Neurochemical and Behavioral Properties of the Novel Synthesized Naltrexone-Related Mor Antagonist At-99

Abstract

The development of new μ-opioid receptor (MOR) antagonists has been stimulated by the opioid crisis. Our earlier in silico investigations on ligand–MOR receptor interaction features indicated that ligand cis-equatorial conformation of the amine and phenol is the most likely orientation to be observed within the receptor. Here, we synthesized and characterized AT-99 [3-(1,3-dimethyl-cis-5-(methyl(phenethyl)amino)cyclohexyl)phenol] as a new in vivo and in vitro MOR antagonist. AT-99 effectively blocked MOR-mediated G protein activation by the agonists fentanyl and DAMGO, similarly to the reference compound naloxone (NAL). Moreover, AT-99 behaves as a neutral MOR antagonist per se, as it failed to stimulate [35S]GTPγS binding but dose-dependently inhibited DAMGO-induced [35S]GTPγS binding. While 0.1 µM NAL significantly reduced DAMGO potency, AT-99 produced a comparable effect only at its highest tested concentration. Further, in radioligand competition binding assays, AT-99 fully displaced specific [3H]DAMGO binding in a concentration-dependent manner, although with lower affinity than NAL. In vivo, AT-99 (1 or 3 mg/kg i.v.) dose-dependently reduced the increase of dialysate dopamine (DA) in the nucleus accumbens (NAc) shell induced by morphine (1 mg/kg i.v.). Notably, AT-99 at the higher dose tested counteracted the reduction of behavioral rating scale (BRS) induced by morphine. Altogether, these data indicate that, although AT-99 interacts with the MOR relatively weakly, it displays interesting MOR antagonist properties since it might serve as a scaffold to develop more potent MOR antagonists.

Keywords: GTPγS binding, MOR binding, Microdialysis, DOPAMINE, opioids, sedation