Automated production of [18F]SiTATE on a Scintomics GRP™ platform for PET/CT imaging of neuroendocrine tumors

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.

Good practices for the automated production of 18F-SiFA radiopharmaceuticals

BACKGROUND

The positron emitting isotope fluorine-18 (18F) possesses almost ideal physicochemical properties for the development of radiotracers for diagnostic molecular imaging employing positron emission tomography (PET). 18F in its nucleophilic anionic 18F- form is usually prepared by bombarding an enriched 18O water target with protons of various energies between 5 and 20 MeV depending on the technical specifications of the cyclotron. Large thick-target yields between 5 and 14 GBq/µA can be obtained, enough to prepare large batches of radiotracers capable to serve a considerable contingent of patients (50 + per clinical batch). The overall yield of the radiotracer however depends on the efficiency of the 18F labeling chemistry. The Silicon Fluoride Acceptor chemistry (SiFA) has introduced a convenient and highly efficient way to provide clinical peptide-based 18F-radiotracers in a kit-like procedure matching the convenience of 99mTc radiopharmaceuticals.

MAIN BODY

A radiotracer's clinical success primarily hinges on whether its synthesis can be automated. Due to its simplicity, the SiFA chemistry, which is based on isotopic exchange (18F for 19F), does not only work in a manual setup but has been proven to be automatable, yielding large batches of 18F-radiotracers of high molar activity (Am). The production of SiFA radiotracer can be centralized and the radiopharmaceutical be distributed via the "satellite" principle, where one production facility economically serves multiple clinical application sites. Clinically validated tracers such as [18F]SiTATE and [18F]Ga-rhPSMA-7/-7.3 have been synthesized in an automated synthesis unit under good manufacturing practice conditions and used in large patient cohorts. Communication of common guidelines and practices is warranted to further the dissemination of SiFA radiopharmaceuticals and to give easy access to this technology.

CONCLUSION

This current review highlights the most recent achievements in SiFA radiopharmaceutical automation geared towards large batch production for clinical application. Best practice advice and guidance towards a facilitated implementation of the SiFA technology into new and already operating PET tracer production facilities is provided. A brief outlook spotlights the future potential of SiFA radiochemistry within the landscape of non-canonical labeling chemistries.

Next-generation PET/CT imaging in meningioma-first clinical experiences using the novel SSTR-targeting peptide [18F]SiTATE

BACKGROUND

Somatostatin-receptor (SSTR)-targeted PET/CT provides important clinical information in addition to standard imaging in meningioma patients. [18F]SiTATE is a novel, 18F-labeled SSTR-targeting peptide with superior imaging properties according to preliminary data. We provide the first [18F]SiTATE PET/CT data of a large cohort of meningioma patients.

METHODS

Patients with known or suspected meningioma undergoing [18F]SiTATE PET/CT were included. Uptake intensity (SUV) of meningiomas, non-meningioma lesions, and healthy organs were assessed using a 50% isocontour volume of interest (VOI) or a spherical VOI, respectively. Also, trans-osseous extension on PET/CT was assessed.

RESULTS

A total of 107 patients with 117 [18F]SiTATE PET/CT scans were included. Overall, 231 meningioma lesions and 61 non-meningioma lesions (e.g., post-therapeutic changes) were analyzed. Physiological uptake was lowest in healthy brain tissue, followed by bone marrow, parotid, and pituitary (SUVmean 0.06 ± 0.04 vs. 1.4 ± 0.9 vs. 1.6 ± 1.0 vs. 9.8 ± 4.6; p < 0.001). Meningiomas showed significantly higher uptake than non-meningioma lesions (SUVmax 11.6 ± 10.6 vs. 4.0 ± 3.3, p < 0.001). Meningiomas showed significantly higher uptake than non-meningioma lesions (SUVmax 11.6±10.6 vs. 4.0±3.3, p<0.001). 93/231 (40.3%) meningiomas showed partial trans-osseous extension and 34/231 (14.7%) predominant intra-osseous extension. 59/231 (25.6%) meningioma lesions found on PET/CT had not been reported on previous standard imaging.

CONCLUSION

This is the first PET/CT study using an 18F-labeled SSTR-ligand in meningioma patients: [18F]SiTATE provides extraordinary contrast in meningioma compared to healthy tissue and non-meningioma lesions, which leads to a high detection rate of so far unknown meningioma sites and osseous involvement. Having in mind the advantageous logistic features of 18F-labeled compared to 68Ga-labeled compounds (e.g., longer half-life and large-badge production), [18F]SiTATE has the potential to foster a widespread use of SSTR-targeted imaging in neuro-oncology.

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

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

Development of [18F]AmBF3 Tetrazine for Radiolabeling of Peptides: Preclinical Evaluation and PET Imaging of [18F]AmBF3-PEG7-Tyr3-Octreotide in an AR42J Pancreatic Carcinoma Model

Radiolabeled peptides have emerged as highly specific agents for targeting receptors expressed in tumors for therapeutic and diagnostic purposes. Peptides developed for positron emission tomography (PET) are typically radiolabeled using prosthetic groups or bifunctional chelators for fast "kit-like" incorporation of the radionuclide into the structure. A novel [18F]alkylammoniomethyltrifluoroborate ([18F]AmBF3) tetrazine (Tz), [18F]AmBF3-Tz, was developed for the [18F]fluorination of trans-cyclooctene (TCO)-modified biomolecules using Tyr3-octreotides (TOCs) as model peptides. [18F]AmBF3-Tz (Am = 15.4 ± 9.2 GBq/μmol, n = 14) was evaluated in healthy mice by ex vivo biodistribution and PET/computed tomography (CT), where the radiolabel in the prosthetic group was found stable in vivo, indicated by the low bone uptake in tibia (0.4 ± 0.1% ID/g, t = 270 min). TCO-TOCs tailored with polyethylene glycol (PEG) linkers were radiolabeled with [18F]AmBF3-Tz, forming two new tracers, [18F]AmBF3-PEG4-TOC (Am = 2.8 ± 1.8 GBq/μmol, n = 3) and [18F]AmBF3-PEG7-TOC (Am of 6.0 ± 3.4 GBq/μmol, n = 13), which were evaluated by cell uptake studies and ex vivo biodistribution in subcutaneous AR42J rat pancreatic carcinoma tumor-bearing nude mice. The tracer demonstrating superior behavior ex vivo, the [18F]AmBF3-PEG7-TOC, was further evaluated with PET/CT, where the tracer provided clear tumor visualization (SUVbaseline = 1.01 ± 0.07, vs SUVblocked = 0.76 ± 0.04) at 25 min post injection. The novel AmBF3-Tz demonstrated that it offers potential as a prosthetic group for rapid radiolabeling of biomolecules in mild conditions using bioorthogonal chemistry.

Synthesis and Preclinical Evaluation of [18F]SiFA-PSMA Inhibitors in a Prostate Cancer Model

Positron emission tomography (PET) imaging of prostate-specific membrane antigen (PSMA) with gallium-68 (⁶⁸Ga) and fluorine-18 (¹⁸F) radiotracers has aroused tremendous interest over the past few years. The use of organosilicon-[¹⁸F]fluoride acceptors (SiFA) conjugated to urea-based peptidomimetic PSMA inhibitors provides a "kit-like" multidose synthesis technology. Nine novel ¹⁸F-labeled SiFA-bearing PSMA inhibitors with different linker moieties were synthesized and analyzed for their in vitro binding against [¹²⁵I]I-TAAG-PSMA in LNCaP cells. IC₅₀ values ranged from 58-570 nM. Among all compounds, [¹⁸F]SiFA-Asp₂-PEG₃-PSMA (IC₅₀ = 125 nM) showed the highest tumor uptake in LNCaP tumors (SUV_60min 0.73). A substantial increase in molar activity (A_m) (from 7.5 ± 0.5 to 86 ± 3 GBq/μmol) led to a significant increase in LNCaP tumor uptake (SUV_60min 1.18; Δ 0.45 corresponding to +62%). In vivo blocking with DCFPyL resulted in -32% uptake after 60 min. The SiFA-isotopic exchange chemistry offers a method that is readily adaptable for a "kit-type" labeling procedure and clinical translation.

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.

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.

Ac-EAZY! Towards GMP-Compliant Module Syntheses of 225Ac-Labeled Peptides for Clinical Application

The application of 225Ac (half-life T1/2 = 9.92 d) dramatically reduces the activity used for peptide receptor radionuclide therapy by a factor of 1000 in comparison to 90Y, 177Lu or 188Re while maintaining the therapeutic outcome. Additionally, the range of alpha particles of 225Ac and its daughter nuclides in tissue is much lower (47-85 μm for alpha energies Eα = 5.8-8.4 MeV), which results in a very precise dose deposition within the tumor. DOTA-conjugated commercially available peptides used for endoradiotherapy, which can readily be labeled with 177Lu or 90Y, can also accommodate 225Ac. The benefits are lower doses in normal tissue for the patient, dose reduction of the employees and environment and less shielding material. The low availability of 225Ac activity is preventing its application in clinical practice. Overcoming this barrier would open a broad field of 225Ac therapy. Independent which production pathway of 225Ac proves the most feasible, the use of automated synthesis and feasible and reproducible patient doses are needed. The Modular-Lab EAZY is one example of a GMP-compliant system, and the cassettes used for synthesis are small. Therefore, also the waste after the synthesis can be minimized. In this work, two different automated setups with different purification systems are presented. In its final configuration, three masterbatches were performed on the ML EAZY for DOTA-TATE and PSMA-I&T, respectively, fulfilling all quality criteria with final radiochemical yields of 80-90% for the 225Ac-labeled peptides.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

Background

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

Results

This commentary of highlights has resulted in 23 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals.

Conclusion

Trends in radiochemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Automated synthesis of [18F]Ga-rhPSMA-7/ -7.3: results, quality control and experience from more than 200 routine productions

Introduction

The radiohybrid (rh) prostate-specific membrane antigen (PSMA)-targeted ligand [¹⁸F]Ga-rhPSMA-7 has previously been clinically assessed and demonstrated promising results for PET-imaging of prostate cancer. The ligand is present as a mixture of four stereoisomers ([¹⁸F]Ga-rhPSMA-7.1, − 7.2, − 7.3 and − 7.4) and after a preclinical isomer selection process, [¹⁸F]Ga-rhPSMA-7.3 has entered formal clinical trials. Here we report on the establishment of a fully automated production process for large-scale production of [¹⁸F]Ga-rhPSMA-7/ -7.3 under GMP conditions (EudraLex).

Methods

[¹⁸F]Fluoride in highly enriched [¹⁸O]H₂O was retained on a strong anion exchange cartridge, rinsed with anhydrous acetonitrile and subsequently eluted with a solution of [K⁺ ⊂ 2.2.2]OH⁻ in anhydrous acetonitrile into a reactor containing Ga-rhPSMA ligand and oxalic acid in DMSO. ¹⁸F-for-¹⁹F isotopic exchange at the Silicon-Fluoride Acceptor (SiFA) was performed at room temperature, followed by dilution with buffer and cartridge-based purification. Optimum process parameters were determined on the laboratory scale and thereafter implemented into an automated synthesis. Data for radiochemical yield (RCY), purity and quality control were analyzed for 243 clinical productions (160 for [¹⁸F]Ga-rhPSMA-7; 83 for [¹⁸F]Ga-rhPSMA-7.3).

Results

The automated production of [¹⁸F]Ga-rhPSMA-7 and the single isomer [¹⁸F]Ga-rhPSMA-7.3 is completed in approx. 16 min with an average RCY of 49.2 ± 8.6% and an excellent reliability of 98.8%. Based on the different starting activities (range: 31–130 GBq, 89 ± 14 GBq) an average molar activity of 291 ± 62 GBq/μmol (range: 50–450 GBq/μmol) was reached for labeling of 150 nmol (231 μg) precursor. Radiochemical purity, as measured by radio-high performance liquid chromatography and radio-thin layer chromatography, was 99.9 ± 0.2% and 97.8 ± 1.0%, respectively.

Conclusion

This investigation demonstrates that ¹⁸F-for-¹⁹F isotopic exchange is well suited for the fast, efficient and reliable automated routine production of ¹⁸F-labeled PSMA-targeted ligands. Due to its simplicity, speed and robustness the development of further SiFA-based radiopharmaceuticals is highly promising and can be of far-reaching importance for future theranostic concepts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-021-00120-5.

Role of PET/CT and Therapy Management of Pancreatic Neuroendocrine Tumors

Pancreatic neuroendocrine neoplasms (panNENs) are heterogeneous neoplasms with neuroendocrine differentiation that show peculiar clinical and histomorphological features, with variable prognosis. In recent years, advances in knowledge regarding the pathophysiology and heterogeneous clinical presentation, as well as the availability of different diagnostic procedures for panNEN diagnosis and novel therapeutic options for patient clinical management, has led to the recognition of the need for an active multidisciplinary discussion for optimal patient care. Molecular imaging with positron emission tomography/computed tomography (PET/CT) has become indispensable for the management of panNENs. Several PET radiopharmaceuticals can be used to characterize either panNEN receptor expression or metabolism. The aim of this review is to offer an overview of all the currently used radiopharmaceuticals and of the new upcoming tracers for pancreatic neuroendocrine tumors (panNETs), and their clinical impact on therapy management. [⁶⁸Ga]Ga-DOTA-peptide PET/CT (SSA-PET/CT) has high sensitivity, specificity, and accuracy and is recommended for the staging and restaging of any non-insulinoma well-differentiated panNEN cases to carry out detection of unknown primary tumor sites or early relapse and for evaluation of in vivo somatostatin receptors expression (SRE) to select patient candidates for peptide receptor radiometabolic treatment (PRRT) with ⁹⁰Y or ¹⁷⁷Lu and/or cold analogs. SSA-PET/CT also has a strong impact on clinical management, leading to a change in treatment in approximately a third of the cases. Its role for treatment response assessment is still under debate due to the lack of standardized criteria, even though some semiquantitative parameters seem to be able to predict response. [¹⁸F]FDG PET/CT generally shows low sensitivity in small growing and well-differentiated neuroendocrine tumors (NET; G1 and G2), while it is of utmost importance in the evaluation and management of high-grade NENs and also provides important prognostic information. When positive, [¹⁸F]FDG PET/CT impacts therapeutical management, indicating the need for a more aggressive treatment regime. Although FDG positivity does not exclude the patient from PRRT, several studies have demonstrated that it is certainly useful to predict response, even in this setting. The role of [¹⁸F]FDOPA for the study of panNET is limited by physiological uptake in the pancreas and is therefore not recommended. Moreover, it provides no information on SRE that has crucial clinical management relevance. Early acquisition of the abdomen and premedication with carbidopa may be useful to increase the accuracy, but further studies are needed to clarify its utility. GLP-1R agonists, such as exendin-4, are particularly useful for benign insulinoma detection, but their accuracy decreases in the case of malignant insulinomas. Being a whole-body imaging technique, exendin-PET/CT gives important preoperative information on tumor size and localization, which is fundamental for surgical planning as resection (enucleation of the lesion or partial pancreatic resection) is the only curative treatment. New upcoming tracers are under study, such as promising SSTR antagonists, which show a favorable biodistribution and higher tumor-to-background ratio that increases tumor detection, especially in the liver. [⁶⁸Ga]pentixafor, an in vivo marker of CXCR4 expression associated with the behavior of more aggressive tumors, seems to only play a limited role in detecting well-differentiated NET since there is an inverse expression of SSTR2 and CXCR4 in G1 to G3 NETs with an elevation in CXCR4 and a decrease in SSTR2 expression with increasing grade. Other tracers, such as [⁶⁸Ga]Ga-PSMA, [⁶⁸Ga]Ga-DATA-TOC, [¹⁸F]SiTATE, and [¹⁸F]AlF-OC, are also under investigation.