STRUCTURE-FUNCTION RELATIONSHIP AMONG QUILLAJA SAPONINS SERVING AS EXCIPIENTS FOR NASAL AND OCULAR DELIVERY OF INSULIN

The purpose of this investigation was to explore the structure-functio n relationship among naturally occurring Quillaja saponins and derivat ives for their ability to stimulate insulin delivery from nosedrops an d eyedrops and to test the hypothesis that stimulation of peptide drug delivery was correlated with surfactant strength. Native saponins, in cluding QS-21, were purified from an aqueous extract of Quillaja sapon aria bark by adsorption chromatography and HPLC. Native saponins were then deacylated by mild alkaline hydrolysis to form DS-1 and DS-2, der ivatives that are smaller and more hydrophilic than their parent compo unds. DS-1 was further treated either to reduce an aldehyde residue to form DS-1(R) or to remove the fucose-containing oligosaccharide to fo rm QH-957. Rats receiving eyedrops or nosedrops formulated with insuli n, but without any Quillaja saponins, showed no hypoglycemic response. Rats receiving eyedrops or nosedrops formulated with insulin plus sap onins showed a dose-dependent hypoglycemic response, with the followin g rank order: QS-21 > DS-1 > DS-1(R) > DS-2 > QH-957. Surfactant stren gth was determined by measurement of the critical micellar concentrati on (cmc) and hemolysis of sheep erythrocytes. The cme was lowest for t he parent saponins QS-21 and QS-18, and increased for the deacylated s aponin derivatives DS-1, DS-2, and QH-957; hemolysis of sheep erythroc ytes was observed at low concentrations (similar to 0.006 mM) of the p arent saponins, QS-21 and QS-18, at intermediate concentrations (0.06- 0.08 mM) of DS-1 and DS-2, and at higher concentrations of DS-1(R) (0. 45 mM) and QH-957 (1.5 mM). Hence, efficacy as an absorption-enhancing agent was greatest in those saponins with the lowest hemolytic titers and cmc values. However, this relationship was not a strict one, beca use DS-1, which differs from DS-2 only in the absence of one glucose r esidue, was significantly more potent than DS-2 in stimulating the abs orption of insulin. DS-1 and DS-2 share a similar cmc and hemolytic ti ter, so this difference in efficacy must be due to some specificity be yond simple surfactant strength. Furthermore, DS-1 does not trigger an immune response when administered to animals, whereas QS-21 is a stro ng immune system activator. Therefore, DS-1 has emerged as an interest ing candidate for inclusion in an eyedrop or nosedrop formulation.