Liposomes, Polymeric-Protein Incorporation

First, a mixture of synthetic or natural phospholipids, polymerizable lipids, and proteins can be converted to liposomes and then be polymerized. Second, polymerizable lipids are introduced into e.g. erythrocyte ghost cells by controlled hemolysis and subsequent polymerization as described by Zimmermann et al.61). This hemolysis technique is based on a reversible dielectric breakdown of the cell membrane. Dielectric breakdown provides a third possible path to the production of bi omembrane models. Zimmermann et al. could show that under certain conditions cells can be fused with other cells or liposomes61). Thus, lipids from artificial liposomes could be incorporated into a cell membrane. A fourth approach has been published by Chapman et al.20). Bacterial cells incorporate polymerizable diacetylene fatty acids into their membrane lipids. The diacetylene units can be photopolymerized in vivo. The investigations discussed in more detail below are based on approaches 1. and 3. 

Mixtures of proteins, natural and polymerizable lipids can be transferred into liposomes and polymerized hereafter. Initial experiments have shown that even very complex proteins such as F F.-ATPase can be incorporated in polymeric liposomes by this method under retention of the activity of the protein (76). 

Diacyllipid-polyethyleneoxide conjugates have been introduced into the controlled drug delivery area as polymeric surface modiLers for liposomes (Klibanov et al., 1990). Being incorporated into the liposome membrane by insertion of their lipidic anchor into the bilayer, such molecules can ster-ically stabilize the liposome against interaction with certain plasma proteins in the blood that results in signiLcant prolongation of the vesicle circulation time. The diacyllipid-PEO molecule itself represents a characteristic amphiphilic polymer with a bulky hydrophilic (PEO) portion and a very short but extremely hydrophobic diacyllipid part. Typically, for other PEO-containing amphiphilic block... 

Over the last decades, several academic and industrial research programs have been focused on the development and production of appropriate biocompatible formulations that provide enhanced therapeutic performance. Three different strategies can be discerned that are applied separately or in combination (i) addition of excipients to proteins, such as protease inhibitors, penetration or absorption enhancers like bile salts, fatty acids, cyclodextrins or surfactants " (ii) modification of the physicochemical properties of proteins, e.g. by attachment of lipophilic or hydrophilic moieties or (iii) incorporation of proteins into polymeric or liposomal delivery carriers. " A variety of polymeric vectors has been developed and exploited for this purpose, including biodegradable nanoparticles, nanogels, micelles, polymer bioconjugates and soluble nanocomposites. These polymeric carriers are more extensively described in the following sub-sections. 

Studies on the biological activity of polydiacetylene veacles were repotted by Wagner et al. F j-F,-ATPase from Rhodospirilium rubrum could easily be incorporated into polymerized liposomes under retention of the protein activity. While monomeric liposomes showed a lower activity than corresponding natural phospholipids, it increased upon polymerization to the value determined for natural liposomes. Different from the natural ones the ATPase containing polymers are stable over long time periods. 

Presently attempts are made to incorporate lectins into polymerized liposomal membranes to mimic vesicle-vesicle recognition between lectin- and sugar carrying systems. The possibility of incorporation of a membrane protein into pol3mieric vesicles has already been demonstrated and will be discussed in the following. 

INCORPORATION OF MEMBRANE PROTEINS INTO POLYMERIC LIPOSOMES... 

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