Radioligands for positron emission tomography imaging of cannabinoid type 2 receptor

The cannabinoid type 2 (CB2) receptor is an immunomodulatory receptor mainly expressed in peripheral cells and organs of the immune system. The expression level of CB2 in the central nervous system under physiological conditions is negligible, however under neuroinflammatory conditions an upregulation of CB2 protein or mRNA mainly colocalized with activated microglial cells has been reported. Consequently, CB2 agonists have been confirmed to play a role in neuroprotective and anti‐inflammatory processes. A suitable positron emission tomography radioligand for imaging CB2 would provide an invaluable research tool to explore the role of CB2 receptor expression in inflammatory disorders. In this review, we provide a summary of so far published CB2 radioligands as well as their in vitro and in vivo binding characteristics. Upregulation of the cannabinoid type 2 (CB2) receptor has been reported in distinct inflammatory diseases, and CB2 has become an attractive biological target for therapeutic as well as diagnostic applications. Many CB2 agonists have shown protective and anti‐inflammatory effects in many pathological disorders both in the central nervous system and in the periphery. Therefore, a suitable radioligand for positron emission tomography imaging of CB2 would provide an important diagnostic as well as research tool.