A Combinatorial Peptoid Library for the Identification of Novel MSH and GRP / Bombesin Receptor Ligands

Abstract

A tripeptoid library was synthesized using 69 different primary amines in initially 69 individual reactions by the mix and split approach. The resulting library consisted of 328,509 (69³) single compounds, divided in 69 subpools each containing 4,761 entities. The 69 subpools were tested in two binding assays, one for α-MSH (α-melanotropin) and one for GRP (gastrin-releasing peptide)/bombesin. The sublibraries with the highest affinity to the MSH receptor (i.e. melanocortin type 1 or MC1 receptor) and, respectively, the GRP-preferring bombesin receptor were identified by an iterative process. Individual tripeptoids with good binding activity were resynthesized, analyzed and their dissociation constants and biological activity determined. The K_D of the most potent MC1 receptor ligand was 1.58 μmol/l and that of the GRP-preferring bombesin receptor 3.40 μmol/l. Extension of this latter tripeptoid by one residue at the N-terminus led to the identification of a tetrapeptoid structure whose K_D value increased to 280 nmol/l. A similar increase in activity was not observed with the most potent MSH tripeptoid ligand when extended by one residue, but a compound suitable for radioiodination and lacking the N-terminal amino group had a slightly higher binding activity than the tripeptoids (K_D 850 nmol/l). These results demonstrate that testing a peptoid library containing 328,509 single compounds led to the successful identification of new ligands for both the MC1 receptor as well as the GRP-preferring bombesin receptor.