Synthesis of 4-[18F]fluorophenyl-4'-nitrophenylcarbamate ([18F]FPNPC) and N-(4-[18F]fluorophenyl)-N-(2-maleimidoethyl)urea ([18F]FPMEU) for fluorine-18 labeling biomolecules

Learning Objectives We developed two different ¹⁸F-tags, [¹⁸F]FPNPC and [¹⁸F]FPMEU, for radiolabeling of biomolecules. [¹⁸F]FPNPC can be used to react randomly with primary amines of lysines, while [¹⁸F]FPMEU can conjugate site-specifically to a thiol group of a cysteine.

Summary: A radiosynthesis of [¹⁸F]FPNPC following a described synthesis was carried out. Starting with the radiofluorination of dinitrobenzene to get 4-[¹⁸F]fluoronitrobenzene, reduction of the remaining nitro group using H₃PO₃ and Pd black resulted in 4-[¹⁸F]fluoroaniline. It was activated with 4-nitrophenylchloroformate to give a radiochemically pure [¹⁸F]FPNPC after three solid phase extraction (SPE) purifications. The RCY is up to 60% referred to [¹⁸F]F^- after 65 min of synthesis. The novel thiophilic ¹⁸F-tag, [¹⁸F]FPMEU, can be obtained one step from [¹⁸F]FPNPC after reaction with N-(2-aminoethyl)maleimide. Within an additional 35 min, the final RCY is 40 % (n>10) after SPE purification. A HER2 targeting peptide (KGSGKCCYSL) was chosen as a model compound to be radiolabeled with [¹⁸F]FPNPC. After 15 min, high RCY can be achieved (70%). [¹⁸F]FPMEU conjugation also give high labeling yield (85%), with freshly prepared HER2-targeting peptide using TCEP as reducant agent for thiol group in 30 min. We are optimizing the entire reaction sequences so that it can be adaptable for automation and microfluidic-based radiolabeling. The biological evaluations are well under progress.