Effect of cholesterol or phospholipid incorporation on the chemical stability of the muramyldpeptide derivative B30-MDP in mixed vesicles

Application of the muramyldipeptide derivative B30-MDP to liposomal vaccines will aid in the development of improved high immunogenicity vaccines. To give full play to the effectiveness of B30-MDP as a liposomal vaccine, it is important to evaluate the effect of cholesterol, dimyristoylphosphatidylcholine (DMPC) or distearoylphosphatidylcholine (DSPC) incorporation on the chemical stability of B30-MDP and physicochemical properties of B30-MDP/lipid mixed vesicles from the view point of pharmaceutics.The observed degradation rate constants of B30-MDP by hydrolysis in B30-MDP/cholesterol mixed vesicles were increased with increasing concentration of cholesterol, however, those in B30-MDP/DMPC and B30-MDP/DSPC mixed vesicles were unchanged with increasing concentration of DMPC and DSPC. The degradation behavior of B30-MDP was then compared with physicochemical properties of B30-MDP/lipid mixed vesicles, such as membrane fluidity and particle size. It was apparent that the degradation of B30-MDP in B30-MDP/cholesterol mixed vesicles was influenced by the particle size, but not by the fluidity of the membranes. In the case of B30-MDP/phospholipid mixed vesicles, MDP/phospholipid mixed vesicles, the degradation of B30-MDP was not influenced by either the membranes' fluidity or the particle size of the mixed vesicles.It is considered that the degradation of B30-MDP in the mixed vesicles is dependent on the membrane state, and the addition of cholesterol to B30-MDP vesicle inhibits the mutual interaction of MDP regions, whereas the addition of phospholipids hardly influences the mutual interaction of MDP regions, possibly owing to phase separation between B30-MDP and phospholipids.     

Integrity of liposomes in presence of various formulation excipients, when dispersed in aqueous media and in hydrogels

The integrity of liposomes when dispersed in presence of various common formulation excipients is studied. Additionally, the effect of the excipients on the release of calcein from the same liposomes when dispersed in hydrogels is investigated and the results of the two sets of experiments are compared. Propyleneglycol (PG), transcutol CG (TR), cremophor EL (CR) and labrafac hydro WL 1219 (LB) are used at 10 or 25% (v/v) and the retention of liposome encapsulated calcein is followed for 24 or 48 h periods. Calcein entrapping multilamellar liposomes composed of phosphatidylcholine (PC) or 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) with or without addition of different amounts of cholesterol (Chol) were prepared by the thin film hydration method.

Experimental results reveal that liposomes are affected more by the excipients in the order: LB > CR > PG  TR. Particularly LB and in some cases also CR result in rapid release of most or the entire vesicle encapsulated dye. Addition of Chol in both PC and DSPC liposomes results in substantial increase of vesicle integrity in all cases. Concerning the release of calcein form the liposomal gels, from DSPC/Chol (1:1) liposomal gels calcein release was not affected by addition of 25% of TR or PG in all gels studied, but LB caused a significant increase in calcein release. However, from PC-liposomal gels even TR and PG (at 25%), increases calcein release.

Conclusively, the results of this study suggest that liposomes are protected from excipients when dispersed in gels compared to aqueous media. This should be taken into account when liposomal drug formulations are designed.     

Liposomal drugs dispersed in hydrogels. Effect of liposome, drug and gel properties on drug release kinetics

Release of calcein and griseofulvin (GRF) from control (gels in which solutes are dissolved in) and liposomal gels was studied using agarose-assisted immobilization as a technique to separate gels from drug-receptor compartments. Liposomes composed of phosphatidylcholine (PC) or distearoyl-glycero-PC and cholesterol (DSPC/Chol), and incorporating calcein or GRF were prepared by thin film hydration. After cleaning the liposomes they were dispersed in different hydrogels (carbopol 974 [1, 1.5 or 2% (w/w)], hydroxylethyl-cellulose (HEC) [4% (w/w)], or a mixture of the two), and release of calcein or GRF was followed by fluorescence or photometric technique, respectively. Results show that calcein release from liposomal gels is slower compared to control gels, and can be further retarded by using rigid-membrane liposomes (faster release from PC-liposome compared to DSPC/Chol-liposome gels). Additionally, calcein release is not affected by the lipid amount loaded (in the range from 2 to 8 mg/ml), therefore solute loading can be controlled according to needs.

Oppositely, GRF release from liposomal gels is determined by drug loading. At high drug loading levels (compared to GRF aqueous solubility), GRF is released with constant rate from liposomal gels irrespective of liposome type (PC or DSPC/Chol). Thereby, for amphiphilic/lipophilic drugs, drug properties (solubility, log P) determine the system behavior.

Calcein and GRF release from control carbopol gels is faster compared to HEC and mixture gels. The same is true for calcein in liposomal gels. Carbopol gel rheological properties were found to be significantly different (compared to the other gels), implying that these characteristics are important for drug diffusion from gels.     

Surface modification of liposomes by saccharides: vesicle size and stability of lactosyl liposomes studied by photon correlation spectroscopy

The cell glycocalyx is an attractive model for surface modification of liposomes, because its hydrated oligosaccharide layer inhibits nonspecific protein adsorption and can provide specificity towards desired sites. Here, we report on the use of lactose as a model saccharide to modify the liposome surface and examine the vesicle size and stability. Two kinds of lactosyl lipids, including lactosyl ether-lipid (6a) and lactosyl ester-lipid (6b), which contain octadecyl and octadecanoyl as the lipid tails, respectively, were synthesized and their liposomes were prepared by the extrusion method. The effects of glycolipid structure, concentration, and the pore size of the extrusion membrane on vesicle size and stability were investigated at room temperature by photon correlation spectroscopy (PCS). All liposomes with 5 or 10 mol% of lactosyl lipids had a narrow size distribution and remained stable at room temperature for at least one month, which is comparable to 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)- and poly(ethylene glycol) (PEG)-liposomes. The maximum incorporation of lactosyl ester-lipid into liposomes was 15 mol%, compared with only 10 mol% for the lactosyl ether-lipid. The lactosyl ester-liposomes had better stability and exhibited less size change than the lactosyl ether-liposomes at 15 or 20 mol% of lactosyl lipids incorporated. This may be attributed to the better structural compatibility of lactosyl ester-lipid with DSPC. The PCS results show that the glycolipid structure and concentrations are major factors that affect vesicle stability, while the pore size of extrusion membranes has no influence.     

Antitumor effect of liposomal histone deacetylase inhibitor-lipid conjugates in vitro

Histone deacetylase inhibitor (HDACI), suberoylanilide hydroxamic acid (SAHA), approved by the Food and Drug Administration (FDA) for the treatment of cutaneous T cell lymphoma, is a promising new treatment strategy for various cancers. In this study, we hypothesized that a liposomal formulation of HDACI might efficiently deliver HDACI into tumors. To incorporate HDACI efficiently into the liposomal membrane, we synthesized six HDACI-lipid conjugates, in which polyethylene glycol(2000) (PEG(2000))-lipid or cholesterol (Chol) was linked with a potent hydroxamic acid, HDACI, SAHA or K-182, by cleavable linkers, such as ester, carbamide and disulfide bonds. Liposomal HDACI-lipid conjugates were prepared with distearoylphosphatidylcholine (DSPC) and HDACI-Chol conjugate or with DSPC, Chol and HDACI-PEG-lipid conjugates, and their cytotoxicities were evaluated for human cervix tumor HeLa and mouse colon tumor Colon 26 cells. Among the liposomes, liposomal oleyl-PEG(2000)-SAHA conjugated with SAHA and oleyl-PEG(2000) via a carbamate linker showed higher cytotoxicity via hyperacetylation of histone H3 and induction of caspase 3/7 activity. These results suggested that liposomal HDACI-lipid conjugates may be a potential tool for cancer therapy.     

Length scale of polyisopirene and dynamic light scattering and structure of sodium-2-diethylhexyl sulfosuccinate/water/decane

The mixture of polyisopirene (PI) and sodium-2-diethylhexyl sulfosuccinate /decane/water microemulsion (ME) at AOT to water molar ratio (X = 30) and droplet mass fraction (m_f,drop = 0.08) was studied with dynamic light scattering and small-angle X-ray scattering (SAXS). The light scattering was used to obtain the diffusion coefficient of Brownian motion of the nano-droplets at different polymer concentrations and molecular weights (1000 and 4700) in the ME. The dynamics of the nano-droplets decreased with the increase of molecular weight (from 1000 to 4700) and concentration (from 0.01 to 0.09) of PI. The study of the structure by SAXS showed that with increase of PI (MW = 1000) mass fraction from 0.01 to 0.09 at ME, the size of the droplets changes from 4.5 to 4.3 nm and with increase of PI (MW = 4700) concentration at ME, the size of droplets changes from 4.8 to 4.4 nm. The size ratio of droplets to polymer decreased with increase of concentration and molecular weight of polymer and also the interaction between the droplets increased with increase of polymer concentration.     

Liposomal structure: A comparative study on light scattering and chromatography techniques

Liposomes play an important role in medical and pharmaceutical science as, nanoscale drug carriers. One of the most important features is their size and size distribution, influencing both their bio-distribution and their targeting efficiency to tumors and also therapeutic activity of liposomal antitumor drugs. In this study, the effect of preparation method and molecular interaction on size and shape of liposome was studied. The size and shape characterization of liposomes was performed by asymmetrical flow field-flow fractionation coupled online with multi-angle light scattering (AF4-MALS). The size distributions obtained by AF4-MALS were compared to mean particle sizes and size distribution measured with other standard method such as Photon Correlation Spectroscopy (PCS). Furthermore, the effect of molecular interaction (hydrophilic and hydrophobic model drugs) on liposomal structure was assessed.     

Formation and Characterization of Reversed Micelles Composed of Phospholipids and Fatty Acids

The formation of reversed micellar systems composed of phosphatidylcholine (PC) and fatty acid was newly demonstrated by a significant increase in water content in the organic ethyl oleate phase when the micelles were prepared by the contact method. The solubilized water concentration in the reversed micellar organic phase reached 3 wt%. The new systems are expected to be used as highly biocompatible reversed micellar systems. The structure of the reversed micelles composed of PC and oleic acid was characterized by determining the water concentration and by small-angle X-ray scattering analysis. The reversed micelles composed of PC and oleic acid formed in ethyl oleate were spherical. The radius of gyration was between 30 and 50 Å. The size of the reversed micelles decreased with an increase in the oleic acid concentration and was independent of the PC concentration. Experimental results indicated that the structure of the reversed micellar system was determined by the oleic acid concentration. An increase in the PC concentration caused an increase in the number of reversed micelles of the same size. These reversed micellar systems are expected to be used as solubilization media in pharmaceutical and food industries because they are not toxic.     

Novel asparagine-derived lipid enhances distearoylphosphatidylcholine bilayer resistance to acidic conditions

A novel asparagine-derived lipid analogue (ALA(11,17)) bearing a tetrahydropyrimidinone headgroup and two fatty chains (11 and 17 indicate the lengths of linear alkyl groups) was synthesized in high yield and purity. The thin film hydration of formulations containing 5 mol % or greater ALA(11,17) in distearoylphosphatidylcholine (DSPC) generated multilamellar vesicles (MLVs) that remained unaggregated according to optical microscopy, while those formed from DSPC only were highly clustered. The MLVs were processed into unilamellar liposomes via extrusion and were characterized by dynamic light scattering (DLS), zeta potential, turbidity, and scanning electron microscopy (SEM) analysis. Results show that the presence of ALA(11,17) in DSPC liposomes significantly alters the morphology, colloidal stability, and retention of encapsulated materials in both acidic and neutral conditions. The ability of ALA(11,17)-hybrid liposomes to encapsulate and retain inclusions under neutral and acidic conditions (pH < 2) was demonstrated by calcein dequenching experiments. DLS and SEM confirmed that ALA(11,17)/DSPC liposomes remained intact under these conditions. The bilayer integrity observed under neutral and acidic conditions and the likely biocompatibility of these fatty amino acid analogues suggest that ALA(11,17) is a promising additive for modulating phosphatidylcholine lipid bilayer properties.     

Properties of mixed DOTAP-DPPC bilayer membranes as reported by differential scanning calorimetry and dynamic light scattering measurements

We have investigated the effect of a cationic lipid [DOTAP] on both the thermotropic phase behavior and the structural organization of aqueous dispersions of dipalmitoyl-phosphatidylcholine [DPPC] by means of high-sensitivity differential scanning calorimetry and dynamic light scattering measurements. We find that the incorporation of increasing quantities of DOTAP progressively reduces the temperature and the enthalpy of the gel-to-liquid crystalline transition. We are further showing that, in mixed DOTAP-DPPC systems, the reduction of the phase transition temperature is accompanied by a reduction of the average size of the structures present in the aqueous mixtures, whatever the DOTAP concentration is. These results, which extend a previous investigation by Campbell et al. (Campbell, R. B.; Balasubramanian, S. V.; Straubinger, R. M.; Biochim. Biosphys. Acta 2001, 27, 1512.) limited to a DOTAP concentration below 20 mol %, confirm that the insertion of cationic head groups in zwitterionic phosphatidylcholine bilayers facilitates the formation of stable, relatively small, unilamellar vesicles. This self-assembling restructuring from an aqueous multilamellar structure toward a liposomal phase is favored by decreasing the phospholipid phase transition temperature and by increasing the temperature of the system. This reduction of the average size and the appearance of a stable liposomal phase is also promoted by a heating and cooling thermal treatment.     

拉链型脂肽的温控开关效应

亮氨酸拉链型脂肽是由两条肽链以螺旋结构依靠疏水作用并列结合形成的二聚体,当温度升至其相变温度时,其螺旋结构解旋继而变为无序链状结构。利用该类脂肽的温敏性能,本文设计、合成得到一组具有温敏性的拉链型脂肽,将其与磷脂混合制备温敏性脂质体。用圆二色谱测定磷脂双分子层上脂肽的二级结构,动态光散射测定脂肽-脂质体的粒径及电位;荧光偏振法测定脂质体膜的流动性;采用紫外分光光度计考察阿霉素(DOX)在37.0、45.0°C下的释放行为。结果表明,含有脂肽的脂质体具备较好的温敏性,胆固醇含量、脂质体膜的流动性,对脂肽的温控开关效应有一定的影响。脂肽-脂质体作为一种新型的温敏性药物载体展现了其较好的应用前景。     

Molecular interactions between phospholipids and mangostin in a lipid bilayer

Molecular interactions between phospholipids and mangostin in a lipid bilayer have been investigated in terms of the maximum additive concentration (MAC) of mangostin in liposomes, the surface potential, particle size, microscopic-viscosity and microscopic-polarity of liposomes, and the permeability of glucose. The mangostin used is a natural product extract: 1,3,6-trihydroxy-7-methoxy-2,8-bis(3-methyl-2-butenyl)-9-xanthenenone.

The MAC of mangostin was fairly dependent upon the nature of the liposomes (uncharged, negatively charged or positively charged). Solubilization of mangostin in the liposomal bilayer resulted in both an increase in the negative charge on the liposomal surface, strenghthening the state of the bilayer membrane, and a depression in the release of the glucose involved. Mangostin was found to temporarily stabilize the liposomal bilayer, although the bilayer membrane is still unstable in the long run.     

Membrane interactions of ternary phospholipid/cholesterol bilayers and encapsulation efficiencies of a RIP II protein

Membrane interactions of liposomes of ternary phospholipid/cholesterol bilayers are investigated. These interactions lead to discoidal deformations and regular aggregations and are strongly enhanced by the presence of mistletoe lectin (ML), a RIP II type protein. The encapsulation of ML into liposomal nanocapsules is studied with a systematic variation of the lipid composition to monitor its effect on the physical properties: entrapment, mean size, morphology, and stability. Extrusion of multilamellar vesicles through filters 80 nm pore size was used for the generation of liposomes. The mean sizes of liposomes ranged between 120 and 200 nm in diameter with narrow size distributions. The increase in flow rate with pressure for three dioleoylphosphatidylcholine (DOPC)/cholesterol (Chol)/dipalmitoylphosphatidylcholine (DPPC) lipid mixtures was linear and allowed to extrapolate to the minimum burst pressure of the liposomal bilayers. From the minimum pressures P(min), the bilayer lysis tensions gamma(l) were determined. The increase in P(min) and gamma(l) with an increasing content of a saturated phosopholipid (DPPC) indicates that DPPC increases the mechanical strength of lipid bilayers. Apparently, DPPC, like cholesterol, leads to a less compressible surface and a more cohesive membrane. After preparation, vesicle solutions were purified by gel permeation chromatography to separate encapsulated ML from free ML in the extravesicular solution. Purified liposomes were then characterized. The content of entrapped and adsorbed ML was measured using ELISA. Repetitive freezing/thawing cycles prior to extrusion significantly increased ML uptake. On the contrary, adsorption was not affected neither by lipid composition, nor concentration and preparation. Differences in experimental encapsulation efficiency only reflect the differences in the mean vesicle sizes of the different samples as is revealed by a comparison to a theoretical estimate. Cryo-transmission electron microscopy (Cryo-TEM) images show that beside spherical, single-walled liposomes, there is a considerable fraction of discoidally deformed vesicles. Based on our results and those found in the literature, we speculate that the flattening of the vesicles is a consequence of lipid phase separation and the formation of condensed complexes and areas of different bending elasticities. This phenomenon eventually leads to agglomeration of deformed liposomal structures, becoming more pronounced with the increase in the relative amount of saturated fatty acids, presumably caused by hydrophobic interaction. For the same lipid mixture aggregation correlated linearly with the ML content. Finally, tested liposomal samples were kept at 4 degrees C to examine their stability. Only slight fluctuations in diameter and the increase in polydispersity after 3 weeks of storage occurred, with no statistically significant evidence of drug leakage during a time period of 12 days, illustrating physical stability of liposomes.     

Influence of cetyltrimethylammonium bromide on physicochemical properties and microstructures of chitosan-TPP nanoparticles in aqueous solutions

The nanoparticles of chitosan (CS) were prepared using pentasodium triphosphate (TPP) as a crosslinking agent and the influences of cetyltrimethylammonium bromide (CTAB) on the physicochemical properties of the CS-TPP nanoparticles were first studied by laser light scattering, zeta potential, and transmission electron microscopy (TEM). The concentration played a significant role in controlling the particle size of CS and the overlap concentration c(*) was testified to be about 1.0 mg/mL. The combination of static light scattering (SLS) and dynamic light scattering (DLS) allowed us to obtain more information about the CS-TPP nanoparticles in the presence of surfactant molecules. The addition of CTAB could reduce the hydrodynamic diameter of nanoparticles effectively in the salt solutions and simultaneously increase the zeta potential of the nanoparticles. The effect of CTAB concentration on the size of CS-TPP nanoparticle was also examined. The critical micelle concentration (CMC) of CTAB was used to interpret the complicated complex formed by the polyelectrolyte and the surfactant. Finally, TEM was used to observe the CS-TPP nanoparticles, which were affected by CTAB, to verify the results obtained by light scattering.     

Determination of the size and composition of multicomponent ethanol/water droplets by cavity-enhanced Raman scattering

Cavity-enhanced Raman scattering is used to determine the size and composition of multicomponent ethanol/water droplets in the concentration range 7.5–19% ethanol by volume. Under the experimental conditions presented here, the integrated CERS signal from ethanol shows an exponential increase with increase in ethanol concentration when compared with the integrated intensity of the water band. The calibration is shown to be invariant with particle size over the droplet radius range 20–35 μm. In addition to providing a method for determining particle size and composition, initial studies show that the evaporation dynamics of these multicomponent droplets can be probed by CERS.     

Length scale effect of PCL-PB-PCL on the light scattering and dynamics of network

The effect of length scale of triblock oil-soluble polymer (poly (ε-caprolactone)–poly butadiene-poly (ε-caprolactone)) (PCL-PB-PCL) on the properties of a water-in-oil (W/O) droplet microemulsion (R ~ 5.5 nm) has been studied as a function of the amount of added telechelic polymer. Small-angle X-ray scattering (SAXS) measurements show that the size of the droplets is not affected by the polymer addition but it induces attractive interactions at low concentration and repulsive ones at high polymer content. Measurements of the diffusion coefficient by dynamic light scattering (DLS) show different relaxations in mixed systems. The fast diffusion coefficient increases with increase in polymer concentration. At higher polymer content, the network formation leads to an additional slow relaxation mode in DLS that can be related to the formation of clusters of microemulsion droplets interconnected by the polymer. The collective diffusion of slow relaxations decreases with increase of polymer concentrations.     

The effect of electrolyte on the encapsulation efficiency of vesicles formed by the nonionic surfactant, 2C18E12

Encapsulation efficiencies of vesicles formed by the nonionic surfactant 1,2-dioctadecyl-rac-glycerol-3-omega-methoxydodecylethylene glycol (abbreviated as 2C18E12) and its phospholipid counterpart, distearoylphosphatidylcholine (DSPC) at 298 K, were determined by the entrapment of the water-soluble dye, carboxyfluorescein (CF) to be 0.045+/-0.001 and 0.03+/-0.04 L mol(-1) for 2C18E12 vesicles prepared using low osmolarity (270 m Osm) Krebs-Henseleit (K-H) buffer and a modified 'high salt' (1600 m Osm) variant of K-H buffer, respectively, and 0.64+/-0.01 and 0.31+/-0.04 Lmol(-1) for DSPC vesicles prepared under the same conditions and in the same buffers. Freeze fracture electron microscopy studies confirmed the presence of vesicles when 2C18E12 and DSPC were dispersed in water and both buffer solutions. Small angle neutron scattering (SANS) studies, using D2O in place of H2O, showed that when 2C18E12 vesicles were prepared in the 'high salt' variant of K-H buffer as opposed to K-H buffer or water, a higher proportion of multilamellar vesicles (MLV) were formed. Furthermore when prepared in the 'high salt' variant of K-H buffer, the 2C18E12 bilayers were thinner, and when present in the form of MLV exhibited a smaller layer of water separating the bilayers. However, even in the absence of electrolyte, 2C18E12 formed surprisingly thin bilayers due to the penetration of the polyoxyethylene chains into the hydrophobic chain region of the bilayer. Due to the dehydrating effect of the high concentration of electrolyte present in the 'high salt' variant of K-H, the polyoxyethylene head groups penetrated further into the hydrophobic region of the bilayer making the bilayer even thinner. In the case of the DSPC vesicles, although the SANS study showed an increase in the relative proportion of multilamellar to unilamellar vesicles when samples were prepared in the 'high salt' variant of K-H buffer, no differences were observed in the thickness and the d-spacing of the vesicle bilayers. Variable temperature turbidity measurements of 2C18E12, and DSPC vesicles prepared in water indicated phase changes at 320+/-0.5 and 327+/-0.5 K, respectively, and were unchanged when the 'high salt' variant of K-H buffer was used as hydrating medium. Taken together, these results suggest that a low phase transition temperature was not the reason for the poor entrapment efficiency of 2C18E12 vesicles but rather the very 'thin' hydrophobic barrier formed by the penetration of the polyoxyethylene chains into the hydrophobic region of the bilayer.     

Physicochemical properties of PEG-grafted liposomes

Egg phosphatidylcholine (EggPC) or dimyristoylphosphatidylcholine (DMPC) liposomes containing polyethylene glycol (PEG)-lipids covering a range of 0-30 mol% have been prepared by Extrusion method. The physicochemical properties including size evolution and calcein permeation were evaluated to investigate the effect of PEG-lipids on bilayer structure. The results from quasielasetic light scattering (QELS), freeze-fracture microscopy, and gel exclusion chromatography revealed that presence of low concentration of PEG-lipid results in decreasing of vesicle size and further increase in the PEG-lipid concentrations lead to a transition from the lamellar membranes to micelles. The permeability for calcein increased with increase in concentration of distearoylphosphatidylethanolamine (DSPE)-PEG. On the other hand, the permeability decreased with low amount of cholesterol-PEG (blow 20% cholesterol-PEG) and increased with high amount of it. The maximum concentration of PEG-lipid that may be incorporated without alteration of the liposome structure depends on the composition of the bilayer. The concentration of DSPE-PEG2000 incorporated into vesicles without damaging vesicle structures were <20 mol% for EggPC and <10% for DMPC.     

The physicochemical/thermodynamic balance of advanced drug liposomal delivery systems

The aim of this work is to study the morphological characteristics via fractal analysis and the alterations of the thermotropic behavior of dipalmitoylphosphatidylcholine (DPPC) liposomes, caused by the incorporation of cholesterol, poly(amidoamine) (PAMAM) dendrimer, and MPOx (poly(2-methyl-2-oxazoline)-grad-poly(2-phenyl-2-oxazoline)) gradient block copolymer (9:1 molar ratio). A gamut of light scattering techniques and differential scanning calorimetry were used in order to extract information on the morphological (in different dispersion media) and thermodynamic characteristics of liposomal drug nanocarriers, respectively. The vesicles’ structure of liposomes has a different thermodynamic content, which corresponds to a different thermotropic behavior, in comparison to pure lipid bilayers. The observed metastable phase only for DPPC liposomes has been considered as a “physical impurity”, which leads to “physical incompatibility” and consequently promotes the aggregation of DPPC liposomes in aqueous media. The incorporation of biomaterials such as PAMAM G4 and MPOx, caused alterations in the thermotropic behavior of DPPC liposomes affecting only the main transition specific enthalpy ΔH. All the other calorimetric parameters remained unaltered. These findings supported the hypothesis that the exceptional stability and transition cooperativity of the chimeric liposomal membrane might be due to the reduction of the vesicle size with the smaller membrane curvature that is indicated by the fractal dimensionality of the system. In conclusion, the results from the thermal analysis of the liposomal systems were in line with the picture of their structural characteristics, as indicated by the interplay between physicochemical and thermodynamical parameters, which determines their fractal morphology.     

Design and synthesis of fluorescence-labeled closo-dodecaborate lipid: its liposome formation and in vivo imaging targeting of tumors for boron neutron capture therapy

The fluorescence-labeled closo-dodecaborane lipid (FL-SBL) was synthesized from (S)-(+)-1,2-isopropylideneglycerol as a chiral starting material. FL-SBL was readily accumulated into the PEGylated DSPC liposomes prepared from DSPC, CH, and DSPE-PEG-OMe by the post insertion protocol. The boron concentrations and the fluorescent intensities of the FL-SBL-labeled DSPC liposomes increased with the increase of the additive FL-SBL, and the maximum emission wavelength of the liposomes appeared at 531 nm. A preliminary in vivo imaging study of tumor-bearing mice revealed that the FL-SBL-labeled DSPC liposomes were delivered to the tumor tissue but not distributed to hypoxic regions.