Simultaneous determination of a novel oral Janus kinase inhibitor ASP015K and its sulfated metabolite in rat plasma using LC-MS/MS.

A sensitive and selective liquid chromatography with tandem mass spectrometry (LC-MS/MS) was developed for determining the concentrations of novel Janus kinase inhibitor ASP015K and its sulfated metabolite M2 in rat plasma. This method involves solid-phase extraction (SPE) from 25 μL of rat plasma. LC separation was performed on an Inertsil PH-3 column (100 mm L ×4.6 mm I.D., 5 µm) with a mobile phase consisting of 10 mM ammonium acetate and methanol under linear gradient conditions. Analytes were introduced to the LC-MS/MS through an electrospray ionization source and detected in positive-ion mode using selected reaction monitoring. Standard curves were linear from 0.25 to 500 ng/mL (r ≥0.9964). This assay enabled quantification of ASP015K and M2 at a concentration as low as 0.25 ng/mL in rat plasma. Validation data demonstrated that the method is selective, sensitive and accurate. Further, we also successfully applied this method to a preclinical pharmacokinetic study in rats.