PRODRUGS OF PEPTIDES .8. INVITRO STUDY OF INTESTINAL METABOLISM AND PENETRATION OF THYROTROPIN-RELEASING-HORMONE (TRH) AND ITS PRODRUGS

The feasibility of improving the poor oral bioavailability of the tripeptide TRH (pGlu-His-Pro-NH2) was examined using the pro-drug approach. The prodrugs studied were various N-alkoxycarbonyl derivatives formed by reacting TRH with the appropriate chloroformates at its imidazole moiety. In vitro metabolism studies with rabbit and rat intestinal homogenates showed that TRH and the prodrugs were rapidly degraded in the rabbit homogenate by virtue of prolyl endopeptidase which cleaves the C-terminal proline amide moiety. Whereas TRH showed a high stability in rat gut homogenates, the prodrug derivatives were readily degraded, partly due to prolyl endopeptidase and partly due to non-specific esterases. In vitro penetration studies using the modified Ussing chamber showed that the prodrugs did not improve the penetration of TRH across the jejunal, ileal and colonic segments of the rat. It is concluded that although the prodrugs are much more lipophilic than TRH in terms of octanol-buffer partition coefficients, the greater susceptibility of the derivatives to undergo enzymatic degradation more than offsets the improved lipophilicity characteristics. Prodrugs suitable for improving the oral absorption of TRH should not only possess a certain lipophilicity, but should also be resistant towards the prolyl endopeptidase enzyme.