Labeling of low-density lipoproteins using the ¹⁸F-labeled thiol-reactive reagent N-[6-(4-[¹⁸F]fluorobenzylidene)aminooxyhexyl]maleimide [¹⁸F]FBAM

The novel thiol group-selective bifunctional ¹⁸F-labeling agent N-[6-(4-[¹⁸F]fluoro-benzylidene)aminooxyhexyl]maleimide ([¹⁸F]FBAM) has been developed. The bifunctional labeling precursor 4 containing a thiol-reactive maleimide group and a carbonyl group-reactive aminooxy group was prepared in only three steps in a total chemical yield of 59%.
Subsequent radiolabeling with 4-[¹⁸F]fluorobenzaldehyde gave the bifunctional ¹⁸F-labeling agent [¹⁸F]FBAM in 29% radiochemical yield. In a typical experiment 3.88 GBq of [¹⁸F]fluoride could be converted into 723 MBq of [¹⁸F]FBAM within 69 min. Conjugation of [¹⁸F]FBAM with thiol groups was exemplified with the cystein-containing tripeptide glutathione and with various apolipoproteins of human low density lipoproteins (LDL) subfractions. The latter one was evaluated with respect to uptake of [¹⁸F]FBAM-LDL subfractions in human hepatoma cells (HepG2) in vitro. In vivo biodistribution studies in rats revealed high stability for the [¹⁸F]FBAM-LDL subfractions. Moreover, the metabolic fate of the [¹⁸F]FBAM-LDL subfractions in vivo was delineated by dynamic PET studies using a dedicated small animal tomograph. Data were compared to former studies using the NH2-reactive 18F-labeling agent N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB). Compound [¹⁸F]FBAM can be considered to be an excellent prosthetic group for the selective and mild ¹⁸F-labeling of thiol group-containing biomolecules suitable for subsequent investigations in vitro and in vivo.