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

Abstract

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 <b>24</b> (hCB₂R p<i>K</i> _i = 7.38 ± 0.05), which had 131-fold selectivity over CB₁R. In a cAMP BRET assay, <b>24</b> behaved as a potent CB₂R inverse agonist. Widefield imaging experiments showed that <b>24</b> 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 <b>24</b> make it a valuable tool for studying CB₂R.