Simultaneous Quantification of Filgotinib and Its Active Metabolite in Human Plasma Using Liquid Chromatography–Tandem Mass Spectrometry: Validation and Clinical Application

Filgotinib (FLG) is a Janus kinase 1 inhibitor and is metabolized to an active metabolite, GS-829845. There is no report on the method for simultaneous quantification of FLG and GS-829845 in clinical samples. We developed a liquid chromatography–tandem mass spectrometry method for simultaneous determination of FLG and GS-829845 in patient plasma. FLG and GS-829845 were extracted from an aliquot of 50 μL of human plasma by simple deproteinization using methanol. Chromatographic separation was performed using a Shim-pack Scepter C18-120 column with a combined mobile phase of water and methanol containing 0.1% formic acid and gradient elution at a flow rate of 0.2 mL/min. Detection was performed by positive electrospray ionization using a QTRAP 4500 mass spectrometer. The method was validated in the concentration range of 2.5–50 ng/mL for FLG and 250–5000 ng/mL for GS-829845. Intra- and inter-day assay accuracy and precision were within 11.4% and 13.9%, respectively. Recoveries and matrix effects were consistent and reproducible. This developed and fully validated method is simple, rapid, and cost-effective and was used successfully for therapeutic drug monitoring in a patient with rheumatoid arthritis.