Constrained H-Phe-Phe-NH2 analogues with high affinity to the substance P 1-7 binding site and with improved metabolic stability and cell permeability.

We recently reported the discovery of H-Phe-Phe-NH2 as a small and high affinity ligand for the substance P 1-7 (SP(1-7), H-Arg-Pro-Lys-Pro-Gln-Gln-Phe-OH) specific binding site and its intriguing ability to reduce neuropathic pain. With the overall aim to develop stable and orally bioavailable SP(1-7) mimetics, the dipeptide was chosen as a lead compound. Herein the structure-activity relationship (SAR) of a set of modified H-Phe-Phe-NH2 analogues is presented together with their potential active uptake by PEPT1 transporter, intestinal permeability, and metabolic stability. Local constraints via peptide backbone methylation or preparation of cyclized analogues based on pyrrolidine were evaluated and were shown to significantly improve the in vitro pharmacokinetic properties. The SAR was rationalized by deriving a plausible binding pose for the high affinity ligands. Rigidification using a 3-phenylpyrrolidine moiety in the C-terminal of H-Phe-Phe-NH2 resulted in high affinity and improved intrinsic clearance and intestinal epithelial permeability.