Nanoparticle transport pathways into tumors

Two transport pathways (interendothelial and transendothelial routes) have long been proposed for entry of nanoparticles from the blood circulation into solid tumors. We examine and discuss available evidence supporting interendothelial and transendothelial transport processes and suggest new avenues for re-evaluating these pathways. Understanding of integrative mechanisms controlling nanoparticle extravasation into tumors is important for improving engineering and performance of anti-cancer nanopharmaceuticals.

Peptide and nucleic acid-directed self-assembly of cationic nanovehicles through giant unilamellar vesicle modification: Targetable nanocomplexes for in vivo nucleic acid delivery

One of the greatest challenges for the development of genetic therapies is the efficient targeted delivery of therapeutic nucleic acids. Towards this goal, we have introduced a new engineering initiative in self-assembly of biologically safe and stable nanovesicle complexes (∼90 to 140nm) derived from giant unilamellar vesicle (GUV) precursors and comprising plasmid DNA or siRNA and targeting peptide ligands. The biological performance of the engineered nanovesicle complexes were studied both in vitro and in vivo and compared with cationic liposome-based lipopolyplexes. Compared with cationic lipopolyplexes, nanovesicle complexes did not show advantages in transfection and cell uptake. However, nanovesicle complexes neither displayed significant cytotoxicity nor activated the complement system, which are advantageous for intravenous injection and tumour therapy. On intravenous administration into a neuroblastoma xenograft mouse model, nanovesicle complexes were found to distribute throughout the tumour interstitium, thus providing an alternative safer approach for future development of tumour-specific therapeutic nucleic acid interventions. On oropharyngeal instillation, nanovesicle complexes displayed better transfection efficiency than cationic lipopolyplexes. The technological advantages of nanovesicle complexes, originating from GUVs, over traditional cationic liposome-based lipopolyplexes are discussed.

STATEMENT OF SIGNIFICANCE

The efficient targeted delivery of nucleic acids in vivo provides some of the greatest challenges to the development of genetic therapies. Giant unilamellar lipid vesicles (GUVs) have been used mainly as cell and tissue mimics and are instrumental in studying lipid bilayers and interactions. Here, the GUVs have been modified into smaller nanovesicles. We have then developed novel nanovesicle complexes comprising self-assembling mixtures of the nanovesicles, plasmid DNA or siRNA, and targeting peptide ligands. Their biophysical properties were studied and their transfection efficiency was investigated. They transfected cells efficiently without any associated cytotoxicity and with targeting specificity, and in vivo they resulted in very high and tumour-specific uptake and in addition, efficiently transfected the lung. The peptide-targeted nanovesicle complexes allow for the specific targeted enhancement of nucleic acid delivery with improved biosafety over liposomal formulations and represent a promising tool to improve our arsenal of safe, non-viral vectors to deliver therapeutic cargos in a variety of disorders.

Cancer nanomedicine and the complement system activation paradigm: anaphylaxis and tumour growth

A wide variety of nanocarriers and particularly cancer nanomedicines activate the complement system, which is the first line of the innate immune defence mechanism. Complement activation may induce inflammatory responses, but such responses arising from uncontrolled complement activation could be life threatening. Accordingly, the role of complement in initiation of adverse reactions to particulate and polymer therapeutics is receiving increasing attention. Furthermore, the involvement of complement-activation products in promoting tumour growth has also been indicated. This could be of serious concern for development of cancer nanomedicines and cancer nanotechnology initiatives. These concepts are reviewed with preliminary evidence that intra-tumoural accumulation of model long circulating nanoparticles could promote tumour growth.

Particulate systems for targeting of macrophages: basic and therapeutic concepts

Particulate systems in the form of liposomes, polymeric micelles, polymeric nano- and microparticles, and many others offer a rational approach for selective delivery of therapeutic agents to the macrophage from different physiological portals of entry. Particulate targeting of macrophages and intracellular drug release processes can be optimized through modifications of the drug carrier physicochemical properties, which include hydrodynamic size, shape, composition and surface characteristics. Through such modifications together with understanding of macrophage cell biology, targeting may be aimed at a particular subset of macrophages. Advances in basic and therapeutic concepts of particulate targeting of macrophages and related nanotechnology approaches for immune cell modifications are discussed.

Factors controlling nanoparticle pharmacokinetics: an integrated analysis and perspective

Intravenously injected nanoparticulate drug carriers provide a wide range of unique opportunities for site-specific targeting of therapeutic agents to many areas within the vasculature and beyond. Pharmacokinetics and biodistribution of these carriers are controlled by a complex array of interrelated core and interfacial physicochemical and biological factors. Pertinent to realizing therapeutic goals, definitive maps that establish the interdependency of nanoparticle size, shape, and surface characteristics in relation to interfacial forces, biodistribution, controlled drug release, excretion, and adverse effects must be outlined. These concepts are critically evaluated and an integrated perspective is provided on the basis of the recent application of nanoscience approaches to nanocarrier design and engineering. The future of this exciting field is bright; some regulatory-approved products are already on the market and many are in late-phase clinical trials. With concomitant advances in extensive computational knowledge of the genomics and epigenomics of interindividual variations in drug responses, the boundaries toward development of personalized nanomedicines can be pushed further.

Tunable 3D and 2D polystyrene nanoparticle assemblies using surface wettability, low volume fraction and surfactant effects

Polymer-based nanopatterning on metal surfaces is of increasing importance to a number of applications, including biosensors, bioelectronic devices and medical implants. Here we show that polycrystalline gold surfaces can be functionalized with monocomponent nanoparticle (NP) assemblies by a simple drop deposition method. Ordered 3D hexagonal close-packed structures consisting of 350 nm polystyrene (PS) NPs on hydrophobically modified gold surfaces from solutions of very low volume fraction (varphi = 0.0006) were obtained as a result of capillary force induced self-assembly, whilst 2D self-assembly of PS NPs was generated over large area on hydrophilic gold and TiO(2) surfaces by spin coating. Furthermore, we show that when Triton X-100 is added to the PS NP suspending medium longer range ordering is obtained. Our observations may initiate interesting applications in the areas of nanoengineering of metal-based sensors and as a means to design new nanostructures for biocompatible implant surfaces.

Concentration dependent structural ordering of poloxamine 908 on polystyrene nanoparticles and their modulatory role on complement consumption

Adsorption of poloxamine 908, a tetrafunctional polyethylene oxide (PEO)-polypropylene oxide ethylenediamine block copolymer, onto the surface of monodispersed polystyrene nanoparticles (232 +/- 0.33 nm) follows a bimodal pattern. Initially, the isotherm follows a Langmuir profile with a plateau observable over a very narrow equilibrium poloxamine concentration (0.0018-0.0031 mM). The isotherm then begins to rise again, reaching a final plateau at equilibrium poloxamine concentrations above 0.0089 mM. Similarly, the profile of the adsorbed layer thickness of poloxamine on the surface of nanoparticles is bimodal. The first plateau corresponds to a thickness of 4.6 +/- 0.07 nm, which occurs over the same range of poloxamine concentrations as in the initial plateau of the adsorption isotherm. The second plateau corresponds to a thickness of 9.53 +/- 0.32 nm, observable at a minimum poloxamine concentration of 0.0067 mM. By using a calculated radius of gyration of a PEO chain in poloxamine as 3.1 nm, these observations reflect dynamic changes in the arrangement of surface projected PEO chains; a mushroom-like conformation at the first plateau region of the adsorption isotherm, followed by a transition into a brush-like conformation. These conformational changes are also reflected in rheological studies; the apparent viscosity of nanoparticles in which the PEO chains are in mushroom conformation is considerably higher than particles displaying the brush conformation. Further, atomic force microscopy studies (height profile and phase lag measurements) corroborated that the proposed poloxamine concentration dependent transition of surface associated PEO chains from mushroom to brush appearance is conserved when nanoparticles are dried under ambient conditions. Finally, we compared the influence of the surface PEO characteristics on complement consumption in human serum. Our results show complement-activating nature of all poloxamine-coated nanoparticles. However, complement consumption is reduced substantially with particles bearing a minimum of 11448 poloxamine molecules on their surface, thus demonstrating the importance of PEO surface density as well as brush conformation in suppressing complement consumption. This relationship between surface characteristics of poloxamine nanoparticles and their in vivo performance is discussed.

The effect of methoxy-PEG chain length and molecular architecture on lymph node targeting of immuno-PEG liposomes

The rate of drainage and lymphatic distribution of subcutaneously injected liposomes is controlled by inclusion of methoxypoly(ethyleneglycol), mPEG-phospholipid into the liposomal bilayer. The effect is most dramatic with liposomes containing 15 mol% mPEG-lipid, with an average PEG molecular mass of 350 Da. These vesicles are drained rapidly from the injection site into the initial lymphatics when compared to unmodified liposomes, and are retained more favourably by the scavengers of the regional lymph node. Liposomes decorated with longer surface mPEG chains (6.7 mol% of mPEG2000-lipid) exhibit faster drainage rates than vesicles having 15 mol% mPEG350-lipid in their lipid bilayer, but their lymph node retention is very poor. The lymph node retention of rapidly drained PEG-bearing vesicles was increased dramatically following conjugation of a non-specific IgG to the distal end of PEG, using a functionalized PEG2000 lipid. Adjusting the molecular architecture of surface mPEG and IgG-PEG chains to a "nearly overlapped mushroom" regime further enhanced target recognition of immuno-PEG2000 liposomes without compromising their drainage rate from the interstitium. The lymph node retention of these vesicles was further optimized by enriching their lipid bilayer with 20 mol% phosphatidylserine. These approaches have established important compositional and structural variables that control lymphatic targeting of immuno-PEG liposomes and their application in experimental medicine and biology is discussed.

Stealth liposomes and long circulating nanoparticles: critical issues in pharmacokinetics, opsonization and protein-binding properties

This article critically examines and evaluates the likely mechanisms that contribute to prolonged circulation times of sterically protected nanoparticles and liposomes. It is generally assumed that the macrophage-resistant property of sterically protected particles is due to suppression in surface opsonization and protein adsorption. However, recent evidence shows that sterically stabilized particles are prone to opsonization particularly by the opsonic components of the complement system. We have evaluated these phenomena and discussed theories that reconcile complement activation and opsonization with prolonged circulation times. With respect to particle longevity, the physiological state of macrophages also plays a critical role. For example, stimulated or newly recruited macrophages can recognize and rapidly internalize sterically protected nanoparticles by opsonic-independent mechanisms. These concepts are also examined.

Modulation of lymphatic distribution of subcutaneously injected poloxamer 407-coated nanospheres: the effect of the ethylene oxide chain configuration

Lymphatic distribution of interstitially injected poloxamer 407-coated nanospheres (45 nm in diameter) is controlled by surface configuration of the ethylene oxide (EO) segments of the adsorbed copolymer. At low poloxamer surface coverage, EO tails spread laterally on a nanosphere surface and assume a 'flat or mushroom-like' configuration. Such entities drain rapidly from the subcutaneous site of injection into the initial lymphatic, when compared to uncoated nanospheres, and subsequently are captured by scavengers of the regional lymph nodes. In vitro experiments have also confirmed that such entities are prone to phagocytosis. When the equilibrium poloxamer concentration is at 75 microg/ml or greater the EO chains become more closely packed and project outward from the nanosphere surface. These surface-engineered nanospheres drain faster than those with EO chains in mushroom configurations into the initial lymphatic, escape clearance by lymph node macrophages, reach the systemic circulation, and remain in the blood for prolonged periods. These experiments provide a rational approach for the design and engineering of nano-vehicles for optimal lymphatic targeting and are discussed.

Endothelial Cells as Therapeutic Targets in Cancer: New Biology and Novel Delivery Systems

The significance of the endothelial cell as a target for antitumor therapy has been recognized for some time, but so far the results of clinical trials exploiting this approach have not been encouraging. The subject is likely to gain new momentum, however, following a number of important recent findings that shed new light on the origins and nature of tumor vasculature. Coupled with rapid developments in the use ofphage-displayed peptide libraries to characterize the human vascular map, as well as highly selective delivery systems, this new understanding of tumor vascular biology should provide many fresh and exciting avenues to explore.

PEGylation of microspheres generates a heterogeneous population of particles with differential surface characteristics and biological performance

Surface PEGylation of polystyrene microspheres with methoxy-poly(ethylene glycol)-5000 (mPEG-5000) generated a heterogeneous population of entities that differed in surface characteristics and in vitro biological performance (phagocytosis and complement activation). Surface heterogeneity was determined by hydrophobic interaction chromatography, measurements of particle electrophoretic mobility in a defined field and adlayer thickness of the projected mPEG chains. The particle population separation by hydrophobic interaction chromatography demonstrated a remarkable linear relationship between the particle zeta potential and phagocytosis by J774 A1 macrophage-like cells. Microsphere populations bearing a predominant surface of mPEG molecules as high-density mushroom-brush intermediate and/or brush configuration were most resistant to phagocytosis and activated the human complement system poorly. Conversely, those populations with predominant surface mPEGs in a mushroom regime were potent activators of the complement system and were prone to phagocytosis. Therefore, surface heterogeneity explains why a fraction of intravenously injected 'long-circulating' nanoparticles is cleared rapidly by macrophages of the reticuloendothelial system. Hydrophobic interaction chromatography can readily assess the extent of surface heterogeneity of PEGylated particulate drug delivery systems and pre-select particles with optimal retention times in the blood. These observations may also be relevant with respect to successful surface camouflaging of cells, drug depots and implantable devices.

Chemical camouflage of nanospheres with a poorly reactive surface: towards development of stealth and target-specific nanocarriers

A two-step approach is described to chemically camouflage the inert surface of model polystyrene nanospheres of 60 nm in diameter against recognition by the body's defenses. The first step was based on the strong protein adsorbing potency of polystyrene, and the second step utilized the chemical reactivity of the adsorbed proteins for conjugation with cyanuric chloride-activated methoxypoly(ethyleneglycol)5000, mPEG5000. Bovine serum albumin (BSA) and rat IgG were used as models of non-immune and immune proteins, respectively. The maximum adsorbance values for both proteins were near expectation for a close-packed monolayer. Adsorption isotherms studies and analysis of the hydrodynamic thickness of the adsorbed protein layer confirmed the close-packed side-on mode of adsorption for BSA and the end-on mode of adsorption for IgG, respectively. Nucleophiles on the adsorbed proteins were then reacted with cyanuric chloride activated mPEG5000. The average poly(ethyleneglycol) (PEG) content for a 60-nm nanospheres was found to be 13.7+/-0.4 micromol PEG/micromol BSA and 3.6+/-0.3 micromol PEG/micromol IgG. The interaction of both PEG-bearing nanospheres with the hydrophobic column material octyl-agarose indicated surface heterogeneity among the nanospheres. Only nanospheres with the most hydrophilic phenotype (approximately 70% of the total population) exhibited stealth properties after intravenous injection to rats. In contrast to the described approach, incubation of uncoated nanospheres with preformed BSA-mPEG5000 conjugates failed to produce long circulating entities. For design of splenotropic particles cyanuric chloride-activated mPEG5000 was conjugated to BSA-coated polystyrene beads of 225 nm in diameter. Despite their stealth property to hepatic Kupffer cell recognition, these nanospheres were cleared by the splenic red pulp macrophages.

Capture of stealth nanoparticles by the body's defences

This article highlights and discusses our experience in the design, handling, and biological evaluation of long-circulating nanoparticles based on polyoxypropylene/polyoxyethylene copolymer nonionic surfactants. A frequently observed but ignored phenomenon following intravenous injection of such polymer-modified long-circulating colloidal systems is their eventual recognition and clearance by macrophages of the reticuloendothelial system. The pharmacokinetics as well as the tissue distribution of such so-called 'stealth" nanoparticles are also altered after repeated intravenous injection in a time-dependent manner. An understanding of immunological and pathological factors that control the pharmacokinetic and biological behavior of long-circulating particles after single or repeated administration is therefore crucial for the design of a system with an optimal diagnostic and/or therapeutic performance. Therefore, we have also examined and discussed the concept of macrophage recognition of a 'stealth-like nature" and factors that initiate this cascade. A critical discussion of the future of this interesting area of nano-biotechnology/engineering is also provided.

Liposome recognition by resident and newly recruited murine liver macrophages

The evidence in this communication indicate that, unlike resident Kupffer cells, newly recruited liver macrophages (following monocyte migration from the blood to the liver) use complement receptors to recognize and internalize stearylamine-incorporated liposomes. Within two weeks of hepatic residency complement receptors no longer participate in liposome recognition and uptake.

Modulation of murine liver macrophage clearance of liposomes by diethylstilbestrol. The effect of vesicle surface charge and a role for the complement receptor Mac-1 (CD11b/CD18) of newly recruited macrophages in liposome recognition

We have studied the blood clearance and reticuloendothelial organ distribution of intravenously injected neutral (egg phosphatidylcholine, egg PC/cholesterol, mol ratio 7:2), anionic (egg PC/cholesterol/dicetylphosphate, mol ratio 7:2:1), and cationic (egg PC/cholesterol/stearylamine, mol ratio 7:2:1) liposomes of approximately the same size distribution in mice 3 days after treatment with the synthetic oestrogen diethylstilbestrol (DES). Male mice administered DES intraperitoneally at a dose of 1 mg per mouse (body weight 22-25 g) manifested an increase in the vascular clearance rate of liposomes irrespective of the initial vesicle surface charge. The enhancement in the vascular clearance of liposomes in DES-treated animals was associated with a concomitant increase in liver weight as well as hepatic phagocytosis. However, DES treatment significantly enhanced the hepatic sequestration (on the basis of % of injected dose of liposomes per g of liver tissue) of positively charged liposomes when compared to both neutral and negatively charged vesicles of similar size distribution. This observation was also confirmed by the 'hepatic-blockade' experiments where blockade was induced by prior intravenous injection of liposomes of the same size distribution and charge to that of test vesicles. The in vitro cell suspension studies suggested that the enhanced liposome uptake (irrespective of the initial vesicle surface charge) by Kupffer cells of DES-treated mice was independent of changes in the blood opsonization processes. Furthermore, in vitro studies also showed the operation of multiple mechanisms and involvement of different populations of liver macrophages (resident and recruited cells) in liposome recognition following DES treatment. For example, in DES-treated animals, the newly recruited liver macrophages were found to play a major role in the clearance of stearylamine incorporated liposomes via complement receptors (Mac-1). The resident Kupffer cells seem to recognize cationic vesicles via other receptors as the expression of Mac-1 is virtually absent in these cells. On the other hand, complement receptors seem to play a minor role in the uptake of anionic DCP vesicles by hepatic macrophages of DES-treated mice. DES appears to offer a new approach in dissecting the mechanisms of liposome-macrophage interaction.

Recognition and clearance of methoxypoly(ethyleneglycol)2000-grafted liposomes by macrophages with enhanced phagocytic capacity. Implications in experimental and clinical oncology

Intravenous injection of an endotoxin-free solution of poloxamine-908 to rats can enhance the phagocytic clearance capacity of tissue macrophages, particularly those of the liver and the spleen. Such stimulated cells were able to clear a significant portion of intravenously injected methoxypoly(ethyleneglycol)2000 liposomes (mean size of 87 nm), labelled with technetium-99m via the N-hydroxysuccinimidyl hydrazine nicotinate hydrochloride derivative of distearoyl phosphatidylethanolamine, within 4 h post administration. These liposomes, otherwise, exhibit long circulatory behaviour in control animals, with poor localization to the liver and spleen. We suggest that such technetium-99m-labelled engineered vesicles may be of aid for detection of the liver and spleen macrophages with enhanced phagocytic clearance capacity by gamma scintigraphy. Alterations in the phagocytic activity of liver and spleen macrophages is known to occur during cancer. Therefore, such diagnostic procedures may prove useful for patient selection or for monitoring the progress of treatment with long circulating nanoparticles carrying anti-cancer agents, thus minimizing damage to this important line of body's defence cells, and are discussed.

Long-circulating and target-specific nanoparticles: theory to practice

The rapid recognition of intravenously injected colloidal carriers, such as liposomes and polymeric nanospheres from the blood by Kupffer cells, has initiated a surge of development for "Kupffer cell-evading" or long-circulating particles. Such carriers have applications in vascular drug delivery and release, site-specific targeting (passive as well as active targeting), as well as transfusion medicine. In this article we have critically reviewed and assessed the rational approaches in the design as well as the biological performance of such constructs. For engineering and design of long-circulating carriers, we have taken a lead from nature. Here, we have explored the surface mechanisms, which affords red blood cells long-circulatory lives and the ability of specific microorganisms to evade macrophage recognition. Our analysis is then centered where such strategies have been translated and fabricated to design a wide range of particulate carriers (e.g., nanospheres, liposomes, micelles, oil-in-water emulsions) with prolonged circulation and/or target specificity. With regard to the targeting issues, attention is particularly focused on the importance of physiological barriers and disease states.

Recognition by macrophages and liver cells of opsonized phospholipid vesicles and phospholipid headgroups

The interaction of liposomes with blood proteins is believed to play a critical role in the clearance pharmacokinetics and tissue distribution of intravenously injected liposomes. In this article we have focused our discussion on the interaction of liposomes with key blood proteins, which include immunoglobulins, complement proteins, apolipoproteins, fetuin, von Willebrand factor, and thrombospondin, and their role in liposome recognition by professional phagocytes and nonmacrophage hepatic cells. Alternatively, macrophages as well as hepatocytes and liver endothelial cells may phagocytose/endocytose liposomes via direct recognition of phospholipid headgroups. A number of plasma membrane receptors such as lectin receptors, CD14, various classes of scavenger receptors (e.g., classes A, B, and D), Fc-gammaRI and FcgammaRII-B2 may participate in phospholipid recognition. These concepts are also discussed.

Poloxamers and poloxamines in nanoparticle engineering and experimental medicine

Poloxamers and poloxamine nonionic surfactants have diverse applications in various biomedical fields ranging from drug delivery and medical imaging to management of vascular diseases and disorders. Although this is a progressive, rapidly advancing field in biotechnology, the future will depend on the recognition and rectification of a range of toxicity issues, which have to be addressed but have frequently been ignored until now.

Re-establishing the long circulatory behaviour of poloxamine-coated particles after repeated intravenous administration: applications in cancer drug delivery and imaging

In accordance with earlier observations, a single intravenous injection of long circulatory poloxamine-908-coated polystyrene particles to rats dramatically affected the circulation half-life and body distribution of a second dose when administered 10 days later. Both liver and spleen macrophages recognised and cleared from the blood the majority of the second dose of poloxamine-908-coated particles. The second dose of poloxamine-908-coated particles, however, regained their long circulatory behaviour when administered 1-3 h after a bolus intravenous injection of 30 mg free poloxamine-908 or poloxamer-407 (in saline)/150 g body weight. When the interval between free copolymer and particle administration was increased to 24 h then macrophage-rich organs were able to extract poloxamine-coated beads from the blood. In contrast to poloxamine-908 and poloxamer-407, prior administration of poloxamer-188 and polyethyleneglycol-20000 also failed to restore the long circulatory behaviour of the second dose of poloxamine-908-coated particles. These observations are of interest in experimental drug delivery, particularly in experimental cancer therapy (diagnostic imaging and drug delivery), involving multiple injections of poloxamine-based long circulating nanosized vehicles.

Subcutaneous and intravenous delivery of diagnostic agents to the lymphatic system: applications in lymphoscintigraphy and indirect lymphography

Lymph node status is important in the staging of many malignancies. Although tissue characterization by histologic analysis of biopsy samples may improve staging, noninvasive staging is more acceptable to both patients and clinicians. Several imaging techniques may serve this goal. Modern noninvasive techniques such as computed tomography and magnetic resonance detect lymph node abnormality by nodal enlargement, but that does not always imply malignant involvement. On the other hand, many nodes are infiltrated or replaced by tumour without change in size. This becomes a serious diagnostic defect by these modalities. Consequently, attention has been focused to develop contrast agents and radiolabelled complexes for better cancer detection as well as characterization of individual tumours in lymph nodes. For delivery of such materials to regional lymph nodes one can take advantage from the distinct physiological function of the lymphatic capillaries. The thin-walled and fenestrated lymphatic microvessel is easily penetrated by particulate and macromolecular agents after injection into the extracellular space. Once inside the vessel, materials that are transported with the lymph either specifically target certain nodal elements (e.g. neoplastic cells) or become cleared by macrophages located in the lymph nodes. Indeed, interstitial delivery of diagnostic agents have been of benefit in determining regional spread of cancer and assessing lymphatic function either by lymphoscintigraphy or indirect lymphography. On the other hand, development of contrast materials that can reach lymph nodes after a single intravenous injection is highly desirable because of the large number of lymph nodes in the body and access being difficult to most of them. Today, a number of contrast agents exist that can reach a vast array of lymph nodes in the body, particularly those that are not readily accessible for histologic evaluation, after a single intravascular injection to help distinguish between normal and tumour-bearing nodes or reactive and metastatic nodes with magnetic resonance. In this article we critically examine advantages and limitations of both subcutaneous and intravenous routes of injection for the delivery of diagnostic agents to the lymphatic system.

Activation of the mononuclear phagocyte system by poloxamine 908: its implications for targeted drug delivery

PURPOSE

To investigate the effect of poloxamine 908 on the MPS activity and the importance of its mode of presentation to the immune system.

METHODS

Solutions of endotoxin free poloxamine 908 were injected daily intravenously to rats, and the effect on the degree of sequestration by the liver of I125 labelled, poloxamine 908-coated 60 nm polystyrene particles was investigated by studying effect of dosing regimen(s) and assessment of opsonic activity.

RESULTS

After 3 or 4 days repeated dosing with poloxamine 908 (0.7 mg) in solution, the poloxamine 908-coated polystyrene particles (60 nm) were rapidly cleared from the circulation. The increase sequestration of the particles by the liver lasted for more than 7 days after last dosing with the poloxamine 908 solution. In subsequent studies, it was found that a single dose of poloxamine 908 (0.7 mg) in solution was sufficient to activate the MPS 4 days after the injection. The increased uptake was found not be mediated by a serum component, nor was it due to proliferation of the Kupffer cells in the liver.

CONCLUSIONS

The results provide evidence that a solution of endotoxin-free poloxamine 908 activates the MPS so that 4 days after injection otherwise long-term circulating poloxamine 908-coated particles are sequestered by the liver. This finding has implications for use of such coated systems in therapeutic situations.

A single dose of intravenously injected poloxamine-coated long-circulating particles triggers macrophage clearance of subsequent doses in rats

1. Adsorption of the block copolymer non-ionic surfactant poloxamine-908 on to the surface of polystyrene nanospheres (60 nm in diameter) produced 'phagocyte-resistant' particles (otherwise known as long-circulating particles). This was reflected by a profound reduction in uptake of such engineered nanospheres by macrophages of the reticuloendothelial system and extended blood circulation time, after intravenous administration to rats. 2. A single intravenous administration of poloxamine-908-coated particles dramatically affected the circulation half-life and body distribution of a second subsequent dose. The degree of alteration depended on the interval between the two doses. At 3 days after a single intravenous injection of poloxamine-coated particles, Kupffer cells and spleen macrophages could clear a second dose of long-circulating beads from the blood. When tested at day 14, the second dose of intravenously injected poloxamine-coated particles avoided rapid uptake by liver and spleen macrophages and remained in the blood. 3. The coating polymer (poloxamine-908) apparently triggered bead clearance by resident Kupffer cells and certain sub-populations of spleen macrophages, since a single intravenous dose of an endotoxin-free solution of poloxamine 3 days before the administration of long-circulating particles induced similar effects. When the interval between the two injections was 2 weeks, poloxamine-coated particles again exhibited long circulation half-life. This cycle could be repeated after intravenous administration of a second poloxamine dose 2 weeks after the first poloxamine injection. 4. The mechanism of particle recognition by resident tissue macrophages was found to be independent of opsonization processes. 5. These studies could have important implications in biomedical application, design and engineering of poloxamine-based long-circulating drug carriers for repeated intravenous administration.

Prolonging the circulation time and modifying the body distribution of intravenously injected polystyrene nanospheres by prior intravenous administration of poloxamine-908. A 'hepatic-blockade' event or manipulation of nanosphere surface in vivo?

Intravenously injected uncoated small (60 nm) and large (250 nm) size model polystyrene particles (which are cleared rapidly from the blood by macrophages of the reticuloendothelial system) can be converted to long-circulatory and splenotropic particles in vivo, respectively, if such particles are injected shortly (up to 3 h) after an appropriate dose of the block polymer non-ionic surfactant, poloxamine-908. Evidence indicates that small and large size polystyrene beads can acquire a coating of poloxamine-908 and/or poloxamine-protein complexes in vivo. The adsorption of such complexes on to the bead surface could explain their altered body distribution since small and large size polystyrene beads that were precoated with poloxamine-908 exhibit similar biodistribution following i.v. injection.

Altered tissue-specific opsonic activities and opsono-recognition of liposomes in tumour-bearing rats

Reticuloendothelial phagocytic and serum opsonic activity was evaluated at terminal stages of tumour growth in rats transplanted subcutaneously with chondrosarcoma in an attempt to evaluate the role of opsonic protein(s) in governing liposome recognition and clearance by the macrophage system. The liver of the tumour-bearing animals manifested a decline in the uptake of multilamellar vesicles composed of egg phosphatidylcholine: cholesterol: dicetyl phosphate (mole ratio 7:2:1) from the blood when compared to healthy animals. In contrast, an increase in splenic clearance of liposomes was encountered in tumour-bearing rats. Studies with isolated liver non-parenchymal cells suggested that liposome recognition in both health and at terminal stages of cancer growth is influenced by a serum opsonin, which can be precipitated by 35-50% ammonium sulphate, as well as the concentration of calcium levels in serum. Serum of healthy animals equally enhanced liposome recognition by the hepatic macrophages of both normal and tumour-bearing rats. In contrast, both cell populations manifested poor liposome recognition in the presence of serum pooled from tumour-bearing animals and the results were comparable to the corresponding liposome-cell interaction in the absence of serum. The opsonic activity of serum derived from tumour-bearing rats could be demonstrated either by prior dialysis of serum against de-ionized water or by addition of EGTA. Liver phagocytes of healthy animals recognized more liposome in the presence of dialysed or EGTA-chelated tumour-serum than that of liver cells derived from tumour transplanted rats. A significant increase in serum calcium concentration was found in all tumour-bearing rats. When the concentration of calcium in the serum of normal animals was increased to the level that is encountered in tumour-bearing rats, a sharp drop in liposome recognition by liver phagocytes was observed. This drop in opsonic activity was not related to changes in the ionic strength of serum. The ammonium sulphate precipitated opsonin was also calcium-sensitive and its opsonic activity was abolished in the presence of calcium. Studies with isolated splenic phagocytes suggested that an increase in the opsonic activity of serum, but not the elevated calcium level, was responsible for hyperphagocytosis of liposomes by the splenic phagocytes of tumour-transplanted animals. The opsonic molecule which enhanced liposome recognition by liver non-parenchymal cells failed to enhance liposome clearance by the splenic phagocytes. These findings suggest that the alteration in macrophage clearance of liposomes during the terminal growth of cancer may be mediated in part by changes in the opsonic capacity of serum.

Surface engineered nanospheres with enhanced drainage into lymphatics and uptake by macrophages of the regional lymph nodes

The concept of steric stabilization as used in colloid science is applied to carefully manipulate the drainage and lymphatic distribution of subcutaneously administered model polystyrene nanospheres. A wide range of synthetic polyoxyethylene (POE)/polyoxypropylene (POP) block co-polymers of poloxamine and poloxamer series have been used to produce sterically stabilized nanospheres. We have found a correlation between the length of the stabilizing POE chains of the block co-polymers and nanosphere drainage and passageway across tissue lymph interface in dermal lymphatic capillaries in the rat footpads; the longer the POE chains, the faster the particle drainage. Nanospheres conditioned with block co-polymers of POE chains of 5-15 ethylene oxide units are effectively opsonized in lymphatics; a process which dramatically enhances sequestration (up to 40% of the administered dose) by macrophages of the regional lymph nodes. If the dimensions of the stabilizing POE chains of the poloxamines and poloxamers exceed the range of the Van der Waals force of attraction, opsonization fails to occur and rapidly drained engineered vehicles escape clearance by macrophages of the regional nodes, reach the systemic circulation and remain in the blood for prolonged periods. These observations suggest that a lymphatic delivery composition based on polymer-coated particles will be advantageous for many applications in clinical and experimental medicine.

Enhanced hepatic clearance of intravenously administered sterically stabilized microspheres in zymosan-stimulated rats

The blood clearance and organ deposition of sterically stabilized (poloxamine-908 coated) polystyrene microspheres of two different sizes (60 and 220 nm in diameter) were compared in control and zymosan-stimulated rats 3 h after intravenous administration. Poloxamine coating dramatically decreased the uptake of 60-nm microspheres by organs of the reticuloendothelial system and, concomitantly, kept microspheres in the blood. Large poloxamine-coated microspheres (220 nm) initially remained in the blood, but eventually a large fraction of these microspheres was filtered by the spleen. Daily administration of zymosan produced a marked increase in the intravascular clearance of the large, but not the small, poloxamine-coated microspheres. The enhanced intravascular clearance of large poloxamine-coated microspheres in zymosan-treated rats was the result of hepatic sequestration. On the other hand, the splenic filtration of these microspheres was depressed by 225% below the control values, despite the dramatic increase in spleen size of zymosan-treated rats. Preincubation of large poloxamine-coated microspheres in serum derived from both the control and zymosan-treated animals suggested that the enhanced hepatic uptake of large sterically stabilized microspheres following zymosan stimulation was not the result of "specific opsonization" processes. Instead, the changes in the proliferative as well as the phagocytic response of Kupffer cells appeared to be responsible for these observations. The preferred hepatic uptake of large poloxamine-coated microspheres, as opposed to smaller particles, is suggested to be due to differences in surface characteristics and the properties of microspheres. These may include differences in polymer density and the surface conformation of the polyoxyethylene segments of the polymer in the biological environment and the way they interact with both plasma components and the macrophage surface. These observations could be of importance in the use of sterically stabilized drug carriers for delivery of therapeutic agents to sites other than the reticuloendothelial system in clinical conditions associated with globally or regionally enhanced reticuloendothelial activity.

Coating particles with a block co-polymer (poloxamine-908) suppresses opsonization but permits the activity of dysopsonins in the serum

The surfaces of polystyrene microspheres (60 nm in diameter) and colloidal gold particles (17 nm in diameter) were coated with a polyoxyethylene (POE)/polyoxypropylene (POP) block co-polymer; poloxamine-908. The polymer adsorb strongly to the microspheres via its relatively hydrophobic POP segments. This leaves the POE chains in a mobile state as they extend outward from the surface and thereby provide stability to the particle suspension by suppressing aggregation. The blood clearance and biodistribution of uncoated vs. poloxamine-908-coated 125I-labelled polystyrene microspheres were compared 1 h after intravenous administration into rats. Poloxamine coating dramatically reduced liver accumulation of microspheres and kept them within the systemic circulation. These observations were further confirmed by electron microscopy, demonstrating that Kupffer cells were loaded with uncoated latex but had ingested few if any of the poloxamine-908-coated particles. The interaction of uncoated and poloxamine-coated gold particles with freshly isolated rat liver sinusoidal cells was examined by electron microscopy. The accumulation in Kupffer cells of gold particles after opsonization with autologous plasma was in accordance with previous observations where the dominant opsonizing activity had been identified as fibronectin. In contrast, coating of gold particles with poloxamine-908 prior to plasma opsonization prevented the adsorption of fibronectin onto their surface. Simultaneously, Kupffer cells failed to recognize poloxamine-908-coated gold particles before and after opsonization. Unlike Kupffer cells, liver endothelial cells endocytosed poloxamine-908-coated gold particles prior to opsonization but failed to recognize them after the opsonization process. This was taken as an indication of the presence of dysopsonic activity in plasma. This dysopsonic activity was studied using polystyrene latex microspheres, where the uptake of such particles by phagocytes is known to be independent of opsonization. The coating of 125I-labelled polystyrene microspheres with poloxamine-908 dramatically reduced their interaction with liver sinusoidal cells. This interaction was further reduced in the presence of either autologous plasma or serum. A heat-stable (60 degrees C for 15 min) serum component of molecular mass > 100 kDa was found to mediate this suppressive effect. Thus, we demonstrate that organ-specific receptors, opsonin activities and plasma dysopsonins regulate the in vivo clearance of particulate materials from the circulation. Poloxamine-908 coating modulates particle clearance by effectively blocking opsonization but still allowing for dysopsonization.

An investigation of the filtration capacity and the fate of large filtered sterically-stabilized microspheres in rat spleen

Earlier we demonstrated that coating the surface of large model polystyrene microspheres (220-300 nm in diameter) with the block co-polymer polyoxyethylene/polyoxypropylene poloxamine-908 triggered their accumulation in the rat spleen by a filtration mechanism following intravenous administration [Moghimi, S.M., Porter, C.J.H., Muir, I.S., Illum, L. and Davis, S.S. (1991) Biochem. Biophys. Res. Commun. 177, 861-866]. We have now demonstrated that the macrophages of the red-pulp can effectively phagocytose the filtered poloxamine-coated microspheres 24 h post-administration. This could be the result of either the loss of the surface absorbed poloxamine, and hence the steric barrier, or 'neutralization' of the effect of the anti-phagocytic material poloxamine-908 within the spleen. In order to assess the capacity of the splenic uptake mechanism(s), rats received daily intravenous administration of unlabelled large poloxamine-908 coated microspheres (220 nm in diameter) for 4 days (daily-dosed animals). Control rats received daily saline injection. On the fifth day all animals were injected with either radiolabelled large (220 nm) or small (60 nm in diameter) poloxamine-coated polystyrene microspheres. Predosing dramatically decreased the splenic uptake of the large test microspheres but had no effect on the uptake of small test-microspheres. The failure of the spleen to take up particles was not associated with an increased circulatory level of microspheres. Surprisingly, both small and large coated microspheres were sequestered by the liver and accumulated in Kupffer cells as demonstrated by electron microscopy in daily-dosed animals. In contrast, the liver of control animals did not effectively sequester poloxamine-coated microspheres. Here, microspheres predominantly remained in blood. Sequestration of poloxamine-908 coated microspheres by Kupffer cells of the liver of daily-dosed animals was the result of opsonization by an unknown serum component.

Effect of splenic congestion associated with haemolytic anaemia on filtration of 'spleen-homing' microspheres

1. The surface of large model 125I-labelled polystyrene microspheres (220 nm in diameter) was coated with the polyoxyethylene/polyoxypropylene block co-polymer poloxamine-908. The coated microspheres were injected intravenously into rats. Up to 40% of the administered dose had accumulated in the spleen by a filtration mechanism, as compared with 5% for uncoated microspheres, at 3 h after administration. The enhanced splenic sequestration of microspheres was accompanied by a decrease in the liver uptake. In contrast, smaller poloxamine-908-coated microspheres (60 nm in diameter) effectively avoided uptake by both the liver and the spleen and remained in systemic circulation. 2. The effect of splenic congestion, associated with phenylhydrazine-induced haemolytic anaemia, on filtration of poloxamine-908-coated microspheres was studied. The enlarged spleen of the anaemic rats was incapable of efficiently filtering large poloxamine-908-coated microspheres when compared with the spleen of normal animals. This was suggested to be the result of extensive 'clogging' of the red pulp by damaged erythrocytes. However, the splenic filtration of large coated microspheres was still five-fold higher than that of uncoated microspheres of similar size in anaemic animals. 3. The potential use of large sterically stabilized colloids and drug carriers for selective spleen scanning and splenic drug delivery in pathological conditions, where anaemia prevails, is discussed. 4. Sterically stabilized microspheres may have potential for re-examining the microcirculatory pathways in healthy spleen and various splenomegalies.

Current progress and future prospects of liposomes in dermal drug delivery

Liposomes were introduced first in 1980 for topical drug delivery and since then have attracted considerable interest and generated many speculative claims concerning their potential utility both as a drug carrier and reservoir for controlled release of drugs within various layers of the skin. A number of clinical studies have now demonstrated the superiority of liposomal drug formulations over conventional delivery systems. In this respect, liposomal formulations have been successful in treatment of a number of dermatological diseases and disorders such as psoriasis, mycoses, idiopathic hirsutism and cutaneous infections. This review emphasizes the evaluation of topically applied liposomal formulation both at experimental and clinical levels. Mechanism(s) by which liposomes facilitate deposition of drugs in various layers of the skin is also discussed.

Opsonophagocytosis of liposomes by peritoneal macrophages and bone marrow reticuloendothelial cells

We investigated the opsonic effect of serum on the phagocytosis of cholesterol-poor and cholesterol-rich liposomes with freshly prepared rat bone marrow and peritoneal phagocytes in suspension. The presence of serum in incubation increased the uptake of cholesterol-rich but not cholesterol-poor liposomes in bone marrow cells, whereas serum enhances phagocytosis of both liposome preparations in peritoneal phagocytes. The opsonic activity of serum on the uptake of liposomes by both cell types is destroyed by heat, ammonium hydroxide, and zymosan treatments of serum, which suggests the involvement of complement in phagocytosis of liposomes. However, bone marrow cells and peritoneal macrophages responded differently when incubated with dialysed or EGTA-chelated serum. These treatments result in complete loss of the opsonic effect of serum on the uptake of liposome in bone marrow cells. Addition of serum dialysate or divalent cations to dialysed serum do not reinstall its lost opsonic activity. Thus these results shade doubt on possibility of involvement of complements in phagocytosis of liposomes by bone marrow cells. Dialysed or EGTA-treated serum also failed to promote liposome uptake by peritoneal macrophages but unlike in the case of bone marrow cells replacement of divalent cations to such sera reinstalls its opsonic activity and enhances phagocytosis of liposomes by peritoneal macrophages. These observations suggest the involvement of different serum components on liposome uptake by bone marrow cells and peritoneal macrophages, and emphasize the complexities associated with RES-mediated blood clearance of the intravenously injected liposomes. The properties of these opsonins are compared with those of 'liver-' and 'spleen-specific' opsonins responsible for phagocytosis of liposomes by Kupffer cells and spleen macrophages, respectively [4]. It is suggested that these tissue specific opsonins are responsible for distribution of intravenously injected liposomes in various organs of the reticuloendothelial system.

The polyoxyethylene/polyoxypropylene block co-polymer poloxamer-407 selectively redirects intravenously injected microspheres to sinusoidal endothelial cells of rabbit bone marrow

Small colloidal particulates (150 nm and below, in diameter) can be redirected specifically to the rabbit bone marrow following intravenous administration by coating their surface with the block co-polymer poloxamer-407, a non-ionic surfactant. The coated colloids are sequestered by the sinusoidal endothelial cells of the bone marrow and are accumulated in dense bodies within these cells. The uptake of poloxamer-407-coated colloids by marrow endothelial cells suggests that the steric repulsive barrier, imposed by the polyoxyethylene segment of the polymer, to particle-cell interaction can apparently be overcome by a specific interaction mechanism(s) with the cell surface. Such a dramatic uptake cannot be achieved with other block co-polymers of similar structure to poloxamer-407. The application of the current model for the site-specific targeting of drug carriers to bone marrow and the prevention of the adherence of metastases of tumours which selectively colonize the bone marrow endothelium is discussed.

Non-phagocytic uptake of intravenously injected microspheres in rat spleen: influence of particle size and hydrophilic coating

A recent development in prolonging the circulation time of drug carriers, such as liposomes and microspheres, has been to minimize their removal by macrophages of the reticuloendothelial system by covering their surface with hydrophilic polymers such as poloxamers, poloxamines and poly(ethyleneglycols). Here we demonstrate that this strategy may not necessarily prolong the circulatory half-life of drug carriers in all animal models. In rats, as opposed to rabbits, a non-phagocytic mechanism in the spleen may be triggered to remove efficiently from the blood drug carriers coated with hydrophilic coatings. Both the size of particle and its hydrophilic coating may act synergistically to trigger this non-phagocytic mechanism. In rats, a remarkable log to log relationship between particle size and spleen uptake was observed for both uncoated and polymeric coated microspheres. The potential implication of these observations in site-specific delivery of drug carriers is discussed.

Calcium as a possible modulator of Kupffer cell phagocytic function by regulating liver-specific opsonic activity

Recently we described some properties of organ-specific serum opsonins which differentiate between liver- and spleen-specific opsonic activities, and reported that, on dialysis of serum, its liver opsonic activity is enhanced by 2- to 3-fold, whereas spleen-specific activity is reduced by 20-30% of that of control serum (Moghimi, S.M. and Patel, H.M. (1989) Biochim. Biophys. Acta 984, 379-383). This observation suggests that serum contains dialysable factors which regulate liver- as well as spleen-specific opsonic activities. Our results from EGTA-treated serum suggest that dialysable factor(s) could be divalent cations such as Ca2+, Mn2+, Mg2+ or Co2+, and among them, calcium may be the key regulatory factor for liver-specific opsonic activity. The regulatory mechanism of spleen-specific opsonic activity seems to be complex, since addition of dialysate or calcium or magnesium to the dialysed serum does not restore its activity; probably the removal of divalent cations has induced an irreversible conformational change in spleen-specific opsonin. In conclusion, we propose that the blood calcium concentration may play an important role in modulating hepatic phagocytic function by modifying liver-specific opsonic activity in serum. An increase in the physiological concentration of calcium will suppress and a decrease will enhance this opsonic activity.

Serum opsonins and phagocytosis of saturated and unsaturated phospholipid liposomes

Recently we reported that serum contains opsonins specific for hepatic and splenic phagocytic cells and that these opsonins have different properties and affinities for cholesterol-rich and cholesterol-free egg phosphatidylcholine liposomes (Moghimi, S.M. and Patel, H.M. (1988) FEBS Lett. 233, 143-147). In the present report we investigate the affinity of these opsonins for the liposomes prepared from sphingomyelin and saturated phospholipids, as measured by their effect on the uptake of these liposomes by hepatic and splenic phagocytic cells. Results presented here suggest that neither liver- nor spleen-specific opsonins have affinity for sphingomyelin or saturated phospholipid liposomes since serum fails to enhance their uptake in liver or splenic cells. On the contrary, these liposomes attract serum dysopsonins which inhibit their uptake by liver cells. Inclusion of cholesterol in these liposome preparations enhances their uptake in splenic cells but not in liver cells. It is suggested that fluidity and hydrophobicity of liposomal membranes play an important role in attracting the right opsonins which determine their phagocytic fate.