Influence of heparin and systemic lysis on coronary blood flow after reperfusion induced by the novel recombinant plasminogen activator BM 06.022 in a canine model of coronary thrombosis

Fibrinolytic Compromise by Synthetic and Recombinant Thrombin Inhibitors: Implications in the Treatment of Thrombotic Disorders

Site-directed synthetic and recombinant antithrombin agents are widely used to prevent reocclusion during thrombolytic therapy in acute myocardial infarction and other vascular occlusive disorders. However, minimal studies have been conducted to examine the interactions between these anticoagulant and thrombolytic agents. Because of the structural homology of serine proteases, some of these newly developed antithrombin agents are also capable of inhibiting fibrinolytic enzymes, which may lead to fibrinolytic compromise during thrombolytic therapy. In addition, inhibition of thrombomodutin-bound thrombin may also result in fibrinolytic deficit. Several thrombin inhibitors were studied in in vitro systems to assess whether their inactivating properties extend to fibrinolytic and profibinolytic enzymes, such as kallikrein, plasmin, urokinase, streptokinase, and tissue plasminogen activator. The thrombin inhibitors studied included hirudin, hirulog-I, argatroban, D-MePhe-Pro-Arg-H, and Ac-D-Phe-Pro-boroArg-OH. Their activities were compared with those of aprotinin, which is currently used clinically as an antifibrinolytic agent. Although argatroban, hirulog-I, and hirudin exhibited minimal inhibition of the nonthrombin enzymes studied, the tripeptide derivatives showed variable inhibitory activities, with the boronic acid derivative being the most potent and universal inhibitor. The in vivo antifibrinolytic activities of these thrombin inhibitors were also studied in a rabbit model of jugular vein clot lysis. In agreement with the in vitro studies, argatroban, hirulog-I, and hirudin exhibited minimal antifibrinolytic activities, while Ac-D-Phe-Pro-boroArg-OH and D-MePhe-Pro-Arg-H showed marked inhibition of the thrombolytic process. The results of these studies indicate that newly developed thrombin inhibitors with a broader serine protease spectrum may exhibit fibrinolytic compromise resulting in diminishment of the expected thrombolytic outcome. Key Words: Thrombolysis—Fibrinolysis—Thrombin inhibitors—Argatroban—Hirudin—Hirulog-I—Aprotinin.

Thrombolytic therapy in acute myocardial infarction

Early reperfusion of thrombotically occluded coronary arteries by thrombolytic therapy has become a routine option in initial therapy of acute myocardial infarction. Many efforts have been made to improve the biological properties of thrombolytic agents in terms of fibrin specificity, plasma half-life and resistance to natural plasma inhibitors, to improve adjuvant therapy and to shorten the 'pain to reperfusion' time. Numerous randomised, multicentre trials have analysed the benefit of the various thrombolytic agents and regimens, which has enabled the creation of a 'current standard of therapy'. This review presents an update on available thrombolytic agents, their biochemical and pharmacological properties and results from clinical trials.

Prevention of microvascular thrombosis with short-term infusion of human tissue-type plasminogen activator

A locally active thrombolytic agent, human tissue-type plasminogen activator (t-PA), given over a finite time period (24 hours) by local infusion, maintains long-term microvascular patency (7 days) in a proven thrombosis model using an arterial inversion graft in the rabbit model. Thirteen rabbits in the control group and 16 rabbits in the experimental group underwent an arterial inversion graft followed by continuous infusion (24 hours) with human tissue-type plasminogen activator (experimental) or normal saline (control). No significant clinical bleeding or alteration of coagulation parameters was noted in hematologic studies in both experimental and control groups. Scanning electron microscopy of the postoperative human tissue-type plasminogen activator-perfused arteries suggests an interaction of the human tissue-type plasminogen activator with specific platelet receptors in reversing microvascular thrombosis by decreasing or preventing further platelet aggregation and adhesion. Human tissue-type plasminogen activator infused locally for a finite period (24 hours) allows adequate time for platelet metamorphosis to occur in converting a thrombogenic to a nonthrombogenic vessel surface. The clinical ramifications in preventing or reversing microvascular thrombosis in free-tissue transfers and replantation surgery are apparent. Further study in this area will enhance our understanding of the pathogenesis and prevention of microvascular thrombosis.

Comparison of desulfatohirudin (REVASC) and heparin as adjuncts to thrombolytic therapy with reteplase in a canine model of coronary thrombosis

1. We compared the direct thrombin inhibitor, desulfatohirudin (REVASC) and the indirect thrombin inhibitor, heparin, as adjuncts to thrombolytic therapy with reteplase in a canine model of coronary artery thrombosis. 2. Reteplase (BM 06.022) is a recombinant unglycosylated variant of human tissue-type plasminogen activator. Thrombus formation in anaesthetized open chest dogs was induced by electrical injury. Left circumflex coronary artery blood flow was monitored for 210 min with an electromagnetic flow probe. Twenty eight dogs were randomized to receive i.v. heparin (120 iu kg-1 bolus plus 80 iu kg-1 per h) or i.v. hirudin (2.0 mg kg-1 bolus plus 2.0 mg kg-1 per h) 10 min before thrombolysis preceded by i.v. acetylsalicyclic acid (20 mg kg-1) 5 min prior to anticoagulation. Every dog received an i.v. double bolus injection of 0.14 + 0.14 u kg-1 ( = 0.24 + 0.24 mg kg-1) reteplase, 30 min apart, 1 h after thrombus formation. 3. At comparable reperfusion rates (12 out of 12 vs. 15 out of 16 dogs), hirudin enhanced time to reperfusion (14.3 +/- 1.4 vs. 23.2 +/- 3.4 min; P < 0.05) and completely prevented reocclusion after reperfusion in contrast to heparin (0 out of 11 vs. 7 out of 11 dogs; P < 0.05). Coronary blood flow quality was improved by hirudin as shown by a higher maximum blood flow after reperfusion (130 +/- 14.3 vs. 83 +/- 9.3% of baseline; P < 0.05), a higher blood flow level at 20, 30, 40, and 50 min after onset of thrombolysis (P < 0.05) and a longer cumulative patency time (195 +/- 1.7 vs. 166 +/- 12 min; P < 0.05). Activated partial thromboplastin time and buccal mucosa bleeding time were prolonged (P < 0.05) by either anticoagulant, but did not differ significantly between groups. 4. The direct thrombin inhibitor, desulfatohirudin, enhanced thrombolysis, prevented reocclusion and increased blood flow as compared with the indirect thrombin inhibitor, heparin, when investigated at one dose level each and used in conjunction with reteplase.

Novel thrombolytic agents

The fibrinolytic system comprises an inactive pro-enzyme, plasminogen, that is converted by plasminogen activators to the active enzyme, plasmin, that degrades fibrin. Two immunologically distinct plasminogen activators have been identified: tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). Plasminogen activation is regulated by specific molecular interactions between its main components, as well as by controlled synthesis and release of plasminogen activator inhibitors, primarily from endothelial cells. The observed association between abnormal fibrinolysis and a tendency toward bleeding or thrombosis demonstrates the (patho)physiological importance of the fibrinolytic system. Transgenic animals are a suitable experimental model to examine the in vivo impact of fibrinolytic components in thrombosis and thrombolysis. Inactivation, by homologous recombination, of the tissue-type plasminogen activator genes in mice impairs thrombolysis in a significant manner whereas inactivation of the plasminogen activator-1 gene enhances the rate of spontaneous lysis. Despite their widespread use all currently available thrombolytic agents suffer from a number of significant limitations, including resistance to reperfusion, the occurrence of acute coronary reocclusion and bleeding complications. Therefore, the quest for thrombolytic agents with a higher thrombolytic potency, specific thrombolytic activity and/or a better fibrin-selectivity continues. Several lines of research toward improvement of thrombolytic agents are being explored, including the construction of mutants and variants of plasminogen activators, chimeric plasminogen activators, conjugates of plasminogen activators with monoclonal antibodies, or plasminogen activators from animal or bacterial origin.