Bio-responsive delivery of tissue plasminogen activator for localized thrombolysis

Fibrinogen binding to activated platelets and its biomimetic thrombus-targeted thrombolytic strategies

Thrombosis is associated with various fatal arteriovenous syndromes including ischemic stroke, myocardial infarction, and pulmonary embolism. However, current clinical thrombolytic treatment strategies still have many problems in targeting and safety to meet the thrombolytic therapy needs. Understanding the molecular mechanism that underlies thrombosis is critical in developing effective thrombolytic strategies. It is well known that platelets play a central role in thrombosis and the binding of fibrinogen to activated platelets is a common pathway in the process of clot formation. Based on this, a concept of biomimetic thrombus-targeted thrombolytic strategy inspired from fibrinogen binding to activated platelets in thrombosis was proposed, which could selectively bind to activated platelets at a thrombus site, thus enabling targeted delivery and local release of thrombolytic agents for effective thrombolysis. In this review, we first summarized the main characteristics of platelets and fibrinogen, and then introduced the classical molecular mechanisms of thrombosis, including platelet adhesion, platelet activation and platelet aggregation through the interactions of activated platelets with fibrinogen. In addition, we highlighted the recent advances in biomimetic thrombus-targeted thrombolytic strategies which inspired from fibrinogen binding to activated platelets in thrombosis. The possible future directions and perspectives in this emerging area are briefly discussed.

Poly-α, β-d, l-Aspartyl-Arg-Gly-Asp-Ser-Based Urokinase Nanoparticles for Thrombolysis Therapy

The most concerning adverse effects of thrombolytic agents are major bleeding and intracranial hemorrhage due to their short half-life, low fibrin specificity, and high dosage. To alleviate bleeding side effects during thrombolytic therapy which would bring about the risk of aggravation, we try to find a novel biodegradable delivery nanosystem to carry drugs to target the thrombus, reduce the dosage of the drug, and system side effects. A novel urokinase/poly-α, β-d, l-aspartyl-Arg-Gly-Asp-Ser complex (UK/PD-RGDS) was synthesized and simply prepared. Its thrombolytic potency was assayed by the bubble-rising method and in vitro thrombolytic activity by the thrombus clot lysis assay separately. The in vivo thrombolytic activity and bleeding complication were evaluated by a rat model of carotid arteriovenous bypass thrombolysis. The thrombolytic potency (1288.19 ± 155.20 U/mg) of the UK/PD-RGDS complex nano-globule (18-130 nm) was 1.3 times that of commercial UK (966.77 ± 148.08 U/mg). In vivo, the UK/PD-RGDS complex (2000 IU/kg) could reduce the dose of UK by 90% while achieving the equivalent thrombolysis effect as the free UK (20,000 IU/kg). Additionally, the UK/PD-RGDS complex decreased the tail bleeding time compared with UK. The organ distribution of the FITC-UK/PD-RGDS complex was explored in the rat model. The UK/PD-RGDS complex could provide a promising platform to enhance thrombolytic efficacy significantly and reduce the major bleeding degree.

H2O2-triggered "off/on signal" nanoparticles target P-selectin for the non-invasive and contrast-enhanced theranostics for arterial thrombosis

Pathological coagulation within an injured artery and the subsequent cardiovascular complications, such as stroke and heart attack, greatly threaten human life. Inspired by the biochemical features of acute arterial thrombosis, such as abundant activated platelets and hydrogen peroxide (H₂O₂), we constructed platelet-targeted theranostic nanoparticles (CyBA/PFM NPs) with H₂O₂-triggered photoacoustic contrast enhancement and antithrombotic capabilities. CyBA/PFM NPs were designed to target platelet-rich clots via fucoidan segment within the carrier, which could be activated by H₂O₂ to produce fluorescent "CyOH" molecules, thus turning on the photoacoustic signal. CyBA/PFM NPs showed obvious amplification of fluorescence following incubation with fresh clots, exhibiting efficient scavenging ability of intracellular reactive oxygen species (ROS). In a FeCl₃-induced mouse model of carotid thrombosis, CyBA/PFM NPs significantly amplified the photoacoustic contrast in thrombogenic tissues, effectively eliminated ROS within the occlusion site, and suppressed the thrombus formation, accompanied by a normalization of the soluble CD40L level. Given their accurate imaging potential, potent antithrombotic activities and acceptable biosafety, CyBA/PFM NPs hold strong potential as nanoscale theranostics for H₂O₂-correlated cardiovascular diseases. STATEMENT OF SIGNIFICANCE: In this study, we developed a platelet-targeted and H₂O₂-triggered nanosystem self-assembled from phenylboronated fucoidan/maltodextrin polymers and responsive near-infrared probes. The fucoidan segment within the carrier could facilitate the specific delivery of the therapeutic polymers and probes to the platelet-rich arterial thrombus. In a mouse model of FeCl₃-induced arterial thrombosis, the system could be activated by H₂O₂ to produce fluorescent "CyOH" molecules, thus turning on the photoacoustic signal and specifically imaging thrombosed tissues. Besides, CyBA/PFM NPs significantly effectively eliminated ROS within the occlusion site and suppressed the thrombus formation. Given their theranostic potential and acceptable biosafety, this system has great potential for H₂O₂-correlated cardiovascular diseases.

Recent Advances in Blood Cell-Inspired and Clot Targeted Thrombolytic Therapies

Myocardial infarction, stroke, and pulmonary embolism are all deadly conditions associated with excessive thrombus formation. Standard treatment for these conditions involves systemic delivery of thrombolytic agents to break up clots and restore blood flow; however, this treatment can impact the hemostatic balance in other parts of the vasculature, which can lead to excessive bleeding. To avoid this potential danger, targeted thrombolytic treatments that can successfully target thrombi and release an effective therapeutic load are necessary. Because activated platelets and fibrin make up a large proportion of clots, these two components provide ample opportunities for targeting. This review will highlight potential thrombus targeting mechanisms as well as recent advances in thrombolytic therapies which utilize blood-cells and clotting proteins to effectively target and lyse clots.

Direct delivery of plasmin using clot-anchoring thrombin-responsive nanoparticles for targeted fibrinolytic therapy

BACKGROUND

Fibrin-rich clot formation in thrombo-occlusive pathologies is currently treated by systemic administration of plasminogen activators (e.g. tPA), to convert fibrin-associated plasminogen to plasmin for fibrinolytic action. However, this conversion is not restricted to clot site only but also occurs on circulating plasminogen, causing systemic fibrinogenolysis and bleeding risks. To address this, past research has explored tPA delivery using clot-targeted nanoparticles.

OBJECTIVES

We designed a nanomedicine system that can (1) target clots via binding to activated platelets and fibrin, (2) package plasmin instead of tPA as a direct fibrinolytic agent, and (3) release this plasmin triggered by thrombin for clot-localized action.

METHODS

Clot-targeted thrombin-cleavable nanoparticles (CTNPs) were manufactured using self-assembly of peptide-lipid conjugates. Plasmin loading and its thrombin-triggered release from CTNPs were characterized by UV-visible spectroscopy. CTNP-targeting to clots under flow was studied using microfluidics. Fibrinolytic effect of CTNP-delivered plasmin was studied in vitro using BioFlux imaging and D-dimer analysis and in vivo in a zebrafish thrombosis model.

RESULTS

Plasmin-loaded CTNPs significantly bound to clots under shear flow and showed thrombin-triggered enhanced release of plasmin. BioFlux studies confirmed that thrombin-triggered plasmin released from CTNPs rendered fibrinolysis similar to free plasmin, further corroborated by D-dimer analysis. In the zebrafish model, CTNP-delivered plasmin accelerated time-to-recanalization, or completely prevented occlusion when infused before thrombus formation.

CONCLUSION

Considering that the very short circulation half-life (<1 second) of plasmin prevents its systemic use but also makes it safer without off-target drug effects, clot-targeted delivery of plasmin using CTNPs can enable safer and more efficacious fibrinolytic therapy.

A Fibrinogen-Mimicking, Activated-Platelet-Sensitive Nanocoacervate Enhances Thrombus Targeting and Penetration of Tissue Plasminogen Activator for Effective Thrombolytic Therapy

The development of a fibrinolytic system with long circulation time, high thrombus targeting, efficient thrombus penetration, effective thrombolysis, and minimal hemorrhagic risk remains a major challenge. Herein, inspired by fibrinogen binding to activated platelets in thrombosis, this article reports a fibrinogen-mimicking, activated-platelet-sensitive nanocoacervate to enhance thrombus penetration of tissue plasminogen activator (tPA) for targeted thrombolytic therapy. This biomimetic nanothrombolytic system, denoted as RGD-Chi@tPA, is constructed by "one-pot" coacervation through electrostatic interactions between positively charged arginine-glycine-aspartic acid (RGD)-grafted chitosan (RGD-Chi) and negatively charged tPA. Flow cytometry and confocal laser scanning microscopy measurements show targeting of RGD-Chi@tPA to activated platelets. Controlled tPA release triggered by activated platelets at a thrombus site is demonstrated. Its targeted fibrinolytic and thrombolytic activities are measured in in vitro models. The pharmacokinetic profiles show that RGD-Chi@tPA can significantly prolong circulation time compared to free tPA. In a mouse tail thrombus model, RGD-Chi@tPA displays efficient thrombus targeting and penetration, enabling a complete vascular recanalization as confirmed by the fluorescence imaging, histochemical assay, and laser speckle contrast imager. Consequently, RGD-Chi@tPA induces a substantial enhancement in thrombolysis with minimal hemorrhagic risk compared to free tPA. This simple, effective, and safe platform holds great promise for the development of thrombolytic nanomedicines.

Platelet-inspired nanomedicine in hemostasis thrombosis and thromboinflammation

Platelets are anucleate cell-fragments derived predominantly from megakaryocytes in the bone marrow and released in the blood circulation, with a normal count of 150 000-40 000 per μl and a lifespan of approximately 10 days in humans. A primary role of platelets is to aid in vascular injury site-specific clot formation to stanch bleeding, termed hemostasis. Platelets render hemostasis by a complex concert of mechanisms involving platelet adhesion, activation and aggregation, coagulation amplification, and clot retraction. Additionally, platelet secretome can influence coagulation kinetics and clot morphology. Therefore, platelet defects and dysfunctions result in bleeding complications. Current treatment for such complications involve prophylactic or emergency transfusion of platelets. However, platelet transfusion logistics constantly suffer from limited donor availability, challenges in portability and storage, high bacterial contamination risks, and very short shelf life (~5 days). To address these issues, an exciting area of research is focusing on the development of microparticle- and nanoparticle-based platelet surrogate technologies that can mimic various hemostatic mechanisms of platelets. On the other hand, aberrant occurrence of the platelet mechanisms lead to the pathological manifestation of thrombosis and thromboinflammation. The treatments for this are focused on inhibiting the mechanisms or resolving the formed clots. Here, platelet-inspired technologies can provide unique platforms for disease-targeted drug delivery to achieve high therapeutic efficacy while avoiding systemic side-effects. This review will provide brief mechanistic insight into the role of platelets in hemostasis, thrombosis and thromboinflammation, and present the current state-of-art in the design of platelet-inspired nanomedicine for applications in these areas.

Bioresponsive Polyphenol-Based Nanoparticles as Thrombolytic Drug Carriers

Thrombolytic (clot-busting) therapies with plasminogen activators (PAs) are first-line treatments against acute thrombosis and ischemic stroke. However, limitations such as narrow therapeutic windows, low success rates, and bleeding complications hinder their clinical use. Drug-loaded polyphenol-based nanoparticles (NPs) could address these shortfalls by delivering a more targeted and safer thrombolysis, coupled with advantages such as improved biocompatibility and higher stability in vivo. Herein, a template-mediated polyphenol-based supramolecular assembly strategy is used to prepare nanocarriers of thrombolytic drugs. A thrombin-dependent drug release mechanism is integrated using tannic acid (TA) to cross-link urokinase-type PA (uPA) and a thrombin-cleavable peptide on a sacrificial mesoporous silica template via noncovalent interactions. Following drug loading and template removal, the resulting NPs retain active uPA and demonstrate enhanced plasminogen activation in the presence of thrombin (1.14-fold; p < 0.05). Additionally, they display lower association with macrophage (RAW 264.7) and monocytic (THP-1) cell lines (43 and 7% reduction, respectively), reduced hepatic accumulation, and delayed blood clearance in vivo (90% clearance at 60 min vs 5 min) compared with the template-containing NPs. Our thrombin-responsive, polyphenol-based NPs represent a promising platform for advanced drug delivery applications, with potential to improve thrombolytic therapies.

Enhanced control of bioactivity of tissue plasminogen activator (tPA) through domain-directed enzymatic oxidation of terminal galactose

Introduction: In targeted enzyme prodrug constructs, it is critical to control the bioactivity of the drug in its prodrug form. The preparation of such constructs often involves conjugation reactions directed to functional groups on amino acid side chains of the protein, which result in random conjugation and incomplete control of bioactivity of a prodrug, which may result in significant nontarget effect. Thus, more specific method of modification is desired. If the drug is a glycoprotein, enzymatic oxidation may offer an alternative approach for therapeutic glycoproteins. Methods: Tissue plasminogen activator (tPA), a model glycoprotein enzyme, was treated with galactose oxidase (GO) and horseradish peroxidase, followed by thiolation reaction and conjugation with low molecular weight heparin (LMWH). The LMWH-tPA conjugate was isolated by ion-exchange chromatography followed by centrifugal filtration. The conjugate was characterized for its fibrinolytic activity and for its plasminogen activation through an indirect amidolytic assay with a plasmin-specific substrate S-2251 when LMWH-tPA conjugate is complexed with protamine-albumin conjugate, followed by triggered activation in the presence of heparin. Results: LMWH-tPA conjugate prepared via enzymatic oxidation retained ~95% of its fibrinolytic activity with respect to native tPA. Upon complexation with protamine-albumin conjugate, the activity of LMWH-tPA was effectively inhibited (~90%) whereas the LMWH-tPA prepared by random thiolation exhibited ~55% inhibition. Addition of heparin fully generated the activities of both conjugates. Conclusion: The tPA was successfully modified via enzymatic oxidation by GO, resulting in enhanced control of its activity in the prodrug construct. This approach can be applied to other therapeutic glycoproteins.

Biotechnology-based therapeutics for management of cerebral stroke

Cerebral stroke, commonly caused due to hindrance in blood flow, is broadly classified into two categories-ischemic and haemorrhagic strokes. The onset of stroke triggers multiple mechanisms causing inflammation, generation of free radicals and protein damage leading to apoptosis of neuronal cells. The current therapies available for cerebral strokes involve use of complex surgical treatments and tissue plasminogen activator which increases the risk of internal bleeding, brain edema and cerebral damage, thereby restricting their use in clinical setting. The alarming need to develop safe, effective, target specific systems which, promote neuronal growth and reduce cerebral inflammation can be accomplished with use of biotechnological approaches. The article gives an insight to biotechnology-based advancements for tissue plasminogen activators, cell penetrating peptides, growth factors, ribonucleic acid systems and monoclonal antibodies for cerebral stroke. We also emphasis on challenges and future perspective of biotechnology-based therapeutics for better management of stroke.

Nanotechnology-based drug delivery for central nervous system disorders

Drug delivery to central nervous system (CNS) diseases is very challenging since the presence of the innate blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier that impede drug delivery. Among new strategies to overcome these limitations and successfully deliver drugs to the CNS, nanotechnology-based drug delivery platform, offers potential therapeutic approach for the treatment of some common neurological disorders like Alzheimer's disease, frontotemporal dementia, amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease. This review aimed to highlight advances in research on the development of nano-based therapeutics for their implications in therapy of CNS disorders. The challenges during clinical translation of nanomedicine from bench to bed side is also discussed.

Fibrinogen-mimicking, multiarm nanovesicles for human thrombus-specific delivery of tissue plasminogen activator and targeted thrombolytic therapy

Clinical use of tissue plasminogen activator (tPA) in thrombolytic therapy is limited by its short circulation time and hemorrhagic side effects. Inspired by fibrinogen binding to activated platelets, we report a fibrinogen-mimicking, multiarm nanovesicle for thrombus-specific tPA delivery and targeted thrombolysis. This biomimetic system is based on the lipid nanovesicle coated with polyethylene glycol (PEG) terminally conjugated with a cyclic RGD (cRGD) peptide. Our experiments with human blood demonstrated its highly selective binding to activated platelets and efficient tPA release at a thrombus site under both static and physiological flow conditions. Its clot dissolution time in a microfluidic system was comparable to that of free tPA. Furthermore, we report a purpose-built computational model capable of simulating targeted thrombolysis of the tPA-loaded nanovesicle and with a potential in predicting the dynamics of thrombolysis in physiologically realistic scenarios. This combined experimental and computational work presents a promising platform for development of thrombolytic nanomedicines.

Functionalized polymeric hybrid micelles as an efficient nanotheranostic agent for thrombus imaging and thrombolysis

Pathological thrombosis within a vessel hampers blood flow and is the mainspring of numerous fatal cardiovascular complications. In order to specifically image and dissolve a thrombus, we rationally designed a functionalized polymeric hybrid micelle (PHM) system self-assembled from amphiphilic polycaprolactone-polyethylenimine (PCL-PEI) and polycaprolactone-polyethylene glycol (PCL-PEG). Based on a biological component of thrombi, activated coagulation factor XIII (FXIIIa), which is responsible for fibrin crosslinking, we further developed FXIIIa-targeted near infrared imaging and thrombolytic nanoparticles, termed IR780/FPHM/LK NPs, through chemical conjugation of peptides to the system. In a ferric chloride (FeCl₃)-induced mouse carotid thrombosis model, IR780/FPHM/LK NPs specifically targeted the thrombus and significantly enhanced the photoacoustic signal for an accurate diagnosis. When loaded with the fibrinolytic drug lumbrokinase (LK), FPHM remarkably dissociated the thrombus accompanied by an increase in the d-dimer level, a fibrin degradation product, and alleviation of fatal nonspecific hemorrhagic risk. Given its thrombus-specific imaging along with potent therapeutic activities, IR780/FPHM/LK NPs hold promise for developing nanotheranostic agents in preclinical thrombotic vascular disease models.

Targeted nano-delivery strategies for facilitating thrombolysis treatment in ischemic stroke

Ischemic stroke is one of the major causes of severe disability and death worldwide. It is mainly caused by a sudden reduction in cerebral blood flow due to obstruction of the supplying vessel by thrombi and subsequent initiation of a complex cascade of pathophysiological changes, which ultimately lead to brain ischemia and even irreversible infarction. Thus, timely and effective thrombolysis therapy remains a mainstay for acute ischemic stroke treatment. Tissue plasminogen activator (tPA), the only thrombolytic agent approved globally, provides substantial benefits by exerting a fibrinolysis effect, recovering the blood supply in occluded vessels and, thereby, salvaging the ischemic tissue. However, the clinical application of tPA was limited because of a few unsolved issues, such as a narrow therapeutic window, hemorrhagic complications, and limited thrombolytic efficacy, especially, for large thrombi. With the prosperous development of nanotechnology, a series of targeted delivery strategies and nanocomposites have been extensively investigated for delivering thrombolytic agents to facilitate thrombolysis treatment. Excitingly, numerous novel attempts have been reported to be effective in extending the half-life, targeting the thrombus site, and improving the thrombolytic efficacy in preclinical models. This article begins with a brief introduction to ischemic stroke, then describes the current state of thrombolysis treatment and, finally, introduces the application of various nanotechnology-based strategies for targeted delivery of thrombolytic agents. Representative studies are reviewed according to diverse strategies and nano-formulations, with the aim of providing integrated and up-to-date information and to improve the development of thrombolysis treatment for stroke patients.

Targeted delivery of thrombolytic enzymes

Although thrombolytic agents have been used for several decades in the treatment of thromboembolic conditions, there is an unmet need for the development of safer thrombolytic agents. The development of new molecules themselves may not be sufficient. This has sparked a growing interest in the design of novel nanoscale drug carrier systems for the delivery of thrombolytic enzymes in an effort to address its fatal side effects. There are numerous proof-of-concept reports on such nanoscale systems that seek to capitalize on the pathophysiologic signatures of thrombosis as well as external biochemical/physical triggers. Although there may be a long road ahead before we have such new nanoscale therapeutics on the bedside, hopes remain high.

Thrombolytic Agents: Nanocarriers in Controlled Release

Thrombosis is a life-threatening pathological condition in which blood clots form in blood vessels, obstructing or interfering with blood flow. Thrombolytic agents (TAs) are enzymes that can catalyze the conversion of plasminogen to plasmin to dissolve blood clots. The plasmin formed by TAs breaks down fibrin clots into soluble fibrin that finally dissolves thrombi. Several TAs have been developed to treat various thromboembolic diseases, such as pulmonary embolisms, acute myocardial infarction, deep vein thrombosis, and extensive coronary emboli. However, systemic TA administration can trigger non-specific activation that can increase the incidence of bleeding. Moreover, protein-based TAs are rapidly inactivated upon injection resulting in the need for large doses. To overcome these limitations, various types of nanocarriers have been introduced that enhance the pharmacokinetic effects by protecting the TA from the biological environment and targeting the release into coagulation. The nanocarriers show increasing half-life, reducing side effects, and improving overall TA efficacy. In this work, the recent advances in various types of TAs and nanocarriers are thoroughly reviewed. Various types of nanocarriers, including lipid-based, polymer-based, and metal-based nanoparticles are described, for the targeted delivery of TAs. This work also provides insights into issues related to the future of TA development and successful clinical translation.

A controlled nucleation and formation rate of self-assembled peptide nanofibers

Self-assembling peptide matrixes are powerful platforms for encouraging tissue regeneration, but are usually formed within seconds and remain relatively static in both structure and function throughout their application. For the first time, we have shown that it is possible to extend the time it takes for peptide self-assembly so as to allow for the dynamic building of a self-assembled system over days, in the presence of an enzyme. Specifically, K₅ and K₁₀ sequences were conjugated, via a thrombin-specific cleavage domain NleTPR/SFL, to prevent the nanofiber formation and form stable nanoparticles composed of (RADA)₄-GG-NleTPR/SFL-K₅ and (RADA)₄-GG-NleTPR/SFL-K₁₀ that act as nucleation sites for reassembling. Upon introduction of thrombin, a model enzyme, this system showed an extremely slow rate of nanofiber formation in a parallel direction that is in sharp contrast to the well-known rapid assembly of (RADA)₄ systems with random networks. These bioresponsive materials may provide a novel platform for utilizing long-term enzymatic profiles to form new nanofibers within an existing matrix over long therapeutic timeframes.

Preparation of Peptide and Recombinant Tissue Plasminogen Activator Conjugated Poly(Lactic-Co-Glycolic Acid) (PLGA) Magnetic Nanoparticles for Dual Targeted Thrombolytic Therapy

Recombinant tissue plasminogen activator (rtPA) is the only thrombolytic agent that has been approved by the FDA for treatment of ischemic stroke. However, a high dose intravenous infusion is required to maintain effective drug concentration, owing to the short half-life of the thrombolytic drug, whereas a momentous limitation is the risk of bleeding. We envision a dual targeted strategy for rtPA delivery will be feasible to minimize the required dose of rtPA for treatment. For this purpose, rtPA and fibrin-avid peptide were co-immobilized to poly(lactic-co-glycolic acid) (PLGA) magnetic nanoparticles (PMNP) to prepare peptide/rtPA conjugated PMNPs (pPMNP-rtPA). During preparation, PMNP was first surface modified with avidin, which could interact with biotin. This is followed by binding PMNP-avidin with biotin-PEG-rtPA (or biotin-PEG-peptide), which was prepared beforehand by binding rtPA (or peptide) to biotin-PEG-maleimide while using click chemistry between maleimide and the single -SH group in rtPA (or peptide). The physicochemical property characterization indicated the successful preparation of the magnetic nanoparticles with full retention of rtPA fibrinolysis activity, while biological response studies underlined the high biocompatibility of all magnetic nanoparticles from cytotoxicity and hemolysis assays in vitro. The magnetic guidance and fibrin binding effects were also confirmed, which led to a higher thrombolysis rate in vitro using PMNP-rtPA or pPMNP-rtPA when compared to free rtPA after static or dynamic incubation with blood clots. Using pressure-dependent clot lysis model in a flow system, dual targeted pPMNP-rtPA could reduce the clot lysis time for reperfusion by 40% when compared to free rtPA at the same drug dosage. From in vivo targeted thrombolysis in a rat embolic model, pPMNP-rtPA was used at 20% of free rtPA dosage to restore the iliac blood flow in vascular thrombus that was created by injecting a blood clot to the hind limb area.

Engineered microparticles and nanoparticles for fibrinolysis

Fibrinolytic agents including plasmin and plasminogen activators improve outcomes in acute ischemic stroke and thrombosis by recanalizing occluded vessels. In the decades since their introduction into clinical practice, several limitations of have been identified in terms of both efficacy and bleeding risk associated with these agents. Engineered nanoparticles and microparticles address some of these limitations by improving circulation time, reducing inhibition and degradation in circulation, accelerating recanalization, improving targeting to thrombotic occlusions, and reducing off-target effects; however, many particle-based approaches have only been used in preclinical studies to date. This review covers four advances in coupling fibrinolytic agents with engineered particles: (a) modifications of plasminogen activators with macromolecules, (b) encapsulation of plasminogen activators and plasmin in polymer and liposomal particles, (c) triggered release of encapsulated fibrinolytic agents and mechanical disruption of clots with ultrasound, and (d) enhancing targeting with magnetic particles and magnetic fields. Technical challenges for the translation of these approaches to the clinic are discussed.

Sequentially Site-Specific Delivery of Thrombolytics and Neuroprotectant for Enhanced Treatment of Ischemic Stroke

Ischemic stroke caused by a thrombus clog and ischemia is one of the most lethal and disabling cerebrovascular diseases. A sequentially targeted delivery system is highly desired to deliver thrombolytics and neuroprotectant to the site of the thrombus and ischemic penumbra, respectively, to pursue a maximized combinational effect. Inspired by the vital roles that platelets play in thrombus formation, herein, we develop a bioengineered "nanoplatelet" (tP-NP-rtPA/ZL006e) for sequentially site-specific delivery of recombinant tissue plasminogen activator (rtPA) and neuroprotectant (ZL006e) for ischemic stroke treatment. The tP-NP-rtPA/ZL006e consists of a ZL006e-loaded dextran derivative polymeric nanoparticle core and platelet membrane shell conjugated with thrombin-cleavable Tat-peptide-coupled rtPA. Mediated by the cloak of the platelet membrane, tP-NP-rtPA/ZL006e targets the thrombus site and rtPA is triggered to release by the upregulated thrombin. Subsequently, the in situ exposed Tat peptide enhanced penetration of the "nanoplatelet" across the blood-brain barrier into ischemic brain for ZL006e site-specific delivery. From the in vitro and in vivo evaluation, tP-NP-rtPA/ZL006e is demonstrated to significantly enhance the anti-ischemic stroke efficacy in the rat model  with middle cerebral artery occlusion, showing a 63 and 72% decrease in ischemic area and reactive oxygen species level compared to that with free drug combination, respectively.

Synthesis and evaluation of cationic polymeric micelles as carriers of lumbrokinase for targeted thrombolysis

To achieve targeted thrombolysis, a targeted delivery system of lumbrokinase (LK) was constructed using RGDfk-conjugated hybrid micelles. Based on the specific affinity of RGDfk to glycoprotein complex of GPⅡb/Ⅲa expressed on the surface of membrane of activated platelet, LK loaded targeted micelles (LKTM) can be delivered to thrombus. The hybrid micelles were composed of polycaprolactone-block-poly (2-(dimethylamino) ethyl methacrylate) (PCL-PDMAEMA), methoxy polyethylene glycol-block- polycaprolactone (mPEG-PCL) and RGDfk conjugated polycaprolactone-block- polyethylene glycol (PCL-PEG-RGDfk). PCL-PDMAEMA was synthesized via ring open polymerization (ROP) and atom transfer radical polymerization (ATRP). PCL-PEG-RGDfk was synthesized via ROP and carbodiimide chemistry. The prepared LKTM was characterized by dynamic light scattering (DLS) and transmission electron microscope (TEM). Colloidal stability assay showed the prepared LKTM was stable. Biocompatibility assay was performed to determine the safe concentration range of polymer. The assay of fluorescent distribution in vivo demonstrated that LKTM can be efficiently delivered to thrombi in vivo. Thrombolysis in vivo indicated the thrombolytic potency of LKTM was optimal in all groups. Notably, the laboratory mice treated with LKTM exhibited a significantly shorter tail bleeding time compared to those treated with LK or LK-loaded micelles without RGDfk, which suggested that the targeted delivery of LK using RGDfk-conjugated hybrid micelles effectively reduced the bleeding risk.

Recent strategies on targeted delivery of thrombolytics

Thrombus formed in blood vessel is a progressive process, which would lead to life-threatening thrombotic diseases such as ischemic stroke. Unlike other diseases, the recognition of thrombus is usually in the late stage where blood vessels are largely blocked. So acute thrombotic diseases have a narrow therapeutic window, and remain leading causes of morbidity and mortality, whereas current thrombolysis therapy has limited therapeutic effects and bleeding complications. Thrombolytic agents in unwanted sites would cause hemorrhage due to the activation of plasminogen. Moreover, untargeted thrombolysis therapy require large amounts of thrombolytic agents, which in return would enhance hemorrhage risk. To improve the efficiency while minimizing the adverse effects of traditional thrombolysis therapy, novel drug delivery systems have been investigated. Various targeting strategies including ultrasound and magnetic field directed targeting, and specific binding, have been designed to deliver thrombolytic drugs to the thrombotic sites. These strategies demonstrate promising results in reducing bleeding risk as well as allowing less dosage of thrombolytic drugs with lowered clot lysis time. In this review, we discuss recent progress on targeted delivery of thrombolytics, and summarize treatment advantages and shortcomings, potentially helping to further promote the development of targeted thrombolysis.

Tissue plasminogen activator-based nanothrombolysis for ischemic stroke

INTRODUCTION

Thrombolysis with intravenous tissue plasminogen activator (tPA) is the only FDA approved treatment for patients with acute ischemic stroke, but its use is limited by narrow therapeutic window, selective efficacy, and hemorrhagic complication. In the past two decades, extensive efforts have been undertaken to extend its therapeutic time window and explore alternative thrombolytic agents, but both show little progress. Nanotechnology has emerged as a promising strategy to improve the efficacy and safety of tPA.

AREAS COVERED

We reviewed the biology, thrombolytic mechanism, and pleiotropic functions of tPA in the brain and discussed current applications of various nanocarriers intended for the delivery of tPA for treatment of ischemic stroke. Current challenges and potential further directions of t-PA-based nanothrombolysis in stroke therapy are also discussed.

EXPERT OPINION

Using nanocarriers to deliver tPA offers many advantages to enhance the efficacy and safety of tPA therapy. Further research is needed to characterize the physicochemical characteristics and in vivo behavior of tPA-loaded nanocarriers. Combination of tPA based nanothrombolysis and neuroprotection represents a promising treatment strategy for acute ischemic stroke. Theranostic nanocarriers co-delivered with tPA and imaging agents are also promising for future stroke management.

Novel Thrombolytic Drug Based on Thrombin Cleavable Microplasminogen Coupled to a Single-Chain Antibody Specific for Activated GPIIb/IIIa

Background

Thrombolytic therapy for acute thrombosis is limited by life‐threatening side effects such as major bleeding and neurotoxicity. New treatment options with enhanced fibrinolytic potential are therefore required. Here, we report the development of a new thrombolytic molecule that exploits key features of thrombosis. We designed a recombinant microplasminogen modified to be activated by the prothrombotic serine‐protease thrombin (HtPlg), fused to an activation‐specific anti–glycoprotein IIb/IIIa single‐chain antibody (SCE5), thereby hijacking the coagulation system to initiate thrombolysis.

Methods and Results

The resulting fusion protein named SCE5‐HtPlg shows in vitro targeting towards the highly abundant activated form of the fibrinogen receptor glycoprotein IIb/IIIa expressed on activated human platelets. Following thrombin formation, SCE5‐HtPlg is activated to contain active microplasmin. We evaluate the effectiveness of our targeted thrombolytic construct in two models of thromboembolic disease. Administration of SCE5‐HtPlg (4 μg/g body weight) resulted in effective thrombolysis 20 minutes after injection in a ferric chloride–induced model of mesenteric thrombosis (48±3% versus 92±5% for saline control, P<0.01) and also reduced emboli formation in a model of pulmonary embolism (P<0.01 versus saline). Furthermore, at these effective therapeutic doses, the SCE5‐HtPlg did not prolong bleeding time compared with saline (P=0.99).

Conclusions

Our novel fusion molecule is a potent and effective treatment for thrombosis that enables in vivo thrombolysis without bleeding time prolongation. The activation of this construct by thrombin generated within the clot itself rather than by a plasminogen activator, which needs to be delivered systemically, provides a novel targeted approach to improve thrombolysis.

Thrombus Degradation by Fibrinolytic Enzyme of Stenotrophomonas sp. Originated from Indonesian Soybean-Based Fermented Food on Wistar Rats

Objective. To evaluate thrombus degrading effect of a fibrinolytic enzyme from food origin Stenotrophomonas sp. of Indonesia. Methods. Prior to animal study, the enzyme safety was tested using cell culture. The effect on expression of tissue plasminogen activator was also analysed in the cell culture. For in vivo studies, 25 Wistar rats were used: normal control, negative control, treatment groups with crude and semipurified enzyme given orally at 25 mg/kg, and positive control group which received Lumbrokinase at 25 mg/kg. Blood clot in the tail was induced by kappa carrageenan injection at 1 mg/kg BW. Results. Experiment with cell culture confirmed the enzyme safety at the concentration used and increased expression of tPA. Decreasing of thrombus was observed in the positive group down to 70.35 ± 23.11% of the negative control animals (100%). The thrombus observed in the crude enzyme treatment was down to 56.99 ± 15.95% and 71.5 ± 15.7% for semipurified enzyme. Scanning electron microscopy showed clearly that bood clots were found in the animals injected with kappa carrageenan; however, in the treatment and positive groups, the clot was much reduced. Conclusions. Oral treatment of enzyme from Stenotrophomonas sp. of Indonesian fermented food was capable of degrading thrombus induced in Wistar rats.

Magnetically controlled release of recombinant tissue plasminogen activator from chitosan nanocomposites for targeted thrombolysis

Ionic cross-linking of water-soluble chitosan with sodium tripolyphosphate in the presence of recombinant tissue plasminogen activator (rtPA) and magnetite (Fe₃O₄) nanoparticles could produce rtPA-encapsulated magnetic chitosan nanoparticles (MCNPs-rtPA). MCNPs do not elicit cytotoxicity and hemolysis in vitro. MCNPs-rtPA showed a negligible release of the rtPA protein when stored in phosphate buffer for 28 days. In contrast, the burst release of rtPA from MCNPs-rtPA was found in the serum with 60% of the original activity released in 30 min. The drug release into the serum is also magnet-sensitive; the release could be turned down with a magnetic field when MCNPs-rtPA was pelleted and reversibly turned on after removing the magnetic field when MCNPs-rtPA was dispersed. An in vitro thrombolytic study by thromboelastometry indicated a controlled release of rtPA from MCNPs-rtPA. In a rat embolic model where a preformed blood clot lodged in the left iliac artery upstream of the pudic epigastric branch, MCNPs-rtPA (0.2 mg kg^-1 rtPA) was administered and guided magnetically to the clot, followed by mobile magnetic guidance for 60 min. Iliac blood flow increased immediately in response to the treatment, and reached a stable level ∼50 min after drug administration and the hind limb perfusion rate was restored from 53% to 75% of the basal level. Effective thrombolysis was therefore successfully demonstrated at an rtPA dose equivalent to 20% of the regular dose when the MCNPs-rtPA pellet was magnet-guided to the blood clot, followed by a triggered release of rtPA when switched to mobile magnetic guidance.

Engineering of plasminogen activators for targeting to thrombus and heightening thrombolytic efficacy

Thrombotic occlusion of the coronary artery, which triggers acute myocardial infarction, is one of the major causes of death in the USA. Currently, arterial occlusions are treated with intravenous plasminogen activators (PAs), which dissolve the clot by activating plasminogen. However, PAs indiscriminately generate plasmin, which depletes critical clotting factors (fibrinogen, factor V, and factor VIII), precipitates a lytic state in the blood, and produces bleeding complications in a large patient population. PAs have been extensively investigated to achieve thrombus specificity, to attenuate the bleeding risk, and to widen their clinical applications. In this review, we discuss various strategies that have been pursued since the beginning of thrombolytic therapy. We review the biotechnological approaches that have been used to develop mutant and chimeric PAs for thrombus selectivity, including the use of specific antibodies for targeting thrombi. We discuss particulate carrier-based systems and triggered-release concepts. We propose new hypotheses and strategies to spur future studies in this research arena. Overall, we describe the approaches and accomplishments in the development of patient-friendly and workable delivery systems for thrombolytic drugs.

Surface modification and endothelialization of biomaterials as potential scaffolds for vascular tissue engineering applications

This review highlights the recent developments of surface modification and endothelialization of biomaterials in vascular tissue engineering applications.

Tissue plasminogen activator-based clot busting: Controlled delivery approaches

Cardiovascular diseases are the leading cause of death worldwide. Thrombosis, the formation of blood clot (thrombus) in the circulatory system obstructing the blood flow, is one of the main causes behind various ischemic arterial syndromes such as ischemic stroke and myocardial infarction, as well as vein syndromes such as deep vein thrombosis, and consequently, pulmonary emboli. Several thrombolytic agents have been developed for treating thrombosis, the most common being tissue plasminogen activator (tPA), administrated systemically or locally via IV infusion directly proximal to the thrombus, with the aim of restoring and improving the blood flow. TPA triggers the dissolution of thrombi by inducing the conversion of plasminogen to protease plasmin followed by fibrin digestion that eventually leads to clot lysis. Although tPA provides powerful thrombolytic activity, it has many shortcomings, including poor pharmacokinetic profiles, impairment of the reestablishment of normal coronary flow, and impairment of hemostasis, leading to life-threatening bleeding consequences. The bleeding consequence is ascribed to the ability of tPA to circulate throughout the body and therefore can lysis all blood clots in the circulation system, even the good ones that prevent the bleeding and promote injury repair. This review provides an overview of the different delivery approaches for tPA including: liposomes, ultrasound-triggered thrombolysis, anti-fibrin antibody-targeted tPA, camouflaged-tPA, tpA-loaded microcarriers, and nano-modulated delivery approaches.

Smart drug delivery systems: from fundamentals to the clinic

Smart materials can endow implantable depots, targetable nanocarriers and insertable medical devices with activation-modulated and feedback-regulated control of drug release.