Biotransformation of [¹⁴C]midazolam in the rat in vitro and in vivo

Abstract

1. Aerobic incubation of the maleate salt of midazolam, 8-chloro-6-(2-fluorophenyl)-1-methyl-4H-imidazo[1,5-a][1,4]benzodiazepine, with rat liver 9000 g supernatant fluid fortified with an NADPH-generating system yielded three metabolites. These were identified as the 1-hydroxymethyl derivative formed by oxidation of the methyl substituent on the imidazo moiety, the derivative hydroxylated at C-4 of the diazepine ring, and the corresponding dihydroxy derivative.

2. Pretreatment of rats with phenobarbital stimulated the oxidative metabolism in vitro of midazolam by 9000 g liver supernatant.

3. Rats given 2.5 mg/kg of [¹⁴C]midazolam maleate i.v. excreted 81% of the administered radioactivity in the faeces and 10% in the urine in 24 h. The highest levels of ¹⁴C were in the liver and the lowest in the brain during the first day.

4. Four phenolic derivatives of midazolam were identified in rat bile. These were the 4′-hydroxy and the 4′-hydroxy-1-hydroxymethyl derivatives of midazolam and two benzophenones. Evidence that the benzophenones were artifacts derived from the 4-hydroxy derivatives of the two biliary benzodiazepine metabolites is presented. These biliary metabolites were excreted as glucuronide and/or sulphate conjugates.

5. The propensity of the rat to excrete phenolic metabolites of midazolam in bile is in agreement with the metabolism of several other benzodiazepines in this species.