Acylated Derivatives of Streptokinase-Plasminogen Activator Complex as Thrombolytic Agents in a Dog Model of Aged Venous 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.

Rates of thrombin acylation and deacylation upon reaction with low molecular weight acylating agents, carbamylating agents and carbonylating agents

Acylated derivatives of thrombin have been made using low molecular weight acylating agents, carbamylating agents and carbonylating agents. The compounds used to acylate the active site serine include isatoic anhydrides, benzoxazinones, benzylisocyanate, N-(benzylcarbonyloxy)succinimide and p-(dimethylamino)benzoylimidazolide. The rates of acylation and deacylation were determined. The best overall inhibitors of thrombin are 2-ethoxy-4H-3,1-benzoxazin-4-one, isatoic anhydride and tert-butyl-2,4-dioxo-2H-3,1-benzoxazine-1(4H)-acetate, which have k2/Ki values of 52,700 M-1s-1, 48,900 M-1s-1 and 5400 M-1s-1, respectively. The carbamyl derivative of thrombin formed with benzylisocyanate had the slowest rate of deacylation (2.3 x 10(-7) s-1), while the ester derivative formed with 2-(N,N-dimethylamino)methylimino-4H-3,1-benzoxazin-4-one had the fastest rate of deacylation (1.9 x 10(-4) s-1).

Use of plasminogen activators in venous thrombosis

A major advance in the treatment of thrombosis has been the development of thrombolytic agents. Streptokinase and urokinase have been the standard agents available for many years, but in recent years the most exciting change in the field has been the development of a new generation of plasminogen activators, the principal one being tissue plasminogen activator. The first generation of plasminogen activators--streptokinase and urokinase--do not have fibrin specificity and predictably induce plasma proteolysis when administered systemically in doses which introduce thrombolysis. The second generation of plasminogen activators are much more fibrin-specific and offer a promise of fewer complications. In a number of major randomized studies, these thrombolytic agents have proved effective clinically. The major complication of thrombolytic therapy, however, is hemorrhage. The risk of hemorrhage increases with the length of infusion and occurs most often from sites of vascular invasion such as needle punctures or cutdown sites from surgical wounds. This can be treated by applying pressure over the wound and discontinuing the thrombolytic agent whose half-life is measured in hours. It is believed that as more experience is acquired with the second-generation plasminogen activators, better control of these drugs will result in fewer complications and more effective and wider application of therapy.

Enhanced streptokinase-induced thrombolysis using heparin in a rabbit model

Heparin potentiation of clot lysis by streptokinase was studied in a rabbit model. Clot was initiated in the rabbit aorta with stasis and thrombin and allowed to naturally propagate proximally until stasis met flow. The clot was allowed to age for 1 h before assigning treatment. Fifteen rabbits (group I) were given streptokinase (10,000 IU/h) and 11 rabbits (group II) were given streptokinase (10,000 IU/h) plus sodium-heparin (120 IU/h). Thrombolytic therapy was continued for 5 h. Clot lysis averaged 30% in group I and 70% in group II. Ten of 11 rabbits in group II had more than 50% clot lysis, whereas only 4 of 15 in group I had this degree of lysis. One group II rabbit and four group I rabbits died prematurely; each was noted to have clot propagation at the time of death. While a trend for amelioration of hypofibrinogenemia was observed in the group receiving both streptokinase and heparin, this difference was not statistically significant. We conclude, in the animal model, that thrombolysis by a combination of heparin and streptokinase is more effective than streptokinase alone. Systemic effects are apparently no worse with the combination.

Low dose intra-arterial streptokinase and acylated plasminogen-streptokinase activator complex: a retrospective review of two thrombolytic regimes in recent peripheral arterial ischaemia

Thrombolytic therapy was used for 57 patients with acute and sub-acute lower limb arterial ischaemia. In the first 34 patients a new thrombolytic agent, acylated plasminogen-streptokinase complex (BRL 26921) was assessed. Following this, 23 patients received low dose intra-arterial streptokinase. The two thrombolytic regimes have been analysed retrospectively. There were differences observed between the two groups in the type of patients treated and in the severity of limb ischaemia. Of the patients receiving BRL 26921, five (15%) had complete, and three (9%) partial lysis of the occluding thrombus. Serious bleeding occurred in six (18%) and minor bleeding in ten (29%) patients. After 30 days, twelve patients (35%) had limb salvage and eleven (32%) had died. Fifteen patients (65%) receiving intra-arterial streptokinase had lysis of the occluding thrombus. Minor bleeding was observed in three patients (13%). After 30 days, 15 (65%) had limb salvage and three (13%) had died. Patients receiving BRL 26921 had a significantly greater reduction in plasma fibrinogen and plasminogen concentrations during treatment which may have accounted for the bleeding complications. At the dose used, BRL 26921 had no demonstrable fibrinogen sparing effect. Improved lysis rates with fewer bleeding complications might be achieved by reducing the dose of BRL 26921. Low dose intra-arterial streptokinase has been confirmed as a safe, effective method of thrombolysis in recent peripheral arterial ischaemia.