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

Pharmacogenetic influences on mycophenolate therapy

Mycophenolic acid (MPA) is a cornerstone immunosuppressant therapy in solid organ transplantation. MPA is metabolized by uridine diphosphate glucuronosyltransferase to inactive 7-O-MPA-glucuronide (MPAG). At least three minor metabolites are also formed, including a pharmacologically active acyl-glucuronide. MPA and MPAG are subject to enterohepatic recirculation. Biliary excretion of MPA/MPAG involves several transporters, including organic anion transporting polypeptides and multidrug resistant protein-2 (MRP-2). MPA metabolites are also excreted via the kidney, at least in part by MRP-2. MPA exerts its immunosuppressive effect through the inhibition of inosine-5-monophosphate dehydrogenase. Several SNPs have been identified in the genes encoding for uridine diphosphate glucuronosyltransferase, organic anion transporting polypeptides, MRP-2 and inosine-5-monophosphate dehydrogenase. This article provides an extensive overview of the known effects of these SNPs on the pharmacokinetics and pharmacodynamics of MPA.

Limited sampling strategies drawn within 3 hours postdose poorly predict mycophenolic acid area-under-the-curve after enteric-coated mycophenolate sodium

Previous studies predicted that limited sampling strategies (LSS) for estimation of mycophenolic acid (MPA) area-under-the-curve (AUC(0-12)) after ingestion of enteric-coated mycophenolate sodium (EC-MPS) using a clinically feasible sampling scheme may have poor predictive performance. Failure of LSS was thought to be due to the slow absorption of MPA causing late and variable times of maximum MPA concentration and variable predose concentrations. The aim of this study was to formally test the performance of LSS by developing and validating LSS for estimation of MPA AUC(0-12) after EC-MPS administration. Pharmacokinetic data from 109 renal transplant recipients collected during the maintenance period after transplantation were analysed retrospectively. LSS were developed separately for renal transplant patients who concurrently used cyclosporine (n = 79) and for patients not concurrently treated with cyclosporine (n = 30). Data were split into an index and a validation data set. For clinical feasibility reasons, a LSS could consist of a maximum of 3 sampling time points with the latest sample drawn 2 hours after drug administration. LSS with the latest sample drawn 3 hours after drug administration or even later were also tested. The validation of the developed LSS showed that MPA AUC(0-12) for patients concurrently treated with cyclosporine was best estimated by AUC(0-12) (mg x h x L(-1)) = 36.536 + 1.642 x C0.5 + 0.569 x C1.5 + 0.905 x C2 (r2 = 0.33, bias = -1.0 mg x h x L(-1), precision = 24 mg x h x L(-1)), whereas AUC(0-12) [mg x h x L(-1)] = 19.801 + 1.827 x C0.5 + 1.111 x C1 + 1.429 x C2 was the best AUC(0-12) estimator for patients not cotreated with cyclosporine (r2 = 0.31, bias = 0.4 mg x h x L(-1), precision = 14.5 mg x h x L(-1)). Both LSS showed poor precision and overestimation of AUC(0-12) values below the therapeutic window and underestimation of AUC(0-12) values above the therapeutic window of MPA. Using C3 as latest sampling time point improved the fit slightly, but not satisfactory, with r2 still <0.40 and precision still >14.0 mg x h x L(-1). Estimation of MPA AUC(0-12) with LSS for EC-MPS drawn within 2 or 3 hours postdose in renal transplant recipients in the maintenance period is likely to result in biased and imprecise results.

New insights into the pharmacokinetics and pharmacodynamics of the calcineurin inhibitors and mycophenolic acid: possible consequences for therapeutic drug monitoring in solid organ transplantation

Although therapeutic drug monitoring (TDM) of immunosuppressive drugs has been an integral part of routine clinical practice in solid organ transplantation for many years, ongoing research in the field of immunosuppressive drug metabolism, pharmacokinetics, pharmacogenetics, pharmacodynamics, and clinical TDM keeps yielding new insights that might have future clinical implications. In this review, the authors will highlight some of these new insights for the calcineurin inhibitors (CNIs) cyclosporine and tacrolimus and the antimetabolite mycophenolic acid (MPA) and will discuss the possible consequences. For CNIs, important relevant lessons for TDM can be learned from the results of 2 recently published large CNI minimization trials. Furthermore, because acute rejection and drug-related adverse events do occur despite routine application of CNI TDM, alternative approaches to better predict the dose-concentration-response relationship in the individual patient are being explored. Monitoring of CNI concentrations in lymphocytes and other tissues, determination of CNI metabolites, and CNI pharmacogenetics and pharmacodynamics are in their infancy but have the potential to become useful additions to conventional CNI TDM. Although MPA is usually administered at a fixed dose, there is a rationale for MPA TDM, and this is substantiated by the increasing knowledge of the many nongenetic and genetic factors contributing to the interindividual and intraindividual variability in MPA pharmacokinetics. However, recent, large, randomized clinical trials investigating the clinical utility of MPA TDM have reported conflicting data. Therefore, alternative pharmacokinetic (ie, MPA free fraction and metabolites) and pharmacodynamic approaches to better predict drug efficacy and toxicity are being explored. Finally, for MPA and tacrolimus, novel formulations have become available. For MPA, the differences in pharmacokinetic behavior between the old and the novel formulation will have implications for TDM, whereas for tacrolimus, this probably will not to be the case.

Performance of the new mycophenolate assay based on IMPDH enzymatic activity for pharmacokinetic investigations and setup of Bayesian estimators in different populations of allograft recipients

A new mycophenolate (MPA) assay based on the enzymatic activity of recombinant type II inosine monophosphate dehydrogenase (the pharmacological target of MPA) with excellent correlation with high-performance liquid chromatography has recently been released for the measurement of MPA plasma levels. This study aimed to (1) compare this new assay with liquid chromatography tandem mass spectrometry (LC-MS/MS) for MPA pharmacokinetic (PK) studies in different populations of allograft recipients given mycophenolate mofetil, (2) develop specific Bayesian estimators for this inhibition assay and test their accuracy, and (3) compare the resulting MPA area under the curve (AUC0-12h) estimates with those of Bayesian estimators developed based on the LC-MS/MS results. Sixty-four adult or pediatric, renal or lung transplant patients who were administered mycophenolate mofetil in association with cyclosporine, tacrolimus, or sirolimus at different post-transplant periods were enrolled as part of different PK studies. Eight hundred ninety-four patients' samples were analyzed in parallel with the enzymatic MPA assay and a reference LC-MS/MS method. Repeated analysis of quality control samples showed a mean difference of 6% between the 2 assays, whereas the results obtained in different populations of transplanted patients showed excellent correlation (r2 > 0.96) and small mean relative differences (2.0%-16.9%). The full profiles obtained with both assays were adequately fitted using either a 2-compartment model with 1 "gamma" absorption phase or a 1-compartment model with 2 gamma inputs. Several PK parameters were significantly affected by the analytical method used. Accurate Bayesian estimators could be specifically developed for the enzymatic MPA assay, using the same 3 concentration-time points (20 minutes, 1 hour, and 3 hours post dose) as with LC-MS/MS, with a median bias versus reference (trapezoidal) AUC0-12h values of -1.3% (range -45.2% to 40.4%), and 83% of the patients within +/-20% of the reference. These Bayesian estimates were significantly higher than those obtained with LC-MS/MS in patients on cyclosporine or sirolimus, but not in patients on tacrolimus.

EC-MPS permits lower gastrointestinal symptom burden despite higher MPA exposure in patients with severe MMF-related gastrointestinal side-effects

Gastrointestinal (GI) adverse events in renal transplant patients are a common cause of mycophenolate mofetil (MMF) dose reductions, which result in an increased risk of graft rejection because of a low immunosuppression. This study investigated whether conversion from MMF to enteric-coated mycophenolate sodium (EC-MPS) in renal transplant patients with serious GI side-effects, alleviated these symptoms and allowed administration of higher doses of EC-MPS. Nineteen renal transplant patients with severe MMF-related GI side-effects underwent a progressive reduction in MMF dose until symptoms disappeared. At this point, 12-h AUC(MMF) was evaluated and patients were shifted to an equimolar dose of EC-MPS. The EC-MPS dose was then progressively increased until the highest recommended dose was reached or GI symptoms re-appeared. Four weeks post-conversion, AUC(EC-MPS) was determined. Conversion led to a mean increase in EC-MPS dose of 68% (P < 0.0001), with a corresponding rise in AUC(0-12) (60.5%, P < 0.0006) associated with significant benefits in terms of both quality of life (Kidney Transplant Questionnaire, P < 0.01) and GI symptoms (Gastrointestinal Symptom Rating Scale, P < 0.0001), using validated questionnaires. In five of 19 patients, the EC-MPS dose could not be increased because of the prompt insurgence of GI symptoms. Renal function and biochemical parameters remained stable post-conversion and no rejection episodes occurred. These findings suggest that, in selected patients, EC-MPS may be better tolerated than MMF when GI symptoms are particularly important and permits higher mycophenolic acid exposure, when required.

Pharmacokinetics of mycophenolic acid and its glucuronide metabolites in stable adult liver transplant recipients with renal dysfunction on a low-dose calcineurin inhibitor regimen and mycophenolate mofetil

Low-dose calcineurin inhibitors (CNIs) in combination with a fixed dose (2 g/d) of mycophenolate mofetil (MMF) are a strategy to minimize exposure to cyclosporine (CSA) or tacrolimus (TAC) and thus reduce CNI-related side effects. This study compared the pharmacokinetics (PK) of mycophenolic acid (MPA) and its glucuronide metabolites in stable adult liver transplant recipients with moderately impaired renal function converted from a standard to a low-dose CNI regimen in combination with a fixed dose of MMF. Full 12-hour PK profiles of MPA, free MPA, the aryl glucuronide (MPAG), and the acyl glucuronide (AcMPAG) were obtained from 30 stable liver transplant patients on low-dose CNI (CSA, n = 12; TAC, n = 18) therapy at least 3 months after initiation of low-dose therapy. Predose CSA and TAC concentrations (quantified by liquid chromatography-tandem mass spectrometry) ranged from 17 to 35 and 1.1 to 3.7 microg/L, respectively. The PK variables for MPA, MPAG, AcMPAG, and free MPA displayed wide interindividual variability. Of note was the observation that there were no significant differences in the exposure to MPA, MPAG, and free MPA between the CSA and TAC groups. MPA area under the concentration-time curves (AUCs) ranged from 31.8 to 102.1 (median: 52.9) mg.h(-1).L(-1) in the CSA group and from 22.9 to 144.8 (median: 55.9) mg.h(-1).L(-1) in the TAC group. The AcMPAG AUC on patients under low-dose CSA therapy was higher than that observed under patients on low-dose TAC therapy, although this did not quite reach statistical significance (P = 0.057). Patients receiving CSA had a significantly higher AcMPAG Cmax but not AcMPAG AUC, suggesting that only peak CSA concentrations on a low-dose CSA regimen are sufficient to impair the biliary excretion of AcMPAG. In summary, the influence of CSA on the exposure to MPA was attenuated in stable adult liver transplant recipients on a low-dose CNI therapy in combination with a fixed dose of MMF as compared with patients on a standard CNI therapy. Dose adjustment according to drug concentration measurements is recommended to optimize dosing of MMF and to maintain adequate immunosuppression in patients converted to low-dose CNI therapy.

A limited sampling strategy for estimating mycophenolic acid area under the curve in adult heart transplant patients treated with concomitant cyclosporine

OBJECTIVE

Heart transplantation studies have shown a relationship between the mycophenolic acid area under the curve (AUC) 0-12 h (MPA AUC(0-12h)) values and risk of acute rejection episodes and fewer side-effects in patient receiving cyclosporine during the first year post-transplant. However, measurement of full AUC is costly and time consuming and in this case it is an impractical approach to drug monitoring. Therefore, the authors describe a limited sampling strategy to estimate the MPA AUC(0-12h) value in adult heart transplant recipients.

METHODS

Ninety MPA pharmacokinetic (PK) profiles were studied. The samples were collected immediately before and 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 9, 12 h after the morning dose of mycophenolate mofetil (MMF) following an overnight fast. PK profiles were determined at 6-8 weeks, 6, 12 months and more than 1 year after transplantation. Using stepwise multiple linear regression analysis a sampling strategy from 60 of PK profiles was obtained and next the bias and precision of the model were evaluated in another 30 PK profiles.

RESULTS

The three-point model using C(0.5h), C(1h), C(2h) was found to be superior to all other models tested (r(2) = 0.841). The regression equation for AUC estimation which gave the best fit to this model is: 9.69 + 0.63C(0.5) + 0.61C(1) + 2.20C(2). Using that model 63 of the 90 (70%) full AUC values were estimated within 15% of their actual value. For the best-fit model, the mean prediction error was 3.2%, with 95% confidence intervals for prediction error to range from -42.2% to 40.3%. All other models which use one, two or three time-points over the first 2 h are poorer predictors of the full AUC than the model above.

CONCLUSION

The proposed three time-point equation to estimate AUC will be helpful in optimizing immunosuppressive therapy in heart transplantation.

Pharmacokinetics, efficacy, and safety of mycophenolate mofetil in combination with standard-dose or reduced-dose tacrolimus in liver transplant recipients

The pharmacokinetics of mycophenolate mofetil (MMF) in liver transplant recipients may change because of pharmacokinetic interactions with coadministered immunosuppressants or because changes in the enterohepatic anatomy may affect biotransformation of MMF to mycophenolic acid (MPA) and enterohepatic recirculation of MPA through the hydrolysis of mycophenolate acid glucuronide to MPA in the gut. In the latter case, the choice of formulation (oral versus intravenous) could have important clinical implications. We randomized liver transplant patients (n = 60) to standard (10-15 ng/mL) or reduced (5-8 ng/mL) trough levels of tacrolimus plus intravenous MMF followed by oral MMF (1 g twice daily) with corticosteroids. Pharmacokinetic sampling was performed after the last intravenous MMF dose, after the first oral MMF dose, and at selected times over 52 weeks. The efficacy and safety of the 2 regimens were also assessed. Twenty-eight and 27 patients in the tacrolimus standard-dose and reduced-dose groups, respectively, were evaluated. No significant differences between the tacrolimus standard-dose and reduced-dose groups were seen in dose-normalized MPA values of the time to the maximum plasma concentration (1.25 versus 1.28 hours), the maximum plasma concentration (15.5 +/- 7.93 versus 13.6 +/- 7.03 microg/mL), or the area under the concentration-time curve from 0 to 12 hours (AUC(0-12); 53.0 +/- 20.6 versus 43.8 +/- 15.5 microg h/mL) at week 26 or at any other time point. No relationship was observed between the tacrolimus trough or AUC(0-12) and MPA AUC(0-12). Exposure to MPA after oral and intravenous administration was similar. Safety and efficacy were similar in the two treatment groups. In conclusion, exposure to MPA is not a function of exposure to tacrolimus. The similar safety and efficacy seen with MMF plus standard or reduced doses of tacrolimus suggest that MMF could be combined with reduced doses of tacrolimus.

Pharmacogenetic effect of the UGT polymorphisms on mycophenolate is modified by calcineurin inhibitors

BACKGROUND

Mycophenolic acid (MPA) is glucuronidated primarily by uridine diphosphate glucuronosyltransferase enzymes (UGT) 1A9 and 1A8. These enzymes are highly polymorphic resulting in low activity and high expression phenotypes. We hypothesized that polymorphisms of UGT1A9 and 1A8 may alter MPA pharmacokinetics in kidney transplantation.

METHODS

One hundred seventeen kidney (n = 93), pancreas (n = 11), or simultaneous kidney and pancreas (SPK) (n = 13) transplant recipients were studied for the effect of UGT1A9 and UGT1A8 polymorphisms on MPA dose-corrected trough concentrations. Individuals were genotyped for UGT1A8 and UGT1A9 polymorphisms (1A8*2, 1A8*3, 1A9*3, 1A9-275 and 1A9-2152). Linear regression was used to estimate the effect of UGT polymorphisms on the individual's mean MPA dose-corrected trough concentration with and without stratification by calcineurin inhibitor. A multiple linear regression analysis was performed to assess the dependence between the average MPA dose-corrected trough concentration and age, gender, UGT genotype (1A8*2, 1A8*3, 1A9*3, 1A9-275, 1A9-2152), serum albumin, hemoglobin (Hgb), hematocrit (HCT), liver transaminases (AST, ALT), serum creatinine, and bilirubin.

RESULTS

Mycophenolic acid dose-corrected trough concentrations were 60% higher in subjects heterozygous or homozygous for UGT1A8*2 than in those with the wild type (p = 0.02); however, this effect was dependent on concomitant calcineurin inhibitor. When subjects were stratified by calcineurin inhibitor status, the UGT1A8*2 effect was only apparent in the tacrolimus group (p < 0.01). Mycophenolic acid dose-corrected trough concentrations were 70% lower in carriers of the UGT1A9 -275T>A/-2152 C>T polymorphism who received cyclosporine (p < 0.01). There was no effect of the UGT1A9 -275T>A/-2152C>T polymorphism in the tacrolimus group.

CONCLUSIONS

The effect of UGT1A8 and UGT1A9 variants on MPA metabolism appears to be modified by concomitant calcineurin inhibitor therapy. Confirmatory in vivo and in vitro studies are needed.

C-440T/T-331C polymorphisms in the UGT1A9 gene affect the pharmacokinetics of mycophenolic acid in kidney transplantation

INTRODUCTION

The immunosuppressive agent mycophenolic acid (MPA) is metabolized by uridine diphosphate glucuronosyltransferase 1A9 (UGT1A9) to 7-O-glucuronide (MPAG) and excreted by multidrug resistance-associated protein 2 in the bile. By contrast, the production of the acyl MPAG, a minor MPA metabolite, was ascribed to UGT2B7 and UGT1A8. Several polymorphisms in the genes encoding for UGT1A9, UGT2B7 and MRP2 proteins have been described. However, their functional role in vivo on MPA metabolism remains poorly defined.

METHODS

A total of 40 Caucasian kidney transplant patients, given induction therapies (with Campath-(1)H or the combination basiliximab/rabbit antithymocyte globulin) and on maintenance immunosuppression with cyclosporine in combination with mycophenolate mofetil (MMF) in a steroid-free regimen, were enrolled in the pharmacogenetic study. Patients had clinical and hematochemical evaluations at month 6 after transplantation, as well as complete MPA pharmacokinetic assessment. They were genotyped for SNPs in UGT1A9 C-2152T, T-1887G, C-665T, C-440T, T-331C, T-275A, T98C, for the nonsynonymous C802T SNP in UGT2B7, and for ABCC2 SNPs C-24T and G1249A. The association of these polymorphisms with MPA pharmacokinetic parameters was investigated.

RESULTS

Differences in the MPA pharmacokinetic profiles confirmed large interpatient variability of MPA exposure, with AUC(0-12) values ranging from 7.9 to 50.1 mg*h/ml. MPA AUC(0-12) was significantly associated with the presence of UGT1A9 -440/-331 genotypes (TT/CC: 61.5 +/- 2.7 mg*h/ml/g MMF; TC/CT: 45.4 +/- 14.0 mg*h/ml/g MMF; CC/TT: 40.8 +/- 10.8 mg*h/ml/g MMF; p = 0.005), whereas MPAG exposure was mainly influenced by renal function. The positive association between MPA AUC and SNPs in position -440/-331 found in kidney transplant patients confirmed previous in vitro findings showing that the abovementioned SNPs had a significant impact on UGT1A9 protein content in the liver. The presence of ABCC2 promoter C-24T and exon 10 G1249A SNPs did not cause any significant variation in MPA and MPAG pharmacokinetic parameters.

CONCLUSION

The study demonstrated a significant impact of C-440T/T-331C SNPs in the promoter region of the UGT1A9 gene on MPA pharmacokinetics in renal allograft recipients.

Targeting mycophenolate mofetil for graft-versus-host disease prophylaxis after allogeneic blood stem cell transplantation

As low trough levels of mycophenolic acid (MPA) have been measured in recipients of allo-SCTs, we performed a pilot study targeting mycophenolate mofetil (MMF) doses according to the MPA area under the concentration (AUC) levels. Twenty-nine patients were transplanted from matched sibling (n=7) and unrelated donors (n=22). Tacrolimus was given orally from day -1 to achieve trough blood levels of 5-10 ng/ml. MMF was started on day 0 at 1500 mg intravenously b.i.d. AUC measurements of MPA by HPLC were scheduled on days 3, 7 and 11 after transplantation. The MMF dose was modified to achieve an MPA AUC of 35-60 microg/ml/h. With the respective adjustments 66 and 75% surpassed the lower AUC target on days 7 and 11, respectively. The cumulative incidence of grade III-IV acute GVHD was 28% (8/29). Eight out of 24 evaluable patients (33%) suffer from limited (n=3) or extensive (n=5) chronic GVHD. Overall, the results of this study suggest that targeting of MPA exposure is feasible early after transplantation. A simplified MMF targeting strategy based on MPA C(max) or C(2h) levels seems to be warranted in future trials involving more patients at later time points in the outpatient setting.

Individualization of mycophenolate mofetil dose in renal transplant recipients

The immunosuppressive agent mycophenolate mofetil has been successfully used over the past 10 years to prevent acute allograft rejection after renal transplantation. It has mainly been administered as a fixed dose of mycophenolate mofetil 1000 mg b.i.d. The pharmacokinetics of mycophenolic acid, the active moiety of the prodrug mycophenolate mofetil, show large between-patient variability, and exposure to mycophenolic acid correlates with the risk for acute rejection. This suggests that already excellent clinical results can be further improved by mycophenolate mofetil dose individualization. This review discusses different arguments in favour of individualization of mycophenolate mofetil dose, as well as strategies for managing mycophenolate mofetil therapy individualization, including pharmacokinetic and pharmacodynamic monitoring and dose individualization based on pharmacogenetic information. It is expected that pharmacokinetic monitoring of mycophenolic acid will offer the most effective and feasible tool for mycophenolate mofetil dose individualization.

Pharmacokinetics of mycophenolic acid and estimation of exposure using multiple linear regression equations in Chinese renal allograft recipients

OBJECTIVES

To investigate the pharmacokinetics of mycophenolic acid (MPA) in Chinese adult renal allograft recipients, and to generate the validated model equations for estimation of the MPA area under the plasma concentration-time curve from 0 to 12 hours (AUC(12)) with a limited sampling strategy.

PATIENTS AND METHODS

The pharmacokinetics in 75 Chinese renal allograft recipients treated with mycophenolate mofetil 2 g/day in combination with cyclosporin and corticosteroids were determined. The MPA concentration was assayed by high-performance liquid chromatography at pre-dose (C(0)) and at 0.5 (C(0.5)), 1 (C(1)), 1.5 (C(1.5)), 2 (C(2)), 4 (C(4)), 6 (C(6)), 8 (C(8)), 10 (C(10)) and 12 (C(12)) hours after dosing on day 14 post-transplant. Patients were randomly divided into: (i) a model group (n = 50) to generate the model equations by multiple stepwise regression analysis for estimation of the MPA AUC by a limited sampling strategy; and (ii) a validation group (n = 25) to evaluate the predictive performance of the model equations.

RESULTS

The mean MPA AUC(12) was 52.97 +/- 15.09 mg . h/L, ranging from 24.0 to 102.3 mg . h/L. The patient's age and serum albumin level had a significant impact on the MPA AUC(12). The correlation between the pre-dose MPA trough level (C(0)) and the MPA AUC(12) was poor (r(2) = 0.02, p = 0.33). Model equations 7 (MPA AUC(12) = 14.81 + 0.80 . C(0.5) + 1.56 . C(2) + 4.80 . C(4), r(2) = 0.70) and 11 (MPA AUC(12) = 11.29 + 0.51 . C(0.5) + 2.13 . C(2) + 8.15 . C(8), r(2) = 0.88) were selected for MPA AUC calculation in Chinese patients, resulting in good agreements between the estimated MPA AUC and the full MPA AUC(12), with a mean prediction error of +/-10.1 and +/-6.9 mg . h/L, respectively.

CONCLUSION

In Chinese renal allograft recipients, MPA pharmacokinetics manifest substantial interindividual variability, and the MPA AUC(12) tends to be higher than that in Caucasian patients receiving the same dose of mycophenolate mofetil. Two validated model equations with three sampling timepoints are recommended for MPA AUC estimation in Chinese patients.

Pharmacokinetics of mycophenolic acid in liver transplant patients after intravenous and oral administration of mycophenolate mofetil

The bioavailability of mycophenolic acid (MPA) after oral administration of mycophenolate mofetil (MMF) has been reported to be more than 90% in healthy volunteers, and in kidney and thoracic organ transplant patients. Such information is limited in liver transplant (LTx) patients. The present study compares the pharmacokinetics of MPA after intravenous (IV) and oral administrations of MMF in LTx recipients. Pharmacokinetic parameters were calculated using WinNonlin software. A total of 12 deceased donor LTx patients initially received IV MMF and were switched to oral MMF after 2-7 days (mean, 3.3 +/- 1.7) when oral feeds were started. Multiple blood samples were drawn immediately prior to and after IV or oral MMF and the plasma concentration of MPA was measured. The mean peak plasma concentrations and the area under the plasma concentration vs. time curve (AUC) were significantly higher after IV MMF compared to oral MMF (peak plasma concentrations of 10.7 +/- 2.1 microg/mL for IV vs. 4.5 +/- 2.8 microg/mL for oral; P = 0.0001; and AUC of 28.9 +/- 7.1 microg . hr/mL for IV vs. 12.8 +/- 4.2 microg . hr/mL for oral; P = 0.0001). The oral bioavailability of MPA was 48.5 +/- 18.7%. The systemic clearance, half-life, and steady state volume of distribution of MPA were 26.9 +/- 6 L/hour, 5.5 hours, and 85 liters, respectively. The terminal disposition half-life was not significantly different between the 2 routes of administration. In conclusion, during the early postoperative period, LTx recipients have MPA exposure with oral MMF of less than half that of IV MMF. Use of IV MMF immediately post-LTx may provide an immunological advantage.

Therapeutic drug monitoring of mycophenolic acid: does it improve patient outcome?

Treatment with the immunosuppressive agent mycophenolate mofetil (MMF) decreases the risk of rejection after renal transplantation and improves graft survival compared with azathioprine. The exposure to the active metabolite mycophenolic acid (MPA) is correlated to the risk of developing acute rejection. The interpatient variability in exposure of MPA is wide relative to the proposed therapeutic window of the MPA AUC(0 12) (30 - 60 mg.h/l). The pharmacokinetics of MPA are influenced by patient characteristics such as gender, time after transplantation, serum albumin concentration, renal function, comedication and pharmacogenetic factors. Therapeutic drug monitoring is likely to reduce inter-patient variability. Limited sampling strategies are used to predict the full AUC(0 12). Three prospective randomised studies compared concentration controlled MMF therapy to a fixed-dose regimen. Preliminary outcomes of these studies showed conflicting results and longer follow up is needed to further clarify the role of therapeutic drug monitoring in increasing the therapeutic potential of MMF.

Clinical pharmacokinetics and pharmacodynamics of mycophenolate in solid organ transplant recipients

This review aims to provide an extensive overview of the literature on the clinical pharmacokinetics of mycophenolate in solid organ transplantation and a briefer summary of current pharmacodynamic information. Strategies are suggested for further optimisation of mycophenolate therapy and areas where additional research is warranted are highlighted. Mycophenolate has gained widespread acceptance as the antimetabolite immunosuppressant of choice in organ transplant regimens. Mycophenolic acid (MPA) is the active drug moiety. Currently, two mycophenolate compounds are available, mycophenolate mofetil and enteric-coated (EC) mycophenolate sodium. MPA is a potent, selective and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH), leading to eventual arrest of T- and B-lymphocyte proliferation. Mycophenolate mofetil and EC-mycophenolate sodium are essentially completely hydrolysed to MPA by esterases in the gut wall, blood, liver and tissue. Oral bioavailability of MPA, subsequent to mycophenolate mofetil administration, ranges from 80.7% to 94%. EC-mycophenolate sodium has an absolute bioavailability of MPA of approximately 72%. MPA binds 97-99% to serum albumin in patients with normal renal and liver function. It is metabolised in the liver, gastrointestinal tract and kidney by uridine diphosphate gluconosyltransferases (UGTs). 7-O-MPA-glucuronide (MPAG) is the major metabolite of MPA. MPAG is usually present in the plasma at 20- to 100-fold higher concentrations than MPA, but it is not pharmacologically active. At least three minor metabolites are also formed, of which an acyl-glucuronide has pharmacological potency comparable to MPA. MPAG is excreted into the urine via active tubular secretion and into the bile by multi-drug resistance protein 2 (MRP-2). MPAG is de-conjugated back to MPA by gut bacteria and then reabsorbed in the colon. Mycophenolate mofetil and EC-mycophenolate sodium display linear pharmacokinetics. Following mycophenolate mofetil administration, MPA maximum concentration usually occurs in 1-2 hours. EC-mycophenolate sodium exhibits a median lag time in absorption of MPA from 0.25 to 1.25 hours. A secondary peak in the concentration-time profile of MPA, due to enterohepatic recirculation, often appears 6-12 hours after dosing. This contributes approximately 40% to the area under the plasma concentration-time curve (AUC). The mean elimination half-life of MPA ranges from 9 to 17 hours. MPA displays large between- and within-subject pharmacokinetic variability. Dose-normalised MPA AUC can vary more than 10-fold. Total MPA concentrations should be interpreted with caution in patients with severe renal impairment, liver disease and hypoalbuminaemia. In such individuals, MPA and MPAG plasma protein binding may be altered, changing the fraction of free MPA available. Apparent oral clearance (CL/F) of total MPA appears to increase in proportion to the increased free fraction, with a reduction in total MPA AUC. However, there may be little change in the MPA free concentration. Ciclosporin inhibits biliary excretion of MPAG by MRP-2, reducing enterohepatic recirculation of MPA. Exposure to MPA when mycophenolate mofetil is given in combination with ciclosporin is approximately 30-40% lower than when given alone or with tacrolimus or sirolimus. High dosages of corticosteroids may induce expression of UGT, reducing exposure to MPA. Other co-medications can interfere with the absorption, enterohepatic recycling and metabolism of mycophenolate. Most pharmacokinetic investigations of MPA have involved mycophenolate mofetil rather than EC-mycophenolate sodium therapy. In population pharmacokinetic studies, MPA CL/F in adults ranges from 14.1 to 34.9 L/h (ciclosporin co-therapy) and from 11.9 to 25.4 L/h (tacrolimus co-therapy). Patient bodyweight, serum albumin concentration and immunosuppressant co-therapy have a significant influence on CL/F. The majority of pharmacodynamic data on MPA have been obtained in patients receiving mycophenolate mofetil therapy in the first year after kidney transplantation. Low MPA AUC is associated with increased incidence of biopsy-proven acute rejection. Gastrointestinal adverse events may be dose related. Leukopenia and anaemia have been associated with high MPA AUC, trough concentration and metabolite concentrations in some, but not all, studies. High free MPA exposure has been identified as a risk factor for leukopenia in some investigations. Targeting a total MPA AUC from 0 to 12 hours (AUC12) of 30-60 mg.hr/L is likely to minimise the risk of acute rejection and may reduce toxicity. IMPDH monitoring is in the early experimental stage. Individualisation of mycophenolate therapy should lead to improved patient outcomes. MPA AUC12 appears to be the most useful exposure measure for such individualisation. Limited sampling strategies and Bayesian forecasting are practical means of estimating MPA AUC12 without full concentration-time profiling. Target concentration intervention may be particularly useful in the first few months post-transplant and prior to major changes in anti-rejection therapy. In patients with impaired renal or hepatic function or hypoalbuminaemia, free drug measurement could be valuable in further interpretation of MPA exposure.

Influence of UGT1A8 and UGT2B7 genetic polymorphisms on mycophenolic acid pharmacokinetics in Japanese renal transplant recipients

OBJECTIVE

UGT1A8 and UGT2B7 are important uridine diphosphate-glucuronosyltransferase isoforms for the glucuronidation of mycophenolic acid (MPA). The aim of this investigation was to elucidate MPA pharmacokinetics in UGT1A8 and UGT2B7 genotypes in Japanese renal transplant recipients.

METHODS

Seventy-two recipients received repeated doses of mycophenolate mofetil and tacrolimus. On day 28 after renal transplantation, plasma MPA concentrations were measured for the next 24 h using high-performance liquid chromatography. UGT1A8*2 (A(173)G) and UGT2B7*2 (Y(268)) were detected using a PCR-RFLP-based procedure.

RESULTS

There were no significant differences in daytime and nighttime pharmacokinetics of MPA between UGT1A8 or UGT2B7 genotypes. The mean daytime dose-adjusted AUC(0-12) of MPA in UGT1A8*1/*1, *1/*2 and *2/*2 were 2.47, 2.33 and 2.57 ng.h/ml/mg/kg (P = 0.7711), and the mean nighttime AUC(0-12) were 2.15, 2.00 and 2.08 ng.h/ml/mg/kg (P = 0.4656). The mean daytime and nighttime dose-adjusted AUC(0-12) of MPA in UGT2B7*1/*1, *1/*2 and *2/*2 were 2.61, 2.24 and 2.03 ng.h/ml/mg/kg and 2.18, 1.94, and 1.45 ng.h/ml/mg/kg, respectively (P = 0.3475 and 0.2575). The mean nighttime C(max), t(max), and AUC(6-12)/AUC(0-12) ratio (enterohepatic circulation and recirculation ratio) of MPA in all UGT1A8 and UGT2B7 genotypes were lower, longer, and higher, respectively, than the daytime values.

CONCLUSIONS

Both UGT1A8 and UGT2B7 allelic variants seem not to affect Japanese interindividual variability for plasma MPA concentration. Regardless of UGT1A8 and UGT2B7 genetic polymorphisms, the absorption of MPA through enterohepatic recirculation is higher at night.

Time-dependent clearance of mycophenolic acid in renal transplant recipients

AIMS

Pharmacokinetic studies of the immunosuppressive compound mycophenolic acid (MPA) have shown a structural decrease in clearance (CL) over time after renal transplantation. The aim of this study was to characterize the time-dependent CL of MPA by means of a population pharmacokinetic meta-analysis, and to test whether it can be described by covariate effects.

METHODS

One thousand eight hundred and ninety-four MPA concentration-time profiles from 468 renal transplant patients (range 1-9 profiles per patient) were analyzed retrospectively by nonlinear mixed effect modelling. Sampling occasions ranged from day 1-10 years after transplantation.

RESULTS

The pharmacokinetics of MPA were described by a two-compartment model with time-lagged first order absorption, and a first-order term for time-dependent CL. The model predicted the mean CL to decrease from 35 l h(-1) (CV = 44%) in the first week after transplantation to 17 l h(-1) (CV = 38%) after 6 months. In a covariate model without a term for time-dependent CL, changes during the first 6 months after transplantation in creatinine clearance from 19 to 71 ml min(-1), in albumin concentration from 35 to 40 g l(-1), in haemoglobin from 9.7 to 12 g dl(-1) and in cyclosporin predose concentration from 225 to 100 ng ml(-1) corresponded with a decrease of CL from 32 to 19 l h(-1). Creatinine clearance, albumin concentration, haemoglobin and cyclosporin predose concentration explained, respectively, 19%, 12%, 4% and 3% of the within-patient variability in MPA CL.

CONCLUSIONS

By monitoring creatinine clearance, albumin concentration, haemoglobin and cyclosporin predose concentration, changes in MPA exposure over time can be predicted. Such information can be used to optimize therapy with mycophenolate mofetil.

Mycophenolic acid trough level monitoring in solid organ transplant recipients treated with mycophenolate mofetil: association with clinical outcome

BACKGROUND

Mycophenolate mofetil (MMF) is widely and successfully used in immunosuppressive regimens for the prophylaxis of organ rejection following transplantation. Conventionally, it is administered at a fixed dose without serial measurements of plasma concentrations of mycophenolic acid (MPA), the active metabolite. Recently, there has been an increased interest in therapeutic drug monitoring (TDM) of MMF therapy to optimize the benefit/risk index of the drug. Predose trough samples of MPA are considered most convenient and economic, thereby allowing an increased use of TDM in the transplant setting. However, the added value of TDM for MMF therapy is still under debate.

OBJECTIVE

This paper reviews (based on a systematic PubMed and EMBASE search, 1995-June 2006) the current evidence of the usefulness and clinical relevance of MPA trough level monitoring during MMF therapy in solid organ transplantation.

FINDINGS AND CONCLUSIONS

Based on data available in the public domain, the contribution of MPA trough level monitoring during MMF therapy in solid organ transplant recipients remains unproven. Available studies have limitations and report conflicting results. There is a lack of prospective randomized trials, particularly in pediatric renal transplant recipients and in cardiac and liver transplantation. While there is a suggestion that there may be a relationship between efficacy and MPA trough levels, the majority of studies showed no correlation between MPA plasma concentrations and adverse effects. Based on current evidence, the adherence to presently recommended target ranges for MPA troughs in solid organ transplantation cannot assure an improved clinical outcome with MMF therapy. Whether MPA trough level monitoring leads to improved efficacy and less toxicity is currently subject to a large randomized trial; final results are eagerly awaited.

Effects of gastric emptying on oral mycophenolic acid pharmacokinetics in stable renal allograft recipients

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

* Strategies that are more elaborate than measuring predose plasma concentrations are required for the therapeutic monitoring of mycophenolic acid (MPA). * Previous studies in healthy subjects and diabetes patients have suggested that MPA pharmacokinetics are influenced by gastric emptying, but this has not been demonstrated directly.

WHAT THIS STUDY ADDS

* This study has investigated the relationship between gastric emptying, measured directly (using the (14)C octanoate and (13)C glycine breath tests) and the steady-state plasma concentration-time profile of MPA. * Delayed gastric emptying was associated with a longer t(max) and lower C(max), but total exposure to MPA was not affected. * The findings suggest that it could be misleading to rely fully on short-term (<2 h) limited sampling strategies for MPA therapeutic monitoring in recipients with gastric emptying disorders, the latter occurring relatively frequently in solid organ transplantation.

AIM

To investigate the effect of gastric emptying on the pharmacokinetics of mycophenolic acid (MPA) in renal transplant patients.

METHODS

We assessed the effect of gastric emptying on the disposition of MPA in 27 stable renal allograft recipients at 2 years after transplantation. Gastric emptying was measured by the (14)C-octanoate and (13)C-glycine breath test.

RESULTS

Delayed gastric emptying was associated with a significantly longer MPA t(max)[1.0 (0.33-2.0) h vs. 0.5 (0.33-1.0) h; mean difference 0.39 h, 95% confidence interval (CI) 0.03, 0.75; P = 0.0289] and with a significant decrease in the maximum MPA concentration after dosing [10.6 (6.5-21.3) mg l(-1)vs. 20.1 (10.7-28.5) mg l(-1); mean difference 6.5 mg l(-1), 95% CI 2.1, 10.9; P = 0.0075]. Despite the substantial effect of delayed gastric emptying rates on MPA C(max) and t(max), total dose-interval exposure, measured by the MPA AUC(0-4), was not affected by the rate of gastric emptying [20.4 (13.9-43.0) mg h(-1) l(-1)vs. 22.4 (13.1-29.8) mg h(-1) l(-1)].

CONCLUSION

Delayed gastric emptying was associated with a slower absorption of MPA, a longer time to reach peak concentrations and lower maximum concentrations. These effects should be taken into account when validating limited (<2 h) sampling strategies to estimate total MPA exposure, which could be unreliable when monitoring patients with gastric emptying disorders.

Total and free mycophenolic acid and its 7-O-glucuronide metabolite in Chinese adult renal transplant patients: pharmacokinetics and application of limited sampling strategies

OBJECTIVE

This study aimed to investigate the pharmacokinetic characteristics of total and free mycophnolic acid (MPA) and its 7-O-glucuronide metabolite (MPAG) in Chinese renal transplant recipients. In addition, limited sampling strategies were developed to estimate the individual area under concentration curve (AUC) of total and free MPA.

METHODS

Total and free MPA and MPAG concentrations were determined by high performance liquid chromatography. Whole 12-h pharmacokinetic profiles were obtained on the 10th day after operation in 12 adult Chinese de novo renal transplant recipients administrated with mycophenolate mofetil (MMF, 750 mg bid), cyclosporine and corticosteroids. Limited sampling strategies with jackknife technique, a resampling method, and Bland-Altman analysis were employed to develop equations to estimate total and free MPA AUC.

RESULTS

The pattern of total and free MPA and MPAG plasma concentration-time curves in the cohort of patients taking lower doses of MMF was consistent with previous reports of Caucasian patients taking MMF 1 g bid, except that dose-normalized exposure of total and free MPAG was much lower in the current study than in those of the Caucasians. The mean C (max) and AUC(0-12h) of total and free MPA were 9.4 +/- 3.4 mg/L, 20.2 +/- 6.5 mg x h/L and 0.4 +/- 0.4 mg/L, 0.7 +/- 0.5 mg x h/L, respectively, whereas mean C (max) and AUC(0-12h) of total and free MPAG were 97.3 +/- 32.6 mg/L, 656.0 +/- 148.0 mg.h/L and 29.9 +/- 8.5 mg/L, 222.0 +/- 58.1 mg x h/L respectively. The mean fractions of free MPA and MPAG were 3.5 +/- 2.0 and 34.6 +/- 8.0%, respectively. No determinant was identified to influence the pharmacokinetics of total and free MPA and MPAG or the free fraction of MPA and MPAG. The combinations of C (2h)-C (4h) and C (1h)-C (2h)-C (3h) were the best to estimate free and total MPA AUC(0-12h) respectively, whereas the combination of C (2h)-C (3h)-C (4h) and C (1h)-C (2h)-C (4h) was the best to estimate both simultaneously.

CONCLUSION

This is the first time that the pharmacokinetics profile of total and free MPA and its main metabolite MPAG has been examined in Chinese adult renal transplant patients. The limited sampling strategies proposed to estimate individual free and total MPA AUC could be useful in optimizing patient care.

Multidrug Resistance Protein 2 Genetic Polymorphisms Influence Mycophenolic Acid Exposure in Renal Allograft Recipients

BACKGROUND

Mycophenolic acid (MPA) is glucuronidated by uridine diphosphate-glucuronosyltransferases (UGTs) to its pharmacologically inactive 7-O-glucuronide metabolite (MPAG). MPAG is excreted into the bile via the multidrug resistance-associated protein 2 (MRP2/ABCC2), which is essential for enterohepatic (re)circulation (EHC) of MPA(G).

METHODS

The objective of this study was to determine the relationship between single nucleotide polymorphisms (SNPs) in the MRP2 (G-1549A, G-1023A, A-1019G, C-24, G1249A, C3972T and G4544A) and UGT1A9 (C-2152T, T-275AandT98C) genes and MPA pharmacokinetics in 95 renal allograft recipients at days 7, 42, 90, and 360 after transplantation. In addition to mycophenolate mofetil, all patients received tacrolimus and corticosteroids as immunosuppression.

RESULTS

At day seven after transplantation, in the absence of the MRP2 C-24T SNP, mild liver dysfunction was associated with significantly lower MPA dose-interval exposure and higher MPA oral clearance, while liver dysfunction did not affect MPA pharmacokinetics in patients with the MRP2 C-24T variant. A similar effect is noted for the C-3972T variant, which is in linkage disequilibrium with C-24T. At later time points after transplantation the MRP2 C-24T SNP was associated with significantly higher dose-corrected MPA trough levels. Patients with the MRP2 C-24T variant had significantly more diarrhea in the first year after transplantation.

CONCLUSIONS

The MRP2 C-24T and C-3972T polymorphisms protect renal transplant recipients from a decrease in MPA exposure associated with mild liver dysfunction. Furthermore, this study suggests that the C-24T SNP is associated with a lower oral clearance of MPA in steady-state conditions.

A limited sampling model for estimation of total and unbound mycophenolic acid (MPA) area under the curve (AUC) in hematopoietic cell transplantation (HCT)

OBJECTIVES

Renal transplant patients with suboptimal mycophenolic acid (MPA) areas under the curves (AUCs) are at greater risk of acute rejection. In hematopoietic cell transplantation, a low MPA AUC is also associated with a higher incidence of acute graft versus host disease. Therefore, a limited sampling model was developed and validated to simultaneously estimate total and unbound MPA AUC0-12 in hematopoietic cell transplantation patients.

METHODS

Intensive pharmacokinetic sampling was performed at steady state between days 3 to 7 posttransplant in 73 adult subjects while receiving prophylactic mycophenolate mofetil 1 g per 12 hours orally or intravenously plus cyclosporine. Total and unbound MPA plasma concentrations were measured, and total and unbound AUC0-12 was determined using noncompartmental analysis. Regression analysis was then performed to build IV and PO, total and unbound AUC0-12 models from the first 34 subjects. The predictive performance of these models was tested in the next 39 subjects.

RESULTS

Trough concentrations poorly estimate observed total and unbound AUC0-12 (r<0.48). A model with 3 concentrations (2-, 4-, and 6-hour post start of infusion) best estimated observed total and unbound AUC0-12 after IV dosing (r>0.99). Oral total and unbound AUC0-12 was more difficult to estimate and required at least 4 concentrations (0-, 1-, 2-, and 6-hour post dose) in the model (r>0.85). The predictive performance of the final models was good. Eighty-three percent of IV and 70% of PO AUC0-12 predictions fell within +/-20% of the observed values without significant bias.

CONCLUSION

Trough MPA concentrations do not accurately describe MPA AUC0-12. Three intravenous (2-, 4-, 6-hour post start of infusion) or 4 oral (0-, 1-, 2-, and 6-hour post dose) MPA plasma concentrations measured over a 12-hour dosing interval will estimate the total and unbound AUC0-12 nearly as well as intensive pharmacokinetic sampling with good precision and low bias. This approach simplifies AUC0-12 targeting of MPA post hematopoietic cell transplantation.

Therapeutic drug monitoring of mycophenolate mofetil in transplantation

A roundtable meeting to discuss the use of therapeutic drug monitoring (TDM) to guide immunosuppression with mycophenolate mofetil was held in New York in December 2004. Existing recommendations for the initial months after transplantation were updated. After ensuring adequate levels of mycophenolic acid (MPA, the active metabolite of mycophenolate mofetil) immediately after transplantation, optimal efficacy may require only a few dose adjustments, because intrapatient variability in exposure seems low. Recommendations based on current knowledge were made for posttransplantation sampling time points and for target MPA concentrations. Algorithms for estimating MPA exposure using limited sampling strategies were presented, and a new assay for MPA discussed. It was agreed that because of interpatient variability and the influence of concomitant immunosuppressants, TDM might help optimize outcomes, especially in patients at higher risk of rejection. The value of TDM in the general transplant population will be assessed from large, ongoing, randomized studies.

Value of therapeutic drug monitoring of MMF therapy in pediatric transplantation

Therapeutic drug monitoring (TDM) is desirable whenever the desired drug effect cannot be predicted from a given dose, or when it is necessary to find a balance between the efficacy and toxicity of the drug. Children and adolescents particularly benefit from TDM, because dosing requirements are often not studied in the same detail as in adults. Also, drug-drug interactions are frequent. The gold standard for assessment of drug exposure is the area-under-the-curve (AUC) for a full pharmacokinetic profile. TDM for mycophenolic acid (MPA) is less well established. Monitoring of trough levels does not suffice because of enterohepatic recirculation of MPA after formation of its main metabolite, a glucoronide termed MPA-G. However, abbreviated sampling schemes specific to mycophenolate mofetil (MMF) correlate well with the AUC for MPA. Cyclosporine interacts with MPA by inhibiting the multidrug resistance-associated protein 2 (MRP2). Higher MPA concentrations result in a decreased two h concentration of cyclosporine, while higher cyclosporine exposure results in a lower MPA exposure. There are no drug interactions between tacrolimus and MPA, and lower doses of MMF are required in combination with tacrolimus. Steroids may induce the clearance of MPA, which could account in part for the increasing MPA exposure following transplantation. TDM has allowed for dosing recommendations of MMF in children, which could lead to improved efficacy and minimization of toxicities. It is important that these provisional target levels are validated in prospective studies. The above points clearly indicate that there is a role for TDM of MPA in pediatric transplant recipients.

Determinants of mycophenolic acid levels after renal transplantation

There are data suggesting an association between mycophenolic acid (MPA) levels and acute rejection and toxicity in renal transplant recipients treated with mycophenolate mofetil (MMF), and therefore, knowledge of factors determining MPA levels may aid in accurate adjustment of MMF dosage. A total of 4970 samples taken 12 hours postdose were analyzed for MPA by immunoassay at regular intervals from the first week posttransplantation in 117 renal transplant patients immunosuppressed with MMF and tacrolimus in a steroid-sparing regimen (prednisolone for the first 7 days only). MPA levels rose in the first 3 months and stabilized thereafter; dose-normalized MPA levels rose throughout the first 12 months and subsequently stabilized. Multivariate analysis by means of a population-averaged linear regression showed positive associations between MPA level and total daily dose (P < 0.001) but not individual dose or total daily dose corrected for body weight. Positive associations were also seen with serum albumin (P = 0.01), tacrolimus trough level (P = 0.01), and female gender (P = 0.002). The association with tacrolimus levels diminished with time. Negative associations were seen between MPA level and higher estimated creatinine clearance (P < 0.001), and also with increasing alanine transaminase levels (P = 0.002), the use of oral antibiotics (P < 0.001), and infective diarrhea (P < 0.001). The latter findings may be related to changes in enterohepatic recirculation of MPA. Many clinical variables show associations with trough MPA levels. An understanding of these factors may aid therapeutic monitoring of MMF.

Impact of mycophenolate mofetil intolerance on early results of kidney transplantation

From 1992 to 2003, 407 kidney transplantations were performed on 403 patients using cyclosporine (CyA) or tacrolimus (Tac), mycophenolate (MMF), and steroid immunosuppression. Patient records examined for adverse events (AE) and MMF dose reductions or discontinuations during 100 days posttransplant were correlated with data on rejections, graft function, and survival. AEs occurred in 79.1% of transplantations. Gastrointestinal (GI) symptoms, infections, and cytopenias were common. Surprisingly, in 50% of all transplantations serum alanine transferase (ALAT) was elevated, among 21% the change was over three times the upper limit of normal. Patients with delayed graft function showed increased incidences of GI symptoms and thrombocytopenias. There were more ALAT increases and thrombocytopenias in patients on CyA and more GI symptoms in patients on Tac. In 34% of transplantations, the MMF dose was reduced or discontinued. In CyA patients with MMF reduction by day 21, rejection incidence during the subsequent 21 days was 10% versus 0.6% in patients with full-dose MMF until day 21 (P < .002). Among Tac patients no increased rejection frequency was seen after reducing MMF.

Comparison of mycophenolic acid pharmacokinetic parameters in kidney transplant patients within the first 3 months post-transplant

OBJECTIVE

The aim of this study was to investigate the effect of time on pharmacokinetic (PK) parameters of mycophenolic acid (MPA) in the early post-transplant period in kidney recipients. MPA is the active metabolite of mycophenolate mofetil (MMF), which was introduced into clinical practice ten years ago.

METHOD

Mycophenolate mofetil was co-administered with cyclosporin (CsA) in a subgroup of 23 patients and with tacrolimus (Tac) in a subgroup of 10 patients. MPA plasma concentration profiles were measured by a validated high performance liquid chromatography method 1 week, 2 and 3 months after transplantation.

RESULTS

Despite a comparable MMF dose, a large inter-patient variability in both MPA area under the curve (AUC) from 0 to 12 h (range 10.03-135.4 microg h/mL) and in predose concentrations (0.31-6.09 microg/mL) was observed. Patients with AUC > 35 microg h/mL showed better (P < 0.1) renal function than patients with AUC < 20 microg h/mL (mean creatinine concentration 1.48 +/- 0.12 vs. 3.35 +/- 0.4 mg/dL respectively). The total MPA trough and AUC did not correlate with biochemical parameters: leucocyte cell count and haematocrit. A higher trough level of the metabolite MPA glucuronide (MPAG) in the 1 week after transplantation was found when compared with the 3-month level (mean 150.1 +/- 146.7; range 17.1 to 560 vs. 75.8 +/- 40.0; range 27.3 to 174.2 microg/mL). The concentration of MPA, and MPA AUC values were significantly lower in patients receiving MMF and CsA than those receiving MMF and Tac during all three periods studied (P < 0.02). The influence of C(0) and MPA AUC values on the risk of graft rejection was investigated using receiver operating characteristic (ROC) curve analysis. The area under the ROC curve for AUC was 0.847, whereas that of C(0) was 0.632.

CONCLUSIONS

The MPA AUC(0-12h) appeared to be the more effective PK parameter for predicting acute rejection. We recommend that routine MPA and MPAG therapeutic drug level monitoring should be an important part of MMF therapy.

Explaining variability in mycophenolic acid exposure to optimize mycophenolate mofetil dosing: a population pharmacokinetic meta-analysis of mycophenolic acid in renal transplant recipients

Large between- and within-patient variability has been observed in the pharmacokinetics of mycophenolic acid (MPA). However, conflicting results exist about the influence of patient characteristics that explain the variability in MPA exposure. This population pharmacokinetic meta-analysis of MPA in renal transplant recipients was performed to explore whether race, renal function, albumin level, delayed graft function, diabetes, and co-medication are determinants of total MPA exposure. A total of 13,346 MPA concentration-time data points from 468 renal transplant patients who participated in six clinical studies were combined and analyzed retrospectively. Sampling occasions ranged from day 1 after transplantation to 10 yr after transplantation. Concentration-time data were analyzed with nonlinear mixed-effect modeling. Exposure to total MPA, as determined by MPA clearance, significantly increased with increasing renal function, albumin level, and hemoglobin as well as decreasing cyclosporine predose level (P<0.001). These variables could explain 18% of the between-patient and 38% of the within-patient variability in MPA exposure. Differences in MPA exposure between patients with or without delayed graft function or between patients of different races are likely to be caused by the effect of renal function on MPA exposure. Diabetes did not have an effect on MPA exposure. The clinical implication is that a change in renal function or albumin level provides an indication for therapeutic drug monitoring as MPA exposure may be altered. Patients in whom cyclosporine and mycophenolate mofetil are combined may need higher mycophenolate mofetil doses, especially during the early phase after transplantation than currently recommended for optimal MPA exposure.

Clinical Pharmacokinetics of Tacrolimus After the First Oral Administration in Combination with Mycophenolate Mofetil and Prednisone in Chinese Renal Transplant Recipients

INTRODUCTION

Data on tacrolimus pharmacokinetics in combination with mycophenolate mofetil and prednisone are scarce in Chinese renal transplantation recipients. The purpose of this study was to detect interpatient pharmacokinetic variability of tacrolimus and to assess the predictability of individual tacrolimus concentrations at various times for the area under the curve (AUC) seeking to find the best sampling time for an abbreviated AUC to predict the total body exposure of tacrolimus after the first oral dose in Chinese renal transplantation recipients.

METHODS

Sixteen primary kidney transplant recipients were treated with methylprednisolone and antilymphocyte globulin for 3 days. The first tacrolimus oral dose (0.075 mg/kg) was given at day 3 posttransplant. Mycophenolate mofetil and prednisone were administered orally posttransplant. Blood samples were obtained at 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, 8.0, and 12.0 hours after taking the first oral dose. Tacrolimus blood concentrations were measured by ELISA. Twelve-hour AUC (AUC12) for each patient was calculated using the linear trapezoid rule. Associations between the blood concentration at each sampling time point and the AUC12 were evaluated by Pearson correlation coefficients. Abbreviated sampling equations were derived by multiple, stepwise regression analyses performed using AUC12 as the dependent variables. The variance in the strength of association between predicted AUC (AUC(P)) and AUC12 was reflected by linear regression coefficients of multiple determinations.

RESULTS

In 16 patients, AUC12 values were within the range of 44.40 ng x h/mL to 158.01 ng x h/mL (mean = 92.23 +/- 34.97 ng x h/mL). The area of the maximum AUC12 was almost fourfold higher than that of the minimum AUC12. C12 significantly correlated with AUC(12) after the first tarcrolimus oral dose (r = .846, P < .001). C5, C8, and C3 showed better correlations: r = .924, .924, and .911, respectively. From stepwise multiple regression, C5 seemed to be the best predictor of total body exposure to tacrolimus (r = .92, r2 = .85). Alternatively, the concentrations at 5 and 1.5 hours or 5, 1.5, and 3 hours as an abbreviated AUC were as good as a full pharmacokinetic study (r = .97, r2 = .94, and r = .99, r2 = .99, respectively).

CONCLUSIONS

Tacrolimus AUC12 show remarkable interindividual variations after the first oral dose in combination with mycophenolate mofetil and prednisone in Chinese renal transplant recipients. Although C12 is a good predictor of efficacy, C5 might be the best predictor of the first AUC12. A two-point sampling method using C5 and C1.5 or three-point sampling method using C5, C1.5, and C3 might be the best abbreviated AUC for a cost-effective tacrolimus monitoring strategy.

Mycophenolate Mofetil Pharmacokinetic Monitoring in Pediatric Kidney Transplant Recipients

This open-label, longitudinal, long-term study of de novo pediatric renal transplant recipients was designed to investigate the pharmacokinetics (PK) of mycophenolic acid (MPA) and its possible interaction with cyclosporine (CsA). Thirty-four children on an immunosuppressive regimen of CsA, prednisone, and mycophenolate mofetil (MMF, 300-400 mg/m2 twice daily) were investigated at 6, 30, 180, and 360 days after transplantation. Considerable interindividual variability in the areas under the concentration curve (AUC(0-12)) of MPA was observed during the follow-up, although the dose of MMF remained the same over the same time. Predose levels (C0) increased significantly during the first 6 months after transplantation: C0 at 6 and 180 days after transplantation was 0.8 +/- 0.6 and 1.9 +/- 1.1 microg/mL (P < .0001). A significant time-dependent increase in the AUC of MPA was also observed during the first 6 posttransplant months: AUC(0-12) at 6 and 180 days after transplantation was 23.3 +/- 10.8 and 40 +/- 11.6 mg*h/L (P = .003). MPA concentrations 3 and 4 hours after MMF intake were the individual time points that best correlated with the full MPA AUC (r = 0.8 and 0.79; P < .001). The abbreviated MPA AUC (0-4 hours) correlated reasonably with the full AUC (r = 0.87; P < .001). Finally, a significant reduction in CsA dose during the first 6 posttransplant months (P < .001) matched the significant increases in both MPA C0 and full MPA AUC, thus demonstrating the interaction of the 2 immunosuppressive drugs. These observations suggest the need for therapeutic drug monitoring when adjusting the dose of MMF in children.

Immunosuppressive drug monitoring - what to use in clinical practice today to improve renal graft outcome

Therapeutic drug monitoring (TDM) of immunosuppressive therapy is becoming an increasingly complex matter as the number of compounds and their respective combinations are continuously expanding. Unfortunately, in clinical practice, monitoring predose trough blood concentrations is often not sufficient for guiding optimal long-term dosing of these drugs. The excellent short-term results obtained nowadays in renal transplantation confer a misleading feeling of safety despite the fact that long-term results have not substantially improved, definitely not to a point where longer graft survival could counteract the increasing need for transplant organs and less toxicity and side-effects could ameliorate patient survival. It is therefore a challenging task to try to tailor immunosuppressive drug therapy to the individual patient profile and this in a time-dependent manner. For the majority of currently used immunosuppressive drugs, measurement of total drug exposure by determination of the dose-interval area under the concentration curve (AUC) seems to provide more useful information for clinicians in terms of concentration-exposure and exposure-response as well as reproducibility. To simplify this laborious way of measuring drug exposure, several validated abbreviated AUC profiles, accurately predicting the dose-interval AUC, have been put forward. Together with an increasing knowledge of the time-related pharmacokinetic behaviour of immunosuppressive drug and their metabolites, studies are focusing on how to apply abbreviated AUC sampling methods in clinical transplantation, taking into account the numerous factors affecting drug pharmacokinetics. Eventually, TDM using abbreviated AUC profiles has to be prospectively tested against classic methods of drug monitoring in terms of cost-effectiveness, feasibility and clinical relevance with the ultimate goal of improving patient and graft survival.