Conformational restrictions in ligand binding to the human intestinal di-/tripeptide transporter: implications for design of hPEPT1 targeted prodrugs

Modulation of Naturally Occurring Linear Dipeptide Chirality to Reduce the Affinity for Oligopeptide Transporter 1 and Increase Intestinal Stability for an Enhanced Colon-Targeting Effect in the Treatment of Inflammatory Bowel Disease: An Application of trans-4-l-Hydroxyprolyl-l-serine

The naturally occurring linear dipeptide JBP923 (trans-4-l-Hyp-l-Ser, HS-tLL) with anti-inflammatory effects showed potential for the treatment of inflammatory bowel disease (IBD). However, colon-specific delivery after oral administration is still a challenge because its absorption is mediated by oligopeptide transporter 1 (PEPT1) in the upper small intestine and because of its instability in the gastrointestinal tract. Therefore, we aimed to enhance the colon-targeting efficiency by modulating HS-tLL chirality to synthesize eight enantiomers. Among these enantiomers, trans-4-d-Hyp-d-Ser, cis-4-l-Hyp-d-Ser, cis-4-d-Hyp-l-Ser, and cis-4-d-Hyp-d-Ser did not work as substrates of PEPT1 and were stable in the gastrointestinal tract, resulting in enhanced colonic accumulation through the paracellular pathway due to the loose tight junctions in IBD. Interestingly, cis-4-d-Hyp-d-Ser exerted the most potent therapeutic effect on IBD. Our findings revealed the impact of chirality on the colonic accumulation of the linear dipeptide, providing strategies for the colon-targeted delivery of the linear dipeptide for the treatment of IBD.

Discovery of 7-hydroxyaporphines as conformationally restricted ligands for beta-1 and beta-2 adrenergic receptors

A series of (-)-nornuciferidine derivatives was synthesized and the non-natural enantiomer of the aporphine alkaloid was discovered to be a potent β1- and β2-adrenergic receptor ligand that antagonized isoproterenol and procaterol induced cyclic AMP increases from adenylyl cyclase, respectively. Progressive deconstruction of the tetracyclic scaffold to less complex cyclic and acyclic analogues revealed that the conformationally restricted (6a-R,7-R)-7-hydroxyaporphine 2 (AK-2-202) was necessary for efficient receptor binding and antagonism.

Bioavailability through PepT1: the role of computer modelling in intelligent drug design

In addition to being responsible for the majority of absorption of dietary nitrogen, the mammalian proton-coupled di- and tri-peptide transporter PepT1 is also recognised as a major route of drug delivery for several important classes of compound, including beta-lactam antibiotics and angiotensin-converting enzyme inhibitors. Thus there is considerable interest in the PepT1 protein and especially its substrate binding site. In the absence of a crystal structure, computer modelling has been used to try to understand the relationship between PepT1 3D structure and function. Two basic approaches have been taken: modelling the transporter protein, and modelling the substrate. For the former, computer modelling has evolved from early interpretations of the twelve transmembrane domain structure to more recent homology modelling based on recently crystallised bacterial members of the major facilitator superfamily (MFS). Substrate modelling has involved the proposal of a substrate binding template, to which all substrates must conform and from which the affinity of a substrate can be estimated relatively accurately, and identification of points of potential interaction of the substrate with the protein by developing a pharmacophore model of the substrates. Most recently, these two approaches have moved closer together, with the attempted docking of a substrate library onto a homology model of the human PepT1 protein. This article will review these two approaches in which computers have been applied to peptide transport and suggest how such computer modelling could affect drug design and delivery through PepT1.

Human PEPT1 Pharmacophore Distinguishes between Dipeptide Transport and Binding

The human intestinal oligopeptide transporter (PEPT1) facilitates the absorption of dipeptides, tripeptides, and many peptidomimetic drugs. In this study, a large number of peptides were selected to investigate the structural features required for PEPT1 transport. Binding affinity was determined in a Gly-Sar uptake inhibition assay, whereas functional transport was ranked in a membrane depolarization assay. Although most of the peptides tested could bind to PEPT1, not all were substrates. As expected, single amino acids and tetrapeptides could not bind to or be transported by PEPT1. Dipeptide transport was influenced by charge, hydrophobicity, size, and side chain flexibility. The extent of transport was variable, and unexpectedly, some dipeptides were not substrates of PEPT1. These included dipeptides with two positive charges or extreme bulk in either position 1 or 2. Our results identify key features required for PEPT1 transport in contrast to most previously described pharmacophores, which are based on the inhibition of transport of a known substrate.

Development of a QSAR Model for Binding of Tripeptides and Tripeptidomimetics to the Human Intestinal Di-/Tripeptide Transporter hPEPT1

PURPOSE

The aim of this study was to develop a three-dimensional quantitative structure-activity relationship (QSAR) model for binding of tripeptides and tripeptidomimetics to hPEPT1 based on a series of 25 diverse tripeptides.

METHODS

VolSurf descriptors were generated and correlated with binding affinities by multivariate data analysis. The affinities for hPEPT1 of the tripeptides and tripeptidomimetics were determined experimentally by use of Caco-2 cell monolayers.

RESULTS

The Ki-values of the 25 tripeptides and tripeptidomimetics ranged from 0.15 to 25 mM and the structural diversity of the compounds was described by VolSurf descriptors. A QSAR model that correlated the VolSurf descriptors of the tripeptides with their experimental binding affinity for hPEPT1 was established.

CONCLUSION

Structural information on tripeptide properties influencing the binding to hPEPT1 was extracted from the QSAR model. This information may contribute to the drug design process of tripeptides and tripeptidomimetics where hPEPT1 is targeted as an absorptive transporter for improvement of intestinal absorption. To our knowledge, this is the first time a correlation between VolSurf descriptors and binding affinities for hPEPT1 has been reported.