Design and Synthesis of an Easily Obtainable Maleimide Reagent N-[2-(4-[ 18 F]fluoro-N-methylbenzenesulfonamido)ethyl]maleimide ([ 18 F]FBSEM) to Radiolabel Thiols in Proteins.

An easily obtainable thiol-selective labeling reagent [18 F]FBSEM (N-[2-(4-[18 F]fluoro-N-methylbenzenesulfonamido)ethyl]maleimide) was developed. The advantage of the design is that the precursor and [18 F]FBSEM have the same backbone and backbone construction is not required; in contrast, known thiol-specific labeling reagents do require backbone construction, and this is thought to be the cause of their complicated synthesis. [18 F]FBSEM was successfully obtained in higher yield (25%) and in a simpler way (two fluorination and deprotection steps in 65 min) than the widely used [18 F]FBEM (N-[2-(4-[18 F]fluorobenzamide)ethyl]maleimide). The labeling efficacy of [18 F]FBSEM was confirmed by conjugation with glutathione. [18 F]FBSEM is a promising labeling agent for proteins.