Sensitivity Comparison of 68Ga-OPS202 and 68Ga-DOTATOC PET/CT in Patients with Gastroenteropancreatic Neuroendocrine Tumors: A Prospective Phase II Imaging Study

Emerging innovations in theranostics for pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (pNETs) often overexpress somatostatin receptor type 2 (SSTR2), making them ideal targets for theranostics, which integrates molecular imaging with targeted radionuclide therapy. 177Lu-DOTATATE significantly extends progression-free survival (22.8 vs. 8.5 months) compared to octreotide LAR. Despite these advances, challenges remain, including treatment resistance and long-term toxicities. In this review, we explore advancements in specialized imaging techniques, rationale combination strategies, and exploring next-generation radiopharmaceuticals.

Superior NET Targeting by Switch From Agonist to Antagonist-mediated Somatostatin Receptor Theranostics Allowing Successful (LM3-based) PRRT of Otherwise Precluded mNET: Intraindividual Proof-of-concept

We present a case of a 69-year-old woman with metastasized small bowel NET G2 initially treated with somatostatin analogs (SSA). Disease progression was observed under SSA, and poor tracer uptake on [68Ga]Ga-DOTATOC PET/CT precluded peptide receptor radionuclide therapy (PRRT). Subsequently, evaluation with [68Ga]Ga-NODAGA-LM3 PET/CT was performed, which showed markedly better and treatment-sufficient uptake, enabling initiation of PRRT with [177Lu]Lu-DOTA-LM3. After 4 therapy cycles, a positive response was observed. This interesting case nicely demonstrates not only the diagnostic superiority of somatostatin receptor antagonist (over agonist) imaging but also the direct translation into successful targeted peptide-mediated radionuclide therapy.

Cancer Targeting Radiopeptidomimetics in Molecular Nuclear Medicine

Peptides are highly receptor-affine molecules exhibiting suitable pharmacokinetics. Additionally, low-cost production, simple protocols allowing easy modifications, and tolerance toward harsh reaction conditions make peptides ideal ligands for preparation of radiopharmaceuticals for cancer detection and treatment. However, natural peptides being substrates for enzymes are susceptible to proteolysis, which limits the in vivo lifetime and the target uptake. Therefore, the majority of peptides are not able to progress beyond preclinical research. Advancement of peptides for clinical analysis needs modification to instill improved features. Continuous increase and further expected rise in cancer cases in the next decade require development of more disease-directed and promising radiopharmaceuticals. Redesigned peptide, mimicking the original peptide with similar or improved affinity and high metabolic stability, shall have significant edge. This review outlines the design of peptidomimetics by incorporation of D-amino acids (inverso); reversal of D-amino acid sequence (retro-inverso), and reversal of L-amino acid sequence (retro). Clinically successful radiopeptidomimetics prepared using the three approaches have been elaborated to elucidate the important role of peptidomimetics in cancer management.

Research process of PET tracers for neuroendocrine tumors diagnosis

Neuroendocrine tumors (NETs) can affect several organ systems and present a variety of clinical symptoms, which are difficult to diagnose by conventional methods. Somatostatin receptor (SSTR) is a group of specific receptors expressed on the well-differentiated NET cell membrane. [68Ga]-labeled somatostatin analogues (SSAs) PET/CT, endogenous ligands targeting SSTR, is widely used in currently clinical NETs diagnosis. The dual-tracer strategy ([68Ga]Ga-SSAs + [18F]FDG) allows for a more detailed evaluation of tumor metabolism and receptor expression. The NETPET score, integrating [68Ga]Ga-SSAs PET/CT and [18F]FDG PET/CT results, enhances the accuracy of predicting treatment response and prognosis. In addition, novel isotopes ([18F]/[64Cu]) labeled SSAs and SSTR antagonists outperformed [68Ga]-SSAs in lesion detection, tumor uptake, and tumor-to-background ratio. Due to undifferentiated or dedifferentiated NETs, SSTR may not be expressed. [68Ga]Ga-Pentixafor and [18F]-FDG PET/CT are applicable for SSTR-negative NET diagnosis. [18F]-MFBG and [18F]-DOPA have a higher sensitivity for identifying non-metastatic pheochromocytoma and paraganglioma (PPGL) than other radiotracers. This review addressed NET diagnosis with conventional imaging techniques, the clinical application of novel radiotracers, and the merits and limitations of the various radiotracers.

Synthesis and preclinical evaluation of [18F]AlF-NOTA-Asp2-PEG2-JR11 as a novel antagonist radioligand for PET imaging of somatostatin receptor

PURPOSE

Somatostatin receptor (SSTR) antagonists have recently emerged as preferable radiotracers for SSTR-targeted imaging and therapy. This study aimed to design a novel SSTR antagonist, [18F]AlF-NOTA-Asp2-PEG2-JR11, and compare its preclinical performance with the previously reported antagonist, [18F]AlF-NOTA-JR11, and the agonist [68Ga]Ga-DOTA-TATE.

METHODS

[18F]AlF-NOTA-Asp2-PEG2-JR11 was synthesized via a one-step radiolabeling process involving [18F]AlF chelation. The binding affinity, internalization, and cellular uptake were evaluated using AR42J/SSTR + cells. Biodistribution and PET/CT imaging were conducted in mice bearing xenografted AR42J/SSTR + or HCT116/SSTR- tumor xenografts.

RESULTS

[18F]AlF-NOTA-Asp2-PEG2-JR11 was manually synthesized within 30 min with an uncorrected radiochemical yield of 39.56 ± 3.25% (n > 5) and radiochemical purity (RCP) exceeding 99% (n > 5). [18F]AlF-NOTA-Asp2-PEG2-JR11 demonstrated excellent in vivo stability over 2 h (RCP > 95%). Among AR42J cells, [18F]AlF-NOTA-Asp2-PEG2-JR11 exhibited high affinity, specific uptake, and low internalization, similar to [18F]AlF-NOTA-JR11. Biodistribution and micro-PET/CT imaging studies revealed comparable tumor uptake between [18F]AlF-NOTA-Asp2-PEG2-JR11 and [18F]AlF-NOTA-JR11 (9.26 ± 0.49 vs. 10.18 ± 0.82%ID/g, p = 0.147) at 60 min post-injection (p.i), both were significantly higher than [68Ga]Ga-DOTA-TATE (6.79 ± 0.29%ID/g, p = 0.001). Co-injecting the corresponding inhibitor significantly reduced the tumor uptake of all three tracers. Notably, [18F]AlF-NOTA-Asp2-PEG2-JR11 reached peak tumor uptake at 30 min p.i. and exhibited the lowest uptake and fastest clearance in most normal organs, including the kidney, bone, liver, and muscle, resulting in the highest and increasing tumor-to-background ratios (TBR) over time among the three tracers.

CONCLUSION

The synthesis of [18F]AlF-NOTA-Asp2-PEG2-JR11 is efficient, with high radiochemical yield and RCP. [18F]AlF-NOTA-Asp2-PEG2-JR11 exhibits excellent in vivo stability, high tumor uptake, and superior TBR, making it a promising potential tracer for imaging SSTR-positive tumors.

PSMA and SSTR2 Dual-Targeting Theranostic Agents for Neuroendocrine-Differentiated Prostate Cancer (NEPC)

Radioactive prostate-specific membrane antigen (PSMA)-targeting agents are clinically useful for the diagnosis and treatment of patients with PSMA-positive metastatic castration-resistant prostate cancer (mCRPC). Neuroendocrine-differentiated prostate cancer (NEPC), a highly aggressive subtype that is strongly associated with a poor clinical prognosis, may present with reduced PSMA expression and evade detection with PSMA-targeted agents. Several studies have shown elevated uptake of somatostatin receptor 2 (SSTR2) ligands in PSMA-negative NEPC. By combining a SSTR2-targeting peptide, JR11, with previously reported PSMA-targeting ligands, P16-093 and P17-087, [68Ga]Ga-1 and [68Ga]Ga/[177Lu]Lu-2 were designed and synthesized. The cell uptake of [68Ga]Ga-1 was comparable to [68Ga]Ga-P16-093 in PSMA-positive cell lines, while [68Ga]Ga-1 and [68Ga]Ga-2 showed a positive but slightly lower uptake than [68Ga]Ga-DOTA-TATE in SSTR2-positive cell lines. In vivo studies in SSTR2+ or PSMA+ tumor-bearing mice demonstrated that [68Ga]Ga-1 and [68Ga]Ga/[177Lu]Lu-2 showed positive uptake for both SSTR2+ and PSMA+ tumors. These dual-targeting radiotracers are potentially valuable for the diagnosis and radioligand therapy of NEPC.

Ectopic Cushing Syndrome Localized by 68 Ga-DOTA-LM3 PET/CT

ABSTRACT

A 53-year-old woman presented with signs of Cushing syndrome with challenges in diagnosis and localization. A novel somatostatin receptor (SSTR)-targeted PET/CT with 68 Ga-DOTA-LM3, an SSTR antagonist, revealed a suspicious focal finding in the pancreatic head, proven to be ectopic Cushing syndrome after surgical resection. This interesting image clearly shows the potential of PET imaging with SSTR antagonists as 68 Ga-DOTA-LM3 in the diagnosis of ectopic Cushing syndrome.

Radiolabeled Somatostatin Analogs for Cancer Imaging

Somatostatin receptors (SSTR) are expressed by many tumours especially those related to neuro-endocrine origin and molecular functional imaging of SSTR expression using radiolabelled somatostatin analogs have revolutionized imaging of patients with these group of malignancies. Coming a long way from the first radiolabelled somatostatin analog 123I-Tyr-3-octreotide, there has been significant developments in terms of radionuclides used, the ligands and somatostatin derivatives. 111In-Pentetreotide extensively employed for imaging NETs at the beginning has now been replaced by 68Ga-SSA based PET-CT. SSA-PET/CT performs superior to conventional imaging modalities and has evolved in the mainframe for NET imaging. The advantages were multiple: (i) superior spatial resolution of PET versus SPECT, (ii) quantitative capabilities of PET aiding in disease activity and treatment response monitoring with better precision, (iii) shorter scan time and (iv) less patient exposure to radiation. The modality is indicated for staging, detecting the primary in CUP-NETs, restaging, treatment planning (along with FDG: the concept of dual-tracer PET-CT) as well as treatment response evaluation and follow-up of NETs. SSA PET/CT has also been incorporated in the guidelines for imaging of Pheochromocytoma-Paraganglioma, Medullary carcinoma thyroid, Meningioma and Tumor induced osteomalacia. At present, there is rising interest on (a) 18F-labelled SSA, (b) 64Cu-labelled SSA, and (c) somatostatin antagonists. 18F offers excellent imaging properties, 64Cu makes delayed imaging feasible which has implications in dosimetry and SSTR antagonists bind with the SST receptors with high affinity and specificity, providing high contrast images with less background, which can be translated to theranostics effectively. SSTR have been demonstrated in non-neuroendocrine tumours as well in the peer-reviewed literature, with studies demonstrating the potential of SSA PET/CT in Neuroblastoma, Nasopharyngeal carcinoma, carcinoma prostate (neuroendocrine differentiation) and lymphoma. This review will focus on the currently available SSAs and their history, different SPECT/PET agents, SSTR antagonists, comparison between the various imaging tracers, and their utility in both neuroendocrine and non-neuroendocrine tumors.

Case report: Pancreatic metastasis from small-cell lung cancer appears as primary G2 pancreatic neuroendocrine tumor on combined contrast PET imaging with three probes

Pancreatic metastasis is a rare malignant tumor; when it comes to multiple cancers, it may be a challenge to identify the primary lesion of new pancreatic metastases. With the continuous advancement of imaging technology, the PET/computed tomography (CT) has been widely used because of its high diagnostic accuracy and non-invasiveness. However, in the present case, the patient had history of limited small-cell lung carcinoma and prostatic cancer; the combined application of the three kinds of PET/CT was used to identify the new metastases of pancreatic and bone metastases, which suggested a high probability of primary G2 pancreatic neuroendocrine tumor with bone metastases. After the needle biopsy, samples were confirmed by diagnostic pathology as small-cell lung cancer metastasizing to the pancreas and bone. The results of our case suggests the irreplaceability of pathology and possibility of misdiagnosis by PET/CT; moreover, it also supplements clinical data for second primary cancers after small-cell lung cancer.

Theranostics in Neuroendocrine Tumors: Updates and Emerging Technologies

Advancements in somatostatin receptor (SSTR) targeted imaging and treatment of well-differentiated neuroendocrine tumors (NETs) have revolutionized the management of these tumors. This comprehensive review delves into the current practice, discussing the use of the various FDA-approved SSTR-agonist PET tracers and the predictive imaging biomarkers, and elaborating on Lu177-DOTATATE peptide receptor radionuclide therapy (PRRT) including the evolving areas of post-therapy imaging practices, PRRT retreatment, and the potential role of dosimetry in optimizing patient treatments. The future directions sections highlight ongoing research on investigational PET imaging radiotracers, future prospects in alpha particle therapy, and combination therapy strategies.

Evaluation of the safety, biodistribution, dosimetry of [18F]AlF-NOTA-LM3 and head-to-head comparison with [68Ga]Ga-DOTATATE in patients with well-differentiated neuroendocrine tumors: an interim analysis of a prospective trial

PURPOSE

18F-labelled somatostatin receptor (SSTR) analogs offer several advantages over 68Ga in terms of yield, cost, spatial resolution and detection rate. This study presents an interim analysis of a prospective trial designed to assess the safety, biodistribution and dosimetry of [18F]AlF-NOTA-LM3, and compare its diagnostic efficacy and clinical management outcomes with [68Ga]Ga-DOTATATE or [68Ga]Ga-NODAGA-LM3 in patients with well-differentiated NETs.

METHODS

Twenty-one patients with histologically confirmed well-differentiated neuroendocrine tumors (G1 and G2) were prospectively recruited. The first eight patients underwent serial PET scans at 5, 15, 30, 45, 60, and 120 min after [18F]AlF-NOTA-LM3 injection to assess biodistribution and dosimetry. The remaining patients underwent whole-body PET/CT scans. [18F]AlF-NOTA-LM3 and [68Ga]Ga-DOTATATE PET/CT were done within a week, with a minimum 24-hour interval between the two scans. Focal uptake above the surrounding background activity and could not be explained by physiologic uptake was considered lesions of NETs. Lesion number, tumor uptake, and tumor-to-background ratio (TBR) were compared. In patients with discrepant findings, the size of the smallest lesions (measured on coregistered CT) detected on [68Ga]Ga-DOTATATE and [18F]AlF-NOTA-LM3 was compared.

RESULTS

[18F]AlF-NOTA-LM3 was safe and well-tolerated. Physiological uptake of [18F]AlF-NOTA-LM3 was significantly lower than that of [68Ga]Ga-DOTATATE in abdominal organs and bone marrow, but higher in blood pool and lung. The mean effective dose was 0.024 ± 0.014 mSv/MBq. [18F]AlF-NOTA-LM3 detected significantly more liver lesions (457 vs. 291, P = 0.006) and lymph node lesions (30 vs. 22, P = 0.011) compared to [68Ga]Ga-DOTATATE. The tumor uptake was comparable, but TBR was significantly higher with [18F]AlF-NOTA-LM3 for lesions from all sites except for the duodenum. The size of the minimum liver lesions (0.54 ± 0.15 vs. 1.01 ± 0.49, P<0.001) and lymph node lesions (0.50 ± 0.19 vs. 1.26 ± 0.86, P = 0.024) detected on [18F]ALF-NOTA-LM3 were significantly smaller than those detected on [68Ga]Ga-DOTATATE.

CONCLUSION

[18F]AlF-NOTA-LM3 shows favorable biodistribution, higher spatial resolution and superior performance than [68Ga]Ga-DOTATATE in detecting liver and lymph node metastases, with higher TBR. Notably, it is the first SSTR analog to show superiority in detecting lymph node lesions when compared to [68Ga]Ga-DOTATATE.

TRIAL REGISTRATION

ClinicalTrials.gov identifier NCT06056362.

Small Bowel Neuroendocrine Neoplasms-A Review

Neuroendocrine neoplasms (NENs) are rapidly evolving small bowel tumors, and the patients are asymptomatic at the initial stages. Metastases are commonly observed at the time of presentation and diagnosis. This review addresses the small bowel NEN (SB-NEN) and its molecular, histological, and imaging features, which aid diagnosis and therapy guidance. Somatic cell number alterations and epigenetic mutations are studied to be responsible for sporadic and familial SB-NEN. The review also describes the grading of SB-NEN in addition to rare histological findings such as mixed neuroendocrine-non-NENs. Anatomic and nuclear imaging with conventional computed tomography, magnetic resonance imaging, computed tomographic enterography, and positron emission tomography are adopted in clinical practice for diagnosing, staging, and follow-up of NEN. Along with the characteristic imaging features of SB-NEN, the therapeutic aspects of imaging, such as peptide receptor radionuclide therapy, are discussed in this review.

Appendiceal Neuroendocrine Neoplasms: A Comprehensive Review

ABSTRACT

Appendiceal neuroendocrine neoplasm (NEN) is the most common adult appendiceal malignant tumor, constituting 16% of gastrointestinal NENs. They are versatile tumors with varying morphology, immunohistochemistry, secretory properties, and cancer genomics. They are slow growing and clinically silent, to begin with, or present with features of nonspecific vague abdominal pain. Most acute presentations are attributed clinically to appendicitis, with most cases detected incidentally on pathology after an appendectomy. Approximately 40% of them present clinically with features of hormonal excess, which is likened to the functional secretory nature of their parent cell of origin. The symptoms of carcinoid syndrome render their presence clinically evident. However, slow growing and symptomatically silent in its initial stages, high-grade neuroendocrine tumors and neuroendocrine carcinomas of the appendix are aggressive and usually have hepatic and lymph node metastasis at presentation. This review article focuses on imaging characteristics, World Health Organization histopathological classification and grading, American Joint Committee on Cancer/Union or International Cancer Control, European Neuroendocrine Tumor Society staging, European Neuroendocrine Tumor Society standardized guidelines for reporting, data interpretation, early-stage management protocols, and advanced-stage appendiceal NENs. Guidelines are also set for the follow-up and reassessment. The role of targeted radiotherapy, chemotherapy, and high-dose somatostatin analogs in treating advanced disease are discussed, along with types of ablative therapies and liver transplantation for tumor recurrence. The search for newer location-specific biomarkers in NEN is also summarized. Regarding the varying aggressiveness of the tumor, there is a scope for research in the field, with plenty of data yet to be discovered.

State-of-the-Art Hybrid Imaging of Neuroendocrine Neoplasms

ABSTRACT

Neuroendocrine neoplasms (NENs) may be challenging to diagnose due to their small size and diverse anatomical locations. Hybrid imaging techniques, specifically positron emission tomography/computed tomography (PET/CT) and positron emission tomography/magnetic resonance imaging (PET/MRI), represent the current state-of-the-art for evaluating NENs. The preferred radiopharmaceuticals for NEN PET imaging are gallium-68 (68Ga) DOTA-peptides, which target somatostatin receptors (SSTR) overexpressed on NEN cells. Clinical applications of [68Ga]Ga-DOTA-peptides PET/CT include diagnosis, staging, prognosis assessment, treatment selection, and response evaluation. Fluorodeoxyglucose-18 (18F-FDG) PET/CT aids in detecting low-SSTR-expressing lesions and helps in patient stratification and treatment planning, particularly in grade 3 neuroendocrine tumors (NETs). New radiopharmaceuticals such as fluorine-labeled SSTR agonists and SSTR antagonists are emerging as alternatives to 68Ga-labeled peptides, offering improved detection rates and favorable biodistribution. The maturing of PET/MRI brings advantages to NEN imaging, including simultaneous acquisition of PET and MRI images, superior soft tissue contrast resolution, and motion correction capabilities. The PET/MRI with [68Ga]Ga-DOTA-peptides has demonstrated higher lesion detection rates and more accurate lesion classification compared to PET/CT. Overall, hybrid imaging offers valuable insights in the diagnosis, staging, and treatment planning of NENs. Further research is needed to refine response assessment criteria and standardize reporting guidelines.

Gallium-68 labeled somatostatin receptor antagonist PET/CT in over 500 patients with neuroendocrine neoplasms: experience from a single center in China

PURPOSE

Somatostatin receptor antagonists have shown promising performance for imaging neuroendocrine neoplasms. However, there is a lack of studies exploring the diagnostic performance of SSTR antagonists or comparing them with agonists in a large cohort of patients with NENs. This study aimed to retrospectively review all SSTR antagonist PET/CT scans conducted at Peking Union Medical College Hospital since November 2018 in patients with confirmed or suspected NENs.

METHODS

Four types of SSTR antagonists were utilized, including [68Ga]Ga-NODAGA-LM3, [68Ga]Ga-DOTA-LM3, [68Ga]Ga-NODAGA-JR11, and [68Ga]Ga-DOTA-JR11. The reference standard was based on a combination of histopathology, clinical evaluation, imaging results, and follow-up. Patient-based sensitivity, specificity, and accuracy were evaluated. The SUVmax and tumor-to-liver ratio (TLR) of the hottest lesions was recorded and compared between antagonists and [68Ga]Ga-DOTATATE.

RESULTS

A total of 622 antagonist scans from 549 patients were included in the analysis. The patient-level sensitivity, specificity, and accuracy of antagonist imaging (all tracers combined) were 91.0% (443/487), 91.9% (57/62), and 91.1% (500/549), respectively. In 181 patients with a comparative [68Ga]Ga-DOTATATE PET/CT scan, the patient-level sensitivity, specificity, and accuracy were 87.5% (147/168), 76.9% (10/13), and 86.7% (157/181), respectively. For the hottest lesions, SSTR antagonists all tracers combined demonstrated an overall comparable SUVmax to [68Ga]Ga-DOTATATE (40.1 ± 32.5 vs. 39.4 ± 23.8, p = 0.772). While [68Ga]Ga-NODAGA-LM3 showed significantly higher uptake than [68Ga]Ga-DOTATATE (57.4 ± 38.5 vs. 40.0 ± 22.8, p<0.001), [68Ga]Ga-NODAGA-JR11 (39.7 ± 26.5 vs. 34.3 ± 23.9, p = 0.108) and [68Ga]Ga-DOTA-LM3 (38.9 ± 32.1 vs. 37.2 ± 22.1, p = 0.858) showed comparable uptake to [68Ga]Ga-DOTATATE, and [68Ga]Ga-DOTA-JR11 showed lower uptake (28.9 ± 26.1 vs. 44.0 ± 25.7, p = 0.001). All antagonists exhibited significantly higher TLR than [68Ga]Ga-DOTATATE (12.1 ± 10.8 vs. 5.2 ± 4.5, p<0.001).

CONCLUSION

Gallium-68 labeled SSTR antagonists could serve as alternatives to SSTR agonists for imaging of NENs. Among various antagonists, [68Ga]Ga-NODAGA-LM3 seems to have the best imaging profile.

Recent advances and future challenges in the diagnosis of neuroendocrine neoplasms

Neuroendocrine neoplasms (NEN) are a heterogeneous group of malignancies with increasing incidence, whose diagnosis is usually delayed, negatively impacting on patients' prognosis. The latest advances in pathological classifications, biomarker identification and imaging techniques may provide early detection, leading to personalized treatment strategies. In this narrative review the recent developments in diagnosis of NEN are discussed including progresses in pathological classifications, biomarker and imaging. Furthermore, the challenges that lie ahead are investigated. By discussing the limitations of current approaches and addressing potential roadblocks, we hope to guide future research directions in this field. This article is proposed as a valuable resource for clinicians and researchers involved in the management of NEN. Update of pathological classifications and the availability of standardized templates in pathology and radiology represent a substantially improvement in diagnosis and communication among clinicians. Additional immunohistochemistry markers may now enrich pathological classifications, as well as miRNA profiling. New and multi-analytical circulating biomarkers, as liquid biopsy and NETest, are being proposed for diagnosis but their validation and availability should be improved. Radiological imaging strives for precise, non-invasive and less harmful technique to improve safety and quality of life in NEN patient. Nuclear medicine may benefit of somatostatin receptors' antagonists and membrane receptor analogues. Diagnosis in NEN still represents a challenge due to their complex biology and variable presentation. Further advancements are necessary to obtain early and minimally invasive diagnosis to improve patients' outcomes.

Recent Progress in Peptide-Based Molecular Probes for Disease Bioimaging

Recent strides in molecular pathology have unveiled distinctive alterations at the molecular level throughout the onset and progression of diseases. Enhancing the in vivo visualization of these biomarkers is crucial for advancing disease classification, staging, and treatment strategies. Peptide-based molecular probes (PMPs) have emerged as versatile tools due to their exceptional ability to discern these molecular changes with unparalleled specificity and precision. In this Perspective, we first summarize the methodologies for crafting innovative functional peptides, emphasizing recent advancements in both peptide library technologies and computer-assisted peptide design approaches. Furthermore, we offer an overview of the latest advances in PMPs within the realm of biological imaging, showcasing their varied applications in diagnostic and therapeutic modalities. We also briefly address current challenges and potential future directions in this dynamic field.

Somatostatin Receptor Subtype-2 Targeting System for Specific Delivery of Temozolomide

Temozolomide (TMZ) is a DNA alkylating agent that produces objective responses in patients with neuroendocrine tumors (NETs) when the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) is inactivated. At high doses, TMZ therapy exhausts MGMT activity but also produces dose-limiting toxicities. To reduce off-target effects, we converted the clinically approved radiotracer 68Ga-DOTA-TOC into a peptide-drug conjugate (PDC) for targeted delivery of TMZ to somatostatin receptor subtype-2 (SSTR2)-positive tumor cells. We used an integrated radiolabeling strategy for direct quantitative assessment of receptor binding, pharmacokinetics, and tissue biodistribution. In vitro studies revealed selective binding to SSTR2-positive cells with high affinity (5.98 ± 0.96 nmol/L), internalization, receptor-dependent DNA damage, cytotoxicity, and MGMT depletion. Imaging and biodistribution analysis showed preferential accumulation of the PDC in receptor-positive tumors and high renal clearance. This study identified a trackable SSTR2-targeting system for TMZ delivery and utilizes a modular design that could be broadly applied in PDC development.

PET/MRI imaging in neuroendocrine neoplasm

Molecular imaging plays a vital role in the management of neuroendocrine neoplasms (NENs). Somatostatin receptor (SSTR) PET is critical for evaluating NENs, ascertaining peptide receptor radionuclide therapy (PRRT) eligibility, and treatment response. SSTR-PET/MRI can provide a one-stop-shop multiparametric evaluation of NENs. The acquisition of complementary imaging information in PET/MRI has distinct advantages over PET/CT and MR imaging acquisitions. The purpose of this manuscript is to provide a comprehensive overview of PET/MRI and a current review of recent PET/MRI advances in the diagnosis, staging, treatment, and surveillance of NENs.

The usefulness of [68 Ga]Ga-DOTA-JR11 PET/CT in patients with meningioma: comparison with MRI

PURPOSE

Clinical studies of PET imaging using SSTR2 agonists have demonstrated high accuracy and correlation with SSTR2 expression in meningiomas. However, the usefulness of the SSTR2 antagonist with [68 Ga]Ga-DOTA-JR11 is uncertain. To evaluate the diagnostic performance of [68 Ga]Ga-DOTA-JR11 PET/CT and to clarify tumor characteristics in patients with suspected meningiomas.

MATERIALS AND METHODS

Patients with suspected de novo or recurrent meningioma in complex locations or atypical images were enrolled from August 2021 to October 2022 in prospective study. All patients underwent contrast-enhanced MRI (CE-MRI), [68 Ga]Ga-DOTA-JR11 PET/CT, and histopathological evaluation. Tumor uptake of [68 Ga]Ga-DOTA-JR11 was measured by SUVmax and tumor-endocranium ratio (TBR). Diagnostic performance was compared between PET and MRI.

RESULTS

Of 36 (50.0 ± 13.0 years of age, 20 women) patients, 32 were histopathologically confirmed meningiomas and four with other tumors. [68 Ga]Ga-DOTA-JR11 uptake was significantly higher in meningioma patients than in those with other tumors (SUVmax: 13.6 ± 7.7 vs. 5.2 ± 3.0, P < 0.001; TBR: 64.2 ± 27.7 vs. 25.0 ± 18.9, P = 0.001). [68 Ga]Ga-DOTA-JR11 PET/CT detected 31 meningiomas, while CE-MRI detected 17 meningiomas of 25 initial diagnosis and 11 recurrent tumors; [68 Ga]Ga-DOTA-JR11 PET had an incremental diagnostic value of 24% (6/25) over MRI in the group of initial diagnosis. There was no statistically significant difference in diagnostic efficacy between PET and MRI (P = 0.45) for all 36 patients. In skull base meningiomas, PET provided a more definitive diagnosis of pituitary involvement (in 12, not in12), compared to MRI (in eight, possible in six, possible not in six, not in four). PET revealed bone involvement in all 14 patients proven by pathology, while MRI identified only 11.

CONCLUSIONS

[68 Ga]Ga-DOTA-JR11 PET/CT provided high image quality and presented an ideal diagnostic performance in detecting meningioma and evaluating the involvement of the pituitary and bone. The study provides valuable evidence for the use of [68 Ga]Ga-DOTA-JR11 PET/CT as a complementary imaging modality to CE-MRI in the evaluation of meningiomas.

Influence of the Molar Activity of 203/212Pb-PSC-PEG2-TOC on Somatostatin Receptor Type 2-Binding and Cell Uptake

(1) Background: In neuroendocrine tumors (NETs), somatostatin receptor subtype 2 is highly expressed, which can be targeted by a radioactive ligand such as [177Lu]Lu-1,4,7,10-tetraazacyclododecane-N,N',N″,N‴,-tetraacetic acid-[Tyr3,Thr8]-octreotide (177Lu-DOTA-TOC) and, more recently, by a lead specific chelator (PSC) containing 203/212Pb-PSC-PEG2-TOC (PSC-TOC). The molar activity (AM) can play a crucial role in tumor uptake, especially in receptor-mediated uptake, such as in NETs. Therefore, an investigation of the influence of different molar activities of 203/212Pb-PSC-TOC on cell uptake was investigated. (2) Methods: Optimized radiolabeling of 203/212Pb-PSC-TOC was performed with 50 µg of precursor in a NaAc/AcOH buffer at pH 5.3-5.5 within 15-45 min at 95° C. Cell uptake was studied in AR42 J, HEK293 sst2, and ZR75-1 cells. (3) Results: 203/212Pb-PSC-TOC was radiolabeled with high radiochemical purity >95% and high radiochemical yield >95%, with AM ranging from 0.2 to 61.6 MBq/nmol. The cell uptake of 203Pb-PSC-TOC (AM = 38 MBq/nmol) was highest in AR42 J (17.9%), moderate in HEK293 sstr (9.1%) and lowest in ZR75-1 (0.6%). Cell uptake increased with the level of AM. (4) Conclusions: A moderate AM of 15-40 MBq/nmol showed the highest cell uptake. No uptake limitation was found in the first 24-48 h. Further escalation experiments with even higher AM should be performed in the future. It was shown that AM plays an important role because of its direct dependence on the cellular uptake levels, possibly due to less receptor saturation with non-radioactive ligands at higher AM.

68Ga-SSO-120 PET for Initial Staging of Small Cell Lung Cancer Patients: A Single-Center Retrospective Study

PET imaging using the somatostatin receptor 2 (SSTR2) antagonist satoreotide trizoxetan (SSO-120, previously OPS-202) could offer accurate tumor detection and screening for SSTR2-antagonist radionuclide therapy in patients with SSTR2-expressing small cell lung cancer (SCLC). The aim of this single-center study was to investigate tumor uptake and detection rates of 68Ga-SSO-120 in comparison to 18F-FDG PET in the initial staging of SCLC patients. Methods: Patients with newly diagnosed SCLC who underwent additional whole-body 68Ga-SSO-120 PET/CT during the initial diagnostic workup were retrospectively included. The mean administered activity was 139 MBq, and the mean uptake time was 60 min. Gold-standard staging 18F-FDG PET/CT was evaluated if available within 2 wk before or after 68Ga-SSO-120 PET if morphologic differences in CT images were absent. 68Ga-SSO-120- or 18F-FDG-positive lesions were reported in 7 anatomic regions (primary tumor, thoracic lymph node metastases, and distant metastases including pleural, contralateral pulmonary, liver, bone, and other) according to the TNM classification for lung cancer (eighth edition). Consensus TNM staging (derived from CT, endobronchial ultrasound-guided transbronchial needle aspiration, PET, and brain MRI) by a clinical tumor board served as the reference standard. Results: Thirty-one patients were included, 12 with limited and 19 with extensive disease according to the Veterans Administration Lung Study Group classification. 68Ga-SSO-120-positive tumor was detected in all patients (100%) and in 90 of the 217 evaluated regions (41.5%). Thirteen patients (42.0%) had intense average 68Ga-SSO-120 uptake (region-based mean SUVmax ≥ 10); 28 patients (90.3%) had average 68Ga-SSO-120 uptake greater than liver uptake (region-based mean peak tumor-to-liver ratio > 1). In 25 patients with evaluable 18F-FDG PET, primary tumor, thoracic lymph node metastases, and distant metastases were detected in 100%, 92%, and 64%, respectively, of all investigated patients by 68Ga-SSO-120 and in 100%, 92%, and 56%, respectively, by 18F-FDG PET. 68Ga-SSO-120 PET detected additional contralateral lymph node, liver, and brain metastases in 1, 1, and 2 patients, respectively (no histopathology available), and 18F-FDG PET detected additional contralateral lymph node metastases in 3 patients (1 confirmed, 1 systematic endobronchial ultrasound-guided transbronchial needle aspiration-negative, and 1 without available histopathology). None of these differences altered Veterans Administration Lung Study Group staging. The region-based monotonic correlation between 68Ga-SSO-120 and 18F-FDG uptake was low (Spearman ρ = 0.26-0.33). Conclusion: 68Ga-SSO-120 PET offers high diagnostic precision with comparable detection rates and additional complementary information to the gold standard, 18F-FDG PET. Consistent uptake in most patients warrants exploration of SSTR2-directed radionuclide therapy.

Head-to-Head Comparison of 68Ga-NODAGA-JR11 and 68Ga-DOTATATE PET/CT in Patients with Metastatic, Well-Differentiated Neuroendocrine Tumors: Interim Analysis of a Prospective Bicenter Study

The current study aimed to compare 68Ga-NODAGA-Cpa-cyclo(d-Cys-amino-Phe-hydroorotic acid-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)-d-Tyr-NH2 (JR11) and 68Ga-DOTATATE PET/CT in patients with metastatic, well-differentiated neuroendocrine tumors. Methods: A prospective bicenter study aimed at enrolling 100 patients with histologically proven, metastatic or unresectable, well-differentiated neuroendocrine tumors was conducted. The first 48 patients represented the study cohort. Each patient received 68Ga-DOTATATE on the first day and 68Ga-NODAGA-JR11 on the second day. Whole-body PET/CT scans were performed at 40-60 min after injection. Normal-organ uptake, lesion numbers, lesion uptake, and sensitivity were compared. The potential impact on clinical management was also determined. Results: Overall, 68Ga-NODAGA-JR11 demonstrated lower background uptake in normal organs. Compared with 68Ga-DOTATATE, 68Ga-NODAGA-JR11 detected significantly more liver lesions (673 vs. 584, P = 0.002). The target-to-background ratio of liver lesions was significantly higher on 68Ga-NODAGA-JR11 (6.4 ± 8.7 vs. 3.1 ±2.6, P = 0.000). Comparable uptake was observed for primary tumors, bone lesions, and lymph node metastases. In total, 180 lesions were detected on conventional imaging in 15 patients; 165 and 139 lesions of them were positive on 68Ga-NODAGA-JR11 and 68Ga-DOTATATE, leading to a sensitivity of 91.7% and 77.2%, respectively. In 14.5% (7/48) of patients, 68Ga-NODAGA-JR11 PET might have a potential impact on clinical management. Conclusion: 68Ga-NODAGA-JR11 shows better sensitivity and a higher target-to-background ratio than 68Ga-DOTATATE. The detection of more lesions by the antagonist may have a potential impact on clinical management in a subgroup of patients.

New horizon of radiopharmaceuticals in management of neuroendocrine tumors

Neuroendocrine neoplasms are rare and heterogenous group of tumors with varying degrees of clinical presentations and involvement of multiple organ systems in the body. In the modern clinical practice somatostatin receptor molecular imaging and targeted radioligand therapy plays a vital role in the diagnosis and management of the disease. Several new and promising radiotracers for NET imaging and theranostics, belonging to various groups and classes are being studied and investigated. This exponential growth of radiotracers poses concerns about the indication, clinical benefit, and safety profile of the agents. We discuss the basis behind these radiotracers clinical use, receptor targeting and intra and inter tumor heterogeneity. Furthermore, role of dual tracer imaging, combination therapy and potential applications of dosimetry in predicting treatment outcome and safety profile is reviewed. Individualized precision medicine with better tumor characterization, maximum therapeutic benefit and minimum toxicity is the way forward for future medicine.

Somatostatin Receptor Targeted PET-CT and Its Role in the Management and Theranostics of Gastroenteropancreatic Neuroendocrine Neoplasms

Somatostatin receptor (SSTR) agonist-based Positron Emission Tomography-Computed Tomography (PET-CT) imaging is nowadays the mainstay for the assessment and diagnostic imaging of neuroendocrine neoplasms (NEN), especially in well-differentiated neuroendocrine tumors (NET) (World Health Organization (WHO) grade I and II). Major clinical indications for SSTR imaging are primary staging and metastatic workup, especially (a) before surgery, (b) detection of unknown primary in metastatic NET, (c) patient selection for theranostics and appropriate therapy, especially peptide receptor radionuclide therapy (PRRT), while less major indications include treatment response evaluation on and disease prognostication. Dual tracer PET-CT imaging using SSTR targeted PET tracers, viz. [⁶⁸Ga]Ga-DOTA-Tyr3-Octreotate (DOTA-TATE) and [⁶⁸Ga]Ga-DOTA-NaI3-Octreotide (DOTA-NOC), and fluorodeoxyglucose (FDG), have recently gained widespread acceptance for better assessment of whole-body tumor biology compared to single-site histopathology, in terms of being non-invasive and the ability to assess inter- and intra-tumoral heterogeneity on a global scale. FDG uptake has been identified as independent adverse risk factor in various studies. Recently, somatostatin receptor antagonists have been shown to be more sensitive and specific in detecting the disease. The aim of this review article is to summarize the clinical importance of SSTR-based imaging in the clinical management of neuroendocrine and related tumors.

18F-Labeled Somatostatin Analogs as PET Tracers for the Somatostatin Receptor: Ready for Clinical Use

Molecular imaging of the somatostatin receptor plays a key role in the clinical management of neuroendocrine tumors. PET imaging with somatostatin analogs (SSAs) labeled with ⁶⁸Ga or ⁶⁴Cu is currently the gold standard in clinical practice. However, widespread implementation of ⁶⁸Ga imaging is often hampered by practical and economic issues related to ⁶⁸Ge/⁶⁸Ga generators. ¹⁸F offers several advantages to tackle these issues. Recent developments in radiochemistry have allowed a shift from ⁶⁸Ga toward ¹⁸F labeling, leading to promising clinical translations of ¹⁸F-labeled SSAs, such as Gluc-Lys-[¹⁸F]FP-TOCA, [¹⁸F]F-FET-βAG-TOCA, [¹⁸F]AlF-NOTA-octreotide, [¹⁸F]SiTATE, and [¹⁸F]AlF-NOTA-JR11. This review gives an update of currently available clinical data regarding ¹⁸F-labeled SSA tracers and provides justification for the clinical application of this class of tracers.

Peptide Radioligands in Cancer Theranostics: Agonists and Antagonists

The clinical success of radiolabeled somatostatin analogs in the diagnosis and therapy-"theranostics"-of tumors expressing the somatostatin subtype 2 receptor (SST₂R) has paved the way for the development of a broader panel of peptide radioligands targeting different human tumors. This approach relies on the overexpression of other receptor-targets in different cancer types. In recent years, a shift in paradigm from internalizing agonists to antagonists has occurred. Thus, SST₂R-antagonist radioligands were first shown to accumulate more efficiently in tumor lesions and clear faster from the background in animal models and patients. The switch to receptor antagonists was soon adopted in the field of radiolabeled bombesin (BBN). Unlike the stable cyclic octapeptides used in the case of somatostatin, BBN-like peptides are linear, fast to biodegradable and elicit adverse effects in the body. Thus, the advent of BBN-like antagonists provided an elegant way to obtain effective and safe radiotheranostics. Likewise, the pursuit of gastrin and exendin antagonist-based radioligands is advancing with exciting new outcomes on the horizon. In the present review, we discuss these developments with a focus on clinical results, commenting on challenges and opportunities for personalized treatment of cancer patients by means of state-of-the-art antagonist-based radiopharmaceuticals.

Development of the 99mTc-Labelled SST2 Antagonist TECANT-1 for a First-in-Man Multicentre Clinical Study

Broad availability and cost-effectiveness of 99Mo/99mTc generators worldwide support the use, and thus the development, of novel 99mTc-labelled radiopharmaceuticals. In recent years, preclinical and clinical developments for neuroendocrine neoplasms patient management focused on somatostatin receptor subtype 2 (SST2) antagonists, mainly due to their superiority in SST2-tumour targeting and improved diagnostic sensitivity over agonists. The goal of this work was to provide a reliable method for facile preparation of a 99mTc-labelled SST2 antagonist, [99mTc]Tc-TECANT-1, in a hospital radiopharmacy setting, suitable for a multi-centre clinical trial. To ensure successful and reproducible on-site preparation of the radiopharmaceutical for human use shortly before administration, a freeze-dried three-vial kit was developed. The final composition of the kit was established based on the radiolabelling results obtained during the optimisation process, in which variables such as precursor content, pH and buffer, as well as kit formulations, were tested. Finally, the prepared GMP-grade batches met all predefined specification parameters together with long-term kit stability and stability of the product [99mTc]Tc-TECANT-1. Furthermore, the selected precursor content complies with micro-dosing, based on an extended single-dose toxicity study, where histopathology NOEL was established at 0.5 mg/kg BW, being more than 1000 times higher than the planned human dose of 20 µg. In conclusion, [99mTc]Tc-TECANT-1 is suitable to be advanced into a first-in-human clinical trial.

Direct comparison of [18F]AlF-NOTA-JR11 and [18F]AlF-NOTA-octreotide for PET imaging of neuroendocrine tumors: Antagonist versus agonist

BACKGROUND

[¹⁸F]AlF-NOTA-octreotide is an ¹⁸F-labeled somatostatin analogue which is a good clinical alternative for ⁶⁸Ga-labeled somatostatin analogues. However, radiolabeled somatostatin receptor (SSTR) antagonists might outperform agonists regarding imaging sensitivity of neuroendocrine tumors (NETs). No direct comparison between the antagonist [¹⁸F]AlF-NOTA-JR11 and the agonist [¹⁸F]AlF-NOTA-octreotide as SSTR PET probes is available. Herein, we present the radiosynthesis of [¹⁸F]AlF-NOTA-JR11 and compare its NETs imaging properties directly with the established agonist radioligand [¹⁸F]AlF-NOTA-octreotide preclinically.

METHODS

[¹⁸F]AlF-NOTA-JR11 was synthesized in an automated synthesis module. The in vitro binding characteristics (IC₅₀) of [^natF]AlF-NOTA-JR11 and [^natF]AlF-NOTA-octreotide were evaluated and the in vitro stability of [¹⁸F]AlF-NOTA-JR11 was determined in human serum. In vitro cell binding and internalization was performed with [¹⁸F]AlF-NOTA-JR11 and [¹⁸F]AlF-NOTA-octreotide using SSTR2 expressing cells and the pharmacokinetics were evaluated using μPET/CT in mice bearing BON1.SSTR2 tumor xenografts.

RESULTS

Excellent binding affinity for SSTR2 was found for [^natF]AlF-NOTA-octreotide (IC₅₀ of 25.7 ± 7.9 nM). However, the IC₅₀ value for [^natF]AlF-NOTA-JR11 (290.6 ± 71 nM) was 11-fold higher compared to [^natF]AlF-NOTA-octreotide, indicating lower affinity for SSTR2. [¹⁸F]AlF-NOTA-JR11 was obtained in a good RCY (50 ± 6 %) but with moderate RCP of 94 ± 1 %. [¹⁸F]AlF-NOTA-JR11 demonstrated excellent stability in human serum (>95 % after 240 min). 2.7-fold higher cell binding was observed for [¹⁸F]AlF-NOTA-JR11 as compared to [¹⁸F]AlF-NOTA-octreotide after 60 min. μPET/CT images demonstrated comparable pharmacokinetics and tumor uptake between [¹⁸F]AlF-NOTA-JR11 (SUV_max: 3.7 ± 0.8) and [¹⁸F]AlF-NOTA-octreotide (SUV_max: 3.6 ± 0.4).

CONCLUSIONS

[¹⁸F]AlF-NOTA-JR11 was obtained in good RCY, albeit with a moderate RCP. The cell binding study showed significant higher binding of [¹⁸F]AlF-NOTA-JR11 compared to [¹⁸F]AlF-NOTA-octreotide, despite the higher IC₅₀ value of AlF-NOTA-JR11. However, pharmacokinetics and in vivo tumor uptake was comparable for both radiotracers. Novel Al¹⁸F-labeled derivatives of JR11 with higher SSTR2 affinity should be developed for increased tumor uptake and NET imaging sensitivity.

Clinical Pharmacology of Radiotheranostics in Oncology

The combined use of diagnostic and therapeutic radioligands with the same molecular target, also known as theranostics, enables accurate patient selection, targeted therapy, and prediction of treatment response. Radioiodine, bone-seeking radioligands and norepinephrine analogs have been used for many years for diagnostic imaging and radioligand therapy of thyroid carcinoma, bone metastases, pheochromocytoma, paraganglioma, and neuroblastoma, respectively. In recent years, radiolabeled somatostatin analogs and prostate-specific membrane antigen ligands have shown clinical efficacy in the treatment of neuroendocrine tumors and prostate cancer, respectively. Several candidate compounds are targeting novel theranostic targets such as fibroblast activation protein, C-X-C chemokine receptor 4, and gastrin-releasing peptide receptor. In addition, several strategies to improve efficacy of radioligand therapy are being evaluated, including dosimetry-based dose optimization, multireceptor targeting, upregulation of target receptors, radiosensitization, pharmacogenomics, and radiation genomics. Design and evaluation of novel radioligands and optimization of dose and dose schedules, within the complex context of individualized multimodal cancer treatment, requires a multidisciplinary approach that includes clinical pharmacology. Significant increases in the use of these radiopharmaceuticals in routine oncological practice can be expected, which will have major impact on patient care as well as (radio)pharmacy utilization.

Feasibility and therapeutic potential of the 68Ga/177Lu-DOTATATE theranostic pair in patients with metastatic medullary thyroid carcinoma

BACKGROUND

This study assessed: 1) the clinical efficacy of imaging with 68Ga-DOTATATE PET/CT (SSTR (somatostatin receptor)-PET) to detect medullary thyroid carcinoma (MTC); and 2) the therapeutic efficacy of peptide receptor radionuclide therapy (PRRT) with 177Lu-DOTATATE in MTC patients.

MATERIALS AND METHODS

Patients with histologically proven MTC and suspected recurrence following thyroidectomy, based on raised serum calcitonin levels, underwent SSTR-PET. In addition, to evaluate the clinical efficacy and safety of PRRT, the patients with intense uptake on SSTR-PET or 99mTc-octreotide scintigraphy underwent PRRT. The Common Terminology Criteria for Adverse Events (version 4.03) was used to grade adverse events after PRRT. Treatment response was classified as complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD).

RESULTS

Twenty MTC patients (10 male, 10 female) with a median age of 48.5 years underwent SSTR-PET. SSTR-PET was positive in 17/20 patients (85%). Four of the 17 patients with positive SSTR-PET were scheduled for PRRT. In addition, 2 patients had positive 99mTc-octreotide scintigraphy results (Krenning score ≥ 2) and were scheduled for PRRT. Two of the 6 patients who underwent PRRT showed PR, 2 SD and 2 PD. Two patients died during the follow-up period. Median overall survival was 19 months (95% CI: 5.52-29.48). There were no cases of significant toxicity.

CONCLUSION

Radiolabeled somatostatin analogs are contributive for the management of recurrent MTC. 68Ga-DOTATAE PET-CT showed a relatively high detection rate in recurrent MTC. In addition, PRRT with 177Lu-DOTATATE was found to be a safe alternative therapeutic option for MTC.

PET/CT and PET/MRI in neuroendocrine neoplasms

Advanced molecular imaging has come to play an integral role in the management of gastro-entero-pancreatic neuroendocrine neoplasms (GEP-NENs). Somatostatin receptor (SSTR) PET has now emerged as the reference standard for the evaluation of NENs and is particularly critical in the context of peptide receptor radionuclide therapy (PRRT) eligibility. SSTR PET/MRI with liver-specific contrast agent has a strong potential for one-stop-shop multiparametric evaluation of GEP-NENs. 18F-FDG is a complementary radiotracer to SSTR, especially in the context of high-grade neuroendocrine neoplasms. Knowledge gaps in quantitative evaluation of molecular imaging studies and their role in assessment of response to PRRT and combination therapies are active research areas. Novel radiotracers have the potential to overcome existing limitations in the molecular imaging of GEP-NENs. The purpose of this article is to provide an overview of the current trends, pitfalls, and recent advancements of molecular imaging for GEP-NENs.

Nuclear Medicine and Radiological Imaging of Pancreatic Neuroendocrine Neoplasms: A Multidisciplinary Update

Pancreatic neuroendocrine neoplasms (panNENs) are part of a large family of tumors arising from the neuroendocrine system. PanNENs show low-intermediate tumor grade and generally high somatostatin receptor (SSTR) expression. Therefore, panNENs benefit from functional imaging with ⁶⁸Ga-somatostatin analogues (SSA) for diagnosis, staging, and treatment choice in parallel with morphological imaging. This narrative review aims to present conventional imaging techniques and new perspectives in the management of panNENs, providing the clinicians with useful insight for clinical practice. The ⁶⁸Ga-SSA PET/CT is the most widely used in panNENs, not only fr diagnosis and staging purpose but also to characterize the biology of the tumor and its responsiveness to SSAs. On the contrary, the ¹⁸F-Fluordeoxiglucose (FDG) PET/CT is not employed systematically in all panNEN patients, being generally preferred in G2-G3, to predict aggressiveness and progression rate. The combination of ⁶⁸Ga-SSA PET/CT and ¹⁸F-FDG PET/CT can finally suggest the best therapeutic strategy. Other radiopharmaceuticals are ⁶⁸Ga-exendin-4 in case of insulinomas and ¹⁸F-dopamine (DOPA), which can be helpful in SSTR-negative tumors. New promising but still-under-investigation radiopharmaceuticals include radiolabeled SSTR antagonists and ¹⁸F-SSAs. Conventional imaging includes contrast enhanced CT and multiparametric MRI. There are now enriched by radiomics, a new non-invasive imaging approach, very promising to early predict tumor response or progression.

Head-to-Head Comparison between Peptide-Based Radiopharmaceutical for PET and SPECT in the Evaluation of Neuroendocrine Tumors: A Systematic Review

We compared head-to-head the most used radiolabeled peptides for single photon computed emission tomography (SPECT) and positron emission tomography (PET) imaging of neuroendocrine tumors (NETs). A comprehensive literature search was performed in PubMed, Web of Science, and Scopus databases. The following words, coupled two by two, were used: 68Ga-DOTATOC; 68Ga-DOTATATE; 68Ga-DOTANOC; 99mTc-EDDA/HYNIC-TOC; 64Cu-DOTATATE; and 111In-DTPA-octreotide. Moreover, a second-step search strategy was adopted by using the following combined terms: "Somatostatin receptor imaging,"; "Somatostatin receptor imaging" and "Functional,"; "Somatostatin receptor imaging" and "SPECT,"; and "Somatostatin receptor imaging" and "PET". Eligible criteria were: (1) original articles focusing on the clinical application of the radiopharmaceutical agents in NETs; (2) original articles in the English language; (3) comparative studies (head-to-head comparative or matched-paired studies). Editorials, letters to the editor, reviews, pictorial essays, clinical cases, or opinions were excluded. A total of 1077 articles were found in the three electronic databases. The full texts of 104 articles were assessed for eligibility. Nineteen articles were finally included. Most articles focused on the comparison between 111In-DTPA-Octreotide and 68Ga-DOTATOC/TATE. Few papers compared 64Cu-DOTATATE and 68Ga-DOTATOC/TATE, or SPECT tracers. The rates of true positivity were 63.7%, 58.5%, 78.4% and 82.4%, respectively, for 111In-DTPA-Octreotide, 99mTc-EDDA/HYNIC-TOC, 68Ga-DOTATATE/TOC and 64Cu-DOTATATE. In conclusion, as highly expected, PET tracers are more suitable for the in vivo identification of NETs. Indeed, in comparative studies, they demonstrated a higher true positive rate than SPECT agents.

A prospective head-to-head comparison of 68 Ga-NOTA-3P-TATE-RGD and 68 Ga-DOTATATE in patients with gastroenteropancreatic neuroendocrine tumours

PURPOSE

The aim of this study was to compare 68 Ga-NOTA-3P-TATE-RGD, a dual somatostatin receptor 2- and integrin αVβ3-targeting tracer, to 68 Ga-DOTATATE in a single group of patients with gastroenteropancreatic (GEP)-neuroendocrine tumours (NETs).

METHODS

Thirty-five patients with histologically confirmed GEP-NETs (5 grade 1, 28 grade 2, and 2 grade 3 tumours) were prospectively enrolled with informed consent. The primary tumour mainly originated from the pancreas and rectum. All patients were scanned with both 68 Ga-NOTA-3P-TATE-RGD PET/CT and 68 Ga-DOTATATE PET/CT within a week and compared on a head-to-head basis. Sixteen patients also had conventional 18F-FDG PET/CT. Images were evaluated semi-quantitatively using maximum standardized uptake values (SUVmax) of tumour and tumour-to-background ratio.

RESULTS

All patients had at least one positive lesion on each of the two scans. A total of 1190 and 1106 lesions were detected on 68 Ga-NOTA-3P-TATE-RGD images and 68 Ga-DOTATATE images, respectively (P = 0.152). 68 Ga-NOTA-3P-TATE-RGD PET/CT revealed significantly more lesions in the liver than 68 Ga-DOTATATE PET/CT (634 vs. 532, P = 0.021). Both tracers produced comparable results for detecting primary tumours (20 vs. 20, P = 1.000), lymph node metastases (101 vs. 102, P = 0.655), and bone metastases (381 vs. 398, P = 0.244). The tumour SUVmax in 12 patients was significantly higher for 68 Ga-NOTA-3P-TATE-RGD than for 68 Ga-DOTATATE (27.2 ± 13.6 vs. 19.5 ± 10.0, P < 0.001); among them, 9 had 18F-FDG PET/CT and all were found to be FDG-positive. The remaining 23 patients had significantly higher 68 Ga-DOTATATE uptake than 68 Ga-NOTA-3P-TATE-RGD uptake (22.3 ± 16.4 vs. 11.9 ± 7.5, P < 0.001); among them, 7 had 18F-FDG PET/CT and 6 were FDG-negative. Generally, 68 Ga-DOTATATE demonstrated higher tumour SUVmax than 68 Ga-NOTA-3P-TATE-RGD (20.8 ± 16.0 vs. 14.2 ± 8.9, P < 0.001), including primary tumours, liver lesions, lymph node lesions, and bone lesions. However, the tumour-to-background ratio of liver lesions was significantly higher when using 68 Ga-NOTA-3P-TATE-RGD compared with that when using 68 Ga-DOTATATE (8.4 ± 5.5 vs. 4.7 ± 3.7, P < 0.001).

CONCLUSION

68 Ga-NOTA-3P-TATE-RGD performed better than 68 Ga-DOTATATE in detection of liver metastases with a higher tumour-to-background ratio. Moreover, 68 Ga-NOTA-3P-TATE-RGD tended to demonstrate higher uptake over 68 Ga-DOTATATE in FDG-avid NETs.

TRIAL REGISTRATION

Dual SSTR2 and Integrin αvβ3 Targeting PET/CT Imaging (NCT02817945, registered 5 November 2018). URL OF REGISTRY: https://clinicaltrials.gov/ct2/show/NCT02817945.

Synthesis and Evaluation of Two Long-Acting SSTR2 Antagonists for Radionuclide Therapy of Neuroendocrine Tumors

Somatostatin receptor subtype 2 (SSTR2) has become an essential target for radionuclide therapy of neuroendocrine tumors (NETs). JR11 was introduced as a promising antagonist peptide to target SSTR2. However, due to its rapid blood clearance, a better pharmacokinetic profile is necessary for more effective treatment. Therefore, two JR11 analogs (8a and 8b), each carrying an albumin binding domain, were designed to prolong the blood residence time of JR11. Both compounds were labeled with lutetium-177 and evaluated via in vitro assays, followed by in vivo SPECT/CT imaging and ex vivo biodistribution studies. [177Lu]Lu-8a and [177Lu]Lu-8b were obtained with high radiochemical purity (>97%) and demonstrated excellent stability in PBS and mouse serum (>95%). [177Lu]Lu-8a showed better affinity towards human albumin compared to [177Lu]Lu-8b. Further, 8a and 8b exhibited binding affinities 30- and 48-fold lower, respectively, than that of the parent peptide JR11, along with high cell uptake and low internalization rate. SPECT/CT imaging verified high tumor accumulation for [177Lu]Lu-8a and [177Lu]Lu-JR11 at 4, 24, 48, and 72 h post-injection, but no tumor uptake was observed for [177Lu]Lu-8b. Ex vivo biodistribution studies revealed high and increasing tumor uptake for [177Lu]Lu-8a. However, its extended blood circulation led to an unfavorable biodistribution profile for radionuclide therapy.

Molecular Imaging of Neuroendocrine Neoplasms

The key for molecular imaging is the use of a radiotracer with a radioactive and a functional component. While the functional component targets a specific feature of the tumor, the radioactive component makes the target visible. Neuroendocrine neoplasms (NEN) are a diverse group of rare tumors that arise from neuroendocrine cells found mainly in the gastroenteropancreatic system, lung, thyroid, and adrenal glands. They are characterized by the expression of specific hormone receptors on the tumor cell surface, which makes them ideal targets for radiolabeled peptides. The most commonly expressed hormone receptors on NEN cells are the somatostatin receptors. They can be targeted for molecular imaging with various radiolabeled somatostatin analogs, but also with somatostatin antagonists, which have shown improved imaging quality. 18F-DOPA imaging has become a second-line imaging modality in NENs, with the exception of the evaluation of advanced medullary thyroid carcinoma. Alternatives for NENs with insufficient somatostatin receptor expression due to poor differentiation involve targeting glucose metabolism, which can also be used for prognosis. For the localization of the often-small insulinoma, glucagon-like peptide-1 (GLP-1) receptor imaging has become the new standard. Other alternatives involve metaiodobenzylguanidine and the molecular target C-X-C motif chemokine receptor-4. In addition, new radiopeptides targeting the fibroblast activation protein, the glucose-dependent insulinotropic polypeptide receptor and cholecystokinin-2 receptors have been identified in NENs and await further evaluation. This mini-review aims to provide an overview of the major molecular imaging modalities currently used in the field of NENs, and also to provide an outlook on future developments.

Surgical Strategy and Prognosis of Pancreatic Neuroendocrine Tumors Based on Smart Medical Imaging

It is imperative to seize the "golden rescue time" and implement new concepts and new skills in modern trauma rescue. Combining with the development background of smart medical image analysis, this topic focuses on surgical strategies and prognostic measures and studies a serious and difficult disease frequently occurring in middle-aged and elderly people: pancreatic neuroendocrine tumors. This article uses the comparative test method and sample collection method to collect the medical records of patients with neuroendocrine tumors diagnosed by pathology from July 2010 to January 2018 in the First Affiliated Hospital of X City and analyze the samples with gender and age. At the same time, routine tumor marker examination and the location of NEN in the digestive system were performed. The distribution analysis of EUS characteristics of neuroendocrine tumor mucosa in each site was performed after operation, and the analysis of survival-related factors was performed during postoperative follow-up. The experimental data showed that among the tumor causes, WHO tumor grade (p < 0.05) and whether the surgical method was R0 resection (p < 0.05) were associated with prognosis. However, factors such as gender, age, and functional status were associated with prognosis. It has successfully completed the subject of surgical strategy and prognosis of pancreatic neuroendocrine tumors based on smart medical image analysis.

Theranostics bei gut bis mäßig differenzierten GEP-NEN

Neuroendokrine Neoplasien (NEN) sind seltene, heterogene und typischerweise langsam wachsende Tumoren. Die häufigsten Lokalisationen finden sich im gastro-entero-pankreatischen System (GEP-NEN). NENs werden nach proliferativer Aktivität (Ki-67-Index) eingeteilt (G1–3). Gut differenzierte Tumoren exprimieren dabei typischerweise Somatostatinrezeptoren (SSTR), die als Zielstruktur in der nuklearmedizinischen Theranostik dienen. Bei diesem Prinzip kann nach einer diagnostischen molekularen Bildgebung, meist mittels Positronenemissionstomografie/Computertomografie (PET/CT), eine individuell zugeschnittene Peptidradiorezeptortherapie (PRRT) mit einem β-Strahler-markierten Radiopharmakon erfolgen. In Metaanalysen zeigte die Diagnostik mittels SSTR-gerichteter PET/CT eine Sensitivität von 93% und eine Spezifität von 96%. Die SSTR-gerichtete Diagnostik kann auch zur radioaktiven Markierung von Tumoren verwendet werden, um eine zielgerichtete Chirurgie zu ermöglichen. Die Indikation zur Einleitung einer PRRT soll stets in einer interdisziplinären Tumorkonferenz getroffen werden. Ein Tumorprogress unter der vorangegangenen Therapie sollte dokumentiert sein. Die Therapie wird intravenös und insgesamt 4-mal in 8-wöchigem Abstand in spezialisierten nuklearmedizinischen Zentren verabreicht. Die Wirksamkeit der PRRT wurde in der NETTER-1-Studie prospektiv untersucht und konnte eine signifikante Verbesserung des progressionsfreien Überlebens (primärer Endpunkt) zeigen. Ausgehend von diesen Studienergebnissen steht mit Lutathera (177Lu-DOTATATE) inzwischen ein in Deutschland zugelassenes Radiopharmazeutikum zu Behandlung von nicht resektablen oder metastasierten bzw. progredienten, gut differenzierten (G1 und G2), SSTR-positiven GEP-NEN zur Verfügung.

Molecular Imaging in neuroendocrine neoplasias

Molecular imaging, which uses molecular targets due to the overexpression of specific peptide hormone receptors on the tumour surface, has become an indispensable diagnostic technique. Neuroendocrine neoplasms (NENs) especially differentiated NENs or neuroendocrine tumours (NETs) are a rare group of heterogeneous tumours, characterized by the expression of hormone receptors on the tumour cell surface. This property makes them receptive to diagnostic and therapeutic approaches (theranostics) using radiolabelled peptides. Amongst the known hormone receptors, somatostatin receptors (SSTR) are expressed on the majority of NETs and are therefore the most relevant receptors for theranostic approaches. Current research aims to medically upregulate their expression, while other focuses are on the use of different radiopeptides (64Cu and 67Cu) or somatostatin-antagonists instead of the established somatostatin agonists. The GLP-1 receptor is another clinically relevant target, as GLP-1-R imaging has become the new standard for the localisation of insulinomas. For staging and prognostic evaluation in dedifferentiated NENs, 18F-FDG-imaging is useful, but lacks a therapeutic counterpart. Further options for patients with insufficient expression of SSTR involve metaiodobenzylguanidine (MIBG) and the molecular target C-X-C motif chemokine receptor-4 (CXCR4). New targets such as the glucose-dependant insulinotropic polypeptide receptor (GIPR) and the fibroblast activation protein (FAP) have been identified in NENs recently and await further evaluation. For medullary thyroid cancer 18-F-DOPA imaging is standard, however this technique is rather second line for other NENs. In this area, the discovery of minigastrin, which targets the cholecystokinin-2 (CCK2) receptors in medullary thyroid carcinoma and foregut NENs, may improve future management. This review aims to provide an overview of the most commonly used functional imaging modalities for theranostics in NENs today and in the possible future.

Current status and future of targeted peptide receptor radionuclide positron emission tomography imaging and therapy of gastroenteropancreatic-neuroendocrine tumors

Theranostics is the highly targeted molecular imaging and therapy of tumors. Targeted peptide receptor radionuclide therapy has taken the lead in demonstrating the safety and effectiveness of this molecular approach to treating cancers. Metastatic, well-differentiated gastroenteropancreatic neuroendocrine tumors may be most effectively imaged and treated with DOTATATE ligands. We review the current practice, safety, advantages, and limitations of DOTATATE based theranostics. Finally, we briefly describe the exciting new areas of development and future directions of gastroenteropancreatic neuroendocrine tumor theranostics.

New PET Radiotracers for the Imaging of Neuroendocrine Neoplasms

OPINION STATEMENT

Neuroendocrine neoplasms (NEN) are a heterogeneous group of tumours derived from cells of neuroendocrine origin and can potentially arise everywhere in the human body. The diagnostic assessment of NEN can be performed using a variety of PET radiopharmaceuticals. Well-differentiated NEN (NET) present a high expression of SSTR (somatostatin receptors) and can therefore be studied with 68Ga-DOTA-peptides ([68Ga]Ga-DOTANOC, [68Ga]Ga-DOTATOC, [68Ga]Ga-DOTATATE). Current guidelines recommend the use of SSTR imaging to assess disease extension at staging/restaging, follow-up, assessment of response to therapy and selection of patients who may benefit from radionuclide therapy (PRRT). [18F]F-FDG is used for the assessment of high-grade tumours (high-grade G2, G3 and NEC) and in every case, there is one or more mismatched lesions between diagnostic CT (positive) and SSTR-PET/CT (negative). [18F]F-DOPA is currently used for the assessment of medullary thyroid carcinoma, neuroblastoma, primary pheochromocytoma and abdominal paraganglioma. In recent years, however, several new tracers were designed exploiting the many potential targets of the neuroendocrine cell and were employed in clinical trials for both imaging and therapy. Currently, the real-life clinical impact of these tracers is still mostly not known; however, the favourable biodistribution (e.g. [68Ga]Ga-FAPI, SSTR antagonists) and the possibility to use new theranostic pairs may provide novel diagnostic as well as therapeutic options (e.g. [68Ga]Ga-PSMA, [64Cu]Cu-SARTATE, [68Ga]Ga-CXCR4) for NEN patients.

Advances in the Diagnosis and Therapeutic Management of Gastroenteropancreatic Neuroendocrine Neoplasms (GEP-NENs)

Neuroendocrine neoplasms (NENs) are an increasingly common cause of neoplastic diseases. One of the largest groups of NENs are neoplasms localized to the gastroenteropancreatic system, which are known as gastroenteropancreatic NENs (GEP-NENs). Because of nonspecific clinical symptoms, GEP-NEN patient diagnosis and, consequently, their treatment, might be difficult and delayed. This situation has forced researchers all over the world to continue progress in the diagnosis and treatment of patients with GEP-NENs. Our review is designed to present the latest reports on the laboratory diagnostic techniques, imaging tests and surgical and nonsurgical treatment strategies used for patients with these rare neoplasms. We paid particular attention to the nuclear approach, the use of which has been applied to GEP-NEN patient diagnosis, and to nonsurgical and radionuclide treatment strategies. Recent publications were reviewed in search of reports on new strategies for effective disease management. Attention was also paid to those studies still in progress, but with successful results. A total of 248 papers were analyzed, from which 141 papers most relevant to the aim of the study were selected. Using these papers, we highlight the progress in the development of diagnostic and treatment strategies for patients with GEP-NENs.

Radiolabeled Somatostatin Analogs-A Continuously Evolving Class of Radiopharmaceuticals

Somatostatin receptors (SSTs) are recognized as favorable molecular targets in neuroendocrine tumors (NETs) and neuroendocrine neoplasms (NENs), with subtype 2 (SST₂) being the predominantly and most frequently expressed. PET/CT imaging with ⁶⁸Ga-labeled SST agonists, e.g., ⁶⁸Ga-DOTA-TOC (SomaKit TOC^®) or ⁶⁸Ga-DOTA-TATE (NETSPOT^®), plays an important role in staging and restaging these tumors and can identify patients who qualify and would potentially benefit from peptide receptor radionuclide therapy (PRRT) with the therapeutic counterparts ¹⁷⁷Lu-DOTA-TOC or ¹⁷⁷Lu-DOTA-TATE (Lutathera^®). This is an important feature of SST targeting, as it allows a personalized treatment approach (theranostic approach). Today, new developments hold promise for enhancing diagnostic accuracy and therapeutic efficacy. Among them, the use of SST₂ antagonists, such as JR11 and LM3, has shown certain advantages in improving image sensitivity and tumor radiation dose, and there is evidence that they may find application in other oncological indications beyond NETs and NENs. In addition, PRRT performed with more cytotoxic α-emitters, such as ²²⁵Ac, or β^- and Auger electrons, such as ¹⁶¹Tb, presents higher efficacy. It remains to be seen if any of these new developments will overpower the established radiolabeled SST analogs and PRRT with β^--emitters.

Radiolabeled Somatostatin Analogues for Diagnosis and Treatment of Neuroendocrine Tumors

Neuroendocrine neoplasms (NENs) are rare and heterogeneous tumors that require multidisciplinary discussion for optimal care. The theranostic approach (DOTA peptides labelled with 68Ga for diagnosis and with 90Y or 177Lu for therapy) plays a crucial role in the management of NENs to assess disease extension and as a criteria for peptide receptor radionuclide therapy (PRRT) eligibility based on somatostatin receptor (SSTR) expression. On the diagnostic side, [68Ga]Ga-DOTA peptides PET/CT (SSTR PET/CT) is the gold standard for imaging well-differentiated SSTR-expressing neuroendocrine tumors (NETs). [18F]FDG PET/CT is useful in higher grade NENs (NET G2 with Ki-67 > 10% and NET G3; NEC) for more accurate disease characterization and prognostication. Promising emerging radiopharmaceuticals include somatostatin analogues labelled with 18F (to overcome the limits imposed by 68Ga), and SSTR antagonists (for both diagnosis and therapy). On the therapeutic side, the evidence gathered over the past two decades indicates that PRRT is to be considered as an effective and safe treatment option for SSTR-expressing NETs, and is currently included in the therapeutic algorithms of the main scientific societies. The positioning of PRRT in the treatment sequence, as well as treatment personalization (e.g., tailored dosimetry, re-treatment, selection criteria, and combination with other alternative treatment options), is warranted in order to improve its efficacy while reducing toxicity. Although very preliminary (being mostly hampered by lack of methodological standardization, especially regarding feature selection/extraction) and often including small patient cohorts, radiomic studies in NETs are also presented. To date, the implementation of radiomics in clinical practice is still unclear. The purpose of this review is to offer an overview of radiolabeled SSTR analogues for theranostic use in NENs.

Cold Kit Labeling: The Future of 68Ga Radiopharmaceuticals?

Over the last couple of decades, gallium-68 (68Ga) has gained a formidable interest for PET molecular imaging of various conditions, from cancer to infection, through cardiac pathologies or neuropathies. It has gained routine use, with successful radiopharmaceuticals such as somatostatin analogs ([68Ga]Ga-DOTATOC and [68Ga]GaDOTATATE) for neuroendocrine tumors, and PSMA ligands for prostate cancer. It represents a major clinical impact, particularly in the context of theranostics, coupled with their 177Lu-labeled counterparts. Beside those, a bunch of new 68Ga-labeled molecules are in the preclinical and clinical pipelines, with some of them showing great promise for patient care. Increasing clinical demand and regulatory issues have led to the development of automated procedures for the production of 68Ga radiopharmaceuticals. However, the widespread use of these radiopharmaceuticals may rely on simple and efficient radiolabeling methods, undemanding in terms of equipment and infrastructure. To make them technically and economically accessible to the medical community and its patients, it appears mandatory to develop a procedure similar to the well-established kit-based 99mTc chemistry. Already available commercial kits for the production of 68Ga radiopharmaceuticals have demonstrated the feasibility of using such an approach, thus paving the way for more kit-based 68Ga radiopharmaceuticals to be developed. This article discusses the development of 68Ga cold kit radiopharmacy, including technical issues, and regulatory aspects.