Lipids poloxamer systems

Besides the asymmetry between monolayers in cytomembranes, two of the more obvious differences between cubic phases and membranes are the unit cell size and the water activity. It has been argued that tire latter must control the topology of the cubic membranes [15], and hence tiiat the cubic membrane structures must be of the reversed type (in the accepted nomenclature of equilibrium phase behaviour discussed in Chapters 4 and 5 type II) rather than normal (type I). All known lipid-water and lipid-protein-water systems that exhibit phases in equilibrium with excess water are of the reversed type. Thus, water activity alone cannot determine the topology of cubic membranes. Cubic phases have recently been observed with very high water activity (75-90 wt.%), in mixtures of lipids [127], in lipid-protein systems [56], in lipid-poloxamer systems [128], and in lipid A and similar lipopolysaccharides [129,130]. 

Most cubic phases in lipid-water systems exhibit unit cell parameters not larger than 20 mn, while the imit cell of cubic membranes is usually larger than 100 nm. Some exceptioi have been apparently found [131, 132] although at this stage such findings should be treated with caution, as the determination of lattice parameters is dependent on the indexing of diffraction patterns, based only on a small niunber of reflections. Further, in lipid-protein-water, lipid-poloxamer-water and lipid-cationic surfactant-water systems, cubic phases with cell parameters of the order of 50 nm have been observed [56,127, 128]. Due to the small number of reports dealing with the... 

Lee, M.-J., Lee, M.-H., and Shim, C.-K. (1995) Inverse targeting of drugs to reticuloendothelial system-rich organs by lipid microemulsion emulsi.ed with poloxamer 3SB.J. Pharm., 113 175-187. 

Different emulsifier compositions might require different homogenization parameters. For example, the maximal degree of dispersing was obtained with 500 bar and 3 cycles for poloxamer 188-stabilized systems [38], Homogenization with pressures of 1000 or 1500 bar did not result in further reduction of the particle size. In contrast, pressures of 1500 bar proved to be the best for lecithin- (Lipoid S75) stabilized systems. A possible explanation for this observation is the different velocity of the coverage of the new lipid surfaces. 

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