Morphology of Cationic Liposome/DNA Complexes in Relation to Their Chemical Composition

Budding and Division of Giant Vesicles Linked to Phospholipid Production

The self-reproduction of supramolecular assemblies based on the synthesis and self-assembly of building blocks is a critical step towards the construction of chemical systems with autonomous, adaptive, and propagation properties. In this report, we demonstrate that giant vesicles can grow and produce daughter vesicles by synthesizing and incorporating phospholipids in situ from ad-hoc precursors. Our model involves acyl chain elongation via copper(I)-catalyzed azide-alkyne [3 + 2] cycloaddition reaction and the ensuing production of synthetic phospholipids to induce budding and division. In addition, the growth and budding of giant vesicles were compatible with the encapsulation and transfer of macromolecules as large as lambda phage DNA to the buds. This chemical system provides a useful model towards the implementation of cell-like compartments capable of propagation and transport of biological materials.

Gene therapy and DNA delivery systems

Gene therapy is a promising new technique for treating many serious incurable diseases, such as cancer and genetic disorders. The main problem limiting the application of this strategy in vivo is the difficulty of transporting large, fragile and negatively charged molecules like DNA into the nucleus of the cell without degradation. The key to success of gene therapy is to create safe and efficient gene delivery vehicles. Ideally, the vehicle must be able to remain in the bloodstream for a long time and avoid uptake by the mononuclear phagocyte system, in order to ensure its arrival at the desired targets. Moreover, this carrier must also be able to transport the DNA efficiently into the cell cytoplasm, avoiding lysosomal degradation. Viral vehicles are the most commonly used carriers for delivering DNA and have long been used for their high efficiency. However, these vehicles can trigger dangerous immunological responses. Scientists need to find safer and cheaper alternatives. Consequently, the non-viral carriers are being prepared and developed until techniques for encapsulating DNA can be found. This review highlights gene therapy as a new promising technique used to treat many incurable diseases and the different strategies used to transfer DNA, taking into account that introducing DNA into the cell nucleus without degradation is essential for the success of this therapeutic technique.

Self-assembly of stable monomolecular nucleic acid lipid particles with a size of 30 nm

The design of efficient nucleic acid complexes is key to progress in genetic research and therapies based on RNA interference. For optimal transport within tissue and across extracellular barriers, nucleic acid carriers need to be small and stable. In this Article, we prepare and characterize mono-nucleic acid lipid particles (mono-NALPs). The particles consist of single short double-stranded oligonucleotides or single siRNA molecules each encapsulated within a closed shell of a cationic-zwitterionic lipid bilayer, furnished with an outer polyethylene glycol (PEG) shield. The particles self-assemble by solvent exchange from a solution containing nucleic acid mixed with the four lipid components DOTAP, DOPE, DOPC, and DSPE-PEG(2000). Using fluorescence correlation spectroscopy, we monitor the formation of mono-NALPs from short double-stranded oligonucleotides or siRNA and lipids into monodisperse particles of approximately 30 nm in diameter. Small angle neutron and X-ray scattering and transmission electron microscopy experiments substantiate a micelle-like core-shell structure of the particles. The PEGylated lipid shell protects the nucleic acid core against degradation by nucleases, sterically stabilizes the mono-NALPs against disassembly in collagen networks, and prevents nonspecific binding to cells. Hence, PEG-lipid shielded mono-NALPs are the smallest stable siRNA lipid system possible and may provide a structural design to be built upon for the development of novel nucleic acid delivery systems with enhanced biodistribution in vivo.

Reversible masking using low-molecular-weight neutral lipids to achieve optimal-targeted delivery

Intravenous injection of therapeutics is required to effectively treat or cure metastatic cancer, certain cardiovascular diseases, and other acquired or inherited diseases. Using this route of delivery allows potential uptake in all disease targets that are accessed by the bloodstream. However, normal tissues and organs also have the potential for uptake of therapeutic agents. Therefore, investigators have used targeted delivery to attempt delivery solely to the target cells; however, use of ligands on the surface of delivery vehicles to target specific cell surface receptors is not sufficient to avoid nonspecific uptake. PEGylation has been used for decades to try to avoid nonspecific uptake but suffers from many problems known as “The PEGylation Dilemma.” We have solved this dilemma by replacing PEGylation with reversible masking using low-molecular-weight neutral lipids in order to achieve optimal-targeted delivery solely to target cells. Our paper will focus on this topic.

High Sensitivity Differential Scanning Calorimetry Study of DNA-Cationic Liposome Complexes

PURPOSE

To investigate plasmid DNA interactions with liposomes prepared from dimyristoylglyceroethylphosphocholine (EDMPC) and DOPE using high sensitivity differential scanning calorimetry (HSDSC).

MATERIALS AND METHODS

Large unilamellar liposomes of EDMPC with DOPE (mol ratio 0-50%) were prepared. Plasmid DNA was added to give a final DNA/lipid (-/+) charge ratio of 0.5. Samples were placed into an HSDSC and cooled to 3 degrees C, held isothermally for 30 min and then the temperature was ramped to 120 degrees C at a rate of 1 degree C/min.

RESULTS

On heating EDMPC liposomes, the main phase transition occurred at 21.2 degrees C, with a low temperature shoulder on the endothermic peak. At low DOPE concentrations the main phase transition temperatures and enthalpies of transition were lower than for pure EDMPC, with a peak corresponding to a pure EDMPC phase occurring at DOPE concentrations of 12-17 mol%. At 50 mol%, no main transition endotherm was observed. DNA solution produced two endothermic peaks with numerous 'satellite' peaks indicating thermal denaturation. DNA binding to EDMPC changed the shape of the thermogram, indicating alteration in lipid packing within the bilayer. DNA induced demixing in the bilayers of DOPE-containing liposomes.

CONCLUSION

HSDSC provided information for characterizing liposome formulations and DNA interactions with such vesicles.

Comparison of the shape parameters of DNA–cationic lipid complexes and model polyelectrolyte–lipid complexes

In this study, we used a rheological method to study the shape of DNA-cationic lipid complexes and model polyelectrolyte-lipid complexes. We introduced two kinds of anionic polyelectrolytes, sodium polygalacturonate (PGU) and sodium dextran sulfate (DSS), of varying size, as models for DNA. The prepared complexes were incubated under laminar flow conditions. The results show the same quantitative relation between the shape parameter of lipoplexes and the length of anionic polyelectrolytes, including DNA. The rheological behavior of PGU and DSS were similar to that of DNA.

Mechanisms of Lipoplex Formation: Dependence of the Biological Properties ofTransfection Complexes on Formulation Procedures

Phospholipid-DNA complexes were made of the cationic triester derivative of phosphatidylcholine, EDOPC (1,2-dioleoyl- sn-glycero-3-ethylphosphocholine), by varying conditions of complex formation, in particular, the rate and direction of mixing, as well as by changing the mode of dispersing the lipid (extrusion or vortexing). The biological effects of variations in the formulation procedure were assessed by measuring transfection activity and cell association in cultures of BHK cells. Formulation procedures generally had little effect on cell association, but had marked effects on transfection efficiency. Transfection varied from effectively nil to extremely efficient with what appeared to be modest changes in formulation procedure. Formulation procedures also had significant effects on average sizes and size distributions of lipoplexes as determined by dynamic light scattering. Among the four possibilities of rapid or slow mixing combined with the two possible directions of mixing, slow addition of DNA to lipid gave results that differed significantly from the other three modes. In the case of vortexed lipid, the latter procedure was much less satisfactory than the other three, whereas in the case of extruded lipid, it was the only mode that produced satisfactory transfection. The factors that determine the difference in lipoplex properties can be identified as both geometric and physical. The geometric factor has to do with the symmetries of the participating units. There are three physical factors that are critical: the difference in vesicle stability upon interaction with DNA, the time dependence of interdiffusion of the components relative to that of vesicle rupture, and difference in input concentrations. These factors determine lipoplex size and, as already also shown by others, lipoplex size influences transfection efficiency.

Mechanism of formation of DNA-cationic vesicle complexes

Cationic vesicles and DNA form complexes that are promising gene delivery systems. Despite the increasing number of publications on their morphology and structure, the mechanism leading to their formation is not yet understood due to a lack of kinetic data. In the present study the kinetics of the interaction between DNA and cationic vesicles were followed using stopped-flow turbidity and small-angle neutron scattering techniques. The neutron real-time experiments were performed on a high-flux diffractometer, the D22 at the ILL, using a stopped-flow set-up. Extruded mixed vesicles of dimethyldioctadecylammonium bromide (DODAB) with various amounts of dioleoylphosphatidylethanolamine (DOPE) were investigated at 25 degrees C. The results show that the transition from unilamellar vesicles to a multilamellar structure upon DNA addition occurs in three steps. The first step, on the millisecond time scale, is currently not accessible to neutron scattering but was observed by stopped-flow turbidity and fluorescence experiments. The second step, on a time scale of seconds, corresponds to the formation of an intermediate with a locally cylindrical structure. As time progresses this unstable intermediate evolves to a multilamellar structure, on a time scale of minutes. An understanding of the mechanisms behind the DNA-cationic vesicle complex formation event will allow the production of more homogeneous, efficient delivery systems in pharmaceutically acceptable forms.

Myths concerning the use of cationic liposomes in vivo

Varied results have been obtained using cationic liposomes for in vivo delivery. Furthermore, optimisation of cationic liposomal complexes for in vivo applications is complicated, involving many diverse components. These components include nucleic acid purification, plasmid design, formulation of the delivery vehicle, administration route and schedule, dosing, detection of gene expression and others. Broad assumptions have frequently been made based on data obtained from focused studies using cationic liposomes. However, these assumptions do not necessarily apply to all delivery vehicles and, most likely, do not apply to many liposomal systems, when considering these other key components which influence the results obtained in vivo. Optimising all the components of the delivery system is pivotal and will allow broad use of liposomal complexes to treat or cure human diseases or disorders. This review will highlight the features of liposomes that contribute to successful delivery, gene expression and efficacy.

Liposomal delivery of nucleic acids in vivo

Optimization of cationic liposomal complexes for in vivo applications and therapeutics is complex involving many distinct components. These components include nucleic acid purification, plasmid design, formulation of the delivery vehicle, administration route and schedule, dosing, detection of gene expression, and others. This review will focus on optimization of these components for use in a variety of in vivo applications. Use of improved liposome formulations for delivery in vivo is valuable for gene therapy and would avoid several problems associated with viral delivery. Delivery of nucleic acids using liposomes is promising as a safe and non-immunogenic approach to gene therapy. Furthermore, gene therapeutics composed of artificial reagents can be standardized and regulated as drugs rather than as biologics. Optimizing all components of the delivery system will allow broad use of liposomal complexes to treat or cure human diseases or disorders.

Oligonucleotide delivery by a cationic derivative of the polyene antibiotic amphotericin B. I: interaction oligonucleotide/vector as studied by optical spectroscopy and electron microscopy

Antisense strategy requires efficient systems for the delivery of oligodeoxyribonucleotides (ODN) into target cells. Cationic amphiphiles have shown good efficiency in vitro and a lot of attention is currently paid to their interaction with nucleic acids. In the present study, this interaction was, for the first time, analysed at the molecular level, taking advantage of the spectroscopic properties of the positively charged chiral polyene molecule amphotericin B 3-dimethylaminopropyl amide (AMA), the efficiency of which, as delivery system, has been demonstrated [Garcia et al., Pharmacol. Ther. (2000), in press]. By UV-visible absorption and circular dichroism (CD) we studied its self-association properties in pure water, saline and RPMI medium. Drastic changes were observed upon ODN addition, stronger in pure water than in media of high ionic strength. At low AMA concentration (<10(-6) M), the strong increase of the CD signal, characteristic of self-association, indicated condensation of AMA on the ODN molecules. At a higher concentration (10(-4) M), and for a nucleic acid negative charge/AMA positive charge ratio higher than 1, spectra were interpreted as a reorganisation of free self-associated AMA species into smaller ones 'decorating' the nucleic acid molecule. Electron microscopy data were interpreted according to this scheme.

The phase behavior of cationic lipid-DNA complexes

We present a theoretical analysis of the phase behavior of solutions containing DNA, cationic lipids, and nonionic (helper) lipids. Our model allows for five possible structures, treated as incompressible macroscopic phases: two lipid-DNA composite (lipoplex) phases, namely, the lamellar (L(alpha)(C)) and hexagonal (H(II)(C)) complexes; two binary (cationic/neutral) lipid phases, that is, the bilayer (L(alpha)) and inverse-hexagonal (H(II)) structures, and uncomplexed DNA. The free energy of the four lipid-containing phases is expressed as a sum of composition-dependent electrostatic, elastic, and mixing terms. The electrostatic free energies of all phases are calculated based on Poisson-Boltzmann theory. The phase diagram of the system is evaluated by minimizing the total free energy of the three-component mixture with respect to all the compositional degrees of freedom. We show that the phase behavior, in particular the preferred lipid-DNA complex geometry, is governed by a subtle interplay between the electrostatic, elastic, and mixing terms, which depend, in turn, on the lipid composition and lipid/DNA ratio. Detailed calculations are presented for three prototypical systems, exhibiting markedly different phase behaviors. The simplest mixture corresponds to a rigid planar membrane as the lipid source, in which case, only lamellar complexes appear in solution. When the membranes are "soft" (i.e., low bending modulus) the system exhibits the formation of both lamellar and hexagonal complexes, sometimes coexisting with each other, and with pure lipid or DNA phases. The last system corresponds to a lipid mixture involving helper lipids with strong propensity toward the inverse-hexagonal phase. Here, again, the phase diagram is rather complex, revealing a multitude of phase transitions and coexistences. Lamellar and hexagonal complexes appear, sometimes together, in different regions of the phase diagram.

Nucleotide chain length and the morphology of complexes with cationic amphiphiles: (31)P-NMR observations

31P-NMR and UV spectroscopies were used to study the interactions between cationic amphiphile-containing lipid bilayers and either a phosphorothioate oligonucleotide (OligoS) (n=21) or polyadenylic acid (PolyA) (n approximately 18,000). Multilamellar vesicles (MLVs) were composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) in binary mixture with either of the cationic lipids, N-[1-(2, 3-dioleoyloxy)propyl]-N',N',N'-trimethylammonium chloride (DOTAP) or cetyltrimethylammonium bromide (CTAB). A UV-difference assay showed that OligoS binding ceased above a 1:1 anion/cation ratio, while PolyA binding continued until a 2:1 ratio was reached, indicating a 'flat' conformation for bound OligoS, but not necessarily for PolyA. Cross-polarization (31)P-NMR of the nucleotide chains bound to 100% DOTAP MLVs produced spectra virtually identical to those of dry powders of OligoS or PolyA, indicating effective immobilization of the surface-bound nucleotide chains. Hahn echo (31)P-NMR showed that MLVs composed of binary mixtures of POPC with DOTAP or CTAB retained a lamellar bilayer architecture upon adding nucleotide chains. At less than stoichiometric anion/cation ratios little or no signal attributable to free nucleotide chains was visible. A narrow signal at the chemical shift expected for phosphorothiodiesters or phosphodiesters became visible at greater levels of added OligoS or PolyA, respectively, indicating the presence of mobile nucleotide chains. Salt addition caused complete desorption of the nucleotide chains. When POPC was replaced with DOPE, binding of OligoS or PolyA produced non-bilayer lipid phases in the presence of DOTAP, but not in the presence of CTAB.

Physical and biological properties of cationic triesters of phosphatidylcholine

The properties of a new class of phospholipids, alkyl phosphocholine triesters, are described. These compounds were prepared from phosphatidylcholines through substitution of the phosphate oxygen by reaction with alkyl trifluoromethylsulfonates. Their unusual behavior is ascribed to their net positive charge and absence of intermolecular hydrogen bonding. The O-ethyl, unsaturated derivatives hydrated to generate large, unilamellar liposomes. The phase transition temperature of the saturated derivatives is very similar to that of the precursor phosphatidylcholine and quite insensitive to ionic strength. The dissociation of single molecules from bilayers is unusually facile, as revealed by the surface activity of aqueous liposome dispersions. Vesicles of cationic phospholipids fused with vesicles of anionic lipids. Liquid crystalline cationic phospholipids such as 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine triflate formed normal lipid bilayers in aqueous phases that interacted with short, linear DNA and supercoiled plasmid DNA to form a sandwich-structured complex in which bilayers were separated by strands of DNA. DNA in a 1:1 (mol) complex with cationic lipid was shielded from the aqueous phase, but was released by neutralizing the cationic charge with anionic lipid. DNA-lipid complexes transfected DNA into cells very effectively. Transfection efficiency depended upon the form of the lipid dispersion used to generate DNA-lipid complexes; in the case of the O-ethyl derivative described here, large vesicle preparations in the liquid crystalline phase were most effective.

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.

New directions in liposome gene delivery

The history of liposomes, progress in liposome gene delivery, and future directions are discussed. Specific characteristics of liposomes and DNA:liposome complexes have been identified that are essential for optimal delivery and gene expression. Of particular interest are the requirements for increased delivery and high levels of gene expression in vivo. At present, significant efforts are focused towards achieving specific delivery and gene expression in target organs and tissues.

Synthesis of cholesterol modified cationic lipids for liposomal drug delivery of antisense oligonucleotides

The paper describes a novel synthesis of cholest-5-en-3 beta-yl-6-aminohexyl ether (AH-Chol). AH-Chol was used to prepare positively charged liposomes. The liposomes consisted of phospholipon 90H and the cationic cholesterol derivative in an equimolar ratio. Liposome preparation was achieved by membrane homogenization after rehydration of a dry lipid film. Oligonucleotides (ODN) were adsorbed to the cationic liposomes very efficiently. At an ODN/liposome ratio of 1:5 (10:50 micrograms/ml) 84.2 +/- 5.4% of the ODNs were bound to the liposomal membrane. Within the range of 1:40 and 1:100 charge neutralization occurred and the liposome dispersion showed an increase in particle size due to aggregation. Below or above this range of charge neutralization the ODN loaded liposome preparation was physically stable, no sedimentation, increase of vesicle size or vesicle aggregation occurred.

Ultrastructural characterization of cationic liposome-DNA complexes showing enhanced stability in serum and high transfection activity in vivo

We have investigated the morphology and transfection activity of cationic liposome-DNA complexes (CLDC) under conditions relevant to both in vivo and in vitro studies. Moreover we have attempted to establish structure-function relationships relevant for high transfection activities under both conditions. CLDC were composed of dimethyldioctadecylammonium bromide with either 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) or cholesterol (Chol) interacting either with pre-condensed DNA or with uncondensed plasmid DNA. Furthermore for steric stabilization 1% poly(ethylene glycol)-phospholipid conjugate was added to CLDC containing Chol and plasmid DNA. The in vivo studies were carried out in mice following i.v. injection, and the in vitro studies were performed on SK-BR-3 human breast cancer cells in the presence of media with serum. The morphology of the CLDC, monitored by freeze-fracture electron microscopy, was investigated after mixing with mouse serum or the medium where the cells were kept. The substitution of DOPE with Chol, and the addition of N-[omega-methoxypoly(oxyethylene)-alpha-oxycarbonyl-DSPE+ ++ are producing CLDC which are stabilized with respect to time and serum, and are relatively small (100-300 nm). These stabilized complexes show high expression of a marker gene in mouse lungs reaching expression values up to 10 ng luciferase per mg tissue protein, but relatively low expression in SK-BR-3 cells in vitro. Additionally, some of the complexes containing pre-condensed DNA look like 'map-pin' structures showing heads of the size of liposomes and short, stiff and tapering tails. The in vivo transfection activity of these preparations is highest. Similar complexes containing DOPE rather than Chol as helper lipid precipitate in the presence of serum and especially of cell medium and convert into hexagonal lipid (HII) phase. Such complexes, despite their high transfection activity in vitro, show very little transfection activity in vivo. These comparisons may help us to understand the fundamental difference between in vitro and in vivo activity of CLDC: high in vitro transfection activity seems to be associated with hexagonal lipid precipitates whereas high in vivo activity seems to be related with small, stabilized complexes, which in our case also exhibit some protrusions (map-pin structures).

Cationic amphiphile interactions with polyadenylic acid as probed via 2H-NMR

2H-NMR spectroscopy was used to investigate the effects of polyadenylic acid (PolyA) on three aminomethyl-deuterated cationic amphiphiles: specifically, N-[1-(2,3-dioleoyloxy)propyl]-N',N',N'-trimethylammonium chloride (DOTAP-gamma-d3), 3beta-[N-(N',N',N'-trimethylaminoethane)carbamoyl] cholesterol (TC-CHOL-gamma-d3), and cetyltrimethylammonium bromide (CTAB-gamma-d9). When mixed with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and incorporated into lipid bilayer membranes, each of the cationic amphiphiles yielded 2H-NMR spectra consisting of a motionally averaged Pake powder pattern. The 2H-NMR quadrupolar splitting generally increased with increasing mole fraction of cationic amphiphile in the lipid bilayer with the exception of CTAB-gamma-d9. Adding PolyA caused the quadrupolar splitting to increase progressively in every case, until a 1:1 cation:anion charge ratio was achieved, after which the quadrupolar splitting changed no further. Deuterium NMR relaxation time measurements showed a parallel increase in T(qe)2 with increasing PolyA. The size of these changes produced by PolyA increased in the order: TC-CHOL < DOTAP < CTAB. NaCl addition reversed much, but not all, of the PolyA-related changes in 2H-NMR quadrupolar splittings and T(qe)2 relaxation times. A UV-based PolyA-membrane binding assay showed that salt addition caused PolyA desorption, and that the salt concentration required to do so increased in the order: TC-CHOL < DOTAP < CTAB. The results are consistent with an electrostatic binding of PolyA to the cationic lipid bilayer surface, accompanied by formation of a stoichiometric charge complex between PolyA and the cationic amphiphile, in which the cationic amphiphile retains considerable motional freedom. The strength of the complex increases in the order: TC-CHOL < DOTAP < CTAB.

Structure, stability, and thermodynamics of lamellar DNA-lipid complexes

We develop a statistical thermodynamic model for the phase evolution of DNA-cationic lipid complexes in aqueous solution, as a function of the ratios of charged to neutral lipid and charged lipid to DNA. The complexes consist of parallel strands of DNA intercalated in the water layers of lamellar stacks of mixed lipid bilayers, as determined by recent synchrotron x-ray measurements. Elastic deformations of the DNA and the lipid bilayers are neglected, but DNA-induced spatial inhomogeneities in the bilayer charge densities are included. The relevant nonlinear Poisson-Boltzmann equation is solved numerically, including self-consistent treatment of the boundary conditions at the polarized membrane surfaces. For a wide range of lipid compositions, the phase evolution is characterized by three regions of lipid to DNA charge ratio, rho: 1) for low rho, the complexes coexist with excess DNA, and the DNA-DNA spacing in the complex, d, is constant; 2) for intermediate rho, including the isoelectric point rho = 1, all of the lipid and DNA in solution is incorporated into the complex, whose inter-DNA distance d increases linearly with rho; and 3) for high rho, the complexes coexist with excess liposomes (whose lipid composition is different from that in the complex), and their spacing d is nearly, but not completely, independent of rho. These results can be understood in terms of a simple charging model that reflects the competition between counterion entropy and inter-DNA (rho < 1) and interbilayer (rho > 1) repulsions. Finally, our approach and conclusions are compared with theoretical work by others, and with relevant experiments.

Electrostatic and structural properties of complexes involving plasmid DNA and cationic lipids commonly used for gene delivery

The present study is aimed to characterize the interactions between plasmid DNA and cationic, large unilamellar vesicles, 110+/-20nm in size, composed of lipids commonly used for transfections including DOTAP/DOPE (mole ratio 1/1), DOTAP/DOPC (mole ratio 1/1), 100% DOTAP, or DC-CHOL/DOPE (mole ratio 1/1). [

ABBREVIATIONS

DOTAP, N-(1-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylammonium chloride; DOPE, 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine; DOPC, 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine; DC-CHOL, 3 beta-[N-(N',N'-dimethylaminoethane)carbamoyl] cholesterol]. A novel approach of combining Gouy-Chapman calculations and fluorescence measurements of the pH at the surface of lipid assemblies by the fluorophore 4-heptadecyl-7-hydroxycoumarin showed that electrostatic parameters played a key role in the instantaneous formation of the DNA-lipid complexes upon addition of different amounts of plasmid DNA to cationic liposomes in 20 mM Hepes buffer (pH 7.4). Addition of large amounts of plasmid DNA leads to neutralization of 60% of the protonated DC-CHOL in DC-CHOL/DOPE (1/1) assemblies and 80% of the DOTAP in lipid assemblies. The characterization of these electrostatic parameters of the complexes suggests better and closer surrounding of plasmid DNA by lipids when DOPE is present. Time-dependent static light-scattering measurements monitored the formation of complexes and also showed that these complexes were highly unstable with respect to size at DNA/cationic lipid molar ratios between 0.2 and 0.8.

Cationic lipids, phosphatidylethanolamine and the intracellular delivery of polymeric, nucleic acid-based drugs (review)

Polymeric, nucleic acid drugs must be protected from endogenous nucleases and delivered to target cell nuclei in order to maximize their activity. Constructs expressing therapeutic genes, antisense oligonucleotides and ribozymes can be delivered into cells by viral vectors, but concerns over safety and clinical utility have led to research into the development of alternative, non-viral delivery systems. Antisense and ribozyme drug development has focused upon modifications to the natural oligonucleotide chemistry which make the molecules resistant to nuclease degradation. These novel oligonucleotides cannot be generated by transgenes and must be administered in similar fashion to conventional drugs. However, oligonucleotides cannot cross membranes by passive diffusion and intracellular delivery for these drugs is very inefficient. Here we review the recent advances in forming lipid-DNA particles designed to mimic viral delivery of DNA. Most evidence now supports the hypothesis that lipid-DNA drugs enter target cells by endocytosis and disrupt the endosomal membrane, releasing nucleic acid into the cytoplasm. The mechanisms of particle formation and endosome disruption are not well understood. Cationic lipids are employed to provide an electrostatic interaction between the lipid carrier and polyanionic nucleic acids, and they are critical for efficient packaging of the drugs into a form suitable for systemic administration. However, their role in endosome disruption and other aspects of successful delivery leading to gene expression or inhibition of mRNA translation are less clear. We discuss the propensity of lipid-nucleic acid particles to undergo lipid mixing and fusion with adjacent membranes, and how phosphatidylethanolamine and other lipids may act as factors capable of disrupting bilayer structure and the endosomal pathway. Finally, we consider the challenges that remain in bringing nucleic acid based drugs into the realm of clinical reality.

Electrostatic parameters of cationic liposomes commonly used for gene delivery as determined by 4-heptadecyl-7-hydroxycoumarin

Cationic liposomes are used to deliver genes into cells in vitro and in vivo. The present study is aimed to characterize the electrostatic parameters of cationic, large unilamellar vesicles, 110 +/- 20 nm in size, composed of DOTAP/DOPE (mole ratio 1/1), DOTAP/DOPC (mole ratio 1/1), 100% DOTAP, DMRIE/DOPE 1/1, or DC-CHOL/DOPE (mole ratio 1/1). {

ABBREVIATIONS

DOTAP, N-(1-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylammonium chloride; DOPE, 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine; DOPC, 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine; DMRIE, 1,2-dimyristyloxypropyl-3-dimethyl-hydroxyethylammonium bromide; DC-CHOL, 3beta[N-(N',N'-dimethylaminoethane)carbamoyl]cholesterol}. The cationic liposomes had a large positive surface potential and a high pH at the liposomal surface in 20 mM Hepes buffer (pH 7.4) as monitored by the pH-sensitive fluorophore 4-heptadecyl-7-hydroxycoumarin. In contrast to DOTAP and DMRIE which were 100% charged, DC-CHOL in DC-CHOL/DOPE (1/1) liposomes was only about 50% charged in 20 mM Hepes buffer (pH 7.4). This might result in an easier dissociation of bilayers containing DC-CHOL from the plasmid DNA (which is necessary to enable transcription), in a decrease of the charge on the external surfaces of the liposomes or DNA-lipid complexes, and in an increase in release of the DNA-lipid complex into the cytosol from the endosomes. Other electrostatic characteristics found were that the primary amine group of DOPE in cationic liposomes dissociated at high (> 7.9) pHbulk and that a salt bridge was likely between the quaternary amine of DOTAP or DMRIE and the phosphate group of DOPE or DOPC, but not between the tertiary amine of DC-CHOL and the phosphate group of DOPE. The liposomes containing DOTAP were unstable upon dilution, probably due to the high critical aggregation concentration of DOTAP, 7 X 10(-5) M. This might also be a mechanism of the dissociation of bilayers containing DOTAP from the plasmid DNA.