Evaluation of 177Lu-DOTA-sst2 Antagonist Versus 177Lu-DOTA-sst2 Agonist Binding in Human Cancers In Vitro

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.

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.

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.

[111In]In/[177Lu]Lu-AAZTA5-LM4 SST2R-Antagonists in Cancer Theranostics: From Preclinical Testing to First Patient Results

Aiming to expand the application of the SST2R-antagonist LM4 (DPhe-c[DCys-4Pal-DAph(Cbm)-Lys-Thr-Cys]-DTyr-NH2) beyond [68Ga]Ga-DATA5m-LM4 PET/CT (DATA5m, (6-pentanoic acid)-6-(amino)methy-1,4-diazepinetriacetate), we now introduce AAZTA5-LM4 (AAZTA5, 1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-[pentanoic-acid]perhydro-1,4-diazepine), allowing for the convenient coordination of trivalent radiometals of clinical interest, such as In-111 (for SPECT/CT) or Lu-177 (for radionuclide therapy). After labeling, the preclinical profiles of [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 were compared in HEK293-SST2R cells and double HEK293-SST2R/wtHEK293 tumor-bearing mice using [111In]In-DOTA-LM3 and [177Lu]Lu-DOTA-LM3 as references. The biodistribution of [177Lu]Lu-AAZTA5-LM4 was additionally studied for the first time in a NET patient. Both [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 displayed high and selective targeting of the HEK293-SST2R tumors in mice and fast background clearance via the kidneys and the urinary system. This pattern was reproduced for [177Lu]Lu-AAZTA5-LM4 in the patient according to SPECT/CT results in a monitoring time span of 4–72 h pi. In view of the above, we may conclude that [177Lu]Lu-AAZTA5-LM4 shows promise as a therapeutic radiopharmaceutical candidate for SST2R-expressing human NETs, based on previous [68Ga]Ga-DATA5m-LM4 PET/CT, but further studies are needed to fully assess its clinical value. Furthermore, [111In]In-AAZTA5-LM4 SPECT/CT may represent a legitimate alternative diagnostic option in cases where PET/CT is not available.

Evaluation of Re/99mTc-labeled somatostatin receptor-targeting peptide complexes synthesized via direct metal cyclization

With interest in the development of somatostatin receptor (SSTR) targeting agents for potential application in diagnostic SPECT imaging (99mTc) or Peptide Radionuclide Receptor Therapy (PRRT, 186Re or 188Re) of neuroendocrine tumors, we present herein 99mTc/Re (radio)complexes synthesized by the integrated (radio)labeling approach of peptide cyclization via metal complexation. In particular, we utilized the potent SSTR2 peptide antagonist sequence DOTA-4-NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH2 (DOTA-sst2-ANT) and report the syntheses and in vitro evaluations of its respective [99mTc]Tc/Re-cyclized peptides ([99mTc]Tc/Re-cyc-DOTA-sst2-ANT). The Re-cyc-DOTA-sst2-ANT complex was synthesized via an on-resin Re(V)-cyclization reaction using the ReOCl3(PPh3)2 precursor and consisted of three isomers characterized by LC–ESI-MS. The [99mTc]Tc-cyclized analogue was prepared via a ligand exchange reaction of the [99mTc][TcO]3+ core through a [99mTc]Tc-glucoheptonate intermediate with linear DOTA-sst2-ANT and was characterized by comparative HPLC studies against Re-cyc-DOTA-sst2-ANT. Good in vitro binding affinity was demonstrated in SSTR-expressing cells (AR42J) by the Re-cyc-DOTA-sst2-ANT major isomer, similar to the potent binder Lu-DOTA-sst2-ANT, in which the Lu metal was complexed by the bifunctional chelator DOTA versus via peptide cyclization. [99mTc]Tc-cyc-DOTA-sst2-ANT was obtained in high radiochemical yield, also with an elution pattern of three isomers observed by HPLC analysis, which were comparable yet not identical to those of Re-cyc-DOTA-sst2-ANT. The [99mTc]Tc-tracer complex was shown to be hydrophilic, and stability studies at 4 h demonstrated that it remained intact in both PBS and in rat serum, with low non-specific rat serum protein binding, while exhibiting more moderate stability in 1 mM cysteine. These findings demonstrate that direct Re/[99mTc]Tc-cyclization of DOTA-sst2-ANT is feasible and may be used as an alternative approach to the bifunctional chelate labeling strategy. However, given that the non-radioactive (Re) and radiotracer (99mTc) analogues are not identical and both form isomeric products in equilibrium, additional design modifications will be necessary prior to in vivo application of [99mTc]Tc/Re-cyc-DOTA-sst2-ANT.

[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.

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.

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.

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.

Peptide Receptor Radionuclide Therapy with [177Lu]Lu-DOTA-TATE in Patients with Advanced GEP NENS: Present and Future Directions

Simple Summary

Neuroendocrine neoplasms have been usually described as infrequent tumors, but their incidence has been rising over time. [¹⁷⁷Lu]Lu-DOTA-TATE (PRRT-Lu) was approved by the European Medicines Agency and by the Food and Drug Administration as the first radiopharmaceutical for peptide receptor radionuclide therapy in progressive gastroenteropancreatic NET. PRRT-Lu is considered a therapeutic option in progressive SSTR-positive NETs with homogenous SSTR expression. The NETTER-1 study demonstrated that PRRT-Lu yielded a statistically and clinically significant improvement in PFS as a primary endpoint (HR: 0.18, p < 0.0001), as well as a clinical trend towards improvement in OS. These results made scientific societies incorporate PRRT-Lu into their clinical guidelines; however, some questions still remain unanswered.

Abstract

This review article summarizes findings published in the last years on peptide receptor radionuclide therapy in GEP NENs, as well as potential future developments and directions. Unanswered questions remain, such as the following: Which is the correct dose and individual dosimetry? Which is the place for salvage PRRT-Lu? Whicht is the role of PRRT-Lu in the pediatric population? Which is the optimal sequencing of PRRT-Lu in advanced GEP NETs? Which is the place of PRRT-Lu in G3 NENs? These, and future developments such as inclusion new radiopharmaceuticals and combination therapy with different agents, such as radiosensitizers, will be discussed.

Peptide Receptor Radionuclide Therapy Targeting the Somatostatin Receptor: Basic Principles, Clinical Applications and Optimization Strategies

Simple Summary

Peptide receptor radionuclide therapy (PRRT) is a systemic treatment consisting of the administration of a tumor-targeting radiopharmaceutical into the circulation of a patient. The radiopharmaceutical will bind to a specific peptide receptor leading to tumor-specific binding and retention. This will subsequently cause lethal DNA damage to the tumor cell. The only target that is currently used in widespread clinical practice is the somatostatin receptor, which is overexpressed on a range of tumor cells, including neuroendocrine tumors and neural-crest derived tumors. Academia played an important role in the development of PRRT, which has led to heterogeneous literature over the last two decades, as no standard radiopharmaceutical or regimen has been available for a long time. This review focuses on the basic principles and clinical applications of PRRT, and discusses several PRRT-optimization strategies.

Abstract

Peptide receptor radionuclide therapy (PRRT) consists of the administration of a tumor-targeting radiopharmaceutical into the circulation of a patient. The radiopharmaceutical will bind to a specific peptide receptor leading to tumor-specific binding and retention. The only target that is currently used in clinical practice is the somatostatin receptor (SSTR), which is overexpressed on a range of tumor cells, including neuroendocrine tumors and neural-crest derived tumors. Academia played an important role in the development of PRRT, which has led to heterogeneous literature over the last two decades, as no standard radiopharmaceutical or regimen has been available for a long time. This review provides a summary of the treatment efficacy (e.g., response rates and symptom-relief), impact on patient outcome and toxicity profile of PRRT performed with different generations of SSTR-targeting radiopharmaceuticals, including the landmark randomized-controlled trial NETTER-1. In addition, multiple optimization strategies for PRRT are discussed, i.e., the dose–effect concept, dosimetry, combination therapies (i.e., tandem/duo PRRT, chemoPRRT, targeted molecular therapy, somatostatin analogues and radiosensitizers), new radiopharmaceuticals (i.e., SSTR-antagonists, Evans-blue containing vector molecules and alpha-emitters), administration route (intra-arterial versus intravenous) and response prediction via molecular testing or imaging. The evolution and continuous refinement of PRRT resulted in many lessons for the future development of radionuclide therapy aimed at other targets and tumor types.

Peptide receptor radionuclide therapy with [177Lu]Lu-DOTA-TATE

This continuing education aims to present in a clear and easy-to-understand way, the biology of neuroendocrine tumors (NETs), the characteristics of somatostatin receptors, the selection of patients for radiolabelled peptide therapy (PRRT), the inclusion criteria to benefit from treatment with the minimum possible adverse effects, the administration protocol, follow-up and response evaluation. The functional imaging studies necessary to explore the biology of the tumor and to individualize the treatment are also carried out, and constitute the cornerstone for the development of teragnosis. Clinical trials are being developed to better define the position of PRRT within the broad therapeutic options, and among the future perspectives, there are several lines of research to improve the objective response rate and survival with PRRT, focused on the development of new agonists and somatostatin receptor antagonists, new radionuclides and radiosensitizing combination therapies. In conclusion, PRRT is a great therapeutic, well-tolerated and safe tool with generally mild and self-limited acute side effects, that must be sequenced at the best moment of the evolution of the disease of patients with NET. Candidate patients for PRRT should always be evaluated by a multidisciplinary clinical committee.

New Directions in Imaging Neuroendocrine Neoplasms

Purpose of Review

Accurate imaging is crucial for correct diagnosis, staging, and therapy of neuroendocrine neoplasms (NENs). The search for the optimal imaging technique has triggered rapid development in the field. This review aims at giving an overview on contemporary imaging methods and providing an outlook on current progresses.

Recent Findings

The discovery of molecular targets due to the overexpression of specific peptide hormone receptors on the NEN’s surface has triggered the development of multiple radionuclide imaging modalities. In addition to the established imaging technique of targeting somatostatin receptors, several alternative radioligands have been developed. Targeting the glucagon-like peptide-1 receptor by exendin-4 has a high sensitivity in localizing insulinomas. For dedifferentiated NENs, new molecular targets such as the C-X-C motif chemokine-receptor-4 have been evaluated. Other new targets involve the fibroblast activation protein and the cholecystokinin-2 receptors, where the ligand minigastrin opens new possibilities for the management of medullary thyroid carcinoma.

Summary

Molecular imaging is an emerging field that improves the management of NENs.

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.

Theranostics in neuroendocrine tumors: an overview of current approaches and future challenges

Neuroendocrine neoplasms (NENs) comprise a heterogeneous group of tumors, mainly localized in the gastrointestinal system. What characterizes NENs is the expression of hormone receptors on the tumor cell surface, making them accessible for diagnostic and therapeutic approaches (theranostics) using radiolabelled peptides. Somatostatin receptors subtype-two (SST2) play an important role in NENs since they are overexpressed and homogeneously distributed at the surface of the majority of NENs. Accordingly, targeting SST2 for diagnostic and therapeutic purposes has been established. Current research aims at upregulating its expression by epigenetic treatment or improving its targeting via use of alternative radioligands. In addition, recent data suggest a future role of SST antagonists as a diagnostic tool and a potential therapeutic option. Another promising target is the glucagon-like peptide-1 (GLP-1) receptor. Targeting GLP-1R using exendin-4 (GLP-1 analogue) has a high sensitivity for the localization of the often SST2-negative sporadic insulinomas and insulinomas in the context of multiple endocrine neoplasia type-1. Further options for patients with insufficient expression of SST2 involve metaiodobenzylguanidine (MIBG) and the molecular target C-X-C motif chemokine receptor-4 (CXCR4), which have been evaluated for potential theranostic approach in symptomatic NENs or dedifferentiated tumors. Recently, new targets such as the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the fibroblast activation protein (FAP) have been identified in NENs. Finally, minigastrin - a ligand targeting the cholecystokinin-2 (CCK2) receptors in medullary thyroid carcinoma and foregut neuroendocrine tumors - may improve future management of these diseases with currently limited therapeutic options. This review summarises the current approaches and future challenges of diagnostic and therapeutic evaluations in neuroendocrine neoplasms.

Strategies Towards Improving Clinical Outcomes of Peptide Receptor Radionuclide Therapy

PURPOSE OF REVIEW

Peptide receptor radionuclide therapy (PRRT) with [177Lu-DOTA0,Tyr3] octreotate is an effective and safe second- or third-line treatment option for patients with low-grade advanced gastroenteropancreatic (GEP) neuroendocrine neoplasms (NEN). In this review, we will focus on possible extensions of the current use of PRRT and on new approaches which could further improve its treatment efficacy and safety.

RECENT FINDINGS

Promising results were published regarding PRRT in other NENs, including lung NENs or high-grade NENs, and applying PRRT as neoadjuvant or salvage therapy. Furthermore, a diversity of strategic approaches, including dosimetry, somatostatin receptor antagonists, somatostatin receptor upregulation, radiosensitization, different radionuclides, albumin binding, alternative renal protection, and liver-directed therapy in combination with PRRT, have the potential to improve the outcome of PRRT. Also, novel biomarkers are presented that could predict response to PRRT. Multiple preclinical and early clinical studies have shown encouraging potential to advance the clinical outcome of PRRT in NEN patients. However, at this moment, most of these strategies have not yet reached the clinical setting of randomized phase III trials.

Consensus on molecular imaging and theranostics in neuroendocrine neoplasms

Nuclear medicine plays an increasingly important role in the management neuroendocrine neoplasms (NEN). Somatostatin analogue (SSA)-based positron emission tomography/computed tomography (PET/CT) and peptide receptor radionuclide therapy (PRRT) have been used in clinical trials and approved by the European Medicines Agency (EMA) and the US Food and Drug Administration (FDA). European Association of Nuclear Medicine (EANM) Focus 3 performed a multidisciplinary Delphi process to deliver a balanced perspective on molecular imaging and radionuclide therapy in well-differentiated neuroendocrine tumours (NETs). NETs form in cells that interact with the nervous system or in glands that produce hormones. These cells, called neuroendocrine cells, can be found throughout the body, but NETs are most often found in the abdomen, especially in the gastrointestinal tract. These tumours may also be found in the lungs, pancreas and adrenal glands. In addition to being rare, NETs are also complex and may be difficult to diagnose. Most NETs are non-functioning; however, a minority present with symptoms related to hypersecretion of bioactive compounds. NETs often do not cause symptoms early in the disease process. When diagnosed, substantial number of patients are already found to have metastatic disease. Several societies' guidelines address Neuroendocrine neoplasms (NENs) management; however, many issues are still debated, due to both the difficulty in acquiring strong clinical evidence in a rare and heterogeneous disease and the different availability of diagnostic and therapeutic options across countries. EANM Focus 3 reached consensus on employing ⁶⁸gallium-labelled somatostatin analogue ([⁶⁸Ga]Ga-DOTA-SSA)-based PET/CT with diagnostic CT or magnetic resonance imaging (MRI) for unknown primary NET detection, metastatic NET, NET staging/restaging, suspected extra-adrenal pheochromocytoma/paraganglioma and suspected paraganglioma. Consensus was reached on employing ¹⁸fluorine-fluoro-2-deoxyglucose ([¹⁸F]FDG) PET/CT in neuroendocrine carcinoma, G3 NET and in G1-2 NET with mismatched lesions (CT-positive/[⁶⁸Ga]Ga-DOTA-SSA-negative). Peptide receptor radionuclide therapy (PRRT) was recommended for second line treatment for gastrointestinal NET with [⁶⁸Ga]Ga-DOTA-SSA uptake in all lesions, in G1/G2 NET at disease progression, and in a subset of G3 NET provided all lesions are positive at [¹⁸F]FDG and [⁶⁸Ga]Ga-DOTA-SSA. PRRT rechallenge may be used for in patients with stable disease for at least 1 year after therapy completion. An international consensus is not only a prelude to a more standardised management across countries but also serves as a guide for the direction to follow when designing new research studies.

Radiolabeled Peptides for SPECT and PET Imaging in the Detection of Breast Cancer: Preclinical and Clinical Perspectives

Breast cancer is the most common cancer in women worldwide. Due to the heterogeneous nature of breast cancer, the optimal treatment and expected response for each patient may not necessarily be universal. Molecular imaging techniques could play an important role in the early detection and targeted therapy evaluation of breast cancer. This review focuses on the development of peptides labeled with SPECT and PET radionuclides for breast cancer imaging. We summarized the current status of radiolabeled peptides for different receptors in breast cancer. The characteristics of radionuclides and major techniques for peptide labeling are also briefly discussed.

Development and Preclinical Evaluation of 99mTc- and 186Re-Labeled NOTA and NODAGA Bioconjugates Demonstrating Matched Pair Targeting of GRPR-Expressing Tumors

PURPOSE

The goal of this work was to develop hydrophilic gastrin-releasing peptide receptor (GRPR)-targeting complexes of the general formula fac-[M(CO)3(L)]+ [M = natRe, 99mTc, 186Re; L: NOTA for 1, NODAGA for 2] conjugated to a powerful GRPR peptide antagonist (DPhe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) via a 6-aminohexanoic acid linker.

PROCEDURES

Metallated-peptides were prepared employing the [M(OH2)3(CO)3]+ [M = Re, 99mTc, 186Re] precursors. Re-1/2 complexes were characterized with HR-MS. IC50 studies were performed for peptides 1/2 and their respective Re-1/2 complexes in a binding assay utilizing GRPR-expressing human PC-3 prostate cancer cells and [125I]I-Tyr4-BBN as the competing ligand. The 99mTc/186Re-complexes were identified by HPLC co-injection with their Re-analogues. All tracers were challenged in vitro at 37 °C against cysteine/histidine (phosphate-buffered saline 10 mM, pH 7.4) and rat serum. Biodistribution and micro-SPECT/CT imaging of [99mTc]Tc-1/2 and [186Re]Re-2 were performed in PC-3 tumor-bearing ICR SCID mice.

RESULTS

High in vitro receptor affinity (IC50 2-3 nM) was demonstrated for all compounds. The 99mTc/186Re-tracers were found to be hydrophilic (log D7.4 ≤ - 1.35) and highly stable. Biodistribution in PC-3 xenografted mice revealed good tumor uptake (%ID/g at 1 h: 4.3 ± 0.7 for [99mTc]Tc-1, 8.3 ± 0.9 for [99mTc]Tc-2 and 4.2 ± 0.8 for [186Re]Re-2) with moderate retention over 24 h. Rapid renal clearance was observed for [99mTc]Tc-2 and [186Re]Re-2 (> 84 % at 4 h), indicating favorable pharmacokinetics. Micro-SPECT/CT images for the 99mTc-tracers clearly visualized PC-3 tumors in agreement with the biodistribution data and with superior imaging properties found for [99mTc]Tc-2.

CONCLUSIONS

[99mTc]Tc-2 shows promise for further development as a GRPR-imaging agent. [186Re]Re-2 demonstrated very similar in vivo behavior to [99mTc]Tc-2, and further studies are therefore justified to explore the theranostic potential of our approach for targeting of GRPR-positive cancers.

Phase 0 Radiopharmaceutical-Agent Clinical Development

The evaluation of antibody-targeted or peptide-targeted radiopharmaceuticals as monotherapy or in oncological drug combinations requires programmatic collaboration within the National Cancer Institute (NCI) clinical trial enterprise. Phase 0 trials provide a flexible research platform for the study of radiopharmaceutical–drug pharmacokinetics, radiation dosimetry, biomarkers of DNA damage response modulation, and pharmacodynamic benchmarks predictive of therapeutic success. In this article, we discuss a phase 0 clinical development approach for human antibody-targeted or peptide-targeted radiopharmaceutical–agent combinations. We expect that early-phase radiopharmaceutical–agent combination trials will become a more tactical and more prevalent part of radiopharmaceutical clinical development in the near-term future for the NCI Cancer Therapy Evaluation Program.

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.

Advances in the Diagnosis and Management of Well-Differentiated Neuroendocrine Neoplasms

Neuroendocrine neoplasms constitute a diverse group of tumors that derive from the sensory and secretory neuroendocrine cells and predominantly arise within the pulmonary and gastrointestinal tracts. The majority of these neoplasms have a well-differentiated grade and are termed neuroendocrine tumors (NETs). This subgroup is characterized by limited proliferation and patients affected by these tumors carry a good to moderate prognosis. A substantial subset of patients presenting with a NET suffer from the consequences of endocrine syndromes as a result of the excessive secretion of amines or peptide hormones, which can impair their quality of life and prognosis. Over the past 15 years, critical developments in tumor grading, diagnostic biomarkers, radionuclide imaging, randomized controlled drug trials, evidence-based guidelines, and superior prognostic outcomes have substantially altered the field of NET care. Here, we review the relevant advances to clinical practice that have significantly upgraded our approach to NET patients, both in diagnostic and in therapeutic options.

Preclinical study of a new 177Lu-labeled somatostatin receptor antagonist in HT-29 human colorectal cancer cells

OBJECTIVES

Somatostatin receptor-positive neuroendocrine tumors have been targeted using various peptide analogs radiolabeled with therapeutic radionuclides for years. The better biomedical properties of radioantagonists as higher tumor uptake make these radioligands more attractive than agonists for somatostatin receptor-targeted radionuclide therapy. In this study, we tried to evaluate the efficiency of Luthetium-177 (177Lu) radiolabeled DOTA-Peptide 2 (177Lu-DOTA-Peptide 2) as a new radioantagonist in HT-29 human colorectal cancer in vitro and in vivo.

METHODS

DOTA conjugated antagonistic peptide with the sequence of p-Cl-Phe-Cyclo(D-Cys-L-BzThi-D-Aph-Lys-Thr-Cys)-D-Tyr-NH2 (DOTA-Peptide 2) was labeled with 177Lu. In vitro assays (saturation binding assay and internalization test) and animal biodistribution were performed in human colon adenocarcinoma cells (HT-29) and HT-29 tumor-bearing nude mice.

RESULTS

177Lu-DOTA-Peptide 2 showed high stability in acetate buffer and human plasma (>97%). Antagonistic property of 177Lu-DOTA-Peptide 2 was confirmed by low internalization in HT-29 cells (<5%). The desired dissociation constant (Kd =11.14 nM) and effective tumor uptake (10.89 percentage of injected dose per gram of tumor) showed high binding affinity of 177Lu-DOTA-Peptide 2 to somatostatin receptors.

CONCLUSION

177Lu-DOTA-Peptide 2 demonstrated selective and high binding affinity to somatostatin receptors overexpressed on the surface of HT-29 cancer cells, which could make this radiopeptide suitable for somatostatin receptor-targeted radionuclide therapy.

Recent Advances in Nuclear Imaging of Receptor Expression to Guide Targeted Therapies in Breast Cancer

Breast cancer remains the most frequent cancer in women with different patterns of disease progression and response to treatments. The identification of specific biomarkers for different breast cancer subtypes has allowed the development of novel targeting agents for imaging and therapy. To date, patient management depends on immunohistochemistry analysis of receptor status on bioptic samples. This approach is too invasive, and in some cases, not entirely representative of the disease. Nuclear imaging using receptor tracers may provide whole-body information and detect any changes of receptor expression during disease progression. Therefore, imaging is useful to guide clinicians to select the best treatments for each patient and to evaluate early response thus reducing unnecessary therapies. In this review, we focused on the development of novel tracers that are ongoing in preclinical and/or clinical studies as promising tools to lead treatment decisions for breast cancer management.

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.

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.

In Vivo Biokinetics of 177Lu-OPS201 in Mice and Pigs as a Model for Predicting Human Dosimetry

Introduction

¹⁷⁷Lu-OPS201 is a high-affinity somatostatin receptor subtype 2 antagonist for PRRT in patients with neuroendocrine tumors. The aim is to find the optimal scaling for dosimetry and to compare the biokinetics of ¹⁷⁷Lu-OPS201 in animals and humans.

Methods

Data on biokinetics of ¹⁷⁷Lu-OPS201 were analyzed in athymic nude Foxn1^nu mice (28 F, weight: 26 ± 1 g), Danish Landrace pigs (3 F-1 M, weight: 28 ± 2 kg), and patients (3 F-1 M, weight: 61 ± 17 kg) with administered activities of 0.19–0.27 MBq (mice), 97–113 MBq (pigs), and 850–1086 MBq (patients). After euthanizing mice (up to 168 h), the organ-specific activity contents (including blood) were measured. Multiple planar and SPECT/CT scans were performed until 250 h (pigs) and 72 h (patients) to quantify the uptake in the kidneys and liver. Blood samples were taken up to 23 h (patients) and 300 h (pigs). In pigs and patients, kidney protection was applied. Time-dependent uptake data sets were created for each species and organ/tissue. Biexponential fits were applied to compare the biokinetics in the kidneys, liver, and blood of each species. The time-integrated activity coefficients (TIACs) were calculated by using NUKFIT. To determine the optimal scaling, several methods (relative mass scaling, time scaling, combined mass and time scaling, and allometric scaling) were compared.

Results

A fast blood clearance of the compound was observed in the first phase (<56 h) for all species. In comparison with patients, pigs showed higher liver retention. Based on the direct comparison of the TIACs, an underestimation in mice (liver and kidneys) and an overestimation in pigs' kidneys compared to the patient data (kidney TIAC: mice = 1.4 h, pigs = 7.7 h, and patients = 5.8 h; liver TIAC: mice = 0.7 h, pigs = 4.1 h, and patients = 5.3 h) were observed. Most similar TIACs were obtained by applying time scaling (mice) and combined scaling (pigs) (kidney TIAC: mice = 3.9 h, pigs = 4.8 h, and patients = 5.8 h; liver TIAC: mice = 0.9 h, pigs = 4.7 h, and patients = 5.3 h).

Conclusion

If the organ mass ratios between the species are high, the combined mass and time scaling method is optimal to minimize the interspecies differences. The analysis of the fit functions and the TIACs shows that pigs are better mimicking human biokinetics.

Personalized 177Lu-octreotate peptide receptor radionuclide therapy of neuroendocrine tumours: initial results from the P-PRRT trial

PURPOSE

Peptide receptor radionuclide therapy (PRRT) is mostly administered using a fixed injected activity (IA) per cycle. This empiric regime results in highly variable absorbed doses to the critical organs and undertreatment of the majority of patients. We conceived a personalized PRRT protocol in which the IA is adjusted to deliver a prescribed absorbed dose to the kidney, with the aim to safely increase tumour irradiation. We herein report on the initial results of our prospective study of personalized PRRT, the P-PRRT Trial (NCT02754297).

METHODS

PRRT-naïve patients with progressive and/or symptomatic neuroendocrine tumour (NET) were scheduled to receive a four-cycle induction course of ¹⁷⁷Lu-octreotate with quantitative SPECT/CT-based dosimetry. The IA was personalized according to the glomerular filtration rate and the body surface area for the first cycle, and according to the prior renal Gy/GBq for the subsequent cycles. The prescribed renal absorbed dose of 23 Gy was reduced by 25-50% in case of significant renal or haematological impairment. Responders were allowed to receive consolidation or maintenance cycles, for each of which 6 Gy to the kidney were prescribed. We simulated the empiric PRRT regime by fixing the IA at 7.4 GBq per cycle, with the same percentage reductions as above. Radiological, molecular imaging, biochemical, and quality of life responses, as well as safety, were assessed.

RESULTS

Fifty-two patients underwent 171 cycles. In 34 patients who completed the induction course, a median cumulative IA of 36.1 (range, 6.3-78.6) GBq was administered, and the median cumulative kidney and maximum tumour absorbed doses were 22.1 (range, 8.3-24.3) Gy and 185.7 (range: 15.2-443.1) Gy respectively. Compared with the simulated fixed-IA induction regime, there was a median 1.26-fold increase (range, 0.47-2.12 fold) in the cumulative maximum tumour absorbed dose, which was higher in 85.3% of patients. In 39 assessable patients, the best objective response was partial response in nine (23.1%), minor response in 14 (35.9%), stable disease in 13 (33.3%) and progressive disease in three patients (7.7%). In particular, 11 of 13 patients (84.6%) with pancreatic NET had partial or minor response. The global health status/quality of life score significantly increased in 50% of patients. Acute and subacute side-effects were all of grade 1 or 2, and the most common were nausea (in 32.7% of patients) and fatigue (in 30.8% of patients) respectively. Subacute grade 3 or 4 toxicities occurred in less than 10% of patients, with the exception of lymphocytopenia in 51.9% of patients, without any clinical consequences however. No patient experienced severe renal toxicity.

CONCLUSIONS

Personalized PRRT makes it possible to safely increase tumour irradiation in the majority of patients. Our first results indicate a favourable tolerance profile, which appears similar to that of the empiric regime. The response rates are promising, in particular in patients with NET of pancreatic origin.

Role of biomarker tests for diagnosis of neuroendocrine tumours

Neuroendocrine tumours (NETs) are neoplasms that arise from neuroendocrine cells. Neuroendocrine cells and their tumours can secrete a wide range of amines and polypeptide hormones into the systemic circulation. This feature has triggered widespread investigation into circulating biomarkers for the diagnosis of NETs as well as for the prediction of the biological behaviour of tumour cells. Classic examples of circulating biomarkers for gastroenteropancreatic NETs include chromogranin A, neuron-specific enolase and pancreatic polypeptide as well as hormones that elicit clinical syndromes, such as serotonin and its metabolites, insulin, glucagon and gastrin. Biomarker metrics of general markers for diagnosing all gastroenteropancreatic NET subtypes are limited, but specific hormonal measurements can be of diagnostic value in select cases. In the past decade, methods for detecting circulating transcripts and tumour cells have been developed to improve the diagnosis of patients with NETs. Concurrently, modern scanning techniques and superior radiotracers for functional imaging have markedly expanded the options for clinicians dealing with NETs. Here, we review the latest research on biomarkers in the NET field to provide clinicians with a comprehensive overview of relevant diagnostic biomarkers that can be implemented in dedicated situations.

Biodistribution and radiation dose estimates for 68Ga-DOTA-JR11 in patients with metastatic neuroendocrine tumors

PURPOSE

Somatostatin receptor antagonists have shown promise for imaging neuroendocrine tumors (NETs) in preclinical studies, but clinical data is still very limited. In this study, we assess the feasibility of using the novel somatostatin antagonist ⁶⁸Ga-DOTA-JR11 for PET imaging of NETs.

METHODS

Twenty patients with advanced NETs underwent whole-body PET/CT imaging 60 min after injection of 169 MBq (median) ⁶⁸Ga-DOTA-JR11 as part of a prospective study. Volumes of interest were drawn around up to four ⁶⁸Ga-DOTA-JR11-avid lesions per patient (with uptake greater than liver) and standardized uptake values were estimated. Additionally, target-to-normal tissue ratios were calculated. A subset of six patients had additional imaging (25-min dynamic scan of the upper abdomen including, at least partly, cardiac left ventricle, liver, spleen, and kidney, and a whole-body PET/CT scan at 30 min post-injection) to determine the time course of tracer distribution and facilitate radiation dose estimates. Absorbed doses were calculated using OLINDA/EXM 1.0.

RESULTS

In contrast to the known biodistribution of somatostatin receptor agonists, little or no uptake above background was seen in the pituitary gland, spleen, adrenals, and uninvolved liver; e.g., median spleen SUV_mean 1.4 (range: 0.7-1.8), liver SUV_mean 1.1 (0.7-1.9). A total of 42 tumor lesions were analyzed with median SUV_max 13.0 (range: 2.9-94), TNR blood 9.3 (1.8-87), TNR spleen 4.9 (1.9-48), TNR kidney 2.2 (0.52-28), and TNR liver 10.5 (2.3-107). Tumor uptake reached plateau levels by 20-30 min post-injection. The highest absorbed dose estimates (mGy/MBq) to normal tissues were: urinary bladder wall (0.30; SD 0.06) and kidneys (0.050; SD 0.013). The effective dose (ICRP 103) was 0.022 (SD 0.003) mSv/MBq.

CONCLUSIONS

⁶⁸Ga-DOTA-JR11 demonstrated rapid tumor uptake, high tumor/background ratios, and rapid clearance from blood. The low liver background is advantageous and may facilitate detection of liver metastases. Dosimetric data compare favorably with published data for ⁶⁸Ga-DOTATATE and ⁶⁸Ga-DOTATOC.

Lu-177-Based Peptide Receptor Radionuclide Therapy for Advanced Neuroendocrine Tumors

Peptide receptor radionuclide therapy (PRRT) is a systemic cytotoxic radiation therapy using a compound of β-emitting radionuclide chelated to a peptide for the treatment of tumor with overexpressed specific cell receptor such as somatostatin receptor subtype 2 (SSTR2) of neuroendocrine tumor (NET). Surgical resection should be performed for the curative treatment for NETs when it is feasible; however, a multi-disciplinary approach is needed when locally advanced or metastasized disease. PRRT with lutetium-177 (Lu-177)-labeled somatostatin analogues, as a new treatment modality targeting metastatic or inoperable NETs expressing the SSTR2, have been developed and successfully used for the past two decades. As Lu-177 emits both β- and γ-radiation, it has the ability as a theragnostic agent for NETs compared with only β-emitting yttrium-90 labeled PRRT. Several recent studies reported that Lu-177 gave an overall positive response and improved the patients' quality of life. To fully exploit its potential, large comparative studies are needed for the assessment of distinct efficacies of Lu-177 labeled PRRT. Additionally, for extending the indications and developing new regimens of Lu-177-based PRRT, more dedicated clinical research is required.

Seek & Destroy, use of targeting peptides for cancer detection and drug delivery

Accounting for 16 million new cases and 9 million deaths annually, cancer leaves a great number of patients helpless. It is a complex disease and still a major challenge for the scientific and medical communities. The efficacy of conventional chemotherapies is often poor and patients suffer from off-target effects. Each neoplasm exhibits molecular signatures - sometimes in a patient specific manner - that may completely differ from the organ of origin, may be expressed in markedly higher amounts and/or in different location compared to the normal tissue. Although adding layers of complexity in the understanding of cancer biology, this cancer-specific signature provides an opportunity to develop targeting agents for early detection, diagnosis, and therapeutics. Chimeric antibodies, recombinant proteins or synthetic polypeptides have emerged as excellent candidates for specific homing to peripheral and central nervous system cancers. Specifically, peptide ligands benefit from their small size, easy and affordable production, high specificity, and remarkable flexibility regarding their sequence and conjugation possibilities. Coupled to imaging agents, chemotherapies and/or nanocarriers they have shown to increase the on-site delivery, thus allowing better tumor mass contouring in imaging and increased efficacy of the chemotherapies associated with reduced adverse effects. Therefore, some of the peptides alone or in combination have been tested in clinical trials to treat patients. Peptides have been well-tolerated and shown absence of toxicity. This review aims to offer a view on tumor targeting peptides that are either derived from natural peptide ligands or identified using phage display screening. We also include examples of peptides targeting the high-grade malignant tumors of the central nervous system as an example of the complex therapeutic management due to the tumor's location. Peptide vaccines are outside of the scope of this review.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

Advances in the Diagnosis of Neuroendocrine Neoplasms

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

SSTR-Mediated Imaging in Breast Cancer: Is There a Role for Radiolabeled Somatostatin Receptor Antagonists?

Recent studies have shown enhanced tumor targeting by novel somatostatin receptor (SSTR) antagonists compared with clinically widely used agonists. However, these results have been obtained mostly in neuroendocrine tumors, and only limited data are available for cancer types with lower SSTR expression, including breast cancer (BC). To date, two studies have reported higher binding of the antagonist than the agonist in BC, but in both studies only a limited number of cases were evaluated. In this preclinical study, we further investigated whether the application of an SSTR antagonist can improve SSTR-mediated BC imaging in a large panel of BC specimens. We also generated an in vivo BC mouse model and performed SPECT/MRI and biodistribution studies. Methods: Binding of ¹¹¹In-DOTA-Tyr³-octreotate (SSTR agonist) and ¹¹¹In-DOTA-JR11 (SSTR antagonist) to 40 human BC specimens was compared using in vitro autoradiography. SSTR2 immunostaining was performed to confirm SSTR2 expression of the tumor cells. Furthermore, binding of the radiolabeled SSTR agonist and antagonist was analyzed in tissue material from 6 patient-derived xenografts. One patient-derived xenograft, the estrogen receptor-positive model T126, was chosen to generate in vivo mouse models containing orthotopic breast tumors for in vivo SPECT/MRI and biodistribution studies after injection with ¹⁷⁷Lu-DOTA-Tyr³-octreotate or ¹⁷⁷Lu-DOTA-JR11. Results:¹¹¹In-DOTA-JR11 binding to human BC tissue was significantly higher than ¹¹¹In-DOTA-Tyr³-octreotate binding (P < 0.001). The median ratio of antagonist binding versus agonist binding was 3.39 (interquartile range, 2-5). SSTR2 immunostaining confirmed SSTR2 expression on the tumor cells. SPECT/MRI of the mouse model found better tumor visualization with the antagonist. This result was in line with the significantly higher tumor uptake of the radiolabeled antagonist than of the agonist as measured in biodistribution studies 285 min after radiotracer injection (percentage injected dose per gram of tissue: 1.92 ± 0.43 vs. 0.90 ± 0.17; P = 0.002). Conclusion: SSTR antagonists are promising candidates for BC imaging.

Evaluation of agonist and antagonist radioligands for somatostatin receptor imaging of breast cancer using positron emission tomography

Background

The somatostatin receptor subtype 2 (sstr2) is expressed on a majority of luminal breast cancers, however SPECT and scintigraphy imaging with agonistic sstr2 probes has been sub-optimal. High affinity antagonists can access more binding sites on the cell surface, resulting in higher tumor uptake and improved sensitivity. We compared the tumor uptake and biodistribution of the antagonist ⁶⁸Ga-NODAGA-JR11 with two agonists ⁶⁸Ga-DOTA-Tyr³-octreotide (⁶⁸Ga-DOTATOC) and ⁶⁸Ga-DOTA-Tyr³-octreotate (⁶⁸Ga-DOTATATE), in the human, sstr2-positive, luminal breast cancer model: ZR-75-1.

Results

Peptides were assayed for binding affinity using a filtration-based competitive assay to sstr2. ^natGa-DOTATOC and ^natGa-DOTATATE had excellent affinity (inhibition constant K_i: 0.9 ± 0.1 nM and 1.4 ± 0.3 nM respectively) compared to ^natGa-NODAGA-JR11 (25.9 ± 0.2 nM). The number of binding sites on ZR-75-1 cells was determined in vitro by saturation assays. Agonist ^67/natGa-DOTATOC bound to 6.64 ± 0.39 × 10⁴ sites/cells, which was 1.5-fold higher than ^67/natGa-NODAGA-JR11 and 2.3-fold higher than ^67/natGa-DOTATATE. All three ⁶⁸Ga-labeled peptides were obtained in good decay-corrected radiochemical yield (61-68%) and were purified by high performance liquid chromatography to ensure high specific activity (137 – 281 MBq/nmol at the end of synthesis). NOD scid gamma mice bearing ZR-75-1 tumors were injected intravenously with the labeled peptides and used for PET/CT imaging and biodistribution at 1 h post-injection. We found that ⁶⁸Ga-DOTATOC had the highest tumor uptake (18.4 ± 2.9%ID/g), followed by ⁶⁸Ga-DOTATATE (15.2 ± 2.2%ID/g) and ⁶⁸Ga-NODAGA-JR11 (12.2 ± 0.8%ID/g). Tumor-to-blood and tumor-to-muscle ratios were also higher for the agonists (>40 and >150 respectively), compared to the antagonist (15.6 ± 2.2 and 45.2 ± 11.6 respectively).

Conclusions

The antagonist ⁶⁸Ga-NODAGA-JR11 had the lowest tumor uptake and contrast compared to agonists ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE in ZR-75-1 xenografts.

The main contributing factor to this result could be the use of an endogenously expressing cell line, which may differ from previously published transfected models in the number of low-affinity, antagonist-specific binding sites. The relative merit of agonists versus antagonists for sstr2 breast cancer imaging warrants further investigation, first in preclinical models with other sstr2-positive breast cancer xenografts, and ultimately in luminal breast cancer patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-017-0023-y) contains supplementary material, which is available to authorized users.

Current Status of Radiopharmaceuticals for the Theranostics of Neuroendocrine Neoplasms

Nuclear medicine plays a pivotal role in the management of patients affected by neuroendocrine neoplasms (NENs). Radiolabeled somatostatin receptor analogs are by far the most advanced radiopharmaceuticals for diagnosis and therapy (radiotheranostics) of NENs. Their clinical success emerged receptor-targeted radiolabeled peptides as an important class of radiopharmaceuticals and it paved the way for the investigation of other radioligand-receptor systems. Besides the somatostatin receptors (sstr), other receptors have also been linked to NENs and quite a number of potential radiolabeled peptides have been derived from them. The Glucagon-Like Peptide-1 Receptor (GLP-1R) is highly expressed in benign insulinomas, the Cholecystokinin 2 (CCK2)/Gastrin receptor is expressed in different NENs, in particular medullary thyroid cancer, and the Glucose-dependent Insulinotropic Polypeptide (GIP) receptor was found to be expressed in gastrointestinal and bronchial NENs, where interestingly, it is present in most of the sstr-negative and GLP-1R-negative NENs. Also in the field of sstr targeting new discoveries brought into light an alternative approach with the use of radiolabeled somatostatin receptor antagonists, instead of the clinically used agonists. The purpose of this review is to present the current status and the most innovative strategies for the diagnosis and treatment (theranostics) of neuroendocrine neoplasms using a cadre of radiolabeled regulatory peptides targeting their receptors.

Review: Receptor Targeted Nuclear Imaging of Breast Cancer

Receptor targeted nuclear imaging directed against molecular markers overexpressed on breast cancer (BC) cells offers a sensitive and specific method for BC imaging. Currently, a few targets such as estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), somatostatin receptor (SSTR), and the gastrin releasing peptide receptor (GRPR) are being investigated for this purpose. Expression of these targets is BC subtype dependent and information that can be gained from lesion visualization is dependent on the target; ER-targeting radiotracers, e.g., can be used to monitor response to anti-estrogen treatment. Here we give an overview of the studies currently under investigation for receptor targeted nuclear imaging of BC. Main findings of imaging studies are summarized and (potential) purposes of lesion visualization by targeting these molecular markers are discussed. Since BC is a very heterogeneous disease and molecular target expression can vary per subtype, but also during disease progression or under influence of treatment, radiotracers for selected imaging purposes should be chosen carefully.

Tactics for preclinical validation of receptor-binding radiotracers

INTRODUCTION

Aspects of radiopharmaceutical development are illustrated through preclinical studies of [125I]-(E)-1-(2-(2,3-dihydrobenzofuran-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-BF-PE-PIPZE), a radioligand for sigma-1 (σ1) receptors, coupled with examples from the recent literature. Findings are compared to those previously observed for [125I]-(E)-1-(2-(2,3-dimethoxy-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-DM-PE-PIPZE).

METHODS

Syntheses of E-IA-BF-PE-PIPZE and [125I]-E-IA-BF-PE-PIPZE were accomplished by standard methods. In vitro receptor binding studies and autoradiography were performed, and binding potential was predicted. Measurements of lipophilicity and protein binding were obtained. In vivo studies were conducted in mice to evaluate radioligand stability, as well as specific binding to σ1 sites in brain, brain regions and peripheral organs in the presence and absence of potential blockers.

RESULTS

E-IA-BF-PE-PIPZE exhibited high affinity and selectivity for σ1 receptors (Ki = 0.43 ± 0.03 nM, σ2/σ1 = 173). [125I]-E-IA-BF-PE-PIPZE was prepared in good yield and purity, with high specific activity. Radioligand binding provided dissociation (koff) and association (kon) rate constants, along with a measured Kd of 0.24 ± 0.01 nM and Bmax of 472 ± 13 fmol/mg protein. The radioligand proved suitable for quantitative autoradiography in vitro using brain sections. Moderate lipophilicity, Log D7.4 2.69 ± 0.28, was determined, and protein binding was 71 ± 0.3%. In vivo, high initial whole brain uptake, >6% injected dose/g, cleared slowly over 24 h. Specific binding represented 75% to 93% of total binding from 15 min to 24 h. Findings were confirmed and extended by regional brain biodistribution. Radiometabolites were not observed in brain (1%).

CONCLUSIONS

Substitution of dihydrobenzofuranylethyl for dimethoxyphenethyl increased radioligand affinity for σ1 receptors by 16-fold. While high specific binding to σ1 receptors was observed for both radioligands in vivo, [125I]-E-IA-BF-PE-PIPZE displayed much slower clearance kinetics than [125I]-E-IA-DM-PE-PIPZE. Thus, minor structural modifications of σ1 receptor radioligands lead to major differences in binding properties in vitro and in vivo.