Characteristics and behaviour of liposomes when incubated with natural bile salt extract: implications for their use as oral drug delivery systems.

The use of liposomes for oral administration of drugs and for food applications is based on their ability to preserve entrapped substances and to increase their bioavailability. Bile salts are one of the agents that affect the liposome structure during intestinal digestion and the main reported studies on liposome/bile salt systems used only one bile salt. The aim of this work is to characterise the interaction of liposomes with a natural bile salt extract (BSE) at physiological pH and temperature. Three types of liposomes (fluid, gel-state and liquid-ordered bilayers) were studied. Phase diagrams were obtained and a very different behaviour was found. Fluid bilayers were completely permeable to an entrapped dye with partial or complete disruption of vesicles (final size 10 nm). Gel-state bilayers released their content but BSE led to the formation of large mixed structures (2000 nm). Liquid-ordered bilayers formed mixed vesicles (1000 nm) and, surprisingly, retained a high percentage of their aqueous content (about 50%). As a consequence, each type of liposome offers singular features to be used in oral applications due to their specific interaction with bile salts.

Preparation and characterization of iron-containing liposomes: their effect on soluble iron uptake by Caco-2 cells.

The aim of this work was to study the iron uptake of Caco-2 cells incubated with five different formulations of liposomes containing iron. The vesicles were also characterized before, during, and after in vitro digestion. Caco-2 cells were incubated with digested and nondigested liposomes, and soluble iron uptake was determined. Nondigested liposomes made with chitosan (CHI) or the cationic lipid, DC-Cholesterol (DC-CHOL), generated the highest iron uptake. However, these two formulations were highly unstable under in vitro digestion, resulting in nonmeasurable iron uptake. Digested conventional liposomes composed of soybean phosphatidylcholine (SPC), hydrogentated phosphatidylcholine (HSPC), or HSPC and cholesterol (CHOL) presented the highest iron-uptake values. These liposomal formulations protected iron from oxidation and improved iron uptake from intestinal cells, compared to an aqueous solution of ferrous sulphate.

Combined strategies for liposome characterization during in vitro digestion.

Three types of pyranine (HPTS)-containing liposomes were prepared by high-pressure homogenization under optimized conditions. At 37 degrees C, they were 1) fluid-state vesicles made from soybean phosphatidylcholine (SPC), 2) gel-state liposomes made from hydrogenated SPC (HSPC), and 3) solid-disordered membranes obtained from HSPC and cholesterol (HSPC-Chol). These liposome formulations were characterized before, during, and after in vitro digestion, which involved the presence of pH gradients, enzymes, and bile salts. Mean sizes and size distributions of the vesicles were determined by DLS; (31)P-NMR (nuclear magnetic resonance) was used to quantify lyso-PC forms; internal pH was monitored throughout digestion with two different fluorescent pH probes; and changes in bilayer permeability and HPTS encapsulation were determined by size-exclusion chromatography and fluorimetry. Differential scanning calorimetry analysis was also performed in order to study the effect of digestion on HSPC vesicles. SPC liposomes were physically stable during digestion; they presented 8% lyso-forms and an HPTS encapsulation around 85% after in vitro digestion. However, they were extremely permeable to ions, so that the internal pH immediately equilibrated with the bulk pH. HSPC liposomes were the most affected by the digestive process. Even though they were chemically stable, as inferred from the low lyso-PC content, very important changes in their size distribution were observed. A final 50% HPTS leakage was quantified after in vitro digestion. Nevertheless, they were the least permeable to protons under pH gradients. HSPC-Chol vesicles presented intermediate permeability to protons, having their internal pH decreased from approximately 6.8 to 4.6 after 1 hour of incubation at pH 2. This was the most chemically stable formulation and showed the highest encapsulation, even after in vitro digestion. Therefore, HSPC-Chol liposomes would be the most adequate choice for the design of lipid products for oral administration.