Outcome and One Year Follow-up of Intra-arterial Staphylokinase in 191 Patients with Peripheral Arterial Occlusion

Acute Limb Ischemia Interventions

Acute limb ischemia (ALI) is a sudden decrease in limb perfusion that threatens limb viability. Using the Rutherford classification, limbs can be categorized as threatened but viable, or irreversibly damaged, which aids clinicians in selecting appropriate therapy. Treatment options for threatened limbs include catheter-directed thrombolysis, percutaneous mechanical thrombectomy, and surgical revascularization. Potential complications from ALI and treatment include ischemia-reperfusion injury, compartment syndrome, systemic inflammatory response syndrome, multiple organ dysfunction syndrome, hyperkalemia, and bleeding.

Cloning and purification of an anti-thrombotic, chimeric Staphylokinase in Pichia pastoris

There has been an increasing prevalence of cardiovascular diseases such as myocardial infarction and stroke in modern societies because of multiple lifestyle related issues like sedentariness and obesity, alcohol consumption and many more "life-style"factors. The FDA-approved thrombolytics such as Tissue Plasminogen Activator, Streptokinase etc. are used to lyse the clots in thrombotic disorders such as myocardial infarction, stroke etc. but re-occlusion and bleeding that are co-incident to their clinical usage are not addressed. Hence, there is need to develop thrombolytics having properties like increased fibrin clot specificity and thrombin inhibition capability to prevent re-occlusion. In the present work, a fusion protein construct containing two components i.e. Staphylokinase (SAK) and Epidermal Growth Factor (EGF) 4, 5, 6-like domains of human thrombomodulin (THBD) was expressed in Pichia pastoris after genetic optimization. SAK isolated from Staphylococcus aureus is a fibrin-specific plasminogen activator while EGF 4, 5, 6-like domains are reported to be responsible for imparting thrombin inhibition to human thrombomodulin, and therefore, expected could help prevent re-occlusion in the novel construct - SAK_EGF, which is a 43 kDa protein. After expression, it was purified (approx. 13-fold) using two-step purification protocol involving ion-exchange followed by Gel Filtration Chromatography (GFC). The functional characterization including plasminogen activation and thrombin inhibition showed that both the fusion partners viz. SAK and 4,5,6 EGF-like domains retained their respective activities after fusion, confirming it to be a bio-active construct. Thus, this engineered protein could be clinically promising due to the combinatorial effect of fibrin-specific thrombus lysis and prevention of re-occulusion.

The role of fibrinogen and fibrinolysis in peripheral arterial disease

Peripheral arterial disease (PAD) is associated with high rates of cerebrovascular and cardiovascular events; PAD is a marker of systemic atherosclerosis. As a result, standard therapy for all PAD patients should be directed at both peripheral and systemic atherosclerosis. Modification of established risk factors in the form of smoking cessation, correcting hypertension, optimizing diabetic control and normalizing lipids is essential. Furthermore, novel risk factors have emerged including fibrinogen and other hemostatic factors. Fibrinogen is a coagulation factor and a marker of the acute phase response (inflammation), a platelet activator, a major determinant of plasma viscosity and a component of the atherosclerotic plaque. Fibrinogen appears not only to predict the severity of PAD, but also serves as a marker for future development of PAD. Whether reducing the levels of fibrinogen and other coagulation factors will decrease the incidence and progression of PAD remains to be resolved. This review summarizes the role of fibrinogen in the pathogenesis of PAD and its association with other hemostatic factors. The role of fibrinolysis in patients with PAD is also considered.

Staphylokinase-annexin XI chimera exhibited efficient in vitro thrombolytic activities

Annexins (ANXs) are a family of calcium dependent phospholipid binding proteins. Phospholipids such as phosphatidylserine are rapidly exposed on the surfaces of injured endothelial cells, activated platelets, and apoptotic cells in a large number of disorders. In this study, annexin V and XI (ANXV and ANXXI) were individually fused to the C-terminal of staphylokinase (SAK), a fibrin-selective thrombolytic protein, to form chimeras for evaluation of their in-vitro thrombolytic activities. The two chimeras were found to have plasminogen activation activity of comparable efficiency. When the chimeras were challenged under higher concentrations of plasmin for 1 h, hydrolysis of them into moieties was not seen on SDS-PAGE. In two thrombolytic assays, SAK-ANXXI was found to resolve both platelet rich plasma (PRP) clots and platelet poor plasma (PPP) clots with an efficiency similar to that of SAK. However, SAK-ANXV showed significantly reduced efficiency. With regard to anticoagulation ability, SAK-ANXXI was also found to have a stronger effect on dose-dependent extension of clotting time among the four tested proteins. The unique long N-terminal tail of ANXXI, composed of 202 residues, in contrast to the 16 residues of ANXV, probably served successfully to dispatch two moieties to function properly in a complicated microenvironment. Hence, a new option other than the most committed ANXV for the ANX based chimera without elaboration of linker construction is presented.

A dose-finding clinical trial of staphylokinase SY162 in patients with long-term venous access catheter thrombotic occlusion

BACKGROUND

We investigated the safety and efficacy of several dosing regimens of catheter-directed staphylokinase (SY162) bolus administration for the treatment of long-term venous access catheter occlusion.

METHODS

This open-label, ascending dose study enrolled 24 subjects. Three doses of SY162 were evaluated in three cohorts (0.15 mg, 0.3 mg and 0.45 mg) with eight subjects each. Catheter function was evaluated 30 min after the first bolus administration. In case of incomplete catheter function restoration, a second bolus was administered with reassessment of catheter function 30 min thereafter. Cathetergram was repeated to assess thrombus resolution.

RESULTS

Complete restoration of catheter withdrawal function was observed in 2 (25%), 1 (13%) and 7 (88%) subjects after the first bolus in the first, second and third cohort respectively and in 4 (50%), 7 (88%) and 7 (88%) patients after the second administration of SY162. There were no bleeding complications nor other adverse events related to SY162.

CONCLUSIONS

The doses tested in this trial were well tolerated and safe. A dose-response effect within the dose range tested was observed. A 2 ml bolus injection of 0.45 mg SY162 resulted in optimal efficacy after 30 min.

Safety of plasmin in the setting of concomitant aspirin and heparin administration in an animal model of bleeding

Plasmin is a direct thrombolytic which has been shown to have a strikingly favorable benefit to risk profile in comparison with plasminogen activators, notably tissue plasminogen activator (t-PA). As heparin is known to increase the risk of hemorrhage when co-administered with a plasminogen activator, we asked whether adjunct antithrombotic agents such as aspirin and heparin would affect the safety of plasmin. Three groups of rabbits were administered plasmin at a dose (4 mg kg-1) designed to induce significant decreases in antiplasmin, fibrinogen and factor (F)VIII, to about 25, 40 and 40%, respectively, of baseline values, but not cause prolongation of the ear puncture bleeding time. In a blinded and randomized trial, the results show that an intravenous aspirin bolus plus heparin administered as a bolus followed by a maintenance continuous infusion did not significantly prolong the bleeding time during plasmin infusion. These data indicate that in the rabbit, concomitant use of aspirin plus heparin does not affect the safety of a therapeutic dose of plasmin.

Engineering of a staphylokinase-based fibrinolytic agent with antithrombotic activity and targeting capability toward thrombin-rich fibrin and plasma clots

Current clinically approved thrombolytic agents have significant drawbacks including reocclusion and bleeding complications. To address these problems, a staphylokinase-based thrombolytic agent equipped with antithrombotic activity from hirudin was engineered. Because the N termini for both staphylokinase and hirudin are required for their activities, a Y-shaped molecule is generated using engineered coiled-coil sequences as the heterodimerization domain. This agent, designated HE-SAKK, was produced and assembled from Bacillus subtilis via secretion using an optimized co-cultivation approach. After a simple in vitro treatment to reshuffle the disulfide bonds of hirudin, both staphylokinase and hirudin in HE-SAKK showed biological activities comparable with their parent molecules. This agent was capable of targeting thrombin-rich fibrin clots and inhibiting clot-bound thrombin activity. The time required for lysing 50% of fibrin clot in the absence or presence of fibrinogen was shortened 21 and 30%, respectively, with HE-SAKK in comparison with staphylokinase. In plasma clot studies, the HE-SAKK concentration required to achieve a comparable 50% clot lysis time was at least 12 times less than that of staphylokinase. Therefore, HE-SAKK is a promising thrombolytic agent with the capability to target thrombin-rich fibrin clots and to minimize clot reformation during fibrinolysis.

Distinct dose-dependent effects of plasmin and TPA on coagulation and hemorrhage

All thrombolytic agents in current clinical usage are plasminogen activators. Although effective, plasminogen activators uniformly increase the risk of bleeding complications, especially intracranial hemorrhage, and no laboratory test is applicable to avoid such bleeding. We report results of a randomized, blinded, dose-ranging comparison of tissue-type plasminogen activator (TPA) with a direct-acting thrombolytic agent, plasmin, in an animal model of fibrinolytic hemorrhage. This study focuses on the role of plasma coagulation factors in hemostatic competence. Plasmin at 4-fold, 6-fold, and 8-fold the thrombolytic dose (1 mg/kg) induced a dose-dependent effect on coagulation, depleting antiplasmin activity completely, then degrading fibrinogen and factor VIII. However, even with complete consumption of antiplasmin and decreases in fibrinogen and factor VIII to 20% of initial activity, excessive bleeding did not occur. Bleeding occurred only at 8-fold the thrombolytic dose, on complete depletion of fibrinogen and factor VIII, manifest as prolonged primary bleeding, but with minimal effect on stable hemostatic sites. Although TPA had minimal effect on coagulation, hemostasis was disrupted in a dose-dependent manner, even at 25% of the thrombolytic dose (1 mg/kg), manifest as rebleeding from hemostatically stable ear puncture sites. Plasmin degrades plasma fibrinogen and factor VIII in a dose-dependent manner, but it does not disrupt hemostasis until clotting factors are completely depleted, at an 8-fold higher dose than is needed for thrombolysis. Plasmin has a 6-fold margin of safety, in contrast with TPA, which causes hemorrhage at thrombolytic dosages.

Thrombolytic therapy: clinical applications

The therapeutic use of thrombolytic agents is the result of the increasing understanding of the pathophysiologic mechanisms underlying normal and deranged thrombosis and fibrinolysis. Plasminogen activators capable of increasing the production of plasmin exhibit considerable efficacy in the treatment of a variety of arterial and venous thrombotic disorders. The ideal thrombolytic agent has not been developed, but the desired clinical result of rapid opening of the thrombosed vessel without reocclusion, without activation of systemic fibrinogenolysis, and without a risk of hemorrhage are defined. Clinical studies clearly demonstrate that the addition of a variety of adjunctive agents to available thrombolytics enhances benefit without inordinate risk. The addition of intravascular angioplasty and stenting to thrombolysis increases the potential long-term benefit. Newer thrombolytic agents and new protocols for the use of existing therapies offer the promise of saving many who would otherwise succumb to coronary or cerebral arterial thrombosis or to venous thromboembolism.

Thrombolytic therapy

The therapeutic use of thrombolytic agents is the natural result of the increasing understanding of the pathophysiologic mechanisms underlying normal and deranged thrombosis and fibrinolysis. Plasminogen activators capable of increasing the production of plasmin exhibit considerable efficacy in the treatment of a variety of arterial and venous thrombotic disorders. The ideal thrombolytic agent has yet to be developed but the desired clinical result of rapid opening of the thrombosed vessel without reocclusion, without activation of systemic fibrinogenolysis, and without a risk of hemorrhage is well defined. Clinical studies clearly demonstrate that the addition of a variety of adjunctive agents to the available thrombolytics enhances benefit without inordinate risk. The addition of intravascular angioplasty and stenting to thrombolysis increases the potential long-term benefit. Newer thrombolytic agents and new protocols for the use of existing therapies offer the promise of saving many who would otherwise succumb to coronary or cerebral arterial thrombosis or to venous thromboembolism.

Fusion proteins with anticoagulant and fibrinolytic properties: functional studies and structural considerations

In an effort to combine the benefits of fibrinolytics, such as staphylokinase, with those of thrombin inhibitors for the prevention of vessel reocclusion after vascular injury, we have produced several chimeric proteins with plasminogen-activating and thrombin-inhibiting properties. Fusion proteins were constructed consisting of the modules staphylokinase (Sak), the factor Xa cleavage site, and various dipetalin (Dip) domains (H(6)-Sak-Dip-I+II, H(6)-Sak-Dip-I, and H(6)-Sak-Dip-II). Sak stimulates fibrinolysis via activation of plasminogen, whereas dipetalin is a two-domain, Kazal-type inhibitor of thrombin. NMR spectroscopy of the fusion proteins revealed that the molecular structures of the modules are retained in the fusion protein and that no significant interactions occur between the modules in terms of their functionally relevant regions. In enzymatic thrombin inhibition tests and blood coagulation assays (thrombin, prothrombin, and activated partial thromboplastin times), no significant differences in anticoagulant capacity were observed between the fusion protein H(6)-Sak-Dip-I+II and isolated Dip-I+II, even at nanomolar concentrations. Similar results (i.e., the inhibition of thrombin-induced platelet aggregation and the inhibition of thrombin-induced vascular relaxation) were obtained when the cellular thrombin effects were studied. The fusion protein containing Dip-I has less but still significant thrombin inhibitory effects compared with those of H(6)-Sak-Dip-I+II. In contrast, the H(6)-Sak-Dip-II protein failed to inhibit thrombin in each of the assays used. The plasminogen-activating and fibrinolytic activities of the fusion proteins are similar to those of wild-type Sak. The individual dipetalin domains do not activate plasminogen. In conclusion, the fusion protein H(6)-Sak-Dip-I+II is a bifunctional molecule able to activate fibrinolysis via plasminogen activation and inhibit blood coagulation via direct inhibition of thrombin.

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.

Treatment of acute occlusion of peripheral arteries

Acute lower-extremity peripheral arterial occlusion is responsible for a wide variety of complications culminating in limb loss or death. The real incidence of acute limb ischemia (ALI) in the general population is not well known even though recent epidemiological data estimated that it occurs in 14 out of a population of 100,000 and that it accounts for 10-16% of the vascular workload. The two main causes of acute occlusion of peripheral arteries are: (i) embolism and (ii) thrombosis, which usually occurs in cases of severe atherosclerotic stenoses. Arterial flow can be restored through operative revascularization or pharmacological dissolution of thrombus. Immediate surgical revascularization is indicated in the profoundly ischemic limb. Catheter embolectomy is also usually preferred for emboli to a non-atherosclerotic limb. Catheter-directed thrombolysis has become a commonly employed technique in the treatment of ALI. It may offer definitive treatment without the need of major surgery in a significant subset of patients with acute occlusion of a native leg artery or a bypass graft. A number or reports from individual centers and three large prospective studies, which compared intra-arterial thrombolysis to surgical intervention, suggest that thrombolytic therapy may be an appropriate initial treatment of ALI, provided that the limb is not immediately or irreversibly threatened. Using this approach, the underlying lesions can be further defined by angiography, and the percutaneous or surgical revascularization procedure can be performed. However, severe bleeding is still a non-rare complication of intra-arterial thrombolysis and the risk of intracranial hemorrhage is 1-2%.

New directions in thrombolytic therapy

Although current thrombolytic agents have proven their clinical benefit, the failure to rapidly reperfuse some patients and the persistent bleeding risk represent areas for improvement in therapy. In the past two years, the field has been advanced by the regulatory approval of agents with greater ease of administration, continued development of new agents and exploration of the use of more advanced antiplatelet therapies in combination with thrombolytic agents. Finally, a new class of directly acting fibrinolytic agents is available.

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.