Organotin derivatives as versatile precursors for the radioiodination and radiofluorination of 1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (TIC(OH)) analogues

Objective: Organotin compounds are well known as efficient precursors for reliable radioiodination of aromatic derivatives, with high molar activities, via electrophilic aromatic substitution reaction. They can be easily prepared from corresponding halogenated derivatives using metalation or palladium-catalyzed reactions. In the last years, organotin derivatives, together with boronic acid or ester precursors, were also successfully applied to the direct radiolabelling of electron-rich aromatic structures from [18F]F- via Cu-mediated radiofluorination. As evidenced with the progresses reported for the radiosynthesis of the [18F]FDOPA for example, these major developments in the fluorine-18 radiochemistry field created new opportunities to produce radiofluorinated arenes that could not be routinely accessed even a few years ago. Surprisingly, the [123I]8-iodo-L-TIC(OH), a promising radiotracer for SPECT imaging of prostatic tumours, did not benefit from these methodological advances and no corresponding radiofluorinated derivatives, which could allow the use of the TIC(OH) scaffold to PET imaging, were reported so far. Methods: A convergent synthetic route was developed to produce radioiodinated [125I]iodo-L-TIC(OH), and radiofluorinated [18F]fluoro-L-TIC(OH) tracers from common organotin intermediates, synthesized from iodinated analogues via palladium catalyzed I/SnMe3 exchange. The [125I]iodo-L-TIC(OH) radiotracers were obtained by electrophilic radioiododestannylation with [125I]I+, while the radiofluorinated analogues [18F]fluoro-L-TIC(OH) were produced from the organotin precursors by a copper-mediated aromatic radiofluorination using nucleophilic [18F]F-. For control of the purity, molar activity and enantiomeric excess, corresponding non-radiolabelled iodinated and fluorinated derivatives from the L and D series were synthesized. Results: Organotin compounds were radiolabelled using no-carrier-added [125I]NaI in the presence of Chloramine-T as mild oxidative