Positron emission tomography to assess drug occupancy at peripheral and central incretin receptors.

BACKGROUND: Incretin mimetics, especially dual/triple agonists, are effective for type 2 diabetes and obesity, though mechanisms remain unclear. This study applied PET using [Ga]Ga-DO3A-Exendin-4 and [Ga]S02-GIP-T4 to assess GLP-1R and GIPR occupancy of SAR441255 (a GLP-1R, GIPR, and GCGR agonist) and tirzepatide in pig pancreas and CNS. METHODS: In vitro binding assays on frozen HEK293 cell sections overexpressing human GLP-1R, GIPR, or GCGR assessed [Ga]Ga-DO3A-Exendin-4 specificity and competition with tirzepatide and SAR441255, and [Ga]S02-GIP-T4 with SAR441255. In vivo, the GLP-1R occupancy by SAR441255 and tirzepatide, and GIPR occupancy by SAR441255, was evaluated in healthy pigs using PET/CT initiated at tracer injection. A subcutaneous dose of the study drug was then administered, and a second scan was performed 2.5 h later. Occupancy was determined by comparing pancreatic and CNS tracer SUV before and after dosing. Two animals were used to compare the tracers directly. FINDINGS: [Ga]Ga-DO3A-Exendin-4 and [Ga]S02-GIP-T4 showed high specificity for GLP-1R and GIPR, respectively, and competed with the study drugs in vitro. In vivo, SAR441255 induced dose-dependent GLP-1R occupancy (>70%) in pancreas and pituitary and (>60%) in CNS, while tirzepatide showed lower occupancy. SAR441255 also reduced pancreatic [Ga]S02-GIP-T4 uptake by 23 ± 8.5%, indicating GIPR engagement. INTERPRETATION: PET imaging in pigs demonstrated in vivo GLP-1R engagement by SAR441255 and tirzepatide, and GIPR engagement by SAR441255 in the pancreas. SAR441255 exhibited dose-dependent GLP-1R occupancy in the pancreas and brain regions linked to appetite regulation. FUNDING: The study was funded by Uppsala Diabetes Center, Diabetesfonden, ExoDiab, Diabetes Wellness Sweden, Barndiabetesfonden, Science for Life Laboratory, and the Swedish Research Council.