Conformationally constrained CCK8 analogues obtained from a rationally designed peptide library as ligands for cholecystokinin type B receptor

A library of 14 cyclic peptide analogues derived from the octapeptide C-terminal sequence of the human cholecystokinin hormone (CCK(26-33), or CCK8) was designed, synthesized, and characterized. The 14 peptide analogues were rationally designed to specifically interact with the CCK type B receptor (CCK₈-R) on the basis of the structure of the bimolecular complex between CCK8 and the third extracellular loop of CCK₈-R, namely CCK₈-R-(352-379). The rational design of new ligands for CCK ₈-R has relied on stabilization by cyclic constraints of the structural motifs that bring the key residues of the ligand (especially Trp 30, Met31, and Phe33) in the proper spatial orientation for optimal interaction with the receptor. The binding affinity of the new ligands for CCK₈-R was assessed by displacement experiments of ¹¹¹In-radiolabeled CCK8 in cells that overexpress the CCK₈ receptor. The new ligands generally showed binding affinities lower than that of parent CCK8, with the best compounds having IC₅₀ values around 10 μM. Structure-activity relationship data show that preservation of the Trp30-Met31 motif is essential and that the Phe33 side chain must be present. NMR conformational studies of the compound with maximal binding affinity (cyclo-B11, IC₅₀ = 11 μM) in DPC micelles shows that this compound presents a turn-like conformation centered at the Trp 30-Met31 segment, as planned by rational design. Such a conformation is stabilized by its interaction with the micelle rather than by the cyclic constraint.