A NOVEL ELECTROPHILIC HIGH-AFFINITY IRREVERSIBLE PROBE FOR THE CANNABINOID RECEPTOR

In order to explore the structural requirements for cannabinoid activi ty we have been involved in the design and synthesis of stereochemical ly defined high affinity probes for the cannabinoid receptor. This eff ort has involved the development of irreversible ligands which will al low us to obtain detailed information on the cannabinoid receptor acti ve site(s). The irreversible ligands, which incorporate highly reactiv e functional groups in a strategic position of the ligand, may form co valent bonds with amino acid residues at the receptor active site or i n the neighborhood of this site. We shall discuss the biochemical prop erties of one of these probes, which incorporates the electrophilic is othiocyanate group into the structure of the highly potent cannabinoid agonist (-)-1',1'-dimethylheptyl-Delta(8)-THC. This ligand, '-isothio cyanato-1',1'-dimethylheptyl-Delta(8)-THC (7'-NCS-DMH-Delta(8)-THC), w as evaluated for its affinity for cannabinoid binding sites using rat forebrain membrane preparations and found to have an apparent IC50 val ue of 660 pM. Incubation of the membrane preparation with a ligand con centration of five times the apparent IC50 resulted in the irreversibl e occupation of nearly all of the receptor specific binding sites.