Improved synthesis of 4-[¹⁸ F]fluoro-m-hydroxyphenethylguanidine using an iodonium ylide precursor

Fluorine-18 labeled hydroxyphenethylguanidines were recently developed in our laboratory as a new class of PET radiopharmaceuticals for quantifying regional cardiac sympathetic nerve density in heart disease patients. Studies of 4-[¹⁸ F]fluoro-m-hydroxyphenethylguanidine ([¹⁸ F]4F-MHPG) and 3-[¹⁸ F]fluoro-p-hydroxyphenethylguanidine ([¹⁸ F]3F-PHPG) in human subjects have shown that these radiotracers can be used to generate high-resolution maps of regional sympathetic nerve density using the Patlak graphical method. Previously, these compounds were synthesized using iodonium salt precursors, which provided sufficient radiochemical yields for on-site clinical PET studies. However, we were interested in exploring new methods that could offer significantly higher radiochemical yields. Spirocyclic iodonium ylide precursors have recently been established as an attractive new approach to radiofluorination of electron-rich aromatic compounds, offering several advantages over iodonium salt precursors. The goal of this study was to prepare a spirocyclic iodonium ylide precursor for synthesizing [¹⁸ F]4F-MHPG and evaluate its efficacy in production of this radiopharmaceutical. Under optimized automated reaction conditions, the iodonium ylide precursor provided radiochemical yields averaging 7.8% ± 1.4% (n = 8, EOS, not decay corrected), around threefold higher than those achieved previously using an iodonium salt precursor. With further optimization and scale-up, this approach could potentially support commercial distribution of [¹⁸ F]4F-MHPG to PET centers without on-site radiochemistry facilities.