A porcine type 1 Diabetes Mellitus model, for non-invasive in vivo imaging of the glucagon-like peptide-1 receptor in the pancreas, using [68Ga]Ga-DO3A-VS-Cys40-conjugated synthetic exendin-4 in PET-CT
Diabetes mellitus is a rising epidemic throughout the world and there is currently great interest in quantifying the beta-cell mass (BCM) in vivo non-invasively. In the present experiment, the feasibility of in vivo imaging of the glucagon-like peptide-1 receptor (GLP-1R) in beta-cells was examined, using the positron emission tomography (PET) tracer [68Ga]Ga-DO3A-VS-Cys40-exendin-4 as a marker, in native pancreatic beta-cells of a porcine diabetic animal model and healthy controls.
Eight Swedish high-health domestic pigs were randomly assigned to be either controls or made diabetic using streptozotocin (STZ). The experiment proceeded during eight weeks, starting with an acclimatisation period. Once the pigs had been socialised they underwent surgery for the insertion of a jugular vein catheter, allowing induction of diabetes with STZ, intravenous (i.v.) injections and stress-free blood sampling. Development of diabetes was confirmed by clinical examinations, blood glucose values and insulin-staining of pancreatic sections post mortem.
The diabetic pigs were insulin treated and responded well. PET-CT (PET-computed tomography) examinations were performed on healthy controls and insulin-treated diabetic pigs. At the beginning of the PET-CT scan, oxygen-15 labelled water ([15O]WAT) was injected i.v. to measure tissue perfusion in the pancreas and kidneys. The specific binding of [68Ga]Ga-DO3A-VS-Cys40-exendin-4 to the GLP-1R in vivo was assessed by i.v. administration of the tracer compound giving a baseline image. This was followed by the administration of a competing high dose of synthetic exendin-4 and a new imaging sequence. The pigs were humanely euthanised 0?6 days after the PET-CT examination and full post mortem examinations were performed in all pigs.
Diabetes was successfully induced, confirmed by immunohistochemical (IHC) staining for insulin. An important incidental finding, during PET-CT examination, was that the tracer and synthetic exendin-4 immediately induced a significant tachycardia in all pigs both at low and high dose. PET scans showed a reduced tissue perfusion in the pancreas and kidneys of the diabetic pigs. GLP-1R-mediated uptake of the tracer was detected in the pancreas of both healthy controls and diabetic pigs and surprisingly, the uptake of the tracer did not differ between the two groups. Thus, the pancreatic tracer uptake of [68Ga]Ga-DO3A-VS-Cys40-exendin-4 was not significantly reduced by selective destruction of beta-cells in diabetic pigs and the GLP-1R is not a suitable target for imaging of native pancreatic beta-cells in pigs. Additionally, this experiment shows how the pig can be made diabetic, insulin-treated and properly anaesthetised for several hours, which makes the pig a suitable animal model for further diabetic research.