Insulin stabilization and gastrointestinal absorption

Insulin and other peptide drugs are vulnerable to the intestinal environment. They are degraded, precipitate or are not allowed to diffuse through the mucosal lining, resulting in low bioavailability. In this study stabilization by low molecular compounds was successfully applied to the gastrointestinal delivery of insulin. In order to better understand the factors of importance for gastrointestinal delivery of insulin, stabilization by alkylmaltosides with varying carbon chain length was studied. Alkylmaltosides were synthesized from saturated alkanols and alpha-acetobromomaltose. The maltosides were isolated after deacetylation and chromatographic purifications. The structures of the maltosides were characterized by spectroscopic and elemental analyses. Critical micelle concentration and micelle size were determined by dynamic laser light scattering. Aggregation studies in vitro and in vivo and surface adsorption experiments in vitro with alkylmaltoside stabilized insulin showed increased stability of insulin. Stabilized insulin did not precipitate on polymer surfaces. Stabilized insulin did not form high molecular aggregates after extensive agitation in vitro. Dodecylmaltoside was found to have superior stabilizing ability of insulin. Dynamic laser light scattering determined the nature of the stabilization. It was concluded that monomeric insulin was stabilized in micelles. Insulin was found to precipitate in the mucin layer of the small intestine, this was not the case for dodecylmaltoside stabilized insulin. In vitro enzymatic degradation experiments by model enzymes of stabilized insulin were performed. Dodecylmaltoside stabilized insulin appeared more enzyme resistant than native insulin, as determined by colorimetric ninhydrin assay. Fast protein liquid chromatography showed that build up of low molecular fragments in solution were not as pronounced for stabilized insulin as for native insulin. Duodenal absorption in rats and dogs was only observed for dodecylmaltoside stabilized insulin. Insulin absorption was assessed by blood glucose measurements in normoglycemic rats, and dogs. Serum and plasma insulin was measured by radioimmunoassay in diabetic rats and normoglycemic dogs. This dissertation demonstrates the importance of stabilization of insulin for intestinal administration. Crucial factors to overcome by stabilization of insulin include aggregation, enzymatic degradation and precipitation in the intestine. The result of this study may be applied to other labile peptide drugs for oral delivery