Phosphorylation of extracellular signal-regulated kinase as a biomarker for cannabinoid receptor 2 activation

Cannabinoid receptor 2 (CB2R) is a therapeutic target in inflammatory diseases; its activation by agonists provides important clinical information, but there are currently no methods to quantify CB2R activation in humans. Chinese hamster ovary (CHO)-K1 cells and mouse and human whole blood cells were used for experiments. CB2R was activated in cells by treatment with the agonist CP55,940. Cells were also pretreated with proprietary Compound A and B (experimental agonists). We developed our method based on the finding that CB2R ligand binding and activation stimulates acute-phase extracellular signal-regulated kinase (ERK) phosphorylation in human and rodent immune cells, after which CB2R becomes unresponsive to stimulation by a second CB2R agonist CP55940 for a certain time period. We detected ERK phosphorylation as a measure of target engagement in mouse and human whole blood cells by flow cytometry. In cells overexpressing human or mouse CB2R, pretreatment with Compound A dose-dependently inhibited ERK phosphorylation for 2 h, prolonging the time window for measuring ERK phosphorylation. Our method enables measurement of CB2R activation by its agonists in human blood cells based on detection of ERK phosphorylation, which is useful for therapeutic drug monitoring and other clinical applications.