Effects of glucose and its oligomers on the stability of freeze-dried liposomes

The Role of Cryoprotective Agents in Liposome Stabilization and Preservation

To improve liposomes’ usage as drug delivery vehicles, cryoprotectants can be utilized to prevent constituent leakage and liposome instability. Cryoprotective agents (CPAs) or cryoprotectants can protect liposomes from the mechanical stress of ice by vitrifying at a specific temperature, which forms a glassy matrix. The majority of studies on cryoprotectants demonstrate that as the concentration of the cryoprotectant is increased, the liposomal stability improves, resulting in decreased aggregation. The effectiveness of CPAs in maintaining liposome stability in the aqueous state essentially depends on a complex interaction between protectants and bilayer composition. Furthermore, different types of CPAs have distinct effective mechanisms of action; therefore, the combination of several cryoprotectants may be beneficial and novel attributed to the synergistic actions of the CPAs. In this review, we discuss the use of liposomes as drug delivery vehicles, phospholipid–CPA interactions, their thermotropic behavior during freezing, types of CPA and their mechanism for preventing leakage of drugs from liposomes.

Characterization techniques: The stepping stone to liposome lyophilized product development

The primary drying is the longest step of the freeze-drying process and becomes one of the focuses for lyophilization cycle development inevitably, which is often approaching through a "trial and error" course and requires a labor-intensive and time-consuming endeavor. Nevertheless, drawing support from characterization techniques to understand the physic-chemical properties changing of the sample during lyophilization and their correlation with process conditions comprehensively, the freeze-drying development and optimization will get more from less. To get the optimal lyophilization cycle in the least time, the instrumental methods assisting primary drying design are summarized. The techniques used for estimating the collapse temperature of products are reviewed at first, aiming to provide a reference on the primary drying temperature setting to guarantee product quality. The instrumental methods for primary drying end prediction are also discussed to get optimal freeze-drying protocol with higher productivity. This review highlights the practicality of the above techniques through expounding basic principles, typical measurement conditions, merits and drawbacks, interpretation of results and practical applications, etc. At last, the techniques used for residual moisture detection of lyophilized products and size determination after liposome rehydration are briefly introduced.

Preserving the Integrity of Surfactant-Stabilized Microbubble Membranes for Localized Oxygen Delivery

Ultrasound contrast agents consist of stabilized microbubbles. We are developing a surfactant-stabilized microbubble platform with a shell composed of Span 60 (Sorbitan monostearate) and an emulsifying agent, water-soluble vitamin E (α-tocopheryl poly(ethylene glycol) succinate, abbreviated as TPGS), named SE61. The microbubbles act both as an imaging agent and a vehicle for delivering oxygen to hypoxic areas in tumors. For clinical use, it is important that a platform be stable under storage at room temperature. To accomplish this, a majority of biologicals are prepared as freeze-dried powders, which also eliminates the necessity of a cold chain. The interfaces among the surfactants, gas, and liquids are subject to disruption in both the freezing and drying phases. Using thermocouples to monitor temperature profiles, differential scanning calorimetry to determine the phase transitions, and acoustic properties to gauge the degree of microbubble disruption, the effects of the freezing rate and the addition of different concentrations of lyoprotectants were determined. Slower cooling rates achieved by freezing the samples in a -20 °C bath were found to be reproducible and produce contrast agents with acceptable acoustical properties. The ionic strength of the solutions and the concentration of the lyoprotectant determined the glass-transition temperature (Tg') of the frozen sample, which determines at what temperature samples can be dried without collapse. Crucially, we found that the shelf stability of surfactant-shelled oxygen microbubbles can be enhanced by increasing the lyoprotectant (glucose) concentration from 1.8 to 5.0% (w/v), which prevents the melt temperature (Tm) of the TPGS phase from rising above room temperature. The increase in glucose concentration results in a lowering of Tm of the emulsifying agent, preventing a phase change in the liquid-crystalline phase and allowing for more stable bubbles. We believe that preventing this phase change is necessary to producing stabilized freeze-dried microbubbles.

Lyophilization of Liposomal Formulations: Still Necessary, Still Challenging

Nowadays, the freeze-drying of liposome dispersions is still necessary to provide a solid dosage form intended for different routes of administration (i.e., parenteral, oral, nasal and/or pulmonary). However, after decades of studies the optimization of process conditions remains still challenging since the freezing and the dehydration destabilize the vesicle organization with the concomitant drug leakage. Starting from the thermal properties of phospholipids, this work reviews the main formulation and process parameters which can guarantee a product with suitable characteristics and increase the efficiency of the manufacturing process. In particular, an overview of the cryo- and/or lyo-protective mechanisms of several excipients and the possible use of co-solvent mixtures is provided. Attention is also focused on the imaging methods recently proposed to characterize the appearance of freeze-dried products and liposome dispersions upon reconstitution. The combination of such data would allow a better knowledge of the factors causing inter-vials variability in the attempt to improve the quality of the final medicinal product.

Adefovir dipivoxil loaded proliposomal powders with improved hepatoprotective activity: formulation, optimization, pharmacokinetic, and biodistribution studies

The present study aimed to prepare proliposomal formulae for improving the oral bioavailability of adefovir dipivoxil (AD), a nucleoside reverse transcriptase inhibitor effective against hepatitis B virus (HBV). The prepared proliposomal formulae were characterized for entrapment efficiency (E.E.%), vesicle size and in vitro drug release after reconstitution to conventional liposomes. The optimized formula (F9) with a maximum desirability value of 0.858 was selected having E.E.% of 71 ± 3.3% with an average vesicle size of 164.6 ± 5 nm. Moreover, the crystallization of AD within the optimized formula investigated via powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) confirmed the presence of the drug in an amorphous state within the lipid vesicles with enhanced stability over a storage period of 12 months. Thioacetamide-induced liver damage in rats evidenced by elevated liver enzymes was significantly improved after treatment with the optimum formula. Pharmacokinetic and biodistribution studies of formula F9 showed a higher accumulation of AD in the liver with enhanced bioavailability compared to AD suspension which highlights its potential advantage for an effective treatment of chronic HBV. Hence, proliposomal drug delivery is considered as a better choice for the oral delivery of AD.

Influence of type and proportion of lyoprotectants on lyophilized ginsenoside Rg3 liposomes

Objectives

To improve stability and shelf life, lyophilized formulations of 20(R)-Ginsenoside Rg3 liposomes (G-Rg3-Ls) were prepared.

Methods

Glucose, trehalose, sucrose, maltose, lactose, mannitol, inositol, hydroxypropyl-β-cyclodextrin and polyethylene glycol were used as single lyoprotectant and then compared in terms of their ability to protect lyophilized G-Rg3-Ls. Further, a glucose–mannitol complex was used to determine the optimal lyophilized preparation. The analysis of lyophilized liposomes or lyoprotectant was further investigated by scanning electron microscopy, thermogravimetry-differential thermal analysis, X-ray diffractometry and Fourier transform infrared spectroscopy. Cytotoxicity assay was used to assess the cyto-inhibition of freshly prepared and lyophilized liposomes.

Key findings

When the ratio of glucose–mannitol to phospholipids was 4 : 2 : 1 (w/w) the lyophilized G-Rg3-Ls exhibited good appearance, high DRR (86.52% ± 5.02%), small change in particle size (45.83 ± 0.50%) and short rehydration reconstruction time (8.3 ± 1.5 s). All indices were considerably better than those of each single protective agent. Results indicated that when the two lyoprotectants were combined, the stabilizing effect of glucose and shaping effect of mannitol were well maintained. The cyto-inhibition of freshly prepared and lyophilized G-Rg3 liposomes showed that lyophilization did not affect the bioactivity of G-Rg3.

Conclusions

The application of glucose–mannitol composite lyoprotectants can obtain a good G-Rg3 lyophilized preparation.

Preparation, characterization and in vitro evaluation of sterically stabilized liposome containing a naphthalenediimide derivative as anticancer agent

The aim of this study was to incorporate a new naphthalenediimide derivative (AN169) with a promising anticancer activity into pegylated liposomes to an extent that allows its in vitro and in vivo testing without use of toxic solvent. AN169-loaded liposomes were prepared using the thin-film hydration method and characterized for size, polydispersity index, drug content and drug release. We examined their lyophilization ability in the presence of cryoprotectants (trehalose, sucrose and lysine) and the long-term stability of the lyophilized products stored at 4 °C for 3 and 6 months by particle size changes and drug leakage. AN169 was successfully loaded into liposomes with an entrapment efficiency of 87.3 ± 2.5%. The hydrodynamic diameter of these liposomes after sonication was ∼ 145 nm with a high degree of monodispersity. Trehalose was found to be superior to the other lyoprotectants. In particular, trehalose 1:10 lipid:cryoprotectant molar ratio may provide stable lyophilized liposomes with the conservation of physicochemical properties upon freeze-drying and long-term storage conditions. We also assessed their in vitro antitumor activity in human cancer cell lines (HTLA-230 neuroblastoma, Mel 3.0 melanoma, OVCAR-3 ovarian carcinoma and SV620 prostate cancer cells). However, only after 72 h incubation, loaded liposomes showed almost the same IC50 as free AN169. In conclusion, we developed a stable lyophilized liposomal formulation for intravenous administration of AN169 as anticancer drug, with the advantage of avoiding the use of potentially toxic solubilizing agents for future in vivo experiments.

Fructan and its relationship to abiotic stress tolerance in plants

Numerous studies have been published that attempted to correlate fructan concentrations with freezing and drought tolerance. Studies investigating the effect of fructan on liposomes indicated that a direct interaction between membranes and fructan was possible. This new area of research began to move fructan and its association with stress beyond mere correlation by confirming that fructan has the capacity to stabilize membranes during drying by inserting at least part of the polysaccharide into the lipid headgroup region of the membrane. This helps prevent leakage when water is removed from the system either during freezing or drought. When plants were transformed with the ability to synthesize fructan, a concomitant increase in drought and/or freezing tolerance was confirmed. These experiments indicate that besides an indirect effect of supplying tissues with hexose sugars, fructan has a direct protective effect that can be demonstrated by both model systems and genetic transformation.

Protection of liposomes against fusion during drying by oligosaccharides is not predicted by the calorimetric glass transition temperatures of the dry sugars

Sugars play an important role in the desiccation tolerance of most anhydrobiotic organisms. It has been shown in previous studies that different structural families of oligosaccharides have different efficacies to interact with phospholipid headgroups and protect membranes from solute leakage during drying. Here, we have compared three families of linear oligosaccharides (fructans (inulins), malto-oligosaccharides, manno-oligosaccharides) for their chain-length dependent protection of egg phosphatidylcholine liposomes against membrane fusion. We found increased protection with chain length up to a degree of polymerization (DP) of 5 for malto-oligosaccharides, and a decrease for inulins and manno-oligosaccharides. Differential scanning calorimetry measurements showed that for all sugars the glass transition temperature (T_g) increased with DP, although to different degrees for the different oligosaccharide families. Higher T_g values resulted in reduced membrane fusion only for malto-oligosaccharides below DP5. Contrary to expectation, for inulins, manno-oligosaccharides and malto-oligosaccharides of a DP above five, fusion increased with increasing T_g, indicating that other physical parameters are more important in determining the ability of different sugars to protect membranes against fusion during drying. Further research will be necessary to experimentally define such parameters.

Amino acids as cryoprotectants for liposomal delivery systems

Liposomes provide an efficient delivery system for solubilisation and delivery of both small and macro molecules. However, they suffer from the disadvantage of instability when stored as aqueous dispersions. Cryoprotection of the liposomal systems provides an effective approach to overcome poor stability whilst maintaining formulation characteristics, although, the formulation of a freeze-dried product requires the consideration of not only the selection of an appropriate cryoprotectant, but also needs careful consideration of the processing parameters including pre-freezing conditions, primary and secondary drying protocols along with optimisation of cryoprotectant concentration. This current work investigates the application of amino acids as potential cryoprotectants for the stabilisation of liposomes, and results indicate that amino acids show biphasic nature of stabilisation with 4 mol of cryoprotectant per mole of the lipid exhibiting optimum cryoprotection. The investigations of process parameters showed that the pre-freezing temperatures below the glass transition of the amino acids followed by drying for over 6h resulted in stable formulations. Studies investigating the efficiency of drug retention showed that the cryoprotection offered by lysine was similar to that shown by trehalose, suggesting that amino acids act as effective stabilizers. ESEM analysis was carried out to monitor morphology of the rehydrated liposomes.

Fructans from oat and rye: composition and effects on membrane stability during drying

Fructans have been implicated in the abiotic stress tolerance of many plant species, including grasses and cereals. To elucidate the possibility that cereal fructans may stabilize cellular membranes during dehydration, we used liposomes as a model system and isolated fructans from oat (Avena sativa) and rye (Secale cereale). Fructans were fractionated by preparative size exclusion chromatography into five defined size classes (degree of polymerization (DP) 3 to 7) and two size classes containing high DP fructans (DP>7 short and long). They were characterized by high performance liquid chromatography (HPLC) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The effects of the fructans on liposome stability during drying and rehydration were assessed as the ability of the sugars to prevent leakage of a soluble marker from liposomes and liposome fusion. Both species contain highly complex mixtures of fructans, with a DP up to 17. The two DP>7 fractions from both species were unable to protect liposomes, while the fractions containing smaller fructans were protective to different degrees. Protection showed an optimum at DP 4 and the DP 3, 4, and 5 fractions from oat were more protective than all other fractions from both species. In addition, we found evidence for synergistic effects in membrane stabilization in mixtures of low DP with DP>7 fructans. The data indicate that cereal fructans have the ability to stabilize membranes under stress conditions and that there are size and species dependent differences between the fructans. In addition, mixtures of fructans, as they occur in living cells may have protective properties that differ significantly from those of the purified fractions.

Tethered bilayer lipid membranes studied by simultaneous attenuated total reflectance infrared spectroscopy and electrochemical impedance spectroscopy

The formation of tethered lipid bilayer membranes (tBLMs) from unilamelar vesicles of egg yolk phosphatidylcholine (EggPC) on mixed self-assembled monolayers (SAMs) from varying ratios of 6-mercaptohexanol and EO(3)Cholesteryl on gold has been monitored by simultaneous attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and electrochemical impedance spectroscopy (EIS). The influence of the lipid orientation (and hence the anisotropy) of lipids on a gold film on the dichroic ratio was studied by simulations of spectra with a matrix method for anisotropic layers. It is shown that for certain tilt angles of the dielectric tensor of the adsorbed anisotropic layer dispersive and negative absorption bands are possible. The experimental data indicate that the structure of the assemblies obtained varies with varying SAM composition. On SAMs with a high content of EO(3)Cholesteryl, tBLMs with reduced fluidity are formed. For SAMs with a high content of 6-mercaptohexanol, the results are consistent with the adsorption of flattened vesicles, and spherical vesicles have been found in a small range of surface compositions. The kinetics of the adsorption process is consistent with the assumption of spherical vesicles as long-living intermediates for surfaces of a high 6-mercaptohexanol content. No long-living spherical vesicles have been detected for surfaces with a large fraction of EO(3)Cholesteryl tethers. The observed differences between the surfaces suggest that for the formation of tBLMs (unlike supported BLMs) no critical surface coverage of vesicles is needed prior to lipid bilayer formation.

Monosaccharide composition, chain length and linkage type influence the interactions of oligosaccharides with dry phosphatidylcholine membranes

Sugars play an important role in the desiccation tolerance of most anhydrobiotic organisms and disaccharides have been extensively investigated for their ability to stabilize model membranes in the dry state. Much less is known about the ability of oligosaccharides to protect dry membranes. However, it has been shown that different structural families of oligosaccharides have different efficacies to interact with and protect membranes during drying. Here, we have compared three families of linear oligosaccharides (fructans, malto-oligosaccharides, manno-oligosaccharides) for their chain-length dependent lyoprotective effect on egg phosphatidylcholine liposomes. We found increased protection with chain length for the fructans, a moderate decrease in protection with chain length for malto-oligosaccharides, and a strong decrease for manno-oligosaccharides. Using Fourier-transform infrared spectroscopy and differential scanning calorimetry, we show that the degree of lyoprotection of the different sugars is closely related to their influence on the gel to liquid-crystalline phase behavior of the dry membranes and to the extent of H-bonding to different groups (C=O, P=O, choline) in the lipids. Possible structural characteristics of the different oligosaccharides that may determine the extent to which they are able to interact with and protect membranes are discussed.

The effects of lyophilization on the stability of liposomes containing 5-FU

Multilamellar liposomes containing 5-fluorouracil (5-FU) were prepared by modified lipid film hydration method and were lyophilized with or without saccharose as cryoprotectant. The effect of lyophilization on the stability of liposomes was evaluated by comparing the vesicle size, encapsulation efficiency and the drug release rate before and after lyophilization/rehydration. The process of lyophilization, without cryoprotectant, resulted in particle size increase and significant content leakage. By the addition of saccharose, the lipid bilayers become more stable and less permeable to the encapsulated drug, saccharose imparted 5-FU retention of about 80% after lyophilization/rehydration. Freeze-drying did not affect the particle size of liposomes containing saccharose as cryoprotectant. The drug release profiles of rehydrated liposomes followed Higuchi's square root model. Also, the obtained release profiles were all biphasic: a rapid initial drug release phase (burst release of the portion of the drug that leaked out of liposomes during the lyophilization) was followed by a slower, approximately constant drug release phase (zero-order kinetics).

Protection of Protein Secondary Structure by Saccharides of Different Molecular Weights during Freeze-Drying

The protective effects of saccharides with various molecular weights (glucose, maltose, maltotriose, maltotetraose, maltopentaose, maltoheptaose, dextran 1060, dextran 4900, and dextran 10200) against lyophilization-induced structural perturbation of model proteins (BSA, ovalbumin) were studied. Fourier transform infrared (FT-IR) analysis of the proteins in initial solutions and freeze-dried solids indicated that maltose conferred the greatest protection against secondary structure change. The structure-stabilizing effect of maltooligosaccharides decreased in increasing the number of saccharide units. Larger molecules of dextran also showed a smaller structure-stabilizing effect. Increasing the effective saccharide molecular size by a borate-saccharide complexation reduced the protein structure-stabilizing effect of all of the saccharides except glucose. The results indicate that the larger saccharide molecules, and/or the complex formation with borate ion, reduce the free and accessible hydroxyl groups to interact with and stabilize the protein structure by a water-substitution mechanism.

Exclusion of maltodextrins from phosphatidylcholine multilayers during dehydration: effects on membrane phase behaviour

The effect of increasing solute size on phosphatidylcholine phase behaviour at a range of hydrations was investigated using differential scanning calorimetry. Dehydration of phospholipid membranes gives rise to a compressive stress within the bilayers that promotes fluid-to-gel phase transitions. According to the Hydration Forces Explanation, sugars in the intermembrane space minimize the compressive stress and limit increases in the fluid-gel transition temperature, T(m), by acting as osmotic and volumetric spacers that hinder the close approach of membranes. However, the sugars must remain between the bilayers in order to limit the rise in T(m). Large polymers are excluded from the interlamellar space during dehydration and do not limit the dehydration-induced rise in T(m). In this study, we used maltodextrins with a range of molecular weights to investigate the size-exclusion limit for polymers between phosphatidylcholine bilayers. Solutes with sizes ranging from glucose to dextran 1000 limited the rise in lipid T(m) during dehydration, suggesting that they remain between dehydrated bilayers. At the lowest hydrations the solutions vitrified, and T(m) was further depressed to about 20 degrees C below the transition temperature for the lipid in excess water, T(o). The depression of T(m) below T(o) occurs when the interlamellar solution vitrifies between fluid phase bilayers. The larger maltodextrins, dextran 5000 and 12,000, had little effect on the T(m) of the PCs at any hydration, nor did vitrification of these larger polymers affect the lipid phase behaviour. This suggests that the larger maltodextrins are excluded from the interlamellar region during dehydration.

Specific effects of fructo- and gluco-oligosaccharides in the preservation of liposomes during drying

The fructan family of oligo- and polysaccharides is a group of molecules that have long been implicated as protective agents in the drought and freezing tolerance of many plant species. However, it has been unclear whether fructans have properties that make them better protectants for cellular structures than other sugars. We compared the effects of fructans and glucans on membrane stability during air-drying. Although glucans of increasing chain length were progressively less able to stabilize liposomes against leakage of aqueous content after rehydration, fructans showed increased protection. On the other hand, glucans became more effective in protecting liposomes against membrane fusion with increasing chain length, whereas fructans became less effective. Fourier transform infrared spectroscopy showed a reduction of the gel to liquid-crystalline phase transition temperature (T(m)) of air-dried liposomes by approximately 25 degrees C in the presence of sucrose and maltose. For the respective pentasaccharides, the reduction of T(m) of the lipids was 9 degrees C lower for samples containing fructan than for those containing glucan, indicating increased sugar--membrane interactions for the fructan compared to the glucan. A reduced interaction of the longer-chain glucans and an increased interaction of the respective fructans with the phospholipid head groups in the dry state was also indicated by dramatic differences in the phosphate asymmetric stretch region of the infrared spectrum. Collectively, our data indicate that the fructo-oligosaccharides accumulated in many plant species under stress conditions could indeed play an important role in cellular dehydration tolerance.

Effects of the acyl chain composition of phosphatidylcholines on the stability of freeze-dried small liposomes in the presence of maltose

The effects of the acyl chain composition of phosphatidylcholines (PCs) on the stability of small unilamellar vesicles during freeze-drying and rehydration in the presence of maltose were studied by monitoring the retention of a trapped marker, calcein, in the internal liposome compartment. In dipalmitoyl PC, beta-oleoyl-gamma-palmitoyl-PC and egg yolk PC liposomes, good or fair retentions (>50%) were observed in the presence of maltose, but maltose was ineffective in preserving retention in the dioleoyl PC (DOPC) liposomes (<10%). The extremely low retention in the DOPC liposome was ascribed to neither a formation of the inverted hexagonal phase of the liposomal membrane nor the fusion/aggregation of the liposomes in the drying-rehydration process. Differential scanning calorimetry measurements suggested that interactions of maltose with PC headgroups were essential to stabilizing the dry liposomes. These interactions were significant in the saturated or mixed chain liposomes but were markedly reduced in the DOPC liposomes.

Non-disaccharide-based mechanisms of protection during drying

Few tissues or organisms can survive the removal of nearly all their intra and extracellular water. These few have developed specialized adaptations to protect their cellular components from the damage caused by desiccation and rehydration. One mechanism, common to almost all such organisms, is the accumulation of disaccharides within cells and tissues at the onset of dehydration. This adaptation has been extensively studied and will not be considered in this review. It has become increasingly clear that true desiccation tolerance is likely to involve several mechanisms working in concert; thus, we will highlight several other important and complimentary adaptations found especially in the dehydration-resistant tissues of higher plants. These include the scavenging of reactive oxygen species, the down-regulation of metabolism, and the accumulation of certain amphiphilic solutes, proteins, and polysaccharides.

Physicochemical evaluation of a stability-driven approach to drug entrapment in regular and in surface-modified liposomes

The traditional mode of encapsulating drugs in liposomes poses risks to drug stability, especially when recognition agents are attached to the liposomal surface to obtain targeted liposomes. To reduce such risks, we devised a simple, novel method to entrap drugs in liposomes, consisting of (i) preparation and lyophilization of drug-free regular and surface-modified liposomes and (ii) drug encapsulation in the course of liposome reconstitution through rehydration in an aqueous solution of the drug. In this paper, we report physicochemical studies in which we compared regular and surface-modified liposomes made by this novel approach (denoted N-liposomes) to respective liposomes made by the traditional mode (denoted T-liposomes). The studies were performed with fluorescein, sucrose, histidine, mitomycin C (MMC), and chloramphenicol (CAM) encapsulated (each) in regular and in bioadhesive liposomes, the latter having hyaluronic acid as the surface-bound ligand. Our major findings are as follows: (1) The drug-specific encapsulation efficiencies spanning the range of 10-90% were, excepting sucrose, either similar in the N- and T-liposomes or better in the N- than in the T-liposomes, for both regular and bioadhesive liposomes. (2) For all liposome types and methods of preparation, fluorescein, histidine, and MMC did not adsorb to the liposomal surface. Sucrose and MMC did adsorb to the liposomal surface irrespective of the liposome preparation mode, sucrose favoring bioadhesive over regular liposomes and MMC having the opposite trend. (3) For both regular and bioadhesive liposomes, the mechanism of drug efflux from the N-liposomes was found to be governed by a single rate constant, as previously found for the T-liposomes. The magnitudes obtained, ranging from 3.5(+/-0.2) x 10(-3) to 400(+/-17) x 10(-3) h(-1), were always drug specific and occasionally also liposome type (i.e., regular or bioadhesive) specific. For MMC and CAM, the novel approach rendered liposomes with improved sustained release. The results reported here attest, overall, to the potential of this novel approach, meriting further investigations. Studies currently underway with MMC indicate N-liposomes also have functional advantages.

An Aqueous Polymer Two-Phase System as Carrier in the Spray-Drying of Biological Material

This investigation describes a novel concept in the formulation of carrier systems for the spray-drying of biological materials. As carrier material a system composed of poly(vinyl pyrrolidone) (PVP) and dextran was used. This system yields an aqueous two-phase system in which each phase is enriched in one of the polymers. By varying the composition of the system, the effective structure of a "stirred" system can be varied, covering the entire range from dextran continuous to PVP continuous. This facilitates encapsulation of either of these polymers in a spray-drying operation. In an attempt to investigate the spray-drying from such a system, the surface composition of the spray-dried powder obtained from various compositions of the two-phase system was analyzed by electron spectroscopy for chemical analysis (ESCA), providing information on the distribution of the polymers in the powder and thus also in the spray droplets. The two-phase system was applied for the spray-drying of live bacteria. The survival rate of the bacteria depended on the composition of the two-phase system. The storage stability of the bacteria in these formulations was investigated after storage at room temperature under dry conditions for 4 weeks, and it was found that the survival rate was 10-45%. The results therefore show that this type of formulation holds promise for future applications for micro-organisms as well as other sensitive biological materials such as proteins. Copyright 2000 Academic Press.

Effects of vitrified and nonvitrified sugars on phosphatidylcholine fluid-to-gel phase transitions

DSC was used to study the ability of glass-forming sugars to affect the gel-to-fluid phase transition temperature, T(m), of several phosphatidylcholines during dehydration. In the absence of sugars, T(m) increased as the lipid dried. Sugars diminished this increase, an effect we explain using the osmotic and volumetric properties of sugars. Sugars vitrifying around fluid phase lipids lowered T(m) below the transition temperature of the fully hydrated lipid, T(o). The extent to which T(m) was lowered below T(o) ranged from 12 degrees to 57 degrees, depending on the lipids' acyl chain composition. Sugars vitrifying around gel phase lipids raised T(m) during the first heating scan in the calorimeter, then lowered it below T(o) in subsequent scans of the sample. Ultrasound measurements of the mechanical properties of a typical sugar-glass indicate that it is sufficiently rigid to hinder the lipid gel-to-fluid transition. The effects of vitrification on T(m) are explained using the two-dimensional Clausius-Clapeyron equation to model the mechanical stress in the lipid bilayer imposed by the glassy matrix. Dextran and polyvinylpyrrolidone (PVP) also vitrified but did not depress T(m) during drying. Hydration data suggest that the large molecular volumes of these polymers caused their exclusion from the interbilayer space during drying.

Coalescence of Lipid Emulsions in Floating and Freeze-Thawing Processes: Examination of the Coalescence Transition State Theory

Lipid emulsions, triacylglycerol droplets covered with single surface monolayers of phospholipid in aqueous medium, were prepared by high-pressure emulsification and successive ultracentrifugation. Egg yolk phosphatidylethanolamine did not stabilize triolein (TO) droplets in aqueous medium. Phosphatidylcholine (PC) of various long fatty acyl chains dispersed TO and tricaprylin (TC) well in aqueous medium as emulsion droplets of 110 nm. After ultracentrifugation, however, oil separation was observed in the floating creamy layer of TO/dioleoyl-PC emulsions. Dioleoyl-PC contains two bulky unsaturated fatty acyl chains, as well as oleoyl chains, and it has a lower spontaneous curvature than egg PC. Egg PC and dipalmitoyl-PC gave stably dispersed emulsion droplets of TO even after the freeze-thawing (F-T) procedure. On the other hand, TO/egg PC emulsion droplets containing cholesterol of 83 mol% PC and TC/egg PC emulsion droplets coalesced after the F-T procedure. Cholesterol and a medium chain triacylglycerol, TC, were distributed into the surface PC monolayers and occupied an appreciable fraction of the surface area. These neutral lipids have small polar groups and thus decrease the mean spontaneous curvature of surface lipids. The relationship between the droplet coalescence and the spontaneous curvature was discussed on the basis of the coalescence transition state theory recently developed by Kabalnov et al. in 1996. In addition, effects of maltose on the emulsion coalescence in the freeze state were briefly discussed. Copyright 1999 Academic Press.

Short term stability of freeze-dried, lyoprotected liposomes

In the present study we examined the short term stability of liposomes in the freeze-dried state for different lipid compositions containing trehalose as a lyoprotectant. The retention of carboxyfluorescein and average vesicle size after rehydration were monitored as a function of the temperature to which the dry cakes were exposed for 0.5 h. The thermal behaviour of the cakes was analysed by modulated temperature differential scanning calorimetry, and acyl chain order and interaction between trehalose molecules and the phospholipid headgroups was studied by Fourier transform infrared spectroscopy. Induction of leakage, suppression of the (onset) bilayer transition temperature (Tm) and enhancement of the interaction between sugar and phospholipid molecules were observed below the glass transition temperature (Tg) for all lipid compositions studied. The above changes concurred with the melting transition of the bilayer. Two out of five lipid compositions showed no significant change in average vesicle size, indicating that leakage was not necessarily caused by vesicle fusion. In addition, leakage could not be explained in terms of a phase transition during rehydration of the liposomes. We conclude that for liposomes freeze-dried in trehalose the temperature range of the bilayer melting process is a better indicator than Tg for the maximal temperature to which liposomes may be exposed for a short period of time (0.5 h) without loss of stability.

Effects of surfactants on the spectral behaviour of calcein (II): a method of evaluation

The spectral behavior of calcein, a water-soluble self quenching fluorescent marker often used in biomedical analysis, can be considerably affected by the presence of surfactants. With this study we intend to obtain further information on the photophysical properties of calcein, in the presence of surfactants and in the concentration range commonly used to investigate the release of such marker from vesicle dispersions. The experiments were carried out both in water and in a physiological buffer (HEPES, pH 7.5), in the presence of Triton X-100, sodium dodecyl sulphate and centyltrimethylammonium bromide, both below and above their critical micelle concentration (c.m.c.). The obtained results confirm that calcein fluorescence can be affected by the presence of surfactants. Thus, environmental conditions must always be carefully checked for the actual quantitative evaluation of this dye. Furthermore, this study sheds some light on the nature and mechanism of calcein quenching.

Temperature change of the lamellar structure of DPPC/disaccharide/water systems with low water content

Temperature change in l-alpha-dipalmitoyl phosphatidylcholine (DPPC)/disaccharide systems with low water content (less than 8 wt. %) was investigated using X-ray diffraction within a range of two transition temperatures. X-ray diffraction above the higher transition temperature showed a broad symmetric peak, indicating the Lalpha phase. Below the higher transition temperature, two overlapping diffraction peaks were observed. After peak separation, temperature change in these systems was analyzed using peak parameters of the two peaks. Peak parameters of the lower angle peak changed continuously up to and above the higher transition temperature, suggesting the systems to be in a liquid crystal phase below the higher transition temperature. Fourier-transform infrared (FT-IR) spectra of the DPPC/trehalose system with 5.5 wt.% water showed the wave number of asymmetric stretching of phosphate groups to change at the lower transition temperature and that of symmetric stretching of CH2 groups, to change between the lower and higher transition temperatures. Thus, below the lower transition temperature, the system is shown to be in a gel phase. Conformational change in phosphate groups occurred at the lower transition temperature. Within the lower and higher transition temperatures, two phases were found to coexist and transition from the gel phase to Lalpha phase to occur continuously. Above the higher transition temperature, the system is in the Lalpha phase.

Thermal analysis of freeze-dried liposome-carbohydrate mixtures with modulated temperature differential scanning calorimetry

In this study we investigated the use of modulated temperature differential scanning calorimetry (MTDSC) for the detection of the glass transition temperature (Tg) in freeze-dried cakes of lyoprotected liposomes and for the analysis of frozen carbohydrate solutions. The glass transition appeared in the reversing heat flow, whereas the bilayer melting endotherm was observed in the nonreversing heat flow. This enabled the detection of Tg even in samples were the glass and bilayer transition overlapped. In addition, relaxation processes occurring in nonannealed freeze-dried carbohydrate-liposome mixtures, which hinder the determination of Tg with conventional DSC, were also separated from the heat capacity related heat flow. Analysis of frozen carbohydrate solutions with MTDSC facilitated the identification of the glass transition, devitrification peak, and "softening" transition, which could help to further optimize freeze-drying conditions by rationale. Sampling and selection of experimental parameters are discussed for the special case of porous, freeze-dried cakes.

Enhanced permeability of freeze-dried liposomal bilayers upon rehydration

Until now, studies on the protection of liposomes against freeze-drying damage have mainly focused on the bilayer integrity during the freezing or drying step of this process. Here, we investigated the bilayer permeability of freeze-dried, lyoprotected liposomes to a nonbilayer interacting compound after rehydration, by monitoring the leak-in kinetics of externally added carboxyfluorescein (CF). The results showed that freeze-drying and rehydration of DPPC:DPPG 10:1 liposomes with sucrose in- and outside the vesicles caused a temporary increase in the bilayer permeability for CF, which leveled off after approximately 20 h. The amount of CF/mol phospholipid which leaked into the vesicles increased with vesicle size (range 0.1-1 micro m) / lamellarity. Reduction of the number of bilayers in 1-1 micro m) vesicles enhanced the permeability to CF after freeze-drying and rehydration. The presence of CHOL decreased CF leak-in rates into 1 micro m MLVs consisting of DPPC:DPPG 10:1, but not into 0.1-micro m unilamellar vesicles. In the absence of sucrose similar leak-in profiles as a function of time were found after rehydration, suggesting that repacking processes of the bilayer were responsible for the enhanced permeability after freeze-drying and dehydration both with and without sucrose. The effect of size and lamellarity on the CF leak-in correlated with the retention of encapsulated CF after freeze-drying and rehydration, but no correlation was found with the effect of lipid composition. Both small (0.1 micro m) lyoprotected liposomes made of DPPC:DPPG 10:1 and DPPC:DPPG:CHOL 10:1:4 were highly permeable during the rehydration step itself. The results indicate that, despite the presence of the lyoprotectant, "repacking" of the bilayer components takes place both during and after rehydration. This eventually leads to regaining of its barrier function.