Opportunities for fundamental physics research with radioactive molecules.

Molecules containing short-lived, radioactive nuclei are uniquely positioned to enable a wide range of scientific discoveries in the areas of fundamental symmetries, astrophysics, nuclear structure, and chemistry. Recent advances in the ability to create, cool, and control complex molecules down to the quantum level, along with recent and upcoming advances in radioactive species production at several facilities around the world, create a compelling opportunity to coordinate and combine these efforts to bring precision measurement and control to molecules containing extreme nuclei. In this manuscript, we review the scientific case for studying radioactive molecules, discuss recent atomic, molecular, nuclear, astrophysical, and chemical advances which provide the foundation for their study, describe the facilities where these species are and will be produced, and provide an outlook for the future of this nascent field.

PET Ligands for Imaging Mutant Huntingtin Aggregates: A Case Study in Non-For-Profit Scientific Management.

Positron emission tomography imaging of misfolded proteins with high-affinity and selective radioligands has played a vital role in expanding our knowledge of neurodegenerative diseases such as Parkinson's and Alzheimer's disease. The pathogenesis of Huntington's disease, a CAG trinucleotide repeat disorder, is similarly linked to the presence of protein fibrils formed from mutant huntingtin (mHTT) protein. Development of mHTT fibril-specific radioligands has been limited by the lack of structural knowledge around mHTT and a dearth of available hit compounds for medicinal chemistry refinement. Over the past decade, the CHDI Foundation, a non-for-profit scientific management organisation has orchestrated a large-scale screen of small molecules to identify high affinity ligands of mHTT, with lead compounds now reaching clinical maturity. Here we describe the mHTT radioligands developed to date and opportunities for further improvement of this radiotracer class.

Mechanistic Insights into the Effect of Sodium Iodide on Copper-Mediated Iododeboronation.

The copper-catalyzed Chan-Evans-Lam (CEL) coupling reaction advances carbon-heteroatom cross-coupling and has facilitated the development of radiohalogenation methodologies in radiochemistry. This study investigated the mechanisms and side reactions of CEL iodination under conditions relevant to radiosynthesis and typical organic synthesis, focusing on the effects of sodium iodide. The concentrations of copper and iodide, as well as the copper-to-iodide ratio, were identified as significant factors for successful copper-mediated CEL iodination, influencing the reaction mechanisms and side reactions. Excess iodide relative to the copper salt led to the formation of poorly soluble iodinated copper(I) complexes that competed with that of the desired aryl iodide. Additionally, the predominant copper complex involved in the catalytic cycle differed between the early and late stages of the reaction, depending on the copper-to-iodide ratio. The results of this study indicate that the specialized radiosynthesis conditions meet the requirements for efficient CEL iodination. In particular, an extremely low concentration of iodide is optimal for CEL iodination. These in-depth mechanistic insights not only provide a detailed comparison of CEL iodination across radiochemistry and synthetic organic chemistry but can also inspire the development of novel (radio)iodination methods.

Bifunctional Hexadentate Pyclen-based Chelating Agent for Mild Radiofluorination in Aqueous Solution at Room Temperature with a Ga-18F Ternary Complex.

Positron Emission Tomography (PET) is used in oncology for tumor diagnosis, commonly relying on fluorine-18 (18F) emission detection. The conventional method of 18F incorporation on to probes by covalent bonding is harsh for sensitive biomolecules, which are nonetheless compounds of choice for the development of targeted probes. This study explores gallium-18F (Ga18F) coordination, a milder alternative method occurring in aqueous media at the final stage of radiosyntheses. Pyclen-based chelating agents were proposed to capture (GaF) species at room temperature and pH ≥ 5 making the radiofluorination process compatible with heat- and acid-sensitive biomolecules. Highly promising results were obtained with the PC2A-based chelating agent LH2 derived from the new bifunctional PC2A-OAE-NCS compound. The solid-state structure of GaF(L) was elucidated by X-ray diffraction and revealed an unconventional heptacoordination of Ga(III). A high radiochemical conversion (RCC) of 86% was achieved at room temperature, in water at pH 5 within 20 minutes. Stability studies showed the robustness of the GaF(L) complex in aqueous media for at least one day and at least one hour for the radiolabeled analog Ga18F(L). These findings demonstrated that PC2A-based compounds are chelating agents of choice for (Ga18F) species, suggesting a real technological breakthrough for PET imaging and precision medicine.

Synthesis of K+ channel radioligand [18F]5-methyl-3-fluoro-4-aminopyridine and PET imaging in mice.

[18F]3-fluoro-4-aminopyridine ([18F]3F4AP) is the first positron emission tomography (PET) radioligand that targets voltage-gated potassium (K+) channels in the brain for imaging demyelination. [18F]3F4AP exhibits high brain penetration, favorable kinetics for PET imaging, and high sensitivity to demyelinating lesions. However, recent studies in awake human subjects indicate lower metabolic stability than in anesthetized animals, resulting in reduced brain uptake. Therefore, there is a need for novel radioligands for K+ channels with suitable pharmacological properties and enhanced metabolic stability. Recent in vitro studies demonstrate that 5-methyl-3-fluoro-4-aminopyridine (5Me3F4AP) exhibits comparable binding affinity to K+ channels, pKa, logD, and membrane permeability as 3F4AP, and a slower enzymatic metabolic rate, suggesting its potential as a K+ channel PET tracer. In this study, we describe the radiochemical synthesis of [18F]5Me3F4AP using an isotope exchange method from the corresponding 3-fluoro-5-methyl-4-nitropyridine N-oxide, followed by a palladium on carbon mediated hydrogenation of the nitro and N-oxide groups. This method yielded [18F]5Me3F4AP with high purity and acceptable molar activity. PET/CT studies using naïve mice demonstrate that [18F]5Me3F4AP effectively crosses the blood-brain barrier and has comparable kinetics to [18F]3F4AP. These findings strongly suggest that [18F]5Me3F4AP is a promising candidate for neuroimaging applications and warrant further studies to investigate its sensitivity to lesions and in vivo metabolic stability.

Uranium contamination of bivalve Mytilus galloprovincialis, speciation and localization.

Uranium is a natural radioelement (also a model for heavier actinides), but may be released through anthropogenic activities. In order to assess its environmental impact in a given ecosystem, such as the marine system, it is essential to understand its distribution and speciation, and also to quantify its bioaccumulation. Our objective was to improve our understanding of the transfer and accumulation of uranium in marine biota with mussels taken here as sentinel species because of their sedentary nature and ability to filter seawater. We report here on the investigation of uranium accumulation, speciation, and localization in Mytilus galloprovincialis using a combination of several analytical (Inductively Coupled Plasma Mass Spectrometry, ICP-MS), spectroscopic (X ray Absorption Spectroscopy, XAS, Time Resolved Laser Induced Fluorescence Spectroscopy, TRLIFS), and imaging (Transmission Electron Microscopy, TEM, µ-XAS, Secondary Ion Mass Spectrometry, SIMS) techniques. Two cohorts of mussels from the Toulon Naval Base and the Villefranche-sur-Mer location were studied. The measurement of uranium Concentration Factor (CF) values show a clear trend in the organs of M. galloprovincialis: hepatopancreas â‰« gill > body ≥ mantle > foot. Although CF values for the entire mussel are comparable for TNB and VFM, hepatopancreas values show a significant increase in those from Toulon versus Villefranche-sur-Mer. Two organs of interest were selected for further spectroscopic investigations: the byssus and the hepatopancreas. In both cases, U(VI) (uranyl) is accumulated in a diffuse pattern, most probably linked to protein complexing functions, with the absence of a condensed phase. While such speciation studies on marine organisms can be challenging, they are an essential step for deciphering the impact of metallic radionuclides on the marine biota in the case of accidental release. Following our assumptions on uranyl speciation in both byssus and hepatopancreas, further steps will include the inventory and identification of the proteins or metabolites involved.

Radiometric approaches with carbon-14-labeled molecules for determining herbicide fate in plant systems.

Weeds cause economic losses in cropping systems, leading to the use of 1.7 million tons of herbicides worldwide for weed control annually. Once in the environment, herbicides can reach non-target organisms, causing negative impacts on the ecosystem. Herbicide retention, transport, and degradation processes determine their environmental fate and are essential to assure the safety of these molecules. Radiometric strategies using carbon-14 herbicides (14C) are suitable approaches for determining herbicide absorption, translocation, degradation, retention, and transport in soil, plants, and water. In this work, we demonstrate how 14C-herbicides can be used from different perspectives. Our work focused on herbicide-plant-environment interactions when the herbicide is applied (a) through the leaf, (b) in the soil, and (c) in the water. We also quantified the mass balance in each experiment. 14C-mesotrione foliar absorption increased with oil and adjuvant addition (5-6 % to 25-46 %), and translocation increased only with adjuvant. More than 80 % of 14C-quinclorac and 14C-indaziflam remained in the soil and cover crops species absorbed less than 20 % of the total herbicides applied. In water systems, Salvinia spp. plants removed 10-18 % of atrazine from the water. Atrazine metabolism was not influenced by the presence of the plants. The radiometric strategies used were able to quantify the fate of the herbicide in different plant systems and the mass balance varied from 70 % to 130 %. Importantly, we highlight a critical and practical view of tracking herbicides in different matrices. This technique can aid scientists to explore other pesticides as environmental contaminants.

Improved Radiosynthesis and Automation of [11C]2-(2,6-Difluoro-4-((2-(N-methylphenylsulfonamido)ethyl)thio)phenoxy)acetamide ([11C]K2) for Positron Emission Tomography of the Glutamate α-Amino-3-hydroxy-5-methyl-4-isoxazole Propionic Acid (AMPA) Receptor.

A new automated radiosynthesis of [11C]2-(2,6-difluoro-4-((2-(N-methylphenylsulfonamido)ethyl)thio)phenoxy)acetamide ([11C]K2), a radiopharmaceutical for the glutamate α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor, is reported. Although manual syntheses have been described, these are unsuitable for routine production of larger batches of [11C]K2 for (pre)clinical PET imaging applications. To meet demands for the imaging agent from our functional neuroimaging collaborators, herein, we report a current good manufacturing practice (cGMP)-compliant synthesis of [11C]K2 using a commercial synthesis module. The new synthesis is fully automated and has been validated for clinical use. The total synthesis time is 33 min from end of bombardment, and the production method provides 2.66 ± 0.3 GBq (71.9 ± 8.6 mCi) of [11C]K2 in 97.7 ± 0.5% radiochemical purity and 754.1 ± 231.5 TBq/mmol (20,382.7 ± 6256.1 Ci/mmol) molar activity (n = 3). Batches passed all requisite quality control testing confirming suitability for clinical use.

Open-source flow setup for rapid and efficient [18 F]fluoride drying for automation of PET tracer syntheses.

One of the key strategies for radiochemical research facilities is the automation of synthesis processes. Unnecessary manual operations increase the radiation exposure of personnel, while simultaneously threatening the reliability of syntheses. We have previously reported an affordable open-source system comprising 3D-printed continuous flow reactors, a custom syringe pump, and a pressure regulator that can be used to perform radiofluorinations. In this paper, we address additional essential processes that are needed for radiotracer development and synthesis, with the aim of making laboratory work safer and research more efficient. We have designed and evaluated a fully automated system for rapidly and effectively processing and drying aqueous [18 F]fluoride that can be directly connected to the cyclotron. This process relies on triflyl fluoride gas generation and allows nucleophilic [18 F]fluoride to be prepared safely in a hotcell within 10 min and an activity recovery of 91.7 ± 1.6% (n = 5). Owing to the need for convenient radiofluorinated prosthetic ligands, we have adapted our continuous flow system to produce [18 F]fluoroethyl tosylate (FEOTs) and [18 F]fluoroethyl triflate (FEOTf), prosthetic groups that are widely used for late-stage fluoroethylation of PET tracers. The processes as well as the radiolabeling of different groups are compared and comprehensively discussed. Having a method providing [18 F]fluoroethyl tosylate (FEOTs) as well as [18 F]fluoroethyl triflate (FEOTf) quickly and highly efficiently is beneficial for radiochemical research.

Exploring microtubule dynamics in Alzheimer's disease: Longitudinal assessment using [11C]MPC-6827 PET imaging in rodent models of Alzheimer's-related pathology.

INTRODUCTION: Microtubule (MT) stability is crucial for proper neuronal function. Understanding MT dysregulation is critical for connecting amyloid beta (Aß) and tau-based degenerative events and early changes in presymptomatic Alzheimer's disease (AD). Herein we present positron emission tomography (PET) imaging properties of our MT-PET radiotracer, [11C]MPC-6827, in multiple established AD mouse models. METHODS: Longitudinal PET, biodistribution, autoradiography, immunohistochemistry, and behavioral studies were conducted at multiple time points in APPswe/PSEN1dE9 (APP/PS1), P301S-PS19 (P301S), 5xFAD, and age-matched control mice. RESULTS: Longitudinal [11C]MPC-6827 brain imaging showed significant increases in APP/PS1, P301S, and 5xFAD mice compared to controls. Longitudinal MT-PET correlated positively with biodistribution, autoradiography, and immunohistochemistry results and negatively with behavior data. DISCUSSION: Our study demonstrated significant longitudinal [11C]MPC-6827 PET increases in multiple AD mouse models for the first time. Strong correlations between PET and biomarker data underscored the interplay of MT destabilization, amyloid, and tau pathology in AD. These results suggest [11C]MPC-6827 PET as a promising tool for monitoring MT dysregulation early in AD progression. HIGHLIGHTS: Longitudinal positron emission tomography (PET) imaging studies using [11C]MPC-6827 in multiple established Alzheimer's disease (AD) mouse models revealed an early onset of microtubule dysregulation, with significant changes in brain radiotracer uptake evident from 2 to 4 months of age. Intra-group analysis showed a progressive increase in microtubule dysregulation with increasing AD burden, supported by significant correlations between PET imaging data and biodistribution, autoradiography, and molecular pathological markers. [11C]MPC-6827 PET imaging demonstrated its efficacy in detecting early microtubule alterations preceding observable behavioral changes in AD mouse models, suggesting its potential for early AD imaging. The inclusion of the 5xFAD mouse model further elucidated the impact of amyloid beta (Aß) toxicity on inducing tau hyperphosphorylation-mediated microtubule dysregulation, highlighting the versatility of [11C]MPC-6827 in delineating various aspects of AD pathology. Our study provides immediate clarity on high uptake of the microtubule-based radiotracer in AD brains in a longitudinal setting, which directly informs clinical utility in Aß/tau-based studies.

Gas Phase Transformations in Carbon-11 Chemistry.

The short-lived positron-emitter carbon-11 (t1/2 = 20.4 min; ß+, 99.8%) is prominent for labeling tracers for use in biomedical research with positron emission tomography (PET). Carbon-11 is produced for this purpose with a cyclotron, nowadays almost exclusively by the 14N(p,α)11C nuclear reaction, either on nitrogen containing a low concentration of oxygen (0.1-0.5%) or hydrogen (~5%) to produce [11C]carbon dioxide or [11C]methane, respectively. These primary radioactive products can be produced in high yields and with high molar activities. However, only [11C]carbon dioxide has some utility for directly labeling PET tracers. Primary products are required to be converted rapidly and efficiently into secondary labeling synthons to provide versatile radiochemistry for labeling diverse tracer chemotypes at molecular positions of choice. This review surveys known gas phase transformations of carbon-11 and summarizes the important roles that many of these transformations now play for producing a broad range of labeling synthons in carbon-11 chemistry.

Hetero-Diels-Alder and CuAAC Click Reactions for Fluorine-18 Labeling of Peptides: Automation and Comparative Study of the Two Methods.

Radiolabeled peptides are valuable tools for diagnosis or therapies; they are often radiofluorinated using an indirect approach based on an F-18 prosthetic group. Herein, we are reporting our results on the F-18 radiolabeling of three peptides using two different methods based on click reactions. The first one used the well-known CuAAC reaction, and the second one is based on our recently reported hetero-Diels-Alder (HDA) using a dithioesters (thia-Diels-Alder) reaction. Both methods have been automated, and the 18F-peptides were obtained in similar yields and synthesis time (37-39% decay corrected yields by both methods in 120-140 min). However, to obtain similar yields, the CuAAC needs a large amount of copper along with many additives, while the HDA is a catalyst and metal-free reaction necessitating only an appropriate ratio of water/ethanol. The HDA can therefore be considered as a minimalist method offering easy access to fluorine-18 labeled peptides and making it a valuable additional tool for the indirect and site-specific labeling of peptides or biomolecules.

Expedient Access to 18F-Fluoroheteroarenes via Deaminative Radiofluorination of Aniline-Derived Pyridinium Salts.

Herein, we disclose that pyridinium salts derived from abundant (hetero)anilines represent a novel precursor class for nucleophilic aromatic substitution reactions with [18F]fluoride. The value of this new 18F-fluorodeamination is demonstrated with the synthesis of over 30 structurally diverse and complex heteroaryl 18F-fluorides, several derived from scaffolds that were yet to be labelled with fluorine-18. The protocol tolerates heteroarenes and functionalities commonly found in drug discovery libraries, and is amenable to scale-up and automation on a commercial radiosynthesiser.

Controlling the Redox Chemistry of Cobalt Radiopharmaceuticals.

The elementally matched 55Co (t1/2 = 17.53 h, Iß+ = 77%)/58mCo (t1/2 = 9.10 h, IC= 100%) radioisotope pair is of interest for development of paired diagnostic/therapeutic radiopharmaceuticals. Due to the accessibility of the Co2+/3+ redox couple, the redox state can be readily modulated. Here, we show that macroscopic and radiochemical redox reactions can be closely monitored and controlled using spectroscopic and radiochemical methods. We employ model systems to inform how to selectively synthesize thermodynamically favored coordination complexes. In addition to exogenous oxidants, our data indicates that 55Co-induced radiolysis of water efficiently and directly drives selective oxidation to the Co3+ species under no-carrier added (n.c.a.) conditions. Our synthetic strategies subsequently stabilize the respective 55Co2+ or 55Co3+ species for targeted Co-compounds' imaging in a mouse tumor model.

Synthesis of 18F-labeled Aryl Trifluoromethyl Sulfones, -Sulfoxides, and -Sulfides for Positron Emission Tomography.

Positron emission tomography (PET) is becoming increasingly important in nuclear medicine and drug discovery. To date, the development of many potential PET tracers is hampered by the lack of suitable synthetic pathways for their preparation. This is particularly true for the highly desired radiolabeling of compounds bearing [18F]CF3-groups. For instance, S(O)nCF3-groups (n=0, 1, 2) serve as structural motif in a range of biologically active compounds, but their radiosynthesis remains largely unprecedented (for n=1, 2). Herein, we describe general methods for the radiosynthesis of 18F-labeled aryl trifluoromethyl sulfones, -sulfoxides, and -sulfides. All three methods are operationally straightforward, start from widely available precursors, i.e., sulfonyl fluorides and thiophenols, and make use of the recently established [18F]Ruppert-Prakash reagent. Further, the syntheses display good functional group tolerance as demonstrated by the 18F-labeling of more than 40 compounds. The applicability of the new method is demonstrated by the radiolabeling of three bioactive molecules, optionally to be used as PET tracers. In a broader context, this work presents a substantial expansion of the chemical space of radiofluorinated structural motifs to be used for the development of new PET tracers.

The 18F-Difluoromethyl Group: Challenges, Impact and Outlook.

The difluoromethyl functionality has proven useful in drug discovery, as it can modulate the properties of bioactive molecules. For PET imaging, this structural motif has been largely underexploited in (pre)clinical radiotracers due to a lack of user-friendly radiosynthetic routes. This Minireview provides an overview of the challenges facing radiochemists and summarises the efforts made to date to access 18F-difluoromethyl-containing radiotracers. Two distinct approaches have prevailed, the first of which relies on 18F-fluorination. A second approach consists of a 18F-difluoromethylation process, which uses 18F-labelled reagents capable of releasing key reactive intermediates such as the [18F]CF2H radical or [18F]difluorocarbene. Finally, we provide an outlook for future directions in the radiosynthesis of [18F]CF2H compounds and their application in tracer radiosynthesis.

"[18F]Trifluoroiodomethane - Enabling Photoredox-mediated Radical [18F]Trifluoromethylation for Positron Emission Tomography.

The development of new tracers for positron emission tomography (PET) is highly dependent on the available synthetic tools for their radiosynthesis. Herein, we present the radiosynthesis and application of [18F]trifluoroiodomethane - the first reagent for broad scope radical [18F]trifluoromethylation chemistry in high molar activity. CF218FI can be prepared from [18F]fluoroform with 67±5% AY and >99% RCP. Its synthetic utility is demonstrated by the radiosynthesis of previously unprecedented 18F-labeled a-trifluoromethyl ketones and trifluoromethyl sulfides, important motifs that are present in a range of bioactive compounds. Both protocols are Ru- and photo-mediated and proceed under mild reaction conditions. They show good functional group tolerance evidenced by the respective reaction scopes and make use of easily obtainable starting materials. The products can be isolated in 8.3-11.1 GBq/µmol (starting from ca. 5 GBq [18F]fluoride). The applicability to PET tracer synthesis is shown by the radiolabeling of bioactive compounds, such as derivatives of probenecid and febuxostat. In a broader context, this work opens the door to the utilization of radical [18F]trifluoromethylation chemistry for the radiolabeling of PET tracers in high molar activity.

Light-Driven Radiochemistry with Fluorine-18, Carbon-11 and Zirconium-89.

This review discusses recent advances in light-driven radiochemistry for three key isotopes: fluorine-18, carbon-11, and zirconium-89, and their applications in positron emission tomography (PET). In the case of fluorine-18, the predominant approach involves the use of cyclotron-produced [18F]fluoride or reagents derived thereof. Light serves to activate either the substrate or the fluorine-18 labeled reagent. Advancements in carbon-11 photo-mediated radiochemistry have been leveraged for the radiolabeling of small molecules, achieving various transformations, including 11C-methylation, 11C-carboxylation, 11C-carbonylation, and 11C-cyanation. Contrastingly, zirconium-89 photo-mediated radiochemistry differs from fluorine-18 and carbon-11 approaches. In these cases, light facilitates a postlabeling click reaction, which has proven valuable for the labeling of large biomolecules such as monoclonal antibodies (mAbs). New technological developments, such as the incorporation of photoreactors in commercial radiosynthesizers, illustrate the commitment the field is making in embracing photochemistry. Taken together, these advances in photo-mediated radiochemistry enable radiochemists to apply new retrosynthetic strategies in accessing novel PET radiotracers.

Zinc-Mediated Radiosynthesis of Unprotected Fluorine-18 Labelled α-Tertiary Amides.

This report describes the development of a Zn(OTf)2 -mediated method for converting α-tertiary haloamides to the corresponding fluorine-18 labelled α-tertiary fluoroamides with no-carrier-added [18 F]tetramethylammonium fluoride. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene is an essential additive for achieving high radiochemical conversion. Under the optimised conditions, radiofluorination proceeds at sterically hindered tertiary sites in high radiochemical conversions, yields, and purities. This method has been successfully automated and applied to access >200 mCi (>7.4 GBq) of several model radiofluorides. Mechanistic studies led to the development of a new, nucleophilic C-H radiofluorination process using N-sulphonyloxyamide substrates.

Base-Mediated Radio-Iodination of Arenes by Using Organosilane and Organogermane as Radiolabelling Precursors.

The radio-iodination of arenes is investigated from organosilane and organogermane precursors using ipso-electrophilic halogenation (IEH). Discovery of a mild base mediated process allows radio-iodination in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) of either aryl silane or germane, with germanes being more reactive. Clinical potential of arylgermanes as radio-iodination precursors is demonstrated through the labelling of [125 I]IMTO (iodometomidate) and [125 I]MIBG (meta-iodobenzylguanidine) thus offering an alternative to radio-iododestannylation processes using non-toxic precursors.