Liposome formation

The manufacture of liposomes (which is, for example, carried out by the cosmetics industry) is a complex technical process. Thus, the question arises as to whether a simple mechanism for liposome formation exists which could have functioned under plausible prebiotic conditions. More than 20 years ago, Deamer and co-workers... 

The use of sphingolipids in liposome formation is possible due to the natural amphipathic properties of the molecules. Some sphingolipids can lend structural advantages to the integrity... 

For the activation of PE prior to liposome formation, it is best to employ a highly purified form of the molecule. While egg PE is abundantly available, it consists of a range of fatty acid derivatives—many of which are unsaturated—and is highly susceptible to oxidation. Synthetic PE, by contrast, can be obtained having a discrete fatty acid composition and is much more stable to oxidative degradation. 

Biotinylation may be done before or after liposome formation, but having a stock supply of biotin-modified PE is an advantage, since it can then be used to test a number of liposomal recipes. In addition, only a very small percent of the total lipid should be biotinylated to prevent avidin-induced aggregation in the absence of antigen. It is difficult to control precisely... 

Liposome Formation. The pioneering investigations of Bang-ham (5) have shown that thin films of natural phospholipids form bilayer assemblies if they are lyophilized in excess water by simple handshaking above the phase transition temperature. While this procedure results in the formation of large, multibilayered spherical structures, by ultrasonication of such lipid dispersions small unilamellar liposomes are formed (16), which are schematically shown in Figure 10. Additional metTiods for liposome preparation are described in a number of reviews (17,44,45,46). 

The sensors discussed so far are based on ligands covalently bound to the polymer backbone. Other methods of detection - often referred to as mix and detect methods - work by simple noncovalent incorporation of the polymer with the ligand of interest. Reichert et al. generated liposomes of polydiacetylene with sialic acid for the same purpose of detection as Charych s surface-bound polymers, but realized that covalent functionalization of the polymer was not necessary [17]. Through simple mixing of the lipid-bound sialic acid with the polymer before sonication and liposome formation, they were able to form a functional colorimetric recognition system (Fig. 8). 

Maurer N, Wong KF, Stark H, et al. Spontaneous entrapment of polynucleotides upon electrostatic interaction with ethanol destabilized cationic liposomes formation of small multilamellar liposomes. Biophys J 2001 80 2310. 

As the solubility of this peptide in water is very low, the peptide can be associated with the liposomal membrane. As the peptide is only sparingly soluble in methanol or chloroform, DMSO had to be used as dissolution medium for mixing the peptide with the lipids for liposome formation. A lOmg/mL stock solution in DMSO of the peptide could be obtained. Appropriate amounts of lipid stock solutions and the peptide stock solution were mixed (lipid peptide ratio = 95 5) and processed as thoroughly described in the... 

Figure 1 Influence of liposome formation method on TRP2 content of liposomes.

Up to 500 pg of plasmid DNA (for the amount of PC shown above) is dissolved in 2mL distilled water, or lOmM sodium phosphate buffer (PB) of pH 7.2 if needed. For liposomes containing both the plasmid DNA and the vaccine protein it encodes (or only the protein), up to 1 mg of the protein is included. The nature of buffer with respect to composition, pH, and molarity can be varied as long as this does not interfere with liposome formation or DNA and protein entrapment yield. Amounts of added DNA and protein can be increased proportionally to the total amount of lipid used. For cationic liposomes, the amount of added DNA can also be increased by employing more cationic lipid. 

Shown in Figure 1 are the principal schemes for micelle and liposome formation and loading with various reporter moieties that might be covalently or noncovalently incorporated into different compartments of these particulate carriers. Although micelles may be loaded with a contrast agent only into the core in the process of micelle assembly, liposomes may incorporate contrast agents in both the internal water compartment and the bilayer membrane. 

The study of liposomes as dmg-delivery agents has been ongoing for decades. The lipids for liposome formation are typically harvested by extraction from egg yolks and soybeans, and a number of recipes exist for generating liposomes of various diameters. Because the shell material in liposomes is not polymeric, we will not discuss them in depth, limiting ourselves only to those aspects that are pertinent to synthetic analogues like the copolymer vesicles of the next section. 

For the activation of PE prior to liposome formation, it is best to employ a highly purified form of the molecule. Although egg PE is abundantly available, it consists of a... 

Biotinylation may be done before or after liposome formation, but having a stock supply of biotin-modified PE is an advantage, since it can then be used to test a number... 

Assadullahi, T.P., R.C. Hider, and A.J. McAuley. 1991. Liposome formation from synthetic polyhydroxyl lipids. Biochim Biophys Acta 1083 271. 

Arien, A., Toulme-Henry, N., and Dupuy, B. Cholate-induced disruption of calcitonin-loaded liposomes Formation of trypsin-resistant lipid-calcitonin-cholate complexes. Pharm. Res. 12 1289-1292, 1995. 

Artificial membranes are used to study the influence of drug structure and of membrane composition on drug-membrane interactions. Artificial membranes that simulate mammalian membranes can easily be prepared because of the readiness of phospholipids to form lipid bilayers spontaneously. They have a strong tendency to self-associate in water. The macroscopic structure of dispersions of phospholipids depends on the type of lipids and on the water content. The structure and properties of self-assembled phospholipids in excess water have been described [74], and the mechanism of vesicle (synonym for liposome) formation has been reviewed [75]. While the individual components of membranes, proteins and lipids, are made up of atoms and covalent bonds, their association with each other to produce membrane structures is governed largely by hydrophobic effects. The hydrophobic effect is derived from the structure of water and the interaction of other components with the water structure. Because of their enormous hydrogen-bonding capacity, water molecules adopt a structure in both the liquid and solid state. 

In this article, some of the fundamental kinetic ideas relating to spontaneous vesicle formation and breakdown by surfactants in aqueous media are described. The work is related to liposome formation and breakdown using phospholipids, and the induced rupture of membranes. 

Liposome Preparation Techniques In most cases, liposomes are named by the preparation method used for their formation, Such as sonicated, dehydrated-rehy-drated vesicle (DRV), reverse-phase evaporation (REV), one step, and extruded. Several reviews have summarized available liposome preparation methods [91,124, 125], Liposome formation happens spontaneously when phospholipids are dispersed in water. However, the preparation of drug-encapsulating liposomes with high drug encapsulation and specific size and lamellarity is not always an easy task. The most important methods are highlighted below. 

Talsma, H., Van Steenbergen, M. J., Borchert, J. C. H., Crommelin, D. J. A. (1994), A novel technique for the one-step preparation of liposomes and nonionic surfactant vesicles without the use of organic solvents. Liposome formation in a continuous gas stream The bubble method,/. Pharm. Sci., 83, 276-280. 

Liposomes are spherical vesioles formed by the aggregation of amphiphilio phospholipid moleoules in a bilayer struoture. Liposome formation ooours when phospholipids are dispersed into an aqueous medium — usually water — as a result of interaotions between phospholipids and water. Thus, liposomes encapsulate part of the aqueous medium in whioh they are suspended. The amphiphilic charaoter of phospholipids allows them to form olosed struotures where hydrophobic and (or) hydrophilio moleoules oan be entrapped or anohored. 

Based on the modes of lipid dispersion, the methods of liposomes formation can be classified into three categories mechanical dispersion, solvent dispersion, and detergent solubilization (175). These generally involve the following stages as evident in Figure 8.23. 

The buffer used can be varied with respect to composition, pH, and molarity, as long as this does not interfere with liposome formation or entrapment yield (see Note 10). 

Nebulization can cause disruption or processing of multilameller vesicles [52]. Fortunately, these issues can be addressed, and, through manipulation of the composition, buffer and environment liposomes have been aerosolized without causing loss of entrapped drug [53-55]. Liposomes have also been prepared as spray-dried and lyophilized powders [56-59]. The former may be aerosolized directly as a powder, but in both cases reconstitution in an aqueous environment results in liposome formation. However, it is not understood if this is just spontaneous reformation of original liposomes (pre-spray-drying) or the creation of de novo liposomes in an aqueous environment. 

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