Aqueous ternary phase systems containing lipids with hydrotropes for liposome synthesis and characterization of conventional and polymer stabilized liposomes.

摘要

Liposomes have been widely investigated for potential applications in drug delivery, gene therapy, medical imaging, cosmetics, and consumer products. However, only a handful of these explorations have led into practical use due to the difficulties in manufacturing vesicles with precise homogeneity, stability, and surface functionality. For example, to date, there have only been a limited number of methods to make vesicles with predictable sizes. One such method involves using a micelle solution composed of lipid, hydrotrope, and aqueous medium to produce vesicles of probable size distribution upon dilution with its aqueous medium. Phase diagrams are fundamentally useful in providing insights to the region where micelles exist when the three components are mixed. This thesis work is aimed at understanding the physical nature of the micelle phase as the precursor to vesicle formation, as well as the properties of different types of liposomes in terms of their size, stability, homogeneity, membrane kinetics, and dynamic behavior.; More specifically, a homogeneous phase referred to as L1 can be obtained by mixing an appropriate ratio of hydrotropes to lipids. This microemulsion region, known as the micelle region, can be used to prepare vesicles by dilution with its aqueous medium. Ternary phase diagrams were constructed using egg lecithin, sodium xylenesulfonate (SXS), and various aqueous media. The L1 regions were utilized to prepare liposomes by diluting the micelle solution with an equal amount of its aqueous medium. When the ratio of lecithin/SXS was adjusted in the original micelle solution the average size of the corresponding vesicles that formed upon dilution of the L1 phase changed accordingly. It was found that vesicle size is proportional to the lipid/hydrotrope ratio, where the addition of hydrotrope to lipid micelle solutions affects the curvature of the vesicles that form.; To further understand the process of vesicle formation and control the physical and chemical properties of synthesized vesicles, it is important to understand the nature of lipid/hydrotrope interaction in the L1 phase. Another set of phase diagrams was constructed using DPPC/hydrotrope/water, where 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) was used as the lipid, and sodium xylenesulfonate (SXS), sodium cumenesulfonate (SCS), and sodium toluenesulfonate (STS) were used as the hydrotropes. Due to their structural difference, the degree of interaction between the hydrotrope and lipid varies. It is further hypothesized that such a subtle difference in lipid-hydrotrope interaction is the origin of vast difference in terms of their ability to solubilize various lipids and cause the formation of vesicles with different physical and chemical properties. The L1 phase for each ternary system was mapped out and different lipid/hydrotrope molar ratios were investigated using dynamic NMR for T1 relaxation times and pulse field gradient NMR for diffusion coefficients. The T1 for water was found to decrease with increasing lipid concentration, suggesting that water gets trapped in the growing lipid bilayers. However, higher hydrotrope concentrations of SCS and STS yielded lower T1 for water, implying that at high concentrations these hydrotropes can self associate. Pulse field gradient NMR revealed that the diffusion coefficient of the lipids decreased as a function of lipid concentration, where the greatest change in diffusion was seen when viscosity was changing the least. The diffusion coefficient was used to determine average micelle size, which was found to range from 6 to 14 nm depending on the concentration and type of hydrotrope. Fluorescence was also used to estimate the aggregation number for the micelles in the L1 region (DPPC, SXS, and water).; In addition to synthesizing liposomes by dilution from the micelle phase, conventional liposomes were also prepared by the thin film hydration method followed by extrusion or sonication. Sonication