AM-251 is an inverse agonist at the CB<sub>1</sub> cannabinoid receptor. AM-251 is structurally very close to rimonabant; both are biarylpyrazole 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<sub>1</sub> receptor (with a K<sub>i</sub> value of 7.5 nM) than rimonabant, which has a K<sub>i</sub> value of 11.5 nM, AM-251 is, however, about two-fold more selective for the CB<sub>1</sub> receptor when compared to rimonabant. Like rimonabant, it is additionally a ü-opioid receptor antagonist that attenuates analgesic effects.
AM251 has shown an in vitro antimelanoma activity against pancreatic and colon cancer cells.