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

Optimization and automation of the radiosynthesis of [18F]Lu-LuFL as a clinically useful PET ligand targeting FAP for tumor imaging

Recently, a novel radiohybrid tracer [18F]Lu-LuFL targeting the fibroblast activation protein (FAP) has been developed for PET imaging of solid tumors. This tracer has shown promising results, prompting us to conduct a first-in-human study to evaluate its efficacy for PET imaging of FAP in human body. In order to facilitate the routine production and clinical application of [18F]Lu-LuFL, a straightforward and efficient automated synthesis is described. The optimum labeling parameters were determined at laboratory scale, and subsequently incorporated into an automated production process. Further studies have demonstrated that clinical doses of [18F]Lu-LuFL can be prepared within 19 min, with excellent radio chemical purity (>99%) and activity yield (23.58% ± 2.20%, non-decay corrected), coupled with solid phase extraction (SPE) purification method. All the quality control results satisfy the required criteria for release. In conclusion, we have successfully synthesized [18F]Lu-LuFL with sufficient radioactivity and superior quality, thereby establishing its potential for further clinical application.

Optimization and scale up of production of the PSMA imaging agent [18F]AlF-P16-093 on a custom automated radiosynthesis platform

BACKGROUND

Recent advancements in positron emission tomograph (PET) using prostate specific membrane antigen (PSMA)-targeted radiopharmaceuticals have changed the standard of care for prostate cancer patients by providing more accurate information during staging of primary and recurrent disease. [68Ga]Ga-P16-093 is a new PSMA-PET radiopharmaceutical that demonstrated superior imaging performance in recent head-to-head studies with [68Ga]Ga-PSMA-11. To improve the availability of this new PSMA PET imaging agent, [18F]AlF-P16-093 was developed. The 18F-analog [18F]AlF-P16-093 has been synthesized manually at low activity levels using [18F]AlF2+ and validated in pre-clinical models. This work reports the optimization of the production of > 15 GBq of [18F]AlF-P16-093 using a custom automated synthesis platform.

RESULTS

The sensitivity of the radiochemical yield of [18F]AlF-P16-093 to reaction parameters of time, temperature and reagent amounts was investigated using a custom automated system. The automated system is a low-cost, cassette-based system designed for 1-pot syntheses with flow-controlled solid phase extraction (SPE) workup and is based on the Raspberry Pi Zero 2 microcomputer/Python3 ecosystem. The optimized none-decay-corrected yield was 52 ± 4% (N = 3; 17.5 ± 2.2 GBq) with a molar activity of 109 ± 14 GBq/µmole and a radiochemical purity of 98.6 ± 0.6%. Run time was 30 min. A two-step sequence was used: SPE-purified [18F]F- was reacted with 80 nmoles of freeze-dried AlCl3·6H2O at 65 °C for 5 min followed by reaction with 160 nmoles of P16-093 ligand at 40 °C for 4 min in a 1:1 mixture of ethanol:0.5 M pH 4.5 NaOAc buffer. The mixture was purified by SPE (> 97% recovery). The final product formulation (5 mM pH 7 phosphate buffer with saline) exhibited a rate of decline in radiochemical purity of ~ 1.4%/h which was slowed to ~ 0.4%/h when stored at 4 °C.

CONCLUSION

The optimized method using a custom automated system enabled the efficient (> 50% none-decay-corrected yield) production of [18F]AlF-P16-093 with high radiochemical purity (> 95%). The method and automation system are simple and robust, facilitating further clinical studies with [18F]AlF-P16-093.

Brain PET Imaging in Small Animals: Tracer Formulation, Data Acquisition, Image Reconstruction, and Data Analysis

Positron emission tomography (PET) is a noninvasive functional imaging modality that involves in vivo detection of spatiotemporal changes in the binding of radioactive pharmaceuticals (a.k.a. PET tracers) to their target sites in different organs. The development of new PET tracers commonly involves their preclinical evaluation in small rodents. Moreover, laboratory animal PET research is now being used with progressively greater frequency to complement human PET studies, to investigate in greater depth the underlying pathophysiology of human diseases, and to monitor the efficiency of novel therapeutic interventions. Here we describe the steps toward a successful small animal PET study, from tracer formulation and image acquisition to data reconstruction and analysis of the acquired scans, with a particular focus on its utility for the brain.

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.

Validation of 18F-rhPSMA-7 and 18F-rhPSMA-7.3 PET Imaging Results with Histopathology from Salvage Surgery in Patients with Biochemical Recurrence of Prostate Cancer

¹⁸F-rhPSMA-7, and its single diastereoisomer form, ¹⁸F-rhPSMA-7.3, are prostate-specific membrane antigen (PSMA)-targeting radiopharmaceuticals. Here, we investigated their accuracy for the assessment of lymph node (LN) metastases validated by histopathology. Methods: Data from 58 patients with biochemical recurrence of prostate cancer after radical prostatectomy receiving salvage surgery after PET imaging with ¹⁸F-rhPSMA-7 or ¹⁸F-rhPSMA-7.3 were retrospectively reviewed. Two nuclear medicine physicians reviewed all PET scans and morphologic imaging in consensus. Readers were masked from the results of histopathology. PET and morphologic imaging were correlated with histopathology from resected LNs. Results: In 75 of 150 resected regions in 54 of 58 patients, tumor lesions were present in histopathology. The template-based specificity of PET (¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 combined) and morphologic imaging was 93.3% and 100%, respectively. However, ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET detected metastases in 61 of 75 histopathologically proven metastatic LN fields (81.3%) whereas morphologic imaging was positive in only 9 of 75 (12.0%). The positive predictive value was 92.4% for ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET and 100% for morphologic imaging. ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET performance was significantly superior to morphologic imaging (difference in the areas under the receiver-operating-characteristic curves, 0.222; 95% CI, 0.147-0.298; P < 0.001). The mean size of PET-positive and histologically confirmed LN metastases was 6.3 ± 3.1 mm (range, 2-15 mm) compared with a mean size of 9.8 ± 2.5 mm (range, 7-15 mm) on morphologic imaging. Conclusion: ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET offer a high positive predictive value comparable to that reported for ⁶⁸Ga-PSMA-11 and represent a valuable tool for guiding salvage lymphadenectomy.

Radiolabeled PSMA Inhibitors

PSMA has shown to be a promising target for diagnosis and therapy (theranostics) of prostate cancer. We have reviewed developments in the field of radio- and fluorescence-guided surgery and targeted photodynamic therapy as well as multitargeting PSMA inhibitors also addressing albumin, GRPr and integrin αvβ3. An overview of the regulatory status of PSMA-targeting radiopharmaceuticals in the USA and Europe is also provided. Technical and quality aspects of PSMA-targeting radiopharmaceuticals are described and new emerging radiolabeling strategies are discussed. Furthermore, insights are given into the production, application and potential of alternatives beyond the commonly used radionuclides for radiolabeling PSMA inhibitors. An additional refinement of radiopharmaceuticals is required in order to further improve dose-limiting factors, such as nephrotoxicity and salivary gland uptake during endoradiotherapy. The improvement of patient treatment achieved by the advantageous combination of radionuclide therapy with alternative therapies is also a special focus of this review.

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.

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 21 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. Also the first contribution in relation to MRI-agents is included.

CONCLUSIONS

Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Feasibility and Outcome of PSMA-PET-Based Dose-Escalated Salvage Radiotherapy Versus Conventional Salvage Radiotherapy for Patients With Recurrent Prostate Cancer

Introduction

Prostate-specific membrane antigen-positron emission tomography-(PSMA-PET) imaging facilitates dose-escalated salvage radiotherapy (DE-SRT) with simultaneous-integrated boost (SIB) for PET-positive lesions in patients with prostate cancer (PC). Therefore, we aimed to compare toxicity rates of DE-SRT with SIB to conventional SRT (C-SRT) without SIB and to report outcome.

Materials and Methods

We evaluated 199 patients who were treated with SRT between June 2014 and June 2020. 101 patients received DE-SRT with SIB for PET-positive local recurrence and/or PET-positive lymph nodes. 98 patients were treated with C-SRT to the prostate bed +/− elective pelvic lymphatic pathways without SIB. All patients received PSMA-PET imaging prior to DE-SRT ([68Ga]PSMA-11: 45.5%; [18F]-labeled PSMA: 54.5%). Toxicity rates for early (<6 months) and late (>6 months) gastrointestinal (GI) toxicities rectal bleeding, proctitis, stool incontinence, and genitourinary (GU) toxicities hematuria, cystitis, urine incontinence, urinary obstruction, and erectile dysfunction were assessed. Further, we analyzed the outcome with disease-free survival (DFS) and prostate-specific antigen (PSA) response.

Results

The overall toxicity rates for early GI (C-SRT: 2.1%, DE-SRT: 1.0%) and late GI (C-SRT: 1.4%, DE-SRT: 5.3%) toxicities ≥ grade 2 were similar. Early GU (C-SRT: 2.1%, DE-SRT: 3.0%) and late GU (C-SRT: 11.0%, DE-SRT: 14.7%) toxicities ≥ grade 2 were comparable, as well. Early and late toxicity rates did not differ significantly between DE-SRT versus C-SRT in all subcategories (p>0.05). PSA response (PSA ≤0.2 ng/ml) in the overall group of patients with DE-SRT was 75.0% and 86.4% at first and last follow-up, respectively.

Conclusion

DE-SRT showed no significantly increased toxicity rates compared with C-SRT and thus is feasible. The outcome of DE-SRT showed good results. Therefore, DE-SRT with a PSMA-PET-based SIB can be considered for the personalized treatment in patients with recurrent PC.

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.