Structural Basis of Species-Dependent Differential Affinity of 6-Alkoxy-5-Aryl-3-Pyridinecarboxamide Cannabinoid-1 Receptor Antagonists

6-Alkoxy-5-aryl-3-pyridincarboxamides, including the brain-penetrant compound 14g [5-(4-chlorophenyl)-6-(cyclopropylmethoxy)-N-[(1R,2R)-2-hydroxy-cyclohexyl]-3-pyridinecarboxamide] and its peripherally restricted analog 14h [5-(4-chlorophenyl)N-[(1R,2R)-2-hydroxycyclohexyl]-6-(2-methoxyethoxy)-3-pyridinecarboxamide], have been recently introduced as selective, high-affinity antagonists of the human cannabinoid-1 receptor (hCB(1)R). Binding analyses revealed two orders of magnitude lower affinity of these compounds for mouse and rat versus human CB1R, whereas the affinity of rimonabant is comparable for all three CB(1)Rs. Modeling of ligand binding to CB1R and binding assays with native and mutant (Ile105Met) hCB(1)Rs indicate that the Ile105 to Met mutation in rodent CB(1)Rs accounts for the species-dependent affinity of 14g and 14h. Our work identifies Ile105 as a new pharmacophore component for developing better hCB(1)R antagonists and invalidates rodent models for assessing the antiobesity efficacy of 14g and 14h.