Validated LC-MS/MS method for quantitation of total and free mycophenolic acid concentration and its application to a pharmacokinetic study in pediatric renal transplant recipients.

A simple and sensitive LC-MS/MS method was established to quantify total and free mycophenolic acid plasma concentrations during immunosuppressive medication for pediatric renal transplantation. The chromatographic separation was performed with the Hypersil GOLD C18 column, using a mobile phase consisting of 0.1 % formic acid in water and acetonitrile (60:40, v/v) at an isocratic flow rate of 0.4 mL/min. Agilent 6420 triple quadrupole mass spectrometer was operated via a positive electrospray ionization interface using the transitions m/z 321.14→206.9 for MPA and m/z 324.15→209.9 for MPA-d3 (internal standard). The linearity was between 0.1 to 50 μg/mL for tMPA and 0.0025 to 0.5 μg/mL for fMPA. The within-run and between-run precision were all <5% and accuracy was within 96.23%-107.63%. The validated method was successfully applied to the pharmacokinetic study in 28 pediatric renal recipients. The mean free fraction of MPA in our patients was 0.89% (ranging from 0.62% to 1.25%) and albumin level plays a major role in the variability of free fraction of MPA, thus, in pediatric patients with hypoproteinemia, close free drug monitoring and dose adjustments should be considered to prevent toxicity.