The history of thrombolytic therapy began in 1933 when it was discovered that filtrates of broth cultures of certain strains of Streptococcus bacteria (beta-hemolytic streptococci) could dissolve a fibrin clot. 1 Streptokinase found its initial clinical application in combating fibrinous pleural exudates, hemothorax, and tuberculous meningitis. 2 In 1958, streptokinase was first used in patients with acute myocardial infarction, and this changed the focus of treatment.

At first, streptokinase infusion produced conflicting results until the Gruppo Italiano per la Sperimentazione della Streptochinasi nell'Infarto Miocardico (GISSI) trial in 1986, which validated streptokinase as an effective therapy and established a fixed protocol for its use in acute myocardial infarction. 2

The fibrinolytic potential of human urine was first described in 1947. 3 The active molecule was named urokinase. Unlike streptokinase, urokinase is not antigenic and directly activates plasminogen to form plasmin. Their ability to catalyze the conversion of plasminogen to plasmin is affected only slightly by the presence or absence of local fibrin clot.

 

Pharmacology & pharmacokinetics 
Non–Fibrin-Specific Agents
Streptokinase (SK) was once the most common fibrinolytic agent used globally .It was usually used as a short-term infusion (30 to 60 minutes) in doses of 1.5x106 U and has a plasma elimination half-life in man of 20 minutes. Within a few days, the anti-SK titer rapidly rises to 50 to 100 times the pre-infusion level, remaining there for many months or even years. This makes repeated administration impractical except very early after initial dosing. A new recombinant preparation of SK has been demonstrated to possess a similar risk/benefit profile in acute myocardial patients compared with wild-type SK.
Anisoylated plasminogen streptokinase activator complex (APSAC) or anistreplase was the first bolus fibrinolytic agent developed for clinical use. It produces a similar fall in fibrinogen and increase in neutralizing antibody equivalent to its SK content.  Angiographic studies indicated the coronary thrombolytic efficacy of APSAC is comparable to or somewhat higher than intravenous SK but lower than intra-coronary SK.  Although APSAC demonstrated promise on the basis of an initial randomized placebo-controlled trial (APSAC Intervention Mortality Study, AIMS), However in the International Study of Infarct Survival (ISIS)-3 study, no mortality advantage was observed over that with SK or rt-PA. 4

Urokinase is a naturally occurring 2-polypeptide chain plasminogen activator derived from human urine and human kidney cells in culture. It produces extensive systemic fibrinolysis, is non-immunogenic, and has achieved coronary patency rates approximating that of  SK, and it is cleared from plasma with an initial half-life of 6 to 9 minutes.
Prourokinase or single-chain urokinase-type plasminogen activator (scu-PA) is a recombinant form of prourokinase and has 2 formulations: a glycosylated form (Abbott-74187) produced in mouse hybridoma cells having greater fibrin specificity and stability at similar doses than its nonglycosylated form (saruplase produced in Escherichia coli). An initial phase 2 study of glycosylated prourokinase suggested promising coronary patency rates in human infarction, but further randomized studies have not been undertaken.
Nonglycosylated prourokinase or saruplase has a biphasic disappearance with an initial half-life of 6 to 9 minutes in patients with acute myocardial infarction. In Prourokinase in Myocardial Infarction [PRIMI] study 5; early TIMI 2 and 3 patency rates at 60 minutes demonstrated a higher rate (71.8%) for scu-PA than for SK (48.0%). In the Study in Europe With Saruplase and Alteplase in Myocardial Infarction (SESAM) 6 conducted 9 years after the PRIMI study, the 90-minute combined TIMI 2 and 3 patency rates were similar for saruplase (79.9%) and a 3-hour infusion of alteplase (81.4%).In the Comparison of Saruplase and Streptokinase (COMPASS) trial, an equivalence study of 3089 patients that evaluated 30-day mortality,. mortality rates were not different, i.e., 5.7% for saruplase versus 6.7% for SK, but the rate of intracranial hemorrhage was higher for saruplase than for SK (0.9% versus 0.3%; P=0.038). The European Medical Evaluation Agency (EMEA) has rejected saruplase for clinical use.

Fibrin-Specific Agents
Wild-Type rt-PA
This prototype fibrin-specific plasminogen activator has high affinity for plasminogen in the presence of fibrin and is a serine proteinase containing a single polypeptide chain of 527 amino acids. Manufactured by recombinant DNA technology, it is converted from a single- to a double-chain form by plasmin.
The recommended dose of rt-PA (alteplase) for the treatment of acute myocardial infarction is 100 mg administered "front loaded," starting with a bolus of 15 mg followed by 50 mg in the next 30 minutes and the remaining 35 mg in the following hour. The initial half-life of rt-PA in plasma is 4 to 8 minutes. In the Global Utilization of Streptokinase and TPA for Occluded Arteries (GUSTO) trial, a 15-mg IV bolus of rt-PA was administered, followed by a weight-adjusted regimen of 0.75 mg/kg over 30 minutes (not to exceed 50 mg) and then 0.50 mg/kg over 60 minutes (not to exceed 35 mg).

The term facilitated PCI refers to the use of an initial pharmacological regimen (high dose heparin, early GPIIb/IIIa inhibitors, full dose or reduced dose fibrinolytics or combinations. The ASSENT 4 PCI was the largest trial to evaluate full dose fibrinolytic (tenecteplase) plus PCI vs. primary PCI alone. 14 The trial was terminated prematurely because of higher in hospital mortality rates and higher primary composite end points with full dose fibrinolytics plus PCI vs. PCI alone. The FINESSE trial reported in European Society of Cardiology in 2007 compared half dose reteplase with abciximab vs. abciximab alone vs. placebo in patients undergoing PCI for STEMI. The composite endpoints were was no different in the various strategies. Moreover the bleeding rates were higher in the facilitated arms. 15 Finally a recent meta-analysis of multiple small trials confirmed that primary PCI is superior to facilitated PCI. 16

Combination of fibrinolytic therapy with GP IIb/IIIa inhibitors (without PCI)
Platelets have paradoxically increased activity after fibrinolysis and are important mediators in the tendency for vessel re-occlusion. Aspirin is pathway specific and therefore a relatively weak ant platelet agent. GP IIb/IIIa inhibitors by virtue of their ability to block the final pathway of platelet aggregation are potent anti-platelet agents & have been studied with half dose fibrinolytics. The GUSTO V & ASSENT 3 compared addition of abciximab with half dose of reteplase & tenecteplase respectively with full dose of the agents alone. 17, 18 However the trials could not demonstrate a definite advantage with this approach.
Thrombolytic therapy for Pulmonary Embolism
Pulmonary embolism (PE) is a common disorder and an important cause of morbidity and mortality. Among patients who are hemo-dynamically unstable at presentation, in-hospital mortality reaches 30%.
Pulmonary emboli often arise from thrombi originating in the deep venous system of the lower extremities or pelvis. A blood clot dislodges and is swept into the pulmonary circulation and lodges there. A large enough clot may obstruct large vessels in the lung, & may cause homodynamic instability, along with right ventricular failure, and possibly death. Currently, thrombolytic therapy in pulmonary embolism is still controversial.
Dong et al searched the literature and were able to combine data from eight randomized controlled clinical trials. The trials involved 679 adult patients who were in a stable condition and randomly assigned to a thrombolytic agent or heparin. 19 Thrombolytics did not show any benefit over heparin in terms of deaths and recurrence of blood clots. Limited information from only three of the trials showed that they were better at improving blood flow through the lungs. Major bleeding events were similar with both therapies. More double-blind trials are needed to show if there is a true benefit of thrombolytic therapy for pulmonary embolism

Pulmonary embolism varies in severity from acute massive pulmonary embolism, acute pulmonary infarction, acute embolism without infarction to multiple emboli. Only patients with acute massive pulmonary embolism, those at the highest risk of immediate death, are eligible for fibrinolytic therapy in absence of contraindications. 20 Anticoagulants or anti-thrombotic therapy suffice for other categories.

Fibrinolytic therapy in patients who are normotensive with RV dysfunction remains controversial. MAPPET-3 the largest trial of thrombolytic therapy versus heparin alone was carried out in patients with normal BP but RV dysfunction or pulmonary hypertension. Tissue plasminogen activator minimized escalation of therapy without an increase risk of bleeding. 21

Patients with pulmonary thromboembolism often decompensate suddenly, and, once hemodynamic compromise has developed, the mortality rate is extremely high. When the decision is made to use thrombolysis, the fastest-acting available thrombolytic agent with an acceptable safety and efficacy profile should be chosen. Many centers prefer off-label regimens to the slower on-label regimens that have been approved by the FDA.

UFH should not be given concomitantly with fibrinolytic therapy in acute massive pulmonary embolism. After fibrinolytic therapy, anticoagulation treatment is recommended to prevent recurrent thrombosis.  Heparin should not be started until the aPTT has decreased to less than twice the normal control value. Cardiac arrest in the event of pulmonary embolism has a mortality of about 70%. Recently, numerous case reports state the use of thrombolytic boluses in cardiac arrest due to pulmonary embolism, with apparent heroic results.

Use of thrombolytic agents in pulmonary embolism

Reteplase

Reteplase has not been labeled by the FDA for any indication except acute MI, but it is widely used for acute deep vein thrombosis and pulmonary embolism. The dosing used is the same as that approved for patients with acute MI: 2 IV boluses of 10 U each, administered 30 minutes apart.
Alteplase   
The FDA-approved regimen for pulmonary thromboembolism is 100 mg as a continuous infusion over 2 hours.
Some centers prefer to use an accelerated 90-minute regimen that appears to be faster acting, safer, and more efficacious than the 2-hour infusion. For  patients weighing less than 67 kg,

the drug is administered as 15 mg IV bolus, followed by 0.75 mg/kg infused over the next 30 minutes (maximum 50 mg) and then 0.50 mg/kg over the next 60 minutes (maximum 35 mg). For patients weighing more than 67 kg, 100 mg is administered as 15 mg IV bolus, followed by 50 mg infused over the next 30 minutes and then 35 mg infused over the next 60 minutes.

Urokinase

The FDA-approved regimen is 4,400 U/kg as a loading dose given at a rate of 90 mL/h over a period of 10 minutes, followed by a continuous infusion of 4,400 U/kg/h at a rate of 15 mL/h for 12-24 hours.

Streptokinase

The FDA-approved regimen for pulmonary embolism is 250,000 U as a loading dose over 30 minutes, followed by 100,000 U/h over 12-24 hours.

Thrombolysis in stroke
 In appropriately selected patients with acute cerebral ischemia thrombolytic therapy is of proven and substantial benefit. The evidence base for thrombolysis in stroke includes 21 completed randomized controlled clinical trials enrolling 7152 patients, using various agents, doses, time windows, and intravenous or intra-arterial modes of administration. Data from these trials are congruent in supporting the following conclusions:

1. Intravenous fibrinolytic therapy at the cerebral circulation done within the first 3 hours of ischemic stroke onset offers substantial net benefits to all patients.
2. Fibrinolytic therapy within 3-4.5 hours offers moderate net benefits when applied to all patients with potentially disabling deficits.
3. MRI of the extent of the infarct core (already irreversibly injured tissue) and the penumbra (tissue at risk but still salvageable) can likely increase the therapeutic yield of lytic therapy, especially in the 3- to 9-hour window.
4. Intra-arterial fibrinolytic therapy in the 3- to 6-hour window offers moderate net benefits when applied to all patients with potentially disabling deficits and large artery cerebral thrombotic occlusions.

Intravenous thrombolytic therapy in the first 3 hours after stroke onset is directly supported by the 2 phase 3 National Institute of Neurological Disorders and Stroke (NINDS) tissue plasminogen activator (tPA) trials, completed in 1995 and reported together.

Most recently, the Middle Cerebral Artery Embolism Local Fibrinolytic Intervention Trial (MELT) investigated intra-arterial urokinase up to 6 hours after onset in 114 subjects. Favorable trends were noted in good functional outcome compared to placebo.

Three prospective randomized trials, ROCHESTER, STILE, and TOPAS, which compared thrombolytic therapy with traditional surgical revascularization for lower limb ischemia, have been published. 27-29 They suggest that thrombolysis, as an initial therapy, reduces the risk of subsequent surgery and improves limb salvage for patients with PAD. Using this approach, the underlying lesions can be identified and treated by trans-luminal balloon angioplasty or stenting, or by elective surgical revascularization. However, severe bleeding is still a non rare complication of intra-arterial thrombolysis and the risk of intracranial hemorrhage is 1-2%.

Patients with limb-threatening ischemia are not candidates for local fibrinolysis. Usually, it takes between 6 and 72 hours to achieve clot lysis. These patients require emergent embolectomy. Catheter-directed thrombolysis is reserved for patients with non–life-threatening limb ischemia due to in situ thrombosis of less than 14 days.  Patients with thrombosis for more than 30 days are not likely to respond to local fibrinolysis.

Initially, streptokinase was the most widely used agent but later was replaced by urokinase and alteplase (tPA). Other drugs that have been studied include prourokinase, reteplase, and tenecteplase.

There have been case reports of successfully treating acute superior mesenteric artery embolism with hydrodynamic thrombectomy and pharmacological thrombolysis. 30

 

 

Thrombolytic Therapy for prosthetic valve thrombosis
History - In 1971, Luluaga, et al were the first to use the thrombolytic therapy in prosthetic valve thrombosis. 31 Streptokinase was used for treating thrombosis of the tricuspid valve prosthesis. Three years later, Baille, et al reported the use of that thrombolytic agent in the aortic valve prosthesis. 32 Since then, several cases of prosthetic valve thrombolysis have been reported, with varied rates of success and complication.
Recent studies have aimed at a better selection of candidates for thrombolysis. Echocardiography, particularly trans-esophageal echocardiography, has proved to be very important for the diagnostic and therapeutic decision.

Prosthetic valve thrombolysis: the rationale
 As already stated, prosthetic valve thrombosis can be treated clinically, with thrombolysis, or surgically, the physician in charge being responsible for this decision. When the thrombosis involves the tricuspid valve, and, more rarely, the pulmonary valve, thrombolysis is the therapy of choice. The studies have reported low complication rates in that group of patients. However, when thrombosis involves the left-sided valves (mitral and aortic), treatment should be indidualized. All patients should undergo trans-esophageal echocardiography before definitive treatment. The characteristics of the thrombus should be assessed. A thrombus <0.8 cm2 predicts success of the thrombolysis. Therefore, for patients in such conditions, independently of the functional class, i.e., from asymptomatic to cardiogenic shock, and independently of the time of thrombosis installation, the thrombolytic therapy can be considered the top choice. This consideration is not valid if the patient has the antecedent of stroke or atrial fibrillation, because the risk of complications is greater. When the thrombus is >0.8 cm2, surgery should be considered the best choice, unless the clinical conditions are unfavorable, such as cardiogenic shock, posing high surgical risk. In such conditions, thrombolysis, although potentially complicated, is justified. Also when cardiac surgery is not available, thrombolysis may be an option, mainly when clinical severity is extreme, and expectant management is not recommended.
Once thrombolysis is chosen, the greater experience with fibrinolytic agents makes streptokinase the first choice. Its dosage should be a bolus of 250,000 U in 30 minutes, followed by 100,000 U/h. Doppler echocardiography should be used to assess the efficacy of the thrombolytic therapy & adjust the duration of infusion. The thrombolytic agent should be interrupted at the 24 th hour of treatment, if no hemodynamic improvement (improvement in the gradient) occurs. It should be interrupted after 72 hours, even if the improvement is partial, or should be interrupted earlier, if the hemodynamic improvement is complete, i.e., the trans-valvular gradient returns to baseline values admitted as normal for mechanical prostheses. After thrombolysis, the patients should undergo anticoagulation with warfarin with an INR between 3 and 4.

Prosthetic valve obstruction is caused by thrombi or fibrous tissue overgrowth, or both. Thrombolysis avoids re-operation-related risks, but is effective only on clots. Renzulli et al reported20 cases of prosthetic thrombosis treated with thrombolysis using recombinant tissue type plasminogen activator (rt-PA). Indication criteria for thrombolysis were: (i) recent onset of symptoms; (ii) trans-esophageal echocardiographic (TEE) evidence of clots on the valve or cardiac chambers; and (iii) a partially preserved disc excursion. All patients were fitted with mechanical valves (four caged balls, 10 tilting discs, six bi-leaflets), with 17 valves located in the mitral and three in the aortic position. Symptoms of obstruction comprised cardiac failure in 11 cases and/or embolism

in 10. He found that after rt-PA infusion, normal prosthetic function was restored in all patients, though one underwent successful re-operation five days later. During infusion, five patients had a transient ischemic attack and one a minor transient peripheral embolism. Recurrence of thrombosis occurred in three patients during follow up; subsequent thrombolysis was successful in two, without complication. 33

Sonothrombolysis:
Reopening of the occluded artery is the primary therapeutic goal in hyper-acute ischemic stroke. The combination of ultrasound with thrombolytic agents may enhance the potential benefit by means of enzyme-mediated thrombolysis. Ultrasound insonation is efficient for accelerating enzymatic thrombolysis within a wide range of intensities, from 0.5W/cm2 (MI ~0.3) to several watts per square centimeter, particularly in the non focused ultrasound field. Insonation with ultrasound increased tPA-mediated thrombolysis up to 20% in a static model, while it enhanced the re-canalization rate from 30 to 90% in a flow model. Results from embolic rat models suggest that low-frequency ultrasound with 0.6W/cm2 significantly reduces infarct volume compared to pure tPA treatment. Safety of ultrasound exposure of the brain for therapeutic purposes has to address hemorrhage, heating, and direct tissue damage. Since animal studies suggested no increase of bleeding rate or harm to the blood-brain barrier, a clinical phase II study applying low-frequency ultrasound at ~300 kHz found a high number of secondary hemorrhages. Heating depends critically on the characteristics of the ultrasound. The most significant heating of the brain tissue itself is >1°C per hour using a 2W/cm2 probe; however, no significant heating could be found when using an emission protocol pulsing the ultrasound. The current experimental data helps to identify the optimal ultrasound characteristics for sono-thrombolysis and supports the hypothesis combined treatment being a perspective in optimizing thrombolytic therapy in acute stroke.

References

1) Sherry S. The origin of thrombolytic therapy. JACC. 1989;14:1085–1092
2) Long Term effects of intravenous thrombolysis in a acute myocardial infarction.; final report of theGISSI study.Gruppo Italiano per lo studio della streptochinasi nell infarto myocardico(GISSI).Lancet 1987;2:871-874
3) Ouriel K. A history of thrombolytic therapy. J Endovasc Ther. Dec 2004;11 Suppl 2:II128-133.
4) ISIS3 A randomized comparison of streptokinase vs tissue plasminogen activator vs anistreplase and of aspirin alone among 41299 cases of suspected acute myocardial infarction.Lancet 1992; 339:753-770
5) . PRIMI Trial Study Group. Randomized double-blind trial of infarction. Lancet. 1989;1:863–868
6) Bär FW, Meyer J, Vermeer F, et al. Comparison of saruplase and alteplase in acute myocardial infarction: the Study in Europe with Saruplase and Alteplase in Myocardial Infarction. Am J Cardiol. 1997; 79:727–732
7) GUSTO Investigators. An international randomized trial comparing four thrombolytic strategies for acute myocardial infarction. N Engl J Med. 1993;329:673–682
8). Smalling RW, Bode C, Kalbfleisch J, et al. More rapid, complete, and stable coronary thrombolysis with bolus administration of reteplase compared with alteplase infusion in acute myocardial infarction: RAPID Investigators.Circulation. 1995;91:2725–2732.
9). Bode C, Smalling RW, Berg G, et al. Randomized comparison of coronary thrombolysis achieved with double-bolus reteplase (recombinant plasminogen activator) and front-loaded, accelerated alteplase (recombinant tissue plasminogen activator) in patients with acute myocardial infarction: the RAPID II Investigators. Circulation. 1996;94:891–898.
10) Paoni NF, Keyt BA, Refino CJ, et al. A slow clearing, fibrin-specific, PAI-1 resistant variant of t-PA (T103N, KHRR296–299AAAA). Thromb Haemost. 1993;70:307–312