AM-251 is an inverse agonist at the CB 1 cannabinoid receptor. AM-251 is structurally very close to rimonabant; both are cannabinoid 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 CB (with a K 1 receptor i value of 7.5 nM) than rimonabant, which has a K i value of 11.5 nM, AM-251 is, however, about two-fold more selective for the CB 1 receptor when compared to rimonabant. Like rimonabant, it is additionally a μ-opioid receptor antagonist. 
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): 769–76. doi: 10.1021/jm980363y. PMID 10052983.
Seely KA, Brents LK, Franks LN, Rajasekaran M, Zimmerman SM, Fantegrossi WE, Prather PL (October 2012). "AM-251 and rimonabant act as direct antagonists at mu-opioid receptors: implications for opioid/cannabinoid interaction studies". Neuropharmacology. 63 (5): 905–15. doi: 10.1016/j.neuropharm.2012.06.046. PMC . 3408547 PMID 22771770.