Targeting of a mutant plasminogen activator to circulating red blood cells for prophylactic fibrinolysis

Drug-loaded erythrocytes: Modern approaches for advanced drug delivery for clinical use

Scientific organizations worldwide are striving to create drug delivery systems that provide a high local concentration of a drug in pathological tissue without side effects on healthy organs in the body. Important physiological properties of red blood cells (RBCs), such as frequent renewal ability, good oxygen carrying ability, unique shape and membrane flexibility, allow them to be used as natural carriers of drugs in the body. Erythrocyte carriers derived from autologous blood are even more promising drug delivery systems due to their immunogenic compatibility, safety, natural uniqueness, simple preparation, biodegradability and convenience of use in clinical practice. This review is focused on the achievements in the clinical application of targeted drug delivery systems based on osmotic methods of loading RBCs, with an emphasis on advancements in their industrial production. This article describes the basic methods used for encapsulating drugs into erythrocytes, key strategic approaches to the clinical use of drug-loaded erythrocytes obtained by hypotonic hemolysis. Moreover, clinical trials of erythrocyte carriers for the targeted delivery are discussed. This article explores the recent advancements and engineering approaches employed in the encapsulation of erythrocytes through hypotonic hemolysis methods, as well as the most promising inventions in this field. There is currently a shortage of reviews focused on the automation of drug loading into RBCs; therefore, our work fills this gap. Finally, further prospects for the development of engineering and technological solutions for the automatic production of drug-loaded RBCs were studied. Automated devices have the potential to provide the widespread production of RBC-encapsulated therapeutic drugs and optimize the process of targeted drug delivery in the body. Furthermore, they can expedite the widespread introduction of this innovative treatment method into clinical practice, thereby significantly expanding the effectiveness of treatment in both surgery and all areas of medicine.

In situ cellular hitchhiking of nanoparticles for drug delivery

Since the inception of the concept of "magic bullet", nanoparticles have evolved to be one of the most effective carriers in drug delivery. Nanoparticles improve the therapeutic efficacy of drugs offering benefits to treating various diseases. Unlike free drugs which freely diffuse and distribute through the body, nanoparticles protect the body from the drug by reducing non-specific interactions while also improving the drug's pharmacokinetics. Despite acquiring some FDA approvals, further clinical application of nanoparticles is majorly hindered by its limited ability to overcome biological barriers resulting in uncontrolled biodistribution and high clearance. The use of cell-inspired systems has emerged as a promising approach to overcome this challenge as cells are biocompatible and have improved access to tissues and organs. One of such is the hitchhiking of nanoparticles to circulating cells such that they are recognized as 'self' components evading clearance and resulting in site-specific drug delivery. In this review, we discuss the concept of nanoparticle cellular hitchhiking, highlighting its advantages, the principles governing the process and the challenges currently limiting its clinical translation. We also discuss in situ hitchhiking as a tool for overcoming these challenges and the considerations to be taken to guide research efforts in advancing this promising technology.

Surface Modification of Erythrocytes with Lipid Anchors: Structure-Activity Relationship for Optimal Membrane Incorporation, in vivo Retention, and Immunocompatibility

Red blood cells (RBCs) are natural carriers for sustained drug delivery, imaging, and in vivo sensing. One of the popular approaches to functionalize RBCs is through lipophilic anchors, but the structural requirements for anchor stability and in vivo longevity remain to be investigated. Using fluorescent lipids with the same cyanine 3 (Cy3) headgroup but different lipid chain and linker, the labeling efficiency of RBCs and in vivo stability are investigated. Short-chain derivatives exhibited better insertion efficiency, and mouse RBCs are better labeled than human RBCs. Short-chain derivatives demonstrate low retention in vivo. Derivatives with ester bonds are especially unstable, due to removal and degradation. On the other hand, long-chain, covalently linked derivatives show remarkably long retention and stability (over 80 days half life in the membrane). The clearance organs are liver and spleen with evidence of lipid transfer to the liver sinusoidal endothelium. Notably, RBCs modified with PEGylated lipid show decreased macrophage uptake. Some of the derivatives promote binding of antibodies in human plasma and mouse sera and modest increase in complement deposition and hemolysis, but these do not correlate with in vivo stability of RBCs. Ultra-stable anchors can enable functionalization of RBCs for drug delivery, imaging, and sensing.

Targeted In Vivo Loading of Red Blood Cells Markedly Prolongs Nanocarrier Circulation

Engineering drug delivery systems for prolonged pharmacokinetics (PK) has been an ongoing pursuit for nearly 50 years. The gold standard for PK enhancement is the coating of nanoparticles with polymers, namely polyethylene glycol (PEGylation), which has been applied in several clinically used products. In the present work, we utilize the longest circulating and most abundant component of blood─the erythrocyte─to improve the PK behavior of liposomes. Antibody-mediated coupling of liposomes to erythrocytes was tested in vitro to identify a loading dose that did not adversely impact the carrier cells. Injection of erythrocyte targeting liposomes into mice resulted in a ∼2-fold improvement in the area under the blood concentration versus time profile versus PEGylated liposomes and a redistribution from the plasma into the cellular fraction of blood. These results suggest that in vivo targeting of erythrocytes is a viable strategy to improve liposome PK relative to current, clinically viable strategies.

Asparaginyl Ligase-Catalyzed One-Step Cell Surface Modification of Red Blood Cells

Red blood cells (RBCs) can serve as vascular carriers for drugs, proteins, peptides, and nanoparticles. Human RBCs remain in the circulation for ∼120 days, are biocompatible, and are immunologically largely inert. RBCs are cleared by the reticuloendothelial system and can induce immune tolerance to foreign components attached to the RBC surface. RBC conjugates have been pursued in clinical trials to treat cancers and autoimmune diseases and to correct genetic disorders. Still, most methods used to modify RBCs require multiple steps, are resource-intensive and time-consuming, and increase the risk of inflicting damage to the RBCs. Here, we describe direct conjugation of peptides and proteins onto the surface of RBCs in a single step, catalyzed by a highly efficient, recombinant asparaginyl ligase under mild, physiological conditions. In mice, the modified RBCs remain intact in the circulation, display a normal circulatory half-life, and retain their immune tolerance-inducing properties, as shown for protection against an accelerated model for type 1 diabetes. We conjugated different nanobodies to RBCs with retention of their binding properties, and these modified RBCs can target cancer cells in vitro. This approach provides an appealing alternative to current methods of RBC engineering. It provides ready access to more complex RBC constructs and highlights the general utility of asparaginyl ligases for the modification of native cell surfaces.

Red Blood Cell Hitchhiking: A Novel Approach for Vascular Delivery of Nanocarriers

Red blood cell (RBC) hitchhiking is a method of drug delivery that can increase drug concentration in target organs by orders of magnitude. In RBC hitchhiking, drug-loaded nanoparticles (NPs) are adsorbed onto red blood cells and then injected intravascularly, which causes the NPs to transfer to cells of the capillaries in the downstream organ. RBC hitchhiking has been demonstrated in multiple species and multiple organs. For example, RBC-hitchhiking NPs localized at unprecedented levels in the brain when using intra-arterial catheters, such as those in place immediately after mechanical thrombectomy for acute ischemic stroke. RBC hitchhiking has been successfully employed in numerous preclinical models of disease, ranging from pulmonary embolism to cancer metastasis. In addition to summarizing the versatility of RBC hitchhiking, we also describe studies into the surprisingly complex mechanisms of RBC hitchhiking as well as outline future studies to further improve RBC hitchhiking's clinical utility.

Surface loading of nanoparticles on engineered or natural erythrocytes for prolonged circulation time: strategies and applications

Nano drug-delivery systems (DDS) may significantly improve efficiency and reduce toxicity of loaded drugs, but a few nano-DDS are highly successful in clinical use. Unprotected nanoparticles in blood flow are often quickly cleared, which could limit their circulation time and drug delivery efficiency. Elongating their blood circulation time may improve their delivery efficiency or grant them new therapeutic possibilities. Erythrocytes are abundant endogenous cells in blood and are continuously renewed, with a long life span of 100-120 days. Hence, loading nanoparticles on the surface of erythrocytes to protect the nanoparticles could be highly effective for enhancing their in vivo circulation time. One of the key questions here is how to properly attach nanoparticles on erythrocytes for different purposes and different types of nanoparticles to achieve ideal results. In this review, we describe various methods to attach nanoparticles and drugs to the erythrocyte surface, and discuss the key factors that influence the stability and circulation properties of the erythrocytes-based delivery system in vivo. These data show that using erythrocytes as a host for nanoparticles possesses great potential for further development.

Mechanisms of thrombosis and research progress on targeted antithrombotic drugs

Globally, the incidence of thromboembolic diseases has increased in recent years, accompanied by an increase in patient mortality. Currently, several targeting delivery strategies have been developed to treat thromboembolic diseases. In this review, we discuss the mechanisms of thrombolysis and current anticoagulant drugs, particularly those with targeting capability, highlighting advances in the accurate treatment of thrombolysis with fewer adverse effects. Such approaches include magnetic drug-loading systems combined with molecular imaging to recanalize blood vessels and systems based on chimeric Arg-Gly-Asp (RGD) sequences that can target platelet glycoprotein receptor. With such progress in targeted antithrombotic drugs, targeted thrombolysis treatment shows significant potential benefit for patients.

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.

Red blood cell hitchhiking enhances the accumulation of nano- and micro-particles in the constriction of a stenosed microvessel

We investigate the circulation of nano- and micro-particles, including spherical particles and filamentous nanoworms, with red blood cells (RBCs) suspension in a constricted channel that mimics a stenosed microvessel. Through three-dimensional simulations using the immersed boundary-based Lattice Boltzmann method, the influence of channel geometries, such as the length and ratio of the constriction, on the accumulation of particles is systematically studied. Firstly, we find that the accumulation of spherical particles with 1 μm diameter in the constriction increases with the increases of both the length and ratio of the constriction. This is attributed to the interaction between spheres and RBCs. The RBCs "carry" the spheres and they accumulate inside the constriction together, due to the altered local hydrodynamics induced by the existence of the constriction. Secondly, nanoworms demonstrate higher accumulation than that of spheres inside the constriction, which is associated with the escape of nanoworms from RBC clusters and their accumulation near the wall of main channel. The accumulated near-wall nanoworms will eventually enter the constriction, thus enhancing their concentration inside the constriction. However, an exceptional case occurs in the case of constrictions with large ratio and long length. In such circumstances, the RBCs aggregate together tightly and concentrate at the center of the channel, which makes the nanoworms hardly able to escape from RBC clusters, leading to a similar accumulation of nanoworms and spheres inside the constriction. This study may provide theoretical guidance for the design of nano- and micro-particles for biomedical engineering applications, such as drug delivery systems for patients with stenosed microvessels.

Vascular Drug Delivery Using Carrier Red Blood Cells: Focus on RBC Surface Loading and Pharmacokinetics

Red blood cells (RBC) have great potential as drug delivery systems, capable of producing unprecedented changes in pharmacokinetics, pharmacodynamics, and immunogenicity. Despite this great potential and nearly 50 years of research, it is only recently that RBC-mediated drug delivery has begun to move out of the academic lab and into industrial drug development. RBC loading with drugs can be performed in several ways-either via encapsulation within the RBC or surface coupling, and either ex vivo or in vivo-depending on the intended application. In this review, we briefly summarize currently used technologies for RBC loading/coupling with an eye on how pharmacokinetics is impacted. Additionally, we provide a detailed description of key ADME (absorption, distribution, metabolism, elimination) changes that would be expected for RBC-associated drugs and address unique features of RBC pharmacokinetics. As thorough understanding of pharmacokinetics is critical in successful translation to the clinic, we expect that this review will provide a jumping off point for further investigations into this area.

Polydopamine-modified dual-ligand nanoparticles as highly effective and targeted magnetic resonance/photoacoustic dual-modality thrombus imaging agents

Background

Platelet activation and subsequent aggregation are the initial stages of thrombosis. A molecular probe that specifically targets activated platelets and remains retained under high shear stress in vivo can enhance the imaging effect to achieve early and accurate diagnosis.

Methods and materials

In this study, we constructed nanoparticles (NPs) using polydopamine to carry two peptides that simultaneously bind integrin αIIbβ3 and P-selectin on activated platelets to enhance the targeting of NPs to thrombus.

Results

The targeting specificity and binding stability of the NPs on red and white thrombi were demonstrated in vitro using a simulated circulatory device and the targeting effect of the NPs on mixed thrombus was studied by  magnetic resonance (MR)/photoacoustic (PA) dual-modality imaging in vivo. NPs that were surface modified with both peptides have higher selectivity and retention to red and white thrombi in vitro than NPs with a single or no peptide, and the targeting effect was closely related to the number and distribution of activated platelets as well as the structure and type of thrombus. The NPs also have MR/PA dual-modality imaging functionality, significantly enhancing the imaging of mixed thrombus in vivo.

Conclusion

These dual-targeted NPs have improved targeting specificity and binding stability to different thrombi under high shear stress and are beneficial for the early diagnosis of thrombosis.

Red Blood Cell Membrane Processing for Biomedical Applications

Red blood cells (RBC) are actually exploited as innovative drug delivery systems with unconventional and convenient properties. Because of a long in vivo survival and a non-random removal from circulation, RBC can be loaded with drugs and/or contrasting agents without affecting these properties and maintaining the original immune competence. However, native or drug-loaded RBC, can be modified decorating the membrane with peptides, antibodies or small chemical entities so favoring the targeting of the processed RBC to specific cells or organs. Convenient modifications have been exploited to induce immune tolerance or immunogenicity, to deliver antibodies capable of targeting other cells, and to deliver a number of constructs that can recognize circulating pathogens or toxins. The methods used to induce membrane processing useful for biomedical applications include the use of crosslinking agents and bifunctional antibodies, biotinylation and membrane insertion. Another approach includes the expression of engineered membrane proteins upon ex vivo transfection of immature erythroid precursors with lentiviral vectors, followed by in vitro expansion and differentiation into mature erythrocytes before administration to a patient in need. Several applications have now reached the clinic and a couple of companies that take advantage from these properties of RBC are already in Phase 3 with selected applications. The peculiar properties of the RBC and the active research in this field by a number of qualified investigators, have opened new exciting perspectives on the use of RBC as carriers of drugs or as cellular therapeutics.

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.

Therapeutics incorporating blood constituents

Blood deficiency and dysfunctionality can result in adverse events, which can primarily be treated by transfusion of blood or the re-introduction of properly functioning sub-components. Blood constituents can be engineered on the sub-cellular (i.e., DNA recombinant technology) and cellular level (i.e., cellular hitchhiking for drug delivery) for supplementing and enhancing therapeutic efficacy, in addition to rectifying dysfunctioning mechanisms (i.e., clotting). Herein, we report the progress of blood-based therapeutics, with an emphasis on recent applications of blood transfusion, blood cell-based therapies and biomimetic carriers. Clinically translated technologies and commercial products of blood-based therapeutics are subsequently highlighted and perspectives on challenges and future prospects are discussed.

STATEMENT OF SIGNIFICANCE

Blood-based therapeutics is a burgeoning field and has advanced considerably in recent years. Blood and its constituents, with and without modification (i.e., combinatorial), have been utilized in a broad spectrum of pre-clinical and clinically-translated treatments. This review article summarizes the most up-to-date progress of blood-based therapeutics in the following contexts: synthetic blood substitutes, acellular/non-recombinant therapies, cell-based therapies, and therapeutic sub-components. The article subsequently discusses clinically-translated technologies and future prospects thereof.

Lipophilic indocarbocyanine conjugates for efficient incorporation of enzymes, antibodies and small molecules into biological membranes

Decoration of cell membranes with biomolecules, targeting ligands and imaging agents is an emerging strategy to improve functionality of cell-based therapies. Compared to covalent chemistry or genetic expression on the cell surface, lipid painting (i.e., incorporation of lipid-conjugated molecules into the cell bilayer) is a fast, non-damaging and less expensive approach. Previous studies demonstrated excellent incorporation and retention of distearyl indocarbocyanine dye DiI in membranes of cells in vitro and in vivo. In order to exploit the membrane stability of DiI, we synthesized an amino-DiI derivative, to which we subsequently conjugated an antibody (cetuximab), an enzyme (superoxide dismutase), and a small molecule (DyLight 800). Red blood cells have long been used as drug delivery vehicles so they were utilized as a model to study the incorporation of DiI conjugates in the plasma membrane. All the DiI constructs demonstrated fast and efficient ex vivo incorporation in the membrane of mouse RBCs, resulting in millions of exogenous molecules per RBC. Following an intravenous injection into mice, the molecules were detected on circulating RBCs for several days. DiI anchored molecules showed longer residence time in blood and significantly higher area under the curve (AUC) compared to free non-conjugated molecules. Thus, cetuximab, SOD and DyLight painted on RBC showed 5.5-fold, 6.5-fold and 78-fold increase in the AUC, respectively, compared to the non-modified molecules. Lipophilic indocarbocyanine anchors are a promising technology for incorporation of biomolecules and small molecules into biological membranes for in vivo applications.

Red blood cells: Supercarriers for drugs, biologicals, and nanoparticles and inspiration for advanced delivery systems

Red blood cells (RBCs) constitute a unique drug delivery system as a biologic or hybrid carrier capable of greatly enhancing pharmacokinetics, altering pharmacodynamics (for example, by changing margination within the intravascular space), and modulating immune responses to appended cargoes. Strategies for RBC drug delivery systems include internal and surface loading, and the latter can be performed both ex vivo and in vivo. A relatively new avenue for RBC drug delivery is their application as a carrier for nanoparticles. Efforts are also being made to incorporate features of RBCs in nanocarriers to mimic their most useful aspects, such as long circulation and stealth features. RBCs have also recently been explored as carriers for the delivery of antigens for modulation of immune response. Therefore, RBC-based drug delivery systems represent supercarriers for a diverse array of biomedical interventions, and this is reflected by several industrial and academic efforts that are poised to enter the clinical realm.

Erythrocytes as Carriers for Drug Delivery in Blood Transfusion and Beyond

Red blood cells (RBCs) are innate carriers that can also be engineered to improve the pharmacokinetics and pharmacodynamics of many drugs, particularly biotherapeutics. Successful loading of drugs, both internally and on the external surface of RBCs, has been demonstrated for many drugs including anti-inflammatory, antimicrobial, and antithrombotic agents. Methods for internal loading of drugs within RBCs are now entering clinical use. Although internal loading can result in membrane disruption that may compromise biocompatibility, surface loading using either affinity or chemical ligands offers a diverse set of approaches for the production of RBC drug carriers. A wide range of surface determinants is potentially available for this approach, although there remains a need to characterize the effects of coupling agents to these surface proteins. Somewhat surprisingly, recent data also suggest that red cell-mediated delivery may confer tolerogenic immune effects. Questions remaining before widespread application of these technologies include determining the optimal loading protocol, source of RBCs, and production logistics, as well as addressing regulatory hurdles. Red blood cell drug carriers, after many decades of progress, are now poised to enter the clinic and broaden the potential application of RBCs in blood transfusion.

Advances of blood cell-based drug delivery systems

Blood cells, including erythrocytes, leukocytes and platelets are used as drug carriers in a wide range of applications. They have many unique advantages such as long life-span in circulation (especially erythrocytes), target release capacities (especially platelets), and natural adhesive properties (leukocytes and platelets). These properties make blood cell based delivery systems, as well as their membrane-derived carriers, far superior to other drug delivery systems. Despite the advantages, the further development of blood cell-based delivery systems was hindered by limitations in the source, storage, and mass production. To overcome these problems, synthetic biomaterials that mimic blood cell and nanocrystallization of blood cells have been developed and may represent the future direction for blood cell membrane-based delivery systems. In this paper, we review recent progress of the rising blood cell-based drug delivery systems, and also discuss their challenges and future tendency of development.

Erythrocyte-mediated delivery of recombinant enzymes

The possibility to clone, express and purify recombinant enzymes have originated the opportunity to dispose of a virtually infinite array of proteins that could be used in the clinics to treat several inherited and acquired pathological conditions. However, the direct administration of these recombinant proteins faces some intrinsic difficulties, such as degradation by circulating proteases and/or inactivation by the patient immune system. The use of drug delivery systems may overcome these limitations. Concerning recombinant enzyme therapy, the present review will mainly focus on the exploitation of erythrocytes as a carrier system for enzymes removing potentially noxious metabolites from the circulation, either as limiting treatment strategy for auxotrophic tumours or as a detoxing approach for some intoxication type inherited metabolic disorders. Moreover, the possibility of using RBCs as a potential delivering system addressing the enzymes to the monocyte-macrophages of reticular endothelial system for the treatment of diseases associated with this cell lineage, e.g. lysosome storage diseases, will be briefly discussed.

Non-affinity factors modulating vascular targeting of nano- and microcarriers

Particles capable of homing and adhering to specific vascular biomarkers have potential as fundamental tools in drug delivery for mediation of a wide variety of pathologies, including inflammation, thrombosis, and pulmonary disorders. The presentation of affinity ligands on the surface of a particle provides a means of targeting the particle to sites of therapeutic interest, but a host of other factors come into play in determining the targeting capacity of the particle. This review presents a summary of several key considerations in nano- and microparticle design that modulate targeted delivery without directly altering epitope-specific affinity. Namely, we describe the effect of factors in definition of the base carrier (including shape, size, and flexibility) on the capacity of carriers to access, adhere to, and integrate in target biological milieus. Furthermore, we present a summary of fundamental dynamics of carrier behavior in circulation, taking into account interactions with cells in circulation and the role of hemodynamics in mediating the direction of carriers to target sites. Finally, we note non-affinity aspects to uptake and intracellular trafficking of carriers in target cells. In total, recent findings presented here may offer an opportunity to capitalize on mitigating factors in the behavior of ligand-targeted carriers in order to optimize targeting.

The Effect of Polymeric Nanoparticles on Biocompatibility of Carrier Red Blood Cells

Red blood cells (RBCs) can be used for vascular delivery of encapsulated or surface-bound drugs and carriers. Coupling to RBC prolongs circulation of nanoparticles (NP, 200 nm spheres, a conventional model of polymeric drug delivery carrier) enabling their transfer to the pulmonary vasculature without provoking overt RBC elimination. However, little is known about more subtle and potentially harmful effects of drugs and drug carriers on RBCs. Here we devised high-throughput in vitro assays to determine the sensitivity of loaded RBCs to osmotic stress and other damaging insults that they may encounter in vivo (e.g. mechanical, oxidative and complement insults). Sensitivity of these tests is inversely proportional to RBC concentration in suspension and our results suggest that mouse RBCs are more sensitive to damaging factors than human RBCs. Loading RBCs by NP at 1:50 ratio did not affect RBCs, while 10–50 fold higher NP load accentuated RBC damage by mechanical, osmotic and oxidative stress. This extensive loading of RBC by NP also leads to RBCs agglutination in buffer; however, addition of albumin diminished this effect. These results provide a template for analyses of the effects of diverse cargoes loaded on carrier RBCs and indicate that: i) RBCs can tolerate carriage of NP at doses providing loading of millions of nanoparticles per microliter of blood; ii) tests using protein-free buffers and mouse RBCs may overestimate adversity that may be encountered in humans.

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.

Delivery of drugs bound to erythrocytes: new avenues for an old intravascular carrier

For several decades, researchers have used erythrocytes for drug delivery of a wide variety of therapeutics in order to improve their pharmacokinetics, biodistribution, controlled release and pharmacodynamics. Approaches include encapsulation of drugs within erythrocytes, as well as coupling of drugs onto the red cell surface. This review focuses on the latter approach, and examines the delivery of red blood cell (RBC)-surface-bound anti-inflammatory, anti-thrombotic and anti-microbial agents, as well as RBC carriage of nanoparticles. Herein, we discuss the progress that has been made in surface loading approaches, and address in depth the issues relevant to surface loading of RBC, including intrinsic features of erythrocyte membranes, immune considerations, potential surface targets and techniques for the production of affinity ligands.

Targeted thrombolysis strategies for neuroprotective effect

Stroke is usually treated by systemic thrombolytic therapy if the patient presents within an appropriate time window. There is also widespread interest in the development of thrombolytic agents that can be used in cases of delayed presentation. Current agents that can be used in cases of delayed presentation of nerve damage by thrombus. Current systemic thrombolytic therapy is associated with adverse effects such as fibrinogenolysis and bleeding. In an attempt to increase the efficacy, safety, and specificity of thrombolytic therapy, a number of targeted thrombolytic agents have been studied in recent years. This review focuses on the concepts underlying targeted thrombolytic therapy and describes recent drug developments in this field.

Erythrocytes-based synthetic delivery systems: transition from conventional to novel engineering strategies

INTRODUCTION

Erythrocytes (red blood cells [RBCs]) and artificial or synthetic delivery systems such as liposomes, nanoparticles (NPs) are the most investigated carrier systems. Herein, progress made from conventional approach of using RBC as delivery systems to novel approach of using synthetic delivery systems based on RBC properties will be reviewed.

AREAS COVERED

We aim to highlight both conventional and novel approaches of using RBCs as potential carrier system. Conventional approaches which include two main strategies are: i) directly loading therapeutic moieties in RBCs; and ii) coupling them with RBCs whereas novel approaches exploit structural, mechanical and biological properties of RBCs to design synthetic delivery systems through various engineering strategies. Initial attempts included coupling of antibodies to liposomes to specifically target RBCs. Knowledge obtained from several studies led to the development of RBC membrane derived liposomes (nanoerythrosomes), inspiring future application of RBC or its structural features in other attractive delivery systems (hydrogels, filomicelles, microcapsules, micro- and NPs) for even greater potential.

EXPERT OPINION

In conclusion, this review dwells upon comparative analysis of various conventional and novel engineering strategies in developing RBC based drug delivery systems, diversifying their applications in arena of drug delivery. Regardless of the challenges in front of us, RBC based delivery systems offer an exciting approach of exploiting biological entities in a multitude of medical applications.

RBC-coupled tPA Prevents Whereas tPA Aggravates JNK MAPK-Mediated Impairment of ATP- and Ca-Sensitive K Channel-Mediated Cerebrovasodilation After Cerebral Photothrombosis

The sole Food and Drug Administration-approved treatment for acute stroke is tissue-type plasminogen activator (tPA), but tPA aggravates impairment of cerebrovasodilation during hypotension in a newborn pig photothrombotic model of stroke. Coupling to carrier red blood cells (RBC) enhances thrombolytic effects of tPA, while reducing its side effects. ATP- and Ca-sensitive K channels (Katp and Kca) are important regulators of cerebrovascular tone and mediate cerebrovasodilation during hypotension. Mitogen-activated protein kinase, a family of at least three kinases, ERK, p38, and c-Jun-N-terminal kinase (JNK), is upregulated after photothrombosis. This study examined the effect of photothrombosis on Katp- and Kca-induced cerebrovasodilation and the roles of tPA and JNK during/after injury. Photothrombosis blunted vasodilation induced by the Katp agonists cromakalim, calcitonin gene-related peptide, and the Kca agonist NS 1619, which was aggravated by injection of tPA. In contrast, both pre- or post-injury thrombosis injection of RBC-tPA and JNK antagonist SP 600125 prevented impairment of Katp- and Kca-induced vasodilation. Therefore, JNK activation in thrombosis impairs K channel-mediated cerebrovasodilation. Standard thrombolytic therapy of central nervous system ischemic disorders using free tPA poses the danger of further dysregulation of cerebrohemodynamics by impairing cation-mediated control of cerebrovascular tone, whereas RBC-coupled tPA both restores reperfusion and normalizes cerebral hemodynamics.

Carrier erythrocytes: recent advances, present status, current trends and future horizons

INTRODUCTION

Carrier erythrocytes, thanks to their main advantages, including biocompatibility, biodegradability, immunocompatibility, simple and well-known structure and physiology, availability for sampling and versatility in loading and use, have been studied as cellular carriers for delivery of drugs and other bioactive agents for more than three decades. Based on this body of knowledge and recent advances in this field, and with the help of novel multidisciplinary sciences and technologies, it seems that this field is becoming renowned and experiencing an outstanding turning point in its developmental history.

AREAS COVERED

In this trendy and timely review, following a short historical review of the story of erythrocytes from oxygen delivery to drug delivery and evaluation of the present status of these biocarriers, recent advances and current experimental, technological and clinical trends, as well as future horizons, and, in particular, translation-prone strategies, are going to be discussed in detail.

EXPERT OPINION

Despite the challenging developmental history of carrier erythrocytes, they now stand closer to clinical use and market entrance due to their unique advantages in drug delivery, proven by recently reported success stories in late-stage clinical trials and progresses made in biotechnology, nanotechnology and biomaterials fields. Translation-prone approaches, like in vivo loading of circulating erythrocytes or semiautomatic loading of erythrocytes, and more realistic study designs by focusing on clinical needs that have not been responded to or erythrocyte biology/fate-inspired study design are among the main trends being focused on by pioneer research groups active in this field of drug delivery.

Distearoyl anchor-painted erythrocytes with prolonged ligand retention and circulation properties in vivo

Red blood cells (RBCs) attract significant interest as carriers of biomolecules, drugs, and nanoparticles. In this regard, versatile technologies to attach molecules and ligands to the RBC surface are of great importance. Reported here is a fast and efficient surface painting strategy to attach ligands to the surface of RBCs, and the factors that control the stability and circulation properties of the modified RBCs in vivo. Distearoyl phosphatidylethanolamine anchor-conjugated immunoglobulin (IgG) efficiently incorporates in the RBC membrane following 15-30 min incubation. The optimized RBCs show prolonged circulation in vivo (70% of the injected dose after 48 h) and efficient retention of IgG in the membrane with terminal half-life of 73 h. The IgG construct is gradually lost from the RBCs mainly due to the transfer to plasma components, liver endothelial cells, and Kupffer cells. The ligand retention efficiency is partially dictated by ligand type, anchor type, and ligand concentration in the membrane, while RBC half-life is determined by initial concentration of the ligand in the membrane and presence of PEG linker between the ligand and the anchor. This work provides important guidance for non-covalent surface painting of RBCs as well as other types of blood borne cells for in vivo therapeutic and targeting applications.

Advanced drug delivery systems for antithrombotic agents

Despite continued achievements in antithrombotic pharmacotherapy, difficulties remain in managing patients at high risk for both thrombosis and hemorrhage. Utility of antithrombotic agents (ATAs) in these settings is restricted by inadequate pharmacokinetics and narrow therapeutic indices. Use of advanced drug delivery systems (ADDSs) may help to circumvent these problems. Various nanocarriers, affinity ligands, and polymer coatings provide ADDSs that have the potential to help optimize ATA pharmacokinetics, target drug delivery to sites of thrombosis, and sense pathologic changes in the vascular microenvironment, such as altered hemodynamic forces, expression of inflammatory markers, and structural differences between mature hemostatic and growing pathological clots. Delivery of ATAs using biomimetic synthetic carriers, host blood cells, and recombinant fusion proteins that are activated preferentially at sites of thrombus development has shown promising outcomes in preclinical models. Further development and translation of ADDSs that spare hemostatic fibrin clots hold promise for extending the utility of ATAs in the management of acute thrombotic disorders through rapid, transient, and targeted thromboprophylaxis. If the potential benefit of this technology is to be realized, a systematic and concerted effort is required to develop clinical trials and translate the use of ADDSs to the clinical arena.

Erythrocyte-inspired delivery systems

Herein recent progress in developing red blood cell (RBC)-inspired delivery systems is reviewed, with an emphasis on how our growing understanding of fundamental biological properties of natural RBCs has been applied in the design and engineering of these delivery systems. Specifically, progress achieved in developing carrier RBCs, a class of delivery vehicles engineered by directly loading natural RBCs with therapeutic agents, will be reviewed. Then alternative approaches to engineering synthetic vehicles through mimicking the mechanobiological and chemico-biological properties of natural RBCs will be considered. The synthesis and application of RBC membrane-derived vesicles, of which the natural RBC membranes are collected and directly utilized to prepare drug carriers, will then be discussed. Finally, a recent approach in engineering RBC membrane-camouflaged nanoparticle systems that combine advantages of natural RBCs and synthetic biomaterials will be highlighted. These developments indicate that RBC-inspired delivery systems will result in next-generation nanomedicine with extensive medical applications.

Erythrocytes as a novel delivery vehicle for biologics: from enzymes to nucleic acid-based therapeutics

Biological drugs are among the most exciting drugs of the future, offering better treatment options for patients than ever before but they need an appropriate delivery vehicle. Carrier erythrocytes are one of the most promising drug-delivery systems. Application of erythrocytes as containers for various drugs minimizes toxicity, decreasing the risk of side effects and pathologic immune reactions against encapsulated agents as well as improving their efficacy, leading to better patient compliance. This review discusses the rationale for the use of erythrocytes as a vehicle for biopharmaceuticals and summarizes the categories of these new encapsulable compounds that are currently under investigation. The authors' intent is to describe the development of this delivery system to give the reader an overview of the remarkable potential of erythrocytes as naturally designed carriers and their versatility in the field of biologics for the treatment of various pathological conditions.

Cardiological biopharmaceuticals in the conception of drug targeting delivery: practical results and research perspectives

The results of the clinical use of thrombolytic and antithrombotic preparations developed on the basis of protein conjugates obtained within the framework of the conception of drug targeting delivery in the organism are considered. A decrease has been noted in the number of biomedical projects focused on these derivatives as a result of various factors: the significant depletion of financial and organizational funds, the saturation of the pharmaceutical market with preparations of this kind, and the appearance of original means for interventional procedures. Factors that actively facilitate the conspicuous potentiation of the efficacy of bioconjugates were revealed: the biomedical testing of protein domains and their selected combinations, the optimization of molecular sizes for the bioconjugates obtained, the density of target localization, the application of cell adhesion molecules as targets, and the application of connected enzyme activities. Enzyme antioxidants and the opportunity for further elaboration of the drug delivery conception via the elucidation and formation of therapeutic targets for effective drug reactions by means of pharmacological pre- and postconditioning of myocardium arouse significant interest.

A humanized single-chain variable fragment antibody against beta3 integrin in Escherichia coli

Patients with HIV-1 immune-related thrombocytopenia (HIV-1-ITP) have a unique antibody (Ab) against platelet GPIIIa49-66, which is capable of inducing oxidative platelet fragmentation in the absence of complement activation. By screening a human phage antibody library with the GPIIIa49-66 peptide as bait, we have developed several humanized phage Abs, which act similarly to the parental Ab. However, the presence of a stop codon in the heavy chain of the obtained phage clones limits their expression in soluble recombinant form. To circumvent this problem, we mutated the stop codon inside clone 11 that exhibits the highest binding activity to platelet GPIIIa49-66, resulting in a soluble scFv format (named A11) in Escherichia coli Rosseta. In in vitro binding assay, A11 exhibited similar binding specificity to parental Ab at various concentrations. Moreover, A11 is able to induce oxidative platelet fragmentation by preferentially binding to activated versus resting platelets. These findings provide a proof-of-principle for the development of a novel approach to inhibit arterial thrombosis by generating a selective scFv for the lysis of platelet-rich thrombi.

Targeting recombinant thrombomodulin fusion protein to red blood cells provides multifaceted thromboprophylaxis

Thrombin generates fibrin and activates platelets and endothelium, causing thrombosis and inflammation. Endothelial thrombomodulin (TM) changes thrombin's substrate specificity toward cleavage of plasma protein C into activated protein C (APC), which opposes its thrombotic and inflammatory activities. Endogenous TM activity is suppressed in pathologic conditions, and antithrombotic interventions involving soluble TM are limited by rapid blood clearance. To overcome this problem, we fused TM with a single chain fragment (scFv) of an antibody targeted to red blood cells. scFv/TM catalyzes thrombin-mediated generation of activated protein C and binds to circulating RBCs without apparent damage, thereby prolonging its circulation time and bioavailability orders of magnitude compared with soluble TM. In animal models, a single dose of scFv/TM, but not soluble TM, prevents platelet activation and vascular occlusion by clots. Thus, scFv/TM serves as a prodrug and provides thromboprophylaxis at low doses (0.15 mg/kg) via multifaceted mechanisms inhibiting platelets and coagulation.

Microthrombosis after experimental subarachnoid hemorrhage: time course and effect of red blood cell-bound thrombin-activated pro-urokinase and clazosentan

Delayed cerebral ischemia (DCI) is a significant cause of morbidity and mortality for patients surviving the rupture of an intracranial aneurysm. Despite an association between vasospasm and DCI, thrombosis and thromboembolism may also contribute to DCI. In this study we investigate the time course of intravascular microclot formation after experimental subarachnoid hemorrhage (SAH) and assess the effects of the following two drugs on microclot burden: mutant thrombin-activated urokinase-type plasminogen activator (scFv/uPA-T), which is bound to red blood cells for use as a thromboprophylactic agent, and clazosentan, an endothelin antagonist. In the first study, adult male C57BL/6 mice were sacrificed at 24 (n=5), 48 (n=6), 72 (n=8), and 96 (n=3) hours after SAH induced by filament perforation of the anterior cerebral artery. Sham animals (n=5) underwent filament insertion without puncture. In the second study, animals received scFv/uPA-T (n=5) 3 hours after hemorrhage, clazosentan (n=5) by bolus and subcutaneous pump after SAH just prior to skin closure, or a combination of scFv/uPA-T and clazosentan (n=4). Control (n=6) and sham (n=5) animals received saline alone. All animals were sacrificed at 48 hours and underwent intra-cardiac perfusion with 4% paraformaldehyde. The brains were then extracted and sliced coronally on a cryostat and processed for immunohistochemistry. An antibody recognizing thrombin-anti-thrombin complexes was used to detect microclots on coronal slices. Microclot burden was calculated for each animal and compared among groups. Following SAH, positive anti-thrombin staining was detected bilaterally in the following brain regions, in order of decreasing frequency: cortex; hippocampus; hypothalamus; basal ganglia. Few microclots were found in the shams. Microclot burden peaked at 48 hours and then decreased gradually. Animals receiving scFv/uPA-T and scFv/uPA-T+clazosentan had a lower microclot burden than controls, whereas animals receiving clazosentan alone had a higher microclot burden (p<0.005). The overall mortality rate in the time course study was 40%; mortality was highest among control animals in the second study. Intravascular microclots form in a delayed fashion after experimental SAH. Microclots may be safely reduced using a novel form of thromboprophylaxis provided by RBC-targeted scFv/uPA-T and represent a potential target for therapeutic intervention in the treatment of DCI.

Red blood cell-coupled tissue plasminogen activator prevents impairment of cerebral vasodilatory responses through inhibition of c-Jun-N-terminal kinase and potentiation of p38 mitogen-activated protein kinase after cerebral photothrombosis in the newborn pig

OBJECTIVE

Pediatric ischemic stroke is a poorly understood, yet clinically important, problem. The sole approved treatment for acute stroke is tissue-type plasminogen activator. However, tissue plasminogen activator vasoactivity aggravates hypoxia/ischemia-induced impairment of cerebrovasodilation in response to hypercapnia and hypotension in newborn pigs. Mitogen-activated protein kinase (a family of 3 kinases, extracellular signal-related kinase, p38, and c-Jun-N-terminal kinase) is upregulated after hypoxia/ischemia. Coupling of tissue plasminogen activator to red blood cells prevented hypoxia/ischemia-induced impairment of dilation and suppressed extracellular signal-related kinase mitogen-activated protein kinase activation. This study investigated the differential roles of mitogen-activated protein kinase isoforms in the effects of red blood cells-tissue plasminogen activator on cerebrovasodilation in a translationally relevant injury model, photothrombosis.

DESIGN

Prospective, randomized animal study.

SETTING

: University laboratory.

SUBJECTS

Newborn (1- to 5-day-old) pigs.

INTERVENTIONS

Cerebral blood flow and pial artery diameter were determined before and after photothrombotic injury (laser 532 nm and erythrosine B) was produced in piglets equipped with a closed cranial window. Cerebral blood flow extracellular signal-related kinase, p38, and c-Jun-N-terminal kinase mitogen-activated protein kinase were determined by enzyme-linked immunosorbent assay.

MEASUREMENTS AND MAIN RESULTS

Tissue plasminogen activator and red blood cells-tissue plasminogen activator alleviated reduction of cerebral blood flow after photothrombotic injury. Cerebrovasodilation was blunted by photothrombotic injury, reversed to vasoconstriction by tissue plasminogen activator, but dilation was maintained by red blood cells-tissue plasminogen activator. Cerebral blood flow c-Jun-N-terminal kinase and p38 mitogen-activated protein kinase but not extracellular signal-related kinase mitogen-activated protein kinase was elevated by photothrombotic injury, an effect potentiated by tissue plasminogen activator. Red blood cells-tissue plasminogen activator blocked c-Jun-N-terminal kinase but potentiated p38 mitogen-activated protein kinase upregulation after photothrombotic injury. A c-Jun-N-terminal kinase mitogen-activated protein kinase antagonist prevented, a p38 mitogen-activated protein kinase antagonist potentiated, whereas an extracellular signal-related kinase mitogen-activated protein kinase antagonist had no effect on dilator impairment after photothrombotic injury.

CONCLUSIONS

These data indicate that in addition to restoring perfusion, red blood cells-tissue plasminogen activator prevents impairment of cerebrovasodilation after photothrombotic injury through blockade of c-Jun-N-terminal kinase and potentiation of p38 mitogen-activated protein kinase. These data suggest tissue plasminogen activator coupling to red blood cells offers a novel approach to increase the benefit/risk ratio of thrombolytic therapy to treat central nervous system ischemic disorders.

Vascular targeting of antithrombotic agents

In this review we discuss the limited efficacy for current pharmacological agents used in prophylaxis and treatment of thrombosis and highlight targeted delivery of anti-thrombotic agents to fibrin, platelets, red blood cells and endothelium.

Flow-dependent channel formation in clots by an erythrocyte-bound fibrinolytic agent

Studies in animal models have shown that plasminogen activators bound to erythrocytes (RBC-PA) have an extended lifetime in the circulation and are safer than free PAs. RBC-PAs incorporate into nascent thrombi, which are focally lysed from within, an attractive thromboprophylactic option. In static systems, RBC-PAs cleave surrounding fibrin fibers, forming pores larger than the cells themselves, and move around the pore edges, enlarging them until eventual clot dissolution. We hypothesized that under flow in blood vessels, RBC-PAs form functional patent channels before clot dissolution. Here we used perfusion chambers to study clot lysis by RBC-PAs under static versus arterial and venous flow conditions. We found that flow decelerates bulk clot lysis but quickly generates patent channels filled with passing RBCs, via pore enlargement and merging in the direction of flow. Formation of such channels by RBC-PAs may help rescue ischemic tissue before bulk dissolution of potentially occlusive clots.

The spatial dynamics of fibrin clot dissolution catalyzed by erythrocyte-bound vs. free fibrinolytics

SUMMARY BACKGROUND

Coupling fibrinolytic plasminogen activators to red blood cells (RBCs) has been proposed as an effective, yet safe method of thromboprophylaxis, because of increased circulation lifetime and reduced propensity to induce hemorrhage by selectivity for nascent thrombi rather than pre-formed hemostatic clots.

OBJECTIVES AND METHODS

We used confocal microscopy of fluorescently labeled fibrin and erythrocytes in plasma-derived clots to study the spatial dynamics of lysis catalyzed by RBC-coupled vs. free plasminogen activators (RBC-PA vs. PA).

RESULTS

Clot lysis catalyzed by free PA progressed gradually and uniformly. In contrast, distinct holes formed surrounding RBC-PA while the rest of the clot remained intact until these holes enlarged sufficiently to merge, causing sudden clot dissolution. Compared with naïve RBCs within clots lysed by free PA, RBC-PA moved faster inside the fibrin network prior to clot dissolution, providing a potential mechanism for spatial propagation of RBC-PA induced lysis. We also showed the focal nature of fibrinolysis by RBC-PA as dense loading of PA onto RBCs initiates more efficient lysis than equal amounts of PA spread sparsely over more RBCs. In an in vitro model of clots exposed to buffer flow, incorporated RBC-PA increased permeability and formed channels eventually triggering clot dissolution, whereas clots containing free PA remained intact.

CONCLUSIONS

Clot lysis by RBC-PA begins focally, has a longer lag phase when measured by residual mass than homogeneous lysis by PA, is propagated by RBC-PA motility and provides more effective clot reperfusion than free PA, making RBC-PA attractive for short-term thromboprophylaxis.