A developed determination of midazolam and 1′-hydroxymidazolam in plasma by liquid chromatography-mass spectrometry: Application of human pharmacokinetic study for measurement of CYP3A activity

This paper describes sensitive and reliable determination of midazolam (MDZ) and its major metabolite 1′-hydroxymidazolam (1-OHMDZ) in human plasma by liquid chromatography-mass spectrometry (LC-MS) with a sonic spray ionization (SSI) interface. MDZ, 1-OHMDZ and diazepam as an internal standard were extracted from 1 ml of alkalinized plasma using n-hexane-chloroform (70:30, v/v). The extract was injected into an analytical column (YMC-Pak Pro C₁₈, 50 mm × 2.0 mm i.d.). The mobile phase for separation consisted of 10 mM ammonium acetate and methanol (50:50, v/v) and was delivered at a flow-rate of 0.2 ml/min. The drift voltage was 100 V. The sampling aperture was heated at 120 °C and the shield temperature was 260 °C. The total time for chromatographic separation was less than 16 min. The validated concentration ranges of this method were 0.25-50 ng/ml for both MDZ and 1-OHMDZ. Mean recoveries were 93.6% for MDZ and 86.6% for 1-OHMDZ. Intra- and inter-day coefficient variations were less than 6.5 and 5.5% for MDZ, and 6.1 and 5.7% for 1-OHMDZ at 0.3, 4, 20 and 40 ng/ml. The limits of quantification were 0.25 ng/ml for both MDZ and 1-OHMDZ. This method was sensitive and reliable enough for pharmacokinetic studies on healthy volunteers, and was applied for the measurement of CYP3A activity in humans after an intravenous (1 mg) and a single-oral administration (2 mg) of subtherapeutic MDZ dose.