Alcohol-Supported Cu-Mediated 18F-Fluorination of Iodonium Salts under "Minimalist" Conditions

Preparation of NIn-Methyl-6-[18F]fluoro- and 5-Hydroxy-7-[18F]fluorotryptophans as Candidate PET-Tracers for Pathway-Specific Visualization of Tryptophan Metabolism

Tryptophan (Trp) is an essential proteinogenic amino acid and metabolic precursor for several signaling molecules that has been implicated in many physiological and pathological processes. Since the two main branches of Trp metabolism-serotonin biosynthesis and kynurenine pathway-are differently affected by a variety of neurological and neoplastic diseases, selective visualization of these pathways is of high clinical relevance. However, while positron emission tomography (PET) with existing probes can be used for non-invasive assessment of total Trp metabolism, optimal imaging agents for pathway-specific PET imaging are still lacking. In this work, we describe the preparation of two 18F-labeled Trp derivatives, NIn-methyl-6-[18F]fluorotryptophan (NIn-Me-6-[18F]FTrp) and 5-hydroxy-7-[18F]fluorotryptophan (5-HO-7-[18F]FTrp). We also report feasible synthetic routes for the preparation of the hitherto unknown boronate radiolabeling precursors and non-radioactive reference compounds. Under optimized conditions, alcohol-enhanced Cu-mediated radiofluorination of the respective precursors afforded NIn-Me-6-[18F]FTrp and 5-HO-7-[18F]FTrp as application-ready solutions in radiochemical yields of 45 ± 7% and 29 ± 4%, respectively. As such, our work provides access to two promising candidate probes for pathway-specific visualization of Trp metabolism in amounts sufficient for their preclinical evaluation.

One-Pot Radiosynthesis of [18F]Anle138b—5-(3-Bromophenyl)-3-(6-[18F]fluorobenzo[d][1,3]dioxol-5-yl)-1H-pyrazole—A Potential PET Radiotracer Targeting α-Synuclein Aggregates

Availability of PET imaging radiotracers targeting α-synuclein aggregates is important for early diagnosis of Parkinson’s disease and related α-synucleinopathies, as well as for the development of new therapeutics. Derived from a pyrazole backbone, 11C-labelled derivatives of anle138b (3-(1,3-benzodioxol-5-yl)-5-(3-bromophenyl)-1H-pyrazole)—an inhibitor of α-synuclein and prion protein oligomerization—are currently in active development as the candidates for PET imaging α-syn aggregates. This work outlines the synthesis of a radiotracer based on the original structure of anle138b, labelled with fluorine-18 isotope, eminently suitable for PET imaging due to half-life and decay energy characteristics (97% β+ decay, 109.7 min half-life, and 635 keV positron energy). A three-step radiosynthesis was developed starting from 6-[18F]fluoropiperonal (6-[18F]FP) that was prepared using (piperonyl)(phenyl)iodonium bromide as a labelling precursor. The obtained 6-[18F]FP was used directly in the condensation reaction with tosylhydrazide followed by 1,3-cycloaddition of the intermediate with 3′-bromophenylacetylene eliminating any midway without any intermediate purifications. This one-pot approach allowed the complete synthesis of [18F]anle138b within 105 min with RCY of 15 ± 3% (n = 3) and Am in the range of 32–78 GBq/µmol. The [18F]fluoride processing and synthesis were performed in a custom-built semi-automated module, but the method can be implemented in all the modern automated platforms. While there is definitely space for further optimization, the procedure developed is well suited for preclinical studies of this novel radiotracer in animal models and/or cell cultures.

Next Generation Copper Mediators for the Efficient Production of 18 F-Labeled Aromatics

Cu-mediated radiofluorination is a versatile tool for the preparation of ¹⁸ F-labeled (hetero)aromatics. In this work, we systematically evaluated a series of complexes and identified several generally applicable mediators for highly efficient radiofluorination of aryl boronic and stannyl substrates. Utilization of these mediators in nBuOH/DMI or DMI significantly improved ¹⁸ F-labeling yields despite use of lower precursor amounts. Impressively, application of 2.5 μmol aryl boronic acids was sufficient to achieve ¹⁸ F-labeling yields of up to 75 %. The practicality of the novel mediators was demonstrated by efficient production of five PET-tracers and transfer of the method to an automated radiosynthesis module. In addition, (S)-3-[¹⁸ F]FPhe and 6-[¹⁸ F]FDOPA were prepared in activity yields of 23±1 % and 30±3 % using only 2.5 μmol of the corresponding boronic acid or trimethylstannyl precursor.

Optimization of Direct Aromatic 18F-Labeling of Tetrazines

Radiolabeling of tetrazines has gained increasing attention due to their important role in pretargeted imaging or therapy. The most commonly used radionuclide in PET imaging is fluorine-18. For this reason, we have recently developed a method which enables the direct aromatic ¹⁸F-fluorination of tetrazines using stannane precursors through copper-mediated fluorinations. Herein, we further optimized this labeling procedure. 3-(3-fluorophenyl)-1,2,4,5-tetrazine was chosen for this purpose because of its high reactivity and respective limited stability during the labeling process. By optimizing parameters such as elution conditions, precursor amount, catalyst, time or temperature, the radiochemical yield (RCY) could be increased by approximately 30%. These conditions were then applied to optimize the RCY of a recently successfully developed and promising pretargeting imaging agent. This agent could be isolated in a decay corrected RCY of 14 ± 3% and Am of 201 ± 30 GBq/µmol in a synthesis time of 70 min. Consequently, the RCY increased by 27%.

Synthesis and Preclinical Evaluation of [18F]SiFA-PSMA Inhibitors in a Prostate Cancer Model

Positron emission tomography (PET) imaging of prostate-specific membrane antigen (PSMA) with gallium-68 (⁶⁸Ga) and fluorine-18 (¹⁸F) radiotracers has aroused tremendous interest over the past few years. The use of organosilicon-[¹⁸F]fluoride acceptors (SiFA) conjugated to urea-based peptidomimetic PSMA inhibitors provides a "kit-like" multidose synthesis technology. Nine novel ¹⁸F-labeled SiFA-bearing PSMA inhibitors with different linker moieties were synthesized and analyzed for their in vitro binding against [¹²⁵I]I-TAAG-PSMA in LNCaP cells. IC₅₀ values ranged from 58-570 nM. Among all compounds, [¹⁸F]SiFA-Asp₂-PEG₃-PSMA (IC₅₀ = 125 nM) showed the highest tumor uptake in LNCaP tumors (SUV_60min 0.73). A substantial increase in molar activity (A_m) (from 7.5 ± 0.5 to 86 ± 3 GBq/μmol) led to a significant increase in LNCaP tumor uptake (SUV_60min 1.18; Δ 0.45 corresponding to +62%). In vivo blocking with DCFPyL resulted in -32% uptake after 60 min. The SiFA-isotopic exchange chemistry offers a method that is readily adaptable for a "kit-type" labeling procedure and clinical translation.

18F-Fluorination Using Tri-Tert-Butanol Ammonium Iodide as Phase-Transfer Catalyst: An Alternative Minimalist Approach

The ¹⁸F syntheses of tracers for positron emission tomography (PET) typically require several steps, including extraction of [¹⁸F]fluoride from H₂[¹⁸O]O, elution, and drying, prior to nucleophilic substitution reaction, being a laborious and time-consuming process. The elution of [¹⁸F]fluoride is commonly achieved by phase transfer catalysts (PTC) in aqueous solution, which makes azeotropic drying indispensable. The ideal PTC is characterized by a slightly basic nature, its capacity to elute [¹⁸F]fluoride with anhydrous solvents, and its efficient complex formation with [¹⁸F]fluoride during subsequent labeling. Herein, we developed tri-(tert-butanol)-methylammonium iodide (TBMA-I), a quaternary ammonium salt serving as the PTC for ¹⁸F-fluorination reactions. The favorable elution efficiency of [¹⁸F]fluoride using TBMA-I was demonstrated with aprotic and protic solvents, maintaining high ¹⁸F-recoveries of 96-99%. ¹⁸F-labeling reactions using TBMA-I as PTC were studied with aliphatic 1,3-ditosylpropane and aryl pinacol boronate esters as precursors, providing ¹⁸F-labeled products in moderate-to-high radiochemical yields. TBMA-I revealed adequate properties for application to ¹⁸F-fluorination reactions and could be used for elution of [¹⁸F]fluoride with MeOH, omitting an additional base and azeotropic drying prior to ¹⁸F-labeling. We speculate that the tert-alcohol functionality of TBMA-I promotes intermolecular hydrogen bonding, which enhances the elution efficiency and stability of [¹⁸F]fluoride during nucleophilic ¹⁸F-fluorination.

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 FX_FN 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 (A_m), respectively.

Lewis Acid-Facilitated Radiofluorination of MN3PU: A LRRK2 Radiotracer

Background: Temperature-sensitive radiopharmaceutical precursors require lower reaction temperatures (<100 °C) during nucleophilic radiofluorination in order to avoid compound thermolysis, often resulting in sub-optimal radiochemical yields (RCYs). To facilitate nucleophilic aromatic substitution (S_NAr) of nucleofuges commonly used in radiofluorination (e.g., nitro group), we explored the use of Lewis acids as nucleophilic activators to accelerate [¹⁸F]fluoride incorporation at lower temperatures, and thereby increasing RCYs for thermolabile activated precursors. Lewis acid-assisted radiofluorination was exemplified on the temperature-sensitive compound 1-(4-(4-morpholino-7-neopentyl-7H-pyrrolo[2,3-d]pyrimidin-2-yl)phenyl)-3-(6-nitropyridin-3-yl)urea (MN3PU, compound 3) targeting leucine-rich repeat kinase 2 (LRRK2), an important target in the study of Parkinson’s disease and various cancers. Methods: To a vessel containing dried K[¹⁸F]F-K222 complex, a solution of precursor MN3PU ((3), 1 mg; 1.8 μmol) and Lewis acid (6 μL of 0.2 μmol: chromium II chloride (A), ferric nitrite (B) or titanocene dichloride (C)) in 500 μL of N,N-dimethylformamide (DMF) (with 10% t-BuOH for B) were added. Reactions were stirred for 25 min at 90 °C. In parallel, reactions were conducted without the addition of Lewis acids for baseline comparison. After purification via preconditioned Sep-Pak C18 plus cartridges, aliquots were analyzed by analytical radio-HPLC. Results: Non-decay corrected radiochemical yields (ndc RCYs) for [¹⁸F]FMN3PU (7) were improved from 1.7 ± 0.7% (no addition of Lewis acids) to 41 ± 1% using Cr(II) and 37 ± 0.7% using Ti(II)-based Lewis acids, with radiochemical purities of ≥96% and molar activities (A_m) of up to 3.23 ± 1.7 Ci/μmol (120 ± 1.7 GBq/μmol). Conclusion: RCYs of [¹⁸F]FMN3PU (7) improved from ~5% using conventional nucleophilic radiofluorination, up to 41 ± 1% using Lewis-acid supported S_NAr.

Nucleophilic Synthesis of 6-l-[18F]FDOPA. Is Copper-Mediated Radiofluorination the Answer?

Positron emission tomography employing 6-l-[18F]fluoro-3,4-dihydroxyphenylalanine (6-l-[18F]FDOPA) is currently a highly relevant clinical tool for detection of gliomas, neuroendocrine tumors and evaluation of Parkinson's disease progression. Yet, the deficiencies of electrophilic synthesis of 6-l-[18F]FDOPA hold back its wider use. To fulfill growing clinical demands for this radiotracer, novel synthetic strategies via direct nucleophilic 18F-radiloabeling starting from multi-Curie amounts of [18F]fluoride, have been recently introduced. In particular, Cu-mediated radiofluorination of arylpinacol boronates and arylstannanes show significant promise for introduction into clinical practice. In this short review these current developments will be discussed with a focus on their applicability to automation.

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.

Selective synthesis of L-2-[18 F]fluoro-alpha-methylphenylalanine via copper-mediated 18 F-fluorination of (mesityl)(aryl)iodonium salt

L-2-[¹⁸ F]fluoro-alpha-methylphenylalanine (2-[¹⁸ F]FAMP) is a promising amino acid tracer for positron emission tomography (PET) imaging, yet the low production yield of direct electrophilic radiofluorination with [¹⁸ F]F₂ necessitates further optimization of the radiolabeling process. This paper describes a two-step preparation method for L-2-[¹⁸ F]fluoro-alpha-methylphenylalanine (2-[¹⁸ F]FAMP) starting from [¹⁸ F]fluoride. The (Mesityl)(L-alpha-methylphenylalanine)-2-iodonium tetrafluoroborate precursors with various protecting groups were prepared. The copper-mediated ¹⁸ F-fluorination of the iodonium salt precursors successfully produced 2-[¹⁸ F]FAMP. The highest radio chemical conversion of 57.6% was noted with N-Piv-protected (mesityl)(aryl)iodonium salt in the presence of 5 equivalent of Cu (OTf)₂ . Subsequent deprotection with 57% hydrogen iodide produced 2-[¹⁸ F]FAMP within 120 min in 21.4 ± 11.7% overall radiochemical yield with >95% radiochemical purity and an enantiomeric excess >99%. The obtained 2-[¹⁸ F]FAMP showed comparable biodistribution profiles in normal mice with that of the carrier-added 2-[¹⁸ F]FAMP. These results indicate that usefulness of copper mediated ¹⁸ F-fluorination for the production of 2-[¹⁸ F]FAMP, which would facilitate clinical translation of the promising tumor specific amino acid tracer. Individual facilities could adopt either production method based on radioactivity demand and equipment availability.