The Search for an Alternative to [68Ga]Ga-DOTA-TATE in Neuroendocrine Tumor Theranostics: Current State of 18F-labeled Somatostatin Analog Development

Design, Preclinical Evaluation, and Clinical Translation of 68Ga-FAPI-LM3, a Heterobivalent Molecule for PET Imaging of Nasopharyngeal Carcinoma

Extensive research has been conducted on radiolabeled fibroblast activation protein (FAP) inhibitors (FAPIs) and p-Cl-Phe-cyclo(d-Cys-Tyr-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)d-Tyr-NH2 (LM3) peptides for imaging of FAP and somatostatin receptor 2 (SSTR2)-positive tumors. In this study, we designed and synthesized a FAPI-LM3 heterobivalent molecule radiolabeled with 68Ga and evaluated its effectiveness in both tumor xenografts and patients with nasopharyngeal carcinoma (NPC). Methods: The synthesis of FAPI-LM3 was based on the structures of FAPI-46 and LM3. After radiolabeling with 68Ga, its dual-receptor-binding affinity was evaluated in vitro and in vivo. Preclinical studies, including small-animal PET and biodistribution evaluation, were conducted on HT-1080-FAP and HT-1080-SSTR2 tumor xenografts. The feasibility of 68Ga-FAPI-LM3 PET/CT in a clinical setting was evaluated in patients with NPC, and the results were compared with those of 18F-FDG. Results: 68Ga-FAPI-LM3 showed high affinity for both FAP and SSTR2. The tumor uptake of 68Ga-FAPI-LM3 was significantly higher than that of 68Ga-FAPI-46 and 68Ga-DOTA-LM3 in HT-1080-FAP-plus-HT-1080-SSTR2 tumor xenografts. In a clinical study involving 6 NPC patients, 68Ga-FAPI-LM3 PET/CT showed significantly higher uptake than did 18F-FDG in primary and metastatic lesions, leading to enhanced lesion detectability and tumor delineation. Conclusion: 68Ga-FAPI-LM3 exhibited FAPI and SSTR2 dual-receptor-targeting properties both in vitro and in vivo, resulting in improved tumor uptake and retention compared with that observed with monomeric 68Ga-FAPI and 68Ga-DOTA-LM3. This study highlights the clinical feasibility of 68Ga-FAPI-LM3 PET/CT for NPC imaging.

Silicon-Fluoride Acceptors (SiFA) for 18F-Radiolabeling: From Bench to Bedside

Fluorine-18 (18F) is undoubtedly one of the most frequently applied radionuclides for the development of new radiotracers for positron emission tomography (PET) in the context of clinical cancer, neurological, and metabolic imaging. Until recently, the available radiochemical methodologies to introduce 18F into organic molecules ranging from small- to medium- and large-sized compounds were limited to a few applicable protocols. With the advent of late-stage fluorination of small aromatic, nonactivated compounds and various noncanonical labeling strategies geared toward the labeling of peptides and proteins, the molecular toolbox for PET radiotracer development was substantially extended. Especially, the noncanonical labeling methodologies characterized by the formation of Si-18F, B-18F, and Al-18F bonds give access to kit-like 18F-labeling of complex and side-group unprotected compounds, some of them already in clinical use. This chapter will particularly focus on silicon-fluoride acceptor (SiFA) chemistry and cover the history of its conceptual design and its translation into the clinical practice.

Production and purification of 43Sc and 47Sc from enriched [46Ti]TiO2 and [50Ti]TiO2 targets

The radioscandium isotopes, 43Sc and 47Sc, compose a promising elementally matched theranostic pair that can be used for the development of imaging and therapeutic radiopharmaceuticals with identical structures. This study aimed to investigate the production of high radionuclidic purity 43Sc from enriched [46Ti]TiO2 targets and 47Sc from enriched [50Ti]TiO2 targets and establish a target recycling technique. Enriched [46Ti]TiO2 targets were irradiated with 18 MeV protons, and enriched [50Ti]TiO2 targets were bombarded with 24 MeV protons. 43Sc and 47Sc were purified using ion chromatography attaining recovery yields of 91.7 ± 7.4% and 89.9 ± 3.9%, respectively. The average radionuclidic purity for 43Sc was 98.8 ± 0.3% and for 47Sc 91.5 ± 0.6%, while the average recovery of enriched titanium target material was 96 ± 4.0%. The highest apparent molar activity for [43Sc]Sc-DOTA was 23.2 GBq/µmol and 3.39 GBq/µmol for [47Sc]Sc-DOTA. This work demonstrates the feasibility of using enriched recycled [46Ti]TiO2 and [50Ti]TiO2 targets to produce high purity 43Sc and 47Sc as an elementally matched theranostic isotope pair.

Typical Zollinger-Ellison syndrome-atypical location of gastrinoma and absence of hypergastrinemia: A case report and review of literature

BACKGROUND

Zollinger-Ellison syndrome (ZES) results from hypersecretion of gastrin from pancreatic or duodenal neuroendocrine tumors, commonly referred to as gastrinomas. The high levels of gastrin lead to a typical presentation involving watery diarrhea and multiple ulcers in the duodenum. Here, we have presented the rare case of a patient with ZES and absence of hypergastrinemia as well as an atypical location of gastrinoma.

CASE SUMMARY

A 72-year-old woman presented with the typical clinical manifestations of ZES, including upper abdominal pain, significant watery diarrhea, and acidic liquid vomitus. Surprisingly, however, she did not have an increased level of serum gastrin. In addition, there was no evidence of gastrinoma or any other ulcerogenic tumor. Esophagogastroduodenoscopy was conducted to examine the upper digestive tract. Revised diagnoses were considered, and an individualized treatment plan was developed. The patient responded to antacid medication while experiencing intermittent, recurring bouts of ZES. 18F-AlF-NOTA-octreotide positron emission tomography (18F-OC PET)/computed tomography (CT) helped locate the tumor. Postoperative pathology and immunohistochemistry results suggested that the tumor was a gastrinoma located at an unconventional site.

CONCLUSION

This present case study demonstrates the possibility of ZES-like manifestation in patients with absence of hypergastrinemia. 18F-OC PET/CT is a relatively new imaging technique that can be applied for diagnosing even tiny gastrinomas that are atypical in terms of location.

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.

N-Benzylethanolammonium Ionic Liquids and Molten Salts in the Synthesis of 68Ga- and Al18F-Labeled Radiopharmaceuticals

Ionic liquids (ILs), due to their structural features, have unique physical and chemical properties and are environmentally friendly. Every year, the number of studies devoted to the use of ILs in medicine and pharmaceutics is growing. In nuclear medicine, the use of ILs with self-buffering capacity in the synthesis of radiopharmaceuticals is extremely important. This research is devoted to obtaining new ionic buffer agents containing N-benzylethanolammonium (BEA) cations and anions of carboxylic acids. A series of new BEA salts was synthesized and identified by NMR (¹H, ¹³C), IR spectroscopy and elemental and thermal analysis. The crystal structures of BEA hydrogen succinate, hydrogen oxalate and oxalate were determined by x-ray diffraction. Newly synthesized compounds were tested as buffer solutions in ⁶⁸Ga- and Al¹⁸F-radiolabeling reactions with a series of bifunctional chelating agents and clinically relevant peptides used for visualization of malignancies by positron emission tomography. The results obtained confirm the promise of using new buffers in the synthesis of ⁶⁸Ga- and Al¹⁸F-labeled radiopharmaceuticals.

Al[18F]F-NOTA-Octreotide Is Comparable to [68Ga]Ga-DOTA-TATE for PET/CT Imaging of Neuroendocrine Tumours in the Latin-American Population

PET imaging of neuroendocrine tumours (NET) is well established for staging and therapy follow-up. The short half-life, increasing costs, and regulatory issues significantly limit the availability of approved imaging agents, such as [68Ga]Ga-DOTA-TATE. Al[18F]F-NOTA-Octreotide provides a similar biodistribution and tumour uptake, can be produced on a large scale and may improve access to precision imaging. Here we prospectively compared the clinical utility of [68Ga]Ga-DOTA-TATE and Al[18F]F-NOTA-Octreotide in the Latin-American population. Our results showed that in patients with stage IV NETs [68Ga]Ga-DOTA-TATE presents higher physiological uptake than Al[18F]F-NOTA-Octreotide in the liver, hypophysis, salivary glands, adrenal glands (all p < 0.001), pancreatic uncinated process, kidneys, and small intestine (all p < 0.05). Nevertheless, despite the lower background uptake of Al[18F]F-NOTA-Octreotide, comparative analysis of tumour-to-liver (TLR) and tumour-to-spleen (TSR) showed no statistically significant difference for lesions in the liver, bone, lymph nodes, and other tissues. Only three discordant lesions in highly-metastases livers were detected by [68Ga]Ga-DOTA-TATE but not by Al[18F]F-NOTA-Octreotide and only one discordant lesion was detected by Al[18F]F-NOTA-Octreotide but not by [68Ga]Ga-DOTA-TATE. Non-inferiority analysis showed that Al[18F]F-NOTA-Octreotide is comparable to [68Ga]Ga-DOTA-TATE. Hence, our results demonstrate that Al[18F]F-NOTA-Octreotide provided excellent image quality, visualized NET lesions with high sensitivity and represents a highly promising, clinical alternative to [68Ga]Ga-DOTA-TATE.

Cyclotron Production of Gallium-68 Radiopharmaceuticals Using the 68Zn(p,n)68Ga Reaction and Their Regulatory Aspects

Designing and implementing various radionuclide production methods guarantees a sustainable supply, which is important for medical use. The use of medical cyclotrons for radiometal production can increase the availability of gallium-68 (⁶⁸Ga) radiopharmaceuticals. Although generators have greatly influenced the demand for ⁶⁸Ga radiopharmaceuticals, the use of medical cyclotrons is currently being explored. The resulting ⁶⁸Ga production is several times higher than obtained from a generator. Moreover, the use of solid targets yields end of purification and end of synthesis (EOS) of up to 194 GBq and 72 GBq, respectively. Furthermore, experiments employing liquid targets have provided promising results, with an EOS of 3 GBq for [⁶⁸Ga]Ga-PSMA-11. However, some processes can be further optimized, specifically purification, to achieve high ⁶⁸Ga recovery and apparent molar activity. In the future, ⁶⁸Ga will probably remain one of the most in-demand radionuclides; however, careful consideration is needed regarding how to reduce the production costs. Thus, this review aimed to discuss the production of ⁶⁸Ga radiopharmaceuticals using Advanced Cyclotron Systems, Inc. (ACSI, Richmond, BC, Canada) Richmond, Canada and GE Healthcare, Wisconsin, USA cyclotrons, its related factors, and regulatory concerns.

Net theranostics

Theranostics is a term coined by combining the words “therapeutics” and “diagnostics,” referring to single chemical entities developed to deliver therapy and diagnosis simultaneously. Neuroendocrine tumors are rare cancers that occur in various organs of the body, and they express neuroendocrine factors such as chromogranin A and somatostatin receptor. Somatostatin analogs bind to somatostatin receptor, and when combined with diagnostic radionuclides, such as gamma‐emitters, are utilized for diagnosis of neuroendocrine tumor. Somatostatin receptor scintigraphy when combined with therapeutic radionuclides, such as beta‐emitters, are effective in treating neuroendocrine tumor as peptide receptor radionuclide therapy. Somatostatin receptor scintigraphy and peptide receptor radionuclide therapy are some of the most frequently used and successful theranostics for neuroendocrine tumor. In Japan, radiopharmaceuticals are regulated under a complex law system, creating a significant drug lag, which is a major public concern. It took nearly 10 years to obtain the approval for somatostatin receptor scintigraphy and peptide receptor radionuclide therapy use by the Japanese government. In 2021, ¹¹¹Lu‐DOTATATE (Lutathera), a drug for peptide receptor radionuclide therapy, was covered by insurance in Japan. In this review, we summarize the history of the development of neuroendocrine tumor theranostics and theranostics in general, as therapeutic treatment for cancer in the future. Furthermore, we briefly address the Japanese point of view regarding the development of new radiopharmaceuticals.

Improved Personalised Neuroendocrine Tumours' Diagnosis Predictive Power by New Receptor Somatostatin Image Processing Quantification

Although neuroendocrine tumours (NETs) are intensively studied, their diagnosis and consequently personalised therapy management is still puzzling due to their tumoral heterogeneity. In their theragnosis algorithm, receptor somatostatin scintigraphy takes the central place, the diagnosis receptor somatostatin analogue (RSA) choice depending on laboratory experience and accessibility. However, in all cases, the results depend decisively on correct radiotracer tumoral uptake quantification, where unfortunately there are still unrevealed clues and lack of standardization. We propose an improved method to quantify the biodistribution of gamma-emitting RSA, using tissular corrected uptake indices. We conducted a bi-centric retrospective study on 101 patients with different types of NETs. Three uptake indices obtained after applying new corrections to areas of interest drawn for the tumour and for three reference organs (liver, spleen and lung) were statistically analysed. For the corrected pathological uptake indices, the results showed a significant decrease in the error of estimating the occurrence of errors and an increase in the diagnostic predictive power for NETs, especially in the case of lung-referring corrected index. In conclusion, these results support the importance of corrected uptake indices use in the analysis of ^99mTcRSA biodistribution for a better personalised diagnostic accuracy of NETs patients.

Dosimetry and optimal scan time of [18F]SiTATE-PET/CT in patients with neuroendocrine tumours

PURPOSE

Radiolabelled somatostatin analogues targeting somatostatin receptors (SSR) are well established for combined positron emission tomography/computer tomography (PET/CT) imaging of neuroendocrine tumours (NET). [18F]SiTATE has recently been introduced showing high image quality, promising clinical performance and improved logistics compared to the clinical reference standard 68Ga-DOTA-TOC. Here we present the first dosimetry and optimal scan time analysis.

METHODS

Eight NET patients received a [18F]SiTATE-PET/CT (250 ± 66 MBq) with repeated emission scans (10, 30, 60, 120, 180 min after injection). Biodistribution in normal organs and SSR-positive tumour uptake were assessed. Dosimetry estimates for risk organs were determined using a combined linear-monoexponential model, and by applying 18F S-values and reference target masses for the ICRP89 adult male or female (OLINDA 2.0). Tumour-to-background ratios were compared quantitatively and visually between different scan times.

RESULTS

After 1 h, normal organs showed similar tracer uptake with only negligible changes until 3 h post-injection. In contrast, tracer uptake by tumours increased progressively for almost all types of metastases, thus increasing tumour-to-background ratios over time. Dosimetry resulted in a total effective dose of 0.015 ± 0.004 mSv/MBq. Visual evaluation revealed no clinically relevant discrepancies between later scan times, but image quality was rated highest in 60 and 120 min images.

CONCLUSION

[18F]SiTATE-PET/CT in NET shows overall high tumour-to-background ratios from 60 to 180 min after injection and an effective dose comparable to 68Ga-labelled alternatives. For clinical use of [18F]SiTATE, the best compromise between image quality and tumour-to-background contrast is reached at 120 min, followed by 60 min after injection.

The aluminium-[18F]fluoride revolution: simple radiochemistry with a big impact for radiolabelled biomolecules

The aluminium-[¹⁸F]fluoride ([¹⁸F]AlF) radiolabelling method combines the favourable decay characteristics of fluorine-18 with the convenience and familiarity of metal-based radiochemistry and has been used to parallel gallium-68 radiopharmaceutical developments. As such, the [¹⁸F]AlF method is popular and widely implemented in the development of radiopharmaceuticals for the clinic. In this review, we capture the current status of [¹⁸F]AlF-based technology and reflect upon its impact on nuclear medicine, as well as offering our perspective on what the future holds for this unique radiolabelling method.

Labelling via [Al18F]2+ Using Precomplexed Al-NODA Moieties

Over the past 20 years, ⁶⁸Ga-labelled radiopharmaceuticals have become an important part in clinical routine. However, the worldwide supply with ⁶⁸Ge/⁶⁸Ga generators is limited as well as the number of patient doses per batch of ⁶⁸Ga radiopharmaceutical. In the recent years, a new technique appeared, making use of the ease of aqueous labelling via chelators as with ⁶⁸Ga but using ¹⁸F instead. This technique takes advantage of the strong coordinative bond between aluminium and fluoride, realized in the aqueous cation [Al¹⁸F]²⁺. Most applications to date make use of one-pot syntheses with free Al(III) ions in the system. In contrast, we investigated the labelling approach split into two steps: generating the Al-bearing precursor in pure form and using this Al compound as a precursor in the labelling step with aqueous [¹⁸F]fluoride. Hence, no free Al³⁺ ions are present in the labelling step. We investigated the impact of parameters: temperature, pH, addition of organic solvent, and reaction time using the model chelator NH₂-MPAA-NODA. With optimized parameters we could stably achieve a 80% radiochemical yield exerting a 30-min reaction time at 100 °C. This technique has the potential to become an important approach in radiopharmaceutical syntheses.

Recent Advances in the Clinical Translation of Silicon Fluoride Acceptor (SiFA) 18F-Radiopharmaceuticals

The incorporation of silicon fluoride acceptor (SiFA) moieties into a variety of molecules, such as peptides, proteins and biologically relevant small molecules, has improved the generation of ¹⁸F-radiopharmaceuticals for medical imaging. The efficient isotopic exchange radiofluorination process, in combination with the enhanced [¹⁸F]SiFA in vivo stability, make it a suitable strategy for fluorine-18 incorporation. This review will highlight the clinical applicability of [¹⁸F]SiFA-labeled compounds and discuss the significant radiotracers currently in clinical use.

From the Magic Bullet to Theragnostics: Certitudes and Hypotheses, Trying to Optimize the Somatostatin Model

Simple Summary

In oncology, the hypothetical “perfect magic bullet” should have a specific target on tumor cells which allows one to target only the tumor, in the absence of uptake in normal and/or non-neoplastic cells. Theragnostics is a strategy that strictly combines diagnosis and therapy, which creates the conditions for an “a priori” definition of an effective therapeutic effect. The most complete theragnostic and “magic bullet” experiences in clinical practice are those associated with radioiodine and somatostatin model. In this paper, we analyze whether it could be possible to improve present clinical results, further extending the survival of a wider number of patients, expanding the recruitment criteria to other types of pathology, and improving the quality of life. The ultimate goal is to transform the theragnostic strategy based on the somatostatin model into a curative therapy in the highest possible number of patients.

Abstract

The first “theragnostic model”, that of radioiodine, was first applied both in diagnosis and therapy in the 1940s. Since then, many other theragnostic models have been introduced into clinical practice. To bring about the closest pharmacokinetic connection, the radiocompound used for diagnosis and therapy should be the same, although at present this is rarely applicable. Today, a widely applied and effective model is also the “DOTA-Ga-68/Lu-177”, used with success in neuroendocrine tumors (NET). In this paper, we analyze the necessary steps from the in vitro evaluation of a target to the choice of radionuclide and chelate for therapy up to in vivo transition and clinical application of most employed radiocompounds used for theragnostic purposes. Possible future applications and strategies of theragnostic models are also highlighted.

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.

SSTR2 Expression in Olfactory Neuroblastoma: Clinical and Therapeutic Implications

Somatostatin receptor 2 (SSTR2) expression has previously been documented in olfactory neuroblastoma (ONB). Here, we fully characterize SSTR2 expression in ONB and correlate staining results with clinicopathologic parameters including Hyams grade. We also assess SSTR2 immunohistochemistry expression in various histologic mimics of ONB to assess its diagnostic functionality. 78 ONBs (51 primary biopsies/excisions and 27 recurrences/metastases) from 58 patients were stained for SSTR2. H-scores based on intensity (0-3 +) and percentage of tumor cells staining were assigned to all cases. 51 histologic mimics were stained and scored in an identical fashion. 77/78 (99%) ONB cases demonstrated SSTR2 staining (mean H-score: 189, range: 0-290). There were no significant differences in staining between primary tumors and recurrences/metastases (mean H-score: 185 vs 198). Primary low-grade ONB had somewhat stronger staining than high-grade tumors (mean H-score: 200 vs 174). SSTR2 expression had no prognostic value when considering disease-free or disease-specific survival. SSTR2 staining is significantly higher in ONB than its histologic mimics (mean H-score: 189 vs 12.9, p < 0.001) suggesting a potential use of the marker in diagnosis of ONB. In conclusion, SSTR2 is consistently expressed in ONB suggesting a role for somatostatin-analog based imaging and therapy in this disease. More generally, SSTR2 may be another marker of neuroendocrine differentiation in ONB.

A Cyanine-Bridged Somatostatin Hybrid Probe for Multimodal SSTR2 Imaging in Vitro and in Vivo: Synthesis and Evaluation

Multimodal imaging probes have attracted the interest of ongoing research, for example, for the surgical removal of tumors. Modular synthesis approaches allow the construction of hybrid probes consisting of a radiotracer, a fluorophore and a targeting unit. We present the synthesis of a new asymmetric bifunctional cyanine dye that can be used as a structural and functional linker for the construction of such hybrid probes. 68 Ga-DOTATATE, a well-characterized radiopeptide targeting the overexpressed somatostatin receptor subtype 2 (SSTR2) in neuroendocrine tumors, was labeled with our cyanine dye, thus providing additional information along with the data obtained from the radiotracer. We tested the SSTR2-targeting and imaging properties of the resulting probe 68 Ga-DOTA-ICC-TATE in vitro and in a tumor xenograft mouse model. Despite the close proximity between dye and pharmacophore, we observed a high binding affinity towards SSTR2 as well as elevated uptake in SSTR2-overexpressing tumors in the positron emission tomography (PET) scan and histological examination.

The chemical tool-kit for molecular imaging with radionuclides in the age of targeted and immune therapy

Nuclear medicine has evolved over the last half-century from a functional imaging modality using a handful of radiopharmaceuticals, many of unknown structure and mechanism of action, into a modern speciality that can properly be described as molecular imaging, with a very large number of specific radioactive probes of known structure that image specific molecular processes. The advances of cancer treatment in recent decades towards targeted and immune therapies, combined with recognition of heterogeneity of cancer cell phenotype among patients, within patients and even within tumours, has created a growing need for personalised molecular imaging to support treatment decision. This article describes the evolution of the present vast range of radioactive probes – radiopharmaceuticals – leveraging a wide variety of chemical disciplines, over the last half century. These radiochemical innovations have been inspired by the need to support personalised medicine and also by the parallel development in development of new radionuclide imaging technologies – from gamma scintigraphy, through single photon emission tomography (SPECT), through the rise of clinical positron emission tomography (PET) and PET-CT, and perhaps in the future, by the advent of total body PET. Thus, in the interdisciplinary world of nuclear medicine and molecular imaging, as quickly as radiochemistry solutions are developed to meet new needs in cancer imaging, new challenges emerge as developments in one contributing technology drive innovations in the others.

How We Do It: A Multidisciplinary Approach to 177Lu DOTATATE Peptide Receptor Radionuclide Therapy

Lutetium 177 (¹⁷⁷Lu) DOTA-0-Tyr3-Octreotate (DOTATATE) peptide receptor radionuclide therapy (PRRT) is an effective treatment for advanced gastroenteropancreatic neuroendocrine tumors. This review presents a clinical practice workflow that has been successful since ¹⁷⁷Lu DOTATATE PRRT was approved by the U.S. Food and Drug Administration. The workflow relies heavily on the input of a multidisciplinary team and involves a nuclear medicine consultation service, tumor board, and specific preparations in advance of therapy and day-of-therapy procedures. A systematic checklist designed to ensure appropriate selection of treatment candidates and identification of any concerns to address to safely administer PRRT is provided. All patients were evaluated with gallium 68 DOTATATE PET/CT, and in cases of high-grade tumors, they were also evaluated with fluorine 18 fluorodeoxyglucose PET/CT, with imaging findings reviewed as part of the systematic checklist before PRRT. Adverse effects are discussed and imaging follow-up regimens are reviewed, including alternative diagnostic contrast materials. Approaches to multiple challenging patient scenarios are illustrated through case examples. Finally, alternative theranostic radionuclides and treatment strategies are discussed.

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.

Targeted Radionuclide Therapy in Patient-Derived Xenografts Using 177Lu-EB-RGD

Currently, most patients with non-small cell lung cancer (NSCLC) are diagnosed in advanced stages with a poor five-year survival rate. Therefore, intensive research aimed at finding novel therapeutic strategies has been ongoing; experimental models that reliably emulate NSCLC disease are greatly needed to predict responses to novel therapeutics. Therefore, we developed patient-derived xenograft (PDX) models of NSCLC, which we then used to evaluate the therapeutic efficacy of ¹⁷⁷Lu-EB-RGD, a peptide-based radiopharmaceutical with improved pharmacokinetics that targets integrin α_vβ₃ In this study, three different groups of NSCLC-PDXs were successfully established, all of which maintained the same IHC and genetic characteristics of the human primary tumor. The two NSCLC-PDX groups with intense and low expression of integrin α_vβ₃ (denoted as PDX_αvβ3+ and PDX_αvβ3-) were chosen as the experimental models to evaluate the in vivo biological behavior of ¹⁷⁷Lu-EB-RGD. In SPECT imaging and biodistribution studies, ¹⁷⁷Lu-EB-RGD showed significantly higher accumulation in PDX_αvβ3+ and PDX_αvβ3- models than its corresponding monomer ¹⁷⁷Lu-RGD. A single dose of 18.5 MBq ¹⁷⁷Lu-EB-RGD was enough to completely eradicate the tumors in PDX_αvβ3+, with no sign of tumor recurrence during the observation period. Such treatment was also efficacious in PDX_αvβ3-: a single dose of 29.6 MBq ¹⁷⁷Lu-EB-RGD led to a significant delay in tumor growth as compared with that in the control or ¹⁷⁷Lu-RGD group. The preclinical data from the use of this model suggest that ¹⁷⁷Lu-EB-RGD may be an effective treatment option for NSCLC and should be further evaluated in human trials.

[18F]FET-βAG-TOCA: The Design, Evaluation and Clinical Translation of a Fluorinated Octreotide

The success of Lutathera™ ([¹⁷⁷Lu]Lu-DOTA-TATE) in the NETTER-1 clinical trial as a peptide receptor radionuclide therapy (PRRT) for somatostatin receptor expressing (SSTR) neuroendocrine tumours (NET) is likely to increase the demand for patient stratification by positron emission tomography (PET). The current gold standard of gallium-68 radiolabelled somatostatin analogues (e.g., [⁶⁸Ga]Ga-DOTA-TATE) works effectively, but access is constrained by the limited availability and scalability of gallium-68 radiopharmaceutical production. The aim of this review is three-fold: firstly, we discuss the peptide library design, biological evaluation and clinical translation of [¹⁸F]fluoroethyltriazole-βAG-TOCA ([¹⁸F]FET-βAG-TOCA), our fluorine-18 radiolabelled octreotide; secondly, to exemplify the potential of the 2-[¹⁸F]fluoroethylazide prosthetic group and copper-catalysed azide-alkyne cycloaddition (CuAAC) chemistry in accessing good manufacturing practice (GMP) compatible radiopharmaceuticals; thirdly, we aim to illustrate a framework for the translation of similarly radiolabelled peptides, in which in vivo pharmacokinetics drives candidate selection, supported by robust radiochemistry methodology and a route to GMP production. It is hoped that this review will continue to inspire the development and translation of fluorine-18 radiolabelled peptides into clinical studies for the benefit of patients.

Pharmacokinetics, radiation dosimetry, acute toxicity and automated synthesis of [18F]AmBF3-TATE

Introduction

[¹⁸F]AmBF₃-TATE is a somatostatin agonist that selectively binds to somatostatin receptor subtype 2 (SSTR2). For clinical translation, pharmacokinetics, radiation dosimetry, and acute toxicity of [¹⁸F]AmBF₃-TATE were assessed with good laboratory practice (GLP) standards.

Methods

ICR mice were intravenously administered 0.8–2.0 MBq of [¹⁸F]AmBF₃-TATE, with one group pre-injected with 100 μg of [¹⁹F]AmBF₃-TATE 30 min before radiopharmaceutical administration to assess uptake specificity. The mice were euthanized at 0.5, 1, 2, or 4 h post-injection (p.i.). Blood and tissues were collected, weighed, and counted on a gamma counter to determine percentage injected dose per gram (%ID/g). Dosimetry was calculated based on biodistribution data using the mouse and human phantoms included in OLINDA. Acute toxicity was assessed in Sprague-Dawley rats at the dose of 0.742 mg/kg [¹⁹F]AmBF₃-TATE, with a 14-day observation/recovery period. Blood chemistry parameters, gross, and histopathology were evaluated. Body weight change and food consumption were monitored. The production of [¹⁸F]AmBF₃-TATE was automated on a Trasis AllinOne synthesis module.

Results

[¹⁸F]AmBF₃-TATE was cleared through the renal and hepatobiliary pathway. At 1 h p.i., the pancreas (F, 15.7 ± 3.72 and M 14.3 ± 1.61 %ID/g), stomach (F, 15.3 ± 3.63 and M, 19.0 ± 3.49 %ID/g), and lungs (F, 9.26 ± 2.24 and M, 6.17 ± 6.04 %ID/g) were the organs with the highest specific uptake. Pre-injection with [¹⁹F]AmBF₃-TATE significantly reduced pancreatic uptake (F, 0.13 ± 0.03 and M, 0.18 ± 0.09 %ID/g) at 1 h p.i. For dosimetry extrapolated to the average adult human, the bladder (0.027–0.030 mGy/MBq), pancreas (0.018–0.028 mGy/MBq), and lungs (0.006–0.013 mGy/MBq) are expected to receive the highest doses. No test-item related effects were observed upon evaluation of clinical observations, body weights, food consumption, clinical pathology, gross pathology, and histopathology for acute toxicity. [¹⁸F]AmBF₃-TATE was produced at activity yields of 15.6 ± 4.59 GBq, average molar activity of 435 ± 162 GBq/μmol, and radiochemical purity of 98.0 ± 1.73% with the automated synthesizer.

Conclusion

[¹⁸F]AmBF₃-TATE binds specifically to SSTR2. At 1000× clinical dose, [¹⁹F]AmBF₃-TATE was well tolerated with no treatment-related adverse effects.

[68Ga]Ga-DOTA-TOC: The First FDA-Approved 68Ga-Radiopharmaceutical for PET Imaging

In the United States, [⁶⁸Ga]Ga-DOTA-TOC has been approved by the Food and Drug Administration (FDA) in 2019 as the first ⁶⁸Ga-radiopharmaceutical for imaging of somatostatin receptor (SSTR) positive gastroenteropancreatic neuroendocrine tumors while employing positron emission tomography (PET). In Europe (Austria, Germany, France), [⁶⁸Ga]Ga-DOTA-TOC was already approved back in 2016. This radiopharmaceutical combines the radionuclide ⁶⁸Ga with the somatostatin analogue DOTA-TOC for specific imaging of tumor cells expressing SSTRs. Such a targeting approach can also be used for therapy planning in the case of both localized as well as disseminated disease and potentially for the evaluation of treatment response.

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

This review deals with the development of peptide-based radiopharmaceuticals for the use with positron emission tomography and peptide receptor radiotherapy. It discusses the pros and cons of this class of radiopharmaceuticals as well as the different labelling strategies, and summarises approaches to optimise metabolic stability. Additionally, it presents different target structures and addresses corresponding tracers, which are already used in clinical routine or are being investigated in clinical trials.

Molecular Imaging and Therapy for Neuroendocrine Tumors

OPINION STATEMENT

Neuroendocrine tumors (NETs) are relatively rare, with 12,000-15,000 new cases diagnosed annually in the USA. Although NETs are a diverse group of neoplasms, they share common molecular targets that can be exploited using nuclear medicine techniques for both imaging and therapy. NETs have traditionally been imaged with SPECT imaging using ¹¹¹In-labeled octreotide analogs to detect neoplasms with somatostatin receptors. In addition, certain NETs (pheochromocytomas, paragangliomas, and neuroblastomas) are also effectively imaged using ¹²³I- or ¹³¹I-labeled metaiodobenzylguanidine (MIBG), an analog of guanethidine. More recently, PET imaging with ⁶⁸Ga-labeled somatostatin receptor (SSR) analogs allows neuroendocrine tumors to be imaged with much higher sensitivity. ⁶⁸Ga-DOTATATE was approved as a PET tracer by the FDA in June 2016. In addition to imaging, both MIBG and DOTATATE can be labeled with therapeutic radionuclides to deliver targeted radiation selectively to macroscopic and microscopic tumor sites. The incorporation of the same molecular probe for imaging and therapy provides a radio-theranostic approach to identifying, targeting, and treating tumors. Over the years, several centers have experience treating NETs with high-dose ¹³¹I-MIBG. ¹⁷⁷Lu-DOTATATE was approved by the FDA in 2018 for treatment of gastroenteropancreatic NETs and constitutes a major advancement in the treatment of these diseases. In this paper, we provide an overview of imaging and treating neuroendocrine tumors using MIBG and SSR probes. Although uncommon, neuroendocrine tumors have provided the largest experience for targeted radionuclide imaging and therapy (with the exception of radioiodine treatment for thyroid disease). In addition to benefitting patients with these rare tumors, the knowledge gained provides a blueprint for the development of future paired diagnostic/therapeutic probes for treating other diseases, such as prostate cancer.