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Furosemide, a loop diuretic, causes hepatic necrosis in mice. Previous evidence suggested hepatotoxicity arises from metabolic bioactivation to a chemically reactive metabolite that binds to hepatic proteins. To define the nature of the toxic metabolite, we examined the relationship between furosemide metabolism in CD-1 mice and Wistar rats. Furosemide (1.21 mmol/kg) was shown to cause toxicity in mice, but not rats, at 24 h, without resulting in glutathione depletion. In vivo covalent binding to hepatic protein was 6-fold higher in the mouse (1.57 Ϯ 0.98 nmol equivalent bound/mg protein) than rat (0.26 Ϯ 0.13 nmol equivalent bound/mg protein). In vivo covalent binding to mouse hepatic protein was reduced 14-fold by a predose of the cytochrome P450 (P450) inhibitor, 1-aminobenzotriazole (ABT; 0.11 Ϯ 0.04 nmol equivalent bound/mg protein), which also reduced hepatotoxicity. Administration of [ 14 C]furosemide to bile duct-cannulated rats demonstrated turnover to glutathione conjugate (8.8 Ϯ 2.8%), ␥-ketocarboxylic acid metabolite (22.1 Ϯ 3.3%), N-dealkylated metabolite (21.1 Ϯ 2.9%), and furosemide glucuronide (12.8 Ϯ 1.8%). Furosemide-glutathione conjugate was not observed in bile from mice dosed with [14 C]furosemide. The novel ␥-ketocarboxylic acid, identified by nuclear magnetic resonance spectroscopy, indicates bioactivation of the furan ring. Formation of ␥-ketocarboxylic acid was P450-dependent. In mouse liver microsomes, a ␥-ketoenal furosemide metabolite was trapped, forming an N-acetylcysteine/N-acetyl lysine furosemide adduct. Furosemide (1 mM, 6 h) became irreversibly bound to primary mouse and rat hepatocytes, 0.73 Ϯ 0.1 and 2.44 Ϯ 0.3 nmol equivalent bound/mg protein, respectively, which was significantly reduced in the presence of ABT, 0.11 Ϯ 0.03 and 0.21 Ϯ 0.1 nmol equivalent bound/mg protein, respectively. Furan rings are part of new chemical entities, and mechanisms underlying species differences in toxicity are important to understand to decrease the drug attrition rate.

Keywords 1′-hydroxymidazolam, 4-hydroxymidazolam, glucuronidation, in vitro metabolism, in vivo metabolism, midazolam ---------------------------------------------------------------------- Received WHAT THIS STUDY ADDS• N-glucuronide of midazolam has been quantified in human urine, indicating for the first time that this route of metabolism occurs in vivo.• Metabolism of 4-hydroxymidazolam has been compared with that of 1′-hydroxymidazolam in vitro.• This study provides further evidence, in vitro and in vivo, of the importance of N-glucuronidation in the metabolism of midazolam and its metabolites. AIMSMidazolam (MDZ) is a benzodiazepine used as a CYP3A4 probe in clinical and in vitro studies. A glucuronide metabolite of MDZ has been identified in vitro in human liver microsome (HLM) incubations. The primary aim of this study was to understand the in vivo relevance of this pathway. METHODSAn authentic standard of N-glucuronide was generated from microsomal incubations and isolated using solid-phase extraction. The structure was confirmed using proton nuclear magnetic resonance (NMR) and RESULTSNMR data confirmed conjugation of midazolam N-glucuronide (MDZG) standard to be on the a-nitrogen of the imidazole ring. In vivo, MDZG in the urine accounted for 1-2% of the administered dose. In vitro incubations confirmed UGT1A4 as the enzyme of interest. The pathway exhibited atypical kinetics and a substrate inhibitory cooperative binding model was applied to determine Km (46 mM, 64 mM), Vmax (445 pmol min -1 mg -1, 427 pmol min -1 mg -1) and Ki (58 mM, 79 mM) in HLM and rUGT1A4, respectively. From incubations with HLM and rUGT enzymes, N-glucuronidation of 1′-OH MDZ and 4-OH MDZ is also inferred. CONCLUSIONSA more complete picture of MDZ metabolism and the enzymes involved has been elucidated. Direct N-glucuronidation of MDZ occurs in vivo. Pharmacokinetic modelling using Simcyp™ illustrates an increased role for UGT1A4 under CYP3A inhibited conditions.

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