Biotransformation of the Flame Retardant 1,2-Dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) in Vitro by Human Liver Microsomes

The technical mixture of 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH or DBE-DBCH) and the pure β-TBECH isomer were subjected to in vitro biotransformation by human liver microsomes (HLM). After 60 min of incubation, 5 potential metabolites of TBECH were identified in microsomal assays of both the TBECH mixture and β-TBECH using ultraperformance liquid chromatography–Q-Exactive Orbitrap mass spectrometry. These include mono- and dihydroxylated TBECH and mono- and dihydroxylated TriBECH as well as an α-oxidation metabolite bromo-(1,2-dibromocyclohexyl)-acetic acid. Our results indicate potential hepatic biotransformation of TBECH via cyctochrome P450-catalyzed hydroxylation, debromination, and α-oxidation. Kinetic studies revealed that the formation of monohydroxy-TBECH, dihydroxy-TBECH, and monohydroxy-TriBECH were best fitted to a Michaelis–Menten enzyme kinetic model. Respective estimated V max values (maximum metabolic rate) for these metabolites were 11.8 ± 4, 0.6 ± 0.1, and 10.1 ± 0.8 pmol min –1 mg protein –1 in TBECH mixture and 4992 ± 1340, 14.1 ± 4.9, and 66.1 ± 7.3 pmol min –1 mg protein –1 in β-TBECH. This indicates monohydroxy-TBECH as the major metabolite of TBECH by in vitro HLM-based assay. The estimated in vitro intrinsic clearance (Cl int ) of TBECH mixture was slower ( P < 0.05) than that of pure β-TBECH. While the formation of monohydroxy-TBECH may reduce the bioaccumulation potential and provide a useful biomarker for monitoring TBECH exposure, further studies are required to fully understand the levels and toxicological implications of the identified metabolites.