Relative importance of thrombin compared with plasmin-mediated platelet activation in response to plasminogen activation with streptokinase.

BACKGROUND. Platelet activation occurs in vivo during pharmacologic thrombolysis and may contribute to recurrent thrombosis. Plasmin does not directly activate platelets except at high concentrations; thus, the mechanisms for platelet activation during thrombolysis remain undefined. Increases in thrombin activity also occur in patients treated with fibrinolytic agents and may contribute to activation of platelets. We have shown that one mechanism for increased thrombin activity is activation of the coagulation system by plasmin. METHODS AND RESULTS. In the present study we sought to determine whether activation of platelets in response to pharmacologic activation of plasminogen in plasma is due primarily to plasmin or mediated by increased thrombin activity. Platelet-rich citrated plasma (PRP) was recalcified and incubated with 1,000 IU/ml of streptokinase or 1.0 caseinolytic units/ml of plasmin. Concentrations of fibrinopeptide A, a marker of thrombin activity, increased markedly over 10 minutes in plasma incubated with streptokinase or plasmin, but not in PRP incubated without plasminogen activator. Platelet activation characterized by the secretion of 14C-serotonin occurred within 2-4 minutes after thrombin activity increased. In stirred recalcified PRP, platelet aggregation was accelerated from 3.6 +/- 0.5 to 2.5 +/- 0.3 minutes (p less than 0.01) when incubated with 1,000 IU/ml of streptokinase. Leupeptin and aprotinin, inhibitors of plasmin activity, markedly attenuated platelet activation in response to pharmacologic activation of plasminogen. However, inhibition of thrombin with heparin, hirudin, or D-Phe-D-Pro-L-Arg-chloromethylketone was more effective in inhibiting the acceleration of platelet activation induced by plasminogen activation, despite the elaboration of plasmin activity. CONCLUSIONS. Activation of platelets during coronary thrombolysis may be due in part to increased procoagulant activity induced by plasminogen activation as well as other factors that promote platelet activation in vivo.     

Issues regarding the use of heparin following streptokinase therapy

The success of thrombolytic therapy is dependent upon the balance of fibrinolytic activity and procoagulant activity. Streptokinase produces fibrin degradation products that have anticoagulant effects and may potentially protect against reocclusion. However, streptokinase also activates platelets and thrombin, and the prothrombotic effects may be more marked than after administration of recombinant tissue plasminogen activator (rt-PA). Administration of highdose, delayed subcutaneous heparin after streptokinase and aspirin has been shown to have some benefits and some risks. The benefits and risks of adding intravenous heparin to aspirin and streptokinase have not been clearly defined.     

Thrombolysis in Myocardial Infarction (TIMI) Trial--phase I: hemorrhagic manifestations and changes in plasma fibrinogen and the fibrinolytic system in patients treated with recombinant tissue plasminogen activator and streptokinase

Two hundred ninety patients with acute myocardial infarction were treated according to random assignment with an intravenous infusion of either 80 mg of recombinant tissue plasminogen activator (rt-PA) over 3 h or 1.5 million units of streptokinase over 1 h. Patients received an intravenous bolus of heparin (5,000 U [USP]) before pretreatment coronary angiography and a continuous infusion (1,000 U/h) starting 3 h later. The frequency of major and minor hemorrhagic events (33% rt-PA, 31% streptokinase) and associated transfusions (22% rt-PA, 20% streptokinase) were comparable in both groups. More than 70% of bleeding episodes in each group occurred at catheterization or vascular puncture sites. Precipitable fibrinogen levels, measured in plasma samples collected in the presence of a protease inhibitor (aprotinin), declined in rt-PA and streptokinase groups by averages of 26 and 57% at 3 h and by 33 and 58% at 5 h, respectively (rt-PA versus streptokinase, p less than 0.001). At 5 h the plasma plasminogen declined by 57% (rt-PA) and 82% (streptokinase) (p less than 0.001); plasma fibrin(ogen) degradation products were higher in streptokinase-treated patients (244 +/- 12 micrograms/ml, mean +/- SE) than in rt-PA-treated patients (97 +/- 9 micrograms/ml, p less than 0.001). At 27 h, plasma fibrinogen and plasminogen levels were lower and fibrin(ogen) degradation products higher than pretreatment levels in both groups. The frequency of hemorrhagic events was higher in patients with greater changes in plasma factors at 5 h; within treatment groups the levels of fibrin(ogen) degradation products correlated with bleeding complications (p less than 0.005). Thus, in the doses administered, rt-PA induces systemic fibrinogenolysis that is substantially less intense than that induced by streptokinase. The high frequency of bleeding encountered is related to the protocol used, including vigorous anticoagulation, arterial punctures and thrombolytic therapy. These findings emphasize the need for avoidance of invasive procedures and for meticulous care in the selection and management of patients subjected to thrombolytic therapy.     

Paradoxic elevation of fibrinopeptide A after streptokinase: evidence for continued thrombosis despite intense fibrinolysis

Elevated levels of fibrinopeptide A, a marker of thrombin activity associated with acute myocardial infarction, have been found to decrease after administration of streptokinase when reperfusion occurs. In contrast, in patients without reperfusion and those with reocclusion after streptokinase therapy, fibrinopeptide A remains elevated. In the present study early serial measurements of fibrinopeptide A were used to further characterize this paradoxic increase in thrombin activity after streptokinase and to characterize its response to heparin. In 19 patients with acute myocardial infarction fibrinopeptide A was elevated to 82.3 +/- 43.5 ng/ml (mean +/- SE) before therapy. Thirty minutes after the initiation of streptokinase, fibrinopeptide A increased to 300.1 +/- 117.4 ng/ml (p less than 0.01), consistent with extensive thrombin activity. Fibrinopeptide A remained elevated until 15 minutes after a heparin bolus injection when levels decreased to 15% of the poststreptokinase value (49.2 +/- 13.3 ng/ml) (p less than 0.001). These data document a prompt paradoxic increase in thrombin activity after administration of streptokinase that may be responsible for failure of therapy in some patients.     

Designing thrombolytic agents: focus on safety and efficacy.

Thrombolytic therapy for evolving acute myocardial infarction (AMI) reduces infarct size, preserves ventricular function, and reduces mortality. Intravenous streptokinase is commonly followed by approximately 50% patency of coronary arteries within 90 minutes and by reduction of mortality by 25%. Recombinant tissue plasminogen activator (rt-PA) is more potent for coronary arterial thrombolysis, producing both more rapid and more frequent recanalization (approximately 75% patency at 90 minutes) with a dose of 100 mg given over 3 hours. Side effects (mainly bleeding) associated with the use of streptokinase and rt-PA are not markedly different. That the higher efficacy of rt-PA would translate into a larger reduction of mortality is suggested by the results of several small trials but remains to be confirmed in well-designed comparative clinical trials. This question has not been adequately answered by the recent International rt-PA/streptokinase mortality trial and the International Study on Infarct Survival (ISIS-3) study, because of concerns with respect to the role of conjunctive intravenous heparin administration and the dose of rt-PA used in ISIS-3. All available thrombolytic agents still have significant shortcomings, including the need for large doses to be maximally efficient, a limited fibrin specificity, and a significant associated bleeding tendency. New developments toward improved efficacy and fibrin-specificity of thrombolytic agents include the use of mutants of rt-PA, chimeric rt-PA or single chain urokinase plasminogen activator molecules, and antibody-targeted thrombolytic agents. Some of these artificial plasminogen activators have a 5- to 10-fold increased potency (thrombolytic activity per unit dose), but whether they are safe enough to be clinically useful remains to be established. The conjunctive use of anticoagulants and antiplatelet agents with thrombolytic agents increases their efficacy to an extent that monotherapy with a plasminogen activator alone is no longer tenable. Heparin and aspirin are only moderately efficient for acceleration of lysis and prevention of reocclusion, but are relatively safe. More selective thrombin inhibitors and antiplatelet agents are more potent, but their safety remains to be confirmed. Continued investigation in this area will provide new insights and promote progress toward the development of the ideal thrombolytic therapy, characterized by maximized stable coronary arterial thrombolysis with minimal bleeding.     

A multicenter, randomized, placebo-controlled trial of a new form of intravenous recombinant tissue-type plasminogen activator (activase) in acute myocardial infarction

A new, predominantly single chain preparation of recombinant tissue-type plasminogen activator was evaluated to determine coronary thrombolytic efficacy in 100 patients with acute myocardial infarction. At 3.6 +/- 1.2 hours (mean +/- SD) from symptom onset, patients received either intravenous tissue plasminogen activator (1.25 mg/kg body weight over 3 hours) or placebo on a 3:1 randomized, double-blind basis. Coronary angiography, performed 68 +/- 13 minutes after initiation of the study drug infusion, demonstrated patency of the infarct-related artery in 40 (57%) of 70 patients in the tissue plasminogen activator group compared with 3 (13%) of 23 patients in the placebo group (p less than 0.001). Patients in the placebo group were then eligible to receive intracoronary streptokinase. At 90 minutes the patency was observed in 49 (69%) of 71 tissue plasminogen activator patients compared with 5 (24%) of 21 placebo patients (p less than 0.001). At 120 minutes patency was observed in 59 (79%) of 75 patients of the tissue plasminogen activator group and in 10 (40%) of 25 in the intracoronary streptokinase/placebo group. A nadir value of less than 100 mg/dl fibrinogen occurred in 8 (11%) of 73 patients receiving tissue plasminogen activator versus 8 (40%) of 20 patients treated with intracoronary streptokinase (p = 0.002). Moderate or severe bleeding episodes occurred in 39% of patients treated with tissue plasminogen activator compared with 32% of patients who received placebo/intracoronary streptokinase (p = NS). Thus, this tissue plasminogen activator preparation achieves a high rate of recanalization and, at the doses employed, exhibits increased fibrinogen sparing compared with intracoronary streptokinase.     

Activation of the hemostatic mechanism during thrombolysis in patients with unstable angina pectoris

In patients with myocardial infarction, thrombolytic therapy induces a paradoxical activation of the hemostatic mechanism. In patients with unstable angina, the effect of thrombolysis on the coagulation cascade is unknown. We prospectively measured the plasma concentrations of prothrombin fragment 1 + 2 and fibrinopeptide A in consecutive patients with unstable angina randomized to receive placebo alone (n = 23), streptokinase 1,500,000 IU over 1 hour followed by a 48-hour placebo infusion (n = 21), or streptokinase 250,000 over 1 hour followed by a continuous infusion of 100,000 IU per hour over 48 hours (n = 20). All the patients received intravenous heparin for 72 hours. The plasma levels of the different markers were measured at baseline, 90 minutes, 24 hours, and 48 hours after the start of therapy. The median baseline plasma concentrations of prothrombin fragment 1 + 2 and fibrinopeptide A were similar in the three treatment groups. In comparison with placebo, an increase in plasma prothrombin fragment 1 + 2 and fibrinopeptide A, was observed after 90 minutes in the two groups receiving thrombolysis. After 24 and 48 hours, the prothrombin fragment 1 + 2 levels remained significantly higher only in the patients receiving the 48-hour streptokinase infusion. In patients with unstable angina, thrombolytic therapy induces an activation of the hemostatic mechanism, despite concomitant heparin administration; in those receiving a prolonged streptokinase infusion, the activation of coagulation persists for as long as the drug is administered.     

Thrombolysis with recombinant tissue plasminogen activator in atherosclerotic thrombotic occlusion

Human tissue plasminogen activator holds promise for the dissolution of coronary thrombi by intravenous administration and without systemic anticoagulation. Prior animal experiments have been conducted only in vessels without disease. To test the thrombolytic efficacy of recombinant tissue plasminogen activator in the presence of diseased intima, an established model of atherosclerosis was utilized. The aorta of 16 New Zealand white rabbits (2 to 3 kg) was made atherosclerotic by balloon endothelial denudation and concurrent 1% cholesterol feeding for 8 weeks. An aged (24 hour) heterologous (human) clot, labeled with I-125 fibrinogen was injected into the distal aorta and produced thrombotic occlusion. After 1 hour of thrombosis (control period), recombinant tissue plasminogen activator (100,000 IU approximately equal to 1 mg protein, n = 6) or streptokinase (100,000 IU, n = 5) or saline solution (n = 5) was systemically infused over 30 minutes. Serial blood samples, obtained to determine fractional change in blood radioactivity over time, showed a fourfold increase of blood radioactivity after tissue plasminogen activator and streptokinase infusion compared with the control period (47,400 +/- 3,300 [mean +/- standard error] versus 11,800 +/- 300 counts/min, p less than 0.001). Time to 50% of maximal thrombolysis was 41 +/- 14 minutes (+/- standard deviation) for tissue plasminogen activator versus 63 +/- 16 minutes for streptokinase (p less than 0.01). In six of six rabbits receiving tissue plasminogen activator and four of five rabbits receiving streptokinase, reestablishment of distal aortic flow was detected via the indwelling catheter within 25 minutes of drug infusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Efficacy and safety of low-dose streptokinase plus desmopressin in acute myocardial infarction: A pilot study

In this pilot study the combined use of desmopressin, which releases tissue plasminogen activator from vascular endothelium, and a low dose of streptokinase as a new thrombolytic regimen for acute myocardial infarction is proposed. Eighteen patients with acute myocardial infarction were treated intravenously with 150,000 U (4 patients) or 250,000 U (14 patients) of streptokinase infused over 10 minutes, followed by 24 g of desmopressin infused over 5–10 minutes. Aspirin and beta-blockers were administered at admission, and heparin and oral anticoagulants were started at the end of the thrombolytic regimen. Hemostatic parameters were studied before and 30, 60, 120, and 240 minutes after starting thrombolytic therapy. Fifteen patients (83.3%) had evidence of clinical reperfusion. Angiography was performed with a mean delay of 8.8 hours (range 1.5–22 hours) from the start of thrombolytic therapy. Fourteen patients (77.8%) had patency of the infarct-related artery: 10 patients (55.6%) achieved TIMI grade 3, and 4 patients (22%) achieved TIMI grade 2. Two patients (one TIMI grade 1 and one TIMI grade 2) underwent coronary angioplasty. No patient died during the in-hospital period. At 18 months follow-up, all patients are alive. No major or minor bleeding was detected. The significant decline in plasma fibrinogen and in the euglobulin lysis time, and the increase in fibrinogen/fibrin degradation products, indicate a plasma lytic state. Crosslinked fibrin degradation products increased from 310±120 ng/ml to 670 ±310 ng/ml (p=0.009), suggesting that fibrin digestion occurred in vivo. This pilot study provides data supporting the feasibility and efficacy of a new and more economic thrombolytic treatment of acute myocardial infarction without hemorrhagic complications.     

Fibrinopeptide A levels indicative of pulmonary vascular thrombosis in patients with primary pulmonary hypertension

Although the mechanisms involved in the pathophysiology of primary pulmonary hypertension have not yet been delineated, thrombosis has been implicated. This study was designed to determine whether thrombin activity as reflected by plasma concentrations of fibrinopeptide A (FPA), a marker of the action of thrombin on fibrinogen, is increased in patients with primary pulmonary hypertension. To evaluate fibrinolytic activity, we measured plasma concentrations of tissue-type plasminogen activator, plasminogen activator inhibitor-1, and cross-linked fibrin degradation products. We studied 31 patients with primary pulmonary hypertension. Plasma FPA concentrations measured by radioimmunoassay, were elevated to 87.4 +/- 36.9 ng/ml (mean +/- SEM). Fifteen minutes after administration of heparin (5,000 U), FPA concentrations decreased to 6.8 +/- 1.4 ng/ml (p less than 0.001 compared with preheparin levels). In 21 of 30 patients (70%), FPA concentrations after heparin administration were less than half the preheparin levels, a response consistent with inhibition of thrombin by heparin and the short half-life of FPA. Despite evidence for marked thrombin activity, plasma concentrations of cross-linked fibrin degradation products were normal in all but four patients. Plasminogen activator inhibitor-1 activity was elevated in 19 of the 27 patients in whom it was measured, potentially limiting the fibrinolytic response. The elevations of FPA indicate that thrombin activity is increased in vivo in patients with primary pulmonary hypertension. Thus, sequential assays of plasma markers of thrombosis and fibrinolysis in vivo may help identify those patients who may benefit from treatment with anticoagulants.     

Clot lysis induced by a monoclonal antibody against alpha 2-plasmin inhibitor

A monoclonal antibody (MoAb) to alpha 2-plasmin inhibitor designated JTPI-1 inhibited antiplasmin activity by interfering with formation of alpha 2-plasmin inhibitor (alpha 2-PI)-plasmin complex. With this MoAb, we observed plasma clot lysis in vitro and evaluated the potential of JTPI-1 to serve as a new therapeutic agent for thrombolysis. After adding 125I-labeled fibrinogen to plasma, clots were made by adding thrombin and calcium and were then resuspended in normal plasma containing various concentrations of JTPI-1. The presence of JTPI-1 enhanced release of the soluble 125I-labeled fibrin degradation fragment from the clots in a dose-dependent manner. With tissue plasminogen activator (t-PA)-depleted plasma, we showed that induction of clot lysis by JTPI-1 was dependent on fibrin-bound endogenous t-PA. Regulation of fibrinolysis initiated on the fibrin surface by fibrin-bound t-PA and plasminogen is mediated by alpha 2-PI cross-linked to fibrin by activated factor XIII. JTPI-1 bound to this cross-linked alpha 2-PI neutralized its activity and induced partial digestion of fibrin by plasmin. This resulted in additional binding of Glu-plasminogen to fibrin during the incubation. When 1.2 mumol/L JTPI-1 and 5 U/mL exogenous t-PA were present in the suspending plasma, the rate of clot lysis was essentially the same as that induced by 60 U/mL exogenous t-PA alone. These results suggest that JTPI-1 may be useful in reducing the amount of t-PA administered for thrombolytic therapy.     

Failure of fixed dose intravenous heparin to suppress increases in thrombin activity after coronary thrombolysis with streptokinase

Objectives. This study was designed to define the extent of inhibition of thrombin activity achieved with conjunctive fixed dose intravenous sodium heparin compared with fixed dose subcutaneous calcium heparin in patients receiving intravenous streptokinase for acute myocardial infarction.Background. The role of heparin therapy during coronary thrombolysis with streptokinase is controversial, in part because the efficacy of different conjunctive heparin regimens in inhibiting early increases of thrombin activity is not known.Methods. Twenty-eight patients treated with 1.5 million U of streptokinase and 165 mg of aspirin for acute myocardial infarction were randomly assigned to receive fixed dose subcutaneous heparin therapy (12,500 U every 12 h delayed until 4 h after the end of streptokinase therapy [n = 14]) or fixed dose intravenous heparin (5,000-U bolus followed by 1,000-U/h infusion [n = 14]). Anticoagulation was assessed with serial measurements of activated partial thromboplastin time, and thrombin activity by measuring fibrinopeptide A and thrombin-antithrombin III complex levels. Plasma concentrations of creatine kinase (CK) MM isoforms were measured for 3 h to determine recanalization (increase la activity > 0.18%/min).Results. Recanalization occurred in 27%, 64% and 79% of patients given subcutaneous heparin versus 43%, 76% and 86% of those given intravenous heparin at 1, 2 and 3 h, respectively (p = 0.6). Concentrations of fibrinopeptide A (mean ± SEM) at 1 h were higher in patients without (n = 5) than in those with (n = 23) CK-MM isofona criteria for recanalization (76.4 ± 25.7 vs. 25.2 ± 52 nmol/liter, p = 0.02), and at 1, 2 and 3 h were significantly lower with fixed dose intravenous heparin (18.4 ± 4.8 vs. 46.7 ± 102 nmol/liter at 1 h, p = 0.004) than without heparin. After fixed dose subcutaneous heparin at 4 h, fibrinopeptide A levels were similar in both groups despite lower activated partial thromboplastin times in patients who received fixed dose subcutaneous heparin. However, fibrinopeptide A was not consistently suppressed in either group (fixed dose subcutaneous heparin 8.7 ± 1.8 nmol/liter vs. fixed dose intravenous heparin 11.8 ± 5.2 nmol/liter) at 48 h (p = 0.4). No significant changes in the concentration of thrombin-antithrombin III complexes were found between the two groups.Conclusions. Fixed dose intravenous heparin attenuates increases in fibrinopeptide A early after streptokinase. Subsequent fixed dose intravenous and subcutaneous heparin have similar effects but are relatively ineffective in suppressing thrombin activity, suggesting a role for more potent antithrombin agents during coronary thrombolysis with streptokinase.     

Cost, uncertainty, and doctors' decisions. The case of thrombolytic therapy.

OBJECTIVE--To examine the prescribing patterns and predictors of physician choice in the selection of intravenous thrombolytic therapy for the treatment of acute myocardial infarction. DESIGN--Survey of a random sample of 250 California cardiologists between July 1989 and February 1990. RESULTS--The adjusted response rate was 66%. For a patient presenting within 30 minutes after the onset of an uncomplicated acute anterior-wall myocardial infarction, 98% of respondents reported that they would prescribe a thrombolytic agent, 79% chose tissue plasminogen activator, and 21% chose streptokinase. Users of tissue plasminogen activator were nine times more likely than streptokinase users to perceive tissue plasminogen activator as superior for early coronary artery recanalization, although most users of tissue plasminogen activator and streptokinase perceived no difference between the two agents for improvement in ejection fraction and mortality. Estimates of side effects did not distinguish the two groups. Users of streptokinase were eight times more likely to practice in a health maintenance organization than were users of tissue plasminogen. For a self-paying patient, 36% of users of tissue plasminogen activator said that they would switch to streptokinase, and 27% would switch for a patient insured by Medicaid. CONCLUSIONS--These results indicate that physicians place great emphasis on surrogate end points. Physicians vary in their willingness to use more expensive therapies over cheaper alternatives, even when their perceptions of the relative risks and benefits are similar. The patient's insurance status and the provider's practice setting may exert an important influence on doctors' clinical choices.     

Fibrin degradation products generation and fibrinopeptide A release in normal plasma incubated with thrombolytic agents: proposed mechanisms.

Clinical data have shown that the evaluation of fibrin degradation products (FbDP) does not reflect the efficiency of thrombolytic therapy in vivo. In this study, we found that the addition of plasminogen activators to normal plasma resulted in generation of FbDP and release of fibrinopeptide A (FpA) as shown by ELISA and HPLC. This FpA release was concomitant with fibrinogen degradation, and was not inhibited by thrombin inhibition or by prothrombin depletion in plasma. Thus, the increase in FpA did not result from coagulation activation and may result from the plasmin-induced release of FpA from fibrinogen degradation product E1. The generation of cross-linked FbDP after tPA addition occurred in normal plasma as well as in factor-XIII-deficient plasma and quickly reached a plateau. It was not inhibited by hirudin. Therefore FbDP in these plasmas probably derived from the plasmin degradation of cellular transglutaminase cross-linked fibrin/fibrinogen derivatives present in plasma.     

Safety of helicopter transport and out-of-hospital intravenous fibrinolytic therapy in patients with evolving myocardial infarction

Of 150 consecutive patients with acute myocardial infarction transported by helicopter for acute intervention, 55 had intravenous thrombolytic therapy (tissue plasminogen activator in 12, streptokinase in 43) initiated prior to transfer. Patients were transported 55 +/- 10 ground miles in 17 +/- 6 minutes and no patient died or experienced bleeding or hemodynamic instability during transfer. Patients receiving thrombolytic therapy had a higher incidence of arrhythmias during transit compared to the untreated group, ventricular tachycardia in six and third-degree atrioventricular block in one compared to ventricular tachycardia in one patient, respectively (p = 0.005). However, these arrhythmias were transient and did not require cardioversion, temporary pacing, or further antiarrhythmic medical treatment. Chest pain was relieved or decreased more frequently in the patients receiving thrombolytic therapy vs. those untreated; 21 of 55 vs. 21 of 95 respectively (p = 0.04). Immediate coronary angiography confirmed a higher incidence and more complete infarct vessel patency (34/55 vs. 30/95) in the patients receiving tissue plasminogen activator or streptokinase (p less than 0.001). Thus, helicopter transfer of patients with evolving myocardial infarction is safe, and early initiation of thrombolytic therapy is associated with increased infarct vessel patency and benign reperfusion arrhythmias.     

Hirudin and Hirulog

A brief outline is presented of a proposed trial of Hirulog versus heparin after thrombolytic therapy with streptokinase (SK). The lower patency rates achieved with SK compared with tissue plasminogen activator (t-PA) suggest that a potent thrombin specific agent may be more important for SK than for t-PA therapy. This study will involve more than 400 patients with two dose levels of Hirulog. Endpoints will be angiographic patency, and ST segment monitoring showing evidence of coronary reperfusion.     

The profibrinolytic effect of activated protein C in clots formed from plasma is TAFI-dependent

Thrombin-activatable fibrinolysis inhibitor (TAFI) is the precursor of an exopeptidase that is identical to plasma procarboxypeptidase B. Upon activation by thrombin, activated TAFI (TAFIa) attenuates fibrinolysis, presumably by catalyzing the removal of C-terminal lysines from partially degraded fibrin. Activated protein C (APC) proteolytically inactivates the essential cofactor in prothrombinase, factor Va, and limits both the formation of thrombin and subsequent activation of TAFI, thereby appearing profibrinolytic. TAFI is able to reconstitute an APC-dependent shortening of lysis time in a purified system; however, it remained to be determined the extent to which TAFI is involved in the profibrinolytic effect of APC in a plasma-based system. To aid in addressing this question, two monoclonal antibodies (MoAbTAFI#16 and #13) and a polyclonal antibody were produced against purified TAFI. MoAbTAFI#16 was shown to inhibit TAFI activation and thereby appears to stimulate fibrinolysis. Furthermore, an enzyme- linked immunosorbent assay was developed using MoAbTAFI#13 and the polyclonal antibody. Through its use, the plasma concentration of TAFI was determined to be 73 nmol/L. In addition, a turbidity assay was used to determine the effect of APC on tissue plasminogen activator-induced fibrinolysis of clots produced from normal human plasma (NHP), plasma immunodepleted of TAFI (TdP), and TdP reconstituted with purified TAFI. APC shortened lysis time of clots produced from NHP in a saturable and concentration-dependent manner. However, APC had no effect on lysis time of clots formed from either TdP or NHP in the presence of 80 nmol/L MoAbTAFI#16. The APC effect could be reconstituted in TdP by the addition of purified TAFI. The lysis time in TdP was increased from 50 to 180 minutes in a TAFI concentration-dependent manner. The EC50 was 15 nmol/L and saturation was approached at physiologically relevant concentrations (60 nmol/L). The profibrinolytic effect of APC was also compared with that of MoAbTAFI#16 and two competitive inhibitors, an inhibitor of the carboxypeptidase A and B family purified from potato tubers and 2-Guanidinoethylmercaptosuccinic acid (GEMSA). All were able to reduce lysis time of clots formed from normal human plasma by 90 minutes, yielding respective EC50 values of 5 nmol/L, 15 nmol/L, 50 nmol/L, and 90 mumol/L. Therefore, the majority of the profibrinolytic effect of APC, in an in vitro plasma system, is dependent on TAFI. Because TAFIa dramatically influences lysis time, inhibitors of TAFIa or TAFI activation may prove to be important adjuvants for thrombolytic therapy.     

Regulation by alpha 2-antiplasmin and fibrin of the activation of plasminogen with recombinant staphylokinase in plasma

The effects of alpha 2-antiplasmin and fibrin on the activation of plasminogen by recombinant staphylokinase (STAR) were studied in an effort to elucidate further the molecular basis of the fibrin- specificity of this fibrinolytic agent. In purified systems consisting of 1.5 mumol/L intact or low-M(r) plasminogen and 3 mumol/L alpha 2- antiplasmin, at 37 degrees C and in the absence of fibrin, STAR did not induce plasminogen activation and plasmin-alpha 2-antiplasmin complex (PAP) formation. Addition of a purified fibrin clot (30% vol at a concentration of 3 mg/mL) to mixtures containing intact plasminogen caused approximately 40% plasminogen activation within 2 hours, whereas in mixtures containing low-M(r) plasminogen, no activation was observed. In contrast, 10 nmol/L streptokinase (SK) induced 74% to 100% plasminogen activation within 2 hours in mixtures containing either intact or low-M(r) plasminogen, in both the absence and the presence of fibrin. In citrated human plasma in the absence of fibrin, 30 nmol/L STAR did not induce measurable plasminogen activation and PAP formation (< 1.5% within 2 hours), whereas addition of a plasma clot (12% vol) resulted in complete clot lysis and conversion of 19% +/- 8% of the plasminogen to PAP within 2 hours. Addition of a second plasma clot produced 23% +/- 2% additional plasminogen activation. Equipotent concentrations for plasma clot lysis of SK (100 nmol/L) induced 54% +/- 11% plasminogen activation in the absence and 49% +/- 16% in the presence of fibrin. Addition of 50 mmol/L 6-aminohexanoic acid (6-AHA) abolished the effect of fibrin on plasminogen activation with STAR, but not on activation with SK. In alpha 2-antiplasmin-depleted human plasma in the absence of fibrin, 30 nmol/L STAR did not induce fibrinogen breakdown (> 90% residual fibrinogen after 6 hours), whereas 30 nmol/L preformed plasmin-STAR complex induced extensive fibrinogen degradation (70% within 20 minutes). Thus, in the absence of fibrin, alpha 2- antiplasmin inhibits the activation of plasminogen by STAR, by preventing generation of active plasmin-STAR complex. Fibrin stimulates plasminogen activation by STAR via mechanisms involving the lysine- binding sites of plasminogen, probably by facilitating the generation of plasmin-STAR complex and by delaying its inhibition at the clot surface.     

Thrombolytic versus fibrinogenolytic activity of rt-PA and streptokinase in patients with acute myocardial infarction

In this study the concentration of plasma breakdown products of cross-linked fibrin (XDP), serum fibrinogen-fibrin degradation products (FDP), and plasma fibrinogen were measured before and at the end of the administration of single-chain recombinant tissue-type plasminogen activator (rt-PA, 100 mg IV over three hours) or streptokinase (1.5 million units over one hour), respectively, in two groups, each composed of 22 patients with acute myocardial infarction. The XDP concentration was not statistically different between the two groups at the end of thrombolytic treatment, whereas FDP and fibrinogen concentrations were significantly different (FDP: streptokinase 396 +/- 287 vs rt-PA 177 +/- 222 micrograms/mL, p less than 0.01; fibrinogen: streptokinase 71 +/- 43 vs rt-PA 181 +/- 49 mg/dL, p less than 0.001). These results indicate that the two drugs have equipotent thrombolytic activity at this administration regimen but that rt-PA causes a markedly more selective lysis of fibrin in comparison with streptokinase.