In vivo imaging of the glucagonlike peptide 1 receptor in the pancreas with 68Ga-labeled DO3A-exendin-4

Introduction of a fatty acid chain modification to prolong circulatory half-life of a radioligand towards glucose-dependent insulinotropic polypeptide receptor

BACKGROUND

The beneficial role of glucose-dependent insulinotropic polypeptide receptor (GIPR) in weight control and maintaining glucose levels has led to the development of several multi-agonistic peptide drug candidates, targeting GIPR and glucagon like peptide 1 receptor (GLP1R) and/or the glucagon receptor (GCGR). The in vivo quantification of target occupancy by these drugs would accelerate the development of new drug candidates. The aim of this study was to evaluate a novel peptide (GIP1234), based on previously reported ligand DOTA-GIP-C803, modified with a fatty acid moiety to prolong its blood circulation. It would allow higher target tissue exposure and consequently improved peptide uptake as well as in vivo PET imaging and quantification of GIPR occupancy by novel drugs of interest.

METHOD

A 40 amino acid residue peptide (GIP1234) was synthesized based on DOTA-GIP-C803, in turn based on the sequences of endogenous GIP and Exendin-4 with specific amino acid modifications to obtain GIPR selectivity. A palmitoyl fatty acid chain was furthermore added at Lys14 via a glutamic acid linker to prolong its blood circulation time by the interaction with albumin. GIP1234 was conjugated with a DOTA chelator at the C-terminal cysteine residue to achieve 68Ga radiolabeling. The resulting PET probe, [68Ga]Ga-DOTA-GIP1234 was evaluated for receptor binding specificity and selectivity using HEK293 cells transfected with human GIPR, GLP1R, or GCGR. Blocking experiments with tirzepatide (2 μM) were conducted using huGIPR HEK293 cells to investigate binding specificity. Ex vivo and in vivo organ distribution of [68Ga]Ga-DOTA-GIP1234 was studied in rats and a pig in comparison to [68Ga]Ga-DOTA-C803-GIP. Binding of [68Ga]Ga-DOTA-GIP1234 to albumin was assessed in situ using polyacrylamide gel electrophoresis (PAGE). The stability was tested in formulation buffer and rat blood plasma.

RESULTS

[68Ga]Ga-DOTA-GIP1234 was synthesized with non-decay corrected radiochemical yield of 88 ± 3.7 % and radiochemical purity of 97.8 ± 0.8 %. The molar activity for the radiotracer was 8.1 ± 1.1 MBq/nmol. [68Ga]Ga-DOTA-GIP1234 was stable and maintained affinity to huGIPR HEK293 cells (dissociation constant (Kd) = 40 ± 12.5 nM). The binding of [68Ga]Ga-DOTA-GIP1234 to huGCGR and huGLP1R cells was insignificant. Pre-incubation of huGIPR HEK293 cell sections with tirzepatide resulted in the decrease of [68Ga]Ga-DOTA-GIP1234 binding by close to 90 %. [68Ga]Ga-DOTA-GIP1234 displayed slow blood clearance in pigs with SUV = 3.5 after 60 min. Blood retention of the tracer in rat was 2-fold higher than that of [68Ga]Ga-DOTA-C803-GIP. [68Ga]Ga-DOTA-GIP1234 also demonstrated strong liver uptake in both pig and rat combined with decreased renal excretion. The concentration dependent binding of [68Ga]Ga-DOTA-GIP1234 to albumin was confirmed in situ by PAGE.

CONCLUSION

[68Ga]Ga-DOTA-GIP1234 demonstrated nanomolar affinity and selectivity for huGIPR in vitro. Addition of a fatty acid moiety prolonged blood circulation time and tissue exposure in both rat and pig in vivo. However, the liver uptake was also increased which may make PET imaging of abdominal tissues such as pancreas challenging. The investigation of the influence of fatty acid moiety on the biological performance of the peptide ligand paved the way for further rational design of GIPR ligand analogues with improved characteristics.

Targeting β Cells with Cathelicidin Nanomedicines Improves Insulin Function and Pancreas Regeneration in Type 1 Diabetic Rats

Type 1 diabetes (T1D) is an incurable condition with an increasing incidence worldwide, in which the hallmark is the autoimmune destruction of pancreatic insulin-producing β cells. Cathelicidin-based peptides have been shown to improve β cell function and neogenesis and may thus be relevant while developing T1D therapeutics. In this work, a cathelicidin-derived peptide, LLKKK18, was loaded in poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs), surface-functionalized with exenatide toward a GLP-1 receptor, aiming the β cell-targeted delivery of the peptide. The NPs present a mean size of around 100 nm and showed long-term stability, narrow size distribution, and negative ζ-potential (-10 mV). The LLKKK18 association efficiency and loading were 62 and 2.9%, respectively, presenting slow and sustained in vitro release under simulated physiologic fluids. Glucose-stimulated insulin release in the INS-1E cell line was observed in the presence of the peptide. In addition, NPs showed a strong association with β cells from isolated rat islets. After administration to diabetic rats, NPs induced a significant reduction of the hyperglycemic state, an improvement in the pancreatic insulin content, and glucose tolerance. Also remarkable, a considerable increase in the β cell mass in the pancreas was observed. Overall, this novel and versatile nanomedicine showed glucoregulatory ability and can pave the way for the development of a new generation of therapeutic approaches for T1D treatment.

Noninvasive evaluation of donor and native pancreases following simultaneous pancreas-kidney transplantation using positron emission tomography/computed tomography

It is crucial to develop practical and noninvasive methods to assess the functional beta-cell mass in a donor pancreas, in which monitoring and precise evaluation is challenging. A patient with type 1 diabetes underwent noninvasive imaging following simultaneous kidney-pancreas transplantation with positron emission tomography/computed tomography (PET/CT) using an exendin-based probe, [18 F]FB(ePEG12)12-exendin-4. Following transplantation, PET imaging with [18 F]FB(ePEG12)12-exendin-4 revealed simultaneous and distinct accumulations in the donor and native pancreases. The pancreases were outlined at a reasonable distance from the surrounding organs using [18 F]FB(ePEG12)12-exendin-4 whole-body maximum intensity projection and axial PET images. At 1 and 2 h after [18 F]FB(ePEG12)12-exendin-4 administration, the mean standardized uptake values were 2.96 and 3.08, respectively, in the donor pancreas and 1.97 and 2.25, respectively, in the native pancreas. [18 F]FB(ePEG12)12-exendin-4 positron emission tomography imaging allowed repeatable and quantitative assessment of beta-cell mass following simultaneous kidney-pancreas transplantation.

Multivalent γ-PGA-Exendin-4 conjugates to target pancreatic β-cells

Targeting of glucagon‐like peptide 1 receptor (GLP‐1R), expressed on the surface of pancreatic β‐cells, is of great interest for the development of advanced therapies for diabetes and diagnostics for insulinoma. We report the conjugation of exendin‐4 (Ex‐4), an approved drug to treat type 2 diabetes, to poly‐γ‐glutamic acid (γ‐PGA) to obtain more stable and effective GLP‐1R ligands. Exendin‐4 modified at Lysine‐27 with PEG4‐maleimide was conjugated to γ‐PGA functionalized with furan, in different molar ratios, exploiting a chemoselective Diels‐Alder cycloaddition. The γ‐PGA presenting the highest number of conjugated Ex‐4 molecules (average 120 per polymeric chain) showed a double affinity towards GLP‐1R with respect to exendin per se, paving the way to improved therapeutic and diagnostic applications.

Effects on weight loss and glycemic control with SAR441255, a potent unimolecular peptide GLP-1/GIP/GCG receptor triagonist

Unimolecular triple incretins, combining the activity of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon (GCG), have demonstrated reduction in body weight and improved glucose control in rodent models. We developed SAR441255, a synthetic peptide agonist of the GLP-1, GCG, and GIP receptors, structurally based on the exendin-4 sequence. SAR441255 displays high potency with balanced activation of all three target receptors. In animal models, metabolic outcomes were superior to results with a dual GLP-1/GCG receptor agonist. Preclinical in vivo positron emission tomography imaging demonstrated SAR441255 binding to GLP-1 and GCG receptors. In healthy subjects, SAR441255 improved glycemic control during a mixed-meal tolerance test and impacted biomarkers for GCG and GIP receptor activation. Single doses of SAR441255 were well tolerated. The results demonstrate that integrating GIP activity into dual GLP-1 and GCG receptor agonism provides improved effects on weight loss and glycemic control while buffering the diabetogenic risk of chronic GCG receptor agonism.

The Current State of Beta-Cell-Mass PET Imaging for Diabetes Research and Therapies

Diabetes is a chronic metabolic disease affecting over 400 million people worldwide and one of the leading causes of death, especially in developing nations. The disease is characterized by chronic hyperglycemia, caused by defects in the insulin secretion or action pathway. Current diagnostic methods measure metabolic byproducts of the disease such as glucose level, glycated hemoglobin (HbA1c), insulin or C-peptide levels, which are indicators of the beta-cell function. However, they inaccurately reflect the disease progression and provide poor longitudinal information. Beta-cell mass has been suggested as an alternative approach to study disease progression in correlation to beta-cell function, as it behaves differently in the diabetes physiopathology. Study of the beta-cell mass, however, requires highly invasive and potentially harmful procedures such as pancreatic biopsies, making diagnosis and monitoring of the disease tedious. Nuclear medical imaging techniques using radiation emitting tracers have been suggested as strong non-invasive tools for beta-cell mass. A highly sensitive and high-resolution technique, such as positron emission tomography, provides an ideal solution for the visualization of beta-cell mass, which is particularly essential for better characterization of a disease such as diabetes, and for estimating treatment effects towards regeneration of the beta-cell mass. Development of novel, validated biomarkers that are aimed at beta-cell mass imaging are thus highly necessary and would contribute to invaluable breakthroughs in the field of diabetes research and therapies. This review aims to describe the various biomarkers and radioactive probes currently available for positron emission tomography imaging of beta-cell mass, as well as highlight the need for precise quantification and visualization of the beta-cell mass for designing new therapy strategies and monitoring changes in the beta-cell mass during the progression of diabetes.

Sodium Glucose Transporter, Type 2 (SGLT2) Inhibitors (SGLT2i) and Glucagon-Like Peptide 1-Receptor Agonists: Newer Therapies in Whole-Body Glucose Stabilization

Diabetes is a worldwide epidemic that is increasing rapidly to become the seventh leading cause of death in the world. The increased incidence of this disease mirrors a similar uptick in obesity and metabolic syndrome, and, collectively, these conditions can cause deleterious effects on a number of organ systems including the renal and cardiovascular systems. Historically, treatment of type 2 diabetes has focused on decreasing hyperglycemia and glycated hemoglobin levels. However, it now is appreciated that there is more to the puzzle. Emerging evidence has indicated that newer classes of diabetes drugs, sodium-glucose co-transporter 2 inhibitors and glucagon-like peptide 1-receptor agonists, improve cardiovascular and renal function, while appropriately managing hyperglycemia. In this review, we highlight the recent clinical and preclinical studies that have shed light on sodium-glucose co-transporter 2 inhibitors and glucagon-like peptide 1-receptor agonists and their ability to stabilize blood glucose levels while offering whole-body protection in diabetic and nondiabetic patient populations.

Glucagon-like Peptide-1 Receptor as Emerging Target: Will It Make It to the Clinic?

The glucagon-like peptide-1 receptor (GLP-1R) is an emerging target due to its high expression in benign insulinomas as well as in islet cell hypertrophia/hyperplasia (nesidioblastosis) and pancreatic β-cells. In 2008, occult insulinomas were localized for the first time in men using the metabolically stable radiolabeled glucagon-like peptide-1 (GLP-1) agonist [Lys40(Ahx-DTPA-111In)NH2]-exendin-4 (111In-DTPA-exendin-4). Afterward, several radiopharmaceuticals for GLP-1R PET/CT imaging were synthesized and evaluated, for example, [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]-exendin-4 (68Ga-DOTA-exendin-4), [Cys40(MAL-NOTA-68Ga)NH2]-exendin-4 (68Ga-NOTA-exendin-4), and [Lys40(NODAGA-68Ga)NH2]-exendin-4 (68Ga-NODAGA-exendin-4). Several prospective comparison studies provided evidence that GLP-1R PET/CT is significantly more sensitive than contrast-enhanced MRI (ceMRI), contrast-enhanced CT (ceCT), GLP-1R SPECT/CT, somatostatin receptor PET/CT, and SPECT/CT in the detection of benign insulinomas, and insulinomas in the context of multiple endocrine neoplasia type 1. As a result, the European Neuroendocrine Tumor Society guidelines recommend GLP-1R imaging or selective intraarterial calcium stimulation and venous sampling (ASVS) in patients for whom there is a clinical suspicion of having an insulinoma but who have a negative ceMRI/ceCT or negative endoscopic ultrasound. Furthermore, there is growing evidence that GLP-1R PET/CT can visualize and localize adult nesidioblastosis. This is clinically relevant as the distinction between focal and diffuse nesidioblastosis is critical in directing a therapeutic strategy in these patients. Prospective studies have proven the clinical relevance of GLP-1R imaging as it is often the only imaging modality able to localize the insulinoma or nesidioblastosis. It is therefore likely that this noninvasive imaging modality will replace the invasive localization of insulinomas using ASVS. More experimental indications for GLP-1R imaging include the diagnosis of an insulinoma/nesidioblastosis in patients with postprandial hypoglycemia after bariatric bypass surgery and monitoring β-cells in patients with brittle type 1 diabetes after islet-cell transplantation. We believe that these indications and possibly future indications will bring GLP-1R imaging to the clinic.

GLP-1 peptide analogs for targeting pancreatic beta cells

Loss or dysfunction of the pancreatic beta cells or insulin receptors leads to diabetes mellitus (DM). This usually occurs over many years; therefore, the development of methods for the timely detection and clinical intervention are vital to prevent the development of this disease. Glucagon-like peptide-1 receptor (GLP-1R) is the receptor of GLP-1, an incretin hormone that causes insulin secretion in a glucose-dependent manner. GLP-1R is highly expressed on the surface of pancreatic beta cells, providing a potential target for bioimaging. In this review, we provide an overview of various strategies, such as the development of GLP-1R agonists (e.g., exendin-4), and GLP-1 sequence modifications for GLP-1R targeting for the diagnosis and treatment of pancreatic beta cell disorders. We also discuss the challenges of targeting pancreatic beta cells and strategies to address such challenges.

Drug Occupancy Assessment at the Glucose-Dependent Insulinotropic Polypeptide Receptor by Positron Emission Tomography

Targeting of the glucose-dependent insulinotropic polypeptide receptor (GIPR) is an emerging strategy in antidiabetic drug development. The aim of this study was to develop a positron emission tomography (PET) radioligand for the GIPR to enable the assessment of target distribution and drug target engagement in vivo. The GIPR-selective peptide S02-GIP was radiolabeled with ⁶⁸Ga. The resulting PET tracer [⁶⁸Ga]S02-GIP-T4 was evaluated for affinity and specificity to human GIPR (huGIPR). The in vivo GIPR binding of [⁶⁸Ga]S02-GIP-T4 as well as the occupancy of a drug candidate with GIPR activity were assessed in nonhuman primates (NHPs) by PET. [⁶⁸Ga]S02-GIP-T4 bound with nanomolar affinity and high selectivity to huGIPR in overexpressing cells. In vivo, pancreatic binding in NHPs could be dose-dependently inhibited by coinjection of unlabeled S02-GIP-T4. Finally, subcutaneous pretreatment with a high dose of a drug candidate with GIPR activity led to a decreased pancreatic binding of [⁶⁸Ga]S02-GIP-T4, corresponding to a GIPR drug occupancy of almost 90%. [⁶⁸Ga]S02-GIP-T4 demonstrated a safe dosimetric profile, allowing for repeated studies in humans. In conclusion, [⁶⁸Ga]S02-GIP-T4 is a novel PET biomarker for safe, noninvasive, and quantitative assessment of GIPR target distribution and drug occupancy.

Feasibility of a scale-down production of [68Ga]Ga-NODAGA-Exendin-4 in a hospital based radiopharmacy

BACKGROUND

Glucagon-like peptide 1 receptor (GLP-1R) is preferentially expressed in β-cells, but it is highly expressed in human insulinomas and gastrinomas. Several GLP-1 receptor-avid radioligands have been developed to image insulin-secreting tumors or to provide a quantitative in vivo biomarker of pancreatic β-cell mass. Exendin-4 is a high affinity ligand of the GLP1-R, which is a candidate for being labeled with a PET isotope and used for imaging purposes.

OBJECTIVE

Here, we report the development and validation results of a semi manual procedure to label [Lys40,Nle14(Ahx-NODAGA)NH2]exendin-4, with Ga-68.

METHODS

A ⁶⁸Ge/⁶⁸Ga Generator (GalliaPharma®,Eckert and Ziegler) was eluted with 0.1M HCl on an automated synthesis module (Scintomics GRP®). The peptide contained in the kit vial (Radioisotope Center POLATOM) in different amounts (10-20-30 µg) was reconstituted with 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethansulfonic acid (HEPES) solution and 68GaCl3 (400-900 MBq), followed by 10 min incubation at 95°C. The reaction solution was then purified through an Oasis HLB column. The radiopharmaceutical product was tested for quality controls (CQs), in accordance with the European Pharmacopoeia standards.

RESULTS

The synthesis of ⁶⁸Ga]Ga-NODAGA-Exendin-4 provided optimal results with 10 µg of peptide, getting the best radiochemical yield (23.53 ± 2.4 %), molar activity (100 GBq/µmol) and radiochemical purity (91.69 %).

CONCLUSION

The study developed an imaging tool [⁶⁸Ga]Ga-NODAGA-Exendin-4, avoiding pharmacological effects of exendin-4, for the clinical community.

Imaging of Insulinoma by Targeting Glucagonlike Peptide-1 Receptor

"Glucagonlike peptide-1 (GLP-1) receptor imaging, using radiolabeled exendin-4, was recently established for detecting insulinoma in patients with hyperinsulinemic hypoglycemia. It has proven to be a sensitive and specific method for preoperative localization of insulinoma. This review introduces the development, clinical research, and perspective of GLP-1 receptor imaging mainly in insulinoma.

News ways of understanding the complex biology of diabetes using PET

The understanding of metabolic disease and diabetes on a molecular level has increased significantly due to the recent advances in molecular biology and biotechnology. However, in vitro studies and animal models do not always translate to the human disease, perhaps illustrated by the failure of many drug candidates in the clinical phase. Non-invasive biomedical imaging techniques such as Positron Emission Tomography (PET) offer tools for direct visualization and quantification of molecular processes in humans. Developments in this area potentially enable longitudinal in vivo studies of receptors and processes involved in diabetes guiding drug development and diagnosis in the near future. This mini-review focuses on describing the overall perspective of how PET can be used to increase our understanding and improve treatment of diabetes. The methodological aspects and future developments and challenges are highlighted.

Non-invasive Beta-cell Imaging: Visualization, Quantification, and Beyond

Pancreatic beta (β)-cell dysfunction and reduced mass play a central role in the development and progression of diabetes mellitus. Conventional histological β-cell mass (BCM) analysis is invasive and limited to cross-sectional observations in a restricted sampling area. However, the non-invasive evaluation of BCM remains elusive, and practical in vivo and clinical techniques for β-cell-specific imaging are yet to be established. The lack of such techniques hampers a deeper understanding of the pathophysiological role of BCM in diabetes, the implementation of personalized BCM-based diabetes management, and the development of antidiabetic therapies targeting BCM preservation and restoration. Nuclear medical techniques have recently triggered a major leap in this field. In particular, radioisotope-labeled probes using exendin peptides that include glucagon-like peptide-1 receptor (GLP-1R) agonist and antagonist have been employed in positron emission tomography and single-photon emission computed tomography. These probes have demonstrated high specificity to β cells and provide clear images accurately showing uptake in the pancreas and transplanted islets in preclinical in vivo and clinical studies. One of these probes, ¹¹¹indium-labeled exendin-4 derivative ([Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4), has captured the longitudinal changes in BCM during the development and progression of diabetes and under antidiabetic therapies in various mouse models of type 1 and type 2 diabetes mellitus. GLP-1R-targeted imaging is therefore a promising tool for non-invasive BCM evaluation. This review focuses on recent advances in non-invasive in vivo β-cell imaging for BCM evaluation in the field of diabetes; in particular, the exendin-based GLP-1R-targeted nuclear medicine techniques.

Glucose Sensing Mediated by Portal Glucagon-Like Peptide 1 Receptor Is Markedly Impaired in Insulin-Resistant Obese Animals

The glucose portal sensor informs the brain of changes in glucose inflow through vagal afferents that require an activated glucagon-like peptide 1 receptor (GLP-1r). The GLP-1 system is known to be impaired in insulin-resistant conditions, and we sought to understand the consequences of GLP-1 resistance on glucose portal signaling. GLP-1-dependent portal glucose signaling was identified, in vivo, using a novel 68Ga-labeled GLP-1r positron-emitting probe that supplied a quantitative in situ tridimensional representation of the portal sensor with specific reference to the receptor density expressed in binding potential units. It also served as a map for single-neuron electrophysiology driven by an image-based abdominal navigation. We determined that in insulin-resistant animals, portal vagal afferents failed to inhibit their spiking activity during glucose infusion, a GLP-1r-dependent function. This reflected a reduction in portal GLP-1r binding potential, particularly between the splenic vein and the entrance of the liver. We propose that insulin resistance, through a reduction in GLP-1r density, leads to functional portal desensitization with a consequent suppression of vagal sensitivity to portal glucose.

Noninvasive Monitoring of Glycemia-Induced Regulation of GLP-1R Expression in Murine and Human Islets of Langerhans

Glucagon-like peptide 1 receptor (GLP-1R) imaging with radiolabeled exendin has proven to be a powerful tool to quantify β-cell mass (BCM) in vivo. As GLP-1R expression is thought to be influenced by glycemic control, we examined the effect of blood glucose (BG) levels on GLP-1R-mediated exendin uptake in both murine and human islets and its implications for BCM quantification. Periods of hyperglycemia significantly reduced exendin uptake in murine and human islets, which was paralleled by a reduction in GLP-1R expression. Detailed mapping of the tracer uptake and insulin and GLP-1R expression conclusively demonstrated that the observed reduction in tracer uptake directly correlates to GLP-1R expression levels. Importantly, the linear correlation between tracer uptake and β-cell area was maintained in spite of the reduced GLP-1R expression levels. Subsequent normalization of BG levels restored absolute tracer uptake and GLP-1R expression in β-cells and the observed loss in islet volume was halted. This manuscript emphasizes the potency of nuclear imaging techniques to monitor receptor regulation noninvasively. Our findings have significant implications for clinical practice, indicating that BG levels should be near-normalized for at least 3 weeks prior to GLP-1R agonist treatment or quantitative radiolabeled exendin imaging for BCM analysis.

Pancreatic GLP-1r binding potential is reduced in insulin-resistant pigs

INTRODUCTION

The insulinotropic capacity of exogenous glucagon like peptide-1 (GLP-1) is reduced in type 2 diabetes and the insulin-resistant obese. We have tested the hypothesis that this response is the consequence of a reduced pancreatic GLP-1 receptor (GLP-1r) density in insulin-resistant obese animals.

RESEARCH DESIGN AND METHODS

GLP-1r density was measured in lean and insulin-resistant adult miniature pigs after the administration of a 68Ga-labeled GLP-1r agonist. The effect of hyperinsulinemia on GLP-1r was assessed using sequential positron emission tomography (PET), both in the fasted state and during a clamp. The impact of tissue perfusion, which could account for changes in GLP-1r agonist uptake, was also investigated using 68Ga-DOTA imaging.

RESULTS

GLP-1r binding potential in the obese pancreas was reduced by 75% compared with lean animals. Similar reductions were evident for fat tissue, but not for the duodenum. In the lean group, induced hyperinsulinemia reduced pancreatic GLP-1r density to a level comparable with that of the obese group. The reduction in blood to tissue transfer of the GLP-1r ligand paralleled that of tissue perfusion estimated using 68Ga-DOTA.

CONCLUSIONS

These observations establish that a reduction in abdominal tissue perfusion and a lower GLP-1r density account for the diminished insulinotropic effect of GLP-1 agonists in type 2 diabetes.

Receptor occupancy of dual glucagon-like peptide 1/glucagon receptor agonist SAR425899 in individuals with type 2 diabetes

Unimolecular dual agonists for the glucagon-like peptide 1 receptor (GLP1R) and glucagon receptor (GCGR) are emerging as a potential new class of important therapeutics in type 2 diabetes (T2D). Reliable and quantitative assessments of in vivo occupancy on each receptor would improve the understanding of the efficacy of this class of drugs. In this study we investigated the target occupancy of the dual agonist SAR425899 at the GLP1R in pancreas and GCGR in liver by Positron Emission Tomography/Computed Tomography (PET/CT). Patients with T2D were examined by [⁶⁸Ga]Ga-DO3A-Tuna-2 and [⁶⁸Ga]Ga-DO3A-Exendin4 by PET, to assess the GCGR in liver and GLP1R in pancreas, respectively. Follow up PET examinations were performed after 17 (GCGR) and 20 (GLP-1R) days of treatment with SAR425899, to assess the occupancy at each receptor. Six out of 13 included patients prematurely discontinued the study due to adverse events. SAR425899 at a dose of 0.2 mg daily demonstrated an average GCGR occupancy of 11.2 ± 14.4% (SD) in N = 5 patients and a GLP1R occupancy of 49.9 ± 13.3%. Fasting Plasma Glucose levels (- 3.30 ± 1.14 mmol/L) and body weight (- 3.87 ± 0.87%) were lowered under treatment with SAR425899. In conclusion, SAR425899 demonstrated strong interactions at the GLP1R, but no clear occupancy at the GCGR. The study demonstrates that quantitative target engagement of dual agonists can be assessed by PET.

Targeted pancreatic beta cell imaging for early diagnosis

Pancreatic beta cells are important in blood glucose level regulation. As type 1 and 2 diabetes are getting prevalent worldwide, we need to explore new methods for early detection of beta cell-related afflictions. Using bioimaging techniques to measure beta cell mass is crucial because a decrease in beta cell density is seen in diseases such as diabetes and thus can be a new way of diagnosis for such diseases. We also need to appraise beta cell purity in transplanted islets for type 1 diabetes patients. Sufficient amount of functional beta cells must also be determined before being transplanted to the patients. In this review, indirect imaging of beta cells will be discussed. This includes membrane protein on pancreatic beta cells whereby specific probes are designed for different imaging modalities mainly magnetic resonance imaging, positron emission tomography and fluorescence imaging. Direct imaging of insulin is also explored though probes synthesized for such function are relatively fewer. The path for successful pancreatic beta cell imaging is fraught with challenges like non-specific binding, lack of beta cell-restricted targets, the requirement of probes to cross multiple lipid layers to bind to intracellular insulin. Hence, there is an urgent need to develop new imaging techniques and innovative probing constructs in the entire imaging chain of bioengineering to provide early detection of beta cell-related pathology.

Synthesis and in vivo behaviour of an exendin-4-based MRI probe capable of β-cell-dependent contrast enhancement in the pancreas

Global rates of diabetes mellitus are increasing, and treatment of the disease consumes a growing proportion of healthcare spending across the world. Pancreatic β-cells, responsible for insulin production, decline in mass in type 1 and, to a more limited degree, in type 2 diabetes. However, the extent and rate of loss in both diseases differs between patients resulting in the need for the development of novel diagnostic tools, which could quantitatively assess changes in mass of β-cells over time and potentially lead to earlier diagnosis and improved treatments. Exendin-4, a potent analogue of glucagon-like-peptide 1 (GLP-1), binds to the receptor GLP-1R, whose expression is enriched in β-cells. GLP-1R has thus been used in the past as a means of targeting probes for a wide variety of imaging modalities to the endocrine pancreas. However, exendin-4 conjugates designed specifically for MRI contrast agents are an under-explored area. In the present work, the synthesis and characterization of an exendin-4-dota(ga)-Gd(iii) complex, GdEx, is reported, along with its in vivo behaviour in healthy and in β-cell-depleted C57BL/6J mice. Compared to the ubiquitous probe, [Gd(dota)]-, GdEx shows selective uptake by the pancreas with a marked decrease in accumulation observed after the loss of β-cells elicited by deleting the microRNA processing enzyme, DICER. These results open up pathways towards the development of other targeted MRI contrast agents based on similar chemistry methodology.

A new perspective of probe development for imaging pancreatic beta cell in vivo

Beta cells assume a fundamental role in maintaining blood glucose homeostasis through the secretion of insulin, which is contingent on both beta cell mass and function, in response to elevated blood glucose levels or secretagogues. For this reason, evaluating beta cell mass and function, as well as scrutinizing how they change over time in a diabetic state, are essential prerequisites in elucidating diabetes pathophysiology. Current clinical methods to measure human beta cell mass and/or function are largely lacking, indirect and sub-optimal, highlighting the continued need for noninvasive in vivo beta cell imaging technologies such as optical imaging techniques. While numerous probes have been developed and evaluated for their specificity to beta cells, most of them are more suited to visualize beta cell mass rather than function. In this review, we highlight the distinction between beta cell mass and function, and the importance of developing more probes to measure beta cell function. Additionally, we also explore various existing probes that can be employed to measure beta cell mass and function in vivo, as well as the caveats in probe development for in vivo beta cell imaging.

Islet Transplantation Imaging in vivo

Although islet transplantation plays an effective and powerful role in the treatment of diabetes, a large amount of islet grafts are lost at an early stage due to instant blood-mediated inflammatory reactions, immune rejection, and β-cell toxicity resulting from immunosuppressive agents. Timely intervention based on the viability and function of the transplanted islets at an early stage is crucial. Various islet transplantation imaging techniques are available for monitoring the conditions of post-transplanted islets. Due to the development of various imaging modalities and the continuous study of contrast agents, non-invasive islet transplantation imaging in vivo has made great progress. The tracing and functional evaluation of transplanted islets in vivo have thus become possible. However, most studies on contrast agent and imaging modalities are limited to animal experiments, and long-term toxicity and stability need further evaluation. Accordingly, the clinical application of the current achievements still requires a large amount of effort. In this review, we discuss the contrast agents for MRI, SPECT/PET, BLI/FI, US, MPI, PAI, and multimodal imaging. We further summarize the advantages and limitations of various molecular imaging methods.

Advances in GLP-1 receptor targeting radiolabeled agent development and prospective of theranostics

In the light of theranostics/radiotheranostics and prospective of personalized medicine in diabetes and oncology, this review presents prior and current advances in the development of radiolabeled imaging and radiotherapeutic exendin-based agents targeting glucagon-like peptide-1 receptor. The review covers chemistry, preclinical, and clinical evaluation. Such critical aspects as structure-activity-relationship, stability, physiological potency, kidney uptake, and dosimetry are discussed.

Noninvasive longitudinal quantification of β-cell mass with [111In]-labeled exendin-4

Currently, quantifying β-cell mass (BCM) requires harvesting the pancreas. In this study, we investigated a potential noninvasive method to quantify BCM changes longitudinally using [Lys12(111In-BnDTPA-Ahx)]exendin-4 ([111In]-Ex4) and single-photon emission computed tomography (SPECT). We used autoradiography and transgenic mice expressing green fluorescent protein under the control of mouse insulin 1 gene promotor to evaluate the specificity of [111In]-Ex4 toward β cells. Using nonobese diabetic (NOD) mice, we injected [111In]-Ex4 (3.0 MBq) intravenously and performed SPECT 30 min later, repeating this at a 2-wk interval. After the second scan, we harvested the pancreas and calculated BCM from immunohistochemically stained pancreatic sections. Specific accumulation of [111In]-Ex4 in β cells was confirmed by autoradiography, with a significant correlation (r = 0.94) between the fluorescent and radioactive signal intensities. The radioactive signal from the pancreas in the second SPECT scan significantly correlated (r = 0.89) with BCM calculated from the immunostained pancreatic sections. We developed a regression formula to estimate BCM from the radioactive signals from the pancreas in SPECT scans. BCM can be quantified longitudinally and noninvasively by SPECT imaging with [111In]-Ex4. This technique successfully demonstrated longitudinal changes in BCM in NOD mice before and after onset of hyperglycemia.-Fujita, N., Fujimoto, H., Hamamatsu, K., Murakami, T., Kimura, H., Toyoda, K., Saji, H., Inagaki, N. Noninvasive longitudinal quantification of β-cell mass with [111In]-labeled exendin-4.

Assessment of glucagon receptor occupancy by Positron Emission Tomography in non-human primates

The glucagon receptor (GCGR) is an emerging target in anti-diabetic therapy. Reliable biomarkers for in vivo activity on the GCGR, in the setting of dual glucagon-like peptide 1/glucagon (GLP-1/GCG) receptor agonism, are currently unavailable. Here, we investigated [⁶⁸Ga]Ga-DO3A-S01-GCG as a biomarker for GCGR occupancy in liver, the tissue with highest GCGR expression, in non-human primates (NHP) by PET. [⁶⁸Ga]Ga-DO3A-S01-GCG was evaluated by dynamic PET in NHPs by a dose escalation study design, where up to 67 µg/kg DO3A-S01-GCG peptide mass was co-injected. The test-retest reproducibility of [⁶⁸Ga]Ga-DO3A-S01-GCG binding in liver was evaluated. Furthermore, we investigated the effect of pre-treatment with acylated glucagon agonist 1-GCG on [⁶⁸Ga]Ga-DO3A-S01-GCG binding in liver. [⁶⁸Ga]Ga-DO3A-S01-GCG bound to liver in vivo in a dose-dependent manner. Negligible peptide mass effect was observed for DO3A-S01-GCG doses <0.2 µg/kg. In vivo K_d for [⁶⁸Ga]Ga-DO3A-S01-GCG corresponded to 0.7 µg/kg, which indicates high potency. The test-retest reproducibility for [⁶⁸Ga]Ga-DO3A-S01-GCG binding in liver was 5.7 ± 7.9%. Pre-treatment with 1-GCG, an acylated glucagon agonist, resulted in a GCGR occupancy of 61.5 ± 9.1% in liver. Predicted human radiation dosimetry would allow for repeated annual [⁶⁸Ga]Ga-DO3A-S01-GCG PET examinations. In summary, PET radioligand [⁶⁸Ga]Ga-DO3A-S01-GCG is a quantitative biomarker of in vivo GCGR occupancy.

[68Ga]Ga-NOTA-MAL-Cys39-exendin-4, a potential GLP-1R targeted PET tracer for the detection of insulinoma

Glucagon-like peptide-1 receptor (GLP-1R) is a kind of G protein coupled receptor which regulates the insulin secretion and serves as potential target in the diagnosis of functional pancreas neuroendocrine tumor. The aim of this study was to evaluate the feasibility of GLP-1R targeted tracer [⁶⁸Ga]Ga-NOTA-MAL-Cys³⁹-exendin-4 in the detection of insulinoma.

METHODS

NOTA-MAL-Cys³⁹-exendin-4 was synthesized and then radiolabeled with gallium-68 in iQS® Ga-68 Fluidic Labeling Module. The in vitro binding affinity and cell uptake studies were evaluated in INS-1 cells. The in vivo micro-PET/CT imaging and biodistribution studies were performed on INS-1 xenograft tumor models.

RESULTS

[⁶⁸Ga]Ga-NOTA-MAL-Cys³⁹-exendin-4 can be efficiently radiolabelled with a yield of about 85% (non-decay corrected) and radiochemical purity of >95% with a favorable stability. The molar activity was at least 145.5 GBq/μmol. The affinity (IC50) for [⁶⁸Ga]Ga-NOTA-MAL-Cys³⁹-exendin-4 was 12.99 ± 0.81 nM. Micro-PET/CT images showed intense tumor uptake with good contrast to background. Biodistribution study showed the predominant uptake was in the kidney, followed by pancreas, and the liver and spleen just showed mild uptake in the blood-pool phase with rapid clearance. At 1 h post- injection, the tumor to blood, muscle and pancreas ratios were 30.64, 40.21 and 6.46, respectively. Blocking studies showed significantly decreased tumor uptake, which further confirmed binding affinity of [⁶⁸Ga]Ga-NOTA-MAL-Cys³⁹-exendin-4 to GLP-1R.

CONCLUSION

[⁶⁸Ga]Ga-NOTA-MAL-Cys³⁹-exendin-4 was easily synthesized with high yield, favorable biodistribution and high affinity to islet tumor cell, making the tracer may have great potential in the detection of GLP-1R positive tumor such as an insulinoma.

Comparison of ELISA and HPLC-MS methods for the determination of exenatide in biological and biotechnology-based formulation matrices

The development of biotechnology-based active pharmaceutical ingredients, such as GLP-1 analogs, brought changes in type 2 diabetes treatment options. For better therapeutic efficiency, these active pharmaceutical ingredients require appropriate administration, without the development of adverse effects or toxicity. Therefore, it is required to develop several quantification methods for GLP-1 analogs products, in order to achieve the therapeutic goals, among which ELISA and HPLC arise. These methods are developed, optimized and validated in order to determine GLP-1 analogs, not only in final formulation of the active pharmaceutical ingredient, but also during preclinical and clinical trials assessment. This review highlights the role of ELISA and HPLC methods that have been used during the assessment for GLP-1 analogs, especially for exenatide.

Evaluation of [68Ga]DO3A-VS-Cys40-Exendin-4 as a PET Probe for Imaging Human Transplanted Islets in the Liver

[⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4, a glucagon-like peptide 1 receptor agonist, was evaluated as a potential PET tracer for the quantitation of human islets transplanted to the liver. The short-lived PET radionuclide ⁶⁸Ga, available on a regular basis from a ⁶⁸Ge/⁶⁸Ga generator, is an attractive choice. Human C-peptide was measured to evaluate human islet function post-transplantation and prior to microPET imaging. [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 was radiosynthesized and evaluated for PET imaging of transplanted human islets in the liver of healthy NOD/SCID mice. The biodistribution of the tracer was evaluated to determine the uptake into various organs, and qPCR of liver samples was conducted to confirm engrafted islet numbers after PET imaging. Measurement of human C-peptide indicated that higher engrafted islet mass resulted in higher human C-peptide levels in post-transplantation. The microPET imaging yielded high resolution images of liver-engrafted islets and also showed significant retention in mouse livers at 8 weeks post-transplantation. Biodistribution studies in mice revealed that liver uptake of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 was approximately 6-fold higher in mice that received 1000 islet equivalent (IEQ) than in non-transplanted mice. qPCR analysis of insulin expression suggested that islet engraftment numbers were close to 1000 IEQ transplanted. In conclusion, human islets transplanted into the livers of mice exhibited significant uptake of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 compared to the livers of untreated mice; and imaging of the mice using PET showed the human islets clearly with high contrast against liver tissue, enabling accurate quantitation of islet mass. Further validation of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 as an islet imaging probe for future clinical application is ongoing.

First-in-class positron emission tomography tracer for the glucagon receptor

Abstract

The glucagon receptor (GCGR) is emerging as an important target in anti-diabetic therapy, especially as part of the pharmacology of dual glucagon-like peptide-1/glucagon (GLP-1/GCG) receptor agonists. However, currently, there are no suitable biomarkers that reliably demonstrate GCG receptor target engagement.

Methods

Two potent GCG receptor peptide agonists, S01-GCG and S02-GCG, were labeled with positron emission tomography (PET) radionuclide gallium-68. The GCG receptor binding affinity and specificity of the resulting radiopharmaceuticals [⁶⁸Ga]Ga-DO3A-S01-GCG and [⁶⁸Ga]Ga-DO3A-S02-GCG were evaluated in HEK-293 cells overexpressing the human GCG receptor and on frozen hepatic sections from human, non-human primate, and rat. In in vivo biodistribution, binding specificity and dosimetry were assessed in rat.

Results

[⁶⁸Ga]Ga-DO3A-S01-GCG in particular demonstrated GCG receptor-mediated binding in cells and liver tissue with affinity in the nanomolar range required for imaging. [⁶⁸Ga]Ga-DO3A-S01-GCG binding was not blocked by co-incubation of a GLP-1 agonist. In vivo binding in rat liver was GCG receptor specific with low non-specific binding throughout the body. Moreover, the extrapolated human effective doses, predicted from rat biodistribution data, allow for repeated PET imaging potentially also in combination with GLP-1R radiopharmaceuticals.

Conclusion

[⁶⁸Ga]Ga-DO3A-S01-GCG thus constitutes a first-in-class PET tracer targeting the GCG receptor, with suitable properties for clinical development. This tool has potential to provide direct quantitative evidence of GCG receptor occupancy in humans.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0482-0) contains supplementary material, which is available to authorized users.

Molecular imaging of β-cells: diabetes and beyond

Since diabetes is becoming a global epidemic, there is a great need to develop early β-cell specific diagnostic techniques for this disorder. There are two types of diabetes (i.e., type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM)). In T1DM, the destruction of pancreatic β-cells leads to reduced insulin production or even absolute insulin deficiency, which consequently results in hyperglycemia. Actually, a central issue in the pathophysiology of all types of diabetes is the relative reduction of β-cell mass (BCM) and/or impairment of the function of individual β-cells. In the past two decades, scientists have been trying to develop imaging techniques for noninvasive measurement of the viability and mass of pancreatic β-cells. Despite intense scientific efforts, only two tracers for positron emission tomography (PET) and one contrast agent for magnetic resonance (MR) imaging are currently under clinical evaluation. β-cell specific imaging probes may also allow us to precisely and specifically visualize transplanted β-cells and to improve transplantation outcomes, as transplantation of pancreatic islets has shown promise in treating T1DM. In addition, some of these probes can be applied to the preoperative detection of hidden insulinomas as well. In the present review, we primarily summarize potential tracers under development for imaging β-cells with a focus on tracers for PET, SPECT, MRI, and optical imaging. We will discuss the advantages and limitations of the various imaging probes and extend an outlook on future developments in the field.

Use of the PET ligand florbetapir for in vivo imaging of pancreatic islet amyloid deposits in hIAPP transgenic mice

AIMS/HYPOTHESIS

Islet amyloid deposits contribute to beta cell dysfunction and death in most individuals with type 2 diabetes but non-invasive methods to determine the presence of these pathological protein aggregates are currently not available. Therefore, we examined whether florbetapir, a radiopharmaceutical agent used for detection of amyloid-β deposits in the brain, also allows identification of islet amyloid in the pancreas.

METHODS

Saturation binding assays were used to determine the affinity of florbetapir for human islet amyloid polypeptide (hIAPP) aggregates in vitro. Islet amyloid-prone transgenic mice that express hIAPP in their beta cells and amyloid-free non-transgenic control mice were used to examine the ability of florbetapir to detect islet amyloid deposits in vitro, in vivo and ex vivo. Mice or mouse pancreases were subjected to autoradiographic, histochemical and/or positron emission tomography (PET) analyses to assess the utility of florbetapir in identifying islet amyloid.

RESULTS

In vitro, florbetapir bound synthetic hIAPP fibrils with a dissociation constant of 7.9 nmol/l. Additionally, florbetapir bound preferentially to amyloid-containing hIAPP transgenic vs amyloid-free non-transgenic mouse pancreas sections in vitro, as determined by autoradiography (16,475 ± 5581 vs 5762 ± 575 density/unit area, p < 0.05). In hIAPP transgenic and non-transgenic mice fed a high-fat diet for 1 year, intravenous administration of florbetapir followed by PET scanning showed that the florbetapir signal was significantly higher in amyloid-laden hIAPP transgenic vs amyloid-free non-transgenic pancreases in vivo during the first 5 min of the scan (36.83 ± 2.22 vs 29.34 ± 2.03 standardised uptake value × min, p < 0.05). Following PET, pancreases were excised and florbetapir uptake was determined ex vivo by γ counting. Pancreatic uptake of florbetapir was significantly correlated with the degree of islet amyloid deposition, the latter assessed by histochemistry (r = 0.74, p < 0.001).

CONCLUSIONS/INTERPRETATION

Florbetapir binds to islet amyloid deposits in a specific and quantitative manner. In the future, florbetapir may be useful as a non-invasive tool to identify islet amyloid deposits in humans.

Species differences in pancreatic binding of DO3A-VS-Cys40-Exendin4

AIMS

Radiolabeled Exendin-4 has been proposed as suitable imaging marker for pancreatic beta cell mass quantification mediated by Glucagon-like peptide-1 receptor (GLP-1R). However, noticeable species variations in basal pancreatic uptake as well as uptake reduction degree due to selective beta cell ablation were observed.

METHODS

In vitro and ex vivo autoradiography studies of pancreas were performed using [¹⁷⁷Lu]Lu-DO3A-VS-Cys⁴⁰-Exendin4, in order to investigate the mechanism of uptake as well as the islet uptake contrast in mouse, rat, pig, and non-human primate. The autoradiography results were compared to the in vivo pancreatic uptake as assessed by [⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin4 Positron Emission Tomography (PET) in the same species. In vitro, ex vivo, and in vivo data formed the basis for calculating the theoretical in vivo contribution of each pancreatic compartment.

RESULTS

[¹⁷⁷Lu]Lu-DO3A-VS-Cys⁴⁰-Exendin4 displayed the highest islet-to-exocrine pancreas ratio (IPR) in rat (IPR 45) followed by non-human primate and mouse at similar levels (IPR approximately 5) while pigs exhibited negligible IPR (1.1). In vivo pancreas uptake was mainly GLP-1R mediated in all species, but the magnitude of uptake under basal physiology varied significantly in decreasing order: non-human primate, mouse, pig, and rat. The theoretical calculation of islet contribution to the total pancreatic PET signal predicted the in vivo observation of differences in pancreatic uptake of [⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin4.

CONCLUSIONS

IPR as well as the exocrine GLP-1R density is the main determinants of the species variability in pancreatic uptake. Thus, the IPR in human is an important factor for assessing the potential of GLP-1R as an imaging biomarker for pancreatic beta cells.

Synthesis and biological evaluation of an 111In-labeled exendin-4 derivative as a single-photon emission computed tomography probe for imaging pancreatic β-cells

A non-invasive method of pancreatic β-cell mass measurement is needed to enhance our understanding of the pathogenesis of diabetes, facilitate the early diagnosis of this disease, and promote the development of novel therapeutics. Here, we described the synthesis of a novel indium-111 (¹¹¹In) exendin-4 derivative, [Lys¹²(In-BnDTPA-Ahx)]exendin-4, through a process involving isothiocyanate-benzyl-DTPA (BnDTPA) and 6-aminohexanoic acid (Ahx) attached to an ɛ-amino group at the lysine-12 residue. We further evaluated the potential use of this derivative as a SPECT probe for pancreatic β-cell imaging. An in vitro binding assay revealed that [Lys¹²(^natIn-BnDTPA-Ahx)]exendin-4 has a high affinity for GLP-1 receptors (IC₅₀=0.43nM). In biodistribution experiments involving normal mice, high [Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4 uptake was observed in the pancreas (21.8 ± 4.0%ID/g) and was maintained for 2h after injection. Pre-injection of excess exendin(9-39) markedly reduced the pancreatic uptake of [Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4 (95.2%), indicating that the uptake of this tracer is specific and mediated by GLP-1 receptors. Ex vivo autoradiography experiments involving pancreatic sections from MIP-GFP mice confirmed the accumulation of [Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4 in pancreatic β-cells. Finally, in mice, [Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4 SPECT/CT yielded clear images of the pancreas at 30min post-injection. In conclusion, SPECT with [Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4 enables to visualize β-cells in vivo non-invasively.

Radiomanganese PET Detects Changes in Functional β-Cell Mass in Mouse Models of Diabetes

The noninvasive measurement of functional β-cell mass would be clinically valuable for monitoring the progression of type 1 and type 2 diabetes as well as the viability of transplanted insulin-producing cells. Although previous work using MRI has shown promise for functional β-cell mass determination through voltage-dependent Ca²⁺ channel (VDCC)-mediated internalization of Mn²⁺, the clinical utility of this technique is limited by the cytotoxic levels of the Mn²⁺ contrast agent. Here, we show that positron emission tomography (PET) is advantageous for determining functional β-cell mass using ⁵²Mn²⁺ (t_1/2: 5.6 days). We investigated the whole-body distribution of ⁵²Mn²⁺ in healthy adult mice by dynamic and static PET imaging. Pancreatic VDCC uptake of ⁵²Mn²⁺ was successfully manipulated pharmacologically in vitro and in vivo using glucose, nifedipine (VDCC blocker), the sulfonylureas tolbutamide and glibenclamide (K_ATP channel blockers), and diazoxide (K_ATP channel opener). In a mouse model of streptozotocin-induced type 1 diabetes, ⁵²Mn²⁺ uptake in the pancreas was distinguished from healthy controls in parallel with classic histological quantification of β-cell mass from pancreatic sections. ⁵²Mn²⁺-PET also reported the expected increase in functional β-cell mass in the ob/ob model of pretype 2 diabetes, a result corroborated by histological β-cell mass measurements and live-cell imaging of β-cell Ca²⁺ oscillations. These results indicate that ⁵²Mn²⁺-PET is a sensitive new tool for the noninvasive assessment of functional β-cell mass.

Venom peptides as pharmacological tools and therapeutics for diabetes

Diabetes mellitus is a chronic disease caused by a deficiency in production of insulin by the beta cells of the pancreas (type 1 diabetes, T1D), or by partial deficiency of insulin production and the ineffectiveness of the insulin produced (type 2 diabetes, T2D). Animal venoms are a unique source of compounds targeting ion channels and receptors in the nervous and cardiovascular systems. In recent years, several venom peptides have also emerged as pharmacological tools and therapeutics for T1D and T2D. Some of these peptides act directly as mimics of endogenous metabolic hormones while others act on ion channels expressed in pancreatic beta cells. Here, we provide an overview of the discovery of these venom peptides, their mechanisms of action in the context of diabetes, and their therapeutic potential for the treatment of this disease. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Current Status of Radiopharmaceuticals for the Theranostics of Neuroendocrine Neoplasms

Nuclear medicine plays a pivotal role in the management of patients affected by neuroendocrine neoplasms (NENs). Radiolabeled somatostatin receptor analogs are by far the most advanced radiopharmaceuticals for diagnosis and therapy (radiotheranostics) of NENs. Their clinical success emerged receptor-targeted radiolabeled peptides as an important class of radiopharmaceuticals and it paved the way for the investigation of other radioligand-receptor systems. Besides the somatostatin receptors (sstr), other receptors have also been linked to NENs and quite a number of potential radiolabeled peptides have been derived from them. The Glucagon-Like Peptide-1 Receptor (GLP-1R) is highly expressed in benign insulinomas, the Cholecystokinin 2 (CCK2)/Gastrin receptor is expressed in different NENs, in particular medullary thyroid cancer, and the Glucose-dependent Insulinotropic Polypeptide (GIP) receptor was found to be expressed in gastrointestinal and bronchial NENs, where interestingly, it is present in most of the sstr-negative and GLP-1R-negative NENs. Also in the field of sstr targeting new discoveries brought into light an alternative approach with the use of radiolabeled somatostatin receptor antagonists, instead of the clinically used agonists. The purpose of this review is to present the current status and the most innovative strategies for the diagnosis and treatment (theranostics) of neuroendocrine neoplasms using a cadre of radiolabeled regulatory peptides targeting their receptors.

Radioiodinated Exendin-4 Is Superior to the Radiometal-Labelled Glucagon-Like Peptide-1 Receptor Probes Overcoming Their High Kidney Uptake

GLP-1 receptors are ideal targets for preoperative imaging of benign insulinoma and for quantifying the beta cell mass. The existing clinical tracers targeting GLP-1R are all agonists with low specific activity and very high kidney uptake. In order to solve those issues we evaluated GLP-1R agonist Ex-4 and antagonist Ex(9-39) radioiodinated at Tyr40 side by side with [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4 (68Ga-Ex-4) used in the clinic. The Kd, Bmax, internalization and binding kinetics of [Nle14,125I-Tyr40-NH2]Ex-4 and [Nle14,125I-Tyr40-NH2]Ex(9-39) were studied in vitro using Ins-1E cells. Biodistribution and imaging studies were performed in nude mice bearing Ins-1E xenografts. In vitro evaluation demonstrated high affinity binding of the [Nle14,125I-Tyr40-NH2]Ex-4 agonist to the Ins-1E cells with fast internalization kinetics reaching a plateau after 30 min. The antagonist [Nle14,125I-Tyr40-NH2]Ex(9-39) did not internalize and had a 4-fold higher Kd value compared to the agonist. In contrast to [Nle14,125I-Tyr40-NH2]Ex(9-39), which showed low and transient tumor uptake, [Nle14,125I-Tyr40-NH2]Ex-4 demonstrated excellent in vivo binding properties with tumor uptake identical to that of 68Ga-Ex-4, but substantially lower kidney uptake resulting in a tumor-to-kidney ratio of 9.7 at 1 h compared to 0.3 with 68Ga-Ex-4. Accumulation of activity in thyroid and stomach for both peptides, which was effectively blocked by irenat, confirms that in vivo deiodination is the mechanism behind the low kidney retention of iodinated peptides. The 124I congener of [Nle14,125I-Tyr40-NH2]Ex-4 demonstrated a similar favourable biodistribution profile in the PET imaging studies in contrast to the typical biodistribution pattern of [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4. Our results demonstrate that iodinated Ex-4 is a very promising tracer for imaging of benign insulinomas. It solves the problem of high kidney uptake of the radiometal-labelled tracers by improving the tumor-to-kidney ratio measured for [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4 by 32 fold.

Targets and probes for non-invasive imaging of β-cells

β-cells, located in the islets of the pancreas, are responsible for production and secretion of insulin and play a crucial role in blood sugar regulation. Pathologic β-cells often cause serious medical conditions affecting blood glucose level, which severely impact life quality and are life-threatening if untreated. With 347 million patients, diabetes is one of the most prevalent diseases, and will continue to be one of the largest socioeconomic challenges in the future. The diagnosis still relies mainly on indirect methods like blood sugar measurements. A non-invasive diagnostic imaging modality would allow direct evaluation of β-cell mass and would be a huge step towards personalized medicine. Hyperinsulinism is another serious condition caused by β-cells that excessively secrete insulin, like for instance β-cell hyperplasia and insulinomas. Treatment options with drugs are normally not curative, whereas curative procedures usually consist of the resection of affected regions for which, however, an exact localization of the foci is necessary. In this review, we describe potential tracers under development for targeting β-cells with focus on radiotracers for PET and SPECT imaging, which allow the non-invasive visualization of β-cells. We discuss either the advantages or limitations for the various tracers and modalities. This article concludes with an outlook on future developments and discuss the potential of new imaging probes including dual probes that utilize functionalities for both a radioactive and optical moiety as well as for theranostic applications.

Low kidney uptake of GLP-1R-targeting, beta cell-specific PET tracer, 18F-labeled [Nle14,Lys40]exendin-4 analog, shows promise for clinical imaging

Background

Several radiometal-labeled, exendin-based tracers that target glucagon-like peptide-1 receptors (GLP-1R) have been intensively explored for β cell imaging. The main obstacle has been the high uptake of tracer in the kidneys. This study aimed to develop a novel GLP1-R-specific tracer, with fluorine-18 attached to exendin-4, to label β cells for clinical imaging with PET (positron emission tomography). We hypothesized that this tracer would undergo reduced kidney uptake. ¹⁸F-labeled [Nle¹⁴,Lys⁴⁰]exendin-4 analog ([¹⁸F]exendin-4) was produced via Cu-catalyzed click chemistry. The biodistribution of [¹⁸F]exendin-4 was assessed with ex vivo organ γ-counting and in vivo PET imaging. We also tested the in vivo stability of the radiotracer. The localization of ¹⁸F radioactivity in rat and human pancreatic tissue sections was investigated with autoradiography. Receptor specificity was assessed with unlabeled exendin-3. Islet labeling was confirmed with immunohistochemistry. The doses of radiation in humans were estimated based on biodistribution results in rats.

Results

[¹⁸F]exendin-4 was synthesized with high yield and high specific activity. Results showed specific, sustained [¹⁸F]exendin-4 uptake in pancreatic islets. In contrast to previous studies that tested radiometal-labeled exendin-based tracers, we observed rapid renal clearance of [¹⁸F]exendin-4.

Conclusions

[¹⁸F]exendin-4 showed promise as a tracer for clinical imaging of pancreatic β cells, due to its high specific uptake in native β cells and its concomitant low kidney radioactivity uptake.

Evaluation of Cu-64 and Ga-68 Radiolabeled Glucagon-Like Peptide-1 Receptor Agonists as PET Tracers for Pancreatic β cell Imaging

PURPOSE

Copper-64 (Cu-64) and Galium-68 (Ga-68) radiolabeled DO3A and NODA conjugates of exendin-4 were used for preclinical imaging of pancreatic β cells via targeting of glucagon-like peptide-1 receptor (GLP-1R).

PROCEDURES

DO3A-VS- and NODA-VS-tagged Cys(40)exendin-4 (DO3A-VS-Cys(40)-exendin-4 and NODA-VS-Cys(40)-exendin-4, respectively) were labeled with Cu-64 and Ga-68 using standard techniques. Biodistribution and dynamic positron emission tomography (PET) were carried out in normal Sprague-Dawley (SD) rats. Ex vivo autoradiography imaging was conducted with freshly frozen pancreatic thin sections.

RESULTS

DO3A-VS- and NODA-VS-Cys(40)-exendin-4 analogues were labeled with Cu-64 and Ga-68 to a specific activity of 518.7 ± 3.7 Ci/mmol (19.19 ± 0.14 TBq/mmol) and radiochemical yield above 98 %. Biodistribution data demonstrated pancreatic uptake of 0.11 ± 0.02 %ID/g for [(64)Cu]DO3A-VS-, 0.14 ± 0.02 %ID/g for [(64)Cu]NODA-VS-, 0.11 ± 0.03 for [(68)Ga]DO3A-VS-, and 0.26 ± 0.03 for [(68)Ga]NODA-VS-Cys(40)-exendin-4. Excess exendin-4 and exendin-(9-39)-amide displaced all four Cu-64 and Ga-68 labeled exendin-4 derivatives in blocking studies.

CONCLUSIONS

[(64)Cu]/[(68)Ga]DO3A-VS-Cys(40)- and [(64)Cu]/[(68)Ga]NODA-VS-Cys(40)-exendin-4 can be used as PET imaging agents specific for GLP-1R expressed on β cells. Here, we report the first evidence of pancreatic uptake visualized with exendin-4 derivative in a rat animal model via in vivo dynamic PET imaging.

Imaging pancreatic islet cells by positron emission tomography

It was estimated that every year more than 30000 persons in the United States - approximately 80 people per day - are diagnosed with type 1 diabetes (T1D). T1D is caused by autoimmune destruction of the pancreatic islet (β cells) cells. Islet transplantation has become a promising therapy option for T1D patients, while the lack of suitable tools is difficult to directly evaluate of the viability of the grafted islet over time. Positron emission tomography (PET) as an important non-invasive methodology providing high sensitivity and good resolution, is able to accurate detection of the disturbed biochemical processes and physiological abnormality in living organism. The successful PET imaging of islets would be able to localize the specific site where transplanted islets engraft in the liver, and to quantify the level of islets remain alive and functional over time. This information would be vital to establishing and evaluating the efficiency of pancreatic islet transplantation. Many novel imaging agents have been developed to improve the sensitivity and specificity of PET islet imaging. In this article, we summarize the latest developments in carbon-11, fluorine-18, copper-64, and gallium-68 labeled radioligands for the PET imaging of pancreatic islet cells.

Improved Quantification of the Beta Cell Mass after Pancreas Visualization with 99mTc-demobesin-4 and Beta Cell Imaging with 111In-exendin-3 in Rodents

OBJECTIVE

Accurate assessment of the ¹¹¹In-exendin-3 uptake within the pancreas requires exact delineation of the pancreas, which is highly challenging by MRI and CT in rodents. In this study, the pancreatic tracer ^99mTc-demobesin-4 was evaluated for accurate delineation of the pancreas to be able to accurately quantify ¹¹¹In-exendin-3 uptake within the pancreas.

METHODS

Healthy and alloxan-induced diabetic Brown Norway rats were injected with the pancreatic tracer ^99mTc-demobesin-4 ([^99mTc-N₄-Pro¹,Tyr⁴,Nle¹⁴]bombesin) and the beta cell tracer ¹¹¹In-exendin-3 ([¹¹¹In-DTPA-Lys⁴⁰]exendin-3). After dual isotope acquisition of SPECT images, ^99mTc-demobesin-4 was used to define a volume of interest for the pancreas in SPECT images subsequently the ¹¹¹In-exendin-3 uptake within this region was quantified. Furthermore, biodistribution and autoradiography were performed in order to gain insight in the distribution of both tracers in the animals.

RESULTS

^99mTc-demobesin-4 showed high accumulation in the pancreas. The uptake was highly homogeneous throughout the pancreas, independent of diabetic status, as demonstrated by autoradiography, whereas ¹¹¹In-exendin-3 only accumulates in the islets of Langerhans. Quantification of both ex vivo and in vivo SPECT images resulted in an excellent linear correlation between the pancreatic uptake, determined with ex vivo counting and ¹¹¹In-exendin-3 uptake, determined from the quantitative analysis of the SPECT images (Pearson r = 0.97, Pearson r = 0.92).

CONCLUSION

^99mTc-demobesin-4 shows high accumulation in the pancreas of rats. It is a suitable tracer for accurate delineation of the pancreas and can be conveniently used for simultaneous acquisition with ¹¹¹In labeled exendin-3. This method provides a straightforward, reliable, and objective method for preclinical beta cell mass (BCM) quantification with ¹¹¹In-exendin-3.

Characterization of 5-(2- 18F-fluoroethoxy)-L-tryptophan for PET imaging of the pancreas

Purpose: In diabetes, pancreatic beta cell mass declines significantly prior to onset of fasting hyperglycemia. This decline may be due to endoplasmic reticulum (ER) stress, and the system L amino acid transporter LAT1 may be a biomarker of this process. In this study, we used 5-(2- ¹⁸F-fluoroethoxy)-L-tryptophan ( ¹⁸F-L-FEHTP) to target LAT1 as a potential biomarker of beta cell function in diabetes.

Procedures: Uptake of ¹⁸F-L-FEHTP was determined in wild-type C57BL/6 mice by ex vivo biodistribution. Both dynamic and static positron emission tomography (PET) images were acquired in wild-type and Akita mice, a model of ER stress-induced diabetes, as well as in mice treated with streptozotocin (STZ). LAT1 expression in both groups of mice was evaluated by immunofluorescence microscopy.

Results: Uptake of ¹⁸F-L-FEHTP was highest in the pancreas, and static PET images showed highly specific pancreatic signal. Time-activity curves showed significantly reduced ¹⁸F-L-FEHTP uptake in Akita mice, and LAT1 expression was also reduced. However, mice treated with STZ, in which beta cell mass was reduced by 62%, showed no differences in ¹⁸F-L-FEHTP uptake in the pancreas, and there was no significant correlation of ¹⁸F-L-FEHTP uptake with beta cell mass.

Conclusions: ¹⁸F-L-FEHTP is highly specific for the pancreas with little background uptake in kidney or liver. We were able to detect changes in LAT1 in a mouse model of diabetes, but these changes did not correlate with beta cell function or mass. Therefore, ¹⁸F-L-FEHTP PET is not a suitable method for the noninvasive imaging of changes in beta cell function during the progression of diabetes.