Radiosynthesis of 1-[18F]fluoroethyl-L-tryptophan as a novel potential amino acid PET tracer

Tryptophan Kynurenine Pathway-Based Imaging Agents for Brain Disorders and Oncology-From Bench to Bedside

Tryptophan (Trp)-based radiotracers have excellent potential for imaging many different types of brain pathology because of their involvement with both the serotonergic and kynurenine (KYN) pathways. However, radiotracers specific to the kynurenine metabolism pathway are limited. In addition, historically Trp-based radiopharmaceuticals were synthesized with the short-lived isotope carbon-11. A newer generation of Trp-based imaging agents using the longer half-lived and commercially available isotopes, such as fluorine-18 and iodine-124, are being developed. The newly developed amino acid-based tracers have been demonstrated to have favorable radiochemical and imaging characteristics in pre-clinical studies. However, many barriers still exist in the clinical translation of KYN pathway-specific radiotracers.

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.

Development of [18F]F-5-OMe-Tryptophans through Photoredox Radiofluorination: A New Method to Access Tryptophan-Based PET Agents

Although various radiolabeled tryptophan analogs have been developed to monitor tryptophan metabolism using positron emission tomography (PET) for various human diseases including melanoma and other cancers, their application can be limited due to the complicated synthesis process. In this study, we demonstrated that photoredox radiofluorination represents a simple method to access novel tryptophan-based PET agents. In brief, 4-F-5-OMe-tryptophans (l/d-T13) and 6-F-5-OMe-tryptophans (l/d-T18) were easily synthesized. The 18F-labeled analogs were produced by photoredox radiofluorination with radiochemical yields ranging from 2.6 ± 0.5% to 32.4 ± 4.1% (3 ≤ n ≤ 5, enantiomeric excess ≥ 99.0%) and over 98.0% radiochemical purity. Small animal imaging showed that l-[18F]T13 achieved 9.58 ± 0.26%ID/g tumor uptake and good contrast in B16F10 tumor-bearing mice (n = 3). Clearly, l-[18F]T13 exhibited prominent tumor uptake, warranting future evaluations of its potential usage in precise immunotherapy monitoring.

A closer look at the synthesis of 2-[18F]fluoroethyl tosylate to minimize the formation of volatile side-products

BACKGROUND

2-[¹⁸F]Fluoroethyltosylate ([¹⁸F]FEtOTs) is a well-known ¹⁸F-fluoroalkylating agent widely used to synthesize radiotracers for positron emission tomography. The widespread use of [¹⁸F]FEtOTs is due in part to its low volatility when compared to other halide and sulfonate building blocks. In this work, the radioactive volatile side-products formed during the synthesis of [¹⁸F]FEtOTs were identified and characterized for the first time, and an optimization of the reaction conditions to minimize their formation was proposed.

RESULTS

In order to characterize the volatiles produced during [¹⁸F]FEtOTs synthesis, the reaction mixtures of both cold FEtOTs and [¹⁸F]FEtOTs were co-injected onto the HPLC system. The radioactive peaks corresponding to the volatile compounds were collected, analyzed through headspace gas chromatography mass spectrometry sampler (HS-GC-MS) and identified as vinyl fluoride ([¹⁹F]VF) and 2-fluoroethanol ([¹⁹F]FEOH). By using a rotatable central composite design with a two-level full factorial core of two factors (2²), it was determined that temperature and time are independent variables which affect the generation of [¹⁸F]VF and [¹⁸F]FEOH during the radiosynthesis of [¹⁸F]FEtOTs. In addition, in order to reduce the formation of the volatiles ([¹⁸F]VF and [¹⁸F]FEOH) and increase the yield of [¹⁸F]FEtOTs, it was demonstrated that the molar ratio of base to precursor must also be considered.

CONCLUSION

[¹⁸F]VF and [¹⁸F]FEOH are volatile side-products formed during the radiosynthesis of [¹⁸F]FEtOTs, whose yields depend on the reaction time, temperature, and the molar ratio of base to precursor. Therefore, special care should be taken during the radiosynthesis and subsequent reactions using [¹⁸F]FEOTs in order to avoid environmental contamination and to improve the yield of the desired products.

Fluorine-18-Labeled PET Radiotracers for Imaging Tryptophan Uptake and Metabolism: a Systematic Review

Due to its metabolism via the serotonin and kynurenine pathways, tryptophan plays a key role in multiple disease processes including cancer. Imaging tryptophan uptake and metabolism in vivo can be achieved with tryptophan derivative positron emission tomography (PET) radiotracers. While human studies with such tracers have been confined to C-11-labeled compounds, preclinical development of F-18-labeled tryptophan-based radiotracers has surged in recent years. We performed a systematic review of studies reporting on such F-18-labeled tryptophan tracers to summarize and compare their biological characteristics and their potential for tumor imaging, with a particular focus on key enzymes of the kynurenine pathway (indoleamine 2,3-dioxygenase [IDO] and tryptophan 2,3-dioxygenase [TDO]), which play an important role in tumoral immune resistance. From a PubMed search, English language articles including data on the preparation and radiochemical and/or biological characteristics of F-18-labeled tryptophan derivative radiotracers were reviewed. A total of 19 original papers included data on 15 unique radiotracers, the majority of which were synthesized with an adequate radiochemical yield. Automated synthesis was reported for 1-(2-[¹⁸F]fluoroethyl)-L-tryptophan, the most extensively evaluated tracer thus far. Biodistribution studies showed high uptake in the pancreas, while the L-type amino acid transporter was the dominant transport mechanism for most of the reviewed tracers. Tracers tested for tumor uptake showed accumulation in tumor cell lines in vitro and in xenografts in vivo, often with favorable tumor-to-background uptake ratios in comparison with clinically used F-18-labeled radiotracers. Five tracers showed promise for imaging IDO activity, including 1-(2-[¹⁸F]fluoroethyl)-L-tryptophan and a F-18-labeled analog of alpha-[¹¹C]methyl-L-tryptophan tested clinically in previous studies. Two radiotracers were metabolized by TDO but showed defluorination in vivo. In summary, most F-18-labeled tryptophan derivative PET tracers share common transport mechanisms and biodistribution characteristics. Several reported tracers could be candidates for further testing and validation toward PET imaging applications in a variety of human diseases.

11C- and 18F-labelled tryptophans as PET-tracers for imaging of altered tryptophan metabolism in age-associated disorders

The ageing of the world’s population is the result of increased life expectancy observed in almost all countries throughout the world. Consequently, a rising tide of ageing-associated disorders, like cancer and neurodegenerative diseases, represents one of the main global challenges of the 21st century. The ability of mankind to overcome these challenges is directly dependent on the capability to develop novel methods for therapy and diagnosis of age-associated diseases. One hallmark of age-related pathologies is an altered tryptophan metabolism. Numerous pathological processes including neurodegenerative and neurological diseases like epilepsy, Parkinson’s and Alzheimer’s diseases, cancer and diabetes exhibit marked changes in tryptophan metabolism. Visualization of key processes of tryptophan metabolic pathways, especially using positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI, can be exploited to early detect the aforementioned disorders with considerable accuracy, allowing appropriate and timely treatment of patients. Here we review the published 11C- and 18F-labelled tryptophans with respect to the production and also preclinical and clinical evaluation as PET-tracers for visualization of different branches of tryptophan metabolism.

The bibliography includes 159 references.

Evaluation of L-1-[18F]Fluoroethyl-Tryptophan for PET Imaging of Cancer

PURPOSE

Fluorine-18 labeled tryptophan analog L-1-[¹⁸F]fluoroethyl-tryptophan (L-1-[¹⁸F]FETrp) was designed for positron emission tomography (PET) imaging of cancer by dual targeting of the overexpressed amino acid transporters and altered indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway of tryptophan metabolism. In our previous study, we described the radiosynthesis and preliminary evaluation of L-1-[¹⁸F]FETrp for PET imaging of breast cancer. The aim of this study was to investigate the in vivo imaging mechanism and further evaluate this radiotracer in more wide range types of cancers including prostate cancer, lung cancer, and glioma.

PROCEDURES

The mice bearing subcutaneous PC-3 prostate cancer, subcutaneous H2009 and H460 lung cancers, subcutaneous MDA-MB-231, orthotopic A549 lung cancer, and intracranial 73C glioma were employed to evaluate L-1-[¹⁸F]FETrp for PET imaging of cancer. The in vivo catabolism of L-1-[¹⁸F]FETrp in the tumor was studied by analysis of PC-3 extracts with radio-HPLC.

RESULTS

Small animal PET/CT imaging of L-1-[¹⁸F]FETrp visualized all tumors in these different mouse models with high accumulations of radioactivity in PC-3 (7.5 ± 0.6 % ID/g), H2009 (5.3 ± 0.8 % ID/g), H460 (9.0 ± 1.4 % ID/g), A549 (4.5 ± 0.5 % ID/g), and 73C (4.1 ± 0.7 % ID/g) tumors. The radio-HPLC analysis of PC-3 tumor extracts revealed that about 30 % of L-1-[¹⁸F]FETrp was converted into a highly polar radioactive metabolite. The uptake in H460 cancer was about 1.7-fold higher than that in H2009 cancer, which indicated L-1-[¹⁸F]FETrp could differentiate these subtypes of lung cancers (H2009 and H460) by imaging quantification. Furthermore, small animal PET/CT imaging in intracranial glioma revealed L-1-[¹⁸F]FETrp could pass blood-brain barrier (BBB) and accumulate in glioma with a favorable imaging contrast (tumor-to-brain 2.9).

CONCLUSIONS

L-1-[¹⁸F]FETrp highly accumulated in a wide range of malignancies including lung cancer, prostate cancer, and glioma. These results suggested that L-1-[¹⁸F]FETrp is a promising radiotracer for PET imaging of cancer.

PET imaging of medulloblastoma with an 18F-labeled tryptophan analogue in a transgenic mouse model

In vivo positron emission tomography (PET) imaging is a key modality to evaluate disease status of brain tumors. In recent years, tremendous efforts have been made in developing PET imaging methods for pediatric brain tumors. Carbon-11 labelled tryptophan derivatives are feasible as PET imaging probes in brain tumor patients with activation of the kynurenine pathway, but the short half-life of carbon-11 limits its application. Using a transgenic mouse model for the sonic hedgehog (Shh) subgroup of medulloblastoma, here we evaluated the potential of the newly developed 1-(2-[¹⁸F]fluoroethyl)-L-tryptophan (1-L-[¹⁸F]FETrp) as a PET imaging probe for this common malignant pediatric brain tumor. 1-L-[¹⁸F]FETrp was synthesized on a PETCHEM automatic synthesizer with good chemical and radiochemical purities and enantiomeric excess values. Imaging was performed in tumor-bearing Smo/Smo medulloblastoma mice with constitutive actvation of the Smoothened (Smo) receptor using a PerkinElmer G4 PET-X-Ray scanner. Medulloblastoma showed significant and specific accumulation of 1-L-[¹⁸F]FETrp. 1-L-[¹⁸F]FETrp also showed significantly higher tumor uptake than its D-enantiomer, 1-D-[¹⁸F]FETrp. The uptake of 1-L-[¹⁸F]FETrp in the normal brain tissue was low, suggesting that 1-L-[¹⁸F]FETrp may prove a valuable PET imaging probe for the Shh subgroup of medulloblastoma and possibly other pediatric and adult brain tumors.

Automated production of 1-(2-[18F]fluoroethyl)-l-tryptophan for imaging of tryptophan metabolism

Automated production of an fluorine-18 labeled tryptophan analogue, 1-(2-[¹⁸F]fluoroethyl)-l-tryptophan (1-L-[¹⁸F]FETrp) in a current Good Manufacturing Practice facility was achieved. 1-L-[¹⁸F]FETrp was produced by a one-pot, two-step strategy with an overall synthesis time of approximately 100 min, a radiochemical yield of 20 ± 5% (decay corrected), radiochemical purity and enantiomeric excess over 90%, and a molar activity of 103 ± 15 GBq/μmol at the end of synthesis (EOS). The dose mass of 1-L-FETrp in four consecutive batches was less than 5 μg. The radiopharmaceutical product met all quality control criteria for clinical use.

Improved Radiosynthesis and Biological Evaluations of L- and D-1-[18F]Fluoroethyl-Tryptophan for PET Imaging of IDO-Mediated Kynurenine Pathway of Tryptophan Metabolism

PURPOSE

Tryptophan metabolism via indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway plays a role in immunomodulation and has been emerging as a plausible target for cancer immunotherapy. Imaging IDO-mediated kynurenine pathway of tryptophan metabolism with positron emission tomography (PET) could provide valuable information for noninvasive assessment of cancer immunotherapy response. In this work, radiotracer 1-(2-[¹⁸F]fluoroethyl)-L-tryptophan (1-L-[¹⁸F]FETrp) and its enantioisomer 1-D-[¹⁸F]FETrp were synthesized and evaluated for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism.

PROCEDURES

Enantiopure 1-L-[¹⁸F]FETrp and 1-D-[¹⁸F]FETrp were prepared by a nucleophilic reaction of N-boc-1-(2-tosylethyl) tryptophan tert-butyl ester with [¹⁸F]Fluoride, followed by acid hydrolysis in a GE Tracerlab FX-N module. In vitro cell uptake assays were performed with a breast cancer cell line MDA-MB-231. Small animal PET/computed tomography (CT) imaging was carried out in a mouse model bearing MDA-MB-231 xenografts.

RESULTS

Automatic radiosynthesis of 1-L-[¹⁸F]FETrp and 1-D-[¹⁸F]FETrp was achieved by a one-pot two-step approach in 19.0 ± 7.0 and 9.0 ± 3.0 % (n = 3) decay-corrected yield with radiochemical purity over 99 %, respectively. In vitro cell uptake study indicated the uptake of 1-D-[¹⁸F]FETrp in MDA-MB-231 cells was 0.73 ± 0.07 %/mg of protein at 60 min, while, the corresponding uptake of 1-L-[¹⁸F]FETrp was 6.60 ± 0.77 %/mg. Further mechanistic assays revealed that amino acid transport systems L-tpye amino acid transporter (LAT) and alanine-, serine-, and cysteine-preferring (ASC), and enzyme IDO expression were involved in cell uptake of 1-L-[¹⁸F]FETrp. Small animal PET/CT imaging study showed the tumor uptake of 1-L-[¹⁸F]FETrp was 4.6 ± 0.4 % ID/g, while, the tumor uptake of 1-D-[¹⁸F]FETrp was low to 1.0 ± 0.2 % ID/g, which were confirmed by ex vivo biodistribution study.

CONCLUSIONS

We have developed a practical method for the automatic radiosynthesis of 1-L-[¹⁸F]FETrp and 1-D-[¹⁸F]FETrp. Our biological evaluation results suggest that 1-L-[¹⁸F]FETrp is a promising radiotracer for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism in cancer.

Challenges in the automated synthesis of [18F]-1-fluoroethyl tryptophan: Formation of both O- and N-alkylated products

[¹⁸F]Fluoroethyl tosylate was synthesized using an automated "Synthra" module using ethylene di-tosylate and [¹⁸F]fluoride/K₂₂₂/K₂CO₃ in acetonitrile. [¹⁸F]Fluoroethyl tosylate was purified by semi-preparative HPLC followed by reformulation using a C18 Sep-Pak cartridge and eluted with DMF. Using this [¹⁸F]fluoroethyl tosylate, we attempted to alkylate protected tryptophan aiming to obtain the N-[¹⁸F]fluoroethyl-t-Boc-tryptophan methyl ester. Initial attempts resulted in the formation of the O-alkylated, rather than N-alkylated product. Manual removal of the cartridge from the automated module, followed by an extended drying of the cartridge under high flow nitrogen, was required to form the desired N-alkylated product. This demonstrates that the drying process in automated modules requires modification for sensitive N-alkylation of compounds and may be essential for compounds like tryptophan methyl ester that have multiple potential sites of alkylation in their chemical structure.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Assessment of Tryptophan Uptake and Kinetics Using 1-(2-18F-Fluoroethyl)-l-Tryptophan and α-11C-Methyl-l-Tryptophan PET Imaging in Mice Implanted with Patient-Derived Brain Tumor Xenografts

Abnormal tryptophan metabolism via the kynurenine pathway is involved in the pathophysiology of a variety of human diseases including cancers. α-¹¹C-methyl-l-tryptophan (¹¹C-AMT) PET imaging demonstrated increased tryptophan uptake and trapping in epileptic foci and brain tumors, but the short half-life of ¹¹C limits its widespread clinical application. Recent in vitro studies suggested that the novel radiotracer 1-(2-¹⁸F-fluoroethyl)-l-tryptophan (¹⁸F-FETrp) may be useful to assess tryptophan metabolism via the kynurenine pathway. In this study, we tested in vivo organ and tumor uptake and kinetics of ¹⁸F-FETrp in patient-derived xenograft mouse models and compared them with ¹¹C-AMT uptake.

METHODS

Xenograft mouse models of glioblastoma and metastatic brain tumors (from lung and breast cancer) were developed by subcutaneous implantation of patient tumor fragments. Dynamic PET scans with ¹⁸F-FETrp and ¹¹C-AMT were obtained for mice bearing human brain tumors 1-7 d apart. The biodistribution and tumoral SUVs for both tracers were compared.

RESULTS

¹⁸F-FETrp showed prominent uptake in the pancreas and no bone uptake, whereas ¹¹C-AMT showed higher uptake in the kidneys. Both tracers showed uptake in the xenograft tumors, with a plateau of approximately 30 min after injection; however, ¹⁸F-FETrp showed higher tumoral SUV than ¹¹C-AMT in all 3 tumor types tested. The radiation dosimetry for ¹⁸F-FETrp determined from the mouse data compared favorably with the clinical ¹⁸F-FDG PET tracer.

CONCLUSION

¹⁸F-FETrp tumoral uptake, biodistribution, and radiation dosimetry data provide strong preclinical evidence that this new radiotracer warrants further studies that may lead to a broadly applicable molecular imaging tool to examine abnormal tryptophan metabolism in human tumors.

Fully automated radiosynthesis of N(1)-[(18)F]fluoroethyl-tryptophan and study of its biological activity as a new potential substrate for indoleamine 2,3-dioxygenase PET imaging

INTRODUCTION

Indoleamine 2,3-dioxygenase (IDO) catalyzes the initial step in the catabolism of l-tryptophan along the kynurenine pathway and exerts immunosuppressive properties in inflammatory and tumor tissues by blocking locally T-lymphocyte proliferation. Recently, 1-(2-[(19)F]fluoroethyl)-dl-tryptophan (1-[(19)F]FE-dl-Trp) was reported as a good and specific substrate of this enzyme. Herein, the radiosynthesis of its radioactive isotopomer (1-[(18)F]FE-dl-Trp, dl-[(18)F]5) is presented along with in vitro enzymatic and cellular uptake studies.

METHODS

The one-pot n.c.a. radiosynthesis of this novel potential PET imaging tracer, including HPLC purification and formulation, has been fully automated on a FASTlab™ synthesizer. Chiral separation of both isomers and their formulation were implemented on a second cassette. In vitro enzymatic and cellular uptake studies were then conducted with the d-, l- and dl-radiotracers.

RESULTS

The radiolabeling of the tosylate precursor was performed in DMF (in 5min; RCY: 57% (d.c.), n=3). After hydrolysis, HPLC purification and formulation, dl-[(18)F]5 was obtained with a global radiochemical yield of 18±3% (not decay corrected, n=7, in 80min) and a specific activity of 600±180GBq/μmol (n=5). The subsequent separation of l- and d-enantiomers was performed by chiral HPLC and both were obtained after formulation with an RCY (d.c.) of 6.1% and 5.8%, respectively. In vitro enzymatic assays reveal that l-[(18)F]5 is a better substrate than d-[(18)F]5 for human IDO. In vitro cellular assays show an IDO-specific uptake of the racemate varying from 30% to 50% of that of l-[(18)F]5, and a negligible uptake of d-[(18)F]5.

CONCLUSION

In vitro studies show that l-[(18)F]5 is a good and specific substrate of hIDO, while presenting a very low efflux. These results confirm that l-[(18)F]5 could be a very useful PET radiotracer for IDO expressing cells in cancer imaging.

Development of 1-N-(11)C-Methyl-L- and -D-Tryptophan for pharmacokinetic imaging of the immune checkpoint inhibitor 1-Methyl-Tryptophan

1-Methyl-tryptophan (1MTrp) is known as a specific inhibitor targeting the immune- checkpoint protein indoleamine-2,3-dioxygenase, in two stereoisomers of levorotary (l) and dextrorotary (d). A long-standing debate exists in immunology and oncology: which stereoisomer has the potential of antitumor immunotherapy. Herein, we developed two novel radioprobes, 1-N-¹¹C-methyl-l- and -d-tryptophan (¹¹C-l-1MTrp and ¹¹C-d-1MTrp), without modifying the chemical structures of the two isomers, and investigated their utility for pharmacokinetic imaging of the whole body. ¹¹C-l-1MTrp and ¹¹C-d-1MTrp were synthesized rapidly with radiochemical yields of 47 ± 6.3% (decay-corrected, based on ¹¹C-CO₂), a radiochemical purity of >98%, specific activity of 47–130 GBq/μmol, and high enantiomeric purity. PET/CT imaging in rats revealed that for ¹¹C-l-1MTrp, the highest distribution of radioactivity was observed in the pancreas, while for ¹¹C-D-1MTrp, it was observed in the kidney. Ex vivo biodistribution confirmed the PET/CT results, indicating the differences in pharmacokinetics between the two isomers. Both ¹¹C-l-1MTrp and ¹¹C-d-1MTrp are therefore useful PET probes for delineating the distribution and action of the checkpoint inhibitor 1MTrp in vivo. This study represents the first step toward using whole-body and real-time insight to disentangle the antitumor potential of the two stereoisomers of 1MTrp, and it can facilitate the development of 1MTrp immunotherapy.

N (1)-Fluoroalkyltryptophan Analogues: Synthesis and in vitro Study as Potential Substrates for Indoleamine 2,3-Dioxygenase

Indoleamine 2,3-dioxygenase (hIDO) is an enzyme that catalyzes the oxidative cleavage of the indole ring of l-tryptophan through the kynurenine pathway, thereby exerting immunosuppressive properties in inflammatory and tumoral tissues. The syntheses of 1-(2-fluoroethyl)-tryptophan (1-FETrp) and 1-((1-(2-fluoroethyl)-1H-1,2,3-triazol-4-yl)methyl)-tryptophan, two N (1)-fluoroalkylated tryptophan derivatives, are described here. In vitro enzymatic assays with these two new potential substrates of hIDO show that 1-FETrp is a good and specific substrate of hIDO. Therefore, its radioactive isotopomer, 1-[(18)F]FETrp, should be a molecule of choice to visualize tumoral and inflammatory tissues and/or to validate new potential inhibitors.

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

18F-labelled intermediates for radiosynthesis by modular build-up reactions: newer developments

This brief review gives an overview of newer developments in (18)F-chemistry with the focus on small (18)F-labelled molecules as intermediates for modular build-up syntheses. The short half-life (<2 h) of the radionuclide requires efficient syntheses of these intermediates considering that multistep syntheses are often time consuming and characterized by a loss of yield in each reaction step. Recent examples of improved synthesis of (18)F-labelled intermediates show new possibilities for no-carrier-added ring-fluorinated arenes, novel intermediates for tri[(18)F]fluoromethylation reactions, and (18)F-fluorovinylation methods.