Fibrin specificity and procoagulant effect related to the kallikrein-contact phase system and to plasmin generation with double-bolus reteplase and front-loaded alteplase thrombolysis in acute myocardial infarction

Fibrinolytic Agents in Thromboembolic Diseases: Historical Perspectives and Approved Indications

Fibrinolytic agents catalyze the conversion of the inactive proenzyme plasminogen into the active protease plasmin, degrading fibrin within the thrombus and recanalizing occluded vessels. The history of these medications dates to the discovery of the first fibrinolytic compound, streptokinase, from bacterial cultures in 1933. Over time, researchers identified two other plasminogen activators in human samples, namely urokinase and tissue plasminogen activator (tPA). Subsequently, tPA was cloned using recombinant DNA methods to produce alteplase. Several additional derivatives of tPA, such as tenecteplase and reteplase, were developed to extend the plasma half-life of tPA. Over the past decades, fibrinolytic medications have been widely used to manage patients with venous and arterial thromboembolic events. Currently, alteplase is approved by the U.S. Food and Drug Administration (FDA) for use in patients with pulmonary embolism with hemodynamic compromise, ST-segment elevation myocardial infarction (STEMI), acute ischemic stroke, and central venous access device occlusion. Reteplase and tenecteplase have also received FDA approval for treating patients with STEMI. This review provides an overview of the historical background related to fibrinolytic agents and briefly summarizes their approved indications across various thromboembolic diseases.

Efficacy evaluation of reteplase in a novel canine acute pulmonary thromboembolism model developed by minimally invasive surgery and digital subtraction angiography

Purpose

In order to evaluate the thrombolytic effects of reteplase in pulmonary thromboembolism (PTE), we developed a novel canine PTE model. The efficacy of reteplase against PTE in comparison to alteplase was clarified for the first time, and this PTE model could be further applied to studies of novel thrombolytic therapies.

Patients and methods

Twenty-four dogs were divided into four groups: sham operation, vehicle, alteplase, and reteplase. Autologous thrombi/saline were injected into the pulmonary artery, and thrombolytic agents were administrated. Thrombus formation and dissolution were monitored by real-time digital subtraction angiography (DSA), and pulmonary pressures were measured simultaneously. Blood coagulation, blood gas, hematology, and histopathologic examinations were used as subsidiary methods.

Results

The canine PTE model was established with a significant decrease of blood flow and ~75% blocking area. Administration of reteplase (0.6 mg/kg) resulted in effective thrombus dissolution with a recovery of over 80% blood flow, as effective as alteplase (1.6 mg/kg). Correspondingly, the elevated pulmonary systolic, diastolic, and mean arterial pressures declined to the normal level. Blood coagulation was changed by reteplase, with a dramatic elongation of prothrombin time, activated partial thromboplastin time, and thrombin time, even longer than alteplase. In contrast to the vehicle group, no obvious pathological changes were found in the two thrombolytic groups. Hematological, blood biochemical, and blood gas results also indicated that reteplase had no adverse reactions in this PTE model.

Conclusion

Reteplase proved to be an effective and safe therapy for PTE for the first time, and a small dosage of reteplase exerted an efficacy comparable to the routine dosage of alteplase. Our findings indicated the potential of reteplase as clinical treatment against PTE. This technically innovative, stability- and validity-proved canine PTE model developed by minimally invasive surgery and DSA resembled major clinical features. This may further facilitate our understanding of thrombotic disorders and development of prophylactic and therapeutic approaches.

Microvascular thrombosis, fibrinolysis, ischemic injury, and death after cerebral thromboembolism are affected by levels of circulating α2-antiplasmin

OBJECTIVE

Ischemic stroke is primarily attributable to thrombotic vascular occlusion. Elevated α2-antiplasmin (a2AP) levels correlate with increased stroke risk, but whether a2AP contributes to the pathogenesis of stroke is unknown. We examined how a2AP affects thrombosis, ischemic brain injury, and survival after experimental cerebral thromboembolism.

APPROACH AND RESULTS

We evaluated the effects of a2AP on stroke outcomes in mice with increased, normal, or no circulating a2AP, as well as in mice given an a2AP-inactivating antibody. Higher a2AP levels were correlated with greater ischemic brain injury (rs=0.88, P<0.001), brain swelling (rs=0.82, P<0.001), and reduced middle cerebral artery thrombus dissolution (rs=-0.93, P<0.001). In contrast, a2AP deficiency enhanced thrombus dissolution, increased cerebral blood flow, reduced brain infarction, and decreased brain swelling. By comparison to tissue plasminogen activator (TPA), a2AP inactivation hours after thromboembolism still reduced brain infarction (P<0.001) and hemorrhage (P<0.05). Microvascular thrombosis, a process that enhances brain ischemia, was markedly reduced in a2AP-deficient or a2AP-inactivated mice compared with TPA-treated mice or mice with increased a2AP levels (all P<0.001). Matrix metalloproteinase-9 expression, which contributes to acute brain injury, was profoundly decreased in a2AP-deficient or a2AP-inactivated mice versus TPA-treated mice or mice with increased a2AP levels (all P<0.001). a2AP inactivation markedly reduced stroke mortality versus TPA (P<0.0001).

CONCLUSIONS

a2AP has profound, dose-related effects on ischemic brain injury, swelling, hemorrhage, and survival after cerebral thromboembolism. By comparison to TPA, the protective effects of a2AP deficiency or inactivation seem to be mediated through reductions in microvascular thrombosis and matrix metalloproteinase-9 expression.

Combination of mild hypothermia with neuroprotectants has greater neuroprotective effects during oxygen-glucose deprivation and reoxygenation-mediated neuronal injury

Co-treatment of neuroprotective reagents may improve the therapeutic efficacy of hypothermia in protecting neurons during ischemic stroke. This study aimed to find promising drugs that enhance the neuroprotective effect of mild hypothermia (MH). 26 candidate drugs were selected based on different targets. Primary cultured cortical neurons were exposed to oxygen-glucose deprivation and reoxygenation (OGD/R) to induce neuronal damage, followed by either single treatment (a drug or MH) or a combination of a drug and MH. Results showed that, compared with single treatment, combination of MH with brain derived neurotrophic factor, glibenclamide, dizocilpine, human urinary kallidinogenase or neuroglobin displayed higher proportion of neuronal cell viability. The latter three drugs also caused less apoptosis rate in combined treatment. Furthermore, co-treatment of those three drugs and MH decreased the level of reactive oxygen species (ROS) and intracellular calcium accumulation, as well as stabilized mitochondrial membrane potential (MMP), indicating the combined neuroprotective effects are probably via inhibiting mitochondrial apoptosis pathway. Taken together, the study suggests that combined treatment with hypothermia and certain neuroprotective reagents provide a better protection against OGD/R-induced neuronal injury.

The impact of fibrinolytic therapy for ST-segment-elevation acute myocardial infarction

As privileged witnesses of the initiation and widespread use of reperfusion therapy the authors review the history of fibrinolytic therapy and of tissue-plasminogen activator (t-PA) more particularly and the current indications for its use in the era of mechanical reperfusion.

Reteplase: a review of its use in the management of thrombotic occlusive disorders

Reteplase (Retavase) is a plasminogen activator, mimicking endogenous tissue plasminogen activator (t-PA), a serine protease, converting plasminogen to plasmin and thereby precipitating thrombolysis. It is a third-generation recombinant form of t-PA that operates in the presence of fibrin (i.e. it is fibrin specific). Reteplase can be administered as a bolus dose (nonweight-based) rather than an infusion, which promotes rapid and safe administration. The ease of administration of this reteplase dosage regimen (two 10U bolus doses, each over 2 minutes, 30 minutes apart) is conducive to prehospital initiation of thrombolytic treatment in patients with ST-segment elevation myocardial infarction (STEMI), which reduces the time to treatment, a critical factor in improving long-term survival. In large randomized clinical trials of patients with STEMI, reteplase was superior to alteplase for coronary artery patency (according to TIMI [thrombolysis in myocardial infarction] flow) at 60 and 90 minutes, but there was no significant difference between agents for mortality rate and incidence of intracranial bleeding. The 35-day mortality rates were equivalent for reteplase and streptokinase recipients; there was reduced incidence of some cardiac events with reteplase versus streptokinase, but a greater incidence of hemorrhagic stroke. Reteplase has also shown thrombolytic efficacy (in nonapproved indications) as a catheter-directed intra-arterial or intravenous infusion for peripheral vessel occlusions, as 5-minute bolus doses (in 1U increments) for acute ischemic stroke, as a low-dose solution for occluded catheters or grafts, and as an intravenous double bolus for massive pulmonary embolism. Across studies in these indications, the incidence of bleeding complications associated with reteplase treatment appeared to be similar to that associated with other thrombolytic agents. With its efficacy, and the ease of administration of the bolus doses potentially minimizing dosage errors when treatment is administered under time pressure, reteplase is a valuable option for pre- or in-hospital thrombolytic treatment in patients with STEMI, and is a useful thrombolytic for the treatment of the other thrombotic occlusive disorders described.

Comparison of C-reactive protein and terminal complement complex in patients with unstable angina pectoris versus stable angina pectoris

Elevated C-reactive protein (CRP) can identify patients with coronary artery disease who are prone to future acute events. We investigated whether elevated CRP is related to the activation of the terminal complement cascade in 66 patients with unstable angina pectoris (UAP), in 45 patients with stable angina pectoris, and in 42 controls. CRP, additional acute phase reactants, the terminal complement complex (sC5b-9), leukocytes, and troponin T were measured. In 47 patients with UAP the CRP values were regarded as elevated (>0.3 mg/dl). In patients with UAP and elevated CRP, the plasma levels of sC5b-9 were markedly higher than in patients with UAP and lower CRP (245 +/- 14 vs 188 +/- 19 ng/ml, p <0.02) and in patients with stable angina pectoris with slightly (0.4 +/- 0.1 mg/dl) increased CRP (sC5b-9 173 +/- 21 vs 130 +/- 7 ng/ml [controls; p <0.05]). A further acute phase reaction was present only in patients with UAP and elevated CRP already on admission (p <0.01). sC5b-9 was not related to troponin release. Thus, elevated CRP levels are associated with activation of the plaque destabilizating terminal complement system in patients with UAP during the acute phase reaction. This may explain the prognostic value of CRP in acute coronary syndromes (ACS).

Thrombolytic therapy. State of the art

Thrombolytic agents have become the corner stone in the treatment of acute myocardial infarction. However, the current agents are far from perfect. New thrombolytic drugs have been designed to overcome these shortcomings. Development of these agents has focused not only on increasing plasma half-life and thus allowing single-bolus administration, but also on improving fibrin specificity and resistance to plasminogen activator inhibitor. The safety and efficacy of several of these promising thrombolytic drugs have been evaluated in large-scale trials, which are discussed in the present review. Parallel to these advances, alternatives to standard thrombolytic regimens have been developed. New trials evaluating the combination of reduced-dose fibrinolytics with different regimens of antithrombotic agents will optimize future reperfusion strategies.

The thrombolytic paradox

Thrombolytic drugs do not only stimulate the plasmin system but also induce thrombin activation additionally to the preexisting hypercoagulative state in patients with acute myocardial infarction. Testing the in vitro-derived hypothesis of a plasmin-mediated activation of the contact phase of the coagulation leading to the procoagulant effect, several thrombolytic regimen have been evaluated. Paradoxical thrombin activation (referred to as "thrombolytic paradox") was related to absence of fibrin specificity. Highly fibrin-specific drugs like tenecteplase did not cause additional thrombin activation, while non-fibrin-specific drugs like streptokinase caused a marked additional activation of the contact phase and of thrombin. It could be shown that the thrombolytic paradox was related to the extent of systemic plasmin activation confirming the hypothesis of a plasmin-mediated factor XII/kallikrein system activation as cause of the thrombolytic paradox.