Elucidating the Structure–Activity Relationship of the Pentaglutamic Acid Sequence of Minigastrin with Cholecystokinin Receptor Subtype 2

Abstract

Derivatized minigastrin analogues make up a promising class of candidates for targeting cholecystokinin receptor subtype 2 (CCK2R), which is overexpressed on cancer cells of various neuroendocrine tumors. The pentaglutamic acid sequence of minigastrin influences its biological properties. In particular, it plays a crucial role in the kidney reuptake mechanism. However, the importance of the binding affinity and interaction of this region with the receptor on a molecular level remains unclear. To elucidate its structure–activity relationship with CCK2R, we replaced this sequence with various linkers differing in their amount of anionic charge, structural characteristics, and flexibility. Specifically, a flexible aliphatic linker, a linker with only three d-Glu residues, and a structured linker with four adjacent β3-glutamic acid residues were evaluated and compared to the lead compound PP-F11N (DOTA-[d-Glu1–6,Nle11]gastrin-13). 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was conjugated to the minigastrin derivatives, which allowed radiolabeling with Lutetium-177. The levels of In vitro internalization into MZ-CRC1 cells and in vivo tumor uptake as well as human blood plasma stability increased in the following order: aliphatic linker < three d-Glu < (β3-Glu)4 < (d-Glu)6. The in vitro and in vivo behavior was therefore significantly improved with anionic charges. Computational modeling of a CCK2 receptor–ligand complex revealed ionic interactions between cationic residues (Arg and His) of the receptor and anionic residues of the ligand in the linker.