Carrier Red Blood Cells Research Articles

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.

Nanoparticle Properties Modulate Their Attachment and Effect on Carrier Red Blood Cells

Attachment of nanoparticles (NPs) to the surface of carrier red blood cells (RBCs) profoundly alters their interactions with the host organism, decelerating NP clearance from the bloodstream while enabling NP transfer from the RBC surface to the vascular cells. These changes in pharmacokinetics of NPs imposed by carrier RBCs are favorable for many drug delivery purposes. On the other hand, understanding effects of NPs on the carrier RBCs is vital for successful translation of this novel drug delivery paradigm. Here, using two types of distinct nanoparticles (polystyrene (PSNP) and lysozyme-dextran nanogels (LDNG)) we assessed potential adverse and sensitizing effects of surface adsorption of NPs on mouse and human RBCs. At similar NP loadings (approx. 50 particles per RBC), adsorption of PSNPs, but not LDNGs, induces RBCs agglutination and sensitizes RBCs to damage by osmotic, mechanical and oxidative stress. PSNPs, but not LDNGs, increase RBC stiffening and surface exposure of phosphatidylserine, both known to accelerate RBC clearance in vivo. Therefore, NP properties and loading amounts have a profound impact on RBCs. Furthermore, LDNGs appear conducive to nanoparticle drug delivery using carrier RBCs.

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.

The effect of polymeric nanoparticles on biocompatibility of carrier red blood cells

The effect of polymeric nanoparticles on biocompatibility of carrier red blood cells

Engineering red‐blood‐cell‐membrane–coated nanoparticles for broad biomedical applications

R ed blood cells (RBCs) are nature’s long-circulating delivery vehicles. They possess various remarkable properties and continue to inspire the design and engineering of man-made delivery systems. Inherently suited for intravascular delivery, RBCs are intrinsically biocompatible, biodegradable, and nonimmunogenic. They form natural compartments capable of protecting encapsulated cargos, and this allows them to circulate in the bloodstream for a long period of time (up to 120 days). In addition, their semipermeable membrane affords sustained release to smallmolecule drugs, yet they are ideal for retaining large proteins while providing them access to the substrates. Delivery vehicles possessing one or several of these properties have long been desired for efficacious therapeutics. A long-sought strategy for RBC-mimicking delivery vehicles is to load natural RBCs with therapeutic agents without compromising the structural integrity and biological functions of the RBCs. Various loading techniques, including automated loading devices, have been developed to enable the encapsulation of payloads with molecular weights of over 180 kDa into RBCs with maintenance of the carriers’ biological competence. In addition to the interior of RBCs, their exterior surface has also been coupled with therapeutic molecules, either covalently or physically, for various delivery applications. These RBC-based delivery vehicles, namely carrier RBCs, have been developed for the delivery of numerous therapeutic agents, including proteins, nucleic acids, and small-molecule drugs. Several of them have entered clinical tests to treat various diseases, including cancers and enzyme deficiencies. Meanwhile, advances in molecular biology have provided unprecedented understanding of the connections between the physicochemical characteristics of RBCs and their biological functions. This understanding, in turn, has inspired researchers to model drug carriers after RBCs. Designs that mimic the physicochemical characteristics and biological functions of RBCs, particularly those that enable their passage through narrow constrictions while maintaining a long in vivo survival, have been integrated into the engineering of drug carriers and have resulted in novel delivery systems with improved drug tolerability, circulation lifetime, and efficacy. Recently, the pursuit of RBC-mimicking nanoparticles led us to develop an intriguing approach for functionalizing synthetic nanoparticles with natural RBC membranes. In this approach, we first collected intact cellular membranes and then coated them onto polymeric nanoparticle cores, such as those made from poly(lactic-co-glycolic acid) (PLGA); this resulted in red-blood-cell-membrane–coated nanoparticles (RBC-NPs; Figure 1). Our aim was to fabricate cellmimicking nanoparticles through a top-down approach that bypassed the labor-intensive processes of protein identification, purification, and conjugation. The natural membranes also provide a bilayered medium for transmembrane protein anchorage while preventing common chemical modifications that could compromise the integrity and functionalities of these proteins. The independent preparation of cellular membranes and particle cores before coating offers a new level of engineering flexibility toward highly functional biomimetic nanoparticles. Since their initial development, RBC-NPs have provided an unprecedented capability for harnessing the natural functionalities of native cells that would otherwise be difficult to replicate. They have since inspired us to develop novel nanotherapeutics for better disease intervention. In this Perspective, we review our recent progress in developing RBC-NPs for three distinct biomedical applications: long-circulating nanocarriers for drug delivery, biomimetic nanosponges for detoxification, and nanotoxoids for safe and effective toxin vaccination.

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.

Drug delivery by red blood cells

Drug delivery is a growing field of interdisciplinary activities that combine the use of new materials with the biochemical properties of selected drugs, with the aim of improving their therapeutic action and reducing their toxicity. In few cases, proper medical devices have been also realized to implement new drug delivery modalities. In this article, we have summarized available information and our experience on the use of autologous Red Blood Cells as carriers for drugs to be released within the vascular system. This is not a comprehensive review, but it focuses on the mechanisms that are available to distribute drugs in circulation by carrier red blood cells and provide illustrative examples on how this is currently obtained. We have not included a summary of clinical data collected in recent years using this technology but simply provided proper references for the interested readers. Finally, a special attention is devoted to the possibility of entrapping, into autologous red blood cells, recombinant drug-binding proteins. This new strategy is opening the way at a new modality to influence the vascular distribution of drugs by realizing a dynamic circulating container (the engineered red cell) capable of reversible binding and transportation of one or more drugs of interest selected on the bases of the red cell entrapped target proteins. This new modality is not yet fully developed and explored but will certainly provide a technical solution to the problem of stabilizing drug concentration in circulation improving drug efficacy and reducing drug toxicity.

Signaling, delivery and age as emerging issues in the benefit/risk ratio outcome of tPA For treatment of CNS ischemic disorders.

Stroke is a leading cause of morbidity and mortality. While tissue-type plasminogen activator (tPA) remains the only FDA-approved treatment for ischemic stroke, clinical use of tPA has been constrained to roughly 3% of eligible patients because of the danger of intracranial hemorrhage and a narrow 3 h time window for safe administration. Basic science studies indicate that tPA enhances excitotoxic neuronal cell death. In this review, the beneficial and deleterious effects of tPA in ischemic brain are discussed along with emphasis on development of new approaches toward treatment of patients with acute ischemic stroke. In particular, roles of tPA-induced signaling and a novel delivery system for tPA administration based on tPA coupling to carrier red blood cells will be considered as therapeutic modalities for increasing tPA benefit/risk ratio. The concept of the neurovascular unit will be discussed in the context of dynamic relationships between tPA-induced changes in cerebral hemodynamics and histopathologic outcome of CNS ischemia. Additionally, the role of age will be considered since thrombolytic therapy is being increasingly used in the pediatric population, but there are few basic science studies of CNS injury in pediatric animals.

Targeting of a Mutant Plasminogen Activator to Circulating Red Blood Cells for Prophylactic Fibrinolysis

Chemical coupling to carrier red blood cells (RBCs) converts tissue type plasminogen activator (tPA) from a problematic therapeutic into a safe agent for thromboprophylaxis. The goal of this study was to develop a more clinically relevant recombinant biotherapeutic by fusing a mutant tPA with a single-chain antibody fragment (scFv) with specificity for glycophorin A (GPA) on mouse RBCs. The fusion construct (anti-GPA scFv/PA) bound specifically to mouse but not human RBCs and activated plasminogen; this led to rapid and stable attachment of up to 30,000 copies of anti-GPA scFv/PA per mouse RBC that were thereby endowed with high fibrinolytic activity. Binding of anti-GPA scFv/PA neither caused RBC aggregation, hemolysis, uptake in capillary-rich lungs or in the reticuloendothelial system nor otherwise altered the circulation of RBCs. Over 40% of labeled anti-GPA scFv/PA injected in mice bound to RBC, which markedly prolonged its intravascular circulation and fibrinolytic activity compared with its nontargeted PA counterpart, anti-GPA scFv/PA, but not its nontargeted PA analog, prevented thrombotic occlusion in FeCl(3) models of vascular injury. These results provide proof-of-principle for the development of a recombinant PA variant that binds to circulating RBC and provides thromboprophylaxis by use of a clinically relevant approach.

Soluble urokinase receptor conjugated to carrier red blood cells binds latent pro-urokinase and alters its functional profile

Coupling plasminogen activators to carrier red blood cells (RBC) prolongs their life-time in the circulation and restricts extravascular side effects, thereby allowing their utility for short-term thromboprophylaxis. Unlike constitutively active plasminogen activators, single chain urokinase plasminogen activator (scuPA) is activated by plasmin proteolysis or binding to its receptor, uPAR. In this study we conjugated recombinant soluble uPAR (suPAR) to rat RBC, forming RBC/suPAR complex. RBC carrying suPAR circulated in rats similarly to naïve RBC and markedly prolonged the circulation time of suPAR. RBC/suPAR carrying ~ 3 × 10 4 suPAR molecules per RBC specifically bound up to 2 × 10 4 molecules of scuPA, retained ~ 75% of scuPA-binding capacity after circulation in rats and markedly altered the functional profile of bound scuPA. RBC carrying directly conjugated scuPA adhered to endothelial cells, while showing no appreciable fibrinolytic activity. In contrast, RBC/suPAR loaded with scuPA did not exhibit increased adhesion to endothelium, while effectively dissolving fibrin clots. This molecular design, capitalizing on unique biological features of the interaction of scuPA with its receptor, provides a promising modality to deliver a pro-drug for prevention of thrombosis.

Cerebrovascular Thromboprophylaxis in Mice by Erythrocyte-Coupled Tissue-Type Plasminogen Activator

Cerebrovascular thrombosis is a major source of morbidity and mortality after surgery, but thromboprophylaxis in this setting is limited because of the formidable risk of perioperative bleeding. Thrombolytics (eg, tissue-type plasminogen activator [tPA]) cannot be used prophylactically in this high-risk setting because of their short duration of action and risk of causing hemorrhage and central nervous system damage. We found that coupling tPA to carrier red blood cells (RBCs) prolongs and localizes tPA activity within the bloodstream and converts it into a thromboprophylactic agent, RBC/tPA. To evaluate the utility of this new approach for preventing cerebrovascular thrombosis, we examined the effect of RBC/tPA in animal models of cerebrovascular thromboembolism and ischemia. Preformed fibrin microemboli were injected into the middle carotid artery of mice, occluding downstream perfusion and causing severe infarction and 50% mortality within 48 hours. Preinjected RBC/tPA rapidly lysed nascent cerebral thromboemboli, providing rapid, durable reperfusion and reducing morbidity and mortality. These beneficial effects were not achieved by preinjection of tPA, even at a 10-fold higher dose, which increased mortality from 50% to 90% by 10 hours after embolization. RBC/tPA injected 10 minutes after tail amputation to simulate postsurgical hemostasis did not cause bleeding from the wound, whereas soluble tPA caused profuse bleeding. RBC/tPA neither aggravated brain damage caused by focal ischemia in a filament model of middle carotid artery occlusion nor caused postthrombotic hemorrhage in hypertensive rats. These results suggest a potential RBC/tPA utility as thromboprophylaxis in patients who are at risk for acute cerebrovascular thromboembolism.

Transparent Adult Zebrafish as a Tool for In Vivo Transplantation Analysis

The zebrafish is a useful model for understanding normal and cancer stem cells, but analysis has been limited to embryogenesis due to the opacity of the adult fish. To address this, we have created a transparent adult zebrafish in which we transplanted either hematopoietic stem/progenitor cells or tumor cells. In a hematopoiesis radiation recovery assay, transplantation of GFP-labeled marrow cells allowed for striking in vivo visual assessment of engraftment from 2 hr-5 weeks posttransplant. Using FACS analysis, both transparent and wild-type fish had equal engraftment, but this could only be visualized in the transparent recipient. In a tumor engraftment model, transplantation of RAS-melanoma cells allowed for visualization of tumor engraftment, proliferation, and distant metastases in as little as 5 days, which is not seen in wild-type recipients until 3 to 4 weeks. This transparent adult zebrafish serves as the ideal combination of both sensitivity and resolution for in vivo stem cell analyses.

The Glycocalyx Protects Erythrocyte-Bound Tissue-Type Plasminogen Activator from Enzymatic Inhibition

Coupling tissue-type plasminogen activator (tPA) to carrier red blood cells (RBC) prolongs its intravascular life span and permits its use for thromboprophylaxis. Here, we studied the susceptibility of RBC/tPA to PA inhibitors including plasminogen activator inhibitor-1 (PAI-1) that constrain its activity and may reduce the duration of its effect. Despite lesser spatial and diffusional limitations, soluble tPA was far less effective than RBC/tPA in dissolving clots formed in vitro from blood of wild-type (WT) mice (40 versus 80% lysis at equal doses of tPA). Furthermore, after i.v. injection, soluble tPA lost activity faster in transgenic mice expressing a high level of PAI-1 than in WT mice, whereas the activity of RBC/tPA was unaffected. PAI-1 inactivated soluble tPA at equimolar ratios in vitro, but it had no effect on the amidolytic or fibrinolytic activity of RBC/tPA. RBC/tPA was also more resistant than soluble tPA to in vitro inhibition by other serpins (alpha2-macroglobulin and alpha1-antitrypsin) and pathologically high levels of glucose. However, coupling to RBC did not protect a truncated tPA mutant, Retavase, from plasma inhibitors. Chemical removal of the RBC glycocalyx negated tPA protection from inhibitors: tPA coupled to glycocalyx-stripped RBC bound twice as much 125I-PAI-1 as did tPA coupled to naive RBC, and susceptibility of the bound tPA to inhibition by PAI-1 was restored. Thus, the RBC glycocalyx protects RBC-coupled tPA against inhibition. Resistance to high levels of inhibitors in vivo contributes to the potential utility of RBC/tPA for thromboprophylaxis.

Erythrocyte Coupled tPA Improves Outcomes of Percussion Brain Trauma in Rats.

Coupling tissue plasminogen activator (tPA) to carrier red blood cells (RBC): i) restricts tPA's permeation into tissues and pre-existing hemostatic clots, minimizing hemorrhage; ii) protects it from plasma inhibitors; and iii) prolongs its circulation, permitting incorporation into nascent clots and their lysis from within. These features support the thromboprophylactic utility of RBC coupled tPA (RBC/tPA). In this study we explored the utility of RBC/tPA in traumatic brain injury (TBI), a disorder in which both cerebral thrombosis leading to cerebral ischemia and intracerebral hemorrhage (ICH) have been implicated. Eleven male, Sprague-Dawley rats (340–400g) were subjected to a moderate (avg. peak pressure 2.6atm) lateral fluid percussion injury to the left hemisphere. Rats were given a single intravenous dose of RBC/tPA (0.05mg/Kg, n=5) or vehicle (n=4), 15 min post-injury. Animals were sacrificed 48h later for histopathology and staining for fibrin. The lesions in control animals occupied 8.3 ± 2.8% (mean±SD) of the hemispheric volume. Animals treated with RBC/tPA had a significant decrease in mean lesion volume (1.4±0.7%; p<0.001). Thrombotic burden was reduced from a mean of 10 clots in vehicle-treated to 1 per RBC/tPA-treated rats p<0.001). These data indicate that RBC/tPA attenuates post-traumatic thrombosis without aggravating hemorrhage induced by TBI. These observations support the safety of RBC/tPA in thromboprophylaxis even in the context of brain trauma, due to intravascular containment of RBC/tPA, among other factors. Durable lysis of post-traumatic thrombi by long-circulating RBC/tPA may alleviate secondary brain ischemia and resultant ICH. Correlation between reduction of thrombotic burden and lesion size implicate thrombosis in the pathophysiology of parenchymal brain damage after head trauma. Supported by PENN Research Foundation, HL66442, HL60169 and in part by NS38104.

Coupling Tissue Type Plasminogen Activator to Carrier Erythrocyte Protects Against Plasma Inhibitors.

Conjugating tissue plasminogen activator (tPA) to carrier red blood cells (RBC) restricts its permeation into tissues and pre-existing hemostatic clots, minimizing side effects, and prolongs its circulation, which permits it to be incorporated within nascent clot which it lyses from inside. We now report that RBC/tPA dissolves clots formed from mouse blood more effectively than free tPA, while they were equipotent against clots formed in vitro from PAI-1 KO mouse blood. To test whether tPA acquires resistance to plasma inhibitors when conjugated to RBC, we compared the activity of RBC/tPA vs free tPA in the presence of purified PAI-1, α2 macroglobulin (α2M) and α1anti-trypsin (α1AT). At equimolar concentrations, PAI-1 completely inhibited the activity of soluble tPA in vitro, whereas RBC/tPA retained 100% and 75% of its amidolytic and fibrinolytic activity, respectively. RBC/tPA, but not free tPA, was also resistant to equimolar concentrations of α2M and α1AT. However, tPA coupled to RBC pre-incubated with a mixture of hyaluronidase, heparinase and neuraminidase was as susceptible to inactivation by PAI-1, α2M and α1AT as free tPA. Further, tPA coupled to glycocalyx-stripped RBC bound two-fold more 125I-labeled PAI-1 than tPA coupled to naïve RBC. We conclude that the RBC glycocalyx protects tPA from inactivation by PA inhibitors, likely by steric hindrance. This unexpected benefit may enhance the utility of RBC/tPA in thromboprophylaxis and suggests a previous under-appreciated role for the glycocalyx in modulating PA activity on the vasculature and other cell surfaces.

Conjugation of Plasminogen Activators to Carrier Erythrocytes Imparts New Biological Properties to Their Soluble Counterparts.

Conjugation of plasminogen activators (PA) to carrier red blood cells (RBC) generates a new agent (RBC/PA), which selectively lyses nascent blood clots. In this work, we studied two types of RBC/PA conjugates, carrying Alteplase (tissue type plasminogen activator, tPA) or Reteplase (an engineered form of tPA, Ret). Compared to non-conjugated 125I-PA counterparts, both 125I-tPA and 125I-Ret coupled to 51Cr-RBC via biocompatible biotin-streptavidin cross-linker showed markedly prolonged circulation in rats and mice. Despite slightly faster blood clearance, RBC/tPA retained significantly higher fibrinolytic activity in circulation than RBC/Ret. In part, the higher fibrinolytic activity of RBC/tPA vs RBC/Ret in the circulation was due to its lessened susceptibility to plasma inhibitors. Analysis of amidolytic activity of RBC-coupled vs free tPA and Ret using chromogenic substrates in vitro revealed that coupling to RBC rendered Ret, but not tPA, insensitive to stimulation of fibrinolytic activity by fibrin. In vitro binding assay in cultural wells showed that RBC/tPA specifically binds to fibrin clot (6±0.2x104 RBC/tPA bound per well vs. 1±0.4x104 RBC/Ret or 7.3±0.6x103 naive RBC). RBC/tPA also bind specifically to immobilized fibrinogen and plasminogen (8±2x104 and 2±0.4x104 RBC/well), but not to non-cleavable plasminogen, collagen, fibronectin or thrombospondin (<1.1±0.3x103RBC/well). Neither RBC/Ret nor naïve RBC bind to these proteins (<1.1±0.2x103 RBC/well). Therefore, RBC/PA complexes display a considerable functional diversity, a result of interplay between the pharmacokinetic features offered by carrier RBC, individual features of a given PA and alterations of the latter caused by RBC coupling. In theory, this diversity may enhance flexibility and utility of potential applications of RBC/PA for thromboprophylaxis.

Blood Clearance and Activity of Erythrocyte-Coupled Fibrinolytics

Conjugating tissue-type plasminogen activator (tPA) to red blood cells (RBCs) endows it with features useful for thromboprophylaxis. However, the optimal intensity and duration of thromboprophylaxis vary among clinical settings. To assess how the intrinsic properties of a plasminogen activator (PA) affect functions of the corresponding RBC/PA conjugate, we coupled equal amounts of tPA or Retavase (rPA; a variant with an extended circulation time, lower fibrin affinity, and greater susceptibility to PA inhibitors). Conjugation to RBC markedly prolonged the circulation of each PA in rats and mice, without detrimental effects on carrier RBC. The initial blood clearance of RBC/tPA was faster than RBC/rPA, yet it exerted greater fibrinolytic activity, in part due to greater resistance of tPA and RBC/tPA to plasma inhibitors versus rPA and RBC/rPA observed in vitro. Soluble and RBC-coupled tPA and rPA exerted the same amidolytic activity, yet RBC/tPA lysed fibrin clots more effectively than RBC/rPA, notwithstanding comparable fibrinolytic activity of their soluble counterparts. Conjugation to RBC suppressed rPA's ability to be activated by fibrin, whereas the fibrin activation of RBC-coupled tPA was not hindered. Therefore, the functional profile of RBC/PA is influenced by: pharmacokinetic features provided by carrier RBC (e.g., prolonged circulation), intrinsic PA features (e.g., clearance rate, resistance to inhibitors), and changes imposed by conjugation to RBC (e.g., loss of cofactor stimulation). These factors, different from those guiding the design of soluble PA for lysis of existing clots, can be exploited in the rational design of RBC/PA tailored for specific prophylactic indications.

In vitro phagocytosis of carrier mouse red blood cells is increased by Band 3 cross-linking or diamide treatment.

Chemical alteration of red blood cells (RBCs) can induce increased phagocytosis of modified cells by macrophages. In this study we have used different chemical treatments for the modification of the mouse red-blood-cell membrane surface, namely oxidant compounds, such as ascorbate/Fe(+2) and diamide [azodicarboxylic acid bis(dimethylamide)], or Band 3-cross-linking reagents. We monitored the phagocytosis of oxidized or Band 3-cross-linked mouse red blood cells by peritoneal macrophages. The extent of phagocytosis of RBCs is not affected by oxidation with ascorbate/Fe(3+), but it is increased (up to 10%) by oxidation with 2 mM diamide. Furthermore, phagocytosis is greatly increased (up to 40%) as a result of cross-linking with either of two Band 3 bifunctional reagents [bis(sulphosuccinimidyl) suberate (BS(3)) and 3,3'-dithiobis(sulphosuccinimidyl propionate) (DTSSP)]. To evaluate targeting towards macrophages of such modified RBCs for therapeutical purposes, we have determined the phagocytosis of Band 3 carrier RBCs loaded with carbonic anhydrase. In this case phagocytosis is high enough (25%) to deliver the enzyme into macrophages. We have also assayed the influence of serum components and IgG on the efficiency of phagocytosis and discuss the possible phagocytosis mechanisms. In the case of BS(3)-cross-linked carrier RBCs, phagocytosis is markedly enhanced (from 12% up to 25%) by serum components. This opens a way for therapeutic application of these carrier RBCs, with special relevance in short-term delivery to cells of the mononuclear phagocytic system.

In vivo survival of selected murine carrier red blood cells after separation by density gradients or aqueous polymer two-phase systems

In order to explore possibilities of using erythrocytes as carrier systems for delivery of pharmacological agents, we have studied the in vivo survival of murine carrier red blood cell populations enriched in young or old cells. Hypotonicisotonic dialysis has been used to modify the cells as carrier systems and Percoll/albumin density gradients or counter-current distribution in aqueous polymer two-phase systems to separate them according to age. Hypotonicisotonic dialysis produces a decrease in the red blood cell populations in vivo survival rate (from 9.5 to 7.8 days). Among the cells modified as carriers, the enriched young red blood cell populations show a higher in vivo survival (half-life 6.5–7.4 days) than populations made up of predominantly old red blood cells (half-life 4.7-6.2 days). Half-life of young or old circulating red blood cells was approximately one day longer when these cells were separated by counter-current distribution rather than by Percoll density gradients. Based on these results, hypotonic-isotonic dialysis of whole and enriched young or old red blood cell populations, with higher or lower survival rates, can be considered as a useful tool for modification of these cells as carriers. The final outcome of such changes can be translated into better control of plasma drug delivery during therapy.

Erythrocytes as carriers for recombinant human erythropoietin.

The aim of this work was to encapsulate recombinant human erythropoietin (rHuEpo) in human and mouse red blood cells (RBCs) to improve the stability of encapsulated rHuEpo. The encapsulation of rHuEpo was achieved by an hypotonic dialysis-isotonic resealing procedure. A radioimmunoassay method was used for the estimation of rHuEpo. The hypoosmotic resistance of carrier erytrhocytes was studied by osmotic fragility measurements. Cell morphology was observed under scanning electron microscopy. Encapsulated rHuEpo was identified by an immunogold labeling assay. Encapsulation yields were 22% for human RBCs and 14% for mouse RBCs. Cell recovery was around 70%. Carrier-RBCs exhibited a tendency to spherocytic morphology, and showed the typical higher hypoosmotic resistance than normal RBCs. The presence of rHuEpo inside carrier RBCs was identified. The stability of encapsulated rHuEpo seems to be related to the experimental conditions used during the encapsulation procedure. An increase with time of released rHuEpo was observed in carrier-RBC suspensions. The encapsulation of rHuEpo in RBCs has been achieved for the first time. These carrier RBC-preparations may serve as an alternative sustained cell delivery system for the in vivo administration of rHuEpo.