Degradation of insulin and calcitonin and their protection by various protease inhibitors in rat caecal contents: implications in peptide delivery to the colon.

The objective of this study was to examine the metabolism of insulin and calcitonin, and their protection by various protease inhibitors, in the large intestine. Fresh caecal contents were prepared from non-fasted rats and the degradation of insulin and calcitonin was studied in a suspension of rat caecal contents, as a model of the content of the large intestine. Both insulin and calcitonin were metabolized in suspensions of rat caecal contents, but the degradation of calcitonin was much faster than that of insulin. The degradation of insulin was fastest at pH 6.8. Protease inhibitors such as camostat and aprotinin inhibited the degradation of insulin and calcitonin in rat caecal contents, which was consistent with the high chymotrypsin activity of these contents. These findings suggest that care should be taken when administering peptide drugs to the large intestine for colon-specific drug delivery because they can be degraded in rat caecal contents. Protease inhibitors might be useful for increasing the stability of these peptides in the large intestine, thereby improving their large-intestinal absorption to the systemic circulation.

Stereoselective reductive metabolism of metyrapone and inhibitory activity of metyrapone metabolites, metyrapol enantiomers, on steroid 11 beta-hydroxylase in the rat.

Pharmacokinetics of metyrapone and metyrapol enantiomers was studied in the rat to determine the stereoselective reductive metabolism of metyrapone. The HPLC method using a chiral column was developed for the stereoselective analysis of metyrapol enantiomers in rat plasma. The AUC ratio of (-)- and (+)-metyrapol appeared in rat plasma after i.v. administration of metyrapone was about 3:1. The interconversion of (-)- or (+)-metyrapol to its antipode was negligible, and the reverse reaction from metyrapol to metyrapone was insignificant. There were similar kinetic parameters of (-)-metyrapol to those of (+)-metyrapol after i.v. administration of racemic metyrapol. These results indicate metyrapone displays product-stereoselective reductive metabolism in the rat. The inhibition of steroid 11 beta-hydroxylase by metyrapone, racemic metyrapol, (-)-metyrapol or (+)-metyrapol was analyzed in rat adrenal homogenates. Metyrapol was equally as potent as metyrapone in the inhibition of steroid 11 beta-hydroxylase and each enantiomer of metyrapol showed similar inhibitory activity on the rat adrenal steroid 11 beta-hydroxylase. These results indicate there is an insignificant difference in the inhibitory effects on steroid 11 beta-hydroxylase of metyrapol enantiomers, and that the inhibitory effects of metyrapol may be involved in the pharmacological activity of metyrapone in vivo.

Metabolism of peptide drugs by the microorganisms in rat cecal contents.

The metabolism of insulin and calcitonin by microorganisms was examined in rat cecal contents. Both insulin and calcitonin were markedly degraded. Calcitonin was more susceptible to proteolysis in rat cecal contents than insulin. Calcitonin was also rapidly degraded in supernatant, while we found few degradation products of insulin. These findings suggest that care should be taken to metabolize the peptide drugs by microorganisms when they are administered to the large intestine for colon-specific drug delivery.