Establishment of High-Performance Liquid Chromatography and Enzyme Multiplied Immunoassay Technology Methods for Determination of Free Mycophenolic Acid and Its Application in Chinese Liver Transplant Recipients

The objective of this study is to investigate the correlation between methods of high-performance liquid chromatography (HPLC) and enzyme multiplied immunoassay technology (EMIT) for determination of total mycophenolic acid (tMPA) and free (fMPA) concentration and to study pharmacokinetics of fMPA in Chinese liver transplant recipients. An HPLC method with fluorometric detection and an EMIT assay were established to determine fMPA in plasma ultrafiltrates. Pharmacokinetic parameters of tMPA and fMPA in 51 patients were estimated. The calibration range of fMPA was 0.0025 to 1.0 mu g/mL for the HPLC method and 0.0050 to 0.50 mu g/mL for the EMIT method. Mean recovery of the two methods was 98.0% and 97.1%, respectively. The intraday and interday coefficient of variations were 0.93% to 3.1% and 1.6% to 2.9% for HPLC and 4.51% to 15.8% and 5.83% to 19.5% for EMIT, respectively. The relationship of the two methods was EMIT = 1.074 x HPLC + 0.582 (r(2) = 0.918, n = 470, P < 0.05) for tMPA and EMIT = 1.068 x HPLC + 0.004 (r(2) = 0.945, n = 297, P < 0.05) for fMPA. There was a positive mean bias of EMIT for tMPA (27.0%) and fMPA (23.3%). The AUC(0-12) of tMPA and fMPA obtained by HPLC in 51 patients was 34.7 +/- 11.1 and 0.72 +/- 0.38 mu g.h/mL, respectively. The free fraction of MPA was 2.3% +/- 1.3%, which was significantly correlated with 7-O-glucuronide conjugate of MPA AUC(0-12) (r = 0.705, P < 0.001), albumin (r = -0.529, P < 0.001), and the clearance of creatinine (r = -0.417, r = 0.005). Both HPLC and EMIT assay are suitable for the determination of fMPA. A considerable interindividual variability exists in pharmacokinetics of fMPA among Chinese liver transplant recipients. 7-O-Glucuronide conjugate of MPA and albumin concentrations are two factors correlated to fMPA variance.