New Radiolabeled Exendin Analogues Show Reduced Renal Retention.

PET imaging of the glucagon-like peptide-1 receptor (GLP-1R) using radiolabeled exendin is a promising imaging method to detect insulinomas. However, high renal accumulation of radiolabeled exendin could hamper the detection of small insulinomas in proximity to the kidneys and limit its use as a radiotherapeutic agent. Here, we report two new exendin analogues for GLP-1R imaging and therapy, designed to reduce renal retention by incorporating a cleavable methionine-isoleucine (Met-Ile) linker. We examined the renal retention and insulinoma targeting properties of these new exendin analogues in a nude mouse model bearing subcutaneous GLP-1R-expressing insulinomas. NOTA or DOTA was conjugated via a methionine-isoleucine linker to the C-terminus of exendin-4 (NOTA-MI-exendin-4 or DOTA-MI-exendin-4). NOTA- and DOTA-exendin-4 without the linker were used as references. The affinity for GLP-1R was determined in a competitive binding assay using GLP-1R transfected cells. Biodistribution of [68Ga]Ga-NOTA-exendin-4, [68Ga]Ga-NOTA-MI-exendin-4, [177Lu]Lu-DOTA-exendin-4, and [177Lu]Lu-DOTA-MI-exendin-4 was determined in INS-1 tumor-bearing BALB/c nude mice, and PET/CT was acquired to visualize renal retention and tumor targeting. For all tracers, dosimetric calculations were performed to determine the kidney self-dose. The affinity for GLP-1R was in the low nanomolar range (<11 nM) for all peptides. In vivo biodistribution revealed a significantly lower kidney uptake of [68Ga]Ga-NOTA-MI-exendin-4 at 4 h post-injection (p.i.) (34.2 ± 4.2 %IA/g), compared with [68Ga]Ga-NOTA-exendin-4 (128 ± 10 %IA/g). Accumulation of [68Ga]Ga-NOTA-MI-exendin-4 in the tumor was 25.0 ± 8.0 %IA/g 4 h p.i., which was similar to that of [68Ga]Ga-NOTA-exendin-4 (24.9 ± 9.3 %IA/g). This resulted in an improved tumor-to-kidney ratio from 0.2 ± 0.0 to 0.8 ± 0.3. PET/CT confirmed the findings in the biodistribution studies. The kidney uptake of [177Lu]Lu-DOTA-MI-exendin-4 was 39.4 ± 6.3 %IA/g at 24 h p.i. and 13.0 ± 2.5 %IA/g at 72 h p.i., which were significantly lower than those for [177Lu]Lu-DOTA-exendin-4 (99.3 ± 9.2 %IA/g 24 h p.i. and 45.8 ± 3.9 %IA/g 72 h p.i.). The uptake in the tumor was 7.8 ± 1.5 and 11.3 ± 2.0 %IA/g 24 h p.i. for [177Lu]Lu-DOTA-MI-exendin-4 and [177Lu]Lu-DOTA-exendin-4, respectively, resulting in improved tumor-to-kidney ratios for [177Lu]Lu-DOTA-MI-exendin-4. The new exendin analogues with a Met-Ile linker showed 2-3-fold reduced renal retention and improved tumor-to-kidney ratios compared with their reference without the Met-Ile linker. Future studies should demonstrate whether [68Ga]Ga-NOTA-MI-exendin-4 results in improved detection of small insulinomas in close proximity to the kidneys with PET/CT. [177Lu]Lu-DOTA-MI-exendin-4 might open a window of opportunity for exendin-based radionuclide therapy.

Monitoring ß-Cell Survival After Intrahepatic Islet Transplantation Using Dynamic Exendin PET Imaging: A Proof-of-Concept Study in Individuals With Type 1 Diabetes.

Intrahepatic transplantation of islets of Langerhans (ITx) is a treatment option for individuals with complicated type 1 diabetes and profoundly unstable glycemic control, but its therapeutic success is hampered by deterioration of graft function over time. To improve ITx strategies, technologies to noninvasively monitor the fate and survival of transplanted islets over time are of great potential value. We used [68Ga]Ga-NODAGA-exendin-4 (68Ga-exendin) positron emission tomography (PET)/computed tomography (CT) imaging to demonstrate the feasibility of quantifying ß-cell mass in intrahepatic islet grafts in 13 individuals with type 1 diabetes, nine after ITx with functional islet grafts and four control patients not treated with ITx. ß-Cell function was measured by mixed-meal tolerance test. With dynamic 68Ga-exendin PET/CT images, we determined tracer accumulation in hepatic hotspots, and intrahepatic fat was assessed using MRI and spectroscopy. Quantification of hepatic hotspots showed a significantly higher uptake of 68Ga-exendin in the ITx group compared with the control group (median 0.55 [interquartile range 0.51-0.63] vs. 0.43 [0.42-0.45]). GLP-1 receptor expression was found in transplanted islets by immunohistochemistry. Intrahepatic fat was not detected in a majority of the individuals. Our study provides the first clinical evidence that radiolabeled exendin imaging can be used to monitor viable transplanted islets after intraportal ITx. ARTICLE HIGHLIGHTS: This clinical study researched the potential of radiolabeled exendin to follow the fate and survival of intrahepatic islet grafts. Is it feasible to quantitatively detect intrahepatic islet transplants with [68Ga]Ga-NODAGA-exendin-4 (68Ga-exendin) positron emission tomography (PET) imaging? Our study findings indicate that the imaging technique 68Ga-exendin PET can be used to monitor viable islet mass after intrahepatic islet transplantation in humans. Alongside functional measures, 68Ga-exendin PET imaging could significantly aid in the evaluation of strategies designed to improve islet engraftment, survival, and function.

Importance of beta cell mass for glycaemic control in people with type 1 diabetes.

AIMS/HYPOTHESIS: The role of beta cell mass in the balance of glucose control and hypoglycaemic burden in people with type 1 diabetes is unclear. We applied positron emission tomography (PET) imaging with radiolabelled exendin to compare beta cell mass among people with type 1 diabetes and either low glucose variability (LGV) or high glucose variability (HGV). METHODS: All participants with either LGV (n=9) or HGV (n=7) underwent a mixed-meal tolerance test to determine beta cell function and wore a blinded continuous glucose monitor for a week. After an i.v. injection with [68Ga]Ga-NODAGA-exendin-4, PET images were acquired for the quantification of pancreatic uptake of radiolabelled exendin. The mean standardised uptake value (SUVmean) of the pancreas was used to determine the amount of beta cell mass. RESULTS: Participants with LGV had lower HbA1c (46.0 mmol/mol [44.5-52.5] [6.4% (6.3-7)] vs 80 mmol/mol [69.0-110] [9.5% (8.5-12.2)], p=0.001) and higher time in range (TIR) (75.6% [73.5-90.3] vs 38.7% [25.1-48.5], p=0.002) than those with HGV. The SUVmean of the pancreas was higher for the LGV than for the HGV group (5.1 [3.6-5.6] vs 2.9 [2.1-3.4], p=0.008). The AUCC-peptide:AUCglucose ratio was numerically, but not statistically, higher in the LGV compared with the HGV group (2.7×10-2 [6.2×10-4-5.3×10-2] vs 9.3×10-4 [4.7×10-4-5.2×10-3], p=0.21). SUVmean correlated with the AUCC-peptide:AUCglucose ratio (Pearson r=0.64, p=0.01), as well as with the TIR (r=0.64, p=0.01) and the SD of interstitial glucose levels (r=-0.66, p=0.007). CONCLUSION/INTERPRETATION: Our data show higher beta cell mass in people with type 1 diabetes and LGV than in those with HGV, independent of beta cell function.