Labeling proteins with fluorine-18 using N-succinimidyl 4-[18F]fluorobenzoate

Two methods were investigated for the no-carrier-added synthesis of N-succinimidyl 4-¹⁸F]fluorobenzoate (S¹⁸F]FB). The first, an attempted nucleophilic aromatic substitution by ¹⁸F]fluoride on N-succinimidyl 4-nitrobenzoate was unsuccessful. The second method involved three steps; ¹⁸F]fluoride for trimethylammonium substitution on 4-formyl-N,N,N-trimethylanilinium triflate, oxidation to 4-¹⁸F]fluorobenzoic acid, followed by reaction with N-hydroxysuccinimide and dicyclohexylcarbodiimide to form S¹⁸F]FB. Total synthesis and purification time was 100 min and the overall radiochemical yield was 25% (decay corrected). A monoclonal antibody F(ab′)₂ fragment could be labeled in 40–60% yield by reaction with S¹⁸F]FB for 15–20 min. The tissue distribution in normal mice and in vitro tumor binding of the antibody F(ab′)₂ labeled by reaction with S¹⁸F]FB were comparable to those observed for the fragment after radioiodination using N-succinimidyl 4-¹²⁵I]iodobenzoate.