Filgotinib is a selective JAK1 (Janus kinase) inhibitor, filed in Japan for the treatment of rheumatoid arthritis. In this paper we are reporting a validated LC-MS/MS (liquid chromatography coupled with tandem with mass spectrometry) for the quantification of filgotinib in rat plasma using tofacitinib as an internal standard (IS) as per the FDA regulatory guideline. Filgotinib and the IS were extracted from rat plasma using ethyl acetate as an extraction solvent and chromatographed using an isocratic mobile phase (0.2% formic acid:acetonitrile; 20:80, v/v) at a flow-rate of 0.9 ml/min on a Gemini C column. Filgotinib and the IS eluted at ~1.31 and 0.89 min, respectively. The MS/MS ion transitions monitored were m/z 426.3→291.3 and m/z 313.2→149.2 for filgotinib and the IS, respectively. The calibration range was 0.78-1924 ng/ml. No matrix effect and carry over were observed. Intra- and inter-day accuracies and precisions were within the acceptance range. Filgotinib was stable for three freeze/thaw cycles, on bench-top up to 6 h, in auto-sampler up to 21 h and for one month at -80°C. This novel method has been applied to a pharmacokinetic study in rats.
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