Oral absorption of peptides through the cobalamin (vitamin B12) pathway in the rat intestine.
OBJECTIVE
This study was aimed at examining the extent and mechanism of uptake of cobalamin (Cbl)-conjugated peptides in vitro and in vivo.


METHODS
To enable acquisition of quantitative absorption data of Cbl-peptides, metabolically stable octapeptides (DP3), with (Cbl-Hex-DP3) or without a hexyl spacer (Cbl-DP3), were coupled to Cbl and radiolabeled. For comparison, LHRH coupled to Cbl was used as metabolically susceptible peptide. Biological recognition of Cbl-peptides was studied in the physiological order: binding by Intrinsic Factor (IF), recognition and transport of the IF-complexes 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 (NIF)-Cbl. Oral administration of these ligands to rats resulted in absorption 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-conjugates were comparable to Cbl.


CONCLUSIONS
The endogenous Cbl uptake pathway can be exploited for oral peptide delivery as indicated by the specific and high (40-45%) uptake of metabolically stable Cbl-coupled octapeptides.
Alsenz
J
J
Preclinical Research Department, F. Hoffman-La Roche Ltd., Basle, Switzerland. jochem.alsenz@roche.com
Russell-Jones
G J
GJ
Westwood
S
S
Levet-Trafit
B
B
de Smidt
P C
PC
D000284:Administration, Oral; D000818:Animals; D018938:Caco-2 Cells / Q000378:metabolism; D006801:Humans; D007408:Intestinal Absorption / Q000502:physiology*; D007413:Intestinal Mucosa / Q000378:metabolism*; D007437:Intrinsic Factor / Q000378:metabolism*; D008297:Male; D010455:Peptides / Q000493:pharmacokinetics*; D051381:Rats; D017208:Rats, Wistar; D014805:Vitamin B 12 / Q000493:pharmacokinetics*
D010455:Peptides; D007437:Intrinsic Factor; D014805:Vitamin B 12