Effect of mycophenolate mofetil on IMP dehydrogenase after the first dose and after long-term treatment in renal transplant recipients

Individualization of Mycophenolic Acid Therapy through Pharmacogenetic, Pharmacokinetic and Pharmacodynamic Testing

Mycophenolic acid (MPA) is a widely used immunosuppressive agent and exerts its effect by inhibiting inosine 5′-monophosphate dehydrogenase (IMPDH), the main regulating enzyme of purine metabolism. However, significant unexplained differences in the efficacy and tolerability of MPA therapy pose a clinical challenge. Therefore, broad pharmacogenetic, pharmacokinetic, and pharmacodynamic approaches are needed to individualize MPA therapy. In this prospective cohort study including 277 renal transplant recipients, IMPDH2 rs11706052 SNP status was assessed by genetic sequencing, and plasma MPA trough levels were determined by HPLC and IMPDH enzyme activity in peripheral blood mononuclear cells (PBMCs) by liquid chromatography–mass spectrometry. Among the 277 patients, 84 were identified with episodes of biopsy-proven rejection (BPR). No association was found between rs11706052 SNP status and graft rejection (OR 1.808, and 95% CI, 0.939 to 3.479; p = 0.076). Furthermore, there was no association between MPA plasma levels and BPR (p = 0.69). However, the patients with graft rejection had a significantly higher predose IMPDH activity in PBMCs compared to the controls without rejection at the time of biopsy (110.1 ± 50.2 vs. 95.2 ± 45.4 pmol/h; p = 0.001), and relative to the baseline IMPDH activity before transplantation (p = 0.042). Our results suggest that individualization of MPA therapy, particularly through pharmacodynamic monitoring of IMPDH activity in PBMCs, has the potential to improve the clinical outcomes of transplant patients.

Getting immunosuppression just right: the role of clinical biomarkers in predicting patient response post solid organ transplantation

INTRODUCTION

Actually, immunosuppression selection isn't based on individual immune alloreactivity, and immunosuppressive drug dosing is mainly based on the development of toxicity and the achievement of specific target concentrations. Since a successful outcome requires optimal patient risk stratification and treatment, several groups have evaluated candidate biomarkers that have shown promise in the assessment of individual immune responses, the prediction of personal pharmacodynamic effects of immunosuppressive drugs and the prognosis and diagnosis of graft outcomes..

AREAS COVERED

This review includes biomarkers that the Scientific Community in Solid Organ Transplantation currently considers to have potential as diagnostic and prognostic biomarkers of graft evolution. We have focused on recent scientific advances and expert recommendations regarding the role of specific and non-specific pharmacodynamic biomarkers that are mainly involved in the T-cell-mediated response.

EXPERT OPINION

Integral pharmacologic monitoring that combines pharmacokinetics, pharmacogenetics and predictive pharmacodynamic biomarkers may provide crucial information and allow personal adjustment of immunosuppressive drugs at an early stage before severe adverse events ensue. Multicentre, randomized, prospective and interventional trials are needed to fine tune the established cut-off values for each biomarker and the optimal monitoring frequency for each biomarker and to accurately evaluate possible clinical confounding factors to enable correct clinical qualification.

Personalized Therapy for Mycophenolate: Consensus Report by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology

ABSTRACT

When mycophenolic acid (MPA) was originally marketed for immunosuppressive therapy, fixed doses were recommended by the manufacturer. Awareness of the potential for a more personalized dosing has led to development of methods to estimate MPA area under the curve based on the measurement of drug concentrations in only a few samples. This approach is feasible in the clinical routine and has proven successful in terms of correlation with outcome. However, the search for superior correlates has continued, and numerous studies in search of biomarkers that could better predict the perfect dosage for the individual patient have been published. As it was considered timely for an updated and comprehensive presentation of consensus on the status for personalized treatment with MPA, this report was prepared following an initiative from members of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT). Topics included are the criteria for analytics, methods to estimate exposure including pharmacometrics, the potential influence of pharmacogenetics, development of biomarkers, and the practical aspects of implementation of target concentration intervention. For selected topics with sufficient evidence, such as the application of limited sampling strategies for MPA area under the curve, graded recommendations on target ranges are presented. To provide a comprehensive review, this report also includes updates on the status of potential biomarkers including those which may be promising but with a low level of evidence. In view of the fact that there are very few new immunosuppressive drugs under development for the transplant field, it is likely that MPA will continue to be prescribed on a large scale in the upcoming years. Discontinuation of therapy due to adverse effects is relatively common, increasing the risk for late rejections, which may contribute to graft loss. Therefore, the continued search for innovative methods to better personalize MPA dosage is warranted.

Therapeutic drug monitoring of mycophenolate mofetil in pediatric patients: novel techniques and current opinion

Introduction: Mycophenolate mofetil (MMF) is an ester prodrug of the immunosuppressant mycophenolic acid (MPA) and is recommended and widely used for maintenance immunosuppressive therapy in solid organ and stem-cell transplantation as well as in immunological kidney diseases. MPA is a potent, reversible, noncompetitive inhibitor of the inosine monophosphate dehydrogenase (IMPDH), a crucial enzyme in the de novo purine synthesis in T- and B-lymphocytes, thereby inhibiting cell-mediated immunity and antibody formation. The use of therapeutic drug monitoring (TDM) of MMF is still controversial as outcome data of clinical trials are equivocal. Areas covered: This review covers in great depth the existing literature on TDM of MMF in the field of pediatric (kidney) transplantation. In addition, the relevance of TDM in immunological kidney diseases, in particular childhood nephrotic syndrome is highlighted. Expert opinion: TDM of MMF has the potential to optimize therapy in pediatric transplantation as well as in nephrotic syndrome. Limited sampling strategies to estimate MPA exposure increase its feasibility. Future perspectives rather encompass approaches reflecting total immunosuppressive load than single drug TDM.

Efficacy and safety of mycophenolate mofetil for IgA nephropathy: An updated meta-analysis of randomized controlled trials

The efficacy and safety of mycophenolate mofetil (MMF) for immunoglobulin A nephropathy (IgAN) remains debatable. Therefore, the present meta-analysis was conducted with randomized controlled trials (RCTs). PubMed/MEDLINE, EMBASE and the Cochrane Central Register of Controlled Trials were analyzed to identify eligible trials. The pooled risk ratio (RR) with 95% confidence interval (CI) was estimated for all the dichotomous outcome measures. A total of eight RCTs with nine publications (n=510 patients) were included. No significant difference was noted between therapeutic regimens with and without MMF for renal remission and end stage renal disease (ESRD) of patients with IgAN (seven trials; RR, 1.250; 95% CI, 0.993–1.574; P=0.057; and four trials; RR, 0.728; 95% CI, 0.164–3.236; P=0.676). To further define the effects of MMF for renal remission, subgroup analysis was performed, demonstrating that MMF was significantly more effective compared with the placebo (three trials; RR, 2.152; 95% CI, 1.198–3.867; P=0.010), although the immunosuppressive regimens with MMF had no significantly different effects compared with those without MMF (four trials; RR, 1.140; 95% CI, 0.955–1.361; P=0.146), indicating that MMF was superior to placebo and had a similar efficacy to other immunosuppressants for renal remission. In addition, subgroup analysis for ESRD revealed no significant differences between MMF and placebo and between the immunosuppressive regimens with and without MMF (three trials; RR, 0.957; 95% CI, 0.160–5.726; P=0.962; and one trial; RR, 0.205; 95% CI, 0.010–4.200; P=0.303). Furthermore, there were no significant differences between the therapeutic regimens with and without MMF in terms of the risk of adverse events. The present meta-analysis demonstrated that MMF was more effective compared with the placebo, may have similar efficacy to other immunosuppressants in terms of inducing renal remission of IgAN and may not increase the risk of adverse events. The long-term effects of MMF on the prognosis of patients with IgAN require verification in further studies.

The pharmacokinetics and pharmacodynamics of mycophenolate mofetil in younger and elderly renal transplant recipients

AIMS

Elderly transplant recipients have a lower incidence of acute rejection, and a higher risk to die from infectious complications. A potential cause may be differences in the pharmacokinetics (PK) or pharmacodynamics (PD) of the immunosuppressive drugs they are taking. This study was designed to comprehensively evaluate the influence of age on the PK and PD of mycophenolic acid (MPA).

METHODS

In this study the PK and PD of MPA was studied in 26 elderly and 54 younger renal transplant recipients treated with mycophenolate mofetil and tacrolimus. Patients were sampled repetitively, both before and during the first 6 months after kidney transplantation. Age-related variability in MPA PK, baseline IMPDH activity, as well as MPA-induced IMPDH inhibition were studied.

RESULTS

The IMPDH activity pre-transplantation did not differ between elderly and younger patients. Neither IMPDH activity pre-transplantation nor maximum IMPDH inhibition was significantly correlated with the patients' age. The area under the MPA plasma concentration-time curve (AUC_0-12h ) and the area under the effect (IMPDH activity)-time curve (AEC_0-12h ) from 0 to 12 h were also not significantly different between the two groups. We found no significant differences in EC₅₀ and E_max between elderly and younger patients.

CONCLUSIONS

Age did not significantly affect the PK or PD of MPA. It is unlikely that the lower incidence of acute rejection in elderly patients, or the higher risk to die from a severe infection in elderly patients is due to different handling of MPA in the elderly.

Mycophenolate mofetil and systemic lupus erythematosus

Mycophenolate mofetil (MMF) is an immunosuppressive agent which provides protection against acute transplant rejection, in patients who undergo kidney, heart and liver transplantation. Recently MMF has been used in various autoimmune conditions, including systemic lupus erythematosus (SLE). In SLE, MMF has been more extensively used in the treatment of proliferative lupus glomerulonephritis (GLN) and following the success in this field, it has also been used to control extra-renal manifestations. However, in the majority of cases MMF was administered to patients with refractory SLE manifestations and, therefore, no definite conclusion could be drawn from these experiences.

In this paper, after a brief description of the mechanisms of action, the pharmacokinetics and metabolism of MMF which are relevant in SLE, and after a short discussion on the utility of performing therapeutic dose monitoring of mycphenolic acid, the experiences with the use of this drug in the different SLE manifestations were summarized and some personal data in patients with GLN were reported. Finally, the hypothetical use of MMF as a preventive strategy against the occurrence of severe manifestations in patients with mild SLE has been put forward and discussed.

Why, when and how should immunosuppressive therapy considered in patients with immunoglobulin A nephropathy?

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. Lifelong mesangial deposition of IgA1 complexes subsist inflammation and nephron loss, but the complex pathogenesis in detail remains unclear. In regard to the heterogeneous course, classical immunosuppressive and specific therapeutic regimens adapted to the loss of renal function will here be discussed in addition to the essential common renal supportive therapy. Renal supportive therapy alleviates secondary, surrogate effects or sequelae on renal function and proteinuria of high intraglomerular pressure and subsequent nephrosclerosis by inhibition of the renin angiotensin system (RAASB). In patients with physiological (ΔGFR < 1·5 ml/min/year) or mild (ΔGFR 1·5-5 ml/min/year) decrease of renal function and proteinuric forms (> 1 g/day after RAASB), corticosteroids have shown a reduction of proteinuria and might protect further loss of renal function. In patients with progressive loss of renal function (ΔGFR > 3 ml/min within 3 months) or a rapidly progressive course with or without crescents in renal biopsy, cyclophosphamide with high-dose corticosteroids as induction therapy and azathioprine maintenance has proved effective in one randomized controlled study of a homogeneous cohort in loss of renal function (ΔGFR). Mycophenolic acid provided further maintenance in non-randomized trials. Differentiated, precise, larger, randomized, placebo-controlled studies focused on the loss of renal function in the heterogeneous forms of IgAN are still lacking. Prospectively, fewer toxic agents will be necessary in the treatment of IgAN.

Mycophenolic acid exposure and complement fraction C3 influence inosine 5'-monophosphate dehydrogenase activity in systemic lupus erythematosus

Background Mycophenolate mofetil has recently been reported to be effective against systemic lupus erythematosus. The influence of the pharmacokinetics of mycophenolic acid, the active form of mycophenolate mofetil and the major inactive mycophenolic acid phenolic glucuronide on the activity of the target enzyme inosine 5'-monophosphate dehydrogenase, is expected to be revealed. The aim of this study was to identify the factors associated with inosine 5'-monophosphate dehydrogenase activity in systemic lupus erythematosus patients. Methods Fifty systemic lupus erythematosus patients in remission maintenance phase (29 received mycophenolate mofetil [MMF+] and 21 did not [MMF-]) were enrolled. Median and interquartile range of dose of mycophenolate mofetil were 1500 and 1000-1500 mg/day, respectively. Stepwise multiple linear regression analysis was performed to assess the dependence between inosine 5'-monophosphate dehydrogenase activity and 25 predictor values including predose plasma concentrations of free mycophenolic acid and mycophenolic acid phenolic glucuronide. Results Median and interquartile range of predose total plasma concentrations of mycophenolic acid and mycophenolic acid phenolic glucuronide were 2.73 and 1.43-5.73 and 25.5 and 13.1-54.7  µg/mL, respectively. Predose inosine 5'-monophosphate dehydrogenase activity was significantly higher in MMF+ than MMF- patients (median 38.3 and 20.6 nmoL xanthosine 5'-monophosphate/g haemoglobin/h, P<0.01). The plasma concentration of free mycophenolic acid phenolic glucuronide, complement fraction C3 and body weight were significant predictors accounting for interindividual variability in the inosine 5'-monophosphate dehydrogenase activity (adjusted R²= 0.52, P < 0.01) in a multivariate analysis. Conclusions Predose inosine 5'-monophosphate dehydrogenase activity was higher in systemic lupus erythematosus patients receiving mycophenolate mofetil therapy. Inosine 5'-monophosphate dehydrogenase activity may be determined by mycophenolic acid exposure and complement fraction C3 in systemic lupus erythematosus patients.

Peripheral Lymphocyte Response to Mycophenolic Acid In Vitro and Incidence of Cytomegalovirus Infection in Renal Transplantation

The lymphocyte immunosuppressant sensitivity test (LIST) with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay procedure has been used to predict the pharmacological efficacy of immunosuppressive agents to prevent acute rejection episodes for renal transplant recipients. In this study, mycophenolic acid (MPA) pharmacological efficacies were evaluated by LIST at both prior to and just after renal transplantation. We compared the efficacies to the clinical outcome of these recipients. MPA's pharmacological efficacy was evaluated by LIST not only before the operation but also at 2, 4, and 6 weeks after transplantation in 16 renal transplant recipients. These recipients were divided into high- and low-sensitivity groups according to peripheral blood mononuclear cell (PBMC) sensitivity to MPA in vitro. The MPA sensitivities were compared to cytomegalovirus (CMV) infection and acute rejection episodes in these recipients under MPA immunosuppressive therapy. The rate of CMV infection episodes in the low-MPA pharmacological efficacy group categorized at 2 weeks after renal transplantation was 5/6 (83.3%), which was significantly higher than the rate of 1/10 (10.0%) (p < 0.01) in the high-MPA sensitivity group. However, the MPA pharmacological efficacy evaluated both before and after transplantation had no relationship with the incidence of rejection episodes. These findings suggest that the MPA pharmacological efficacy evaluated by LIST at 2 weeks after operation is a useful biomarker for predicting the following occurrence of CMV infection episodes in renal transplant recipients.

Monitoring of inosine monophosphate dehydrogenase activity and expression during the early period of mycophenolate mofetil therapy in de novo renal transplant patients

Measurement of inosine-monophosphate dehydrogenase (IMPDH) activity or gene expression was used as a further approach in pharmacokinetics (PK)/pharmacodynamic (PD)-guided mycophenolate mofetil (MMF) therapy. Forty-four de novo kidney transplant patients were enrolled; 35 of these completed the study, and were followed for 24 weeks for clinical status, PK parameters, IMPDH activity and IMPDH1/2 gene expression. IMPDH activity and expression were measured in peripheral blood mononuclear cells before transplant and at week 2,4,12 and 24, drawn before (t0) and 2 h (t2 h) after MMF administration. No significant correlation was found between IMPDH activity/expression and PK parameters. For both genes, significant enhancement in t2 h expression was observed, then decreases towards week 24 with a trend following steroid dosages. Seven patients experienced acute rejection (AR) and exhibited significantly higher pre-transplant expression of both IMPDH1 (median 3.42 vs. 0.84; p=0.0025), and IMPDH2 genes (135 vs. 104; p=0.0218) with respect to non-rejecting patients. A significant association was also found between pre-transplant IMPDH1 mRNA and haematological complications (p=0.032). This study suggests that high steroid dosages may influence IMPDH1/2 expression, hampering their use as a PD biomarker, particularly during the early post-transplant period. The measurement of pre-transplant levels of IMPDH1/2 may contribute to prediction of individual drug responsiveness to improve the clinical management of patients in MMF therapy.

Liquid chromatography-coupled tandem mass spectrometry based assay to evaluate inosine-5'-monophosphate dehydrogenase activity in peripheral blood mononuclear cells from stem cell transplant recipients

Combinations of immunosuppressive drugs are routinely used post-transplantation to prevent rejection and/or other complications and optimize outcomes. The prodrug ester mycophenolate mofetil (MMF) is frequently used in solid-organ and stem cell transplantation settings. A growing body of evidence supports therapeutic monitoring of this immunosuppressant to optimize its efficacy and reduce toxicity. Thus, pharmacodynamic monitoring of MMF is a strategy that could potentially improve patient outcomes. Pharmacodynamic measurements require evaluation of inosine-5'-monophosphate dehydrogenase (IMPDH) activity, the target enzyme of the active moiety mycophenolic acid. Various nonradioactive methods using chromatographic separations have been used to quantify xanthosine monophosphate, the catalytic product of the enzyme, to indirectly evaluate IMPDH activity. However, no methods have used mass spectrometry based detection, which provides more specificity and sensitivity. Here, we describe a liquid chromatography-coupled tandem mass spectrometry (LC-MS/MS) method for the quantification of xanthosine monophosphate and adenosine monophosphate (for normalization) in lysates of peripheral blood mononuclear cells (PBMCs) from hematopoietic stem cell transplant (HSCT) recipients. Linearity, precision, and accuracy were validated over a large range of concentrations for each compound. The method could measure analytes with high sensitivity, accuracy (93-116%), and reproducibility (CV < 7.5%). Its clinical application was validated in PBMC lysates obtained from healthy individuals (n = 43) and HSCT recipients (n = 19). This reliable and validated LC-MS/MS method could be a useful tool for pharmacodynamic monitoring of patients treated with MMF.

Inosine 5'-monophosphate dehydrogenase activity as a biomarker in the field of transplantation

Inosine 5'monophosphate dehydrogenase (IMPDH) is the rate limiting enzyme in the de novo synthesis of guanine nucleotides. The direct determination of target enzyme activity as a biomarker of mycophenolic acid (MPA) may help to estimate better the individual response to the immunosuppressant. However, the assessment of the clinical utility of this approach is limited by the diversity of the assay systems, which has not yet allowed the prospective assessment of this enzyme in larger patient cohorts. A recently validated and standardized assay allows the investigation of IMPDH activity in larger clinical studies. Although descriptive results from observational studies hold promise for a more individualized therapy in transplant medicine, more studies are needed to prospectively validate this approach.

SLCO1B1 genetic polymorphism influences mycophenolic acid tolerance in renal transplant recipients

AIMS

This study aimed to determine the influence of gene candidates on mycophenolic acid (MPA) response during the first year of renal transplantation.

MATERIALS & METHODS

A total of 218 renal transplant recipients who received MPA from the first day of transplantation at a fixed dose of 2 g/day were genotyped for ABCB1, ABCC2, UGT2B7, UGT1A9, SLCO1B1, SLCO1B3 and IMPDH1 polymorphisms. Clinical end points were MPA-related adverse drug reactions (ADRs) and acute rejection episodes during the first year post-transplantation.

RESULTS

After correction for multiple statistical testing, SLCO1B1 (encoding the hepatic uptake transporter OATP1B1) was the only gene associated with MPA-related ADRs, showing a 75% risk reduction in favor of a protective effect of the SLCO1B1*5 allele (p = 0.002). In vitro experiments showed that MPA metabolites MPA-phenyl-glucuronide and MPA-acyl-glucuronide are substrates of OATP1B1. Their transport was decreased in the presence of the variant transporter (OATP1B1*5).

CONCLUSION

These results suggest for the first time that carriers of the SLCO1B1*5 allele seem to be protected from MPA-related ADRs.

Biomarkers as a tool for management of immunosuppression in transplant patients

Therapeutic drug monitoring is a well-established approach in transplantation medicine to guide immunosuppressive therapy. However, it cannot always predict the effects of immunosuppressive drugs on immune cells, because it does not reflect any aspect of an individual patient's immune system. Pharmacodynamic monitoring is a more recent strategy to provide information about the biologic effect of a specific drug or drug combination on the individual transplant patient. Currently, there is a large number of different biomarkers that either directly (specific markers) or indirectly (global markers) relate to the pharmacodynamic effects of immunosuppressive drugs and are under investigation as potential candidates to be introduced in clinical practice. Such biomarkers may be useful to identify patients at risk of developing acute rejection, infection, or cancer as well as patients who are suitable for minimization of immunosuppressant therapy and may be helpful to manage the timing and rate of immunosuppressant weaning. Serial longitudinal monitoring may allow maintenance of an individualized immunosuppressive regimen. Thus, biomarker monitoring is a potential complementary tool to therapeutic drug monitoring. This review summarizes the current state of knowledge about the use of a number of global or drug-specific pharmacodynamic biomarkers. It is not a comprehensive overview of the literature available, but rather an evidence-based reflection by experts who are intensively involved in scientific work in this field.

Impact of changing from cyclosporine to tacrolimus on pharmacokinetics of mycophenolic acid in renal transplant recipients with diabetes

The rate of mycophenolic acid (MPA) absorption after oral administration of mycophenolate mofetil (MMF) is delayed in patients with diabetes. Cyclosporine (CsA) decreases MPA exposure by inhibiting enterohepatic recirculation of MPA/MPA glucuronide, and tacrolimus (TRL) may alter the rate and extent of MPA absorption due to its prokinetic properties especially in patients with diabetic gastroparesis. This study evaluated the effect of changing from CsA to TRL on pharmacokinetics of MPA in stable renal transplant recipients with long-standing diabetes. Eight patients were switched from a stable dose of CsA to TRL while taking MMF 1 g twice daily. The 12-hour steady-state total plasma concentration-time profiles of MPA and MPA glucuronide were obtained after oral administration of MMF on 2 occasions: first while taking CsA and second after changing to TRL. Pharmacokinetic parameters of MPA were calculated by the noncompartmental method. Changing from CsA to TRL resulted in significantly increased MPA exposure (area under the concentration-time curve from 0 to 12 hours, AUC0-12) by 46 +/- 32% (P = 0.012) and MPA predose concentration (C0) by 121 +/- 67% (P = 0.008). The magnitude of change in MPA exposure did not correlate well with MPA-C0 or CsA trough concentration. Switching to TRL had minimal impact on peak concentration of MPA (15.0 +/- 6.9 mg/L with CsA versus 16.1 +/- 9.7 mg/L with TRL, P = 0.773) and time to reach the peak concentration (1.0 +/- 0.4 hours with CsA versus 1.2 +/- 0.8 hours with TRL, P = 0.461). Highly variable and unpredictable changes in MPA exposure among renal transplant patients with diabetes do not support a strategy of preemptively adjusting MMF dose when switching calcineurin inhibitors in this population.

Target enzyme activity as a biomarker for immunosuppression

Pharmacokinetic drug monitoring has been used for many years to relate immunosuppressant dose to drug exposure in vivo. However, this conventional therapeutic drug monitoring of blood immunosuppressant levels may not necessarily predict the pharmacologic effects on immune cells. The direct determination of target enzyme activity (eg, calcineurin activity, inosine-5'-monophospahte dehydrogenase [IMPDH] activity, p70S6 kinase) may help to better assess the individual response to the immunosuppressant. However, its use is limited by the difficulties of the assay systems, which did not allow yet the prospective assessment of these enzymes in larger patient cohorts with the establishment of validated pharmacodynamic drug monitoring. The most progress regarding a robust and reproducible test system has been achieved with the determination of IMPDH activity as a specific pharmacodynamic parameter of mycophenolic acid activity. This recently validated and standardized assay allows the investigation of IMPDH activity in larger clinical studies. Although the determination of target enzyme activity, eg, by the determination of IMPDH activity, holds promise for a more individualized therapy in transplant medicine, more studies are needed to prospectively validate this approach.

Pharmacokinetics and pharmacodynamics of mycophenolic acid and their relation to response to therapy of childhood-onset systemic lupus erythematosus

OBJECTIVES

Mycophenolic acid (MPA) is the active form of mycophenolate mofetil (MMF), which is currently used off-label as immunosuppressive therapy in childhood-onset SLE (cSLE). The objectives of this study were to (1) characterize the pharmacokinetics (MPA-PK) and pharmacodynamics (MPA-PD) of MPA and (2) explore the relationship between MPA-PK and cSLE disease activity.

METHODS

MPA-PK [area under the curve from 0-12 hours (AUC(0-12))] and MPA-PD [inosine-monophosphate dehydrogenase (IMPDH) activity] were evaluated in cSLE patients on stable MMF dosing. Change in SLE disease activity while on MMF therapy was measured using the British Isles Lupus Assessment Group (BILAG) index.

RESULTS

A total of 19 AUC(0-12) and 10 IMPDH activity profiles were included in the analysis. Large interpatient variability in MPA exposure (AUC(0-12)) was observed (mean ± SE: 32 ± 4.2 mg h/L; coefficient of variation: 57%). Maximum MPA serum concentrations coincided with maximum IMPDH inhibition. AUC(0-12) and weight-adjusted MMF dosing were only moderately correlated (r = 0.56, P = 0.01). An AUC(0-12) of ≥30 mg h/L was associated with decreased BILAG scores while on MMF therapy (P = 0.002).

CONCLUSION

Weight-adjusted MMF dosing alone does not reliably allow for the prediction of exposure to biologically active MPA in cSLE. Individualized dosing considering MPA-PK appears warranted as this allows for better estimation of immunologic suppression (IMPDH activity). Additional controlled studies are necessary to confirm that an MPA AUC(0-12) of at least 30 mg h/L is required for cSLE improvement.

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.

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.

Improved assay for the nonradioactive determination of inosine 5'-monophosphate dehydrogenase activity in peripheral blood mononuclear cells

Mycophenolic acid (MPA) inhibits the enzyme inosine 5'-monophosphate dehydrogenase (IMPDH). Thus, the measurement of IMPDH activity could serve as a specific pharmacodynamic (PD) tool for monitoring MPA therapy. At present, however, monitoring of pharmacokinetic parameters is preferred over that of PD parameters because, in general, PD assays are labor-intensive and poorly reproducible. Currently, cell count or protein concentration is widely accepted as methods to normalize enzyme activity. In the present study, we have attempted to further improve a method for the determination of IMPDH activity to increase the robustness and reproducibility of the IMPDH activity assay itself, without making the assay more labor-intensive. Therefore, several aspects of the IMPDH method were investigated regarding their influence on the reproducibility and also modified to increase the feasibility and consistency of the assay. The isolation of peripheral blood mononuclear cells (PBMCs) of whole blood samples was found to be the most variable step. Normalization on cell count is labor-intensive and at the same time has a poor reproducibility. Determination of the protein content in cell extracts is impaired by contamination with extracellular proteins and non-PBMCs. Alternatively, the intracellular substance adenosine monophosphate (AMP) was investigated to normalize the newly generated xanthosine monophosphate. Among various subject groups, no significant differences in mean AMP concentration were found. To simplify the procedure, PBMCs were diluted to a fixed volume after isolation from sample of whole blood, and the IMPDH activity was normalized to the AMP concentration quantified in the same high-performance liquid chromatography run as xanthosine monophosphate was quantified. The within-run and total imprecision (coefficient of variation) ranged from 4.2% to 10.6% and from 6.6% to 11.9%, respectively. In conclusion, the modified method described here for the measurement of IMPDH activity can be used reliably in multicenter trials and in longitudinal studies to evaluate the additional value of any PD monitoring among a diversity of patients treated with MPA.

Interpatient variability in IMPDH activity in MMF-treated renal transplant patients is correlated with IMPDH type II 3757T > C polymorphism

OBJECTIVES

The active metabolite of mycophenolate mofetil (MMF), mycophenolic acid, inhibits the activity of the target enzyme inosine monophosphate dehydrogenase (IMPDH). The aim of this study was to correlate eight different single nucleotide polymorphisms of the IMPDH type II gene to the activity of the IMPDH enzyme to explain between-patient differences in IMPDH activity.

METHODS AND RESULTS

In a prospective study, we measured IMPDH activity, mycophenolic acid plasma concentrations, and eight polymorphisms of IMPDH type II in de novo kidney transplant recipients, 6 days posttransplantation while on MMF treatment. Polymorphisms in the IMPDH type II gene were only observed for the IMPDH type II 3757T > C (rs11706052) single nucleotide polymorphism. Ten of 101 patients (10%) were heterozygous and two of 101 patients (2%) homozygous for IMPDH type II 3757T > C. The allele frequency was 6.9%. The IMPDH activity over 12 h (AUC(act)) was 49% higher for patients with an IMPDH type II 3757C variant [n = 12 vs. n = 68; 336 (95% confidence interval: 216-521) vs. 227 (95% confidence interval: 198-260) hmicromol/s/mol adenosine monophosphate; P = 0.04]. The IMPDH activity measured before transplantation (Act(pre-Tx)) was not significantly different between IMPDH type II 3757TT wild-type and variant carrier patients (P = 0.99).

CONCLUSION

We report that the IMPDH type II 3757T > C polymorphism is associated with an increased IMPDH activity in MMF-treated renal transplant patients. This polymorphism explains 8.0% of the interpatient variability in IMPDH activity.

Pharmacodynamic evaluation of the first dose of mycophenolate mofetil before kidney transplantation

BACKGROUND AND OBJECTIVES

The effect of mycophenolate mofetil (MMF) on T cell function has not been evaluated in patients undergoing kidney transplantation. The aim of this study was to assess the effect of 1g of MMF on T cell function, that is, intralymphocyte cytokine expression, T cell activation (CD25 and CD71), and T cell proliferation, as well as inosine monophosphate dehydrogenase (IMPDH) activity, to better understand the relationship between pharmacokinetic and pharmacodynamic markers in patients receiving the first dose of MMF before kidney transplantation.

PATIENTS

Twenty-four patients undergoing a kidney transplantation from a living donor were enrolled in this study.

RESULTS

Compared with baseline (before MMF intake), T cell proliferation (93%), IMPDH activity (74%), CD25 (46%), and CD71 (50%) expression significantly decreased during the first hour after MMF intake, in parallel to the rise in MPA concentration. Thereafter, all pharmacodynamic markers, except IMPDH activity, returned back to baseline level. There was a complex inverse relationship between pharmacokinetic and pharmacodynamic markers. The inhibition of T cell proliferation was highly correlated to IMPDH activity, but also to T cell activation markers.

CONCLUSION

The administration of MMF to patients is associated not only with a dramatic decrease in both T cell proliferation and IMPDH activity, but also with in a decrease in CD25 and CD71 expression.

Pharmacodynamics of mycophenolic acid in CD4+ cells: a single-dose study of IMPDH and purine nucleotide responses in healthy individuals

Mycophenolate mofetil is used in rejection prophylaxis after allograft transplantation. The highly variable pharmacokinetics and pharmacodynamics (PD) of the active moiety mycophenolic acid (MPA) render this drug attractive for therapeutic monitoring. The aim of this study was to characterize the exposure-response relationship for MPA to guide future strategies for individualized therapy based on PD monitoring. A single-dose (100, 250, 500, and 1000 mg mycophenolate mofetil) crossover exposure-response study of MPA PD in CD4 cells was performed in 5 healthy individuals. The activity of inosine 5'-monophosphate dehydrogenase (IMPDH) at time 0 ranged from 1.2 to 7.2 pmol per 10 cells/min. IMPDH was strongly inhibited by MPA; MPA EC50 (concentration required for 50% inhibition) of 2.3 mg/L was determined by a pooled data analysis. Decreased IMPDH gene expression was associated with the exposure to MPA. There were no immediate reductions of guanine nucleotides. On the contrary, a trend toward increased guanosine triphosphate was observed. IMPDH activity AUC0-12h approached maximum reduction at MPA AUC0-12h 22 mg x h/L (corresponding to the 500 mg dose), whereas plasma concentrations exceeding approximately 6 mg/L did not further increase the IMPDH inhibition. The results suggest that guanine nucleotides in circulating lymphocytes may not serve as immediate response biomarkers to MPA. Strategies for preventing over- or underexposure to MPA may be developed by means of IMPDH activity combined with MPA concentration measurement.

Expression of IMPDH1 and IMPDH2 after transplantation and initiation of immunosuppression

BACKGROUND

Mycophenolic acid (MPA) mediates immunosuppressive effects by inhibiting inosine monophosphate dehydrogenase (IMPDH). Induction of IMPDH activity has been observed in whole blood and erythrocyte samples during immunosuppressive therapy. Information concerning the mechanisms for increased IMPDH activity is limited and the potential implications of induction have been debated.

METHODS

Whole blood, CD4+ cell, and reticulocyte samples were collected from 30 renal transplant patients pre- and posttransplantation. The expressions of two IMPDH isoforms, type 1 and 2, were analyzed by real-time reverse-transcription polymerase chain reaction and quantified using a housekeeping gene index. The IMPDH activity was determined by ultraviolet high-performance liquid chromatography.

RESULTS

Transplantation and the initiation of immunosuppressive therapy was associated with increased IMPDH1 (50-88%, P<0.0005) and decreased IMPDH2 (42-56%, P<0.0005) expression. In CD4+ cells, however, IMPDH2 increased (15%, P=0.009). These changes are probably related to glucocorticoid effects. Two weeks posttransplant, MPA-treated patients displayed elevated IMPDH 1 and 2 in reticulocytes, suggesting enzyme induction in these cells during prolonged MPA therapy. Patients with acute rejection during follow-up demonstrated higher IMPDH2 expression in CD4+ cells pretransplant than nonrejecting patients (median expression 1.26 vs. 0.87 respectively, P=0.017).

CONCLUSIONS

Knowledge of changes in IMPDH 1 and 2 expression after transplantation and initiation of immunosuppression is important considering the action of MPA on IMPDH and the potential for pharmacodynamic monitoring of MPA by measuring IMPDH activity. The expression of IMPDH2 in CD4+ cells pretransplant may be an indicator of immune activation.

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.

Monitoring of inosine monophosphate dehydrogenase activity as a biomarker for mycophenolic acid effect: potential clinical implications

Mycophenolic acid (MPA) is a reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH) and, in combination with other immunosuppressive drugs, effectively inhibits rejection in solid organ transplant recipients. MPA has a relatively narrow therapeutic window and exhibits wide inter- and intrapatient pharmacokinetic (PK) variability. This has stimulated the use of therapeutic drug monitoring as a strategy to tailor the MPA exposure to each patient's individual needs. Despite increasing therapeutic drug monitoring use, PK-assisted dosing is not universally adopted in part because of MPA's complex PK behavior. Targeting inosine monophosphate IMPDH activity as a surrogate pharmacodynamic (PD) marker of MPA-induced immunosuppression may allow for increased precision when used in an integrated PK-PD fashion, providing a more accurate assessment of efficacy and aid in limiting toxicity. IMPDH activity displays wide interpatient variability but relatively small intrapatient variability even after long-term administration of MPA. The advent of calcineurin and corticosteroid-sparing regimens necessitates more patient-specific PK-PD parameters, which can be used throughout the posttransplant period to optimize MPA exposure and immediate and long-term graft and patient outcomes. Quantification of IMPDH posttransplant may serve as a stable, surrogate PD marker of MPA-induced immunosuppression when combined with current PK and monitoring strategies.

Pharmacokinetics and pharmacodynamics of mycophenolic acid after enteric-coated mycophenolate versus mycophenolate mofetil in patients with progressive IgA nephritis

Mycophenolic acid can be administered orally using mycophenolate mofetil or enteric-coated mycophenolate. In renal transplant patients on immunosuppressant combination therapy, the overall mycophenolic acid exposure after oral dosing with mycophenolate mofetil and enteric-coated mycophenolate is similar. This study compared pharmacokinetics and pharmacodynamics of mycophenolic acid after equivalent doses of enteric-coated mycophenolate (360 mg twice daily) or mycophenolate mofetil (500 mg twice daily) in 7 patients with progressive IgA nephritis (glomerular filtration rate 20-35 mL/min) using a randomized crossover design. The pharmacokinetics of mycophenolic acid concentrations and pharmacodynamics (using inosine 5'-monophosphate dehydrogenase activity as a bio-marker) were sequentially monitored for 12 hours. After enteric-coated mycophenolate treatment, the mycophenolic acid peak concentration (Cmax = 12.8 vs 6.0 microg/mL, P < .05) and the overall exposure were significantly higher (AUC = 60.9 vs 40.7 microg.h/mL, P < .05), and the apparent clearance was significantly lower (CL/F = 7.9 vs 10.7 L/h, P < .05) as compared to that after mycophenolate mofetil. Paradoxically, inosine 5'-monophosphate dehydrogenase activity was not significantly different. In conclusion, the steady-state mycophenolic acid exposure was higher during treatment with enteric-coated mycophenolate as compared to mycophenolate mofetil, which might be explained by more extensive enterohepatic recycling of mycophenolic acid after administration of enteric-coated mycophenolate, whereas inosine 5'-monophosphate dehydrogenase suppression was not different.

Pharmacokinetic and pharmacodynamic comparison of enteric-coated mycophenolate sodium and mycophenolate mofetil in maintenance renal transplant patients

The aim of this single-center crossover substudy was to assess pharmacokinetics and pharmacodynamics [inosine 5'-monophosphate dehydrogenase (IMPDH) activity] of enteric-coated mycophenolate sodium (EC-MPS) and mycophenolate mofetil (MMF) at steady-state conditions. Stable maintenance renal transplant patients on 1 g MMF b.i.d. participating in a double-blind, multicenter study, were randomized to receive EC-MPS (720 mg b.i.d.) or continue receiving MMF (1000 mg b.i.d.) for 12 months. Thereafter, all patients (n = 18) received 720 mg EC-MPS b.i.d. Area under the plasma mycophenolic acid (MPA) concentration-time curve with EC-MPS (57.4 +/- 15.0 microg h/mL) fulfilled bioequivalence criteria (geometric mean 0.98 (90% CI: 0.87-1.11) compared to MMF (58.4 +/- 14.1 microg h/mL). Consistent with the delayed release characteristics of EC-MPS, peak MPA concentration (geometric mean 0.89; 90% CI: 0.70-1.13) occurred approximately 0.5 h later (p < 0.05) and predose MPA levels (geometric mean 2.10; 90% CI: 1.51-2.91) were higher and more variable, not fulfilling bioequivalence criteria. IMPDH activity inversely followed MPA concentrations and was inhibited to a similar degree (approximately 85%) by both formulations. The calculated value for 50% IMPDH inhibition was identical for both drugs. In conclusion, equimolar doses of EC-MPS and MMF produce equivalent MPA exposure, while the delayed release formulation of EC-MPS exhibits more variable predose levels and T(max). Overall, IMPDH activity reflected MPA pharmacokinetics.

Circadian and time-dependent variability in tacrolimus pharmacokinetics

Tacrolimus (TAC) is considered a critical dose drug. The purpose of our study was to investigate circadian and time-dependent changes in TAC pharmacokinetics over the first year after kidney transplantation. Pharmacokinetic (PK) studies were performed in 26 recipients of first living donor kidney transplants at day 7 after morning (a.m.) and evening (p.m.) doses of TAC. Additional serial PK studies were carried out in nine patients at month 6 (M6) and month 12 (M12). Blood samples were collected before 1, 1.5, 2, 2.5, 3, 4, 6, 8 and 12 h after TAC administration. Demographics, TAC and adjunctive immunosuppressive doses, hematology, and biochemistry were recorded in each PK study. Mean age was 37 years, body mass index 23 kg/m(2), 58% males, and 85% Caucasian. Higher AUC (231.4 vs. 220 ng.h/mL, P = 0.06) and C(max) (34.1 +/- 12.6 vs. 24.4 +/- 9.8 ng/mL, P < 0.001), and lower T(max) (1.6 +/- 0.8 vs. 2.7 +/- 2.0 h, P = 0.05) values were observed comparing a.m. and p.m. administrations. Comparing D7, M6 and M12, there was a significant increase in dose-normalized AUC (31.4 +/- 22.2 vs. 50.1 +/- 33 vs. 39.2 +/- 24.4 ng.h/mL/mg, P = 0.005), C(max) (4.4 +/- 2.4 vs. 7.8 +/- 3.5 vs. 6.0 +/- 3.3 ng/mL/mg, P < 0.001) and T(max) (1.6 +/- 1.1 vs. 1.7 +/- 0.4 vs. 1.8 +/- 0.8 h, P = 0.006), respectively. Over the first year the intraindividual variability of dose-normalized AUC, C(max) and C(0) were 82%, 72%, and 90%, respectively. No significant changes were observed comparing inter-individual variability of dose-normalized AUC (21%, 24%, 33%), C(max) (46%, 45%, 55%), C(0) (49%, 83%, 81%) at D7, M6 and M12, respectively. We observed a good correlation between a.m. and p.m. TAC AUC (r(2) = 0.90) and C(0) (r(2) = 0.88). Tacrolimus pharmacokinetics display circadian variation suggesting a slower and delayed absorption phase at nighttime. Tacrolimus also showed time-dependent PK changes, suggesting an improvement in absorption during the first 6 months. Despite circadian variation we observed good correlations between a.m. and p.m. TAC AUC (r(2) = 0.90) and C(0) (r(2) = 0.88) and between C(0) and total daily TAC exposure (a.m. + p.m. AUC) suggesting that trough-guided therapeutic monitoring is still a reliable and simple strategy to optimize the clinical use of TAC.

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.

Inosine monophosphate dehydrogenase activity in renal allograft recipients during mycophenolate treatment

OBJECTIVE

Mycophenolic acid (MPA) exerts its immunosuppression by inhibiting inosine 5'-monophosphate dehydrogenase (IMPDH), depleting activated lymphocytes of guanine nucleotides and retarding their proliferation. An optimal strategy for monitoring has not been established for mycophenolate mofetil (MMF) in renal transplantation, and clinical investigations of the pharmacokinetic-pharmacodynamic relationship are warranted.

MATERIAL AND METHODS

Mycophenolic acid pharmacokinetics and whole blood cell IMPDH activity were investigated in two separate groups of renal allograft recipients. One group was studied within the 12-h dose interval, while the second group was examined by pre-dose samples pre-transplant and then repeatedly during 8 weeks post-transplant.

RESULTS

An inverse relationship between plasma MPA and IMPDH activity within the dose interval was demonstrated. Minimum IMPDH activity was a median 8 % of values pre-MMF dose, coinciding with the MPA peak. Six hours post-dose, IMPDH activity had returned to pre-dose values. Patients receiving MMF had a 4.5-fold higher pre-dose enzyme activity than transplanted patients without MMF. During the 8 weeks post-transplant, the median MPA trough concentration was fairly stable. Following an initial decrease during the first 4 days post-transplant, IMPDH activity gradually increased during the 40 days post-transplant, reaching 5-fold the pre-transplant values.

CONCLUSIONS

Provided that the changes in IMPDH activity in whole blood cells predict the clinical effect, these pharmacokinetic-pharmacodynamic findings may prove useful in the attempts to identify optimal timing and range for the monitoring of mycophenolate in renal transplantation. The question of whether MPA concentrations or measurements of IMPDH activity per se will be the optimal way of monitoring this immunosuppressant remains open and will only be answered by prospective clinical testing.

The Rationale for and Limitations of Therapeutic Drug Monitoring for Mycophenolate Mofetil in Transplantation

The addition of mycophenolate mofetil (MMF) to calcineurin inhibitor-based regimens reduces the incidence of acute rejection after kidney transplantation. The interpatient variability, changes over time of pharmacokinetic parameters, and the potential for drug interactions make the systemic exposure of mycophenolic acid (MPA) unpredictable at a fixed-dose regimen. An increase in plasma concentration of MPA significantly correlates with a decreased likelihood of an acute rejection after kidney or heart transplantation; therefore, a strategy of therapeutic drug monitoring for MMF therapy could improve outcome. Two large randomized, multicenter, prospective trials investigating the added value of therapeutic drug monitoring for MPA, by comparing fixed-dose treatment with concentration-controlled MMF treatment in kidney transplant recipients, are currently ongoing. More data are needed to fully establish the meaning of the reported prognostic value of preoperative inosine monophosphate dehydrogenase (IMPDH) activity, and longitudinal studies monitoring IMPDH activity after transplantation are eagerly awaited.

Therapeutic drug monitoring of mycophenolic acid in renal transplant recipients

Immunosuppressive regimens including mycophenolate mofetil (MMF, Cellcept) were used in a renal transplant transplant program since May 2000 including 67 patients in whom it was the primary drug. Acute rejection (AR) occurred in 9 cases (13%) with 1-year graft survival rate of 96.8%. Pharmacokinetic (PK) studies of mycophenolic acid (MPA) were performed in 46 recent patients (total, 127 times). There was no correlation between dose (mg/kg) and blood concentration (AUC0-9: r2= 0.27). AUC0-9 was well correlated with AUC0-4 (r2= 0.91), but not with a single timepoint concentration. MPA AUC0-9 level was significantly higher among the AR-negative group (n = 33; 34.2 +/- 16.8 ng.hr/mL) compared with AR-positive group (n = 3; 28.2 +/- 1.9 ng.hr/mL; P = .04085) over the 2 weeks after transplantation. MPA AUC0-9 level was higher among the adverse event (AE-positive) group (n = 15; 39.2 +/- 22.8 ng.hr/mL) compared with the negative group (n = 21; 30.1 +/- 8.0 ng.hr/mL; P = .08772) within 2 weeks after transplantation. These results suggest the necessity of measuring AUC for therapeutic drug monitoring (TDM) of MMF-containing immunosuppressive therapy. The possible target level of MPA AUC0-9 would be approximately 30 ng.hr/mL using the present immunosuppressive regimen.

Mycophenolate mofetil slows progression in anti-thy1-induced chronic renal fibrosis but is not additive to a high dose of enalapril

Tubulointerstitial inflammation and fibrosis are hallmarks of chronic progressive renal diseases. To characterize the functional interaction between cell infiltration and matrix expansion, this study compared the immunosuppressant mycophenolate mofetil (MMF), intended as primarily anti-inflammatory intervention, the angiotensin-converting enzyme inhibitor enalapril, intended as primarily an anti-fibrotic drug, and a combination of both as anticipated anti-inflammatory/anti-fibrotic intervention. The model used was anti-thy1-induced chronic-progressive glomerulosclerosis (cGS) in the rat, where a brief anti-thy1-induced glomerular injury progresses spontaneously toward tubulointerstitial fibrosis and renal insufficiency. cGS was induced by injection of anti-thy1 antibody into uninephrectomized Wistar rats. One week after disease induction, animals were randomly assigned to the following groups: cGS, cGS plus MMF (20 mg·kg body wt−1·day−1), cGS plus high-dose enalapril (12 mg·kg body wt−1·day−1), and cGS plus both. At week 16 after disease induction, MMF or enalapril alone reduced signs of chronic renal disease significantly and similarly compared with the untreated cGS group. Variables measured included proteinuria, blood pressure, tubulointerstitial and glomerular matrix accumulation, expression of transforming growth factor-β1, fibronectin, and plasminogen activator inhibitor-1, infiltration of lymphocytes and macrophages, plasma creatinine and urea levels, and glomerular filtration rate. Combined MMF and enalapril treatment was not superior to single therapy. In conclusion, MMF slows the progression of chronic renal fibrosis and renal insufficiency as effectively as high-dose enalapril in the anti-thy1-induced chronic-progressive glomerulosclerosis model. The dual anti-inflammatory/anti-fibrotic intervention does not yield additive renoprotective effects, indicating that MMF and enalapril interfere with similar or very closely related pathways involved in progression of renal disease.

Mycophenolate mofetil as a treatment for refractory idiopathic thrombocytopenic purpura

AIM

To determine whether mycophenolate mofetil (MMF) has beneficial effects on refractory idiopathic thrombocytopenic purpura (ITP) and the corresponding cellular mechanism.

METHODS

Twenty refractory ITP patients resistant to corticosteroid and/or splenectomy and chemical therapy were given MMF 1.5-2.0 g/d orally for a 2 to 4-month period. Serum immunoglobulin was detected by rate nephelometry. Platelet-associated antibodies (PAIgG) were assayed by enzyme-linked immunosorbant assay. The immunophenotypic analysis was performed on a flow cytometer and cell apoptosis was detected with transferase mediated dUTP biotin nick end labeling (TUNEL) method.

RESULTS

Sixteen of the 20 (80%) patients had responses to MMF treatment; 9 (45%) achieved a complete response, 4 (20%) achieved a partial response, and 3 (15%) achieved a minor response. The therapeutic effects were found to be better in male patients than female patients. The number of CD3+ peripheral blood cells (PBCs) and CD4+ PBCs increased and the number of CD8+ PBCs decreased. The plasma level of IgG, IgM, IgA and platelet associated IgG (PAIgG) decreased in 86% of the patients. TUNEL assay showed that mycophenolate acid (MPA) 0.1 mmol/L induced apoptosis of peripheral blood mononuclear cells isolated from refractory ITP patients. The apoptosis rate was increased in male patients after treatment with MPA, but was unchanged in female patients.

CONCLUSION

Therapy for a period of 8 to 16 weeks with median-dose of MMF was valuable for the treatment of refractory ITP.

Pre-transplant inosine monophosphate dehydrogenase activity is associated with clinical outcome after renal transplantation

Mycophenolate mofetil (MMF), an inhibitor of inosine monophosphate dehydrogenase (IMPDH) activity, is usually administered as a standard dose of 1 g b.i.d. after renal transplantation. Because MMF dose reductions are associated with inferior outcome, we investigated pre-transplant IMPDH activity, MMF dose reductions and outcome. IMPDH activity was determined in isolated peripheral mononuclear cells immediately prior to renal transplantation. We observed considerable inter-individual variability in pre-transplant IMPDH activity (9.35 +/- 4.22 nmol/mg/h). Thirty of 48 patients (62.5%) with standard MMF dose (1 g b.i.d.) had dose reductions within 3 years post-transplant; these patients also had significantly lower IMPDH activity. The area under the receiver-operating characteristics curve (AUC-ROC) for prediction of dose reduction within 6 months post-transplant was 0.75 (95% CI, 0.61-0.89; p < 0.004). IMPDH activity above the cut-off value, MMF dose reduction and age of recipient were significant contributors for the occurrence of acute rejection in the multivariate logistic regression. Patients with high IMPDH activity and MMF dose reduction had the highest rejection rate (81.8% vs. 36.4%; p < 0.01).

CONCLUSION

Patients with low IMPDH activity experienced more complications of MMF therapy. High pre-transplant IMPDH activity and MMF dose reductions were associated with rejection. Determination of IMPDH activity prior to transplantation may help to improve MMF therapy after renal transplantation.

Age-dependency of mycophenolate mofetil dosing in combination with tacrolimus after pediatric renal transplantation

BACKGROUND

Dosing of mycophenolate mofetil (MMF) must be lower in combination therapy with Tacrolimus (Tac) than with Cyclosporine. One study with mostly adolescent recipients recommended an MMF dose of 250 mg/m2 BID. Because this dose resulted in low area-under-the-curve (AUC) in our infant population, we retrospectively analyzed all available pharmacokinetic (PK) profiles in pediatric renal transplant patients on MMF plus Tac therapy to propose appropriate MMF dosing in pediatric patients of all ages.

PATIENTS AND METHODS

Forty-four PK profiles were performed in 27 patients (median age, 11.6 years; range, 1.8-20.7 years). The investigations were performed at a median of 299 days (range, 24-3424) after transplantation. Ten patients were converted to Tac plus MMF, all others received this as primary therapy. For patients with repeated measurements, we calculated the average AUC and doses. We used first-order PK modeling to calculate the doses for a mycophenolic acid (MPA) AUC of 60 ug*h/mL and a Tac AUC of 150 ng*h/mL.

RESULTS

The mean Tac dose was 2.6 +/- 1.2 mg/m2/d or 0.086 +/- 0.038 mg/kg/d, resulting in an average AUC of 120.6 +/- 30.4 ng*h/mL. The MMF dose was not normally distributed; the median dose was 549 mg/m2/d (range, 146-1413) and the median MPA AUC was 49.8 ug*h/mL (range, 26.7-156.0). The mean dose for a Tac AUC of 150 ng*h/mL was 3.50 +/- 1.77 mg/m2/d (0.117 +/- 0.058 mg/kg) and was independent of age or time after transplantation. By contrast, we found a negative relationship between the dose per m2 (r2 = 0.29; P = 0.0038) or per kg (r2 = 0.58; P < .0001) required for an MPA AUC of 60 ug*h/mL and patient age. Converted and primary patients behaved identically. The dosing requirement decreased from 500 mg/m2 BID in 2-year-old patients to 250 mg/m2 in adolescents. There was substantial interpatient variability of 44%.

CONCLUSIONS

Higher MMF doses are required for young children. Our data suggest a starting dose for infants of 500 mg/m2 BID, with PK monitoring of MPA due to substantial interpatient variability.