Design, synthesis and in vitro characterization of Glucagon-Like Peptide-1 derivatives for pancreatic beta cell imaging by SPECT.

Novel Glucagon-Like Peptide-1 (GLP-1) derivatives containing the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and naturally occurring Indium ((113/115)In) were prepared using solid-phase Fmoc methods. All synthesized peptides contained d-Ala-8, a modification known to improve resistance towards degradation by dipeptidyl peptidase-IV. The effect of increased distance between DOTA and the peptide chain was investigated using an (aminoethyl) ethoxy acetyl linker, in order to reduce steric effects imposed by DOTA. Placement of linker and DOTA moieties were also varied within the GLP-1 sequence to test for optimal metal-complex location. The binding affinity of the peptide derivatives was determined in vitro with Chinese hamster ovary cells stably transfected with a human GLP-1 receptor (CHO/GLP-1R) cell line and was shown to be in the nM range. Gamma camera imaging of an insulinoma cell line was carried out using (111)In-labeled peptides. Our results suggest that the prepared GLP-1 derivatives are suitable imaging probes for studying pancreatic islet function in vivo.