Alkyl indole-based cannabinoid type 2 receptor tools: Exploration of linker and fluorophore attachment.

Cannabinoid type 2 (CB2) receptor continues to emerge as a promising drug target for many diseases and conditions. New tools for studying CB2 receptor are required to further inform how this receptor functions in healthy and diseased states. The alkyl indole scaffold is a well-recognised ligand for cannabinoid receptors, and in this study the indole C5-7 positions were explored for linker and fluorophore attachment. A new high affinity, CB2 receptor selective inverse agonist was identified (16b) along with a general trend of C5-substituted indoles acting as agonists versus C7-substituted indoles acting as inverse agonists. The indole C7 position was found to be the most tolerant to linker extension and resulted in a high affinity inverse agonist with a medium length linker (19). Although a high affinity fluorescent ligand for CB2 receptor was not identified in this study, the indole C7 position shows great promise for fluorophore or probe attachment.

Development of selective, fluorescent cannabinoid type 2 receptor ligands based on a 1,8-naphthyridin-2-(1H)-one-3-carboxamide scaffold.

Cannabinoid type 2 (CB2) receptor has been implicated in several diseases and conditions, however no CB2 receptor selective drugs have made it to market. The aim of this study was to develop fluorescent ligands as CB2 receptor tools, to enable an increased understanding of CB2 receptor expression and signalling and thereby accelerate drug discovery. Fluorescent ligands have been successfully developed for other receptors, however none with adequate subtype selectivity or imaging properties have been reported for CB2 receptor. A series of 1,8-naphthyridin-2-(1H)-one-3-carboxamides with linkers and fluorophores appended in the N1 and C3-positions were developed. Molecular modelling indicated the C3 cis-cyclohexanol-linked compounds directed the linker out of the CB2 receptor between transmembrane helices 1 and 7. Herein we report fluorescent ligand 32 (hCB2 pK i = 6.33 ± 0.02) as one of the highest affinity, selective CB2 receptor fluorescent ligands reported. Despite 32 displaying poor specific labelling of CB2 receptor, the naphthyridine scaffold with this linker remains highly promising for future development of CB2 receptor tools.