High affinity electrophilic and photoactivatable covalent endocannabinoid probes for the CB1 receptor.

We have designed and synthesized the first two high affinity covalent anandamide probes for the CB1 receptor by introducing either an electrophilic isothiocyanato or a photoactivatable azido group at the terminal carbon of the arachidonic acid moiety. The headgroup of these anandamide analogues was optimized by using a cyclopropylamide substituent to impart optimal CB1 affinity. Both 20-isothiocyanato-eicosa-5,8,11,14-tetraenoic acid cyclopropylamide (1, AM3677) and 20-azido-eicosa-5,8,11,14-tetraenoic acid cyclopropylamide (2, AM3661) exhibited high selectivities for the CB1 receptor with K(i) values of 1.3 and 0.9 nM, respectively. Using suitable experimental conditions, both ligands were shown to covalently label the CB1 receptor with high efficiency. These two covalent probes for the endocannabinoid CB1 binding site open the door for exploring the ligand binding motifs involved in the activation of the CB1 receptor by its endogenous ligand, anandamide.