Tissue plasminogen activator (t-PA). Will it fulfill its promise?

Analytic Reviews : Potential New Therapies for Acute Ischemic Stroke

Evidence from animal models of acute stroke suggests ischemia may be reversible if blood flow is restored in the first few hours. Studies of human stroke using posi tron emission tomography demonstrate areas with re duced blood flow and relatively preserved metabolism, indicating potentially reversible ischemic brain. Resto ration of blood flow during this reversible phase should improve outcome after stroke.

Many therapeutic strategies for treatment of acute ischemic stroke have been proposed, including increas ing collateral flow, removing vascular obstructions, and interfering with the intracellular cascade of events that lead to neuronal cell death. Hypervolemic hemodilution reduces viscosity and increases cerebral blood flow, and this may hopefully raise blood flow above the critical threshold of irreversible ischemia. Naloxone, calcium channel blockers, and glutamate antagonists alter blood flow and influence intracellular events during and after acute ischemia. Thrombolytic therapy restores blood flow by lysis of obstructing clot. These therapies show promise in preliminary studies, but additional ran domized controlled studies are needed.

Acute stroke

Disruption of cerebral blood flow may influence brain energy metabolism to produce reversible or irreversible neurologic deficits. The emergency physician is in a unique position to provide timely treatment during the first few hours of an acute stroke. He or she must be facile with unique pharmacologic and non-pharmacologic treatment designed for the stroke patient concerning ventilation, blood pressure, and circulation.

Thrombolytic activity of a novel modified tissue-type plasminogen activator, YM866, in a canine model of coronary artery thrombosis

The thrombolytic activity of a novel modified t-PA, YM866, was compared with that of a recombinant t-PA in a canine model of copper coil-induced coronary thrombosis. The coronary thrombus was allowed to age for 1, 3 or 6 hr before either drug was administered. YM866 was administered by i.v. bolus injection, while t-PA was given by the same method, as well as by 60-min i.v. infusion. YM866 showed thrombolytic activity 2 to 4 times as potent as that of t-PA when administered by bolus injection, the difference in thrombolytic effect being obvious in the 3- and 6-hr-old thrombi. Coronary reperfusion was achieved more rapidly with YM866 than with i.v. infusion of t-PA. In animals injected with doses of more than 0.1 mg/kg of YM866, no acute reocclusion occurred. Depletion of plasma fibrinogen to 70% of baseline levels was observed in animals given 0.2 mg/kg YM866, 0.4 mg/kg t-PA by bolus, and 0.6 mg/kg t-PA via infusion. The residual plasma YM866 and t-PA antigen 30 min after bolus injection was 25% and 3% of the peak levels, respectively. YM866, administered by i.v. bolus injection, was thus confirmed to exert a thrombolytic effect superior to that of bolus injection and infusion of t-PA, without systemic fibrinolytic activation. These results suggest the potential clinical applicability of YM866 as a thrombolytic agent that can be administered by i.v. bolus injection for acute myocardial infarction.

Creation of the recombinant tissue plasminogen activator (rt-PA) image and its influence on practice habits

American physicians have commonly practiced thrombolytic therapy for acute myocardial infarction with the recombinant form of tissue plasminogen activator (rt-PA), although its cost is much higher than that of streptokinase. The greater popularity of rt-PA is based on the belief that it is a more effective and a safer drug for achieving myocardial salvage and mortality reduction. However, a series of studies testing this assumption have not substantiated its greater efficacy or safety with respect to not only streptokinase but also urokinase and anisoylated plasminogen-streptokinase activator complex (APSAC). This editorial reviews the sequence of events that led to the creation of the rt-PA image, the mistaken premises on which it was based and the questions that need to be addressed if we are to strengthen the scientific method for evaluating similar types of drugs and its influence on practice habits including the costs to the health system.

Hemodilution and hypertension effects on cerebral hemorrhage in cerebral ischemia in rats

We determined the effect of hemodilution and hypertension on cerebral hemorrhage and brain injury in 32 rats subjected to 180 minutes of middle cerebral artery occlusion and 120 minutes of reperfusion. We divided the rats into four groups. In the control group (n = 8) neither hematocrit nor blood pressure was manipulated during occlusion, in the hemodilution group (n = 8) 5% albumin was administered to maintain a hematocrit of 30% during occlusion, in the hypertension group (n = 8) mean arterial blood pressure was increased to 30 mm Hg above baseline during occlusion with phenylephrine, and in the hemodilution/hypertension group (n = 8) albumin and phenylephrine were employed simultaneously during occlusion. We assessed the amount of cerebral hemorrhage (as concentration of extravasated hemoglobin) spectrophotometrically and the extent of ischemic injury (as percentage of the hemisphere with deficient staining) histochemically using 2,3,5-triphenyltetrazolium chloride. Mean +/- SD hemoglobin concentration in the hemisphere ipsilateral to the occlusion in the hemodilution/hypertension group (71 +/- 14 micrograms/g brain tissue) was significantly (p less than 0.05) greater than that in the hemodilution and hypertension groups (25 +/- 5 and 29 +/- 7 micrograms/g, respectively), hemoglobin concentrations in these two groups were in turn significantly (p less than 0.05) greater than that in the control group (2 +/- 3 micrograms/g). Mean +/- SD percentage of the ipsilateral hemisphere with deficient staining was significantly (p less than 0.05) less in the hypertension and hemodilution hypertension groups (8 +/- 3% and 11 +/- 6%, respectively) than in the control and hemodilution groups (26 +/- 8% and 26 +/- 7%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of oral pretreatment with zofenopril, an angiotensin-converting enzyme inhibitor, on early reperfusion and subsequent electrophysiologic stability in the pig

The effects of oral zofenopril pretreatment were investigated in a chronic closed-chest pig model of ischemia and reperfusion. Pigs (25-35 kg) were pretreated orally with zofenopril (15 mg/day) on the 2 days prior to ischemia, which was evoked by the inflation of a catheter balloon in the left anterior descending coronary artery over 45 minutes. The catheter was then removed and the myocardium was reperfused. After 2 weeks, infarct properties were assessed by signal averaging of the body surface electrocardiogram and the inducibility of malignant ventricular tachyarrhythmias was tested with a programmed electrical stimulation protocol. A significant increase in the pressure-rate product (43 +/- 11%, mean +/- SEM), indicating the oxygen demand of the heart, was prevented by zofenopril (19 +/- 8%, p less than 0.05). Zofenopril reduced the peak efflux of adrenaline (1302 +/- 213 vs. 3201 +/- 760 pg/ml; p less than 0.05), noradrenaline (402 +/- 54 vs. 902 +/- 282 pg/ml; p less than 0.05), and of the adenosine catabolites inosine and hypoxanthine (56 +/- 4 vs. 78 +/- 9, pg/ml; p less than 0.05) in the coronary venous effluent. The efflux of the cytoplasmatic enzyme creatine phosphokinase was not significantly reduced after zofenopril (p = 0.08). No difference in plasma renin levels between the groups were found. After 2 weeks, late potentials were found only in the surviving animals from the untreated group, i.e., the voltage vector magnitude was more reduced, and a prolongation of the QRS duration and of the terminal low-amplitude part of the high-frequency QRS were found.(ABSTRACT TRUNCATED AT 250 WORDS)

Favorable early and long-term prognosis following coronary bypass surgery therapy for myocardial infarction: results of a multicenter trial. TAMI Study Group

Coronary bypass surgery was performed before hospital discharge on 82 (21%) of 386 consecutive patients enrolled in the Thrombolysis and Angioplasty in Myocardial Infarction (TAMI) multicenter trial of intravenous tissue plasminogen activator and coronary angioplasty for acute myocardial infarction. Time from infarct symptom onset to coronary bypass surgery was 7.3 +/- 1.9 hours for 24 patients operated upon on an emergency basis and 9.3 +/- 5.2 days for 58 patients having late in-hospital surgery. There were no operative deaths and five in-hospital deaths in the surgical group, all of which occurred in patients with preoperative cardiogenic shock. Although patients in the surgical group were older (59.7 +/- 10.4 years versus 54.9 +/- 10.2 years; p = 0.03), had more extensive coronary artery disease (42% three-vessel disease versus 11%; p = 0.001), and had a higher incidence of anterior wall myocardial infarction (48% versus 39%; p = 0.02), in-hospital mortality for the surgical group (6%) was similar to that in 301 patients not undergoing surgery (7%) in this trial. For patients discharged from the hospital, mortality at 1 year was 2.5% in the surgical group and 1.8% in patients not having coronary bypass surgery before hospital discharge. At a 1 year follow-up, there were no significant differences in the frequency of cardiac or noncardiac-related hospitalizations or in event-free survival between surgical and nonsurgical groups. The majority of patients in both groups considered themselves to be in excellent or good condition. Coronary bypass surgery can be performed with low morbidity and mortality rates in close temporal association to acute myocardial infarction.(ABSTRACT TRUNCATED AT 250 WORDS)

[Antibody mediated thrombolysis. A new therapeutic principle]

Thrombosis of a coronary artery is the most common cause of myocardial infarction. Thrombolytic therapy, when instituted timely, has been shown capable of reducing morbidity and mortality. However, the use of presently available thrombolytic agents is associated with a bleeding tendency and efficacy is not optimal. This article reviews one of several lines of investigation that are presently being pursued in order to improve efficacy and specificity of thrombolytic therapy. The chemical conjugation of a fibrin specific monoclonal antibody and urokinase or tissue plasminogen activator results in markedly enhanced thrombolytic potency, both in vitro and in vivo. Specificity of the conjugates is greater than that of the parent plasminogen activators as reflected by conservation of fibrinogen, plasminogen and alpha-2 antiplasmin. A bispecific antibody, with specificity for both, fibrin and tissue plasminogen activator, has the potential of concentrating endogenous tissue plasminogen activator at the site of a thrombus. In the presence of the bispecific antibody, efficacy and specificity of tissue plasminogen activator are markedly enhanced in vitro and in vivo. The tools of molecular biology are presently being applied in order to translate these findings into better thrombolytic therapy.

Clinical disorders of fibrinolysis: a critical review

Much progress has recently been made in understanding the biochemistry and physiology of endogenous fibrinolysis. As a result, a better understanding of the mechanisms and clinical consequences of disordered fibrinolysis has emerged. Increased fibrinolytic activity is an uncommon but important cause of hemorrhagic disease. Congenital disorders of fibrinolysis which cause bleeding include increased plasma plasminogen activator activity and deficiency of alpha-2 antiplasmin. Acquired disorders associated with increased fibrinolytic activity and bleeding include liver cirrhosis, amyloidosis, acute promyelocytic leukemia, some solid tumors, and certain snake envenomation syndromes. Increased fibrinolysis is important to recognize because epsilon-aminocaproic acid (EACA) may be required to prevent or control bleeding. Diminished fibrinolytic activity has been associated with a variety of thrombotic disorders, but a direct cause-and-effect relationship has yet to be established. Congenital abnormalities of fibrinolysis associated with thrombosis include plasminogen deficiency, decreased endothelial generation of plasminogen activator activity, and certain abnormal fibrinogens. Thrombosis in these disorders is effectively managed with warfarin. Diminished fibrinolysis has also been reported in "idiopathic" venous thrombosis, oral contraceptive-induced and post-operative venous thrombosis, coronary artery disease, cerebrovascular disease, systemic lupus erythematosus, and thrombotic thrombocytopenic purpura, but the significance of abnormal fibrinolysis in these disorders is uncertain. Large, prospective studies of fibrinolytic variables as risk factors for vascular and thrombotic disease are needed to determine whether pharmacologic augmentation of impaired fibrinolysis could be useful in the prevention or treatment of these disorders.

Plasma plasminogen concentrations in clinically normal horses: the effect of age, sex and pregnancy

Plasma concentrations of plasminogen were determined in 28 clinically normal horses, including 13 adult geldings, five non-pregnant mares, five pregnant mares and five yearlings (two fillies, three geldings). Plasminogen was quantitated by a chromogenic assay based on activation of plasmin by excess urokinase. The overall mean plasma plasminogen for these horses was 2.94 +/- 0.54 CTA units (casein units, as defined by the Committee on Thrombolytic Agents) per ml. There were no significant differences in mean plasma plasminogen values among adult geldings, non-pregnant mares, pregnant mares or yearling horses (P greater than 0.05).

Is aeromedical transport of patients during acute myocardial infarction safe?

The emergence of coronary artery thrombolysis and angioplasty have raised new questions about the transfer of patients with acute myocardial infarction (AMI). Since these modalities are generally limited in their availability, and since the success of thrombolysis is strongly time-dependent, the interfacility transfer of patients during AMI has become more common. Study of a relatively small number of patients indicates that aeromedical helicopter transport can be conducted in a safe manner, and that the outcome of management is of benefit to the patients. Reperfusion events, however, must be anticipated during transport of patients in whom thrombolysis is initiated preflight. Furthermore, initiation of thrombolytic therapy may be problematic if begun in patients with events mimicking AMI, particularly aortic dissection or Prinzmetal's angina.

Investigational use of tPA in acute stroke

Significant current interest has focused on the possible value of fibrin-selective thrombolytic agents in acute stroke. Acute thrombosis contributes to carotid and vertebrobasilar arterial occlusions in the majority of acute stroke patients. Hence, fibrin(ogen)olytic agents may produce arterial recanalization and clinical benefit in thrombotic stroke. There are, however, unique features of cerebral tissue that suggest caution with the use of fibrin-selective agents in cerebral ischemia. The specific vascular anatomy and collateral flow suggest that salvage of the "ischemic penumbra" following vascular recanalization in focal ischemia is more likely to be successful than attempts in global ischemia. Recanalization may be associated with reperfusion injury and, more importantly, the risk of hemorrhagic transformation. There is little concrete information regarding the relative contribution of either event to post-thrombolysis cerebral injury. Early studies with exogenous fibrinolytic agents (urokinase, streptokinase) in completed stroke were regarded as inconclusive, demonstrating only an increased risk of intracerebral hemorrhage. Subsequent pilot studies in carotid and in vertebrobasilar territory thrombotic stroke have demonstrated that recanalization can result when exogenous agents are infused just proximal to the cerebral artery occlusion by interventional neuroradiological techniques. This experience and the advent of fibrin-selective agents (tissue plasminogen activator [tPA] and single-chain urokinase plasminogen activator) have led to the development of a multicenter prospective safety/dose-ranging study of tPA in acute (less than eight hours from symptom onset) thrombotic stroke. Following initial clinical assessment, computed tomography scan, and angiography, each patient with a documented cerebral artery occlusion appropriate to the clinical syndrome receives a preassigned intravenous dose of tPA over 60 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)

Six- and twelve-month follow-up of the phase I Thrombolysis in Myocardial Infarction (TIMI) trial

The Thrombolysis in Myocardial Infarction (TIMI) trial Phase I was designed to compare the efficacy and side effects of intravenous recombinant tissue-type plasminogen activator (rt-PA) and intravenous streptokinase (SK) in patients with acute myocardial infarction (AMI). As previously reported, rt-PA led to a reperfusion rate of 62% of totally occluded coronary arteries compared with 31% for SK (p less than 0.001). This study was not designed to determine if intravenous thrombolytic therapy decreases the mortality of AMI; however, the findings in these patients after 1 year of follow-up do permit certain insights into the impact of early reperfusion and reocclusion on the clinical course of patients with AMI. The mortality rate at 6 and 12 months was not significantly different in patients treated with rt-PA compared with SK (7.7% and 10.5% rt-PA vs 9.5% and 11.6% for SK). The frequency of recurrent AMI, coronary artery bypass grafting (CABG) and percutaneous transluminal coronary angioplasty (PTCA) was similar in the 2 treatment groups. There was no significant difference in 6- and 12-month mortality or in the rate of recurrent AMI in patients who received thrombolytic therapy before compared with after 4 hours of the onset of AMI symptoms. When the results were analyzed on the basis of the patency of the infarct-related artery, irrespective of thrombolytic agent used, for those patients with patent arteries 90 minutes after the initiation of therapy, there was a trend toward a lower 6-month (5.6% vs 12.5%) and 12-month mortality (8.1% vs 14.8%) (p = 0.07).(ABSTRACT TRUNCATED AT 250 WORDS)

Advances in thrombolytic therapy for acute myocardial infarction

There has been rapid proliferation of understanding and experience with thrombolytic therapy for acute myocardial infarction. Over the last few years, selective intracoronary infusion of lytic therapy has been replaced by intravenous administration because of the fundamental importance of time to reperfusion. Newer thrombolytic agents, such as tissue plasminogen activator (t-PA) and acylated streptokinase (APSAC), with properties distinct from streptokinase (SK) and urokinase, have been developed and have undergone extensive clinical trial evaluation. This review will focus primarily on the recent advances in thrombolytic therapy, with particular attention to efficacy, safety, and comparative aspects of the various agents currently or soon to be available.

Current practice patterns in the management of acute myocardial infarction. Survey of the American College of Chest Physicians

Over the past decade, the management of patients with acute myocardial infarction has undergone remarkable change due in part to the availability of new diagnostic and therapeutic modalities. Many of these techniques have gained widespread acceptance and use without the benefit of studies demonstrating efficacy. A survey of 391 members of the Section on Clinical Cardiology of the American College of Chest Physicians was conducted to assess current practice patterns in the management of acute myocardial infarction. Significant differences in the routine (greater than or equal to 50 percent of the time) use of various management approaches were found, with exercise tolerance testing, echocardiography, Holter monitoring, and cardiac rehabilitation frequently employed, whereas electrophysiologic studies, pyrophosphate scans, pulmonary artery catheterization, and cardiac pacing were infrequently used. Significant differences in the use of these procedures were noted with regard to the age of the physician and his or her geographic location of practice. These differences in practice patterns indicate that physicians have not reached consensus with regard to the value of these new diagnostic procedures and therapeutic approaches in the treatment of patients with acute myocardial infarction. Given the incremental costs of these procedures, clinical trials to assess their efficacy are clearly indicated.

Use and limitations of thrombolytic therapy in the treatment of peripheral arterial ischemia: results of a multi-institutional questionnaire

In an attempt to assess the efficacy of thrombolytic infusions for arterial ischemia, a questionnaire was distributed to 142 vascular surgeons. Data from 45 respondents who had experience with thrombolytic infusion in 623 patients were reviewed. A successful outcome was obtained in 313 of 623 patients (50.2%). Morbidity was significant, with hemorrhage requiring transfusion or operation in 125 cases (20.1%) and major amputation in 103 cases (16.5%). There were nine strokes associated with thrombolytic infusion (1.4%), six of which were fatal. Sixteen deaths were associated with thrombolytic therapy, for a mortality rate of 2.5% (16 of 623 patients). Analysis of results by grouping centers according to numbers of lytic infusions failed to show significant correlation with center experience. Furthermore, morbidity and mortality were seen in centers with both limited and extensive experience with thrombolytic infusion. The initial enthusiasm for thrombolytic infusion to treat arterial ischemia is not substantiated by our data. Carefully controlled prospective trials are needed before this method can be offered as an alternative to arterial reconstruction.

Thrombolysis in Myocardial Infarction (TIMI) Trial, Phase I: A comparison between intravenous tissue plasminogen activator and intravenous streptokinase. Clinical findings through hospital discharge

Intravenous administration of 80 mg of recombinant tissue plasminogen activator (rt-PA, 40, 20, and 20 mg in successive hours) and streptokinase (SK, 1.5 million units over 1 hr) was compared in a double-blind, randomized trial in 290 patients with evolving acute myocardial infarction. These patients entered the trial within 7 hr of the onset of symptoms and underwent baseline coronary arteriography before thrombolytic therapy was instituted. Ninety minutes after the start of thrombolytic therapy, occluded infarct-related arteries had opened in 62% of 113 patients in the rt-PA and 31% of 119 patients in the SK group (p less than .001). Twice as many occluded infarct-related arteries opened after rt-PA compared with SK at the time of each of seven angiograms obtained during the first 90 min after commencing thrombolytic therapy. Regardless of the time from onset of symptoms to treatment, more arteries were opened after rt-PA than SK. The reduction in circulating fibrinogen and plasminogen and the increase in circulating fibrin split products at 3 and 24 hr were significantly less in patients treated with rt-PA than in those treated with SK (p less than .001). The occurrence of bleeding events, administration of blood transfusions, and reocclusion of the infarct-related artery was comparable in the two groups. Thus, in patients with acute myocardial infarction, rt-PA elicited reperfusion in twice as many occluded infarct-related arteries as compared with SK at each of seven serial observations during the first 90 min after onset of treatment.

Unilateral blurred vision and dilated retinal veins

A 39-year-old woman presented to her ophthalmologist complaining of floaters. This was associated with elevation of the optic disc and late staining of the retinal veins. The differential diagnosis of venous occlusive disease is discussed.

Crescentic glomerulonephritis: a possible complication of streptokinase treatment for myocardial infarction

Twenty days after a streptokinase infusion given for myocardial infarction, a patient developed a group G streptococcal throat infection. Thirteen days later he presented with a serum sickness type illness and progressive renal failure. Renal biopsy showed crescentic glomerulonephritis.

Coagulability and Rheology: Hematologic Benefits From Exercise, Fish, and Aspirin. Implications for Athletes and Nonathletes

In brief: Physical activity makes the blood more fluid and less likely to clot. The healthy hematologic adaptations to exercise (enhanced fibrinolysis, expanded plasma volume, decreased hematocrit, increased red cell deformability, and decreased blood viscosity) seem to enhance the delivery of oxygen and decrease the risk of thrombosis. Regular exercise, then, by changing the blood, may offer the elite athlete enhanced performance and the general population reduced risk of heart attack. Increased amounts of fish in the diet and-for selected persons-low-dose aspirin, may be useful antithrombotic adjuncts to exercise.

Pulmonary embolism

Contemporary concepts in the diagnosis, treatment and natural history of pulmonary embolism are reviewed, and a practical rationale for investigation and therapy is outlined.

Fibrinogen and fibrin: biochemistry and pathophysiology

Fibrinogen is a thrombin-coagulable glycoprotein occurring in the blood of vertebrates. The primary structure of the alpha, beta, and gamma polypeptide chains of human fibrinogen is known from amino acid and nucleic acid sequencing. The intact molecule has a trinodular, dimeric structure and is functionally bivalent. Thrombin cleaves short peptides from the amino termini of the alpha and beta chains exposing polymerization sites that are responsible for the formation of fibrin fibers and appearance of a clot. The major physiological function of fibrinogen is the formation of fibrin that binds together platelets and some plasma proteins in a hemostatic plug. In pathological situations, the network entraps large numbers of erythrocytes and leukocytes forming a thrombus that may occlude a blood vessel. Fibrinogen and fibrin are multifunctional proteins. Fibrinogen is indispensable for platelet aggregation; it also binds to several plasma proteins, however, the biological function of this interaction is not completely understood. Fibrin is an essential matrix for regulation of fibrinolysis and for facilitation of cell attachment in wound healing.