Targets and probes for non-invasive imaging of β-cells

beta-cells, located in the islets of the pancreas, are responsible for production and secretion of insulin and play a crucial role in blood sugar regulation. Pathologic beta-cells often cause serious medical conditions affecting blood glucose level, which severely impact life quality and are life-threatening if untreated. With 347 million patients, diabetes is one of the most prevalent diseases, and will continue to be one of the largest socioeconomic challenges in the future. The diagnosis still relies mainly on indirect methods like blood sugar measurements. A non-invasive diagnostic imaging modality would allow direct evaluation of beta-cell mass and would be a huge step towards personalized medicine. Hyperinsulinism is another serious condition caused by beta-cells that excessively secrete insulin, like for instance beta-cell hyperplasia and insulinomas. Treatment options with drugs are normally not curative, whereas curative procedures usually consist of the resection of affected regions for which, however, an exact localization of the foci is necessary. In this review, we describe potential tracers under development for targeting beta-cells with focus on radiotracers for PET and SPECT imaging, which allow the non-invasive visualization of beta-cells. We discuss either the advantages or limitations for the various tracers and modalities. This article concludes with an outlook on future developments and discuss the potential of new imaging probes including dual probes that utilize functionalities for both a radioactive and optical moiety as well as for theranostic applications.