The glucagon-like peptide-1 receptor (GLP-1R) is highly and specifically expressed on the pancreatic β-cells. It plays an important role in glucose metabolism as well as in β-cell-derived diseases like diabetes, insulinoma, or congenital and adult hyperinsulinemic hypoglycemia. Radiolabeled exendin-4, a ligand of GLP-1R, has routinely been used in clinics to image insulinomas. However, its major drawback is the high kidney accumulation. Here, we show that the addition of an albumin-binding moiety (ABM) to radiolabeled exendin-4 results in a significant reduction of kidney uptake while retaining its high affinity and specificity to GLP-1R. The four tested peptides were shown to have high affinity to the GLP-1 receptor (IC50 of 3.7 ± 0.6 to 15.1 ± 0.8 nM). The radiolabeled derivatives were taken up into cells efficiently, internalizing between 39 ± 2 and 56 ± 2% after 2 h. Thus, the derivatives with ABM outperformed the reference peptide with its IC50 of 22.5 ± 2.9 nM and internalization of 41 ± 4%. Stability in human blood plasma was slightly enhanced by the addition of the albumin binder. In biodistribution studies, the radioligands exhibited an improved target-to-kidney ratio in comparison to the reference peptide of up to seven-fold. This was confirmed qualitatively in single-photon-emission computed tomography (SPECT)/CT imaging. This study demonstrated in vitro and in vivo that the addition of an ABM to radiolabeled exendin-4 strongly decreased kidney accumulation while retaining affinity to GLP-1R. Thus, exendin-4 derivatives with an albumin-binding moiety could present a viable class of diagnostic tracers for the detection of insulinomas and other GLP-1R-positive tissue in clinical application.
Keywords: GLP-1R; albumin binder; exendin-4; insulinoma; β-cell imaging; β-cells.