Transport characteristics of peptides and peptidomimetics: II. Hydroxyethylamine bioisostere-containing peptidomimetics as substrates for the oligopeptide transporter and P-glycoprotein in the intestinal mucosa

Peptide bond bioisosteres, such as hydroxyethylamine (Hea), have frequently been used to stabilize metabolically labile peptide bonds in peptidomimeti c drug design in an effort to increase the oral bioavailability of drug can didates. However, the impact of the peptide bond bioisosteres on the cell p ermeation characteristics of peptidomimetics is not well understood, partic ularly with respect to the effects on the substrate activity for proteins t hat can restrict (e.g. P-glycoprotein, P-gp) or facilitate (e.g. the oligop eptide transporter, OPT) intestinal mucosal permeation of peptidomimetics. in this study, terminally free and terminally modified (N-acetylated and C- amidated) peptidomimetics of H-Ala-Phe-OH and H-Ala-Phe-Ala-OH with the Ala -Phe peptide bonds replaced by Hea bioisosteres were synthesized. Transport characteristics of these peptidomimetics were investigated using Caco-2 ce ll monolayers as an in vitro model of the intestinal mucosa. The study show ed that the Hea bioisostere stabilized the peptidomimetics to protease meta bolism in Caco-2 cells. All terminally free peptidomimetics showed signific ant affinity and substrate activity for OPT. The affinity and substrate act ivity for OPT were stereoselective for peptidomimetics containing an SS-con figuration for the two adjacent chiral centers related to the Hea bioisoste re. Three of the four terminally modified peptidomimetics showed significan t substrate activity for P-gp and, interestingly, the substrate activity fo r P-gp was also stereoselective; however, it was in favor of an R,R-configu ration for the two adjacent chiral centers related to the Hea bioisostere.