In vitro studies on flubromazolam metabolism and detection of its metabolites in authentic forensic samples

Flubromazolam is a triazole benzodiazepine with high potency and long-lasting central nervous system depressant effects; however, limited data about its pharmacokinetics are available. Here, we report in vitro studies of the human flubromazolam metabolism analyzed by liquid chromatography high-resolution mass spectrometry (LC-HRMS). In vitro investigations were carried out in pooled human liver microsomes (pHLM) and recombinant cytochrome P450 (CYP)-enzymes. To confirm those metabolites detected in vitro, authentic samples obtained from two forensic cases were also analyzed by LC-HRMS. Additionally, determination of the unbound fraction of flubromazolam in pHLM and in plasma was performed by equilibrium dialysis with subsequent prediction of its hepatic clearance (CL_H ) using well-stirred and parallel-tube models. Additional findings obtained by routine screening methods of these forensic cases are also reported. Studies using incubations with nicotinamide adenine dinucleotide phosphate-fortified pHLM with or without uridine 5'-diphosphoglucuronic acid and incubations with CYP-enzymes identified the main metabolic pathway of flubromazolam as hydroxylation on the α- and/or 4-position mediated by CYP3A4 and CYP3A5, with subsequent glucuronidation of the hydroxylated metabolites as well as of the parent drug. Further, α-hydroxy-flubromazolam and its corresponding glucuronide were detected in vivo together with the N-glucuronide of flubromazolam. The predicted CL_H of flubromazolam using the well-stirred and parallel-tube models were 0.42 and 0.43 mL/min/kg, respectively. Based on the data presented here, flubromazolam is primarily metabolized by CYP3A4/5 with a high protein-binding and a predicted low clearance. Analysis of authentic samples suggested that analytical targets for flubromazolam should be the compound itself and α-hydroxy-flubromazolam. Copyright © 2016 John Wiley & Sons, Ltd.