Poly(ethylene glycol)--lipid conjugates regulate the calcium-induced fusion of liposomes composed of phosphatidylethanolamine and phosphatidylserine

Characterization and transfection properties of lipoplexes stabilized with novel exchangeable polyethylene glycol-lipid conjugates

The positive charge of cationic-lipid/DNA complexes (lipoplexes) renders them highly susceptible to interactions with the biological milieu, leading to aggregation and destabilization, and rapid clearance from the blood circulation. In this study we synthesized and characterized a set of novel amphiphiles, based on N-methyl-4-alkylpyridinium chlorides (SAINTs), to which a PEG moiety is coupled. Plasmids were fully protected in lipoplexes prepared from cationic SAINT-2 lipid and stabilized with SAINT-PEGs. Our results demonstrate that SAINT-PEG stabilization is transient, and permits DNA to be released from these lipoplexes. The rate of SAINT-PEG transfer from lipoplexes to acceptor liposomes was determined by the nature of the lipid anchor. Increased hydrophobicity, by lengthening the alkyl chain, resulted in a decrease of the rate of DNA release from the lipoplexes. Chain unsaturation had the opposite effect. Similarly, the in vitro transfection potency of lipoplexes containing PEG-SAINT derivatives was sensitive to the length and (un)saturation of the alkyl chain. However, the internalization of SAINT-PEG stabilized lipoplexes is determined by their charge, rather than by the concentration of the polymer conjugate. Lipoplexes targeted to cell-surface epithelial glycoprotein 2, by means of a covalently coupled monoclonal antibody, were specifically internalized by cells expressing this antigen.

Interactions between pH-sensitive liposomes and model membranes

The structure and dynamics of two different pH-sensitive liposome systems were investigated by means of cryo-transmission electron microscopy and different photophysical techniques. Both systems consisted of dioleoylphosphatidylethanolamine (DOPE) and contained either oleic acid (OA) or a novel acid-labile polyethylene glycol-conjugated lipid (DHCho-MPEG5000) as stabiliser. Proton induced leakage, lipid mixing and structural changes were studied in the absence and presence of EPC liposomes, as well as in the presence of liposomes designed to model the endosome membrane. Neither DHCho-MPEG5000- nor OA-stabilised liposomes showed any tendency for fusion with pure EPC liposomes or endosome-like liposomes composed of EPC/DOPE/SM/Cho (40/20/6/34 mol.%). Our investigations showed, however, that incorporation of lipids from the pH-sensitive liposomes into the endosome membrane may lead to increased permeability and formation of non-lamellar structures. Taken together the results suggest that the observed ability of DOPE-containing liposomes to mediate cytoplasmic delivery of hydrophilic molecules cannot be explained by a mechanism based on a direct, and non-leaky, fusion between the liposome and endosome membranes. A mechanism involving destabilisation of the endosome membrane due to incorporation of DOPE, seems more plausible.

Mechanism of pH-triggered collapse of phosphatidylethanolamine liposomes stabilized by an ortho ester polyethyleneglycol lipid

The mechanism of pH-triggered destabilization of liposomes composed of a polyethyleneglycol-orthoester-distearoylglycerol lipid (POD) and phosphatidyl ethanolamine (PE) has been studied using an ANTS/DPX leakage and a lipid-mixing assay. We developed a kinetic model that relates POD hydrolysis to liposome collapse. This minimum-surface-shielding model describes the kinetics of the pH-triggered release of POD/PE liposomes. In the model, when acid-catalyzed hydrolysis lowers the mole percentage of POD on the liposome surface to a critical level, intervesicular lipid mixing is initiated, resulting in a burst of contents release. Two phases of content leakage are observed: a lag phase and a burst phase. During the lag phase, less than 20% of liposomal contents are released and the leakage begins to accelerate when approaching to the transition point. During the burst phase, the leakage rate is dependent on interbilayer contact. The burst phase occurs when the surface density of the PEG lipid is 2.3 +/- 0.6 mol%, regardless of the pH. Vesicles containing 4 mol% of a pH-insensitive PEG-lipid conjugate and 10% POD did not leak contents or collapse at any pH. These data are consistent with the stalk theory to describe the lamellar-to-inverted hexagonal phase transition and set a lower bound of approximately 16 PE lipids on the external monolayer as the contact site required for lipid mixing between two bilayers.

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.

Triggered release of doxorubicin following mixing of cationic and anionic liposomes

In many applications, an ability of liposomes to retain drug and then rapidly release it at some later time would be of benefit. In this work, we investigate the ability of cationic large unilamellar vesicles (LUV) to promote rapid release of doxorubicin from anionic LUV. It is shown that the addition of cationic liposomes containing cholesterol, dioleoylphosphatidylethanolamine (DOPE), distearoylphosphatidylcholine (DSPC) and the cationic lipid N,N-dioleyl-N,N-dimethylammonium chloride (DODAC) to doxorubicin-containing LUV composed of cholesterol, DOPE, DSPC and the anionic lipid dioleoyphosphatidylglycerol (DOPG) can result in release of more than 90% of the drug in times of 30 s or less. Further, it is shown that these release characteristics are exquisitely dependent on the presence of DOPE and cholesterol. In the absence of DOPE, much slower release rates are observed, with maximum release levels of 50% after a 2-h incubation at 20 degrees C. Remarkably, threshold levels of more than 10 mol% cholesterol are required before any appreciable release is observed. [31P]NMR spectroscopy and freeze-fracture electron microscopy studies reveal that systems giving rise to rapid release of doxorubicin exhibit limited formation of inverted hexagonal (H(II)) phase, suggesting that these lipids facilitate drug release by formation of local regions of non-bilayer structure. It is concluded that drug release triggered by mixing anionic and cationic liposomes could be of utility in drug delivery applications.

Subcellular trafficking of antisense oligonucleotides and down-regulation of bcl-2 gene expression in human melanoma cells using a fusogenic liposome delivery system

Antisense oligonucleotides (ODN) targeted to specific genes have shown considerable potential as therapeutic agents. The polyanionic charges carried by these molecules, however, present a barrier to efficient cellular uptake and consequently their biological effects on gene regulation are compromised. To overcome this obstacle, a rationally designed carrier system is desirable for antisense delivery. This carrier should assist antisense ODN penetrate the cell membrane and, once inside the cell, then release the ODN and make them available for target binding. We have developed a carrier formulation employing programmable fusogenic vesicles (PFV) as the antisense delivery mediator. This study investigates the intracellular fate of PFV-ODN and bioavailability of antisense ODN to cells. The subcellular distribution of PFV and ODN was examined by monitoring the trafficking of FITC-labeled ODN and rhodamine/phosphatidylethanolamine (Rh-PE)-labeled PFV using confocal microscopy. Fluorescently tagged ODN were first co-localized with the liposomal carrier in the cytoplasm, presumably in endosome/lysosome compartments, shortly after incubation of PFV-ODN with HEK 293 and 518A2 cells. Between 24 and 48 h incubation, however, separation of FITC-ODN from the carrier and subsequent accumulation in the nucleus was observed. In contrast, the Rh-PE label was localized to the cell cytoplasm. The enhanced cellular uptake achieved using the PFV carrier, compared to incubation of free ODN with cells, and subsequent release of ODN from the carrier resulted in significant down-regulation of mRNA expression. Specifically, G3139, an antisense construct targeting the apoptotic antagonist gene bcl-2, was examined in the human melanoma cell line 518A2. Upon exposure to PFV-encapsulated G3139, cells displayed a time-dependent reduction in bcl-2 message levels. The bcl-2 mRNA level was reduced by 50% after 24 h treatment and by approximately 80% after 72 h when compared to cells treated with free G3139, empty PFV or PFV-G3622, a control ODN sequence. Our results establish that ODN can be released from PFV after intracellular uptake and can then migrate to the nucleus and selectively down-regulate target mRNA.

Interactions of adriamycin, cytochrome c, and serum albumin with lipid monolayers containing poly(ethylene glycol)-ceramide

Poly(ethylene glycol)(2000)C(20)ceramide (PEG-Cer) containing monolayers at an air/water interface were characterized by measuring their surface pressure versus area/molecule (pi-A) and surface potential versus area/molecule (Delta V-A) isotherms. The behavior of pi-A as well as Delta V versus lipid density (Delta V-n) and Delta V-pi isotherms for PEG-Cer are in keeping with two transitions of the lipopolymer, starting at pi approximately equal to 9 and 21 mN/m. We also investigated the effects of PEG-Cer on the binding of adriamycin, cytochrome c and bovine serum albumin to monolayers containing varying mole fractions X of PEG-Cer. PEG-Cer impedes the penetration of these ligands into lipid monolayers with similar effects at both X = 0.04 and 0.08. This effect of PEG-Cer depends on the conformation of the lipopolymer and the interactions between the lipid surface and the surface-interacting molecule as well as the size of the latter.

Stabilized plasmid-lipid particles: a systemic gene therapy vector

The ability of a systemically administered gene therapy vector to exhibit extended circulation lifetimes, accumulate at a distal tumor site, and enable transgene expression is unique to SPLP. The flexibility and low toxicity of SPLP as a platform technology for systemic gene therapy allows for further optimization of tumor transfection properties following systemic administration. For example, the PEG coating of SPLP is necessary to engender the long circulation lifetimes required to achieve tumor delivery. However, PEG coatings have also been shown to inhibit cell association and uptake required for transfection. The dissociation rate of the PEG coating from SPLP can be modulated by varying the acyl chain length of the ceramide anchor, suggesting the possibility of developing PEG-Cer molecules that remain associated with SPLP long enough to promote tumor delivery, but which dissociate quickly enough to allow transfection. Alternatively, improvements may be expected from inclusion of cell-specific targeting ligands in SPLP to promote cell association and uptake. Finally, the nontoxic properties of SPLP allow the possibility of higher doses. A dose of 100 micrograms plasmid DNA per mouse corresponds to a dose of approximately 5 mg plasmid DNA per kg body weight. This compares well to small molecules used for cancer therapy, which typically are used at dose levels of 10 to 50 mg per kg body weight. In summary, SPLP consist of plasmid encapsulated in a lipid vesicle that, in contrast to naked plasmid or complexes, exhibit extended circulation lifetimes following intravenous injection, resulting in accumulation and transgene expression at a distal tumor site in a murine model. The pharmacokinetics, biodistribution, and tumor transfection properties of SPLP are highly sensitive to the nature of the ceramide anchor employed to attach the PEG to the SPLP surface. The SPLP-CerC20 system in which the PEG-Cer does not readily dissociate exhibits good serum stability, long circulation lifetimes, and high levels of tumor accumulation and mediates marker gene expression at the tumor site. The flexibility of the SPLP system offers the potential of further optimization to achieve therapeutically effective levels of gene transfer and clearly has considerable potential as a nontoxic systemic gene therapy vehicle with general applicability. These features of SPLP contrast favorably with previous plasmid encapsulation procedures. Plasmid DNA has been encapsulated by a variety of methods, including reverse phase evaporation, ether injection, detergent dialysis in the absence of PEG stabilization, lipid hydration and dehydration-rehydration techniques, and sonication, among others. The characteristics of these protocols are summarized in Table I. None of these procedures yields small, serum-stable particles at high plasmid concentrations and plasmid-to-lipid ratios in combination with high plasmid-encapsulation efficiencies. Trapping efficiencies comparable with the SPLP procedure can be achieved employing methods relying on sonication. However, sonication is a harsh technique that can shear nucleic acids. Size ranges of 100 mm diameter or less can be achieved by reverse-phase techniques; however, this requires an extrusion step through filters with 100 nm or smaller pore size which can often lead to significant loss of plasmid. Finally, it may be noted that the plasmid DNA-to-lipid ratios that can be achieved for SPLP are significantly higher than those achievable by any other encapsulation procedure.

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.

Effects of phosphatidylserine on membrane incorporation and surface protection properties of exchangeable poly(ethylene glycol)-conjugated lipids

Liposomes containing the acidic phospholipid phosphatidylserine (PS) have been shown to avidly interact with proteins involved in blood coagulation and complement activation. Membranes with PS were therefore used to assess the shielding properties of poly(ethylene glycol 2000)-derivatized phosphatidylethanolamine (PE-PEG(2000)) with various acyl chain lengths on membranes containing reactive lipids. The desorption of PE-PEG(2000) from PS containing liposomes was studied using an in vitro assay which involved the transfer of PE-PEG(2000) into multilamellar vesicles, and the reactivity of PS containing liposomes was monitored by quantifying interactions with blood coagulation proteins. The percent inhibition of clotting activity of PS liposomes was dependent on the PE-PEG(2000) content. 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine (DSPE)-PEG(2000) which transferred out slowly from PS liposomes was able to abolish >80% of clotting activity of PS liposomes at 15 mol%. This level of DSPE-PEG(2000) was also able to extend the mean residence time of PS liposomes from 0.2 h to 14 h. However, PE-PEG(2000) with shorter acyl chains such as 1,2-dimyristyl-sn-glycero-3-phosphoethanolamine-PEG(2000) were rapidly transferred out from PS liposomes, which resulted in a 73% decrease in clotting activity inhibition and 45% of administered intravenously liposomes were removed from the blood within 15 min after injection. Thus, PS facilitates the desorption of PE-PEG(2000) from PS containing liposomes, thereby providing additional control of PEG release rates from membrane surfaces. These results suggest that membrane reactivity can be selectively regulated by surface grafted PEGs coupled to phosphatidylethanolamine of an appropriate acyl chain length.

Membrane-active properties of alpha-MSH analogs: aggregation and fusion of liposomes triggered by surface-conjugated peptides

Reaction of the melanotropin hormone analogs [Nle(4),D-Phe(7)]-alpha-MSH and [Nle(4),D-Phe(7)]-alpha-MSH(4-10), which were extended at their N-terminus by a thiol-functionalized spacer arm, with preformed liposomes containing thiol-reactive (phospho)lipid derivatives resulted in the aggregation of the vesicles and in a partial leakage of their inner contents. This aggregation/leakage effect, which was only observed when the peptides were covalently conjugated to the surface of the liposomes, was correlated with the fusion of the vesicles as demonstrated by the observed decrease in resonance energy transfer between probes in a membrane lipid mixing assay. A limited fusion was confirmed by monitoring the mixing of the liposome inner contents (formation of 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylene bis(pyridinium bromide) complex). The membrane-active properties of the peptides could be correlated with changes in the fluorescence emission spectra of their tryptophan residue, which suggested that after their covalent binding to the outer surface of the liposomes they can partition within the core of the bilayers. A blue shift of 10 nm was observed for [Nle(4),D-Phe(7)]-alpha-MSH which was correlated with an increase in fluorescence anisotropy and with changes in the accessibility of the coupled peptide as assessed by the quenching of fluorescence of its tryptophan residue by iodide (Stern-Volmer plots). These results should be related to the previously described capacity of alpha-MSH, and analogs, to interact with membranes and with the favored conformation of these peptides which, via a beta-turn, segregate their central hydrophobic residues into a domain that could insert into membranes and, as shown here, trigger their destabilization.

Characterization of the inhibitory effect of PEG-lipid conjugates on the intracellular delivery of plasmid and antisense DNA mediated by cationic lipid liposomes

Poly(ethylene glycol)-lipid (PEG-lipid) conjugates are widely used in the field of liposomal drug delivery to provide a polymer coat that can confer favorable pharmacokinetic characteristics on particles in the circulation. More recently these lipids have been employed as an essential component in the self-assembly of cationic and neutral lipids with polynucleic acids to form small, stable lipid/DNA complexes that exhibit long circulation times in vivo and accumulate at sites of disease. However, the presence of a steric barrier lipid might be expected to inhibit the transfection activity of lipid/DNA complexes by reducing particle-membrane contact. In this study we examine what effect varying the size of the hydrophobic anchor and hydrophilic head group of PEG-lipids has on both gene and antisense delivery into cells in culture. Lipid/DNA complexes were made using unilamellar vesicles composed of 5 mole% PEG-lipids in combination with equimolar dioleoylphosphatidylethanolamine and the cationic lipid dioleyldimethylammonium chloride. Using HeLa and HepG2 cells we show that under the conditions employed PEG-lipids had a minimal effect on the binding and subsequent endocytosis of lipid/DNA complexes but they severely inhibited active gene transfer and the endosomal release of antisense oligodeoxynucleotides into the cytoplasm. Decreasing the size of the hydrophobic anchor or the size of the grafted hydrophilic PEG moiety enhanced DNA transfer by the complexes.

Enzyme-activated targeting of liposomes

It has become increasingly evident that tissues utilize specific localization of enzymes to perform certain tasks, often associated with various types of tissue remodeling. The ubiquitous presence of such enzymes, along with their specific localizations, provides an ideal opportunity to elicit specific delivery via an enzyme-triggered mechanism. A survey of some of the recent progress in enzyme-activated targeting of delivery systems, with a focus on a few liposomal systems, is presented.

Targeted delivery and triggered release of liposomal doxorubicin enhances cytotoxicity against human B lymphoma cells

Dioleoylphosphatidylethanolamine (DOPE)-containing liposomes that demonstrated pH-dependent release of their contents were stabilized in the bilayer form through the addition of a cleavable lipid derivative of polyethylene glycol (PEG) in which the PEG was attached to a lipid anchor via a disulfide linkage (mPEG-S-S-DSPE). Liposomes stabilized with either a non-cleavable PEG (mPEG-DSPE) or mPEG-S-S-DSPE retained an encapsulated dye at pH 5.5, but treatment at pH 5.5 of liposomes stabilized with mPEG-S-S-DSPE with either dithiothreitol or cell-free extracts caused contents release due to cleavage of the PEG chains and concomitant destabilization of the DOPE liposomes. While formulations loaded with doxorubicin (DXR) were stable in culture media, DXR was rapidly released in human plasma. pH-Sensitive liposomes, targeted to the CD19 epitope on B-lymphoma cells, showed enhanced DXR delivery into the nuclei of the target cells and increased cytotoxicity compared to non-pH-sensitive liposomes. Pharmacokinetic studies suggested that mPEG-S-S-DSPE was rapidly cleaved in circulation. In a murine model of B-cell lymphoma, the therapeutic efficacy of an anti-CD19-targeted pH-sensitive formulation was superior to that of a stable long-circulating formulation of targeted liposomes despite the more rapid drug release and clearance of the pH-sensitive formulation. These results suggest that targeted pH-sensitive formulations of drugs may be able to increase the therapeutic efficacy of entrapped drugs.

Developments in liposomal drug delivery systems

Liposomes are the leading drug delivery systems for the systemic (iv.) administration of drugs. There are now liposomal formulations of conventional drugs that have received clinical approval and many others in clinical trials that bring benefits of reduced toxicity and enhanced efficacy for the treatment of cancer and other life-threatening diseases. The mechanisms giving rise to the therapeutic advantages of liposomes, such as the ability of long-circulating liposomes to preferentially accumulate at disease sites including tumours, sites of infection and sites of inflammation are increasingly well understood. Further, liposome-based formulations of genetic drugs such as antisense oligonucleotides and plasmids for gene therapy that have clear potential for systemic utility are increasingly available. This paper reviews the liposomal drug delivery field, summarises the success of liposomes for the delivery of small molecules and indicates how this success is being built on to design effective carriers for genetic drugs.

Enhanced immune response to T-independent antigen by using CpG oligodeoxynucleotides encapsulated in liposomes

Immunostimulatory CpG oligodeoxynucleotides (ODN) have been tested as immunoadjuvants for various vaccines including T-cell independent (TI) antigens. Findings from previous reports suggest that close physical association of CpG ODN to the antigen could enhance its adjuvant effect. As an alternative to chemical conjugation of CpG ODN to the antigen, the current study is aimed at determining the benefit of using liposomes as a carrier for CpG ODN to improve the immune response to biotinylated liposomes (Bx-liposomes), a model of a TI antigen. Liposomes with suboptimal concentration of hapten (1% biotin) were not immunogenic. However, when CpG ODN encapsulated in Bx-liposomes were used to immunize mice, a hapten-specific response was obtained as indicated by antibody-mediated elimination of re-administered Bx-liposomes. CpG ODN co-administered with empty Bx-liposomes could not achieve the same effect, indicating the requirement for encapsulation of the adjuvant. Using both intravenous (i.v.) and subcutaneous (s.c.) immunization methods, it was found that IgM levels, but not IgG levels were elevated. Immunization in nude mice confirmed that the immune response obtained was TI. The use of non-CpG ODN and an ODN with alternatively flanked CpG motifs showed no adjuvant effect. Incorporation of poly(ethylene)glycol (PEG)-modified lipid in liposomes enhanced the immune response even further. In conclusion, our data shows that liposomes are a useful delivery vehicle for CpG ODN as an immune adjuvant for TI antigens.

Programmable fusogenic vesicles for intracellular delivery of antisense oligodeoxynucleotides: enhanced cellular uptake and biological effects

Programmable fusogenic vesicles (PFV) are liposomes composed of non-bilayer lipid components stabilized by the inclusion of an exchangeable poly(ethylene glycol) (PEG)-lipid conjugate. Vesicle destabilization by loss of the PEG-lipid results in recovery of the inherent fusogenic character. As a result, PFV can be designed to display a long circulation lifetime after i.v. administration, high accumulation at disease sites and full bioavailability of an encapsulated compound. In the present study, we investigated the potential application of PFV as carriers for intracellular delivery of antisense oligodeoxynucleotides (ODN). Antisense phosphorothioate ODN were encapsulated into PFV containing dioleoylphosphatidylethanolamine, cholesterol, dioleyldimethylammonium chloride and PEG-ceramides with different carbon chain length (C(8), C(14) and C(20)). In vitro fluorescent microscopy and flow cytometry analysis demonstrated that PFV containing PEG-ceramide C(14) provided enhanced intracellular delivery of FITC-labelled antisense ODN compared to PFV displaying faster or slower rates of destabilization (containing PEG-ceramide C(8) or C(20), respectively). Therapeutic efficacy of PFV-encapsulated antisense ODN against two proto-oncogenes, c-myc and bcl-2, was examined in various cell lines. At antisense concentrations of 0.5 microM, no significant downregulation of c-myc mRNA levels was observed in HEK293, B16 and MCA207 cells. However, treatment of 518A2 melanoma cells with PFV-encapsulated antisense targeting bcl-2 at concentrations of 0.5 microM and 1.0 microM resulted in reduced bcl-2 mRNA level by about 20% and 25% after 48 h incubation. Free antisense ODN did not affect bcl-2 mRNA expression at the concentrations used in this study and encapsulated control antisense (reverse polarity) led to a non-specific increase in mRNA levels. Our results suggest that PFV carriers displaying appropriate rates of destabilization have the potential to act as intracellular delivery vehicles and may improve the bioavailability and potency of antisense oligonucleotides.

Intermembrane transfer of polyethylene glycol-modified phosphatidylethanolamine as a means to reveal surface-associated binding ligands on liposomes

In order to explore the use of exchangeable poly(ethylene glycol) (PEG)-modified diacylphosphatidylethanolamines (PE) to temporarily shield binding ligands attached to the surface of liposomes, a model reaction based on inhibition and subsequent recovery of biotinylated liposome binding to streptavidin immobilized on superparamagnetic iron oxide particles (SA magnetic particles) was developed. PEG-lipid incorporation into biotinylated liposomes decreased liposome binding to SA magnetic particles in a non-linear fashion, where as little as 0.1 mol% PEG-PE resulted in a 20% decrease in binding. Using an assay based on inhibition of binding, PEG(2000)-PE transfer from donor liposomes to biotinylated acceptor liposomes could be measured. The influence of temperature and acyl chain composition on the transfer of PEG-diacyl PEs from donor liposomes to acceptor liposomes, consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine, cholesterol and N-((6-biotinoyl)amino)hexanoyl)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine (54.9:45:0.1 mole ratio), was measured. Donor liposomes were prepared using 1,2-distearoyl-sn-glycero-3-phosphocholine (50 mol%), cholesterol (45 mol%) and 5 mol% of either PEG-derivatized 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE-PEG(2000)), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE-PEG(2000)), or 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE-PEG(2000)). Transfer of DSPE-PEG(2000) to the donor liposomes was not detected under the conditions employed. In contrast, DMPE-PEG(2000) was transferred efficiently even at 4 degrees C. Using an acceptor to donor liposome ratio of 1:4, the time required for DMPE-PEG(2000) to become evenly distributed between the two liposome populations (T(EQ)) at 4 degrees C and 37 degrees C was approx. 2 and <0.5 h, respectively. An increase in acyl chain length from C14:0 to C16:0 of the PEG-lipid resulted in a significant reduction in the rate of transfer as measured by this assay. The transfer of PEG-lipid out of biotinylated liposomes was also studied in mice following intravenous administration. The relative rates of transfer for the various PEG-lipids were found to be comparable under in vivo and in vitro conditions. These results suggest that it is possible to design targeted liposomes with the targeting ligand protected while in the circulation through the use of PEG-lipids that are selected on the basis of exchange characteristics which result in exposure of the shielded ligand following localization within a target tissue.

Protein adsorption to polyethylene glycol modified liposomes from fibrinogen solution and from plasma

Unmodified and polyethylene glycol (PEG) modified neutral and negatively charged liposomes were prepared by freeze-thaw and extrusion followed by chromatographic purification. The effects of PEG molecular weight (PEG 550, 2000, 5000), PEG loading (0-15 mol%), and liposome surface charge on fibrinogen adsorption were quantified using radiolabeling techniques. All adsorption isotherms increased monotonically over the concentration range 0-3 mg/ml and adsorption levels were low. Negatively charged liposomes adsorbed significantly more fibrinogen than neutral liposomes. PEG modification had no effect on fibrinogen adsorption to neutral liposomes. An inverse relationship was found between PEG loading of negatively charged liposomes and fibrinogen adsorption. PEGs of all three molecular weights at a loading of 5 mol% reduced fibrinogen adsorption to negatively charged liposomes. Protein adsorption from diluted plasma (10% normal strength) to four different liposome types (neutral, PEG-neutral, negatively charged, and PEG-negatively charged) was investigated using gel electrophoresis and immunoblotting. The profiles of adsorbed proteins were similar on all four liposome types, but distinctly different from the profile of plasma itself, indicating a partitioning effect of the lipid surfaces. alpha2-macroglobulin and fibronectin were significantly enriched on the liposomes whereas albumin, transferrin, and fibrinogen were depleted compared to plasma. Apolipoprotein AI was a major component of the adsorbed protein layers. The blot of complement protein C3 adsorbed on the liposomes suggested that the complement system was activated.

Roles of lipid polymorphism in intracellular delivery

Lipids, which adopt nonbilayer phases, have fascinated researchers as to the functional roles of these components in biomembranes. In particular, lipids capable of adopting the hexagonal H(II) phase have received considerable attention because of the observation that such lipids can promote membrane fusion. In the rational design of lipid-based delivery systems, H(II) phase lipids have been employed to endow systems with fusogenic, membrane-destabilizing properties. We will outline the molecular basis for the polymorphic phase behavior of lipids and highlight some of the uses of nonbilayer lipids in the preparation of lipid-based delivery systems. In addition, a distinction will be drawn between lipid-based systems which rely on the inclusion of nonbilayer lipids for activity, and systems which contain components which actively promote formation of nonbilayer structure within biological membranes.

Efficient encapsulation of antisense oligonucleotides in lipid vesicles using ionizable aminolipids: formation of novel small multilamellar vesicle structures

Typical methods used for encapsulating antisense oligodeoxynucleotides (ODN) and plasmid DNA in lipid vesicles result in very low encapsulation efficiencies or employ cationic lipids that exhibit unfavorable pharmacokinetic and toxicity characteristics when administered intravenously. In this study, we describe and characterize a novel formulation process that utilizes an ionizable aminolipid (1,2-dioleoyl-3-dimethylammonium propane, DODAP) and an ethanol-containing buffer system for encapsulating large quantities (0.15--0.25 g ODN/g lipid) of polyanionic ODN in lipid vesicles. This process requires the presence of up to 40% ethanol (v/v) and initial formulation at acidic pH values where the DODAP is positively charged. In addition, the presence of a poly(ethylene glycol)-lipid was required during the formulation process to prevent aggregation. The 'stabilized antisense-lipid particles' (SALP) formed are stable on adjustment of the external pH to neutral pH values and the formulation process allows encapsulation efficiencies of up to 70%. ODN encapsulation was confirmed by nuclease protection assays and (31)P NMR measurements. Cryo-electron microscopy indicated that the final particles consisted of a mixed population of unilamellar and small multilamellar vesicles (80--140 nm diameter), the relative proportion of which was dependent on the initial ODN to lipid ratio. Finally, SALP exhibited significantly enhanced circulation lifetimes in mice relative to free antisense ODN, cationic lipid/ODN complexes and SALP prepared with quaternary aminolipids. Given the small particle sizes and improved encapsulation efficiency, ODN to lipid ratios, and circulation times of this formulation compared to others, we believe SALP represent a viable candidate for systemic applications involving nucleic acid therapeutics.

Phase behavior and aggregate structure in mixtures of dioleoylphosphatidylethanolamine and poly(ethylene glycol)-lipids

Cryo-transmission electron microscopy has been used to investigate the phase behavior and aggregate structure in dilute aqueous mixtures of dioleoylphosphatidylethanolamine (DOPE) and poly(ethylene glycol)-phospholipids (PEG-lipids). It is shown that PEG-lipids (micelle-forming lipids) induce a lamellar phase in mixtures with DOPE (inverted hexagonal forming lipid). The amount of PEG-lipid that is needed to induce a pure dispersed lamellar phase, at physiological conditions, depends on the size of the PEG headgroup. In the transition region between the inverted hexagonal phase and the lamellar phase, particles with dense inner textures are formed. It is proposed that these aggregates constitute dispersed cubic phase particles. Above bilayer saturating concentration of PEG-lipid, small disks and spherical micelles are formed. The stability of DOPE/PEG-lipid liposomes, prepared at high pH, against a rapid drop of the pH was also investigated. It is shown that the density of PEG-lipid in the membrane, sufficient to prevent liposome aggregation and subsequent phase transition, depends on the size of the PEG headgroup. Below a certain density of PEG-lipid, aggregation and phase transition occurs, but the processes involved proceed relatively slow, over the time scale of weeks. This allows detailed studies of the aggregate structure during membrane fusion.

Encapsulation of plasmid DNA in stabilized plasmid-lipid particles composed of different cationic lipid concentration for optimal transfection activity

In previous work (Wheeler et al. (1999) Gene Therapy 6, 271-281) we have shown that plasmid DNA can be entrapped in "stabilized plasmid lipid particles" (SPLP) using low levels (5-10 mol%) of cationic lipid, the fusogenic lipid dioleoylphosphatidylethanolamine (DOPE), and a polyethyleneglycol (PEG) coating for stabilization. The PEG moieties are attached to a ceramide anchor containing an arachidoyl acyl group (PEG-CerC20). However, these SPLP exhibit low transfection potencies in vitro as compared to plasmid/cationic lipid complexes formed with liposomes composed of cationic and neutral lipid at a 1:1 lipid ratio. The objective of this study was to construct SPLPs with increased cationic lipid contents that result in maximum transfection levels. A phosphate buffer detergent dialysis technique is described resulting in formation of SPLP containing 7-42.5 mol% DODAC with reproducible encapsulation efficiency of up to 80%. An octanoyl acyl group was used as anchor for the PEG moiety (PEG-CerC8) permitting a quick exchange out of the SPLP to further optimize the in vitro and in vivo transfection. We have demonstrated that this technique can be used to encapsulate either linearized DNA or supercoiled plasmids ranging from 3-20 kb. The SPLP formed could be isolated from empty vesicles by sucrose density gradient centrifugation, and exhibited a narrow size distribution of approximately 75 +/- 6 nm as determined by cryo-electron microscopy. The high plasmid-to-lipid ratio observed corresponded to one plasmid per particle. The SPLP consist of a lipid bilayer surrounding the plasmid DNA as visualized by cryo-electron microscopy. SPLP containing a range of DODAC concentrations were tested for in vitro and in vivo transfection. In vitro, in COS-7 cells transfection reached a maximum after 48 h. The transfection efficiency increased when the DODAC concentration in the SPLP was decreased from 42.5 to 24 mol% DODAC. Decreasing the cationic lipid concentration improved transfection in part due to decreased toxicity. In vivo studies using an intraperitoneal B16 tumor model and intraperitoneal administration of SPLP showed maximum transfection activity for SPLP containing 24 mol% DODAC. Gene expression observed in tumor cells was increased by approximately one magnitude as compared to cationic lipid/DNA complexes. The SPLP were stable and upon storage at 4 degrees C no significant change in the transfection activity was observed over a one-year period. Thus this phosphate buffer detergent dialysis technique can be used to generate SPLP formulations containing a wide range of cationic lipid concentrations to determine optimal SPLP composition for high transfection activity and low toxicity.

Aggregation and fusion of vesicles composed of N-palmitoyl derivatives of membrane phospholipids

N-Acylphosphatidylethanolamines and N-acylphosphatidylserines have been isolated from mammalian cells and have been associated with some tissue degenerative changes, although the relationship between their synthesis and the uncontrolled sequence of events that ends in irreversible tissue damage is not completely established. Our results show that monovalent and divalent cations induce aggregation and fusion of liposomes constituted by N-palmitoylphosphatidylethanolamine (NPPE) and N-palmitoylphosphatidylserine (NPPS). The effectiveness of cations to induce the aggregation of NPPE and NPPS liposomes is Ca2+ > Mg2+ >> Na+. NPPS liposomes aggregate at lower concentrations of divalent cations than NPPE liposomes, but with sodium NPPE liposomes aggregate to a higher extent than NPPS liposomes. The reaction order for the aggregation processes depends on the lipid and the cation nature and range from 1.04 to 1.64. Dynamic light scattering shows an irreversible increase of the size of the aggregates in the presence of all cations tested. The irreversibility of the aggregation process and the intermixing of bilayer lipids, as studied by resonance energy transfer assay, suggest that fusion, rather than aggregation, occurs. The existence of a real fusion was demonstrated by the coalescence of the aqueous contents of both NPPS and NPPE liposomes in the presence of either monovalent or divalent cations. The different binding sensitivity of Ca2+ to NPPS and NPPE liposomes, determined by zeta potential measurements, agrees with the results obtained in the aggregation and fusion assays. Our results suggest that the synthesis in vivo of N-acylated phospholipids can introduce important changes in membrane-mediated processes.

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.

In vitro characterization of a novel polymeric-based pH-sensitive liposome system

This study demonstrates rapid and pH-sensitive release of a highly water-soluble fluorescent aqueous content marker, pyranine, from egg phosphatidylcholine liposomes following incorporation of N-isopropylacrylamide (NIPA) copolymers in liposomal membranes. The pH-sensitivity of this system correlates with the precipitation of the copolymers at acidic pH. In vitro release can be significantly improved by increasing the percentage of anchor in the copolymer and thus favoring its binding to the liposomal bilayer. In the case of liposomes containing a poly(ethylene glycol)-phospholipid conjugate, the insertion of the pH-sensitive copolymer in the liposomal membrane appears to be sterically inhibited. Dye release from these formulations at acidic pH can still be achieved by varying the anchor molar ratio and/or molecular mass of the polymers or by including the latter during the liposome preparation procedure. Removal of unbound polymer results in decreased leakage only when the copolymer is inserted by incubation with preformed liposomes, but can be overcome by preparing liposomes in the presence of polymer. Aqueous content and lipid mixing assays suggest contents release can occur without membrane fusion. The results of this study indicate that the addition of pH-sensitive copolymers of NIPA represents promising strategy for improving liposomal drug delivery.

Immunoliposomes for the targeted delivery of antitumor drugs

This review presents an overview of the field of immunoliposome-mediated targeting of anticancer agents. First, problems that are encountered when immunoliposomes are used for systemic anticancer drug delivery and potential solutions are discussed. Second, an update is given of the in vivo results obtained with immunoliposomes in tumor models. Finally, new developments on the utilization of immunoliposomes for the treatment of cancer are highlighted.

Biological barriers to cellular delivery of lipid-based DNA carriers

Although lipid-based DNA delivery systems are being assessed in gene therapy clinical trials, many investigators in this field are concerned about the inefficiency of lipid-based gene transfer technology, a criticism directed at all formulations used to enhance transfer of plasmid expression vectors. It is important to recognize that many approaches have been taken to improve transfection efficiency, however because of the complex nature of the formulation technology being developed, it has been extremely difficult to define specific carrier attributes that enhance transfection. We believe that these optimization processes are flawed for two reasons. First, a very defined change in formulation components affects the physical and chemical characteristics of the carrier in many ways. As a consequence, it has not been possible to define structure/activity relationships. Second, the primary endpoint used to assess plasmid delivery has been transgene expression, an activity that is under the control of cellular processes that have nothing to do with delivery. Gene expression following administration of a plasmid expression vector involves a number of critical steps: (i) DNA protection, (ii) binding to a specific cell population, (iii) DNA transfer across the cell membrane, (iv) release of DNA into the cytoplasm, (v) transport through the cell and across the nuclear membrane as well as (vi) transcription and translation of the gene. The objective of this review is to describe lipid-based DNA carrier systems and the attributes believed to be important in regulating the transfection activity of these formulations. Although membrane destabilization activity of the lipid-based carriers plays an important role, we suggest here that a critical element required for efficient transfection is dissociation of lipids bound to the plasmid expression vector following internalization.

Cytosolic drug delivery using pH- and light-sensitive liposomes

A growing body of literature describes the development and applications of novel targeting and/or contents release triggering schemes to improve the therapeutic index of drugs encapsulated within liposomes. This review focuses on literature appearing between January 1995-December 1997 that report 1) antibody and receptor-mediated targeting approaches for improving drug localization and 2) acid, enzymatic, thermal or photochemical triggering processes that destabilize membranes and improve drug bioavailability via cytoplasmic delivery of liposomal contents.

Stabilized plasmid-lipid particles for regional gene therapy: formulation and transfection properties

Previous work (Wheeler et al, Gene Therapy 1999; 6: 271-281) has shown that plasmid DNA can be entrapped in 'stabilized plasmid-lipid particles' (SPLP) containing the fusogenic lipid dioleoylphosphatidylethanolamine (DOPE), low levels (5-10 mol%) of cationic lipid, and stabilized by a polyethyleneglycol (PEG) coating. The PEG moieties are attached to a ceramide anchor containing an arachidoyl acyl group (PEG-CerC20). These SPLP exhibit low transfection potencies in vitro, due in part to the long residence time of the PEG-CerC20 on the SPLP surface. In this work we employed SPLP stabilized by PEG attached to ceramide containing an octanoyl acyl group (PEG-CerC8), which is able to quickly exchange out of the SPLP, to develop systems that give rise to optimized in vitro and in vivo (regional) transfection. A particular objective was to achieve cationic lipid contents that give rise to maximum transfection levels. It is shown that by performing the dialysis procedure in the presence of increasing concentrations of citrate, SPLP containing up to 30 mol% of the cationic lipid dioleoydimethylammonium chloride (DODAC) could be generated. The SPLP produced could be isolated from empty vesicles by sucrose density gradient centrifugation, and exhibited a narrow size distribution (62 +/- 8 nm, as determined by freeze-fracture electron microscopy) and a high plasmid-to-lipid ratio of 65 microg/micromol (corresponding to one plasmid per particle) regardless of the DODAC content. It was found that isolated SPLP containing 20-24 mol% DODAC resulted in optimum transfection of COS-7 and HepG2 cells in vitro, with luciferase expression levels comparable to those achieved for plasmid DNA-cationic lipid complexes. In vivo studies employing an intraperitoneal B16 tumor model and intraperitoneal administration of SPLP also demonstrated maximum luciferase expression for DODAC contents of 20-24 mol% and significantly improved gene expression in tumor tissue as compared with complexes. We conclude that SPLP stabilized by PEG-CerC8 and containing 20-24 mol% cationic lipid are attractive alternatives to plasmid DNA-cationic lipid complexes for regional gene therapy applications.

Controlled destabilization of a liposomal drug delivery system enhances mitoxantrone antitumor activity

Programmable fusogenic vesicles (PFVs) are lipid-based drug-delivery systems that exhibit time-dependent destabilization. The rate at which this destabilization occurs is determined by the exchange rate of a bilayer-stabilizing component, polyethylene glycol-phosphatidylethanolamine (PEG-PE) from the vesicle surface. This exchange rate is controlled, in turn, by the acyl chain composition of the PEG-PE. We describe in vitro and in vivo studies using PFVs as delivery vehicles for the anticancer drug mitoxantrone. We demonstrate that the PEG-PE acyl composition determined the rate at which PFVs are eliminated from plasma after intravenous administration, and the rate of mitoxantrone leakage from PFV. The nature of the PEG-PE component also determined the antitumor efficacy of mitoxantrone-loaded PFV in murine and human in murine and human xenograft tumor models. Increased circulation time and improved activity were obtained for PFV containing PEG-PE with an 18-carbon acyl chain length, as a result of slower vesicle destabilization.

Stabilized plasmid-lipid particles: factors influencing plasmid entrapment and transfection properties

Previous work has shown that plasmid DNA can be encapsulated in small 'stabilized plasmid-lipid particles' (SPLP) composed of 1, 2-dioleyl-3-phosphatidylethanolamine (DOPE), the cationic lipid N, N-dioleyl-N,N-dimethylammonium chloride (DODAC) and poly(ethylene glycol) (PEG) conjugated ceramides (PEG-Cer), employing a detergent dialysis procedure. These SPLP have potential as vectors for in vivo gene therapy. This study is aimed at characterizing the influence of the cationic lipid and PEG-Cer species on SPLP formation and in vitro transfection properties. It is shown that the transfection potency of SPLP is sensitive to the cationic lipid species employed, the size of the PEG polymer incorporated in the PEG-ceramide and the length of the acyl chain contained in the ceramide anchor. With regard to the influence of cationic lipid, the transfection levels achieved were highest for SPLP containing N-[2, 3-(dioleyloxy)propyl]-N,N-dimethyl-N-cyanomethylammonium chloride (DODMA-AN) and lowest for SPLP containing 3-beta-[N-(N', N'-dimethylaminoethyl)carbamoyl]-cholesterol (DC-CHOL), according to the series DODMA-AN>N-[2,3-(dioleyloxy)propyl]-N,N, N-trimethylammonium chloride (DOTMA)>DODAC>N,N-distearyl-N, N-dimethylammonium chloride (DSDAC)>DC-CHOL. Incorporation of short (PEG(750)) PEG polymers in the PEG-ceramide components resulted in modest improvements in transfection levels over PEG(2000) and PEG(5000) polymers, however variation of the length of the acyl chain contained in the hydrophobic ceramide anchor from octanoyl (PEG-CerC(8)) to myristoyl (PEG-CerC(14)) to arachidoyl (PEG-CerC(20)) had the most dramatic effects. Transfection levels achieved for SPLP containing PEG-CerC(8) were substantially larger than observed for SPLP containing PEG-CerC(14) or PEG-CerC(20), consistent with a requirement for the PEG-ceramide to dissociate from the SPLP surface for maximum transfection potency. It is also shown that the ability of SPLP to be accumulated into cells is a dominant factor influencing transfection potency, and that the transfection potency of SPLP that are accumulated is at least equivalent to that of cationic lipid-plasmid DNA complexes.

Electrostatically mediated interactions between cationic lipid-DNA particles and an anionic surface

In an effort to model the interaction of lipid-based DNA delivery systems with anionic surfaces, such as a cell membrane, we have utilized microelectrophoresis to characterize how electrokinetic measurements can provide information on surface charge and binding characteristics. We have established that cationic lipids, specifically N-N-dioleoyl-N,N-dimethylammonium chloride (DODAC), incorporated into liposomes prepared with 1, 2-dioleoyl-i-glycero-3-phosphoethanolamine (DOPE) or 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 50 mol%, change the inherent electrophoretic mobility of anionic latex polystyrene beads. Self-assembling lipid-DNA particles (LDPs), prepared at various cationic lipid to negative DNA phosphate charge ratios, effected no changes in bead mobility when the LDP charge ratio (+/-) was equal to or less than 1. Increasing the LDP concentration in a solution of 0.1% (w/v) anionic beads resulted in a charge reversal effect when a net charge of LDP to total bead charge ratio (+/-) of 1:1 was observed. LDP formulations, utilizing either DOPE or DOPC, showed similar titration profiles with a charge reversal observed at a 1:1 net LDP to bead charge ratio (+/-). It was confirmed through centrifugation studies that the DNA in the LDP was associated with the anionic latex beads through electrostatic interactions. LDP binding, rather than the binding of dissociated cationic lipids, resulted in the observed electrophoretic mobility changes of the anionic latex beads.

Separation of liposomes from plasma components using fast protein liquid chromatography

We describe an efficient method for separating liposomes (large unilamellar vesicles, 120-150 nm diameter) from plasma lipoproteins employing fast protein liquid chromatography (FPLC). This method resolves very low density lipoprotein (VLDL), low-density lipoprotein, high-density lipoprotein, and other plasma components. Selective detection of liposomes (large unilamellar vesicles, 120-150 nm diameter) was achieved using either radiolabeled or fluorescent lipid probes. The liposomes were found to coelute with the earliest FPLC-eluting lipoprotein fraction, VLDL. The remaining plasma lipoprotein and protein components eluted at later times and were resolved from liposomes and VLDL. In order to separate VLDL from liposomes, we selectively precipitated the VLDL fraction from plasma using tungstophosphoric acid and magnesium chloride, prior to separation by FPLC. Furthermore, we demonstrate that this technique can be used to separate liposomes from lipoproteins in plasma samples collected after intravenous administration of liposomes to mice. This technique has wide application in studies of liposome stability in blood and, in particular, for the characterization of liposomal drug carrier systems.

Interaction of cationic liposomes and their DNA complexes with monocytic leukemia cells

Cationic liposomes complexed with DNA have been used extensively as non-viral vectors for the intracellular delivery of reporter or therapeutic genes in culture and in vivo. We examined the relationship between the characteristics of the lipoplexes, their mode of interaction with monocytic THP-1 cells and their ability to transfect these cells. We determined the size and zeta potential of cationic liposomes (composed of 1,2-dioleoyl-3-(trimethylammonium) propane (DOTAP) and its mixtures with neutral lipids), and lipoplexes at different (+/-) charge ratios. As the (+/-) charge ratio of the lipoplexes decreased to (1/1), a significant reduction in zeta potential and an increase in size was observed. The increase in size resulted from fusion between liposomes promoted by DNA, as demonstrated by a lipid mixing assay, and from aggregation of the complexes. Interaction of liposomes and lipoplexes with THP-1 cells was assessed by monitoring lipid mixing ('fusion') as well as binding and cell association. While no lipid mixing was observed with the 1/2 (+/-) lipid/DNA complexes, lipoplexes with higher (+/-) charge ratios underwent significant fusion in conjunction with extensive cell binding. Liposome binding to cells was dependent on the positive charge of the liposomes, and their fusion could be modulated by the co-lipid. DOTAP/phosphatidylethanolamine (1:1) liposomes fused with THP-1 cells, unlike DOTAP/phosphatidylcholine (1:1) liposomes, although both liposome types bound to the cells to a similar extent. The use of inhibitors of endocytosis indicated that fusion of the cationic liposomes with cells occurred mainly at the plasma membrane level. The presence of serum increased the size of the cationic liposomes, but not that of the lipoplexes. Low concentrations of serum (3%) completely inhibited the fusion of cationic liposomes with cells, while inhibiting binding by only 20%. Our results suggest that binding of cationic liposomes and lipoplexes to cells is governed primarily by electrostatic interactions, whereas their fusion is regulated by the lipid composition and sterically favorable interactions with cell surface molecules. In addition our results indicate no correlation between fusion of the lipoplexes with the plasma membrane and the levels of transfection.

Influenza hemagglutinin mediated fusion of membranes containing poly(ethylene-glycol) grafted lipids: new insights into the fusion mechanism

The influence of a hydrophilic layer covering the membrane on influenza hemagglutinin (HA) mediated fusion was investigated using membranes containing poly(ethylene-glycol) grafted phosphatidylethanolamine (PEG-2000-PE). Steric inhibition of HA-membrane interactions by these lipids affected virus fusion (half-maximal inhibition at 0.8 mol% for lipids with 114 ethylene glycol residues, or at 3.2 mol% for 45 residues (PEG-2000-PE), concentrations at which the PEG moieties adopt a random coil structure). Reconstituted viral membranes containing 3 mol% PEG-2000-PE retained 40% of their fusion activity. Therefore, efficient fusion is possible with membranes completely covered by a hydrophilic layer of several nanometers, and fusogenic virosomes containing PEG-PE are feasible.

Stabilized plasmid-lipid particles: construction and characterization

A detergent dialysis procedure is described which allows encapsulation of plasmid DNA within a lipid envelope, where the resulting particle is stabilized in aqueous media by the presence of a poly(ethyleneglycol) (PEG) coating. These 'stabilized plasmid-lipid particles' (SPLP) exhibit an average size of 70 nm in diameter, contain one plasmid per particle and fully protect the encapsulated plasmid from digestion by serum nucleases and E. coli DNase I. Encapsulation is a sensitive function of cationic lipid content, with maximum entrapment observed at dioleoyldimethylammonium chloride (DODAC) contents of 5 to 10 mol%. The formulation process results in plasmid-trapping efficiencies of up to 70% and permits inclusion of 'fusigenic' lipids such as dioleoylphosphatidylethanolamine (DOPE). The in vitro transfection capabilities of SPLP are demonstrated to be strongly dependent on the length of the acyl chain contained in the ceramide group used to anchor the PEG polymer to the surface of the SPLP. Shorter acyl chain lengths result in a PEG coating which can dissociate from the SPLP surface, transforming the SPLP from a stable particle to a transfection-competent entity. It is suggested that SPLP may have utility as systemic gene delivery systems for gene therapy protocols.

Liposomes coated with chemically modified dextran interact with human endothelial cells

Some liposomal formulations are now in clinical use. New applications in biology and medicine using targeted liposomes remain an intensive research area. In this context, liposomes constituted of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and cholesterol (70/10/20 mol %) were prepared by detergent dialysis and coated with dextran (Dx) or functionalized dextran (FDx), both hydrophobized by a cholesterol anchor which penetrates the lipid bilayer during the vesicle formation. The coating of liposomes with these polysaccharides was performed because chemically modified dextran but not native Dx interacted with vascular cells. The liposome uptake by human endothelial cells was followed using uncoated and coated liposomes radiolabeled with a neutral lipid (3H-cholesterol) and a polar phospholipid (14C-PC). The results indicated for both radiolabels a preferential uptake by endothelial cells of FDx-coated liposomes compared to uncoated or Dx-coated liposomes. Addition to the culture medium of calcium up to 10 mM further enhanced the level and rate of incorporation of FDx-coated liposomes, whereas interaction of endothelial cells with uncoated liposomes or liposomes coated with Dx was poorly affected. Liposome membranes were then labeled with N-(lissamine rhodamine B sulfonyl)diacyl-PE and liposome uptake by endothelial cells was observed by fluorescence microscopy. The punctate intracellular fluorescence of cells incubated at 37 degrees C with fluorolabeled liposomes is indicative of the liposome localization within the endocytotic pathway of the cells. Altogether, these data demonstrate that coating of liposomes with FDx enable specific interactions with human endothelial cells in culture. Consequently, these liposomes coated with bioactive polymers represent an attractive approach as materials for use as drug delivery vehicles targeting vascular cells.

Lipid-based systems for the intracellular delivery of genetic drugs

Currently available delivery systems for genetic drugs have limited utility for systemic applications. Cationic liposome/plasmid DNA or oligonucleotide complexes are rapidly cleared from circulation, and the highest levels of activity are observed in 'first pass' organs, such as the lungs, spleen and liver. Engineered viruses can generate an immune response, which compromises transfection resulting from subsequent injections and lack target specificity. A carrier, which can accumulate at sites of diseases such as infections, inflammations and tumours, has to be a small, neutral and highly serum-stable particle, which is not readily recognized by the fixed and free macrophages of the reticuloendothelial system (RES). This review summarizes lipid-based technologies for the delivery of nucleic acid-based drugs and introduces a new class of carrier systems, which solve, at least in part, the conflicting demands of circulation longevity and intracellular delivery. Plasmid DNA and oligonucleotides are entrapped into lipid particles that contain small amounts of a positively charged lipid and are stabilized by the presence of a polythylene glycol (PEG) coating. These carriers protect nucleic acid-based drugs from degradation by nucleases, are on average 70 nm in diameter, achieve long circulation lifetimes and are capable of transfecting cells.

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

Complement activation causes opsonization of foreign particles leading to particle elimination from the blood. Complement-mediated opsonization of charged and large liposomes presents a fundamental problem in their use to deliver therapeutic agents in vivo. To prolong the circulation half-lives of such liposomes, complement activation must be curtailed. The aim of this study was to assess the ability of poly(ethylene glycol)-lipids (PEG-lipids) to inhibit the in vitro activation of the classical pathway of complement in human serum by anionic liposomes. Incorporation of cholesterol-PEG600 (CH-PEG600), cholesterol-PEG1000 (CH-PEG1000), or phosphatidylethanolamine-PEG2000 (PE-PEG2000) resulted in dose-dependent inhibition of C1q binding and complement activation. The dose of PEG-lipid at which complement activation was blocked was inversely related to the PEG chain length. Complement activation was strongly inhibited when 15 mole% of CH-PEG600, 10 mole% CH-PEG1000, or 5 mole% PE-PEG2000 was incorporated into 100-nm anionic liposomes. PEG-lipid incorporation into larger liposomes (240 nm) was also successful in blocking C1q binding and complement activation. Radiolabeled cholesterol-PEG approximately 1400 was prepared and used to determine both the percentage of CH-PEG incorporated into the liposomes and the percentage maintained in the liposomes in the presence of 50% human serum at 37 degrees C for up to 24 h.

Characterization of lipid DNA interactions. I. Destabilization of bound lipids and DNA dissociation

We have recently described a method for preparing lipid-based DNA particles (LDPs) that form spontaneously when detergent-solubilized cationic lipids are mixed with DNA. LDPs have the potential to be developed as carriers for use in gene therapy. More importantly, the lipid-DNA interactions that give rise to particle formation can be studied to gain a better understanding of factors that govern lipid binding and lipid dissociation. In this study the stability of lipid-DNA interactions was evaluated by measurement of DNA protection (binding of the DNA intercalating dye TO-PRO-1 and sensitivity to DNase I) and membrane destabilization (lipid mixing reactions measured by fluorescence resonance energy transfer techniques) after the addition of anionic liposomes. Lipid-based DNA transfer systems were prepared with pInexCAT v.2.0, a 4.49-kb plasmid expression vector that contains the marker gene for chloramphenicol acetyltransferase (CAT). LDPs were prepared using N-N-dioleoyl-N,N-dimethylammonium chloride (DODAC) and either 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). For comparison, liposome/DNA aggregates (LDAs) were also prepared by using preformed DODAC/DOPE (1:1 mole ratio) and DODAC/DOPC (1:1 mole ratio) liposomes. The addition of anionic liposomes to the lipid-based DNA formulations initiated rapid membrane destabilization as measured by the resonance energy transfer lipid-mixing assay. It is suggested that lipid mixing is a reflection of processes (contact, dehydration, packing defects) that lead to formulation disassembly and DNA release. This destabilization reaction was associated with an increase in DNA sensitivity to DNase I, and anionic membrane-mediated destabilization was not dependent on the incorporation of DOPE. These results are interpreted in terms of factors that regulate the disassembly of lipid-based DNA formulations.

Controlling liposome blood clearance by surface-grafted polymers

The incorporation of polymer-lipid conjugates, initially using PEG and subsequently other selected flexible, hydrophilic polymers, into lipid bilayers gives rise to sterically stabilized liposomes that exhibit reduced blood clearance and concomitant changes in tissue distribution largely because of reduced, but not eliminated, phagocytic uptake. Changes in tissue distribution includes 'passive' targeting localization into sites of tumors, infection, inflammation characterized by presence of a 'leaky' vasculature which represent useful applications for drug delivery. The polymer forms a surface coating which has been characterized by physical measurements and it appears to function through steric inhibition of the protein binding and cellular interactions leading to phagocytic uptake. The current understanding of the physical and biological properties are reviewed. Ongoing work in the field involves interests to increase complexity such as addition of (1) selective targeting ligands by chemical conjugation to the exterior surface of the polymer coating, (2) capabilities for intracellular release of encapsulated agents into the cytoplasm, and (3) both simultaneously.

N-stearoyl-phosphatidylserine: synthesis and role in divalent-cation-induced aggregation and fusion

N-Acylphosphatidylserines have been isolated from intact and injured tissues, but the participation of such acidic phospholipids in membrane aggregation and fusion has not been demonstrated. We have synthesized N-stearoylphosphatidylserine (NSPS) and examined divalent-cation-induced aggregation of NSPS-liposomes, which leads to membrane destabilization and fusion. The purified lipid was characterized by its chromatographic and spectroscopic (infrared and 1H nuclear magnetic resonance) properties and by its chemical degradation pattern. Aggregation of unilamellar NSPS-liposomes was studied as a function of calcium and magnesium concentration. The ability of calcium and magnesium to induce vesicle aggregation is higher for phosphatidylserine (PS)-liposomes (threshold concentration 1.5 mM for calcium and 4.6 mM for magnesium) than for NSPS-liposomes (threshold concentration 2.8 mM for calcium and 6.6 mM for magnesium). The irreversibility of the aggregation reactions after adding EDTA suggests that vesicle fusion might occur in the presence of calcium and magnesium. Preliminary studies, based on mixing of both lipid and internal aqueous contents, show that fusion rather than aggregation of NSPS-liposomes occurs in the presence of calcium ions. The tendency of NSPS-liposomes to aggregate at higher cation concentrations than PS-liposomes suggests that N-acylation of phosphatidylserine protects the membrane against degenerative damage caused by aggregation and fusion.

Recent advances in liposome technologies and their applications for systemic gene delivery

The recent clinical successes experienced by liposomal drug delivery systems stem from the ability to produce well-defined liposomes that can be composed of a wide variety of lipids, have high drug-trapping efficiencies and have a narrow size distribution, averaging less than 100 nm in diameter. Agents that prolong the circulation lifetime of liposomes, enhance the delivery of liposomal drugs to specific target cells, or enhance the ability of liposomes to deliver drugs intracellularly can be incorporated to further increase the therapeutic activity. The physical and chemical requirements for optimum liposome drug delivery systems will likely apply to lipid-based gene delivery systems. As a result, the development of liposomal delivery systems for systemic gene delivery should follow similar strategies.

Mechanism of calcium ion induced multilamellar vesicle-DNA interaction

The effect of Ca2+ on the DNA interaction with anionic and neutral multilamellar vesicles (MLV) has been investigated. DNA from wheat (Triticum aestivum L. Gerek) was introduced to a suspension of MLV, composed of phosphatidylcholine (PC):dicetylphosphate (DCP):cholesterol (CHOL) at different molar ratios, to which Ca2+ (5-75 mM) was subsequently added. Indication of aggregation and/or fusion was obtained via light-scattering examination following the addition of Ca2+ and DNA to the MLV medium. Using a UV spectrophotometric assay, it was observed that although DNA alone has no effect on negatively charged MLV, it enhances liposomal interaction in the presence of calcium ions. The minimal Ca2+ concentration required to promote the interaction was detected to be 10 mM, and the highest level of interaction was observed at 75 mM. The aggregation/fusion of vesicles was detected for uncharged MLV (with no DCP in their structure), as well as for the anionic ones containing c. 10% CHOL, but not for anionic MLV containing 40% CHOL. This is explained in terms of cholesterol decreasing the membrane fluidity (above the Tc of components) as a result of which more rigid vesicles become less prone to aggregation/fusion interactions.

Poly(ethylene glycol)-lipid conjugates inhibit phospholipase C-induced lipid hydrolysis, liposome aggregation and fusion through independent mechanisms

Poly(ethylene glycol)-phosphatidylethanolamine (PEG-PE) conjugates have been introduced in liposomal compositions. The resulting large unilamellar vesicles were subjected to the action of phospholipase C. Enzyme-promoted vesicle aggregation and fusion were assayed in liposomes containing various proportions of PEG-PE. At PEG-PE concentrations above 1 mol% the rate of phospholipid hydrolysis decreases, perhaps because the PEG moiety hinders the enzyme from reaching the membrane surface. At concentrations above 0.1 mol% vesicle aggregation occurs at a slower rate, presumably because of the repulsive barrier properties or surface-grafted PEG. Lipid mixing decreases in parallel with vesicle aggregation. Finally, liposomal fusion rates measured as mixing of vesicle aqueous contents are decreased at or even below 0.1 mol%. The latter inhibition is due, apart from the reduced rates of lipid hydrolysis, vesicle aggregation and lipid mixing, to a PEG-PE-based stabilization of the lipid bilayer structure. Thus the observed low rates of contents mixing arise from three combined and independent inhibitory effects of PEG-PE.