Polymerized liposomes

In the 1980s, polymerization was introduced to overcome the limited stability of synthetic vesicles (2-4). It was found that the stability of the polymerized vesicles was improved dramatically compared to the unpolymerized vesicle and that entrapped substances are released to a much smaller extent from polymerized liposomes than from monomeric ones. 

Monomers (15) and (16) have been used for the investigation of the recognition of polymeric liposomes by lectins (62). 

In the previous chapters it has been shown that stable cell membrane models can be realized via polymerization of appropriate lipids in planar monolayers at the gas-water interface as well as in spherical vesicles. Moreover, initial experiments demonstrate that polymeric liposomes carrying sugar moieties on their surface can be recognized by lectins, which is a first approach for a successful targeting of stabilized vesicles being one of the preconditions of their use as specific drug carriers in vivo. 

An additional possibility could be the transport of polymeric tensides, which are themselves not able to destroy the polymeric liposomes, to the membrane of the cancer cell. For the release of the incorporated tenside the membrane of the carrier liposome must contain destabilizable areas comparable to cork-stoppers (11). The destabilizable areas could eventually be opened by pTiotochemical destabilization of the membrane ( 73), variation of pH (74), increase of temperature ( 75), or enzymatic processes. 

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). 

Figure 14. Scanning electron micrograph of large multilamellar polymeric liposomes from (5) (9).

Fig. 8 (top) Liposome with polydiacetylene linked monolayer mixed with ligand for receptor detection. (bottom) Colorimetric detection of influenza virus using polymerized liposomes to which have been added increasing amounts of influenza virus from left to right. [18]... 

Hayward JA, Levine DM, Neufeld L, et al. Polymerized liposomes as stable oxygen-carriers. FEBS Lett 1985 187 261. 

Figure 12.4 Colorimetric detection of influenza virus using polymerized liposomes containing sialic acid, (a) Photograph of liposomes to which have been added increasing amounts (from left to right) of influenza virus. Liposomes were 1 and 95% 5. To each well was added the following amounts of influenza virus (left to right) 0, 8, 16, and 32 hemagglutinin units (HAU). Reprinted from Charych et al. (1996). Copyright 1996 Elsevier Science. (See color insert.)...

A variety of methods for immobilizing polymerized liposomes have been reported. Multiple layers of vesicles were deposited on quartz slides by layer-by-layer assembly (Su 2005). The slides were first coated with a layer of poly(ethylene imine), a... 

S- [51], and C- [49,52] a-sialosides have been prepared using copolymerization or grafting methods. More complex sialylohgosaccharide-containing polymers have also been prepared, including GM3 [53], sialyl Lewis [54], sialyl Lewis [55], and a related 3 -sulfo-Lewis analog [56, 57], including polymerized liposomes [58]. 

Fig. 18. Scheme of formation of polymeric liposomes from polymerizable lipids 19-33)... 

An even more striking effect is observed by addition of the surfactant sodiumdodecyl sulfate (SDS) to vesicles (Fig. 23). While monomeric vesicles of (19) and dipalmitoylle-cithin are destroyed by low SDS concentrations, the polymerized vesicles are stable up to 2 - 10 3 mol/1 SDS25). Polymerized vesicle dispersions can be diluted with ethanol without precipitation.231 Polymeric liposomes of (20) are stable in 80% ethanol for weeks. This could also be shown by Regen et al. for polymerized vesicles of the methacryloylic lipids (4) and (6)13141 (Fig. 24) by monitoring the turbidity (absor-... 

In contrast, the phase transition of polymeric liposomes is retained if the polymer chain is more flexible or located on the surface of the vesicles instead within the hydrophobic core. Polymerized vesicles of methacrylamide (29) show a phase transition temperature which is slightly lower than the one for the corresponding monomeric vesicles (Fig. 26). This can be explained by a disordering influence of the polymer chain on the head group packing 15). 

Polycondensation reactions in oriented monolayers and bilayers proceed without catalysis, and simply occur due to the high packing density of the reactive groups and their orientation in these layers. Bulk condensation of the a-amino acid esters at higher temperatures does not lead to polypeptides but to 2,5-diketopiperazines. No diketopiperazines are found in polycondensed monolayers or liposomes. Polycondensation in monolayers and liposomes leading to oriented polyamides represents a new route for stabilizing model membranes under mild conditions. In addition, polypeptide vesicles may be cleavable by enzymes in the blood vessels. In this case, they would represent the first example of stable but biodegradable polymeric liposomes. 

Fig. 41. ATPase activity of ATP synthetase incorporated into liposomes (-O-) incubation of the enzyme with polymerized liposomes (-+-) incubation of the enzyme with monomeric liposomes of (22) followed by polymerization (- ) activity of ATP synthetase in soybean lecithin liposomes24. ATPase activity was measured in 100 mM Tris-HCl (pH 8.0) in the presence of 1 mM ATP at 37 °C by determining the liberated orthophosphate...

Reviews have already been published by J. H. Fendler on Polymerized Surfactant Vesicles 91 92,931 which refer to Novel Membrane Mimetic Systems , synthetic strategies leading to them and their characterization and potential utilization in various areas such as solar energy conversion and reactivity control. It is the intend of this appendix to bring the reader up to date on the state of the art of polymerized liposomes. 

In such phase-separated partially polymerized liposomes the corks consisting of cleavable lipids can be visualized by freeze-fracture electron microscopy101) and photobleaching 100). 

Chen, H., Torchilin, V., and Langer, R. Lectin-bearing polymerized liposomes as potential oral vaccine carriers. Pharm. Res. 13 1378-1383, 1996. 

Clark, M. A., Blair, H., Liang L., et al. Targeting polymerized liposome vaccine carriers to intestinal M cells. Vaccine 20 208-217, 2001. 

Mineral Salts Immunostimulatory adjuvants Lipid particles Particulate adjuvants Mucosal adjuvants Aluminium hydroxide, aluminium phosphate, calcium phosphate Saponins (e.g., QS21), MDP derivatives, bacterial DNA (CpG oligos), LPS, MPL and synthetic derivatives, lipopeptides, cytokines (e.g., GM-CSF, IL-2, IL-12) Liposomes, virosomes, iscoms, cochleates, emulsions (e.g., Freunds adjuvant, SAF, MF59 ) Poloxamer particles, virus-like particles, PLG microparticles Cholera toxin (CT), mutant toxin (e.g., LTK63, LTR72), heat labile enterotoxin (LT), microparticles, polymerized liposomes, chitosan... 

Note All of these adjuvants have been evaluated in clinical trials with the exception of cochleates and polymerized liposomes. Only those adjuvants marked are currently licensed as adjuvants in approved vaccine products. 

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