Selective Photoaffinity Probe That Enables Assessment of Cannabinoid CB(2) Receptor Expression and Ligand Engagement in Human Cells

[Image: see text] Chemical tools and methods that report on G protein-coupled receptor (GPCR) expression levels and receptor occupancy by small molecules are highly desirable. We report the development of LEI121 as a photoreactive probe to study the type 2 cannabinoid receptor (CB(2)R), a promising GPCR to treat tissue injury and inflammatory diseases. LEI121 is the first CB(2)R-selective bifunctional probe that covalently captures CB(2)R upon photoactivation. An incorporated alkyne serves as ligation handle for the introduction of reporter groups. LEI121 enables target engagement studies and visualization of endogenously expressed CB(2)R in HL-60 as well as primary human immune cells using flow cytometry. Our findings show that strategically functionalized probes allow monitoring of endogenous GPCR expression and engagement in human cells using tandem photoclick chemistry and hold promise as biomarkers in translational drug discovery.