68Ga-labelled exendin-3, a new agent for the detection of insulinomas with PET

Combined Optical Coherence and Fluorescence Microscopy to assess dynamics and specificity of pancreatic beta-cell tracers

The identification of a beta-cell tracer is a major quest in diabetes research. However, since MRI, PET and SPECT cannot resolve individual islets, optical techniques are required to assess the specificity of these tracers. We propose to combine Optical Coherence Microscopy (OCM) with fluorescence detection in a single optical platform to facilitate these initial screening steps from cell culture up to living rodents. OCM can image islets and vascularization without any labeling. Thereby, it alleviates the need of both genetically modified mice to detect islets and injection of external dye to reveal vascularization. We characterized Cy5.5-exendin-3, an agonist of glucagon-like peptide 1 receptor (GLP1R), for which other imaging modalities have been used and can serve as a reference. Cultured cells transfected with GLP1R and incubated with Cy5.5-exendin-3 show full tracer internalization. We determined that a dose of 1 μg of Cy5.5-exendin-3 is sufficient to optically detect in vivo the tracer in islets with a high specificity. In a next step, time-lapse OCM imaging was used to monitor the rapid and specific tracer accumulation in murine islets and its persistence over hours. This optical platform represents a versatile toolbox for selecting beta-cell specific markers for diabetes research and future clinical diagnosis.

(111)In-exendin uptake in the pancreas correlates with the β-cell mass and not with the α-cell mass

Targeting of the GLP-1 receptor with (111)In-labeled exendin is an attractive approach to determine the β-cell mass (BCM). Preclinical studies as well as a proof-of-concept study in type 1 diabetic patients and healthy subjects showed a direct correlation between BCM and radiotracer uptake. Despite these promising initial results, the influence of α-cells on the uptake of the radiotracer remains a matter of debate. In this study, we determined the correlation between pancreatic tracer uptake and β- and α-cell mass in a rat model for β-cell loss. The uptake of (111)In-exendin (% ID/g) showed a strong positive linear correlation with the BCM (Pearson r = 0.82). The fraction of glucagon-positive cells in the total endocrine mass was increased after alloxan treatment (26% ± 4%, 43% ± 8%, and 69% ± 21% for 0, 45, and 60 mg/kg alloxan, respectively). The uptake of (111)In-exendin showed a negative linear correlation with the α-cell fraction (Pearson r = -0.76). These data clearly indicate toward specificity of (111)In-exendin for β-cells and that the influence of the α-cells on (111)In-exendin uptake is negligible.

F-18 fluorodeoxyglucose positron emission tomography for differential diagnosis of pancreatic tumors

Positron emission tomography with 2-deoxy-2-[¹⁸F]fluoro-D-glucose (FDG-PET) has been proven useful for differentiating pancreatic ductal cancer from mass-forming chronic pancreatitis. However, there are particular pancreatic tumors having various grades of malignancy such as intraductal papillary mucinous neoplasm (IPMN) or pancreatic neuroendocrine tumor. We examined whether the cut-off value of maximum standardized uptake value (SUV_max) determined by pancreatic ductal cancers is also applicable for other pancreatic tumors.

One hundred thirty six patients with pancreatic tumors underwent FDG-PET imaging. We first analyzed the cut-off value to differentiate pancreatic ductal cancers from mass-forming chronic pancreatitis. Secondly, we determined the cut-off value between malignant IPMN and benign IPMN. Thirdly, we computed a cut-off value between malignant pancreatic tumors and benign tumors irrespective of tumor type.

The optimal cut-off value to differentiate ductal cancers from mass-forming chronic pancreatitis was 2.5. The optimal cut-off value for differentiating malignant IPMN from benign IPMN was also 2.5, similar to that of reported studies. In all types of pancreatic tumors, the cut-off value was also 2.5. The accuracy for detecting malignancy was 93.4% for all tumors.

In the FDG-PET study for pancreatic tumors, an SUV_max of 2.5 would be justified as a cut-off value to differentiate malignant lesions.

Preliminary evaluation of [18F]AlF-NOTA-MAL-Cys39-exendin-4 in insulinoma with PET

BACKGROUND

High expression of glucagon-like peptide-1 receptor (GLP-1R) in insulinoma supplies a potential drug target for tumor imaging. Exendin-4 can specifically bind to GLP-1R as an agonist and its analogs are extensively used in receptor imaging studies.

PURPOSE

A new GLP-1R imaging agent, [(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4, was designed and prepared for insulinoma imaging.

METHODS

Cys(39)-exendin-4 was conjugated with NOTA-MAL, then the compound was radiolabeled with [(18)F]AlF complex to obtained [(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4. The tumor-targeting characters of the tracer were evaluated in INS-1 cells and BALB/c nude mice models.

RESULTS

[(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4 can be efficiently produced with a yield of 17.5 ± 3.2% (non-decay corrected) and radiochemical purity of >95%. The IC50 value of displacement [(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4 with Cys(39)-exendin-4 was 13.52 ± 1.36 nM. PET images showed excellent tumor visualization with high uptake (9.15 ± 1.6%ID/g at 30 min and 7.74 ± 0.87%ID/g at 60 min). The tumor to muscle, pancreas and liver ratios were 63.25, 3.85 and 7.29 at 60 min after injection. GLP-1R binding specificity was demonstrated by co-injection with an excess of unlabeled Cys(39)-exendin-4 and the tumor uptake was found to be reduced significantly.

CONCLUSION

[(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4 shows favorable characteristics for insulinoma imaging and may be translated to clinical studies.

Synthesis and biological evaluation of [¹¹C]AZ12504948; a novel tracer for imaging of glucokinase in pancreas and liver

INTRODUCTION

Glucokinase (GK) is potentially a target for imaging of islets of Langerhans. Here we report the radiosynthesis and preclinical evaluation of the GK activator, [(11)C]AZ12504948, for in vivo imaging of GK.

METHODS

[(11)C]AZ12504948 was synthesized by O-methylation of the precursor, AZ125555620, using carbon-11 methyl iodide ([(11)C]CH₃I). Preclinical evaluation was performed by autoradiography (ARG) of human tissues and PET/CT studies in pig and non-human primate.

RESULT

[(11)C]AZ12504948 was produced in reproducible good radiochemical yield in 28-30 min. Radiochemical purity of the formulated product was >98% for up to 2 h with specific radioactivities 855 ± 209 GBq/μmol (n=8). The preclinical evaluation showed some specificity for GK in liver, but not in pancreas.

CONCLUSION

[(11)C]AZ12504948 images GK in liver, but the low specificity impedes the visualization of GK in pancreas. Improved target specificity is required for further progress using PET probes based on this class of GK activators.

Glucagon-like peptide-1 (GLP-1) analogs: recent advances, new possibilities, and therapeutic implications

Glucagon-like peptide-1 (GLP-1) is an incretin that plays important physiological roles in glucose homeostasis. Produced from intestine upon food intake, it stimulates insulin secretion and keeps pancreatic β-cells healthy and proliferating. Because of these beneficial effects, it has attracted a great deal of attention in the past decade, and an entirely new line of diabetic therapeutics has emerged based on the peptide. In addition to the therapeutic applications, GLP-1 analogs have demonstrated a potential in molecular imaging of pancreatic β-cells; this may be useful in early detection of the disease and evaluation of therapeutic interventions, including islet transplantation. In this Perspective, we focus on GLP-1 analogs for their studies on improvement of biological activities, enhancement of metabolic stability, investigation of receptor interaction, and visualization of the pancreatic islets.

64Cu- and 68Ga-labelled [Nle(14),Lys(40)(Ahx-NODAGA)NH2]-exendin-4 for pancreatic beta cell imaging in rats

PURPOSE

Glucagon-like peptide-1 receptor (GLP-1R) is a molecular target for imaging of pancreatic beta cells. We compared the ability of [Nle(14),Lys(40)(Ahx-NODAGA-(64)Cu)NH2]-exendin-4 ([(64)Cu]NODAGA-exendin-4) and [Nle(14),Lys(40)(Ahx-NODAGA-(68)Ga)NH2]-exendin-4 ([(68)Ga]NODAGA-exendin-4) to detect native pancreatic islets in rodents.

PROCEDURES

The stability, lipophilicity and affinity of the radiotracers to the GLP-1R were determined in vitro. The biodistribution of the tracers was assessed using autoradiography, ex vivo biodistribution and PET imaging. Estimates for human radiation dosimetry were calculated.

RESULTS

We found GLP-1R-specific labelling of pancreatic islets. However, the pancreas could not be visualised in PET images. The highest uptake of the tracers was observed in the kidneys. Effective dose estimates for [(64)Cu]NODAGA-exendin-4 and [(68)Ga]NODAGA-exendin-4 were 0.144 and 0.012 mSv/MBq, respectively.

CONCLUSION

[(64)Cu]NODAGA-exendin-4 might be more effective for labelling islets than [(68)Ga]NODAGA-exendin-4. This is probably due to the lower specific radioactivity of [(68)Ga]NODAGA-exendin-4 compared to [(64)Cu]NODAGA-exendin-4. The radiation dose in the kidneys may limit the use of [(64)Cu]NODAGA-exendin-4 as a clinical tracer.

18F-FDOPA PET/CT imaging of insulinoma revisited

PURPOSE

(18)F-FDOPA PET imaging is increasingly used in the work-up of patients with neuroendocrine tumours. It has been shown to be of limited value in localizing pancreatic insulin-secreting tumours in adults with hyperinsulinaemic hypoglycaemia (HH) mainly due to (18)F-FDOPA uptake by the whole pancreatic gland. The objective of this study was to review our experience with (18)F-FDOPA PET/CT imaging with carbidopa (CD) premedication in patients with HH in comparison with PET/CT studies performed without CD premedication in an independent population.

METHODS

A retrospective study including 16 HH patients who were investigated between January 2011 and December 2013 using (18)F-FDOPA PET/CT (17 examinations) in two academic endocrine tumour centres was conducted. All PET/CT examinations were performed under CD premedication (200 mg orally, 1 - 2 h prior to tracer injection). The PET/CT acquisition protocol included an early acquisition (5 min after (18)F-FDOPA injection) centred over the upper abdomen and a delayed whole-body acquisition starting 20 - 30 min later. An independent series of eight consecutive patients with HH and investigated before 2011 were considered for comparison. All patients had a reference whole-body PET/CT scan performed about 1 h after (18)F-FDOPA injection. In all cases, PET/CT was performed without CD premedication.

RESULTS

In the study group, (18)F-FDOPA PET/CT with CD premedication was positive in 8 out of 11 patients with histologically proven insulinoma (73 %). All (18)F-FDOPA PET/CT-avid insulinomas were detected on early images and 5 of 11 (45 %) on delayed ones. The tumour/normal pancreas uptake ratio was not significantly different between early and delayed acquisitions. Considering all patients with HH, including those without imaging evidence of disease, the detection rate of the primary lesions using CD-assisted (18)F-FDOPA PET/CT was 53 %, showing 9 insulinomas in 17 studies performed. In the control group (without CD premedication, eight patients), the final diagnosis was benign insulinoma in four, nesidioblastosis in one, and no definitive diagnosis in the remainder. (18)F-FDOPA PET/CT failed to detect any tumour in these patients.

CONCLUSION

According to our experience, CD administration before (18)F-FDOPA injection leads to low residual pancreatic (18)F-FDOPA activity preserving tumoral uptake with consequent insulinoma detection in more than half of adult patients with HH and more than 70 % of patients with a final diagnosis of insulinoma. If (18)F-FDOPA PET/CT is indicated, we strongly recommend combining CD premedication with early acquisition centred over the pancreas.

Preclinical studies of SPECT and PET tracers for NET

Radiolabeled somatostatin analogues are routinely used for the detection of neuroendocrine tumors (NETs), exploiting the expression of somatostatin-receptor subtypes on the cell membrane. Because of the heterogeneity of the origin of these tumors, the performance of radiolabeled somatostatin analogues in certain types of NETs is limited due to the low incidence or low levels of receptor expression. In this review, the most recent developments and in vitro and in vivo characterization of these radiolabeled peptide analogues are discussed.

A comparison of three (67/68)Ga-labelled exendin-4 derivatives for β-cell imaging on the GLP-1 receptor: the influence of the conjugation site of NODAGA as chelator

Background

Various diseases derive from pathologically altered β-cells. Their function can be increased, leading to hyperinsulinism, or decreased, resulting in diabetes. Non-invasive imaging of the β-cell-specific glucagon-like peptide receptor-1 (GLP-1R) would allow the assessment of both β-cell mass and derived tumours, potentially improving the diagnosis of various conditions. We tested three new ^67/68Ga-labelled derivatives of exendin-4, an agonist of GLP-1R, in vitro and in vivo. We determined the influence of the chelator NODAGA conjugated to resident lysines either at positions 12 and 27 or the C-terminally attached lysine at position 40 on the binding and kinetics of the peptide.

Methods

Binding and internalisation of ⁶⁷Ga-labelled Ex4NOD12, Ex4NOD27 and Ex4NOD40 were tested on Chinese hamster lung (CHL) cells stably transfected to express the GLP-1 receptor (GLP-1R). In vivo biodistribution of ⁶⁸Ga-labelled peptides was investigated in CD1 nu/nu mice with subcutaneous CHL-GLP-1R positive tumours; the specificity of the binding to GLP-1R was determined by pre-injecting excess peptide.

Results

All peptides showed good in vitro binding affinities to GLP-1R in the range of 29 to 54 nM. ^67/68Ga-Ex4NOD40 and ^67/68Ga-Ex4NOD12 show excellent internalisation (>30%) and high specific uptake in GLP-1R positive tissue, but high activity was also found in the kidneys.

Conclusions

We show that of the three peptides, Ga-Ex4NOD40 and Ga-Ex4NOD12 demonstrate the most favourable in vitro properties and in vivo binding to GLP-1R positive tissue. Therefore, we conclude that the lysines at positions 12 and 40 might preferentially be utilised for modifying exendin-4.

(64)Cu labeled sarcophagine exendin-4 for microPET imaging of glucagon like peptide-1 receptor expression

The Glucagon-like peptide 1 receptor (GLP-1R) has become an important target for imaging due to its elevated expression profile in pancreatic islets, insulinoma, and the cardiovascular system. Because native GLP-1 is degraded rapidly by dipeptidyl peptidase-IV (DPP-IV), several studies have conjugated different chelators to a more stable analog of GLP-1 (such as exendin-4) as PET or SPECT imaging agents with various advantages and disadvantages. Based on the recently developed Sarcophagin chelator, here, we describe the construction of GLP-1R targeted PET probes containing monomeric and dimeric exendin-4 subunit. The in vitro binding affinity of BarMalSar-exendin-4 and Mal2Sar-(exendin-4)2 was evaluated in INS-1 cells, which over-express GLP-1R. Mal2Sar-(exendin-4)2 demonstrated around 3 times higher binding affinity compared with BaMalSar-exendin-4. After (64)Cu labeling, microPET imaging of (64)Cu-BaMalSar-exendin-4 and (64)Cu-Mal2Sar-(exendin-4)2 were performed on subcutaneous INS-1 tumors, which were clearly visualized with both probes. The tumor uptake of (64)Cu-Mal2Sar-(exendin-4)2 was significantly higher than that of (64)Cu-BaMaSarl-exendin-4, which could be caused by polyvalency effect. The receptor specificity of these probes was confirmed by effective blocking of the uptake in both tumor and normal positive organs with 20-fold excess of unlabeled exendin-4. In conclusion, sarcophagine cage conjugated exendin-4 demonstrated persistent and specific uptake in INS-1 insulinoma model. Dimerization of exendin-4 could successfully lead to increased tumor uptake in vivo. Both (64)Cu-BaMalSar-exendin-4 and (64)Cu-Mal2Sar-(exendin-4)2 hold a great potential for GLP-1R targeted imaging.

Detection of metastatic insulinoma by positron emission tomography with [(68)ga]exendin-4-a case report

CONTEXT

Insulinomas are the most common cause of endogenous hyperinsulinemic hypoglycemia in nondiabetic adult patients. They are usually benign, and curative surgery is the "gold standard" treatment if they can be localized. Malignant insulinomas are seen in less than 10% of patients, and their prognosis is poor. The glucagon like peptide-1 receptor (GLP-1R) is markedly up-regulated in insulinomas-especially benign lesions, which are difficult to localize with current imaging techniques.

OBJECTIVE

The aim of the study was to assess the possibility of the detection of primary and metastatic insulinoma by positron emission tomography (PET) using [(68)Ga]Ga-DO3A-VS-Cys(40)-Exendin-4 ([(68)Ga]Exendin-4) in a patient with severe hypoglycemia.

DESIGN AND SETTING

Dynamic and static PET/computed tomography (CT) examination of a patient was performed using [(68)Ga]Exendin-4 at Uppsala University Hospital, Uppsala, Sweden.

PATIENTS

A patient presented with hypoglycemia requiring continuous iv glucose infusions. A pancreatic insulinoma was suspected, and an exploratory laparotomy was urgently performed. At surgery, a tumor in the pancreatic tail with an adjacent metastasis was found, and a distal pancreatic resection (plus splenectomy) and removal of lymph node were performed. Histopathology showed a World Health Organization classification grade II insulinoma. Postoperatively, hypoglycemia persisted, but a PET/CT examination using the neuroendocrine marker [(11)C]-5-hydroxy-L-tryptophan was negative.

INTERVENTIONS

The patient was administered [(68)Ga]Exendin-4 and was examined by dynamic PET over the liver and pancreas.

RESULTS

The stable GLP-1 analog Exendin-4 was labeled with (68)Ga for PET imaging of GLP-1R-expressing tumors. The patient was examined by [(68)Ga]Exendin-4-PET/CT, which confirmed several small GLP-1R-positive lesions in the liver and a lymph node that could not be conclusively identified by other imaging techniques. The results obtained from the [(68)Ga]Exendin-4-PET/CT examination provided the basis for continued systemic treatment.

CONCLUSION

The results of the [(68)Ga]Exendin-4-PET/CT examination governed the treatment strategy of this particular patient and demonstrated the potential of this technique for future management of patients with this rare but potentially fatal disease.

Inside the pancreas: progress and challenges of human beta cell mass quantification

The accurate quantification of beta cell mass in humans is one of the key challenges in understanding the role of beta cell loss and dysfunction in the pathogenesis of diabetes mellitus. Autopsy studies indicate that beta cell loss is not only a hallmark of autoimmune diabetes but also plays a pivotal role in type 2 diabetes, owing to the toxic effects of lipids, glucose and cytokines. Thus, there is an urgent need for non-invasive clinical techniques for beta cell mass quantification, which should be optimally integrated into standard diagnostic equipment in hospitals. In this issue of Diabetologia (Brom et al DOI 10.1007/s00125-014-3166-3) it is reported that single photon emission computed tomography (SPECT) data with (111)indium-labelled glucagon-like peptide-1 (GLP-1) receptor agonist exendin-3 correlate with the morphometric analysis of beta cell mass in a rat model of alloxan-induced diabetes. With this validation, the authors were able to demonstrate a significant loss of beta cell mass in C-peptide-negative type 1 diabetic patients. Thus, (111)indium-labelled exendin-3 could serve as a model tracer for future studies of larger cohorts of diabetic patients to monitor the dynamics of beta cell loss and regeneration. Despite the recent progress from SPECT imaging data there remain open questions that await clarification in the near future such as variations in GLP-1 receptor density and physiological variation of beta cell mass in relation to beta cell function. The use of GLP-1-based tracer analysis may open new clinical avenues for non-invasive quantification of beta cell mass in patients with newly diagnosed type 1 diabetes and prediabetic individuals with high titres of autoantibodies.

GEP-NETS update: functional localisation and scintigraphy in neuroendocrine tumours of the gastrointestinal tract and pancreas (GEP-NETs)

For patients with neuroendocrine tumours (NETs) of the gastrointestinal tract and pancreas (GEP) (GEP-NETs), excellent care should ideally be provided by a multidisciplinary team of skilled health care professionals. In these patients, a combination of nuclear medicine imaging and conventional radiological imaging techniques is usually mandatory for primary tumour visualisation, tumour staging and evaluation of treatment. In specific cases, as in patients with occult insulinomas, sampling procedures can provide a clue as to where to localise the insulin-hypersecreting pancreatic NETs. Recent developments in these fields have led to an increase in the detection rate of primary GEP-NETs and their metastatic deposits. Radiopharmaceuticals targeted at specific tumour cell properties and processes can be used to provide sensitive and specific whole-body imaging. Functional imaging also allows for patient selection for receptor-based therapies and prediction of the efficacy of such therapies. Positron emission tomography/computed tomography (CT) and single-photon emission CT/CT are used to map functional images with anatomical localisations. As a result, tumour imaging and tumour follow-up strategies can be optimised for every individual GEP-NET patient. In some cases, functional imaging might give indications with regard to future tumour behaviour and prognosis.

Non-invasive quantification of the beta cell mass by SPECT with ¹¹¹In-labelled exendin

AIMS/HYPOTHESIS

A reliable method for in vivo quantification of pancreatic beta cell mass (BCM) could lead to further insight into the pathophysiology of diabetes. The glucagon-like peptide 1 receptor, abundantly expressed on beta cells, may be a suitable target for imaging. We investigated the potential of radiotracer imaging with the GLP-1 analogue exendin labelled with indium-111 for determination of BCM in vivo in a rodent model of beta cell loss and in patients with type 1 diabetes and healthy individuals.

METHODS

The targeting of (111)In-labelled exendin was examined in a rat model of alloxan-induced beta cell loss. Rats were injected with 15 MBq (111)In-labelled exendin and single photon emission computed tomography (SPECT) acquisition was performed 1 h post injection, followed by dissection, biodistribution and ex vivo autoradiography studies of pancreatic sections. BCM was determined by morphometric analysis after staining with an anti-insulin antibody. For clinical evaluation SPECT was acquired 4, 24 and 48 h after injection of 150 MBq (111)In-labelled exendin in five patients with type 1 diabetes and five healthy individuals. The tracer uptake was determined by quantitative analysis of the SPECT images.

RESULTS

In rats, (111)In-labelled exendin specifically targets the beta cells and pancreatic uptake is highly correlated with BCM. In humans, the pancreas was visible in SPECT images and the pancreatic uptake showed high interindividual variation with a substantially lower uptake in patients with type 1 diabetes.

CONCLUSIONS/INTERPRETATION

These studies indicate that (111)In-labelled exendin may be suitable for non-invasive quantification of BCM.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01825148, EudraCT: 2012-000619-10.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Gallium-68: chemistry and radiolabeled peptides exploring different oncogenic pathways

Abstract Early and specific tumor detection and also therapy selection and response evaluation are some challenges of personalized medicine. This calls for high sensitive and specific molecular imaging such as positron emission tomography (PET). The use of peptides for PET molecular imaging has undeniable advantages: possibility of targeting through peptide-receptor interaction, small size and low-molecular weight conferring good penetration in the tissue or at cellular level, low toxicity, no antigenicity, and possibility of wide choice for radiolabeling. Among β(+)-emitter radioelements, Gallium-68 is a very attractive positron-emitter compared with carbon-11 or fluorine-18 taking into account its easy production via a (68)Ge/(68)Ga generator and well established radiochemistry. Gallium-68 chemistry is based on well-defined coordination complexes with macrocycle or chelates having strong binding properties, particularly suitable for linking peptides that allow resistance to in vivo transchelation of the metal ion. Understanding specific and nonspecific molecular mechanisms involved in oncogenesis is one major key to develop new molecular imaging tools. The present review focuses on peptide signaling involved in different oncogenic pathways. This peptide signalization might be common for tumoral and non-tumoral processes or could be specific of an oncological process. This review describes gallium chemistry and different (68)Ga-radiolabeled peptides already in use or under development aiming at developing molecular PET imaging of different oncological processes.

Development of a new thiol site-specific prosthetic group and its conjugation with [Cys(40)]-exendin-4 for in vivo targeting of insulinomas

A new tracer, N-5-[(18)F]fluoropentylmaleimide ([(18)F]FPenM), for site-specific labeling of free thiol group in proteins and peptides was developed. The tracer was synthesized in three steps ((18)F displacement of the aliphatic tosylate, di-Boc removal by TFA to expose free amine, and incorporation of the free amine into a maleimide). The radiosynthesis was completed in 110 min with 11-17% radiochemical yield (uncorrected), and specific activity of 20-49 GBq/μmol. [(18)F]FPenM showed comparable labeling efficiency with N-[2-(4-[(18)F]fluorobenzamido)ethyl]maleimide ([(18)F]FBEM). Its application was demonstrated by conjugation with glucagon-like peptide type 1 (GLP-1) analogue [cys(40)]-exendin-4. The cell uptake, binding affinity, imaging properties, biodistribution, and metabolic stability of the radiolabeled [(18)F]FPenM-[cys(40)]-exendin-4 were studied using INS-1 tumor cells and INS-1 xenograft model. Positron emission tomography (PET) results showed that the new thiol-specific tracer, [(18)F]FPenM-[cys(40)]-exendin-4, had high tumor uptake (20.32 ± 4.36%ID/g at 60 min postinjection) and rapid liver and kidney clearance, which was comparable to the imaging results with [(18)F]FBEM-[cys(40)]-exendin-4 reported by our group.

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

The glucagonlike peptide 1 receptor (GLP-1R) is mainly expressed on β-cells in the islets of Langerhans and is therefore an attractive target for imaging of the β-cell mass. In the present study, (68)Ga-labeled exendin-4 was evaluated for PET imaging and quantification of GLP-1R in the pancreas.

METHODS

Dose escalation studies of (68)Ga-labeled 1,4,7-tris(carboxymethylaza)cyclododecane-10-azaacetyl (DO3A)-exendin-4 were performed in rats (organ distribution) and cynomolgus monkeys (PET/CT imaging) to determine the GLP-1R-specific tissue uptake in vivo. Pancreatic uptake (as determined by organ distribution) in healthy rats was compared with that in diabetic rats. GLP-1R occupancy in the cynomolgus pancreas was quantified with a 1-tissue-compartment model.

RESULTS

In rodents, uptake in the pancreas was decreased from the baseline by up to 90% (P < 0.0001) by coadministration of DO3A-exendin-4 at 100 μg/kg. Pancreatic uptake in diabetic animals was decreased by more than 80% (P < 0.001) compared with that in healthy controls, as measured by organ distribution. GLP-1R occupancy in the cynomolgus pancreas after coinjection of DO3A-exendin-4 at 0.15-20 μg/kg ranged from 49% to 97%, as estimated by compartment modeling.

CONCLUSION

These results strongly support the notion that (68)Ga-DO3A-exendin-4 uptake in the pancreas is mediated by specific receptor binding. In addition, pancreatic uptake was decreased by selective destruction of β-cells. This result suggests that GLP-1R can be quantified in vivo, which has major implications for the prospect of imaging of native β-cells.

The role of exendin-4-conjugated superparamagnetic iron oxide nanoparticles in beta-cell-targeted MRI

Noninvasive targeted visualization of pancreatic beta cells or islets is becoming the focus of molecular imaging application in diabetes and islet transplantation studies, but it is currently unsuccessful due to the lack of specific beta cell biomarkers. Glucagon-like peptide 1 receptor (GLP-1R) is highly expressed in beta cells and considered as a promising target. We here developed a targeted superparamagnetic iron oxide (SPIO) nanoparticle using GLP-1 analog-exendin-4 which is conjugated to polyethylene glycol coated SPIO (PEG-SPIO). The results demonstrated that exendin-4 functionalized SPIO was able to specifically bind to and internalized by GLP-1R-expressing INS-1 cells, with the higher labeling efficiency than non-targeted nanoparticles. Notably, SPIO-exendin4 could differentially label islets in pancreatic slices or beta cell grafts in vitro. Systemic delivery of SPIO-exendin4 into nude mice bearing s.c. insulinomas (derived from INS-1 cells) leads to the accumulation of the nanoparticles in tumors, generating a strong magnetic resonance imaging contrast detectable by a clinical MRI scanner at field strength of 3.0 T, and the iron deposition in tumors was further confirmed by Prussian blue staining. Furthermore, preliminary biodistribution study indicated that SPIO-exendin4 had a tendency to accumulate in pancreas. Toxicity assessments demonstrated good biocompatibility in vivo. These results suggest that SPIO-exendin4 has potential as molecularly targeted imaging agents for in vivo imaging of insulinoma, and possibly for future beta cell imaging.

Development and evaluation of 18F-TTCO-Cys40-Exendin-4: a PET probe for imaging transplanted islets

Because islet transplantation has become a promising treatment option for patients with type 1 diabetes, a noninvasive imaging method is greatly needed to monitor these islets over time. Here, we developed an (18)F-labeled exendin-4 in high specific activity for islet imaging by targeting the glucagonlike peptide-1 receptor (GLP-1R).

METHODS

Tetrazine ligation was used to radiolabel exendin-4 with (18)F. The receptor binding of (19/18)F-tetrazine trans-cyclooctene (TTCO)-Cys(40)-exendin-4 was evaluated in vitro with INS-1 cell and in vivo on INS-1 tumor (GLP-1R positive) and islet transplantation models.

RESULTS

(18)F-TTCO-Cys(40)-exendin-4 was obtained in high specific activity and could specifically bind to GLP-1R in vitro and in vivo. Unlike the radiometal-labeled exendin-4, (18)F-TTCO-Cys(40)-exendin-4 has much lower kidney uptake. (18)F-TTCO-Cys(40)-exendin-4 demonstrated its great potential for transplanted islet imaging: the liver uptake value derived from small-animal PET images correlated well with the transplanted β-cell mass determined by immunostaining. Autoradiography showed that the localizations of radioactive signal indeed corresponded to the distribution of islet grafts in the liver of islet-transplanted mice.

CONCLUSION

(18)F-TTCO-Cys(40)-exendin-4 demonstrated specific binding to GLP-1R. This PET probe provides a method to noninvasively image intraportally transplanted islets.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

Molecular Imaging using PET/CT Applying 68Ga-Labeled Tracers and Targeted Radionuclide Therapy: Theranostics on the Way to Personalized Medicine

Theranostics is an acronym, which exemplifies the togetherness of diagnostics and therapeutics in the individualized management of disease. The key to personalized medicine in cancer is to determine the molecular phenotypes of neoplasms, so that specific probes can be selected to target the tumor and its microenvironment. Molecular imaging and radionuclide therapy using a particular probe is based on this premise. Neuroendocrine neoplasms express somatostatin receptors, enabling the use of somatostatin analogs for molecular imaging, when labeled with the positron-emitter 68Ga for receptor positron emission tomography/computed tomography (PET/CT), and targeted radionuclide therapy, when labeled with beta-emitters 90Y and 177Lu.

How to cite this article

Kulkarni HR, Baum RP. Molecular Imaging using PET/CT Applying 68Ga-Labeled Tracers and Targeted Radionuclide Therapy: Theranostics on the Way to Personalized Medicine. J Postgrad Med Edu Res 2013; 47(1):47-53.

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

PURPOSE

The objective of this article is to present a new method for the diagnosis of insulinoma with the use of [Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH2]-exendin-4.

METHODS

Studies were performed in 11 patients with negative results of all available non-isotopic diagnostic methods (8 with symptoms of insulinoma, 2 with malignant insulinoma and 1 with nesidioblastosis). In all patients glucagon-like peptide-1 (GLP-1) receptor imaging (whole-body and single photon emission computed tomography/CT examinations) after the injection of 740 MBq of the tracer was performed.

RESULTS

Both sensitivity and specificity of GLP-1 receptor imaging were assessed to be 100 % in patients with benign insulinoma. In all eight cases with suspicion of insulinoma a focal uptake in the pancreas was found. In six patients surgical excision of the tumour was performed (type G1 tumours were confirmed histopathologically). In one patient surgical treatment is planned. One patient was disqualified from surgery. In one case with malignant insulinoma pathological accumulation of the tracer was found only in the region of local recurrence. The GLP-1 study was negative in the other malignant insulinoma patient. In one case with suspicion of nesidioblastosis, a focal accumulation of the tracer was observed and histopathology revealed coexistence of insulinoma and nesidioblastosis.

CONCLUSION

[Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH2]-exendin-4 seems to be a promising diagnostic tool in the localization of small insulinoma tumours, but requires verification in a larger series of patients.

Imaging of β-cell mass and insulitis in insulin-dependent (Type 1) diabetes mellitus

Insulin-dependent (type 1) diabetes mellitus is a metabolic disease with a complex multifactorial etiology and a poorly understood pathogenesis. Genetic and environmental factors cause an autoimmune reaction against pancreatic β-cells, called insulitis, confirmed in pancreatic samples obtained at autopsy. The possibility to noninvasively quantify β-cell mass in vivo would provide important biological insights and facilitate aspects of diagnosis and therapy, including follow-up of islet cell transplantation. Moreover, the availability of a noninvasive tool to quantify the extent and severity of pancreatic insulitis could be useful for understanding the natural history of human insulin-dependent (type 1) diabetes mellitus, to early diagnose children at risk to develop overt diabetes, and to select patients to be treated with immunotherapies aimed at blocking the insulitis and monitoring the efficacy of these therapies. In this review, we outline the imaging techniques currently available for in vivo, noninvasive detection of β-cell mass and insulitis. These imaging techniques include magnetic resonance imaging, ultrasound, computed tomography, bioluminescence and fluorescence imaging, and the nuclear medicine techniques positron emission tomography and single-photon emission computed tomography. Several approaches and radiopharmaceuticals for imaging β-cells and lymphocytic insulitis are reviewed in detail.

PET of glucagonlike peptide receptor upregulation after myocardial ischemia or reperfusion injury

Glucagonlike peptide (GLP-1) and its receptor (GLP-1R) exhibit cardioprotective effects after myocardial ischemia and reperfusion (MI/R) in both animal studies and clinical trials. However, the kinetics of GLP-1R expression in the infarcted/ischemic myocardium has not yet been explored. The purpose of this study was to monitor the presence and time course of regional myocardial GLP-1R expression after MI/R with noninvasive PET.

METHODS

Male Sprague-Dawley rats underwent a 45-min transient left coronary artery occlusion, followed by reperfusion. The myocardial infarction was confirmed by electrocardiogram and cardiac ultrasound. In vivo PET was performed to determine myocardial uptake of (18)F-FBEM-Cys(40)-exendin-4 at different time points after reperfusion. The localization of (18)F-FBEM-Cys(40)-exendin-4 accumulation was determined by coregistering (18)F-FDG PET and CT images. Ex vivo autoradiography, GLP-1R immunohistochemical staining, and Western blot analysis were performed to confirm the PET results.

RESULTS

Myocardial origin and infarcted/ischemic area localization of (18)F-FBEM-Cys(40)-exendin-4 accumulation was confirmed by coregistration of small-animal CT and (18)F-FDG images. At 8 h after MI/R, tracer uptake in the infarcted/ischemic region was 0.37 ± 0.05 percentage injected dose per gram, significantly higher than that in the control group (P < 0.01). The localized tracer uptake decreased, relative to the 8-h time point, but was still significantly higher than the control group on days 1 and 3 after MI/R. At 2 wk after MI/R, the tracer uptake in the affected area showed no significant difference, compared with that in the healthy myocardium. Autoradiography showed the same trend of (18)F-FBEM-Cys(40)-exendin-4 uptake in the myocardial infarcted/ischemic area. The specificity of tracer uptake into ischemic myocardium was supported by decreased tracer uptake after the rats were pretreated with an excess amount of unlabeled exendin-4. Immunohistochemical staining and Western blotting of GLP-1R protein of excised cardiac sections confirmed that the change in uptake observed by PET corresponded to a change in GLP-1R expression.

CONCLUSION

Noninvasive PET using (18)F-FBEM-Cys(40)-exendin-4 revealed a dynamic pattern of GLP-1R upregulation in the infarcted/ischemic area after MI/R. The imaging results will deepen our understanding of the mechanism of the cardioprotective effect of GLP-1 and its analogs and potentially provide guidance for optimization of the time frame of therapeutic intervention.

The untapped potential of Gallium 68-PET: the next wave of ⁶⁸Ga-agents

(68)Gallium-PET ((68)Ga-PET) agents have significant clinical promise. The radionuclide can be produced from a (68)Ge/(68)Ga generator on site and is a convenient alternative to cyclotron-based PET isotopes. The short half-life of (68)Ga permits imaging applications with sufficient radioactivity while maintaining patient dose to an acceptable level. Furthermore, due to superior resolution, (68)Ga-PET agents have the ability to replace current SPECT agents in many applications. This article outlines the upcoming agents and challenges faced during the translational development of (68)Ga agents.

Evaluation of an [(18)F]AlF-NOTA Analog of Exendin-4 for Imaging of GLP-1 Receptor in Insulinoma

INTRODUCTION

The GLP-1 receptor plays an important role in glucose homeostasis and thus is a very important target for diabetes therapy. The receptor is also overexpressed in insulinoma, a tumor of pancreatic beta-cells. We previously evaluated two fluorine-18-labeled analogs of exendin-4 prepared by conjugation with [(18)F]FBEM (N-[2-(4-[(18)F]fluorobenzamide)ethyl]maleimide). Both compounds demonstrated good tumor uptake, but the synthesis of the radiotracers was time consuming. To overcome this challenge, we developed a NOTA analog and performed radiolabeling using aluminum [(18)F]fluoride complexation.

METHODS

Cys(40)-exendin-4 was conjugated with NOTA mono N-ethylmaleimide. [(18)F]AlF conjugation was conducted and the radiolabeled product purified by preparative HPLC. Dynamic and static PET imaging scans were conducted on nude mice with established INS-1 xenografts. Uptake of tumor and other major organs in static images was quantitated (%ID/g) and comparison with blocking studies was made. PET quantification was also compared with ex vivo biodistribution results.

RESULTS

The radiosynthesis provided [(18)F]AlF-NOTA-MAL-cys(40)-exendin-4 in 23.6 ± 2.4 % radiochemical yield (uncorrected, n = 3) after HPLC; the process required about 55 min. The specific activity at time of injection ranged from 19.6 to 31.4 GBq (0.53-0.85 Ci)/µmol. Tumor uptake had reached its maximum (16.09 ± 1.18% ID/g, n = 4) by 5 min and remained nearly constant for the duration of the study. Kidney uptake continued to increase throughout the entire one hour time course. Pre-injection of exendin-4 caused a marked reduction in tissue uptake with the major exception of liver and kidneys, in which uptake was not affected. HPLC analysis of the radioactive components in extracts of the tumor and plasma showed primarily parent compound at 60 min post-injection, whereas extracts of kidney and urine contained exclusively one polar radioactive component.

CONCLUSION

The radiotracer is prepared in a simple one-step procedure and obtained in high specific activity after HPLC purification. [(18)F]AlF-NOTA-MAL-exendin-4 shows high tumor uptake and highly selective GLP-1 tissue uptake (INS-1 tumor, lung, pancreas), but still suffers from high kidney uptake.

Imaging of the β-cells of the islets of Langerhans

The major aim of this paper is to review the present status of the techniques for the non-invasive imaging and quantification of insulin-producing pancreatic islet β-cells. Emphasis is placed on both the expansion of prior work already considered in a prior review and novel achievements. Thus, the use of d-mannoheptulose analogs, hypoglycemic sulfonylureas and glinides, neural imaging agents, neuro-hormonal receptor ligands and nanoparticles is first dealt with. Thereafter, consideration is given on optical imaging technologies, the identification of new β-cells specific binding and target proteins, the functional imaging of islets transplanted into the eye anterior chamber and in vivo manganese-enhanced magnetic resonance imaging.

Efficient 18F-Labeling of Synthetic Exendin-4 Analogues for Imaging Beta Cells

A number of exendin derivatives have been developed to target glucagon-like peptide 1 (GLP-1) receptors on beta cells in vivo. Modifications of exendin analogues have been shown to have significant effects on pharmacokinetics and, as such, have been used to develop a variety of therapeutic compounds. Here, we show that an exendin-4, modified at position 12 with a cysteine conjugated to a tetrazine, can be labeled with 18F-trans-cyclooctene and converted into a PET imaging agent at high yields and with good selectivity. The agent accumulates in beta cells in vivo and has sufficiently high accumulation in mouse models of insulinomas to enable in vivo imaging.

Radiolabelled GLP-1 analogues for in vivo targeting of insulinomas

Internalizing agonists are usually selected for peptide receptor targeting. There is increasing evidence that non-internalizing receptor antagonists can be used for this purpose. We investigated whether the glucagon-like peptide-1 receptor (GLP-1R) antagonist exendin(9-39) can be used for in vivo targeting of GLP-1R expressing tumours and compared the in vitro and in vivo characteristics with the GLP-1R agonists exendin-3 and exendin-4. The binding and internalization kinetics of labelled [Lys(40) (DTPA)]exendin-3, [Lys(40) (DTPA)]exendin-4 and [Lys(40) (DTPA)]exendin(9-39) were determined in vitro using INS-1 cells. The in vivo targeting properties of [Lys(40) ((111) In-DTPA)]exendin-3, [Lys(40) ((111) In-DTPA)]exendin-4 and [Lys(40) ((111) In-DTPA)]exendin(9-39) were examined in BALB/c nude mice with subcutaneous INS-1 tumours. (nat) In-labelled [Lys(40) (DTPA)]exendin-3, [Lys(40) (DTPA)]exendin-4 and [Lys(40) (DTPA)]exendin(9-39) exhibited similar IC(50) values (13.5, 14.4 and 13.4 n m, respectively) and bound to 26 × 10(3) , 41 × 10(3) and 37 × 10(3) receptors per cell, respectively. [Lys(40) ((111) In-DTPA)]exendin-3 and [Lys(40) ((111) In-DTPA)]exendin-4 showed rapid in vitro binding and internalization kinetics, whereas [Lys(40) ((111) In-DTPA)]exendin(9-39) showed lower binding and minimal internalization in vitro. In mice, high specific uptake of [Lys(40) ((111) In-DTPA)]exendin-3 [25.0 ± 6.0% injected dose (ID) g(-1) ] in the tumour was observed at 0.5 h post-injection (p.i.) with similar uptake up to 4 h p.i. [Lys(40) ((111) In-DTPA)]exendin-4 showed higher tumour uptake at 1 and 4 h p.i. (40.8 ± 7.0 and 41.9 ± 7.2% ID g(-1), respectively). Remarkably, [Lys(40) ((111) In-DTPA)]exendin(9-39) showed only low specific uptake in the tumour at 0.5 h p.i. (3.2 ± 0.7% ID g(-1)), rapidly decreasing over time. In conclusion, the GLP-1R agonists [Lys(40) (DTPA)]exendin-3 and [Lys(40) (DTPA)]exendin-4 labelled with (111) In could be useful for in vivo GLP-1R targeting, whereas [Lys(40) (DTPA)]exendin(9-39) is not suited for in vivo targeting of the GLP-1R.

Radiolabelled peptides for oncological diagnosis

Radiolabelled receptor-binding peptides targeting receptors (over)expressed on tumour cells are widely under investigation for tumour diagnosis and therapy. The concept of using radiolabelled receptor-binding peptides to target receptor-expressing tissues in vivo has stimulated a large body of research in nuclear medicine. The ¹¹¹In-labelled somatostatin analogue octreotide (OctreoScan™) is the most successful radiopeptide for tumour imaging, and was the first to be approved for diagnostic use. Based on the success of these studies, other receptor-targeting peptides such as cholecystokinin/gastrin analogues, glucagon-like peptide-1, bombesin (BN), chemokine receptor CXCR4 targeting peptides, and RGD peptides are currently under development or undergoing clinical trials. In this review, we discuss some of these peptides and their analogues, with regard to their potential for radionuclide imaging of tumours.

Radiopharmaceutical development of radiolabelled peptides

Receptor targeting with radiolabelled peptides has become very important in nuclear medicine and oncology in the past few years. The overexpression of many peptide receptors in numerous cancers, compared to their relatively low density in physiological organs, represents the molecular basis for in vivo imaging and targeted radionuclide therapy with radiolabelled peptide-based probes. The prototypes are analogs of somatostatin which are routinely used in the clinic. More recent developments include somatostatin analogs with a broader receptor subtype profile or with antagonistic properties. Many other peptide families such as bombesin, cholecystokinin/gastrin, glucagon-like peptide-1 (GLP-1)/exendin, arginine-glycine-aspartic acid (RGD) etc. have been explored during the last few years and quite a number of potential radiolabelled probes have been derived from them. On the other hand, a variety of strategies and optimized protocols for efficient labelling of peptides with clinically relevant radionuclides such as (99m)Tc, M(3+) radiometals ((111)In, (86/90)Y, (177)Lu, (67/68)Ga), (64/67)Cu, (18)F or radioisotopes of iodine have been developed. The labelling approaches include direct labelling, the use of bifunctional chelators or prosthetic groups. The choice of the labelling approach is driven by the nature and the chemical properties of the radionuclide. Additionally, chemical strategies, including modification of the amino acid sequence and introduction of linkers/spacers with different characteristics, have been explored for the improvement of the overall performance of the radiopeptides, e.g. metabolic stability and pharmacokinetics. Herein, we discuss the development of peptides as radiopharmaceuticals starting from the choice of the labelling method and the conditions to the design and optimization of the peptide probe, as well as some recent developments, focusing on a selected list of peptide families, including somatostatin, bombesin, cholecystokinin/gastrin, GLP-1/exendin and RGD.

Radiolabeled peptides: valuable tools for the detection and treatment of cancer

Human cancer cells overexpress many peptide receptors as molecular targets. Radiolabeled peptides that bind with high affinity and specificity to the receptors on tumor cells hold great potential for both diagnostic imaging and targeted radionuclide therapy. The advantage of solid-phase peptide synthesis, the availability of different chelating agents and prosthetic groups and bioconjugation techniques permit the facile preparation of a wide variety of peptide-based targeting molecules with diverse biological and tumor targeting properties. Some of these peptides, including somatostatin, bombesin, vasoactive intestinal peptide, gastrin, neurotensin, exendin and RGD are currently under investigation. It is anticipated that in the near future many of these peptides may find applications in nuclear oncology. This article presents recent developments in the field of small peptides, and their applications in the diagnosis and treatment of cancer.

Obstacles on the way to the clinical visualisation of beta cells: looking for the Aeneas of molecular imaging to navigate between Scylla and Charybdis

For more than a decade, researchers have been trying to develop non-invasive imaging techniques for the in vivo measurement of viable pancreatic beta cells. However, in spite of intense research efforts, only one tracer for positron emission tomography (PET) imaging is currently under clinical evaluation. To many diabetologists it may remain unclear why the imaging world struggles to develop an effective method for non-invasive beta cell imaging (BCI), which could be useful for both research and clinical purposes. Here, we provide a concise overview of the obstacles and challenges encountered on the way to such BCI, in both native and transplanted islets. We discuss the major difficulties posed by the anatomical and cell biological features of pancreatic islets, as well as the chemical and physical limits of the main imaging modalities, with special focus on PET, SPECT and MRI. We conclude by indicating new avenues for future research in the field, based on several remarkable recent results.

18F-radiolabeled analogs of exendin-4 for PET imaging of GLP-1 in insulinoma

PURPOSE

Glucagon-like peptide type 1 (GLP-1) is an incretin peptide that augments glucose-stimulated insulin release following oral consumption of nutrients. Its message is transmitted via a G protein-coupled receptor called GLP-1R, which is colocalized with pancreatic β-cells. The GLP-1 system is responsible for enhancing insulin release, inhibiting glucagon production, inhibiting hepatic gluconeogenesis, inhibiting gastric mobility, and suppression of appetite. The abundance of GLP-1R in pancreatic β-cells in insulinoma, a cancer of the pancreas, and the activity of GLP-1 in the cardiovascular system have made GLP-1R a target for molecular imaging.

METHODS

We prepared (18)F radioligands for GLP-1R by the reaction of [(18)F]FBEM, a maleimide prosthetic group, with [Cys(0)] and [Cys(40)] analogs of exendin-4. The binding affinity, cellular uptake and internalization, in vitro stability, and uptake and specificity of uptake of the resulting compounds were determined in an INS-1 xenograft model in nude mice.

RESULTS

The [(18)F]FBEM-[Cys(x)]-exendin-4 analogs were obtained in good yield (34.3 ± 3.4%, n = 11), based on the starting compound [(18)F]FBEM), and had a specific activity of 45.51 ± 16.28 GBq/μmol (1.23 ± 0.44 Ci/μmol, n = 7) at the end of synthesis. The C-terminal isomer, [(18)F]FBEM-[Cys(40)]-exendin-4, had higher affinity for INS-1 tumor cells (IC(50) 1.11 ± 0.057 nM) and higher tumor uptake (25.25 ± 3.39 %ID/g at 1 h) than the N-terminal isomer, [(18)F]FBEM-[Cys(0)]-exendin-4 (IC(50) 2.99 ± 0.06 nM, uptake 7.20 ± 1.26 %ID/g at 1 h). Uptake of both isomers into INS-1 tumor, pancreas, stomach, and lung could be blocked by preinjection of nonradiolabeled [Cys(x)]-exendin-4 (p < 0.05).

CONCLUSION

[(18)F]FBEM-[Cys(40)]-exendin-4 and [(18)F]FBEM-[Cys(0)]-exendin-4 have high affinity for GLP-1R and display similar in vitro cell internalization. The higher uptake into INS-1 xenograft tumors exhibited by [(18)F]FBEM-[Cys(40)]-exendin-4 suggests that this compound would be the better tracer for imaging GLP-1R.

Improved labelling of DTPA- and DOTA-conjugated peptides and antibodies with 111In in HEPES and MES buffer

BACKGROUND

In single photon emission computed tomography [SPECT], high specific activity of 111In-labelled tracers will allow administration of low amounts of tracer to prevent receptor saturation and/or side effects. To increase the specific activity, we studied the effect of the buffer used during the labelling procedure: NaAc, NH4Ac, HEPES and MES buffer. The effect of the ageing of the 111InCl3 stock and cadmium contamination, the decay product of 111In, was also examined in these buffers.

METHODS

Escalating amounts of 111InCl3 were added to 1 μg of the diethylene triamine pentaacetic acid [DTPA]- and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid [DOTA]-conjugated compounds (exendin-3, octreotide and anti-carbonic anhydrase IX [CAIX] antibody). Five volumes of 2-(N-morpholino)ethanesulfonic acid [MES], 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid [HEPES], NH4Ac or NaAc (0.1 M, pH 5.5) were added. After 20 min at 20°C (DTPA-conjugated compounds), at 95°C (DOTA-exendin-3 and DOTA-octreotide) or at 45°C (DOTA-anti-CAIX antibody), the labelling efficiency was determined by instant thin layer chromatography. The effect of the ageing of the 111InCl3 stock on the labelling efficiency of DTPA-exendin-3 as well as the effect of increasing concentrations of Cd2+ (the decay product of 111In) were also examined.

RESULTS

Specific activities obtained for DTPA-octreotide and DOTA-anti-CAIX antibody were five times higher in MES and HEPES buffer. Radiolabelling of DTPA-exendin-3, DOTA-exendin-3 and DTPA-anti-CAIX antibody in MES and HEPES buffer resulted in twofold higher specific activities than that in NaAc and NH4Ac. Labelling of DTPA-exendin-3 decreased with 66% and 73% for NaAc and NH4Ac, respectively, at day 11 after the production date of 111InCl3, while for MES and HEPES, the maximal decrease in the specific activity was 10% and 4% at day 11, respectively. The presence of 1 pM Cd2+ in the labelling mixture of DTPA-exendin-3 in NaAc and NH4Ac markedly reduced the labelling efficiency, whereas Cd2+ concentrations up to 0.1 nM did not affect the labelling efficiency in MES and HEPES buffer.

CONCLUSIONS

We showed improved labelling of DTPA- and DOTA-conjugated compounds with 111In in HEPES and MES buffer. The enhanced labelling efficiency appears to be due to the reduced competitive chelation of cadmium. The enhanced labelling efficiency will allow more sensitive imaging of the biomarkers with SPECT.

The role of cell lines in the study of neuroendocrine tumors

Cell lines originating from neuroendocrine tumors (NETs) represent useful experimental models to assess the control of synthesis and release of different hormones and hormone-like peptides, to evaluate the mechanisms of action of these agents in target tissues at the cellular and subcellular levels, and to study cell proliferation and tumor development, as well as the effect of different drugs on these complex processes. To date, the understanding of NET biology (with regard to their mechanisms of hormone secretion, cell proliferation and metastatic spread) has been hampered by the lack of appropriate animal models or cell lines for their study. In the present review, we aim to summarize the recent in vitro/in vivo data regarding cell lines derived from NETs which are most frequently employed in experimental neuroendocrinology.

Glucagon-like peptide-1 receptor overexpression in cancer and its impact on clinical applications

Peptide hormones of the glucagon-like peptide (GLP) family play an increasing clinical role, such as GLP-1 in diabetes therapy. Moreover, GLP receptors are overexpressed in various human tumor types and therefore represent molecular targets for important clinical applications. In particular, virtually all benign insulinomas highly overexpress GLP-1 receptors (GLP-1R). Targeting GLP-1R with the stable GLP-1 analogs (111)In-DOTA/DPTA-exendin-4 offers a new approach to successfully localize these small tumors. This non-invasive technique has the potential to replace the invasive localization of insulinomas by selective arterial stimulation and venous sampling. Malignant insulinomas, in contrast to their benign counterparts, express GLP-1R in only one-third of the cases, while they more often express the somatostatin type 2 receptors. Importantly, one of the two receptors appears to be always expressed in malignant insulinomas. The GLP-1R overexpression in selected cancers is worth to be kept in mind with regard to the increasing use of GLP-1 analogs for diabetes therapy. While the functional role of GLP-1R in neoplasia is not known yet, it may be safe to monitor patients undergoing GLP-1 therapy carefully.

Striated muscle as implantation site for transplanted pancreatic islets

Islet transplantation is an attractive treatment for selected patients with brittle type 1 diabetes. In the clinical setting, intraportal transplantation predominates. However, due to extensive early islet cell death, the quantity of islets needed to restore glucose homeostasis requires in general a minimum of two donors. Moreover, the deterioration of islet function over time results in few insulin-independent patients after five-year followup. Specific obstacles to the success of islet transplantation include site-specific concerns for the liver such as the instant blood mediated inflammatory reaction, islet lipotoxicity, low oxygen tension, and poor revascularization, impediments that have led to the developing interest for alternative implantation sites over recent years. Within preclinical settings, several alternative sites have now been investigated and proven favorable in various aspects. Muscle is considered a very promising site and has physiologically properties and technical advantages that could make it optimal for islet transplantation.

Imaging the islet graft by positron emission tomography

Clinical islet transplantation is being investigated as a permanent cure for type 1 diabetes mellitus (T1DM). Currently, intraportal infusion of islets is the favoured procedure, but several novel implantation sites have been suggested. Noninvasive longitudinal methodologies are an increasingly important tool for assessing the fate of transplanted islets, their mass, function and early signs of rejection. This article reviews the approaches available for islet graft imaging by positron emission tomography and progress in the field, as well as future challenges and opportunities.

PET of insulinoma using ¹⁸F-FBEM-EM3106B, a new GLP-1 analogue

Derived from endocrine pancreatic beta cells, insulinomas express glucagon-like peptide-1 (GLP-1) receptor with high density and incidence. In this study, we labeled a novel GLP-1 analogue, EM3106B, with (18)F and performed PET imaging to visualize insulinoma tumors in an animal model. A GLP-1 analogue that contains multiple lactam bridges, EM3106B, was labeled with (18)F through a maleimide-based prosthetic group, N-2-(4-(18)F-fluorobenzamido)ethylmaleimide ((18)F-FBEM). The newly developed radiotracer was characterized by cell based receptor-binding assay, cell uptake and efflux assay. The stability in serum was evaluated by radio-HPLC analysis. In vivo PET imaging was performed in nude mice bearing subcutaneous INS-1 insulinoma tumors and MDA-MB-435 tumors of melanoma origin. Ex vivo biodistribution study was performed to confirm the PET imaging data. EM3106B showed high binding affinity (IC(50) = 1.38 nM) and high cell uptake (5.25 ± 0.61% after 120 min incubation). (18)F-FBEM conjugation of EM3106B resulted in high labeling yield (24.9 ± 2.4%) and high specific activity (>75 GBq/μmol at the end of bombardment). EM3106B specifically bound and was internalized by GLP-1R positive INS-1 cells. After intravenous injection of 3.7 MBq (100 μCi) of (18)F-FBEM-EM3106B, the INS-1 tumors were clearly visible with high contrast in relation to the contralateral background on PET images, and tumor uptake of (18)F-FBEM-EM3106B was determined to be 28.5 ± 4.7 and 25.4 ± 4.1% ID/g at 60 and 120 min, respectively. (18)F-FBEM-EM3106B showed low uptake in MB-MDA-435 tumors with low level of GLP-1R expression. Direct tissue sampling biodistribution experiment confirmed high tracer uptake in INS-1 tumors and receptor specificity in both INS-1 tumor and pancreas. In conclusion, (18)F-FBEM-EM3106B exhibited GLP-1R-receptor-specific targeting properties in insulinomas. The favorable characteristics of (18)F-FBEM-EM3106B, such as high specific activity and high tumor uptake, and high tumor to nontarget uptake, demonstrate that it is a promising tracer for clinical insulinoma imaging.