In vitro and in vivo glucuronidation of midazolam in humans

AIMS

Midazolam (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.

METHODS

An 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 ¹H-¹³C long range correlation experiments. The metabolite was quantified in vivo in human urine samples. Enzyme kinetic behaviour of the pathway was investigated in HLM and recombinant UGT (rUGT) enzymes. Additionally, preliminary experiments were performed with 1′-OH midazolam (1′-OH MDZ) and 4-OH-midazolam (4-OH MDZ) to investigate N-glucuronidation.

RESULTS

NMR data confirmed conjugation of midazolam N-glucuronide (MDZG) standard to be on the α-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 K_m (46 µM, 64 µM), V_max (445 pmol min⁻¹ mg⁻¹, 427 pmol min⁻¹ mg⁻¹) and K_i (58 µM, 79 µM) 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.

CONCLUSIONS

A 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.