Title: Novel bicyclic and tricyclic cannabinergic ligands as therapeutic agents
Abstract: The endocannabinoid system (ECS) encompasses of cannabinoid receptors CB1 and CB2, their endogenous ligands (endocannabinoids), enzymes, proteins and FAAH-like anandamide transporter (FLAT), involved in endocannabinoid formation, transportation and bio-inactivation.1-3 The ECS is shown to be involved in an ever increasing number of pathological conditions including pain, immune-suppression and appetite disorders and has become an important target for the development of novel medications. Δ9-THC, the major psychoactive ingredient of marijuana, produces its physiological response through interaction with two well-characterized cannabinoid receptors (CB1 and CB2).4,5 Extensive structure-activity relationship (SAR) studies on tricyclic classical cannabinoids have established that the C-3 side chain plays a pivotal role in the ligand's affinity, selectivity, as well as potency towards these two receptors. Earlier our laboratory reported that AM411, a tricyclic cannabinoid bearing a pendant 1-adamantyl group at the C-3 position exhibits improved affinity and selectivity for CB1 as compared to Δ-9-THC.6 Our present work involves optimizing other structural features within the tricyclic template to further improve CB1 affinity, selectivity, potency and in vivo profile. Variations in the Northern Aliphatic Hydroxyl (NAH) region and the Southern Aliphatic Hydroxyl (SAH) region of the optimized template were carried out to optimize molecule's interaction with CB1 and CB2 receptors. This work has identified AM8607 as very potent and highly efficacious CB1-selective ligand. It is also functionally biased ligand with high potency in cAMP assay with no activity in β-arrestin assay. Also, AM8607 is much more potent in in vivo studies such as edema and allodynia experiments when compared with THC. This compound showed minimum tolerance development as compared to THC in tetrad experiments. When compared with diclofenac, a well-known painkiller, AM8607 was found to be about 50-fold more potent as analesic in an anti-allodynia experiment. With this potent and selective ligand in hand, we further explored SAR in the southern binding pocket. Also, to advance AM8607 for further in vivo studies, this compound was needed in larger quantities. With the original 23-steps synthesis it was not amenable to explore further SAR and synthesize AM8607 on large scale to enhance the project further. A shorter and more efficient synthetic route was optimized and developed to make AM8607 in good yield. Overall yield was improved from 1.8% in the earlier route to 2.2% in the new synthetic route. This synthetic route yielded a mixture of two enantiomers which can be fully separated on chiral HPLC to give the desired SAH enantiomer.