A comparison of three 67/68Ga-labelled exendin-4 derivatives for β-cell imaging on the GLP-1 receptor: the influence of the conjugation site of NODAGA as chelator

Background: Various diseases derive from pathologically altered beta-cells. Their function can be increased, leading to hyperinsulinism, or decreased, resulting in diabetes. Non-invasive imaging of the beta-cell-specific glucagon-like peptide receptor-1 (GLP-1R) would allow the assessment of both beta-cell mass and derived tumours, potentially improving the diagnosis of various conditions. We tested three new Ga-67/68-labelled derivatives of exendin-4, an agonist of GLP-1R, in vitro and in vivo. We determined the influence of the chelator NODAGA conjugated to resident lysines either at positions 12 and 27 or the C-terminally attached lysine at position 40 on the binding and kinetics of the peptide. Methods: Binding and internalisation of Ga-67-labelled Ex4NOD12, Ex4NOD27 and Ex4NOD40 were tested on Chinese hamster lung (CHL) cells stably transfected to express the GLP-1 receptor (GLP-1R). In vivo biodistribution of Ga-68-labelled peptides was investigated in CD1 nu/nu mice with subcutaneous CHL-GLP-1R positive tumours; the specificity of the binding to GLP-1R was determined by pre-injecting excess peptide. Results: All peptides showed good in vitro binding affinities to GLP-1R in the range of 29 to 54 nM. Ga-67/68-Ex4NOD40 and Ga-67/68-Ex4NOD12 show excellent internalisation (>30%) and high specific uptake in GLP-1R positive tissue, but high activity was also found in the kidneys. Conclusions: We show that of the three peptides, Ga-Ex4NOD40 and Ga-Ex4NOD12 demonstrate the most favourable in vitro properties and in vivo binding to GLP-1R positive tissue. Therefore, we conclude that the lysines at positions 12 and 40 might preferentially be utilised for modifying exendin-4.