Intestinal absorption of the octapeptide SMS 201–995 visualized by fluorescence derivatization

The absorption of an intact oligopeptide was investigated in rat and dog small intestine using the metabolically stable somatostatin analogue SMS 201–995. The synthetic octapeptide was coupled to 4-nitrobenzo-2-oxa-1,3-diazol to have a fluorescent label for the direct visualization. The 4-nitrobenzo-2-oxa-1,3-diazol-labeled peptide was active in displacing the corresponding hormone 125I-Tyr3-SMS 201–995 (Sandostatin; Sandoz Pharmaceuticals, Basel, Switzerland) from its high-affinity binding site in rat cortex membranes with an IC50 = 4.6 × 10−10 mol/L. The release of growth hormone from cultured anterior pituitary cells was inhibited by the fluorescent somatostatin analogue with the same potency as by somatostatin 14 (IC50 = 6 × 10−10 mol/L). Incubation with mucosal scrapings followed by high-performance thin-layer chromatography analysis showed that the peptide was stable against proteolysis. 4-Nitrobenzo-2-oxa-1,3-diazol SMS 201–995 was well absorbed from enterocytes of rat small intestine. The absorption was highest into jejunal cells and it could be inhibited by an excess of unlabeled peptide. A significantly lower absorption was detected in crypts compared with villus tips. No fluorescence could be seen in intestinal mucin and goblet cells. After oral administration, the 4-nitrobenzo-2-oxa-1,3-diazol-labeled peptide rapidly appeared in the blood of rats and dogs, reaching a bioavailability of 4.3% and maintaining pharmacological activity. This suggests that enterocytes are able to absorb intact oligopeptides being stabilized against proteolytic degradation through a transcellular mechanism.