Cardiovascular side-effects and insulin secretion after intravenous administration of radiolabeled Exendin-4 in pigs.

INTRODUCTION: Radiolabeled Exendin-4, a synthetic glucagon-like peptide-1 (GLP-1) analog, is used as a tracer for diagnostic purposes of ß-cells and in experimental animal research. Exendin-4 can be radiolabeled with (68)Ga, (111)In or (99m)Tc and used for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging to diagnose insulinomas, visualization of pancreatic ß-cell mass and transplanted Islets of Langerhans. In humans, Exendin-4 is widely used as a therapeutic agent for treatment of type 2 diabetes (T2D). The compound, which is administered subcutaneously (SC) may cause nausea, vomiting and a minor increase in the heart rate (HR). However, possible side-effects on cardiovascular functions after intravenous (IV) administration have not been reported. This study describes the Exendin-4 dose at which cardiovascular side-effects occur in pigs and cynomolgus monkeys. The IV effect of the tracer on insulin secretion is also investigated in pigs. METHODS: Seven clinically healthy littermate pigs (40days old) were used; three of them were made diabetic by streptozotocin (STZ). All pigs underwent PET imaging under general anesthesia to examine the glucagon-like peptide-1 receptor (GLP-1R) in ß-cells with radiolabeled Exendin-4. A baseline tracer dose IV [(68)Ga]Exendin-4 (0.025±0.010µg/kg) followed by a competition dose IV [(68)Ga]Exendin-4 (3.98±1.33µg/kg) 60min later were administered. Blood samples were taken and analyzed for insulin secretion by using ELISA. Cardiovascular and respiratory variables were monitored throughout the experiment. RESULTS: Immediately after administration of the high dose [(68)Ga]Exendin-4 the HR rose from 122±14 to 227±40bpm (p<0.01) and from 100±5 to 181±13bpm (p<0.01) in healthy non-diabetic and diabetes-induced pigs, respectively. The tachycardia was observed for >2h and one healthy non-diabetic pig suffered cardiac arrest 3h after the IV [(68)Ga]Exendin-4. Arrhythmia was detected by listening to the heart with a stethoscope up to 4days after the [(68)Ga]Exendin-4 injection. In all animals, no effect on the cardiovascular system was registered after the low dose of IV [(68)Ga]Exendin-4. Insulin secretion increased (p<0.05) when IV [(68)Ga]Exendin-4 was given in dosages ≥0.14µg/kg. CONCLUSIONS: Intravenous administration of ≥2.8µg/kg [(68)Ga]Exendin-4 resulted in severe tachycardia and arrhythmias in healthy non-diabetic and diabetes-induced pigs, and the insulin secretion was stimulated in healthy non-diabetic animals when ≥0.14µg/kg [(68)Ga]Exendin-4 was given.

Positron emission tomography imaging of the glucagon-like peptide-1 receptor in healthy and streptozotocin-induced diabetic pigs.

PURPOSE: The glucagon-like peptide-1 receptor (GLP-1R) has been proposed as a target for molecular imaging of beta cells. The feasibility of non-invasive imaging and quantification of GLP-1R in pancreas using the positron emission tomography (PET) tracer [(68)Ga]Ga-DO3A-VS-Cys(40)-Exendin-4 in non-diabetic and streptozotocin (STZ)-induced diabetic pigs treated with insulin was investigated. METHODS: Non-diabetic (n = 4) and STZ-induced diabetic pigs (n = 3) from the same litter were examined. Development of diabetes was confirmed by blood glucose values, clinical examinations and insulin staining of pancreatic sections post mortem. Tissue perfusion in the pancreas and kidneys was evaluated by [(15)O]water PET/computed tomography (CT) scans. The in vivo receptor specificity of [(68)Ga]Ga-DO3A-VS-Cys(40)-Exendin-4 was assessed by administration of either tracer alone or by competition with 3-6.5 µg/kg of Exendin-4. Volume of distribution and occupancy in the pancreas were quantified with a single tissue compartment model. RESULTS: [(15)O]water PET/CT examinations showed reduced perfusion in the pancreas and kidneys in diabetic pigs. [(68)Ga]Ga-DO3A-VS-Cys(40)-Exendin-4 uptake in the pancreas of both non-diabetic and diabetic pigs was almost completely abolished by co-injection of unlabeled Exendin-4 peptide. [(68)Ga]Ga-DO3A-VS-Cys(40)-Exendin-4 uptake did not differ between non-diabetic and diabetic pigs. In all animals, administration of the tracer resulted in an immediate increase in the heart rate (HR). CONCLUSION: Pancreatic uptake of [(68)Ga]Ga-DO3A-VS-Cys(40)-Exendin-4 was not reduced by destruction of beta cells in STZ-induced diabetic pigs.