Cationic O-ethylphosphatidylcholines and their lipoplexes: phase behavior aspects, structural organization and morphology

The interplay of quaternary ammonium lipid structure and protein corona on lung-specific mRNA delivery by selective organ targeting (SORT) nanoparticles

Messenger RNA (mRNA) can treat genetic disease using protein replacement or genome editing approaches but requires a suitable carrier to circumnavigate biological barriers and access the desired cell type within the target organ. Lipid nanoparticles (LNPs) are widely used in the clinic for mRNA delivery yet are limited in their applications due to significant hepatic accumulation because of the formation of a protein corona enriched in apolipoprotein E (ApoE). Our lab developed selective organ targeting (SORT) LNPs that incorporate a supplementary component, termed a SORT molecule, for tissue-specific mRNA delivery to the liver, spleen, and lungs of mice. Mechanistic work revealed that the biophysical class of SORT molecule added to the LNP forms a distinct protein corona that helps determine where in the body mRNA is delivered. To better understand which plasma proteins could drive tissue-specific mRNA delivery, we characterized a panel of quaternary ammonium lipids as SORT molecules to assess how chemical structure affects the organ-targeting outcomes and protein corona of lung-targeting SORT LNPs. We discovered that variations in the chemical structure of both the lipid alkyl tail and headgroup impact the potency and specificity of mRNA delivery to the lungs. Furthermore, changes to the chemical structure alter the quantities and identities of protein corona constituents in a manner that correlates with organ-targeting outcomes, with certain proteins appearing to promote lung targeting whereas others reduce delivery to off-target organs. These findings unveil a nuanced relationship between LNP chemistry and endogenous targeting, where the ensemble of proteins associated with an LNP can play various roles in determining the tissue-specificity of mRNA delivery, providing further design criteria for optimization of clinically-relevant nanoparticles for extrahepatic delivery of genetic payloads.

The replacement of helper lipids with charged alternatives in lipid nanoparticles facilities targeted mRNA delivery to the spleen and lungs

The broad clinical application of mRNA therapeutics has been hampered by a lack of delivery vehicles that induce protein expression in extrahepatic organs and tissues. Recently, it was shown that mRNA delivery to the spleen or lungs is possible upon the addition of a charged lipid to a standard four-component lipid nanoparticle formulation. This approach, while effective, further complicates an already complex drug formulation and has the potential to slow regulatory approval and adversely impact manufacturing processes. We were thus motivated to maintain a four-component nanoparticle system while achieving shifts in tropism. To that end, we replaced the standard helper lipid in lipidoid nanoparticles, DOPE, with one of eight alternatives. These lipids included the neutral lipids, DOPC, sphingomyelin, and ceramide; the anionic lipids, phosphatidylserine (PS), phosphatidylglycerol, and phosphatidic acid; and the cationic lipids, DOTAP and ethyl phosphatidylcholine. While neutral helper lipids maintained protein expression in the liver, anionic and cationic lipids shifted protein expression to the spleen and lungs, respectively. For example, replacing DOPE with DOTAP increased positive LNP surface charge at pH 7 by 5-fold and altered the ratio of liver to lung protein expression from 36:1 to 1:56. Similarly, replacing DOPE with PS reduced positive charge by half and altered the ratio of liver to spleen protein expression from 8:1 to 1:3. Effects were consistent across ionizable lipidoid chemistries. Regarding mechanism, nanoparticles formulated with neutral and anionic helper lipids best transfected epithelial and immune cells, respectively. Further, the lung-tropic effect of DOTAP was linked to reduced immune cell infiltration of the lungs compared to neutral or anionic lipids. Together, these data show that intravenous non-hepatocellular mRNA delivery is readily achievable while maintaining a four-component formulation with modified helper lipid chemistry.

Mucus-producing epithelial models for investigating the activity of gene delivery systems in the lung

Inhaled transfection particles have to penetrate the mucus layer lining the airways to successfully deliver their therapeutic nucleic acid payload to target cells in the underlying epithelium. However, the in vitro models used for evaluating gene carrier efficiency often disregard this viscous defensive barrier. In this study, the two mucus-secreting cell lines NCI-H292 and Calu-3 were selected to develop a series of epithelial models displaying gradual mucus production. In NCI-H292 models, a gradual increase in the MUC5AC mucin was obtained after cell exposure to inducers. In Calu-3 models, MUC5AC production increased as a function of culture duration (3, 7, 14 days) at the air-liquid interface (ALI). Six DOPC-derived cationic lipids were designed and their pDNA delivery activity was evaluated to validate these cellular models. The strongest impairment of the lipid delivery activity was observed in the Calu-3 14-d ALI model. The MUC5AC production in this model was the greatest and the mucus layer was 20 µm thick. The mucus exhibited a solid viscoelastic behaviour, and represented a major hindrance to lipoplex diffusion. The Calu-3 14-d ALI model will be highly useful for accurate evaluation of gene carriers intended for airway administration and characterization of their interactions with the mucus.

The studies on the membrane activity of triester of phosphatidylcholine in artificial membrane systems

Due to the increasing number of infections together with the appearance of bacteria exhibiting multi-drug resistance, new antibiotics are being sought. In this context the interest of the cationic lipoids increases because of their amphiphilic structure and positive charge that can stimulates the antibacterial action of these compounds. Thus, in this work we have performed the studies on the effect of one selected triesters of phosphatidylcholine, namely 1,2-dipalmitoyl-sn-glycero-3-ethylphosphocholine (EDPPC), on the model lipid membranes. The investigations included the analysis of the impact of EDPPC on multicomponent monolayers and bilayers consisting of the lipids naturally occurring in bacterial membranes (phosphatidylethanolamines (PE), phosphatidylglycerols (PG) and cardiolipin (CL)), mixed in proportions reflecting the lipid composition of these biomembranes. In the study, the Langmuir monolayers (registered on water and PBS buffer) and liposomes as model bacterial biomembranes were applied. The obtained results demonstrate that the presence of cationic lipoid in PE/PG and PE/PG/CL systems significantly modifies their properties and molecular organization. The incorporation of EDPPC into model bacterial membranes primarily impact on the intermolecular interactions. It was shown that the strength of the interaction between the cationic lipid and the components of the model membranes depends both on the composition of the membrane as well as on the type of subphase. Furthermore, the investigated cationic lipoid leads to the decrease of the ordering of acyl chains and thus to the increase of fluidity of membranes. The obtained results allow one to propose that EDPPC may behave as antibiotic active at the level of membrane.

Microfluidic-directed self-assembly of liposomes: Role of interdigitation

Microfluidics has been used to process self-assembling liposomal systems that are commonly considered for drug delivery applications. However, it has been found that the parameters of the process are not universally suited for all lipid types. We hypothesize here that size aggregation and instability of microfluidic liposomes are a direct consequence of the presence of interdigitation in these liposomes. Interdigitation refers to the phenomenon where two opposing leaflets of a bilayer interpenetrate into one another and form a single layer. When this happens, aggregation results as the single layer is not thermodynamically stable. Such interdigitation can be induced by pressure, chemicals or by the type of lipid structure. In this study, we systematically investigate the role of lipid composition on membrane interdigitation in order to understand the dependency of lipid interdigitation on liposome formation by microfluidics. By doing so, we use nano DSC and SAXS to probe the extent of lipid interdigitation by measuring the changes in thermodynamics and membrane thickness of the lipid bilayers. Our results show that microfluidic-fabricated liposomes undergo chemical interdigitation in the presence of ethanol, in particular saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Strategies to prevent interdigitation is to either remove ethanol above the lipid's main transition temperature (T_m), preventing the formation of interdigitated structures and subsequent aggregated states or by the incorporation of the inhibiting additives, such as cholesterol.

The influence of cationic lipoid - 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine - on model lipid membranes

The triesters of phosphatidylcholine as the derivatives of natural phosphatidylcholines are less cytotoxic than the other cationic lipoids, therefore they can be applied in lipofection and in drug delivery. However, a successful and effective use of these compounds requires detailed information of their mechanism of action, which is probably highly complex and multi-stages. However, the first barrier in the way to cell and thus the first side of action of these compounds is the cellular membrane. The aim of this work was to investigate the effect of one cationic lipoid, namely 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine (EPOPC) on model POPC/SM/Chol = 1:1:1 membranes. The experiments were performed on monolayer and bilayer systems and they involved the surface pressure measurements, Brewster angle microscopy studies, dynamic light scattering and zeta potential measurements and the experiments with the surfactant solution and steady-state fluorescence anisotropy of DPH and TMA-DPH. Moreover, to perform the studies systematically also the properties of the binary (POPC/EPOPC, SM/EPOPC, Chol/EPOPC) and ternary (POPC/Chol/EPOPC, SM/Chol/EPOPC) model systems were investigated. The obtained results indicated that even low concentration of EPOPC alters properties and organization of model membranes. Namely, EPOPC makes the interactions in model membrane weaker and increases fluidity and permeability of the lipid system. Finally, based on these data it can be proposed that the mechanism of action of EPOPC in lipofection/drug delivery involves the modifications in membrane organization, which facilitates the incorporation of drug or other material into the cell.

Determination of the Main Phase Transition Temperature of Phospholipids by Nanoplasmonic Sensing

Our study demonstrates that nanoplasmonic sensing (NPS) can be utilized for the determination of the phase transition temperature (T_m) of phospholipids. During the phase transition, the lipid bilayer undergoes a conformational change. Therefore, it is presumed that the T_m of phospholipids can be determined by detecting conformational changes in liposomes. The studied lipids included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). Liposomes in gel phase are immobilized onto silicon dioxide sensors and the sensor cell temperature is increased until passing the T_m of the lipid. The results show that, when the system temperature approaches the T_m, a drop of the NPS signal is observed. The breakpoints in the temperatures are 22.5 °C, 41.0 °C, and 55.5 °C for DMPC, DPPC, and DSPC, respectively. These values are very close to the theoretical T_m values, i.e., 24 °C, 41.4 °C, and 55 °C for DMPC, DPPC, and DSPC, respectively. Our studies prove that the NPS methodology is a simple and valuable tool for the determination of the T_m of phospholipids.

Malonic acid based cationic lipids - The way to highly efficient DNA-carriers

Cationic lipids play an important role as non-viral nucleic acid carriers in gene therapy since 3 decades. This review will introduce malonic acid derived cationic lipids as nucleic acid carriers which appeared in the literature dealing with lipofection 10years ago. The family of amino-functionalized branched fatty acid amides will be presented as well as different generations of malonic acid diamides. Both groups of cationic lipids yield lipid mixtures with highly efficient nucleic acid transfer activities in in-vitro cell culture models. The DNA transfer screening of lipid libraries with directed structural variations in the lipophilic as well as in the hydrophilic part of the amphiphiles yields structure/activity relationships. Furthermore, the detailed characterizations of selected lipid composites at the air/water interface and in bulk systems are summarized with regard to transfection determining physical-chemical properties. The findings are also discussed in comparison to results obtained with other families of cationic lipids.

Conserved Molecular Superlattices in a Series of Homologous Synthetic Mycobacterial Cell-Wall Lipids Forming Interdigitated Bilayers

Synthetic analogues of the cell-wall lipid monomycoloyl glycerol (MMG) are promising as next-generation vaccine adjuvants. In the present study, the thermotropic phase behavior of an array of synthetic MMG analogues was examined by using simultaneous small- and wide-angle X-ray scattering under excess water conditions. The MMG analogues differed in the alkyl chain lengths and in the stereochemistry of the polar glycerol headgroup or of the lipid tails (native-like versus alternative compounds). All MMG analogues formed poorly hydrated lamellar phases at low temperatures and inverse hexagonal (H₂) phases at higher temperatures prior to melting. MMG analogues with a native-like lipid acid configuration self-assembled into noninterdigitated bilayers whereas the analogues displaying an alternative lipid acid configuration formed interdigitated bilayers in a subgel (L_c') state. This is in contrast to previously described interdigitated phases for other lipids, which are usually in a gel (L_β) state. All investigated MMG analogues displayed an abrupt direct temperature-induced phase transition from L_c' to H₂. This transition is ultimately driven by the lipid chain melting and the accompanying change in molecular shape. No intermediate structures were found, but the entire array of MMG analogues displayed phase coexistence during the lamellar to H₂ transition. The structural data also showed that the headgroups of the MMG analogues adopting the alternative lipid acid configuration were ordered and formed a two-dimensional molecular superlattice, which was conserved regardless of the lipid tail length. To our knowledge, the MMG analogues with an alternative lipid acid configuration represent the first example of a lipid system showing both interdigitation and superlattice formation, and as such could serve as an interesting model system for future studies. The MMG analogues are also relevant from a subunit vaccine perspective because they are well-tolerated and display promising immunopotentiating activity. The structural characterization described here will serve as a prerequisite for the rational design of nanoparticulate adjuvants with specific and tailored structural features.

Characterization of Titratable Amphiphiles in Lipid Membranes by Fluorescence Spectroscopy

Understanding the ionization behavior of lipid membranes is a key parameter for successful development of lipid-based drug delivery systems. Accurate determination of the ionization state of a titratable species incorporated in a lipid bilayer however requires special care. Herein we investigated the behavior of titratable lipids in liposomes by fluorescence spectroscopy and determined which extrinsic parameters-i.e., besides those directly related to their molecular structure-determine their ionization state. Two fluorescent dyes, TNS and R18, have been used to investigate basic and acidic titratable lipids, respectively. Our results suggest that the titration behavior of the ionizable lipid in the membrane is more sensitive to the composition of the membrane and to its physical state than to the presence of solutes in the aqueous phase. Essentially overlooked in earlier studies on ionizable lipid assemblies, the concentration of the titratable lipid in the membrane was found to have a major effect on the ionization state of the lipid polar head. This may result in a shift in the apparent pKa value which may be as large as two pKa units and cannot be satisfactorily predicted.

Bridging small interfering RNA with giant therapeutic outcomes using nanometric liposomes

The scope of RNAi based therapeutics is unquestionable. However, if we dissect the current trend of clinical trials for afore mentioned drug class, some stark trends appear: 1) naked siRNA only exerts influence in topical mode whilst systemic delivery requires a carrier and 2) even after two decades of extensive efforts, not even a single siRNA containing product is commercially available. It was therefore felt that a perspective simplifying the unique intricacies of working with a merger of siRNA and liposomes from a pharmaceutical viewpoint could draw the attention of a wider array of interested researchers. We begin from the beginning and attempt to conduit the gap between theoretical logic and experimental/actual constraints. This, in turn could stimulate the next generation of investigators, gearing them to tackle the conundrum, which is siRNA delivery.

Cationic liposomes as adjuvants for influenza hemagglutinin: more than charge alone

Cationic liposomes are known as potent adjuvants for subunit vaccines. The purpose of this work was to study whether the content and the physicochemical properties of the positively charged compound affect the adjuvanticity of cationic liposomes. Cationic liposomes containing a cationic compound (DDA, DPTAP, DC-Chol, or eDPPC) and a neutral phospholipid (DPPC) were prepared by the film hydration-extrusion method and loaded with influenza hemagglutinin (HA) by adsorption. The liposomes were characterized (hydrodynamic diameter, zeta potential, membrane fluidity, HA loading) and their adjuvanticity was tested in mice. The formulations were administered twice subcutaneously and mouse sera were analyzed for HA-specific antibodies by ELISA and for HA-neutralizing antibodies by hemagglutination inhibition (HI) assay. First, the influence of cationic lipid concentration in the DC-Chol/DPPC liposomes (10 vs. 50 mol%) was investigated. The DC-Chol/DPPC (50:50) liposomes showed a higher zeta potential and HA loading, resulting in stronger immunogenicity of the HA/DC-Chol/DPPC (50:50) liposomes compared to the corresponding (10:90) liposomes. Next, we used liposomes composed of 50 mol% cationic lipids to investigate the influence of the nature of the cationic compound on the adjuvant effect. Liposomes made of the four cationic compounds showed similar hydrodynamic diameters (between 100 and 170 nm), zeta potentials (between +40 and +50 mV), HA loading (between 55% and 76%) and melting temperatures (between 40 and 55 °C), except for the DC-Chol liposomes, which did not show any phase transition. HA adjuvanted with the DC-Chol/DPPC (50:50) liposomes elicited significantly higher total IgG1 and IgG2a titers compared to the other liposomal HA formulations and non-adjuvanted HA. A similar trend was observed for the HI titers. These results show that the adjuvanticity of cationic liposomes depends on both the content and the physicochemical properties of the charged compound.

Analysis of lipoplex structure and lipid phase changes

Efficient delivery of genetic material to cells is needed for tasks of utmost importance in the laboratory and clinic, such as gene transfection and gene silencing. Synthetic cationic lipids can be used as delivery vehicles for nucleic acids and are now considered the most promising nonviral gene carriers. They form complexes (lipoplexes) with the polyanionic nucleic acids. A critical obstacle for clinical application of the lipid-mediated DNA delivery (lipofection) is its unsatisfactory efficiency for many cell types. Understanding the mechanism of lipid-mediated DNA delivery is essential for their successful application, as well as for a rational design and synthesis of novel cationic lipoid compounds for enhanced gene delivery. A viewpoint now emerging is that the critical factor in lipid-mediated transfection is the structural evolution of lipoplexes within the cell, upon interacting and mixing with cellular lipids. In particular, recent studies showed that the phase evolution of lipoplex lipids upon interaction and mixing with membrane lipids appears to be decisive for transfection success: specifically, lamellar lipoplex formulations, which were readily susceptible to undergoing lamellar-nonlamellar phase transition upon mixing with cellular lipids and were found rather consistently associated with superior transfection potency, presumably as a result of facilitated DNA release. Thus, understanding the lipoplex structure and the phase changes upon interacting with membrane lipids is important for the successful application of the cationic lipids as gene carriers.

Cationic lipids: molecular structure/ transfection activity relationships and interactions with biomembranes

Abstract Synthetic cationic lipids, which form complexes (lipoplexes) with polyanionic DNA, are presently the most widely used constituents of nonviral gene carriers. A large number of cationic amphiphiles have been synthesized and tested in transfection studies. However, due to the complexity of the transfection pathway, no general schemes have emerged for correlating the cationic lipid chemistry with their transfection efficacy and the approaches for optimizing their molecular structures are still largely empirical. Here we summarize data on the relationships between transfection activity and cationic lipid molecular structure and demonstrate that the transfection activity depends in a systematic way on the lipid hydrocarbon chain structure. A number of examples, including a large series of cationic phosphatidylcholine derivatives, show that optimum transfection is displayed by lipids with chain length of approximately 14 carbon atoms and that the transfection efficiency strongly increases with increase of chain unsaturation, specifically upon replacement of saturated with monounsaturated chains.

Lipid Phases Eye View to Lipofection. Cationic Phosphatidylcholine Derivatives as Efficient DNA Carriers for Gene Delivery

Efficient delivery of genetic material to cells is needed for tasks of utmost importance in laboratory and clinic, such as gene transfection and gene silencing. Synthetic cationic lipids can be used as delivery vehicles for nucleic acids and are now considered the most promising non-viral gene carriers. They form complexes (lipoplexes) with the polyanionic nucleic acids. A critical obstacle for clinical application of the lipid-mediated DNA delivery (lipofection) is its unsatisfactory efficiency for many cell types. Understanding the mechanism of lipid-mediated DNA delivery is essential for their successful application, as well as for rational design and synthesis of novel cationic lipoid compounds for enhanced gene delivery. According to the current understanding, the critical factor in lipid-mediated transfection is the structural evolution of lipoplexes within the cell, upon interacting and mixing with cellular lipids. In particular, recent studies with cationic phosphatidylcholine derivatives showed that the phase evolution of lipoplex lipids upon interaction and mixing with membrane lipids appears to be decisive for transfection success: specifically, lamellar lipoplex formulations, which were readily susceptible to undergoing lamellar-nonlamellar (precisely lamellar-cubic) phase transition upon mixing with cellular lipids, were found rather consistently associated with superior transfection potency, presumably as a result of facilitated DNA release subsequent to lipoplex fusion with the cellular membranes. Further, hydrophobic moiety of the cationic phospholipids was found able to strongly modulate liposomal gene delivery into primary human umbilical artery endothelial cells; superior activity was found for cationic phosphatidylcholine derivatives with two 14-carbon atom monounsaturated hydrocarbon chains, able to induce formation of cubic phase in membranes. Thus, understanding the lipoplex structure and the phase changes upon interacting with membrane lipids is important for the rational design and successful application of cationic lipids as superior nucleotide delivery agents.

Synergy in lipofection by cationic lipid mixtures: superior activity at the gel-liquid crystalline phase transition

Some mixtures of two cationic lipids including phospholipid compounds (O-ethylphosphatidylcholines) as well as common, commercially available cationic lipids, such as dimethylammonium bromides and trimethylammonium propanes, deliver therapeutic DNA considerably more efficiently than do the separate molecules. In an effort to rationalize this widespread "mixture synergism", we examined the phase behavior of the cationic lipid mixtures and constructed their binary phase diagrams. Among a group of more than 50 formulations, the compositions with maximum delivery activity resided unambiguously in the solid-liquid crystalline two-phase region at physiological temperature. Thus, the transfection efficacy of formulations exhibiting solid-liquid crystalline phase coexistence is more than 5 times higher than that of formulations in the gel (solid) phase and over twice that of liquid crystalline formulations; phase coexistence occurring at physiological temperature thus appears to contribute significantly to mixture synergism. This relationship between delivery activity and physical property can be rationalized on the basis of the known consequences of lipid-phase transitions, namely, the accumulation of defects and increased disorder at solid-liquid crystalline phase boundaries. Packing defects at the borders of coexisting solid and liquid crystalline domains, as well as large local density fluctuations, could be responsible for the enhanced fusogenicity of mixtures. This study leads to the important conclusion that manipulating the composition of the lipid carriers so that their phase transition takes place at physiological temperature can enhance their delivery efficacy.

Lipoplex formulation of superior efficacy exhibits high surface activity and fusogenicity, and readily releases DNA

Lipoplexes containing a mixture of cationic phospholipids dioleoylethylphosphatidylcholine (EDOPC) and dilauroylethylphosphatidylcholine (EDLPC) are known to be far more efficient agents in transfection of cultured primary endothelial cells than are lipoplexes containing either lipid alone. The large magnitude of the synergy permits comparison of the physical and physico-chemical properties of lipoplexes that have very different transfection efficiencies, but minor chemical differences. Here we report that the superior transfection efficiency of the EDLPC/EDOPC lipoplexes correlates with higher surface activity, higher affinity to interact and mix with negatively charged membrane-mimicking liposomes, and with considerably more efficient DNA release relative to the EDOPC lipoplexes. Observations on cultured cells agree with the results obtained with model systems; confocal microscopy of transfected human umbilical artery endothelial cells (HUAEC) demonstrated more extensive DNA release into the cytoplasm and nucleoplasm for the EDLPC/EDOPC lipoplexes than for EDOPC lipoplexes; electron microscopy of cells fixed and embedded directly on the culture dish revealed contact of EDLPC/EDOPC lipoplexes with various cellular membranes, including those of the endoplasmic reticulum, mitochondria and nucleus. The sequence of events outlining efficient lipofection is discussed based on the presented data.

An intracellular lamellar-nonlamellar phase transition rationalizes the superior performance of some cationic lipid transfection agents

Two cationic phospholipid derivatives with asymmetric hydrocarbon chains were synthesized: ethyl esters of oleoyldecanoyl-ethylphosphatidylcholine (C18:1/C10-EPC) and stearoyldecanoyl-ethylphosphatidylcholine (C18:0/C10-EPC). The former was 50 times more effective as a DNA transfection agent (human umbilical artery endothelial cells) than the latter, despite their similar chemical structure and virtually identical lipoplex organization. A likely reason for the superior effectiveness of C18:1/C10-EPC relative to C18:0/C10-EPC (and to many other cationic lipoids) was suggested by the phases that evolved when these lipoids were mixed with negatively charged membrane lipid formulations. The saturated C18:0/C10-EPC remained lamellar in mixtures with biomembrane-mimicking lipid formulations [e.g., dioleoyl-phosphatidylcholine/dioleoyl-phosphatidylethanolamine/dioleoyl-phosphatidylserine/cholesterol at 45:20:20:15 (wt/wt)]; in contrast, the unsaturated C18:1/C10-EPC exhibited a lamellar-nonlamellar phase transition in such mixtures, which took place at physiological temperatures, approximately 37 degrees C. As is well known, lipid vehicles exhibit maximum leakiness and contents release in the vicinity of phase transitions, especially those involving nonlamellar phase formation. Moreover, nonlamellar phase-forming compositions are frequently highly fusogenic. Indeed, FRET experiments showed that C18:1/C10-EPC exhibits lipid mixing with negatively charged membranes that is several times more extensive than that of C18:0/C10-EPC. Thus, C18:1/C10-EPC lipoplexes are likely to easily fuse with membranes, and, as a result of lipid mixing, the resultant aggregates should exhibit extensive phase coexistence and heterogeneity, thereby facilitating DNA release and leading to superior transfection efficiency. These results highlight the phase properties of the carrier lipid/cellular lipid mixtures as a decisive factor for transfection success and suggest a strategy for the rational design of superior cationic lipid carriers.

Transfection activity of binary mixtures of cationic o-substituted phosphatidylcholine derivatives: the hydrophobic core strongly modulates physical properties and DNA delivery efficacy

A combination of two cationic lipid derivatives having the same headgroup but tails of different chain lengths has been shown to have considerably different transfection activity than do the separate molecules. Such findings point to the importance of investigating the hydrophobic portions of cationic amphiphiles. Hence, we have synthesized a variety of cationic phosphatidylcholines with unusual hydrophobic moieties and have evaluated their transfection activity and that of their mixtures with the original molecule of this class, dioleoyl-O-ethylphosphatidylcholine (EDOPC). Four distinct relationships between transfection activity and composition of the mixture (plotted as percent of the new compound added to EDOPC) were found, namely: with a maximum or minimum; with a proportional change; or with essentially no change. Relevant physical properties of the lipoplexes were also examined; specifically, membrane fusion (by fluorescence resonance energy transfer between cationic and anionic lipids) and DNA unbinding (measured as accessibility of DNA to ethidium bromide by electrophoresis and by fluorescence resonance energy transfer between DNA and cationic lipid), both after the addition of negatively charged membrane lipids. Fusibility increased with increasing content of second cationic lipid, regardless of the transfection pattern. However, the extent of DNA unbinding after addition of negatively charged membrane lipids did correlate with extent of transfection. The phase behavior of cationic lipids per se as well as that of their mixtures with membrane lipids revealed structural differences that may account for and support the hypothesis that a membrane lipid-triggered, lamellar-->nonlamellar phase transition that facilitates DNA release is critical to efficient transfection by cationic lipids.

Lipid transfer between cationic vesicles and lipid–DNA lipoplexes: Effect of serum

Differential scanning calorimetry was used to examine the lipid exchange between model lipid systems, including vesicles of the cationic lipoids ethyldimyristoylphosphatidylcholine (EDMPC), ethyldipalmitoylphosphatidylcholine (EDPPC) or their complexes with DNA (lipoplexes), and the zwitterionic lipids (DMPC, DPPC). The changes of the lipid phase transition parameters (temperature, enthalpy, and cooperativity) upon consecutive temperature scans was used as an indication of lipid mixing between aggregates. A selective lipid transfer of the shorter-chain cationic lipoid EDMPC into the longer-chain aggregates was inferred. In contrast, transfer was hindered when EDMPC (but not EDPPC) was bound to DNA in the lipoplexes. These data support a simple molecular lipid exchange mechanism, but not lipid bilayer fusion. Exchange via lipid monomers is considerably more facile for the cationic ethylphosphatidylcholines than for zwitterionic phosphatidylcholines, presumably due to the higher monomer solubility of the charged lipids. With the cationic liposomes, lipid transfer was strongly promoted by the presence of serum in the dispersing medium. Serum proteins are presumed to be responsible for the accelerated transfer, since the effect was strongly reduced upon heating the serum to 80 degrees C. The effect of serum indicates that even though much lipoplex lipid is inaccessible due to the multilayered structure, the barrier due to buried lipid can be easily overcome. Serum did not noticeably promote the lipid exchange of zwitterionic liposomes. The phenomenon is of potential importance for the application of cationic liposomes to nonviral gene delivery, which often involves the presence of serum in vitro, and necessarily involves serum contact in vivo.

DNA release from lipoplexes by anionic lipids: correlation with lipid mesomorphism, interfacial curvature, and membrane fusion

DNA release from lipoplexes is an essential step during lipofection and is probably a result of charge neutralization by cellular anionic lipids. As a model system to test this possibility, fluorescence resonance energy transfer between DNA and lipid covalently labeled with Cy3 and BODIPY, respectively, was used to monitor the release of DNA from lipid surfaces induced by anionic liposomes. The separation of DNA from lipid measured this way was considerably slower and less complete than that estimated with noncovalently labeled DNA, and depends on the lipid composition of both lipoplexes and anionic liposomes. This result was confirmed by centrifugal separation of released DNA and lipid. X-ray diffraction revealed a clear correlation of the DNA release capacity of the anionic lipids with the interfacial curvature of the mesomorphic structures developed when the anionic and cationic liposomes were mixed. DNA release also correlated with the rate of fusion of anionic liposomes with lipoplexes. It is concluded that the tendency to fuse and the phase preference of the mixed lipid membranes are key factors for the rate and extent of DNA release. The approach presented emphasizes the importance of the lipid composition of both lipoplexes and target membranes and suggests optimal transfection may be obtained by tailoring lipoplex composition to the lipid composition of target cells.

Polymorphism of pyridinium amphiphiles for gene delivery: influence of ionic strength, helper lipid content, and plasmid DNA complexation

Two double-tailed pyridinium cationic amphiphiles, differing only in the degree of unsaturation of the alkyl chains, have been selected for a detailed study of their aggregation behavior, under conditions employed for transfection experiments. The transfection efficiencies of the two molecules are remarkably different, especially when combined with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) as helper lipid. The phase behavior of the cationic amphiphile/DOPE mixtures have been studied using (31)P- and (2)H-NMR (on deuterated cationic amphiphiles) as main techniques, to monitor independently the behavior of the two components. In water, the lamellar organization is dominant for both the surfactants in their mixtures with the helper lipid. In HEPES saline buffer (HBS), the mixtures of the unsaturated surfactant form inverted phases and, in particular, stable H(II) phases for DOPE contents > or =30 mol %. By contrast, the saturated surfactant does not form homogeneously mixed inverted phases in mixtures with DOPE at room temperature. However, mixed inverted phases are observed for this system at higher temperatures and, after mixing has been achieved by heating, the metastable mixed phases remain present for several hours at 5 degrees C. At 35 degrees C the dominant phase is the cubic phase. The lipoplex composed of equimolar mixtures of the unsaturated surfactant with DOPE and plasmid DNA was found to be organized in highly curved bilayers.

Novel fluorescent cationic phospholipid, O-4-napthylimido-1-butyl-DOPC, exhibits unusual foam morphology, forms hexagonal and cubic phases in mixtures, and transfects DNA

The novel cationic triester of phosphatidylcholine, O-4-napthylimido-1-butyl-dioleoylphosphatidylcholine (NB-DOPC), has been synthesized: 1-amino-4-butanol was reacted with napthylic anhydride to form 4-hydroxybutyl-1-napthylamide, which was then reacted with triflic anhydride; the resultant triflate was reacted with dioleoylphosphatidylcholine so as to transfer the napthylimido-butyl group to the unsubstituted phosphate oxygen. The resultant compound is thus not only positively charged, but also has a bulky hydrophobic moiety attached to the headgroup. This novel cationic phospholipid exhibits a peculiar long-living foam-like morphology upon hydration, which could have applications in encapsulation and delivery. It is characterized by high adhesiveness to hydrophobic surfaces. X-ray diffraction showed that it forms a lamellar structure of rather short repeat period, indicative of an unusually small interlamellar separation and low hydration level. It readily incorporates DNA and organizes into lamellar lipoplexes. These DNA-lipid complexes effectively transfect DNA into cells. In an equimolar mixture of this lipid with the anionic dioleoylphosphatidylglycerol lamellar arrays coexist with disordered uncorrelated structures, however, these transform into a bicontinuous cubic phase, Pn3m, upon addition of DNA. When mixed with another anionic lipid, cardiolipin, at a NB-DOPC/ cardiolipin 2:1 molar ratio, it forms the inverted hexagonal phase which is of potential interest for nanotechnology applications.