Insights into Elution of Anion Exchange Cartridges: Opening the Path toward Aliphatic 18F-Radiolabeling of Base-Sensitive Tracers

Sultones for Radiofluorination and Bioconjugation: Fluorine-18 Labeling of Native Proteins and Glycoproteins

Aliphatic nucleophilic substitution of a sulfonate ester group (such as triflate, mesylate, tosylate, or nosylate) represents a prominent reaction in fluorine-18 chemistry, as illustrated by the radiosynthesis of [18F]FDG (fluorodeoxyglucose) routinely produced for clinical imaging by positron emission tomography (PET). In prior studies, radiofluorination of sultones (i.e., cyclic sulfonate esters) was shown to easily afford, by ring opening, [18F]fluorosulfocompounds as a new class of promising hydrophilic radiophamaceuticals. Herein, we first depict a further exploration of the 18F-radiochemistry of sultones, including a comparative study with acyclic sulfonate esters. Propane sultones were found to be highly reactive toward [18F]TBAF (tetra-n-butylammonium fluoride) under mild anhydrous conditions and far more reactive than acyclic analogues (mesylate and tosylate) and butane sultones. We then developed the 18F-labeling of protein (human serum albumin) and glycoprotein (recombinant human erythropoietin) according to a double ring opening strategy from a bispropane sultone involving radiofluorination followed by subsequent bioconjugation in aqueous buffer solution to the ε-amino group in lysine residues. Overall, the results highlight the distinction of propane sultones vs acyclic analogues for radiofluorination, and they confirm the viability of the bispropane sultone as a novel key precursor for the 18F-radiolabeling of biopolymers under biocompatible conditions. In addition, these findings open the way to the development of innovative radiopharmaceuticals that are especially appropriate for in vivo imaging by taking advantage of the anionic sulfo group.

Toward Realization of Bioorthogonal Chemistry in the Clinic

In the last decade, the use of bioorthogonal chemistry toward medical applications has increased tremendously. Besides being useful for the production of pharmaceuticals, the efficient, nontoxic reactions open possibilities for the development of therapies that rely on in vivo chemistry between two bioorthogonal components. Here we discuss the latest developments in bioorthogonal chemistry, with a focus on their use in living organisms, the translation from model systems to humans, and the challenges encountered during preclinical development. We aim to provide the reader a broad presentation of the current state of the art and demonstrate the numerous possibilities that bioorthogonal reactions have for clinical use, now and in the near future.

An 211At-labeled Tetrazine for Pretargeted Therapy

Pretargeted radioimmunoimaging has been shown to enhance tumor-to-background ratios by up to 125-fold at early time points, leading to more efficient and less toxic radionuclide therapies, particularly with shorter half-lives such as astatine-211 (211At). The tetrazine ligation is the most utilized bioorthogonal reaction in these strategies, making tetrazines ideal for 211At labeling and controlling the biodistribution. We developed a 211At-labeled pretargeting agent for alpha-radionuclide therapy, achieving a radiochemical yield of approximately 65% and purity over 99%. Our results showed higher tumor-to-blood ratios within the first 24 h compared to directly labeled monoclonal antibodies. This suggests that pretargeted therapy may deliver better tumor doses than conventional methods, although the deastatination observed will need to be addressed in future tetrazine developments.

Automation of Copper-Mediated 18F-Fluorination of Aryl Pinacol Boronates Using 4-Dimethylaminopyridinium Triflate

Currently, the copper-mediated radiofluorination of aryl pinacol boronates (arylBPin) using the commercially available, air-stable Cu(OTf)2Py4 catalyst is one of the most efficient synthesis approaches, greatly facilitating access to a range of radiotracers, including drug-like molecules with nonactivated aryl scaffolds. Further adjustment of this methodology, in particular, the [18F]fluoride recovery step for the routine preparation of radiotracers, has been the focus of recent research. In our recent study, an organic solution of 4-dimethylaminopyridinium trifluoromethanesulfonate (DMAPOTf) was found to be an efficient PTC for eluting radionuclides retained on the weak anion exchange cartridge, Oasis WAX 1cc, employing the inverse sorption-elution protocol. Notably, the following Cu-mediated radiofluorination of arylBPin precursors in the presence of the Cu(OTf)2(Py)4 catalyst can be performed with high efficiency in the same solvent, bypassing not only the conventional azeotropic drying procedure but any solvent replacement. In the current study, we aimed to translate this methodology, originally developed for remote-controlled operation with manual interventions, into the automated synthesis module on the TRACERlab automation platform. The adjustment of the reagent amounts and solvents allowed for high efficiency in the radiofluorination of a series of model arylBPin substrates on the TRACERlab FXFE Pro synthesis module, which was adapted for nucleophilic radiofluorinations. The practical applicability of the developed radiofluorination approach with DMAPOTf elution was demonstrated in the automated synthesis of 6-L-[18F]FDOPA. The radiotracer was obtained with an activity yield (AY; isolated, not decay-corrected) of 5.2 ± 0.5% (n = 3), with a synthesis time of ca. 70 min on the TRACERlab FX N Pro automation platform. The obtained AY was comparable with one reported by others (6 ± 1%) using the same boronate precursor, while a slightly higher AY of 6-L-[18F]FDOPA (14.5 ± 0.5%) was achieved in our previous work using commercially available Bu4NOTf as the PTC.

A facile synthesis of precursor for the σ-1 receptor PET radioligand [18 F]FTC-146 and its radiofluorination

The σ-1 receptor is a non-opioid transmembrane protein involved in various human pathologies including neurodegenerative diseases, inflammation, and cancer. The previously published ligand [18 F]FTC-146 is among the most promising tools for σ-1 molecular imaging by positron emission tomography (PET), with a potential for application in clinical diagnostics and research. However, the published six- or four-step synthesis of the tosyl ester precursor for its radiosynthesis is complicated and time-consuming. Herein, we present a simple one-step precursor synthesis followed by a one-step fluorine-18 labeling procedure that streamlines the preparation of [18 F]FTC-146. Instead of a tosyl-based precursor, we developed a one-step synthesis of the precursor analog AM-16 containing a chloride leaving group for the SN 2 reaction with 18 F-fluoride. 18 F-fluorination of AM-16 led to a moderate decay-corrected radiochemical yield (RCY = 7.5%) with molar activity (Am ) of 45.9 GBq/μmol. Further optimization of this procedure should enable routine radiopharmaceutical production of this promising PET tracer.

Evaluation of F-537-Tetrazine in a model for brain pretargeting imaging. Comparison to N-(3-[18F] fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine

Brain pretargeted nuclear imaging for the diagnosis of various neurodegenerative diseases is a quickly developing field. The tetrazine ligation is currently the most explored approach to achieve this goal due to its remarkable properties. In this work, we evaluated the performance of F-537-Tetrazine, previously developed by Biogen, and N-(3-[18F]fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine, previously developed in our group, thereby allowing for the direct comparison of these two imaging probes. The evaluation included synthesis, radiolabeling and a comparison of the physicochemical properties of the compounds. Furthermore, their performance was evaluated by in vitro and in vivo pretargeting models. This study indicated that N-(3-[18F] fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine might be more suited for brain pretargeted imaging.

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

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.

Synthesis, Fluorine-18 Radiolabeling, and In Vivo PET Imaging of a Hydrophilic Fluorosulfotetrazine

The development of ¹⁸F-fluorotetrazines, suitable for the radiolabeling of biologics such as proteins and antibodies by IEDDA ligation, represents a major challenge, especially for pre-targeting applications. The hydrophilicity of the tetrazine has clearly become a crucial parameter for the performance of in vivo chemistry. In this study, we present the design, the synthesis, the radiosynthesis, the physicochemical characterization, the in vitro and in vivo stability, as well as the pharmacokinetics and the biodistribution determined by PET imaging in healthy animals of an original hydrophilic ¹⁸F-fluorosulfotetrazine. This tetrazine was prepared and radiolabelled with fluorine-18 according to a three-step procedure, starting from propargylic butanesultone as the precursor. The propargylic sultone was converted into the corresponding propargylic fluorosulfonate by a ring-opening reaction with ^18/19F-fluoride. Propargylic ^18/19F-fluorosulfonate was then subject to a CuACC reaction with an azidotetrazine, followed by oxidation. The overall automated radiosynthesis afforded the ¹⁸F-fluorosulfotetrazine in 29-35% DCY, within 90-95 min. The experimental LogP and LogD_7.4 values of -1.27 ± 0.02 and -1.70 ± 0.02, respectively, confirmed the hydrophilicity of the ¹⁸F-fluorosulfotetrazine. In vitro and in vivo studies displayed a total stability of the ¹⁸F-fluorosulfotetrazine without any traces of metabolization, the absence of non-specific retention in all organs, and the appropriate pharmacokinetics for pre-targeting applications.

[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.

Copper-Mediated Radiobromination of (Hetero)Aryl Boronic Pinacol Esters

A copper-mediated radiobromination of (hetero)aryl boronic pinacol esters is described. Cyclotron-produced [76/77Br]bromide was isolated using an anion exchange cartridge, wherein the pre-equilibration and elution solutions played a critical role in downstream deboro-bromination. The bromination tolerates a broad range of functional groups, labeling molecules with ranging electronic and steric effects. Bologically active radiopharmaceuticals were synthesized, including two radiobrominated inhibitors of poly ADP ribose polymerase, a clinically relevant chemotherapeutic target for ovarian, breast, and prostate cancers.

Pretargeted Imaging beyond the Blood-Brain Barrier-Utopia or Feasible?

Pretargeting is a promising nuclear imaging technique that allows for the usage of antibodies (Abs) with enhanced imaging contrast and reduced patient radiation burden. It is based on bioorthogonal chemistry with the tetrazine ligation-a reaction between trans-cyclooctenes (TCOs) and tetrazines (Tzs)-currently being the most popular reaction due to its high selectivity and reactivity. As Abs can be designed to bind specifically to currently 'undruggable' targets such as protein isoforms or oligomers, which play a crucial role in neurodegenerative diseases, pretargeted imaging beyond the BBB is highly sought after, but has not been achieved yet. A challenge in this respect is that large molecules such as Abs show poor brain uptake. Uptake can be increased by receptor mediated transcytosis; however, it is largely unknown if the achieved brain concentrations are sufficient for pretargeted imaging. In this study, we investigated whether the required concentrations are feasible to reach. As a model Ab, we used the bispecific anti-amyloid beta (Aβ) anti-transferrin receptor (TfR) Ab 3D6scFv8D3 and conjugated it to a different amount of TCOs per Ab and tested different concentrations in vitro. With this model in hand, we estimated the minimum required TCO concentration to achieve a suitable contrast between the high and low binding regions. The estimation was carried out using pretargeted autoradiography on brain sections of an Alzheimer's disease mouse model. Biodistribution studies in wild-type (WT) mice were used to correlate how different TCO/Ab ratios alter the brain uptake. Pretargeted autoradiography showed that increasing the number of TCOs as well as increasing the TCO-Ab concentration increased the imaging contrast. A minimum brain concentration of TCOs for pretargeting purposes was determined to be 10.7 pmol/g in vitro. Biodistribution studies in WT mice showed a brain uptake of 1.1% ID/g using TCO-3D6scFv8D3 with 6.8 TCO/Ab. According to our estimations using the optimal parameters, pretargeted imaging beyond the BBB is not a utopia. Necessary brain TCO concentrations can be reached and are in the same order of magnitude as required to achieve sufficient contrast. This work gives a first estimate that pretargeted imaging is indeed possible with antibodies. This could allow the imaging of currently 'undruggable' targets and therefore be crucial to monitor (e.g., therapies for intractable neurodegenerative diseases).

Optimization of Direct Aromatic 18F-Labeling of Tetrazines

Radiolabeling of tetrazines has gained increasing attention due to their important role in pretargeted imaging or therapy. The most commonly used radionuclide in PET imaging is fluorine-18. For this reason, we have recently developed a method which enables the direct aromatic ¹⁸F-fluorination of tetrazines using stannane precursors through copper-mediated fluorinations. Herein, we further optimized this labeling procedure. 3-(3-fluorophenyl)-1,2,4,5-tetrazine was chosen for this purpose because of its high reactivity and respective limited stability during the labeling process. By optimizing parameters such as elution conditions, precursor amount, catalyst, time or temperature, the radiochemical yield (RCY) could be increased by approximately 30%. These conditions were then applied to optimize the RCY of a recently successfully developed and promising pretargeting imaging agent. This agent could be isolated in a decay corrected RCY of 14 ± 3% and Am of 201 ± 30 GBq/µmol in a synthesis time of 70 min. Consequently, the RCY increased by 27%.

Recent Advances in the Development of Tetrazine Ligation Tools for Pretargeted Nuclear Imaging

Tetrazine ligation has gained interest as a bio-orthogonal chemistry tool within the last decade. In nuclear medicine, tetrazine ligation is currently being explored for pretargeted approaches, which have the potential to revolutionize state-of-the-art theranostic strategies. Pretargeting has been shown to increase target-to-background ratios for radiopharmaceuticals based on nanomedicines, especially within early timeframes. This allows the use of radionuclides with short half-lives which are more suited for clinical applications. Pretargeting bears the potential to increase the therapeutic dose delivered to the target as well as reduce the respective dose to healthy tissue. Combined with the possibility to be applied for diagnostic imaging, pretargeting could be optimal for theranostic approaches. In this review, we highlight efforts that have been made to radiolabel tetrazines with an emphasis on imaging.

Development and Biological Evaluation of the First Highly Potent and Specific Benzamide-Based Radiotracer [18F]BA3 for Imaging of Histone Deacetylases 1 and 2 in Brain

The degree of acetylation of lysine residues on histones influences the accessibility of DNA and, furthermore, the gene expression. Histone deacetylases (HDACs) are overexpressed in various tumour diseases, resulting in the interest in HDAC inhibitors for cancer therapy. The aim of this work is the development of a novel 18F-labelled HDAC1/2-specific inhibitor with a benzamide-based zinc-binding group to visualize these enzymes in brain tumours by positron emission tomography (PET). BA3, exhibiting high inhibitory potency for HDAC1 (IC50 = 4.8 nM) and HDAC2 (IC50 = 39.9 nM), and specificity towards HDAC3 and HDAC6 (specificity ratios >230 and >2080, respectively), was selected for radiofluorination. The two-step one-pot radiosynthesis of [18F]BA3 was performed in a TRACERlab FX2 N radiosynthesizer by a nucleophilic aliphatic substitution reaction. The automated radiosynthesis of [18F]BA3 resulted in a radiochemical yield of 1%, a radiochemical purity of >96% and a molar activity between 21 and 51 GBq/µmol (n = 5, EOS). For the characterization of BA3, in vitro and in vivo experiments were carried out. The results of these pharmacological and pharmacokinetic studies indicate a suitable inhibitory potency of BA3, whereas the applicability for non-invasive imaging of HDAC1/2 by PET requires further optimization of the properties of this compound.

Development of 18F-Labeled Bispyridyl Tetrazines for In Vivo Pretargeted PET Imaging

Pretargeted PET imaging is an emerging and fast-developing method to monitor immuno-oncology strategies. Currently, tetrazine ligation is considered the most promising bioorthogonal reaction for pretargeting in vivo. Recently, we have developed a method to ¹⁸F-label ultrareactive tetrazines by copper-mediated fluorinations. However, bispyridyl tetrazines—one of the most promising structures for in vivo pretargeted applications—were inaccessible using this strategy. We believed that our successful efforts to ¹⁸F-label H-tetrazines using low basic labeling conditions could also be used to label bispyridyl tetrazines via aliphatic nucleophilic substitution. Here, we report the first direct ¹⁸F-labeling of bispyridyl tetrazines, their optimization for in vivo use, as well as their successful application in pretargeted PET imaging. This strategy resulted in the design of [¹⁸F]45, which could be labeled in a satisfactorily radiochemical yield (RCY = 16%), molar activity (A_m = 57 GBq/µmol), and high radiochemical purity (RCP > 98%). The [¹⁸F]45 displayed a target-to-background ratio comparable to previously successfully applied tracers for pretargeted imaging. This study showed that bispyridyl tetrazines can be developed into pretargeted imaging agents. These structures allow an easy chemical modification of ¹⁸F-labeled tetrazines, paving the road toward highly functionalized pretargeting tools. Moreover, bispyridyl tetrazines led to near-instant drug release of iTCO-tetrazine-based ‘click-to-release’ reactions. Consequently, ¹⁸F-labeled bispyridyl tetrazines bear the possibility to quantify such release in vivo in the future.

[11 C]Carboxylated Tetrazines for Facile Labeling of Trans-Cyclooctene-Functionalized PeptoBrushes

Functionalization of macromolecules (antibodies, polymers, nanoparticles) with click-reactive groups greatly enhances the versatility of their potential applications. Click chemistry based on tetrazine - trans-cyclooctene (TCO) ligation is especially promising and is already widely applied for pretargeted imaging and therapy. Indirect radiolabeling of TCO-functionalized macromolecules with substoichiometric amounts of radioactive tetrazines is a convenient way to monitor the fate of those macromolecules by means of positron emission tomography (PET) imaging after their administration into the test subject. In this work, the preparation is reported of TCO-containing graft copolymers, namely PeptoBrushes (polyglutamic acid-graft-polysarcosine), novel [¹¹ C]carboxylated tetrazines, and their combined use in radiolabeling the polymer by inverse electron demand Diels Alder reaction, to investigate it is potential for an application in pretarget imaging or injectable brachytherapy. The procedure for [¹¹ C]tetrazine production is easy and scalable, while indirect TCO-PeptoBrushes labeling with these [¹¹ C]tetrazines is mild, fast, and quantitative. This strategy allows facile ¹¹ C-labeling of diverse TCO-functionalized macromolecules, so that their localization and distribution shortly after injection can be assessed by PET.

Development of the First Aliphatic 18F-Labeled Tetrazine Suitable for Pretargeted PET Imaging-Expanding the Bioorthogonal Tool Box

Pretargeted imaging of nanomedicines have attracted considerable interest because it has the potential to increase imaging contrast while reducing radiation burden to healthy tissue. Currently, the tetrazine ligation is the fastest bioorthogonal reaction for this strategy and, consequently, the state-of-art choice for in vivo chemistry. We have recently identified key properties for tetrazines in pretargeting. We have also developed a method to ¹⁸F-label reactive tetrazines using an aliphatic nucleophilic substitution strategy. Here, we combined this knowledge and developed an ¹⁸F-labeled tetrazine for pretargeted imaging. In order to develop this ligand, a small SAR study was performed. The most promising compound was selected for labeling and subsequent positron-emission-tomography in vivo imaging. Radiolabeling was achieved in satisfactory yields, molar activities, and high radiochemical purities. [¹⁸F]15 displayed favorable pharmacokinetics and remarkable target-to-background ratios-as early as 1 h post injection. We believe that this agent could be a promising candidate for translation into clinical studies.