Syntheses of 5-fluoro-d/l-dopa and [18F]5-fluoro-l-dopa

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.

Sequential Ir/Cu-Mediated Method for the Meta-Selective C-H Radiofluorination of (Hetero)Arenes

This article describes a sequential Ir/Cu-mediated process for the meta-selective C-H radiofluorination of (hetero)arene substrates. In the first step, Ir-catalyzed C(sp2)-H borylation affords (hetero)aryl pinacolboronate (BPin) esters. The intermediate organoboronates are then directly subjected to copper-mediated radiofluorination with [18F]tetrabutylammonium fluoride to afford fluorine-18 labeled (hetero)arenes in high radiochemical yield and radiochemical purity. This entire process is performed on a benchtop without Schlenk or glovebox techniques and circumvents the need to isolate (hetero)aryl boronate esters. The reaction was automated on a TracerLab FXFN module with 1,3-dimethoxybenzene and a meta-tyrosine derivative. The products, [18F]1-fluoro-3,5-dimethoxybenzene and an 18F-labeled meta-tyrosine derivative, were obtained in 37 ± 5% isolated radiochemical yield and >99% radiochemical purity and 25% isolated radiochemical yield and 99% radiochemical purity, and 0.52 Ci/μmol (19.24 GBq/μmol) molar activity (Am), respectively.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

Fluorine-18 radiochemistry, labeling strategies and synthetic routes

Fluorine-18 is the most frequently used radioisotope in positron emission tomography (PET) radiopharmaceuticals in both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.7 min half-life, 635 keV positron energy), along with high specific activity and ease of large scale production, make it an attractive nuclide for radiochemical labeling and molecular imaging. Versatile chemistry including nucleophilic and electrophilic substitutions allows direct or indirect introduction of (18)F into molecules of interest. The significant increase in (18)F radiotracers for PET imaging accentuates the need for simple and efficient (18)F-labeling procedures. In this review, we will describe the current radiosynthesis routes and strategies for (18)F labeling of small molecules and biomolecules.

Syntheses of meta-[F]Fluorobenzaldehyde and meta-[F]Fluorobenzylbromide from Phenyl(3-Formylphenyl) Iodonium Salt Precursors

(18)F-labeled fluorobenzaldehydes and fluorobenzylbromides are useful synthons for the preparation of PET radiopharmaceuticals. Whereas ortho- and para-[(18)F]fluorobenzaldehydes can easily be prepared with high yields, the corresponding meta- derivatives are more problematic. In order to improve the yield of meta-[(18)F]fluorobenzaldehyde we used the corresponding diaryliodonium salt precursors, since diaryliodonium salts had already been used as precursors in preparations of (18)F-labeled electron rich, as well as electron deficient, aromatic rings. Diaryliodonium salts with different counter ions [PhIPhCHO]X (X = Cl, Br, OTs, OTf) were synthesized. (18)F radiolabeling was performed using different bases at different temperatures in the presence of a radical scavenger, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). The best conversion (~80%) to meta-[(18)F] fluorobenzaldehyde was obtained using CsHCO(3) base at a reaction temperature of 110 °C. To study iodonium salt counter ion effects on radiofluorination, each precursor was separately treated with CsF[(18)F]/CsHCO(3) in DMF at 110 °C for 5 min in the presence of TEMPO. Our observed reactivity order was OTs Collapse

Key Words

• ¹⁸f radiolabeling
• iodonium salt precursors
• meta-[¹⁸f]fluorobenzaldehyde
• meta-[¹⁸f]fluorobenzylbromide

Collapse

MESH Headings Collapse

Grants

• ZIA HL005090-05 Intramural NIH HHS

Collapse

Affiliation(s)

Falguni Basuli Imaging Probe Development Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Rockville, MD 20850
Haitao Wu Imaging Probe Development Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Rockville, MD 20850
Gary L. Griffiths Imaging Probe Development Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Rockville, MD 20850

Collapse

COMT inhibitors and metabolism of fluorodopa enantiomers in aggregating cell cultures

Organotypic primary cell cultures of fetal rat brain were used as a model system to study the effect of COMT inhibitors on the cerebral metabolic conversions of fluoro-DOPA enantiomers. The selective COMT inhibitors OR 486 and CGP 28014 were used in conjunction with 5F-L-DOPA, 6F-L-DOPA and 6F-D-DOPA as substrates. Methylation can be clearly reduced by application of OR 486 at nanomolar level, without inhibition of AADC and MAO. The uptake of the substrate is unchanged. CGP 28014, already known to be active only in vivo, has no influence on the metabolic conversion rates of the fluoro-DOPA isomers. These results show that use of this culture system allows statement concerning the in vitro activity of COMT inhibitors. It has not been possible to show an increase of absolute levels of decarboxylation products due to inhibition of COMT, however, but the reduction in levels of methylated product itself may have significance for PET studies of the human brain.