AM-251 is an inverse agonist at the CB1cannabinoid receptor. AM-251 is structurally very close to rimonabant; both are biarylpyrazolecannabinoid receptor antagonists. In AM-251, the p-chloro group attached to the phenyl substituent at C-5 of the pyrazole ring is replaced with a p-iodo group. The resulting compound exhibits slightly better binding affinity for the CB1 receptor (with a Ki value of 7.5 nM) than rimonabant, which has a Ki value of 11.5 nM, AM-251 is, however, about two-fold more selective for the CB1 receptor when compared to rimonabant.[1] Like rimonabant, it is additionally a μ-opioid receptor antagonist[2] that attenuates analgesic effects.[3]
^Lan R, Liu Q, Fan P, Lin S, Fernando SR, McCallion D, et al. (February 1999). "Structure-activity relationships of pyrazole derivatives as cannabinoid receptor antagonists". Journal of Medicinal Chemistry. 42 (4): 769–776. doi:10.1021/jm980363y. PMID10052983.