Base/Cryptand/Metal-Free Automated Nucleophilic Radiofluorination of [18 F]FDOPA from Iodonium Salts: Importance of Hydrogen Carbonate Counterion

Automated synthesis of [89Zr]ZrCl4, [89Zr]ZrDFOSquaramide-bisPh(PSMA) and [89Zr]ZrDFOSquaramide-TATE

BACKGROUND

Automated [89Zr]Zr-radiolabeling processes have the potential to streamline the production of [89Zr]Zr-labelled PET imaging agents. Most radiolabeling protocols use [89Zr][Zr(ox)4]4- as the starting material and oxalate is removed after radiolabeling. In some instances, radiolabeling with [89Zr]ZrCl4 as starting material gives better radiochemical yields at lower reaction temperatures. In this work, a fully-automated process for production of [89Zr]ZrCl4 is reported and its use for the synthesis of [89Zr]ZrDFOSq-bisPhPSMA and [89Zr]ZrDFOSq-TATE.

RESULTS

A simple automated process for the isolation of [89Zr]ZrCl4 by trapping [89Zr][Zr(ox)4]4- on a bicarbonate-activated strong anion exchange cartridge followed by elution with 0.1 M HCl in 1 M NaCl was developed. [89Zr]ZrCl4 was routinely recovered from [89Zr][Zr(ox)4]4- in > 95% yield in mildly acidic solution of 0.1 M HCl in 1 M NaCl using a fully-automated process. The [89Zr]ZrCl4 was neutralized with sodium acetate buffer (0.25 M) removing the requirement for cumbersome manual neutralization with strong base. The mixture of [89Zr]ZrCl4 was used for direct automated radiolabeling reactions to produce [89Zr]Zr-DFOSquaramide-bisPhPSMA and [89Zr]ZrDFOSquaramide-TATE in 80-90% over all RCY in > 95% RCP.

CONCLUSIONS

This method for the production of [89Zr]ZrCl4 does not require removal of HCl by evaporation making this process relatively fast and efficient. The fully automated procedures for the production of [89Zr]ZrCl4 and its use in radiolabeling are well suited to support the centralized and standardized manufacture of multiple dose preparations of zirconium-89 based radiopharmaceuticals.

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.

Ionic Liquids as Organocatalysts for Nucleophilic Fluorination: Concepts and Perspectives

Besides their extremely useful properties as solvent, ionic liquids (ILs) are now considered to be highly instructive tools for enhancing the rates of chemical reactions. The ionic nature of the IL anion and cation seems to be the origin of this fascinating function of ILs as organocatalyst/promoter through their strong Coulombic forces on other ionic species in the reaction and also through the formation of hydrogen bonds with various functional groups in substrates. It is now possible to tailor-make ILs for specific purposes as solvent/promoters in a variety of situations by carefully monitoring these interactions. Despite the enormous potentiality, it seems that the application of ILs as organocatalysts/promoters for chemical reactions have not been fully achieved so far. Herein, we review recent developments of ILs for promoting the nucleophilic reactions, focusing on fluorination. Various aspects of the processes, such as organocatalytic capability, reaction mechanisms and salt effects, are discussed.

Nucleophilic Radiofluorination Using Tri-tert-Butanol Ammonium as a Bifunctional Organocatalyst: Mechanism and Energetics

We present a quantum chemical analysis of the ¹⁸F-fluorination of 1,3-ditosylpropane, promoted by a quaternary ammonium salt (tri-(tert-butanol)-methylammonium iodide (TBMA-I) with moderate to good radiochemical yields (RCYs), experimentally observed by Shinde et al. We obtained the mechanism of the S_N2 process, focusing on the role of the –OH functional groups facilitating the reactions. We found that the counter-cation TBMA⁺ acts as a bifunctional promoter: the –OH groups function as a bidentate ‘anchor’ bridging the nucleophile [¹⁸F]F⁻ and the –OTs leaving group or the third –OH. These electrostatic interactions cooperate for the formation of the transition states of a very compact configuration for facile S_N2 ¹⁸F-fluorination.

A Convenient Route to New (Radio)Fluorinated and (Radio)Iodinated Cyclic Tyrosine Analogs

The use of radiolabeled non-natural amino acids can provide high contrast SPECT/PET metabolic imaging of solid tumors. Among them, radiohalogenated tyrosine analogs (i.e., [¹²³I]IMT, [¹⁸F]FET, [¹⁸F]FDOPA, [¹²³I]8-iodo-L-TIC(OH), etc.) are of particular interest. While radioiodinated derivatives, such as [¹²³I]IMT, are easily available via electrophilic aromatic substitutions, the production of radiofluorinated aryl tyrosine analogs was a long-standing challenge for radiochemists before the development of innovative radiofluorination processes using arylboronate, arylstannane or iodoniums salts as precursors. Surprisingly, despite these methodological advances, no radiofluorinated analogs have been reported for [¹²³I]8-iodo-L-TIC(OH), a very promising radiotracer for SPECT imaging of prostatic tumors. This work describes a convenient synthetic pathway to obtain new radioiodinated and radiofluorinated derivatives of TIC(OH), as well as their non-radiolabeled counterparts. Using organotin compounds as key intermediates, [¹²⁵I]5-iodo-L-TIC(OH), [¹²⁵I]6-iodo-L-TIC(OH) and [¹²⁵I]8-iodo-L-TIC(OH) were efficiently prepared with good radiochemical yield (RCY, 51–78%), high radiochemical purity (RCP, >98%), molar activity (Am, >1.5–2.9 GBq/µmol) and enantiomeric excess (e.e. >99%). The corresponding [¹⁸F]fluoro-L-TIC(OH) derivatives were also successfully obtained by radiofluorination of the organotin precursors in the presence of tetrakis(pyridine)copper(II) triflate and nucleophilic [¹⁸F]F⁻ with 19–28% RCY d.c., high RCP (>98.9%), Am (20–107 GBq/µmol) and e.e. (>99%).

Insights into Elution of Anion Exchange Cartridges: Opening the Path toward Aliphatic 18F-Radiolabeling of Base-Sensitive Tracers

Aliphatic nucleophilic substitution (S_N2) with [¹⁸F]fluoride is the most widely applied method to prepare ¹⁸F-labeled positron emission tomography (PET) tracers. Strong basic conditions commonly used during ¹⁸F-labeling procedures inherently limit or prohibit labeling of base-sensitive scaffolds. The high basicity stems from the tradition to trap [¹⁸F]fluoride on anion exchange cartridges and elute it afterward with basic anions. This sequence is used to facilitate the transfer of [¹⁸F]fluoride from an aqueous to an aprotic organic, polar reaction medium, which is beneficial for S_N2 reactions. Furthermore, this sequence also removes cationic radioactive contaminations from cyclotron-irradiated [¹⁸O]water from which [¹⁸F]fluoride is produced. In this study, we developed an efficient elution procedure resulting in low basicity that permits S_N2 ¹⁸F-labeling of base-sensitive scaffolds. Extensive screening of trapping and elution conditions (>1000 experiments) and studying their influence on the radiochemical yield (RCY) allowed us to identify a suitable procedure for this. Using this procedure, four PET tracers and three synthons could be radiolabeled in substantially higher RCYs (up to 2.5-fold) compared to those of previously published procedures, even from lower precursor amounts. Encouraged by these results, we applied our low-basicity method to the radiolabeling of highly base-sensitive tetrazines, which cannot be labeled using state-of-art direct aliphatic ¹⁸F-labeling procedures. Labeling succeeded in RCYs of up to 20%. We believe that our findings facilitate PET tracer development by opening the path toward simple and direct S_N2 ¹⁸F fluorination of base-sensitive substrates.

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.

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.

Harnessing Ionic Interactions and Hydrogen Bonding for Nucleophilic Fluorination

We review recent works for nucleophilic fluorination of organic compounds in which the Coulombic interactions between ionic species and/or hydrogen bonding affect the outcome of the reaction. S_N2 fluorination of aliphatic compounds promoted by ionic liquids is first discussed, focusing on the mechanistic features for reaction using alkali metal fluorides. The influence of the interplay of ionic liquid cation, anion, nucleophile and counter-cation is treated in detail. The role of ionic liquid as bifunctional (both electrophilic and nucleophilic) activator is envisaged. We also review the S_NAr fluorination of diaryliodonium salts from the same perspective. Nucleophilic fluorination of guanidine-containing of diaryliodonium salts, which are capable of forming hydrogen bonds with the nucleophile, is exemplified as an excellent case where ionic interactions and hydrogen bonding significantly affect the efficiency of reaction. The origin of experimental observation for the strong dependence of fluorination yields on the positions of -Boc protection is understood in terms of the location of the nucleophile with respect to the reaction center, being either close to far from it. Recent advances in the synthesis of [¹⁸F]F-dopa are also cited in relation to S_NAr fluorination of diaryliodonium salts. Discussions are made with a focus on tailor-making promoters and solvent engineering based on ionic interactions and hydrogen bonding.

One-pot synthesis of high molar activity 6-[18F]fluoro-l-DOPA by Cu-mediated fluorination of a BPin precursor

A one-pot two-step synthesis of 6-[18F]fluoro-l-DOPA ([18F]FDOPA) has been developed involving Cu-mediated radiofluorination of a pinacol boronate ester precursor. The method is fully automated, provides [18F]FDOPA in good activity yield (104 ± 16 mCi, 6 ± 1%), excellent radiochemical purity (>99%) and high molar activity (3799 ± 2087 Ci mmol-1), n = 3, and has been validated to produce the radiotracer for human use.

The first radiosynthesis of 2-amino-5-[18F]fluoropyridines via a "minimalist" radiofluorination/palladium-catalyzed amination sequence from anisyl(2-bromopyridinyl)iodonium triflate

The synthesis of 2-amino-5-[¹⁸F]fluoropyridines was achieved in 8-85% yields by palladium-catalyzed reaction of 2-bromo-5-[¹⁸F]fluoropyridine with piperidine, dimethylamine, butylamine, methylpiperazine, benzylamine, aniline and 3-aminopyridine. 2-Bromo-5-[¹⁸F]fluoropyridine was obtained by radiofluorination of anisyl(2-bromopyridinyl-5)iodonium triflate (88% yield). The radiofluorination step was performed under "minimalist" conditions to guarantee a successful subsequent amination reaction.