Variability In Mycophenolic Acid Pharmacokinetics Research Articles

A Single Nucleotide Polymorphism in the SLCO1B3 Gene is Associated with Dose-Normalized Mycophenolic Acid AUC12

<h3>Purpose</h3> Lung transplant recipients are commonly maintained on mycophenolic acid (MPA). However, research has shown that transplant recipients experience a wide variability in MPA pharmacokinetics. We hypothesize that some of this variability is due to genetic factors. To this end we analyzed the relationship between MPA concentration and a hypofunctional polymorphism in the SLCO1B3 gene, rs7311358, which we have previously reported is associated with MPA-related clinical outcomes. <h3>Methods</h3> We performed full pharmacokinetic testing in 30 adult bilateral lung transplant recipients who were maintained on MPA and who had previously enrolled in our lung transplant genetic database. MPA levels were assessed using a high-performance liquid chromatography peak area quantitation. We calculated the 12-hour area under the curve (AUC₁₂) using the linear trapezoidal method. We used linear regression to evaluate the relationship between the SLCO1B3 polymorphism and the AUC₁₂. We also performed multivariate linear regression analysis to evaluate potential confounders. <h3>Results</h3> The SLCO1B3 rs7311358 polymorphism was present in 8 of 30 patients. These patients had a significantly lower dose normalized MPA AUC₁₂ than patients without the polymorphism (rs7311358 GG/GA 40.11 mg*h/L vs. rs7311358 AA 68.84 mg*h/L, p=0.041, Figure 1). Additionally, in multivariate testing the presence of the polymorphism remained significantly associated with MPA AUC₁₂. <h3>Conclusion</h3> We provide the first evidence that SLCO1B3 polymorphism rs7311358 is significantly associated with MPA dose normalized AUC₁₂ in lung recipients. Integration of this pharmacogenetic data with traditional pharmacokinetic testing could lead to more effective MPA management strategies.

Influence of Calcineurin Inhibitor and Sex on Mycophenolic Acid Pharmacokinetics and Adverse Effects Post-Renal Transplant.

Tacrolimus or cyclosporine is prescribed with mycophenolic acid posttransplant and contributes to interpatient variability in mycophenolic acid pharmacokinetics and response. Cyclosporine inhibits enterohepatic circulation of the metabolite mycophenolic acid glucuronide, which is not described with tacrolimus. This study investigated mycophenolic acid pharmacokinetics and adverse effects in stable renal transplant recipients and the association with calcineurin inhibitors, sex, and race. Mycophenolic acid and mycophenolic acid glucuronide area under the concentration-time curve from 0 to 12hours (AUC0-12h ) and apparent clearance were determined at steady state in 80 patients receiving cyclosporine with mycophenolate mofetil and 67 patients receiving tacrolimus with mycophenolate sodium. Gastrointestinal adverse effects and hematologic parameters were evaluated. Statistical models evaluated mycophenolic acid pharmacokinetics and adverse effects. Mycophenolic acid AUC0-12h was 1.70-fold greater with tacrolimus (68.9±30.9mg·h/L) relative to cyclosporine (40.8±17.6mg·h/L); P<.001. Target mycophenolic acid AUC0-12h of 30-60mg·h/L was achieved in 56.3% on cyclosporine compared with 34.3% receiving tacrolimus (P<.001). Mycophenolic acid clearance was 48% slower with tacrolimus (10.6±4.7L/h) relative to cyclosporine (20.5±10.0L/h); P<.001. Enterohepatic circulation occurred less frequently with cyclosporine (45%) compared with tacrolimus (78%); P<0.001; with a 2.9-fold greater mycophenolic acid glucuronide AUC0-12h to mycophenolic acid AUC0-12h ratio (P<.001). Race did not affect mycophenolic acid pharmacokinetics. Gastrointestinal adverse effect scores were 2.2-fold higher with tacrolimus (P<.001) and more prominent in women (P=.017). Lymphopenia was more prevalent with tacrolimus (52.2%) than cyclosporine (22.5%); P<0.001. Calcineurin inhibitors and sex contributed to interpatient variability in mycophenolic acid pharmacokinetics and adverse effects post-renal transplant, which could be attributed to differences in enterohepatic circulation.

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

Elderly transplant recipients have a lower incidence of acute rejection, and a higher risk to die from infectious complications. A potential cause may be differences in the pharmacokinetics (PK) or pharmacodynamics (PD) of the immunosuppressive drugs they are taking. This study was designed to comprehensively evaluate the influence of age on the PK and PD of mycophenolic acid (MPA). In this study the PK and PD of MPA was studied in 26 elderly and 54 younger renal transplant recipients treated with mycophenolate mofetil and tacrolimus. Patients were sampled repetitively, both before and during the first 6 months after kidney transplantation. Age-related variability in MPA PK, baseline IMPDH activity, as well as MPA-induced IMPDH inhibition were studied. The IMPDH activity pre-transplantation did not differ between elderly and younger patients. Neither IMPDH activity pre-transplantation nor maximum IMPDH inhibition was significantly correlated with the patients' age. The area under the MPA plasma concentration-time curve (AUC0-12h ) and the area under the effect (IMPDH activity)-time curve (AEC0-12h ) from 0 to 12h were also not significantly different between the two groups. We found no significant differences in EC50 and Emax between elderly and younger patients. Age did not significantly affect the PK or PD of MPA. It is unlikely that the lower incidence of acute rejection in elderly patients, or the higher risk to die from a severe infection in elderly patients is due to different handling of MPA in the elderly.

Effects of Plasmapheresis on Mycophenolic Acid Concentrations

Although plasmapheresis (PP) is a widely used intervention in immunologic disease states, there is an absence of literature on its effects on the pharmacokinetics of mycophenolic acid (MPA) (1). Supplemental dosing is often necessary for highly protein-bound drugs with low volumes of distribution (Vd) as pharmacokinetics can be significantly altered by PP. Based on the pharmacokinetic information on Vd and protein binding provided in mycophenolate mofetil and mycophenolate sodium package inserts, clearance by PP would theoretically be expected to be minimal; however, there exists wide variance in reported Vd and protein binding in the literature with reported unbound free fraction of up to 29%, which suggests that drug removal by PP may be possible (2, 3). This was a prospective case series. Institutional review board approval and written informed consent were obtained before study enrollment. Patient 1 was a 63-year-old white female, weighing 63 kg, with myasthenia gravis admitted for intermittent PP. She had been maintained on mycophenolate mofetil 1000 mg in the morning and 1500 mg in the evening for approximately 1 month before the study. Patient 2 was a 55-year-old, African American female, weighing 71 kg, admitted for three times weekly PP for biopsy-confirmed recurrent focal segmental glomerulosclerosis approximately 9 months after renal transplantation. Her mycophenolate sodium dose of 360 mg twice daily had been stable since the time of transplantation. PP was performed using continuous flow centrifugation to separate red blood cells from plasma. Wasted plasma is referred to as plasmapheresate. For patient 1, three MPA concentrations were obtained during each weekly session for a total of 3 weeks, and for patient 2, three MPA concentrations were obtained during each thrice-weekly session for a total of 1 week. For each patient, 5 mL blood samples were collected immediately preceding and after PP. A 10 mL plasmapheresate sample was collected at the conclusion of PP with MPA concentrations assayed within 24 hr. Concentrations of MPA in serum and plasmapheresate were determined using high-pressure liquid chromatography with diode array detection (Agilent, Santa Clara, CA) and assayed at the hospital laboratory. Duration of PP, total blood volume, hematocrit, plasma volume, and volume of plasma exchanged were collected (Table 1).TABLE 1: Pharmacometrics of MPA with plasmapheresisPharmacometric Analysis The amount of MPA removed by PP was calculated by the equation: The proportion of total body MPA removed by PP was estimated by the equation: To approximate the percentage of daily MPA dose removed by PP, we assumed 94% absorption (4, 5). Using this assumption and knowing that PP was conducted under steady-state conditions, the approximation of daily MPA dose removed by PP was: To evaluate plasma concentration profiles of MPA in the time interval between pre-PP and post-PP, we calculated the elimination rate constant (KE), terminal half-life based on the following equation: Minor variations in serum concentrations between PP sessions was likely due to inherent variability in MPA pharmacokinetics related to secondary peaks as a consequence of enterohepatic circulation, but may also reflect redistribution from tissue. Because of patient constraints, we were unable to assess AUC measurements, and we acknowledge the controversy of utility of trough concentrations compared with AUC measurements as an assessment of total MPA exposure. However, we sought to determine whether PP affected MPA concentrations, and this is the first report to show that exchange of approximately 3 L of plasma did not significantly alter post-PP serum concentrations of MPA during PP sessions in two female patients. Based on typical trough steady-state calculations before initiation of PP of 1 to 5 mg/L, it was estimated that 10.5 to 12 mg of MPA would be removed per PP. This is similar to the average amount actually removed: 11.6 mg per PP, equivalent to approximately 0.5% of the bioavailable daily MPA dose or 2.5% of body stores. Dose escalation was, therefore, unnecessary to counteract the loss of MPA during PP. Angela Q. Maldonado1,2 Neal M. Davies3 Stacy A. Crow1 Cindy Little4 Okechukwu N. Ojogho2 Douglas L. Weeks1 1Department of Pharmacotherapy Washington State University Spokane, WA 2Kidney Transplant Program Providence Sacred Heart Medical Center and Children's Hospital Spokane, WA 3Department of Pharmaceutical Sciences Washington State University Pullman, WA 4Inland Northwest Blood Center Spokane, WA

The Evolution of Population Pharmacokinetic Models to Describe the Enterohepatic Recycling of Mycophenolic Acid in Solid Organ Transplantation and Autoimmune Disease

With the increasing use of mycophenolic acid (MPA) as an immunosuppressant in solid organ transplantation and in treating autoimmune diseases such as systemic lupus erythematosus, the need for strategies to optimize therapy with this agent has become increasingly apparent. This need is largely based on MPA's significant between-subject and between-occasion (within-subject) pharmacokinetic variability. While there is a strong relationship between MPA exposure and effect, the relationship between drug dose, plasma concentration and exposure (area under the concentration-time curve [AUC]) is very complex and remains to be completely defined. Population pharmacokinetic models using various approaches have been proposed over the past 10 years to further evaluate the pharmacokinetic and pharmacodynamic behaviour of MPA. These models have evolved from simple one-compartment linear iterations to complex multi-compartment versions that try to include various factors, which may influence MPA's pharmacokinetic variability, such as enterohepatic recycling and pharmacogenetic polymorphisms. There have been major advances in the understanding of the roles transport mechanisms, metabolizing and other enzymes, drug-drug interactions and pharmacogenetic polymorphisms play in MPA's pharmacokinetic variability. Given these advances, the usefulness of empirical-based models and the limitations of nonlinear mixed-effects modelling in developing mechanism-based models need to be considered and discussed. If the goal is to individualize MPA dosing, it needs to be determined whether factors which may contribute significantly to variability can be utilized in the population pharmacokinetic models. Some pharmacokinetic models developed to date show promise in being able to describe the impact of physiological processes such as enterohepatic recycling. Most studies have historically been based on retrospective data or poorly designed studies which do not take these factors into consideration. Modelling typically has been undertaken using non-controlled therapeutic drug monitoring data, which do not have the information content to support the development of complex mechanistic models. Only a few recent modelling approaches have moved away from empiricism and have included mechanisms considered important, such as enterohepatic recycling. It is recognized that well thought-out sampling schedules allow for better evaluation of the pharmacokinetic data. It is not possible to undertake complex absorption modelling with very few samples being obtained during the absorption phase (which has often been the case). It is important to utilize robust AUC monitoring which is now being propagated in the latest consensus guideline on MPA therapeutic drug monitoring. This review aims to explore the biological factors that contribute to the clinical pharmacokinetics of MPA and how these have been introduced in the development of population pharmacokinetic models. An overview of the processes involved in the enterohepatic recycling of MPA will be provided. This will summarize the components that complicate absorption and recycling to influence MPA exposure such as biotransformation, transport, bile physiology and gut flora. Already published population pharmacokinetic models will be examined, and the evolution of these models away from empirical approaches to more mechanism-based models will be discussed.

New Insights Into the Pharmacokinetics and Pharmacodynamics of the Calcineurin Inhibitors and Mycophenolic Acid: Possible Consequences for Therapeutic Drug Monitoring in Solid Organ Transplantation

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

Simultaneous determination of mycophenolic acid and its glucuronides in human plasma using isocratic ion pair high-performance liquid chromatography

Therapeutic drug monitoring (TDM) of mycophenolic acid (MPA) following administration of mycophenolate mofetil (MMF) or the enteric-coated sodium salt of MPA formulations, seems beneficial because of the large intra- and inter-individual variability in MPA pharmacokinetics. MPA is an active component from these oral formulations and are further metabolised to inactive phenolic glucuronide (MPAG) and active acyl glucuronide (AcMPAG). This study aims to determine simultaneously these three metabolites of MMF using isocratic ion pair HPLC and to evaluate the short-term stability of AcMPAG in human plasma. Samples were prepared using solid phase extraction. Chromatographic separation was achieved over an RP column (TSKgel ® ODS-80Ts, 150 mm × 4.6 mm i.d., 5 μm particle size) with acetonitrile and 30 mM tetra- n-butylammonium bromide containing 5 mM ammonium acetate at pH 9.0 (33/67, v/v) as the mobile phase. The flow rate of the mobile phase was 1 ml/min, and the wavelength of determination by UV detection was 250 nm (run time, 16 min). Calibration curves for MPA, MPAG and AcMPAG in human plasma were linear over a concentration range of 0.05–50, 0.1–400 and 0.08–8 μg/ml, respectively. Intra- and inter-assay R.S.D. were < 6.5%. Extraction efficiencies were more than 85% for all analytes. Since AcMPAG was unstable in human plasma, plasma acidification was needed for the quantification of AcMPAG. Large interindividual variability was observed in the AcMPAG pharmacokinetics in the early period after renal transplantation. In conclusion, a simple, accurate and reproducible HPLC method to measure simultaneously these three MMF metabolites has been established. The method will be helpful in evaluating pharmacokinetics of MPA and its glucuronides.

Limited Sampling Strategy for Estimating Area under the Concentration Curve for Mycophenolic Acid in Renal Transplant Recipients with Co-administration of Tacrolimus.

To obtain an adequate immunosuppressive effect of mycophenolate mofetil (MMF), various equations have been used for estimating the area under the concentration curve (AUC) for mycophenolic acid (MPA) under a limited sampling strategy. Most of them, however, involved cyclosporine (CYA) -based immunosuppression. In this study, we measured chronological blood concentrations of MPA when included in tacrolimus (TAC, alternative calcineurin inhibitor) -based triple-drug immunosuppression in renal transplant recipients and investigated the effectiveness of a limited sampling strategy which minimized patients' inconvenience.Measurement of MPA concentrations in consecutive samplings over a 24-hour period in 6 renal transplant recipients receiving TAC-based triple-drug immunosuppression therapy (MMF dose : 1250-2000 mg/day, twice a day), showed substantial inter-patient and intra-patient variability in MPA pharmacokinetics. The second MPA peak was observed in 8 concentration profiles. There were no significant differences in Tmax, Tlag, dose-normalized Cmax, pre-dose concentration (C0), and AUC (0-12) (AUC (τ)) between morning and evening dosing.Regarding a relationship between AUC (τ) and the MPA concentrations for each sampling point, despite the substantial variation in MPA pharmacokinetics, a significant correlation was found between C0 and AUC (τ) (r2= 0.734). The highest r2 value was obtained at C8 (r2=0.852). The optimum combination of sampling time points for estimating AUC for MPA using stepwise linear regression was C0, C3, C8 (r2= 0.964) and this combination had the lowest maximum prediction error (22.6%) in the estimation of AUC (τ). From the above findings, we recommend the use of the formula AUC (τ) (μg·h/ mL) = 7.82 +3.58· C0 + 1.87· C3 + 7.15 ·C8 for estimating the AUC for MPA in renal transplant recipients also receiving TAC.

Automated determination of free mycophenolic acid and its glucuronide in plasma from renal allograft recipients.

Mycophenolic acid, the active moiety of mycophenolate mofetil, inhibits the enzyme inosine monophosphate dehydrogenase. The main metabolite, mycophenolic acid glucuronide, has no immunosuppressive effect. Reported protein bindings are 97% for mycophenolic acid and 82% for mycophenolic acid glucuronide. Considerable intraindividual and interindividual variability in mycophenolic acid pharmacokinetics has been observed. Data on the variability of mycophenolic acid free fraction in plasma are sparse but may be relevant when discussing whether therapeutic drug monitoring of this drug is warranted. The authors describe a fully automated method for the determination of free concentrations by dialysis across a membrane followed by concentration of the dialysate on a trace enrichment column and liquid chromatography. Total concentrations are measured by protein precipitation and direct injection on the trace enrichment column. Plasma concentrations as low as 6 ng/mL free mycophenolic acid and 1 microg/mL free mycophenolic acid glucuronide can be measured with between-day coefficient of variation less than 15% and 6%, respectively. Stability testing confirmed that plasma samples could be stored for 14 days at 4 degrees C or -20 degrees C and at room temperature for approximately 12 hours without significant changes in free concentrations. Predose total and free concentrations of mycophenolic acid and mycophenolic acid glucuronide were determined in 27 samples from stable renal allograft recipients treated with mycophenolate mofetil, cyclosporin, and steroids. Total concentrations ranged from 0.57 to 16.2 microg/mL mycophenolic acid and 36 to 199 microg/mL mycophenolic acid glucuronide. Free concentrations ranged from 13 to 210 ng/mL mycophenolic acid and 8 to 58 microg/mL mycophenolic acid glucuronide. The method presented here has been successfully applied to measure free mycophenolic acid and free mycophenolic acid glucuronide in clinical samples. Further investigations may provide important data to support the identification of principles and target ranges for the monitoring of mycophenolic acid in the immunosuppressive therapy of organ transplant recipients.