Structure-Activity Relationships and Pharmacokinetics of In-Labeled Glucagon-like Peptide-1 Receptor-Targeting Exendin-4 Derivatives Conjugated with Albumin Binder Moieties.

Insulinomas are neuroendocrine tumors that are derived from pancreatic β-cells, and they often overexpress the glucagon-like peptide-1 receptor (GLP-1R). Radiolabeled exendin-4 derivatives have been used to noninvasively detect the GLP-1R during the diagnosis and preoperative localization of insulinomas; however, their marked renal accumulation can hinder the imaging of pancreatic tail lesions. In this study, we designed and synthesized In-labeled exendin-4 derivatives that possessed 4-(4-substituted phenyl)-moieties as albumin binder (ALB) moieties ([In]In-E4DA2-4), and studied their structure-activity relationships and pharmacokinetics (as well as those of [In]In-E4DA1, which we previously reported) to determine their usefulness as radioligands for GLP-1R imaging. In-labeling was performed by reacting maleimide precursors with [In]InCl in 2-(-morpholino)ethanesulfonic acid buffer, and then, the products were conjugated with exendin-4-Cys. A saturation binding assay using GLP-1R-expressing INS-1 cells was carried out to evaluate the affinity of the radioligands for the cells. In addition, the affinity of the In-labeled derivatives for human serum albumin (HSA) was evaluated in an HSA-binding assay. Furthermore, an biodistribution study and single-photon emission computed tomography (SPECT) imaging were performed using INS-1 tumor-bearing mice. [In]In-E4DA1-4 were prepared at radiochemical yields of 6-17%. In the saturation binding assay, [In]In-E4DA1-4 showed a similar affinity for the INS-1 cells, indicating that the kind of ALB moiety used had no effect on the affinity of the exendin-4 derivatives for the cells. In the HSA-binding assay, [In]In-E4DA1-4 all bound to HSA. In the biodistribution assay, [In]In-E4DA1-4 exhibited marked tumor accumulation and retention. In addition, they showed lower renal accumulation than previously reported exendin-4-based radioligands without ALB moieties. The pharmacokinetics of the In-labeled exendin-4 derivatives varied markedly according to the kind of ALB moiety used. In particular, [In]In-E4DA2, which contained a 4-(4-bromophenyl)butyric acid derivative as an ALB moiety, showed the highest tumor accumulation. SPECT imaging with [In]In-E4DA2 clearly visualized INS-1 tumors with no marked accumulation in normal organs. These results provide important information that will aid the design of novel exendin-4-based radioligands targeting the GLP-1R.