Spontaneous liposome formation induced by grafted poly(ethylene oxide) layers: theoretical prediction and experimental verification

Preparation, characterization, and biodistribution of glutathione PEGylated nanoliposomal doxorubicin for brain drug delivery with a post-insertion approach

Objectives

Brain cancer treatments have mainly failed due to their inability to cross the blood-brain barrier. Several studies have confirmed the presence of glutathione (GSH) receptors on BBB's surface, as a result, products like 2B3-101, which contain over 5% pre-inserted GSH PEGylated liposomal doxorubicin, are being tested in clinical trials. Here we conducted the PEGylated nanoliposomal doxorubicin particles that are covalently attached to the glutathione using the post-insertion technique. Compared with the pre-insertion approach, the post-insertion method is notably simpler, faster, and more cost-effective, making it ideal for large-scale pharmaceutical manufacturing.

Materials and Methods

The ligands of the DSPE PEG(2000) Maleimide-GSH were introduced in the amounts of 25, 50, 100, 200, and 400 on the available Caelyx. Following physicochemical evaluations, animal experiments such as biodistribution, fluorescence microscopy, and pharmacokinetics were done.

Results

In comparison with Caelyx, the 200L and 400L treatment arms were the most promising formulations. We showed that nanocarriers containing 40 times fewer GSH micelles than 2B3-101 significantly increased blood-brain barrier penetrance. Due to the expressed GSH receptors on tissues as an endogenous antioxidant, doxorubicin will likely concentrate in the liver, spleen, heart, and lung in comparison with Caelyx, according to other tissue analyses.

Conclusion

The post-insertion technique was found a successful approach with more pharmaceutical aspects for large-scale production. Moreover, further investigations are highly recommended to determine the efficacy of 5% post-inserted GSH targeted nanoliposomes versus 2B3-101 as a similar formulation with a different preparation method.

Tailored nanostructured platforms for boosting transcorneal permeation: Box-Behnken statistical optimization, comprehensive in vitro, ex vivo and in vivo characterization

Ocular drug delivery systems suffer from rapid drainage, intractable corneal permeation and short dosing intervals. Transcorneal drug permeation could increase the drug availability and efficiency in the aqueous humor. The aim of this study was to develop and optimize nanostructured formulations to provide accurate doses, long contact time and enhanced drug permeation. Nanovesicles were designed based on Box-Behnken model and prepared using the thin film hydration technique. The formed nanodispersions were evaluated by measuring the particle size, polydispersity index, zeta potential, entrapment efficiency and gelation temperature. The obtained desirability values were utilized to develop an optimized nanostructured in situ gel and insert. The optimized formulations were imaged by transmission and scanning electron microscopes. In addition, rheological characters, in vitro drug diffusion, ex vivo and in vivo permeation and safety of the optimized formulation were investigated. The optimized insert formulation was found to have a relatively lower viscosity, higher diffusion, ex vivo and in vivo permeation, when compared to the optimized in situ gel. So, the lyophilized nanostructured insert could be considered as a promising carrier and transporter for drugs across the cornea with high biocompatibility and effectiveness.

Comparative study of (Asp)7-CHOL-modified liposome prepared using pre-insertion and post-insertion methods for bone targeting in vivo

Specific delivery of drugs to bone tissue is very challenging due to the architecture and structure of bone tissue. A seven-repeat sequence of aspartate, a representative bone-targeting oligopeptide, is preferentially used for targeted therapy for bone diseases. In this study, Asp7-cholesterol((Asp)7-CHOL) was synthesized and (Asp)7-CHOL-modified liposome loaded with doxorubicin (DOX) was successfully prepared using both pre-insertion (pre-L) and post-insertion (post-L) methods. The formulation was optimized according to particle size, zeta potential and the drug-loading efficiency of the liposome. In addition, the bone affinity of the (Asp)7-CHOL-modified liposome was evaluated using a hydroxyapatite (HA) absorption method. The results suggested that (Asp)7-CHOL-modified liposome show excellent HA absorption; pre-L showed slightly higher HA binding than post-L. However, post-L had a higher DOX entrapment efficiency than pre-L. In vivo imaging further demonstrated that pre-L showed a higher bone-targeting efficiency than post-L, which was consistent with in vitro results. In all, (Asp)7-CHOL-modified liposome showed excellent bone-targeting activity, suggesting their potential for use as a drug delivery system for bone disease-targeted therapies.

Curvature elasticity of a grafted polyelectrolyte brush

The curvature elasticity of a polyelectrolyte brush monolayer attached to curved surface is investigated theoretically. An analytical method based on the strong-stretching theory for a Gaussian chain is developed to calculate the elastic modulus induced by a polyelectrolyte brush. In particular, the scaling relations for the bending or Gaussian modulus with respect to system parameters related to the electrostatic interaction (degree of ionization and salt concentration) are derived. Using the numerical self-consistent-field theory, the inner structural, free-energy, and elastic moduli are computed for the polyelectrolyte brush with excluded-volume interactions. Compared to the analytical result, the curvature elasticity has a weaker dependence on the system parameters, which is attributed to the linearization for the Poisson-Boltzmann equation in the analytical treatment. Furthermore, our results are compared to the curvature elasticity of a bare charged surface, wherefrom the unique polyelectrolyte brush effect on the surface elasticity is clarified clearly. The scaling relations derived in our paper can serve as a guide to experimental studies on the related systems.

Membrane rigidity induced by grafted polymer brush

The contribution of neutral polymer brush to the curvature elasticity of the grafting surface is investigated theoretically. Using self-consistent field theory, we accurately evaluate the dependence of bending modulus on parameters including chain length, Flory-Huggins parameter and grafting density and reveal the importance of solvent. The results show that the brush-induced bending modulus follows a complex dependence on grafting density and Flory-Huggins parameter, while it obeys a simple power law with chain length as N(3). The method is further applied to calculate the polymer brush's contribution to the elastic properties of PEG-grafted lipid monolayers.

Recent progress in designing shell cross-linked polymer capsules for drug delivery

This tutorial review highlights the progress made during recent years in the development of the shell cross-linked (SCL) polymer nanocapsules and the impact of the most important scientific ideas on this field of knowledge.

Surface engineering of liposomes for stealth behavior

Liposomes are used as a delivery vehicle for drug molecules and imaging agents. The major impetus in their biomedical applications comes from the ability to prolong their circulation half-life after administration. Conventional liposomes are easily recognized by the mononuclear phagocyte system and are rapidly cleared from the blood stream. Modification of the liposomal surface with hydrophilic polymers delays the elimination process by endowing them with stealth properties. In recent times, the development of various materials for surface engineering of liposomes and other nanomaterials has made remarkable progress. Poly(ethylene glycol)-linked phospholipids (PEG-PLs) are the best representatives of such materials. Although PEG-PLs have served the formulation scientists amazingly well, closer scrutiny has uncovered a few shortcomings, especially pertaining to immunogenicity and pharmaceutical characteristics (drug loading, targeting, etc.) of PEG. On the other hand, researchers have also begun questioning the biological behavior of the phospholipid portion in PEG-PLs. Consequently, stealth lipopolymers consisting of non-phospholipids and PEG-alternatives are being developed. These novel lipopolymers offer the potential advantages of structural versatility, reduced complement activation, greater stability, flexible handling and storage procedures and low cost. In this article, we review the materials available as alternatives to PEG and PEG-lipopolymers for effective surface modification of liposomes.

Influence of shell composition on the resonance frequency of microbubble contrast agents

The effect of variations in microbubble shell composition on microbubble resonance frequency is revealed through experiment. These variations are achieved by altering the mole fraction and molecular weight of functionalized polyethylene glycol (PEG) in the microbubble phospholipid monolayer shell and measuring the microbubble resonance frequency. The resonance frequency is measured via a chirp pulse and identified as the frequency at which the pressure amplitude loss of the ultrasound wave is the greatest as a result of passing through a population of microbubbles. For the shell compositions used herein, we find that PEG molecular weight has little to no influence on resonance frequency at an overall PEG mole fraction (0.01) corresponding to a mushroom regime and influences the resonance frequency markedly at overall PEG mole fractions (0.050-0.100) corresponding to a brush regime. Specifically, the measured resonance frequency was found to be 8.4, 4.9, 3.3 and 1.4 MHz at PEG molecular weights of 1000, 2000, 3000 and 5000 g/mol, respectively, at an overall PEG mole fraction of 0.075. At an overall PEG mole fraction of just 0.01, on the other hand, resonance frequency exhibited no systematic variation, with values ranging from 5.7 to 4.9 MHz. Experimental results were analyzed using the Sarkar bubble dynamics model. With the dilatational viscosity held constant (10(-8) N·s/m) and the elastic modulus used as a fitting parameter, model fits to the pressure amplitude loss data resulted in elastic modulus values of 2.2, 2.4, 1.6 and 1.8 N/m for PEG molecular weights of 1000, 2000, 3000 and 5000 g/mol, respectively, at an overall PEG mole fraction of 0.010 and 4.2, 1.4, 0.5 and 0.0 N/m, respectively, at an overall PEG mole fraction of 0.075. These results are consistent with theory, which predicts that the elastic modulus is constant in the mushroom regime and decreases with PEG molecular weight to the inverse 3/5 power in the brush regime. Additionally, these results are consistent with inertial cavitation studies, which revealed that increasing PEG molecular weight has little to no effect on inethe rtial cavitation threshold in the mushroom regime, but that increasing PEG molecular weight decreases inertial cavitation markedly in the brush regime. We conclude that the design and synthesis of microbubbles with a prescribed resonance frequency is attainable by tuning PEG composition and molecular weight.

Mode specific elastic constants for the gel, liquid-ordered, and liquid-disordered phases of DPPC/DOPC/cholesterol model lipid bilayers

Using microscopic molecular theory, we determine the bending and saddle-splay constants of three-component lipid bilayers. The membrane contains cholesterol, dipalmitoyl-phosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC) and the predictions of the theory have been shown to qualitatively reproduce phase diagrams of giant unilamellar vesicles (GUVs) of the same three components. The bending and saddle-splay constants were calculated for the gel, liquid-ordered (lo) and liquid-disordered (ld) phases. By proper expansion of the free energy, the molecular theory enables us to determine the effects of the mode of membrane bending deformation on the value of the elastic constants for different phases. In particular, we refer to the ability of the molecules to arrange the composition between the two monolayers upon deformation. The bending and saddle-splay constants obtained from the free energy expansion can be expressed in terms of moments of the local lateral pressures and their derivatives, all evaluated for a symmetric planar bilayer. The effect of blocked vs. free exchange of lipids across the two monolayers on the values of the bending constant is as high as 50 k(B)Tin the ld phase to as high as 200 k(B)T in the lo phase. These results show that one must strongly consider the mode of deformation in determining the mechanical properties of lipid bilayers. We discuss how the different contributions to the lateral pressures affect the values of the elastic constants, including the effects of the cholesterol concentration and temperature on the membrane elastic constants. We also calculate the equilibrium binding concentrations of lipid tail anchors as a function of membrane curvature by explicitly determining the chemical potential difference of species across a curved bilayer. Our results are in excellent agreement with recent experimental results.

Bursting bubbles and bilayers

This paper discusses various interactions between ultrasound, phospholipid monolayer-coated gas bubbles, phospholipid bilayer vesicles, and cells. The paper begins with a review of microbubble physics models, developed to describe microbubble dynamic behavior in the presence of ultrasound, and follows this with a discussion of how such models can be used to predict inertial cavitation profiles. Predicted sensitivities of inertial cavitation to changes in the values of membrane properties, including surface tension, surface dilatational viscosity, and area expansion modulus, indicate that area expansion modulus exerts the greatest relative influence on inertial cavitation. Accordingly, the theoretical dependence of area expansion modulus on chemical composition - in particular, poly (ethylene glyclol) (PEG) - is reviewed, and predictions of inertial cavitation for different PEG molecular weights and compositions are compared with experiment. Noteworthy is the predicted dependence, or lack thereof, of inertial cavitation on PEG molecular weight and mole fraction. Specifically, inertial cavitation is predicted to be independent of PEG molecular weight and mole fraction in the so-called mushroom regime. In the “brush” regime, however, inertial cavitation is predicted to increase with PEG mole fraction but to decrease (to the inverse 3/5 power) with PEG molecular weight. While excellent agreement between experiment and theory can be achieved, it is shown that the calculated inertial cavitation profiles depend strongly on the criterion used to predict inertial cavitation. This is followed by a discussion of nesting microbubbles inside the aqueous core of microcapsules and how this significantly increases the inertial cavitation threshold. Nesting thus offers a means for avoiding unwanted inertial cavitation and cell death during imaging and other applications such as sonoporation. A review of putative sonoporation mechanisms is then presented, including those involving microbubbles to deliver cargo into a cell, and those - not necessarily involving microubbles - to release cargo from a phospholipid vesicle (or reverse sonoporation). It is shown that the rate of (reverse) sonoporation from liposomes correlates with phospholipid bilayer phase behavior, liquid-disordered phases giving appreciably faster release than liquid-ordered phases. Moreover, liquid-disordered phases exhibit evidence of two release mechanisms, which are described well mathematically by enhanced diffusion (possibly via dilation of membrane phospholipids) and irreversible membrane disruption, whereas liquid-ordered phases are described by a single mechanism, which has yet to be positively identified. The ability to tune release kinetics with bilayer composition makes reverse sonoporation of phospholipid vesicles a promising methodology for controlled drug delivery. Moreover, nesting of microbubbles inside vesicles constitutes a truly “theranostic” vehicle, one that can be used for both long-lasting, safe imaging and for controlled drug delivery.

Shaping vesicles-controlling size and stability by admixture of amphiphilic copolymer

The production of structurally well-defined unilamellar vesicles and the control of their stability are of utmost importance for many of their applications but still a largely unresolved practical issue. In the present work we show that by admixing small amounts of amphiphilic copolymer to the original components of a spontaneously vesicle-forming surfactant mixture we are able to control the self-assembly process in a systematic way. For this purpose we employed a zwitanionic model system of zwitterionic TMDAO and anionic LiPFOS. As the copolymer reduces the line tension of the intermediately formed disks, this translates directly into a longer disk growth phase and formation of correspondingly larger vesicles. By this approach we are able to vary their size over a large range and produce vesicles of extremely low polydispersity. Furthermore, the temporal stability of the formed vesicles is enhanced by orders of magnitude in proportion to the concentration of copolymer added. This is achieved by exerting kinetic control that allows engineering the vesicle structure via a detailed knowledge of the formation pathway as obtained by highly time-resolved SAXS experiments. Synthesis of such very well-defined vesicles by the method shown should in general be applicable to catanionic or zwitanionic amphiphiles and will have far reaching consequences for controlled nanostructure formation and application of these self-assembled systems.

Tunable diacetylene polymerized shell microbubbles as ultrasound contrast agents

Monodisperse gas microbubbles, encapsulated with a shell of photopolymerizable diacetylene lipids and phospholipids, were produced by microfluidic flow focusing, for use as ultrasound contrast agents. The stability of the polymerized shell microbubbles against both aggregation and gas dissolution under physiological conditions was studied. Polyethylene glycol (PEG) 5000, which was attached to the diacetylene lipids, was predicted by molecular theory to provide more steric hindrance against aggregation than PEG 2000, and this was confirmed experimentally. The polymerized shell microbubbles were found to have higher shell-resistance than nonpolymerizable shell microbubbles and commercially available microbubbles (Vevo MicroMarker). The acoustic stability under 7.5 MHz ultrasound insonation was significantly greater than that for the two comparison microbubbles. The acoustic stability was tunable by varying the amount of diacetylene lipid. Thus, our polymerized shell microbubbles are a promising platform for ultrasound contrast agents.

The functional roles of poly(ethylene glycol)-lipid and lysolipid in the drug retention and release from lysolipid-containing thermosensitive liposomes in vitro and in vivo

Triggered release of liposomal contents following tumor accumulation and mild local heating is pursued as a means of improving the therapeutic index of chemotherapeutic drugs. Lysolipid-containing thermosensitive liposomes (LTSLs) are composed of dipalmitoylphosphatidylcholine (DPPC), the lysolipid monostearoylphosphatidylcholine (MSPC), and poly(ethylene glycol)-conjugated distearoylphosphatidylethanolamine (DSPE-PEG(2000)). We investigated the roles of DSPE-PEG(2000) and lysolipid in the functional performance of the LTSL-doxorubicin formulation. Varying PEG-lipid concentration (0-5 mol%) or bilayer orientation did not affect the release; however, lysolipid (0-10 mol%) had a concentration-dependent effect on drug release at 42 degrees C in vitro. Pharmacokinetics of various LTSL formulations were compared in mice with body temperature controlled at 37 degrees C. As expected, incorporation of the PEG-lipid increased doxorubicin plasma half-life; however, PEG-lipid orientation (bilayer vs. external leaflet) did not significantly improve circulation lifetime or drug retention in LTSL. Approximately 70% of lysolipid was lost within 1 h postinjection of LTSL, which could be due to interactions with the large membrane pool of the biological milieu. Considering that the present LTSL-doxorubicin formulation exhibits significant therapeutic activity when used in conjunction with mild heating, our current study provided critical insights into how the physicochemical properties of LTSL can be tailored to achieve better therapeutic activity.

Paclitaxel-loaded Pluronic P123/F127 mixed polymeric micelles: formulation, optimization and in vitro characterization

The objective of this study was to optimize and characterize a novel polymeric mixed micelle composed of Pluronic P123 and F127 loaded with paclitaxel (PTX). A Doehlert matrix design was utilized to investigate the effect of four variables, namely P123 mass fraction, amount of water, feeding of PTX and hydration temperature on the responses including drug-loading coefficient (DL %), encapsulation ratio (ER %) and the percentage of PTX precipitated from the drug-loaded mixed micelles after 48 h at 37 (PTX precipitated %) for improvement of drug solubilization efficiency and micelle stability. PTX-loaded P123/F127 mixed micelles were prepared by thin-film hydration method. The optimized formulation showed a particle size of about 25 nm with ER %>90%, and a sustained release behavior compared to Taxol. Micelle formation was confirmed by NMR spectroscopy. The mixed micelles had a low CMC of 0.0059% in water. In addition, micelle stability studies implied that introduction of Pluronic F127 (33 wt%) into P123 micelle system significantly increased the stability of PTX-loaded micelles. More importantly, in vitro cytotoxicity was assessed using human lung adenocarcinoma cell lines SPC-A1 and A-549 and was compared to Taxol and the free drug. The cell viability assay against A-549 cells exhibited the 50% inhibition concentration (IC50) of PTX-loaded P123/F127 mixed micelles (0.1 microg/ml) was much lower than those of Taxol injection (0.4 microg/ml) and the free PTX (1.7 microg/ml). Therefore, PTX-loaded P123/F127 mixed micelles may be considered as an effective anticancer drug delivery system for cancer chemotherapy.

Polymer-vesicle association

Mixed polymer-surfactant systems have been intensively investigated in the last two decades, with the main focus on surfactant micelles as the surfactant aggregate in interaction. The main types of phase behavior, driving forces and structural/rheological effects at stake are now fairly well understood. Polymer-vesicle systems, on the other hand, have received comparatively less attention from a physico-chemical perspective. In this review, our main goal has been to bridge this gap, taking a broad approach to cover a field that is in clear expansion, in view of its multiple implications for colloid and biological sciences and in applied areas. We start by a general background on amphiphile self-assembly and phase separation phenomena in mixed polymer-surfactant solutions. We then address vesicle formation, properties and stability not only in classic lipids, but also in various other surfactant systems, among which catanionic vesicles are highlighted. Traditionally, lipid and surfactant vesicles have been studied separately, with little cross-information and comparison, giving duplication of physico-chemical interpretations. This situation has changed in more recent times. We then proceed to cover more in-depth the work done on different aspects of the associative behavior between vesicles (of different composition and type of stability) and different types of polymers, including polysaccharides, proteins and DNA. Thus, phase behavior features, effects of vesicle structure and stability, and the forces/mechanisms of vesicle-macromolecule interaction are addressed. Such association may generate gels with interesting rheological properties and high potential for applications. Finally, special focus is also given to DNA, a high charge polymer, and its interactions with surfactants, and vesicles, in particular, in the context of gene transfection studies.

Biophysical approaches to protein-induced membrane deformations in trafficking

Membrane traffic requires membrane deformation to generate vesicles and tubules. Strong evidence suggests that assembly of curvature-active proteins can drive such membrane shape changes. Well-documented pathways often involve protein scaffolds, in particular coats (clathrin or COP). However, membrane curvature should, in principle, be influenced by any protein binding asymmetrically on a membrane; large membrane morphological changes could result from their aggregation. In the case of Shiga toxin or viral matrix proteins, tubules and buds appear to result from the cargo-driven formation of protein-lipid nanodomains, showing that collective protein behaviour is crucial in the process. We argue here that a combination of in vitro experiments on giant unilamellar vesicles and theoretical modelling based on statistical physics is ideally suited to tackle these collective effects.

Budding and asymmetric protein microcompartmentation in giant vesicles containing two aqueous phases

We report the effect of external osmolarity on giant lipid vesicles containing an aqueous two-phase system (ATPS GVs). The ATPS, which is comprised of poly(ethyleneglycol) [PEG], dextran, and water, serves as a primitive model of the macromolecularly crowded environment of the cytoplasm. Coexisting PEG-rich and dextran-rich aqueous phases provide chemically dissimilar microenvironments, enabling local differences in protein concentration to be maintained within single ATPS GVs. The degree of biomolecule microcompartmentation can be increased by exposing the ATPS GVs to a hypertonic external solution, which draws water out of the vesicles, concentrating the polymers. Enrichment of a protein, soybean agglutinin, in the dextran-rich phase improves from 2.3-fold to 10-fold with an increase in external osmolarity from 100 to 200 mmol/kg. In some cases, budding occurs, with the bud(s) formed by partial expulsion of one of the two polymer-rich aqueous phases. Budding results in asymmetry in the internal polymer and biomolecule composition, giving rise to polarity in these primitive model cells. Budding is observed with increasing frequency as external ionic strength increases, when membrane elasticity permits, and can be reversed by decreasing external osmolarity. We note that the random symmetry-breaking induced by simple osmotic shrinkage resulted in polarity in both the structure and internal protein distribution in these primitive model cells. Budding in ATPS-containing GVs thus offers an experimental model system for investigating the effects of biochemical asymmetry on the length scale of single cells.

Structure and size of spontaneously formed aggregates in Aerosol OT/PEG mixtures: Effects of polymer size and composition

Dynamic light scattering and Cryo-TEM measurements have allowed us to obtain the size and structure of spontaneous aggregates formed by mixtures of Aerosol OT, AOT, and ethylene glycol polymers of different molecular mass. The results presented in this work show that small unilamellar vesicles predominate in pure Aerosol OT solutions and in dilute polymer solutions mixed with AOT. In the latter case, elongated micelles coexist with unilamellar vesicles. When polymer concentration increases above a certain concentration, the small vesicles disappear and the size of the elongated micelles decreases to a radius compatible with spherical micelles. For PEG concentrations above the overlapping ones, spherical micelles coexist with very large aggregates probably formed by large rod like micelles or by superstructures of elongated micelles embedded in a polymer network. This behavior is consistent with theoretical models based in molecular mean-field theory [M. Rovira-Bru, D.H. Thompson, I. Szleifer, Biophys. J. 83 (2002) 2419]. The properties of the different types of aggregates are obtained by fluorescence spectroscopy and electrophoretic mobility measurements.

Effects of poly (ethylene glycol) chains conformational transition on the properties of mixed DMPC/DMPE-PEG thin liquid films and monolayers

Foam thin liquid films (TLF) and monolayers at the air-water interface formed by DMPC mixed with DMPE-bonded poly (ethylene glycol)s (DMPE-PEG(550), DMPE-PEG(2000) and DMPE-PEG(5000)) were obtained. The influence of both (i) PEG chain size (evaluated in terms of Mw) and mushroom-to-brush conformational transition and (ii) of the liposome/micelle ratio in the film-forming dispersions, on the interfacial properties of mixed DMPC/DMPE-PEG films was compared. Foam film studies demonstrated that DMPE-PEG addition to foam TLFs caused (i) delayed kinetics of film thinning and black spot expansion and (ii) film stabilization. At the mushroom-to-brush transition, due to steric repulsion increased DMPE-PEG films thickness reached 25 nm while pure DMPC films were only 8 nm thick Newton black films. It was possible to differentiate DMPE-PEG(2000/5000) from DMPE-PEG(550) by the ability to change foam TLF formation mechanism, which could be of great importance for "stealth" liposome design. Monolayer studies showed improved formation kinetics and equilibrium surface tension decrease for DMPE-PEG monolayers compared with DMPC pure films. SEM observations revealed "smoothing" and "sealing" of the defects in the solid-supported layer surface by DMPE-PEGs adsorption, which could explain DMPE-PEGs ability to stabilize TLFs and to decrease monolayer surface tension. All effects in monolayers, foam TLFs and solid-supported layers increased with the increase of PEG Mw and DMPE-PEG concentration. However, at the critical DMPE-PEG concentration (where mushroom-to-brush conformational transition occurred) maximal magnitude of the effects was reached, which only slightly changed at further DMPE-PEG content and micelle/liposome ratio increase.

Synthetic Polymers and Biomembranes. How Do They Interact?: Atomistic Molecular Dynamics Simulation Study of PEO in Contact with a DMPC Lipid Bilayer

The understanding of interactions of poly(ethylene glycol) (PEG) or poly(ethylene oxide) (PEO) with biological interfaces has important technological application in industry and in medicine. In this paper, structural and dynamical properties of PEO at the dimyristoylphospatidylcholine (DMPC) bilayer/water interface have been investigated by molecular dynamics (MD) and steered molecular dynamics (SMD) simulations. The structural properties of a PEO chain in bulk water, at the water/vacuum interface, and in the presence of the membrane were compared with available experimental data. The presence of a barrier for the PEO penetration into the DMPC bilayer has been found. A qualitative estimation of the barrier provided a value equal to approximately 19 kJ/mol, that is, 7 times the value of kT at 310 K.

Thin liquid films and monolayers of DMPC mixed with PEG and phospholipid linked PEG

In this work thin liquid films (TLFs) and monolayers at the air/water interface formed by dimyristoylphosphatidylcholine (DMPC) and by DMPC mixed with poly ethylene glycols (PEGs) and dimyristoylphosphatidylethanolamine (DMPE) linked PEGs were studied. Film forming dispersions were composed of two types of particles: liposomes and micelles. TLFs stability, threshold concentration C(t) (i.e., the minimum one for stable film formation), and hydrodynamic behavior were measured. At equivalent conditions, DMPC films were Newton black films (real bilayers), while DMPE-PEGs films were much thicker with free water between the monolayers. DMPE-PEG addition to DMPC films caused both C(t) decrease (depending on PEG moiety length and Mw) and change of TLF formation mechanism. TLFs' hydrodynamic behavior also strongly depended on DMPE-PEG content and Mw. It was observed that thinning of the DMPC and DMPE-PEGs films continued to different film types and thickness, being much thicker for the latter films. Addition of free PEGs (PEG-200/6000) did not alter TLF type or stability, but changed TLF thinning time, confirming that free PEGs with Mw<8000 could not penetrate in the membrane and alter "near-membrane" water layer viscosity. Monolayer studies showed improved formation kinetics of both adsorbed and spread films, decrease of surface tension (equilibrium and dynamic), and of film compression/decompression histeresis area in DMPE-PEGs monolayers compared with DMPC pure films. Our study shows that combining the models of phospholipid TLFs and monolayers provide the opportunity to investigate the properties of membrane surface and to clarify some mechanisms of its interactions with membrane-active agents.

Rigid rod anchored to infinite membrane

We investigate the shape deformation of an infinite membrane anchored by a rigid rod. The density profile of the rod is calculated by the self-consistent-field theory and the shape of the membrane is predicted by the Helfrich membrane elasticity theory [W. Helfrich, Z. Naturforsch. 28c, 693 (1973)]. It is found that the membrane bends away from the rigid rod when the interaction between the rod and the membrane is repulsive or weakly attractive (adsorption). However, the pulled height of the membrane at first increases and then decreases with the increase of the adsorption strength. Compared to a Gaussian chain with the same length, the rigid rod covers much larger area of the membrane, whereas exerts less local entropic pressure on the membrane. An evident gap is found between the membrane and the rigid rod because the membrane's curvature has to be continuous. These behaviors are compared with that of the flexible-polymer-anchored membranes studied by previous Monte Carlo simulations and theoretical analysis. It is straightforward to extend this method to more complicated and real biological systems, such as infinite membrane/multiple chains, protein inclusion, or systems with phase separation.

Vesicle Formation from Temperature Jumps in a Nonionic Surfactant System

When heating a dilute sample of the binary system of tetraethyleneglycol dodecyl ether (C12E4) and water from the micellar phase (L1) into the two-phase region of a lamellar phase (L(alpha)), and excess water (W) vesicles are formed. During heating, one passes a region of phase separation in the micellar phase (L1' + L1'') where the initial micelles rapidly fuse into larger aggregates forming the concentrated L1 phase (L1'') with a structure of branched cylindrical micelles, a so-called "living network". The static correlation length of the micelles are increasing with increasing concentration, from ca. 10 nm to 80 nm in the concentration range of 0.0001 g/cm3-0.0035 g/cm3. The overlap concentration was determined to 0.0035 g/cm3. When the temperature reaches the L1' + L(alpha) region the network particles transform into bilayer vesicles with a z-average apparent hydrodynamic radius in the order of 200 nm depending on the composition. The size of the final vesicles depends on the extent of aggregation/fusion in the L1' + L1'' region and hence on the rate of heating. The aggregation/fusion in the L1' + L1'' is slower than diffusion-limited aggregation, and it is shown that 1/100 of the collisions are sticky results in the fusion event.

Micellar formulations for drug delivery based on mixtures of hydrophobic and hydrophilic Pluronic® block copolymers

Micelles formed by Pluronic block copolymers (PBC) have been studied in multiple applications as drug delivery systems. Hydrophobic PBC form lamellar aggregates with a higher solubilization capacity than spherical micelles formed by hydrophilic PBC. However, they also have a larger size and low stability. To overcome these limitations, binary mixtures from hydrophobic PBC (L121, L101, L81, and L61) and hydrophilic PBC (F127, P105, F87, P85, and F68) were prepared. In most cases, PBC mixtures were not stable, revealing formation of large aggregates and phase separation within 1-2 day(s). However, stable aqueous dispersions of the particles were obtained upon (1). sonication of the PBC mixtures for 1 or 2 min or (2). heating at 70 degrees C for 30 min. Among all combinations, L121/F127 mixtures (1:1% weight ratio) formed stable dispersions with a small particle size. The solubilizing capacity of this system was examined using a model water-insoluble dye, Sudan (III). Mixed L121/F127 aggregates exhibited approximately 10-fold higher solubilization capacity compared to that of F127 micelles. In conclusion, stable aqueous dispersions of nanoscale size were prepared from mixtures of hydrophobic and hydrophilic PBC by using the external input of energy. The prepared mixed aggregates can efficiently incorporate hydrophobic compounds.

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.

Synthesis and Characterization of Oligomaltose-Grafted Lipids with Application to Liposomes

Novel glycolipids, which contain 2 and 15 oligomaltose units and a phosphatidylethanolamine, were synthesized and characterized by FTIR and (1)H-NMR spectroscopies. The well-defined linear structure of the glycolipids was assured by an end-point conjugation strategy using selective oxidation of the reducing end groups of maltose oligosaccharides, followed by aminolysis with distearoylphosphatidylethanolamine. The intermediate acids react selectively with amines to form amide linkages, catalyzed by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxysuccinimide. Conformations of the glycolipids at the air-water interface were proposed based on the film balance measurements. The unique conformations of glycolipids at interfaces may offer advantages over traditional PEO-derived lipids in regard to their applications for sterically stabilizing liposomes. The glycolipids demonstrated the ability for sterically stabilizing liposome dispersions, as determined by turbidity measurements.

Stable polymeric nanoballoons: lyophilization and rehydration of cross-linked liposomes

The cross-linking of supramolecular assemblies of hydrated lipids is an effective method to stabilize these assemblies to disruption by surfactants or aqueous alcohol. The heterobifunctional lipids, Acryl/DenPC(16,18) and Sorb/DenPC(18,21), are examples of a new class of polymerizable lipid designed for the creation of cross-linked lipid structures. The robust nature of cross-linked liposomes was demonstrated by lyophilization of the liposomes followed by their essentially complete redispersion in water. The resulting liposomes were compared to the original sample by quasi-elastic light scattering and transmission electron microscopy. There was no major change in the size or structure of the cross-linked liposomes after rehydration of the freeze-dried powder of liposomes. Moreover, the rehydrated cross-linked liposomes continued to be resistant to surfactant solubilization. Neutral cross-linked liposomes were predominantly redispersed after freeze-drying with the aid of bath sonication. The small amount of residual liposome aggregation observed with neutral liposomes could be prevented by incorporating a surface charge into the liposome or attaching hydrophilic polymers, for example, poly(ethylene glycol), onto the liposome.

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.

Lipid chain length effect on the phase behaviour of PCs/PEG:2000-PEs mixtures. A spin label electron spin resonance and spectrophotometric study

Spin-label electron spin resonance (ESR) spectroscopy and spectrophotometry at fixed wavelength are used to study fully hydrated aqueous dispersions of phosphatidylcholines (PCs) with poly(ethylene glycol:2000)-phosphatidylethanolamines (PEG:2000-PEs). PEG:2000-PE is a micelle-forming polymer-lipid that is extensively used for increasing the lifetime of PC liposomes in the blood circulation through a steric stabilisation effect. The PC lipids and the PEG:2000-PE polymer-lipids have the same acyl chain length of either dimiristoyl (DM) or distearoyl (DS) chains. DMPC/PEG:2000-DMPE and DSPC/PEG:2000-DSPE mixtures were investigated over the entire range of relative compositions (0-100 mol%). In both dispersions, the low-temperature conventional spin label ESR spectra and the temperature dependence of the absorbance at 400 nm give an indication of the conversion from lamellae to micelles with increasing PEG:2000-PEs content. The physical state of the lipid assemblies, lamellar or micellar, is dependent not only on PEG:2000-PEs content, but also on the length of hydrocarbon chain of the lipid matrix. Micellisation is attained more readily in dispersions with longer hydrocarbon chains (i.e. in DSPC/PEG:2000-DSPE mixtures) than in those with shorter acyl chains (i.e. in DMPC/PEG:2000-DMPE mixtures). Saturation transfer ESR (ST-ESR) and absorbance measurements reflect the disaggregation of the bilayers and a reduction in the size of the lipid aggregates by PEG:2000-PEs at low content.

Spontaneous vesiculation

The thermodynamics of vesicle formation was analyzed by using the elastic bending energy approach. Several different possibilities of spontaneous vesiculation, due to soft bilayers, non-zero spontaneous curvature and Gaussian curvature, respectively, were presented and discussed. Intermediate structures in the closed vesicle-disklike mixed micelle phase transition could be either cup-like particles or open bilayers partially rolled into lipid tubules.

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.

Molecular and mesoscopic properties of hydrophilic polymer-grafted phospholipids mixed with phosphatidylcholine in aqueous dispersion: interaction of dipalmitoyl N-poly(ethylene glycol)phosphatidylethanolamine with dipalmitoylphosphatidylcholine studied by spectrophotometry and spin-label electron spin resonance

Spin-label electron spin resonance (ESR) spectroscopy, together with optical density measurements, has been used to investigate, at both the molecular and supramolecular levels, the interactions of N-poly(ethylene glycol)-phosphatidylethanolamines (PEG-PE) with phosphatidylcholine (PC) in aqueous dispersions. PEG-PEs are micelle-forming hydrophilic polymer-grafted lipids that are used extensively for steric stabilization of PC liposomes to increase their lifetimes in the blood circulation. All lipids had dipalmitoyl (C16:0) chains, and the polymer polar group of the PEG-PE lipids had a mean molecular mass of either 350 or 2000 Da. PC/PEG-PE mixtures were investigated over the entire range of relative compositions. Spin-label ESR was used quantitatively to investigate bilayer-micelle conversion with increasing PEG-PE content by measurements at temperatures for which the bilayer membrane component of the mixture was in the gel phase. Both saturation transfer ESR and optical density measurements were used to obtain information on the dependence of lipid aggregate size on PEG-PE content. It is found that the stable state of lipid aggregation is strongly dependent not only on PEG-PE content but also on the size of the hydrophilic polar group. These biophysical properties may be used for optimized design of sterically stabilized liposomes.