Affinity and translocation relationships via hPEPT1 of H-Xaa-Ser-OH dipeptides: Evaluation of H-Phe-Ser-OH as a pro-moiety for ibuprofen and benzoic acid prodrugs

The intestinal di/tri-peptide transporter 1 (hPEPT1) has been suggested as a drug delivery target for peptide-based prodrugs. The aim of the study was to synthesize a series of 11 serine-containing dipeptides (H-X_aa-Ser-OH) and to investigate the relationship between binding to and transport via hPEPT1. An additional aim was to design a dipeptide which could serve as a pro-moiety for prodrugs targeted to hPEPT1. X_aa was chosen from the 20 proteogenic amino acids. The dipeptides were synthesized using solid phase peptide synthesis. The K_i-values of H-X_aa-Ser-OH dipeptides for hPEPT1 in MDCK/hPEPT1 cells ranged from 0.14 mM (log IC₅₀ = −0.85 ± 0.06) for H-Tyr-Ser-OH to 0.89 mM (log IC₅₀ = −0.09 ± 0.02) for H-Gly-Ser-OH, as measured in a competition assay with [¹⁴C]Gly-Sar. The dipeptides were translocated via hPEPT1 with K_m-values in the range of 0.20 (log IC₅₀ = −0.69 ± 0.04) for H-Met-Ser-OH to 1.04 (log IC₅₀ = 0.02 ± 0.04) mM for H-Gly-Ser-OH. The relationship between ligand and transportate properties indicated that the initial binding of the ligand to hPEPT1 is the major determinant for translocation of the investigated dipeptides. H-Phe-Ser-OH was selected as a pro-moiety, and two prodrugs were synthesized, i.e. H-Phe-Ser(Ibuprofyl)-OH and H-Phe-Ser(Bz)-OH. Both H-Phe-Ser(Ibuprofyl)-OH and H-Phe-Ser(Bz)-OH had high affinity for hPEPT1 with K_i-values of 0.07 mM (log IC₅₀ = −0.92 ± 0.12) and 0.12 mM (log IC₅₀ = −1.17 ± 0.40), respectively. However, none of the prodrugs were translocated via hPEPT1. This indicated that the coupling of the drug compounds to the peptide backbone did not decrease transporter binding, but abolished translocation, and that high affinity of prodrugs does not necessarily translate into favourable permeation properties.