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Long-Term Changes in Mycophenolic Acid Exposure in Combination with Tacrolimus and Corticosteroids Are Dose Dependent and Not Reflected by Trough Plasma Concentration: A Prospective Study in 100 De Novo Renal Allograft Recipients

From the Department of Nephrology and Renal Transplantation (Dr. Kuypers, Dr. Claes, Dr. Evenepoel, Dr. Maes, Dr. Vanrenterghem) and the Department of Abdominal Transplant Surgery (Dr. Coosemans, Dr. Pirenne), University Hospitals Leuven, Belgium.

Tacrolimus and cyclosporine A have different effects on exposure to concomitantly administered mycophenolate mofetil (MMF), measured as the mycophenolic acid (MPA) dose interval area under the plasma concentration versus time curve (AUC_{0-12 h}) or the plasma MPA predose concentration (C₀). This has led to recommendations in using a 50% lower dose of MMF in combination with tacrolimus compared to cyclosporin A. At present, no long-term data are available regarding the pharmacokinetics (PK) of different dosages of MMF in combination with tacrolimus and the clinical variables that influence the dose-exposure relationship of MPA. A prospective 12-month pharmacokinetic study was performed in 100 de novo renal transplant recipients treated with two different MMF dosages (1 g/day vs. 2 g/day) in combination with tacrolimus and corticosteroids. MPA AUC data were collected 7 days, 6 weeks, and 3 and 12 months posttransplantation, and model-independent PK parameters were calculated. Clinical variables that could possibly influence MPA PK were evaluated. The MPA AUC_{0-12 h} significantly increased toward 6 weeks (p < 0.05) but only in the 2-g MMF dosing group. The MPA AUC_{0-12 h} in the 1-g MMF group reached its nadir at 3 months, while in the 2-g MMF group, it remained elevated until 3 months, returning to baseline values by 12 months. This differential evolution in exposure was not only inadequately reflected by the corresponding MPA C₀ concentrations, but the MPA C₀ concentrations also were not significantly different between the two dosing groups at early postgrafting (day 7) and at 12 months. Using multiple stepwise regression analysis, C₀ (r = 0.51, p< 0.0001) and end-of-dose interval MPA plasma concentration (C₁₂: r² = 0.61, p < 0.0001) were found to poorly predict MPA AUC_{0-12 h}, while MPA plasma concentrations at 4 hours (C₄: r² = 0.85, p < 0.0001) and 6 hours postdosing (C₆: r² = 0.83, p < 0.0001) were superior but hampered by a large prediction bias and imprecision. An abbreviated 2-hour AUC measurement (r² = 0.78), using three sampling points (C₀,C₄₀ [MPA plasma concentration 40 min postdosing], C₂), provided the best compromise between a monitoring tool that is theoretically ideal and practically feasible. MPA pharmacokinetics were not influenced by recipient age, gender, and body weight or by serum albumin concentrations, allograft function, or corticosteroid or tacrolimus dose. Mild hepatic dysfunction early after grafting did result in significantly reduced MPA exposure (MPA AUC_{0-12 h}, p = 0.01 and C₀, p = 0.03). In this study, it was demonstrated for the first time that the dynamics of long-term MPA pharmacokinetics in combination with tacrolimus differ according to the daily MMF dose and that this effect is not adequately reflected by MPA trough concentrations. Using the latter as a routine measure for therapeutic drug monitoring might mislead clinicians into drawing wrong conclusions in terms of relating questions of efficacy or toxicity to MPA exposure.

Key Words: Mycophenolic acid • mycophenolate mofetil • clinical pharmacokinetics • tacrolimus • kidney transplantation

Address for reprints: Dirk R. J. Kuypers, Department of Nephrology and Renal Transplantation, University Hospitals Leuven, Herestraat 49, B-3000 Leuven, Belgium.

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