Identification of the Putative Binding Site of a Novel Benzimidazole (Etazene) and its Metabolites on µ-Opioid Receptor: A Systematic Computational Study and Detection using Human Liver Microsomes

The abuse of novel psychoactive substances (NPS) has risen dramatically worldwide, especially among recreational users and regular abusers. The absence of pharmacological and toxicological data on new, illicitly used, and abused opioids, results in serious adverse effects, including death. The synthetic benzimidazole opioid, Etazene (70 times higher analgesic activity than morphine), gained popularity as a recreational drug on the illegal/darknet market; however, no experimental information is available at the molecular level on the binding mechanism and putative binding site of Etazene and its metabolites at the µ-opioid receptor (MOR). In the present study, we investigated the possibilities of MOR activation by Etazene and its metabolites by studying their binding mechanism and interaction profiles at the active-state MOR using homology modeling, molecular docking, binding free-energy calculations, and all-atom molecular dynamics (MD) simulations. The putative metabolites of Etazene were also predicted using ADMET Predictor 10.1. Etazene was metabolized by human liver microsomes, and an AB SCIEX 3200 mass spectrometer (LC/MS/MS) in multiple reaction monitoring (MRM) mode was used to characterize the metabolites. The molecular docking studies and free-energy calculations showed that Etazene and its metabolites (M1, M2, and M6-M7)) exhibited strong predicted binding affinity at MOR and showed overlapped binding orientation with MOR bound agonist BU72 co-crystalized in the MOR X-ray crystal structure (PDB ID: 5C1M). These results suggest that Etazene and its metabolites may act as a strong MOR agonist, highlighting the necessity of experimental validation. The results of the liver microsomal assay are still a work in progress. Identifying key interactions between Etazene and its metabolites and the MOR is a step towards a better understanding of the target protein and the design of new analogs