Liposomes biological stability

Efficient drug delivery systems based on liposomes need to possess a large number of special qualities. First, good colloidal, chemical, and biological stability is required. The fact that liposomes are non-equilibrium structures does not necessarily mean that they are unsuitable for drug delivery. On the contrary, a colloidally stable non-equilibrium structure is less sensitive to external changes than equilibrium structures, such as micelles. Hence, colloidally stable liposomes often work well in pharmaceutical applications. 

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Models for biological membranes have either been realized as planar lipid monolayers at the gas-water interface (3) or as bi-molecular lipid membranes (BLM) (4) and spherical liposomes (vesicles), respectively (5 6) (Figure 2.). All these models that are only composed of lipid molecules exhibit a diminished stability compared to natural cell membranes. Obviously the protein part besides being functionally important plays a role in terms of stability of biomembranes. This is the case not only for the integral but especially for the boundary proteins ( 7). 

Woodle MC, Newman MS, Cohen JA. Sterically stabilized liposomes physical and biological properties. J Drug Target 1994 2 397. 

Woodle MC, Newman MS, Working PK. Biological properties of sterically stabilized liposomes. In Lasic DD, Martin F, eds. Stealth Liposomes. Boca Raton CRC Press, 1995 103. 

Woodle, M. C., Newman, M. S., and Cohn, J. A. (1994). Sterically stabilized liposomes physical and biological propertiesJ. Drug Target., 2, 397-403. 

In this chapter we will provide information about the basic characteristics of liposomes staring from their building blocks, that is, phospholipids. After this, liposome structure, physicochemical properties, and stability, which are most important for their in vivo performance, will be discussed as well as methods used for liposome preparation, characterization, and stabilization. Following this first part which is more technological, we will move into the biological part and talk about the fate of conventional liposomes and sterically stabilized liposomes, as well as liposomal drugs, after in vivo administration by different routes [mainly intravenous (i.v.), intraperitoneal (i.p.), or subcutaneous (s.c.)] and also give some information about other possible routes for in vivo administration of liposomes. Finally, specific applications of liposomes in therapeutics will be presented, some in more detail, mainly for the therapy of different types of cancer. 

Surface Charge of Liposomes The electrical properties of liposomal surfaces can influence the physical stability of liposomal dispersions during storage as well as the behavior of liposomes in the biological milieu and their interaction with cells [47,81],... 

Pharmaceuticals Antioxidant, emulsifier or surfactant, liposomal encapsulating agent, machining aid, nutritional supplement or vitamin aid, stabilizer, wetting agent, biologically active agent... 

The rote of cholesterol in the fluidity of biological membranes is characterized as essential. Cholesterol is a silent molecule and in the case of lipid bilayers and liposomes it is included into bilayers to control the rate of the release of encapsulated molecules or to influence the stability of liposomes. The addition of cholesterol in lipid bilayers composed of DPPC, at concentrations more than 20%, results in the decrease of the Tin and elimination of the pretransition temperature. 

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