Mycophenolic acid concentrations are associated with cardiac allograft rejection

Using Established Immunosuppressant Therapy Effectively

Getting the most effective use of immunosuppressant medications in transplant patients continues to be a major challenge in clinical practice. This need applies to established immunosuppressants just as it does to new agents. In this review this principle is illustrated for mycophenolic acid, the active metabolite of mycophenolate mofetil, the most commonly used immunosuppressant, in various combinations with other immunosuppressants, in current clinical practice. Defining, as rigorously as possible, the requirements for effective therapeutic monitoring of MPA is an important goal given all of the changes in immunosuppressive drug regimens. This review will focus on the major factors known to influence MPA clearance including: UDP-glucuronyltransferases, enterohepatic circulation, MPA free fraction, the effect of time posttransplantation, concomitant administration of immunosuppressant drugs. The significant variability of MPA pharmacokinetics and the need to deepen our understanding of the influence of these factors on MPA clearance are strong reasons why additional clinical trials are needed to define best practice therapeutic drug monitoring of this drug.

Population pharmacokinetics and Bayesian estimation of mycophenolic acid concentrations in stable renal transplant patients

BACKGROUND

Therapeutic drug monitoring of mycophenolic acid (MPA) may minimise the risk of acute rejection after transplantation. Area under the curve (AUC) rather than trough concentration-based monitoring is recommended and models for AUC estimation are needed.

OBJECTIVES

To develop a population pharmacokinetic model suitable for Bayesian estimation of individual AUC in stable renal transplant patients.

PATIENTS AND METHODS

The population pharmacokinetics of MPA were studied using nonlinear mixed effects modelling (NONMEM) in 60 patients (index group) receiving MPA on a twice-daily basis. Ten blood samples were collected at fixed timepoints from ten patients and four blood samples were collected at sparse timepoints from 50 patients. Bayesian estimation of individual AUC was made on the basis of three blood concentration measurements and covariates. The predictive performances of the Bayesian procedure were evaluated in an independent group of patients (test group) comprising ten subjects in whom ten blood samples were collected at fixed timepoints.

RESULTS

A two-compartment model with zero-order absorption best fitted the data. Covariate analysis showed that bodyweight was positively correlated with oral clearance. However, the weak magnitude of the reduction in variability (from 34.8 to 28.2%) indicates that administration on a per kilogram basis would be of limited value in decreasing interindividual variability in MPA exposure. Bayesian estimation of pharmacokinetic parameters using samples drawn at 20 minutes and 1 and 3 hours enabled estimation of individual AUC with satisfactory accuracy (bias 7.7%, range of prediction errors 0.43-15.1%) and precision (root mean squared error 12.4%) as compared with the reference value obtained using the trapezoidal method.

CONCLUSION

This paper reports for the first time population pharmacokinetic data for MPA in stable renal transplant patients, and shows that Bayesian estimation can allow accurate prediction of AUC with only three samples. This method provides a tool for therapeutic drug monitoring of MPA or for concentration-effect studies. Its application to MPA monitoring in the early period post-transplantation needs to be evaluated.

Monitoring mycophenolate in liver transplant recipients: toward a therapeutic range

Predose plasma mycophenolic acid (MPA) concentrations measured with a semi-automated enzyme-multiplied immunoassay were related to adverse events (e.g., rejection, leukopenia, infection), drug dose, and clinical status in 147 adult and 63 pediatric liver allograft recipients receiving adjunctive immunosuppression with mycophenolate mofetil (MMF). In 12 of 13 acute rejection episodes, predose MPA levels were below the 1 mg/L cut-off defined using receiver operating characteristic (ROC) curve analysis. The relative risk of developing infection or leukopenia increased more than 3-fold above predose MPA levels of 3 to 4 mg/L. Plasma MPA levels correlated weakly (r2 = 0.081) with MMF dose and the dose / level relationship was variably influenced by age, the indication for MMF, concentrations of serum albumin and creatinine, and comedication with tacrolimus or cyclosporine. The median mycophenolate dose required per unit mycophenolate level was 50% lower in children than in adults. Comparable drug requirements were also decreased by renal dysfunction (by 40 and 43% in adults and children, respectively), and in patients prescribed MMF alone rather than with tacrolimus or cyclosporine. However, in patients with serum albumin less than 35 g/L, MMF dose requirements were higher than in those with normal albumin levels (by 2.1- and 2.6-fold in adults and children, respectively). In adults, 44.7% achieved clinically acceptable therapeutic MPA concentrations at a dose less than 1 g MMF twice daily and only 6.3% required 1.5 g twice daily as suggested by the manufacturer. The immunoassay was a rapid, reliable, and acceptably precise technique in which only 10.8% of measurements were unproductive. In conclusion, our data suggests that MPA predose level monitoring is both clinically- and cost-effective and that a therapeutic range of 1 to 3.5mg/L (by immunoassay) is applicable in liver allograft recipients given adjunctive MMF.

Mycophenolate Pharmacokinetics in Early Period Following Lung or Heart Transplantation

BACKGROUND

The available pharmacokinetic and pharmacodynamic data on mycophenolic acid (MPA), the pharmacologically active metabolite of mycophenolate mofetil (MMF), are derived largely from renal transplant patients, not thoracic transplant recipients.

OBJECTIVE

To evaluate, in a pilot study, the pharmacokinetics of MPA at 3 different times in the early period (up to the first 9 mo) following lung or heart transplantation.

METHODS

Nine patients were entered into this open-label study. Upon administration of a steady-state morning MMF dose, blood samples were collected at 0, 20, 40, 60, and 90 minutes and at 2, 4, 6, 8, 10, and 12 hours after the dose at 3 times (denoted as sampling periods 1, 2, and 3) in the early posttransplant period. Total MPA concentrations were measured by a validated HPLC method with ultraviolet detection and followed by ultrafiltration of pooled samples for unbound MPA concentrations. Pharmacokinetic parameters (maximal concentration [C(max)], dose-normalized C(max), time to C(max), minimum concentration, predose concentration, AUC, dose-normalized AUC, free fraction, free AUC) were calculated by traditional noncompartmental methods.

RESULTS

Patient characteristics included 7 men and 2 women, 5 lung and 4 heart transplant recipients, mean +/- SD age 53 +/- 11 years, and weight 77 +/- 14 kg. All patients were receiving prednisone and cyclosporine (with the exception of 2 pts. on tacrolimus during sampling periods 2 and 3). Sampling periods 1, 2, and 3 occurred on posttransplant days 15 +/- 13, 56 +/- 33, and 125 +/- 73, respectively. No significant differences were found between sampling periods in any pharmacokinetic parameter. Drug exposure as evaluated by AUC was 39.95 +/- 44.86, 25.24 +/- 25.68, and 43.96 +/- 38.67 micro g*h/mL during sampling periods 1, 2, and 3, respectively, (p > 0.05).

CONCLUSIONS

As of September 26, 2003, this is the first study to systematically evaluate MPA pharmacokinetics in thoracic transplant recipients at 3 different time points during the early posttransplant period. Wide interpatient variability in MPA pharmacokinetics was observed, thus emphasizing the need to individualize dosing of MMF and to further evaluate important pharmacokinetic/pharmacodynamic parameters and endpoints that impact on clinical outcomes. Further studies involving more patients and pharmacodynamic outcomes are underway to help identify optimal MMF strategies.

The impact of mycophenolate mofetil dosing patterns on clinical outcome after renal transplantation

BACKGROUND

Mycophenolate mofetil (MMF) has proven to be a very effective drug for the prevention of acute rejection following renal transplantation when dosed as prescribed at 2 or 3 g/d. However, circumstances arise in clinical transplantation where the dose must be lowered, either to avoid drug toxicity or because of concurrent infection. The impact on the incidence of acute rejection and graft survival when the MMF dose must be lowered has not previously been investigated.

METHODS

In this study, a cohort of 721 kidney transplant recipients who received immunosuppression using MMF in conjunction with cyclosporine and prednisone and OKT3 (n = 425) or Simulect (n = 296) induction were evaluated. Clinical outcomes were compared and contrasted between patients with and without MMF dose changes within the first year post-transplantation.

RESULTS

The majority of patients (70.3%, n = 507) had at least one dose change within the first post-transplant year. Compared with the 214 patients who did not have a dose change, these patients had a much higher incidence of acute rejection within the first post-transplant year (23.3% vs. 3.7%, p < 0.001). This resulted in a significantly decreased 3-yr death-censored graft survival (76.3% vs. 88.3%, p = 0.003). The incidence of acute rejection for patients who had a dose change was highest if the dose change occurred within the first post-transplant month (34.4%). The incidence of acute rejection for the dose change patients was influenced by recipient ethnicity (African-American vs. Caucasian) and the type of induction agent used (OKT3 vs. Simulect).

CONCLUSION

Altering the dose of MMF within the first post-transplant year correlated with a significantly worse clinical outcome in this cohort of renal transplant recipients. These data suggest that avoidance of MMF dose changes within the first year after renal transplantation would result in improved graft survival.

Mycophenolate mofetil for solid organ transplantation: does the evidence support the need for clinical pharmacokinetic monitoring?

The need for clinical pharmacokinetic monitoring (CPM) of the immunosuppressant mycophenolate mofetil (MMF) has been debated. Using a previously developed algorithm, the authors reviewed the evidence to support or refute the utility of CPM of MMF. First, MMF has proven efficacy for prevention of organ rejection in renal and cardiac transplant populations. In addition, the pharmacologically active form of MMF, mycophenolic acid (MPA), can be measured readily in plasma, and relationships between the incidence of rejection and MPA predose concentrations and MPA area under the curve (AUC) have been reported. A lower limit of the therapeutic range (MPA predose concentrations >1.55 microg/mL, as measured by enzyme multiplied immunoassay technique [EMIT], or MPA AUC >30 or 40 microg. h/mL, as measured by high-performance liquid chromatography [HPLC]) has been suggested to prevent rejection in renal allograft patients. Similarly, in cardiac transplant patients, decreased incidences of organ rejection have been reported in patients with MPA concentrations >2 or 3 microg/mL (using EMIT) and total AUC values >42.8 microg. h/mL (using HPLC). However, the relationship between pharmacokinetic parameters and adverse events in renal and cardiac transplant patients remains unclear. Due to the nature of antirejection therapy, the pharmacologic response of MMF is not readily assessable, and therapy is life-long. MPA pharmacokinetics exhibit large inter- and intrapatient variability and may be altered in specific patient populations due to changes in protein binding, concomitant disease states, or interactions with concurrent immunosuppressants. Therefore, on the basis of current evidence, CPM can provide more information regarding efficacy of MMF than clinical judgment alone in select patient populations. However, further randomized, prospective trials are required to clarify unresolved issues. Specifically, an upper limit of the therapeutic range, above which the risk of side effects is increased, needs to be elucidated for MMF therapy. Other future directions for research include determining a practical limited sampling strategy for MPA AUC; clarifying the relationship between free MPA concentrations, efficacy, and toxicity; and defining the pharmacodynamic relationship between activity of inosine monophosphate dehydrogenase (the enzyme inhibited by MPA) and risk of rejection or adverse effects.

Limited Sampling Strategy for the Estimation of Mycophenolic Acid Area under the Curve in Adult Renal Transplant Patients Treated with Concomitant Tacrolimus

Background: Significant relationships between the mycophenolic acid (MPA) area under the concentration–time curve (AUC0–12h) and the risks for acute rejection and side effects have been reported. We developed a practical method for estimation of MPA AUCs. Regression equations were developed using repeated cross-validation for randomly chosen subsets, characterized statistically, and verified for acceptable performance.

Methods: Twenty-one renal transplant patients receiving 0.5 or 1.0 g of mycophenolate mofetil twice daily and concomitant tacrolimus provided a total of 50 pharmacokinetic profiles. MPA concentrations were measured by a validated HPLC method in 12 plasma samples collected at predose and at 30 and 60 min; 2, 3, 4, 6, 8, 9, 10, 11, and 12 h; 1 and 2 weeks; and 3 months after transplantation. Twenty-six 1-, 2-, or 3-sample estimation models were fit (r2 = 0.341–0.862) to a randomly selected subset of the profiles using linear regression and were used to estimate AUC0–12h for the profiles not included in the regression fit, comparing those estimates with the corresponding AUC0–12h values, calculated with the linear trapezoidal rule, including all 12 timed MPA concentrations. The 3-sample models were constrained to include no samples past 2 h.

Results: The model using c0h, c0.5h, and c2h was superior to all other models tested (r2 = 0.862), minimizing prediction error for the AUC0–12h values not included in the fit (i.e., the cross-validation error). The regression equation for AUC estimation that gave the best performance for this model was: 7.75 + 6.49c0h + 0.76c0.5h + 2.43c2h. When we applied this model to the full data set, 41 of the 50 (82%) estimated AUC values were within 15% of the value of AUC0–12h calculated using all 12 concentrations.

Conclusions: This limited sampling strategy provides an effective approach for estimation of the full MPA AUC0–12h in renal transplant patients receiving concomitant tacrolimus therapy.

Therapeutic monitoring of mycophenolate mofetil in organ transplant recipients: is it necessary?

Adequate immunosuppression minimising the risk of organ rejection with acceptable tolerability of the used drugs is a crucial step in organ transplantation. The primary goal is to maintain a consistent time-dependent target concentration by tailoring individual dosage leading to the best efficacy and tolerability combination. The use of therapeutic drug monitoring (TDM) to optimise immunosuppressive therapy is routinely employed for maintenance drugs such as cyclosporin and tacrolimus. The question whether therapeutic monitoring of mycophenolic acid (MPA) in organ transplant recipients treated with mycophenolate mofetil is necessary is not definitely answered. The correlation of mycophenolic acid pharmacokinetic parameters with efficacy and toxicity makes the therapeutic monitoring of this drug promising. However, further studies are mandatory to draw the best guidelines in order to achieve higher levels of evidence that MPA-TDM may improve patient outcome.

Atypical pharmacokinetics and metabolism of mycophenolic acid in a young kidney transplant recipient with impaired renal function

A juvenile, female renal transplant recipient suffered two acute rejection episodes: the first on posttransplant day 31 while taking cyclosporine, prednisone, and mycophenolate mofetil (MMF); and the second on posttransplant day 67, when she was taking tacrolimus, prednisone, and MMF. Dosage of MMF was initially started at 2 g/d (corresponding to 600 mg MMF/m(2) twice daily.), but was reduced to 250 mg/d to 500 mg/d after severe diarrhea and a paralytic ileus on posttransplant day 16. During therapy with tacrolimus, prednisone, and MMF, predose plasma mycophenolic acid (MPA) concentrations varied from 1.1 mg/L to 8.2 mg/L (median 3.0 mg/L). On posttransplant day 91, a 12-hour pharmacokinetic profile was obtained. The concentrations of MPA and its metabolites were determined with a validated high-performance liquid chromatography (HPLC) procedure. After oral MMF (250 mg) administration, the MPA concentration showed an atypical decline from a predose concentration of 6.0 mg/L to a value of 3.8 mg/L at 75 minutes postdose, and 3.4 mg/L at 6 hours postdose, before returning to 6.0 mg/L after 12 hours. The 12-hour area under the concentration-time curve (AUC) values for MPA and its major metabolite the phenolic glucuronide MPAG were 55.1 mg.h/L and 800 mg.h/L, respectively. An unusually high concentration (12-h AUC, 165 mg.h/L) of the phenolic glucose conjugate of MPA was found. The apparent renal clearance of MPAG was only 2.2 mL/min. Her creatinine clearance was 30 mL/min. MPAG clearances have been reported to range from approximately. 5.5 mL/min to 35 mL/min at a creatinine clearance of approximately 30 mL/min in renal transplant recipients. The authors' findings suggest that conjugation and clearance of MPA through the kidney is strongly impaired in this patient. The relatively high predose MPA concentrations could result from an enhanced enterohepatic circulation of MPA and its metabolites.

Mycophenolic acid concentrations in long-term heart transplant patients: relationship with calcineurin antagonists and acute rejection

BACKGROUND

When used in conjunction with steroids and cyclosporin, mycophenolate mofetil (MMF) has been shown to significantly reduce mortality and incidence of rejection in the first year after heart transplantation. It also appears that in this early post-transplantation period, the monitoring of immunosuppressive therapies may be warranted. The current study was undertaken to determine if such monitoring is still useful more than 1 yr after heart transplantation.

METHODS

Twenty-six patients who had survived the first year after orthotopic heart transplantation and had been on MMF therapy for more than 3 months were prospectively followed. At the time of their routine endomyocardial biopsy blood samples were taken to monitor immunosuppressive therapy. Most patients had two samples taken, on average 109 d apart.

RESULTS

There were 22 episodes of asymptomatic rejection documented on a total of 48 biopsies. Of these, only two were of ISHLT (International Society for Heart and Lung Transplantation) grade 3A the remainder being of ISHLT grades 1 or 2. There was no relation between immunosuppressive regimen (tacrolimus and MMF or cyclosporin and MMF) and rejection. There was no relation between monitored immunosuppressive levels and rejection. Patients with the combination of MMF and tacrolimus had significantly higher plasma mycophenolic acid levels despite significantly lower daily MMF dose.

CONCLUSION

There does not appear to be a benefit in continued monitoring of plasma mycophenolic acid levels beyond the first year of heart transplantation. There were significant differences in plasma mycophenolic acid levels depending on the type of calcineurin inhibitor concomitantly used.

Therapeutic drug monitoring of immunosuppressant drugs in clinical practice

BACKGROUND

Therapeutic drug monitoring (TDM) is essential to maintain the efficacy of many immunosuppressant drugs while minimizing their toxicity. TDM has become more refined with the development of new monitoring techniques and more specific assays.

OBJECTIVE

This article summarizes current data on TDM of the following immunosuppressant drugs used in organ transplantation: cyclosporine, tacrolimus, sirolimus, everolimus, and mycophenolate mofetil.

METHODS

Published data were identified by a MEDLINE search of the English-language literature through March 2001 using the terms therapeutic drug monitoring, cyclosporine, tacrolimus, sirolimus, everolimus, and mycophenolate mofetil. Relevant conference abstracts were also included.

RESULTS

TDM of cyclosporine has been well studied, and recent findings indicate that monitoring of drug levels 2 hours after dosing is a more sensitive predictor of outcome than trough (C0) monitoring. C0 levels are being used more widely in TDM of tacrolimus; however, the relationship between C0 and area under the curve has varied widely in clinical trials, with correlations ranging from 0.11 to 0.92. The use of TDM of sirolimus, everolimus, and mycophenolate mofetil is evolving rapidly.

CONCLUSIONS

TDM of immunosuppressant drugs that have a narrow therapeutic index is an increasingly useful tool for minimizing drug toxicity while maximizing prevention of graft loss and organ rejection.

International Federation of Clinical Chemistry/International Association of Therapeutic Drug Monitoring and Clinical Toxicology working group on immunosuppressive drug monitoring

Issues surrounding the measurement and interpretation of immunosuppressive drug concentrations have been summarized in a number of consensus documents. The Scientific Division of the International Federation of Clinical Chemistry has formed a working group in collaboration with the International Association of Therapeutic Drug Monitoring and Clinical Toxicology. This paper sets out the goals of the working group in light of the developments that have occurred in the field of immunosuppressive drug monitoring since the publication of the last consensus documents.