Analytical applications of liposomes

• Main Author: Frost, S. J.
• Published: University of Surrey 1994
• Subjects: 615.1; Drug delivery
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240201

Liposomes have established roles in drug delivery and cell membrane studies. Amongst other applications; they can also be used as analytical reagents, particularly in immunoassays. Liposomal immunoassays have potential advantages over alternatives; including sensitivity, speed, simplicity and relative reagent stability. The aim of these studies was to develop and evaluate novel examples of these assays. When liposomes entrapped the dye, Sulphorhodamine B, a shift in its maximum absorption wavelength compared to free dye was observed. This was attributed to dimerization of the dye at high concentrations. If the liposomes were disrupted, the released dye was diluted into the external buffer, and the dye's absorption spectrum reverted to that of free dye. After optimization of dye entrapment, immunoassays were developed using these liposomes. Albumin-coated liposomes were used in a model assay to measure serum albumin. This assay employed complement-mediated immunolysis, commonly used in liposomal immunoassays. The liposomes were lysed by anti-albumin and complement, and this could be competitively inhibited by serum albumin. To improve sensitivity, Fab' anti-albumin liposomes were prepared. These enabled measurement of urinary albumin by a complement-mediated immunoassay, but using a sandwich technique. Anti-albumin (intact) liposomes were shown to precipitate on gentle centrifugation after reaction with albumin. They were applied as a solid phase reagent in an heterogeneous immunoassay, using radioimmunoassay for urinary microalbumin as a model assay. Liposomes containing Sulphorhodamine B were also used in a more novel assay; for serum anticardiolipin antibodies. Cardiolipin-containing liposomes were prepared. These were lysable using magnesium ions. Anticardiolipin antibodies (IgG) were found to augment this lysis, enabling their estimation. Similar imprecision and acceptable correlation with a commercial enzyme-linked immunosorbent assay (ELISA) were obtained. The findings demonstrate Sulphorhodamine B release can be used as a marker in homogeneous colorimetric liposomal immunoassays; both in model assays and in potentially more useful clinical biochemistry applications.