Allosteric modulation of the Cannabinoid CB₁ receptor

We investigated the pharmacology of three novel compounds, Org 27569 (5-chloro-3-ethyl-1H-indole-2-carboxylic acid [2-(4-piperidin-1-yl-phenyl)- ethyl]-amide), Org 27759 (3-ethyl-5-fluoro-1H-indole-2-carboxylic acid [2-94-dimethylamino-phenyl)-ethyl]-amide), and Org 29647 (5-chloro-3-ethyl-1H- indole-2-carboxylic acid (1-benzyl-pyrrolidin-3-yl)-amide, 2-enedioic acid salt), at the cannabinoid CB₁ receptor. In equilibrium binding assays, the Org compounds significantly increased the binding of the CB1 receptor agonist [³H]CP 55,940 [(1R,3R,4R)-3-[2-hydroxy-4-(1,1- dimethylheptyl)phenyl]-4-(3-hydroxypropyl) cyclohexan-1-ol], indicative of a positively cooperative allosteric effect. The same compounds caused a significant, but incomplete, decrease in the specific binding of the CB ₁ receptor inverse agonist [³H]SR 141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl) -4-methyl-1H-pyrazole-3-carboximide hydrochloride], indicative of a limited negative binding cooperativity. Analysis of the data according to an allosteric ternary complex model revealed that the estimated affinity of each Org compound was not significantly different when the radioligand was [³H]CP 55,940 or [³H]SR 141716A. However, the estimated cooperatively factor for the interaction between modulator and radioligand was greater than 1 when determined against [³H]CP 55,940 and less than 1 when determined against [³H]SR 141716A. [³H]CP 55,940 dissociation kinetic studies also validated the allosteric nature of the Org compounds, because they all significantly decreased radioligand dissociation. These data suggest that the Org compounds bind allosterically to the CB₁ receptor and elicit a conformational change that increases agonist affinity for the orthosteric binding site. In contrast to the binding assays, however, the Org compounds behaved as insurmountable antagonists of receptor function; in the reporter gene assay, the guanosine 5′-O-(3-[³⁵S]thio)triphosphate binding assay and the mouse vas deferens assay they elicited a significant reduction in the E_max value for CB₁ receptor agonists. The data presented clearly demonstrate, for the first time, that the cannabinoid CB ₁ receptor contains an allosteric binding site that can be recognized by synthetic small molecule ligands.