New strategy of a two-step radiosynthesis of [F-18]fluoropyridine-based maleimide-containing prosthetic groups for labelling of peptides and proteins

Abstract

Objectives The use of thiol-reactive groups allows the introduction of fluorine-18 in a peptide or protein with cysteine residues, and different radiofluorinated maleimide-containing prosthetic groups have been described in the literature. All compounds were, however, prepared by a two- or three-step synthesis with an overall reaction time of at least 70 minutes. The aim of this work was to improve the radiosynthesis of maleimide-containing compounds with reduced reaction steps by using a new synthetic route via protection of the maleimide function. This was first done here with the example of 1-[3-(2-[18F]fluoropyridin-3-oxy)propyl]pyrrol-2,5-dione ([18F]FPyME) which is usually prepared in three steps [1]. Methods The first step for the preparation of the precursor was a Williamson reaction to form the ether from 3-hydroxy-2-nitropyridine with 1,3-dibromopropane. In a parallel process maleimide was protected with 2,5-dimethylfurane via a Diels-Alder-reaction. These two products were combined by N-alkylation to form the precursor. The radiosynthesis includes the introduction of fluorine-18 by nucleophilic substitution of the nitro group followed by deprotection of the maleimide function (see fig. 1). The reaction steps were optimized with regard to temperature, time and solvents. Results The optimal conditions for the n.c.a. radiofluorination were identified at a temperature of 80 °C in DMSO and a reaction time of 5 minutes, resulting in a radiochemical yield of about 29 ± 3 %. At higher temperatures the deprotection of the precursor and the labelled compound come to the fore and thereby the decomposition of the unprotected maleimide occurs due to the basic reaction conditions. The deprotection step was quantitatively carried out within 15 minutes. [18F]FPyME was isolated by HPLC to provide the pure prosthetic group which was directly used for effective peptide labelling. The overall synthesis time was about 60 minutes and the overall radiochemical yield was about 20 %. Conclusions The described synthetic route provides the possibility to gain a variety of further [18F]fluoropyridine-based maleimide-containing compounds in two steps only. In addition, this method offers to be performed as one-pot synthesis. Acknowledgements References [1] de Bruin B. et al. (2006) Bioconjugate Chem., 16, 406-420