A new recombinant thrombolytic and antithrombotic agent with higher fibrin affinity--a staphylokinase variant. I. In vitro study

Platelet Membrane-Coated r-SAK Improves Thrombolytic Efficacy by Targeting Thrombus

Thrombolytic therapy is one of the most effective treatments for thrombus dissolution and recanalization of blocked vessels in thrombotic diseases. However, the application of the thrombolytic strategy has been limited due to unsatisfactory thrombolytic efficacy, relatively higher bleeding complications, and consequently restricted indications. Recombinant staphylokinase (r-SAK) is a third-generation thrombolytic agent produced by genetic engineering technology, which exhibits a better thrombolytic efficacy than urokinase and recombinant streptokinase. Inspired by the natural affinity of platelets in hemostasis and pathological thrombosis, we developed a platelet membrane (PM)-coated r-SAK (PM-r-SAK). Results from animal experiments and human in vitro studies showed that the PM-r-SAK had a thrombolytic efficacy equal to or better than its 4-fold dose of r-SAK. In a totally occluded rabbit femoral artery thrombosis model, the PM-r-SAK significantly shortened the initial recanalization time compared to the same dose and 4-fold dose of r-SAK. Regarding the recanalized vessels, the PM-r-SAK prolonged the time of reperfusion compared to the same dose and 4-fold dose of r-SAK, though the differences were not significant. An in vitro thrombolytic experiment demonstrated that the thrombolytic efficacy of PM-r-SAK could be inhibited by platelet-poor plasma from patients taking aspirin and ticagrelor. PM coating significantly improves the thrombolytic efficacy of r-SAK, which is related to the thrombus-targeting activity of the PM-r-SAK and can be inhibited by aspirin- and ticagrelor-treated plasma.

Thrombolytic Enzymes of Microbial Origin: A Review

Enzyme therapies are attracting significant attention as thrombolytic drugs during the current scenario owing to their great affinity, specificity, catalytic activity, and stability. Among various sources, the application of microbial-derived thrombolytic and fibrinolytic enzymes to prevent and treat vascular occlusion is promising due to their advantageous cost-benefit ratio and large-scale production. Thrombotic complications such as stroke, myocardial infarction, pulmonary embolism, deep venous thrombosis, and peripheral occlusive diseases resulting from blood vessel blockage are the major cause of poor prognosis and mortality. Given the ability of microbial thrombolytic enzymes to dissolve blood clots and prevent any adverse effects, their use as a potential thrombolytic therapy has attracted great interest. A better understanding of the hemostasis and fibrinolytic system may aid in improving the efficacy and safety of this treatment approach over classical thrombolytic agents. Here, we concisely discuss the physiological mechanism of thrombus formation, thrombo-, and fibrinolysis, thrombolytic and fibrinolytic agents isolated from bacteria, fungi, and algae along with their mode of action and the potential application of microbial enzymes in thrombosis therapy.

Development and Testing of Thrombolytics in Stroke

Despite recent advances in recanalization therapy, mechanical thrombectomy will never be a treatment for every ischemic stroke because access to mechanical thrombectomy is still limited in many countries. Moreover, many ischemic strokes are caused by occlusion of cerebral arteries that cannot be reached by intra-arterial catheters. Reperfusion using thrombolytic agents will therefore remain an important therapy for hyperacute ischemic stroke. However, thrombolytic drugs have shown limited efficacy and notable hemorrhagic complication rates, leaving room for improvement. A comprehensive understanding of basic and clinical research pipelines as well as the current status of thrombolytic therapy will help facilitate the development of new thrombolytics. Compared with alteplase, an ideal thrombolytic agent is expected to provide faster reperfusion in more patients; prevent re-occlusions; have higher fibrin specificity for selective activation of clot-bound plasminogen to decrease bleeding complications; be retained in the blood for a longer time to minimize dosage and allow administration as a single bolus; be more resistant to inhibitors; and be less antigenic for repetitive usage. Here, we review the currently available thrombolytics, strategies for the development of new clot-dissolving substances, and the assessment of thrombolytic efficacies in vitro and in vivo.

Integration of VEK-30 peptide enhances fibrinolytic properties of staphylokinase

Staphylokinase (SAK), a 136 amino acid bacterial protein with profibrinolytic properties, has emerged as an important thrombolytic agent because of its fibrin specificity and reduced inhibition by α-2 antiplasmin. In an attempt to enhance the clot dissolution ability of SAK, a 30 amino acid peptide (VEK-30) derived from a plasminogen (Pg) binding protein (PAM), was fused at the C-terminal end of SAK with a RGD (Arg-Gly-Asp) linker. The chimeric protein, SAKVEK, was expressed in E. coli and purified as a soluble protein. Pg activation by equimolar complexes of SAKVEK and SAK with plasmin revealed that the fusion of VEK-30 peptide has significantly enhanced the catalytic activity of SAK. The kinetic constant, k_cat /K_m , of SAKVEK for the substrate Pg appeared 2.7 times higher than that of SAK and the time required for the fibrin and platelet rich clot lysis was shortened by 30% and 50%, respectively. The binary activator complex of SAKVEK with plasmin gets inhibited by α2- antiplasmin but remains protected in the presence of fibrin, very similar to SAK. Thus, the present study suggests that SAKVEK is more potent and effective as a thrombolytic agent due to its higher catalytic activity for Pg activation in a fibrin-specific manner and its ability to clear platelet-rich plasma clot faster than SAK.

Designing a multifunctional staphylokinase variant (SAK-2RGD-TTI) with appropriate thrombolytic activity in vitro

OBJECTIVE

Thrombin, platelets, and plasmin are three key factors involved in hemostasis and thrombolysis. Thrombolytic therapy with clinically approved drugs is often followed by recurrent thrombosis caused by thrombin-induced platelet aggregation from the clot debris. In order to minimize these problems, new constructs were designed for the expression of recombinant staphylokinase (rSAK) and also a fusion protein composed of staphylokinase, 20 amino acids containing 2 RGD followed by tsetse thrombin Inhibitor (SAK-2RGD-TTI) in Pichia pastoris.

RESULT

Modeling the tertiary structure of SAK-2RGD-TTI showed that the linker containing RGD and TTI did not interfere with proper folding of SAK. In laboratory testing, the purified SAK-2RGD-TTI (420 μg/mL) dissolved an average of 45% of the blood clot. The activity of the SAK-2RGD-TTI was also confirmed in various tests including human plasminogen activation assay, fibrin clot lysis assay, well diffusion method, activated partial thromboplastin time and platelet rich clot lysis assay.

CONCLUSION

Our findings suggest that SAK-2RGD-TTI has improved therapeutic properties preventing reocclussion. It further confirms that it is practicable to assemble and produce a hybrid multifunctional protein that targets hemostatic process at various stages.

Bacterial staphylokinase as a promising third-generation drug in the treatment for vascular occlusion

Vascular occlusion is one of the major causes of mortality and morbidity. Blood vessel blockage can lead to thrombotic complications such as myocardial infarction, stroke, deep venous thrombosis, peripheral occlusive disease, and pulmonary embolism. Thrombolytic therapy currently aims to rectify this through the administration of recombinant tissue plasminogen activator. Research is underway to design an ideal thrombolytic drug with the lowest risk. Despite the potent clot lysis achievable using approved thrombolytic drugs such as alteplase, reteplase, streptokinase, tenecteplase, and some other fibrinolytic agents, there are some drawbacks, such as high production cost, systemic bleeding, intracranial hemorrhage, vessel re-occlusion by platelet-rich and retracted secondary clots, and non-fibrin specificity. In comparison, bacterial staphylokinase, is a new, small-size plasminogen activator, unlike bacterial streptokinase, it hinders the systemic degradation of fibrinogen and reduces the risk of severe hemorrhage. A fibrin-bound plasmin-staphylokinase complex shows high resistance to a₂-antiplasmin-related inhibition. Staphylokinase has the potential to be considered as a promising thrombolytic agent with properties of cost-effective production and the least side effects.

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 evolution of recombinant thrombolytics: Current status and future directions

Cardiovascular disorders are on the rise worldwide due to alcohol abuse, obesity, hypertension, raised blood lipids, diabetes and age-related risks. The use of classical antiplatelet and anticoagulant therapies combined with surgical intervention helped to clear blood clots during the inceptive years. However, the discovery of streptokinase and urokinase ushered the way of using these enzymes as thrombolytic agents to degrade the fibrin network with an issue of systemic hemorrhage. The development of second generation plasminogen activators like anistreplase and tissue plasminogen activator partially controlled this problem. The third generation molecules, majorly t-PA variants, showed desirable properties of improved stability, safety and efficacy with enhanced fibrin specificity. Plasmin variants are produced as direct fibrinolytic agents as a futuristic approach with targeted delivery of these drugs using liposome technlogy. The novel molecules from microbial, plant and animal origin present the future of direct thrombolytics due to their safety and ease of administration.

In vitro characterization of a multifunctional staphylokinase variant with reduced reocclusion, produced from salt inducible E. coli GJ1158

The thrombolytic therapy with clinically approved drugs often ensues with recurrent thrombosis caused by thrombin-induced platelet aggregation from the clot debris. In order to minimize these problems, a staphylokinase (SAK)-based bacterial friendly multifunctional recombinant protein SRH (staphylokinase (SAK) linked with tripeptide RGD and dodecapeptide Hirulog (SRH)) was constructed to have Hirulog as an antithrombin agent and RGD (Arg-Gly-Asp) as an antiplatelet agent in the present study. This multifunctional fusion protein SRH was expressed in osmotically inducible E. coli GJ1158 as soluble form and purified with a yield of 0.27 g/L and functionally characterized in vitro. SRH retained the fibrinolytic activity and plasminogen activation rate comparable to the parental counterpart SAK. The antithrombin activity of SRH was significantly higher than SAK. The platelet rich clot lysis assay indicated that SRH had enhanced platelet binding activity and T_50% and C₅₀ of SRH were significantly lower than that of SAK. Furthermore, SRH inhibited the ADP-induced platelet aggregation in dose-dependent manner while SAK had no significant effect on platelet aggregation. Thus, the current study suggests that the SAK variant produced from osmotically inducible GJ1158 is more potent thrombolytic agent with antithrombin and antiplatelet aggregation activities for reduction of reocclusion in thrombolytic therapy.

Evaluation of a multifunctional staphylokinase variant with thrombin inhibition and antiplatelet aggregation activities produced from salt-inducible E. coli GJ1158

Reocclusion is one of the major root causes for secondary complications that arise during thrombolytic therapy. A multifunctional staphylokinase variant SRH (staphylokinase (SAK) linked with tripeptide RGD and didecapeptide Hirulog) with antiplatelet and antithrombin activities in addition to clot specific thrombolytic function, was developed to address the reocclusion problem. We preferred to use Escherichia coli GJ1158 as the host in this study for economic production of SRH by osmotic (0.3 mol/L sodium chloride) induction, to overcome the problems associated with the yeast expression system. The therapeutic potential of SRH was evaluated in the murine model of vascular thrombosis. The SAK protein (1 mg/kg body mass) and SRH protein (1 mg/kg and 2 mg/kg) were administered intravenously to the different treatment groups. The results have shown a dose-dependent antithrombotic effect in carrageenan-induced mouse tail thrombosis. The thrombin time, activated partial thromboplastin time, and prothrombin time were significantly prolonged (p < 0.05) in the SRH-infused groups. Moreover, SRH inhibited platelet aggregation in a dose-dependent manner (p < 0.05), while the bleeding time was significantly (p < 0.05) prolonged. All of these results inferred that the osmotically produced multifunctional fusion protein SRH (SAK-RGD-Hirulog) is a promising thrombolytic agent, and one which sustained its multifunctionality in the animal models.

Expression of recombinant staphylokinase, a fibrin-specific plasminogen activator of bacterial origin, in potato (Solanum tuberosum L.) plants

One of the most dynamically developing sectors of green biotechnology is molecular farming using transgenic plants as natural bioreactors for the large scale production of recombinant proteins with biopharmaceutical and therapeutic values. Such properties are characteristic of certain proteins of bacterial origin, including staphylokinase. For many years, work has been carried out on the use of this protein in thrombolytic therapy. In this study, transgenic Solanum tuberosum plants expressing a CaMV::sak-mgpf-gusA gene fusion, were obtained. AGL1 A. tumefaciens strain was used in the process of transformation. The presence of the staphylokinase gene was confirmed by PCR in 22.5% of the investigated plants. The expression of the fusion transgene was detected using the β-glucuronidase activity assay in 32 putative transgenic plants. Furthermore, on the basis of the GUS histochemical reaction, the transgene expression pattern had a strong, constitutive character in seven of the transformants. The polyacrylamide gel electrophoresis of a protein extract from the SAK/PCR-positive plants, revealed the presence of a119 kDa protein that corresponds to that of the fusion protein SAK-mGFP-GUSA. Western blot analysis, using an antibody against staphylokinase, showed the presence of the staphylokinase domain in the 119 kDa protein in six analyzed transformants. However, the enzymatic test revealed amidolytic activity characteristic of staphylokinase in the protein extract of only one plant. This is the first report on a Solanum tuberosum plant producing a recombinant staphylokinase protein, a plasminogen activator of bacterial origin.

One-step thickness shear mode acoustic assay for plasminogen activators

A new procedure is presented for the measurement of plasminogen activators using a thickness shear mode sensor and a modified version of the fibrin plate assay at the micro-scale. Separate, well-mixed solutions of the substrates fibrinogen and plasminogen, and enzymes thrombin and the plasminogen activator sample were mixed together and placed on the sensor surface. The temperature and evaporation were controlled during the assay. The clot dissolution time correlated well with the quantity of the plasminogen activator in the sample. The average relative standard deviation was 12.5%.

Characterization of a novel bifunctional mutant of staphylokinase with platelet-targeted thrombolysis and antiplatelet aggregation activities

Background

Although staphylokianse (SAK) is among the most promising blood dissolving agents, it is far from ideal. It is interesting to hypothesize that the clot lysis efficacy of SAK can be enhanced with direct active platelet binding ability, and at the same time the rethrombosis complication after successful recanalization can be minimized with an antiplatelet aggregation activity. The present study was performed to characterize the functional properties of RGD-SAK, a novel mutant of staphylokinase (SAK).

Results

By using site-directed mutagenesis, an Arg-Gly-Asp (RGD) motif was engineered in the staphylokinase (SAK). This mutant of SAK designated RGD-SAK was expressed, purified and characterized. Biochemical analysis indicated that RGD-SAK maintained the similar structure and the fibrinolytic function of SAK. Measurement of platelet binding activity in vitro demonstrated that RGD-SAK had a much higher affinity with platelets than SAK. In vitro platelet-rich clot lysis assay demonstrated that the engineered mutant outperformed the non-manipulated SAK. The time required for 50% platelet-rich clot lysis and the concentration required to obtain 50% clot lysis (C₅₀) were reduced significantly across different concentrations of RGD-SAK comparing with SAK. Meanwhile, RGD-SAK was found to inhibit ADP-induced platelet aggregation in a concentration-dependent manner while SAK had negligible effect on platelet aggregation.

Conclusion

RGD-SAK possessed the bifunction to target platelet-rich clots and to block platelets aggregation, and thus may serve as a more potential thrombolytic agent with platelet-targeted thrombolytic and antiplatelet aggregation activities in compared with SAK.

Locally activity-released bifunctional fusion protein enhances antithrombosis and alleviates bleeding risk

Despite the fact that lytic therapy of thromboembolic disorder has been achieved, reocclusion of the damaged vessels and bleeding complication frequently reduce the therapeutic effect. In order to prevent the vessel reocclusion and enhance the therapeutic effect, combining the anticoagulant with the thrombolytic was assumed. Herein, we propose that restraining but locally releasing anticoagulant activity in the vicinity of thrombus is a way to alleviate the bleeding risk. A bifunctional fusion protein, termed as SFH (Staphylokinase (SAK) linked by FXa recognition peptide at N-terminus of Hirudin (HV)), was designed. SFH retained thrombolytic activity but no anticoagulant activity in thrombus-free blood due to the extension of the N-terminus of HV. However, it could locally liberate intact HV and exhibit anticoagulant activity when FXa or fresh thrombus was present. At equimolar dose, both improved antithrombotic and thrombolytic effects of SFH were observed in kappa-carrageenin inducing mouse-tail thrombosis model and rat inferior vena cava thrombosis model, respectively. Moreover, we observed significantly lower bleeding risk in mice and rats treated with SFH than with the mixture of SAK and HV with monitoring TT (P < 0.01), aPTT (P < 0.05) and PT (P < 0.05), and bleeding time (P < 0.05). In conclusion, SFH is a promising bifunctional therapeutic candidate with lower bleeding risk.

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.

Thrombolytic efficacy of native recombinant staphylokinase on femoral artery thrombus of rabbits

AIM

To investigate the thrombolytic efficacy, ideal dosage and administration of native recombinant staphylokinase (r-SAK).

METHODS

Forty New Zealand rabbits were randomly assigned into the control, r-SAK low-dose, medial-dose, high dose, single bolus, allied therapy, recombinant streptokinase (r-SK) and urokinase (UK) groups. The right femoral artery thrombosis models were made by balloon injury, and 120 min after the injury, the thrombolytic agents were infused through the rabbits' parallel-ear vein.

RESULTS

(1) 2 h after balloon injury, the pulse pressures of the right femoral arteries reduced to 0 or less than 10% of that of left femoral arteries in all groups; (2) after thrombolytic therapy, the pulse pressures in some of the femoral arteries markedly enhanced to more than 50% of that of left femoral arteries; (3) the reopening rates in the r-SAK medial and high dose groups were significantly higher than that of the control. The reopening rate of the same dose native r-SAK was significant higher than that of UK and r-SK; (4) the patency score of the right femoral arteries tended to be better in the r-SAK medial and high-dose groups than that of the low-dose group, and the time to reopening in the allied therapy group tended to be shorter.

CONCLUSION

(1) r-SAK has a definite thrombolytic effect on the femoral artery thrombus of rabbits; (2) single bolus is an effective manner of r-SAK therapy, and r-SAK allied therapy with heparin may shorten the time to recanalization; (3) the efficacy of the same dose native r-SAK was superior to that of r-SK and UK.

Design of a novel plasminogen activator based on the structure of hirudin

Using a phage library, seven peptide sequences with high affinity to human microplasminogen were obtained. Caseinolytic assay indicated that only the synthesized peptide P07 had slight fibrinolytic activity. To enhance its plasminogen activation ability, peptide P07 was fused into loop 32-35 of hirudin. In vitro assay demonstrated that this hirudin-like fusion protein can activate human plasminogen and retain the function of thrombin inhibition. Fusing the sequence ''SPDASRL'' into hirudin generated a plasminogen activation activity 100 times higher than peptide P07 in chromogenic and radial caseinolytic assay. This significant functional improvement might originate from a more specific active structure due to the hirudin scaffold.

Novel method of expression and purification of hirudin based on pBAD TOPO, pTYB12 vectors and gene synthesis

To express recombinant hirudins in Escherichia coli cells, a fragment of chemically synthesized DNA was used, containing codons for the individual amino acids preferred by the host cells. Gene synthesis was based on the design of two DNA fragments, so-called mega primers H1 and H2 with a complementary fragment, and their incubation with Taq polymerase. The gene obtained in this fashion was multiplied using the PCR, and then expressed in E. coli cells with the use of TOPO vectors pBAD and pTYB12. Using this method, hirudins were obtained in the amount of 17 mg/l E. coli strain, with the activity of 17 antithrombin units (ATU)/mg protein. The method can be considered as an easy and inexpensive route to small protein synthesis.