GLP1R and pancreatic insulinoma: Although the tracers 18F-TTCO-Cys40-Exendin-4 and 64Cu-DO3A-VS-Cys40-Exendin-4 displayed highly specific binding to GLP-1R in the mouse insulinoma model, they did not provide sufficient contrast for PET islet imaging in a mouse model of human islet transplantation.