Solubilization of multilamellar liposomes of egg yolk lecithin by the bile salt sodiumtaurodeoxycholate and the effect of cholesterol--a rapid-ultrafiltration study

Bolalipid-Doped Liposomes: Can Bolalipids Increase the Integrity of Liposomes Exposed to Gastrointestinal Fluids?

The use of archaeal lipids and their artificial analogues, also known as bolalipids, represents a promising approach for the stabilization of classical lipid vesicles for oral application. In a previous study, we investigated the mixing behavior of three single-chain alkyl-branched bolalipids PC-C32(1,32Cn)-PC (n = 3, 6, 9) with either saturated or unsaturated phosphatidyl-cholines. We proved, that the bolalipids PC-C32(1,32C6)-PC and PC-C32(1,32C9)-PC show miscibility with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). In the present work, we extended our vesicle system to natural lipid mixtures using phosphatidylcholine from soy beans, and we investigated the effect of incorporated bolalipids on the integrity of these mixed liposomes (bolasomes) in different gastrointestinal fluids using a dithionite assay and a calcein release assay in combination with particle size measurements. Finally, we also studied the retention of calcein within the bolasomes during freeze-drying. As a main result, we could show that in particular PC-C32(1,32C6)-PC is able to increase the stability of bolasomes in simulated gastric fluid-a prerequisite for the further use of liposomes as oral drug delivery vehicles.

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.

Calcitonin-loaded liposomes: stability under acidic conditions and bile salts-induced disruption resulting in calcitonin-phospholipid complex formation

Calcitonin-loading in liposomes composed of phosphatidylcholine, cholesterol and stearylamine or dipalmitoyl phosphatidylglycerol was studied at low pH values and in the presence of bile salts to check whether liposomal entrapment could be a possible means of protecting the peptide against the aggressive conditions present in the gastrointestinal tract. The association of calcitonin with the lipidic vesicles was monitored using radioactive labelling of the peptide and gel-filtration separation of the free and liposome-associated fractions. The results show that for all phospholipid compositions tested, loading was preserved in light acidic or basic buffers, and that only a slight disruption was observed at pH 2.5. Cholate caused a significant but only partial release of calcitonin even when the cholate-to-phospholipid ratio was increased. To understand the mode of calcitonin entrapment in the vesicles, the release of liposome-entrapped calcein was monitored concomitantly and taken as a stability criterion. Liposome integrity appears to be resistant at low pHs but to be totally destroyed by 4 mM cholate in a manner quasi-independent of the phospholipid concentration. These results strongly suggest that bile salts induce a disruption of the liposomes which results in the formation of new lipidic structures involving calcitonin and probably cholate.