Radiofluorination of non-activated aromatic prosthetic groups for synthesis and evaluation of fluorine-18 labelled ghrelin(1–8) analogues

Abstract

The growth hormone secretagogue receptor 1a (GHSR) is differentially expressed in various disease states compared to healthy tissues and thus is a target for molecular imaging. The endogenous ligand for the GHSR is ghrelin, a 28 amino acid peptide with a unique octanoyl group on the serine-3 residue. A recently reported ghrelin analogue revealed the successful use of fluorine-containing, polycyclic aromatic groups in place of the octanoyl side chain, thereby providing potential access to new ¹⁸F-PET imaging probes. The peptide [Inp¹,Dpr³(6-FN),1Nal⁴,Thr⁸]ghrelin(1–8) amide (1) showed sub-nanomolar receptor affinity (IC₅₀ = 0.11 nM) toward the GHSR making it the strongest affinity ghrelin analogue reported to date. However, attempts to label such non-activated aromatic groups with fluoride-18 through conventional substitution methods resulted in low radiochemical yields, impractical for use in vivo. Since larger, non-activated aromatic groups appear to be of value for incorporating fluorine into ghrelin(1–8) analogues, an additional peptide bearing a 4′-fluorobiphenyl-4-carboxyl (4′-FBC) group in place of the octanoyl side chain was also of interest. Herein, we describe the radiosynthesis of [Inp¹,Dpr³([¹⁸F]6-FN),1Nal⁴,Thr⁸]ghrelin(1–8) amide ([¹⁸F]1) and [Inp¹,Dpr³([¹⁸F]4′-FBC),1Nal⁴,Thr⁸]ghrelin(1–8) amide ([¹⁸F]2) using a prosthetic group approach from iodonium ylide precursors as well as initial in vitro and in vivo evaluation of [¹⁸F]1 as a potential PET tracer for targeted imaging of the GHSR.