Synthesis of high-molar-activity [18F]6-fluoro-L-DOPA suitable for human use via Cu-mediated fluorination of a BPin precursor

Step-by-step optimisation of the radiosynthesis of the brain HDAC6 radioligand [18F]FSW-100 for clinical applications

BACKGROUND

Histone deacetylase 6 (HDAC6) is an emerging target for the treatment and diagnosis of proteinopathies. [18F]FSW-100 was recently developed as a promising brain-penetrating radioligand for HDAC6 PET imaging and the process validation of [18F]FSW-100 radiosynthesis for clinical use is complete, but no detailed synthetic strategy nor process optimisation has been reported. Here, we describe the optimisation of several processes in [18F]FSW-100 radiosynthesis, including the 18F-fluorination reaction, semipurification of the 18F-intermediate, and purification of the product by high-performance liquid chromatography (HPLC), to achieve a radiochemical yield (RCY) adequate for clinical applications of the radioligand. Our findings will aid optimisation of radiosynthesis processes in general.

RESULTS

In the 18F-fluorination reaction, the amount of copper reagent was reduced without reducing the nonisolated RCY of the intermediate (50%), thus reducing the risk of copper contamination in the product injection solution. Optimising the solid-phase extraction (SPE) conditions for semipurification of the intermediate improved its recovery efficiency. The addition of anti-radiolysis reagents to the mobile phase for the HPLC purification of [18F]FSW-100 increased its activity yield in radiosynthesis using a high [18F]fluoride radioactivity of approximately 50 GBq. The SPE-based formulation method and additives for the injection solution were optimised, and the resulting [18F]FSW-100 injection solution was stable for over 2 h with a radiochemical purity of greater than 95%.

CONCLUSIONS

Of all the reconsidered processes, we found that optimisation of the SPE-based semipurification of the intermediate and of the mobile phase for HPLC purification in particular improved the RCY of [18F]FSW-100, doubling it compared to that of the original protocol. The radioactivity of [18F]FSW-100 synthesized using the optimized protocol was sufficient for multiple doses for a clinical study.

Manufacturing 6-[18F]Fluoro-L-DOPA via Flow Chemistry-Enhanced Photoredox Radiofluorination

In this study, we introduce a practical methodology for the synthesis of PET probes by seamlessly combining flow chemistry with photoredox radiofluorination. The clinical PET tracer 6-[18F]FDOPA was smoothly prepared in a 24.3% non-decay-corrected yield with over 99.0% radiochemical purity (RCP) and enantiomeric excess (ee), notably by a simple cartridge-based purification. The flow chemistry-enhanced photolabeling method supplies an efficient and versatile solution for the synthesis of 6-[18F]FDOPA and for more PET tracer development.

Tandem Iridium-Catalyzed C-H Borylation/Copper-Mediated Radiofluorination of Aromatic C-H Bonds with [18F]TBAF

Direct C-H functionalization of (hetero)aromatic C-H bonds with iridium-catalyzed borylation followed by copper-mediated radiofluorination of the in situ generated organoboronates affords fluorine-18 labeled aromatics in high radiochemical conversions and meta-selectivities. This protocol describes the benchtop reaction assembly of the C-H borylation and radiofluorination steps, which can be utilized for the fluorine-18 labeling of densely functionalized bioactive scaffolds.

One-Pot Formal Carboradiofluorination of Alkenes: A Toolkit for Positron Emission Tomography Imaging Probe Development

We report the first one-pot formal alkene carboradiofluorination reaction employing easily accessible alkenes as both prosthetic group precursors and coupling partners. The methodology features rapid sequential Markovnikov-selective iodofluorination and photoinduced Pd(0/I/II)-catalyzed alkyl Heck reaction as a mild and robust fluorine-18 (18F) radiochemical approach for positron emission tomography (PET) imaging probe development. A new class of prosthetic groups for PET imaging probe synthesis was isolated as iodofluorinated intermediates in moderate to excellent yields. The one-pot formal alkenylfluorination reaction was carried out to produce over 30 analogues of a wide range of bioactive molecules. Further application of the Pd(0/I/II) manifold in PET probe development was illustrated by the direct carbo(radio)fluorination of electron-rich alkenes. The methods were successfully translated to radiolabel a broad scope of medicinally relevant small molecules in generally good radiochemical conversion. The protocol was further optimized to accommodate no-carrier-added conditions with similar efficiency for future (pre)clinical translation. Moreover, the radiosynthesis of prosthetic groups was automated in a radiochemistry module to facilitate its practical use in multistep radiochemical reactions.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Next Generation Copper Mediators for the Efficient Production of 18 F-Labeled Aromatics

Cu-mediated radiofluorination is a versatile tool for the preparation of ¹⁸ F-labeled (hetero)aromatics. In this work, we systematically evaluated a series of complexes and identified several generally applicable mediators for highly efficient radiofluorination of aryl boronic and stannyl substrates. Utilization of these mediators in nBuOH/DMI or DMI significantly improved ¹⁸ F-labeling yields despite use of lower precursor amounts. Impressively, application of 2.5 μmol aryl boronic acids was sufficient to achieve ¹⁸ F-labeling yields of up to 75 %. The practicality of the novel mediators was demonstrated by efficient production of five PET-tracers and transfer of the method to an automated radiosynthesis module. In addition, (S)-3-[¹⁸ F]FPhe and 6-[¹⁸ F]FDOPA were prepared in activity yields of 23±1 % and 30±3 % using only 2.5 μmol of the corresponding boronic acid or trimethylstannyl precursor.

Mild, Organo-Catalysed Borono-Deamination as a Key to Late-Stage Pharmaceutical Precursors and 18F-Labelled Radiotracers

A tris(pentafluorophenyl)borane catalysed method for the synthesis of boronic acid esters from aromatic amines in yields of up to 93% was devised. Mild conditions, benign reagents, short reaction times, low temperatures and a wide substrate scope characterize the method. The reaction was found applicable to the synthesis of boronic acid ester derivatives of complex drug molecules in up to 86% isolated yield and high purity suitable for labelling. These boronates were subsequently labelled with [¹⁸F]fluoride ion in radiochemical yields of up to 55% with and even without isolation of the boronate-intermediate.

Recent Advances in Synthetic Methodologies to Form C-18F Bonds

Positron emission tomography (PET) is an important technique for the early diagnosis of disease. Due to the specific physical and chemical properties of Fluorine-18, this important isotope is widely used in PET for labelling and molecular imaging, and its introduction into medicine molecules could produce PET tracers. Developing with the development of organic synthetic methodologies, the introduction of Fluorine-18 into drug molecules efficiently and rapidly under mild conditions, and the formation of C-¹⁸F chemical bonds, has become one of the leading topics in both organic synthetic chemistry and radiochemistry. In this mini-review, we review a series of recent advances in the organic synthesis of C-¹⁸F bonds (2015–2021), including non-catalytic radiofluorinations via good leaving functional groups, transition metal-catalyzed radiofluorinations, and photo- or electro-catalytic synthetic radiofluorinations. As a result of the remarkable advancements in this field, organic synthetic methods for forming C-¹⁸F bonds are expected to continue growing.

multi-patient dose synthesis of [18F]Flumazenil via a copper-mediated 18F-fluorination

Background

Flumazenil (FMZ) is a functionally silent imidazobenzodiazepine which binds to the benzodiazepine binding site of approximately 75% of the brain γ-aminobutyric acid-A receptors (GABA_ARs). Positron Emission Tomography (PET) imaging of the GABAARs with [¹¹C]FMZ has been used to evidence alterations in neuronal density, to assess target engagement of novel pharmacological agents, and to study disorders such as epilepsy and Huntington’s disease. Despite the potential of FMZ PET imaging the short half-life (t_1/2) of carbon-11 (20 min) has limited the more widespread clinical use of [¹¹C]FMZ. The fluorine-18 (¹⁸F) isotopologue with a longer t_1/2 (110 min) is ideally suited to address this drawback. However, the majority of current radiochemical methods for the synthesis of [¹⁸F]FMZ are non-trivial and low yielding. We report a robust, automated protocol that is good manufacturing practice (GMP) compatible, and yields multi-patient doses of [¹⁸F]FMZ.

Results

The fully automated synthesis was developed on the Trasis AllinOne (AIO) platform using a single-use cassette. [¹⁸F]FMZ was synthesized in a one-step procedure from [¹⁸F]fluoride, via a copper-mediated ¹⁸F-fluorination of a boronate ester precursor. Purification was performed by semi-preparative radio-HPLC and the collected fraction formulated directly into the final product vial. The overall process from start of synthesis to delivery of product is approximately 55 min. Starting with an initial activity of 23.6 ± 5.8 GBq (n = 3) activity yields of [¹⁸F]FMZ were 8.0 ± 1 GBq (n = 3). The synthesis was successfully reproduced at two independent sites, where the product passed quality control release criteria in line with the European Pharmacopoeia standards and ICH Q3D(R1) guidelines to be suitable for human use.

Conclusion

Reported is a fully automated cassette-based synthesis of [¹⁸F]FMZ that is Good Manufacturing Practice (GMP) compatible and produces multi-patient doses of [¹⁸F]FMZ.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00158-z.

Targeting Amyloids with [18F]AV-45 for Medullary Thyroid Carcinoma Positron Emission Tomography/Computed Tomography Imaging: A Pilot Clinical Study

Medullary thyroid carcinoma (MTC) is a malignant neuroendocrine tumor with a high recurrence rate. Amyloid plaques formed from the misfolding of calcitonin are the key characteristics of MTC. Herein, we conducted a first-in-human pilot clinical study by applying a β-amyloid-specific radiotracer, [¹⁸F]AV-45, to positron emission tomography (PET)/computed tomography (CT) imaging of MTC. The presence of amyloid plaques in the tumor tissue sections from five MTC patients was confirmed by hematoxylin and eosin (H&E) and Congo Red staining. [¹⁸F]AV-45 selectively accumulated in the amyloid plaques in the continued tumor tissue sections with similar distribution patterns to the H&E and Congo Red staining. In addition, the [¹⁸F]AV-45 uptake can be largely blocked by its nonradioactive reference compound. The [¹⁸F]AV-45 accumulation in the thyroid, neck lymph nodes, and muscles in healthy human subjects is close to the background indicated by PET/CT imaging. In the comparison PET/CT imaging study of a recurrent MTC patient, 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) showed an elevated uptake by multiple neck lymph nodes. In contrast, only one of these neck lymph nodes had increased [¹⁸F]AV-45 uptake. Postoperative histopathological analysis confirmed the [¹⁸F]AV-45 PET-positive lymph node as MTC with amyloid deposition, while other [¹⁸F]FDG positive lymph nodes were free from MTC and amyloid plaques. Thus, [¹⁸F]AV-45 showed the promise for the clinical PET/CT imaging of MTC.

Arene radiofluorination enabled by photoredox-mediated halide interconversion

Positron emission tomography (PET) is a powerful imaging technology that can visualize and measure metabolic processes in vivo and/or obtain unique information about drug candidates. The identification of new and improved molecular probes plays a critical role in PET, but its progress is somewhat limited due to the lack of efficient and simple labelling methods to modify biologically active small molecules and/or drugs. Current methods to radiofluorinate unactivated arenes are still relatively limited, especially in a simple and site-selective way. Here we disclose a method for constructing C-¹⁸F bonds through direct halide/¹⁸F conversion in electron-rich halo(hetero)arenes. [¹⁸F]F^- is introduced into a broad spectrum of readily available aryl halide precursors in a site-selective manner under mild photoredox conditions. Notably, our direct ¹⁹F/¹⁸F exchange method enables rapid PET probe diversification through the preparation and evaluation of an [¹⁸F]-labelled O-methyl tyrosine library. This strategy also results in the high-yielding synthesis of the widely used PET agent L-[¹⁸F]FDOPA from a readily available L-FDOPA analogue.

Enzymatic synthesis of fluorinated compounds

Fluorinated compounds are widely used in the fields of molecular imaging, pharmaceuticals, and materials. Fluorinated natural products in nature are rare, and the introduction of fluorine atoms into organic compound molecules can give these compounds new functions and make them have better performance. Therefore, the synthesis of fluorides has attracted more and more attention from biologists and chemists. Even so, achieving selective fluorination is still a huge challenge under mild conditions. In this review, the research progress of enzymatic synthesis of fluorinated compounds is summarized since 2015, including cytochrome P450 enzymes, aldolases, fluoroacetyl coenzyme A thioesterases, lipases, transaminases, reductive aminases, purine nucleoside phosphorylases, polyketide synthases, fluoroacetate dehalogenases, tyrosine phenol-lyases, glycosidases, fluorinases, and multienzyme system. Of all enzyme-catalyzed synthesis methods, the direct formation of the C-F bond by fluorinase is the most effective and promising method. The structure and catalytic mechanism of fluorinase are introduced to understand fluorobiochemistry. Furthermore, the distribution, applications, and future development trends of fluorinated compounds are also outlined. Hopefully, this review will help researchers to understand the significance of enzymatic methods for the synthesis of fluorinated compounds and find or create excellent fluoride synthase in future research.Key points• Fluorinated compounds are distributed in plants and microorganisms, and are used in imaging, medicine, materials science.• Enzyme catalysis is essential for the synthesis of fluorinated compounds.• The loop structure of fluorinase is the key to forming the C-F bond.

SNAr Radiofluorination with In Situ Generated [18F]Tetramethylammonium Fluoride

This report describes a method for the nucleophilic radiofluorination of (hetero)aryl chlorides, (hetero)aryl triflates, and nitroarenes using a combination of [¹⁸F]KF·K_2.2.2 and Me₄NHCO₃ for the in situ formation of a strongly nucleophilic fluorinating reagent (proposed to be [¹⁸F]Me₄NF). This method is applied to 24 substrates bearing diverse functional groups, and it generates [¹⁸F](hetero)aryl fluoride products in good to excellent radiochemical yields in the presence of ambient air/moisture. The reaction is applied to the preparation of ¹⁸F-labeled HQ-415 for potential (pre)clinical use.

Closing the gap between 19F and 18F chemistry

Positron emission tomography (PET) has become an invaluable tool for drug discovery and diagnosis. The positron-emitting radionuclide fluorine-18 is frequently used in PET radiopharmaceuticals due to its advantageous characteristics; hence, methods streamlining access to 18F-labelled radiotracers can make a direct impact in medicine. For many years, access to 18F-labelled radiotracers was limited by the paucity of methodologies available, and the poor diversity of precursors amenable to 18F-incorporation. During the last two decades, 18F-radiochemistry has progressed at a fast pace with the appearance of numerous methodologies for late-stage 18F-incorporation onto complex molecules from a range of readily available precursors including those that do not require pre-functionalisation. Key to these advances is the inclusion of new activation modes to facilitate 18F-incorporation. Specifically, new advances in late-stage 19F-fluorination under transition metal catalysis, photoredox catalysis, and organocatalysis combined with the availability of novel 18F-labelled fluorination reagents have enabled the invention of novel processes for 18F-incorporation onto complex (bio)molecules. This review describes these major breakthroughs with a focus on methodologies for C-18F bond formation. This reinvigorated interest in 18F-radiochemistry that we have witnessed in recent years has made a direct impact on 19F-chemistry with many laboratories refocusing their efforts on the development of methods using nucleophilic fluoride instead of fluorination reagents derived from molecular fluorine gas.

HPLC-free and cassette-based nucleophilic production of [18F]FDOPA for clinical use

Radiotracer 3,4-dihydroxy-6-[¹⁸F]fluoro-L-phenylalanine (L-6-[¹⁸F]fluorodopa or [¹⁸F]FDOPA) is widely used for PET imaging of dopamine metabolism in several diseases including Parkinson's Disease, brain tumor, neuroendocrine tumors, and focal hyperinsulinism of infancy. In 2019, [¹⁸F]FDOPA was approved by US FDA for detection of dopaminergic nerve terminals in the striatum of adult patients with suspected Parkinsonian Syndromes. A convenient and reliable method is desired for fully automated production of [¹⁸F]FDOPA under cGMP compliance to meet the increasing clinical need. In this study, we reported a cassette-based automated production of [¹⁸F]FDOPA using a GE Fastlab 2 module and the quality control (QC) under fully cGMP compliant environment. Briefly, automated radiosynthesis of [¹⁸F]FDOPA was processed via nucleophilic radio-fluorination using FDOPA Fastlab cassette and solid phase extraction (SPE) purification. The QC tests of [¹⁸F]FDOPA, including appearance, pH, half-life, radiochemical purity and identity, enantiomeric purity, chemical impurities, molecular activity, radioactive concentration, filter integrity, endotoxin, and sterility, were conducted at the end of synthesis (EOS) and 8 h after EOS during the validation runs. Three consecutive productions of [¹⁸F]FDOPA were reliably achieved with desired radiochemical yield and high radiochemical/enantiomeric purities and molar activity. The uncorrected radiochemical yields of [¹⁸F]FDOPA were 9.3-9.8% with a total synthesis time of ~140 min. Both radiochemical and enantiomeric purities of [¹⁸F]FDOPA were >99.9% and the molar activities were 2.1-3.9 Ci/μmole at EOS. The full QC results at EOS and 8 h after EOS showed that the produced [¹⁸F]FDOPA met all release criteria for clinical use within 8 hours of expiration time. Three consecutive validation runs and QC results demonstrated the efficacy of cassette-based production of [¹⁸F]FDOPA for routine clinical use.

Radionuclide Imaging for Neuroscience: Current Opinion and Future Directions

This meeting report summarizes a Consultants Meeting that was held at International Atomic Energy Agency headquarters in Vienna to provide an update on radionuclide imaging for neuroscience applications.

GMP production of 6-[18F]Fluoro-L-DOPA for PET/CT imaging by different synthetic routes: a three center experience

BACKGROUND

The radiofluorinated levodopa analogue 6-[¹⁸F]F-L-DOPA (3,4-dihydroxy-6-¹⁸F-L-phenylalanine) is a commonly employed radiotracer for PET/CT imaging of multiple oncological and neurological indications. An unusually large number of different radiosyntheses have been published to the point where two different Ph. Eur. monographs exist depending on whether the chemistry relies on electrophilic or nucleophilic radiosubstitution of appropriate chemical precursors. For new PET imaging sites wishing to adopt [¹⁸F]FDOPA into clinical practice, selecting the appropriate production process may be difficult and dependent on the clinical needs of the site.

METHODS

Data from four years of [¹⁸F]FDOPA production at three different clinical sites are collected and compared. These three sites, Aarhus University Hospital (AUH), Odense University Hospital (OUH), and Herlev University Hospital (HUH), produce the radiotracer by different radiosynthetic routes with AUH adopting an electrophilic strategy, while OUH and HUH employ two different nucleophilic approaches. Production failure rates, radiochemical yields, and molar activities are compared across sites and time. Additionally, the clinical use of the radiotracer over the time period considered at the different sites are presented and discussed.

RESULTS

The electrophilic substitution route suffers from being demanding in terms of cyclotron operation and maintenance. This challenge, however, was found to be compensated by a production failure rate significantly below that of both nucleophilic approaches; a result of simpler chemistry. The five-step nucleophilic approach employed at HUH produces superior radiochemical yields compared to the three-step approach adopted at OUH but suffers from the need for more comprehensive synthesis equipment given the multi-step nature of the procedure, including HPLC purification. While the procedure at OUH furnishes the lowest radiochemical yield of the synthetic routes considered, it produces the highest molar activity. This is of importance across the clinical applications of the tracer discussed here, including dopamine synthesis in striatum of subjects with schizophrenia and congenital hyperinsulinism in infants.

CONCLUSION

For most sites either of the two nucleophilic substitution strategies should be favored. However, which of the two will depend on whether a given site wishes to optimize the radiochemical yield or the ease of the use.

Sequential Ir/Cu-Mediated Method for the Meta-Selective C-H Radiofluorination of (Hetero)Arenes

This article describes a sequential Ir/Cu-mediated process for the meta-selective C-H radiofluorination of (hetero)arene substrates. In the first step, Ir-catalyzed C(sp²)-H borylation affords (hetero)aryl pinacolboronate (BPin) esters. The intermediate organoboronates are then directly subjected to copper-mediated radiofluorination with [¹⁸F]tetrabutylammonium fluoride to afford fluorine-18 labeled (hetero)arenes in high radiochemical yield and radiochemical purity. This entire process is performed on a benchtop without Schlenk or glovebox techniques and circumvents the need to isolate (hetero)aryl boronate esters. The reaction was automated on a TracerLab FX_FN module with 1,3-dimethoxybenzene and a meta-tyrosine derivative. The products, [¹⁸F]1-fluoro-3,5-dimethoxybenzene and an ¹⁸F-labeled meta-tyrosine derivative, were obtained in 37 ± 5% isolated radiochemical yield and >99% radiochemical purity and 25% isolated radiochemical yield and 99% radiochemical purity, and 0.52 Ci/μmol (19.24 GBq/μmol) molar activity (A_m), respectively.

11C- and 18F-Radiotracers for In Vivo Imaging of the Dopamine System: Past, Present and Future

The applications of positron emission tomography (PET) imaging to study brain biochemistry, and in particular the aspects of dopamine neurotransmission, have grown significantly over the 40 years since the first successful in vivo imaging studies in humans. In vivo PET imaging of dopaminergic functions of the central nervous system (CNS) including dopamine synthesis, vesicular storage, synaptic release and receptor binding, and reuptake processes, are now routinely used for studies in neurology, psychiatry, drug abuse and addiction, and drug development. Underlying these advances in PET imaging has been the development of the unique radiotracers labeled with positron-emitting radionuclides such as carbon-11 and fluorine-18. This review focuses on a selection of the more accepted and utilized PET radiotracers currently available, with a look at their past, present and future.

Cyclotron-based production of 68Ga, [68Ga]GaCl3, and [68Ga]Ga-PSMA-11 from a liquid target

PURPOSE

To optimize the direct production of 68Ga on a cyclotron, via the 68Zn(p,n)68Ga reaction using a liquid cyclotron target. We Investigated the yield of cyclotron-produced 68Ga, extraction of [68Ga]GaCl3 and subsequent [68Ga]Ga-PSMA-11 labeling using an automated synthesis module.

METHODS

Irradiations of a 1.0 M solution of [68Zn]Zn(NO3)2 in dilute (0.2-0.3 M) HNO3 were conducted using GE PETtrace cyclotrons and GE 68Ga liquid targets. The proton beam energy was degraded to a nominal 14.3 MeV to minimize the co-production of 67Ga through the 68Zn(p,2n)67Ga reaction without unduly compromising 68Ga yields. We also evaluated the effects of varying beam times (50-75 min) and beam currents (27-40 μA). Crude 68Ga production was measured. The extraction of [68Ga]GaCl3 was performed using a 2 column solid phase method on the GE FASTlab Developer platform. Extracted [68Ga]GaCl3 was used to label [68Ga]Ga-PSMA-11 that was intended for clinical use.

RESULTS

The decay corrected yield of 68Ga at EOB was typically > 3.7 GBq (100 mCi) for a 60 min beam, with irradiations of [68Zn]Zn(NO3)2 at 0.3 M HNO3. Target/chemistry performance was more consistent when compared with 0.2 M HNO3. Radionuclidic purity of 68Ga was typically > 99.8% at EOB and met the requirements specified in the European Pharmacopoeia (< 2% combined 66/67Ga) for a practical clinical product shelf-life. The activity yield of [68Ga]GaCl3 was typically > 50% (~ 1.85 GBq, 50 mCi); yields improved as processes were optimized. Labeling yields for [68Ga]Ga-PSMA-11 were near quantitative (~ 1.67 GBq, 45 mCi) at EOS. Cyclotron produced [68Ga]Ga-PSMA-11 underwent full quality control, stability and sterility testing, and was implemented for human use at the University of Michigan as an Investigational New Drug through the US FDA and also at the Royal Prince Alfred Hospital (RPA).

CONCLUSION

Direct cyclotron irradiation of a liquid target provides clinically relevant quantities of [68Ga]Ga-PSMA-11 and is a viable alternative to traditional 68Ge/68Ga generators.

Use of 55 PET radiotracers under approval of a Radioactive Drug Research Committee (RDRC)

BACKGROUND

In the US, EU and elsewhere, basic clinical research studies with positron emission tomography (PET) radiotracers that are generally recognized as safe and effective (GRASE) can often be conducted under institutional approval. For example, in the United States, such research is conducted under the oversight of a Radioactive Drug Research Committee (RDRC) as long as certain requirements are met. Firstly, the research must be for basic science and cannot be intended for immediate therapeutic or diagnostic purposes, or to determine the safety and effectiveness of the PET radiotracer. Secondly, the PET radiotracer must be generally recognized as safe and effective. Specifically, the mass dose to be administered must not cause any clinically detectable pharmacological effect in humans, and the radiation dose to be administered must be the smallest dose practical to perform the study and not exceed regulatory dose limits within a 1-year period. In our experience, the main barrier to using a PET radiotracer under RDRC approval is accessing the required information about mass and radioactive dosing.

RESULTS

The University of Michigan (UM) has a long history of using PET radiotracers in clinical research studies. Herein we provide dosing information for 55 radiotracers that will enable other PET Centers to use them under the approval of their own RDRC committees.

CONCLUSIONS

The data provided herein will streamline future RDRC approval, and facilitate further basic science investigation of 55 PET radiotracers that target functionally relevant biomarkers in high impact disease states.

A spot test for determination of residual TBA levels in 18F-radiotracers for human use using Dragendorff reagent

When utilizing [¹⁸F]tetrabutylammonium fluoride ([¹⁸F]TBAF) in the synthesis of ¹⁸F-labeled radiotracers for clinical positron emission tomography (PET) imaging, it is necessary to confirm that residual TBA levels in formulated doses do not exceed established specifications (≤2.6 mg per patient dose). Historically this has been accomplished using HPLC, but this is time consuming for short-lived PET radiotracers and limited by the need for expensive equipment. This motivated us to introduce a TLC spot test for determining residual TBA, and we have developed a new method which employs the Dragendorff reagent. Herein we report details of the TLC method and use it to quantify residual TBA in different formulations of 6-[¹⁸F]fluoro-DOPA.

Nucleophilic Synthesis of 6-l-[18F]FDOPA. Is Copper-Mediated Radiofluorination the Answer?

Positron emission tomography employing 6-l-[18F]fluoro-3,4-dihydroxyphenylalanine (6-l-[18F]FDOPA) is currently a highly relevant clinical tool for detection of gliomas, neuroendocrine tumors and evaluation of Parkinson's disease progression. Yet, the deficiencies of electrophilic synthesis of 6-l-[18F]FDOPA hold back its wider use. To fulfill growing clinical demands for this radiotracer, novel synthetic strategies via direct nucleophilic 18F-radiloabeling starting from multi-Curie amounts of [18F]fluoride, have been recently introduced. In particular, Cu-mediated radiofluorination of arylpinacol boronates and arylstannanes show significant promise for introduction into clinical practice. In this short review these current developments will be discussed with a focus on their applicability to automation.

Copper-Mediated Late-stage Radiofluorination: Five Years of Impact on Pre-clinical and Clinical PET Imaging

Purpose

Copper-mediated radiofluorination (CMRF) is emerging as the method of choice for the formation of aromatic C-¹⁸F bonds. This minireview examines proof-of-concept, pre-clinical, and in-human imaging studies of new and established imaging agents containing aromatic C-¹⁸F bonds synthesized with CMRF. An exhaustive discussion of CMRF methods is not provided, although key developments that have enabled or improved upon the syntheses of fluorine-18 imaging agents are discussed.

Methods

A comprehensive literature search from April 2014 onwards of the Web of Science and PubMed library databases was performed to find reports that utilize CMRF for the synthesis of fluorine-18 radiopharmaceuticals, and these represent the primary body of research discussed in this minireview. Select conference proceedings, previous reports describing alternative methods for the synthesis of imaging agents, and preceding fluorine-19 methodologies have also been included for discussion.

Conclusions

CMRF has significantly expanded the chemical space that is accessible to fluorine-18 radiolabeling with production methods that can meet the regulatory requirements for use in Nuclear Medicine. Furthermore, it has enabled novel and improved syntheses of radiopharmaceuticals and facilitated subsequent PET imaging studies. The rapid adoption of CMRF will undoubtedly continue to simplify the production of imaging agents and inspire the development of new radiofluorination methodologies.