A series of fluorescent dibenzodiazepinone-type muscarinic acetylcholine M2 receptor (M2R) ligands was synthesized using various fluorescent dyes (5-TAMRA, λex/λem ≈ 547/576 nm; BODIPY 630/650, λex/λem ≈ 625/640 nm; pyridinium dye Py-1, λex/λem ≈ 611/665 nm and pyridinium dye Py-5, λex/λem ≈ 465/732 nm). All fluorescent probes exhibited high M2R affinity (pKi (radioligand competition binding): 8.75-9.62, pKd (flow cytometry): 8.36–9.19), a very low preference for the M2R over the M1 and M4 receptors and moderate to pronounced M2R selectivity compared to the M3 and M5 receptors. The presented fluorescent ligands are considered useful molecular tools for future studies using methods such as fluorescence anisotropy and BRET based MR binding assays.
