Concomitant expression of several peptide receptors in neuroendocrine tumours: molecular basis for in vivo multireceptor tumour targeting

68Ga-labelled NOTA-RGD-GE11 peptide for dual integrin and EGFR-targeted tumour imaging

INTRODUCTION

Multiple peptide receptors are co-expressed in many types of cancers. Arg-Gly-Asp (RGD) and GE11 peptides specifically target integrin α_Vβ₃ and EGFR, respectively. Recently, we designed and synthesized a heterodimer peptide NOTA-c(RGDyK)-GE11 (NOTA-RGD-GE11). The aim of this study was to investigate the characteristics of NOTA-RGD-GE11 for dual receptor imaging.

METHODS

NOTA-RGD-GE11 heterodimer was labelled with ⁶⁸Ga. The dual receptor binding affinity was investigated by antibody competition binding assay. The in vitro and in vivo characteristics of [⁶⁸Ga]Ga-NOTA-RGD-GE11 were investigated and compared with that of monomeric peptides [⁶⁸Ga]Ga-NOTA-RGD and [⁶⁸Ga]Ga-NOTA-GE11.

RESULTS

NOTA-RGD-GE11 had binding affinities with both integrin α_Vβ₃ and EGFR. The dual receptor targeting property of [⁶⁸Ga]Ga-NOTA-RGD-GE11 was validated by blocking studies in a NCI-H292 tumour model. [⁶⁸Ga]Ga-NOTA-RGD-GE11 showed higher tumour uptake than [⁶⁸Ga]Ga-NOTA-RGD and [⁶⁸Ga]Ga-NOTA-GE11 in biodistribution and PET/CT imaging studies.

CONCLUSION

The dual receptor targeting and enhanced tumour uptake of [⁶⁸Ga]Ga-NOTA-RGD-GE11 warrant its further investigation for dual integrin α_Vβ₃ and EGFR-targeted tumour imaging.

Tumor targeting with 99m Tc radiolabeled peptides: Clinical application and recent development

Targeting overexpressed receptors on the cancer cells with radiolabeled peptides has become very important in nuclear oncology in the recent years. Peptides are small and have easy preparation and easy radiolabeling protocol with no side-effect and toxicity. These properties made them a valuable tool for tumor targeting. Based on the successful imaging of neuroendocrine tumors with ¹¹¹ In-octreotide, other receptor-targeting peptides such as bombesin (BBN), cholecystokinin/gastrin analogues, neurotensin analogues, glucagon-like peptide-1, and RGD peptides are currently under development or undergoing clinical trials. The most frequently used radionuclides for tumor imaging are ^99m Tc and ¹¹¹ In for single-photon emission computed tomography and ⁶⁸ Ga and ¹⁸ F for positron emission tomography imaging. This review presents some of the ^99m Tc-labeled peptides, with regard to their potential for radionuclide imaging of tumors in clinical and preclinical application.

A Patient-derived Xenograft Model of Pancreatic Neuroendocrine Tumors Identifies Sapanisertib as a Possible New Treatment for Everolimus-resistant Tumors

Patients with pancreatic neuroendocrine tumors (PNET) commonly develop advanced disease and require systemic therapy. However, treatment options remain limited, in part, because experimental models that reliably emulate PNET disease are lacking. We therefore developed a patient-derived xenograft model of PNET (PDX-PNET), which we then used to evaluate two mTOR inhibitor drugs: FDA-approved everolimus and the investigational new drug sapanisertib. PDX-PNETs maintained a PNET morphology and PNET-specific gene expression signature with serial passage. PDX-PNETs also harbored mutations in genes previously associated with PNETs (such as MEN1 and PTEN), displayed activation of the mTOR pathway, and could be detected by Gallium-68 DOTATATE PET-CT. Treatment of PDX-PNETs with either everolimus or sapanisertib strongly inhibited growth. As seen in patients, some PDX-PNETs developed resistance to everolimus. However, sapanisertib, a more potent inhibitor of the mTOR pathway, caused tumor shrinkage in most everolimus-resistant tumors. Our PDX-PNET model is the first available, validated PDX model for PNET, and preclinical data from the use of this model suggest that sapanisertib may be an effective new treatment option for patients with PNET or everolimus-resistant PNET.

Design, Synthesis, In Vitro, and Initial In Vivo Evaluation of Heterobivalent Peptidic Ligands Targeting Both NPY(Y₁)- and GRP-Receptors-An Improvement for Breast Cancer Imaging?

Heterobivalent peptidic ligands (HBPLs), designed to address two different receptors independently, are highly promising tumor imaging agents. For example, breast cancer has been shown to concomitantly and complementarily overexpress the neuropeptide Y receptor subtype 1 (NPY(Y₁)R) as well as the gastrin-releasing peptide receptor (GRPR). Thus, radiolabeled HBPLs being able to bind these two receptors should exhibit an improved tumor targeting efficiency compared to monospecific ligands. We developed here such bispecific HBPLs and radiolabeled them with ⁶⁸Ga, achieving high radiochemical yields, purities, and molar activities. We evaluated the HBPLs and their monospecific reference peptides in vitro regarding stability and uptake into different breast cancer cell lines and found that the ⁶⁸Ga-HBPLs were efficiently taken up via the GRPR. We also performed in vivo PET/CT imaging and ex vivo biodistribution studies in T-47D tumor-bearing mice for the most promising ⁶⁸Ga-HBPL and compared the results to those obtained for its scrambled analogs. The tumors could easily be visualized by the newly developed ⁶⁸Ga-HBPL and considerably higher tumor uptakes and tumor-to-background ratios were obtained compared to the scrambled analogs in and ex vivo. These results demonstrate the general feasibility of the approach to use bispecific radioligands for in vivo imaging of breast cancer.

Pasireotide in the treatment of neuroendocrine tumors: a review of the literature

Somatostatin analogs have an important role in the medical therapy of neuroendocrine tumors (NETs). Octreotide and lanreotide, both somatostatin analogs binding with high affinity for the somatostatin receptor (SSTR)2, can control symptoms in functional NETs. In addition, these compounds, because of their antiproliferative effects, can stabilize growth of well-differentiated NETs. Pasireotide is a novel multireceptor-targeted somatostatin analog with high affinity for SSTR1, 2, 3, and 5. This review provides an overview of the state of the art of pasireotide in the treatment of NETs, with the aim of addressing clinical relevance and future perspectives for this molecule in the management of NETs.

Biology and Systemic Treatment of Advanced Gastroenteropancreatic Neuroendocrine Tumors

In recent years, there have been important scientific advances in the biologic characterization of neuroendocrine neoplasms and in their treatment. This review will describe these scientific advances, the evolving systemic treatment approaches, and important topics to be addressed in future research.

Diabetes and pancreatic neuroendocrine tumours: Which interplays, if any?

Pancreatic neuroendocrine tumours (PanNETs) represent an uncommon type of pancreatic neoplasm, whose incidence is increasing worldwide. As per exocrine pancreatic cancer, a relationship seems to exist between PanNETs and glycaemic alterations. Diabetes mellitus (DM) or impaired glucose tolerance often occurs in PanNET patients as a consequence of hormonal hypersecretion by the tumour, specifically affecting glucose metabolism, or due to tumour mass effects. On the other hand, pre-existing DM may represent a risk factor for developing PanNETs and is likely to worsen the prognosis of such patients. Moreover, the surgical and/or pharmacological treatment of the tumour itself may impair glucose tolerance, as well as antidiabetic therapies may impact tumour behaviour and patients outcome. Differently from exocrine pancreatic tumours, few data are available for PanNETs as yet on this issue. In the present review, the bidirectional association between glycaemic disorders and PanNETs has been extensively examined, since the co-existence of both diseases in the same individual represents a further challenge for the clinical management of PanNETs.

Impact of 68Ga-DOTA-Peptide PET/CT on the Management of Gastrointestinal Neuroendocrine Tumour (GI-NET): Malaysian National Referral Centre Experience

Purpose

The National Cancer Institute is the only referral centre in Malaysia that provides ⁶⁸Ga-DOTA-peptide imaging. The purpose of this study is to determine the impact of ⁶⁸Ga-DOTA-peptide PET/CT on the management of gastrointestinal neuroendocrine tumours (GI-NET).

Materials and Methods

A cross-sectional study was performed to review the impact of ⁶⁸Ga-DOTA-peptide (⁶⁸Ga-DOTATATE or ⁶⁸Ga-DOTATOC) PET/CT on patients with biopsy-proven GI-NET between January 2011 and December 2015. Suspected NET was excluded. Demographic data, tumoral characteristics, change of disease stage, pre-PET intended management and post-PET management were evaluated.

Results

Over a 5-year period, 82 studies of ⁶⁸Ga-DOTA-peptide PET/CT were performed on 44 GI-NET patients. The most common primary site was the rectum (50.0%) followed by the small bowel, stomach and colon. Using WHO 2010 grading, 40.9% of patients had low-grade (G1) tumour, 22.7% intermediate (G2) and 4.5% high (G3). Of ten patients scheduled for pre-operative staging, ⁶⁸Ga-DOTA-peptide PET/CT only led to therapeutic change in three patients. Furthermore, false-negative results of ⁶⁸Ga-DOTA-peptide PET/CT were reported in one patient after surgical confirmation. However, therapeutic changes were seen in 20/36 patients (55.6%) scheduled for post-surgical restaging or assessment of somatostatin analogue (SSA) eligibility. When ⁶⁸Ga-DOTA-peptide PET/CT was used for monitoring disease progress during systemic treatment (sandostatin, chemotherapy, everolimus and PRRT) in metastatic disease, impact on management modification was seen in 19/36 patients (52.8%), of which 84.2% had inter-modality change (switch to everolimus, chemotherapy or PRRT) and 15.8% had intra-modality change (increased SSA dosage).

Conclusions

⁶⁸Ga-DOTA-peptide PET/CT has a significant impact on management decisions in GI-NET patients as it can provide additional information on occult metastasis/equivocal lesions and supply the clinician an opportunity to select patients for targeted therapy.

Characterization of 111In-labeled Glucose-Dependent Insulinotropic Polypeptide as a Radiotracer for Neuroendocrine Tumors

Somatostatin receptor targeting is considered the standard nuclear medicine technique for visualization of neuroendocrine tumors (NET). Since not all NETs over-express somatostatin receptors, the search for novel targets, visualizing these NETs, is ongoing. Many NETs, expressing low somatostatin receptor levels, express glucose-dependent insulinotropic polypeptide (GIP) receptors (GIPR). Here, we evaluated the performance of [Lys³⁷(DTPA)]N-acetyl-GIP_1-42, a newly synthesized GIP analogue to investigate whether NET imaging via GIPR targeting is feasible. Therefore, [Lys³⁷(DTPA)]N-acetyl-GIP_1-42 was radiolabeled with ¹¹¹In with specific activity up to 1.2 TBq/µmol and both in vitro and in vivo receptor targeting properties were examined. In vitro, [Lys³⁷(¹¹¹In-DTPA)]N-acetyl-GIP_1-42 showed receptor-mediated binding to BHK-GIPR positive cells, NES2Y cells and isolated islets. In vivo, both NES2Y and GIPR-transfected BHK tumors were visualized on SPECT/CT. Furthermore, co-administration of an excess unlabeled GIP_1-42 lowered tracer uptake from 0.7 ± 0.2%ID/g to 0.6 ± 0.01%ID/g (p = 0.78) in NES2Y tumors and significantly lowered tracer uptake from 3.3 ± 0.8 to 0.8 ± 0.2%ID/g (p = 0.0001) in GIPR-transfected BHK tumors. In conclusion, [Lys³⁷(¹¹¹In-DTPA)]N-acetyl-GIP_1-42 shows receptor-mediated binding in various models. Furthermore, both GIPR-transfected BHK tumors and NES2Y tumors were visible on SPECT/CT using this tracer. Therefore, [Lys³⁷(¹¹¹In-DTPA)]N-acetyl-GIP_1-42 SPECT seems promising for visualization of somatostatin receptor negative NETs.

Design, Synthesis, and Biological Evaluation of 68Ga-DOTA-PA1 for Lung Cancer: A Novel PET Tracer for Multiple Somatostatin Receptor Imaging

Most of the radiolabeled somatostatin analogues (SSAs) are specific for subtype somatostatin receptor 2 (SSTR₂). Lack of ligands targeting other subtypes of SSTRs, especially SSTR_1, SSTR₃, and SSTR₅, limited their applications in tumors of low SSTR₂ expression, including lung tumor. In this study, we aimed to design and synthesize a positron emission tomography (PET) radiotracer targeting multi-subtypes of SSTRs for PET imaging. PA1 peptide and its conjugate with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator or fluorescein isothiocyanate (FITC) at the N-terminal of the lysine position were synthesized. ⁶⁸Ga was chelated to DOTA-PA1 to obtain ⁶⁸Ga-DOTA-PA1 radiotracer. The stability, lipophilicity, binding affinity, and binding specificity of ⁶⁸Ga-DOTA-PA1 and FITC-PA1 were evaluated by various in vitro experiments. Micro-PET imaging of ⁶⁸Ga-DOTA-PA1 was performed in nude mice bearing A549 lung adenocarcinoma, as compared with ⁶⁸Ga-DOTA-(Tyr3)-octreotate (⁶⁸Ga-DOTA-TATE). Histological analysis of SSTR expression in A549 tumor tissues and human tumor tissues was conducted using immunofluorescence staining and immunohistochemical assay. ⁶⁸Ga-DOTA-PA1 had high radiochemical yield and radiochemical purity of over 95% and 99%, respectively. The radiotracer was stable in vitro in different buffers over a 2 h incubation period. Cell uptake of ⁶⁸Ga-DOTA-PA1 was 1.31-, 1.33-, and 1.90-fold that of ⁶⁸Ga-DOTA-TATE, which has high binding affinity only for SSTR₂, after 2 h incubation in H520, PG, and A549 lung cancer cell lines, respectively. Micro-PET images of ⁶⁸Ga-DOTA-PA1 showed that the PET imaging signal correlated with the total expression of SSTRs, instead of SSTR₂ only, which was measured by Western blotting and immunofluorescence analysis in mice bearing A549 tumors. In summary, a novel PET radiotracer, ⁶⁸Ga-DOTA-PA1, targeting multi-subtypes of SSTRs, was successfully synthesized and was confirmed to be useful for PET imaging. It may have potential as a noninvasive PET radiotracer for imaging SSTR-positive tumors.

Radiosynthesis and evaluation of an 18F-labeled silicon containing exendin-4 peptide as a PET probe for imaging insulinoma

Background

Analogues of exendin-4 have been radiolabeled for imaging the glucagon-like peptide type 1 receptors (GLP-1R) which are overexpressed in insulinoma. The aim of this research was to synthesize an ¹⁸F–labeled silicon containing exendin-4 peptide (¹⁸F-2) and to evaluate its in vitro and in vivo behavior in CHL-GLP-1 receptor positive tumor-bearing mice.

¹⁸F–labeled silicon containing exendin-4 peptide (¹⁸F-2) was prepared via one-step nucleophilic substitution of a silane precursor with ¹⁸F–fluoride in the presence of acetic acid and K222. ¹⁸F-2 was then administered to tumor-bearing mice for PET imaging and ex vivo biodistribution experiments.

Results

¹⁸F-2 was produced in a radiochemical yield (decay corrected) of 1.5% and a molar activity of max. 16 GBq/μmol. The GLP-1R positive tumors were clearly visualized by PET imaging. Biodistribution studies showed reduced uptake of ¹⁸F-2 in the kidneys compared to radiometal labeled exendin-4 derivatives. The radiotracer showed specific tumour uptake which remained steady over 2 h.

Conclusions

This exendin-4 analogue, ¹⁸F-2, is a potential probe for imaging GLP-1R positive tumors.

The utility of 68Ga-DOTATATE positron-emission tomography/computed tomography in the diagnosis, management, follow-up and prognosis of neuroendocrine tumors

Neuroendocrine tumors (NETs) are rare neoplasms that emerge mainly from the GI tract, pancreas and respiratory tract. The incidence of NETs has increased more than sixfold in the last decades. NETs typically express somatostatin receptors on their cell surface, which can be targeted by 'cold' somatostatin analogs for therapy or by 'hot' radiolabeled somatostatin analogs for tumor localization and treatment. 68-Gallium-DOTA peptides (DOTATATE, DOTATOC, DOTANOC) positron emission tomography/computed tomography is a highly accurate imaging modality for NETs that has been found to be more sensitive for NET detection than other imaging modalities. In the current review, we will discuss the clinical utility of 68-Gallium-DOTATATE positron emission tomography/computed tomography for the diagnosis and management of patients with NETs.

Enhanced Specific Activity by Multichelation of Exendin-3 Leads To Improved Image Quality and In Vivo Beta Cell Imaging

Glucagon-like peptide-1 receptor (GLP-1R) targeting using radiolabeled exendin is a promising approach to noninvasively visualize and determine beta cell mass (BCM), which could help to unravel the pathophysiology of diabetes. However, saturation of the GLP-1R on beta cells occurs at low peptide doses, since the number of receptors expressed under physiological conditions is low. Therefore, tracers with high specific activities are required to sensitively image small variations in BCM. Here, we describe a novel exendin-3-based radiotracer with multiple chelators and determine its potential for in vivo beta cell imaging. Exendin-3 was modified by adding six lysine residues C-terminally conjugated with one, two, or six DTPA moieties. All compounds were labeled with ¹¹¹In and their GLP-1R affinity was determined in vitro using GLP-1R expressing cells. The in vivo behavior of the ¹¹¹In-labeled tracers was examined in BALB/c nude mice with a subcutaneous GLP-1R expressing tumor (INS-1). Brown Norway rats were used for SPECT visualization of the pancreatic BCM. Addition of six lysine and six DTPA residues (hexendin(40-45)) resulted in a 7-fold increase in specific activity (from 0.73 GBq/nmol to 5.54 GBq/nmol). IC₅₀ values varied between 5.2 and 69.5 nM. All compounds with two or six lysine and DTPA residues had a significantly lower receptor affinity than [Lys⁴⁰(DTPA)]exendin-3 (4.4 nM, p < 0.05). The biodistribution in mice revealed no significant decrease in pancreatic uptake after addition of six lysine and DTPA molecules. Hexendin(40-45) showed a 6-fold increase in absolute ¹¹¹In uptake in the pancreas of Brown Norway rats compared to [Lys⁴⁰(DTPA)]exendin-3 (182.7 ± 42.3 kBq vs 28.8 ± 6.0 kBq, p < 0.001). Visualization of the pancreas on SPECT was improved using hexendin(40-45), due to the higher count rate, achieved at the same peptide dose. In conclusion, hexendin(40-45) showed an improved visualization of the pancreas with SPECT. This tracer holds promise to sensitively and specifically detect small variations in BCM.

The role of peptide receptor radionuclide therapy in advanced/metastatic thoracic neuroendocrine tumors

Bronchopulmonary (BP) neuroendocrine tumors (NETs) comprise a spectrum of tumors that develop from respiratory neuroendocrine cells and represent ~20% of all lung neoplasia and ~30% of all NETs. The only curative treatment is surgical resection. For well-differentiated forms (typical and atypical carcinoids), medical therapy ranges from bioactive agents (e.g., somatostatin analogs), to biotherapy (e.g., everolimus), standard chemotherapy and peptide receptor radionuclide therapy (PRRT). PRRT with radiolabeled somatostatin analogs is an innovative treatment for inoperable or metastasized, well/moderately differentiated, NET. Initially developed for gastroenteropancreatic tumors, it is also used in BP-NET because these tumors express the target receptor. Two decades of clinical trials with either ⁹⁰Y-octreotide or ¹⁷⁷Lu-octreotate, have demonstrated the efficacy of PRRT, as measured by tumor response, symptom relief and quality of life (QoL) improvement. PRRT with ⁹⁰Y- and ¹⁷⁷Lu-peptides is generally well-tolerated and adverse events (kidney and bone marrow) are modest. The paper illustrates the history, technique and results of this treatment in the few dedicated studies and the many BP NET cases embedded within larger NET series. The limitations of the present body of information are addressed, and the future perspectives, in terms of prospective studies required to define the position of PRRT in the therapeutic algorithm of BP-NETs and the need for predictive molecular biomarkers to guide future studies, are discussed.

Medical Therapy of Gastrointestinal Neuroendocrine Tumors

Intestinal neuroendocrine tumors (NETs) constitute a heterogeneous group with duodenal, small intestinal, colonic and rectal NETs. They constitute more than half of all NETs, with the highest frequencies in the rectum, small intestine, and colon. The tumor biology varies with the location of the primary tumor as well as with the grade and staging of the tumor. Small intestinal NETs usually present low proliferation and are treated in the first line with somatostatin analogs according to current guidelines. If progression occurs, one can add interferon alpha or change the treatment to everolimus. Peptide receptor radionuclide therapy (PRRT) with Lutetium¹⁷⁷-DOTATATE can be an option in the future after registration of the compound. Rectal tumors are usually small when they metastasize; they can be treated with somatostatin analogs but more so with PRRT, while another option is of course everolimus. Colonic NETs are more aggressive than the rest of intestinal NETs and will be treated with everolimus, sometimes in combination with somatostatin analogs based on positive scintigraphy. Another option is a cytotoxic agent such as streptozotocin plus 5-fluorouracil (5-FU) or temozolomide plus capecitabine. The most aggressive tumors, i.e. neuroendocrine carcinoma G3, are treated with a platin-based therapy plus etoposide; if they present with a lower proliferation, i.e. <50%, temozolomide plus capecitabine plus bevacizumab can also be attempted. Duodenal NETs are mostly treated similar to pancreatic NETs, either with cytotoxic agents, streptozotocin plus 5-FU, or temozolomide plus capecitabine, or with targeted agents such as everolimus.

Incidentally Detected Thyroid Follicular Neoplasm on Somatostatin Receptor Imaging and Post-therapy Scan

Peptide receptor radionuclide therapy (PRRT) either using Lu-177 or Y-90 peptide radiopharmaceuticals has emerged as promising treatment modality in patients with inoperable metastatic neuroendocrine tumour (NET) including medullary thyroid cancer, because of overexpression of somatostatin receptor 2 (sstr-2) on these cells. The several investigators have used PRRT in non-iodine avid differentiated thyroid cancer patients with limited success, where other treatment modalities have failed, probably due to faint sstr-2 expression in these lesions. However Hurthle cell neoplasms being predominantly non-iodine avid lesions have shown sstr-2 over-expression. The present case of inoperable NET patient imaged and treated with radiolabelled somatostatin analogue showed incidentally detected thyroid lesion highlighting the its importance in imaging and treatment in these type of thyroid malignancies.

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.

In liver metastases from small intestinal neuroendocrine tumors, SSTR2A expression is heterogeneous

We examined somatostatin receptor type 2A (SSTR2A) expression in primary and metastatic small intestinal neuroendocrine tumors (SI-NETs). We retrieved 156 liver metastases from 26 patients (10 males, 16 females) who had two or more liver lesions resected. A representative formalin-fixed paraffin-embedded section of tumor tissue from each liver metastasis and from the primary tumor, when available, were immunohistochemically stained for SSTR2A. SSTR2A expression was evaluated by the Her2/neu-scoring system and the scoring system proposed by Volante et al. Based on the Her2/neu-scoring system, moderate to strong SSTR2A expression was observed in 121 of 156 (78%) liver metastases. In 15 (58%) subjects, all liver metastases showed moderate to strong SSTR2A expression, whereas in 11 (42%) one or more liver tumors had weak or no expression. Of the 16 stained primaries, 11 (69%) showed heterogeneous SSTR2A expression. The corresponding liver metastases showed only weak to no expression in one, moderate to strong in five, and both weak to no and moderate to strong expression in five of the 11 cases. Using the Volante scoring system, no tumor was scored 0 (0%), two were scored 1 (1%), 38 were scored 2 (24%), and 116 were scored 3 (74%). No statistically significant association was observed between SSTR2A expression and Ki67 index (p = 0.56). Fifteen of 18 (83%) metastatic tumors with a Ki67 index >20% showed moderate to strong SSTR2A. Most liver tumors with weak SSTR2A expression or an IHC score of 2 were detected by OctreoScan. SSTR2A expression in liver metastases of SI-NETs can be variable, even between lesions in the same patient. Expression in metastatic lesions is not always similar to that in the primary tumor. SSTR2A expression is not associated with the Ki67 index.

Management of the hormonal syndrome of neuroendocrine tumors

Gastroenteropancreatic neuroendocrine tumors (GEP/NET) are unusual and rare neoplasms that present many clinical challenges. They characteristically synthesize store and secrete a variety of peptides and neuroamines which can lead to the development of distinct clinical syndrome, however many are clinically silent until late presentation with mass effects. Management strategies include surgery cure and cytoreduction with the use of somatostatin analogues. Somatostatin have a broad range of biological actions that include inhibition of exocrine and endocrine secretions, gut motility, cell proliferation, cell survival and angiogenesis. Five somatostatin receptors (SSTR1-SSTR5) have been cloned and characterized. Somatostatin analogues include octreotide and lanreotide are effective medical tools in the treatment and present selectivity for SSTR2 and SSTR5. During treatment is seen disapperance of flushing, normalization of bowel movements and reduction of serotonin and 5-hydroxyindole acetic acid (5-HIAA) secretion. Telotristat represents a novel approach by specifically inhibiting serotonin synthesis and as such, is a promising potential new treatment for patients with carcinoid syndrome. To pancreatic functionig neuroendocrine tumors belongs insulinoma, gastrinoma, glucagonoma and VIP-oma. Medical management in patients with insulinoma include diazoxide which suppresses insulin release. Also mTOR inhibitors may inhibit insulin secretion. Treatment of gastrinoma include both proton pump inhibitors (PPIs) and histamine H2 - receptor antagonists. In patients with glucagonomas hyperglycaemia can be controlled using insulin and oral blood glucose lowering drugs. In malignant glucagonomas smatostatin analogues are effective in controlling necrolytic migratory erythemia. Severe cases of the VIP-oma syndrome require supplementation of fluid losses. Octreotide reduce tumoral VIP secretion and control secretory diarrhoea.

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.

Proof of Therapeutic Efficacy of a 177Lu-Labeled Neurotensin Receptor 1 Antagonist in a Colon Carcinoma Xenograft Model

Increased expression of neurotensin receptor 1 (NTR1) has been shown in a large number of tumor entities such as pancreatic or colon carcinoma. Hence, this receptor is a promising target for diagnostic imaging and radioligand therapy. Using the favorable biodistribution data of the NTR1-targeting agent ¹¹¹In-3BP-227, we investigated the therapeutic effect of its ¹⁷⁷Lu-labeled analog on the tumor growth of NTR1-positive HT29 colon carcinoma xenografts. Methods: 3BP-227 was labeled with ¹⁷⁷Lu. To assess its biodistribution properties, SPECT and CT scans of HT29-xenografted nude mice injected with ¹⁷⁷Lu-3BP-227 were acquired, and ex vivo tissue activity was determined. To evaluate therapeutic efficacy, 2 groups of mice received the radiopharmaceutical in a median dose of either 165 MBq (129-232 MBq, n = 10) or 110 MBq (82-116 MBq, n = 10), whereas control mice were injected with vehicle (n = 10). Tumor sizes and body weights were monitored for up to 49 d. Renal function and histologic morphology were evaluated. Results: Whole-body SPECT/CT images allowed clear tumor visualization with low background activity and high tumor-to-kidney and -liver ratios. Ex vivo biodistribution data confirmed high and persistent uptake of ¹⁷⁷Lu-3BP-227 in HT29 tumors (19.0 ± 3.6 vs. 2.7 ± 1.6 percentage injected dose per gram at 3 and 69 h after injection, respectively). The application of ¹⁷⁷Lu-3BP-227 resulted in a distinct delay of tumor growth. Median tumor doubling time for controls was 5.5 d (interquartile range [IQR], 2.8-7.0), compared with 17.5 d (IQR, 5.5-22.5 d) for the 110-MBq and 41.0 d (IQR, 27.5-55.0) for the 165-MBg group. Compared with controls, median relative tumor volume at day 23 after injection was reduced by 55% (P = 0.034) in the 110-MBq and by 88% (P < 0.01) in the 165-MBq group. Renal histology and clinical chemistry results did not differ between radiotherapy groups and controls, suggesting absence of therapy-induced acute renal damage. Conclusion: These data demonstrate that the novel NTR1-targeting theranostic agent 3BP-227 is an effective and promising candidate for radioligand therapy, with a favorable preliminary safety profile and high potential for clinical translation.

One Coin, No Need to Flip: Shared PET Targets in Cancer and Coronary Artery Disease

OBJECTIVE

The purposes of this article are to review the common biologic features of cancer and coronary artery disease assessed with PET tracers, focusing on those already used in the clinic and those with translational potential, and to discuss the current value and expected contribution of PET in diagnosis, risk stratification, and treatment monitoring.

CONCLUSION

PET using a wide variety of radiotracers enhances understanding of pathophysiologic changes shared by cancer and coronary artery disease, helps establish an accurate diagnosis, and aids in prognostic assessment and management decisions. It is likely that with the evolution of therapeutic strategies for blocking the development and progression of both diseases and with the introduction of novel, specific ligands in clinical practice, PET will play an ever stronger role in diagnosis, risk stratification, and monitoring of therapy.

Cross sectional and nuclear medicine imaging of pancreatic insulinomas

Insulinomas are rare neuroendocrine tumors which occur predominantly in the pancreas. Although majority of the insulinomas are benign, over-secretion of insulin by the tumor leads to debilitating hypoglycemic symptoms. The diagnosis is based on clinical and biochemical findings. After the diagnosis is made, the principal challenge lies in locating the tumor because most tumors are solitary and small in size. Locating the tumor is of paramount importance as complete surgical excision is the only curative treatment, and incomplete resection leads to persistence of symptoms. Different preoperative and intraoperative imaging techniques have been used with varying success rates for the insulinoma imaging. Besides localizing the tumor, imaging also helps to guide biopsy, detect metastatic lesions, and perform image-guided therapeutic procedures. This review will discuss the role of different Cross sectional and nuclear medicine imaging modalities in insulinomas.

Accuracy and precision assessment for activity quantification in individualized dosimetry of 177Lu-DOTATATE therapy

Background

In order to obtain a reliable ¹⁷⁷Lu-DOTATATE therapy dosimetry, it is crucial to acquire accurate and precise activity measurements with the radionuclide calibrator, the SPECT/CT camera, and the NaI(Tl) well counter. The aim of this study was to determine, in a clinical context, the accuracy and the precision of their activity quantification over a range of activities and time.

Ninety-three ¹⁷⁷Lu sources from the manufacturer were measured in the radionuclide calibrator over 2.5 years to evaluate its calibration accuracy and precision compared to the manufacturer’s value. A NEMA 2012/IEC 2008 phantom was filled with a ¹⁷⁷Lu activity concentration sphere-to-background ratio of five. It was acquired with the SPECT/CT camera to determine the reconstruction parameters offering the best compromise between partial volume effect and signal-to-noise ratio. The calibration factor was computed accordingly. The calibration quality was monitored over 2.5 years with 33 phantom acquisitions with activities ranging from 7040 to 0.6 MBq. Home-made sources were used to calibrate the well counter. Its reliability was evaluated with activities ranging from 150 to 0.2 kBq measured 34 times over 2.5 years.

Results

For the radionuclide calibrator, median [interquartile range] for the error on activity measurement was −0.99 [1.31] %. The optimal SPECT reconstruction parameters were obtained with 16 iterations, 16 subsets and a 12-mm Gaussian post-filter. The calibration factor was 9.87 cps/MBq with an error of −1.05 [2.12] %. The well counter was calibrated with 31.5 cps/kBq, and the error was evaluated to −12.89 [16.55] %.

Conclusions

The accuracy and the precision of activity quantification using dedicated quality control were found to be sufficient for use in dosimetry implemented in clinical routine. The proposed methodology could be implemented in other centres to obtain reproducible ¹⁷⁷Lu-based treatment dosimetry.

Somatostatin receptor expression on von Hippel-Lindau-associated hemangioblastomas offers novel therapeutic target

Von Hippel-Lindau (VHL)-associated hemangioblastomas (VHL-HB) arise in the central nervous system (CNS), and are a leading cause of morbidity and mortality in VHL disease. Currently, surgical resection is the most effective way to manage symptomatic VHL-HBs. Surgically unresectable VHL-HBs or those in frail patients are challenging problems. Therapies targeting oncologic and vascular endothelial growth factor (VEGF) pathways have failed to demonstrate tumor control. Our experience and previous reports on VHL-HB avidity to somatostatin analogues suggested somatostatin receptor (SSTR) expression in VHL-HBs, offering an alternative therapeutic strategy. We explored this possibility by demonstrating consistent histologic expression of SSTR1, 2a, 4, and 5 in VHL-HBs. We found that somatostatin analogue octreotide induces apoptosis in VHL-HB stromal cells in a dose-dependent fashion by BAX – caspase-3 pathway unrelated to canonical VHL pathway. When administered to a patient with unresectable symptomatic suprasellar hemangioblastoma, octreotide resulted in tumor volume reduction, symptom stabilization, and tumor cytopenia on repeat ⁶⁸Ga-DOTA-TATE positron emission tomography (PET) within 6 months, suggesting tumor infarction. We conclude that VHL-HBs harbor multiple SSTR subtypes that offer actionable chemo-therapeutic strategy for management of symptomatic, unresectable tumors by somatostatin analogue therapy.

Clinical utility of lanreotide Autogel® in gastroenteropancreatic neuroendocrine tumors

Somatostatin analogs (SSAs), which were initially used to control hormonal syndromes associated with neuroendocrine neoplasms (NENs), have been successfully proposed as antiproliferative agents, able to control tumor growth in patients affected by gastroenteropancreatic (GEP)-NENs. The development of long-acting formulations of SSAs which require only weekly or monthly injections can improve patient compliance. In particular, lanreotide (LAN) Autogel^®, which is a viscous aqueous formulation supplied in ready-to-use prefilled syringes, can be administered every 28–56 days. Since its introduction in the clinical practice, several studies evaluated the clinical utility of LAN Autogel in the medical treatment of GEP-NENs. Although there is no evidence of an overall survival benefit, these studies confirm the efficacy of LAN Autogel in terms of benefit in progression-free survival, and in more than half of cases, a reduction of tumor markers can be observed during treatment with this drug. Moreover, LAN Autogel is widely recognized to be effective in controlling tumor-related symptoms in the majority of patients affected by GEP tumors, especially in patients affected by carcinoid syndrome, improving considerably patients’ quality of life.

99mTc Labeled Glucagon-Like Peptide-1-Analogue (99mTc-GLP1) Scintigraphy in the Management of Patients with Occult Insulinoma

INTRODUCTION

The aim of this study was to assess the utility of [Lys40(Ahx-HYNIC-99mTc/EDDA)NH2]-exendin-4 scintigraphy in the management of patients with hypoglycemia, particularly in the detection of occult insulinoma.

MATERIALS AND METHODS

Forty patients with hypoglycemia and increased/confusing results of serum insulin and C-peptide concentration and negative/inconclusive results of other imaging examinations were enrolled in the study. In all patients GLP-1 receptor imaging was performed to localise potential pancreatic lesions.

RESULTS

Positive results of GLP-1 scintigraphy were observed in 28 patients. In 18 patients postsurgical histopathological examination confirmed diagnosis of insulinoma. Two patients had contraindications to the surgery, one patient did not want to be operated. One patient, who presented with postprandial hypoglycemia, with positive result of GLP-1 imaging was not qualified for surgery and is in the observational group. Eight patients were lost for follow up, among them 6 patients with positive GLP-1 scintigraphy result. One patient with negative scintigraphy was diagnosed with malignant insulinoma. In two patients with negative scintigraphy Munchausen syndrome was diagnosed (patients were taking insulin). Other seven patients with negative results of 99mTcGLP-1 scintigraphy and postprandial hypoglycemia with C-peptide and insulin levels within the limits of normal ranges are in the observational group. We would like to mention that 99mTc-GLP1-SPECT/CT was also performed in 3 pts with nesidioblastosis (revealing diffuse tracer uptake in two and a focal lesion in one case) and in two patients with malignant insulinoma (with the a focal uptake in the localization of a removed pancreatic headin one case and negative GLP-1 1 scintigraphy in the other patient).

CONCLUSIONS

99mTc-GLP1-SPECT/CT could be helpful examination in the management of patients with hypoglycemia enabling proper localization of the pancreatic lesion and effective surgical treatment. This imaging technique may eliminate the need to perform invasive procedures in case of occult insulinoma.

Paget's Disease Mimicking Bone Metastasis in a Patient with Neuroendocrine Tumor on 68Ga-DOTANOC PET/CT

Somatostatin (SST) is a neuropeptide present in neurons, endocrine cells, and a wide range of neuroendocrine tumors (NETs). ⁶⁸Ga-DOTATOC, ⁶⁸Ga-DOTANOC, and ⁶⁸Ga-DOTATATE are current SST analogues used for PET/CT which bind to SST receptors expressed in NETs. These SST analogues have been used successfully for diagnosis of SST-expressing tumors with a more sensitive detection technique than conventional scintigraphy. However, there is a lack of clinical data on the differentiation between NETs and other malignant tumors or benign pathological conditions. Here, we report a case of Paget’s disease mimicking bone metastasis of NET on PET/CT due to increased ⁶⁸Ga-DOTANOC uptake and review examples of similar cases in the literature.

Glucagon-Like Peptide-1 Receptor PET/CT with 68Ga-NOTA-Exendin-4 for Detecting Localized Insulinoma: A Prospective Cohort Study

Preoperative localization of insulinoma is a clinical dilemma. We aimed to investigate whether glucagon-like peptide-1 receptor (GLP-1R) PET/CT with (68)Ga-NOTA-MAL-cys(40)-exendin-4 ((68)Ga-NOTA-exendin-4) is efficient in detecting insulinoma.

METHODS

In our prospective cohort study, patients with endogenous hyperinsulinemic hypoglycemia were enrolled. CT, MRI, endoscopic ultrasound, and (99m)Tc-hydrazinonicotinamide-TOC SPECT/CT were done according to standard protocols. GLP-1R PET/CT was performed 30-60 min after the injection of (68)Ga-NOTA-exendin-4. The gold standard for diagnosis was the histopathologic results after surgery.

RESULTS

Of 52 recruited patients, 43 patients with histopathologically proven insulinomas were included for the imaging studies. Nine patients did not undergo surgical intervention. (68)Ga-NOTA-exendin-4 PET/CT correctly detected insulinomas in 42 of 43 patients with high tumor uptake (mean SUVavg ± SD, 10.2 ± 4.9; mean SUVmax ± SD, 23.6 ± 11.7), resulting in sensitivity of 97.7%. In contrast, (99m)Tc-hydrazinonicotinamide-TOC SPECT/CT showed a low sensitivity of 19.5% (8/41) in this group of patients; however, it successfully localized the tumor that was false-negative with GLP-1R PET/CT. The sensitivities of CT, MR, and endoscopic ultrasonography were 74.4% (32/43), 56.0% (14/25), and 84.0% (21/25), respectively.

CONCLUSION

(68)Ga-NOTA-exendin-4 PET/CT is a highly sensitive imaging technique for the localization of insulinoma.

Evaluation of a Fluorescent and Radiolabeled Hybrid Somatostatin Analog In Vitro and in Mice Bearing H69 Neuroendocrine Xenografts

In the treatment of neuroendocrine tumors (NETs), complete surgical removal of malignancy is generally desirable, because it offers curative results. Preoperative guidance with radiolabeled somatostatin analogs, commonly used for NET diagnosis and preoperative planning, is limited by its low resolution, with the risk that tumor margins and small metastases will be incompletely resected with subsequent recurrence. A single hybrid probe combining radiotracer and optical dye would enable high-resolution optical guidance, also during surgery. In the current study, the hybrid labeled somatostatin analog Cy5-DTPA-Tyr(3)-octreotate (DTPA is diethylene triamine pentaacetic acid) was synthesized and evaluated for its ability to specifically trace NET cells in vitro and in an animal model. The performance of the hybrid tracer was compared with that of octreotate with only radiolabel or only optical label.

METHODS

The binding affinity and internalization capacity of Cy5-DTPA-Tyr(3)-octreotate were assessed in vitro. Biodistribution profiles and both nuclear and optical in vivo imaging of Cy5-(111)In -DTPA-Tyr(3)-octreotate were performed in NET-bearing mice and compared with the performance of (111)In-DTPA-Tyr(3)-octreotate.

RESULTS

In vitro studies showed a low receptor affinity and internalization rate for Cy5-DTPA-Tyr(3)-octreotate. The dissociation constant value was 387.7 ± 97.9 nM for Cy5-DTPA-Tyr(3)-octreotate, whereas it was 120.5 ± 18.1 nM for DTPA-Tyr(3)-octreotate. Similarly, receptor-mediated internalization reduced from 33.76% ± 1.22% applied dose for DTPA-Tyr(3)-octreotate to 1.32% ± 0.02% applied dose for Cy5-DTPA-Tyr(3)-octreotate. In contrast, in vivo and ex vivo studies revealed similar tumor uptake values of Cy5-(111)In-DTPA-Tyr(3)-octreotate and (111)In -DTPA-Tyr(3)-octreotate (6.93 ± 2.08 and 5.16 ± 1.27, respectively). All organs except the kidneys showed low background radioactivity, with especially low activities in the liver, and high tumor-to-tissue ratios were achieved-both favorable for the tracer's toxicity profile. Hybrid imaging in mice confirmed that the nuclear and fluorescence signals colocalized.

CONCLUSION

The correlation between findings with the optical and the nuclear probes underlines the potential of combining SPECT imaging with fluorescence guidance and shows the promise of this novel hybrid peptide for preoperative and intraoperative imaging of NET.

Preoperative localization of an insulinoma: selective arterial calcium stimulation test performance

PURPOSE

Preoperative localization of an insulinoma is recommended to improve the cure rate, but non-invasive procedures can fail to detect the tumour. The objective of the study was to assess the performance of a selective arterial calcium stimulation test in the preoperative localization of insulinomas that were not detected by conventional imaging procedures.

METHODS

We conducted a monocenter retrospective case review of 13 patients who had endogenous hyperinsulinism and were treated between 1994 and 2013. Patients were selected on the basis of negative or doubtful non-invasive preoperative imaging. A selective arterial calcium stimulation test was performed by pancreatic and hepatic arteriography with selective intra-arterial calcium stimulation and hepatic venous sampling in order to obtain the plasma insulin measurement. We evaluated the efficacy of the test by comparing the results with an endoscopic ultrasound.

RESULTS

Twelve of the 13 patients underwent surgery, and the presence of an insulinoma was proven in 11 patients by pathological analysis of the tumour. An endoscopic ultrasound was consistent with surgery in 71.4 % of cases, while selective arterial calcium stimulation was consistent with surgery in 90.9 % and allowed detection of an insulinoma in two additional patients with a negative endoscopic ultrasound. One false-negative and one false-positive arterial calcium test were observed. No adverse events were recorded except transient skin flush following calcium injection in one patient.

CONCLUSION

The selective arterial calcium stimulation test is a sensitive diagnostic procedure for localizing insulinomas and may be considered when non-invasive radiological imaging does not allow the detection of an occult insulinoma.