A combinatorial peptoid library for the identification of novel MSH and GRP/bombesin receptor ligands

A tripeptoid library was synthesized using 69 different primary amines in i nitially 69 individual reactions by the mix and split approach. The resulti ng library consisted of 328,509 (69(3)) single compounds, divided in 69 sub pools each containing 4,761 entities. The 69 subpools were tested in two bi nding assays, one for alpha-MSH (alpha-melanotropin) and one for GRP (gastr in-releasing peptide)/bombesin. The sublibraries with the highest affinity to the MSH receptor (i.e. melanocortin type 1 or MC1 receptor) and, respect ively, the GRP-preferring bombesin receptor were identified by an iterative process. Individual tripeptoids with good binding activity were resynthesi zed, analyzed and their dissociation constants and biological activity dete rmined. The K-D of the most potent MC1 receptor ligand was 1.58 mu mol/l an d that of the GRP-preferring bombesin receptor 3.40 mu mol/l. Extension of this latter tripeptoid by one residue at the N-terminus led to the identifi cation 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 t ripeptoid ligand when extended by one residue, but a compound suitable for radioiodination and lacking the N-terminal amino group had a slightly highe r binding activity than the tripeptoids (K-D approximate to 850 nmol/l). Th ese results demonstrate that testing a peptoid library containing 328,509 s ingle compounds led to the successful identification of new ligands for bot h the MC1 receptor as well as the GRP-preferring bombesin receptor.