A hamster pulmonary embolism model for the evaluation of the thrombolytic and pharmacokinetic properties of thrombolytic agents

Method to measure in vivo blood fibrinolytic activity with a (125)I-fibrin coated aorta loop validated with agents which affect blood fibrinolytic activity

A functional animal model to measure in vivo the blood fibrinolytic activity and pharmacological-induced changes thereof are described. A (125)I-fibrin coated plastic loop is inserted in the rat aorta; the rate of label disappearance (sigmoid curve) is directly registered outside the animal with a gamma scintillation probe. The time needed to let disappear 50% of the removable-labeled fibrin is used as measure for the blood fibrinolytic activity. The direct advantage of this model is the absence of a blood or plasma clot: a thin labeled fibrin layer attached to the inner wall of the loop is in direct contact with the blood and is therefore sensitive to increased or decreased blood fibrinolytic activity. The total experiment needs about 60 min. Experiments with nontreated rats showed that, after an initial lag phase of about 10 min, the labeled fibrin started to disappear from the loop. A sigmoid pattern was obtained showing that about 20-30% of the coated-labeled fibrin is resistant to removal. Registration of the total curve of a nontreated (control or placebo) rat required about 30-40 min. The clinically used thrombolytics (intravenously administered) urokinase and t-PA showed a dose-dependent fibrinolytic activity resulting in increased removal of the bound (125)I-fibrin. Streptokinase was not active, which is in agreement with literature. Tranexamic acid, dexamethasone and endotoxin (inhibitors of fibrinolysis) showed dose-dependent inhibition of removal of the coated fibrin. Retinoic acid was tested as compound, which may enhance the blood fibrinolytic activity; retinoic acid was not found to be significantly active in this model. The disappearance of labeled fibrin is not sensitive to inhibitors of coagulation or platelet aggregation. This technically simple and fast model can thus be used to measure in vivo quantitatively the effects of pharmacological active compounds, which increase or decrease the blood fibrinolytic activity.

Deficiency or inhibition of Gas6 causes platelet dysfunction and protects mice against thrombosis

The growth arrest-specific gene 6 product (Gas6) is a secreted protein related to the anticoagulant protein S but its role in hemostasis is unknown. Here we show that inactivation of the Gas6 gene prevented venous and arterial thrombosis in mice, and protected against fatal collagen/epinephrine-induced thrombo embolism. Gas6-/- mice did not, however, suffer spontaneous bleeding and had normal bleeding after tail clipping. In addition, we found that Gas6 antibodies inhibited platelet aggregation in vitro and protected mice against fatal thrombo embolism without causing bleeding in vivo. Gas6 amplified platelet aggregation and secretion in response to known agonists. Platelet dysfunction in Gas6-/- mice resembled that of patients with platelet signaling transduction defects. Thus, Gas6 is a platelet-response amplifier that plays a significant role in thrombosis. These findings warrant further evaluation of the possible therapeutic use of Gas6 inhibition for prevention of thrombosis.

Polyethylene glycol-derivatized cysteine-substitution variants of recombinant staphylokinase for single-bolus treatment of acute myocardial infarction

BACKGROUND

Thrombolytic therapy of acute myocardial infarction (AMI) is evolving toward bolus administration. Derivatization of proteins with polyethylene glycol (PEG) may reduce their clearance.

METHODS AND RESULTS

A staphylokinase (SakSTAR) variant with 12 amino acid substitutions to reduce its antigenicity, SakSTAR (K35A, E65Q, K74R, E80A, D82A, T90A, E99D, T101S, E108A, K109A, K130T, K135R), and with Ser in position 3 mutated into Cys (code SY161), was derivatized with maleimide-PEG with M:(r) of 5,000 (P5), 10,000 (P10), or 20,000 (P20). The PEGylated variants recognized only one third of the antibodies elicited with wild-type SakSTAR in AMI patients. In experimental animals, plasma clearances were reduced 2. 5- to 5-fold with P5, 5- to 20-fold with P10, and 20-fold with P20, and bolus injection induced pulmonary plasma clot lysis at doses inversely related to their clearance. Intravenous bolus injection of 5 mg of the P5, P10, or P20 variants in AMI patients was associated with plasma half-lives (t(1/2alpha)) of 13, 30, and 120 minutes and clearances of 75, 43, and 8 mL/min, respectively, compared with 3 minutes and 360 mL/min for SakSTAR. Injection of 5 mg P5 variant restored TIMI-3 flow within 60 minutes in 14 of 18 AMI patients (78%, 95% CI 55% to 91%) and of 2.5 mg in 7 of 11 patients (63%, 95% CI 35% to 85%), both in the absence of fibrinogen degradation. The immunogenicity of the variants was significantly (P:<0.002) reduced.

CONCLUSIONS

The staphylokinase variant SY161-P5, derivatized with one linear polyethylene glycol molecule of M:(r) 5000, is a promising fibrin-selective agent for single-bolus coronary thrombolysis.

Recombinant staphylokinase variants with reduced antigenicity due to elimination of B-lymphocyte epitopes

Site directed mutagenesis (350 variants) of recombinant staphylokinase (SakSTAR), a potent fibrin-selective thrombolytic agent, was undertaken in order to reduce its antigenicity while maintaining its potency. Variants with K35A, (ie, Lys[K] in position 35 substituted with Ala[A]), E65D or E65Q, K74R or K74Q, E80A+D82A, K130T, and K135R displayed increased enzymatic activity or reduced binding of human staphylokinase-specific antibodies. Additive mutagenesis identified 8 variants with intact thrombolytic potencies, which absorbed down to less than a third of SakSTAR-specific antibodies. Intra-arterial administration in 61 patients with peripheral arterial occlusion caused no significant allergic reactions. Median neutralizing antibody titers (with 15 to 85 percentiles), expressed as microgram (μg) compound neutralized per milliliter plasma, were 4.4 (0.3 to 49) for the variants, compared with 12 (4 to 100) in 70 patients given wild-type SakSTAR (P = .002 by Mann-Whitney rank sum test). Overt neutralizing antibody induction (more than 5 μg compound neutralized per milliliter plasma) was observed in 57 of 70 patients (81%) given wild-type SakSTAR, but only in 28 of 60 patients (47%) treated with variants (P < .0001 by Fisher exact test). On the basis of this study, the variant SakSTAR (K35A, E65Q, K74R, D82A, S84A, T90A, E99D, T101S, E108A, K109A, K130T, K135R) (code SY155) has been selected for further clinical development.

Recombinant staphylokinase variants with reduced antigenicity due to elimination of B-lymphocyte epitopes

Site directed mutagenesis (350 variants) of recombinant staphylokinase (SakSTAR), a potent fibrin-selective thrombolytic agent, was undertaken in order to reduce its antigenicity while maintaining its potency. Variants with K35A, (ie, Lys[K] in position 35 substituted with Ala[A]), E65D or E65Q, K74R or K74Q, E80A+D82A, K130T, and K135R displayed increased enzymatic activity or reduced binding of human staphylokinase-specific antibodies. Additive mutagenesis identified 8 variants with intact thrombolytic potencies, which absorbed down to less than a third of SakSTAR-specific antibodies. Intra-arterial administration in 61 patients with peripheral arterial occlusion caused no significant allergic reactions. Median neutralizing antibody titers (with 15 to 85 percentiles), expressed as microgram (μg) compound neutralized per milliliter plasma, were 4.4 (0.3 to 49) for the variants, compared with 12 (4 to 100) in 70 patients given wild-type SakSTAR (P = .002 by Mann-Whitney rank sum test). Overt neutralizing antibody induction (more than 5 μg compound neutralized per milliliter plasma) was observed in 57 of 70 patients (81%) given wild-type SakSTAR, but only in 28 of 60 patients (47%) treated with variants (P &lt; .0001 by Fisher exact test). On the basis of this study, the variant SakSTAR (K35A, E65Q, K74R, D82A, S84A, T90A, E99D, T101S, E108A, K109A, K130T, K135R) (code SY155) has been selected for further clinical development.

Comparative effects of tissue plasminogen activator, streptokinase, and staphylokinase on cerebral ischemic infarction and pulmonary clot lysis in hamster models

BACKGROUND

The effects of alteplase (rtPA), streptokinase, and staphylokinase (rSak) on focal cerebral ischemia (FCI) and on pulmonary clot lysis (PCL) were studied in hamsters.

METHODS AND RESULTS

++FCI was produced by ligation of the left middle cerebral artery (MCA) and common carotid artery (CCA) and a 10-minute occlusion of the right CCA. FCI was measured after 24 hours by 2,3, 5-triphenyltetrazolium chloride staining. (125)I-fibrin-labeled plasma clots were injected via the jugular vein, and clot lysis was determined from residual radioactivity at 90 minutes. Study drugs were given intravenously over 60 minutes. FCI increased from 1.2 (0. 27 to 2.3) mm(3) (median and 17th to 83rd percentile range, n=24) in controls to 19 to 27 mm(3) with thrombolytic agent, with maximal rates at 0.13+/-0.05 mg/kg rtPA, 0.23+/-0.09 mg/kg streptokinase, and 0.037+/-0.025 mg/kg rSak. PCL increased from 18+/-2% (mean+/-SEM, n=27) in controls to approximately 85% with thrombolytics, with maximal rates at 0.12+/-0.03 mg/kg rtPA, 0.17+/-0.05 mg/kg streptokinase, and 0.018+/-0.002 mg/kg rSak. All agents caused maximal FCI and PCL rates at similar doses without alpha(2)-antiplasmin and fibrinogen depletion. Injection of 6 mg/kg human plasminogen combined with streptokinase caused a "systemic fibrinolytic state" with fibrinogen depletion. Maximal rates of FCI were obtained with 0.097+/-0.077 mg/kg streptokinase (P=0.26 versus streptokinase alone) and of PCL with 0.010+/-0.002 mg/kg (P=0.006 versus streptokinase alone).

CONCLUSIONS

Thrombolytic agents cause similar dose-related extension of FCI after MCA ligation and PCL, irrespective of the agent or systemic plasmin generation.

Recombinant staphylokinase variants with altered immunoreactivity. II: Thrombolytic properties and antibody induction

BACKGROUND

The substitution variants K35A,E38A,K74A, E75A,R77A (SakSTAR.M38) and K74A,E75A,R77A,E80A, D82A (SakSTAR.M89) of recombinant staphylokinase (SakSTAR) with reduced antibody reactivity were assayed for thrombolytic potency and antibody induction in animal models and in patients.

METHODS AND RESULTS

In a 125I-fibrin-labeled pulmonary embolism model in the hamster, the doses giving 50% clot lysis in 90 minutes were 25 micrograms/kg for SakSTAR, 85 micrograms/kg for SakSTAR.M38, and 90 micrograms/kg for SakSTAR.M89. In rabbits with 125I-fibrin-labeled plasma clots incorporated into extracorporeal arteriovenous loops, lysis within 2 hours was 76 +/- 18% (mean +/- SD, n = 28) with 400 micrograms/kg SakSTAR, 53 +/- 13% (n = 8) with 1000 micrograms/kg SakSTAR.M38, and 39 +/- 13% (n = 6) with 800 micrograms/kg SakSTAR.M89. When groups of eight rabbits were immunized by intravenous administration of 0.2 to 1.0 micrograms/kg compound followed by subcutaneous injection of 0.4 mg in Freund's adjuvant at 2, 3, and 5 weeks, SakSTAR.M38 and SakSTAR.M89 elicited markedly less circulating neutralizing activity, compared with SakSTAR, when determined at 6 weeks (neutralizing 6.1 +/- 3.0 and 4.9 +/- 1.3 micrograms compound/mL plasma, respectively, versus 20 +/- 17 micrograms/mL; P = .02 and P = .01, respectively) and induced significantly less resistance to thrombolysis (residual thrombolytic potency producing 59 +/- 25% and 39 +/- 12% lysis, respectively, versus 8.5 +/- 5.7%; P = .008 and P = .006, respectively). In patients with peripheral arterial occlusion, intra-arterial administration of SakSTAR.M38 (n = 4) or SakSTAR.M89 (n = 4) induced significantly fewer circulating neutralizing antibodies (P = .03) and specific IgG (P = .01) at 2 to 3 weeks than SakSTAR (n = 8).

CONCLUSIONS

SakSTAR.M38 and SakSTAR.M89 induce less antibody formation and might constitute preferred agents for thrombolytic therapy in humans.

Comparative antigenicity of recombinant wild-type staphylokinase (SakSTAR) and a selected mutant (SakSTAR.M38) in a baboon thrombolysis model

Staphylokinase, a bacterial plasminogen activator, is a potent, highly fibrin-specific but antigenic thrombolytic agent in humans. In an effort to attenuate the antigenicity of wild-type staphylokinase (SakSTAR variant), 2 of its 3 immunodominant epitopes were altered by substituting clusters of 2 or 3 charged amino acids with alanine, yielding the mutant SakSTAR.M38 (K35A, E38A, K74A, E75A, R77A), which was less antigenic in inbred New Zealand White rabbits. In the present study, groups of 6 baboons (Papio hamadryas) were randomized to SakSTAR (group 1) or SakSTAR.M38 (group 2). The thrombolytic potencies of 50 micrograms/kg compound at baseline, assessed in an extracorporeal thrombosis model, were similar: 77 +/- 2.9% (mean +/- SEM) clot lysis in group 1 and 83 +/- 3.6% in group 2. Groups 1 and 2 were immunized subcutaneously at 2, 3, and 5 weeks with 500 micrograms SakSTAR or SakSTAR.M38, respectively. From 6 weeks, group 1 developed significantly more antibody-related neutralizing activity than group 2 (maximal titer at 8 weeks of 100 +/- 23 micrograms SakSTAR and of 22 +/- 7.1 micrograms SakSTAR.M38 neutralized per milliliter of plasma, respectively). Neutralizing activities subsequently decreased gradually to 10-20% of peak values at 18 weeks. At 6 weeks, both groups were resistant to thrombolysis with 50 micrograms/kg of either compound. Rechallenge at 18 weeks with 250 micrograms/kg of the immunizing compound showed a significantly better recovery of the thrombolytic potency of SakSTAR.M38 (68 +/- 4.5% clot lysis) than of SakSTAR (39 +/- 5.3% clot lysis). Neither agent degraded fibrinogen or depleted alpha 2-antiplasmin. Therefore, SakSTAR.M38 is comparably active and fibrin-specific but less antigenic than wild-type SakSTAR. These findings in outbred primates confirm and extend earlier observations in inbred rabbits and provide a basis for the further development of staphylokinase variants with reduced antigenicity in humans.

Effects of disruption of the plasminogen gene on thrombosis, growth, and health in mice

BACKGROUND

Circumstantial evidence suggests that the plasminogen/plasmin system plays a role in many biological processes, including hemostasis, cell migration, and development.

METHODS AND RESULTS

The in vivo function of the plasminogen/plasmin system was studied by generation of plasminogen-deficient (Plg-/-) mice. Inactivation of the murine plasminogen-gene (Plg) was achieved by replacing, via homologous recombination in embryonic stem cells, genomic sequences encoding the exons containing the catalytic site amino acids His605 and Asp648 with a neomycin phosphotransferase expression cassette. Germline transmission of the mutated allele, as determined by Southern blot hybridization and polymerase chain reaction, was obtained via blastocyst injection. Mendelian inheritance of the inactivated plasminogen allele was observed, and homozygous-deficient mice (Plg-/-) displayed normal viability but retarded growth up to at least 12 weeks of age. At 8 weeks of age, body weight was 21.8 +/- 1.2 g (n = 10) for wild-type (Plg+/+) mice, 21.0 +/- 1.1 g (n = 16) for heterozygous-deficient (Plg+/-) mice, and 17.4 +/- 1.3 g (n = 12) for Plg-/- mice; P < .05 versus Plg+/+ or Plg+/-. None of 36 Plg+/+ or 65 Plg+/- mice but 7 of 37 Plg-/- mice (19%) developed rectal prolapse at 7.4 +/- 0.6 weeks of age (P = .03 versus Plg+/+ and P = .003 versus Plg+/-); 4 of 37 Plg-/- mice (11%) became runted and apathic at 5.3 +/- 0.3 weeks of age (P = .041 versus Plg+/-); and 6 of 37 Plg-/- mice (16%) died prematurely at 8.8 +/- 1.7 weeks of age (P = .057 versus Plg+/+ and P = .029 versus Plg+/-). Although male and female Plg-/- mice were able to sire offspring, the fertility of Plg-/- female mice was reduced, possibly owing to their impaired health. Levels of plasminogen-related antigen in plasma, measured by ELISA, were 84 +/- 8 micrograms/mL (n = 4) in Plg+/+, 35 +/- 2 micrograms/mL (n = 3) in Plg+/-, and 0.076 +/- 0.032 microgram/mL (n = 6) in Plg-/- mice (P < .001 versus Plg+/- and Plg+/+). Plasmin activity generated by urokinase activation was unmeasurable in Plg-/- mice (< 5% of Plg+/+ mice). Plasminogen-specific immunoreactivity was observed in hepatocytes from Plg+/+ mice but not from Plg-/- mice (< 10% of Plg+/+ mice). Neither native nor variant plasminogen mRNA nor translation products could be identified by Northern or Western blot of liver extracts from Plg-/- mice. Spontaneous lysis within 24 hours of a 125I-fibrin-labeled pulmonary plasma clot was 85 +/- 5% (n = 5) in Plg+/+ mice, 62 +/- 7% (n = 3) in Plg+/- mice, and -2 +/- 1% (n = 3) in Plg-/- mice (P < .001 versus Plg+/- and Plg+/+). Delayed clot lysis within 72 hours was 33 +/- 1% (n = 3) in tPA-/- mice and 26 +/- 2% (n = 3) in Plg-/- mice (P = .054). Histological examination of several organs revealed fibrin deposition in the liver; lung; and in the stomach, associated with gastric ulcers, in 6- to 12-week-old Plg-/- mice but not in Plg+/+ or Plg+/- littermates.

CONCLUSIONS

Plasminogen-deficient mice survive embryonic development but develop spontaneous fibrin deposition due to impaired thrombolysis and suffer retarded growth and reduced fertility and survival. The Plg-/- phenotype is reminiscent of the combined tPA-/-:uPA-/- phenotype, which suggests that there is no significant additional pathway for physiological plasminogen activation in mice.

Antithrombotic effects of thrombolytic agents in a platelet-rich femoral vein thrombosis model in the hamster

BACKGROUND

The extent and mechanism of the antithrombotic properties of fibrin-selective and non-fibrin-selective thrombolytic agents have not yet been established.

METHODS AND RESULTS

The antithrombotic, thrombolytic, fibrinogenolytic, and pharmacokinetic properties of the following substances were determined in hamsters in the absence of conjunctive anticoagulant or antiplatelet therapy: recombinant tissue-type plasminogen activator (rTPA), recombinant single-chain urokinase-type plasminogen activator (rscu-PA), two-chain urokinase-type plasminogen activator (UK), with a rTPA deletion mutant lacking amino acids 6 to 173 and a mutation N184E (K2Pt), a rTPA/rscu-PA chimeric plasminogen activator consisting of amino acids 1 to 3 and 87 to 274 of rTPA and amino acids 138 to 411 of rscu-PA (K1K2Pu), streptokinase (SK), and recombinant staphylokinase (STAR). The anti-thrombotic effect, defined as the intravenous dose required to reduce mural thrombus formation to 50% in a platelet-mediated femoral vein thrombosis model in the hamster, was 6 +/- 1, 5 +/- 2, 1 +/- 0.05, 2.5 +/- 0.2, 0.02 +/- 0.002, 1 +/- 0.09, and 2 +/- 0.3 mg/kg (mean +/- SEM), respectively. The amounts, given as intravenous infusion over 60 minutes that induced 50% clot lysis in a hamster pulmonary embolism model, were 0.18 +/- 0.03, 1.1 +/- 0.05, 0.9 +/- 0.13, 0.34 +/- 0.03, 0.04 +/- 0.003, 0.05 +/- 0.005, and 0.04 +/- 0.001 mg/kg, respectively, indicating that for most thrombolytic agents the antithrombotic dose is much higher than their thrombolytic dose. The fibrinogen levels, measured 40 minutes after bolus injection, were reduced to 50% of baseline with 3.1 +/- 0.2, 2.5 +/- 0.3, 1.2 +/- 0.08, 2.0 +/- 0.14, 1.7 +/- 0.65, 0.54 +/- 0.03, and 1.2 +/- 0.11 mg/kg, respectively. Mean residence times following intravenous bolus injection were: 18 +/- 1, 14 +/- 1, 100 +/- 10, 80 +/- 2, 20 +/- 3, and 34 +/- 5 minutes for rTPA, rscu-PA, K2Pt, K1K2Pu, SK, and STAR, respectively. Regression analysis revealed a significant correlation of the antithrombotic effect with the fibrinogen breakdown (P = .006) but not with the thrombolytic potency or with the mean residence time.

CONCLUSIONS

These observations support the hypothesis that thrombolytic therapy with fibrinogen-sparing agents requires the conjunctive use of anticoagulant and/or antiplatelet agents.

Physiological consequences of loss of plasminogen activator gene function in mice

Indirect evidence suggests a crucial role for the fibrinolytic system and its physiological triggers, tissue-type (t-PA) and urokinase-type (u-PA) plasminogen activator, in many proteolytic processes. Inactivation of the t-PA gene impairs clot lysis and inactivation of the u-PA gene results in occasional fibrin deposition. Mice with combined t-PA and u-PA deficiency suffer extensive spontaneous fibrin deposition, with its associated effects on growth, fertility and survival.

High yield production and purification of recombinant staphylokinase for thrombolytic therapy

Recombinant plasmids were constructed in which the signal sequence of the sak42D and the sakSTAR staphylokinase genes were replaced by an ATG start codon and which express staphylokinase under the control of a tac promoter and two Shine-Dalgarno sequences in tandem. Induction of transfected E. coli TGl cells in a bacterial fermentor produced intracellular staphylokinase representing 10 to 15% of total cell protein. Gram quantities of highly purified recombinant staphylokinase were obtained from cytosol fractions by chromatography, at room temperature, on SP-Sepharose and on phenyl-Sepharose columns, with yields of 50 to 70 percent. The material, at a dose of 4 mg/kg, did not produce acute reactions or affect body weight in mice. Intravenous administration of 10 mg SakSTAR over 30 minutes in five patients with acute myocardial infarction induced complete coronary artery recanalization, without associated fibrinogen degradation. However, neutralizing antibodies appeared in the plasma of all patients within 12 to 20 days. Thus, the present expression and purification method for recombinant staphylokinase yields large amounts of highly purified mature protein (approximately 200 mg per liter fermentation broth) suitable for a more detailed clinical investigation of its potential as a thrombolytic agent.

Comparative immunogenicity and thrombolytic properties toward arterial and venous thrombi of streptokinase and recombinant staphylokinase in baboons

BACKGROUND

Streptokinase is a routinely used thrombolytic agent that is immunogenic and relatively inefficient toward platelet-rich thrombus, whereas staphylokinase is a poorly studied fibrinolytic agent. Here, the comparative immunogenicity and thrombolytic properties toward arterial platelet-rich thrombus and venous whole blood clots of streptokinase and recombinant staphylokinase were studied in baboons.

METHODS AND RESULTS

The inhibitory capacity of baboon plasma (in a human plasma-based clot lysis assay) was 0.39 +/- 0.25 microgram streptokinase and 0.04 +/- 0.05 microgram recombinant staphylokinase per milliliter of plasma (mean +/- SD, n = 9). Intravenous infusion over 1 hour of 0.300 mg/kg of streptokinase at 0, 1, 2, 3, and 5 weeks in five baboons given heparin and the antiplatelet agent ridogrel increased the streptokinase-neutralizing titer from 0.22 +/- 0.18 microgram/mL plasma at baseline to 3.8 +/- 4.4 micrograms/mL after 2 weeks (p = 0.043 versus baseline by Wilcoxon signed rank test) and to 4.4 +/- 4.6 micrograms/mL after 5 weeks, whereas the thrombolytic potency toward a 125I-fibrin-labeled plasma clot inserted into an extracorporeal arteriovenous loop was reduced from 84 +/- 7% at baseline to 45 +/- 8% after 2 weeks and to 36 +/- 8% after 5 weeks (p < 0.01 versus baseline). Administration over 1 hour of 0.065 mg/kg recombinant staphylokinase at 0, 1, 2, 3, and 5 weeks in four baboons did not induce measurable staphylokinase-neutralizing activity in three of the four animals after 5 weeks. In the fourth baboon, a staphylokinase-neutralizing activity of 0.8 and 1.5 micrograms/mL was found at 3 and 5 weeks, respectively. Repeated staphylokinase administration was not associated with inhibition of clot lysis (43 +/- 4% lysis at baseline, 52 +/- 9% at 3 weeks, and 61 +/- 14% at 5 weeks; p = NS versus baseline). Repeated administration of streptokinase but not of staphylokinase caused a marked (> 50%) decrease in blood pressure, requiring administration of steroids and intravenous fluids, and a marked increase in leukocyte count and hemoglobin concentration. Intravenous infusion of streptokinase or recombinant staphylokinase over 1 hour in doses ranging between 0 and 1.0 mg/kg in three groups of four baboons each induced dose-dependent lysis of a 125I-fibrin-labeled autologous jugular vein blood clot (50% lysis requiring 0.140 mg/kg streptokinase and 0.058 mg/kg recombinant staphylokinase, representing equimolar amounts of 3.25 nmol/kg) without systemic fibrinogen depletion. The thrombolytic potency toward platelet-rich arterial thrombus of streptokinase and recombinant staphylokinase were studied in a femoral arterial eversion graft model. Arterial recanalization with recombinant staphylokinase was more frequent and more persistent than with streptokinase (all p < 0.05). Intravenous infusion of 1.0 mg/kg streptokinase or 0.25 mg/kg recombinant staphylokinase in two groups of four baboons each given intravenous heparin (200-unit bolus and 50 units.kg-1 x hr-1) and aspirin (10 mg/kg) did not produce a significant prolongation of the median template bleeding time.

CONCLUSIONS

Recombinant staphylokinase has a thrombolytic potency toward jugular vein blood clots in baboons comparable to that of streptokinase, but it is less immunogenic and less allergenic and it does not induce resistance to lysis upon repeated administration; it is significantly more efficient than streptokinase for the dissolution of platelet-rich arterial eversion graft thrombi. Recombinant staphylokinase, which can be easily obtained in active form by expression in Escherichia coli, may constitute a potentially useful alternative to streptokinase for the treatment of acute myocardial infarction.

Thrombolytic profiles of clot-targeted plasminogen activators. Parameters determining potency and initial and maximal rates

BACKGROUND

Targeting of plasminogen activators to the thrombus by means of fibrin-specific monoclonal antibodies may enhance their thrombolytic potency. The kinetics of clot binding of two human fibrin-specific monoclonal antibodies (MA-12B3 and MA-15C5) and of clot lysis with their chemical 1:1 stoichiometric complexes with recombinant single-chain urokinase-type plasminogen activator (rscu-PA) (rscu-PA/MA-12B3 and rscu-PA/MA-15C5) were determined in hamsters and rabbits. Thrombolytic potencies, maximal rates of clot lysis, and the duration of the lag phases before clot lysis of the antibody/rscu-PA conjugates were compared with those of rscu-PA and tissue-type plasminogen activator (rt-PA).

METHODS AND RESULTS

Bolus injection of 7.5 micrograms of 125I-labeled antibody in rabbits with an extracorporeal arteriovenous loop containing a 0.3-mL human plasma clot produced clot-to-blood ratios of 6.6 +/- 1.0 (mean +/- SEM) for MA-12B3 and 1.1 +/- 0.15 for MA-15C5 (p < 0.001 versus MA-12B3) within 6 hours. Progressive digestion of the clot did not alter the binding of MA-12B3 but resulted in as much as a 10-fold increase of the binding of MA-15C5. The conjugates infused intravenously over 90 minutes in hamsters with a human plasma clot in the pulmonary artery produced dose-related in vivo clot lysis. Thrombolytic potencies (maximal slope of the percent lysis versus dose in milligrams of u-PA equivalent per kilogram body weight) were 2,500 +/- 440 for rscu-PA/MA-12B3, 3,600 +/- 640 for rscu-PA/MA-15C5 (p = NS vs. rscu-PA/MA-12B3), 60 +/- 8 for rscu-PA (p < 0.001 versus both conjugates), and 380 +/- 66 for rt-PA (p < 0.001 versus both conjugates). The plasma clearances of the conjugates were fourfold to sixfold slower than those of rscu-PA and rt-PA. Maximal rates of clot lysis, determined by continuous external radioisotope scanning over the thorax, were 0.90 +/- 0.13%, 0.91 +/- 0.17%, 0.84 +/- 0.12%, and 1.1 +/- 0.16% lysis per minute for rscu-PA/MA-12B3, rscu-PA/MA-15C5, rscu-PA, and rt-PA, respectively; these maximal rates were obtained with 0.016, 0.016, 1.0, and 0.25 mg/kg, respectively, and were associated with minimal lag phases of 18 +/- 3.2, 28 +/- 4.9, 34 +/- 3.7, and 25 +/- 3.9 minutes, respectively.

CONCLUSIONS

The thrombolytic potency of the rscu-PA/antifibrin conjugates is determined by their clearance, as well as by rate and extent of initial binding to clots and by changes in binding during clot lysis. Clot targeting of rscu-PA with fibrin-specific antibodies increases its thrombolytic potency but does not alter the maximal rate or the minimal lag phase of clot lysis. These parameters appear to be independent of the nature of the plasminogen activator and of targeting.

Comparative thrombolytic properties of tissue-type plasminogen activator (t-PA), single-chain urokinase-type plasminogen activator (u-PA) and K1K2Pu (a t-PA/u-PA chimera) in a combined arterial and venous thrombosis model in the dog

The chimeric molecule K1K2Pu, comprising the two kringle domains (K1 and K2) of tissue-type plasminogen activator (t-PA) and the COOH-terminal region with the serine protease domain (Pu) of urokinase-type plasminogen activator (u-PA), was previously shown to have a 5- to 10-fold reduced clearance rate with maintained specific thrombolytic activity, resulting in an increased thrombolytic potency in animal models of venous and arterial thrombosis. To document the thrombolytic potential of K1K2Pu, the thrombolytic potency and fibrin specificity were studied in a combined platelet-rich arterial eversion graft thrombosis and venous whole blood clot model in heparinized dogs (100 U/kg bolus and 50 U/kg per h infusion). Dose-response effects of bolus injections of K1K2Pu (0.032 to 0.25 mg/kg) were compared with those of recombinant t-PA (rt-PA) and of recombinant single chain u-PA (rscu-PA) (0.25 to 1.0 mg/kg each) in groups of five or six dogs, each given heparin with or without the thromboxane synthase inhibitor/prostaglandin endoperoxide receptor antagonist ridogrel. Heparin and ridogrel in the absence of a thrombolytic agent did not produce arterial reflow or venous clot lysis in five dogs. Addition of K1K2Pu, rt-PA or rscu-PA resulted in a dose-dependent induction of arterial reflow and of venous clot lysis in the absence of systemic fibrinolytic activation and fibrinogen breakdown. Consistent arterial reflow required 0.063 mg/kg of K1K2Pu and 0.5 mg/kg of rt-PA or of rscu-PA. The thrombolytic potency for venous clot lysis, expressed as percent lysis per mg compound administered per kg body weight, was (mean +/- SEM) 750 +/- 160 for K1K2Pu, 68 +/- 17 for rscu-PA (p less than 0.001 vs. K1K2Pu) and 110 +/- 29 for rt-PA (p less than 0.001 vs. K1K2Pu). The plasma clearance rates were significantly lower for K1K2Pu than for rscu-PA and rt-PA. In the absence of ridogrel, arterial reflow was significantly slower and was followed by cyclic reocclusion and reflow; however, venous clot lysis was unaffected. Template bleeding times were not significantly altered in the absence but were markedly prolonged in the presence of ridogrel. These results confirm and establish that, when given as a bolus injection, K1K2Pu has an approximately 10-fold higher thrombolytic potency for arterial and venous thrombolysis than does rt-PA or rscu-PA. Thrombolysis with K1K2Pu is obtained in the absence of systemic fibrinolytic activation and fibrinogen breakdown. These properties suggest that K1K2Pu offers potential for thrombolytic therapy by bolus administration in patients with thromboembolic disease.

Thrombolytic and pharmacokinetic properties of chimeric tissue-type and urokinase-type plasminogen activators

BACKGROUND

Chimeric molecules comprising the A-chain of tissue-type plasminogen activator (t-PA) and the catalytic domain of urokinase-type plasminogen activator (u-PA) have intact enzymatic characteristics of u-PA, partial fibrin-binding properties of t-PA, and thrombolytic properties in animal models comparable with but not superior to those of single-chain u-PA (scu-PA). Deletion of the finger and growth factor domains (t-PA-delta FE/scu-PA-e) in such chimeras further reduces their affinity for fibrin.

METHODS AND RESULTS

A detailed investigation of the thrombolytic potency and the pharmacokinetics of t-PA and u-PA chimeras was performed in quantitative animal models for thrombolysis. In hamsters with pulmonary embolism, in rabbits with jugular vein thrombosis, and in baboons with femoral vein thrombosis, the thrombolytic potency (percent lysis per milligram of compound administered per kilogram of body weight) of t-PA-delta FE/scu-PA-e was significantly higher than that of recombinant scu-PA (rscu-PA, Saruplase) as shown by a maximal rate of 720 +/- 170% versus 45 +/- 5% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in hamsters, 210 +/- 18% versus 49 +/- 3% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in rabbits, and 310 +/- 73% versus 90 +/- 0.3% lysis per milligram of compound per kilogram of body weight (p less than 0.01) in baboons. However, the specific thrombolytic activity (percent lysis per microgram per milliliter steady-state plasma antigen level) of t-PA-delta FE/scu-PA-e was not significantly different from that of rscu-PA in hamsters (210 +/- 57% versus 160 +/- 27% lysis per microgram per milliliter antigen level) and was lower than that of rscu-PA in rabbits (37 +/- 4% versus 130 +/- 5% lysis per microgram per milliliter antigen level; p less than 0.01). In dogs with a combined femoral vein blood clot and a platelet-rich femoral arterial eversion graft thrombosis, 0.25 mg/kg body wt bolus injections of t-PA-delta FE/scu-PA-e produced significantly more venous clot lysis (90 +/- 5%, n = 10) than 0.25 mg/kg rscu-PA (26 +/- 3%, n = 10) (p less than 0.001) and, at the arterial side, more frequent (10 of 10 dogs versus three of 10 dogs) and more persistent (six of 10 dogs versus none of 10 dogs) recanalization (p = 0.002). After bolus injection in hamsters, rabbits, or baboons, t-PA-delta FE/scu-PA-e had a fourfold to sixfold longer initial half-life than rscu-PA and a slower plasma clearance of sixfold in hamsters, 10-fold in rabbits, and more than 10-fold in baboons.

CONCLUSIONS

These results indicate that t-PA-delta FE/scu-PA-e has a markedly enhanced thrombolytic potency toward venous and arterial thrombi caused by a delayed in vivo clearance with relatively maintained specific thrombolytic activity. These properties suggest that the chimera may be clinically useful for thrombolytic therapy by bolus administration in patients with thromboembolic disease.