Synthesis of a New Heterobifunctional Linker, N-[4-(Aminooxy)butyl]maleimide, for Facile Access to a Thiol-Reactive 18F-Labeling Agent

Abstract

A new heterobifunctional linker containing an aldehyde-reactive aminooxy group and a thiol-reactive maleimide group, namely N-[4-(aminooxy)butyl]maleimide, was synthesized as a stable HCl salt by O-alkylation of either N-hydroxyphthalimide or N-(4-monomethoxytrityl)hydroxylamine, followed by N-alkylation of maleimide, in an overall yield of 18% (seven steps) or 29% (five steps), respectively. This heterobifunctional linker allowed a simple and efficient synthesis of a maleimide-containing thiol-reactive ¹⁸F-labeling agent. Thus, N-{4-[(4-[¹⁸F]fluorobenzylidene)aminooxy]butyl}maleimide (specific activity: ∼3000 Ci/mmol at end of synthesis) was synthesized in two steps involving the preparation of 4-[¹⁸F]fluorobenzaldehyde, followed by its aminooxy-aldehyde coupling reaction to the heterobifunctional linker, with an overall radiochemical yield of ∼35% (decay corrected) within ∼60 min from end of bombardment. Initial ¹⁸F-labeling experiments were carried out using a thiol-containing tripeptide glutathione (GSH) and a 5‘-thiol-functionalized oligodeoxynucleotide (5‘-S-ODN) in phosphate-buffered saline (PBS, pH 7.5). After standing at room temperature for 10 min, the ¹⁸F-labeled GSH and 5‘-S-ODN were obtained in ¹⁸F-labeling yields of ∼70% and ∼5% (decay-corrected), respectively. The heterobifunctional linker is easy to synthesize and provides a facile access to the maleimide-containing thiol-reactive ¹⁸F-labeling agent, which could be advantageously employed in the development of ¹⁸F-labeled biomomolecules for use with positron emission tomography.