Liposomes reverse phase evaporation vesicles

Nagata, T., Okabe, K., Takebe, I. and Matsui, C. (1981). Delivery of tobacco mosaic virus RNA into plant protoplasts mediated by reverse-phase evaporation vesicles (liposomes). Mol. Genet. Genomics 184, 161-5. 

Based on size and lamellarity, liposomes can be categorized into four groups (175, 205) (as indicated in Figure 8.22) (i) multilanellar vesicles (MLVs) (ii) large unilamellar vesicles (LUVs) (iii) small unilamellar vesicles (SUVs) and (iv) intermediate-size unilamellar vesicles (lUVs), which are also called reverse-phase evaporation vesicles (REV). 

Canova-Davis, E., Redemann, C. T., Vollmer, Y. P., and Kung, V. T. (1986). Use of a reversed-phase evaporation vesicle formulation for a homogeneous liposome immunoassay. Clin. Chem. 32 1687-1691. 

Qi, X.-R. Maitani, Y. Shimoda, N. Sakaguchi, K. Nagai, T. Evaluation of liposomal erythropoietin prepared with reverse-phase evaporation vesicle method by subcutaneous administration in rats. Chem.Pharm.Bull, 1995, 43, 295-299... 

Figure 10.10 Transmission electron micrograph of ferritin entrapped in POPC liposomes (palmitoyloleoylphosphatidylcholine). Cryo-TEM micrographs of (a) ferritin-containing POPC liposomes prepared using the reverse-phase evaporation method, followed by a sizing down by extrusion through polycarbonate membranes with 100 nm pore diameters ([POPC] = 6.1 mM) and (b) the vesicle suspension obtained after addition of oleate to pre-formed POPC liposomes ([POPC] = 3 mM, [oleic acid - - oleate] = 3 mM). (Adapted from Berclaz et al, 2001a, b.)...

In this case, unilamellar vesicles with a large capture volume were prepared by the reverse phase evaporation technique and alginate was used to microencapsulate the liposome s. The alginate spheres were double coated, first with poly-L-lysine and then with polyvinyl amine (Wheatley and Langer in press). 

One of the major drawbacks of liposomes is related to their preparation methods [3,4]. Liposomes for topical delivery are prepared by the same classic methods widely described in the literature for preparation of these vesicles. The majority of the liposome preparation methods are complicated multistep processes. These methods include hydration of a dry lipid film, emulsification, reverse phase evaporation, freeze thaw processes, and solvent injection. Liposome preparation is followed by homogenization and separation of unentrapped drug by centrifugation, gel filtration, or dialysis. These techniques suffer from one or more drawbacks such as the use of solvents (sometimes pharmaceutically unacceptable), an additional sizing process to control the size distribution of final products (sonication, extrusion), multiple-step entrapment procedure for preparing drug-containing liposomes, and the need for special equipment. 

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. 

The formation of liposomes [or better arsonoliposomes (ARSL)], composed solely of arsonolipids (Ars with R=lauric acid (C12) myristic acid (C14) palmitic acid (C16) and stearic acid (C18) (Fig. 1) have been used for ARSL construction), mixed or not with cholesterol (Choi) (plain ARSL), or composed of mixtures of Ars and phospholipids (as phosphatidylcholine [PC] or l,2-distearoyl- -glyceroyl-PC [DSPC]) and containing or not Choi (mixed ARSL), was not an easy task. Several liposome preparation techniques (thin-film hydration, sonication, reversed phase evaporation, etc.) were initially tested, but were not successful to form vesicles. Thereby a modification of the so called one step or bubble technique (8), in which the lipids (in powder form) are mixed at high temperature with the aqueous medium, for an extended period of time, was developed. This technique was successfiil for the preparation of arsonoliposomes (plain and mixed) (9). If followed by probe sonication, smaller vesicles (compared to those formed without any sonication [non-sonicated]) could be formed [sonicated ARSL] (9). Additionally, sonicated PEGylated ARSL (ARSL that contain polyethyleneglycol [PEG]-conjugated phospholipids in their lipid bilayers) were prepared by the same modified one-step technique followed by sonication (10). 

Over 40 years since it what found that phospholipids can form closed bilayered structures in aqueous systems, liposomes have made a long way to become a popular pharmaceutical carrier for numerous practical applications. Liposomes are phospholipid vesicles, produced by various methods from lipid dispersions in water. Liposome preparation, their physicochemical properties and possible biomedical application have already been discussed in several monographs. Many different methods exist to prepare liposomes of different sizes, structure and size distribution. The most frequently used methods include ultrasonication, reverse phase evaporation and detergent removal from mixed lipid-detergent micelles by dialysis or gel-filtration. To increase liposome stability towards the physiological environment, cholesterol is incorporated into the liposomal membrane (up to 50% mol). The size of liposomes depends on their composition and preparation method and can vary from... 

The size and smface properties of liposomes vary with types of lipids, their compositions, their modification, and methods of preparation. For example, multilamellar vesicles (MLVs) several hundred nanometers in size can be produced by a reverse phase evaporation and extrusion, but smaller unilamellar vesicles (SUVs), whose size is less than 100 mn, can be produced by a sonication process [15]. Further, the membrane state of a bilayer is of primary interest not only for surface treatment but also for recognition of a cell surface and delivery of active ingredients. We will briefly review the microfluidity of bilayers and the interaction of liposomes with a cell surface. 

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