Structure—Function Relationship among Quillaja Saponins Serving as Excipients for Nasal and Ocular Delivery of Insulin

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Abstract
The purpose of this investigation was to explore the structure-function relationship among naturally occurring Quillaja saponins and derivatives for their ability to stimulate insulin delivery from nosedrops and eyedrops and to test the hypothesis that stimulation of peptide drug delivery was correlated with surfactant strength. Native saponins, including QS-21, were purified from an aqueous extract of Quillaja saponaria bark by adsorption chromatography and HPLC. Native saponins were then deacylated by mild alkaline hydrolysis to form DS-1 and DS-2, derivatives that are smaller and more hydrophilic than their parent compounds. DS-1 was further treated either to reduce an aldehyde residue to form DS-1(R) or to remove the fucose-containing oligosaccharide to form QH-957. Rats receiving eyedrops or nosedrops formulated with insulin, but without any Quillaja saponins, showed no hypoglycemic response. Rats receiving eyedrops or nosedrops formulated with insulin plus saponins showed a dose-dependent hypoglycemic response, with the following rank order: QS-21 > DS-1 > DS-1(R) > DS-2 > QH-957. Surfactant strength was determined by measurement of the critical micellar concentration (cmc) and hemolysis of sheep erythrocytes. The cmc was lowest for the parent saponins QS-21 and QS-18, and increased for the deacylated saponin derivatives DS-1, DS-2, and QH-957; hemolysis of sheep erythrocytes was observed at low concentrations (approximately 0.006 mM) of the parent 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, because DS-1, which differs from DS-2 only in the absence of one glucose residue, was significantly more potent than DS-2 in stimulating the absorption of insulin. DS-1 and DS-2 share a similar cmc and hemolytic titer, so this difference in efficacy must be due to some specificity beyond simple surfactant strength. Furthermore, DS-1 does not trigger an immune response when administered to animals, whereas QS-21 is a strong immune system activator. Therefore, DS-1 has emerged as an interesting candidate for inclusion in an eyedrop or nosedrop formulation.