Influence of Quil A on liposomal membranes

Quil A is the purified saponin fraction extracted from the bark of Quillaja saponaria Molina. Besides its utilisation as a surfactant, it is commonly used in a pseudo-ternary system with cholesterol and phospholipid to form colloidal structures known as ISCOMs (immunostimulating complexes). Their appropriateness as immune stimulating drug carriers has been widely demonstrated, albeit the evaluation of physico-chemical properties of the ISCOM matrix still draws a heterogeneous picture. The aim of our study was to elucidate the effects of Quil A on liposomal phosphatidylcholine/cholesterol dispersions as this interaction is regarded as the major step for the formation of the ISCOM matrix. Transmission electron microscopy was applied to observe structural changes of liposomal dispersions upon addition of Quil A. A formation of ISCOM matrices readily out of the liposomal membrane was proven. The entrapment efficiency (EE) of Arsenazo III as well as differential thermal analysis (DSC) also demonstrated an interaction between the components above a critical concentration of Quil A. To further clarify the effects of interaction, Langmuir trough experiments of insoluble monolayers of both cholesterol and PC and their interaction with Quil A were performed. Measurable effects even below the critical concentration of Quil A (derived from DSC and EE) were shown. Cholesterol had a major impact on the formation and stabilisation of the ISCOM matrix.