Validation of LC/MS/MS method for assessment of the "in vitro" activity of selected rat cytochrome P450 isoenzymes : application to early drug metabolism screening

A sensitive and specific liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for simultaneous determination of seven metabolites of CYP450 model substrates (acetaminophen, 4-hydroxytolbutamide, 4'-hydroxymephenytoin, 1-hydroxybufuralol, 6-hydroxychlorzoxazone. 1'-and 4-hydroxymidazolam) in rat liver microsomes was developed. The assay used Kinetex analytical column and a gradient mobile phase consistent of acetonitrile and water with addition of 0.1\% formic acid. The analysis was performed in selected reaction monitoring (SRM) mode both in positive and negative (for 6-hydroxychlorzoxazone) mode. The method was validated over the concentration ranges of 10-2000 ng/mL for 4-hydroxymephenytoin and 4-hydroxytolbutamide, 50-2000 ng/mL for 1-hydroxybufuralol and 25-2000 ng/mL for the rest of the analytes. The intra- and inter-day precision (2-12%) and accuracy (93-119%) were within the limits set by the FDA and EMA guidelines. The developed method was successfully applied to assess the activity of selected CYP450 isoenzymes in rat liver microsomes after addition of ketoconazole