Enhancement of the thrombolytic efficacy of prourokinase by lys-plasminogen in a dog model of arterial thrombosis

2022 Update of the Consensus on the Rational Use of Antithrombotics and Thrombolytics in Veterinary Critical Care (CURATIVE) Domain 6: Defining rational use of thrombolytics

OBJECTIVES

To systematically review available evidence and establish guidelines related to the use of thrombolytics for the management of small animals with suspected or confirmed thrombosis.

DESIGN

PICO (Population, Intervention, Control, and Outcome) questions were formulated, and worksheets completed as part of a standardized and systematic literature evaluation. The population of interest included dogs and cats (considered separately) and arterial and venous thrombosis. The interventions assessed were the use of thrombolytics, compared to no thrombolytics, with or without anticoagulants or antiplatelet agents. Specific protocols for recombinant tissue plasminogen activator were also evaluated. Outcomes assessed included efficacy and safety. Relevant articles were categorized according to level of evidence, quality, and as to whether they supported, were neutral to, or opposed the PICO questions. Conclusions from the PICO worksheets were used to draft guidelines, which were subsequently refined via Delphi surveys undertaken by the Consensus on the Rational Use of Antithrombotics and Thrombolytics in Veterinary Critical Care (CURATIVE) working group.

RESULTS

Fourteen PICO questions were developed, generating 14 guidelines. The majority of the literature addressing the PICO questions in dogs is experimental studies (level of evidence 3), thus providing insufficient evidence to determine if thrombolysis improves patient-centered outcomes. In cats, literature was more limited and often neutral to the PICO questions, precluding strong evidence-based recommendations for thrombolytic use. Rather, for both species, suggestions are made regarding considerations for when thrombolytic drugs may be considered, the combination of thrombolytics with anticoagulant or antiplatelet drugs, and the choice of thrombolytic agent.

CONCLUSIONS

Substantial additional research is needed to address the role of thrombolytics for the treatment of arterial and venous thrombosis in dogs and cats. Clinical trials with patient-centered outcomes will be most valuable for addressing knowledge gaps in the field.

Towards safer thrombolytic therapy

Plasminogen activators (PA) are unique agents that are currently applied as thrombolytic therapy to achieve rapid vascular reperfusion. Regimens of PA plus anticoagulants and antiplatelet drugs have attained a high degree of sophistication and predictable rates of positive clinical outcomes for acute myocardial infarction (MI), ischemic stroke, pulmonary embolism (PE), deep vein thrombosis (DVT), and thrombosed catheters. Included in the repertoire are newly approved mutants of tissue plasminogen activator (TPA), which have biochemical advantages that allow for bolus administration. Yet, despite tremendous effort devoted to enormous trials to establish the clinical efficacy of these agents in acute MI, mortality results are not superior to those with native TPA or streptokinase (SK). Furthermore, all PAs have the potential for hemorrhagic complication, most critically intracranial hemorrhage (ICH), occurring in 0.9% of patients treated with native or mutant TPA. It is possible that a limit of clinical effectiveness has been reached, beyond which more potent PAs do not achieve greater benefit without a serious increase in risk of bleeding. A breakthrough is possible, however, if the risk of ICH could be avoided. One solution is the application of the direct-acting thrombolytic enzyme, plasmin. While intravenous plasmin is not effective when administered systemically, regional infusion to a thrombus induces local thrombolysis. Unlike the PAs, plasmin treatment should not cause hemorrhage from vascular trauma sites, as it is neutralized by antiplasmin in the blood. Animal studies are fully consistent with this approach, which offers potential for achieving a truly regional thrombolytic treatment.

Differential effects of Lys- and mini-plasminogen on clot lysis induced by recombinant urokinase and recombinant pro-urokinase in a canine thrombosis model

These studies were conducted to examine the lytic efficacy of recombinant urokinase (r-UK) and pro-urokinase (r-proUK) in the presence and absence of truncated forms of plasminogen. Due to differences in their structures, these modified proteins are more readily activated to plasmin than the circulating form of plasminogen. Use of such modified substrates for plasminogen activators may improve the clinical outcome in patients treated for a variety of thrombotic diseases. Lys-plasminogen (46 units) or mini-plasminogen (in units of equivalent chromogenic activity), in conjunction with r-UK (7,500 units), were administered in the absence of heparin to dogs (9-11 kg) in which a radiolabelled thrombus was formed in a femoral artery. Fibrinolysis was measured as a loss of radioactivity from the clot. After intra-arterial administration of the agents, clot lysis was 48 +/- 8%, 50 +/- 9% and 75 +/- 2% in the presence of r-UK + vehicle, r-UK + lys-plasminogen, and r-UK + mini-plasminogen, respectively. When these treatment groups were examined in the presence of heparin (500 units + 350 units/hour) in a second study, r-UK (2,000 units) produced clot lysis of 54 +/- 3%; addition of lys- or mini-plasminogen to the regimen resulted in lysis of 62 +/- 9% and 46 +/- 10%, respectively. A third phase of the study examined r-proUK (1,000 units) with heparin; in this case, lysis was 51 +/- 9% in the presence of vehicle, but 55 +/- 17% and 10 +/- 5% when lys- and mini-plasminogen were administered, respectively. Flow restoration, measured in the femoral artery in each experiment, generally paralleled the lytic profile. The results indicate that supplementation with mini-plasminogen is only useful when added to a lytic regimen in the absence of heparin, and that lys-plasminogen, in conjunction with either of the lytic agents, does not improve clot lysis in this canine model.

Recombinant lys-plasminogen, but not glu-plasminogen, improves recombinant tissue-type plasminogen activator-induced coronary thrombolysis in dogs

OBJECTIVES

This study examined the modification of recombinant tissue-type plasminogen activator (rt-PA)-induced thrombolysis by recombinant lys-plasminogen.

BACKGROUND

Recombinant tissue-type plasminogen activator restores flow in the thrombosed coronary artery, but the artery often reoccludes. The rt-PA-induced thrombolysis is a result of activation of plasminogen bound to fibrin in the thrombus and results in generation of the fibrinolytic enzyme plasmin. Small amounts of lys-plasminogen are formed when rt-PA is used. Lys-plasminogen binds to fibrin with a 10-fold greater affinity than the predominant native glu-plasminogen, leading to a loose fibrin structure.

METHODS

Dogs with electrically induced occlusive intracoronary thrombus were treated with saline solution (n = 9), glu-plasminogen (2 mg/kg body weight, n = 5) or lys-plasminogen (2 mg/kg, n = 5), followed by infusion of rt-PA (1 mg/kg over 20 min) 10 min later.

RESULTS

Reperfusion rates were similar in all groups of dogs, but the time to reflow was lowest in dogs given lys-plasminogen compared with those given saline solution or glu-plasminogen before rt-PA (mean [+/- SE] 14 +/- 2 vs. 22 +/- 2 and 23 +/- 3 min, respectively, p < 0.05). None of the reperfused coronary arteries reoccluded in the lys-plasminogen plus rt-PA group, whereas 75% reoccluded in dogs given saline solution plus rt-PA, and 50% reoccluded in those given glu-plasminogen plus rt-PA. Accordingly, duration of reflow was greater in the lys-plasminogen plus rt-PA group (> 120 vs. 39 +/- 7 and 82 +/- 21 min, respectively, p < 0.05). Plasminogen activator inhibitor-1 activity decreased during rt-PA infusion and thereafter increased in all dogs, but less so in dogs given lys-plasminogen (p < 0.05 vs. those given saline solution before rt-PA).

CONCLUSIONS

Treatment with recombinant lys-plasminogen before rt-PA reduces time to reflow and sustains reflow after thrombolysis, whereas glu-plasminogen has no such effect.

Lys- and glu-plasminogen potentiate the inhibitory effect of recombinant tissue plasminogen activator on human platelet aggregation

To examine the basis of enhanced thrombolytic effect of tissue-type plasminogen activator (t-PA) in the presence of lys- or glu-plasminogen, studies were performed with human platelet-rich plasma (PRP) and washed platelets (WP). t-PA inhibited platelet aggregation in PRP and this effect was potentiated by lys-plasminogen as well as glu-plasminogen. t-PA inhibited WP aggregation only in the presence of lys- or glu-plasminogen. The potentiation of the effects of t-PA was greater (P < 0.05) with lys-plasminogen than with glu-plasminogen. t-PA alone also decreased 14C-serotonin release from WP, and lys- as well as glu-plasminogen reversed this effect of low concentrations of t-PA in WP. Aggregation of WP was also inhibited by plasmin, a proteolytic product of plasminogen. Low, but not high concentrations, of plasmin increased the release of 14C-serotonin. Anti-aggregatory effects of plasmin and lys-plasminogen plus t-PA on platelets were attenuated by preincubation of PRP or WP suspension with aprotinin. These observations suggest enhanced inhibitory effect of t-PA on platelet function in the presence of lys-plasminogen as potential basis of salutary interaction in models of arterial thrombosis.

Plasminogen acceleration of urokinase thrombolysis

PURPOSE

A relative deficiency of plasminogen within the thrombus may be the rate limiting factor in clot lysis.

METHODS

To investigate this hypothesis, we used an in vitro perfusion system and expanded polytetrafluoroethylene graft segments filled with radiolabeled human thrombus. Three groups of five perfusions were compared: (1) urokinase infusion (333 IU/min) into clots laced with buffer, (2) urokinase infusion (333 IU/min) into clots laced with plasminogen (44 CU), and (3) control, D5W infusion into clots laced with buffer. Two end points were measured over time: the amount of lysed thrombus and the flow through the graft.

RESULTS

Urokinase infusion resulted in augmented flow through the graft when compared with control (p < 0.05). Lacing with plasminogen resulted in more rapid restoration of flow when compared with urokinase infusion alone (p < 0.05). Similarly, the rate of clot dissolution was significantly greater in plasminogen-laced thrombi (p < 0.05) when compared with the control and urokinase groups. Embolization of particles of thrombus was uniformly observed in the urokinase group, resulting in a temporary decrease in flow through the thrombosed graft. This event characteristically occurred after 60 minutes of infusion but was never seen in the urokinase/plasminogen treatment group.

CONCLUSIONS

These results suggest that plasminogen supplementation of urokinase thrombolysis may result in significant clinical benefits with respect to the rate of clot lysis and the uniformity of clot dissolution with a lower likelihood of secondary embolization.