Liposomes surface modification

Torchilin VP et al (2001) Amphiphilic poly-27-vinylpyrrolidones Synthesis, properties and liposome surface modification. Biomaterials 22 3035-3044... 

The specific applications require different bulk and surface properties of liposomes. Among others, for example, the unique properties of stealth, targeted, and cationic liposomes could be achieved by surface modification. Figure 1 illustrates examples of liposomal surface modifications for such specific applications. The goals of these surface modifications are (1) to increase liposome longevity and stability in the circulation, (2) to change liposome biodistribution, (3) to achieve targeting effect, and (4) to impart to liposomes some unusual properties such as pH or thermal sensitivity. 

The fate of injected liposomes is drastically altered by administration route, dose and size, lipid composition, surface modification, and encapsulated drugs. Liposomes encapsulating drugs are often administered iv, therefore, the stability of liposomes in plasma is important. When liposomes composed of PC with unsaturated fatty acyl chains are incubated in the presence of serum, an efflux of internal solute from the liposomes is observed. This increase in permeability is caused by the transfer of phospholipids to high density lipoprotein (HDL) in serum (55). To reduce the efflux of liposomal contents, cholesterol is added as a liposomal component... 

The concept of de-PEGylation can be applied to the development of nanoparticle-based drug delivery systems. PEG is used for the modification of liposomes to increase their blood circulation time [38], However, it also prevents cellular uptake, resulting in a decrease in therapeutic efficiency thus, modifications of the liposome surface with PEG interfere with membrane fusion to the cell membrane and liposome decomposition [39]. One of the possible strategies to solve this problem is to cleave the PEG chains after the nanoparticle reaches the target site (Fig. 9). This system of de-PEGylation of liposomes is also useful in avoiding the immune... 

Biotinylated liposomes usually are created by modification of PE components with an amine-reactive biotin derivative, for example NHS-LC-Biotin (Chapter 11, Section 1). The NHS ester reacts with the primary amine of PE residues, forming an amide bond linkage (Figure 22.19). A better choice of biotinylation agent may be to use the NHS-PEG -biotin compounds (Chapter 18), because the hydrophilic PEG spacer provides better accessibility in the aqueous environment than a hydrophobic biotin spacer. Since the modification occurs at the hydrophilic end of the phospholipid molecule, after vesicle formation the biotin component protrudes out from the liposomal surface. In this configuration, the surface-immobilized biotins are able to bind (strept)avidin molecules present in the outer aqueous medium. 

Torchilin VP, Weissig V, Martin FJ, Heath TD, New RRC. Surface modification of liposomes. In Torchilin VP, Weissig V, eds. Liposomes A Practical Approach. Oxford, U.K. Oxford University Press, 2003 193. 

Since none of the liposomal immunoassay approaches described in the scientific literature thus far took advantage of surface immobilization techniques, one could envision a double-amplification biosensor in which surface modification plays an important role [35]. For example, consider a dehydrogenase enzyme marker system which requires an electroactive cofactor such as NAD+. In the enzymatic reaction scheme ... 

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