Design, synthesis and biological evaluation of small molecule-based PET radioligands for the 5-hydroxytryptamine 7 receptor

Abstract

The 5-HT₇ receptor is a recently cloned G-protein-coupled receptor (GPCR) that is important in regulating sleep, depression, and circadian rhythms. However, the potential pathophysiological roles of 5-HT₇ have not been fully elucidated, and no 5-HT₇ positron emission tomography (PET) radioligands are available, thus limiting imaging studies of this receptor in humans. Here, we present the radiosynthesis and biological evaluation of 5-(4-([¹¹C]methoxyphenyl)-1-methyl-4-nitro-1H-imidazole ([¹¹C]1) as a new PET ligand for 5-HT₇. Three-dimensional pharmacophore evaluation and docking studies confirmed its high affinity for 5-HT₇, and in vitro binding assays showed that the binding affinity was 16.8 ± 0.9 nM. The specific activity was found to be 48 ± 29 GBq μmol⁻¹ for [¹¹C]1 in a synthetic time of 26 ± 3 min (n = 8), having 38 ± 7% radiochemical yield (decay-corrected) based on [¹¹C]CO₂. Ligand interactions with human serum albumin were studied by fluorescence quenching to obtain a Stern–Volmer plot, which showed a binding constant of 1.15 × 10⁴ M⁻¹. Whole-body biodistribution patterns were evaluated in normal mice by 1 h dynamic PET imaging; this analysis showed rapid clearance of radioactivity from the main peripheral organs, with the exception of the liver. Preliminary PET studies in rat brains showed rapid accumulation of radioactivity in the brain. The regional radioactivity reached a maximum within 0–2 min after the radioligand injection and then decreased rapidly, resulting in minimal radiation burden in the brain during the scan. In summary, this specific biaryl system has shown potential as a 5-HT₇ ligand and further optimization and longitudinal studies may yield the first small molecule-based PET ligand for 5-HT₇ in clinical settings.