The effect of aging on the physical stability of liposome dispersions

Evidence for phospholipid self-organisation in concentrated ammonia-water environments

Titan, the largest moon of Saturn is thought to have the potential to support primordial life. The surface of Titan contains bodies of liquid hydrocarbons, and modelling suggests that an ammonia-water ocean resides deep beneath the surface, both of which have been speculated to support primordial chemistry. Here we present the first evidence that both preformed and self-organised phospholipid vesicles remain stable and can maintain concentration gradients in ammonia-water environments; a fundamental requirement for primordial chemistry and biology to originate. We further reveal the remarkable stability of a diether phospholipid, such as those found in extremophilic bacteria, under these conditions and demonstrate that electron microscopy and tomography are useful tools to investigate macromolecular structure under diverse physico-chemical environments.

Exosome-Coated Prussian Blue Nanoparticles for Specific Targeting and Treatment of Glioblastoma

Glioblastoma is one of the most aggressive and invasive types of brain cancer with a 5-year survival rate of 6.8%. With limited options, patients often have poor quality of life and are moved to palliative care after diagnosis. As a result, there is an extreme need for a novel theranostic method that allows for early diagnosis and noninvasive treatment as current peptide-based delivery standards may have off-target effects. Prussian Blue nanoparticles (PBNPs) have recently been investigated as photoacoustic imaging (PAI) and photothermal ablation agents. However, due to their inability to cross the blood-brain barrier (BBB), their use in glioblastoma treatment is limited. By utilizing a hybrid, biomimetic nanoparticle composed of a PBNP interior and a U-87 cancer cell-derived exosome coating (Exo:PB), we show tumor-specific targeting within the brain and selective thermal therapy potential due to the strong photoconversion abilities. Particle characterization was carried out and showed a complete coating around the PBNPs that contains exosome markers. In vitro cellular uptake patterns are similar to native U-87 exosomes and when exposed to an 808 nm laser, show localized cell death within the specified region. After intravenous injection of Exo:PB into subcutaneously implanted glioblastoma mice, they have shown effective targeting and eradication of tumor volume compared to PEG-coated PBNPs (PEG:PB). Through systemic administration of Exo:PB particles into orthotopic glioblastoma-bearing mice, the PBNP signal was detected in the brain tumor region through PAI. It was seen that Exo:PB had preferential tumor accumulation with less off-targeting compared to the RGD:PB control. Ex vivo analysis validated specific targeting with a direct overlay of Exo:PB with the tumor by both H&E staining and Ki67 labeling. Overall, we have developed a novel biomimetic material that can naturally cross the BBB and act as a theranostic agent for systemic targeting of glioblastoma tissue and photothermal therapeutic effect.

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Evaluation of a static mixer as a new microfluidic method for liposome formulation

Introduction: Microfluidic formulation of liposomes has been extensively studied as a potential replacement for batch methods, which struggle with problems in scalability and difficulty in modulating conditions. Although microfluidic devices are considered to be able to combat these issues, an adequate replacement method has yet to be established. Methods: This paper examines the potential of a static mixer (SM) by comparing the encapsulation efficiency, loading, lamellarity, and user-friendliness with a commonly used microfluidic device, a staggered herringbone micromixer (SHM). Results: In both devices, it was found that as the initial lipid concentration increased, the particle size increased; however, the overall particle size was seen to be significantly larger in the liposomes prepared with SM. PDI remained significantly smaller in SM, however, signifying that better control of the particle size was accomplished in SM. In addition, the encapsulation efficiency was slightly smaller in SM compared to SHM, and in both devices, the values increased as the initial lipid concentration increased. The increase in encapsulation efficiencies was significantly smaller than that of the theoretical encapsulation efficiency, and this was found to be due to the increase in lamellarity as the initial lipid concentration increased. Discussion: In terms of user-friendliness, SM demonstrated significant advantages. The mixing elements could be taken out from the device, allowing for thorough cleaning of the element and device before and after experiments and ensuring experiments are conducted at virgin state in every round. Consequently, it was found that SM not only can produce uniformly distributed liposomes but has the potential to become a more practical method for liposome formulation with modifications in the mixing elements.

Critical Role of Molecular Packing in Lo Phase Membrane Solubilization

Membrane solubilization induced by Triton X-100 (TX-100) was investigated. Different membrane compositions and phase states were studied along the detergent titration. Expected solubilization profiles were obtained but new information is provided. The fluorescence of nitrobenzoxadiazole (NBD)-labeled lipids indicates that the liquid-ordered (Lo)/liquid-disordered (Ld) phase coexistence is barely unaffected at sub-solubilizing detergent concentrations and highlights the vesicle-to-micelle transition. Moreover, the location of the NBD group in the bilayer emphasizes a detergent-membrane interaction in the case of the insoluble Lo phase membrane. It has also been shown that the molecular packing of the membrane loosens in the presence of TX-100, regardless of the solubilization profile. Motivated by studies on GPMVs, the solubilization of less ordered Lo phase membranes was considered in order to improve the effect of molecular packing on the extent of solubilization. Membranes composed of SM and Chol in an equimolar ratio doped with different amounts of PC were studied. The more ordered the Lo phase membrane is in the absence of detergent, the less likely it is to be solubilized. Furthermore, and in contrast to what is observed for membranes exhibiting an Lo/Ld phase coexistence, a very small decrease in the molecular packing of the Lo phase membrane radically modifies the extent of solubilization. These results have implications for the reliability of TX-100 insolubility as a method to detect ordered domains.

Evaluation of suitability and detection range of fluorescent dye-loaded nanoliposomes for sensitive and rapid sensing of wide ranging osmolarities

Measurement of osmolarity is critical for optimizing bioprocesses including antibody production and detecting pathologies. Thus, rapid, sensitive, and in situ sensing of osmolarity is desirable. This study aims to develop and assess the suitability of calcein- and sulforhodamine-loaded nanoliposomes for ratiometric sensing of osmolarity by fluorescence spectroscopy and evaluate the range of detection. The detection is based on concentration-dependent self-quenching of calcein fluorescence (sensor dye at 6-15 mM) and concentration-independent fluorescence of sulforhodamine (reference dye) due to osmotic shrinkage of the nanoliposomes when exposed to hyperosmotic solutions. Using mathematical modeling, 6 mM calcein loading was found to be optimal to sense osmolarity between 300 and 3000 mOsM. Calcein (6 mM)- and sulforhodamine (2 mM)-loaded nanoliposomes were produced by thin-film hydration and serial extrusion. The nanoliposomes were unilamellar, spherical (108 ± 9 nm), and uniform in size (polydispersity index [PDI] 0.12 ± 0.04). Their shrinkage induced by exposure to hyperosmotic solutions led to rapid self-quenching of calcein fluorescence (F_Green), but no effect on sulforhodamine fluorescence (F_Red) was observed. F_Green/F_Red decreased linearly with increasing osmolarity, obeying Boyle van't Hoff's relationship, thus proving that the nanoliposomes are osmosensitive. A calibration curve was generated to compute osmolarity based on F_Green/F_Red measurements. As a proof-of-concept, dynamic changes in osmolarity in a yeast-based fermentation process was demonstrated. Thus, the nanoliposomes have great potential as sensors to rapidly and sensitively measure wide-ranging osmolarities.

Use of supercritical CO2 for the sterilization of liposomes: Study of the influence of sterilization conditions on the chemical and physical stability of phospholipids and liposomes

The effects of four potential supercritical carbon dioxide (ScCO₂) sterilization conditions on the chemical stability of 9 phospholipids and on the physicochemical characteristics of liposomes consisting of stable phospholipids, as well as their sterilization efficiency were evaluated. These conditions were : C1 (ScCO₂/70 °C/150 bar/240 min), C2 (ScCO₂/0.25 % water/ 0.15% H₂O₂/ 0.5% acetic anhydride/38° C/85 bar/45 min), C3 (ScCO₂/0.08 % peracetic acid/35° C/104 bar/180 min) and C4 (ScCO₂/200 ppm H₂O₂/40 °C/270 bar/90 min). The results showed for phospholipids, a significant increase in hydrolysis products of 3.77 to 14.50 % and an increase in oxidation index of 6.10 to 430.50 % with unsaturated phospholipids for all tested conditions while with saturated phospholipids, no significant degradation was observed. Concerning the liposome formulation, no change in dispersion color and no phospholipid degradation were observed. However, a decrease in liposome size from 126.90 nm to 111.80 nm, 96.27 nm, 99.60 nm and 109.13 nm and an increase in the PdI from 0.208 to 0.271, 0.233, 0.285, and 0.298 were found with conditions C1, C2, C3 and C4 respectively. For the sterilization efficiency, conditions C1, C2 and C3 achieved the required sterility assurance level (SAL) of 10^-6 for liposomes.

A Phospholipase-A Activity in Soluble Leishmania Antigens Causes Instability of Liposomes

Aim:

This study aimed to investigate the existence of phospholipase-A (PLA) activity in Soluble L. major Antigens (SLA) because of no reports for it so far. Liposomes were used as sensors to evaluate PLA activity.

Objective:

Liposomal SLA consisting of Egg Phosphatidylcholine (EPC) or Sphingomyelin (SM) were prepared by two different methods in different pH or temperatures and characterized by Dynamic Light Scattering (DLS) and Thin Layer Chromatography (TLC).

Methods:

Lipid hydrolysis led to the disruption of EPC liposomal SLA in both methods but the Film Method (FM) produced more stable liposomes than the Detergent Removal Method (DRM).

Results:

The preparation of EPC liposomal SLA at pH 6 via FM protected liposomes from hydrolysis to some extent for a short time. EPC liposomes but not SM liposomes were disrupted in the presence of SLA.

Conclusion:

Therefore, a phospholipid without ester bond such as SM should be utilized in liposome formulations containing PLA as an encapsulating protein.

Preparation and Properties of Semi-Self-Assembled Lipopeptide Vesicles

Novel lipopeptide vesicles are prepared from self-assembled nanomembranes through an extrusion method. The size of vesicles can be controlled by the pore diameter of the extrusion filter. The vesicles are rather stable because hydrogen bonds exist among the peptide headgroups. When doxorubicin hydrochloride (DOX·HCl) is encapsulated in the vesicles, it could be released sustainably, and its side effect would also be reduced due to encapsulation. The leakage rate of DOX·HCl depends on the pH via charge regulation. As drug carriers, lipopeptide vesicles have been proved to have nontoxicity to normal cells. A magnetic surfactant CH₃(CH₂)₁₄CH₂N(CH₃)₃⁺ [FeCl₃Br]^- (CTAFe) was mixed with lipopeptide to modify the vesicles. Also, the results demonstrated that the vesicles is endowed with magnetic property after the addition of CTAFe. We believe that the strategy of lipopeptide vesicle preparation would enrich the drug carrier family and expand the application of lipopeptide materials.

Thermal Sensitive Liposomes Improve Delivery of Boronated Agents for Boron Neutron Capture Therapy

PURPOSE

Boron neutron capture therapy (BNCT) has the potential to become a viable cancer treatment modality, but its clinical translation requires sufficient tumor boron delivery while minimizing nonspecific accumulation.

METHODS

Thermal sensitive liposomes (TSLs) were designed to have a stable drug payload at physiological temperatures but engineered to have high permeability under mild hyperthermia.

RESULTS

We found that TSLs improved the tumor-specific delivery of boronophenylalanine (BPA) and boronated 2-nitroimidazole derivative B-381 in D54 glioma cells. Uniquely, the 2-nitroimidazole moiety extended the tumor retention of boron content compared to BPA.

CONCLUSION

This is the first study to show the delivery of boronated compounds using TSLs for BNCT, and these results will provide the basis of future clinical trials using TSLs for BNCT.

Phospholipid components of the synthetic pulmonary surfactant CHF5633 probed by fluorescence spectroscopy

CHF5633 (Chiesi Farmaceutici, Italy) is a synthetic pulmonary surfactant currently under clinical development for the treatment of Respiratory Distress Syndrome in premature infants. The product is composed of phospholipids in liposomal organization, together with two peptide analogues of human surfactant proteins B and C. Phospholipids in liposomes can undergo oxidation of unsaturated lipids and hydrolysis, with formation of fatty acids and lysolipids, both affecting the physico-chemical properties of the formulation. We exploited two fluorescence probes, Prodan and ADIFAB, to evaluate the stability of the phospholipid components of CHF5633. While Prodan enters the phospholipid bilayer and probes the polarity of this environment, ADIFAB binds free fatty acids in the aqueous phase, allowing to determine their concentration. Changes of Prodan fluorescence emission indicated an increase in the polarity of the phospholipid bilayer as a function of time. This behavior is coupled with an increase in fatty acids concentration in the aqueous phase, as determined by ADIFAB, and an increase in lysolipids concentration, as determined by HPLC-MS. Prodan and ADIFAB resulted efficient probes to monitor phospholipids hydrolysis in liposomes, reporting an increased stability of CHF5633 at pH values higher than 6.5.

Characterizations on the Stability and Release Properties of β-ionone Loaded Thermosensitive Liposomes (TSLs)

Liposomes with phase transition temperatures, T_m, near pathogenic site temperature are potential chemoprophylactic delivery vehicles. We prepared and characterized the thermal properties of liposomes composed of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and hydrogenated soy phosphatidylcholine (HSPC) incorporating β-ionone with T_m at 42 °C. Liposomes with β-ionone/lipid ratio (w/w) of 1:20 and 1:8 had the necessary stability and released most of the β-ionone. The molecular architecture surrounding T_m was studied by fluorescent probes, Raman spectroscopy, and differential scanning calorimeter (DSC). β-Ionone was found to be preferentially located in the deep regions of the lipid bilayer (toward the long chain alkyl of the lipid) at moderate loading. The results showed that β-ionone encapsulated liposomes have a superior release at higher loading amount. Increasing β-ionone leads to disorder in the liquid crystalline state and accelerates the release rate. These studies provide information on the membrane structural properties of β-ionone loaded liposomes that guide rational bioactive molecular delivery systems design for health products.

Development of Houttuynia cordata Extract-Loaded Solid Lipid Nanoparticles for Oral Delivery: High Drug Loading Efficiency and Controlled Release

Houttuynia cordata (H. cordata) has been used for diuresis and detoxification in folk medicine as well as a herbal medicine with antiviral and antibacterial activities. H. cordata extract-loaded solid lipid nanoparticles (H-SLNs) were prepared with various concentration of poloxamer 188 or poloxamer 407 by a hot homogenization and ultrasonication method. H-SLNs dispersion was freeze-dried with or without trehalose as a cryoprotectant. The physicochemical characteristics of H-SLNs were evaluated by dynamic laser scattering (DLS), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). Additionally, the in vitro release and in vitro cytotoxicity of H-SLNs were measured. Encapsulation efficiencies of H-SLNs (as quercitrin) were 92.9–95.9%. The SEM images of H-SLNs showed that H-SLNs have a spherical morphology. DSC and FT-IR showed that there were no interactions between ingredients. The increased extent of particle size of freeze-dried H-SLNs with trehalose was significantly lower than that of H-SLNs without trehalose. H-SLNs provided sustained release of quercitrin from H. cordata extracts. Cell viability of Caco-2 cells was over 70% according to the concentration of various formulation. Therefore, it was suggested that SLNs could be good carrier for administering H. cordata extracts.

"Staying Out" Rather than "Cracking In": Asymmetric Membrane Insertion of 12:0 Lysophosphocholine

Interactions between detergents and model membranes are well described by the three-stage model: saturation and solubilization boundaries divide bilayer-only, bilayer-micelle coexistence, and micelle-only ranges. An underlying assumption of the model is the equilibration of detergent between the two membrane leaflets. However, many detergents partition asymmetrically at room temperature due to slow flip-flop, such as sodium dodecyl sulfate (SDS) and lysolipids. In this work, we use isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) to investigate the solubilization of unilamellar POPC vesicles by 12:0 lysophosphocholine (12:0 LPC). Flip-flop of 12:0 LPC occurs beyond the time scale of our experiments, which establish a characteristic nonequilibrated state with asymmetric distribution: 12:0 LPC partitions primarily into the outer leaflet. Increasing asymmetry stress in the membrane does not lead to membrane failure, i.e., "cracking in" as seen for alkyl maltosides and other surfactants; instead, it reduces further membrane insertion which leads to the "staying out" of 12:0 LPC in solution. At above the critical micellar concentration of 12:0 LPC in the presence of the membrane, micelles persist and accommodate further LPC but take up lipid from vesicles only very slowly. Ultimately, solubilization proceeds via the micellar mechanism (Kragh-Hansen et al., 1995). With a combination of demicellization and solubilization experiments, we quantify the molar ratio partition coefficient (0.6 ± 0.1 mM^-1) and enthalpy of partitioning (6.1 ± 0.3 kJ·mol^-1) and estimate the maximum detergent/lipid ratio reached in the outer leaflet (<0.13). Despite the inapplicability of the three-stage model to 12:0 LPC at room temperature, we are able to extract quantitative information from ITC solubilization experiments and DLS that are important for the understanding of asymmetry-dependent processes such as endocytosis and the gating of mechanosensitive channels in vitro.

Change of Multiple-Layered Phospholipid Vesicles Produced by Electrostatic Deposition of Polymers during Storage

In this study, 1 wt% lecithin (–), chitosan (+), and λ-carrageenan (–) were prepared to manufacture multiple-layered liposomes with optimal formulations developed in a previous study by using layer-by-layer electrostatic deposition. We observed their particle size, ζ-potential, sedimentation behavior, and microstructure for 6 weeks. Multiple-layered liposomes were quenched with calcein to evaluate stability in terms of factors such as encapsulation efficiency and released amount of calcein. The particle size of multi-layered liposomes increased with storage periods and the ζ-potential of multiple-layered liposomes gained a neutral charge. Interestingly, negatively charged layered liposomes were smaller than positively charged layered liposomes and showed a lower polydispersity index. Moreover, the ζ-potential did not apparently change compared to positively charged layered liposomes. For the calcein release study, multiple-layered liposomes significantly sustained quenched calcein more than that observed using non-layered liposomes. This study showed that it was possible to increase the thickness of the liposome surface and to manipulate its charge using chitosan and λ-carrageenan through electrostatic deposition. Results showed that manufacturing negatively charged multiple-layer (over 4-layer) liposomes with charged biopolymer improved the physicochemical stability of liposomes.

Lipid-based carriers for pulmonary products: preclinical development and case studies in humans

A number of lipid-based technologies have been applied to pharmaceuticals to modify their drug release characteristics, and additionally, to improve the drug loading for poorly soluble drugs. These technologies, including solid-state lipid microparticles, many of which are porous in nature, liposomes, solid lipid nanoparticles and nanostructured lipid carriers, are increasingly being developed for inhalation applications. This article provides a review of the rationale for the use of these technologies in the pulmonary delivery of drugs, and summarizes the manufacturing processes and their limitations, the in vitro and in vivo performance of these systems, the safety of these lipid-based systems in the lung, and their promise for commercialization.

Dynamic, nonsink method for the simultaneous determination of drug permeability and binding coefficients in liposomes

Drug release from liposomal formulations is governed by a complex interplay of kinetic (i.e., drug permeability) and thermodynamic factors (i.e., drug partitioning to the bilayer surface). Release studies under sink conditions that attempt to mimic physiological conditions are insufficient to decipher these separate contributions. The present study explores release studies performed under nonsink conditions coupled with appropriate mathematical models to describe both the release kinetics and the conditions in which equilibrium is established. Liposomal release profiles for a model anticancer agent, topotecan, under nonsink conditions provided values for both the first-order rate constant for drug release and the bilayer/water partition coefficient. These findings were validated by conducting release studies under sink conditions via dynamic dialysis at the same temperature and buffer pH. A nearly identical rate constant for drug release could be obtained from dynamic dialysis data when appropriate volume corrections were applied and a mechanism-based mathematical model was employed to account for lipid bilayer binding and dialysis membrane transport. The usefulness of the nonsink method combined with mathematical modeling was further explored by demonstrating the effects of topotecan dimerization and bilayer surface charge potential on the bilayer/water partition coefficient at varying suspension concentrations of lipid and drug.

The role of pH and ring-opening hydrolysis kinetics on liposomal release of topotecan

The use of liposomal delivery systems for the treatment of cancer has been extensively researched because of their passive targeting to the vasculature of solid tumors. While their potential to provide prolonged retention and high drug encapsulation is desirable for anticancer agents, a mechanistic understanding is required to optimize and design liposomal drug delivery systems capable of controllable release tailored to tumor type and patient. Topotecan (TPT) is a topoisomerase I inhibitor that undergoes reversible, pH-sensitive ring-opening hydrolysis. TPT may benefit from liposomal formulation using active loading strategies to generate low intravesicular pH to prolong drug retention and increase drug encapsulation. This paper develops a mathematical model to describe TPT's permeability as a function of pH by accounting for the drug's ionization state, membrane binding, and ring-opening interconversion kinetics. Studies were conducted to determine the acid dissociation constant of TPT's phenolic -OH and interconversion kinetics between TPT's lactone and carboxylate forms. Using the constants determined from these studies and release studies conducted at varying pH, permeability coefficients and membrane binding constants for each species of TPT were determined. Based on this model, three permeable species were observed. Interestingly, the two most permeable species were zwitterionic forms of TPT, and the permeability of the lactone zwitterion was comparable to that of the neutral form of another camptothecin analogue. Furthermore, release was affected by based-catalyzed interconversion kinetics between TPT's lactone and carboxylate forms. At neutral pH, release was rate-limited by formation of the TPT lactone from the ring-opened carboxylate form. Based on these findings, the developed model describing liposomal release of TPT may be used in the future to evaluate and optimize loading and subsequent release of liposomal TPT formulations utilizing active loading strategies.

Recent Progress in Advanced Nanobiological Materials for Energy and Environmental Applications

In this review, we briefly introduce our efforts to reconstruct cellular life processes by mimicking natural systems and the applications of these systems to energy and environmental problems. Functional units of in vitro cellular life processes are based on the fabrication of artificial organelles using protein-incorporated polymersomes and the creation of bioreactors. This concept of an artificial organelle originates from the first synthesis of poly(siloxane)-poly(alkyloxazoline) block copolymers three decades ago and the first demonstration of protein activity in the polymer membrane a decade ago. The increased value of biomimetic polymers results from many research efforts to find new applications such as functionally active membranes and a biochemical-producing polymersome. At the same time, foam research has advanced to the point that biomolecules can be efficiently produced in the aqueous channels of foam. Ongoing research includes replication of complex biological processes, such as an artificial Calvin cycle for application in biofuel and specialty chemical production, and carbon dioxide sequestration. We believe that the development of optimally designed biomimetic polymers and stable/biocompatible bioreactors would contribute to the realization of the benefits of biomimetic systems. Thus, this paper seeks to review previous research efforts, examine current knowledge/key technical parameters, and identify technical challenges ahead.

Phospholipase A(2)-susceptible liposomes of anticancer double lipid-prodrugs

A novel approach to anticancer drug delivery is presented based on lipid-like liposome-forming anticancer prodrugs that are susceptible to secretory phospholipase A(2) (sPLA(2)) that is overexpressed in several cancer types. The approach provides a selective unloading of anticancer drugs at the target tissues, as well as circumvents the necessity for "conventional" drug loading. In our attempts to improve the performance of the liposomes in vivo, several PEGylated and non-PEGylated liposomal formulations composed of a retinoid prodrug premixed with the sPLA(2)-hydrolyzable DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) were prepared. Besides favorably modifying the physicochemical properties of the liposomes, the incorporation of DPPC and PEG-lipids in the liposomes should substantially enhance the enzymatic activity, as concluded from literature. In addition, one can reap benefits from the presumed permeability enhancing effect of the liberated fatty acids and lysolipids. The size distribution of the prepared liposomes as well as their phase behavior, enzymatic hydrolysis, and cytotoxicity, in the presence and absence of sPLA(2), were determined. The liposomes were around 100nm in diameter and in the gel/fluid coexistence region at 37°C. The enzymatic hydrolysis of the prodrug was pronouncedly accelerated upon the premixing with DPPC, and the hydrolysis was further enhanced by PEGylation. Interestingly, the faster hydrolysis of the prodrug and the released fatty acids and lysolipids from DPPC did not improve the cytotoxicity of the mixture; the effect of combining the prodrug with DPPC was additive and not synergistic. The data presented here question the significance of the permeability enhancing effects claimed for fatty acids and lysolipids at the target cell membrane, and whether these effects can be achieved using physiologically achievable concentrations of fatty acids and lysolipids.

Thermodynamics of lipid interactions with cell-penetrating peptides

Cationic peptides are efficiently taken up by biological cells through different pathways, which can be exploited for delivery of intracellular drugs. For example, their endocytosis is known since 1967, and this typically produces entrapment of the peptides in endocytotic vesicles. The resulting peptide (and cargo) degradation in lysosomes is of little therapeutic interest. Beside endocytosis (and various subtypes thereof), cationic cell-penetrating peptides (CPPs) may also gain access to cytosol and nucleus of livings cells. This process is known since 1988, but it is poorly understood whether the cytosolic CPP appearance requires an active cellular machinery with membrane proteins and signaling molecules, or whether this translocation occurs by passive diffusion and thus can be mimicked with model membranes devoid of proteins or glycans. In the present chapter, protocols are presented that allow for testing the membrane binding and disturbance of CPPs on model membranes with special focus on particular CPP properties. Protocols include vesicle preparation, lipid quantification, and analysis of membrane leakage, lipid polymorphism ((31)P NMR), and membrane binding (isothermal titration calorimetry). Using these protocols, a major difference among CPPs is observed: At low micromolar concentration, nonamphipathic CPPs, such as nona-arginine (WR(9)) and penetratin, have only a poor affinity for model membranes with a lipid composition typical of eukaryotic membranes. No membrane leakage is induced by these compounds at low micromolar concentration. In contrast, their amphipathic derivatives, such as acylated WR(9) (C(14), C(16), C(18)) or amphipathic penetratin mutant p2AL (Drin et al., Biochemistry 40:1824-1834, 2001), bind and disturb lipid model membranes already at low micromolar peptide concentration. This suggests that the mechanism for cytosolic CPP delivery (and potential toxicity) differs among CPPs despite their common name.

A dry powder formulation of liposome-encapsulated recombinant secretory leukocyte protease inhibitor (rSLPI) for inhalation: preparation and characterisation

Inhaled recombinant secretory leukocyte protease inhibitor (rSLPI) has shown potential for the treatment of inflammatory lung conditions. Rapid inactivation of rSLPI by cathepsin L (Cat L) and rapid clearance from the lungs has limited clinical efficacy to date. Previous studies by us have shown that encapsulation of rSLPI within1,2-dioleoyl-sn-glycero-3-[phospho-L-serine]/cholesterol (DOPS/Chol) liposomes protects rSLPI against Cat L inactivation in vitro. Liquid DOPS-rSLPI preparations were found to be unstable upon long-term storage and nebulisation. The aim of this study was therefore to develop a method of manufacture for preparing DOPS-rSLPI liposomes as a dry powder for inhalation. DOPS-rSLPI dry powders were lyophilised and subsequently micronised with a novel micronisation aid. The effects of formulation and processing on rSLPI stability, activity, and uniformity of content within the powders were characterised. Using D-mannitol as the micronisation aid, dry powder particles in the inhalable size range (<5 μm) were prepared. By optimising process parameters, up to 54% of rSLPI was recovered after micronisation, of which there was no significant loss in anti-neutrophil elastase activity and no detectable evidence of protein degradation. Aerosolisation was achieved using a dry powder inhaler, and mass median aerodynamic diameter (MMAD) was evaluated after collection in a cascade impactor. Aerosolisation of the DOPS-rSLPI dry powder yielded 38% emitted dose, with 2.44 μm MMAD. When challenged with Cat L post-aerosolisation, DOPS-rSLPI dry powder was significantly better at retaining a protective function against Cat L-induced rSLPI inactivation compared to the aqueous DOPS-rSLPI liposome dispersion and was also more stable under storage.

Influence of Intravesicular pH Drift and Membrane Binding on the Liposomal Release of a Model Amine‐Containing Permeant

Accurate determination of intrinsic permeability coefficients is critical to the development of structure-permeability relationships and liposomal delivery systems. The apparent release rate of a drug from liposomes may reflect not only its intrinsic permeability coefficient and barrier properties but also a variety of underlying equilibria including drug ionization, membrane binding or complexation, and kinetic processes such as buffer exchange. Additionally, transport of ionizable drugs that are initially at high concentrations in liposomes can generate or dissipate pH gradients across the barrier causing deviations from classical pH-permeability profiles. In this study, the liposomal release of a model amine (tyramine) is determined as a function of drug loading, intravesicular pH, and buffer composition. Kinetic models are derived to study effects of such equilibria (e.g., ionization, membrane binding) and kinetic processes (e.g., pH drift and acid/base carriers). All equilibrium constants needed for the models were independently measured and used. The barrier properties of the lipid bilayers under the experimental conditions were assessed by monitoring the transport of mannitol and bretylium as a function of pH. A corrected intrinsic permeability coefficient of 0.04 cm/s was in close agreement with the value predicted from the barrier domain model for bilayer permeability, suggesting that all perturbing factors were properly addressed.

Nanofabricated particles for engineered drug therapies: a preliminary biodistribution study of PRINT nanoparticles

A novel method for the fabrication of polymeric particles on the order of tens of nanometers to several microns is described. This imprint lithographic technique called PRINT (Particle Replication In Non-wetting Templates), takes advantage of the unique properties of elastomeric molds comprised of a low surface energy perfluoropolyether network, allowing the production of monodisperse, shape-specific nanoparticles from an extensive array of organic precursors. This engineered nature of particle production has a number of advantages over the construction of traditional nanoparticles such as liposomes, dendrimers, and colloidal precipitates. The gentle "top down" approach of PRINT enables the simultaneous and independent control over particle size and shape, composition, and surface functionality, and permits the loading of delicate cargos such as small organic therapeutics and biological macromolecules. Thus, this single tool serves as a comprehensive platform for the rational design and investigation of new nanocarriers in medicine, having applications ranging from therapeutics to advanced diagnostics. Preliminary in vitro and in vivo studies were conducted, demonstrating the future utility of PRINT particles as delivery vectors in nanomedicine. Monodisperse 200 nm poly(ethylene glycol)-based (PEG) particles were fabricated using PRINT methodology and characterized via scanning electron microscopy and dynamic light scattering. Incubation with HeLa cells showed very little cytotoxicity, even at high concentrations. The biodistribution and pharmacokinetics of [(125)I]-labeled particles were studied in healthy mice following bolus tail vein administration. The particles were distributed mainly to the liver and the spleen with an apparent distribution t(1/2) of approximately 17 min followed by slow redistribution with a t(1/2) of 3.3 h. The volume of distribution for the central and peripheral compartments was found to be approximately 3 mL and 5 mL, respectively.

Physical stability of liposomes prepared from milk fat globule membrane and soya phospholipids

Previous research has shown that liposomes prepared from a milk fat globule membrane (MFGM) phospholipid fraction had a significantly higher phase transition temperature, thicker membrane, and lower membrane permeability than liposomes prepared from soya phospholipid material. Subsequent investigations into the relative stability of the two liposome dispersions have found that the MFGM phospholipid liposomes are more stable than their soya counterparts in a range of pH conditions, at a variety of storage and processing temperatures, and in the presence of mono- and divalent cations. These results illustrate some potential advantages in the use of MFGM phospholipids for the manufacture of liposomes for use in food systems.

Effects of phospholipid hydrolysis on the aggregate structure in DPPC/DSPE-PEG2000 liposome preparations after gel to liquid crystalline phase transition

Upon storage of phospholipid liposome samples, lysolipids, fatty acids, and glycerol-3-phosphatidylcholine are generated as a result of acid- or base-catalyzed hydrolysis. Accumulation of hydrolysis products in the liposome membrane can induce fusion, leakage, and structural transformations of the liposomes, which may be detrimental or beneficial to their performance depending on their applications as, e.g., drug delivery devices. We investigated in the present study the influence of phospholipid hydrolysis on the aggregate morphology of DPPC/DSPE-PEG2000 liposomes after transition of the phospholipid membrane from the gel phase to liquid crystalline phase using high performance liquid chromatography (HPLC) in combination with static light scattering, dynamic light scattering, and cryo-transmission electron microscopy (cryo-TEM). The rates of DPPC hydrolysis in DPPC/DSPE-PEG2000 liposomes were investigated at a pH of 2, 4, or 6.5 and temperatures of 22 degrees C or 4 degrees C. Results indicate that following phase transition, severe structural reorganizations occurred in liposome samples that were partially hydrolyzed in the gel phase. The most prominent effect was an increasing tendency of liposomes to disintegrate into membrane discs in accordance with an increasing degree of phospholipid hydrolysis. Complete disintegration occurred when DPPC concentrations had decreased by, in some cases, as little as 3.6%. After extensive phospholipid hydrolysis, liposomes and discs fused to form large bilayer sheets as well as other more complex bilayer structures apparently due to a decreased ratio of lysolipid to palmitic acid levels in the liposome membrane.

Phospholipid hydrolysis in a pharmaceutical emulsion assessed by physicochemical parameters and a new analytical method

The aim of this work was to develop a simple high-performance liquid chromatography (HPLC) technique with evaporative light scattering detection (ELSD) for the separation and quantification of the major phospholipid (PL) and lysophospholipid (LPL) classes contained in a pharmaceutical phospholipid-based emulsion. In the established method, phosphatidylcholine (PC), phosphatidylethanolamine (PE), sphingomyeline (SM), lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) were separated with a PVA-Sil stationary phase and a binary gradient from pure chloroform to methanol:water (94:6 v/v) at 3.4%/min. The ELSD detection was enhanced using 0.1% triethylamine and formic acid in each gradient mobile phases. Factors such as stationary phase and ELSD drift tube temperature were optimized, concluding in optimal temperatures of 25 degrees C for separation and 50 degrees C for evaporation. This HPLC-ELSD method was then applied to a PL-emulsion exposed to autoclaving and accelerated thermal conditions at 50 degrees C. Hydrolysis of PC and PE followed first-order kinetics, representing only 45% of the total lipid mass after 3 months. The chemical stability was correlated to commonly measured formulation physical and physico-chemical parameters such as droplet size, emulsion pH and zeta-potential.

Atomic force microscopy and photon correlation spectroscopy: two techniques for rapid characterization of liposomes

The direct evaluation of the heterogeneity of the particle population of nanometric drug delivery systems as liposomes is difficult to achieve owing to the dimension and the carrier characteristics. The influence of the lipidic ratio and composition on the physical stability of liposomes during their storage was investigated using atomic force microscopy (AFM) and photon correlation spectroscopy (PCS). Liposomes were made by a mixture of different lipids and obtained using distinct methods of preparation. AFM images, acquired immediately after the deposition of the sample on mica surface, clearly showed the spherical shape of the lipidic vesicles. In all the 7 months of the experiment, the average sizes of the different liposomes evaluated using the two techniques were comparable. According to PCS analysis, AFM images confirmed that almost all the diversified vesicular systems tended to form aggregates during their storage; this loss of stability was strengthened by the increase of polydispersity index value. The different behaviours observed were to ascribe to the lipidic composition more than the methods of liposome preparation. In conclusion, AFM technique owing to the relative simplicity cold be useful for the technological control of size distribution profile according to the preparative factors and moreover to the batch-to-batch reproducibility.

Camptothecin-catalyzed phospholipid hydrolysis in liposomes

Hydrolysis of phospholipid (PL) within camptothecin (CPT)-containing liposomes was studied systematically, after elevated lyso-phosphatidylcholine (LPC)-concentrations in pH 5, CPT-containing liposomes (22.1+/-0.9 mol%) relative to control-liposomes (7.3+/-0.5 mol%) occasionally had been observed after four months storage in fridge. Liposomes were prepared by dispersing freeze-dried PL/CPT mixtures in 25 mM phosphate buffered saline (PBS) of varying pH (5.0-7.8) and CPT concentrations (0, 3 and 6 mM). PL-hydrolysis was monitored by HPTLC, quantifying LPC. In an accelerated stability study (60 degrees C), a catalytic effect of CPT on PL-hydrolysis was observed after 40 h, but not up to 30 h of incubation. The pH profile of the hydrolysis indicated a stability optimum at pH 6.0 for the liposomes independent of CPT. The equilibrium point between the more active lactone- and the carboxylate-form of CPT was found to be pH 6.8. As a compromise, pH 6.0 was chosen, assuring >85% CPT to be present in the lactone form. At this pH, both control- and CPT-liposomes showed only minor hydrolysis after autoclaving (121 degrees C, 15 min). Storage at room temperature and in fridge (2 months), as well as accelerated ageing (70 degrees C, 25 h), gave a significant elevation of LPC content in CPT-liposomes relative to control-liposomes. This study demonstrates a catalytic effect of CPT on PL-hydrolysis, the onset of which seems to require a certain threshold level of hydrolytic degradation.

Synergistic permeability enhancing effect of lysophospholipids and fatty acids on lipid membranes

The permeability-enhancing effects of the two surfactants, 1-palmitoyl-2-lyso-sn-gycero-3-phosphocholine (lysoPPC) and palmitic acid (PA), on lipid membranes that at physiological temperatures are in the gel, fluid, and liquid-ordered phases were determined using the concentration-dependent self-quenching properties of the hydrophilic marker, calcein. Adding lysoPPC to lipid membranes in the gel-phase induced a time-dependent calcein release curve that can be described by the sum of two exponentials, whereas PA induces a considerably more complex release curve. However, when lysoPPC and PA were added simultaneously in equimolar concentrations, a dramatic synergistic permeability-enhancing effect was observed. In contrast, when both lysoPPC and PA are added to liposomal membranes that are in the fluid or liquid-ordered phases, no effect on the transmembrane permeation of calcein was observed.

Filter extrusion of liposomes using different devices: comparison of liposome size, encapsulation efficiency, and process characteristics

Liposomes were prepared by stepwise extrusion through 5, 1, 0.4, 0.2, 0.1 and 0.05 microm pore sizes using two different filter-extruders, the continuous high pressure device Dispex Maximator (CE) or alternatively the discontinuous Avestin LiposoFast (DE). The liposome dispersions obtained were compared in terms of particle size, lamellarity and encapsulation efficiency of calcein. The liposomes were smaller with CE than DE at all stages due to higher flow rates and pressure drops, except for final filter pore size (0.05 microm) where both preparations had similar sizes. The particle size analysis technique itself had a strong influence on the liposome sizes measured. For bigger liposomes (extruded through 0.4 microm filters) the Nicomp 370 revealed bigger volume-based mean particle sizes along with more stringent differences between volume-based and number-based diameters than the Malvern Zetasizer. In contrast, for small liposomes extruded through 0.05 microm filters, similar liposome sizes were found no matter which of the two PCS techniques or cryo-transmission electron microscopy was used. In congruence to the liposome sizes measured, encapsulation efficiencies were smaller for CE than DE at all filter stages except the final (0.05 microm). No lipid loss occurred and lyso-phosphatidylcholine formation was negligible irrespective of which extrusion technique was used.

Effect of nucleoside analogues and oligonucleotides on hydrolysis of liposomal phospholipids

The hydrolysis of the bilayer forming phospholipids resulting first of all in lysophospholipids and fatty acids is one limiting factor determining the shelf-life of liposomes. In several studies the influence of pH, buffer, lipid composition and other parameters on the hydrolysis of phospholipids have been demonstrated, but the influence of drugs has not yet been investigated systematically. In this study the influence of nucleoside analogues, especially 2', 2'-difluoro 2'-deoxycytidine (gemcitabine, dFdC) on the degradation of phospholipids was elucidated in more detail. It could be demonstrated that the interaction of dFdC with phospholipid bilayers promotes the hydrolysis of phospholipids in a concentration-dependent manner. Obviously two parts of the molecule, the amino group bound to the pyrimidine moiety and the 2', 2'-difluoro-2'-deoxyribose, seem to be responsible for the forced phospholipid hydrolysis. The dFdC-induced hydrolysis of phospholipids was influenced by pH, buffer, lipid composition and different anions. Optimization of the above parameters resulted in prolonged shelf-life of dFdC liposome dispersions, which is an important prerequisite for clinical practice.

Damage to liposomal lipids: protection by antioxidants and cholesterol-mediated dehydration

Liposomes composed of egg phosphatidylcholine (EPC) (13.4%, of the acyl chains being polyunsaturated fatty acids (PUFA)) and EPC/cholesterol (10:1 mol/mol) were studied for factors that affect liposomal lipid oxidative damage and hydrolysis upon long-term (16 months) storage. Factors studied include: (1) levels of lipid/water interface hydration, related to the presence of cholesterol in the lipid bilayer; (2) the membrane-associated antioxidant vitamin E; (3) the water-soluble antioxidant Tempol; and (4) exposure to light. Liposomal dispersions were stored at room temperature, either exposed to or protected from daylight, for a period of 16 months. Chemical and physical changes were monitored at several time points to assess oxidative and hydrolytic degradation of liposomal lipids. The conclusions of the study are: (1) PUFA are the most sensitive component of the liposome bilayer to oxidative degradation damage during long-term storage; (2) EPC liposomes are more sensitive to degradation during storage than EPC cholesterol liposomes, the presence of cholesterol in the lipid bilayer having a protective effect, probably due to its effect in decreasing the lipid-bilayer hydration; (3) oxidative degradation is the major process during long-term storage, having an earlier onset than the hydrolytic degradation: and (4) Tempol provided significantly better protection than vitamin E to EPC liposomal PUFA against oxidative damage during long-term storage. The relevance of cholesterol's presence, as a 'drying agent', in membranes containing PUFA to resistance of biological membranes to oxidative damage is discussed.

Cytosolic drug delivery using pH- and light-sensitive liposomes

A growing body of literature describes the development and applications of novel targeting and/or contents release triggering schemes to improve the therapeutic index of drugs encapsulated within liposomes. This review focuses on literature appearing between January 1995-December 1997 that report 1) antibody and receptor-mediated targeting approaches for improving drug localization and 2) acid, enzymatic, thermal or photochemical triggering processes that destabilize membranes and improve drug bioavailability via cytoplasmic delivery of liposomal contents.

Freeze-drying of drug-free and drug-loaded solid lipid nanoparticles (SLN)

Solid lipid nanoparticles (SLN) of a quality acceptable for i.v. administration were freeze-dried. Dynasan 112 and Compritol ATO 888 were used as lipid matrices for the SLN, stabilisers were Lipoid S 75 and poloxamer 188, respectively. To study the protective effect of various types and concentrations of cryoprotectants (e.g. carbohydrates), freeze-thaw cycles were carried out as a pre-test. The sugar trehalose proved to be most effective in preventing particle growth during freezing and thawing and also in the freeze-drying process. Changes in particle size distribution during lyophilisation could be minimised by optimising the parameters of the lyophilisation process, i.e. freezing velocity and redispersion method. Lyophilised drug-free SLN could be reconstituted in a quality considered suitable for i.v. injection with regard to the size distribution. Loading with model drugs (tetracaine, etomidate) impairs the quality of reconstituted SLN. However, the lyophilisate quality is sufficient for formulations less critical to limited particle growth, e.g. freeze-dried SLN for oral administration.