AM-251 is an inverse agonist at the CB1 cannabinoid receptor. AM-251 is structurally very close to SR141716A (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 CB1 receptor (with a Ki value of 7.5nM) than SR141716A, which has a Ki value of 11.5nM, AM-251 is, however, about two-fold more selective for the CB1 receptor when compared to SR141716A.[1]