Structure-Activity Relationship and Functional Evaluation of Cannabinoid Type-1 Receptor

Abstract

The type-1 cannabinoid receptor (CB₁R) is a potential therapeutic target in several pathological conditions, including neuropsychological disorders and neurodegenerative diseases. Owing to their structural diversity, it is not easy to derive general structure-activity relationships (SARs) for CB₁R ligands. In this study, CB₁R ligands were classified into six structural families, and the corresponding SAR was determined for their affinities for CB₁R. In addition, we determined their functional activities for the activation of extracellular signal-regulated kinases (ERKs). Among derivatives of indol-3-yl-methanone, the highest ligand affinity was observed when a pentyl and a naphthalenyl group were attached to the N1 position of the indole ring and the carbon site of the methanone moiety, respectively. In the case of adamantane indazole-3-carboxamide derivatives, the presence of fluorine in the pentyl group, the substituent at the N1 position of the indazole ring, strongly increased the affinity for CB₁R. For (naphthalen-1-yl) methanone derivatives, the presence of 4-alkoxynaphthalene in the methanone moiety was more beneficial for the affinity to CB₁R than that of a heterocyclic ring. The functional activities of the tested compounds, evaluated through ERK assay, were correlated with their affinity for CB₁R, suggesting their agonistic nature. In conclusion, this study provides valuable insight for designing novel ligands for CB₁R, which can be used to control psychiatric disorders and drug abuse.

Keywords: Cannabinoid type 1 receptor; ERK; G protein; Ligand affinity; Structure-activity relationship.

Fig. 1

Characterization of CB1R-mediated ERK activation. HEK-293 cells were transiently transfected with the plasmid encoding CB1R, and the receptor expression levels were maintained between 1.7-2.1 pmol/mg protein. (A) HEK-293 cells expressing CB1R were treated with 100 nM THC for 0-10 min. *p<0.05, **p<0.01 compared to 0 min group (n=3). (B) HEK-293 cells expressing CB1R were treated with 0-1,000 nM THC for 2 min. *p<0.05, **p<0.01 compared to 0 nM group (n=3). (C) HEK-293 cells expressing CB1R were treated with 100 ng/mL PTX in serum-free medium, followed by 100 nM THC for 2 min. ***p<0.001 compared to other groups (n=3).

Fig. 2

ERK activation effects of selected ligands. HEK-293 cells were transiently transfected with the plasmid encoding CB1R, and the receptor expression levels were maintained between 1.7-2.1 pmol/mg protein. (A) Effects of (1H-indol-3-yl)(aryl)methanone derivatives (JWH-018, JWH-210, JWH-073, and AM1248) on ERK activation. CB1R-expressing HEK-293 cells were treated with 100 nM JWH-018, JWH-210, JWH-073, and AM1248 for 2 min. ***p<0.001 compared to each vehicle-treated group (n=3). (B) Effects of JWH-018, AM2233, STS-135, and FUBIMINA on ERK activation. CB1R-expressing HEK-293 cells were treated with 100 nM JWH-018, AM2233, STS-135, and FUBIMINA for 2 min. *p<0.05, ***p<0.001 compared to each vehicle-treated group (n=3). (C) Effects of JWH-018, CB-13, and EG-018 on ERK activation. CB1R-expressing HEK-293 cells were treated with 100 nM JWH-018, CB-13, and EG-018 for 2 min. *p<0.05, **p<0.01 compared to each vehicle-treated group (n=3). (D) Effects of JWH-018, mepirapim, and A-836,339 on ERK activation. CB1R-expressing HEK-293 cells were treated with 100 nM JWH-018, mepirapim, and A-836,339 for 2 min. ***p<0.001 compared to each vehicle-treated group (n=3).