[18 F]Ethenesulfonyl Fluoride as a Practical Radiofluoride Relay Reagent

Microfluidic synthesis of radiotracers: recent developments and commercialization prospects

Positron emission tomography (PET) is a powerful diagnostic tool that holds incredible potential for clinicians to track a wide variety of biological processes using specialized radiotracers. Currently, however, a single radiotracer accounts for over 95% of procedures, largely due to the cost of radiotracer synthesis. Microfluidic platforms provide a solution to this problem by enabling a dose-on-demand pipeline in which a single benchtop platform would synthesize a wide array of radiotracers. In this review, we will explore the field of microfluidic production of radiotracers from early research to current development. Furthermore, the benefits and drawbacks of different microfluidic reactor designs will be analyzed. Lastly, we will discuss the various engineering considerations that must be addressed to create a fully developed, commercially effective platform that can usher the field from research and development to commercialization.

[18F]fluoride Activation and 18F-Labelling in Hydrous Conditions-Towards a Microfluidic Synthesis of PET Radiopharmaceuticals

18F-labelled radiopharmaceuticals are indispensable in positron emission tomography. The critical step in the preparation of 18F-labelled tracers is the anhydrous F-18 nucleophilic substitution reaction, which involves [18F]F- anions generated in aqueous media by the cyclotron. For this, azeotropic drying by distillation is widely used in standard synthesisers, but microfluidic systems are often not compatible with such a process. To avoid this step, several methods compatible with aqueous media have been developed. We summarised the existing approaches and two of them have been studied in detail. [18F]fluoride elution efficiencies have been investigated under different conditions showing high 18F-recovery. Finally, a large scope of precursors has been assessed for radiochemical conversion, and these hydrous labelling techniques have shown their potential for tracer production using a microfluidic approach, more particularly compatible with iMiDEV™ cassette volumes.

Recent Advances of S-18F Radiochemistry for Positron Emission Tomography

The click chemistry of sulfur(VI) fluoride exchange (SuFEx) has facilitated the widespread application of sulfur-fluoride compounds such as sulfonyl fluorides, fluorosulfates, and sulfamoyl fluorides in various fields, especially in the development of 18F ligands for PET (positron emission tomography) imaging. In recent years, the prominent progress of sulfur-[18F]fluoride compounds has been achieved through the combination of 18F and sulfur-fluoride chemistry. These compounds serve as potential 18F tracers, 18F synthons, and reagents for 18F-fluorination, thereby complementing the range of 18F ligands, typically C-18F structures, used in PET studies. This review aims to provide an overview of S-18F labeling reactions through examples of relevant 18F compounds and highlight the advancements and breakthroughs achieved in the past decade.

State of the art procedures towards reactive [18F]fluoride in PET tracer synthesis

BACKGROUND

Positron emission tomography (PET) is a powerful, non-invasive preclinical and clinical nuclear imaging technique used in disease diagnosis and therapy assessment. Fluorine-18 is the predominant radionuclide used for PET tracer synthesis. An impressive variety of new 'late-stage' radiolabeling methodologies for the preparation of 18F-labeled tracers has appeared in order to improve the efficiency of the labeling reaction.

MAIN BODY

Despite these developments, one outstanding challenge into the early key steps of the process remains: the preparation of reactive [18F]fluoride from oxygen-18 enriched water ([18O]H2O). In the last decade, significant changes into the trapping, elution and drying stages have been introduced. This review provides an overview of the strategies and recent developments in the production of reactive [18F]fluoride and its use for radiolabeling.

CONCLUSION

Improved, modified or even completely new fluorine-18 work-up procedures have been developed in the last decade with widespread use in base-sensitive nucleophilic 18F-fluorination reactions. The many promising developments may lead to a few standardized drying methodologies for the routine production of a broad scale of PET tracers.

[18F]Tosyl fluoride as a versatile [18F]fluoride source for the preparation of 18F-labeled radiopharmaceuticals

Positron emission tomography (PET) is an in vivo imaging technology that utilizes positron-emitting radioisotope-labeled compounds as PET radiotracers that are commonly used in clinic and in various research areas, including oncology, cardiology, and neurology. Fluorine-18 is the most widely used PET-radionuclide and commonly produced by proton bombardment of ¹⁸O-enriched water in a cyclotron. The [¹⁸F]fluoride thus obtained generally requires processing by azeotropic drying in order to completely remove H₂O before it can be used for nucleophilic radiofluorination. In general, the drying step is important in facilitating the radiofluorination reactions and the preparation of ¹⁸F-labeled PET radiotracers. In this communication, we have demonstrated the feasibility of using [¹⁸F]tosyl fluoride ([¹⁸F]TsF) as a versatile [¹⁸F]fluoride source for radiofluorination to bypass the azeotropic drying step, and we have developed a continuous flow solid-phase radiosynthesis strategy to generate [¹⁸F]TsF in a form that is excellent for radiofluorination. [¹⁸F]TsF shows high reactivity in radiofluorination and provides the features suitable for preparing PET radiotracers on a small scale and exploring novel radiolabeling technologies. Thus, using [¹⁸F]TsF as a [¹⁸F]fluoride source is a promising strategy that facilitates radiofluorination and provides a convenient and efficient solution for the preparation of ¹⁸F-labeled radiopharmaceuticals that is well matched to the emerging trends in PET imaging technologies.

An Investigation into the In Vitro Metabolic Stability of Aryl Sulfonyl Fluorides for their Application in Medicinal Chemistry and Radiochemistry

Molecules that feature a sulfonyl fluoride (SO₂F) moiety have been gaining increasing interest due to their unique reactivity and potential applications in synthetic chemistry, medicinal chemistry, and other biological uses. A particular interest is towards ¹⁸F-radiochemistry where sulfonyl fluorides can be used as a method to radiolabel biomolecules or can be used as radiofluoride relay reagents that facilitate radiolabeling of other molecules. The low metabolic stability of sulfonyl fluoride S-F bonds, however, presents an issue and limits the applicability of sulfonyl fluorides. The aim of this work was to increase understanding of what features contribute to the metabolic instability of the S-F bond in model aryl sulfonyl fluorides and identify approaches to increasing sulfonyl fluoride stability for ¹⁸F-radiochemistry and other medicinal, synthetic chemistry and biological applications. To undertake this, 14 model aryl sulfonyl fluorides compounds with varying functional groups and substitution patterns were investigated, and their stabilities were examined in various media, including phosphate-buffered saline and rat serum as a model for biological conditions. The results indicate that both electronic and steric factors affect the stability of the S-F bond, with the 2,4,6-trisubstituted model aryl sulfonyl fluorides examined displaying the highest in vitro metabolic stability.

Synthesis and 18F Labeling of Alkenyl Sulfonyl Fluorides via an Unconventional Elimination Pathway

A successful Cu-catalyzed addition of both Cl and SO₂OCF₂H groups into alkenes allows us to discover the unusual reactivity of the SO₂OCF₂H group. As opposed to common sulfonic esters (RSO₂-O-R'), in which the R' group is highly electrophilic, the SO₂ moiety demonstrates higher electrophilicity in RSO₂-OCF₂H. The unexpected reactivity is further developed not only as a synthetic tool for well-functionalized alkenyl sulfonyl fluorides but also for the first ¹⁸F labeling of alkenyl sulfonyl fluorides.

Sulfonyl fluorides as targets and substrates in the development of new synthetic methods

The advent of sulfur(VI)-fluoride exchange (SuFEx) processes as transformations with click-like reactivity has invigorated research into electrophilic species featuring a sulfur-fluorine bond. Among these, sulfonyl fluorides have emerged as the workhorse functional group, with diverse applications being reported. Sulfonyl fluorides are used as electrophilic warheads by both medicinal chemists and chemical biologists. The balance of reactivity and stability that is so attractive for these applications, particularly the resistance of sulfonyl fluorides to hydrolysis under physiological conditions, has provided opportunities for synthetic chemists. New synthetic approaches that start with sulfur-containing substrates include the activation of sulfonamides using pyrilium salts, the deoxygenation of sulfonic acids, and the electrochemical oxidation of thiols. Employing non-sulfur-containing substrates has led to the development of transition-metal-catalysed processes based on palladium, copper and nickel, as well as the use of SO₂F₂ gas as an electrophilic hub. Selectively manipulating molecules that already contain a sulfonyl fluoride group has also proved to be a popular tactic, with metal-catalysed processes again at the fore. Finally, coaxing sulfonyl fluorides to engage with nucleophiles, when required, and under suitable reaction conditions, has led to new activation methods. This Review provides an overview of the challenges in the efficient synthesis and manipulation of these intriguing functional groups.

Aliphatic 18 F-Radiofluorination: Recent Advances in the Labeling of Base-Sensitive Substrates*

Aliphatic fluorine-18 radiolabeling is the most commonly used method to synthesize tracers for PET-imaging. With an increasing demand for ¹⁸ F-radiotracers for clinical applications, new labeling strategies aiming to increase radiochemical yields of established tracers or, more importantly, to enable ¹⁸ F-labeling of new scaffolds have been developed. In recent years, increased attention has been focused on the direct aliphatic ¹⁸ F-fluorination of base-sensitive substrates in this respect. This minireview gives a concise overview of the recent advances within this field and aims to highlight the advantages and limitations of these methods.

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

Synthesis of 18F-Labeled Aryl Fluorosulfates via Nucleophilic Radiofluorination

Sulfuryl fluoride gas is a key reagent for SO₂F transfer. However, conventional SO₂F transfer reactions have limited ¹⁸F-radiochemistry translation, due to the inaccessibility of gaseous [¹⁸F]SO₂F₂. Herein, we report the first SO₂F₂-free synthesis of aryl [¹⁸F]fluorosulfates from both phenolic and isolated aryl imidazylate precursors with cyclotron-produced ¹⁸F^-. The radiochemical yields ranged from moderate to good with excellent functional group tolerance. The reliability of our approach was validated by the automated radiosynthesis of 4-acetamidophenyl [¹⁸F]fluorosulfate.

Rhenium(i) complexation–dissociation strategy for synthesising fluorine-18 labelled pyridine bidentate radiotracers

A novel fluorine-18 radiolabelling method employing rhenium(i) mediation is described herein. In less than 1 minute, fluorine-18 labelled complexes and ligands were synthesised in greater than 80% and 60% radiochemical yields (RCY), respectively.

Elsinga P. Late-Stage Copper-Catalyzed Radiofluorination of an Arylboronic Ester Derivative of Atorvastatin

There is an unmet need for late-stage 18F-fluorination strategies to label molecules with a wide range of relevant functionalities to medicinal chemistry, in particular (hetero)arenes, aiming to obtain unique in vivo information on the pharmacokinetics/pharmacodynamics (PK/PD) using positron emission tomography (PET). In the last few years, Cu-mediated oxidative radiofluorination of arylboronic esters/acids arose and has been successful in small molecules containing relatively simple (hetero)aromatic groups. However, this technique is sparsely used in the radiosynthesis of clinically significant molecules containing more complex backbones with several aromatic motifs. In this work, we add a new entry to this very limited database by presenting our recent results on the 18F-fluorination of an arylboronic ester derivative of atorvastatin. The moderate average conversion of [18F]F- (12%), in line with what has been reported for similarly complex molecules, stressed an overview through the literature to understand the radiolabeling variables and limitations preventing consistently higher yields. Nevertheless, the current disparity of procedures reported still hampers a consensual and conclusive output.