Discovery of Dipeptides with High Affinity to the Specific Binding Site for Substance P1-7

Substance P 1-7 (SP1-7, H-Arg-Pro-Lys-Pro-Gln-Gln-Phe-OH) is the major bioactive metabolite of substance P. The interest in this heptapeptide originates from the observation that it modulates, and in certain cases opposes the effects of the parent peptide. e.g., the nociceptive effect. The p-opioid receptor agonist endomorphin-2 (EM-2, H-Tyr-Pro-Phe-Phe-NH2) has been found to also interact with the specific binding site of SP1-7 with only a 10-fold lower affinity compared to the native peptide. Considering the smaller size of EM-2 compared to the target heptapeptide, it was selected as a lead compound in the development of low-molecular-weight ligands to the SP1-7 binding site. An alanine scan and truncation study led to the unexpected discovery of the dipeptide H-Phe-Phe-NH2 (K-i = 1.5 nM), having equal affinity as the endogenous heptapeptide SP1-7. Moreover, the studies show that the C-terminal phenylalanine amide is crucial for the affinity of the dipeptide.