Validation of a rapid and sensitive liquid chromatography-tandem mass spectrometry method for free and total mycophenolic acid

Background: Because. mycophenolic acid (MPA) is highly protein bound and because the free fraction is the pharmacologically active portion, a rapid, reliable, and sensitive procedure is required to study the relationship between free MPA and treatment efficacy/toxicity. Liquid chromatography-tandem mass spectrometry is ideally suited for such a method. Methods: Free MPA was isolated from plasma by ultrafiltration. An online extraction cartridge with a column-switching technique, analytical liquid chromatography over an Aqua Perfect C-18 column, and electrospray tandem mass spectrometry was used to quantify free and total MPA. To investigate ion suppression, a continuous infusion of MPA was introduced into the effluent from the HPLC column, and different ultrafiltrates and extracted plasma samples were injected on the column. Results: A chromatographic run time of 4 min separated MPA from metabolites and internal standard, thereby avoiding interference from in-source fragmentation. Ion suppression occurred well before elution of MPA and internal standard. The lower limit of quantification for free MPA was 0.5 mug/L, and the method was linear to 1000 mug/L. Interassay imprecision (CV) was <10% for free MPA (0.5-333 mug/L). Agreement was good for free MPA (n = 52) and total MPA (n = 106) between the proposed method and a validated HPLC method with ultraviolet detection. The Passing-Bablok regression line was: y = 0.95x + 0.27 mug/L for free MPA and y 0.98x + 0.03 mg/L for total MPA. Conclusions: The presented method allows the accurate, precise, and rapid determination of free and total MPA in plasma over a wide analytical range covering the concentrations relevant to pharmacokinetic studies and routine monitoring of this drug. (C) 2004 American Association for Clinical Chemistry.