Liquid chromatography–tandem mass spectrometry assay for therapeutic drug monitoring of the tyrosine kinase inhibitor, midostaurin, in plasma from patients with advanced systemic mastocytosis

•First LC–MS/MS assay for the protein kinase inhibitor, midostaurin.•Easy method of sample pre-treatment using protein precipitation with methanol.•The assay was validated in a 75–2500ng/mL calibration range.•The method is robust and satisfactory for trueness, precision and accuracy.•The assay has successfully been used for therapeutic drug monitoring. We developed and validated quantitative bioanalytical liquid chromatography–tandem mass spectrometry assay for the protein kinase inhibitor, midostaurin. Plasma samples were pre-treated using a protein precipitation with methanol containing midostaurin-d5 as an internal standard. After centrifugation, 5μL of the supernatant was injected into the chromatographic system. The system consisted of a 3.5μm particle bonded octadecyl silica column, with gradient elution using a mixture of 0.1% (v/v) formic acid in acetonitrile and 10mM ammonium formate in water with 0.1% formic acid. The analyte was quantified using the selected reaction-monitoring mode of a triple quadrupole mass spectrometer equipped with a heated electrospray interface. The assay was validated in a 75–2500ng/mL calibration range. For quality control, within-day and between-day precisions were 1.2–2.8%, and 1.2–6.9%, respectively. The β-expectation tolerance limit (accuracy) met the limits of acceptance ±15% (±20% for the LLQ). The drug was sufficiently stable under all relevant analytical conditions. The assay has successfully been used to assess drug levels for therapeutic drug monitoring in patients presenting advanced systemic mastocytosis and treated with the promising midostaurin.