Effects of cholesterol on the miscibility of synthetic glucosamine diesters in lipid bilayers and the entrapment of superoxide dismutase into the positively charged liposomes

Cationic phospholiposomes: efficient delivery vehicles of anticancer derivatives of ATP to multiple myeloma cells

Analogs of adenosine triphosphate (ATP) with substitutions at the 8-position have been shown to be cytotoxic to multiple myeloma, one of the most prevalent and serious blood cancers. However, these drugs do not readily cross biological membranes and are very sensitive to phosphatases present in body fluids. To circumvent these disadvantages, 8-substituted ATPs were encapsulated into cationic phospholiposomes generated from cationic phosphatidylcholines (EDOPC; 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine, and EDPPC, the corresponding dipalmitoyl homolog), compounds with low toxicity that readily form liposomes. Vortexing was an efficient encapsulation procedure, more so than freeze-thawing. At the lipid:drug ratio of 5:1 (mol/mol), 20% of 8-Br-ATP was encapsulated within EDOPC liposomes. Efficient encapsulation and retention of 8-NH₂-ATP required the inclusion of cholesterol. Liposomes of EDOPC:cholesterol (55:45 mole/mole), at a lipid:drug mole ratio of 10:1, captured ~40% of the drug presented. Cytotoxicity assays of this formulation on multiple myeloma cells in culture showed encapsulated drug to be up to 10-fold more effective than free drug, depending upon dose. Intracellular distribution studies (based on fluorescent derivatives of lipids and of ATP) revealed that both liposomes and drug were taken up by multiple myeloma cells, and that uptake of a fluorescent ATP derivative was significantly greater when encapsulated than when free. Liposomes prepared from EDPPC, having a higher phase-transition temperature than EDOPC, captured 8-NH₂-ATP satisfactorily and released it more slowly than the unsaturated formulations, but were also less cytotoxic. The superior encapsulation efficiencies of the positively charged liposomes can be understood in terms of the electrostatic double layer due to a very high positive charge density on their inner surface. Electrostatic augmentation of encapsulation for small vesicles can be dramatic, easily exceeding an order of magnitude.

Therapeutic uses of antioxidant liposomes

This review will focus on the therapeutic uses of antioxidant liposomes. Antioxidant liposomes have a unique ability to deliver both lipid- and water-soluble antioxidants to tissues. This review will detail the varieties of antioxidants which have been incorporated into liposomes, their modes of administration, and the clinical conditions in which antioxidant liposomes could play an important therapeutic role. Antioxidant liposomes should be particularly useful for treating diseases or conditions in which oxidative stress plays a significant pathophysiological role because this technology has been shown to suppress oxidative stress. These diseases and conditions include cancer, trauma, irradiation, retinotherapy or prematurity, respiratory distress syndrome, chemical weapon exposure, and pulmonary infections.

High efficiency entrapment of superoxide dismutase into mucoadhesive chitosan-coated liposomes

Superoxide dismutase (SOD), antioxidative enzyme and potential anti-inflammatory agent, was encapsulated into mucoadhesive chitosan-coated liposomes in order to increase its releasing time and to facilitate its cellular penetration. Positively, neutrally and negatively charged liposomes were prepared using soybean lecithin, stearylamine, phosphatidyl glycerol and cholesterol. The effects of liposomal lipid composition and protein to lipid ratio on the encapsulation parameters were studied in three preparation methods: dehydration-rehydration, hydration and proliposome methods. The highest efficiency of SOD entrapment, 39-65%, was achieved by the proliposome method. Vesicles prepared by the hydration method entrapped 1-13% and vesicles prepared by dehydration-rehydration entrapped 2-3% of SOD. Stability tests for SOD-loaded liposomes prepared by the proliposome method showed no significant loss of the enzyme activity within 1 month at 4 degrees C or within 2 days at 37 degrees C. Positively, neutrally and negatively charged liposomes, prepared by the proliposome method, were successfully coated with two types of low and medium molecular weight chitosans. Both types of chitosan coating increased the mucoadhesive characteristics of all three types of vesicles. Using the proliposome method and subsequent chitosan coating, highly efficient SOD-loaded vesicles for drug targeting on mucosal tissues could be produced.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Therapeutic uses of antioxidant liposomes

This chapter focuses on the use of antioxidant liposomes in the general area of free radical biology and medicine. The term antioxidant liposome is relatively new and refers to liposomes containing lipid-soluble chemical antioxidants, water-soluble chemical antioxidants, enzymatic antioxidants, or combinations of these various antioxidants. The role of antioxidants in health and disease has been extensively discussed, and many excellent reviews and books are available (1–3). Antioxidant liposomes hold great promise in the treatment of many diseases in which oxidative stress plays a prominent role. Oxidative stress is a physiological condition in which the production of damaging free radicals exceeds the in vivo capacity of antioxidant protection mechanisms to prevent pathophysiology. Free radicals are molecules with unpaired electrons, often highly reactive and damaging to biological systems. The biological membranes of subcellular organelles are a major site of free radical damage but proteins and DNA are also significant targets. Moreover, free radicals can alter cellular signal transduction pathways and stimulate the synthesis of inflammatory cytokines. Oxygen radicals and other reactive oxygen species (ROS) arise from the single electron reductions of oxygen.

Increased permeability of phase-separated liposomal membranes with mixtures of ethanol-induced interdigitated and non-interdigitated structures

It has been suggested by many workers using model membranes that the interdigitated structure formation, in which the acyl chains fully interpenetrate the hydrocarbon chains of the opposing monolayer, plays an important role in regulating many functions of biomembranes. In the present study the control of permeability was focused on as one of the biomembrane functions, and the effects of ethanol on the permeability of large unilamellar vesicles made by the extrusion technique (LUVET) (average diameter: about 250 nm), composed of dipalmitoyl or egg yolk phosphatidylcholines, were studied by monitoring the leakage of fluorescent dye, calcein, entrapped in the inner aqueous phase of the LUVET. The permeability was estimated from the apparent rate constant of calcein leakage at 25 degrees C. Large permeabilities were observed in the region of 0.6 M to 1.3 M ethanol, with a concentration dependence. In this range of ethanol concentrations the normal bilayer and interdigitated structure coexist and the membrane is in a phase-separated state. The large permeability is due to the instability of the boundary regions, the interdigitated membrane being characterized by a thinner structure and more rigid hydrocarbon regions in the layer than its non-interdigitated counter part. These results suggest the possibility of biomembrane-permeability regulation by interdigitated membrane formation.

Ethanol-induced aggregation and fusion of small phosphatidylcholine liposome: participation of interdigitated membrane formation in their processes

The mechanism for the ethanol-induced aggregation/fusion of uniform-sized small liposomes comprised of dipalmitoyl (DPPC) or egg yolk (eggPC) phosphatidylcholines was studied by measuring the average size using a photon correlation spectroscopy, by observing directly the states in the liposomal solutions using freeze-fracture electron microscopy and by attempting resonance energy transfer using flurophore-labeled phospholipids. Abrupt increases in the apparent size of DPPC liposomes were observed in the presence of above 44 mg/ml ethanol, where microscopically plateau membranes form interdigitated structure, in which the acyl chains fully interpenetrate the hydrocarbon chains of the apposing monolayer. On the contrary, in the eggPC liposome, where the membranes cannot form interdigitated structures even in the presence of high concentration of ethanol, such intense aggregation and fusion were not observed, suggesting their intimate relation to the interdigitated structure formation. The formation of interdigitated structures in the adhering region leads to an increase in the interfacial area and an exposure of hydrophobic acyl chain terminal on the surface area, and enhances hydrophobic interactions between two interdigitated bilayers. Thus, the resultant interdigitated structure makes the aggregated state stable and partially initiates the bilayer mixing between the two apposed membranes, leading to fusion.