Comparison of desferrioxamine and NODAGA for the gallium-68 labeling of exendin-4.

Introduction: Radiolabeled exendin-4 (Ex4) derivatives are used to target the glucagon-like peptide-1 receptor (GLP-1R) for the clinical diagnosis of insulinomas, a rare type of neuroendocrine tumor. Gallium-68 is an ideal diagnostic nuclide for this application and a study evaluating an exendin-4-NODAGA conjugate is currently underway. However, in complexion with the chelator DFO, its in vivo stability has been a matter of dispute. The aim of this work was to directly compare [ ⁶⁸ Ga]Ga-Ex4NOD with [ ⁶⁸ Ga]Ga-Ex4DFO in vitro and in vivo.
Methods: In our approach, we directly compared N'-[5-(acetyl-hydroxy-amino)pentyl]-N-[5-[3-(5-aminopentyl-hydroxy-carbamoyl)propanoylamino]pentyl]-N-hydroxy-butane diamide (desferriox-amine B, DFO) and 2-(4,7-bis (carboxymethyl)-1,4,7-triazonan-1-yl) pentanedioic acid (NODAGA) conjugated to exendin-4 in vitro and in vivo. We radiolabeled the peptides with gallium-68, followed by HPLC quality control. In vitro characterization was performed in CHL cells overexpressing the GLP-1R and in vivo studies were conducted with CD1 nu/nu mice carrying tumors derived from these cells.
Results: We found that both peptides could be radiolabeled with a molar activity of about 9.33 MBq/nmol without further purification. They internalized equally well into GLP-1R-expressing cells and their IC ₅₀ was similar with 15.6 ± 7.8 nM and 18.4 ± 3.0 nM for [ ^nat Ga]Ga-Ex4NOD and [ ^nat Ga]Ga-Ex4DFO, respectively. In vivo, [ ⁶⁸ Ga]Ga-Ex4NOD accumulated more in all tissue, while [ ⁶⁸ Ga]Ga-Ex4DFO exhibited a more favorable target-to-kidney ratio.
Conclusion and Relevance: DFO is a suitable chelator for the radiolabeling of exendin-4 derivatives with gallium-68 for in vitro and preclinical in vivo studies. DFO performed better in vivo due to its significantly lower kidney accumulation (p < 0.0001). It was also found to be stable in vivo in mice, contrary to earlier reports. Based on our results, the DFO chelating system in combination with exendin-4 would be an interesting option for clinical imaging of insulinomas.