In this study the intestinal degradation and absorption of a synthetic pentapeptide, metkephamid, were investigated in the rat by determination of its wall permeabilities in the small and large intestine and the extent and mechanism of its intestinal degradation. The peptide was metabolized in the gut wall through contact with membrane-bound enzymes in the brush border membrane. The extent of metabolic inactivation depended on the intestinal segment investigated and decreased in the axial direction. No metabolism was found in the colon. The dimensionless wall permeabilities (P-w*), determined by single-pass perfusion, were also site dependent. P-w* was highest in the ileum [1.91 +/- 0.24, (SE); n = 4], followed by the jejunum (1.64 +/- 0.34; n = 4) and the colon (0.67 +/- 0.38; n = 4). Based on the permeability data alone and under the assumption of no presystemic metabolism, complete bioavailability would be predicted for metkephamid. However, following oral administration, the mean absolute bioavailability was only 0.22 +/- 0.065% (n = 3), indicating the overall dominance of degradation in the absorption process. Thus future strategies in oral peptide delivery should focus on increasing the stability of the peptide in the intestine by modifying the peptide structure and/or delivering the compound to an intestinal segment showing little or no enzymatic degradation.
