Identification of MT-45 metabolites: in silico prediction, in vitro incubation with rat hepatocytes and in vivo confirmation

MT-45 is a synthetic opioid with a pharmacological activity comparable to morphine and it has been involved in intoxications and fatalities reported in Europe and in USA. It was recently subject to control measures, but to date the metabolic pathways of the substance are still unknown. Using rat hepatocytes and LC-HRMS, 14 novel Phase I and II MT-45 metabolites were identified, products of monohydroxylation, dihydroxylation and N-dealkylation; glucuronide conjugation of mono- and dihydroxylated metabolites also occurred. The detected metabolites were firstly predicted in silico, then incubation of the drug with rat hepatocytes was carried out and the obtained metabolites were identified by LC-HRMS, with retention times, mass shift between theoretical mass and observed mass (<5 ppm), peak abundance and fragmentation pattern. Hydroxylated MT-45 was found to be the major metabolite of MT-45 in vitro experiments. The presence of all metabolites was confirmed by in vivo experiments in urine samples of CD-1 male mice; in these samples hydroxy-MT-45-glucuronide and di-hydroxy-MT-45-glucuronide are the most abundant metabolites, while the parent drug is found at concentration <10 ng mL-1after 300 min. The knowledge of Phase I and II MT-45 metabolite structure is then crucial to develop analytical methods toidentify MT-45 consumption in clinical and forensic testing.