Colonic absorption and bioavailability of the pentapeptide metkephamid in the rat

The concept of delivering systemically active peptide drugs to the colon in order to improve their oral absorption requires reasonable peptide permeability of the large intestinal wall and stability against the activity of the colonic microflora. In addition, the role of hepatic extraction needs to be addressed. In this study the absorption of the pentapeptide metkephamid following single pass perfusion of rat ascending colon was investigated by monitoring its disappearance from the large intestine and simultaneous appearance in the portal vein, the hepatic vein and the aorta. In addition its stability against colonic microflora was tested in vitro using pig caecal contents. Metkephamid was absorbed from the large intestine and appeared in the blood circulation; peptide concentrations in the portal vein increased over-proportionally with increasing perfusate concentrations (0.1-4.6 mmol/L) from 0.19 microgram/mL +/- 0.12 (SD, n = 7) to 31.6 micrograms/mL + 20.65 (SD, n = 4), respectively, and thus suggesting a saturable transport or metabolism. Concentrations in the hepatic vein were significantly lower than in the portal vein, hepatic extraction ratios were 0.35 +/- 0.14, 0.61 +/- 0.18 and 0.62 +/- 0.28 (SD, n = 4) for 0.1, 0.5 and 1.0 mM metkephamid perfusate concentrations, respectively. In the anaerobic colon metabolism model the degradation half-life of the peptide was 14.9 hours, thus, indicating relative stability in the bacterial environment of the colon. The results of the present study encourage further investigations on colonic delivery of peptide drugs.