Drug permeability across a phospholipid vesicle-based barrier 2. Characterization of barrier structure, storage stability and stability towards pH changes.

Recently we reported on the development of a phospholipid vesicle-based barrier as a medium throughput method for screening of drug permeability. The aim of this present study is to characterize the barrier structure, including an estimation of the amount of phospholipid within it, its storage stability and its stability over various pH ranges found in different parts of the gastrointestinal tract. The amount of lipid in the barrier was quantified using a colorimetric phospholipase D-based assay. The total amount averaged 3.30mg phospholipid per barrier. The preparation process comprises the consecutive deposition of two types of liposomes on a filter support. We estimated that the smallest liposomes, with a mean diameter of 298nm, would fill the pore volume of the filter when tightly packed. The volume of the bigger liposomes, deposited on top of the filter, was calculated to generate a 0.1mm thick layer. Visualisation of fluorescently labelled liposomes by confocal laser-scanning microscopy confirmed that the pores of the filter were completely filled with liposomes and that there was a liposome layer on top. Small angle X-ray scattering (SAXS) analysis was used to study the lamellarity of the liposomes. The liposomes contained oligo- and/or multilamellar structures before and after deposition. The functionality of the barriers during storage at three different temperatures was examined for a period of up to 4 weeks by measuring the permeability of the hydrophilic marker calcein across them. The conclusion was that the phospholipid vesicle-based barriers could be stored at -80 degrees Celsius for up to 2 weeks without significant changes. The stability of the barriers in a pH range from 2.0 to 8.0 was investigated by performing permeation studies with fluorescein at different pH values. It was found that the phospholipid vesicle-based barrier did not lose its integrity within this range. Thus, the barriers appear suitable for further studies to provide insight into segmental absorption in the human gastrointestinal tract. Furthermore, because the phospholipid vesicle-based barrier can be stored, larger batches can be produced. This makes the phospholipid vesicle-based barrier more appropriate for high throughput screening.