Automated Synthesis of 4-[18F]fluorobenzaldehyde and 4-[18F]fluorobenzoic Acid as Synthons for Reproducible and Reliable 18F-radiolabelling of Macromolecules
Objectives: 4-[18F]Fluorobenzaldehyde ([18F]FBZ) and 4-[18F]fluorobenzoic acid ([18F]FBA) are important intermediates for the indirect radiolabelling of sensitive peptides and proteins. The objective of this study was to establish a reproducible method for the automated synthesis of these synthons suitable for conjugation to peptides, proteins or amines.
Methods: The radiosyntheses were performed on a TRACERlabFX-FN synthesizer linked to an FDG synthesizer to provide multi-step synthesis capability.
[18F]FBA was prepared in a two-step one-pot reaction of [18F]fluoride, K2CO3, Kryptofix[2.2.2] and tert-butyl-4’-N,N,N-trimethylammonium benzoate triflate in ACN (90 oC, 10 minutes). Hydrolysis with 1M HCl (90 oC, 5 min) followed by SEP-PAK® purification gave the [18F]FBA which was eluted with ACN into a second vial for activation with TSTU to give 4’-[18F]fluorosuccinimidyl benzoate ([18F]FSB).
[18F]FBZ was prepared by the reaction of [18F]fluoride, K2CO3, Kryptofix[2.2.2] and 4’-N,N,N-trimethylammoniumbenzaldehyde triflate in DMF (100oC, 15 min). The [18F]FBZ was diluted with 1M HCl, purified by SEP-PAK® and eluted with methanol into a second reactor vial. Sodium cyanoborohydride, acetic acid and an amine were added, heated to 100oC for 30 min and HPLC purified to obtain the reductively alkylated [18F]fluorobenzylamino derivative.
Results: Two commercial synthesizers were used to prepare [18F]FBA and [18F]FBZ in dual reaction multi-step processes. [18F]FBA was obtained in 25-30% yield, whilst [18F]FBZ could be obtained in 58-70% yield uncorrected. [18F]FBA was further transformed to [18F]FSB suitable for peptide labelling.
Conclusions: The reaction conditions of two significant [18F]F-synthons were optimised on two commercial [18F]F-synthesizers linked in series to provide large scale, reproducible [18F]F-labelled proteins and peptides.