The CB(1) receptor is found principally in the central nervous system and is responsible for the overt physiological effects of cannabinoids. In contrast, the CB(2) receptor is expressed primarily in the immune system and is responsible for few, if any, obvious behavioral effects. Although many cannabinoid receptor ligands show little, or at best modest, selectivity for either receptor, a number of synthetic compounds are known which have significant selectivity for the CB(2) receptor. These include cannabimimetic indoles, such as 1-propyl-2-methyl-3-(1-naphthoyl)indole (JWH-015) and 1-(2,3-dichlorobenzoyl)-2-methyl-3-(2-[1-morpho-lino]ethyl)-5-methoxyindole (L768242), both of which have good affinity for the CB(2) receptor, but weak affinity for the CB(1) receptor. Efforts have been made to develop structure-activity relationships (SAR) at CB(2) for cannabimimetic indoles, but with limited success. Several derivatives of traditional dibenzopyran based cannabinoids have also been found to have significant selectivity for the CB(2) receptor. These include 1-methoxy-Delta(8)-THC derivatives, 1-methoxy-Delta(8)-THC-DMH (L759633), 1-methoxy-Delta(9(11))-THC-DMH (L759656), and 1-methoxy-3-(1',1'-dimethylhexyl)-Delta(8)-THC (JWH-229), plus a number of 1-deoxy-Delta(8)-THC analogues. In particular, 1-deoxy-3-(1',1'-dimethylbutyl)-Delta(8)-THC (JWH-133) shows two hundred-fold selectivity for the CB(2) receptor. Very recently several compounds belonging to other structural groups have also shown selectivity for the CB(2) receptor. This review will describe the current status of the results of these studies and discuss the SAR for these classes of ligands.

Following the identification of the CB2 receptor several groups explored the development of selective ligands for this receptor which occurs principally in the periphery. This led to the discovery that two cannabimimetic indoles, 1-(2, 3-dichlorobenzoyl)-2-methyl-3-(2-[1-morpholino]ethyl)-5-methoxyind ole (L768242) and 2-methyl-1-propyl-3-(1-naphthoyl)indole (JWH-015) have high affinity for the CB2 receptor with low affinity for the CB1 receptor. Shortly thereafter two 1-methoxy-delta8-THC analogues, 1-methoxy-delta8-THC-DMH (L759633) and 1-methoxy-delta9(11)-THC-DMH (L759656), were also found to have high affinity for the CB2 receptor and very little affinity for the CB1 receptor. Almost simultaneously two 1-deoxy-delta8-THC analogues, 1-deoxy-11-hydroxy-delta8-THC-DMH (JWH-051) and 1-deoxy-delta8-THC-DMH (JWH-057) were reported to have high affinity for the CB1 receptor, but even greater affinity for the CB2 receptor. These discoveries gave rise to a concerted effort by Huffman and co-workers to explore the structure-activity relationships (SAR) at CB2 of cannabimimetic indoles and 1-deoxy-delta8-THC analogues. These efforts have resulted in the synthesis and pharmacological evaluation of a number of derivatives of 3-(1-naphthoyl)indoles and 1-deoxy-delta8-THC analogues with various side chains. This review will describe the current status of the results of these studies and discuss the SAR for both these classes of ligands.