Intestinal mucosal transport and metabolism of insulin
Abstract
Insulin was chosen as a model protein drug to investigate its absorption from the small intestine and determine the location where permeability was maximal and degradation was minimal. Everted gut sac experiments showed that the apparent permeability (P$\sb{\rm app}$) of insulin varied with the intestinal region, being greatest in the distal jejunum (7.80 $\times$ 10$\sp{-7}$ cm/s). In situ insulin absorption studies showed a corresponding greater bioavailability and glucose depression resulted from administration to this region than compared to a more proximal region. No measurable degradation of insulin by the brush border enzymes was observed. In vitro degradation of insulin by $\alpha$-chymotrypsin was significantly faster and more extensive than by trypsin. Identification of the initial sites of chymotryptic cleavage showed a total of five bonds in insulin were rapidly cleaved, while two or three other susceptible sites which were more internal in the insulin molecule were not cleaved in these studies of initial degradation. Since insulin bioavailability was low (0.133%), mixed micellar solutions of sodium glycocholate (NaGC) and linoleic acid (LA) were examined as absorption enhancers. Regional differences in absorption enhancement were observed, with the greatest P$\sb{\rm app}$ in the distal jejunum. NaGC and LA exhibited synergistic absorption enhancement as mixed micelles in in situ absorption studies in the distal jejunum/proximal ileum. Optimal insulin absorption resulted with 30 mM NaGC/40 mM LA mixed micelles, yielding 41.256% bioavailability. Insulin doses of 5 U/kg with these mixed micelles produced profound and prolonged hypoglycemia. Experiments with sodium glyco-24,25-dihydrofusidate micelles produced profound and prolonged hypoglycemia. Experiments with sodium glyco-24,25-dihydrofusidate (SGDHF) exhibited a more variable and slightly lower bioavailability (34.107 $\pm$ 9.366%). Acute histopathological studies showed that these adjuvants produced only minimal damage to the mucosal membrane which was expected to be reversible. The greatest changes to the intestinal mucosa resulted from exposure to 30 mM SGDHF. Further studies of long-term exposure to mixed micelles are required. Development of an enteric-coated oral dosage form to initially protect, then release insulin with 30 mM NaGC/40 mM LA mixed micelles in the middle or distal jejunum may be a viable method of oral insulin delivery.
Degree
Ph.D.
Advisors
Mitra, Purdue University.
Subject Area
Pharmaceuticals
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