Discovering the major metabolites of the three novel fentanyl analogues 3-methylcrotonylfentanyl, furanylbenzylfentanyl, and 4-fluorocyclopropylbenzylfentanyl for forensic case work

The highly potent opioid analgesic fentanyl and its analogues are involved in an increasing number of overdose deaths worldwide. New fentanyl analogues are continuously emerging, and there is a lack of knowledge concerning the metabolism of these compounds. The determination of fentanyl analogues can be challenging due to their low circulating concentrations and rapid and extensive metabolism, making metabolite identification necessary for confirming drug intake. The aim of this study was to discover and elucidate the structures of the major metabolites of the three novel fentanyl analogues 3-methylcrotonylfentanyl (3-MCF), furanylbenzylfentanyl (FBF), and 4-fluorocyclopropylbenzylfentanyl (4-FCBF), which were all seized at European borders in 2018. 3-MCF, FBF, or 4-FCBF was incubated with human liver microsomes and human hepatocytes for up to 4 h. The metabolites formed were separated by ultra-high-performance liquid chromatography using an octadecyl silica column employing solvent gradient elution with a mobile phase consisting of ammonium formate and methanol. The compounds were detected by quadrupole time-of-flight mass spectrometry. The major metabolites of 3-MCF were formed by N-dealkylation, carboxylation, oxidation, or hydroxylation of the 3-methyl-2-butene, and hydroxylation of both the 3-methyl-2-butene and the piperidine ring. FBF was metabolized through N-dealkylation, amide hydrolysis with/without subsequent hydroxylation at the N-phenyl, and dihydrodiol formation at the furan ring. 4-FCBF metabolism was dominated by N-dealkylation and N-oxidation at the piperidine ring. In the present study, we successfully discovered and elucidated the structures of the major metabolites of 3-MCF, FBF, and 4-FCBF which could be used as markers to confirm intake of these compounds in forensic case work.