Chromenopyrazole-based High Affinity, Selective Fluorescent Ligands for Cannabinoid Type 2 Receptor

Cannabinoid type 2 receptor (CB2R) is an attractive target for the treatment of pain and inflammatory disorders. Availability of a selective CB2R fluorescent ligand to study CB2R expression and localization in healthy and disease conditions would greatly contribute to improving our understanding of this receptor. Herein, we report a series of chromenopyrazole-based CB2R fluorescent ligands. The highest affinity fluorescent ligand was Cy5-containing 24 (hCB2R pKi = 7.38 ± 0.05), which had 131-fold selectivity over CB1R. In a cAMP BRET assay, 24 behaved as a potent CB2R inverse agonist. Widefield imaging experiments showed that 24 binds to CB2R in live cells with good selectivity and low levels of nonspecific fluorescence. The high affinity, selectivity, and suitable imaging properties of fluorescent ligand 24 make it a valuable tool for studying CB2R.