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

Cannabinoid type 2 receptor (CB 2 R) is an attractive target for the treatment of pain and inflammatory disorders. Availability of a selective CB 2 R fluorescent ligand to study CB 2 R 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 CB 2 R fluorescent ligands. The highest affinity fluorescent ligand was Cy5-containing 24 (hCB 2 R p K i = 7.38 ± 0.05), which had 131-fold selectivity over CB 1 R. In a cAMP BRET assay, 24 behaved as a potent CB 2 R inverse agonist. Widefield imaging experiments showed that 24 binds to CB 2 R 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 CB 2 R.