A Novel Method for Determination of Methadone in the Serum by High-Performance Liquid Chromatography with Electrochemical Detection

: In March 2013, the clinical use of oral methadone tablets was initiated in Japan. There are many factors responsible for the change in blood concentrations of methadone, and its pharmacokinetics is very complex. Therefore, a simple and accurate measurement method for methadone blood concentrations was developed using HPLC/electrochemical detector (ECD). An eluent of 10 mM Na2HPO4/CH3CN/CH3OH (20 : 19 : 3) was used as the mobile phase. The column was used the XTerra® RP18, and the voltage of the ECD was set at 400 to 800 mV. As a result, the calibration curve was linear in the ranges of 10 to 100 ng/mL (y=5012.7x+1041.1, r=0.999). The intra- and inter-day coefficients of variation were <5.2 and <5.8%, respectively. Therefore, this method was considered to be useful for the measurement of methadone blood levels in cancer patients. Also, using this method, blood methadone concentration was measured over time in a patient with cancer-associated pain who was treated with methadone. The estimated clearance (CL/F) and distribution volume (Vd/F) of methadone were 2.84 L/h and 502.8 L, respectively, and took about two weeks to reach steady state.