Thrombolytic therapy in acute myocardial infarction

Possible erythrocyte contributions to and exacerbation of the post-thrombolytic no-reflow phenomenon

BACKGROUND

Reperfusion injury often occurs with therapeutic intervention addressing the arterial occlusions causing acute myocardial infarction and stroke. The no-reflow phenomenon has been ascribed to leukocyte plugging and blood vessel constriction in the microcirculation.

OBJECTIVE

To assess possible red cell contributions to post-thrombolytic no-reflow phenomenon.

METHODS

Blood clots were formed by recalcifying 1 ml of citrated fresh human venous blood and then lysed by adding 1,000 units of streptokinase (SK) at several intervals within 1 hour. Red cell deformability was tested by both a microscopic photometric and a filtration technique, viscosity by a cone and plate viscometer, and erythrocyte aggregation by an optical aggregometer.

RESULTS

Two sampling methods were devised for the microscopic photometric test, both of which indicated increases of erythrocyte stiffness after being lysed from the clot by SK. In accompanying experiments, the viscosity, aggregation and filterability of the post-lytic erythrocytes were assessed. Results indicated increased viscosity in Ringer's, decreased aggregation index and filterability through a 5 μm pore size Nuclepore membrane.

CONCLUSION

Findings demonstrated that post-lytic changes in red cell deformability do occur which could contribute to the no-reflow phenomenon.

Signal and propeptide processing of human tissue plasminogen activator: activity of a pro-tPA derivative

We have explored the heterogeneity in the proteolytic processing of the N-terminus of human tissue plasminogen activator. We demonstrate that normal propeptide processing occurs following Arg-4, preceding the sequence Gly-Ala-Arg-Ser+1. Generation of the previously designated Ser+1 occurs via secondary proteolysis following secretion. By site-directed mutagenesis, we have eliminated this cleavage site resulting in a derivative containing the propeptide sequence. N-terminal sequence analysis of this form indicated that signal peptide cleavage occurs following Ser-13. The pro-tPA derivative had near normal serine protease and plasminogen activating activities, and could be stimulated by fibrin. An additional derivative, containing the tribasic sequence from the human protein C propeptide preceding Ser+1, was secreted with full processing of the propeptide. Our data have defined the cleavages for the signal peptide and propeptide and demonstrate that a tribasic sequence can be used to eliminate N-terminal heterogeneity in this molecule. In addition, we demonstrate that, unlike several other serine proteases, a propeptide sequence does not alter the activity of this enzyme.

Thrombolytic activity of a novel plasminogen activator, LY210825, compared with recombinant tissue-type plasminogen activator in a canine model of coronary artery thrombosis

LY210825, a recombinant tissue-type plasminogen activator (rt-PA), which contains the kringle-2 and serine protease functional domains of native tissue-type plasminogen activator, was previously produced by site-directed mutagenesis in a Syrian hamster cell line. We studied the thrombolytic potential of this molecule in a canine thrombosis model. Male hounds (16-22 kg) were anesthetized; a 2.0-cm segment of the left circumflex coronary artery (LCX) was isolated proximal to the first main branch, and the dogs were instrumented with an electromagnetic flow probe to measure coronary blood flow. An occlusive thrombus was formed after injury of the intimal surface of the LCX with an electrical current applied by a needle-tipped anode placed distal to the electromagnetic flow probe. After 1 hour of occlusion, either LY210825 or rt-PA was administered intravenously according to the following protocols: 1) a 1-hour infusion of either 0.25 mg/kg LY210825 or 0.4 mg/kg rt-PA, 2) single injections of 0.15-0.6 mg/kg LY210825, and 3) a single injection of 0.45 mg/kg LY210825 and a 3-hour infusion of 1.0 or 1.7 mg/kg rt-PA. Plasma half-lives of LY210825 and rt-PA were 58 +/- 7 and 3.3 +/- 0.3 minutes, respectively. LY210825 produced more rapid reperfusion of the LCX than did rt-PA. In the third study, 90% of the rt-PA-treated vessels reoccluded within 1 hour after cessation of drug, whereas only 25% of the LY210825-treated vessels reoccluded during a 4-hour washout period. There were significant, but relatively small, reductions produced by both plasminogen activators on plasma fibrinogen and plasminogen (25-35% decreases). Because of its longer plasma half-life, LY210825 could be administered intravenously as a single injection. In a canine model of coronary artery thrombosis, LY210825 was a more effective thrombolytic agent than was rt-PA.

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.