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

Incretin mimetics, especially dual/triple agonists, are effective for type 2 diabetes and obesity, though mechanisms remain unclear. This study applied PET using [⁶⁸Ga]Ga-DO3A-Exendin-4 and [⁶⁸Ga]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.

In vitro binding assays on frozen HEK293 cell sections overexpressing human GLP-1R, GIPR, or GCGR assessed [⁶⁸Ga]Ga-DO3A-Exendin-4 specificity and competition with tirzepatide and SAR441255, and [⁶⁸Ga]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.

[⁶⁸Ga]Ga-DO3A-Exendin-4 and [⁶⁸Ga]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 [⁶⁸Ga]S02-GIP-T4 uptake by 23 ± 8.5%, indicating GIPR engagement.

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.

The study was funded by Uppsala Diabetes Center, Diabetesfonden, ExoDiab, Diabetes Wellness Sweden, Barndiabetesfonden, Science for Life Laboratory, and the Swedish Research Council.