ENHANCED TRANSEPITHELIAL TRANSPORT OF PEPTIDES BY CONJUGATION TO CHOLIC-ACID

The potential of the intestinal bile acid transporter to serve as a sh uttle for small peptide molecules was investigated. Eleven peptides wi th a 2-6 amino acid backbone were conjugated to the 24-position of 3 a lpha,7 alpha,12 alpha-trihydroxy-5 beta-cholan-24-oic acid (cholic aci d) via an amide bond using an automated peptide synthesizer. In a huma n intestinal cell line (CaCo-2), cholic acid-peptide conjugates were a ble to inhibit the transepithelial transport of [H-3]taurocholic acid, a natural substrate for the bile acid carrier, at a 100:1 conjugate/s ubstrate ratio. Affinity for the carrier decreased significantly when the conjugate in the 24-position increased from 1 to 2 amino acids. Fu rther increase in the amino acid chain length caused only minor decrea se in affinity. A tetrapeptide-bile acid conjugate, [H-3]- ChEAAA (Ch = cholic acid), was transported by the bile acid transporter, showing markedly higher apical (AP)-to-basolateral (BL) compared to BL-to-AP t ransport and inhibition by a 100-fold excess taurocholic acid. Another conjugate with 6 amino acids (ChEASASA) was transported by a passive diffusion pathway but still showed higher transport rates than the pas sive permeability marker mannitol, suggesting the possibility that the cholic acid moiety aids the passive membrane transfer of peptide mole cules by increasing its lipophilicity. Metabolism of bile acid-peptide conjugates in CaCo-2 cells was 3% over 3 h. In conclusion, these stud ies show that the coupling of peptides to the 24-position of the stero l nucleus in cholic acid results in a combination of decreased metabol ism and increased intestinal absorption, either by a carrier-mediated pathway or by accelerated passive diffusion.