INTESTINAL-ABSORPTION OF PEPTIDES BY COUPLING TO BILE-ACIDS

Poor intestinal absorption of peptides greatly limits their use as dru gs for the treatment of chronic diseases. Since bile acids are efficie ntly absorbed by an active, Na+-dependent transport system in the ileu m of mammals, model peptides of different chain length were attached t o the 3-position of modified 3 beta-(omega-amino-alkoxy)-7 alpha,12 al pha-dihydroxy-5 beta-cholan-24-oic acid. These peptide-bile acid conju gates inhibited Na+-dependent [H-3]taurocholate uptake into brush-bord er membrane vesicles isolated from rabbit ileum in a concentration-dep endent manner. Furthermore, photoaffinity labeling of the bile acid-bi nding proteins of M(r) 93,000 and 14,000, identified as the protein co mponents of the ileal Na+-dependent bile acid transport system in rabb it ileum (Kramer, W., Girbig, F., Gutjahr, U., Kowalewski, S., Jouvena l, K., Muller, G., Tripler D., and Wess, G. (1993) J. Biol. Chem. 268, 18035-18046) by the photoreactive taurocholate analogue, (3,3-azo-7 a lpha,12 alpha-dihydroxy-5 beta[7 beta,-12 beta-H-3]cholan-24-oyl)-2-am inoethanesulfonic acid, was inhibited by the peptide-bile acid conjuga tes. In contrast, the parent peptides and amino acids neither had a si gnificant effect on [H-3]taurocholate uptake by ileal brush-border mem brane vesicles nor on photoaffinity labeling of the ileal bile acid-bi nding membrane proteins. The inhibitory effect of peptide-bile acid co njugates on [H-3]taurocholate transport and photoaffinity labeling of the bile acid-binding proteins in rabbit ileal vesicles decreased with increasing chain length of the attached peptide radical. By in vivo i leum perfusion in anesthetized rats an intestinal absorption of the bi le acid conjugate S3744 of the fluorescent oxaprolylpeptide nitrobenzo -2-ora-1,3-diazol-beta-Ala-Phe-5-Opr-Gly (S1037) and secretion of the intact compound into bile could be demonstrated, whereas the parent pe ptide S1037 or its t-butylester S4404 were not absorbed. The intestina l absorption of S3744 showed a similar temperature dependence as [H-3] taurocholate absorption and was inhibited by the presence of taurochol ate indicating a carrier-mediated uptake of S3744 via the ileal bile a cid transporter. In conclusion, these results indicate that oligopepti des can be made enterally absorbable by coupling to modified bile acid molecules making use of the specific intestinal absorption pathway fo r bile acids. This finding may be of great importance for the design a nd development of orally active peptide drugs.