Preparation of No-Carrier-Added 6-[18F]Fluoro-l-tryptophan via Cu-Mediated Radiofluorination

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.

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.

Preparation of labeled aromatic amino acids via late-stage 18F-fluorination of chiral nickel and copper complexes

A general protocol for the preparation of 18F-labeled AAAs and α-methyl-AAAs applying alcohol-enhanced Cu-mediated radiofluorination of Bpin-substituted chiral complexes using Ni/Cu-BPX templates as double protecting groups is reported. The chiral auxiliaries are easily accessible from commercially available starting materials in a few synthetic steps. The versatility of the method was demonstrated by the high-yielding preparation of a series of [18F]F-AAAs and the successful implementation of the protocol into automated radiosynthesis modules.

Preparation of a First 18F-Labeled Agonist for M1 Muscarinic Acetylcholine Receptors

M₁ muscarinic acetylcholine receptors (mAChRs) are abundant in postsynaptic nerve terminals of all forebrain regions and have been implicated in the cognitive decline associated with Alzheimer's disease and other CNS pathologies. Consequently, major efforts have been spent in the development of subtype-selective positron emission tomography (PET) tracers for mAChRs resulting in the development of several ¹¹C-labeled probes. However, protocols for the preparation of ¹⁸F-labeled mAChR-ligands have not been published so far. Here, we describe a straightforward procedure for the preparation of an ¹⁸F-labeled M₁ mAChR agonist and its corresponding pinacol boronate radiolabeling precursor and the non-radioactive reference compound. The target compounds were prepared from commercially available aryl fluorides and Boc protected 4-aminopiperidine using a convergent reaction protocol. The radiolabeling precursor was prepared by a modification of the Miyaura reaction and labeled via the alcohol-enhanced Cu-mediated radiofluorination. The developed procedure afforded the radiotracer in a non-decay-corrected radiochemical yield of 17 ± 3% (n = 3) and in excellent radiochemical purity (>99%) on a preparative scale. Taken together, we developed a straightforward protocol for the preparation of an ¹⁸F-labeled M₁ mAChR agonist that is amenable for automation and thus provides an important step towards the routine production of a ¹⁸F-labeled M₁ selective PET tracer for experimental and diagnostic applications.

Synthesis and Preclinical Evaluation of 6-[18F]Fluorine-α-methyl-l-tryptophan, a Novel PET Tracer for Measuring Tryptophan Uptake

The positron emission tomography (PET) radioligand α-[¹¹C]methyl-l-tryptophan ([¹¹C]AMT) has been used to assess tryptophan metabolism in cancer, epilepsy, migraine, and autism. Despite its extensive application, the utility of this tracer is currently hampered by the short half-life of the radionuclide used for its labeling (¹¹C, t_1/2 = 20.4 min). We herein report the design, synthesis, radiolabeling, and initial in vivo evaluation of a fluorine-18 (¹⁸F, t_1/2 = 109.7 min) labeled analogue that is fluorinated in the 6-position of the aromatic ring ([¹⁸F]6-F-AMTr). In a head-to-head comparison between [¹⁸F]6-F-AMTr and [¹¹C]AMT in mice using PET, peak brain radioactivity, regional brain distribution, and kinetic profiles were similar between the two tracers. [¹⁸F]6-F-AMTr was however not a substrate for IDO1 or TPH as determined in in vitro enzymatic assays. The brain uptake of the tracer is thus more likely related to LAT1 transport over the blood-brain barrier than metabolism along the serotonin or kynurenine pathways.

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.

Synthesis of high-molar-activity [18F]6-fluoro-L-DOPA suitable for human use via Cu-mediated fluorination of a BPin precursor

[18F]6-fluoro-L-DOPA ([18F]FDOPA) is a diagnostic radiopharmaceutical for positron emission tomography (PET) imaging that is used to image Parkinson's disease, brain tumors, and focal hyperinsulinism of infancy. Despite these important applications, [18F]FDOPA PET remains underutilized because of synthetic challenges associated with accessing the radiotracer for clinical use; these stem from the need to radiofluorinate a highly electron-rich catechol ring in the presence of an amino acid. To address this longstanding challenge in the PET radiochemistry community, we have developed a one-pot, two-step synthesis of high-molar-activity [18F]FDOPA by Cu-mediated fluorination of a pinacol boronate (BPin) precursor. The method is fully automated, has been validated to work well at two separate sites (an academic facility with a cyclotron on site and an industry lab purchasing [18F]fluoride from an outside vendor), and provides [18F]FDOPA in reasonable radiochemical yield (2.44 ± 0.70 GBq, 66 ± 19 mCi, 5 ± 1%), excellent radiochemical purity (>98%) and high molar activity (76 ± 30 TBq/mmol, 2,050 ± 804 Ci/mmol), n = 26. Herein we report a detailed protocol for the synthesis of [18F]FDOPA that has been successfully implemented at two sites and validated for production of the radiotracer for human use.

Manual and automated Cu-mediated radiosynthesis of the PARP inhibitor [18F]olaparib

Positron emission tomography (PET) is a diagnostic nuclear imaging modality that relies on automated protocols to prepare agents labeled with a positron-emitting radionuclide (e.g., 18F). In recent years, new reactions have appeared for the 18F-labeling of agents that are difficult to access by applying traditional radiochemistry, for example those requiring 18F incorporation into unactivated (hetero)arenes. However, automation of these new methods for translation to the clinic has progressed slowly because extensive modification of manual protocols is typically required when implementing novel 18F-labeling methodologies within automated modules. Here, we describe the workflow that led to the automated radiosynthesis of the poly(ADP-ribose) polymerase (PARP) inhibitor [18F]olaparib. First, we established a robust manual protocol to prepare [18F]olaparib from the protected N-[2-(trimethylsilyl)ethoxy]methyl (SEM) arylboronate ester precursor in a 17% ± 5% (n = 15; synthesis time, 135 min) non-decay-corrected (NDC) activity yield, with molar activity (Am) up to 34.6 GBq/µmol. Automation of the process, consisting of copper-mediated 18F-fluorodeboronation followed by deprotection, was achieved on an Eckert & Ziegler Modular-Lab radiosynthesis platform, affording [18F]olaparib in a 6% ± 5% (n = 3; synthesis time, 120 min) NDC activity yield with Am up to 319 GBq/µmol.

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

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

Indolylboronic Acids: Preparation and Applications

Indole derivatives are associated with a variety of both biological activities and applications in the field of material chemistry. A number of different strategies for synthesizing substituted indoles by means of the reactions of indolylboronic acids with electrophilic compounds are considered the methods of choice for modifying indoles because indolylboronic acids are easily available, stable, non-toxic and new reactions using indolylboronic acids have been described in the literature. Thus, the aim of this review is to summarize the methods available for the preparation of indolylboronic acids as well as their chemical transformations. The review covers the period 2010-2019.

MCR Scaffolds Get Hotter with 18F-Labeling

Imaging techniques, such as positron emission tomography (PET), represent great progress in the clinical development of drugs and diagnostics. However, the efficient and timely synthesis of appropriately labeled compounds is a largely unsolved problem. Numerous small drug-like molecules with high structural diversity can be synthesized via convergent multicomponent reactions (MCRs). The combination of PET labeling with MCR synthesis of biologically active compounds can greatly simplify radioanalytical and imaging-based analysis. In a proof-of-concept study, we optimized robust on-site radiolabeling conditions that were subsequently applied to several structurally different drug-like MCR scaffolds (e.g., arenes, β-lactam, tetrazole, and oxazole). These labeled scaffolds were synthesized via pinacol-derived aryl boronic esters (arylBPin) by copper-mediated oxidative ¹⁸F-fluorination with radiochemical conversions (RCCs) from 15% to 76%.

A Practical Method for the Preparation of 18F-Labeled Aromatic Amino Acids from Nucleophilic [18F]Fluoride and Stannyl Precursors for Electrophilic Radiohalogenation

In a recent contribution of Scott et al., the substrate scope of Cu-mediated nucleophilic radiofluorination with [¹⁸F]KF for the preparation of ¹⁸F-labeled arenes was extended to aryl- and vinylstannanes. Based on these findings, the potential of this reaction for the production of clinically relevant positron emission tomography (PET) tracers was investigated. To this end, Cu-mediated radiofluorodestannylation using trimethyl(phenyl)tin as a model substrate was re-evaluated with respect to different reaction parameters. The resulting labeling protocol was applied for ¹⁸F-fluorination of different electron-rich, -neutral and -poor arylstannyl substrates in RCCs of 16-88%. Furthermore, this method was utilized for the synthesis of ¹⁸F-labeled aromatic amino acids from additionally N-Boc protected commercially available stannyl precursors routinely applied for electrophilic radiohalogenation. Finally, an automated synthesis of 6-[¹⁸F]fluoro-l-m-tyrosine (6-[¹⁸F]FMT), 2-[¹⁸F]fluoro-l-tyrosine (2-[¹⁸F]F-Tyr), 6-[¹⁸F]fluoro-l-3,4-dihydroxyphenylalanine (6-[¹⁸F]FDOPA) and 3-O-methyl-6-[¹⁸F]FDOPA ([¹⁸F]OMFD) was established furnishing these PET probes in isolated radiochemical yields (RCYs) of 32-54% on a preparative scale. Remarkably, the automated radiosynthesis of 6-[¹⁸F]FDOPA afforded an exceptionally high RCY of 54 ± 5% (n = 5).

Derisking the Cu-Mediated 18F-Fluorination of Heterocyclic Positron Emission Tomography Radioligands

Molecules labeled with fluorine-18 (18F) are used in positron emission tomography to visualize, characterize and measure biological processes in the body. Despite recent advances in the incorporation of 18F onto arenes, the development of general and efficient approaches to label radioligands necessary for drug discovery programs remains a significant task. This full account describes a derisking approach toward the radiosynthesis of heterocyclic positron emission tomography (PET) radioligands using the copper-mediated 18F-fluorination of aryl boron reagents with 18F-fluoride as a model reaction. This approach is based on a study examining how the presence of heterocycles commonly used in drug development affects the efficiency of 18F-fluorination for a representative aryl boron reagent, and on the labeling of more than 50 (hetero)aryl boronic esters. This set of data allows for the application of this derisking strategy to the successful radiosynthesis of seven structurally complex pharmaceutically relevant heterocycle-containing molecules.

Preparation and evaluation of L- and D-5-[18F]fluorotryptophan as PET imaging probes for indoleamine and tryptophan 2,3-dioxygenases

Indoleamine and tryptophan 2,3-dioxygenases (IDO1 and TDO2) are pyrrolases catalyzing the oxidative cleavage of the 2,3-double bond of L-tryptophan in kynurenine pathway. In the tumor microenvironment, their increased activity prevents normal immune function, i.e. tumor cell recognition and elimination by cytotoxic T-cells. Consequently, inhibition of the kynurenine pathway may enhance the activity of cancer immunotherapeutics by reversing immune dysfunction. We sought to investigate the properties of radiolabeled 5-[¹⁸F]fluorotryptophan with respect to its ability for measuring IDO1 and TDO2 activity by positron emission tomography (PET).

RESULTS

L-5-[¹⁸F]fluorotryptophan and D-5-[¹⁸F]fluorotryptophan were synthesized by Cu(I) catalyzed [¹⁸F]fluorodeboronylation of Boc/tBu protected precursors in moderate yields (1.5±0.6%) sufficient for pre-clinical studies. The specific activity of the product was 407-740GBq/μmol, radiochemical purity >99% and enantiomeric excess 90-99%. Enzymatic assay confirmed that L-5-fluorotryptophan is an IDO1 and TDO2 substrate whereas the D-isomer is not. In-vitro cell uptake experiments using CT26 cells with doxycycline-induced overexpression of human-IDO1 and human-TDO2 revealed an elevated cell uptake of L-5-[¹⁸F]fluorotryptophan upon induction of IDO1 or TDO2 enzymes compared to baseline; however, the uptake was observed only in the presence of low L-tryptophan levels in media. PET imaging experiments performed using tumor bearing mouse models expressing IDO1 at various levels (CT26, CT26-hIDO1, 17082A, 17095A) showed tumor uptake of the tracer elevated up to 8%ID/g; however, the observed tumor uptake could not be attributed to IDO1 activity in the tumor tissue. The metabolism of L- and D- isomers was markedly different in vivo, the D-isomer was excreted by a combination of hepatobiliary and renal routes, the L-isomer underwent extensive metabolism to [¹⁸F]fluoride.

CONCLUSION

The observed in vivo tumor uptake of the tracer could not be attributed to IDO1 or TDO2 enzyme activity in the tumor, presumably due to competition with endogenous tryptophan as well as rapid tracer metabolism.