First Clinical Evidence That Imaging with Somatostatin Receptor Antagonists Is Feasible

Targeted Radionuclide Therapy in Glioblastoma

Despite the development of various novel therapies, glioblastoma (GBM) remains a devastating disease, with a median survival of less than 15 months. Recently, targeted radionuclide therapy has shown significant progress in treating solid tumors, with the approval of Lutathera for neuroendocrine tumors and Pluvicto for prostate cancer by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA). This achievement has shed light on the potential of targeted radionuclide therapy for other solid tumors, including GBM. This review presents the current status of targeted radionuclide therapy in GBM, highlighting the commonly used therapeutic radionuclides emitting alpha, beta particles, and Auger electrons that could induce potent molecular and cellular damage to treat GBM. We then explore a range of targeting vectors, including small molecules, peptides, and antibodies, which selectively target antigen-expressing tumor cells with minimal or no binding to healthy tissues. Considering that radiopharmaceuticals for GBM are often administered locoregionally to bypass the blood-brain barrier (BBB), we review prominent delivery methods such as convection-enhanced delivery, local implantation, and stereotactic injections. Finally, we address the challenges of this therapeutic approach for GBM and propose potential solutions.

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.

Radiolabeled peptides and their expanding role in clinical imaging and targeted cancer therapy

There is an expanding body of evidence showing that synthetic peptides in combination with radioactive isotopes can be utilized for medical purposes. This area is of particular interest in oncology where applications in diagnosis and therapy are at different stages of development. We review the contributions in this area by the group originally founded by Carlo Pedone in Naples many years ago. We highlight the work of this group in the context of other developments in this area, focusing on three biologically relevant receptor systems: somatostatin, gastrin-releasing peptide, and cholecystokinin-2/gastrin receptors. We focus on key milestones, state of the art, and challenges in this area of research as well as the current and future outlook for expanding clinical applications.

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.

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.

Clinical Advances and Perspectives in Targeted Radionuclide Therapy

Targeted radionuclide therapy has become increasingly prominent as a nuclear medicine subspecialty. For many decades, treatment with radionuclides has been mainly restricted to the use of iodine-131 in thyroid disorders. Currently, radiopharmaceuticals, consisting of a radionuclide coupled to a vector that binds to a desired biological target with high specificity, are being developed. The objective is to be as selective as possible at the tumor level, while limiting the dose received at the healthy tissue level. In recent years, a better understanding of molecular mechanisms of cancer, as well as the appearance of innovative targeting agents (antibodies, peptides, and small molecules) and the availability of new radioisotopes, have enabled considerable advances in the field of vectorized internal radiotherapy with a better therapeutic efficacy, radiation safety and personalized treatments. For instance, targeting the tumor microenvironment, instead of the cancer cells, now appears particularly attractive. Several radiopharmaceuticals for therapeutic targeting have shown clinical value in several types of tumors and have been or will soon be approved and authorized for clinical use. Following their clinical and commercial success, research in that domain is particularly growing, with the clinical pipeline appearing as a promising target. This review aims to provide an overview of current research on targeting radionuclide therapy.

Evolving strategies and application of proteins and peptide therapeutics in cancer treatment

Several proteins and peptides have therapeutic potential and can be used for cancer therapy. By binding to cell surface receptors and other indicators uniquely linked with or overexpressed on tumors compared to healthy tissue, protein biologics enhance the active targeting of cancer cells, as opposed to the passive targeting of cells by conventional small-molecule chemotherapeutics. This study focuses on peptide medications that exist to slow or stop tumor growth and the spread of cancer, demonstrating the therapeutic potential of peptides in cancer treatment. As an alternative to standard chemotherapy, peptides that selectively kill cancer cells while sparing healthy tissue are developing. A mountain of clinical evidence supports the efficacy of peptide-based cancer vaccines. Since a single treatment technique may not be sufficient to produce favourable results in the fight against cancer, combination therapy is emerging as an effective option to generate synergistic benefits. One example of this new area is the use of anticancer peptides in combination with nonpeptidic cytotoxic drugs or the combination of immunotherapy with conventional therapies like radiation and chemotherapy. This review focuses on the different natural and synthetic peptides obtained and researched. Discoveries, manufacture, and modifications of peptide drugs, as well as their contemporary applications, are summarized in this review. We also discuss the benefits and difficulties of potential advances in therapeutic peptides.

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.

[68Ga]Ga-DATA5m-LM4, a PET Radiotracer in the Diagnosis of SST2R-Positive Tumors: Preclinical and First Clinical Results

Radiolabeled somatostatin subtype 2 receptor (SST2R)-antagonists have shown advantageous profiles for cancer theranostics compared with agonists. On the other hand, the newly introduced hybrid chelator (6-pentanoic acid)-6-(amino)methyl-1,4-diazepinetriacetate (DATA^5m) rapidly binds Ga-68 (t_1/2: 67.7 min) at much lower temperature, thus allowing for quick access to "ready-for-injection" [⁶⁸Ga]Ga-tracers in hospitals. We herein introduce [⁶⁸Ga]Ga-DATA^5m-LM4 for PET/CT imaging of SST₂R-positive human tumors. LM4 was obtained by 4Pal³/Tyr³-substitution in the known SST₂R antagonist LM3 (H-DPhe-c[DCys-Tyr-DAph(Cbm)-Lys-Thr-Cys]-DTyr-NH₂) and DATA^5m was coupled at the N-terminus for labeling with radiogallium (Ga-67/68). [⁶⁷Ga]Ga-DATA^5m-LM4 was evaluated in HEK293-SST₂R cells and mice models in a head-to-head comparison with [⁶⁷Ga]Ga-DOTA-LM3. Clinical grade [⁶⁸Ga]Ga-DATA^5m-LM4 was prepared and injected in a neuroendocrine tumor (NET) patient for PET/CT imaging. DATA^5m-LM4 displayed high SST₂R binding affinity. [⁶⁷Ga]Ga-DATA^5m-LM4 showed markedly higher uptake in HEK293-SST₂R cells versus [⁶⁷Ga]Ga-DOTA-LM3 and was stable in vivo. In HEK293-SST₂R xenograft-bearing mice, it achieved longer tumor retention and less kidney uptake than [⁶⁷Ga]Ga-DOTA-LM3. [⁶⁸Ga]Ga-DATA^5m-LM4 accurately visualized tumor lesions with high contrast on PET/CT. In short, [⁶⁸Ga]Ga-DATA^5m-LM4 has shown excellent prospects for the PET/CT diagnosis of SST₂R-positive tumors, further highlighting the benefits of Ga-68 labeling in a hospital environment via the DATA^5m-chelator route.

Imaging-guided targeted radionuclide tumor therapy: From concept to clinical translation

Since the first introduction of sodium iodide I-131 for use with thyroid patients almost 80 years ago, more than 50 radiopharmaceuticals have reached the markets for a wide range of diseases, especially cancers. The nuclear medicine paradigm also shifts from solely molecular imaging or radionuclide therapy to imaging-guided radionuclide therapy, which is deemed a vital component of precision cancer therapy and an emerging medical modality for personalized medicine. The imaging-guided radionuclide therapy highlights the systematic integration of targeted nuclear diagnostics and radionuclide therapeutics. Regarding this, nuclear imaging serves to "visualize" the lesions and guide the therapeutic strategy, followed by administration of a precise patient specific dose of radiotherapeutics for treatment according to the absorbed dose to different organs and tumors calculated by dosimetry tools, and finally repeated imaging to predict the prognosis. This strategy leads to significantly enhanced therapeutic efficacy, improved patient outcomes, and manageable adverse events. In this review, we provide an overview of imaging-guided targeted radionuclide therapy for different tumors such as advanced prostate cancer and neuroendocrine tumors, with a focus on development of new radioligands and their preclinical and clinical results, and further discuss about challenges and future perspectives.

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.

Is a single late SPECT/CT based kidney 177Lu-dosimetry superior to hybrid dosimetry with sequential multiple time-point whole-body planar scans in combination with an early SPECT/CT?

PURPOSE

The aim is to assess the impact of different imaging-protocols on image-based kidney dosimetry in ¹⁷⁷Lu labelled peptide receptor radiotherapies.

METHODS

Kidney data of five [¹⁷⁷Lu]Lu-OPS201 injected pigs and a 3D printed phantom were used for comparing the absorbed doses and time-integrated activity coefficients calculated based on the following imaging-protocols: A-) multiple time-point SPECT/CTs, B-) multiple time-point planar scans in combination with one SPECT/CT, C-) single time-point SPECT/CT. In addition, the influence of late scan time-points on kidney dosimetry was investigated by sequentially eliminating scan data at > 100 h from the pig/phantom datasets for imaging-protocols A and B.

RESULTS

Compared to imaging-protocol A, absorbed doses based on imaging-protocols B and C (scans at > 24 h post-injection) were always lower (differences > 34%). The best agreement in absorbed dose was achieved by imaging-protocol C at ∼ 100 h post-injection (difference: 4%). Regarding the phantom/pig experiments, eliminating scan data at > 100 h post-injection increased the time-integrated activity coefficients calculated based on imaging-protocols A and B by up to 83%.

CONCLUSION

While imaging-protocol A is accurate if scans at >∼100 h are included, it is time-consuming. In addition to being time-consuming, imaging-protocol B shows high differences associated with organ-count overlay, a lack of accuracy concerning the geometric mean based 2D attenuation correction, and 2D background subtraction due to the inhomogeneous and time-varying background contributions. Our findings indicate that dosimetry based on imaging-protocol C, if appropriately performed, provides similar kidney absorbed doses compared to imaging-protocol A, while only a single scan time-point is necessary.

Diagnostic Value of Radiolabelled Somatostatin Analogues for Neuroendocrine Tumour Diagnosis: The Benefits and Drawbacks of [64Cu]Cu-DOTA-TOC

Simple Summary

One of the most incredible advances in nuclear medicine is early detection of neuroendocrine tumors, which leads to appropriate and expedient treatment pathways. Advances made with somatostatin analogue derivatives radiolabeled with Gallium-68 clarified the paths of diagnosis and treatment properly. Despite the significant improvements, widespread efforts are in progress to attain the most specific radiopharmaceutical for this purpose. In this literature review, we will provide a short overview on the role of nuclear medicine in the diagnosis of neuroendocrine tumors focusing on [⁶⁴Cu]Cu-DOTA-TOC as a new radiopharmaceutical with promising clinical results.

Abstract

Neuroendocrine tumours (NETs) arise from secondary epithelial cell lines in the gastrointestinal or respiratory system organs. The rate of development of these tumours varies from an indolent to an aggressive course, typically being initially asymptomatic. The identification of these tumours is difficult, particularly because the primary tumour is often small and undetectable by conventional anatomical imaging. Consequently, diagnosis of NETs is complicated and has been a significant challenge until recently. In the last 30 years, the advent of novel nuclear medicine diagnostic procedures has led to a substantial increase in NET detection. Great varieties of exclusive single photon emission computed tomography (SPECT) and positron emission tomography (PET) radiopharmaceuticals for detecting NETs are being applied successfully in clinical settings, including [¹¹¹In]In-pentetreotide, [^99mTc]Tc-HYNIC-TOC/TATE, [⁶⁸Ga]Ga-DOTA-TATE, and [⁶⁴Cu]Cu-DOTA-TOC/TATE. Among these tracers for functional imaging, PET radiopharmaceuticals are clearly and substantially superior to planar or SPECT imaging radiopharmaceuticals. The main advantages include higher resolution, better sensitivity and increased lesion-to-background uptake. An advantage of diagnosis with a radiopharmaceutical is the capacity of theranostics to provide concomitant diagnosis and treatment with particulate radionuclides, such as beta and alpha emitters including Lutetium-177 (¹⁷⁷Lu) and Actinium-225 (²²⁵Ac). Due to these unique challenges involved with diagnosing NETs, various PET tracers have been developed. This review compares the clinical characteristics of radiolabelled somatostatin analogues for NET diagnosis, focusing on the most recently FDA-approved [⁶⁴Cu]Cu-DOTA-TATE as a state-of-the art NET-PET/CT radiopharmaceutical.

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.

Radiotheranostics - Precision Medicine in Nuclear Medicine and Molecular Imaging

'See what you treat and treat what you see, at a molecular level', could be the motto of theranostics. The concept implies diagnosis (imaging) and treatment of cells (usually cancer) using the same molecule, thus guaranteeing a targeted cytotoxic approach of the imaged tumor cells while sparing healthy tissues. As the brilliant late Sam Gambhir would say, the imaging agent acts like a 'molecular spy' and reveals where the tumoral cells are located and the extent of disease burden (diagnosis). For treatment, the same 'molecular spy' docks to the same tumor cells, this time delivering cytotoxic doses of radiation (treatment). This duality represents the concept of a 'theranostic pair', which follows the scope and fundamental principles of targeted precision and personalized medicine. Although the term theranostic was noted in medical literature in the early 2000s, the principle is not at all new to nuclear medicine. The first example of theranostic dates back to 1941 when Dr. Saul Hertz first applied radioiodine for radionuclide treatment of thyroid cells in patients with hyperthyroidism. Ever since, theranostics has been an integral element of nuclear medicine and molecular imaging. The more we understand tumor biology and molecular pathology of carcinogenesis, including specific mutations and receptor expression profiles, the more specific these 'molecular spies' can be developed for diagnostic molecular imaging and subsequent radionuclide targeted therapy (radiotheranostics). The appropriate selection of the diagnostic and therapeutic radionuclide for the 'theranostic pair' is critical and takes into account not only the type of cytotoxic radiation emission, but also the linear energy transfer (LET), and the physical half-lives. Advances in radiochemistry and radiopharmacy with new radiolabeling techniques and chelators are revolutionizing the field. The landscape of cytotoxic systemic radionuclide treatments has dramatically expanded through the past decades thanks to all these advancements. This article discusses present and promising future theranostic applications for various types of diseases such as thyroid disorders, neuroendocrine tumors (NET), pediatric malignancies, and prostate cancer (PC), and provides an outlook for future perspectives.

Peptide Receptor Radionuclide Therapy in Thyroid Cancer

The treatment options that are currently available for management of metastatic, progressive radioactive iodine (RAI)-refractory differentiated thyroid cancers (DTCs), and medullary thyroid cancers (MTCs) are limited. While there are several systemic targeted therapies, such as tyrosine kinase inhibitors, that are being evaluated and implemented in the treatment of these cancers, such therapies are associated with serious, sometimes life-threatening, adverse events. Peptide receptor radionuclide therapy (PRRT) has the potential to be an effective and safe modality for treating patients with somatostatin receptor (SSTR)+ RAI-refractory DTCs and MTCs. MTCs and certain sub-types of RAI-refractory DTCs, such as Hürthle cell cancers which are less responsive to conventional modalities of treatment, have demonstrated a favorable response to treatment with PRRT. While the current literature offers hope for utilization of PRRT in thyroid cancer, several areas of this field remain to be investigated further, especially head-to-head comparisons with other systemic targeted therapies. In this review, we provide a comprehensive outlook on the current translational and clinical data on the use of various PRRTs, including diagnostic utility of somatostatin analogs, theranostic properties of PRRT, and the potential areas for future research.

A prospective randomized, double-blind study to evaluate the diagnostic efficacy of 68Ga-NODAGA-LM3 and 68Ga-DOTA-LM3 in patients with well-differentiated neuroendocrine tumors: compared with 68Ga-DOTATATE

PURPOSE

The purpose of this study is to evaluate the diagnostic efficacy of ⁶⁸ Ga-NODAGA-LM3 and ⁶⁸ Ga-DOTA-LM3 and compare them with ⁶⁸ Ga-DOTATATE in patients with well-differentiated neuroendocrine tumors.

METHODS

Patients were prospectively recruited and equally randomized into two arms: Arm A, patients would undergo a whole-body ⁶⁸ Ga-NODAGA-LM3 PET/CT scan on the 1st day and ⁶⁸ Ga-DOTATATE PET/CT scan on the 2nd day; Arm B, patients would undergo a whole-body ⁶⁸ Ga-DOTA-LM3 PET/CT scan on the 1st day and ⁶⁸ Ga-DOTATATE PET/CT scan on the 2nd day. Biodistribution in normal organs, lesion detection ability, and tumor uptake were compared between antagonist and agonist in each arm.

RESULTS

A total of 40 patients with well-differentiated NETs, 20 in each arm, were recruited in the study. ⁶⁸ Ga-NODAGA-LM3 showed a similar pattern as ⁶⁸ Ga-DOTATATE, while ⁶⁸ Ga-DOTA-LM3 demonstrated significantly lower uptake in almost all normal organs compared to ⁶⁸ Ga-DOTATATE. Both ⁶⁸ Ga-NODAGA-LM3 and ⁶⁸ Ga-DOTA-LM3 showed superiority in lesion detection compared to ⁶⁸ Ga-DOTATATE on lesion-based and patient-based comparison. ⁶⁸ Ga-NODAGA-LM3 showed a significantly higher tumor uptake (median SUVmax 29.1 versus 21.6, P < 0.05) and tumor-to-background ratio (median tumor-to-liver ratio 5.0 versus 2.9, P < 0.05) compared to ⁶⁸ Ga-DOTATATE. ⁶⁸ Ga-DOTA-LM3 showed comparable uptake (median SUVmax 16.1 versus 17.8, P = 0.714) and higher tumor-to-background ratio (median tumor-to-liver ratio 5.2 versus 2.1, P < 0.05).

CONCLUSION

Both ⁶⁸ Ga-NODAGA-LM3 and ⁶⁸ Ga-DOTA-LM3 are promising SSTR2 antagonists for neuroendocrine tumors. They demonstrated superiority in diagnostic efficacy compared to agonist ⁶⁸ Ga-DOTATATE.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT04318561.

Somatostatin and Somatostatin Receptors: From Signaling to Clinical Applications in Neuroendocrine Neoplasms

Neuroendocrine neoplasms (NENs) are heterogeneous neoplasms which arise from neuroendocrine cells that are distributed widely throughout the body. Although heterogenous, many of them share their ability to overexpress somatostatin receptors (SSTR) on their cell surface. Due to this, SSTR and somatostatin have been a large subject of interest in the discovery of potential biomarkers and treatment options for the disease. The aim of this review is to describe the molecular characteristics of somatostatin and somatostatin receptors and its application in diagnosis and therapy on patients with NENs as well as the use in the near future of somatostatin antagonists.

First-in-Humans Study of the SSTR Antagonist 177Lu-DOTA-LM3 for Peptide Receptor Radionuclide Therapy in Patients with Metastatic Neuroendocrine Neoplasms: Dosimetry, Safety, and Efficacy

The objective of this study was to assess the safety, dosimetry, and efficacy of the 177Lu-labeled somatostatin receptor (SSTR) antagonist DOTA-p-Cl-Phe-cyclo(d-Cys-Tyr-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)d-Tyr-NH2 (177Lu-DOTA-LM3) in patients with metastatic neuroendocrine neoplasms (NENs). Methods: Fifty-one patients (aged 27-76 y; mean, 51.6 ± 13.9 y) with metastatic NENs underwent peptide receptor radionuclide therapy (PRRT) with 177Lu-DOTA-LM3 between August 2017 and December 2019. The median administered activity per cycle was 6.1 ± 0.88 GBq (range, 2.8-7.4 GBq). 68Ga-NODAGA-LM3 PET/CT was used for patient selection and follow-up after 177Lu-DOTA-LM3 PRRT. Morphologic and molecular responses were evaluated in accordance with RECIST 1.1 and the criteria of the European Organisation for Research and Treatment of Cancer (EORTC). Treatment-related adverse events were graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 5.0. Dosimetry was performed on 11 patients and compared with the SSTR agonist 177Lu-DOTATOC in 247 patients undergoing PRRT on the same dosimetry protocol. Results: Higher uptake and a longer effective half-life were found for 177Lu-DOTA-LM3 than for the agonist 177Lu-DOTATOC in the whole body and in the kidneys, spleen, and metastases, resulting in higher mean absorbed organ and tumor doses. All patients tolerated therapy without any serious acute adverse effects. Mild nausea without vomiting was observed in 5 (9.8%) patients; no other symptoms were reported. The most severe delayed adverse event was Common Terminology Criteria (CTC)-3 thrombocytopenia in 3 (5.9%) patients. Neither CTC-4 thrombocytopenia nor CTC-3-4 anemia or leukopenia was observed after treatment. No significant decline in renal function was observed, nor was hepatotoxicity. According to RECIST 1.1, disease control could be reached in 40 patients (disease control rate, 85.1%) of the 47 patients monitored after 177Lu-DOTA-LM3 PRRT, with a partial response in 17 (36.2%) and stable disease in 23 (48.9%), whereas 7 patients (14.9%) had progressive disease, and by EORTC criteria, there was complete remission in 2 patients (4.3%), partial remission in 21 (44.7%), stable disease in 18 (38.3%), and progressive disease in 6 (12.8%). Conclusion: The antagonist PRRT with 177Lu-DOTA-LM3 could be administered without severe adverse effects and was well tolerated by most patients, with thrombocytopenia occurring in only a few. No other severe adverse effects were observed; in particular, there was no nephrotoxicity. The SSTR antagonist 177Lu-DOTA-LM3 appears to be promising for PRRT, provides a favorable biodistribution and higher tumor radiation doses than SSTR agonists, and was effective in treating advanced metastatic NENs, especially in patients with low or no SSTR agonist binding, even achieving complete remission in some patients.

A Prospective, Randomized, Double-Blind Study to Evaluate the Safety, Biodistribution, and Dosimetry of 68Ga-NODAGA-LM3 and 68Ga-DOTA-LM3 in Patients with Well-Differentiated Neuroendocrine Tumors

68Ga-NODAGA-LM3 (where LM3 is p-Cl-Phe-cyclo(d-Cys-Tyr-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)d-Tyr-NH2) and 68Ga-DOTA-LM3 are somatostatin receptor subtype 2 (SSTR2)-specific antagonists used for PET/CT imaging. The purpose of this study was to evaluate the safety, biodistribution, and dosimetry of 68Ga-NODAGA-LM3 and 68Ga-DOTA-LM3 in patients with well-differentiated neuroendocrine tumors. Methods: Patients were equally randomized into 2 arms, with arm A receiving 68Ga-NODAGA-LM3 and arm B receiving 68Ga-DOTA-LM3. Serial PET scans were acquired at 5, 15, 30, 45, 60, and 120 min after 68Ga-NODAGA-LM3 (200 MBq ± 11 MBq/40 μg of total peptide mass) or 68Ga-DOTA-LM3 (172 MBq ± 21 MBq/40 μg of total peptide mass) injection. The biodistribution in normal organs, tumor uptake, and safety were assessed. Radiation dosimetry was calculated using OLINDA/EXM (version 1.0). Results: Sixteen patients, 8 in each arm, were recruited in the study. Both tracers were well tolerated in most patients. Two patients in arm B had nausea (grade 2), and one of them had vomiting (grade 1). The PET images of the other 14 patients were further analyzed. Significantly lower organ uptake was observed in the pituitary, parotids, liver, spleen, pancreas, adrenal, stomach, small intestine, and kidneys with 68Ga-DOTA-LM3 than with 68Ga-NODAGA-LM3. In total, 38 lesions were analyzed, including 18 with 68Ga-NODAGA-LM3 and 20 with 68Ga-DOTA-LM3. Both tracers showed good tumor uptake and retention. With 68Ga-NODAGA-LM3, the tracer accumulation in tumor lesions increased by 138%, from an average SUVmax of 31.3 ± 19.7 at 5 min to 74.6 ± 56.3 at 2 h. With 68Ga-DOTA-LM3, the tumor uptake rapidly reached a high level at 5 min after injection, with an average SUVmax of 36.6 ± 23.6, and continued to increase to 45.3 ± 29.3 until 30 min after injection. The urinary bladder wall was the organ receiving the highest absorbed dose in both arms. The mean effective dose was 0.026 ± 0.003 mSv/MBq for 68Ga-NODAGA-LM3 and 0.025 ± 0.002 mSv/MBq for 68Ga-DOTA-LM3. Conclusion: Both 68Ga-NODAGA-LM3 and 68Ga-DOTA-LM3 show favorable biodistribution, high tumor uptake, and good tumor retention, resulting in high image contrast. The dosimetric data are comparable to those for other 68Ga-labeled SSTR2 antagonists. Further studies are required to look into the potential antagonistic effects of 68Ga-NODAGA-LM3 and 68Ga-DOTA-LM3.

A randomised, factorial phase II study to determine the optimal dosing regimen for 68Ga-satoreotide trizoxetan as an imaging agent in patients with gastroenteropancreatic neuroendocrine tumours

⁶⁸Ga-satoreotide trizoxetan is a novel somatostatin receptor antagonist associated with high sensitivity and reproducibility in neuroendocrine tumour (NET) detection and localisation. However, the optimal peptide mass and radioactivity ranges for ⁶⁸Ga-satoreotide trizoxetan have not yet been established. We therefore aimed to determine its optimal dosing regimen in patients with metastatic gastroenteropancreatic NETs in a prospective, randomised, 2×3 factorial, multicentre, phase II study. Methods: Patients received ⁶⁸Ga-satoreotide trizoxetan at a peptide mass of 5-20 µg on day 1 of the study and of 30-45 µg on day 16-22, at one of three gallium-68 radioactivity ranges (40-80, 100-140, or 160-200 MBq). Whole-body PET/CT imaging was performed 50-70 minutes after each injection. The primary endpoint was the detection rate of NET lesions imaged by ⁶⁸Ga-satoreotide trizoxetan relative to contrast-enhanced CT (CECT) (for each of the six peptide mass/radioactivity range combinations). Results: Twenty-four patients were evaluated in the per-protocol analysis. The median number of lesions detected by ⁶⁸Ga-satoreotide trizoxetan PET/CT or PET only was at least twice as high as the number of lesions detected by CECT across the six studied peptide mass dose/radioactivity range combinations. There were no differences between the two peptide mass ranges and between the three radioactivity ranges in the number of identified lesions. However, a trend towards a lower relative lesion count was noted in the liver for the 40-80 MBq range. No relationship was observed between the radioactivity range per patient's body weight (MBq/kg) and the number of lesions detected by ⁶⁸Ga-satoreotide trizoxetan. Median diagnostic sensitivity of ⁶⁸Ga-satoreotide trizoxetan PET/CT, based on the number of lesions per patient, ranged from 85% to 87% across the different peptide mass and radioactivity ranges. Almost all reported adverse events were mild and self-limiting. Conclusion: A radioactivity of 100-200 MBq with a peptide mass up to 50 μg were confirmed as the optimal dosing regimen for ⁶⁸Ga-satoreotide trizoxetan to be used in future phase III studies.

Emerging trends of receptor-mediated tumor targeting peptides: A review with perspective from molecular imaging modalities

Natural peptides extracted from natural components such are known to have a relatively short in-vivo half-life and can readily metabolize by endo- and exo-peptidases. Fortunately, synthetic peptides can be easily manipulated to increase in-vivo stability, membrane permeability and target specificity with some well-known natural families. Many natural as well as synthetic peptides target to their endogenous receptors for diagnosis and therapeutic applications. In order to detect these peptides externally, they must be modified with radionuclides compatible with single photon emission computed tomography (SPECT) or positron emission tomography (PET). Although, these techniques mainly rely on physiological changes and have profound diagnostic strength over anatomical modalities such as MRI and CT. However, both SPECT and PET observed to possess lack of anatomical reference frame which is a key weakness of these techniques, and unfortunately, cannot be available freely in most clinical centres especially in under-developing countries. Hence, it is need of the time to design and develop economic, patient friendly and versatile strategies to grapple with existing problems without any hazardous side effects. Optical molecular imaging (OMI) has emerged as a novel technique in field of medical science using fluorescent probes as imaging modality and has ability to couple with organic drugs, small molecules, chemotherapeutics, DNA, RNA, anticancer peptide and protein without adding chelators as necessary for radionuclides. Furthermore, this review focuses on difference in imaging modalities and provides ample knowledge about reliable, economic and patient friendly optical imaging technique rather radionuclide-based imaging techniques.

SPECT Imaging of SST2-Expressing Tumors with 99mTc-Based Somatostatin Receptor Antagonists: The Role of Tetraamine, HYNIC, and Spacers

[^99mTc]Tc-HYNIC-TOC is the most widely used ^99mTc-labeled somatostatin receptor (SST) agonist for the SPECT imaging of SST-expressing tumors, such as neuroendocrine tumors. Recently, radiolabeled SST antagonists have shown improved diagnostic efficacy over agonists. ^99mTc-labeled SST antagonists are lacking in clinical practice. Surprisingly, when [^99mTc]Tc-HYNIC was conjugated to the SST2 antagonist SS01, SST2 imaging was not feasible. This was not the case when [^99mTc]Tc-N4 was conjugated to SS01. Here, we assessed the introduction of different spacers (X: β-Ala, Ahx, Aun and PEG₄) among HYNIC and SS01 with the aim of restoring the affinity of HYNIC conjugates. In addition, we used the alternative antagonist JR11 for determining the suitability of HYNIC with ^99mTc-labeled SST2 antagonists. We performed a head-to-head comparison of the N4 conjugates of SS01 and JR11. [^99mTc]Tc-HYNIC-TOC was used as a reference, and HEK-SST2 cells were used for in vitro and in vivo evaluation. EDDA was used as a co-ligand for all [^99mTc]Tc-HYNIC conjugates. The introduction of Ahx restored, to a great extent, the SST2-mediated cellular uptake of the [^99mTc]Tc-HYNIC-X conjugates (X: spacer), albeit lower than the corresponding [^99mTc]Tc-N4-conjugates. SPECT/CT images showed that all ^99mTc-labeled conjugates accumulated in the tumor and kidneys with [^99mTc]Tc-HYNIC-PEG₄-SS01, [^99mTc]Tc-N4-SS01 and [^99mTc]Tc-N4-JR11 having notably higher kidney uptake. Biodistribution studies showed similar or better tumor-to-non-tumor ratios for the [^99mTc]Tc-HYNIC-Ahx conjugates, compared to the [^99mTc]Tc-N4 counterparts. The [^99mTc]Tc-HYNIC-Ahx conjugates of SS01 and JR11 were comparable to [^99mTc]Tc-HYNIC-TOC as imaging agents. HYNIC is a suitable chelator for the development of ^99mTc-labeled SST2 antagonists when a spacer of appropriate length, such as Ahx, is used.

Somatostatin Receptor Imaging PET in Neuroendocrine Neoplasm

PET/computed tomography (CT) imaging increasingly is used in neuroendocrine neoplasms (NENs) for diagnosis, staging, monitoring, prognostication, and choosing treatment. Somatostatin PET analog tracers have added to the specificity by obtaining higher affinity to somatostatin receptors with ⁶⁸Ga-labeled or ⁶⁴Cu-labeled DOTA peptides compared with single-photon emission CT imaging isotopes. PET uptake correlates to tumor grade and is an essential part of theranostics with peptide receptor radionuclide treatment. This article focuses on the literature on head-to-head studies and meta-analyses of different combinations of peptide agonists and a few antagonists. Overall, the published data support the diagnostic capability of PET/CT imaging in NENs.

Direct labeling of a somatostatin receptor antagonist via peptide cyclization with Re, 99mTc and 186Re metal centers: Radiochemistry and in vitro evaluation

INTRODUCTION

With the long-term goal of developing a diagnostic (99mTc) and therapeutic (186Re) agent pair for targeting somatostatin receptor (SSTR)-positive neuroendocrine tumors (NETs), we developed novel metal-cyclized peptides through direct labeling of the potent SSTR2 antagonist Ac-4-NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH2 (1) with Re (in Re-1), 99mTc (in [99mTc]Tc-1) and 186Re (in [186Re]Re-1).

METHODS

Re-1 was characterized by LC-ESI-MS and HR-ESI-MS and was tested for receptor affinity in SSTR-expressing cells (AR42J). Radiolabeling of the peptide was achieved via ligand exchange from 99mTc-labeled glucoheptonate or [186Re]ReOCl3(PPh3)2, yielding [99mTc]Tc-1 or [186Re]Re-1, respectively. In vitro stability of [99mTc]Tc-1/[186Re]Re-1 in PBS (10 mM) at pH 7.4 and 37 °C was determined by HPLC analysis. Moreover, [99mTc]Tc-1 stability was tested in cysteine (1 mM) and rat serum under the same conditions.

RESULTS

Re-1 consisted of two isomers, confirmed by LC-ESI-MS, with good SSTR2 affinity (IC50 = 43 ± 6 nM). Optimization of the 99mTc labeling through varying reaction parameters such as pH, reaction time, and Sn2+ and ligand concentrations resulted in high radiochemical yield (RCY ≥92%). Similarly, [186Re]Re-1 was prepared in reasonable RCY (≥50%). Both 99mTc/186Re-tracers consisted of two product isomers as identified by HPLC co-injection with Re-1. While [99mTc]Tc-1 was sufficiently stable in vitro (≥71% intact through 4 h in PBS, cysteine and rat serum), [186Re]Re-1 exhibited more moderate in vitro stability (58% intact after 1 h in PBS).

CONCLUSIONS

Novel 99mTc/186Re-cyclized SSTR2 antagonist peptides were synthesized and characterized using the Re-cyclized analogue as a reference. Due to the nanomolar SSTR2 affinity of Re-1 and good in vitro stability of [99mTc]Tc-1, the latter shows early promise for development as a radiodiagnostic agent for SSTR-expressing NETs.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The 99mTc-cyclized complex showed promising in vitro properties, and future in vivo studies will determine the potential for translating such a design into the human clinic.

Somatostatin Receptors and Analogs in Pheochromocytoma and Paraganglioma: Old Players in a New Precision Medicine World

Neuroendocrine tumors overexpress somatostatin receptors, which serve as important and unique therapeutic targets for well-differentiated advanced disease. This overexpression is a well-established finding in gastroenteropancreatic neuroendocrine tumors which has guided new medical therapies in the administration of somatostatin analogs, both "cold", particularly octreotide and lanreotide, and "hot" analogs, chelated to radiolabeled isotopes. The binding of these analogs to somatostatin receptors effectively suppresses excess hormone secretion and tumor cell proliferation, leading to stabilization, and in some cases, tumor shrinkage. Radioisotope-labeled somatostatin analogs are utilized for both tumor localization and peptide radionuclide therapy, with ⁶⁸Ga-DOTATATE and ¹⁷⁷Lu-DOTATATE respectively. Benign and malignant pheochromocytomas and paragangliomas also overexpress somatostatin receptors, irrespective of embryological origin. The pattern of somatostatin receptor overexpression is more prominent in succinate dehydrogenase subunit B gene mutation, which is more aggressive than other subgroups of this disease. While the Food and Drug Administration has approved the use of ⁶⁸Ga-DOTATATE as a radiopharmaceutical for somatostatin receptor imaging, the use of its radiotherapeutic counterpart still needs approval beyond gastroenteropancreatic neuroendocrine tumors. Thus, patients with pheochromocytoma and paraganglioma, especially those with inoperable or metastatic diseases, depend on the clinical trials of somatostatin analogs. The review summarizes the advances in the utilization of somatostatin receptor for diagnostic and therapeutic approaches in the neuroendocrine tumor subset of pheochromocytoma and paraganglioma; we hope to provide a positive perspective in using these receptors as targets for treatment in this rare condition.

Role of Somatostatin Receptor in Pancreatic Neuroendocrine Tumor Development, Diagnosis, and Therapy

Pancreatic neuroendocrine tumors (pNETs) are rare and part of the diverse family of neuroendocrine neoplasms (NENs). Somatostatin receptors (SSTRs), which are widely expressed in NENs, are G-protein coupled receptors that can be activated by somatostatins or its synthetic analogs. Therefore, SSTRs have been widely researched as a diagnostic marker and therapeutic target in pNETs. A large number of studies have demonstrated the clinical significance of SSTRs in pNETs. In this review, relevant literature has been appraised to summarize the most recent empirical evidence addressing the clinical significance of SSTRs in pNETs. Overall, these studies have shown that SSTRs have great value in the diagnosis, treatment, and prognostic prediction of pNETs; however, further research is still necessary.

Systemic Therapy Improvements Will Render Locoregional Treatments Obsolete for Patients with Cancer with Liver Metastases

Hepatic metastases are a major cause of morbidity and mortality for patients with cancer. Apart from curative resection, which offers patients the potential for long-term survival, an array of locoregional therapies, with limited evidence of improving survival, are used to treat them. The authors use examples from the realm of gastrointestinal cancer, largely focusing on the experience of patients with neuroendocrine cancer, hepatobiliary cancer, and colorectal cancer, to suggest that current systemic therapies offer, at minimum, similar survival outcomes for patients compared with these locoregional approaches.

Selection of the First 99mTc-Labelled Somatostatin Receptor Subtype 2 Antagonist for Clinical Translation-Preclinical Assessment of Two Optimized Candidates

Recently, radiolabelled antagonists targeting somatostatin receptors subtype 2 (SST2) in neuroendocrine neoplasms demonstrated certain superior properties over agonists. Within the ERA-PerMED project "TECANT" two 99mTc-Tetramine (N4)-derivatized SST2 antagonists (TECANT-1 and TECANT-2) were studied for the selection of the best candidate for clinical translation. Receptor-affinity, internalization and dissociation studies were performed in human embryonic kidney-293 (HEK293) cells transfected with the human SST2 (HEK-SST2). Log D, protein binding and stability in human serum were assessed. Biodistribution and SPECT/CT studies were carried out in nude mice bearing HEK-SST2 xenografts, together with dosimetric estimations from mouse-to-man. [99mTc]Tc-TECANT-1 showed higher hydrophilicity and lower protein binding than [99mTc]-TECANT-2, while stability was comparable. Both radiotracers revealed similar binding affinity, while [99mTc]Tc-TECANT-1 had higher cellular uptake (>50%, at 2 h/37 °C) and lower dissociation rate (<30%, at 2 h/37 °C). In vivo, [99mTc]Tc-TECANT-1 showed lower blood values, kidney and muscles uptake, whereas tumour uptake was comparable to [99mTc]Tc-TECANT-2. SPECT/CT imaging confirmed the biodistribution results, providing the best tumour-to-background image contrast for [99mTc]Tc-TECANT-1 at 4 h post-injection (p.i.). The estimated radiation dose amounted to approximately 6 µSv/MBq for both radiotracers. This preclinical study provided the basis of selection of [99mTc]Tc-TECANT-1 for clinical translation of the first 99mTc-based SST2 antagonist.

Non-FDG PET/CT

The major applications for molecular imaging with PET in clinical practice concern cancer imaging. Undoubtedly, 18F-FDG represents the backbone of nuclear oncology as it remains so far the most widely employed positron emitter compound. The acquired knowledge on cancer features, however, allowed the recognition in the last decades of multiple metabolic or pathogenic pathways within the cancer cells, which stimulated the development of novel radiopharmaceuticals. An endless list of PET tracers, substantially covering all hallmarks of cancer, has entered clinical routine or is being investigated in diagnostic trials. Some of them guard significant clinical applications, whereas others mostly bear a huge potential. This chapter summarizes a selected list of non-FDG PET tracers, described based on their introduction into and impact on clinical practice.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

Identified in 1973, somatostatin (SST) is a cyclic hormone peptide with a short biological half-life. Somatostatin receptors (SSTRs) are widely expressed in the whole body, with five subtypes described. The interaction between SST and its receptors leads to the internalization of the ligand-receptor complex and triggers different cellular signaling pathways. Interestingly, the expression of SSTRs is significantly enhanced in many solid tumors, especially gastro-entero-pancreatic neuroendocrine tumors (GEP-NET). Thus, somatostatin analogs (SSAs) have been developed to improve the stability of the endogenous ligand and so extend its half-life. Radiolabeled analogs have been developed with several radioelements such as indium-111, technetium-99 m, and recently gallium-68, fluorine-18, and copper-64, to visualize the distribution of receptor overexpression in tumors. Internal metabolic radiotherapy is also used as a therapeutic strategy (e.g., using yttrium-90, lutetium-177, and actinium-225). With some radiopharmaceuticals now used in clinical practice, somatostatin analogs developed for imaging and therapy are an example of the concept of personalized medicine with a theranostic approach. Here, we review the development of these analogs, from the well-established and authorized ones to the most recently developed radiotracers, which have better pharmacokinetic properties and demonstrate increased efficacy and safety, as well as the search for new clinical indications.

Multimodal Imaging of 2-Cycle PRRT with 177Lu-DOTA-JR11 and 177Lu-DOTATOC in an Orthotopic Neuroendocrine Xenograft Tumor Mouse Model

Peptide receptor radionuclide therapy (PRRT) using radiolabeled somatostatin receptor (SSTR) analogs is a common approach in advanced neuroendocrine neoplasms. Recently, SSTR antagonists have shown promising results for imaging and therapy due to a higher number of binding sites than in commonly used agonists. We evaluated PRRT with SSTR agonist ¹⁷⁷Lu-DOTATOC and antagonist ¹⁷⁷Lu-DOTA-JR11 longitudinally in an orthotopic murine pancreatic neuroendocrine neoplasm model expressing human SSTR2. Morphologic and metabolic changes during treatment were assessed using multimodal imaging, including hybrid PET/MRI and SPECT/CT. Methods: In vitro radioligand binding and internalization assays and cell-cycle analysis were performed. SSTR2-transfected BON cells (BON-SSTR2) were used for in vivo experiments. Tumor-bearing mice received 2 intravenous injections of 100 μL of saline, 30 MBq of ¹⁷⁷Lu-DOTATOC, or 20 MBq of ¹⁷⁷Lu-DOTA-JR11 with an interval of 3 wk. Weekly T2-weighted MRI was performed for tumor monitoring. Viability of the tumor tissue was assessed by ¹⁸F-FDG PET/MRI once after PRRT. Tumor and kidney uptake of the respective radiopharmaceuticals was measured 24 h after injection by SPECT/CT. Results: Compared with ¹⁷⁷Lu-DOTATOC, ¹⁷⁷Lu-DOTA-JR11 treatment resulted in an increased accumulation of cells in G2/M phase. Animals treated with the SSTR antagonist showed a significant reduction in tumor size (P < 0.001) and an increased median survival (207 d; interquartile range [IQR], 132-228) compared with ¹⁷⁷Lu-DOTATOC (126 d; IQR, 118-129). SPECT/CT revealed a 4-fold higher median tumor uptake for the antagonist and a 3-fold higher tumor-to-kidney ratio in the first treatment cycle. During the second therapy cycle, tumor uptake of ¹⁷⁷Lu-DOTATOC was significantly lower (P = 0.01) whereas ¹⁷⁷Lu-DOTA-JR11 uptake remained stable. Imaging of tumor morphology indicated comparatively larger necrotic fractions for ¹⁷⁷Lu-DOTA-JR11 despite further tumor growth. These results were confirmed by ¹⁸F-FDG PET, revealing the least amount of viable tumor tissue in ¹⁷⁷Lu-DOTA-JR11-treated animals, at 6.2% (IQR, 2%-23%). Conclusion: ¹⁷⁷Lu-DOTA-JR11 showed a higher tumor-to-kidney ratio and a more pronounced cytotoxic effect than did ¹⁷⁷Lu-DOTATOC. Additionally, tumor uptake was more stable over the course of 2 treatment cycles.

Future of Theranostics: An Outlook on Precision Oncology in Nuclear Medicine

Molecular alterations in malignant disease result in the expression or upregulations of various targets that can be used for imaging and treatment with radiopharmaceuticals. This theranostic principle has acquired greater importance in personalized medicine in recent years, particularly in oncology, where advanced tumors can be treated effectively with low side effects. Since the pioneering use of ¹³¹I in differentiated thyroid cancer in the 1940s, remarkable achievements in nuclear medicine endoradiotherapy have been demonstrated, mainly in the treatment of neuroendocrine neoplasms by using ¹⁷⁷Lu-labeled somatostatin analogs or in the treatment of advanced prostate cancer using prostate-specific membrane antigen-directed radionuclide therapy. Besides that, this review focuses on promising novel radiopharmaceuticals and describes their preclinical and clinical status. Radiolabeled antibodies, such as ¹³¹I-omburtamab directed against the B7-H3 protein on the surface of neuroblastoma cells; HuMab-5B1, a ⁸⁹Zr/¹⁷⁷Lu-labeled antibody for the treatment of CA19-9-expressing malignancies; and ¹⁷⁷Lu-lilotomab, a CD37 antibody for the treatment of B-cell lymphomas, are being highlighted. The neurotensin receptor ligand ¹¹¹In/¹⁷⁷Lu-3B-227 has demonstrated high potential in imaging and therapy for several malignancies (e.g., pancreatic adenocarcinomas). Targeting of the fibroblast activation protein is currently being explored for different tumor entities using PET imaging with the fibroblast activation protein inhibitor (FAPI) ⁶⁸Ga-FAPI-04, and the first therapeutic applications of ⁹⁰Y-FAPI-04 have been applied. After 2 decades of rapid development in theranostics, a variety of new targets are available for further clinical investigation.

Comparison of 68Ga-DOTA-JR11 PET/CT with dosimetric 177Lu-satoreotide tetraxetan (177Lu-DOTA-JR11) SPECT/CT in patients with metastatic neuroendocrine tumors undergoing peptide receptor radionuclide therapy

PURPOSE

Paired imaging/therapy with radiolabeled somatostatin receptor (SSTR) antagonists is a novel approach in neuroendocrine tumors (NETs). The aim of this study was to compare tumor uptake of ⁶⁸Ga-DOTA-JR11 and ¹⁷⁷Lu-satoreotide tetraxetan (¹⁷⁷Lu-DOTA-JR11) in patients with NETs.

METHODS

As part of a prospective clinical trial, 20 patients with metastatic NETs underwent ⁶⁸Ga-DOTA-JR11 PET/CT and serial imaging with ¹⁷⁷Lu-satoreotide tetraxetan. PET/CT and SPECT/CT parameters for lesion uptake and absorbed dose of ¹⁷⁷Lu-satoreotide tetraxetan in lesions were compared using linear regression analysis and Pearson correlation.

RESULTS

A total of 95 lesions were analyzed on ⁶⁸Ga-DOTA-JR11 PET/CT and ¹⁷⁷Lu-satoreotide tetraxetan SPECT/CT. SUVs and tumor-to-normal-tissue ratios on PET/CT and SPECT/CT were significantly correlated (p < 0.01), but the degree of correlation was modest with Pearson correlation coefficients ranging from 0.3 to 0.7. Variation in intrapatient lesional correlation was observed. Nevertheless, in all patients, the lesion SUVpeak uptake ratio for ¹⁷⁷Lu-satoreotide tetraxetan vs. ⁶⁸Ga-DOTA-JR11 was high; even in those with low uptake on ⁶⁸Ga-DOTA-JR11 PET/CT (SUVpeak ≤ 10), a ratio of 8.0 ± 5.2 was noted. Correlation of SUVpeak of ⁶⁸Ga-DOTA-JR11 with projected ¹⁷⁷Lu-satoreotide tetratexan-absorbed dose (n = 42) was modest (r = 0.5, p < 0.01), while excellent correlation of SUVpeak of ¹⁷⁷Lu-satoreotide tetraxetan with projected ¹⁷⁷Lu-satoreotide tetraxetan-absorbed dose was noted (r = 0.9, p < 0.0001).

CONCLUSION

Our study shows that ⁶⁸Ga-DOTA-JR11 PET can be used for patient selection and PRRT and that low tumor uptake on PET should not preclude patients from treatment with ¹⁷⁷Lu-satoreotide tetraxetan. The ability to use single time-point SPECT/CT for absorbed dose calculations could facilitate dosimetry regimens, save costs, and improve patient convenience.

Radionuclide imaging of neuroendocrine tumors

Neuroendocrine tumors (NETs) are relatively rare and heterogeneous with a variaty of clinical expression. They derive from the sensory and secretory neuroendocrine cells mainly within the pulmonary and gastrointestinal tract. They comprise less than 2% of all malignancies. On the basis of clinical behavior, histology, and proliferation rate, they are devided into well differentiated (low grade to intermediate grade) and poorly differentiated (high grade) neuroendocrine carcinoma. Tumor stage and grade have the impact on treatment and prognosis. The treatment of choice is surgery. More than 50% of the patients present metastatic disease at the time of diagnosis, thus the systemic treatment should be considered including somatostatin analogs, chemotherapy, targeted therapy, immunotherapy and peptide receptor radionuclide therapy (PRRT). For the diagnosis and follow-up of these tumors, various radiological methods are used (computed tomography, magnetic resonance imaging, ultrasound) as well as endoscopy. Nuclear medicine methods are used in order to exploit their unique properties mainly amine precursor uptake and decarboxylation system characteristics, as well as the expression of somatostatin receptors. These methods enable whole body examination, staging, selection of patient for PRRT and treatment monitoring as well. Imaging can be performed with gamma camera (SPECT, SPECT/CT) or positron emission tomography (PET/ CT). Radiopharmaceuticals used for imaging with gamma camera are usually 99mTc-(V)-DMSA, 99mTc-MIBI, 99mTc-HYNIC TOC, 111In-pentetreotide and 131I-MIBG/123I-MIBG. Positron emitting radiopharmaceuticals has superior spatial resolution and faster imaging, such as 68Ga-DOTA-somatostatin analogues, 18F-FDG (particularly for high-grade tumors), 18F-L-DOPA/11C-L-DOPA and 11C-5-hydroxytryptophan that have demonstrated excellent imaging results. The new targeted agents present a challenge in the evaluation procedure of treatment and, therefore, new imaging techniques and an improvement of currently available techniques are mandatory. In this mini-review, the most frequent methods and radiopharmaceuticals are presented, as well as potential development.

Head-to-Head Comparison of 68Ga-DOTA-JR11 and 68Ga-DOTATATE PET/CT in Patients with Metastatic, Well-Differentiated Neuroendocrine Tumors: A Prospective Study

⁶⁸Ga-DOTA-JR11 is a somatostatin receptor subtype 2-specific antagonist used for PET/CT imaging. The purpose of this study was to compare ⁶⁸Ga-DOTA-JR11 and ⁶⁸Ga-DOTATATE PET/CT in patients with metastatic, well-differentiated neuroendocrine tumors. Methods: Patients with histologically proven, metastatic or unresectable, well-differentiated neuroendocrine tumors were prospectively recruited to this study. Each patient received an intravenous injection of ⁶⁸Ga-DOTATATE (155 ± 52 MBq) on the first day and ⁶⁸Ga-DOTA-JR11 (148 ± 52 MBq) on the second day. Whole-body PET/CT scans were performed at 40-60 min after injection on the same scanner. Physiologic normal-organ uptake, lesion numbers, and lesion uptake were compared. Results: Thirty-one patients were prospectively enrolled in the study. The SUV_max of the spleen, renal cortex, adrenal glands, pituitary glands, stomach wall, normal liver parenchyma, small intestine, pancreas, and bone marrow was significantly lower on ⁶⁸Ga-DOTA-JR11 than on ⁶⁸Ga-DOTATATE PET/CT (P < 0.001). ⁶⁸Ga-DOTA-JR11 detected significantly more liver lesions (552 vs. 365, P = 0.001) but fewer bone lesions (158 vs. 388, P = 0.016) than ⁶⁸Ga-DOTATATE. The target-to-background ratio of liver lesions was significantly higher on ⁶⁸Ga-DOTA-JR11 (7.7 ± 5.4 vs. 3.4 ± 2.0, P < 0.001). ⁶⁸Ga-DOTA-JR11 and ⁶⁸Ga-DOTATATE PET/CT showed comparable results for primary tumors and lymph node metastases on both patient-based and lesion-based comparisons. Conclusion: ⁶⁸Ga-DOTA-JR11 performs better in detecting liver metastases, with a better tumor-to-background ratio, whereas ⁶⁸Ga-DOTATATE may outperform ⁶⁸Ga-DOTA-JR11 in the detection of bone metastases. However, the lower somatostatin receptor subtype 2 affinity of ⁶⁸Ga-DOTA-JR11 than of ¹⁷⁷Lu-DOTA-JR11 may limit its role as a diagnostic pair for the theranostic approach with ¹⁷⁷Lu-DOTA-JR11.

Phase I Trial of Well-Differentiated Neuroendocrine Tumors (NETs) with Radiolabeled Somatostatin Antagonist 177Lu-Satoreotide Tetraxetan

PURPOSE

Radiolabeled somatostatin receptor 2 (SSTR2) antagonists have shown higher tumor uptake and tumor-to-organ ratios than somatostatin agonists in preclinical models of neuroendocrine tumors (NETs). We performed a phase I study to evaluate the safety and efficacy of SSTR2 antagonist ¹⁷⁷Lu-satoreotide tetraxetan.

PATIENTS AND METHODS

Twenty patients with advanced SSTR2-positive NETs were treated with ¹⁷⁷Lu-satoreotide tetraxetan. Patients first underwent a dosimetry study with ¹⁷⁷Lu-satoreotide tetraxetan to determine the therapeutic activity that could be safely administered. This activity was split into two equal cycles to be delivered 3 months apart. The maximum activity was 7.4 GBq per cycle.

RESULTS

Of 20 patients with NETs (one lung, seven small bowel, nine pancreatic, one gastric, one rectal, one kidney; mean prior treatments: three), six received one cycle of ¹⁷⁷Lu- satoreotide tetraxetan and 14 received two cycles. Hematologic toxicity after cycle 1 was mild-moderate and reversed before cycle 2. However, grade 4 hematologic toxicity occurred in four of seven (57%) patients after cycle 2 of ¹⁷⁷Lu-satoreotide tetraxetan. The study was suspended, and the protocol modified to limit the cumulative absorbed bone marrow dose to 1 Gy and to reduce prescribed activity for cycle 2 by 50%. The best overall response rate was 45% [5% complete response (1/20), 40% partial response (8/20)]; with 40% stable disease (8/20) and 15% progression of disease (3/20). Median progression-free survival (PFS) was 21.0 months (95% CI, 13.6-NR).

CONCLUSIONS

In this trial of heavily treated NETs, preliminary data are promising for the use of ¹⁷⁷Lu-satoreotide tetraxetan. Additional studies are ongoing to determine optimal therapeutic dose/schedule.

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

Background:

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

Objective:

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

Methods:

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

Results:

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

Conclusion:

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

Design and development of the theranostic pair 177 Lu-OPS201/68 Ga-OPS202 for targeting somatostatin receptor expressing tumors

Radiolabeled somatostatin receptor (sstr) antagonists have shown superiority in different preclinical and clinical settings compared with the well-established and clinically used agonists for targeting sstr-expressing tumors, with regard to pharmacokinetics, tumor uptake, and retention. The theranostic pair ¹⁷⁷ Lu-OPS201/⁶⁸ Ga-OPS202, based on the sstr2 antagonist JR11 (Cpa-c[d-Cys-Aph(Hor)-d-Aph(Cbm)-Lys-Thr-Cys]-d-Tyr-NH₂ ), is the most advanced pair of the antagonist family in terms of preclinical development and is currently under clinical evaluation. OPS201 and OPS202 share the same amino acid sequence (JR11) but feature different conjugated chelators needed for radiolabeling, DOTA for OPS201 and NODAGA for OPS202. In this review, the design and development of the peptidic analog, JR11, and the selection of chelators and radiometals that led to ¹⁷⁷ Lu-OPS201/⁶⁸ Ga-OPS202 are discussed. Furthermore, the preclinical evaluation of both radiolabeled analogs from bench to bedside and the clinical trials involving the theranostic pair are presented.

Biomedical applications of radioiodinated peptides

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

Receptor radionuclide targeting for neuroendocrine tumors (NET) diagnostic and therapy

Neuroendocrine tumors (NET) represent a heterogeneous group of tumors originating from cells of neuroendocrine origin, which express somatostatin receptors (SSTR). This property allowed the successful development of radionuclides for diagnostic and peptide radionuclide radiation therapy (PRRT). This is the paradigm for the theragnostic concept in NET personalized medicine. The only phase III study to date (NETTER-1) clearly demonstrated the ability of ¹⁷⁷Lutetium-based PRRT to improve progression-free survival in advanced intestinal NETs. In clinical practice, the indications are limited to G1-G2 well-differentiated NETs with high expression of SSTR. NETs with a low tumor burden and slow progression are probably the optimal indication. This treatment is now available in France. However, its precise position in the treatment algorithm remains to be explored. We provide an overview of receptor radionuclide utilization and mechanism in diagnostic and pretherapeutic imaging and we focus on PRRT for endocrine tumors.

[68Ga]-Dota Peptide PET/CT in Neuroendocrine Tumors: Main Clinical Applications

Objective:

Neuroendocrine Neoplasms (NENs) are generally defined as rare and heterogeneous tumors. The gastrointestinal system is the most frequent site of NENs localization, however they can be found in other anatomical regions, such as pancreas, lungs, ovaries, thyroid, pituitary, and adrenal glands. Neuroendocrine neoplasms have significant clinical manifestations depending on the production of active peptide.

Methods:

Imaging modalities play a fundamental role in initial diagnosis as well as in staging and treatment monitoring of NENs, in particular they vastly enhance the understanding of the physiopathology and diagnosis of NENs through the use of somatostatin analogue tracers labeled with appropriate radioisotopes. Additionally, the use of somatostatin analogues provides the ability to in-vivo measure the expression of somatostatin receptors on NEN cells, a process that might have important therapeutic implications.

Results:

A large body of evidences showed improved accuracy of molecular imaging based on PET/CT radiotracer with SST analogues (e.g. [68Ga]-DOTA peptide) for the detection of NEN lesions in comparison to morphological imaging modalities. So far, the role of imaging technologies in assessing treatment response is still under debate.

Conclusion:

This review offers the systems of classification and grading of NENs and summarizes the more useful recommendations based on data recently published for the management of patients with NENs, with special focus on the role of imaging modalities based on SST targeting with PET / CT radiotracers.