Therapeutic drug monitoring of mycophenolic acid: comparison of HPLC and immunoassay reveals new MPA metabolites

Model-informed dose optimization of mycophenolic acid in pediatric kidney transplant patients

PURPOSE

We aimed to develop and evaluate a population PK model of mycophenolic acid (MPA) in pediatric kidney transplant patients to aid MPA dose optimization.

METHODS

Data were collected from pediatric kidney transplant recipients from a Dutch academic hospital (Radboudumc, the Netherlands). Pharmacokinetic model-building and model-validation analyses were performed using NONMEM. Subsequently, we externally evaluated the final model using data from another academic hospital. The final model was used to develop an optimized dosing regimen.

RESULTS

Thirty pediatric patients were included of whom 266 measured MPA plasma concentrations, including 20 full pharmacokinetic (PK) curves and 24 limited sampling curves, were available. A two-compartment model with a transition compartment for Erlang-type absorption best described the data. The final population PK parameter estimates were Ktr (1.48 h-1; 95% CI, 1.15-1.84), CL/F (16.0 L h-1; 95% CI, 10.3-20.4), Vc/F (24.9 L; 95% CI, 93.0-6.71E25), Vp/F (1590 L; 95% CI, 651-2994), and Q/F (36.2 L h-1; 95% CI, 9.63-74.7). The performance of the PK model in the external population was adequate. An optimized initial dose scheme based on bodyweight was developed. With the licensed initial dose, 35% of patients were predicted to achieve the target AUC, compared to 42% using the optimized scheme.

CONCLUSION

We have successfully developed a pharmacokinetic model for MPA in pediatric renal transplant patients. The optimized dosing regimen is expected to result in better target attainment early in treatment. It can be used in combination with model-informed follow-up dosing to further individualize the dose when PK samples become available.

High-throughput detection of mycophenolic acid in human plasma based on sensitive and rapid fluorescence nitrogen-doped carbon dots sensing platform

In this experiment, a water-soluble, nitrogen-doped yellow-green fluorescent N-doped carbon dots (N-CDs) were synthesized by one-step hydrothermal method using β-cyclodextrin as carbon source and L-phenylalanine as nitrogen source. The fluorescence quantum yield of the obtained N-CDs was as high as 9.96%, and the N-CDs exhibited photostability at different pH, ionic strength and temperature. The morphology of the N-CDs was approximately spherical with an average particle size of about 9.4 nm. Based on the fluorescence enhancement effect of mycophenolic acid (MPA) on N-CDs, a quantitative detection method of MPA was established. This method had good selectivity and high sensitivity for MPA. The fluorescence sensing system was applied to the detection of MPA in human plasma. The linear range of MPA were 0.06-3 μg·mL-1 and 3-27 μg·mL-1 with a detection limit of 0.016 μg·mL-1, and the recoveries were 97.03∼100.64 % with the RSDs of 0.13∼2.90 %. The interference experiment results showed that the interference of other coexisting substances, including Fe3+, can be ignored in the actual detection. Comparing the results measured by the established method with the EMIT method, it was found that the results obtained by the two methods were similar, and the relative error was within ± 5 %. This study provided a simple, rapid, sensitive, selective and effective method for the quantitative analysis of MPA, and was expected to be applied to clinical MPA blood concentration monitoring.

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.

Lack of concordance between EMIT assay and LC-MS/MS for Therapeutic Drug Monitoring of Mycophenolic Acid: Potential increased risk for graft rejection?

Therapeutic drug monitoring (TDM) of immunosuppressive drugs is crucial in organ-transplanted patients to prevent rejection or toxic effects due to inadequate dosage. Mycophenolic acid (MPA) is a commonly used immunosuppressant in this setting. Nowadays, MPA concentrations are monitored by Enzyme Multiplied Immunoassay Technology (EMIT), and Liquid Chromatography (LC)-based techniques, particularly coupled to Tandem Mass Spectrometry (LC-MS/MS). This study evaluates the concordance between TDM results for MPA obtained through CE-IVD EMIT and LC-MS/MS assays in plasma samples. LC-MS/MS quantification was based on a commercial kit and the analytical performance in terms of accuracy was tested through external proficiency tests and inter-laboratory comparison with a home-made HPLC-UV method. Both these evaluations confirmed the reliability of the LC-MS/MS method (1.6 % and 9.0 % of bias, respectively). Conversely, the comparison between EMIT and LC-MS/MS showed overestimation by EMIT of 33.5 %. This bias resulted concentration-dependent, ranging from 46.4 % in the concentration range of 1-2 mg/L, to 21.4 % over 4 mg/L. Considering the theoretical clinical impact of this overestimation, a fraction comprised between 12.4 % and 31.4 % of samples which resulted over three different minimum effective concentration values by EMIT (no indication for dose adjustment) had discordant indications by LC-MS/MS (dose adjustment needed). Concluding, this study highlights a clinically relevant systematic overestimation of MPA concentration by EMIT, supporting the switch to LC-MS/MS techniques for TDM purpose. However, further prospective studies are needed in order to evaluate the clinical impact of switching the TDM activity from EMIT to LC-MS/MS in a larger cohort in a long period.

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.

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.

Acute mycophenolate overdose: case series and systematic literature analysis

BACKGROUND

Literature regarding acute human toxicity of mycophenolate mofetil (MMF) and enteric-coated mycophenolate sodium (EC-MPS) is limited.

OBJECTIVES

Our objectives were to describe all cases of overdose with MMF or EC-MPS reported to the Swiss Toxicological Information Centre (STIC) or in the literature between 1995 and 2013. Therefore, we performed an observational case-series and systematic literature search to determine circumstances, magnitude, management and outcome of overdose with MMF or EC-MPS.

RESULTS

Of 152,762 reports to STIC, 15 (7 pediatric) involved overdose with MMF (n = 13) or EC-MPS (n = 2). Three cases from other centers were identified from a systematic literature search. The magnitude of overdose ranged from 1.2 to 16.7 (median 2.9) times usual dose. Six (33%) MMF overdoses had attributable symptoms, which included abdominal pain, vomiting, headache and dizziness. The majority of findings were minor, although a 9-fold MMF overdose caused hypotension 8 h after ingestion and a 12.5-fold overdose caused leukopenia after 5 days. Symptoms did not occur in patients who took 2.5 times or less of their usual MMF dose. Gastrointestinal decontamination measures with activated charcoal were undertaken in one-third of cases.

CONCLUSIONS

Acute MMF and EC-MPS overdoses had a favorable outcome in all cases reported to STIC and in the literature.

Analytical validation of a homogeneous immunoassay for determination of mycophenolic acid in human plasma

Mycophenolic acid (MPA) is an immunosuppression agent for the prophylaxis of organ rejection in patients receiving allogeneic transplants. The drug is administered based in 2 formulations, mycophenolate mofetil (MMF) and enteric-coated mycophenolate sodium (EC-MPS). MPA acts by specific, reversible, uncompetitive inhibition of inosine monophosphate dehydrogenase (IMPDH) and thus blocks the proliferation of both T- and B-activated lymphocytes. Therapeutic drug monitoring (TDM) constitutes an important part of immunosuppressive treatment because of the demonstrated significant intraindividual and interindividual variability of its pharmacokinetic behavior. TDM is required to optimize immunosuppressive efficacy. We present the analytical validation of a homogeneous particle-enhanced turbidimetric inhibition immunoassay (PETINIA) technique for determination of MPA in human plasma, and compare with a homogeneous enzyme immunoassay technique (EMIT; reference method), both methods adapted on a Dimension analyzer (Siemens). We examined 50 human plasma samples from kidney transplant recipients treated with MMF or EC-MPA, which were analyzed simultaneously by both methods. The interassay precision was 5.95% at a concentration of 1.0 μg/mL, 3.47% at 7.5 μg/mL, and 3.75% at 12.0 μg/mL. The bias of PETINIA-MPA for each of the 3 quality control sample was <3.0%. Least squares linear regression yielded an r-value of 0.994 with the following linear regression equation: PETINIA = 0.939 * EMIT - 0.063. Bland-Altman comparison presented a mean negative difference of -0.312 μg/mL (standard deviation [SD], 0.441), namely, -7.6% for PETINIA-MPA. The PETINIA assay for monitoring MPA concentrations is an acceptable method for routine clinical use, with interassay imprecision (% coefficient of variation) ranging from 5.9% to 3.7% below and above the therapeutic concentration range, respectively. In conclusion, MPA-EMIT and PETINIA-MPA methods on Dimension analyzer have a good correlation (r = 0.994), but PETINIA-MPA method demonstrates a negative average difference of -7.6% in comparison with EMIT-MPA method.

Optimization of the dosing regimen of mycophenolate mofetil in pediatric liver transplant recipients

Mycophenolate mofetil (MMF) is now commonly used in pediatric liver transplant recipients, but no clear recommendations about the dosing regimen have been made for this population. The aim of this study was to determine the MMF dosage required for pediatric liver transplant recipients to achieve an area under the plasma concentration-time curve from 0 to 12 hours (AUC(0-12) ) for mycophenolic acid (MPA) greater than 30 mg hour/L. A pharmacokinetic study of 15 children (median age = 8.3 years, range = 1.1-15.2 years) was performed at a median of 11.0 months (range = 0.5-88.0 months) after liver transplantation. MMF was initially introduced at a median starting dose of 300 mg/m(2) twice a day (range = 186-554 mg/m(2) twice a day). Thirteen of the 15 patients had an MPA AUC(0-12) value less than 30 mg hour/L. The MMF dosage had to be increased in all patients except 1. The MMF dosage required to reach an MPA AUC(0-12) value greater than the defined target of 30 mg hour/L ranged from 371 to 1014 mg/m(2) /day. For 2 patients who received rifampin in addition to MMF, the MPA AUC(0-12) value remained low despite a 2-fold increase in the MMF dosage. In conclusion, an initial MMF dose of 600 mg/m(2) twice a day led to MPA AUC(0-12) values greater than the 30 mg hour/L threshold except when rifampin was coadministered. Because of the important interindividual variability of MPA pharmacokinetics, therapeutic drug monitoring is recommended for optimizing the daily MMF dosage. Furthermore, these results suggest that the coadministration of MPA with rifampin should be avoided.

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.

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

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

Random pharmacokinetic profiles of EC-MPS in children with autoimmune disease

BACKGROUND

: Therapy with mycophenolate mofetil (MMF) has become a valuable therapeutic option in children with autoimmune disease. MMF prescription in children with autoimmune diseases differs from that in transplant recipients in terms of different dosing regimen, and concomitant administration of other immunosuppressive medications. Recently, another formulation of the same active compound, mycophenolic acid (MPA), has become available as enteric-coated Mycophenolate Sodium (EC-MPS). Dosing and pharmacokinetics of EC-MPS in pediatric autoimmune disease have never been studied.

METHOD

S: We therefore performed a pilot study on 6 patients, who were treated with EC-MPS. All patients underwent 1-2 full 10-point pharmacokinetic (PK) profiles over a 12-hour dosing interval. We compared the results with that of 22 similar patients on MMF therapy.

RESULTS

: Median EC-MPS dose was 724 mg/m2 (range 179-933 mg/m2). The MPA Area-Under-The-(Time-Concentration)-Curves (AUCs) on MMF and EC-MPS were comparable (54.4 mg x h/L on MMF and 44.0 mg x h/L on EC-MPS, n.s., Mann Whitney). After correcting for bioequivalence, the dose-normalized AUCs were also similar on both the formulations. However, PK profiles on EC-MPS were quite random, and time to maximum concentration varied from 30 minutes to 720 minutes. The concentration at six-hour correlated best with the AUC. This was different from a homogenous PK-profile on MPA.

CONCLUSIONS

: EC-MPS has a different PK profile from MMF. The data suggest that patients on EC-MPS must undergo a complete PK profile to assess adequate exposure. The 6-hour concentration provides an estimate of the exposure and should be targeted between 3-4 mg/L.

Limited sampling strategies for therapeutic drug monitoring of mycophenolate mofetil therapy in patients with autoimmune disease

Mycophenolate mofetil (MMF) is increasingly used for the treatment of autoimmune diseases (AID). In renal transplant recipients, it has been demonstrated that adjustment of the MMF dose according to the area under the plasma concentration versus time curve (AUC) of mycophenolic acid (MPA), the active moiety of MMF, improves clinical outcome. The aim of this study was to develop a maximum a posteriori Bayesian estimator (MAP-BE) to estimate MPA AUC(0-12) in patients with AID using a limited number of samples. The predictive performance of the MAP-BE was compared with a multiple linear regression method. Full MPA concentration versus time curves were available from 38 patients with AID treated with MMF. Nonlinear mixed-effect modeling was used to develop a population pharmacokinetic model. Patients were divided in an index and a validation data set. The pharmacokinetic model derived from the index data set was used to develop several MAP-BEs. The Bayesian estimators were used to predict AUC(0-12) in the validation data set on the basis of a limited number of blood samples. The bias and precision of these predictions were compared with those of limited sampling strategies developed with multiple linear regression. The absorption of MPA was described with 2 first-order processes with a short and a long lag time and a subsequent first-order elimination. The 2-compartment model accounted for the enterohepatic recirculation of MPA as well. Using 1-4 samples, MPA AUC(0-12) was adequately estimated by the MAP-BE. Bias (-5.5%) was not significantly different from zero, and precision was below 27%. The predictive performance of the multiple linear regression method was comparable. In conclusion, MAP-BEs were developed for the estimation of MPA AUC(0-12) in patients with AID. The predictive performance was good and comparable to those of the multiple linear regression method. Due to its flexibility with respect to sample times, the MAP-BE may be preferred over the multiple linear regression method.

Clinical utility of a new enzymatic assay for determination of mycophenolic acid in comparison with an optimized LC-MS/MS method

The goal of this study was to investigate the clinical utility of a new enzymatic assay for use on COBAS INTEGRA systems (Roche Total MPA assay). From 134 patients, plasma mycophenolic acid (MPA) concentrations were measured with both the enzymatic method and a validated liquid chromatography with tandem mass spectrometry (LC-MS/MS) procedure, to compare these assays. The test principle of the enzymatic assay is inhibition of inosine monophosphate dehydrogenase. Method comparison studies revealed good agreement of results (r > 0.99), overall and in patients with delayed graft function or hypoalbuminemia. MPA area under the concentration-time curve (AUCs) obtained with LC-MS/MS (x) and the enzymatic method (y) compared excellent in patients on cyclosporine (y = 1.04x - 1.05, r = 0.992) or tacrolimus (y = 1.02x - 0.63, r = 0.987). MPA exposure determined with either method at different time points after transplantation agreed well (eg, 25th/50th/75th percentile of day 10 AUCs-LC-MS/MS: 25.8/33.8/45.2 versus enzymatic assay: 26.2/34.4/45.3 mg.h/L). AUCs calculated for both methods were lower at the first 3 time points in patients on cyclosporine compared with tacrolimus (week 4 median cyclosporine/tacrolimus: LC-MS/MS 39.6/56.4 versus enzymatic assay 40.5/56.0 mg.h/L). Both LC-MS/MS and the enzymatic methods revealed a tendency toward lower AUCs and predose levels in patients with biopsy-proven acute rejection (BPAR) (day 10 median: 0.9 mg/L with BPAR and 1.7 mg/L without BPAR). The Roche Total MPA assay is a reliable alternative to LC-MS/MS. It can be applied in the clinical setting allowing for easy, fast, and optimized patient management.

Comparison of a new enzymatic assay with a high-performance liquid chromatography/ ultraviolet detection method for therapeutic drug monitoring of mycophenolic acid in adult liver transplant recipients

Mycophenolic acid (MPA) is used to prevent graft rejection. The methods used for determining the plasma MPA concentration in liver transplant recipients are the enzyme-multiplied immunoassay technique (EMIT), high-performance liquid chromatography with ultraviolet detection (HPLC-UV), and most recently mass spectrometry. EMIT has been reported to overestimate the MPA concentration by 30% to 35% in comparison with HPLC-UV. Recently, a new automated enzymatic assay based on inosine monophosphate dehydrogenase inhibition has been designed. The aim of the present investigation was to compare this technique with validated HPLC-UV in adult liver transplant recipients treated with tacrolimus or cyclosporine. One hundred seventy-six samples from 50 adult liver transplant recipients were analyzed with both techniques. Patients received mycophenolate mofetil (2 or 3 times daily) coadministered with cyclosporine microemulsion (n = 18) or tacrolimus (n = 32). Samples were drawn over an interdose interval during the early or late posttransplantation period. The Passing-Bablok regression and Bland-Altman plot were used to compare the 2 techniques. The Passing-Bablock regression, calculated from 166 samples, showed very good agreement between the enzymatic assay and the HPLC-UV method: enzymatic assay = 1.0204 (95% confidence interval, 0.9942, 1.0478) x HPLC-UV + 0.0201 (-0.0442, 0.0882). No significant bias was found between the techniques (Bland-Altman plot), and the median relative difference was 2.7% (95% confidence interval, -0.4, 6.6). In conclusion, the enzymatic assay showed an excellent correlation with HPLC-UV. Therefore, this method was proved valid and reliable for the monitoring of the plasma MPA concentration in adult liver transplant recipients treated with cyclosporine microemulsion or tacrolimus.

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.

Automated monitoring of C2 and C0 blood levels of mycophenolic acid and cyclosporine on the Abbott Architect c8000

OBJECTIVES

Evaluation of the performance of the EMIT 2000 Cyclosporin assay using 2 sets of assay calibrators and the EMIT 2000 mycophenolic acid assay to measure C0 and C2 concentrations on the Abbott Architect c8000 analyzer.

DESIGN AND METHODS

Imprecision studies were performed. Cyclosporin concentration was assayed by EMIT on the c8000, by ACMIA on the Dimension and by LC-MS/MS while mycophenolic acid was analyzed by EMIT on c8000 and on Dimension and by HPLC.

RESULTS

Agreement between cyclosporin and mycophenolic acid concentrations assayed on the c8000 and on the Dimension was very good. Method comparison between the c8000 and LC-MS/MS resulted in a relative bias of 15.7% for C0 and 11.5% for C2 concentrations. Relative bias of the mycophenolic acid concentrations assayed on the c8000 and the HPLC was 37.7%.

CONCLUSIONS

When reported properly to the clinician mycophenolic acid and cyclosporine blood levels can be monitored using the EMIT assays on the c8000 consolidating standard routine workflow and reducing reagent costs significantly.

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.

Therapeutic drug measurement of mycophenolic acid derivatives in transplant patients

INTRODUCTION

Mycophenolic acid, the active metabolite of the prodrug mycophenolate mofetil, is widely used as an immunosuppressive agent in transplant patients for the prophylaxis of acute rejection. Recent prospective trials suggested the need for therapeutic drug monitoring, which raises the necessity to acquire accurate methods to measure MPA and its metabolites.

OBJECTIVE

Present an overview of the reasons to monitor MPA and its metabolites as well as a review of the currently available methods for their determination.

METHODS

Articles published from January 1992 to December 2006 were reviewed.

RESULTS

Most of the cited references use either chromatographic or immunoassay techniques. Basic information about biological samples used for the analysis, sample preparation, stationary phase, mobile phase, detection mode and validation data are discussed. Current information suggests the feasibility to set up method(s) to monitor MPA and its metabolites in most centers.

Influence of the UGT2B7 promoter region and exon 2 polymorphisms and comedications on Acyl-MPAG production in vitro and in adult renal transplant patients

INTRODUCTION

The polymorphic enzyme UGT2B7 metabolizes mycophenolic acid into acyl-mycophenolic acid-glucuronide (AcMPAG), a presumably toxic metabolite. This study aimed at investigating in vitro and in vivo the impact on AcMPAG production of: (i) the UGT2B7 gene G-842A single nucleotide polymorphism, in complete linkage disequilibrium with most other known single nucleotide polymorphisms in the promoter region of this gene and with the C802T single nucleotide polymorphism in exon 2 (UGT2B*2); and (ii) of the other immunosuppressants given to renal transplant patients in association with mycophenolate mofetil.

METHODS

We compared the production of AcMPAG by human liver microsomes genotyped for the UGT2B7 G-842A and C802T single nucleotide polymorphisms, and plasma AcMPAG concentrations in genotyped renal transplant patients administered mycophenolate mofetil associated with sirolimus (n=40), tacrolimus (n=24) or cyclosporin (n=28) and decreasing doses of corticosteroids, over the first 3 months after transplant. The effect of corticosteroids was also investigated in vitro using rats' liver microsomes.

RESULTS

The two polymorphisms studied were in complete reverse linkage disequilibrium. AcMPAG production was 1.25 and 1.56-fold higher in G-842A and -842AA human liver microsomes, respectively, compared with GG-842 human liver microsomes (P=0.01). Enzyme kinetics showed 1.4 and 3.7-fold higher Vmax in the respective pools of human liver microsomes. Km values were 0.20, 0.25 and 0.44 mmol/l for the GG-842, G-842A and -842AA genotypes, respectively. This clear increase in Vmax is in favor of the implication of the promoter region polymorphisms, whereas the slighter increase in Km might be due to the UGT2B7*2 single nucleotide polymorphism. Consistently, the UGT2B7 genotype significantly influenced AcMPAG area under the curve (AUC0-9 h)/dose in patients on sirolimus at months 1 and 3 after transplant (P=0.04 for both). No effect was observed in patients on tacrolimus and possibly also on cyclosporin, maybe owing to pharmacokinetic interaction with mycophenolate. AcMPAG production was increased in corticosteroid-induced rat liver microsomes, consistent with the observed in-vivo decrease of mycophenolic acid metabolites AUC0-9 h/dose with time after transplant.

CONCLUSION

Both UGT2B7 polymorphisms and co-medications significantly influenced AcMPAG production, but cyclosporin and tacrolimus hindered the phenotypic impact of this trait.

Optimization of Immunosuppressive Drug Monitoring in Children

BACKGROUND

The metabolism of drugs in children differs from adults, although pediatric pharmacokinetic (PK) studies remain scarce. Many of the drugs are metabolized by polymorphically expressed enzymes (cytochrome P450 [CYP450]; glucuronyl transferase [GT]) and/or transported by drug transporters (ABC and SLC families). In children, there is added complexity because of the age dependency of drug metabolism. This review addresses the age dependency of drug metabolism in childhood on the basis of routine PK monitoring.

METHODS

Standard pharmacokinetic studies in pediatric renal transplant recipients were analyzed to study drug-drug interactions between mycophenolic acid and cyclosporine on the one hand, and tacrolimus and sirolimus on the other hand. The exposure was compared with age. We also studied sirolimus metabolites, both by mass spectrometry as well as using human liver microsomes.

RESULTS

We demonstrated age dependency for MPA exposure. Independent of the concomitant medication, infants required approximately twice as much drug for the same exposure. The drug-drug interaction between sirolimus and tacrolimus demonstrated age dependency. Sirolimus metabolites showed a remarkably different pattern in children. Whereas 39-O-desmethyl sirolimus is the most prevalent metabolite in adults, we found 77.5% hydroxylated metabolites in children. Similarly, pediatric human liver microsomes produced 86.1% hydroxylated metabolites.

CONCLUSIONS

Our long-term objective is to develop evidence-based guidelines for age-appropriate drug dosing of all drugs commonly used during childhood and adolescence, based on pharmacokinetically/pharmacogenetically determined drug exposure to maximize therapeutic yield while minimizing toxicity. The potential need for lifelong medications warrants efforts to minimize toxicity in chronically ill pediatric patients.

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.

CEDIA® Mycophenolic Acid Assay Compared With HPLC-UV in Specimens From Transplant Recipients

Routine monitoring of mycophenolic acid (MPA) has been accepted as an essential tool in the management of this therapy in transplant recipients. The availability of simple, sensitive assays that measure MPA in plasma permits individualization of dosing regimens according to pharmacokinetic principles. We report the results of an evaluation of the CEDIA Mycophenolic Acid Immunoassay (Microgenics Corporation, Fremont, California) for the measurement of plasma MPA concentrations in a range of transplant indications and compare its performance and specificity to an established HPLC/UV method. Precision and accuracy were determined both within and between runs using the quality control materials provided with the CEDIA MPA assay, which produced within run (n = 21) coefficients of variation (CV%) and biases of less than 5%. The between run analyses, performed over consecutive days following daily calibration of the assay, showed CVs and biases of less than 7%. Routine patient samples (n = 298) from 142 patients of varying transplant type were analyzed using the CEDIA MPA kit and HPLC/UV methods. Regression analysis of the patient samples gave an equation of CEDIA = 1.18 HPLC/UV + 0.45 (r = 0.83). According to the manufacturer's product information, there is 192% cross reactivity with the active mycophenolate acyl glucuronide. The data presented suggest that the CEDIA MPA immunoassay, run on the Hitachi 911 analyzer, over-estimates plasma MPA concentrations with a magnitude that is influenced by transplant type. Hence, users must interpret the immunoassay results with caution and not assume that the metabolite fraction is constant in recipients of the same organ type or in different organ transplant populations.

Value of therapeutic drug monitoring of MMF therapy in pediatric transplantation

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

Unexpectedly high exposure to enteric-coated mycophenolate sodium upon once-daily dosing

Enteric-coated mycophenolate sodium (EC-MPS) has a mean half-life of 11.7 hours, which encouraged hope of using this drug once daily in a nonadherent adolescent SLE patient. This is a case report on a 17-year-old adolescent with a history of noncompliance who was switched from twice-daily mycophenolate mofetil (MMF) to once-daily EC-MPS. The EC-MPS dose was equimolar to the daily MMF dose (1 g MMF BID and 1.44 g of EC-MPS OD). The active compound of both drugs, mycophenolic acid, was measured using a commercially available EMIT assay. Both drugs were well-tolerated and maintained remission of the SLE. The average of three 12-hour areas under the time-concentration curves (AUC) on 1 g of MMF BID was 59.0 mgxh/L. In contrast, the 24-hour AUC after 1.44 g EC-MPS OD was 283.2 mgxh/L, more than double the expected 118.0 mgxh/L of two MMF dosing intervals. A repeat 24-hour AUC after 1.08 g of EC-MPS was 218.2 mgxh/L. EC-MPS once daily may be a well-tolerated therapeutic option for nonadherent adolescent lupus patients, but may be associated with a significantly higher exposure than the equivalent MMF BID dose.

Determination of Mycophenolic Acid Plasma Levels in Renal Transplant Recipients Co-administered Sirolimus: Comparison of an Enzyme Multiplied Immunoassay Technique (EMIT) and Liquid Chromatography–Tandem Mass Spectrometry

This study was designed to compare the enzyme multiplied immunoassay technique (EMIT) and a specific ;liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the determination of mycophenolic acid (MPA) concentrations in plasma samples collected from renal transplant patients receiving mycophenolate mofetil in association with sirolimus and corticosteroids. Ten-point blood concentration-time profiles, between pre-dose (C0) and 9-hour post-dose, were obtained on day 7, day 14, months 1, 2, and 3 posttransplantation in 8 patients. MPA plasma concentrations were measured simultaneously with both EMIT and LC-MS/MS. Higher concentrations were obtained with EMIT. The mean (+/-SD) absolute difference was +0.84 +/- 1.93 mg/L, which corresponds to a significant overestimation of 18.7 +/- 2 6.8% with EMIT, with variations depending on the time elapsed since transplantation. An EMIT overestimation of 16.7 +/- 22.5% also was obtained for the MPA areas under the curve calculated using the trapezoidal rule.

Influence of co-medication with sirolimus or cyclosporine on mycophenolic acid pharmacokinetics in kidney transplantation

The pharmacokinetics of mycophenolic acid (MPA)--the active metabolite of mycophenolate mofetil (MMF)--is significantly influenced by co-medications. The impact of sirolimus on daily MPA exposure, however, has not been investigated so far. As a part of the study aimed at investigating the efficacy of Campath-1H induction therapy in a steroid-free regimen in kidney transplantation, MPA plasma levels were serially measured in 21 patients treated with low-dose sirolimus (SRL) or low-dose CsA both in addition to low-dose MMF over 12 months post-operatively. Full pharmacokinetic profiles were compared at month 6 and 12 post-surgery. Mean dose-adjusted MPA trough levels were 4.4-fold higher in patients on combined SRL and MMF than in those given CsA and MMF. Pharmacokinetic studies demonstrated that mean MPA C(max) and T(max) were comparable in the two groups, while mean MPA AUC(0-12) was higher in SRL than CsA treated patients. The pharmacokinetic profile of SRL- but not of CsA-group showed a second peak consistent with the enterohepatic recirculation of MPA. These findings suggest that SRL and CsA have different effects on MPA metabolism and/or excretion eventually affecting its immunosuppressive property and/or toxicity. CsA, but not SRL, inhibits MPA enterohepatic recirculation, reducing MPA daily exposure.

Validation of a high-performance liquid chromatography method for the measurement of mycophenolic acid and its glucuronide metabolites in plasma

OBJECTIVES

The need for therapeutic drug monitoring of the immunosuppressant mycophenolic acid is becoming more evident. This paper describes a simple high-performance liquid chromatography procedure for the simultaneous quantitation of mycophenolic acid (MPA) and its glucuronide metabolites in plasma using protein precipitation followed by HPLC analysis with isocratic elution and UV detection.

DESIGN AND METHODS

The performance of this method is compared to the EMIT 2000 MPA immunoassay (Dade Behring Diagnostics Inc., Cupertino, California, USA).

RESULTS AND CONCLUSION

Intra-assay precision and accuracy of calibrators were determined for MPA at 0.5 and 20 mg/L, MPAGe at 5 and 200 mg/L, and MPAGa at 2.5 and 100 mg/L and showed coefficients of variation of less than 5.0% and biases of less than 14.0%. Inter-assay precision and accuracy of quality control samples were determined for MPA at 2 and 15 mg/L, MPAGe at 20 and 150 mg/L and showed CVs of less than 5.0% and biases of less than 14%. The lower limit of quantitation of the method was determined for MPA at 0.25 mg/L, MPAGe at 0.5 mg/L, and MPAGa at 0.25 mg/L and showed CVs of less than 19% and biases of less than 20%. This method, compared to the EMIT 2000 MPA immunoassay, showed a linear regression analysis relationship of EMIT = 0.973 HPLC + 0.55 (r(2) = 0.851), and was determined to be suitable for therapeutic drug monitoring and pharmacokinetic studies of MPA.

Therapeutic Mycophenolic Acid Monitoring by Means of Limited Sampling Strategy in Orthotopic Heart Transplant Patients

Therapeutic drug monitoring (TDM) is essential to maintain the efficacy of many immunosuppressant drugs while minimizing their toxicity. TDM of mycophenolate mofetil requires area under the curve AUC determinations but appears laborious, costly, and clinically impractical. To overcome these problems, limited sampling strategies (LSS) have been proposed in adult and pediatric renal transplant patients. The purpose of this study was to develop an LSS in heart transplant patients. Forty-four mycophenolic acid (MPA) full AUC(0-12h) profiles were generated by high-performance liquid chromatography in nine heart transplant patients during the first 12 weeks posttransplant. Each patient received concomitant cyclosporine and prednisone therapy. Multiple stepwise regression analysis was used to define the time points of MPA levels to explain the MPA AUC(0-12h). Agreement between abbreviated AUC and the full AUC(0-12h) was tested by means of a Bland and Altman analysis. The highest coefficient of determination r(2) among MPA AUC and single concentrations (r(2) = .610) was observed with C(2), while C(12) provided the lowest one (r(2) = .003). Stepwise linear regression showed that the minimal model with the best estimation of MPA AUC(0-12h) was obtained at timed values of 1.25, 2, and 6 hours. The corresponding estimated model was AUC = 5.568 + 0.902 * C(1.25) + 2.022 * C(2) + 4.594 * C(6) (r(2) = .926). Bland and Altman analysis revealed good agreement between predicted AUC and full AUC. A further interesting model equation obtained by four samples was AUC = 3.800 + 1.015 * C(1.25) + 1.819 * C(2) + 1.566 * C(4) + 3.479 * C(6) (r(2) = .948).

Comparison of liquid chromatography-tandem mass spectrometry with a commercial enzyme-multiplied immunoassay for the determination of plasma MPA in renal transplant recipients and consequences for therapeutic drug monitoring

Mycophenolic acid (MPA) is an immunosuppressive drug partly metabolized to MPA-glucuronide (MPAG), which is pharmacologically inactive. The currently available enzyme-multiplied immunoassay technique (EMIT) has been reported to overestimate MPA plasma concentration in clinical samples when compared with HPLC techniques. The aims of this study were to design and validate a specific liquid chromatography-tandem mass spectrometry (LC-MS/MS) technique for the determination of MPA and MPAG using a low plasma volume and a simple sample preparation procedure; then to compare it with EMIT for the determination of MPA in plasma samples collected over an interdose interval at different posttransplantation periods (days 3, 7, and 30 and after 3 months) in 25 renal transplant recipients orally administered cyclosporine and mycophenolate mofetil twice daily, to investigate the origins of the differences between techniques. The LC-MS/MS technique developed showed limits of quantification (LOQs) of 0.1 mg/L and 1 mg/L for MPA and MPAG, respectively, and was linear, accurate, and precise from these LOQs up to 30 mg/L for MPA and 300 mg/L for MPAG. EMIT gave similar results to LC-MS/MS for spiked quality control samples (in a synthetic matrix or in drug-free plasma) but significantly overestimated MPA levels in clinical samples: EMIT - LC-MS/MS = +61.39% +/- 57.94%, with large variations depending on patients, time elapsed since transplantation, sampling time, and concentration levels. These results confirmed the known overestimation of the EMIT assay compared with a specific method and showed that the magnitude of this overestimation depended on sampling time and time after transplantation.

To what extent does the understanding of pharmacokinetics of mycophenolate mofetil influence its prescription

Within a short period, we have witnessed a dramatic increase in the use of mycophenolate mofetil (MMF) in pediatric renal transplantation, with the drug often replacing azathioprine in combination with calcineurin inhibitor therapy. When the drug was introduced, the manufacturer considered therapeutic drug monitoring (TDM) unnecessary. However, TDM studies revealed substantial inter- and intra-individual variability and drug interactions. There is a substantial drug interaction between MMF and cyclosporine, and lower doses are required in combination with tacrolimus (~500-800 mg/m(2) per day) than with cyclosporine (~1,200 mg/m(2) per day). Patients with autoimmune disease require an intermediate dose when receiving no concomitant calcineurin inhibitor (~900 mg/m(2) per day). It has been possible to detect drug interactions and to minimize adverse events only with TDM. This is especially important with increasing use of combination therapies. Pharmacodynamic monitoring (measuring the biological response to a drug) coupled with pharmacokinetics allow optimization of drug dosing, with maximum efficacy and minimal toxicity. More work is required to establish specific target ranges with the various drug combinations--especially for the pediatric population.

Liquid chromatographic method for simultaneous determination of mycophenolic acid and its phenol- and acylglucuronide metabolites in plasma

A simple, sensitive and reproducible HPLC method is presented for the simultaneous determination of mycophenolic acid (MPA) and its metabolites phenolic MPA-glucuronide (MPAG) and acyl glucuronide (AcMPAG) in human plasma. Sample purification requires protein precipitation with 0.1 M phosphoric acid/acetonitrile in the presence of Epilan D as an internal standard (IS). Separation was performed by reversed-phase HPLC, using a Zorbax SB-C18 column, 32% acetonitrile and a 40 mM phosphoric acid buffer at pH 3.0 as mobile phase; column temperature was 50 degrees C, flow rate 1.4 ml/min, and measurement by UV detection was at 215 nm (run time 12 min). The method requires only 50 microl plasma. Detection limits were 0.1 microg/ml for MPA and AcMPAG, and 2.0 microg/ml for MPAG, respectively. Mean absolute recovery of all three analytes was >95%. This analytical method for the determination of MPA and its metabolites is a reliable and convenient procedure that meets the criteria for application in routine clinical drug monitoring and pharmacokinetic studies.

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.