelet activation was detected with 145 samples, but varied in intensity and pattern (depending on the donors), although there was no systematic hierarchy; it was presumably due to IgG (inhibited by an IgG Fc receptor-blocking antibody and recovered in the IgG fraction) and only partially affected by aspirin. Marked platelet activation could be detected in samples with concentration as low as 2 microg mL(-1), and/or no detectable neutralizing titers. The way of immunization to SK was not found to influence the functional profile of antibodies.

CONCLUSION

Anti-SK platelet-activating antibodies are widespread, heterogeneous, poorly predictable on the basis of their antifibrinolytic effect and strong enough to trigger procoagulant activities. Their clinical relevance should be formally assessed, using patients' own platelets for detection owing to the variation of platelet reactivity.

Platelet activity and antibody titers after exposure to streptokinase or streptococcal infection

INTRODUCTION

Streptokinase use, in acute myocardial infarction, is hindered by failure to reperfuse (60%) and early reocclusion (16%). This phenomenon may, among other causes, be due to systemic inactivation of streptokinase, as well as streptokinase-induced platelet aggregation and clot propagation from antibodies to streptokinase produced after streptokinase administration or streptococcal infections. The purpose of this study was to determine the incidence of streptokinase-induced, antibody-mediated, platelet activation and aggregation after administration of SK or development of a streptococcal infection.

MATERIALS AND METHODS

We included 45 normal volunteers (Control group), as well as 45 patients who had received streptokinase (Streptokinase group) and 13 who had suffered a severe streptococcal infection (Streptococcal infection group) within the past 3 years. Extent of streptokinase-induced, antibody-mediated, platelet activation and aggregation, as well as anti-streptokinase antibody and streptokinase resistance titers (lowest streptokinase concentration to cause clot lysis within 10 min) were measured.

RESULTS

Whereas streptokinase-induced, antibody-mediated, platelet activation was observed in 49% of streptokinase patients and in only 17% and 15% of streptococcal infection patients and normal volunteers (p<0.05 Streptokinase vs. Control and Streptokinase vs. Streptococcal infection), streptokinase-induced platelet aggregation was observed in 23% of streptokinase patients and streptococcal infection patients, and in none of the control patients (p<0.05).

CONCLUSIONS

Streptokinase-induced, antibody-mediated, platelet activation and aggregation occur in patients with high titers of anti-streptokinase antibody and may play a role in failure of streptokinase therapy. Streptococcal infection patients behave like streptokinase patients in terms of the reactivity of their platelets to subsequent streptokinase dose in vitro.

Streptokinase-induced platelet activation involves antistreptokinase antibodies and cleavage of protease-activated receptor-1

Streptokinase activates platelets, limiting its effectiveness as a thrombolytic agent. The role of antistreptokinase antibodies and proteases in streptokinase-induced platelet activation was investigated. Streptokinase induced localization of human IgG to the platelet surface, platelet aggregation, and thromboxane A2production. These effects were inhibited by a monoclonal antibody to the platelet Fc receptor, IV.3. The platelet response to streptokinase was also blocked by an antibody directed against the cleavage site of the platelet thrombin receptor, protease-activated receptor-1 (PAR-1), but not by hirudin or an active site thrombin inhibitor, Ro46-6240. In plasma depleted of plasminogen, exogenous wild-type plasminogen, but not an inactive mutant protein, S741A plasminogen, supported platelet aggregation, suggesting that the protease cleaving PAR-1 was streptokinase-plasminogen. Streptokinase-plasminogen cleaved a synthetic peptide corresponding to PAR-1, resulting in generation of PAR-1 tethered ligand sequence and selectively reduced binding of a cleavage-sensitive PAR-1 antibody in intact cells. A combination of streptokinase, plasminogen, and antistreptokinase antibodies activated human erythroleukemic cells and was inhibited by pretreatment with IV.3 or pretreating the cells with the PAR-1 agonist SFLLRN, suggesting Fc receptor and PAR-1 interactions are necessary for cell activation in this system also. Streptokinase-induced platelet activation is dependent on both antistreptokinase-Fc receptor interactions and cleavage of PAR-1.

The role of angioplasty in acute myocardial infarction

The role of percutaneous transluminal coronary angioplasty (PTCA) in the management of acute myocardial infarction (AMI) has not yet been precisely defined. The longest experience with PTCA in this setting has been in patients who are not candidates for thrombolytic therapy and in patients in whom thrombolysis has failed. Clinical interest has recently focused on direct use of PTCA (instead of thrombolysis) as the initial approach to reperfusion in AMI. We review the conceptual bases for both thrombolytic therapy and PTCA in AMI, and we then detail the clinical experience with PTCA in a variety of patient populations with AMI to guide use of both therapies in this setting.

In vitro effect of plasmin on human platelet function in plasma. Inhibition of aggregation caused by fibrinogenolysis

BACKGROUND

Plasmin has been reported both to activate platelets and to inhibit platelet functions. The latter effect was thought to be caused by proteolysis of the main membrane glycoproteins.

METHODS AND RESULTS

We found that incubation of citrated human platelet-rich plasma with streptokinase (SK) (300 IU/ml) does not produce any detectable activation but leads to a time-dependent inhibition of ADP-induced aggregation accompanied by substantial fibrinogenolysis. These effects were abrogated by previous addition of a plasmin inhibitor, aprotinin. Crossover experiments (SK-treated or control platelets mixed with SK-treated or control plasma) demonstrated that the platelets remained functional and that the aggregation defect was caused by fibrinogenolysis. Further experiments (addition of purified fibrinogen to fibrinogen-depleted plasma with either SK or thrombin) suggested that in addition to the low residual level of fibrinogen, fibrinogen degradation products had an inhibitory effect. Under the same conditions, tissue-type plasminogen activator (t-PA) (3,000 ng/ml) had no effect on platelet aggregation, and plasma fibrinogen was not significantly lowered. The effects on glycoproteins IIb-IIIa of incubation with SK, t-PA, or plasmin were assessed with immunoblots with murine monoclonal antibodies directed against either part of the complex, which is the receptor for fibrinogen. Proteolysis was detected only in the presence of EDTA, a potent chelator of divalent cations.

CONCLUSIONS

The incubation of human platelets in citrated plasma with SK concentrations obtained during therapy leads to an aggregation defect that is related to the decrease in fibrinogen, the adhesive protein involved in this function, and to the impeding effect of fibrinogen degradation products on its binding onto platelets but not to an alteration of the corresponding platelet receptor, the heterodimer glycoproteins IIb-IIIa.

Importance of reperfusion on thromboxane A2 metabolite excretion after thrombolysis

Fibrinolytic therapy is a major advance in the treatment of coronary artery disease. A marked elevation in plasma and urinary metabolites of thromboxane A2 (TXA2) after administration of thrombolytic therapy has been observed and has been related to a direct effect of thrombolytic drugs on platelets. To test this hypothesis we evaluated the 11-dehydro-thromboxane B2 (11-d-TXB2) level, as an index of platelet activation, in 20 healthy subjects and in 30 patients with acute myocardial infarction (AMI). Patients with infarction received streptokinase (n = 8), recombinant tissue-type plasminogen activator (rt-PA) (n = 8), or thrombolytic therapy preceded by acetylsalicylic acid (n = 7) or were treated without thrombolytic therapy (n = 7). The urinary 11-d-TXB2 level in healthy control subjects was 327 +/- 126 pg/mg creatinine. A significant increase was observed in patients with AMI with no difference between those who received no thrombolytic therapy (673 +/- 283 pg/mg creatinine in the first 12 hours) and those who received streptokinase (833 +/- 613 pg/mg creatinine) or rt-PA (836 +/- 653 pg/mg creatinine). Patients pretreated with acetylsalicylic acid had urinary 11-d-TXB2 values ranging between 361 and 155 pg/mg creatinine. A significant difference in 11-d-TXB2 values was observed only when patients who were reperfused were separated from those who remained occluded according to angiographic criteria (1085 +/- 498 vs 391 +/- 227 pg/mg creatinine in the first 12 hours, p less than 0.001). We conclude that reperfusion and not thrombolytic agents per se appears to be the factor that induces platelet activation and consequently facilitates reocclusion.

Ischemic stroke and intracranial hemorrhage following thrombolytic therapy for acute myocardial infarction: a risk-benefit analysis

Stroke is a potentially serious complication of acute myocardial infarction (AMI). In the prethrombolytic era, most strokes were attributed to cerebral embolism. On the basis of available information, the occurrence of stroke in the thrombolytic era appears to be less than in the prethrombolytic era. In the thrombolytic era, the occurrence of various forms of intracranial hemorrhage has increasingly been documented in addition to cerebral embolism, with intriguing features. In general, however, the delineation of specific stroke subtypes has been imprecise and must take into account factors that are unique to this setting. Age is a risk factor for both ischemic and hemorrhagic stroke. Potential risk factors for intracranial hemorrhage include hypertension, dosage of fibrinolytic agents, and prior neurologic disease. Potential causes of intracranial hemorrhage include combined fibrinolytic/adjunctive therapies, various cerebrovascular lesions, and head trauma. Existing data suggest that mortality related to stroke complicating AMI is on the decline as well. More research is needed in order to quantify precisely the occurrence and proportions of stroke subtypes, risk factors, and causes in order to define mechanisms and preventive measures.

New developments in thrombolytic therapy

Pharmacologic lysis of occlusive, ischemia-producing thrombi has become widely accepted during the past decade. New developments in this field have centered around increasing the efficacy of the known plasminogen activators while employing methods to minimize the risk of hemorrhage and decrease the incidence of rethrombosis. Such methods have included the use of thrombus-directed antibodies linked to plasminogen activators, increased plasminogen (substrate) concentration at the thrombus site, anticoagulant and antiplatelet therapy to prevent thrombus propagation and reformation following lysis, and combination plasminogen activator therapy designed to increase efficacy and safety. These new strategies have been extensively tested in vitro and in a variety of animal models. As we have indicated, extrapolation of such results to human patients cannot be done with confidence. However, the strategies are based on sound rationale and the reported findings should serve as the basis for controlled human trials.

Prolonged bleeding time as a marker of venous clot lysis during streptokinase therapy

Despite their expense and inconvenience, serial ultrasound or venographic examinations are currently the only available methods to assess the efficacy of thrombolysis for deep venous thrombosis (DVT). We therefore tested whether the bleeding time (BT), a simple and inexpensive test, is useful in assessing lytic efficacy and might thus be a valuable adjunct in guiding the dose and duration of the thrombolytic agent. Serial BTs were obtained daily in 16 patients (eight men and eight women, average age 45.5 years, range 19 to 70) receiving streptokinase (SK) for proximal lower extremity DVT (n = 5), for upper extremity DVT (n = 10), or for renal vein thrombosis (n = 1). Duration of treatment averaged 89.9 +/- 43.6 hours (range 35 to 198 hours). Clot lysis on ultrasound, venogram, or magnetic resonance imaging (MRI) was defined as: complete (greater than or equal to 90%), moderate (50% to 90%), minimal (less than 50%), or none. Important (complete or moderate) clot lysis occurred in 9 of 15 patients for whom follow-up studies were available. BT prolongation was defined as greater than 9.5 minutes and, during SK therapy, had a sensitivity of 0.67 and a specificity of 0.67 for important clot lysis. The positive predictive value of prolonged BT for clot lysis was 0.75. Calculated likelihood ratios revealed that a patient with BT prolongation was twice as likely to have important clot lysis rather than minimal or no lysis. These findings suggest that the BT should undergo further investigation as a simple, adjunctive, noninvasive marker of thrombolytic efficacy among DVT patients treated with SK.

Streptokinase-induced platelet aggregation. Prevalence and mechanism

BACKGROUND

Streptokinase (SK) is a bacteria-derived protein and one of the plasminogen activators that is currently available for therapeutic use. Exposure to SK induces synthesis of specific antibodies that may initiate platelet aggregation and paradoxical clot propagation during treatment.

METHODS AND RESULTS

Using platelet-rich plasma (PRP), we found that SK (5,000 units/ml) but not urokinase (2,500 units/ml) or recombinant tissue-type plasminogen activator (2,500 units/ml) caused platelet aggregation in PRP from 14 of 100 normal volunteers. In 13 consecutive patients treated with SK for acute myocardial infarction, SK-mediated platelet aggregation was induced in five patients within 1 week after treatment. SK-mediated platelet aggregation was associated with significantly increased titers of both anti-SK antibodies and SK-neutralizing activity in plasma; it was partially inhibited by aspirin (1 mM) and by aprotinin (500 kallikrein inhibitor units/ml) and completely inhibited by tranexamic acid (1 mM) and by prostaglandin E1 (9 microM). Addition of SK (1,000 or 5,000 units/ml) induce a statistically significant dose-dependent thromboxane B2 release in mixtures of PRP with plasma from subjects with SK-induced aggregation but not in samples of PRP mixed with plasma from nonresponders; addition of recombinant tissue-type plasminogen activator (1 or 50 micrograms/ml) did not induce thromboxane B2 release. Mixing experiments with PRP and immunoglobulin G from reactive and nonreactive donors revealed that SK-induced aggregation requires the presence of anti-SK antibodies. When 125I-SK (50 nM) was used, platelets preincubated with plasminogen (0.5 microM) bound 9,500 +/- 600 (mean +/- SEM, n = 6) molecules SK/platelet, which increased to 25,000 +/- 3,100 molecules/platelet after thrombin stimulation. Tranexamic acid (1 mM) blocked specific binding of SK to resting platelets.

CONCLUSIONS

These data demonstrate that SK-induced platelet aggregation is initiated by the binding of anti-SK antibodies to the SK-plasminogen complex located on the platelet surface. SK-induced platelet activation may limit the therapeutic effectiveness of the drug, and in view of the high prevalence of aggregation in a normal population, prospective evaluation of the effects of platelet aggregation during treatment with SK is warranted.

Partial resistance to anticoagulation after streptokinase treatment for acute myocardial infarction

This study examines the response of 3 different groups of patients to anticoagulants: 50 patients previously treated with streptokinase for acute myocardial infarction (AMI) (group 1), 24 patients with AMI who had received anticoagulants without prior thrombolysis (group 2) and 11 subjects who received anticoagulants for noncoronary indications (group 3). No significant differences were detected between groups 2 and 3; therefore, they were combined for analysis. After streptokinase, patients required 37,755 +/- 1,516 (mean +/- standard error of the mean) U of heparin per day to achieve the desired activated partial thromboplastin time (APTT). The dosage was 30,294 +/- 1,089 U/day in patients without antecedent thrombolysis (p less than 0.001). Group 1 patients required 5 +/- 0.4 days until adequate anticoagulation was achieved, compared with 3 +/- 0.2 days in the control group (p = 0.01). Despite higher heparin requirements, group 1 patients had a lower APTT value than the other subjects (87 +/- 5 vs 101 +/- 6 seconds, p = 0.08). Group 1 patients required 5 +/- 0.3 days to reach anticoagulation with warfarin versus 4 +/- 0.2 days in groups 2 + 3 (p = 0.05). Comparison of groups 1 and 2 yielded similar, although smaller, differences. Patients treated with streptokinase for AMI seem to be partially resistant to anticoagulation, which may increase the risk of reocclusion.

Bleeding time and other laboratory tests to monitor the safety and efficacy of thrombolytic therapy

Widespread use of thrombolytic agents in a variety of settings has highlighted the need for measures of safety and efficacy. Previously used laboratory parameters, such as decreasing levels of fibrinogen and increasing levels of fibrin(ogen) degradation products (FDPs), have failed to correlate consistently with hemorrhagic events and have not yet been useful in predicting patients at risk for bleeding. Although the bleeding time (BT) has been considered primarily to reflect platelet function, it also reflects the interaction of platelets with vessel wall and coagulation pathways. Recently, the BT has been considered as a potential predictor of clinical bleeding during thrombolysis. The BT, as a measure of in vivo hemostatic competence, may be particularly well-suited for this application. Serial BTs during thrombolytic therapy may provide valuable information regarding safety and efficacy, but further studies are needed to confirm preliminary findings.

The bleeding time: its potential utility among patients receiving thrombolytic therapy

The BT as a test of hemostatic function was first described 80 years ago. It has generally been considered a primitive and tedious test. Improvements in technique and standardization have increased the BT's reliability and led to its consideration as a preoperative screening measure. Current use has not been widespread, however, except for patients undergoing neurosurgery and organ biopsy. Recently, though, there has been a renewed interest in the BT for patients receiving thrombolytic therapy because levels of fibrinogen and fibrin(ogen) degradation products have been only weak predictors of hemorrhagic complications. The rationale for using the BT in this setting is that thrombolysis appears to impair platelet function, either through depletion of platelet granules or through direct proteolytic actions on platelets. Further research will determine whether these platelet effects are manifest as BT prolongation; whether increased BT will correlate with hemorrhagic complications; and, finally, whether patients who fail to achieve clot lysis or those at risk for bleeding can be identified prospectively.

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.

Tissue plasminogen activator and plasmin independently decrease human neutrophil activation

Tissue-plasminogen activator (t-PA) and plasmin both decrease platelet aggregation, which may contribute to thrombolysis and tissue salvage. Since neutrophils may contribute to reperfusion injury, we examined the effects of t-PA and plasmin on human neutrophil function. t-PA (1 to 100 micrograms/ml) decreased f-MLP-induced chemotaxis and ionophore A23187-induced superoxide and LTB4 release in isolated neutrophils, and these effects were not blocked by the plasmin-inhibitor epsilon-aminocaproic acid (epsilon-ACA). On the other hand, plasmin (0.05 to 0.5 units/ml) also decreased these neutrophil functions but its effects were blocked in the presence of epsilon-ACA. Thus, while both t-PA and plasmin decrease neutrophil functions, effects of t-PA are independent of plasmin generation. Cumulative effects of t-PA and plasmin on neutrophil functions may relate to the overall efficacy of t-PA in thrombotic disorders.

Primary percutaneous coronary angioplasty: a protagonist's view

The wide variety of reperfusion strategies now available has led to confusion regarding the optimal therapy of patients with acute myocardial infarction. In assessing the comparative efficacy of reperfusion strategies, safety factors, recanalization rates, reocclusion rates, myocardial salvage and improvement in survival must be compared. In this analysis, primary percutaneous transluminal coronary angioplasty (PTCA) compares favorably to intravenous thrombolytic therapy or combination PTCA and thrombolytic therapy as a reperfusion strategy. Primary PTCA has not been reported to cause intracerebral hemorrhage, whereas thrombolytic strategies are associated with an incidence of a 0.2 to 1.4% rate of intracerebral bleeding. Strategies using PTCA have recanalization rates of only 33 to 75%. Disappointingly, a major failure of all reperfusion strategies (including primary PTCA) is a high rate of reocclusion and reinfarction. Primary PTCA is associated with a 7 to 16% increase in global ejection fraction. Similar improvement in ejection fraction has not been demonstrated in patients treated with thrombolytic therapy alone. Although in-hospital mortality is improved, a substantial 1-year mortality persists for intravenous thrombolytic therapy. Long-term survival is greater when PTCA therapy is used. In addition, PTCA does favorably improve survival in high-risk patients with cardiogenic shock. Although it is unlikely that most patients with acute myocardial infarction will have primary PTCA as a treatment option, this therapy remains extremely attractive for centers where primary PTCA is readily available.

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.