Comparison between [18F]fluorination and [18F]fluoroethylation reactions for the synthesis of the PDE10A PET radiotracer [18F]MNI-659

Introduction: 2-(2-(3-(4-(2-[18F]Fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([18F]MNI-659, [18F]1) is a useful PET radiotracer for imaging phosphodiesterase 10A (PDE10A) in human brain. [18F]1 has been previously prepared by direct [18F]fluorination of a tosylate precursor 2 with [18F]F-. The aim of this study was to determine the conditions for the [18F]fluorination reaction to obtain [18F]1 of high quality and with sufficient radioactivity for clinical use in our institute. Moreover, we synthesized [18F]1 by [18F]fluoroethylation of a phenol precursor 3 with [18F]fluoroethyl bromide ([18F]FEtBr), and the outcomes of [18F]fluorination and [18F]fluoroethylation were compared. Methods: We performed the automated synthesis of [18F]1 by [18F]fluorination and [18F]fluoroethylation using a multi-purpose synthesizer. We determined the amounts of tosylate precursor 2 and potassium carbonate as well as the reaction temperature for direct [18F]fluorination.Results: The efficiency of the [18F]fluorination reaction was strongly affected by the amount of 2 and potassium carbonate. Under the determined reaction conditions, [18F]1 with 0.82 ± 0.2 GBq was obtained in 13.6 ± 3.3 % radiochemical yield (n = 8, decay-corrected to EOB and based on [18F]F-) at EOS, starting from 11.5 ± 0.4 GBq of cyclotron-produced [18F]F-. On the other hand, the [18F]fluoroethylation of 3 with [18F]FEtBr produced [18F]1 with 1.0 ± 0.2 GBq and in 22.5 ± 2.5 % radiochemical yields (n = 7, decay-corrected to EOB and based on [18F]F-) at EOS, starting from 7.4 GBq of cyclotron-produced [18F]F-. Clearly, [18F]fluoroethylation resulted in a higher radiochemical yield of [18F]1 than [18F]fluorination.Conclusion: [18F]1 of high quality and with sufficient radioactivity was successfully radiosynthesized by two methods. [18F]1 synthesized by direct [18F]fluorination has been approved and will be provided for clinical use in our institute