Constitutive Activity of Cannabinoid‐2 (CB2) Receptors Plays an Essential Role in the Protean Agonism of (+)AM1241 and L768242

Background and purpose: Cannabinoid‐2 (CB 2 ) receptor‐selective agonists have shown anti‐nociceptive activity in models of neuropathic and inflammatory pain, and the two agonists most widely used, (+/−)AM1241 [(2‐iodo‐5‐nitrophenyl)‐[1‐(1‐methylpiperidin‐2‐ylmethyl)‐1H‐indol‐3‐yl‐methanone] and L768242 [(2,3‐dichloro‐phenyl)‐[5‐methoxy‐2‐methyl‐3‐(2‐morpholin‐4‐yl‐ethyl)‐indol‐1‐yl]‐methanone] (GW405833), have been suggested to be protean agonists. Here we investigated the role of the constitutive activity of CB 2 receptors in (+)AM1241 and L768242 protean agonism. Experimental approach: Pharmacological profiles of CB 2 receptor ligands were evaluated in Chinese hamster ovary cells expressing recombinant human (hCB 2 ) or rat (rCB 2 ) receptors, by measuring modulation of cAMP. To assess the influence of constitutive activity on pharmacological profile, constitutive activity was abolished by pretreatment with AM630 [(6‐iodo‐2‐methyl‐1‐[2‐(4‐morpholinyl)ethyl]‐1H‐indol‐3‐yl](4‐methoxyphenyl) methanone)], followed by extensive washing. Key results: In cell lines expressing either hCB 2 or rCB 2 receptors, (+)AM1241 did not reverse forskolin stimulation of cAMP levels. Conversely, L768242 was an inverse agonist at both hCB 2 and rCB 2 receptors. Abolition of constitutive activity disclosed (+)AM1241 and L768242 agonist activity, while activity of CP55940 [5‐(1,1‐dimethylheptyl)‐2‐[(1R,2R,5R)‐5‐hydroxy‐2‐(3‐hydroxy‐propyl)‐cyclohexyl]‐phenol] was unaffected and AM630 became a neutral antagonist. In presence of constitutively active CB 2 receptors, (+)AM1241 antagonized CP55940, but when constitutive activity was abolished, it acted as a partial agonist with additive or antagonistic behaviour, depending on concentration. Conclusions and implications: These results show that (+)AM1241 and L768242 are protean agonists at both hCB 2 and rCB 2 receptors. Abolition of constitutive activity reveals the agonist activity of these compounds. Thus, differences between in vivo and in vitro profiles of CB 2 receptor agonists could be due to different levels of constitutive activity in recombinant versus native CB 2 receptors.