Topiroxostat is a recently identified safe and selective xanthine oxidoreductase inhibitor and has been used as a urate-lowing agent for the treatment of hyperuricemia and gout in Japan. In this study, we developed a sensitive, selective and high-throughput method that determines topiroxostat in human plasma using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) and it was firstly validated. The samples were prepared by protein precipitation with acetonitrile and dilution of the supernatant in water. The chromatographic separation was conducted on an ACE Excel 5 C18-PFP column (2.1 x 50.0 mm) with gradient elution using a mixture of buffer (2 mM ammonium acetate in 0.1 % formic acid) and acetonitrile as the mobile phase at a flow rate of 0.45 mL/min. The total run time lasted 4.0 min. Tandem mass spectrometry, was performed in positive ionization and multiple reaction monitoring (MRM) modes, with the transitions of m/z 249.2 -> 221.1 for topiroxostat and m/z 253.2 -> 225.1 for topiroxostat-d(4), respectively. The method was validated and the selectivity, carry-over effect, linearity, accuracy, precision, extraction recovery, matrix effect, and stability were acceptable according to the US Food and Drug Administration and the European Medicines Agency regulations. The method was successfully applied to a clinico-pharmacokinetic study involving twelve healthy Chinese subjects under fasting condition. (C) 2020 Elsevier B.V. All rights reserved.