Improved automated one-pot two-step radiosynthesis of (S)-[18F]FETrp, a radiotracer for PET imaging of indoleamine 2,3-dioxygenase 1 (IDO1)

BACKGROUND

(S)-[18F]FETrp is a promising PET radiotracer for imaging IDO1 activity, one of the main enzymes involved in the tryptophan metabolism that plays a key role in several diseases including cancers. To date, the radiosynthesis of this tryptophan analogue remains highly challenging due to partial racemization occurring during the nucleophilic radiofluorination step. This work aims to develop a short, epimerization-free and efficient automated procedure of (S)-[18F]FETrp from a corresponding enantiopure tosylate precursor.

RESULTS

Enantiomerically pure (S)- and (R)-FETrp references as well as tosylate precursors (S)- and (R)-3 were obtained from corresponding Na-Boc-(L and D)-tryptophan in 2 and 4 steps, respectively. Manual optimisation of the radiolabelling conditions resulted in > 90% radiochemical conversion with more than 99% enantiomeric purity. Based on these results, the (S)-[18F]FETrp radiosynthesis was fully automated on a SynChrom R&D EVOI module to produce the radiotracer in 55.2 ± 7.5% radiochemical yield, 99.9% radiochemical purity, 99.1 ± 0.5% enantiomeric excess, and molar activity of 53.2 ± 9.3 GBq/µmol (n = 3).

CONCLUSIONS

To avoid racemisation and complicated purification processes, currently encountered for the radiosynthesis of (S)-[18F]FETrp, we report herein significant improvements, including a versatile synthesis of enantiomerically pure tosylate precursor and reference compound and a convenient one-pot two-step automated procedure for the radiosynthesis of (S)-[18F]FETrp. This optimised and robust production method could facilitate further investigations of this relevant PET radiotracer for imaging IDO1 activity.

Silicon-Fluoride Acceptors (SiFA) for 18F-Radiolabeling: From Bench to Bedside

Fluorine-18 (18F) is undoubtedly one of the most frequently applied radionuclides for the development of new radiotracers for positron emission tomography (PET) in the context of clinical cancer, neurological, and metabolic imaging. Until recently, the available radiochemical methodologies to introduce 18F into organic molecules ranging from small- to medium- and large-sized compounds were limited to a few applicable protocols. With the advent of late-stage fluorination of small aromatic, nonactivated compounds and various noncanonical labeling strategies geared toward the labeling of peptides and proteins, the molecular toolbox for PET radiotracer development was substantially extended. Especially, the noncanonical labeling methodologies characterized by the formation of Si-18F, B-18F, and Al-18F bonds give access to kit-like 18F-labeling of complex and side-group unprotected compounds, some of them already in clinical use. This chapter will particularly focus on silicon-fluoride acceptor (SiFA) chemistry and cover the history of its conceptual design and its translation into the clinical practice.

Fluorination of silyl prosthetic groups by fluorine mediated silyl ether linker cleavage: a concept study with conditions applicable in radiofluorination

BACKGROUND

Positron emission tomography (PET) is a powerful tool in medical imaging, especially in combination with the PET radionuclide fluorine-18 that possesses optimal characteristics. For labelling of biomolecules and low-molecular weight tracers, fluorine-18 can be covalently bound to silicon by either nucleophilic replacements of leaving groups (like ethers) or by isotope exchange of fluorine-19. While nucleophilic substitutions require additional purification steps for the removal of contaminants, isotope exchange with fluorine-18 results in low molar activity. Both challenges can be addressed with a detagging-fluorination of an immobilized silyl ether motif.

RESULTS

By overcoming the susceptibility towards hydrolysis, optimized detagging conditions (improved reaction time, fluorination reagent, linker, and resin) could afford the highly sterically hindered silyl fluoride motifs, that are commonly applied in radiochemistry in small and semipreparative scales. The described reaction conditions with fluorine-19 are transferrable to conditions with [¹⁸F]fluoride and silyl fluorides were obtained after approx. 10 min reaction time and in high-purity after mechanical filtration.

CONCLUSIONS

We present a proof-of-concept study for a detagging-fluorination of two silyl ethers that are bound to an optimized amino alcohol resin. We show with our model substrate that our solid-phase linker combination is capable of yielding the desired silicon fluoride in amounts sufficient for biological studies in animals or humans under standard fluorination conditions that may also be transferred to a radiolabelling setting. In conclusion, our presented approach could optimize the molar activity and simplify the preparation of radiofluorinated silyl fluorides.

Recent advancements in 18F-labeled PSMA targeting PET radiopharmaceuticals

Prostate specific membrane antigen (PSMA) is an attractive target for molecular imaging of prostate cancer and several other solid tumors because of its overexpression in prostate carcinoma and tumor neovasculature, respectively. While currently most commonly used PSMA PET radioligands are ⁶⁸Ga-labeled compounds, the short half-life and relatively low available radioactivity of gallium-68 have led to a steep increase in the development of ¹⁸F-labeled PSMA ligands. Several ¹⁸F-PSMA tracers such as [¹⁸F]DCFPyL and [¹⁸F]PSMA-1007 are already established in clinical practice, but there are still several drawbacks to be considered. Radiofluorination is often a multistep and time-consuming process requiring harsh labeling conditions. The limited sensitivity in the lower PSA ranges raises the need for improving the binding affinity of the ligands. Due to the metallic character of therapeutic radionuclides, there is very limited experience with ¹⁸F-PSMA tracers that can be applied for a theranostic approach. However, developments in the past few years have brought forward several improvements in these fields. These include the application of new radiosynthesis pathways for radiofluorination that reduces the process complexity, new approaches for the design of the pharmacophore, improving target interaction and the introduction of radiohybrid ligands, allowing labeling of the ligand with both diagnostic and therapeutic radionuclides. In this review, we will give an overview of these recent advancements of ¹⁸F-labeled PSMA PET radioligands.

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.

18F-labelled BODIPY dye as a dual imaging agent: Radiofluorination and applications in PET and optical imaging

Dual Positron emission tomography (PET)/optical imaging techniques have captured scientific interest for clinical applications due to their potential as an effective tool for visualizing in vivo information such as disease processes. 4,4'-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye has been considered an ideal platform strategy to achieve dual PET/optical imaging due to its photochemical nature and chemical structure. Various radiofluorination methods to prepare [¹⁸F]BODIPY dye have been developed and established, ranging from nucleophilic substitution reactions to isotope exchange reactions. In addition, ¹⁸F-labelled BODIPY dyes for biologically important targets have been used for in vivo and ex vivo studies. These studies proved the practicality of [¹⁸F]BODIPY dyes as a hybrid PET/optical imaging probe. In this review, recent advances in the synthesis and biological evaluation of ¹⁸F-labelled BODIPY dyes are described.

Automated synthesis of 18F radiolabelled indole containing Oncrasin-like molecules; a comparison of iodonium salts and boronic ester chemistry

Background

Oncrasin-1 is a small molecule which was identified from a screen of KRAS mutant cancer cells and has shown specificity for KRAS mutant cell killing. We aimed to develop a radiolabelled form of Oncrasin-1 to enable in-vivo imaging of mutant KRAS expression in malignant tumours. This work outlines the synthesis of 3 fluorinated derivatives and development of iodonium salt and boronic ester precursors for radiolabelling with the ¹⁸F isotope.

Results

In our hands, synthesis of iodonium salts were not easily accessible due to the 3-carbaldehyde indole structure being preferentially oxidized by conditions required for iodonium salt formation, rather than benzyl iodide. Synthesis and radiolabelling of boronic acid pinacol ester precursors were successful, with the products being obtained in yields of 10.76% ± 0.96% (n = 5), 14.7% ±8.58% (n = 3) and 14.92% ±3.9% (n = 3) for ¹⁸F KAM001, ¹⁸F KAM002 and ¹⁸F KAM003 respectively, with radiochemical purity of greater than 99%.

Conclusions

The successful synthesis of these tracers has been undertaken utilizing boronic ester radio-fluorination methods and will allow for investigation of Oncrasin based molecules as potential diagnostics for cancers expressing mutant KRAS protein.

Supplementary Information

Supplementary information accompanies this paper at 10.1186/s41181-020-00104-x.

Hexavalent lactoside labeled with [18F]AlF for PET imaging of asialoglycoprotein receptor

Several methods have been developed to label compounds with ¹⁸F. However, in general these are laborious and require a multistep synthesis. A method based on the chelation of ¹⁸F-aluminum fluoride ([¹⁸F]AlF) by 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) was developed recently. The present work was aimed to radiolabel hexavalent lactoside (NOTA-HL) by [¹⁸F]AlF method for PET imaging of asialoglycoprotein receptor (ASGPR).

METHODS

hexavalent lactoside was conjugated with the NOTA chelate and labeled with ¹⁸F in a one-pot method. The labeling procedure was investigated with different amounts of NOTA-HL and aluminum concentration. Radiochemical yield and radiochemical purity were determined by radio-TLC and radio-HPLC respectively. In vitro stability study of [¹⁸F]AlF-HL were carried out. PET/CT imaging of normal mice injected with [¹⁸F]AlF-NOTA-HL was performed.

RESULTS

The Radiochemical yield of [¹⁸F]AlF-NOTA-HL was higher with more precursor and optimal Al⁺ concentration. The radiochemical purity of labeled product was >95% after purified by Sep-Pak cartridge to remove unbound [¹⁸F]AlF. The radiolabeling, including purification, was performed in 30 min [¹⁸F]AlF-NOTA-HL exhibited good in vitro stability. PET studies in normal mice revealed high specific accumulation of activity in the liver.

CONCLUSION

NOTA-HL could be labeled rapidly and efficiently with aqueous ¹⁸F using AlF method. [¹⁸F]AlF-NOTA-HL would provide another efficient approach for PET imaging of ASGPR.

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.

Enzymatically Catalyzed Radiofluorination of Biomolecules

There has been significant and rapid growth in the development of amino acid-based molecular imaging agents (e.g., peptides, proteins, and antibody constructs) largely due to facile library preparation and high throughput screening. Positron-emitting fluorine-18 (half-life = 109.7 min) has a unique set of properties that match well with the pharmacokinetics of smaller sized constructs. Several indirect fluorine-18 labeling approaches have been developed yet only a few have advanced to human trials. Enzymatically catalyzed radiofluorination utilizing lipoic acid ligase shows promise as a mild site-specific method for coupling fluorine-18-labeled carboxylate substrates with biomolecules. Methods for preparation of two [¹⁸F]fluorocarboxylates and their ligation to a specific peptide sequence (LAP peptide) are presented herein.

Peripheral ganglia in healthy rats as target structures for the evaluation of PSMA imaging agents

Background

The recent implementation of PET with prostate specific membrane antigen (PSMA)-specific radiotracers into the clinical practice has resulted in the significant improvement of accuracy in the detection of prostate carcinoma (PCa). PSMA-expression in ganglia has been regarded as an important pitfall in prostate carcinoma-PET diagnostics but has not found any practical use for diagnosis or therapy.

Methods

We explored this phenomenon and demonstrated the applicability of peripheral ganglia in healthy rats as surrogates for small PSMA positive lesions for the preclinical evaluation of diagnostic PCa PET probes. Healthy rats were measured with PET/CT using the tracers [¹⁸F]DCFPyL, [Al¹⁸F]PSMA-11 and [⁶⁸Ga]PSMA-11. Sections of ganglia were stained with an anti-PSMA antibody. [¹⁸F]DCFPyL uptake in ganglia was compared to that in LNCaP tumor xenografts in mice.

Results

Whereas [¹⁸F]DCFPyL and [⁶⁸Ga]PSMA-11 were stable in vivo and accumulated in peripheral ganglia, [Al¹⁸F]PSMA-11 suffered from fast in vivo deflourination resulting in high bone uptake. Ganglionic PSMA expression was confirmed by immunohistochemistry. [¹⁸F]DCFPyL uptake and signal-to-noise ratio in the superior cervical ganglion was not significantly different from LNCaP xenografts.

Conclusions

Our results demonstrated the non-inferiority of the novel model compared to conventionally used tumor xenografts in immune compromised rodents with regard to reproducibility and stability of the PSMA signal. Furthermore, the model involves less expense and efforts while it is permanently available and avoids tumor-growth associated animal morbidity and distress. To the best of our knowledge, this is the first tumor-free model suitable for the in vivo evaluation of tumor imaging agents.

Synthesis and preliminary preclinical evaluation of fluorine-18 labelled isatin-4-(4-methoxyphenyl)-3-thiosemicarbazone ([18F]4FIMPTC) as a novel PET tracer of P-glycoprotein expression

Background

Several P-glycoprotein (P-gp) substrate tracers are available to assess P-gp function in vivo, but attempts to develop a tracer for measuring expression levels of P-gp have not been successful. Recently, (Z)-2-(5-fluoro-2-oxoindolin-3-ylidene)-N-(4-methoxyphenyl)hydrazine-carbothioamide was described as a potential selective P-gp inhibitor that is not transported by P-gp. Therefore, the purpose of this study was to radiolabel two of its analogues and to assess their potential for imaging P-gp expression using PET.

Results

[¹⁸F]2-(4-fluoro-2-oxoindolin-3-ylidene)-N-(4-methoxyphenyl)hydrazine-carbothioamide ([¹⁸F]5) and [¹⁸F]2-(6-fluoro-2-oxoindolin-3-ylidene)-N-(4-methoxyphenyl)hydrazine-carbothioamide ([¹⁸F]6) were synthesized and both their biodistribution and metabolism were evaluated in rats. In addition, PET scans were acquired in rats before and after tariquidar (P-gp inhibitor) administration as well as in P-gp knockout (KO) mice.

Both [¹⁸F]5 and [¹⁸F]6 were synthesized in 2–3% overall yield, and showed high brain uptake in ex vivo biodistribution studies. [¹⁸F]6 appeared to be metabolically unstable in vivo, while [¹⁸F]5 showed moderate stability with limited uptake of radiolabelled metabolites in the brain. PET studies showed that transport of [¹⁸F]5 across the blood-brain barrier was not altered by pre-treatment with the P-gp inhibitor tariquidar, and uptake was significantly lower in P-gp KO than in wild-type animals and indeed transported across the BBB or bound to P-gp in endothelial cells.

Conclusion

In conclusion, [¹⁸F]5 and [¹⁸F]6 were successfully and reproducibly synthesized, albeit with low radiochemical yields. [¹⁸F]5 appears to be a radiotracer that binds to P-gp, as showed in P-gp knock-out animals, but is not a substrate for P-gp.

Synthesis and radiofluorination of novel fluoren-9-one based derivatives for the imaging of α7 nicotinic acetylcholine receptor with PET

By structure-activity relationship studies on the tilorone scaffold, the 'one armed' substituted dibenzothiophenes and the fluoren-9-ones were identified as the most potential α₇ nAChR ligands. While the suitability of dibenzothiophene derivatives as PET tracers is recognized, the potential of fluoren-9-ones is insufficiently investigated. We herein report on a series of fluoren-9-one based derivatives targeting α₇ nAChR with compounds 8a and 8c possessing the highest affinity and selectivity. Accordingly, with [¹⁸F]8a and [¹⁸F]8c we designed and initially evaluated the first fluoren-9-one derived α₇ nAChR selective PET ligands. A future application of these radioligands is facilitated by the herein presented successful implementation of fully automated radiosynthesis.

Synthesis and evaluation of 18F-hexafluorophosphate as a novel PET probe for imaging of sodium/iodide symporter in a murine C6-glioma tumor model

Noninvasive imaging of iodide uptake via the sodium/iodide symporter (NIS) has received great interest for evaluation of thyroid cancer and reporter imaging of NIS-expressing viral therapies. In this study, we investigate ¹⁸F-labeled hexafluorophosphate (HFP or PF₆^-) as a high-affinity iodide analog for NIS imaging. ¹⁸F-HFP was synthesized by radiofluorination of phosphorus pentafluoride·N-methylpyrrolidine complex and evaluated in human NIS (hNIS)-expressing C6 glioma cells and a C6 glioma xenograft mouse model. ¹⁸F-HFP was obtained in radiochemical yield of 10 ± 5%, radiochemical purity of >96% and specific radioactivity of 604 ± 18 MBq/µmol. Specific uptake of ¹⁸F-HFP and high affinity of ¹⁹F-HFP were observed in hNIS+ C6-glioma cells. PET imaging showed robust uptake of ¹⁸F-HFP in NIS-expressing tissues (thyroid, stomach, and hNIS+ C6 glioma xenografts), and the uptake of ¹⁸F-HFP was blocked by NaClO₄ pretreatment. Specific accumulation in hNIS-expressing xenograft (hNIS+) was observed relative to isogenic control tumor (hNIS-). Clearance of ¹⁸F-HFP was predominantly through renal excretion. The biodistribution showed consistent results with PET imaging. Minimal bone uptake was observed over 2 h period post-injection, indicating excellent in vivo stability of ¹⁸F-HFP. Although improvement in specific radioactivity is desirable, the results indicate that ¹⁸F-HFP is a promising candidate radiotracer for further evaluation for NIS imaging.

Selective targeting of melanoma using N-(2-diethylaminoethyl) 4-[18F]fluoroethoxy benzamide (4-[18F]FEBZA): a novel PET imaging probe

Background

The purpose of this study was to develop a positron emission tomography (PET) imaging probe that is easy to synthesize and selectively targets melanoma in vivo. Herein, we report the synthesis and preclinical evaluation of N-(2-diethylaminoethyl) 4-[¹⁸F]Fluoroethoxy benzamide (4-[¹⁸F]FEBZA).

A one-step synthesis was developed to prepare 4-[¹⁸F]FEBZA in high radiochemical yields and specific activity. The binding affinity, the in vitro binding, and internalization studies were performed using B16F1 melanoma cell line. The biodistribution studies were performed in C57BL/6 normal mice, C57BL/6 mice bearing B16F1 melanoma tumor xenografts, and nu/nu athymic mice bearing HT-29 human adenocarcinoma tumor and C-32 amelanotic melanoma tumor xenografts. MicroPET studies were performed in mice bearing B16F1 and HT-29 tumor xenografts.

Results

4-[¹⁸F]FEBZA was prepared in 53 ± 14% radiochemical yields and a specific activity of 8.7 ± 1.1 Ci/μmol. The overall synthesis time for 4-[¹⁸F]FEBZA was 54 ± 7 min. The in vitro binding to B16F1 cells was 60.03 ± 0.48% after 1 h incubation at 37 °C. The in vivo biodistribution studies show a rapid and high uptake of F-18 in B16F1 tumor with 8.66 ± 1.02%IA/g in this tumor at 1 h. In contrast, the uptake at 1 h in HT-29 colorectal adenocarcinoma and C-32 amelanotic melanoma tumors was significantly lower with 3.68 ± 0.47%IA/g and 3.91 ± 0.23%IA/g in HT-29 and C-32 tumors, respectively. On microPET images, the melanoma tumor was clearly visible by 10 min post-injection and the intensity in the tumor continued to increase with time. In contrast, the HT-29 tumor was not visible on the microPET scans.

Conclusions

A rapid and facile synthesis of 4-[¹⁸F]FEBZA is developed. This method offers a reliable production of 4-[¹⁸F]FEBZA in high radiochemical yields and specific activity. A high binding affinity to melanoma cells and high uptake in tumor was noted. The microPET scan clearly delineates the melanoma tumor by 10 min post-injection. The results from these preclinical studies support the potential of 4-[¹⁸F]FEBZA as an effective probe to image melanoma.

Radiosynthesis of (S)-[18F]T1: The first PET radioligand for molecular imaging of α3β4 nicotinic acetylcholine receptors

Recent pharmacologic data revealed the implication of α3β4 nicotinic acetylcholine receptors (nAChRs) in nicotine and drug addiction. To image α3β4 nAChRs in vivo, we aimed to establish the synthesis of a [¹⁸F]-labelled analog of the highly affine and selective α3β4 ligand (S)-3-(4-(4-fluorophenyl)-1H-1,2,3-triazol-1-yl)quinuclidine ((S)-T1). (S)-[¹⁸F]T1 was synthesized from ethynyl-4-[¹⁸F]fluorobenzene ([¹⁸F]5) and (S)-azidoquinuclidine by click reaction. After a synthesis time of 130min (S)-[¹⁸F]T1 was obtained with a radiochemical yield (non-decay corrected) of 4.3±1.3%, a radiochemical purity of >99% and a molar activity of >158 GBq/μmol. The brain uptake and the brain-to-blood ratio of (S)-[¹⁸F]T1 in mice at 30min post injection were 2.02 (SUV) and 6.1, respectively. According to an ex-vivo analysis, the tracer remained intact (>99%) in brain. Only one major radiometabolite was detected in plasma and urine samples. In-vitro autoradiography on pig brain slices revealed binding of (S)-[¹⁸F]T1 to brain regions associated with the expression of α3β4 nAChRs, which could be reduced by the α3β4 nAChR selective drug AT-1001. These findings make (S)-[¹⁸F]T1 a potential tool for the non-invasive imaging of α3β4 nAChRs in the brain by PET.

Radiosynthesis of 4-[(18)F]fluoro-L-tryptophan by isotopic exchange on carbonyl-activated precursors

Several (18)F-labeled aromatic amino acids have been developed primarily for tumor imaging with positron-emission-tomography (PET). Also, (18)F-labeled tryptophan derivatives were synthesized by electrophilic (18)F-fluorination or by introducing a [(18)F]fluoroalkyl group. Here, a 3-step method for a nucleophilic radiosynthesis of 4-[(18)F]fluoro-L-tryptophan was developed. A carbonyl activated precursor containing a chiral amino acid building block was radiofluorinated by isotopic exchange, followed by removal of the activating formyl group by reductive decarbonylation and subsequent cleavage of the building block under acidic conditions. The title compound was obtained within 100 min with a radiochemical yield of about 13%, a molar activity of >70 MBq/mmol and an enantiomeric excess of >99%.

Improved synthesis of [¹⁸F]FS-PTAD as a new tyrosine-specific prosthetic group for radiofluorination of biomolecules

A novel prosthetic group, 4-(p-([(18)F]fluorosulfonyl)phenyl)-1,2,4-triazoline-3,5-dione ([(18)F]FS-PTAD) for site-specific radiofluorination of tyrosine residue in small molecules is described. Coupling of [(18)F]FS-PTAD with L-tyrosine, N-acetyl-L-tyrosine methyl amide and phenol as model compounds were achieved in buffered aqueous solution at room temperature, resulting in the corresponding fluorinated tyrosine and phenol derivatives. The total synthesis time including radiosynthesis, HPLC purification and formulation was less than 60 min (n=15) with ≥98% radio chemical purity. An initial in vitro evaluation of [(18)F]FS-PTAD-tyrosine in glioma cell lines revealed moderate uptake.

Design, synthesis and preliminary evaluation of (18)F-labelled 1,8-naphthyridin- and quinolin-2-one-3-carboxamide derivatives for PET imaging of CB2 cannabinoid receptor

In the present work, we report the synthesis of new aryliodonium salts used as precursors of single-stage nucleophilic (18)F radiofluorination. The corresponding unlabelled fluorinated derivatives showed to be CB2 cannabinoid receptor specific ligands, with Ki values in the low nanomolar range and high CB2/CB1 selectivity. The radiolabelled compound [(18)F]CB91, was successfully formulated for in vivo administration, and its preliminary biodistribution was assessed with microPET/CT. This tracer presented a reasonable in vivo stability and a preferential extraction in the tissues that constitutionally express CB2 cannabinoid receptor. The results obtained indicate [(18)F]CB91 as a possible candidate marker of CB2 cannabinoid receptor distribution. This study would open the way to further validation of this tracer for assessing pathologies for which the expression of this receptor is modified.

A high-yield automated radiosynthesis of the alpha-7 nicotinic receptor radioligand [18F]NS10743

[¹⁸F]NS10743, a promising and highly competitive α7 nAChR radioligand has been synthesized so far by microwave irradiation using a manual single-mode device followed by a palladium-catalyzed reduction of remaining nitro-precursor for HPLC separation purposes. For further preclinical and clinical use, regulated production of [¹⁸F]NS10743 by fully automated radiosynthesis is a crucial requirement. Therefore, we chose a commercial synthesis module and developed the automated radiosynthesis of [¹⁸F]NS10743. Besides evaluation of several radiosynthesis procedures, we performed an extensive HPLC study for quantitative separation of [¹⁸F]NS10743 from the corresponding nitro precursor. After implementation of the optimized procedure on a TRACERlab^TM FX F-N synthesis module, [¹⁸F]NS10743 was obtained in high radiochemical purity (≥99%) with an overall radiochemical yield of 32.2±7% (n=3). The specific activities at the end of the synthesis were 571±17GBq/µmol (n=3).