Structure-activity relationships defining the ACD-tricyclic cannabinoids: cannabinoid receptor binding and analgesic...

Previous studies of the structure-activity relationships (SAR) for binding of a series of AC-bicyclic cannabinoid structures to the cannabinoid receptors in rat brain (believed to comprise the CB1 subtype) demonstrated the importance of the A-ring aryl C-3 side chain and phenolic hydroxyl substituents, and elucidated the importance of a C-ring hydroxyalkyl substituent [Melvin et al. Mol. Pharmacol. 44, 1008-1015 (1993)]. The present investigation examines the SAR surrounding this region (D-ring) of the molecule that is not present in the structure of delta(9)-THC and other classical cannabinoid compounds. Both rigid fused ring benzo and cyclohexyl derivatives (creating the D-ring) retained binding affinity for the cannabinoid receptor. Extension of ketone or hydroxyl substituents from the C2 position of the D-ring resulted in a 3-fold increase in binding affinity over the unsubstituted structure. However, the fused ring structure is not critical for the interaction with the receptor in as much as opening the ring did not decrease the potency. Extension of the D-ring C-2 alcohol by one carbon in length resulted in a pair of structures, for which the greatest affinity for the CB1 receptor occurred for the hydroxymethyl group in the axial conformation [(+/-)-CP-55,244]. Upon resolution, the latter provided a pair of enantiomers: (-)-CP-55,244 was approximately 3-fold more potent than the racemic mixtures, and (+)-CP-55,244 failed to bind to the CB1 receptor with an IC50 below 1 mM. Opening of the D-ring of these structures resulted in a loss of binding affinity. This study demonstrates that the potency could be optimized in (-)-CP-55,244 for both binding to the CB1 receptor and the biological activity of analgesia. In addition, the rigid positioning of the hydroxypropyl moiety of CP-55,940 enforced by the decalin ring structure of CP-55,244 increased the enantioselectivity by greater than 100-fold. These data define the critical stereochemistry for a region of the nonclassical ACD-tricyclic cannabinoid structure that contributes a potential hydrogen bonding component to the ligand-receptor interaction mechanism. Inasmuch as this region of the molecule is not present on classical ABC-tricyclic cannabinoid compounds, these studies elucidate a unique agonist recognition site on the CB1 receptor.