Oral absorption of peptides through the cobalamin (vitamin B12) pathway inthe rat intestine

Purpose. This study was aimed at examining the extent and mechanism of upta ke of cobalamin (Cbl)-conjugated peptides in vitro and in vivo. Methods. To enable acquisition of quantitative absorption data of Cbl-pepti des, metabolically stable octapeptides (DP3), with (Cbl-Hex-DP3) or without a hexyl spacer (Cbl-DP3), were coupled to Cbl and radiolabeled. For compar ison, LHRH coupled to Cbl was used as metabolically susceptible peptide. Bi ological recognition of Cbl-peptides was studied in the physiological order : binding by Intrinsic Factor (IF), recognition and transport of the IF-com plexes by IF-Cbl receptors (IFCR) on Caco-2 monolayers and oral absorption of the Cbl-conjugates in the rat. Results. All Cbl-peptides bound to IF and the IF-complexes were recognized by IFCR receptors on Caco-2 monolayers. Binding was saturable and could be inhibited by a 20-fold excess of IF-Cbl, but not of Non-intrinsic Factor (N IF)-Cbl. Oral administration of these ligands to rats resulted in absorptio n of 53%, 45%, 42%, and 23% of the applied radioactivity for Cbl, Cbl-LHRH, Cbl-Hex-DP3, and Cbl-DP3, respectively. Simultaneous administration of a > 10(5)-fold excess of unlabeled Cbl reduced uptake of all compounds to <4%. Tissue distribution and elimination of the metabolically stable Cbl-conjuga tes were comparable to Cbl. Conclusions. The endogenous Cbl uptake pathway can be exploited for oral pe ptide delivery as indicated by the specific and high (40-45%) uptake of met abolically stable Cbl-coupled octapeptides.