Inhibition of liposome-induced complement activation by incorporated poly(ethylene glycol)-lipids

EFFECTS OF POLY(ETHYLENEGLYCOL)-MODIFIED HEMOGLOBIN VESICLES ON AGONIST-INDUCED PLATELET AGGREGATION AND RANTES RELEASE IN VITRO

We studied the effects of hemoglobin-vesicles modified with PEG (PEG-HbV), a type of liposome-encapsulated hemoglobin (LEH), on human platelet functions in vitro. The effect of a low concentration of PEG-HbV (Hb; 5.8 mg/dl) was assessed by examining an agonist-induced aggregation response, and that of relatively high concentrations of PEG-HbV (Hb; 0.29, 1 and 2 g/dl) by measuring the release of RANTES (Regulated upon activation, normal T-cell expressed and presumably secreted) from platelets, which is regarded as a marker of platelet activation. The preincubation of platelets with PEG-HbV at 5.8 mg/dl of Hb did not affect platelet aggregation induced by collagen, thrombin and ristocetin. The pretreatment of platelet-rich plasma (PRP) with PEG-HbV at concen trations up to 2 g/dl of Hb had no aberrant effects on the collagen-induced RANTES release. Furthermore, the collagen-induced release of RANTES from PRP was not affected by longer incubation with PEG-HbV at 2 g/dl of Hb. The basal levels of RANTES from PRP were unchanged in the presence of PEG-HbV. These results suggest that PEG-HbV, at the concentrations studied, have no aberrant effects on platelet functions in the presence of plasma.

The encapsulation of DNA molecules within biomimetic lipid nanocapsules

Most of DNA synthetic complexes result from the self-assembly of DNA molecules with cationic lipids or polymers in an aqueous controlled medium. However, injection of such self-assembled complexes in medium like blood that differ from that of their formulation leads to strong instability. Therefore, DNA vectors that have physico-chemical properties and structural organisation that will not be sensitive to a completely different medium in terms of ionic and protein composition are actively sought. To this end, the goal here was to discover and optimize a nanostructured system where DNA molecules would be encapsulated in nanocapsules consisting in an oily core and a shell covered by PEG stretches obtained through a nanoemulsion process in the absence of organic solvent. This encapsulation form of DNA molecules would prevent interactions with external hostile biological fluid. The results show the entrapment of lipoplexes into lipid nanocapsules, leading to the formation of neutral 110 nm-DNA nanocapsules. They were weakly removed by the immune system, displaying an increased blood half-life, and improved carcinoma cell transfection, in comparison to the parent lipoplexes. Our results demonstrate that the fabrication of nanocapsules encapsulating hydrophilic DNA in an oily core that meet criteria for blood injection is possible.

Liposome Opsonization

Adsorption of serum proteins to the liposomal surface plays a critical role in the clearance of liposomes from the blood circulation. In this review, we will discuss the role of the liposomal opsonins proposed so far in liposome clearance. Additional, related topics that will be addressed are the cell-surface receptors that might be involved in liposome elimination from the blood compartment and the effect of poly(ethylene glycol) (PEG) modification on prevention of liposome opsonization.

Physicochemical characterization and study of in vitro interactions of pH-sensitive liposomes with the complement system

Complement activation is an important step in the acceleration of liposome clearance. The anaphylatoxins released following complement activation may motivate a wide variety of physiologic changes. We performed physicochemical characterization and in vitro studies of the interaction of complement system with both noncirculating and long-circulating pH-sensitive and nonpH-sensitive liposomes. The liposomes were characterized by diameter, zeta potential, and atomic force microscopy (AFM). The study of liposome interactions with complement system was conducted using hemolytic assay in rat serum. All liposomes presented a similar mean diameter (between 99.8 and 124.3 nm). The zeta potential was negative in all liposome preparations, except in liposomes modified with aminopoly (ethyleneglycol) 2000-distearoylphosphatidylethanolamine (aPEG(2000)-DSPE), which presented positive zeta potential. Atomic force microscopy images showed that non-long-circulating pH-sensitive liposomes are prone to vesicles aggregation. Non-pH-sensitive liposomes complement system activates, while pH-sensitive liposomes showed to be poor complement activators in rat serum.

[Accelerated blood clearance (ABC) phenomenon induced by administration of PEGylated liposome]

PEGylated liposomes (approximately 100 nm in diameter) lose their long-circulating characteristic upon repeated injection at certain intervals in the same animal (referred to as the "accelerated blood clearance (ABC) phenomenon"), as described by our group and by researchers in the Netherlands. Recently, it was demonstrated by our group that anti-PEG IgM, induced by the first dose of PEGylated liposomes, is responsible for the ABC phenomenon. The IgM produced in this manner then selectively bound to the surface of subsequently injected PEGylated liposomes, leading to substantial complement activation. It is generally believed that nanocarriers coated with a polymer, such as PEG, have no immunogenicity. However, unexpected immune responses occurred even in response to polymer-coated liposomes. This immunogenicity to PEGylated liposomes presents a serious concern in the development and clinical use of liposomal formulations. In this review, we demonstrate our recent observations regarding with the ABC phenomenon against liposomes.

Targeting of stabilized plasmid lipid particles to hepatocytes in vivo by means of coupled lactoferrin

For non-viral gene delivery we prepared stabilized plasmid lipid particles (SPLPs), to which lactoferrin (LF) was coupled as a hepatocyte specific targeting ligand. LF-SPLPs and untargeted SPLPs labeled with [3H]cholesteryloleyl-ether were injected into rats. About 87% of the LF-SPLPs were eliminated from the blood within 5 min, while 80% of untargeted SPLPs were still circulating after 2 h. Fifty-two percent of the LF-SPLPs were taken up by hepatocytes, while non-parenchymal liver cells accounted for 16% of the uptake. Despite the efficient targeting of LF-SPLPs to hepatocytes and their capacity to transfect HepG2 and COS-7 cells in vitro, expression of a reporter gene was not detected in vivo. Overall, covalent coupling of LF to SPLPs leads to massive delivery in hepatocytes after systemic administration. However, these LF-SPLPs are not able to transfect these cells in vivo.

Cholesterol as a bilayer anchor for PEGylation and targeting ligand in folate-receptor-targeted liposomes

Phospholipids have been extensively evaluated as an anchor for both PEGylation and receptor-targeting in liposomal formulations. However, cholesterol, another important component in biomembranes, has not been fully investigated as an alternative anchor. In this study, the potential role of cholesterol for anchoring PEG and folate was investigated. Cholesterol derivatives were synthesized for PEGylation (mPEG-cholesterol) and folate receptor (FR) targeting (folate-PEG-cholesterol) and incorporated into the bilayer of FR-targeted liposomal doxorubicin. The colloidal stability of these cholesterol derivative-containing liposomes was superior to non-PEGylated liposomes, indicating that steric barrier provided by mPEG-cholesterol can efficiently inhibit aggregation of liposomes. FR-targeting activity of these liposomes was demonstrated by in vitro cell-binding studies on FR-overexpressing KB cells. In addition, in vivo circulation of cholesterol-anchored liposomes was prolonged compared to non-PEGylated liposomes. These studies suggest that cholesterol is a viable bilayer anchor for synthesis of PEGylated and FR-targeted liposomes.

Effect of liposome characteristics and dose on the pharmacokinetics of liposomes coated with poly(amino acid)s

Long-circulating liposomes, such as PEG-liposomes, are frequently studied for drug delivery and diagnostic purposes. In our group, poly(amino acid) (PAA)-based coatings for long-circulating liposomes have been developed. These coatings provide liposomes with similar circulation times as compared to PEG-liposomes, but have the advantage of being enzymatically degradable. For PEG-liposomes it has been reported that circulation times are relatively independent of their physicochemical characteristics. In this study, the influence of factors such as PAA grafting density, cholesterol inclusion, surface charge, particle size, and lipid dose on the circulation kinetics of PAA-liposomes was evaluated after intravenous administration in rats. Prolonged circulation kinetics of PAA-liposomes can be maintained upon variation of liposome characteristics and the lipid dose given. However, the use of relatively high amounts of strongly charge-inducing lipids and a too large mean size is to be avoided. In conclusion, PAA-liposomes represent a versatile drug carrier system for a wide variety of applications.

Emerging Applications of Polymersomes in Delivery: from Molecular Dynamics to Shrinkage of Tumors

Polymersomes are self-assembled shells of amphiphilic block copolymers that are currently being developed by many groups for fundamental insights into the nature of self-assembled states as well as for a variety of potential applications. While recent reviews have highlighted distinctive properties - particularly stability - that are strongly influenced by both copolymer type and polymer molecular weight, here we first review some of the more recent developments in computational molecular dynamics (MD) schemes that lend insight into assembly. We then review polymersome loading, in vivo stealthiness, degradation-based disassembly for controlled release, and even tumor-shrinkage in vivo. Comparisons of polymersomes with viral capsids are shown to encompass and inspire many aspects of current designs.

Effect of Headgroup on DNA−Cationic Surfactant Interactions

The interaction behavior of DNA with different types of hydroxylated cationic surfactants has been studied. Attention was directed to how the introduction of hydroxyl substituents at the headgroup of the cationic surfactants affects the compaction of DNA. The DNA-cationic surfactant interaction was investigated at different charge ratios by several methods like UV melting, ethidium bromide exclusion, and gel electrophoresis. Studies show that there is a discrete transition in the DNA chain from extended coils (free chain) to a compact form and that this transition does not depend substantially on the architecture of the headgroup. However, the accessibility of DNA to ethidium bromide is preserved to a significantly larger extent for the more hydrophilic surfactants. This was discussed in terms of surfactant packing. Observations are interpreted to reflect that the surfactants with more substituents have a larger headgroup and therefore form smaller micellar aggregates; these higher curvature aggregates lead to a less efficient, "patch-like" coverage of DNA. The more hydrophilic surfactants also presented a significantly lower cytotoxicity, which is important for biotechnological applications.

Liposomal drug delivery

Liposomes are composed of phospholipids, the basic components of human cell walls. Liposome encapsulation improves a medication's bioavailability, which can extend treatment effects and reduce drug dosing. The therapeutic advantages of liposomal drug delivery, such as the ability of long-circulating liposomes to accumulate preferentially at disease sites, including tumors and sites of inflammation, are well recognized. In cases in which a single active has more than one liposome product available, formulation changes leading to differences in pharmacokinetics, toxicity, and clinical efficacy are described.

Methylation of the phosphate oxygen moiety of phospholipid‐methoxy(polyethylene glycol) conjugate prevents PEGylated liposome‐mediated complement activation and anaphylatoxin production

Methoxy(polyethylene glycol), mPEG, -grafted liposomes are known to exhibit prolonged circulation time in the blood, but their infusion into a substantial percentage of human subjects triggers immediate non-IgE-mediated hypersensitivity reactions. These reactions are strongly believed to arise from anaphylatoxin production through complement activation. Despite the general view that vesicle surface camouflaging with mPEG should dramatically suppress complement activation, here we show that bilayer enrichment of noncomplement activating liposomes [dipalmitoylphosphatidylcholine (DPPC) vesicles] with phospholipid-mPEG conjugate induces complement activation resulting in vesicle recognition by macrophage complement receptors. The extent of vesicle uptake, however, is dependent on surface mPEG density. We have delineated the likely structural features of phospholipid-mPEG conjugate responsible for PEGylated liposome-induced complement activation in normal as well as C1q-deficient human sera, using DPPC vesicles bearing the classical as well as newly synthesized lipid-mPEG conjugates. With PEGylated DPPC vesicles, the net anionic charge on the phosphate moiety of phospholipid-mPEG conjugate played a key role in activation of both classical and alternative pathways of complement and anaphylatoxin production (reflected in significant rises in SC5b-9, C4d, and C3a-desarg levels in normal human sera as well as SC5b-9 in EGTA-chelated/Mg2+ supplemented serum), since methylation of the phosphate oxygen of phospholipid-mPEG conjugate, and hence the removal of the negative charge, totally prevented complement activation. To further corroborate on the role of the negative charge in complement activation, vesicles bearing anionic phospholipid-mPEG conjugates, but not the methylated phospholipid-mPEG, were shown to significantly decrease serum hemolytic activity and increase plasma thromboxane B2 levels in rats. In contrast to liposomes, phospholipid-mPEG micelles had no effect on complement activation, thus suggesting a possible role for vesicular zwitterionic phospholipid head-groups as an additional factor contributing to PEGylated liposome-mediated complement activation. Our findings provide a rational conceptual basis for development of safer vesicles for site-specific drug delivery and controlled release at pathological sites.

Polymersomes

Polymersomes are self-assembled polymer shells composed of block copolymer amphiphiles. These synthetic amphiphiles have amphiphilicity similar to lipids, but they have much larger molecular weights, so for this reason--along with others reviewed here--comparisons of polymersomes with viral capsids composed of large polypeptide chains are highly appropriate. We summarize the wide range of polymers used to make polymersomes along with descriptions of physical properties such as stability and permeability. We also elaborate on emerging studies of in vivo stealthiness, programmed disassembly for controlled release, targeting in vitro, and tumor-shrinkage in vivo. Comparisons of polymersomes with viral capsids are shown to encompass and inspire many aspects of current designs.

Parameters influencing the stealthiness of colloidal drug delivery systems

Over the last few decades, colloidal drug delivery systems (CDDS) such as nano-structures have been developed in order to improve the efficiency and the specificity of drug action. Their small size permits them to be injected intravenously in order to reach target tissues. However, it is known that they can be rapidly removed from blood circulation by the immune system. CDDS are removed via the complement system and via the cells of the mononuclear phagocyte system (MPS), after their recognition by opsonins and/or receptors present at the cell surface. This recognition is dependent on the physicochemical characteristics of the CDDS. In this study, we will focus on parameters influencing the interactions of opsonins and the macrophage plasma membrane with the surface of CDDS, whereby parameters of the polymer coating become necessary to provide good protection.

Characterization and cytotoxicity of mixed polyethyleneglycol modified liposomes containing doxorubicin

Liposomes are recognized as one of the useful drug carriers, but have many problems to overcome before their clinical application. Liposomes, bonding peculiarly with serum protein (opsonization), are taken up by reticuloendothelial system (RES) cells in the liver and spleen. It is known that polyethyleneglycol (PEG) modification of the liposome surface induces the formation of a fixed aqueous layer around the liposomes due to the interaction between the PEG-polymer and water molecule, and thus prevents the attraction of opsonins. Namely, PEG-modified liposomes are able to escape trapping by the RES cells, and have a prolonged circulation time. In this study, the effects of different anchors with the same PEG molecular weight on the cell uptake and cytotoxicity of mixed PEG-modified liposomal doxorubicin (DOX) were examined. The fixed aqueous layer thickness (FALT) of liposomes covered with mixtures of PEG-molecules which differ in their chain length were increased, compared to that of the single PEG2000-modified liposome. Mixed PEG-modification of liposomes with different anchors (PEG2000-(1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (DSG): cholesterol (CHO)=1:1)-modified liposome) led to an increase in the FALT, compared to that of each single PEG-modification. The uptake of DOX into Ehrlich ascites carcinoma cells by the liposomes covered with PEG-CHO was higher than the other liposomes. Thus, liposomes covered with PEG-DSG and PEG-CHO have an enhanced cytotoxicity. In conclusion, it was confirmed that mix-modified liposomes using PEG-lipid with different anchors were superior.

Polyethylene Glycol (PEG) Modified 99mTc-HMPAOLiposome for Improving Blood Circulation and Biodistribution: The Effect of the Extent of PEGylation

Modification of liposomes using polyethylene glycol (PEG) results in steric hindrance to the phagocyte system and prolongation of blood circulation time. However, PEGylation can reduce radiolabeling efficiency (RE) when using the glutathione method for radiolabeling the liposomes. Therefore, we investigated the effect of the extent of PEGylation (PEG extent (PEGExt): 0, 5, 9.6, and 13.7 mol%) on the in vivo biodistribution of liposomes in Wistar rats, and RE with technetium-(99m) ((99m)Tc). PEGylated liposomes were prepared with egg phosphatidylcholine (egg PC, 1.85 mol%), cholesterol (1.0 mol%), and distearoylphosphatidylethanolamine-N-[polyethylene glycol] (DSPE-PEG; 0, 5, 9.6, and 13.7 mol%, respectively). The size distribution of the PEGylated liposomes was analyzed by a dynamic light scattering. The (99m)Tc-hexamethylpropylene-amine oxime ((99m)Tc-HMPAO) complexes were used for radiolabeling of preformed liposomes. The labeling efficiency and stability was analyzed with Sephadex G-15 column, and the biodistribution studies of (99m)Tc-liposomes after intravenous (i.v.) injection were also investigated with Wistar rats. The sizes of PEGylated liposomes decreased by increasing the PEGExt to 9.6 mol%, whereas sizes increased at 13.7 mol%. RE of (99m)Tc were greater than 90% for all PEGExt tested, and radiolabeling stability in human plasma was enhanced as a function of PEGExt. Liposomes without PEG were cleared rapidly from the blood and accumulated preferentially in the liver and the spleen. When PEGExt was increased, the accumulation in the organs decreased. This accumulation of PEG was maximized at 9.6 mol%. Accumulation of the liposomes in the spleen was increased again when PEGExt increased to 13.7 mol%. The splenic uptake of liposomes seemed to be dependent not only on PEGExt but also on the size of the liposomes. In conclusion, the PEG chains on the surface of liposome have no influence on the labeling efficiency, and the prolongation of circulation time was maximized at the 9.6 mol% of PEGylation.

Cationic lipid–DNA complexes for non-viral gene therapy: relating supramolecular structures to cellular pathways

Cationic liposomes (CLs) are used as nonviral vectors in worldwide human clinical trials of gene therapy. Among other advantages, lipid-DNA complexes have the ability to transfer very large genes into cells, but their efficiency is much lower than that of viruses. Recent studies combining structural and biological techniques are beginning to unravel the relationship between the distinctly structured CL-DNA complexes and their transfection efficiency. Most CL-DNA complexes form a multilayered structure with DNA sandwiched between the cationic lipids (lamellar complexes, LalphaC). On rare occasions, an inverted hexagonal structure (HIIC) is observed. An important recent insight is that the membrane charge density (sigmaM) of the CL-vector is a universal parameter governing the transfection efficiency of LalphaC (but not HIIC) complexes. This has led to a new model of the cellular uptake of LalphaC complexes through activated fusion with endosomal membranes. Surface-functionalised complexes with poly(ethylene glycol)-lipids, potentially suitable for transfection invivo, have also been investigated, and the novel aspects of these complexes are discussed.

Lipoplex Structures and Their Distinct Cellular Pathways

Cationic liposomes (CLs) are used as non-viral vectors in worldwide clinical trials of gene therapy. Among other advantages, CL-DNA complexes have the ability to transfer very large genes into cells. However, since the understanding of their mechanisms of action is still incomplete, their transfection efficiencies remain low compared to those of viruses. We describe recent studies which have started to unravel the relationship between the distinct structures and physicochemical properties of CL-DNA complexes and their transfection efficiency by combining several techniques: synchrotron X-ray diffraction for structure determination, laser-scanning confocal microscopy to probe the interactions of CL-DNA particles with cells, and luciferase reporter-gene expression assays to measure transfection efficiencies in mammalian cells. Most CL-DNA complexes form a multilayered structure with DNA sandwiched between the cationic lipids (lamellar complexes, L(alpha)(C)). Much more rarely, an inverted hexagonal structure (H(II)(C)) with single DNA strands encapsulated in lipid tubules is observed. An important recent insight is that the membrane charge density sigma(M) of the CL-vector, rather than, for example, the charge of the cationic lipid, is a universal parameter governing the transfection efficiency of L(alpha)(C) complexes. This has led to a new model of the intracellular release of L(alpha)(C) complexes, through activated fusion with endosomal membranes. In contrast to L(alpha)(C) complexes, H(II)(C) complexes exhibit no dependence on sigma(M), since their structure leads to a distinctly different mechanism of cell entry. Surface-functionalized complexes with poly(ethyleneglycol)-lipids (PEG-lipids), potentially suitable for transfection in vivo, have also been investigated, and the novel aspects of these complexes are discussed.

Surface functionalized cationic lipid-DNA complexes for gene delivery: PEGylated lamellar complexes exhibit distinct DNA-DNA interaction regimes

Cationic lipid-DNA (CL-DNA) complexes are abundantly used in nonviral gene therapy clinical applications. Surface functionality is the next step in developing these complexes as competent, target-specific gene carriers. Poly(ethylene glycol) (PEG) is the natural choice to serve as a protective coat or act as a tether for a specific ligand on the surface of these complexes due to its biocompatibility and ability to convey stealth-like properties. Understanding the effect of PEG on the internal structure and surface properties of CL-DNA complexes is essential in developing vectors with more complex derivatives of PEG, such as Arg-Gly-Asp (RGD)-based peptide-PEG-lipids. We report on x-ray diffraction studies to probe the internal structure of CL-DNA complexes consisting of a ternary mixture of cationic lipids, neutral lipids, and PEG-lipids. The PEG-coated complexes are found to exhibit a structure consistent with the lamellar phase. In addition, three distinct DNA interchain interaction regimes were found to exist, due to a), repulsive long-range electrostatic forces; b), short-range repulsive hydration forces; and c), novel polymer-induced depletion attraction forces in two dimensions. Optical microscopy and reporter gene assays further demonstrate the incorporation of the PEG-lipids into the lamellar CL-DNA complexes under biologically relevant conditions, revealing surface modification. Both techniques show that PEG-lipids with a polymer chain of molecular weight 400 do not provide adequate shielding of the PEGylated CL-DNA complexes, whereas PEG-lipids with a polymer chain of molecular weight 2000 confer stealth-like properties. This surface functionalization is a crucial initial step in the development of competent vectors for in vivo systemic gene delivery and suggests that a second type of surface functionality can be added specifically for targeting by the incorporation of peptide-PEG-lipids.

The effect of chain length on protein solubilization in polymer-based vesicles (polymersomes)

Using a mean-field analysis we derive a consistent model for the perturbation of a symmetric polymeric bilayer due to the incorporation of transmembrane proteins, as a function of the polymer molecular weight and the protein dimensions. We find that the mechanism for the inhibition of protein incorporation in polymeric bilayers differs from that of their inclusion in polymer-carrying lipid vesicles; in polymersomes, the equilibrium concentration of transmembrane proteins decreases as a function of the thickness mismatch between the protein and the bilayer core, whereas in liposomes the presence of polymer chains affects the protein adsorption kinetics. Despite the increased stiffness of polymer bilayers (when compared to lipid ones), their perturbation decay length and range of protein-protein interaction is found to be relatively long. The energetic penalty due to protein adsorption increases relatively slowly as a function of the polymer chain length due to the self-assembled nature of the polymer bilayer. As a result, we predict that transmembrane proteins may be incorporated in significant numbers even in bilayers where the thickness mismatch is large.

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.

Polymer vesicles in vivo: correlations with PEG molecular weight

PEG-modified lipid vesicles have already shown considerable utility in delaying vesicle clearance from the circulation. They are, however, limited in their ability to stably integrate high molar ratios of PEG-lipid due to the high curvature and micellar preference of the very large hydrophilic PEG chain. Polymersomes, by contrast, are vesicles composed entirely of PEG-based block copolymer amphiphiles that are not only more proportionately designed, but also have already been shown to considerably broaden the range of vesicle properties (e.g. stability). Here, polymersomes composed of varying length copolymer chains were injected into rats and found to have in vivo circulation times, tau(1/2), up to about two-fold longer than PEGylated, or Stealth, liposomes. The dependence of tau(1/2) on PEG molecular weight is nonetheless limited by uptake into the liver and spleen-as with liposomes. In vitro incubations of polymersomes in plasma indicate gradual opsonization through plasma protein adsorption, such that, when vesicles are held in an optical trap and presented to a phagocyte, rapid engulfment occurs only after incubation times of similar magnitude to tau(1/2). The stealthiness introduced to liposomes through PEGylation is thus extended here with completely synthetic polymersomes.

Folate receptor-targeted liposomes as vectors for therapeutic agents

The folate receptor is a cell surface protein that has recently been identified as a tumor marker, due to its differential overexpression in several malignancies. Current research indicates that folate can be covalently attached to the surface of liposomes to mediate their selective internalization by tumor cells through the folate receptor-mediated endocytic pathway. Optimized liposome formulations, characterized by improvements in drug loading, extended residence times in the circulation and improved drug release, have been developed to improve the biodistribution of therapeutic molecules. Theoretically, folate receptor-targeting can be combined with liposome encapsulation to synergistically affect disease outcome by enhancing the delivery of chemotherapeutic agents to neoplastic cells, while reducing systemic toxicities to normal tissues. The purpose of this chapter is to characterize the components of folate receptor-targeted liposomes, and summarize their applications in gene and drug delivery.

Circulation and biodistribution profiles of long-circulating PEG-liposomes of various sizes in rabbits

To determine the largest size of liposomes that can retain stealth behavior conferred by poly(ethylene glycol)-DSPE, neutral liposomes were studied in rabbits for their circulation and distribution. Five sizes (136.2, 165.5, 209.2, 275 and 318 nm) of liposomes (DSPC, Cholesterol, PEG-DSPE and alpha-tocopherol, 90:80:4.5:3.9 molar ratio) were made by extrusion technique and radiolabeled with technetium-99m (Tc-99m) to follow their distribution through 24 h. Although all liposomes showed prolonged circulation in blood, the amount still in circulation at 24 h was dependent on their size. Radioactivity accumulation in spleen progressively increased with increase in size of the liposomes. In the size range of approximately 160-220 nm, liver uptake was minimum, spleen uptake was moderate while the amount of circulating liposomes was maximum. Gamma camera scintigraphy corroborated the distribution pattern of liposomes on necropsy. Images within 1h showed high blood pool activities for liposomes of all sizes. However, at 24h, the blood pool activity was diminished for 275 nm and negligible for 308 nm liposomes; the smaller sized liposomes (136.2-209.2 nm) continued to show high blood pool activity. The amounts of radioactivity still circulating at 24h were 46.4, 50.4, 46.8, 36.2 and 14.5% for 136.2, 165.5, 209.2, 275 and 318 nm liposomes, respectively. Corresponding circulation T(1/2)s were 21.7, 26.5, 24.9, 18.7 and 8.9h, respectively. Thus, the optimum size of PEG-liposomes for prolonged circulation in rabbits is 160-220 nm. Beyond this range, the stealth property of PEG-liposomes is significantly compromised and the distribution is characterized by high RES accumulation.

Size and structure of spontaneously forming liposomes in lipid/PEG-lipid mixtures

The optimal size and structure of spontaneous liposomes formed from lipid/polymer-lipid mixtures was calculated using a molecular mean-field theory. The equilibrium properties of the aggregate are obtained by expanding the free energy of a symmetric planar bilayer up to fourth order in curvature and composition of lipid and polymer. The expansion coefficients are obtained from a molecular theory that explicitly accounts for the conformational degrees of freedom of the hydrophobic tails of the lipid and of the polymer chains. The polar headgroup interactions are treated using the opposing forces model. The onset of stability of the symmetric planar film is obtained from the expansion up to quadratic order. For unstable planar films the equilibrium size and structure of the spherical aggregates is obtained from the second- and fourth-order terms in curvature and composition of lipid and polymer. The driving force for the formation of spontaneous vesicles is the asymmetric distribution of polymers between the inner and outer monolayer. The composition asymmetry between the two monolayers in the aggregates is much larger for the polymer component than for the lipid, and it depends upon the size of the aggregate. The smaller the aggregate, the more asymmetric the distribution of polymer and lipid. The tendency of the polymer chains to be tethered on the outer surface of the aggregate is very strong, and it limits the range of polymer loading for which spherical liposomes are stable. A very small excess of polymer loading causes small spherical micelles to be the optimal aggregates. In these cases spontaneous liposomes can form as metastable aggregates, showing as a local minima in the free energy. Even for metastable aggregates the asymmetric distribution of polymers is very large. The elastic constants of the asymmetric bilayer in the spherical aggregate are found to be the same as those that are calculated from the planar symmetric film. Therefore, the stable structure of the aggregate is not needed to determine its mechanical properties. The range of stable liposomes is very narrow in the range of molecular weights studied, which include the experimental relevant domain of aggregates used in drug delivery. It is found that the stability of the spherical aggregates results from a very fine balance between the tendency of the polymer chains and lipid tails to pack in an asymmetric spherical aggregate and the tendency of the hydrophobic-water interface to keep the area per molecule fixed. The changes in free energy per molecules that are responsible for liposome formation are very small and are very sensitive to detailed molecular properties. The theoretical description of the aggregates requires a theory capable of incorporating these detailed molecular properties. The findings are discussed in the context of vesicle formation and liposome design for drug delivery.

Prevention of antibody-mediated elimination of ligand-targeted liposomes by using poly(ethylene glycol)-modified lipids

One of the major obstacles in the development of ligand-targeted liposomes is poor liposome circulation longevity as a result of antibody-mediated elimination of these highly immunogenic carriers. Because studies from our laboratory suggest that it is not possible to reduce the immunogenicity of ligand-conjugated liposomes by using surface-grafted poly(ethylene glycol) (PEG), we investigated the usefulness of PEG in protecting hapten-conjugated liposomes from elimination by an existing immune response that was previously established against the hapten. Using biotin as a model hapten, a strong biotin-specific antibody response was generated in mice by using bovine serum albumin-biotin. When these animals were challenged with liposomes containing biotin-conjugated lipid (1 or 0.1%), these liposomes were rapidly eliminated. Incorporation of PEG-lipids into these liposomes substantially reduced biotin-specific antibody binding as measured using an in vitro antibody consumption assay. However, depending on the hapten concentration, significant reductions in antibody binding through the use of PEG-lipids may not be sufficient to protect these liposomes from rapid elimination in vivo. Complete protection of liposomes was only achieved when the biotin concentration on liposome surface was low (0.1%) and with 5 mol% of either 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy(polyethylene glycol)-2000] or 1,2-dipalmatoyl-sn-glycero-3-phosphoethanolamine-n-methoxy(polyethylene glycol)-2000]. The use of 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy(polyethylene glycol)-2000] (up to 15 mol%) was not effective in protecting liposomes from rapid elimination in vivo, indicating the limited usefulness of this highly exchangeable PEG-lipid. In conclusion, our in vivo and in vitro data indicate that liposomes can be protected from antibody-mediated elimination by using the right type and concentration of PEG-lipids. This result has important implication in the development of ligand-targeted liposomes.

Preparation of long-circulating immunoliposomes using PEG-cholesterol conjugates: effect of the spacer arm between PEG and cholesterol on liposomal characteristics

Poly(ethylene glycol)-coated liposomes were prepared with two new synthesised pegylated cholesterol (Chol) derivatives linked via carbamate bond. Poly(ethylene glycol) (PEG) was directly linked to Chol (PEG-Chol) or through a space arm of diaminebutane (PEG-L-Chol). In buffer, the physicochemical properties of PC/Chol liposomes (2/1, molar ratio) containing up to 10 mol% of pegylated Chol derivatives did not change significantly and the PEG layer at liposome surface inhibited the agglutination of biotin-liposomes induced by streptavidin. On the other hand, in serum, PEG-L-Chol seemed to reduce the interactions of liposomes with serum proteins, much more than PEG-Chol. The low steric hindrance of PEG-Chol derivative may be due to the slow conformational transition rate of the polymer, since PEG may be deeper located in the membrane. The coupling efficiency of the ligand to the functionalised amino group at the polymer end was also affected, but, its antigen-binding activity was preserved. The basic physical-chemical characteristics studied in this work are relevant to assess the application of pegylated Chol liposomes as drug delivery systems.

Interactions of IgM ABO antibodies and complement with methoxy-PEG-modified human RBCs

BACKGROUND

RBCs modified with cyanuric chloride activated methoxy-PEG (CmPEG; 5000 Da) are less immunogenic than untreated RBCs, and their use thus may reduce the risk of alloimmunization in chronically transfused patients.

STUDY DESIGN AND METHODS

To further examine the potential utility of CmPEG-RBCs, the effects of derivatization on an arm of the immune system that plays an important role in transfusion rejection-the complement system--were determined.

RESULTS

When CmPEG-RBCs were incubated in autologous or heterologous ABO-matched serum, no classical or alternative pathway consumption was found, no C3a was generated, no cell-bound C3b or C9 was detected, and no cell lysis occurred. Cell-bound complement regulation was normal for CmPEG-RBCs, as determined by acidified serum or reactive lysis assays. CmPEG-RBCs differed from control RBCs only when incubated in ABO-mismatched serum. In that case, CmPEG modification failed to protect against ABO antibody-dependent complement-mediated lysis. Indeed, cell lysis was actually enhanced at CmPEG concentrations >1.0 mM.

CONCLUSION

The enhanced lysis of CmPEG-RBCs in ABO-mismatched serum correlated with increased IgM binding and C3a generation and elevated C3b and C9 membrane deposition. While PEG modification effectively blocks non-ABO antigens, these data show that ABO matching is still required. Once ABO-matched, these modified RBCs retain great potential for the prevention of alloimmunization.

Hemoglobin-vesicles as oxygen carriers: influence on phagocytic activity and histopathological changes in reticuloendothelial system

Hemoglobin-vesicles (HbV) have been developed for use as artificial oxygen carriers (particle diameter, 250 nm) in which a purified Hb solution is encapsulated with a phospholipid bilayer membrane. The influence of HbV on the reticuloendothelial system was studied by carbon clearance measurements and histopathological examination. The HbV suspension ([Hb] = 10 g/dl) was intravenously infused in male Wistar rats at dose rates of 10 and 20 ml/kg, and the phagocytic activity was measured by monitoring the rate of carbon clearance at 8 hours and at 1, 3, 7, and 14 days after infusion. The phagocytic activity transiently decreased one day after infusion by about 40%, but it recovered and was enhanced at 3 days, showing a maximum of about twice the quiescent level at 7 days, and then returned to the normal value at 14 days. The initial transient decreased activity indicates a partly, but not completely, suppressed defensive function of the body. The succeeding increased phagocytic activity corresponds to the increased metabolism of HbV. The histopathological examination with anti-human Hb antibody, hematoxylin/eosin, and oil red O stainings showed that HbV was metabolized within 7 days. Hemosiderin was very slightly confirmed with Berlin blue staining at 3 and 7 days in liver and spleen, though they completely disappeared at 14 days, indicating that the heme metabolism, excretion or recycling of iron proceeded smoothly and iron deposition was minimal. Electron microscopic examination of the spleen and liver tissues clearly demonstrated the particles of HbV with a diameter of about 1/40 of red blood cells in capillaries, and in phagosomes as entrapped in the spleen macrophages and Kupffer cells one day after infusion. The vesicular structure could not be observed at 7 days. Even though the infusion of HbV modified the phagocytic activity for 2 weeks, it does not seem to cause any irreversible damage to the phagocytic organs. These results offer important information for evaluating the safety issues of HbV for clinical use.

Surface-associated serum proteins inhibit the uptake of phosphatidylserine and poly(ethylene glycol) liposomes by mouse macrophages

Serum proteins, acting as opsonins, are believed to contribute significantly to liposome-macrophage cell association and thus regulate liposome uptake by cells of the mononuclear phagocytic system (MPS). We studied the effect of serum protein on binding and uptake of phosphatidylglycerol-, phosphatidylserine-, cardiolipin-, and N,N-dioleyl-N,N-dimethylammonium chloride- (DODAC) containing as well as poly(ethylene glycol)- (PEG) containing liposomes by mouse bone marrow macrophages in vitro. Consistent with the postulated surface-shielding properties of PEG, protein-free uptake of liposomes containing 5 mol% PEG and either 20 mol% anionic phosphatidylserine or 20 mol% cationic DODAC was equivalent to uptake of neutral liposomes. In contrast to previous reports indicating that protein adsorption to liposomes increases uptake by macrophages, the presence of bound serum protein did not increase the uptake of these liposomes by cultured macrophages. Rather, we found that pre-incubating liposomes with serum reduced the uptake of liposomes containing phosphatidylserine. Surprisingly, serum treatment of PEG-containing liposomes also significantly reduced liposome uptake by macrophages. It is postulated that, in the case of phosphatidylserine liposomes, the bound serum protein can provide a non-specific surface-shielding property that reduces the charge-mediated interactions between liposomes and bone marrow macrophage cells. In addition, incubation of PEG-bearing liposomes with serum can result in a change in the properties of the PEG, resulting in a surface that is better protected against interactions with cells.

Superoxide generation from human polymorphonuclear leukocytes by liposome-encapsulated hemoglobin

We investigated the interactions between liposome-encapsulated hemoglobin (Neo Red Cells: NRC) and human polymorphonuclear leukocytes as assessed by superoxide generation. NRC triggered superoxide generation from neutrophils in a dose-dependent manner. Empty liposomes also induced superoxide production of neutrophils. Superoxide generation of neutrophils induced by phorbol myristate acetate (PMA) was delayed but intensified both by NRC and empty liposomes. The intensity of superoxide generation induced by NRC was smaller than that by the empty liposomes. As NRC contained superoxide dismutase (SOD) that was copurified with hemoglobin from red blood cells and its activity remained, SOD contained in NRC may partially eliminate superoxide.

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.

Comprehensive analysis of the acute toxicities induced by systemic administration of cationic lipid:plasmid DNA complexes in mice

A major limitation associated with systemic administration of cationic lipid:plasmid DNA (pDNA) complexes is the vector toxicity at the doses necessary to produce therapeutically relevant levels of transgene expression. Systematic evaluation of these toxicities has revealed that mice injected intravenously with cationic lipid:pDNA complexes develop significant, dose-dependent hematologic and serologic changes typified by profound leukopenia, thrombocytopenia, and elevated levels of serum transaminases indicative of hepatocellular necrosis. Vector administration also induced a potent inflammatory response characterized by complement activation and the induction of the cytokines IFN-gamma, TNF-alpha, IL-6, and IL-12. These toxicities were found to be transient, resolving with different kinetics to pretreatment levels by 14 days posttreatment. The toxic syndrome observed was independent of the cationic lipid:pDNA ratio, the cationic lipid species, and the level of transgene expression attained. Mechanistic studies determined that neither the complement cascade nor TNF-alpha were key mediators in the development of these characteristic toxicities. Administration of equivalent doses of the individual vector components revealed that cationic liposomes or pDNA alone did not generate the toxic responses observed with cationic lipid:pDNA complexes. Only moderate leukopenia was associated with administration of cationic liposomes or pDNA alone, while only mild thrombocytopenia was noted in pDNA-treated animals. These results establish a panel of objective parameters that can be used to quantify the acute toxicities resulting from systemic administration of cationic lipid:pDNA complexes, which in turn provides a means to compare the therapeutic indices of these vectors.

Use of poly(ethylene glycol)-lipid conjugates to regulate the surface attributes and transfection activity of lipid-DNA particles

We evaluated the use of poly(ethylene glycol) (PEG)-modified lipids to control the surface properties of a lipid-based gene transfer system. The lipid-DNA particles (LDPs) used form spontaneously when plasmid DNA is added to mixed detergent lipid micelles consisting of the non-ionic detergent n-octyl-D-glucopyranoside, the cationic lipid dioleyldimethylammonium chloride (DODAC), the zwitterionic lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and selected PEG-modified phosphatidylethanolamines. The inclusion of DODAC is required to form the hydrophobic lipid-DNA complex. DOPE is included to facilitate dissociation of DNA from the cationic lipid and the PEG-modified lipids are added in an effort to stabilize the surface attributes of the resulting lipid-DNA particles. We used PEG-lipids that varied in acyl chain composition because of recent results demonstrating acyl chain dependent transfer of PEG-lipids from lipid vesicles, providing the potential to allow a transformation of the surface properties due to loss of surface grafted PEG. The addition of PEG-modified lipids does not interfere in LDP formation and its presence favors formation of smaller particles (75 nm in contrast to 130 nm in the absence of the PEG-modified lipid). PEG-lipid incorporation causes a concentration dependent reduction in LDP-mediated transfection of B16/BL6 melanoma cells, a result that can be partially attributed to a reduction in particle binding to cells. However, significant LDP binding to B16/BL6 cells was still observed under conditions where LDP transfection activity was reduced by more than 85%. The potential for PEG to interfere with LDP processing following cell binding is discussed.

Camouflaged blood cells: low-technology bioengineering for transfusion medicine?

The small number of studies done on the covalent modification of RBC with PEG, or PEG-derivatives, suggests that the immunocamouflage of intact cells significantly reduces the antigenicity and immunogenicity of the foreign cell. Importantly, this protective immunologic effect can be accomplished without adversely affecting the structure, function, or viability of the modified cell (e.g., RBCs and lymphocytes). As a consequence, PEG-RBC may have significant practical value in the treatment of the chronically transfused patient as a prophylactic measure against allosensitization. The PEG-RBC also may be useful in treating the already allosensitized individual. As shown, preexisting antibodies do not effectively recognize nor bind to the modified donor cells. A finding of further interest to transfusion medicine is that pegylation of contaminating lymphocytes within RBC products may prove efficacious in preventing graft-versus-host disease in the immunocompromised patient. However, the main emphasis of our research continues to be the immunocamouflage of RBC for use in chronic transfusion therapy of the SCD and thalassemic patient.

Synthesis of polyamide oligomers based on 14-amino-3,6,9, 12-tetraoxatetradecanoic acid

A series of oligomers of polyamides based on 14-amino-3,6,9, 12-tetraoxatetradecanoic acid monomers (ATTAn) was synthesized. These materials were designed as monodisperse analogues of poly(ethylene glycol) for use in biomedical applications where reproducible behavior is important. Synthesis of the monomer was evaluated using two routes. For small-scale preparations, tetraethylene glycol (TEG) was monoprotected with dihydropyran, converted to an alkoxide, and alkylated with ethyl bromoacetate. On larger scales, TEG was alkylated directly by treatment with sodium, followed by ethyl bromoacetate. The amine function was introduced by mesylation followed by treatment with sodium azide. Reduction of the azide to amino groups was performed over Pd/C using either hydrogen or formic acid as proton sources. Assembly of the oligomers was accomplished using standard DCC/NHS chemistry and an iterative dimerization sequence after appropriate deprotection of a pair of monomers. The amino group was protected by retaining the azido group as a latent amine. A series of ATTAn oligomers was prepared (n = 1-8). A lipid conjugate of the octamer, ATTA8-DSPE, was synthesized.