Overestimation of mycophenolic acid by EMIT correlates with MPA metabolite

Comparison of a Point-of-Care Testing with Enzyme-Multiplied Immunoassay Technique and Liquid Chromatography Combined With Tandem Mass Spectrometry Methods for Therapeutic Drug Monitoring of Mycophenolic Acid: A Preliminary Study

BACKGROUND

For mycophenolic acid (MPA), therapeutic drug monitoring is an essential tool for dosage optimization in transplant recipients and autoimmune diseases. In China, a new commercial kit using an immunochromatographic assay (FICA) with a point-of-care testing system was approved for therapeutic drug monitoring of MPA. However, corroboration between FICA and clinically used assays remains unknown. The authors evaluated MPA concentrations in heart transplant recipients obtained by FICA, high-performance liquid chromatography combined with tandem mass spectrometry (LC-MS/MS), and enzyme-multiplied immunoassay technique (EMIT).

METHODS

Nine heart transplant recipients administered a single mycophenolate mofetil (MMF) dose, and 4 administered multiple MMF doses were enrolled. MPA samples were collected before administration, and after 0.5, 1, 1.5, 2, 4, 6, 8, 10, and 12 hours, and assessed by 2 immunoassays (EMIT and FICA) and LC-MS/MS. Consistency between methods was evaluated using Passing-Bablok regression and Bland-Altman analysis.

RESULTS

For Passing-Bablok regression between FICA and LC-MS/MS, FICA = 0.784 LC-MS/MS + 0.360 (95% CI slope: 0.739 to 0.829, 95% CI intercept: 0.174-0.545). Regardless of a significant observed correlation coefficient (R2 = 0.9126), statistical analyses revealed a significant difference between FICA and the reference LC-MS/MS method. The mean absolute bias was 0.69 mcg/mL between FICA and LC-MS/MS. Bland-Altman plots showed a mean bias of -0.23 mcg/mL (±1.96 SD, -2.19 to 1.72 mcg/mL) and average relative bias of 14.73% (±1.96 SD, -67.91% to 97.37%) between FICA and LC-MS/MS. Unsatisfactory consistency was observed between EMIT and LC-MS/MS, and FICA and EMIT. Differences between pharmacokinetic parameters after a single or 7 days of MMF administration, by LC-MS/MS and FICA, were not statistically significant.

CONCLUSIONS

The consistency of the new FICA using a point-of-care testing device with LC-MS/MS and EMIT was inadequate, and the accuracy of EMIT and LC-MS/MS was inappropriate. Clinicians should be informed when switching MPA detection methods to avoid misleading results.

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.

Development of a Formula to Correct Particle-Enhanced Turbidimetric Inhibition Immunoassay Values so That it More Precisely Reflects High-Performance Liquid Chromatography Values for Mycophenolic Acid

Background

Mycophenolic acid (MPA) concentration measured by homogeneous particle-enhanced turbidimetric inhibition immunoassay (PETINA) may be overestimated due to its cross-reactivity with pharmacologically inactive MPA glucuronide (MPAG), as well as other minor metabolites, accumulated with renal function impairment or co-administered cyclosporine A. In contrast, high-performance liquid chromatography (HPLC) is precise because it can exclude the cross-reactivity. In this study, we assumed HPLC values for MPA (HPLC-MPA) as a reference and aimed to develop a formula correcting PETINA values for MPA (PETINA-MPA) to more precisely reflect HPLC-MPA.

Methods

MPA trough concentrations were measured both by HPLC-UV and PETINA in 39 samples issued from 39 solid-organ transplant recipients. MPAG concentrations were also measured using HPLC UV assay. We determined the impacts of renal function and coadministered calcineurin inhibitor on concentrations of MPA and MPAG measured by HPLC. Then, we evaluated the difference between PETINA-MPA and HPLC-MPA. Finally, we develop a formula to reflect HPLC-MPA by using multilinear regression analysis.

Results

MPAG concentration was negatively correlated with estimated glomerular filtration rate (eGFR) (R² = 0.376, P < 0.001), although MPA was not correlated with eGFR. There were no significant differences in MPA or MPAG concentrations per dose between the patients who were co-administered tacrolimus versus cyclosporine A. Finally, we developed the formulas to reflect HPLC-MPA:Formula 1: Estimated MPA concentration = 0.048 + 0.798 × PETINA-MPAFormula 2: Estimated MPA concentration = - 0.059 + 0.800 × PETINA-MPA + 0.002 × eGFRHowever, there was no significant improvement in the coefficient of determination with addition of eGFR in the formula, suggesting that HPLC-MPA can be well predicted by only 1 variable, PETINA-MPA.

Conclusions

This study developed a formula so that PETINA-MPA can be corrected to more precisely reflect HPLC-MPA.

The Roche Total Mycophenolic Acid® assay: An application protocol for the ABX Pentra 400 analyzer and comparison with LC-MS in children with idiopathic nephrotic syndrome

Background

For TDM of mycophenolate acid (MPA), the Roche Total Mycophenolic Acid® assay based on the inhibition of recombinant inosine monophosphate dehydrogenase (IMPDH) has been shown to be a simple and reliable alternative to chromatographic methods. We have adapted this assay on the ABX Pentra 400 analyzer (HORIBA).

Objective

To investigate the analytical performances of the Roche Total Mycophenolic Acid® assay on the ABX Pentra 400 and to compare it to an LC-MS method using samples from children with nephrotic syndrome treated with mycophenolate mofetil (MMF).

Material and methods

Configuration of the open-channel on the ABX Pentra 400 was based on the Roche MPA assay package insert. Precision was determined as described in the CLSI protocol EP5-A2. Comparison with the LC-MS method was performed using 356 plasma samples from 42 children with nephrotic syndrome (8 h pharmacokinetic profiles).

Results

The enzymatic assay demonstrated high precision. The %CV for Within Run Imprecision ranged from 5.5% at 1.2 mg/L to 1.5% at 14.1 mg/L and Total Imprecision ranged from 9.3% to 2.5%. The method comparison with plasma samples from children yielded overall a good correlation and a good agreement between both methods. The Passing Bablok regression analysis showed the following results: [Roche MPA assay]=1.058 [MPA LC-MS] −0.06; rho=0.996.

Conclusion

The Roche Total Mycophenolic Acid® assay is adaptable to the ABX Pentra 400 analyzer, and demonstrates accurate and precise measurement of MPA in plasma obtained from children with nephrotic syndrome.

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Adaptation of the Roche Total Mycophenolic Acid® assay to the Pentra 400 analyzer.

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Comparison with LC-MS in children with idiopathic nephrotic syndrome.

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Therapeutic drug monitoring of mycophenolate mofetil.

A Comparison of the Immunochemical Methods, PETINIA and EMIT, With That of HPLC-UV for the Routine Monitoring of Mycophenolic Acid in Heart Transplant Patients

BACKGROUND

The aim of this study was to evaluate particle enhanced turbidimetric inhibition immunoassay (PETINIA) recently developed for mycophenolic acid (MPA) determination in plasma and to compare it with a reference high-performance liquid chromatography (HPLC) method, using samples from heart transplant recipients. The results are presented in the context of PETINIA being compared with enzyme multiplied immunoassay technique (EMIT).

METHODS

PETINIA evaluation was performed using 194 routine trough plasma samples at steady state. EMIT was evaluated using 677 samples from 61 steady-state 12-hour profiles obtained from 35 heart transplant patients. Evaluation was undertaken on a Dimension EXL 200 analyzer (PETINIA) and on a Viva-E analyzer (EMIT).

RESULTS

The mean MPA concentration measured by PETINIA was significantly higher than that measured by high-performance liquid chromatography combined with UV detector (2.36 ± 1.30 mcg/mL versus 1.82 ± 1.23 mcg/mL, respectively, P < 0.0001). Bland-Altman analysis revealed a mean bias of 0.54 mcg/mL [95% confidence interval (CI), 0.49-0.59] comprising 33.48% (95% CI, 30.34-36.61). Passing-Bablok regression was: y = 1.100x + 0.38 (95% CI for slope: 1.044-1.154 and for intercept: 0.30-0.47). Regardless of a significant observed correlation (r = 0.9230, P < 0.0001), the statistical analyses showed a significant difference between PETINIA and the reference chromatographic method. The mean MPA concentration measured by EMIT was significantly higher than that measured by HPLC (7.48 ± 8.34 mcg/mL versus 5.57 ± 6.61 mcg/mL, respectively, P < 0.0001) with a mean bias of 1.91 mcg/mL (95% CI, 1.75-2.07) comprising 35.91% (95% CI, 34.37-37.45). The significant difference between EMIT and HPLC was confirmed by Passing-Bablok regression: y = 1.300x + 0.24 (95% CI for slope: 1.279-1.324 and for intercept: 0.18-0.29). The analysis of the determinations, grouped by sampling time, revealed positive bias between EMIT and HPLC ranging from 24.54% to 42.77% and inversely proportional to MPA concentrations with r = 0.9122 (P < 0.001).

CONCLUSIONS

The new immunochemical PETINIA method was associated with significantly higher MPA concentrations in routine therapeutic drug monitoring samples from heart transplant patients. The magnitude of the MPA overestimation was similar to that observed by use of the EMIT method.

Clinical mycophenolic acid monitoring in liver transplant recipients

In liver transplantation, the efficacy of mycophenolate mofetil (MMF) has been confirmed in clinical trials and studies. However, therapeutic drug monitoring for mycophenolic acid (MPA) has not been fully accepted in liver transplantation as no long-term prospective study of concentration controlled vs fixed-dose prescribing of MMF has been done. This review addressed MPA measurement, pharmacokinetic variability and reasons of this variation, exposure related to acute rejection and MMF-associated side effects in liver transplant recipients. Limited sampling strategies to predict MPA area under the concentration-time curve have also been described, and the value of clinical use needs to be investigated in future. The published data suggested that a fixed-dosage MMF regimen might not be suitable and monitoring of MPA exposure seems helpful in various clinical settings of liver transplantation.

How delayed graft function impacts exposure to mycophenolic acid in patients after renal transplantation

INTRODUCTION

Mycophenolic acid (MPA) plasma concentrations are highly variable on standard-dose mycophenolate mofetil therapy. At creatinine clearances below 25 mL/min, MPA clearance increases as a result of a higher nonprotein-bound fraction. Patients with delayed graft function (DGF) after renal transplantation are exposed to low total MPA concentrations, when risk of rejection is highest. This study investigated the influence of DGF on MPA exposure and on clinical outcome.

METHODS

Adult renal transplantation patients treated with mycophenolate mofetil, corticosteroids, and either microemulsified cyclosporine (n = 459) or tacrolimus (n = 371) participated in a randomized controlled trial (the Fixed-Dose Concentration-Controlled [FDCC] Study). Abbreviated MPA areas under the curve (AUCs) were obtained on Day 3, Day 10, Week 4, and Month 3, to calculate MPA AUC₀₋₁₂. Free MPA AUC values were available for a subgroup of patients (n = 269).

RESULTS

The overall incidence of DGF was 187 of 830 (23%) and did not differ between cyclosporine-treated (24%) and tacrolimus- (21%) treated patients. The incidence of biopsy-proven acute rejection at 12 months was significantly higher in patients with DGF (13.8% versus 21.4%). Patients with DGF had significantly lower dose-corrected MPA AUC on Day 3 and Day 10. Free MPA fraction and dose-corrected free MPA AUC were significantly higher in patients with DGF, from Day 3 until Month 3. The total number of patients with at least one opportunistic infection was significantly higher in patients with DGF (33.2%) compared with patients without DGF (25.8%) (P = 0.048). Patients with DGF developing opportunistic infections did not have higher total MPA AUC nor higher free MPA AUC compared with those without opportunistic infections.

CONCLUSION

Patients with DGF have significantly lower dose-corrected MPA AUC in the first month after renal transplantation, presumably as a result of enhanced MPA clearance on account of the elevated MPA free fraction. Because patients with DGF have a higher rate of acute rejection and lower MPA exposure, higher dosing of mycophenolate mofetil in such patients may improve outcome. However, the already increased incidence of opportunistic infections in patients with DGF is a concern.

Establishment of high-performance liquid chromatography and enzyme multiplied immunoassay technology methods for determination of free mycophenolic acid and its application in Chinese liver transplant recipients

The objective of this study is to investigate the correlation between methods of high-performance liquid chromatography (HPLC) and enzyme multiplied immunoassay technology (EMIT) for determination of total mycophenolic acid (tMPA) and free (fMPA) concentration and to study pharmacokinetics of fMPA in Chinese liver transplant recipients. An HPLC method with fluorometric detection and an EMIT assay were established to determine fMPA in plasma ultrafiltrates. Pharmacokinetic parameters of tMPA and fMPA in 51 patients were estimated. The calibration range of fMPA was 0.0025 to 1.0 μg/mL for the HPLC method and 0.0050 to 0.50 μg/mL for the EMIT method. Mean recovery of the two methods was 98.0% and 97.1%, respectively. The intraday and interday coefficient of variations were 0.93% to 3.1% and 1.6% to 2.9% for HPLC and 4.51% to 15.8% and 5.83% to 19.5% for EMIT, respectively. The relationship of the two methods was EMIT = 1.074 × HPLC + 0.582 (r2 = 0.918, n = 470, P < 0.05) for tMPA and EMIT = 1.068 × HPLC + 0.004 (r2 = 0.945, n = 297, P < 0.05) for fMPA. There was a positive mean bias of EMIT for tMPA (27.0%) and fMPA (23.3%). The AUC0-12 of tMPA and fMPA obtained by HPLC in 51 patients was 34.7 ± 11.1 and 0.72 ± 0.38 μg·h/mL, respectively. The free fraction of MPA was 1.60 ± 1.21% (Median:1.36%, interquartile: 0.72, 2.22), [corrected] which was significantly correlated with 7-O-glucuronide conjugate of MPA AUC0-12 (r2 = 0.705, P < 0.001), albumin (r2 = -0.529, P < 0.001), and the clearance of creatinine (r2 = -0.417, r2 = 0.005). Both HPLC and EMIT assay are suitable for the determination of fMPA. A considerable interindividual variability exists in pharmacokinetics of fMPA among Chinese liver transplant recipients. 7-O-Glucuronide conjugate of MPA and albumin concentrations are two factors correlated to fMPA variance.

Investigation of the crossreactivity of mycophenolic acid glucuronide metabolites and of mycophenolate mofetil in the Cedia MPA assay

The immunosuppressant mycophenolic acid (MPA) used for solid organ transplantation is predominantly metabolized to a pharmacologically inactive phenolic glucuronide (MPAG) and, to a lesser extent, to the pharmacologically active acyl glucuronide (AcMPAG). The recently introduced CEDIA Mycophenolic Acid Assay from Microgenics has been reported to overestimate MPA in clinical samples and crossreactivity with AcMPAG has been suspected. A detailed investigation of the crossreactivity of AcMPAG and the prodrug mycophenolate mofetil (MMF) in the CEDIA assay is presented using pure substances. In addition, MPA concentrations in plasma were compared with a validated high-performance liquid chromatography-ultraviolet method. Plasma samples from kidney (KTx, n = 50), heart (HTx, n = 50), and liver (LTx, n = 50) transplant recipients were analyzed by the CEDIA (MPA) and a high-performance liquid chromatography-ultraviolet method (MMF, MPA, MPAG, AcMPAG). Crossreactivity of MMF (0.93-46.3 mg/L), MPAG (50-1000 mg/L), and AcMPAG (0.5-10 mg/L) was investigated using spiked drug-free plasma. Method comparison was performed using Bland & Altman and Passing & Bablok analysis. The method bias was correlated to AcMPAG concentrations using Spearman's rank correlation. Crossreactivity with AcMPAG and MMF was concentration-dependent and reached 215% and 143%, respectively. There was no crossreactivity with MPAG. The CEDIA assay showed a mean positive bias of 36.3% in patient samples. The mean bias was lowest with HTx samples (15%), 41.7% with KTx samples, and highest with LTx samples (52.3%). There was a positive correlation between the method bias and AcMPAG concentrations (r = 0.829; P < 0.001). No MMF was detected in patient samples. The CEDIA overestimates MPA concentrations on average by 36%. This bias is mainly the result of AcMPAG as previously observed with the EMIT MPA assay. It should be considered that the putative therapeutic range for MPA with the CEDIA assay will be higher than the range using high-performance liquid chromatography.

Comparison of high-performance liquid chromatography and enzyme-multiplied immunoassay technique to monitor mycophenolic acid in paediatric renal recipients

Therapeutic drug monitoring (TDM) of mycophenolate mofetil (MMF) is recommended to guide immunosuppression. High-performance liquid chromatography with ultraviolet (HPLC-UV) or the enzyme-multiplied immunoassay technique (EMIT), used to measure mycophenolic acid (MPA) were compared in an exclusive paediatric renal transplant population. Twenty patients were included as part of the pharmacokinetics study of MMF, and 88 additional samples were drawn for TDM. Agreement between HPLC-UV and EMIT was assessed by the Bland-Altman method. With the two methods, pre-dose concentrations were not normally distributed. After logarithmic transformation, their mean was 0.79 +/- 1.16 microg ml(-1) and their mean difference was 0.34 +/- 0.16 microg ml(-1) [95% confidence interval (95%CI 0.30-0.38 microg ml(-1), with antilogarithmic values of these limits of 1.34-1.46 microg ml(-1)). Area under the curve (AUC)(HPLC) and AUC(EMIT) were normally distributed. Their mean was 52.42 +/- 25.91 mg x h/l and their mean difference was 15.22 +/- 8 mg x h/l (95%CI 11.99-18.45 mg x h/l), the Bland-Altman plot showing a bias proportional to the mean. Our data showed the absence of agreement between the HPLC and EMIT methods, with an average positive bias of 15% with the EMIT. Further studies are required to determine which method is best appropriate for TDM of MMF in children.

Monitoring mycophenolic acid pharmacokinetic parameters in liver transplant recipients: prediction of occurrence of leukopenia

Mycophenolate mofetil (MMF) is a very powerful immunosuppressive drug used in preventing acute rejection in liver transplantation. However, MMF has some serious side effects, including hematologic and gastrointestinal disorders. This study was designed to investigate the relationship between the clinical events and the pharmacokinetics of mycophenolic acid (MPA) in Chinese liver transplant recipients. Sixty-three adult liver transplant recipients receiving 1.0 g of MMF twice daily in combination with tacrolimus were prospectively included. The MPA pharmacokinetic profiles (blood sampling time points: before the dose and 0.5, 1, 1.5, 2, 4, 6, 8, 10, and 12 hours after the dose) were monitored after transplantation. Every clinical event, including acute and MMF-related side effects, was monitored in all patients within 3 months. Two patients (3.2%) had an episode of acute rejection. Forty-two patients (66.7%) had 52 episodes of MMF-related side effects, including leukopenia, diarrhea, and infection. The 0-hour concentration (C(0h)), maximum (peak) concentration (C(max)), and area under the curve from 0 to 12 hours (AUC(0-12h)) in patients with side effects were significantly higher than those in patients without side effects (P < 0.05). The thresholds of side effects from receiver operating characteristic analysis were 2 mg/L (sensitivity, 52.4%; specificity, 90.5%) for C(0h), 10 mg/L (sensitivity, 45.2%; specificity, 85.7%) for C(max), and 40 mg h/L (sensitivity, 71.4%; specificity, 61.9%) for AUC(0-12h) (P < 0.05). Leukopenia was discriminated effectively in C(0h) and in C(max) (P < 0.05). These results demonstrate the close relationship between leukopenia and MPA pharmacokinetic parameters in the early period after liver transplantation. C(0h) and AUC(0-12h) of MPA could predict the subsequent occurrence of leukopenia. These values may be used in routine monitoring for MMF therapy.

Sensitive high-performance liquid chromatography–tandem mass spectrometry method for quantitative analysis of mycophenolic acid and its glucuronide metabolites in human plasma and urine

A method to determine total and free mycophenolic acid (MPA) and its metabolites, the phenolic (MPAG) and acyl (AcMPAG) glucuronides, using HPLC and mass spectrometry was developed. Mean recoveries in plasma and urine samples were >85%, and the lower limits of quantification for MPA, MPAG and AcMPAG were 0.05, 0.05 and 0.01 mg/L, respectively. For plasma, the assay was linear over 0.05-50 mg/L for MPA and MPAG, and from 0.01 to 10mg/L for AcMPAG. A validation study demonstrated good inter- and intra-day precision (CV<or=11%) and accuracy (bias<or=16%) and satisfactory specificity and stability. Pharmacokinetic parameters were assessed in plasma and urine from healthy volunteers after an oral dose of mycophenolate mofetil.

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.

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.

Mycophenolate mofetil in organ transplantation: focus on metabolism, safety and tolerability

Mycophenolate mofetil (MMF) received its first approval for the prevention of renal allograft rejection in 1995 and has now become the most frequently used antiproliferative agent in maintenance immunosuppressive therapy for kidney, pancreas, liver and heart transplantation. In addition, its use for the treatment of autoimmune diseases steadily increases. This review focuses on the miscellaneous pharmacodynamic properties of the drug, its pharmacokinetics in healthy subjects, recipients of different organ transplants and combination therapy with other pharmaceuticals, as well as its safety profile. The immunosuppressive activity of MMF is thought to derive mainly from the potent and selective inhibition of purine synthesis in both T and B lymphocytes. In contrast to other immunosuppressants on the market, it is metabolised primarily by glucuronidation and lacks nephrotoxicity, cardiovascular toxicity or diabetogenic potential, thus making it a suitable candidate for combination regimens. The most important side effects under MMF include gastrointestinal disorders, of which the underlying mechanisms are not yet fully understood, but seem to be complex and related to both effects of mycophenolic acid and its acyl glucuronide, as well as to decreased -immunity due to general immunosuppression after transplantation.

Fully Automated Analytical Method for Mycophenolic Acid Quantification in Human Plasma Using On-line Solid Phase Extraction and High Performance Liquid Chromatography With Diode Array Detection

We have developed a new fully automated method for mycophenolic (MPA) acid quantification in plasma to optimize therapeutic drug monitoring of tranplant patients. This method involved solid-phase extraction on disposable extraction cartridges and reversed-phase high-performance liquid chromatography with diode array detection. Solid-phase extraction was performed automatically by an automated sample with extraction catridges system. After washing, MPA was eluted from the cartridge onto a Chromolith RP-18e column. MPA and the internal standard were detected at 306 nm. The retention time of MPA was 6.3 minutes. The developed method was linear from 0.2 to 20 microg/mL. The limit of quantification was 0.2 microg/mL. The method showed a good precision with intraday and interday variation coefficient less than 6%. The intraday accuracy ranged from 97.6% to 100.4% and the interday accuracy varied from 97.1% to 100.8%. The extraction efficiency was greater than 90%. This method is simple and shows a good specificity with respect to commonly co-prescripted drugs.

Mycophenolic acid pharmacokinetics and related outcomes early after renal transplant

AIMS

The pharmacokinetics of mycophenolic acid and its glucuronide are complex. This study investigated the pharmacokinetics, pharmacodynamics and protein binding of mycophenolic acid and its glucuronide metabolite, early post-transplant in renal allograft recipients.

METHODS

Forty-two de novo renal transplant recipients receiving mycophenolate mofetil and concomitant cyclosporin (n = 32) or tacrolimus (n = 10) participated in the study. Blood samples were taken on day 5 post-transplant for measurement of free and total concentrations of mycophenolic acid, mycophenolic acid glucuronide and relevant biochemistry. Associations between free fraction and biochemistry were investigated. Free and total 6-h area under the concentration-time curve (AUC0-6) of mycophenolic acid was assessed relative to clinical outcomes in the first month post-transplant.

RESULTS

Kinetic variability of free and total mycophenolic acid and its glucuronide was greater in patients on cyclosporin (12- to 18-fold variation) than on tacrolimus (four- to fivefold) cotherapy. Cyclosporin-treated patients also had significantly lower predose total mycophenolic acid concentrations than tacrolimus-treated patients (median 0.8 mg l(-1) and 1.6 mg l(-1), respectively, P = 0.002). Mycophenolic acid glucuronide predose concentration correlated positively with mycophenolic acid glucuronide AUC0-6 (r > 0.95). Mycophenolic acid free fraction varied 11-fold, from 1.6% to 18.3%, whilst the glucuronide free fraction varied threefold, from 17.4% to 54.1%. Urea and creatinine concentrations correlated positively (r > 0.46), whilst albumin correlated negatively (r = -0.54) with free fraction of mycophenolic acid. Similar relationships were found for the free fraction of mycophenolic acid glucuronide. Mycophenolic acid free fraction was on average 70% higher in patients with albumin concentrations below a specified albumin cut-off concentration of 31 g l(-1)[free fraction = 7 +/- 4% for lower albumin and 4 +/- 3% for higher albumin, respectively; P = 0.001; 95% confidence interval (CI) for the difference 1.9, 4.2]. Neither free nor total mycophenolic acid AUC0-6 was related to rejection (P > 0.07). Free AUC0-6 was significantly higher in those patients with thrombocytopenic, leukopenic and/or infectious outcomes than in those without (mean +/- SD 1.9 +/- 0.3 mg h(-1) l(-1) and 1.1 +/- 0.1 mg h(-1) l(-1), P = 0.0043; 95% CI for the difference 0.3, 1.4).

CONCLUSIONS

The marked variability in mycophenolic acid/glucuronide pharmacokinetics occurring early post-transplant during the current study was greater in cyclosporin (12-18-fold) than in tacrolimus (four- to fivefold) treated patients. Concomitant cyclosporin was associated with total mycophenolic acid concentrations approximately half that of tacrolimus. Patients with marked renal impairment had the highest free fractions reported to date. The exposure to unbound mycophenolic acid was significantly related to infections and haematological toxicity.

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.

Immunosuppressant Drug Monitoring: Is the Laboratory Meeting Clinical Expectations?

OBJECTIVE:

To review the literature relating to immunosuppressant drug measurement as performed in therapeutic drug monitoring laboratories associated with transplantation centers and consider whether the assay methods widely used for patient dosage management achieve acceptable quality criteria in the context of other sources of variability with these drugs.

DATA SOURCES:

Articles used were accessed primarily through MEDLINE, as well as references cited in related publications. Searches were restricted to organ transplantation in humans.

STUDY SELECTION AND DATA EXTRACTION:

Emphasis was placed on the literature relating to the quality of immunosuppressant drug assays, their limitations, and evidence of clinical benefit in dosage individualization.

DATA SYNTHESIS:

There is a dilemma evident between the quality of the analytical services offered by some diagnostic immunoassay manufacturers and the ability of a significant number of clinical laboratories globally to select only appropriate assay methods.

CONCLUSIONS:

In many cases, clinical laboratories fail to meet the reasonable clinical expectations required for interpretation of immunosuppressant drug assay results as an adjunct to optimal dosage individualization and patient care.

Twelve-month evaluation of the clinical pharmacokinetics of total and free mycophenolic acid and its glucuronide metabolites in renal allograft recipients on low dose tacrolimus in combination with mycophenolate mofetil

BACKGROUND

The establishment of a rationale for therapeutic drug monitoring for mycophenolic acid (MPA) and outlining a therapeutic window remains a challenging task in renal transplantation. Furthermore, the pharmacokinetic characteristics of free and total MPA and its glucuronides depend directly or indirectly on graft function and the type of co-administered calcineurin-inhibitor.

METHODS

The authors conducted a prospective 12-month multicenter pharmacokinetic study on MPA (MPA, free MPA, free fraction MPA) and its metabolites (MPAG, Acyl-MPAG). The aim of this study was to examine the long-term pharmacokinetic characteristics of MMF when combined with tacrolimus in renal allograft recipients and to identify a possible relationship between these pharmacokinetic parameters and clinical outcome parameters.

RESULTS

They have demonstrated that in renal transplant recipients MPA, free MPA, Acyl-MPAG and MPAG have a particular pharmacokinetic profile when combined with tacrolimus which differs from the combination with CsA. They could not establish a relationship between pre-dose trough concentration of MPA and its metabolites and clinical efficacy endpoints and drug-related adverse events, except for anemia.

CONCLUSIONS

These findings suggest that trough plasma concentration monitoring of MPA and its metabolites might not provide a useful clinical tool for guiding MMF dose adjustments to avoid drug-related toxicity. More extensive pharmacokinetic measurements like area under the concentration curves might be necessary for routine therapeutic drug monitoring of MMF.

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.

Simple determination of mycophenolic acid in human serum by column-switching high-performance liquid chromatography

A column-switching high-performance liquid chromatographic analysis was established to monitor the serum concentration of mycophenolic acid, the active metabolite from mycophenolate mofetil administered for the prophylaxis of acute organ rejection in renal transplantation. The system consisted of two pumps for solvent delivery, a column-switching valve, a precolumn, and a reversed-phase analytical column. The present method enabled us to determine MPA by injecting serum samples directly into HPLC without any pretreatment. The mobile phases with different amounts of organic solvent were delivered to the precolumn and analytical column by separate lines, and samples were applied to the precolumn. The column switching valves were switched automatically following the processes for the elimination of protein and the drug analysis. The peak heights of MPA were linearly related to the concentrations (r=0.999) in the range of 0.1-20 micro g/ml, and the limit of quantification was 0.1 micro g/ml (S/N ratio=3). This method was accurate and reproducible on the basis of the results of recovery (94.0-98.0%) and small coefficient of variations of intra and inter-assay (less than 8.3%).

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.

Comparison of the Emit Immunoassay with HPLC for Therapeutic Drug Monitoring of Mycophenolic Acid in Pediatric Renal-Transplant Recipients on Mycophenolate Mofetil Therapy

Background: HPLC is currently the preferred method for accurate measurement of mycophenolic acid (MPA). This study was designed to validate the Emit compared with HPLC in relation to clinical outcome measurements.

Methods: Pediatric renal-transplant recipients (n = 50) on an immunosuppressive triple regimen consisting of cyclosporin A, prednisone, and mycophenolate mofetil (600 mg/m2 twice per day) were investigated in an open-label prospective study. Pharmacokinetic profiles over 12 h were obtained at 1 week, 3 weeks, 3 months, and 6 months posttransplant. Plasma MPA was measured by both reversed-phase HPLC and the Emit immunoassay.

Results: There was an association between the risk of acute rejection episodes and low area under the curve values from t0 to t12h (AUC0–12) for MPA (MPA-AUC0–12) or predose concentrations of MPA derived from both HPLC and Emit measurements. According to ROC analysis, an AUC value of 33.8 mg · h/L for MPA from t0 to t12h (MPA-AUC0–12) determined by HPLC had a diagnostic sensitivity of 80% and a diagnostic specificity of 57%. The corresponding value of the Emit was 36.1 mg · h/L. For the predose concentration (MPA-c12), a concentration of 1.2 mg/L determined by HPLC and 1.4 mg/L determined by Emit gave a sensitivity of 80% and a specificity of 60%, respectively. There was no association of any pharmacokinetic variables derived from total MPA measurements with an increased risk of side effects related to mycophenolate mofetil.

Conclusions: The Emit assay appears to have a comparable diagnostic efficacy to HPLC for assessing the risk of acute rejection in pediatric renal-transplant recipients. However, because of the cross-reactivity of the antibody used in the Emit assay with the active MPA acyl glucuronide metabolite, the decision thresholds for the Emit were higher than those calculated from HPLC measurements.

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.

Effect of cyclosporine on mycophenolic acid area under the concentration-time curve in pediatric kidney transplant recipients

Mycophenolic acid (MPA), the active compound of mycophenolate mofetil (MMF), shows substantial interindividual and intraindividual variability. It was recently shown that in vitro calcineurin inhibitors alter the bioavailability of MPA by dose-dependent inhibition of MPA glucuronidation. The authors retrospectively analyzed full 10-point profiles for both MPA and cyclosporine (CyA) in 23 pediatric patients receiving MMF and cyclosporine microemulsion (Neoral; Novartis Pharmaceuticals Canada; Dorval, Quebec, Canada). Mycophenolic acid was measured using a commercially available EMIT (Novartis Pharmaceuticals, Canada) assay. As the majority of patients were treated with low doses of cyclosporine after adding MMF, the area under the concentration-time curve (AUC) for cyclosporine showed a wide scatter ranging from 296 to 6400 ng x h/mL. The mean cyclosporine dose was 100 +/- 76 mg/m2 per day (range: 28 to 331). There was no correlation between MPA AUC and MPA dose, and there was substantial interindividual variation. However, there was a significant negative correlation between dose-normalized MPA AUC and cyclosporine AUC ( r2 = 0.23, p < 0.0220). When dividing the MPA profiles into two groups (11 and 12 patients) with a CyA AUC less than or greater than 1600 ng x h/mL, there was a significantly higher 8-hour concentration in the patients with the lower CyA AUC, secondary to a higher second peak. The data demonstrate that the cyclosporine AUC is a determining factor for the MPA AUC and that MPA dose should be reduced when cyclosporine dose is reduced to achieve the same MPA AUC. The significantly higher peak in the group with a lower CyA profile supports the concept of a dose-dependent cyclosporine-induced inhibition of MPA glucuronidation.

Pharmacokinetic Basis for the Efficient and Safe Use of Low-Dose Mycophenolate Mofetil in Combination with Tacrolimus in Kidney Transplantation

Background: Mycophenolate mofetil (MMF) is an effective posttransplantation immunosuppressive agent used in combination with cyclosporin A (CsA) or tacrolimus (Tc). An increase in plasma mycophenolic acid (MPA) has been shown in patients receiving Tc-MMF combination therapy compared with CsA-MMF combination therapy at the same dose of MMF. The aim of this prospective study was to assess the pharmacokinetic/pharmacodynamic (PK/PD) relationship for MPA in kidney transplant patients receiving low-dose MMF (500 mg twice a day) in combination with Tc.

Methods: Adult kidney transplant recipients (n = 51) were included. MPA-PK profiles (blood sampling at 0, 0.5, 1, 2, 4, 6, and 12 h after MMF oral dose) were obtained within the first 2 weeks after transplantation, 3 months after grafting, and at every adverse clinical event [side effect or acute rejection (AR)]. All patients received Tc, MMF (500 mg twice a day), and steroids.

Results: Thirty patients (59%) had uneventful outcomes, and 21 patients had 33 episodes of MPA-related side effects; only 3 patients had AR. A total of 78 MPA-PK profiles were obtained. The following PK parameters were increased in the side-effects group compared with the non-side effects group: mean MPA cmin, 2.63 ± 1.58 vs 1.75 ± 0.82 mg/L (P = 0.016); mean c30, 10.47 ± 6.27 vs 7.66 ± 8.95 mg/L (P = 0.009); mean c60, 9.67 ± 5.42 vs 5.83 ± 2.6 mg/L (P = 0.0002); mean area under the MPA time-concentration curve from 0 to 12 h [MPA-AUC(0–12)], 48.38 ± 18.5 vs 36.04 ± 10.82 mg · h/L (P = 0.0006); mean dose-normalized MPA-AUC, 0.16 ± 0.05 vs 0.12 ± 0.04 (mg · h/L)/(mg/m2) (P = 0.0015). For the three AR patients, MPA concentrations obtained at the time of AR revealed MPA cmin values of 1.86, 1.76, and 3.83 mg/L, respectively, and MPA-AUC(0–12) values of 37.7, 24.9, and 104.9 mg · h/L. The threshold of toxicity was 3 mg/L (sensitivity, 38.7%; specificity, 91.5%) for cmin, 8.09 mg/L for maximum MPA concentration during the first hour (sensitivity, 77.8%; specificity, 67.4%), and 37.6 mg · h/L for MPA-AUC(0–12) (sensitivity, 83.3%; specificity, 59.6%).

Conclusions: These results demonstrate the relationship between plasma MPA concentrations and toxicity. High cmin, c30, and c60 values as well as AUC(0–12) are associated with increased risk for side effects. These values may have an importance in a routine monitoring program.

Comparison of the effects of tacrolimus and cyclosporine on the pharmacokinetics of mycophenolic acid

Mycophenolate mofetil (MMF) is almost completely absorbed from the gut and is rapidly de-esterified into its active drug, mycophenolic acid (MPA). The main metabolite is glucuronidated MPA (MPAG), which is excreted into bile and undergoes enterohepatic recirculation. Studies in healthy volunteers treated with cholestyramine show that interruption of the enterohepatic recirculation decreases MPA exposure by approximately 40%. Published data show a difference in mycophenolic acid plasma concentrations between kidney transplant recipients treated with MMF plus cyclosporine (CsA) and those treated with MMF plus tacrolimus (TRL). However, the interpretation of these data is complicated by interpatient differences in variables that may influence MMF pharmacokinetics (e.g., underlying disease, co-medication, and time since transplantation). To understand the influence of TRL and CsA on MMF pharmacokinetics (PK) more completely, the authors eliminated confounding variables in clinical studies by performing drug interaction studies in inbred rats. To achieve a steady state, 3 groups of Lewis rats (n = 8 per group) were treated once daily with oral CsA (8 mg/kg), TRL (4 mg/kg), or placebo on days 0-6 before all rats began once-daily oral treatment with MMF (20 mg/kg) on day 7. Combined treatment with either MMF + CsA, MMF + TRL, or MMF + placebo was continued for 1 week (days 8-14). Thereafter, CsA and TRL treatments were stopped but MMF treatment was continued on days 14-21. Blood was sampled during the 24 hours subsequent to dosing on day 7 (after the first MMF dose), on day 14 (after multiple MMF doses) and on day 21 (after CsA/TRL washout). Rats in the MMF + TRL group and in the MMF + placebo group showed a second peak in the MPA-PK profiles consistent with enterohepatic recirculation of MPA. The MPA-PK profiles for the MMF + CsA-treated animals did not show a second MPA peak. On Day 14, the mean plasma MPA-AUC(0-24 hours) for the CsA-treated animals was significantly less than MPA exposures for rats in the MMF + TRL- and the MMF + placebo-treated groups. Furthermore, in contrast to results from other investigators, co-administration of CsA and MMF significantly increased MPAG-AUC(0-24 hours). Serum creatinines did not differ among rats in the three groups. CsA but not TRL decreased MPA plasma levels and increased MPAG-AUC(0-24 hours). These data suggest that CsA inhibits MPAG excretion into bile and offer an explanation for the well-known increased MPA exposure in organ transplant patients caused by conversion from CsA- to TRL-based immunosuppression.

Determination of the Acyl Glucuronide Metabolite of Mycophenolic Acid in Human Plasma by HPLC and Emit

Background: The acyl glucuronide (AcMPAG) of mycophenolic acid (MPA) has been found to possess pharmacologic and potentially proinflammatory activity in vitro. To establish its pharmacologic and toxicologic relevance in vivo, a reversed-phase HPLC method was modified to simultaneously determine MPA, the phenolic MPA-glucuronide (7-O-MPAG), and AcMPAG. In addition, cross-reactivity of AcMPAG in the Emit assay for MPA was investigated.

Methods: The procedure used simple sample preparation, separation with a Zorbax Eclipse-XDB-C8 column, and gradient elution. AcMPAG was quantified as 7-O-MPAG-equivalents.

Results: The assay was linear up to 50 mg/L for MPA, 250 mg/L for 7-O-MPAG, and 10 mg/L for AcMPAG (r &gt;0.999). Detection limits were 0.01, 0.03, and 0.04 mg/L for MPA, 7-O-MPAG, and AcMPAG, respectively. The recoveries were 99–103% for MPA, 95–103% for 7-O-MPAG, and 104–107% for AcMPAG. The within-day imprecision was &lt;5.0% for MPA (0.2–25 mg/L), &lt;4.4% for 7-O-MPAG (10–250 mg/L), and ≤14% for AcMPAG (0.1–5 mg/L). The between-day imprecision was &lt;6.2%, &lt;4.5%, and ≤14% for MPA, 7-O-MPAG, and AcMPAG, respectively. When isolated from microsomes, purified AcMPAG (1–10 mg/L) revealed a concentration-dependent cross-reactivity in an Emit assay for the determination of MPA ranging from 135% to 185%. This is in accordance with the bias between HPLC and Emit calculated in 270 samples from kidney transplant recipients receiving mycophenolate mofetil therapy, which was greater (median, 151.2%) than the respective AcMPAG concentrations determined by HPLC. AcMPAG was found to undergo hydrolysis when samples were stored up to 24 h at room temperature or up to 30 days at 4 °C or −20 °C. Acidified samples (pH 2.5) were stable up to 30 days at −20 °C.

Conclusions: The HPLC and Emit methods for AcMPAG described here may allow investigation of its relevance for the immunosuppression and side effects associated with mycophenolate mofetil therapy.

Pharmacokinetic and metabolic investigations of mycophenolic acid in pediatric patients after renal transplantation: implications for therapeutic drug monitoring. German Study Group on Mycophenolate Mofetil Therapy in Pediatric Renal Transplant Recipients

The need for mycophenolic acid (MPA) monitoring is still under discussion. Key issues for the PK/PD relationships of this drug are: the role of metabolites, the usefulness of AUC versus predose levels, and the need to monitor the free concentration of MPA (f-MPA). Recent advances have revealed that, in addition to 7-O-MPAG, three additional MPA metabolites are present in the plasma of transplant recipients. One of these metabolites (M-2), identified as an acyl glucuronide of MPA, was found to inhibit IMPDH-II in vitro. This active metabolite was also found to cross-react in the Emit assay for MPA. In an ongoing multicenter study, the authors are evaluating the relevance of monitoring total (t-MPA) and free mycophenolic acid (f-MPA) in pediatric renal transplant recipients. As in adults, a time-dependent increase of t-MPA-AUC(0-12h) within the first 3 months posttransplant (35 versus 64 mg x h/L, [corrected] 3 weeks versus 3 months respectively; daily dosage: 0.6 g/m2 bid) was seen. Receiver operating characteristics curve analyses were used to test the ability of predose levels or AUC(0-12h) to discriminate between cases with no complications and those with acute rejection, adverse events (severe infections, leukopenia), or gastrointestinal disorders observed during the early posttransplant course. In agreement with observations in adults, a significant (p = 0.001) association was observed between AUC(0-12h) and acute rejection. A t-MPA-AUC(0-12h) of approximately 30-60 mg x h/L [corrected], as determined by HPLC, seems to be a reasonable target for the early posttransplant period. It remains to be elucidated whether regular predose level monitoring may be of more practical value. A higher incidence of rejection was observed at predose MPA concentrations < or = 1 mg/L, as measured by HPLC. In contrast to t-MPA, f-MPA-AUC(0-12h) was significantly related to severe infections and leukopenia. The risk for severe adverse events was increased at f-MPA- AUC(0-12h) values > or =600 microg x h/L [corrected]. On the basis of these data and the observed variability in the pharmacokinetics of MPA, the development of monitoring strategies for this drug appears to be promising.

Identification of glucoside and carboxyl-linked glucuronide conjugates of mycophenolic acid in plasma of transplant recipients treated with mycophenolate mofetil

1. Mycophenolic acid (MPA), is primarily metabolized in the liver to 7-O-MPA-beta-glucuronide (MPAG). Using RP-h.p.l.c. we observed three further MPA metabolites, M-1, M-2, M-3, in plasma of transplant recipients on MMF therapy. To obtain information on the structure and source of these metabolites: (A) h.p.l.c. fractions containing either metabolite or MPA were collected and analysed by tandem mass spectrometry; (B) the metabolism of MPA was studied in human liver microsomes in the presence of UDP-glucuronic acid, UDP-glucose or NADPH; (C) hydrolysis of metabolites was investigated using beta-glucosidase, beta-glucuronidase or NaOH; (D) cross-reactivity of each metabolite was tested in an immunoassay for MPA (EMIT). 2. Mass spectrometry of M-1, M-2, MPA and MPAG in the negative ion mode revealed molecular ions of m/z 481, m/z 495, m/z 319 and m/z 495 respectively. 3. Incubation of microsomes with MPA and UDP-glucose produced M-1, with MPA and UDP-glucuronic acid MPAG and M-2 were formed, while with MPA and NADPH, M-3 was observed. 4. Beta-Glucosidase hydrolysed M-1 completely. Beta-Glucuronidase treatment led to a complete disappearance of MPAG whereas the amount of M-2 was reduced by approximately 30%. Only M-2 was labile to alkaline treatment. 5. M-2 and MPA but not M-1 and MPAG cross-reacted in the EMIT assay. 6. These results suggest that: (i) M-1 is the 7-OH glucose conjugate of MPA; (ii) M-2 is the acyl glucuronide conjugate of MPA; (iii) M-3 is derived from the hepatic CYP450 system.