Trough levels of mycophenolic acid and its glucuronidated metabolite in renal transplant recipients

OBJECTIVE: The prophylactic use of the immunosuppressant prodrug, mycophenolate mofetil (MMF) to prevent graft rejection in renal transplant patients is continuing to increase. We measured trough levels of the active metabolite, mycophenolic acid (MPA) and its inactive glucuronide (MPAG) in renal recipients with the aim of characterizing individual variability and of ascertaining factors influencing trough levels, in particular the effect of differences in renal function and the effect of drugs given concurrently. METHODS: Laboratory and clinical data obtained in 35 renal recipients treated with triple therapy (MMF, cyclosporin A (CsA), steroids) were included in this retrospective study. Trough levels of MPA and MPAG were obtained after transplantation and up to 16 months post transplantation where the mean observation period was 5.7 months. Plasma levels were measured using a validated HPLC assay. RESULTS: A total of 212 plasma concentrations of MPA and 209 of MPAG were measured. There was considerable intra- and interindividual variability in MPA and MPAG trough levels especially in the early post-transplantation phase. At a fixed dose of 2 g/d MMF, the mean MPA level during the first 30 days averaged 1.46 +/- 1.31 microg/ml vs. 1.87 +/- 0.89 microg/ml after 30 days and later (p = 0.130) and the mean MPAG concentration averaged 188.1 = 142.8 [microg/ml vs. 98.09 +/- 52.4 microlg/ml (p 0.003). The MPAG levels were positively correlated with the serum creatinine concentrations (r = 0.815, p < 0.001), and in the case of MPA there was a correlation with the serum protein concentrations (r = 0.258, p = 0.001). Concomitant drug treatment using CsA, steroids and furosemide were without effect of the measured plasma concentrations, but in the case of xipamide (+) and diltiazem (-) an effect on MPA and MPAG levels and a co-effect depending on the serum creatinine could not be excluded. Neither CsA trough levels nor hemoglobin levels were related to MPA and MPAG trough levels. CONCLUSIONS: The data of this study demonstrate that there is substantial individual variability in the trough levels of MPA and MPAG after renal transplantation which may be associated with the functional status of the graft and the serum protein level. Whether comedication with xipamide and diltiazem affects the plasma levels of MPA and MPAG remains to be clarified in further investigations.