SR 144528, the First Potent and Selective Antagonist of the CB2 Cannabinoid Receptor

TLDR

The study introduces SR 144528 as the first highly potent, selective, orally active antagonist for the CB2 receptor. The compound was characterized by binding assays showing Ki 0.6 nM for CB2 and Ki 400 nM for CB1, and functional assays demonstrating antagonism of CP 55,940‑induced signaling in CB2‑expressing cells. SR 144528 binds CB2 with Ki 0.6 nM, is 700‑fold more selective than CB1, shows no activity at >70 other targets, antagonizes CP 55,940‑induced signaling and MAPK activation in CB2 cells, inhibits B‑cell activation, and orally displaces CP 55,940 binding in mouse spleen without affecting brain CB1, making it a potent tool for studying CB2‑mediated immune functions.

Abstract

Based on both binding and functional data, this study introduces SR 144528 as the first, highly potent, selective and orally active antagonist for the CB2 receptor. This compound which displays subnanomolar affinity (Ki = 0.6 nM) for both the rat spleen and cloned human CB2 receptors has a 700-fold lower affinity (Ki = 400 nM) for both the rat brain and cloned human CB1 receptors. Furthermore it shows no affinity for any of the more than 70 receptors, ion channels or enzymes investigated (IC50 > 10 microM). In vitro, SR 144528 antagonizes the inhibitory effects of the cannabinoid receptor agonist CP 55,940 on forskolin-stimulated adenylyl cyclase activity in cell lines permanently expressing the h CB2 receptor (EC50 = 10 nM) but not in cells expressing the h CB1 (no effect at 10 microM). Furthermore, SR 144528 is able to selectively block the mitogen-activated protein kinase activity induced by CP 55,940 in cell lines expressing h CB2 (IC50 = 39 nM) whereas in cells expressing h CB1 an IC50 value of more than 1 microM is found. In addition, SR 144528 is shown to antagonize the stimulating effects of CP 55,940 on human tonsillar B-cell activation evoked by cross-linking of surface Igs (IC50 = 20 nM). In vivo, after oral administration SR 144528 totally displaced the ex vivo [3H]-CP 55,940 binding to mouse spleen membranes (ED50 = 0.35 mg/kg) with a long duration of action. In contrast, after the oral route it does not interact with the cannabinoid receptor expressed in the mouse brain (CB1). It is expected that SR 144528 will provide a powerful tool to investigate the in vivo functions of the cannabinoid system in the immune response.