Species-specific in vitro pharmacological effects of the cannabinoid receptor 2 (CB2) selective ligand AM1241 and its resolved enantiomers

Background and purpose: Racemic (R,S) AM1241 is a cannabinoid receptor 2 (CB2)-selective aminoalkylindole with antinociceptive efficacy in animal pain models. The purpose of our studies was to provide a characterization of R, S-AM1241 and its resolved enantiomers in vitro and in vivo. Experimental approach: Competition binding assays were performed using membranes from cell lines expressing recombinant human, rat, and mouse CB2 receptors. Inhibition of cAMP was assayed using intact CB2-expressing cells. A mouse model of visceral pain (para-phenylquinone, PPQ) and a rat model of acute inflammatory pain ( carrageenan) were employed to characterize the compounds in vivo. Key results: In cAMP inhibition assays, R, S-AM1241 was found to be an agonist at human CB2, but an inverse agonist at rat and mouse CB2 receptors. R-AM1241 bound with more than 40-fold higher affinity than S-AM1241, to all three CB2 receptors and displayed a functional profile similar to that of the racemate. In contrast, S-AM1241 was an agonist at all three CB2 receptors. In pain models, S-AM1241 was more efficacious than either R-AM1241 or the racemate. Antagonist blockade demonstrated that the in vivo effects of S-AM1241 were mediated by CB2 receptors. Conclusions and implications: These findings constitute the first in vitro functional assessment of R, S-AM1241 at rodent CB2 receptors and the first characterization of the AM1241 enantiomers in recombinant cell systems and in vivo. The greater antinociceptive efficacy of S-AM1241, the functional CB2 agonist enantiomer of AM1241, is consistent with previous observations that CB2 agonists are effective in relief of pain.