GLP1R and pancreatic insulinoma: A head-to-head comparison of [Lys40([111In]In-DTPA)]Ex-4 and [Lys40([111In]In-DTPA)]Ex(9-39) revealed similar affinity for the GLP-1R and the same number of binding sites; however, the receptor antagonist displayed disappointingly low tumor uptake in rat insulinomas [189].