Glucagon-like peptide-1 receptor imaging with [Lys40(Ahx-HYNIC- 99mTc/EDDA)NH2]-exendin-4 for the detection of insulinoma

Peptide-Coated Polycaprolactone-Benzalkonium Chloride Nanocapsules for Targeted Drug Delivery to the Pancreatic β-Cell

Targeting current therapies to treat or prevent the loss of pancreatic islet β-cells in Type 1 Diabetes (T1D) may provide improved efficacy and reduce off-target effects. Current efforts to target the β-cell are limited by a lack of β-cell-specific targets and the inability to test multiple targeting moieties with the same delivery vehicle. Here, we fabricate a tailorable polycaprolactone nanocapsule (NC) in which multiple different targeting peptides can be interchangeably attached for β-cell-specific delivery. Incorporation of a cationic surfactant in the NC shell allows for the attachment of Exendin-4 and an antibody for ectonucleoside triphosphate diphosphohydrolase 3 (ENTPD3) for β-cell-specific targeting. The average NC size ranges from 250 to 300 nm with a polydispersity index under 0.2. The NCs are nontoxic, stable in media culture, and can be lyophilized and reconstituted. NCs coated with a targeting peptide were taken up by human cadaveric islet β-cells and human stem cell-derived β-like cells (sBC) in vitro with a high level of specificity. Furthermore, NCs successfully delivered both hydrophobic and hydrophilic cargo to human β-cells. Additionally, Exendin-4-coated NCs were stable and targeted the mouse pancreatic islet β-cell in vivo. Overall, our tailorable NCs have the potential to improve cell-targeted drug delivery and can be utilized as a screening platform to test the efficacy of cell-targeting peptides.

Peptide Coated Polycaprolactone-Benzalkonium Chloride Nanocapsules for Targeted Drug Delivery to the Pancreatic β-Cell

Targeting of current therapies to treat or prevent loss of pancreatic islet β-cells in Type 1 Diabetes (T1D) may provide improved efficacy and reduce off target effects. Current efforts to target the β-cell are limited by a lack of β-cell specific targets and the inability to test multiple targeting moieties with the same delivery vehicle. Here we fabricate a novel tailorable polycaprolactone nanocapsule (NC) where multiple different targeting peptides can be interchangeably attached for β-cell specific delivery. Incorporation of a cationic surfactant in the NC shell allows for the attachment of Exendin-4 and an antibody for ectonucleoside triphosphate diphosphohydrolase 3 (ENTPD3) for β-cell specific targeting. The average NC size ranges from 250-300nm with a polydispersity index under 0.2. The NCs are non-toxic, stable in media culture, and can be lyophilized and reconstituted. NCs coated with targeting peptide were taken up by human cadaveric islet β-cells and human stem cell-derived β-like cells (sBC) in vitro with a high level of specificity. Furthermore, NCs successfully delivered both hydrophobic and hydrophilic cargo to human β-cells. Finally, Exendin-4 coated NCs were stable and targeted the mouse pancreatic islet β-cell in vivo . Our unique NC design allows for the interchangeable coating of targeting peptides for future screening of targets with improved cell specificity. The ability to target and deliver thera-peutics to human pancreatic β-cells opens avenues for improved therapies and treatments to help the delay onset, prevent, or reverse T1D.

Recent developments in the diagnosis of pancreatic neuroendocrine neoplasms

INTRODUCTION

Pancreatic Neuroendocrine Neoplasms (PanNENs) are characterized by a highly heterogeneous clinical and biological behavior, making their diagnosis challenging. PanNENs diagnostic work-up mainly relies on biochemical markers, pathological examination, and imaging evaluation. The latter includes radiological imaging (i.e. computed tomography [CT] and magnetic resonance imaging [MRI]), functional imaging (i.e. 68Gallium [68 Ga]Ga-DOTA-peptide PET/CT and Fluorine-18 fluorodeoxyglucose [18F]FDG PET/CT), and endoscopic ultrasound (EUS) with its associated procedures.

AREAS COVERED

This review provides a comprehensive assessment of the recent advancements in the PanNENs diagnostic field. PubMed and Embase databases were used for the research, performed from inception to October 2023.

EXPERT OPINION

A deeper understanding of PanNENs biology, recent technological improvements in imaging modalities, as well as progresses achieved in molecular and cytological assays, are fundamental players for the achievement of early diagnosis and enhanced preoperative characterization of PanNENs. A multimodal diagnostic approach is required for a thorough disease assessment.

Theranostic in GLP-1R molecular imaging: challenges and emerging opportunities

Theranostic in nuclear medicine combines diagnostic imaging and internal irradiation therapy using different therapeutic nuclear probes for visual diagnosis and precise treatment. GLP-1R is a popular receptor target in endocrine diseases, non-alcoholic steatohepatitis, tumors, and other areas. Likewise, it has also made breakthroughs in the development of molecular imaging. It was recognized that GLP-1R imaging originated from the study of insulinoma and afterwards was expanded in application including islet transplantation, pancreatic β-cell mass measurement, and ATP-dependent potassium channel-related endocrine diseases. Fortunately, GLP-1R molecular imaging has been involved in ischemic cardiomyocytes and neurodegenerative diseases. These signs illustrate the power of GLP-1R molecular imaging in the development of medicine. However, it is still limited to imaging diagnosis research in the current molecular imaging environment. The lack of molecular-targeted therapeutics related report hinders its radiology theranostic. In this article, the current research status, challenges, and emerging opportunities for GLP-1R molecular imaging are discussed in order to open a new path for theranostics and to promote the evolution of molecular medicine.

Diffuse, Adult-Onset Nesidioblastosis/Non-Insulinoma Pancreatogenous Hypoglycemia Syndrome (NIPHS): Review of the Literature of a Rare Cause of Hyperinsulinemic Hypoglycemia

Differential diagnosis of hypoglycemia in the non-diabetic adult patient is complex and comprises various diseases, including endogenous hyperinsulinism caused by functional β-cell disorders. The latter is also designated as nesidioblastosis or non-insulinoma pancreatogenous hypoglycemia syndrome (NIPHS). Clinically, this rare disease presents with unspecific adrenergic and neuroglycopenic symptoms and is, therefore, often overlooked. A combination of careful clinical assessment, oral glucose tolerance testing, 72 h fasting, sectional and functional imaging, and invasive insulin measurements can lead to the correct diagnosis. Due to a lack of a pathophysiological understanding of the condition, conservative treatment options are limited and mostly ineffective. Therefore, nearly all patients currently undergo surgical resection of parts or the entire pancreas. Consequently, apart from faster diagnosis, more elaborate and less invasive treatment options are needed to relieve the patients from the dangerous and devastating symptoms. Based on a case of a 23-year-old man presenting with this disease in our department, we performed an extensive review of the medical literature dealing with this condition and herein presented a comprehensive discussion of this interesting disease, including all aspects from epidemiology to therapy.

Role of Exendin-4 Functional Imaging in Diagnosis of Insulinoma: A Systematic Review

BACKGROUND

Insulinomas are the most common neuroendocrine neoplasms of the pancreas. Diagnosis is made through patient clinical presentation with hypoglycemia symptoms and imaging, such as EUS, CT, MRI, and functional imaging. Exendin-4 PET/CT (and SPECT/CT) is a new prominent radiotracer developed to image insulinomas. The aim of the study is to evaluate whether exendin-4 imaging is a useful tool in imaging for insulinoma patients when other imaging methods do not reach them.

METHODS

MEDLINE research conducted on PubMed, Scopus, and Web of Science gathered a total of 501 papers. Studies that evaluated exendin-4 SPECT and PET in insulinoma patients were screened and assessed through QUADAS-2 for risk of bias and applicability concerns' assessment. Sensitivity, specificity, and accuracy were reported when available.

RESULTS

A total of 13 studies were deemed eligible for a QUADAS 2 review. Studies included ranged from 2009 to 2022. The most-used tracer was ⁶⁸Ga-DOTA-exendin-4 in PET and ¹¹¹In-DTPA-exendin-4 in SPECT. Exendin-4 labeled with ⁹⁹mTc was also reported. The QUADAS-2 risk of bias assessment was overall low, with some unclear reports in the reference and index domains. Only two domains were at high risk of bias because of an explicated non-blind imaging review. Applicability concerns for bias were low in all domains. Reported sensitivities ranged from 95% to 100% and specificities from 20% to 100%.

CONCLUSIONS

exendin-4 imaging is a sensitive functional imaging tracer in both SPECT and PET applications, especially in suspicion of benign insulinomas located where endoscopic ultrasound cannot reach, being more sensitive than morfostructural imaging.

Adult focal β-cell nesidioblastosis: A case report

BACKGROUND

Nesidioblastosis usually refers to a series of clinical manifestations caused by the proliferation of β-cells in pancreatic islets, and these clinical manifestations are hyperinsulinemia and persistent hypoglycemia. According to the size of the lesion, nesidioblastosis is divided into focal nesidioblastosis, diffuse nesidioblastosis and atypical nesidioblastosis, and its pathogenesis is still unclear. Nesidioblastosis is mainly seen in infants and rarely reported in adults, especially focal nesidioblastosis, which is difficult to distinguish from insulinoma.

CASE SUMMARY

We report a case of adult focal β-cell nesidioblastosis in which the preoperative diagnosis was insulinoma. The patient was a 48-year-old male who suffered from repeated morning and fasting palpitations, sweating, and severe disturbance of consciousness for 5 years. His blood glucose was found to be as low as 1.79 mmol/L during an attack. However, abdominal computed tomography showed no abnormalities. Magnetic resonance imaging and endoscopic ultrasonography demonstrated a nodular mass in the head of the pancreas, combined with hyperinsulinemia and high serum C-peptide. The patient was diagnosed with insulinoma and underwent Beger surgery; however, the postoperative pathological results showed nesidioblastosis.

CONCLUSION

Although surgical resection is the preferred option for nesidioblastosis, some cases can be treated non-surgically. In order to increase clinicians' understanding of nesidioblastosis, it is necessary to review the pathogenesis, diagnosis and treatment of this disease.

68Ga-DOTATATE PET/CT imaging for insulinoma in MEN1 patient with endogenous hyperinsulinemic hypoglycemia: A case report

RATIONALE

Multiple endocrine neoplasia type 1 (MEN1) syndrome is a rare and complicated disease that is associated with several endocrine tumors. Here, we report a case of MEN1 associated with insulinoma, parathyroid, and pituitary tumors by 68Ga-DOTATATE positron emission tomography/computed tomography (PET/CT).

PATIENT CONCERNS

A 49-year-old woman presented with intermittent hypoglycemia for more than a year and developed indistinct consciousness without an apparent trigger.

DIAGNOSES

Biochemical results showed abnormally high serum insulin and parathyroid hormone levels. She underwent an Abdominal magnetic resonance imaging revealed a small nodule in the uncinate process of the pancreas, but it did not clarify the nature of the small nodule. Pituitary magnetic resonance imaging scan revealed a micropituitary tumor, and parathyroid imaging showed no abnormalities. 18F-FDG PET/CT showed no apparent abnormal 18F-FDG uptake in the whole body. In contrast, 68Ga-DOTATATE PET/CT imaging showed pathological radiotracer uptake in the pancreatic uncinate process, accompanied by mild radiotracer uptake in the pituitary gland, and no apparent abnormal radiotracer uptake in the parathyroid area.

INTERVENTIONS

The patient underwent echoendoscopy for pancreatic uncinate process lesions and surgical resection.

OUTCOMES

Histological analysis was suggested of insulinoma of pancreatic neuroendocrine tumor, the Ki-67 index was low (only 1% being positive).

LESSONS

This case demonstrates that 68Ga-DOTATATE can be used for the detection of MEN1-related tumors and preoperative localization of small and low-grade insulinomas by PET/CT.

Identification and Validation of a New Peptide Targeting Pancreatic Beta Cells

Noninvasive targeted visualization of pancreatic beta cells or islets is becoming the focus of molecular imaging application in diabetes and islet transplantation studies. In this study, we aimed to produce the beta-cell-targeted peptide for molecular imaging of islet. We used phage display libraries to screen a beta-cell-targeted peptide, LNTPLKS, which was tagged with fluorescein isothiocyanate (FITC). This peptide was validated for targeting beta-cell with in vitro and in vivo studies. Immunocytochemistry (ICC) and fluorescence-activated cell sorting (FACS) analysis were used to validate the target specificity of the peptide. FITC-LNTPLKS displayed much higher fluorescence in beta cells vs. control cells in ICC. This discrimination was consistently observed using primary rodent islet. FACS analysis showed right shift of peak point in beta cells compared to control cells. The specific bind to in situ islet was verified by in vitro experiments using rodent and human pancreatic slices. The peptide also showed high affinity of islet grafts under the renal capsule. In the insulinoma animal model, we could find FITC-LNTPLKS accumulated specifically to the tumor, thus indicating a potential clinical application of molecular imaging of insulinoma. In conclusion, LNTPLKS showed a specific probe for beta-cells, which might be further utilized in targeted imaging/monitoring beta cells and theragnosis for beta-cells-related disease (diabetes, insulinoma, etc.).

Development and Validation of an Analytical HPLC Method to Assess Chemical and Radiochemical Purity of [68Ga]Ga-NODAGA-Exendin-4 Produced by a Fully Automated Method

Background: Glucagon-like peptide 1 receptor (GLP-1R) is preferentially expressed in pancreatic islets, especially in β-cells, and highly expressed in human insulinomas and gastrinomas. In recent years several GLP-1R-avid radioligands have been developed to image insulin-secreting tumors or to provide a tentative quantitative in vivo biomarker of pancreatic β-cell mass. Exendin-4, a 39-amino acid peptide with high binding affinity to GLP-1R, has been labeled with Ga-68 for imaging with positron emission tomography (PET). Preparation conditions may influence the quality and in vivo behavior of tracers. Starting from a published synthesis and quality controls (QCs) procedure, we have developed and validated a new rapid and simple UV-Radio-HPLC method to test the chemical and radiochemical purity of [68Ga]Ga-NODAGA-exendin-4, to be used in the clinical routine. Methods: Ga-68 was obtained from a 68Ge/68Ga Generator (GalliaPharma®) and purified using a cationic-exchange cartridge on an automated synthesis module (Scintomics GRP®). NODAGA-exendin-4 contained in the reactor (10 µg) was reconstituted with HEPES and ascorbic acid. The reaction mixture was incubated at 100 °C. The product was purified through HLB cartridge, diluted, and sterilized. To validate the proposed UV-Radio-HPLC method, a stepwise approach was used, as defined in the guidance document released by the International Conference on Harmonization of Technical Requirements of Pharmaceuticals for Human Use (ICH), adopted by the European Medicines Agency (CMP/ICH/381/95 2014). The assessed parameters are specificity, linearity, precision (repeatability), accuracy, and limit of quantification. Therefore, a range of concentrations of Ga-NODAGA-exendin-4, NODAGA-exendin-4 (5, 4, 3.125, 1.25, 1, and 0.75 μg/mL) and [68Ga]Ga-NODAGA-exendin-4 were analyzed. To validate the entire production process, three consecutive batches of [68Ga]Ga-NODAGA-exendin-4 were tested. Results: Excellent linearity was found between 5-0.75 μg/mL for both the analytes (NODAGA-exendin-4 and 68Ga-NODAGA-exendin-4), with a correlation coefficient (R2) for calibration curves equal to 0.999, average coefficients of variation (CV%) < 2% (0.45% and 0.39%) and average per cent deviation value of bias from 100%, of 0.06% and 0.04%, respectively. The calibration curve for the determination of [68Ga]Ga-NODAGA-exendin-4 was linear with a R2 of 0.993 and CV% < 2% (1.97%), in accordance to acceptance criteria. The intra-day and inter-day precision of our method was statistically confirmed using 10 μg of peptide. The mean radiochemical yield was 45 ± 2.4% in all the three validation batches of [68Ga]Ga-NODAGA-exendin-4. The radiochemical purity of [68Ga]Ga-NODAGA-exendin-4 was >95% (97.05%, 95.75% and 96.15%) in all the three batches. Conclusions: The developed UV-Radio-HPLC method to assess the radiochemical and chemical purity of [68Ga]Ga-NODAGA-exendin-4 is rapid, accurate and reproducible like its fully automated production. It allows the routine use of this PET tracer as a diagnostic tool for PET imaging of GLP-1R expression in vivo, ensuring patient safety.

Exendin-4-Conjugated Manganese Magnetism-Engineered Iron Oxide Nanoparticles as a Potential Magnetic Resonance Imaging Contrast Agent for Tracking Transplanted β-Cells

To specifically detect and trace transplanted islet β-cells by magnetic resonance imaging (MRI), we conjugated manganese magnetism-engineered iron oxide nanoparticles (MnMEIO NPs) with exendin-4 (Ex4) which specifically binds glucagon-like peptide-1 receptors on the surface of β-cells. The size distribution of MnMEIO and MnMEIO-Ex4 NPs were 67.8 ± 1.3 and 70.2 ± 2.3 nm and zeta potential 33.3 ± 0.5 and 0.6 ± 0.1 mV, respectively. MnMEIO and MnMEIO-Ex4 NPs with iron content ≤ 40 μg/mL did not affect MIN6 β-cell viability and insulin secretion. Positive iron staining was found in MIN6 β-cells loaded with MnMEIO-Ex4 NPs but not in those with MnMEIO NPs. A transmission electron microscope confirmed MnMEIO-Ex4 NPs were distributed in the cytoplasm of MIN6. In vitro MR images revealed a loss of signal intensity in MIN6 β-cells labeled with MnMEIO-Ex4 NPs but not with MnMEIO NPs. After transplantation of islets labeled with MnMEIO-Ex4, the graft under kidney capsule could be visualized on MRI as persistent hypointense areas up to 17 weeks. Moreover, histology of the islet graft showed positive staining for insulin, glucagon and iron. Our results indicate MnMEIO-Ex4 NPs are safe and effective for the detection and long-term monitoring of transplanted β-cells by MRI.

Diagnostic performance of noninvasive imaging modalities for localization of insulinoma: A meta-analysis

BACKGROUND

Insulinoma is the most common functional neuroendocrine tumor found only in the pancreas. The early detection of insulinoma is of importance. Studies comparing the performance of noninvasive modalities were limited by sample size and heterogeneity between studies. The aim of this meta-analysis was to evaluate the diagnostic performance of PET/CT, SPECT/CT, CT and MRI for the localization of insulinoma, and to provide evidence for clinical practice.

METHODS

PubMed, Embase, Cochrane Library, Wanfang Data and China National Knowledge Infrastructure were searched from inception to May 31, 2021. Pooled sensitivity, specificity, positive Likelihood Ratio (+LR) and negative Likelihood Ratio (-LR), diagnostic odds ratio (DOR), and concordance rate were calculated.

RESULTS

A total of 19 studies including 708 patients of insulinoma reached the inclusion criteria. PET/CT imaging demonstrated a pooled sensitivity of 0.79 (95% CI: 0.54-0.92) and a pooled specificity of 0.84 (95% CI: 0.20-0.99). The pooled sensitivity and specificity of SPECT/CT were 0.77 (95% CI: 0.46-0.93) and 0.45 (95% CI: 0.22-0.70). CT showed an overall sensitivity of 0.54 (95% CI: 0.35-0.72) and specificity of 0.75 (95% CI: 0.54-0.88). The pooled sensitivity and specificity for MRI were 0.54 (95% CI: 0.31-0.75) and 0.65 (95% CI: 0.39-0.84), respectively. The concordance rates of PET, SPECT, CT, and MRI were 78% (95% CI: 66-90%), 74% (95% CI: 52-97%), 56% (95% CI: 41-72%), and 53% (95% CI: 33-73%), respectively.

CONCLUSION

Results of this study indicate that PET/CT demonstrated superior performance than SPECT/CT, CT and MRI for the localization of insulinoma. GLP-1R based PET/CT manifested better diagnostic performance in comparison with SSTR based PET/CT imaging modality.

Current update on imaging for pancreatic neuroendocrine neoplasms

Pancreatic neuroendocrine neoplasms (panNEN) are a heterogeneous group of tumors with differing pathological, genetic, and clinical features. Based on clinical findings, they may be categorized into functioning and nonfunctioning tumors. Adoption of the 2017 World Health Organization classification system, particularly its differentiation between grade 3, well-differentiated pancreatic neuroendocrine tumors (panNET) and grade 3, poorly-differentiated pancreatic neuroendocrine carcinomas (panNEC) has emphasized the role imaging plays in characterizing these lesions. Endoscopic ultrasound can help obtain biopsy specimen and assess tumor margins and local spread. Enhancement patterns on computed tomography (CT) and magnetic resonance imaging (MRI) may be used to classify panNEN. Contrast enhanced MRI and diffusion-weighted imaging have been reported to be useful for characterization of panNEN and quantifying metastatic burden. Current and emerging radiotracers have broadened the utility of functional imaging in evaluating panNEN. Fluorine-18 fluorodeoxyglucose positron emission tomography (PET)/CT and somatostatin receptor imaging such as Gallium-68 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-octreotate PET/CT may be useful for improved identification of panNEN in comparison to anatomic modalities. These new techniques can also play a direct role in optimizing the selection of treatment for individuals and predicting tumor response based on somatostatin receptor expression. In addition, emerging methods of radiomics such as texture analysis may be a potential tool for staging and outcome prediction in panNEN, however further investigation is required before clinical implementation.

The β Cell in Diabetes: Integrating Biomarkers With Functional Measures

The pathogenesis of hyperglycemia observed in most forms of diabetes is intimately tied to the islet β cell. Impairments in propeptide processing and secretory function, along with the loss of these vital cells, is demonstrable not only in those in whom the diagnosis is established but typically also in individuals who are at increased risk of developing the disease. Biomarkers are used to inform on the state of a biological process, pathological condition, or response to an intervention and are increasingly being used for predicting, diagnosing, and prognosticating disease. They are also proving to be of use in the different forms of diabetes in both research and clinical settings. This review focuses on the β cell, addressing the potential utility of genetic markers, circulating molecules, immune cell phenotyping, and imaging approaches as biomarkers of cellular function and loss of this critical cell. Further, we consider how these biomarkers complement the more long-established, dynamic, and often complex measurements of β-cell secretory function that themselves could be considered biomarkers.

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.

NHS-Functionalized THP Derivative for Efficient Synthesis of Kit-Based Precursors for 68Ga Labeled PET Probes

Hexadentate tris(3,4-hydroxypyridinone) ligands (THP) complex Fe3+ at very low iron concentrations and their high affinities for oxophilic trivalent metal ions have led to their development for new applications as bifunctional chelators for the radiometal gallium-68 (68Ga). THP-peptide bioconjugates rapidly and quantitatively complex 68Ga at room temperature, neutral pH, and micromolar ligand concentrations, making them amenable to kit-based radiosynthesis of 68Ga PET radiopharmaceuticals. With the aim to produce an N-hydroxysuccinimide-(NHS)-THP reagent for kit-based 68Ga-labeling and PET imaging, THP-derivatives were designed and synthesized to exploit the advantages of NHS chemistry for coupling with peptides, proteins, and antibodies. The more stable five-carbon atoms linker product was selected for a proof-of-concept conjugation and radiolabeling study with an anti-programmed death ligand 1 (PD-L1) camelid single domain antibody (sdAb) under mild conditions and further evaluated for site-specific amide bond formation with a synthesized glucagon-like peptide-1 (GLP-1) targeting peptide using solid-phase synthesis. The obtained THP-GLP-1 conjugate was tested for its 68Ga chelating ability, demonstrating to be a promising candidate for the detection and monitoring of GLP-1 aberrant malignancies. The obtained sdAb-THP conjugate was radiolabeled with 68Ga under mild conditions, providing sufficient labeling yields after 5 min, demonstrating that the novel NHS-THP bifunctional chelator can be widely used to easily conjugate the THP moiety to different targeting molecules (e.g., antibodies, anticalins, or peptides) under mild conditions, paving the way to the synthesis of different imaging probes with all the advantages of THP radiochemistry.

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.

Pitfalls in the Detection of Insulinomas With Glucagon-Like Peptide-1 Receptor Imaging

PURPOSE

Physiological pancreaticoduodenal uptake of radiolabeled exendin-4 in Brunner glands of the proximal duodenum is the most common pitfall for false interpretation of glucagon-like peptide-1 receptor (GLP-1R) imaging. The aim of this study was to analyze the pancreaticoduodenal uptake in GLP-1R PET/CT and SPECT/CT images and to identify additional potential reading pitfalls in patients with suspected insulinoma.

METHODS

A post hoc analysis of a prospective study, including 52 consecutive patients, was performed. All patients underwent 1 Ga-exendin-4 PET/CT and 2 In-exendin-4 SPECT/CT scans (4 and 72 hours postinjection) in a randomized crossover order. Three board-certified nuclear medicine physicians read all scans independently. They were unaware of other results. Reference standard was surgery with histopathological confirmation of an insulinoma/nesidioblastosis and normalization of blood glucose levels after surgery.

RESULTS

There were no false-positive readings. However, there were a number of false-negative PET/CT and SPECT/CT readings, respectively: (1) due to false interpretation of uptake in the pancreaticoduodenal region (falsely interpreted as physiological uptake in Brunner glands instead of an insulinoma in 0.6% vs 9.0%), (2) due to ectopic insulinoma (0% vs 2.6%), (3) due to small insulinoma (1.9% vs 5.1%), (4) due to insulinoma overlap with kidneys (1.9% vs 4.5%), and (5) due to nesidioblastosis (0.6% and 1.9%). Pitfalls were identified in all GLP-1R PET/CT and SPECT/CT scans.

CONCLUSIONS

Peripancreatic uptake, small size of an insulinoma, insulinoma overlap with kidneys, and presence of nesidioblastosis are potential pitfalls in GLP-1R imaging, which can lead to false reading results.

Detection of Insulinomas Using Dual-Time-Point 68Ga-DOTA-Exendin 4 PET/CT

PURPOSE

Insulinomas are predominantly benign neuroendocrine tumors originating from beta cells within the islets of Langerhans of the endocrine pancreas. Because surgical resection represents the only curative therapy option, exact tumor localization and discrimination of insulinomas from focal or diffuse manifestations of congenital hyperinsulinism are crucial for optimal treatment strategies. We investigated the diagnostic value of glucagon-like peptide 1 receptor PET/CT using Ga-DOTA-exendin 4 for detecting insulinomas and compared the diagnostic value of PET scans performed at 2 time points.

METHODS

In 10 patients with clinically and biochemically suspected insulinoma, PET/CT was performed at 1 hour (PET1) and 2 hours (PET2) after injection of Ga-DOTA-exendin 4. In this retrospective analysis, tracer uptake was visually assessed in both scans by 2 independent readers. SUVmax and tumor-to-background ratio (TBR) of focal lesions were assessed. Imaging results were compared with histopathologic findings, if patients underwent resection.

RESULTS

Increased focal Ga-DOTA-exendin 4 uptake was observed in 8 of 10 patients concordantly by both readers. Seven patients with focal uptake underwent surgery with tumor enucleation and histopathologic proof of insulinoma (7/8). Two of 10 patients without focal uptake were considered to suffer from diffuse form of congenital hyperinsulinism and consequently received medical treatment. A significant increase of tumoral SUVmax on PET2 (PET1: SUVmax 20.2 ± 8.2 g/mL; PET2: SUVmax 24.7 ± 7.9 g/mL; P = 0.0018) did not result in a significant improvement in TBR (PET1: TBR 4.9 ± 1.7; PET2: TBR 4.3 ± 1.2; P = 0.2892).

CONCLUSIONS

Focal uptake of Ga-DOTA-exendin 4 reliably indicated insulinomas as histopathologically confirmed in all patients undergoing consecutive surgery. The diagnostic value of PET2 was not found to be superior to PET1, indicating that a single 1-hour Ga-DOTA-exendin 4 PET/CT scan is a sufficient and convenient approach for patient care.

Exendin-4 analogs in insulinoma theranostics

Insulinomas, neuroendocrine tumors arising from pancreatic beta cells, often show overexpression of the glucagon‐like peptide‐1 receptor. Therefore, imaging with glucagon‐like peptide analog exendin‐4 can be used for diagnosis and preoperative localization. This review presents an overview of the development and clinical implementation of exendin‐based tracers for nuclear imaging, and the potential use of exendin‐4 based tracers for optical imaging and therapeutic applications such as peptide receptor radionuclide therapy or targeted photodynamic therapy.

Innovative imaging of insulinoma: the end of sampling? A review

Receptors for the incretin glucagon-like peptide-1 (GLP-1R) have been found overexpressed in selected types of human tumors and may, therefore, play an increasingly important role in endocrine gastrointestinal tumor management. In particular, virtually all benign insulinomas express GLP-1R in high density. Targeting GLP-1R with indium-111, technetium-99m or gallium-68-labeled exendin-4 offers a new approach that permits the successful localization of small benign insulinomas. It is likely that this new non-invasive technique has the potential to replace the invasive localization of insulinomas by selective arterial stimulation and venous sampling. In contrast to benign insulinomas, malignant insulin-secreting neuroendocrine tumors express GLP-1R in only one-third of the cases, while they more often express the somatostatin subtype 2 receptors. Importantly, one of the two receptors appears to be always overexpressed. In special cases of endogenous hyperinsulinemic hypoglycemia (EHH), that is, in the context of MEN-1 or adult nesidioblastosis GLP-1R imaging is useful whereas in postprandial hypoglycemia in the context of bariatric surgery, GLP-1R imaging is probably not helpful. This review focuses on the potential use of GLP-1R imaging in the differential diagnosis of EHH.

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.

Superior Diagnostic Performance of the GLP-1 Receptor Agonist [Lys40(AhxHYNIC-[99mTc]/EDDA)NH2]-Exendin-4 over Conventional Imaging Modalities for Localization of Insulinoma

PURPOSE

Insulinomas are the most common functioning neuroendocrine neoplasms of the pancreas, typically diagnosed due to characteristic symptoms. In the vast majority, the treatment is surgical and curative, requiring accurate localization of the tumour; conventional imaging, including somatostatin receptor molecular imaging, is negative in up to 10 % of cases. Recently, labelled glucagon-like peptide receptor (GLP-1R) analogues were introduced as a sensitive diagnostic method for localization of insulinomas. The aim of this study was to assess the diagnostic accuracy of a Tc-99m-labelled GLP-1R agonist [Lys40(AhxHYNIC-[99mTc]EDDA)NH2]-exendin-4 for localization of occult insulinoma.

PROCEDURES

Eight patients (all females; age range 35-75 years) with biochemically proven insulinoma and with negative or inconclusive conventional imaging (consisting of somatostatin receptor scintigraphy, computed tomography, endoscopic ultrasound and magnetic resonance imaging) were enrolled. Whole-body single-photon emission tomography/computed tomography (SPECT/CT) imaging was performed 4 h post-injection of 740 MBq of [Lys40(AhxHYNIC-[99mTc]EDDA)NH2]-exendin-4. Surgical treatment was performed based on imaging findings. Histology of the removed lesions and biochemical and clinical symptom resolution was considered as the gold standard for analysis of the imaging results.

RESULTS

Focal uptake of [Lys40(AhxHYNIC-[99mTc]EDDA)NH2]-exendin-4 was found in all patients, leading to successful removal of the offending lesion and complete biochemical and symptomatic resolution. Histological analysis confirmed insulinoma in all included patients.

CONCLUSIONS

[Lys40(AhxHYNIC-[99mTc]EDDA)NH2]-exendin-4 SPECT/CT appears to be an excellent molecular imaging method for preoperative localization of an occult insulinoma, surpassing conventional imaging methods. If routinely available, it could be considered as a method of choice due to its favorable combination of imaging characteristics.

Nesidioblastosis and Insulinoma: A Rare Coexistence and a Therapeutic Challenge

Background: Nesidioblastosis and insulinoma are disorders of the endocrine pancreas causing endogenous hyperinsulinemic hypoglycemia. Their coexistence is very unusual and treatment represents a still unresolved dilemma. Case Description: The patient was a 43-year-old Caucasian woman, with a 2-year history of repeated severe hypoglycemic events. The diagnostic work-up was strongly suggestive of insulinoma and the patient was submitted to surgical treatment carried out laparoscopically under robotic assistance. However, surgical exploration and intraoperative ultrasonography failed to detect a pancreatic tumor. Resection was therefore carried out based on the results of selective intra-arterial calcium stimulation test, following a step-up approach, eventually leading to a pancreatoduodenectomy at the splenic artery. The histopathology examination and the immunohistochemical staining were consistent with adult-onset nesidioblastosis. After surgery, the patient continued to experience hypoglycemia with futile response to medical treatments (octreotide, calcium antagonists, diazoxide, and prednisone). Following multidisciplinary evaluation and critical review of a repeat abdominal computed tomography scan, a small nodular lesion was identified in the tail of the pancreas. The nodule was enucleated laparoscopically and the pathological examination revealed an insulinoma. In spite of the insulinoma resection, glycemic values were only partially restored, with residual nocturnal hypoglycemia. Administration of uncooked cornstarch (1.25 g/kg body weight) at bedtime was associated with significant improvement of interstitial glucose levels (p < 0.0001) and reduction of nocturnal hypoglycemia episodes (p = 0.0002). Conclusions: This report describes a rare coexistence of adult-onset nesidioblastosis and insulinoma, suggesting the existence of a wide and continuous spectrum of proliferative β-cell changes. Moreover, we propose that uncooked cornstarch may offer an additional approach to alleviate the hypoglycemic episodes when surgery is impracticable/unaccepted.

99mTc-EDDA/HYNIC-TOC is a New Opportunity in Neuroendocrine Tumors of the Lung (and in other Malignant and Benign Pulmonary Diseases)

Neuroendocrine tumors (NETs) consist of a relatively rare spectrum of malignancies that can arise from neuroendocrine cells; lung NETs (L-NETs) represent about 25% of primary lung neoplasm and 10% of all carcinoid tumors. Diagnostic algorithm usually takes into consideration chest Xray, contrast-enhanced CT and MRI. Nuclear medicine plays a crucial role in the detection and correct assessment of neoplastic functional status as it provides in vivo metabolic data related to the overexpression of Somatostatin Receptors (SSTRs) and also predicting response to peptide receptor radionuclide therapy (PRRT). 111In-Pentreotide (Octreoscan®) is commercially available for imaging of neuroendocrine tumors, their metastases and the management of patients with NETs. More recently, 99mTc-EDDA/HYNIC-TOC(Tektrotyd®) was introduced into the market and its use has been approved for imaging of patients with L-NETs and other SSTR-positive tumors. 99mTc-EDDA/HYNIC-TOC could also represent a good alternative to 68Ga-DOTA-peptides (DOTA-TOC, DOTA-NOC, DOTATATE) in hospitals or centers where PET/CT or 68Ge/68Ga generators are not available. When compared to 111In-Pentetreotide, Tektrotyd® showed slightly higher sensitivity, in the presence of higher imaging quality and lower radiation exposure for patients. Interesting perspectives depending on the kinetic analysis allowed by Tektrotyd® may be obtained in differential diagnosis of non-small cells lung cancer (NSCLC) versus small cells lung cancer (SCLC) and NETs. An interesting perspective could be also associated with a surgery radio-guided by Tektrotyd® in operable lung tumors, including either NETs and NSCLC.

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.

68Ga-Exendin-4 PET/CT Detects Insulinomas in Patients With Endogenous Hyperinsulinemic Hypoglycemia in MEN-1

CONTEXT

Surgical intervention is advised in patients with multiple endocrine neoplasia type-1 (MEN-1) and nonfunctioning pancreatic neuroendocrine tumors (PanNETs) with a size ≥20 mm. Functioning PanNETs, such as in patients with endogenous hyperinsulinemic hypoglycemia (EHH) due to (one or multiple) insulinomas, should be treated surgically independent of size. Preoperative localization of insulinomas is critical for surgery.

OBJECTIVE

To evaluate the feasibility and sensitivity of 68Ga-DOTA-exendin-4 positron emission tomography (PET)/CT in the detection of clinically relevant lesions in patients with MEN-1 and EHH in combination with MRI.

DESIGN

Post hoc subgroup analysis of a larger prospective imaging study with 52 patients with EHH.

PATIENTS

Six of 52 consecutive patients with EHH and genetically proven MEN-1 mutation were included.

INTERVENTIONS

All patients received one 68Ga-DOTA-exendin-4 PET/CT and one MRI scan within 3 to 4 days. Thereafter, surgery was performed based on all imaging results.

MAIN OUTCOME MEASURES

Lesion-based sensitivity of PET/CT and MRI for detection of clinically relevant lesions was calculated. Readers were unaware of other results. The reference standard was surgery with histology and treatment outcome. True positive (i.e., clinically relevant lesions) was defined as PanNETs ≥20 mm or insulinoma.

RESULTS

In six patients, 37 PanNETs were confirmed by histopathology. Sensitivity (95% CI) in the detection of clinically relevant lesions for combined PET/CT plus MRI, MRI, and PET/CT was 92.3% (64% to 99.8%), 38.5% (13.9% to 68.4%), and 84.6% (54.6% to 98.1%), respectively (P = 0.014 for the comparison of PET/CT plus MRI vs MRI). Postsurgery, EHH resolved in all patients.

CONCLUSION

68Ga-DOTA-exendin-4 PET/CT is feasible in patients with MEN-1 and EHH. The combination with MRI is superior to MRI alone in the detection of insulinomas and may guide the surgical strategy.

Diagnostic value of ASVS for insulinoma localization: A systematic review and meta-analysis

BACKGROUND

Previous studies on the diagnostic value of arterial calcium stimulation with hepatic venous sampling (ASVS) for the localization of insulinoma have reported inconsistent results. Here, we performed a meta-analysis of the relevant published studies.

METHODS

PubMed, Embase, Web of Science, the Cochrane Library, and Wanfang Data were searched for studies on the diagnostic value of ASVS in insulinoma localization published up to May 2019. We calculated the sensitivity, specificity, positive likelihood ratio (PLR), negative likelihood ratio (NLR), diagnostic odds ratio (DOR), and receiver operating characteristic (ROC) curve of ASVS in the localization of insulinoma.

RESULTS

We included ten studies involving 337 patients in the study. The pooled sensitivity, specificity, PLR, and NLR were 0.93 (95% confidence interval [CI]: 0.83-0.97), 0.86 (95%CI: 0.75-0.93), 6.8(95%CI: 3.7-12.7), and 0.08 (95%CI: 0.03-0.19), respectively. The DOR was 84 (95%CI: 30-233), and the area under the ROC curve was 0.96 (95%CI: 0.94-0.97).The results of the heterogeneity of the studies (P = 0.00, I2 = 80.17) were calculated using forest plots of the DOR.

CONCLUSION

ASVS is of significant value in localization of insulinoma. If a qualitative diagnosis of insulinoma is definite and the imaging examination results are negative, ASVS should be performed to confirm the localization of insulinoma.

Preliminary evaluation of 18F‑AlF‑NOTA‑MAL‑Cys40‑Exendin‑4 in rodent heart after myocardial ischemia and reperfusion

Glucagon-like peptide-1 (GLP-1) and its receptor (GLP-1R) exert cardioprotective effects after myocardial ischemia and reperfusion (MI/R) in animal models and human clinical trials. Receptor imaging with positron emission tomography (PET) provides a non-invasive method for monitoring GLP-1R expression. In the present study, a fluorine-18-labeled aluminum fluoride exendin-4 analog [¹⁸F-AlF conjugated with 1,4,7-triazacyclononanetriacetic acid (NOTA)-maleimide (MAL)-Cys⁴⁰-exendin-4] was synthesized and evaluated in a rat MI/R model for GLP-1R imaging. NOTA-MAL-Cys⁴⁰-exendin-4 was synthesized by coupling Cys⁴⁰-exendin-4 with NOTA-MAL. NOTA-MAL-Cys⁴⁰-exendin-4 was then conjugated with ¹⁸F-AlF to obtain ¹⁸F-AlF-NOTA-MAL-Cys⁴⁰-exendin-4. The yield of ¹⁸F-AlF-NOTA-MAL-Cys⁴⁰-exendin-4 was 18.5±3.4% (not decay corrected). The process was completed within ~30 min. In rat MI/R models, the tracer exhibited specific binding to GLP-1R and an appropriate signal-to-noise ratio. At 8 h post-MI/R, tracer uptake reached its peak [0.35±0.053% of injected dose (%ID)/g; n=6] in ischemic myocardium. Localized tracer uptake decreased 1 day (0.20±0.032 %ID/g; n=6) and 3 days (0.16±0.017 %ID/g; n=6) post-MI/R compared with 8 h post-MI/R, but still remained higher compared with sham-operated groups (0.06±0.012 %ID/g; n=6). Pre-injected unlabeled exendin-4 effectively blocked tracer accumulation (0.09±0.041 %ID/g; n=6). In conclusion, ¹⁸F-AlF-NOTA-MAL-Cys⁴⁰-exendin-4 demonstrated favorable characteristics for GLP-1R imaging following MI/R. PET imaging using ¹⁸F-AlF-NOTA-MAL-Cys⁴⁰-exendin-4 in rodent hearts after MI/R revealed a dynamic pattern of GLP-1R upregulation.

Physiopathological Premises to Nuclear Medicine Imaging of Pancreatic Neuroendocrine Tumours

Background:

Pancreatic Neuroendocrine Tumors (P-NETs) are a challenge in terms of both diagnosis and therapy; morphological studies need to be frequently implemented with nonstandard techniques such as Endoscopic Ultrasounds, Dynamic CT, and functional Magnetic Resonance.

Discussion:

The role of nuclear medicine, being scarcely sensitive F-18 Fluorodeoxyglucose, is mainly based on the over-expression of Somatostatin Receptors (SSTR) on neuroendocrine tumor cells surface. Therefore, SSTR can be used as a target for both diagnosis, using radiotracers labeled with gamma or positron emitters, and therapy. SSTRs subtypes are capable of homo and heterodimerization in specific combinations that alter both the response to ligand activation and receptor internalization.

Conclusion:

Although agonists usually provide efficient internalization, also somatostatin antagonists (SS-ANTs) could be used for imaging and therapy. Peptide Receptor Radionuclide Therapy (PRRT) represents the most successful option for targeted therapy. The theranostic model based on SSTR does not work in insulinoma, in which different radiotracers such as F-18 FluoroDOPA or tracers for the glucagon-like peptide-1 receptor have to be preferred.

State of the Art and Recent Developments of Radiopharmaceuticals for Pancreatic Neuroendocrine Tumors Imaging

Background:

Neuroendocrine Tumors (NETs) are relatively rare tumors, mainly originating from the digestive system, that tend to grow slowly and are often diagnosed when metastasised. Surgery is the sole curative option but is feasible only in a minority of patients. Among them, pancreatic neuroendocrine tumors (pancreatic NETs or pNETs) account for less than 5% of all pancreatic tumors. Viable therapeutic options include medical treatments such as biotherapies and more recently Peptide Receptor Radionuclide Therapies (PRRT) with radiolabeled somatostatin analogues. Molecular imaging, with main reference to PET/CT, has a major role in patients with pNETs.

Objective:

The overexpression of specific membrane receptors, as well as the ability of cells to take up amine precursors in NET, have been exploited for the development of specific targeting imaging agents.

Methods:

SPECT/CT and PET/CT with specific isotopes such as [68Ga]-1,4,7,10-tetra-azacyclododecane- N,N’,N’’,N’’’-tetra-acetic acid (DOTA)-somatostatin analogs, [18F]-FDG and [18F]-fluorodopa have been clinically explored.

Results:

To overcome the limitations of SSTR imaging, interesting improvements are connected with the availability of new radiotracers, activating with different mechanisms compared to somatostatin analogues, such as glucagon-like peptide 1 receptor (GLP-1 R) agonists or antagonists.

Conclusion:

This paper shows an overview of the RPs used so far in the imaging of pNETs with insight on potential new radiopharmaceuticals currently under clinical evaluation.

The role of multimodal imaging in guiding resectability and cytoreduction in pancreatic neuroendocrine tumors: focus on PET and MRI

Pancreatic neuroendocrine tumors (pNETs) are rare neoplasms that secrete peptides and neuro-amines. pNETs can be sporadic or hereditary, syndromic or non-syndromic with different clinical presentations and prognoses. The role of medical imaging includes locating the tumor, assessing its extent, and evaluating the feasibility of curative surgery or cytoreduction. Pancreatic NETs have very distinctive phenotypes on CT, MRI, and PET. PET have been demonstrated to be very sensitive to detect either well-differentiated pNETs using ⁶⁸Gallium somatostatin receptor (SSTR) radiotracers, or more aggressive undifferentiated pNETS using ¹⁸F-FDG. A comprehensive interpretation of multimodal imaging guides resectability and cytoreduction in pNETs. The imaging phenotype provides information on the differentiation and proliferation of pNETs, as well as the spatial and temporal heterogeneity of tumors with prognostic and therapeutic implications. This review provides a structured approach for standardized reading and reporting of medical imaging studies with a focus on PET and MR techniques. It explains which imaging approach should be used for different subtypes of pNET and what a radiologist should be looking for and reporting when interpreting these studies.

Biomedical applications of radioiodinated peptides

The overexpression of peptide receptors in certain tumors as compared to endogeneous expression levels represents the molecular basis for the design of peptide-based tools for targeted nuclear imaging and therapy. Receptor targeting with radiolabelled peptides became a very important imaging and/or therapeutic approach in nuclear medicine and oncology. A great variety of peptides has been radiolabelled with clinical relevant radionuclides, such as radiometals and radiohalogens. However, to the best of our knowledge concise and updated reviews providing information about the biomedical application of radioiodinated peptides are still missing. This review outlines the synthetic efforts in the preparation of radioiodinated peptides highlighting the importance of radioiodine in nuclear medicine, giving an overview of the most relevant radioiodination strategies that have been employed and describes relevant examples of their use in the biomedical field.

Molecular Imaging to the Surgeons Rescue: Gallium-68 DOTA-Exendin-4 Positron Emission Tomography-Computed Tomography in Pre-operative Localization of Insulinomas

Background

Insulinoma is an islet-cell neoplasm that secretes insulin. It is usually localized to the pancreas and is often the most common cause of endogenous hyperinsulinemic hypoglycaemia in non-diabetic adult patients. Surgical excision with a curative intent is the standard modality of treatment, and it requires precise localization of tumor tissue. Ga-68 DOTA-exendin-4 PET/CT scan is a clinically reasonable and sensitive scan for the identification of insulinoma. The aim of this prospective cohort study was to determine the overall accuracy of Ga-68 DOTA-exendin-4 PET/CT scan in the detection of insulinoma.

Materials and Methods

Eight patients with fasting hyperinsulinemic hypoglycaemia with neuroglycopenic symptoms were enrolled in this study which was conducted during October 2016 to October 2017. Whole body PET/CT scan was performed on a Philips time of flight PET/CT scanner, 60 minutes after injection of Ga-68 DOTA-exendin-4 (and also Ga-68 DOTANOC). The imaging findings were compared to the histopathological diagnosis in six out of eight patients and to subsequent follow up in the remaining two patients who did not undergo surgery.

Results

The sensitivity of Ga-68 DOTA-Exendin-4 PET/CT scan in insulinoma detection was found to be 75%.

Conclusion

Ga-68 DOTA-Exendin-4 PET/CT scan is highly sensitive for identification and exact localization of insulinoma which can guide better surgical exploration. However, randomised controlled trials are needed to assess the accuracy of Ga-68 DOTA-Exendin PET/CT scan in localization of insulinoma.

PET/MR Imaging of the Pancreas

PET/MR imaging has the potential to markedly alter pancreatic care in both the malignant, and premalignant states with the ability to perform robust, high-resolution, quantitative molecular imaging. The ability of PET/MR imaging to monitor disease processes, potentially correct for motion in the upper abdomen, and provide novel biomarkers that may be a combination of MR imaging and PET biomarkers, offers a unique, precise interrogation of the pancreatic milieu going forward.

Preoperative imaging and pathologic classification for pancreatic neuroendocrine tumors

The management of patients with pancreatic neuroendocrine tumor (PNET), whether hormonally secretory or not, is multidisciplinary and often multimodal. Surgical treatment plays a central role because complete resection is the only potentially curative treatment. The choice of the therapeutic plan for a PNET requires precise localization of the primary tumor (which may sometimes be multiple in case of genetic predisposition), confirmation of the diagnosis of PNET, a search for metastases (mainly hepatic), and identification of the main histoprognostic factors. This update focuses on the WHO 2017 histological classification and recent innovations in the preoperative assessment of PNET using conventional and isotopic imaging. The aim is to not only allow the mapping of primary and metastatic lesions but also to predict tumor aggressiveness.

Fully automated GMP production of [68Ga]Ga-DO3A-VS-Cys40-Exendin-4 for clinical use

[68Ga]Ga-DO3A-VS-Cys40-Exendin-4/PET-CT targeting glucagon like peptide-1 receptor (GLP-1R) has previously demonstrated its potential clinical value for the detection of insulinomas. The production and accessibility of this radiopharmaceutical is one of the critical factors in realization of clinical trials and routine clinical examinations. Previously, the radiopharmaceutical was prepared manually, however larger scale of clinical trials and healthcare requires automation of the production process in order to limit the operator radiation dose as well as improve tracer manufacturing robustness and on-line documentation for enhanced good manufacturing practice (GMP) compliance. A method for 68Ga-labelling of DO3A-VS-Cys40-Exendin-4 on a commercially available synthesis platform was developed. Equipment such as 68Ge/68Ga generator, synthesis platform, and disposable cassettes for 68Ga-labelling used in the study was purchased from Eckert & Ziegler. DO3A-VS-Cys40-Exendin-4 was synthesized in-house. The parameters such as time, temperature, precursor concentration, radical scavenger, buffer concentration, pH, product purification step were investigated and optimised. Reproducible and GMP compliant automated production of [68Ga]Ga-DO3A-VS-Cys40-Exendin-4 was developed. Exendin-4 comprising methionine amino acid residue was prone to oxidation which was strongly influenced by the elevated temperature, radioactivity amount, and precursor concentration. The suppression of the oxidative radiolysis was achieved by addition of ethanol, dihydroxybenzoic acid and ascorbic acid to the reaction buffer as well as by optimizing heating temperature. The non-decay corrected radiochemical yield was 43±2% with radiochemical purity of over 90% wherein the individual impurity signals in HPLC chromatogram did not exceed 5%. Automated production and quality control methods were established for paving the pathway for broader clinical use of [68Ga]Ga-DO3A-VS-Cys40-Exendin-4.

Molecular imaging in the investigation of hypoglycaemic syndromes and their management

There has been recent progress in molecular imaging using a variety of cellular targets for the investigation of adult non-diabetic hypoglycaemic syndromes and its integration into patient management. These targets include peptide receptors (somatostatin receptors (SSTRs) and glucagon-like peptide-1 receptor (GLP-1R)) the amine precursor uptake and decarboxylation system utilising the diphydroxyphenylaline (DOPA) analogue 6-[¹⁸F]-l-fluoro-l-3,4-dihydroxyphenylalanine (¹⁸F-FDOPA), and glycolytic metabolism with 2-[¹⁸F]fluoro-2-deoxy-d-glucose (FDG). Accurate preoperative localisation and staging is critical to enable directed surgical excision or enucleation with minimal morbidity and preservation of residual pancreatic function. Benign insulinoma has near ubiquitous dense GLP-1R expression enabling accurate localisation with radiolabelled-exendin-4 compounds (e.g. ⁶⁸Ga-NOTA-exendin-4 PET/CT), whilst the rarer and more difficult to manage metastatic insulinoma typically express SSTR and is preferably imaged with radiolabelled-SSTR analogues such as ⁶⁸Ga-DOTA-octreotate (DOTATATE) PET/CT for staging and assessment of suitability for peptide receptor radionuclide therapy (PRRT). Similar to other metastatic neuroendocrine tumours, FDG PET/CT is used in the setting of higher-grade metastatic insulinoma to provide important prognostic information that can guide treatment and determine suitability for PRRT. Interestingly, these three tracers appear to represent a spectrum of differentiation, which we conceptually describe as the 'triple-flop' phenomenon, with GLP-1R > SSTR > FDG in benign insulinoma and the opposite in higher-grade disease. This paper will review the clinical syndromes of adult hypoglycaemia (including a practical overview of the differential diagnoses to be considered), comparison of techniques for insulinoma localisation with emphasis on molecular imaging before discussing its implications for management of metastatic insulinoma.

Preclinical evaluation of PAC1 targeting with radiolabeled Maxadilan

There is an ongoing search for new tracers to optimize imaging of beta cell-derived tumors (insulinomas). The PAC1 receptor, expressed by insulinomas, can be used for targeting of these tumors. Here, we investigated whether radiolabeled maxadilan could be used for insulinoma imaging. Maxadilan was C- or N-terminally conjugated with DTPA (termed maxadilan-DPTA or DTPA-maxadilan respectively). BALB/c nude mice bearing subcutaneous INS-1 tumors were injected with either In-111-labeled maxadilan-DTPA or In-111-DTPA-maxadilan. Biodistribution studies were carried out at 1, 2 and 4 hours after injection and SPECT/CT imaging 1 and 4 hours after injection of maxadilan-DTPA-¹¹¹In. Radiolabeling of maxadilan-DTPA (680 MBq/nmol) was more efficient than of DTPA-maxadilan (55 MBq/nmol). Conjugation with DTPA slightly reduced receptor binding affinity in vitro: IC₅₀ values were 3.2, 21.0 and 21.0 nM for maxadilan, ^natIn-DTPA-maxadilan and maxadilan-DTPA-^natIn respectively. Upon i.v. injection maxadilan-DTPA-¹¹¹In accumulated specifically in INS-1 tumors (7.30 ± 1.87%ID/g) and in the pancreas (3.82 ± 0.22%ID/g). INS-1 tumors were clearly visualized by small animal SPECT/CT. In conclusion, this study showed that the high affinity of maxadilan to the PAC1 receptor was maintained after DTPA conjugation. Furthermore, radiolabeled maxadilan-DTPA accumulated specifically in INS-1 tumors and, therefore, may qualify as a useful tracer to image insulinomas.

Advances in the Diagnosis of Neuroendocrine Neoplasms

Somatostatin receptor PET/CT using (68)Ga-labeled somatostatin analogs, is a mainstay for the evaluation of the somatostatin receptor status in neuroendocrine neoplasms. In addition, the assessment of glucose metabolism by (18)F-FDG PET/CT at diagnosis can overcome probable shortcomings of histopathologic grading. This offers a systematic theranostic approach for the management of neuroendocrine neoplasms, that is, patient selection for the appropriate treatment-surgery, somatostatin analogs, peptide receptor radionuclide therapy, targeted therapies like everolimus and sunitinib, or chemotherapy-and also for therapy response monitoring. Novel targets, for example, the chemokine receptor CXCR4 in higher-grade tumors and glucagon like peptide-1 receptor in insulinomas, appear promising for imaging. Scandium-44 and Copper-64, especially on account of their longer half-life (for pretherapeutic dosimetry) and cyclotron production (which favors mass production), might be the potential alternatives to (68)Ga for PET/CT imaging. The future of molecular imaging lies in Radiomics, that is, qualitative and quantitative characterization of tumor phenotypes in correlation with tumor genomics and proteomics, for a personalized cancer management.

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.