Investigation on pharmacokinetics of mycophenolic acid in Chinese adult renal transplant patients

A Systematic Review of Multiple Linear Regression-Based Limited Sampling Strategies for Mycophenolic Acid Area Under the Concentration-Time Curve Estimation

Background and Objective

One approach of therapeutic drug monitoring in the case of mycophenolic acid (MPA) is a limited sampling strategy (LSS), which allows the evaluation of the area under the concentration–time curve (AUC) based on few concentrations. The aim of this systematic review was to review the MPA LSSs and define the most frequent time points for MPA determination in patients with different indications for mycophenolate mofetil (MMF) administration.

Methods

The literature was comprehensively searched in July 2021 using PubMed, Scopus, and Medline databases. Original articles determining multiple linear regression (MLR)-based LSSs for MPA and its free form (fMPA) were included. Studies on enteric-coated mycophenolic sodium, previously established LSS, Bayesian estimator, and different than twice a day dosing were excluded. Data were analyzed separately for (1) adult renal transplant recipients, (2) adults with other than renal transplantation indication, and (3) for pediatric patients.

Results

A total of 27, 17, and 11 studies were found for groups 1, 2, and 3, respectively, and 126 MLR-based LSS formulae (n = 120 for MPA, n = 6 for fMPA) were included in the review. Three time-point equations were the most frequent. Four MPA LSSs: 2.8401 + 5.7435 × C0 + 0.2655 × C0.5 + 1.1546 × C1 + 2.8971 × C4 for adult renal transplant recipients, 1.783 + 1.248 × C1 + 0.888 × C2 + 8.027 × C4 for adults after islet transplantation, 0.10 + 11.15 × C0 + 0.42 × C1 + 2.80 × C2 for adults after heart transplantation, and 8.217 + 3.163 × C0 + 0.994 × C1 + 1.334 × C2 + 4.183 × C4 for pediatric renal transplant recipients, plus one fMPA LSS, 34.2 + 1.12 × C1 + 1.29 × C2 + 2.28 × C4 + 3.95 × C6 for adult liver transplant recipients, seemed to be the most promising and should be validated in independent patient groups before introduction into clinical practice. The LSSs for pediatric patients were few and not fully characterized. There were only a few fMPA LSSs although fMPA is a pharmacologically active form of the drug.

Conclusions

The review includes updated MPA LSSs, e.g., for different MPA formulations (suspension, dispersible tablets), generic form, and intravenous administration for adult and pediatric patients, and emphasizes the need of individual therapeutic approaches according to MMF indication. Five MLR-based MPA LSSs might be implemented into clinical practice after evaluation in independent groups of patients. Further studies are required, e.g., to establish fMPA LSS in pediatric patients.

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.

Effect of Protein Binding on Exposure of Unbound and Total Mycophenolic Acid: A Population Pharmacokinetic Analysis in Chinese Adult Kidney Transplant Recipients

OBJECTIVES

The population pharmacokinetic (popPK) characteristics of total mycophenolic acid (tMPA) have been investigated in various ethnic populations. However, investigations of popPK of unbound MPA (uMPA) are few. Thus, a popPK analysis was performed to: (1) characterize the PK of uMPA and tMPA and its 7-O-mycophenolic acid glucuronide (MPAG) metabolite in kidney transplant patients cotreated with cyclosporine (CsA), and (2) identify the clinically significant covariates that explain variability in the dose-exposure relationship.

METHODS

A total of 740 uMPA, 741 tMPA, and 734 total MPAG (tMPAG) concentration-time data from 58 Chinese kidney transplant patients receiving MPA in combination with CsA were analyzed using NONMEM® software with the stochastic approximation expectation maximization (SAEM) followed by the important sampling (IMP) method. The influence of covariates was tested using a stepwise procedure.

RESULTS

The PK of uMPA and unbound MPAG (uMPAG) were characterized by a two- and one-compartment model with first-order elimination, respectively. A linear protein binding model was used to link uMPA and tMPA. Apparent clearance (CL/F) and central volume of distribution (VC/F) of uMPA (CLuMPA/F and VCuMPA/F, respectively) and protein binding rate constant (k B) were estimated to be 851 L/h [relative standard error (RSE), 7.1%], 718 L (18.5%) and 53.4/h (2.3%), respectively. For uMPAG, the population values (RSE) of CL/F (CLuMPAG) and VC/F (VCuMPAG/F) were 5.71 L/h (4.4%) and 29.9 L (7.7%), respectively. Between-subject variability (BSVs) on CLuMPA/F, VCuMPA/F, CLuMPAG/F, and VCuMPAG/F were 51.0, 80.0, 31.8 and 48.4%, respectively, whereas residual unexplained variability (RUVs) for uMPA, tMPA, and uMPAG were 47.0, 45.9, and 22.0%, respectively. Significant relationships were found between k B and serum albumin (ALB) and between CLuMPAG/F and glomerular filtration rate (GFR). Additionally, model-based simulation showed that changes in ALB concentrations substantially affected tMPA but not uMPA exposure.

CONCLUSIONS

The established model adequately described the popPK characteristics of the uMPA, tMPA, and MPAG. The estimated CLuMPA/F and unbound fraction of MPA (FUMPA) in Chinese kidney transplant recipients cotreated with CsA were comparable to those published previously in Caucasians. We recommend monitoring uMPA instead of tMPA to optimize mycophenolate mofetil (MMF) dosing for patients with lower ALB levels.

Population pharmacokinetics and Bayesian estimation of mycophenolic acid concentrations in Chinese adult renal transplant recipients

Mycophenolate mofetil (MMF) is an important immunosuppressant used in renal transplantation, and mycophenolic acid (MPA) is the active component released from the ester prodrug MMF. The objective of this study was to investigate the population pharmacokinetics of mycophenolic acid (MPA) following oral administration of MMF in Chinese adult renal transplant recipients and to identify factors that explain MPA pharmacokinetic variability. Pharmacokinetic data for MPA and covariate information were retrospectively collected from 118 patients (79 patients were assigned to the group for building the population pharmacokinetic model, while 39 patients were assigned to the validation group). Population pharmacokinetic data analysis was performed using the NONMEM software. The pharmacokinetics of MPA was best described by a two-compartment model with a first-order absorption rate with no lag time. Body weight and serum creatinine level were positively correlated with apparent clearance (CL/F). The polymorphism in uridine diphosphate glucuronosyltransferase gene, UGT2B7, significantly explained the interindividual variability in the initial volume of distribution (V₁/F). The estimated population parameters (and interindividual variability) were CL/F 18.3 L/h (34.2%) and V₁/F 27.9 L (21.3%). The interoccasion variability was 13.7%. These population pharmacokinetic data have significant clinical value for the individualization of MMF therapy in Chinese adult renal transplant patients.

Limited sampling strategy for the estimation of mycophenolic acid area under the curve in Tunisian renal transplant patients

Mycophenolate mofetil is a prodrug widely used in renal transplantation to prevent organ rejection. It is hydrolyzed to its active compound mycophenolic acid (MPA). MPA area under the curve (AUC_0-12h) is considered the best pharmacokinetic parameter for the estimation of MPA exposition and for prediction of rejection. MPA-AUC requires several blood samples, making it impractical for clinical practice. Therefore, development of a limited sampling strategy (LSS) to estimate MPA AUC_0-12h using three blood samples is very helpful for MPA individual dose adjustment. Results of LSS differ according to the patient background and to the drug formulation. Therefore, the purpose of this study was to develop a LSS for the estimation of MPA AUC_0-12h in Tunisian renal transplant patients treated with the generic formulation of mycophenolate mofetil (MMF^®, MEDIS). The best correlation was achieved by a profile based on three time points C_0.5h, C_1.5h, and C_4h after drug intake: AUC_0-12h = 0.414 + 1.210 × C_0.5 + 2.256 × C_1.5 + 4.134 × C₄ (mei = 1.65% and rmse = 5.81%). The correlation between full AUC_0-12h and abbreviated AUC_0-12h was 0.917. In conclusion, this model provides a reliable and simple equation to estimate MPA AUC_0-12h for the generic formulation of mycophenolate mofetil (MMF^®).

Short-term therapeutic drug monitoring of mycophenolic acid reduces infection: a prospective, single-center cohort study in Chinese living-related kidney transplantation

BACKGROUND

The role of therapeutic drug monitoring (TDM) of mycophenolic acid (MPA) in kidney transplant recipients (KTRs) is not clear. We performed a prospective cohort study to evaluate the efficiency of MPA TDM in the Chinese population.

METHODS

A total of 183 living-related KTRs were studied; 101 KTRs received controlled-dose mycophenolate mofetil (MMF) (the CD group), and 82 patients received fixed-dose MMF (the FD group). MPA exposure was measured at days 3, 7, 14, and 30 in the CD group, and at day 30 in the FD group. The primary endpoint was treatment failure (a composite of acute rejection, graft loss, death, or MMF discontinuation) at 12 months post transplantation.

RESULTS

In the CD group, with a starting MMF dose of 2 g/day, approximately 35% of patients had high MPA levels, which were >60 mg × h/L, and mean MPA levels were 59.17 mg × h/L and 61.38 mg × h/L for the CD and FD groups, respectively (P = 0.588). After adjusting MMF dose, MPA exposures in the CD group at day 30 were lower than those in the FD group at day 30 (54.06 vs. 61.38, P = 0.004). At month 12, the CD group had fewer infections (16.8% vs. 31.7%, P = 0.018) with no difference in treatment failure, acute rejection, diarrhea, or anemia.

CONCLUSIONS

KTRs can benefit from short-term TDM of MPA in reducing infection, without increasing acute rejection.

Limited sampling strategy models for estimating the AUC of gliclazide in Chinese healthy volunteers

The aim of this work is to reduce the cost of required sampling for the estimation of the area under the gliclazide plasma concentration versus time curve within 60 h (AUC0-60t ). The limited sampling strategy (LSS) models were established and validated by the multiple regression model within 4 or fewer gliclazide concentration values. Absolute prediction error (APE), root of mean square error (RMSE) and visual prediction check were used as criterion. The results of Jack-Knife validation showed that 10 (25.0 %) of the 40 LSS based on the regression analysis were not within an APE of 15 % using one concentration-time point. 90.2, 91.5 and 92.4 % of the 40 LSS models were capable of prediction using 2, 3 and 4 points, respectively. Limited sampling strategies were developed and validated for estimating AUC0-60t of gliclazide. This study indicates that the implementation of an 80 mg dosage regimen enabled accurate predictions of AUC0-60t by the LSS model. This study shows that 12, 6, 4, 2 h after administration are the key sampling times. The combination of (12, 2 h), (12, 8, 2 h) or (12, 8, 4, 2 h) can be chosen as sampling hours for predicting AUC0-60t in practical application according to requirement.

Development and validation of limited sampling strategies for tacrolimus and mycophenolate in steroid-free renal transplant regimens

PURPOSE

1) To develop and validate limited sampling strategies (LSSs) for tacrolimus (TAC) and mycophenolic acid (MPA) in renal transplant recipients not receiving corticosteroids; and 2) to evaluate predictive performance of published LSSs (for steroid-based regimens) in a steroid-free population.

METHODS

On administration of steady-state morning TAC and mycophenolate mofetil doses, 12-hour serial blood samples from 28 stable renal transplant recipients were collected and measured by validated high-performance liquid chromatography methods and area under the curve (AUC) by trapezoidal rule. TAC LSSs were developed and validated by multiple regression analysis by a two-group method (index n = 18; validation n = 10) and MPA LSSs by the jackknife method (n = 28). Potential LSSs were those with r ≥ .8 (TAC) or r ≥ 0.7 (MPA) and < 3 time points within 2 hours (TAC) or 4 hours (MPA) postdose. Predictive performance was calculated and other published TAC and MPA LSSs tested using preset criteria for bias and precision of within ± 15%.

RESULTS

For TAC, three three-concentration, one two-concentration, and one one-concentration model met preset criteria. The best equations were: TAC AUC = 10.338 + 7.739C0 + 3.589C2 (r = 0.956, bias = -3.4%, precision = 4.7%) and TAC AUC = 29.479 + 5.016C2 (r = 0.862, bias = 3.2%, precision = 9.7%). For MPA, only one model was identified: MPA AUC = 9.328 + 1.311C1 + 1.455C2 + 2.901C4 (r = 0.838, bias = -3.8%, precision = 14.9%). One published TAC (and no MPA) LSS in renal transplant recipients on steroid-based regimens met criteria.

CONCLUSIONS

To the authors' knowledge, these LSSs are the first to be developed and validated in steroid-free renal transplant recipients and can be used to accurately predict TAC and MPA AUCs for steroid-free regimens. Because the commonly used MPA LSS is based on a steroid regimen and not predictive for steroid-free patients, the newly derived MPA LSS is being applied at the authors' institution. Other renal transplant centers may also wish to validate this equation in their own patients.

Bioequivalence and pharmacokinetic comparison of two mycophenolate mofetil formulations in healthy Chinese male volunteers: an open-label, randomized-sequence, single-dose, two-way crossover study

BACKGROUND

Mycophenolate mofetil (MMF) is an ester prodrug of mycophenolic acid (MPA), so clinical studies measure the circulating plasma MPA concentration instead of MMF. MPA is extensively glucuronidated by several uridine diphosphate glycosyltransferases into an inactive 7-O-glucuronide and a pharmacologically active acylglucuronide. Considering the effect of racial differences and genetic factors on the pharmacokinetic (PK) properties of drugs, it is necessary to study them in Chinese populations.

OBJECTIVES

The aim of this study was to compare the clinical bioequivalence and PK properties of a test (dispersible tablets) and reference (capsules) formulation of MMF 1.0 g in healthy Chinese volunteers. We also established a validated HPLC method for the determination and quantification of MPA in human plasma. The study was required to obtain Chinese regulatory approval for the test formulation.

METHODS

This open-label, randomized-sequence, single-dose, 2-way crossover study was conducted at the First Hospital of Nanjing Medical University, Nanjing, China. Eligible subjects were healthy male volunteers who were randomly assigned at a 1:1 ratio to receive a single 1.0-g dose of the test or reference formulation, followed by a 1-week washout period and administration of the alternate formulation. The plasma concentration of MPA, which is the active metabolite of MMF, was determined using a validated HPLC method. For analysis of PK properties, blood samples were collected at 0, 10, 20, 30, and 45 minutes, and 1, 1.5, 3, 5, 8, 11, 18, 36, and 48 hour(s). The PK parameters, including C(max), T(max), t((1/2)), AUC(0-48), and AUC(0-infinity), were determined from the plasma concentrations of the 2 formulations by noncompartmental analysis. Tolerability was assessed at baseline (be- fore administration) and at 30 minutes and 1, 5, 18, and 48 hours after administration by monitoring vital signs. Laboratory tests (hematology, blood biochemistry, hepatic function, and urinalysis) were performed for the identification of adverse events (AEs) (eg, leukopenia, thrombocytopenia, anemia). Patient interviews were conducted to assess the occurrence of AEs such as diarrhea, abdominal pain, nausea, vomiting, and secondary infections. The formulations were considered to meet the regulatory requirements of bioequivalence if the log-transformed ratios of C(max) and AUC were within the predetermined equivalence range (80%-125%) as established by the US Food and Drug Administration (FDA).

RESULTS

Eighteen healthy Chinese male volunteers (mean [range] age, 23.5 [22-30] years; weight, 63.3 [56-68] kg; height, 171 [165-184] cm) were enrolled and completed the study. The main PK parameters of the MMF test and reference formulations were as follows: mean (SD) T(max), 0.68 (0.21) and 0.81 (0.18) hour, respectively; C(max), 25.58 (4.79) and 26.47 (3.67) mg/L; AUC(0-48), 59.19 (9.23) and 58.32 (9.28) mg/L/h; t((1/2)), 15.12 (3.17) and 16.04 (4.22) hours; AUC(0-infinity)), 63.28 (10.23) and 62.41 (10.28) mg/L/h. The mean (SD) relative bioavailability was 101.5% (10.3%). No statistically significant differences were found based on ANOVA. The ratios of C(max) (0.97) and AUC (1.01) for the test and reference formulations were within the FDA bioequivalence definition intervals of 80% to 125%. No AEs were reported by subjects or found on analysis of vital signs or laboratory tests.

CONCLUSIONS

In this study in healthy Chinese male volunteers, results from the PK analysis suggested that a single dose of the test and reference formulations of MMF 1.0 g met the regulatory requirements of bioequivalence, based on the FDA regulatory definition (rate and extent of absorption). Both formulations were well tolerated.

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.

Early phase limited sampling strategy characterizing tacrolimus and mycophenolic acid pharmacokinetics adapted to the maintenance phase of renal transplant patients

The aim of this study was to examine whether a limited sampling strategy (LSS) to allow the simultaneous estimation of the area under the concentration-time curves (AUCs) of tacrolimus and mycophenolic acid (MPA) calculated in the early stage after renal transplantation could be applied to maintenance phase pharmacokinetics. Seventy Japanese patients were enrolled. One year after transplantation, samples were collected just before and 1, 2, 3, 4, 6, 9, and 12 hours after tacrolimus and mycophenolate mofetil administration at 9:00 am and at 9:00 pm. The prediction formulas on day 28 (tacrolimus AUC 0-12 = 7.04 x C 0h + 1.71 x C 2h + 3.23 x C 4h + 15.19 and 2.25 x C 2h + 1.92 x C 4h + 7.27 x C 9h + 6.61, and MPA AUC 0-12 = 0.26 x C 0h + 2.06 x C 2h + 3.82 x C 4h + 20.38 and 1.77 x C 2h + 2.34 x C 4h + 4.76 x C 9h + 15.94) were applied to pharmacokinetic data obtained at 1 year. Three error indices [percent mean prediction error (ME), % mean absolute error, and percent root mean squared prediction error (RMSE)] were used to evaluate the predictive bias, accuracy, and precision. The predicted AUC 0-12 of tacrolimus and MPA at 3 time points, C 2h-C 4h-C 9h, showed higher correlation with the measured AUC 0-12 of tacrolimus and MPA (r2 = 0.817 and 0.789, respectively) in comparison with those at C 0h-C 2h-C 4h. The values for the prediction formulas for tacrolimus AUC at 1 year using the C 2h-C 4h-C 9h combination yielded less than 5% for %ME and 15% for %RMSE. The %ME and %RMSE values of the prediction formulas for tacrolimus AUC using the C 0h-C 2h-C 4h combination were 6.3% and 15.9%, respectively. The %ME and %RMSE values of the prediction formulas for MPA AUC at 1 year using the C 0h-C 2h-C 4h combination were 5.9% and 25.8%, respectively, and those for the C 2h-C 4h-C 9h combination were 4.9% and 21.2%, respectively. AUC 6-12/AUC 0-12 of MPA 1 year after transplantation was significantly lower than 28 days after transplantation. An LSS using C 2h-C 4h-C 9h seems to be applicable for predicting the AUC of tacrolimus and MPA at either posttransplantation stage. The enterohepatic circulation of MPA was significantly reduced 1 year after transplantation. Therefore, 1 year after transplantation, the estimation of the AUC 0-12 of MPA for the C 0h-C 2h-C 4h equations was imprecise. It is important that the LSS includes C 9h because it contains information on the secondary plasma peak of MPA.

Limited sampling strategies for mycophenolic acid in solid organ transplantation: a systematic review

BACKGROUND

Mycophenolic acid (MPA) is the active metabolite of mycophenolate mofetil, a widely used immunosuppressant. Numerous studies have developed limited sampling strategies (LSSs) to predict MPA AUC in solid organ transplant recipients.

OBJECTIVES

To systematically review and assess quality of literature pertaining to MPA LSSs, evaluate clinical implications and provide suggestions for future research.

METHODS

Literature searches of MEDLINE (1966 - May 2009) and EMBASE (1980 - May 2009) for English articles in solid organ transplantation, along with manual review of article references were conducted. Included articles were categorized according to criteria adapted from levels of evidence of the US Preventative Services Task Force.

RESULTS

Of a total of 29 studies identified, 20 were in kidney, 4 in heart, 4 in liver and 1 in lung transplantation and 7 were in pediatrics. A total of 14 studies were deemed to be Level I evidence studies, 3 were Level II-1, 1 was Level II-2 and 11 were Level III.

CONCLUSIONS

Although various LSSs that are well correlated to MPA AUC while being relatively unbiased and precise to predict MPA AUC have been developed, further research is needed to determine validity of these LSSs in a variety of patient populations and to determine if these LSSs improve patient outcomes.

Limited sampling strategies for predicting area under the concentration-time curve of mycophenolic acid in islet transplant recipients

BACKGROUND

Mycophenolate mofetil is widely used in islet transplant recipients and its active metabolite, mycophenolic acid (MPA), exhibits wide pharmacokinetic variability. However, to our knowledge, no limited sampling strategy (LSS) exists for monitoring MPA in this subpopulation.

OBJECTIVE

To define optimal LSSs for MPA monitoring and to test their predictive performance in islet transplant recipients.

METHODS

After written informed consent was obtained and upon administration of a steady-state morning mycophenolate mofetil dose, blood samples were collected at 0, 0.3, 0.6, 1, 1.5, 2, 3, 4, 6, 8, 10, and 12 hours from 16 stable islet transplant recipients. MPA concentrations were measured by a validated high-performance liquid chromatography method with ultraviolet detection and pharmacokinetic parameters analyzed by noncompartmental modeling. All 16 patients' profiles were used to develop the LSSs via multiple regression analysis. Potential LSSs were restricted to ones having R(2) 0.90 or greater and 3 or fewer time points within the first 4 hours postdose. Resulting equations were validated for their predictive performance using the jackknife method, with acceptable criteria for bias and precision preset to within +/-15%. In addition, 14 published LSSs (in the renal transplant population) were tested in our islet transplant patients.

RESULTS

Five LSSs met preset criteria and had conventional sampling times: AUC = 1.783 + 1.248C1 + 0.888C2 + 8.027C4 (R2 = 0.98, bias = -3.09%, precision = 9.53%) AUC = 2.778 + 1.413C1 + 0.963C3 + 7.511C4 (R2 = 0.97, bias = -3.22%, precision = 11.02%) AUC = 1.448 + 1.239C1 + 0.271C1.5 + 9.108 C4 (R2 = 0.96, bias = -1.90%, precision = 11.46) AUC = 1.410 - 0.259C0 + 1.443C1 + 9.622C4 (R2 = 0.96, bias = -2.68%, precision = 11.53%) AUC = 1.547 + 1.417C1 + 9.448C4 (R2 = 0.96, bias = -2.46%, precision = 11.14%) where AUC = area under the concentration-time curve. None of the other published LSSs in the renal transplant population met the preset criteria for bias and precision.

CONCLUSIONS

To our knowledge, these are the first precise and accurate LSSs for predicting MPA AUC developed specifically for islet transplant recipients. The LSS that we recommend is the one utilizing 2 concentrations: AUC = 1.547 + 1.417C1 + 9.448C4. This equation is convenient and clinically feasible. Other islet transplant centers may wish to validate our equation in their population or use our template as a guide to develop accurate and precise LSSs specific to their patient population.

Limited sampling strategy for simultaneous estimation of the area under the concentration-time curve of tacrolimus and mycophenolic acid in adult renal transplant recipients

The aim of this study was to develop a limited sampling strategy to allow the simultaneous estimation of the area under the concentration-time curves (AUCs) of tacrolimus and mycophenolic acid (MPA), the active metabolite of the prodrug mycophenolate mofetil, using a small number of samples from patients undergoing renal transplantation. Fifty Japanese patients were enrolled. On day 28 after transplantation, samples were collected just before and 1, 2, 3, 4, 6, 9, and 12 hours after tacrolimus and mycophenolate mofetil administration at 9:00 am and 9:00 pm. The full pharmacokinetic profiles obtained from these timed concentration data were used to choose the best sampling times. Three error indices (percent mean error, percent mean absolute error, and percent relative mean square error) were used to evaluate the predictive bias, accuracy, and precision. The predicted AUC0-12 of MPA calculated at the three time points of C2h-C4h-C9h best approximated the actual AUC0-12 of MPA (r = 0.877), and the AUC0-12 of tacrolimus calculated at the same time points predicted a good correlation with the actual AUC (r = 0.928). When the three sampling times of trough level (C0h) and two other points within 4 hours after administration were used, the three points of C0h-C2h-C4h were the best points for estimation of the AUC0-12 tacrolimus and MPA (AUC0-12 = 7.04.C0 + 1.71.C2 + 3.23.C4 + 15.19, r = 0.799, P < 0.001 and AUC0-12 = 0.26.C0 + 2.06.C2 + 3.82.C4 + 20.38, r = 0.693, P < 0.001, respectively). The percent mean error, percent mean absolute error, and percent relative mean square error of the prediction formula using the three time points of C0h-C2h-C4h were -0.3%, 8.8%, and 13.5% for tacrolimus and 2.9%, 17.1%, and 21.5% for MPA, respectively. A limited sampling strategy using C2h-C4h-C9h provides the most reliable and accurate simultaneous estimation of the AUC0-12 of tacrolimus and MPA in patients undergoing renal transplantation. In addition, a limited sampling strategy using C0h-C2h-C4h is recommended for the simultaneous estimation of the AUC0-12 of tacrolimus and MPA when focused on samples collected within 4 hours after administration for clinical expediency.

Evaluation of the practicability of limited sampling strategies for the estimation of mycophenolic acid exposure in Chinese adult renal recipients

The immunosuppressive potential of mycophenolic acid (MPA) correlates well with MPA exposure [area under the concentration-time curve (AUC)]. Monitoring MPA AUC is important and helpful for maintaining the efficacy of mycophenolate mofetil while minimizing its side effects, but full MPA AUC monitoring is laborious, cost prohibitive, and impractical. Limited sampling strategies have been proposed as an alternative method for estimating MPA exposure. The objective of this study was to evaluate the practicability of different limited sampling strategies for the estimation of MPA exposure. A total of 56 pharmacokinetic profiles from 53 adult renal recipients were used to evaluate the practicability of 10 published models. Standard correlation and linear regression analysis were used to compare the estimated MPA AUCs and corresponding full MPA AUCs, and the percentage of profiles for which prediction error fell within +/-20% was also used to assess the practicability of these models. Agreement between the estimated MPA AUCs and full MPA AUCs was further tested by Bland and Altman analysis. The model, based on four sampling time points, used the formula AUC = 12.61 + 0.37 x C0.5 + 0.49 x C1 + 3.22 x C4 + 8.17 x C10, was superior to all other evaluated models, with the highest coefficient of determination (r = 0.88), a low percentage prediction error (2.79%), and good agreement according to Bland and Altman analysis. Prediction errors of 87.5% (49/56) of profiles were within 20%, which was the highest of all the models. This algorithm can be reliably used for estimating MPA exposure in adult renal transplant patients treated with cyclosporine as concomitant immunosuppressant. Another model based on the formula AUC = 8.22 + 3.16 x C0 + 0.99 x C1 + 1.33 x C2 + 4.18 x C4 also has acceptable predictive performance, and it may also be practical, especially in outpatient settings, in view of its distribution of time points.

Limited Sampling Strategy for the Estimation of Mycophenolic Acid Area Under the Plasma Concentration-Time Curve in Adult Patients Undergoing Liver Transplant

Mycophenolate mofetil (MMF), the oral prodrug of mycophenolic acid (MPA), is increasingly used in liver transplantation and plays a central role in the immunosuppressive regimen in liver transplantation. To study pharmacokinetic-pharmacodynamic relationships and therapeutic drug monitoring of MPA in the clinical setting, limited sampling strategies have been investigated for the estimation of MPA areas under the curves (AUCs). Thirty-eight adult patients undergoing liver transplant (31 males, seven females) receiving 1.0 g MMF twice daily and concomitant tacrolimus provided a total of 72 pharmacokinetic profiles. Multiple stepwise regression analysis was used to determine the algorithms for limited sampling strategies. Twenty-eight one-, two-, three-, and four-sampling estimation models were fitted (r = 0.288-0.964) to all the profiles using linear regression and were used to estimate MPA AUC0-12h comparing those estimates with the corresponding AUC0-12h values calculated with the linear trapezoidal rule, including all 10 timed MPA concentrations. The four-point estimates at C1h, C2h, C6h, and C8h resulted in the best correlation between estimated AUC and true AUC when using the formula AUC = 6.03 + 0.89C1h + 1.94C2h + 2.24C6h + 4.64 C8h (r = 0.911). Bland and Altman analysis revealed good agreement between estimated AUC and AUC from the full profile. This limited sampling strategy provides an effective approach for estimation of full MPA AUC0-12h in patients undergoing liver transplant receiving concomitant tacrolimus therapy.