Enantioselective synthesis of no-carrier-added (NCA) 6-[18F]fluoro-L-DOPA

Advances in the automated synthesis of 6-[18F]Fluoro-L-DOPA

The neurotracer 6-[18F] FDOPA has been, for many years, a powerful tool in PET imaging of neuropsychiatric diseases, movement disorders and brain malignancies. More recently, it also demonstrated good results in the diagnosis of other malignancies such as neuroendocrine tumours, pheochromocytoma or pancreatic adenocarcinoma.The multiple clinical applications of this tracer fostered a very strong interest in the development of new and improved methods for its radiosynthesis. The no-carrier-added nucleophilic 18F-fluorination process has gained increasing attention, in recent years, due to the high molar activities obtained, when compared with the other methods although the radiochemical yield remains low (17-30%). This led to the development of several nucleophilic synthetic processes in order to obtain the product with molar activity, radiochemical yield and enantiomeric purity suitable for human PET studies.Automation of the synthetic processes is crucial for routine clinical use and compliance with GMP requirements. Nevertheless, the complexity of the synthesis makes the production challenging, increasing the chance of failure in routine production. Thus, for large-scale clinical application and wider use of this radiopharmaceutical, progress in the automation of this complex radiosynthesis is of critical importance.This review summarizes the most recent developments of 6-[18F]FDOPA radiosynthesis and discusses the key issues regarding its automation for routine clinical use.

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).

Evaluation of two nucleophilic syntheses routes for the automated synthesis of 6-[18F]fluoro-l-DOPA

Two different strategies for the nucleophilic radiosynthesis of [¹⁸F]F-DOPA were evaluated regarding their applicability for an automated routine production on an Ecker&Ziegler Modular-Lab Standard module. Initially, we evaluated a promising 5-step synthesis based on a chiral, cinchonidine-derived phase-transfer catalyst (cPTC) being described to give the product in high radiochemical yields (RCY), high specific activities (A_S) and high enantiomeric excesses (ee). However, the radiosynthesis of [¹⁸F]F-DOPA based on this strategy showed to be highly complex, giving the intermediate products as well as the final product in insufficient yields for automatization. Furthermore, the automatization proved to be problematic due to incomplete radiochemical conversions and the formation of precipitates during the enantioselective reaction step. Furthermore, the required use of HI at 180°C during the last reaction step led to partial decomposition of lines and seals of the module which further counteracts an automatization. Further on, we evaluated a 3-step synthesis using the commercially available, enantiomerically pure precursor AB1336 for automatization. This synthesis approach gave much better results and [¹⁸F]F-DOPA could be produced fully automated within 114min in RCYs of 20±1%, ee of >96%, a radiochemical purity (RCP) of >98% and specific activities of up to 2.2GBq/μmol.

6-[18F]fluoro-L-DOPA: a well-established neurotracer with expanding application spectrum and strongly improved radiosyntheses

For many years, the main application of [(18)F]F-DOPA has been the PET imaging of neuropsychiatric diseases, movement disorders, and brain malignancies. Recent findings however point to very favorable results of this tracer for the imaging of other malignant diseases such as neuroendocrine tumors, pheochromocytoma, and pancreatic adenocarcinoma expanding its application spectrum. With the application of this tracer in neuroendocrine tumor imaging, improved radiosyntheses have been developed. Among these, the no-carrier-added nucleophilic introduction of fluorine-18, especially, has gained increasing attention as it gives [(18)F]F-DOPA in higher specific activities and shorter reaction times by less intricate synthesis protocols. The nucleophilic syntheses which were developed recently are able to provide [(18)F]F-DOPA by automated syntheses in very high specific activities, radiochemical yields, and enantiomeric purities. This review summarizes the developments in the field of [(18)F]F-DOPA syntheses using electrophilic synthesis pathways as well as recent developments of nucleophilic syntheses of [(18)F]F-DOPA and compares the different synthesis strategies regarding the accessibility and applicability of the products for human in vivo PET tumor imaging.

Efficient preparation of 6-[18F]fluoroveratraldehyde, the precursor of the multi-step synthesis of [18F]FDOPA

As a clinically well established PET tracer, 6-[18F]fluoro-L-dopa is prepared by a multi-step synthesis if [18F]fluoride is to be used for labelling. The [18F] label is introduced in the first step, i.e. 6-[18F]fluoroveratraldehyde is prepared first with yields reported in the literature ranging between 20 and 50%. To obtain higher over-all yields in the synthesis of 6-[18F]fluoro-L-dopa, the 18F incorporation in the first step was optimised by determining the yield dependencies on solvent, concentration of the precursor, temperature and time of the reaction. Thus, the product is formed with a yield of 89 ± 2% (n = 15) when using 20 mg of 6-nitroveratraldehyde in 1 mL DMF at 140 °C within a reaction time of 10 min.

Synthesis and biological evaluation of a fluorine-18-labeled nonsteroidal androgen receptor antagonist, N-(3-[18F]fluoro-4-nitronaphthyl)-cis-5-norbornene-endo-2,3-dicarboxylic imide

INTRODUCTION

Androgen receptor (AR), which is overexpressed in most prostate cancers, is the target of androgen ablation and antiandrogen therapies: it is also the target for the receptor-mediated imaging of AR-positive prostate cancer using radiolabeled ligands. Previous AR imaging agents were based on a steroidal core labeled with fluorine. To develop a novel class of nonsteroidal imaging agents, with binding and pharmacological characteristics that are more similar to those of clinically used AR antagonists, we synthesized N-(3-fluoro-4-nitronaphthyl)-cis-5-norbornene-endo-2,3-dicarboxylic imide (3-F-NNDI), an analog of recently reported AR antagonist ligands.

METHODS

3-F-NNDI was synthesized in six steps starting with 1-nitronaphthalene, with fluorine incorporation as the final step. The labeling of 3-F-NNDI with fluorine-18 was achieved through a novel, extremely mild, S(N)Ar displacement reaction of an o-nitro-activated arene trimethylammonium salt, and 3-[(18)F]F-NNDI was prepared in high specific activity.

RESULTS AND DISCUSSION

3-F-NNDI was found to have an AR-binding affinity similar to that of its parent compound. In vitro assays demonstrated high stability of the labeled compound under physiological conditions in buffer and in the blood. Androgen target tissue uptake in diethylstilbestrol-pretreated male rats, however, was minimal, probably because of extensive metabolic defluorination the radiolabeled ligand.

CONCLUSIONS

This study is part of our first look at a novel class of nonsteroidal AR antagonists as positron emission tomography (PET) imaging agents that are alternatives to steroidal AR agonist-based imaging agents. Although 3-[(18)F]F-NNDI has significant affinity for AR, it showed limited promise as a PET imaging agent because of its poor target tissue distribution properties.

NCA nucleophilic radiofluorination on substituted benzaldehydes for the preparation of [18F]fluorinated aromatic amino acids

Nucleophilic aromatic substitution is a challenging task in radiochemistry. Therefore, a thorough evaluation and optimisation of this step is needed to provide a satisfactory tool for the routine preparation of [(18)F]fluorinated aromatic amino acids. Two methods, already proposed elsewhere, were evaluated and improved. The yields for the radiofluorination were increased whereas activity loss during solid phase extraction was observed. Radiochemical yields for the two methods were 92.7+/-5.5% (method 1) and 92.1+/-12.3% (method 2) for conversion and 11.1+/-2.8% (method 1) and 34.8+/-0.6% (method 2) for purification, respectively. In total, we demonstrate an optimised method for the preparation of this important class of [(18)F]fluorinated synthons for PET.

Catalytic enantioselective synthesis of 18F-fluorinated α-amino acids under phase-transfer conditions using (s)-NOBIN

We describe a new method for the asymmetric synthesis of [(18)F]fluorinated aromatic alpha-amino acids (FAA) under phase transfer conditions using achiral glycine derivative NiPBPGly and (S)-NOBIN as a novel substrate/catalyst pair. The key alkylation step proceeds under mild conditions. Substituted [(18)F]fluorobenzylbromides were prepared using nucleophilic [(18)F]fluoride and were used as alkylation agents. Two important FAA, 2-[(18)F]fluoro-L-tyrosine (2-FTYR) and 6-[(18)F]fluoro-L-3,4-dihydroxyphenylalanine (6-FDOPA), were synthesized with an ee of 92 and 96%, respectively. The total synthesis time was 110-120 min and radiochemical yields (d.c.) were 25+/-6% for 2-FTYR and 16+/-5% for 6-FDOPA.