Plasminogen replacement therapy for the treatment of children and adults with congenital plasminogen deficiency.

Congenital plasminogen deficiency is caused by mutations in PLG, the gene coding for production of the zymogen plasminogen, and is an ultrarare disorder associated with abnormal accumulation or growth of fibrin-rich pseudomembranous lesions on mucous membranes. Left untreated, these lesions may impair organ function and impact quality of life. Plasminogen replacement therapy should provide an effective treatment of the manifestations of congenital plasminogen deficiency. An open-label phase 2/3 study of human Glu-plasminogen administered IV at 6.6 mg/kg every 2 to 4 days in 15 patients with congenital plasminogen deficiency is ongoing. Reported here are data on 14 patients who completed at least 12 weeks of treatment. The primary end point was an increase in trough plasminogen activity levels by at least an absolute 10% above baseline. The secondary end point was clinical success, defined as ≥50% improvement in lesion number/size or functionality impact from baseline. All patients achieved at least an absolute 10% increase in trough plasminogen activity above baseline. Clinical success was observed in all patients with clinically visible (conjunctiva and gingiva), nonvisible (nasopharynx, bronchus, colon, kidney, cervix, and vagina), and wound-healing manifestations of the disease. Therapeutic effects were rapid, as all but 2 lesions resolved or improved after 4 weeks of treatment. Human Glu-plasminogen was well tolerated in both children and adults. This study provides critical first evidence of the clinical utility of ongoing replacement therapy with human Glu-plasminogen for the treatment of children and adults with congenital plasminogen deficiency. This trial was registered at www.clinicaltrials.gov as #NCT02690714.

Fibrin chain cross-linking, fibrinolysis, and in vivo sealing efficacy of differently structured fibrin sealants.

In this study, we compared the sealing characteristics and efficacy of a fibrin sealant with reduced plasminogen (FS-rplg) and a fibrin sealant with aprotinin as a fibrinolysis inhibitor (FS-apr). The relevant sealing characteristics including clot structure, fibrin chain cross-linking, and clot lysis were tested in the laboratory. The sealing efficacy was then investigated in a follow-up animal model to determine differences in the in vivo sealing properties. A total of 46 animals were available for the final analysis with 23 animals in each treatment arm. In conclusion, we saw differences in vitro between FS-rplg and FS-apr in ultrastructure and α-chain cross-linking rates as well as in the rate of fibrinolysis. These differences may explain the significantly enhanced sealing efficacy in FS-apr compared to FS-rplg shown in vivo in a rabbit intestinal model.

Modification of cyclic NGR tumor neovasculature-homing motif sequence to human plasminogen kringle 5 improves inhibition of tumor growth.

BACKGROUND: Blood vessels in tumors express higher level of aminopeptidase N (APN) than normal tissues. Evidence suggests that the CNGRC motif is an APN ligand which targets tumor vasculature. Increased expression of APN in tumor vascular endothelium, therefore, offers an opportunity for targeted delivery of NGR peptide-linked drugs to tumors. METHODS/PRINCIPAL FINDINGS: To determine whether an additional cyclic CNGRC sequence could improve endothelial cell homing and antitumor effect, human plasminogen kringle 5 (hPK5) was modified genetically to introduce a CNGRC motif (NGR-hPK5) and was subsequently expressed in yeast. The biological activity of NGR-hPK5 was assessed and compared with that of wild-type hPK5, in vitro and in vivo. NGR-hPK5 showed more potent antiangiogenic activity than wild-type hPK5: the former had a stronger inhibitory effect on proliferation, migration and cord formation of vascular endothelial cells, and produced a stronger antiangiogenic response in the CAM assay. To evaluate the tumor-targeting ability, both wild-type hPK5 and NGR-hPK5 were (99 m)Tc-labeled, for tracking biodistribution in the in vivo tumor model. By planar imaging and biodistribution analyses of major organs, NGR-hPK5 was found localized to tumor tissues at a higher level than wild-type hPK5 (approximately 3-fold). Finally, the effects of wild-type hPK5 and NGR-modified hPK5 on tumor growth were investigated in two tumor model systems. NGR modification improved tumor localization and, as a consequence, effectively inhibited the growth of mouse Lewis lung carcinoma (LLC) and human colorectal adenocarcinoma (Colo 205) cells in tumor-bearing mice. CONCLUSIONS/SIGNIFICANCE: These studies indicated that the addition of an APN targeting peptide NGR sequence could improve the ability of hPK5 to inhibit angiogenesis and tumor growth.

Rhenium-188 labeled recombinant human plasminogen kringle5 (rhk5) and preliminary biodistribution. Evaluation in mice bearing A549 tumours.

AIM: Angiogenesis plays a critical role in tumour formation and metastasis. Suitable radiolabeled angiogenesis inhibitor can be used for noninvasive imaging of angiogenesis and radionuclide therapy. Here we prepare rhenium-188 labeled recombinant human plasminogen kringle5 (188Re-rhk5) in a convenient manner than evaluate its properties in A549 lung adenocarcinoma. METHODS: 188Re-rhk5 was obtained by conjugating His group at the C end of rhk5 with fac-[188Re(H2O)3(CO)3]+. Chelating efficiency of fac-[188Re(H2O)3(CO)3]+ and radiolabeling efficiency of 188Re-rhk5 were measured by radio thin-layer chromatography (RTLC). In vitro stability of 188Re-rhk5 was determined in human serum at 37°C and analyzed by RTLC. Competition test was also performed to verify the specificity of binding. A biodistribution study was carried out in nude mice bearing A549 lung adenocarcinoma. RESULTS: 188Re-rhk5 was obtained with a radiolabel efficiency of 66.1%, the radiochemical purity (RCP) can reach 95.2% after purification. 188Re-rhk5 showed high stability in human serum, the RCP was more than 80% even 12 h after incubation. Competition test showed a high binding specificity. Furthermore, this radio-complex was excreted mainly through kidneys and showed specific tumour uptake in mice bearing A549 tumours. CONCLUSION: 188Re-rhk5 was prepared by a simple method. Preliminary biodistribution results showed its potential as an agent for possible tumour imaging, therapy and encouraged further investigation.

Dose escalation trial of the efficacy, safety, and pharmacokinetics of a novel fibrinolytic agent, BB-10153, in patients with ST elevation MI: results of the TIMI 31 trial.

BACKGROUND: Currently available fibrinolytic agents are limited by their ability to restore normal blood flow in only half of patients, the risk of reocclusion, and the risk of intracranial hemorrhage. The genetically engineered agent BB-10153 is activated by thrombin, not plasminogen activator enzymes, which limits its activity to the site of thrombus which may in turn reduce the risk of systemic bleeding. BB-10153 also has a relatively long half-life of 3-4 hours, which may also limit the potential for early reocclusion [1, 2]. METHODS: The study was a phase II, open-label, multi-center, dose escalation, single-dose administration study to determine the efficacy, safety, tolerability, pharmacokinetics and pharmacodynamics of BB-10153 in ST segment elevation MI (STEMI). STEMI patients (n = 50) received a single dose of BB-10153 at one of six dose levels (1.0, 2.0, 3.0, 5.0, 7.5 and 10 mg/kg). The primary endpoint was TIMI flow grade (TFG) 3 at 60 minutes following the intravenous bolus of BB-10153. RESULTS: Mean area under the curve for drug concentration ranged from 48.0 microg.h/mL in the 1 mg/kg dose group to 788.6 microg.h/mL in the 10 mg/kg dose group. Likewise, mean Cmax generally increased with dose over the entire dose range, from 4.9 microg/mL in the 1 mg/kg dose group to 139.6 microg/mL in the 10 mg/kg dose group. The mean apparent terminal half-life (t1/2) was 4.4 hours (range 2.2 to 7.6 hours). Few patients in the 1-3 mg/kg dosage groups achieved TFG 3 on the one-hour post-dose angiogram (4/20, 20%), and no patients achieved complete ST segment resolution. The 5, 7.5 and 10 mg/kg doses were associated with similar rates of TIMI grade 3 flow of approximately three per seven patients. Pooling TFG 3 data from the 5, 7.5 and 10 mg/kg groups yielded a TIMI grade 3 flow rate of 34% (n = 10/29; range 29-43%). No patients experienced 30-day death, recurrent acute MI, cardiogenic shock, stroke or anaphylaxis during the study. One patient experienced recurrent angina and developed recurrent myocardial ischemia requiring urgent revascularization. Three patients sustained TIMI major bleeding events (one in 1 mg/kg group, two in 7.5 mg/kg group), six patients sustained TIMI minor bleeds (one in the 2, 3, 7.5 and 10 mg/kg groups, two in the 5 mg/kg group), twp patients sustained TIMI minimal bleeds (one in each of the 2 and 10 mg/kg groups) and no patients sustained intracranial hemorrhage (ICH). CONCLUSION: In a dose escalation study of a single intravenous bolus, the novel fibrinolytic agent, BB-10153 was associated with a rise in the mean area under the curve and Cmax for drug concentration over the dose range 1 to 10 mg/kg. Higher doses were associated with a range of TIMI grade 3 flow of 29-43%, and no patients experienced 30-day death, recurrent acute MI, cardiogenic shock, stroke or anaphylaxis during the study. In a dose escalation study of a single intravenous bolus, the novel fibrinolytic agent, BB-10153 was associated with a rise in the mean area under the curve and Cmax for drug concentration over the dose range 1 to 10 mg/kg. Higher doses were associated with a range of TIMI grade 3 flow of 29-43%, and no patients experienced 30-day death, recurrent acute MI, cardiogenic shock, stroke or anaphylaxis during the study.

Pharmacokinetics and pharmacodynamics of BB-10153, a thrombin-activatable plasminogen, in healthy volunteers.

BACKGROUND: BB-10153 is an engineered variant of human plasminogen that is activated to plasmin by thrombin. Thrombus-selective induction of reperfusion and prevention of reocclusion have been demonstrated following bolus administration in animal models of thrombosis. OBJECTIVE AND METHODS: The objective of the study was to examine the pharmacokinetics and pharmacodynamics of BB-10153 administered as an intravenous bolus to healthy male human volunteers. Cohorts of four were dosed with BB-10153 (n = 3) or placebo (n = 1). In total, placebo was received by eight volunteers and 0.08, 0.2, 0.6, 1.2, 1.8, 2.4, 3.6 and 4.8 mg kg(-1) BB-10153 by three volunteers each. RESULTS: There was a linear relationship between AUC/Cmax and dose. The half-life of BB-10153 was approximately 3-4 h and all the BB-10153 in the circulation retained the ability to be activated by thrombin. There was a dose-related increase in plasma fibrin D-dimers. Ex vivo plasma clot lysis was observed at doses of 3.6 and 4.8 mg kg(-1), whereas lysis of clots formed from euglobulin-fractionated plasma was first evident at 0.6 mg kg(-1) and activity increased with dose. This activity decreased with time in line with the half-life. BB-10153 had no effect on plasma alpha2-antiplasmin or fibrinogen levels, coagulation assays or bleeding time. An increase in plasminogen was observed as BB-10153 was detected by the enzyme-linked immunosorbent assay (ELISA) for human plasminogen. CONCLUSIONS: BB-10153 was well tolerated and had a 3-4-h plasma half-life. Fibrinolytic activity was demonstrated by dose-related ex vivo clot lysis and in vivo production of fibrin D-dimers. These effects were not accompanied by consumption of alpha2-antiplasmin or fibrinogen.

Metabolism of rabbit angiostatin glycoforms I and II in rabbits: angiostatin-I leaves the intravascular space faster and appears to have greater anti-angiogenic activity than angiostatin-II.

Plasminogen (PLG) exists in the circulation as two glycoforms, I and II. Angiostatin (AST) is a polypeptide that has been cleaved from the kringle region of PLG and has strong anti-angiogenic properties. AST-I and AST-II, which consisted only of kringles 1 through 3, were prepared by the action of urokinase on purified rabbit PLG-I and PLG-II, respectively, in the presence of N-acetyl cysteine, followed by affinity chromatography on lysine-Sepharose. Purified AST-I and AST-II were tested for functional activity with a chick chorioallantoic membrane (CAM) model; when similar amounts were applied to a 6-day CAM, AST-I was substantially more effective than AST-II in decreasing vascular supply to the CAM over a 72-hour period; this activity correlated with a loss of capillaries, probably through apoptosis of endothelial cells. Radiolabeled AST-I and AST-II (iodine 125 and iodine 131) were co-injected intravenously into healthy rabbits to determine their clearances from plasma measured over 3 days. Over a dose range of 0.08 to 2.7 microg/kg, the fractional catabolic rate within the intravascular space (j(3)) indicated that AST-I was cleared 3-fold to 4-fold more rapidly than AST-II (P < .001). The catabolic half-life of AST-I (2.01 +/- 0.19 days) was significantly less than that of AST-II (2.62 +/- 0.20 days). The faster clearance of AST-I from the intravascular space was matched by its more rapid passage than AST-II to the extravascular space of various organs over 60 minutes in vivo. This property of AST-I as compared with AST-II may partially explain its greater anti-angiogenic potential. From the plasma concentrations of PLG-I and PLG-II and their relative behaviors toward rabbit VX-2 lung tumors in vivo, we predict that substantially greater quantities of AST-II than AST-I may be released into the extravascular space of tumors.

Pharmacokinetics and whole body distribution of elastase derived angiostatin (K1-3) in rats.

In the current study, we determined short-term pharmacokinetics and whole body distribution of elastase derived angiostatin [angiostatin(k1-3)] in rats after i.v. injection of radiolabelled protein. Since in gamma-camera studies, no tumor specific angiostatin(k1-3) accumulation was observed, general pharmacokinetics were studied in tumor free rats. By one-compartment model fitting of the data, Km 7.3 +/- 1.7 microg x ml(-1), Vmax 0.94 +/- 0.19 microg x min(-1), V, 10.9 +/- 2.5 ml and intrinsic clearance (Vmax/Km) 0.128 ml x min(-1) were calculated. Of the injected dose (I.D.) of angiostatin(k1-3), 12.1 +/- 2.1% per gram tissue was present in the kidneys 10 min after injection. Accumulation of angiostatin(k1-3) was detectable in spleen, liver, lungs and heart 10 min after injection. Sixty minutes after injection, kidney associated angiostatin(k1-3) had decreased, whereas in stomach and small intestines a small increase was seen. Immunohistochemical analysis demonstrated specific staining of interstitial cells of the kidney, liver Kupffer cells and endothelium of larger blood vessels of the lungs. Renal clearance of angiostatin(k1-3) and/or fragments is a major route of elimination, whereas lack of accumulation of radioactivity in the faeces indicates little hepatic elimination or hepatic elimination followed by enterohepatic cycling of the protein's degradation products. Instant blood coagulation at the site of vascular activation and the occurrence of respiratory problems upon administration of higher doses of angiostatin(k1-3) warrants further investigation of the protein's potential side effects. The data presented can be applied to study the relation between angiostatin(k1-3) treatment regimens, blood concentration levels, anti-tumor activity and harmful effects.

Adsorption of plasminogen from human plasma to lysine-containing surfaces.

The objective of this work is to develop blood-contacting surfaces that will dissolve nascent clots that may begin to form on them. Surfaces were prepared consisting of a polyurethane to which a coating reagent was attached covalently by photochemical methods. The coating reagent was a polyacrylamide with lysine and benzophenone (for photochemical attachment) moieties pendant to the chains. It was hypothesized that via the lysine moieties such surfaces would show specific binding affinity for plasminogen, the principal component of the fibrinolytic system in blood. Surfaces of varying lysine content in which the lysine was bound through the alpha-amino groups, leaving the epsilon-amino groups free, were investigated. A control surface in which the lysine was bound through the epsilon-amino groups was also examined. Advancing water contact angles showed the surfaces to be hydrophilic. Hydrophilicity was found to decrease as the lysine content increased. Adsorption of plasminogen from plasma was studied using radioiodinated plasminogen as a tracer. For the epsilon-lysine surfaces, adsorption increased with increasing lysine content and reached a value of 1.2 microg/cm(2) for the surface with the highest lysine content, that is, in the range expected for a compact monolayer of plasminogen. The control surfaces, which contained either no lysine or lysine in which the epsilon-amino groups were unavailable, adsorbed very small amounts of plasminogen. Immunoblots were obtained for the proteins eluted from the surfaces after incubation with plasma. For the control surfaces, most of the proteins tested for (some 20 in all) were present. However, for the surface containing the highest concentration of epsilon-lysine, only plasminogen was detected in a significant amount. It is concluded that the epsilon-lysine surface adsorbs plasminogen to the exclusion of the other plasma proteins. Studies to examine the fibrinolytic properties of these surfaces will constitute the next phase of this work.

Reduction in stent and vascular graft thrombosis and enhancement of thrombolysis by recombinant Lys-plasminogen in nonhuman primates.

BACKGROUND: To enhance thrombolytic responses without increasing hemorrhagic risks, the antithrombotic effects of recombinant Lys-plasminogen (r-LysPgn), a prothrombolytic plasminogen intermediate, were examined in baboon models of thrombus formation and dissolution. METHODS AND RESULTS: The dose-response effects of r-LysPgn, alone or in combination with subthreshold dosing of tissue plasminogen activator (TPA), were measured with respect to the accumulation of (111)In-labeled platelets and (125)I-fibrin in thrombus forming on endovascular metallic stents or thrombogenic segments of vascular graft interposed in exteriorized long-term arteriovenous (AV) femoral shunts. Thrombolytic losses have also been determined for preformed, stable, (111)In-platelet- and (125)I-fibrin-labeled graft thrombus and corresponding propagated thrombotic tails, together with changes in blood tests of thrombosis, thrombolysis, and hemostasis. Bolus intravenous r-LysPgn in escalating doses (2, 4, or 8 mg/kg) increased circulating plasminogen levels in a dose-dependent manner, was removed by log-linear clearance with a T50 of 120 minutes, and reciprocally decreased the accumulating thrombus on metallic stents and segments of vascular graft (P<.001 in all cases for 8-mg/kg doses). r-LysPgn also impaired platelet aggregatory responses to physiological agonists in vitro but not ex vivo. Prethrombosis administration of low-dose r-LysPgn (2 mg/kg) greatly enhanced the lysis of radiolabeled nonoccluding thrombus by a subthreshold dose of TPA (0.1 mg/kg) compared with TPA-only controls (P=.03). CONCLUSIONS: Elective bolus injections of r-LysPgn before stent deployment decrease the amount of thrombus formed without compromising hemostasis by facilitating endogenous TPA thrombolysis. r-LysPgn may provide effective and safe antithrombotic therapy for interventional vascular procedures.

Efficiency of Ginkgo biloba extract (EGb 761) in antioxidant protection against myocardial ischemia and reperfusion injury.

The cardio-protective mechanisms of EGb 761, an extract of Ginkgo biloba leaves, on myocardial ischemia-reperfusion injury were investigated using rabbits subjected to 30 minutes of regional cardiac ischemia and 120 min of reperfusion under anesthesia. Compared to the saline perfused group, EGb 761 treatment (10 mg/kg, injected into the coronary artery) significantly inhibited the increase in lipid peroxidation and maintained total and CuZn-SOD levels in both plasma and tissue during and at the end of reperfusion. Both the decrease in tissue type plasminogen activator (t-PA) and the increase in plasminogen activator inhibitor-1 (PAI-1) caused by ischemia-reperfusion were also significantly suppressed by EGb 761 treatment. Furthermore, the ultrastructure of the myocytes of the EGb 761 treated heart was slightly damaged after ischemia-reperfusion, while the control ischemic-reperfused hearts demonstrated severe histological damages such as swelling and vacuolization of the mitochondria. These results suggest that EGb 761 protects hearts by its antioxidant properties and by its ability to adjust fibrinolytic activity.

Adsorption of plasminogen from plasma to lysine-derivatized polyurethane surfaces.

The adsorption of plasminogen, the principal protein of the fibrinolytic pathway in blood, to a number of solid surfaces from plasma was investigated. This study forms part of a larger project to develop a fibrinolytic surface for blood-contacting applications. Polyurethanes incorporating lysine residues were developed in an attempt to promote selective adsorption of plasminogen from plasma through lysine-binding sites in the plasminogen molecule. The adsorption of plasminogen to these surfaces as well as to glass, 'conventional' polyurethanes and precursor sulphonated polyurethanes was investigated. Adsorption from citrated human plasma diluted with isotonic Tris buffer (pH 7.4) was measured under static conditions at room temperature using radioiodinated plasminogen. The following trends were observed. (1) Adsorption increases monotonically with increasing plasma concentration and there is no suggestion of transient adsorption (Vroman effect) on any of the surfaces studied. (2) Sulphonate groups appear to have a strong effect on plasminogen adsorption as was found previously for adsorption from buffer. (3) The lysine-derivatized material having the highest lysine content may show a slight increase in plasminogen binding affinity compared to its sulphonated precursor.

Inhibition of tumor implantation at sites of trauma by plasminogen activators.

The authors report on the influence of plasminogen activators (PA) on implantation of TA3Ha mammary tumor cells in the healing hepatic wounds of syngeneic strain A mice. Intravenously injected TA3Ha cells, although they rarely metastasize to the liver, formed tumors in the hepatic wounds of a significant percent (42%, P less than 0.0001) of mice. The frequency of tumor formation declined as the interval between surgery and tumor cell inoculation was increased. Furthermore, preexposure of cells to fibrinogen, fibronectin, laminin, or peptides containing the arginine-glycine-aspartic acid-serine residues dramatically reduced the frequency of tumor formation in the hepatic wounds. These results indicate that TA3Ha cells interact with fibrinogen-related proteins in the wound to aid their attachment and growth. Because these proteins are susceptible to digestion by plasmin, PA were used in this study to examine whether administration of these drugs to the mice would modulate tumor formation in the liver wounds. Among the PA tested, human plasmin B-chain-streptokinase complex (B-SK) and recombinant tissue plasminogen activator (t-PA) inhibited tumor implantation in a dose-related manner. Administration of 900 units (U) of B-SK or 3300 U of t-PA per mouse reduced the frequency of tumor formation from 42% to 0% (P = 0.02) and 11% (P = 0.02), respectively. The B-SK was complexed with p-nitrophenyl-p-guanidinobenzoate; it did not activate the plasminogen or inhibit tumor formation in the hepatic wounds. Although urokinase activated the plasminogen, it did not inhibit tumor implantation in the hepatic wound. Heparin, an anticoagulant that prevents conversion of fibrinogen to fibrin without being fibrinolytic, had no influence on tumor formation in the hepatic wounds. The PA can generate plasmin that digests the cell attachment proteins in wounds and consequently inhibits tumor cell attachment.

The role of carbohydrate in the function of human plasminogen: comparison of the protein obtained from molecular cloning and expression in Escherichia coli and COS cells.

A cDNA library was constructed in the phage lambda gt11 from human liver mRNA enriched for plasminogen mRNA by chromatography on Sepharose 4B. A full-length cDNA clone of human plasminogen was isolated. The 2.7 kb cDNA encoded the entire plasminogen molecule, a signal peptide sequence and two start codons with a 5'-untranslated region of about 80 base pairs. In the 3'-non coding region of 280 base pairs a consensus signal AATAAA was found at a distance of 46 base pairs upstream of the poly(A) tail. The plasminogen cDNA was subcloned in the eukaryotic expression vector p91023 (B), and human plasminogen was expressed in monkey kidney (COS m6) cells and in Escherichia coli. The recombinant molecule obtained from COS cells has physicochemical and biological properties similar to native human plasminogen I, indicating that it has folded in a manner similar to plasminogen synthesized by liver. By contrast, plasminogen expressed in E. coli could not be activated and showed biological properties which are very different from glycosylated forms of plasminogen. However, the non-glycosylated plasminogen was bound by lysine-Sepharose and reacted with a conformation dependent monoclonal antibody to kringles 1 to 3. These data suggest that the protein has properly folded kringle domains. Our studies suggest that the carbohydrate domains may play an important role in the function of the plasminogen molecule.

Preclinical pharmacological evaluation of anisoylated plasminogen streptokinase activator complex.

An ideal thrombolytic (or fibrinolytic) agent is one which would generate the formation of plasmin only where it is required, i.e. bound to fibrin within the thrombus. However, the capacity of even the newer thrombolytic agents to achieve localised plasmin generation within the thrombus is relative and depends on the concentration of the agent administered. For all available activators, the concentration required for effective clinical thrombolysis is also capable of converting plasminogen to plasmin within the circulation (plasminaemia). Since the action of plasmin is not specific to fibrin, plasminaemia results in dissolution not only of fibrin but also of several other clotting factors. For example, plasmin can degrade fibrinogen and cause impaired haemostasis. The plasminogen activators which are available, or have been developed to date, include streptokinase, urokinase, pro-urokinase, anisoylated plasminogen-streptokinase activator complex (APSAC) and tissue plasminogen activator (t-PA). All of these agents have the same biochemical mechanism of action, cleaving an arginine-valine bond in the plasminogen molecule to form plasmin, but they differ with regard to other important properties. The first property to be considered is clot specificity; the ability to dissolve fibrin as opposed to fibrinogen, and also to dissolve the clot as opposed to a haemostatic plug. Unfortunately, fibrin specificity does not equate entirely with thrombus specificity, and all currently developed plasminogen activators, by dissolving fibrin, will induce the destruction of haemostatic extravascular plugs as well as intravascular thrombi. Thus, no agent is thrombus-specific in this respect. The degree of fibrinogenolysis does vary between plasminogen activators. Those which have the least effect on haemostasis or clotting capability would seem, at first, to be preferable. However, a short term reduction in fibrinogen could also be beneficial, since it may reduce the incidence of early reocclusion and, by reducing blood viscosity, improve microcirculation to the infarct zone. The intrinsic efficiency of the plasminogen activators is a second important property. In vitro, under conditions pertaining to the circulation, urokinase is about 10 times more efficient than t-PA at converting glu-plasminogen to plasmin (on the basis of the Vmax to Km ratio), while streptokinase-plasmin is 20 times more efficient. The efficiency of these activators is increased in the presence of fibrin and lys-plasminogen, 1800-fold for t-PA, 8-fold for urokinase and 180-fold for streptokinase-plasmin.(ABSTRACT TRUNCATED AT 400 WORDS)