Drug-drug interactions involving cytochrome P450 (CYP450s) are an important factor for evaluation of a new chemical entity (NCE) in drug development. To evaluate the potential inhibitory effects of a NCE on the pharmacokinetics of a cocktail of representative probes of CYP enzymes (midazolam for CYP3A4, tolbutamide for CYP2C9, omeprazole for CYP2C19 and dextromethorphan for CYP2D6) and the safety and tolerability of the NCE in the presence of probe substrates, a high throughput liquid chromatography/tandem mass spectrometry (LC-MS/MS) method was developed and validated for the simultaneous determination of tolbutamide, omeprazole, midazolam and dextromethorphan in human plasma using tolbutamide-d(9), midazolam-d(4), (+/-)-omeprazole-d(3), and dextromethorphan-d(3) as the internal standards (ISs). Human plasma samples of 50 microL were extracted by a simple protein-precipitation procedure and analyzed using a high performance liquid chromatography electrospray tandem mass spectrometer system. Reversed-phase HPLC separation was achieved with a Hypersil GOLD AQ column (50 mm x 4.6 mm, 5 microm). MS/MS detection was set at mass transitions of 271-->172 m/z for tolbutamide, 346-->198 m/z for omeprazole, 326-->291 m/z for midazolam, 272-->171 m/z for dextromethorphan, 280-->172 m/z for tolbutamide-d(9) (IS), 349-->198 m/z for (+/-)-omeprazole-d(3) (IS), 330-->295 m/z for midazolam-d(4) (IS), and 275-->171 m/z for dextromethorphan-d(3) (IS) in positive mode. The high throughput LC-MS/MS method was validated for accuracy, precision, sensitivity, stability, recovery, matrix effects, and calibration range. Acceptable intra-run and inter-run assay precision (<10%) and accuracy (<10%) were achieved over a linear range of 50-50,000 ng/mL for tolbutamide, 1-1000 ng/mL for omeprazole, 0.1-100 ng/mL for midazolam and 0.05-50 ng/mL for dextromethorphan in human plasma. Method robustness was demonstrated by the 100% pass rate of 24 incurred sample analysis runs and all of the 50 clinical study samples used for incurred sample reproducibility (ISR) test having met the acceptance criterion (%Diff within 20%). The overall ISR results for all compounds showed that over 95% of the samples had a %Diff of less than 10%. The method is simple, rapid and rugged, and has been applied successfully to sample analysis in support of a drug-drug interaction study.
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