Correlation of IMPDH1 Gene Polymorphisms with Subclinical Acute Rejection and Mycophenolic Acid Exposure Parameters on Day 28 after Renal Transplantation

Abstract: The risk of acute rejection in patients with higher exposure to mycophenolic acid (MPA), the active metabolite of mycophenolate mofetil (MMF), might be due to inosine 5′‐monophosphate dehydrogenase (IMPDH) polymorphisms. The correlations with subclinical acute rejection, IMPDH1 polymorphisms and MPA exposure on day 28 post‐transplantation were investigated in 82 Japanese recipients. Renal transplant recipients were given combination immunosuppressive therapy consisting of tacrolimus and 1.0, 1.5 or 2.0 g/day of MMF in equally divided doses every 12 hr at designated times. There were no significant differences in the incidence of subclinical acute rejection between IMPDH1 rs2278293 or rs2278294 polymorphisms (p = 0.243 and 0.735, respectively). However, in the high MPA night‐time exposure range (AUC >60 μg·h/ml and C₀ ≥ 1.9 μg/ml), there was a significant difference in the incidence of subclinical acute rejection between IMPDH1 rs2278293 A/A, A/G and G/G genotypes (each p = 0.019), but not the IMPDH1 rs2278294 genotype. In the higher daytime MPA exposure range, patients with the IMPDH1 rs2278293 G/G genotype also tended to develop subclinical acute rejection. In patients with the IMPDH rs2278293 A/A genotype, the risk of subclinical acute rejection episode tends to be low and the administration of MMF was effective. The risk of subclinical acute rejection for recipients who cannot adapt in therapeutic drug monitoring (TDM) of MPA seems to be influenced by IMPDH1 rs2278293 polymorphism. The prospective analysis of IMPDH1 rs2278293 polymorphism as well as monitoring of MPA plasma concentration after transplantation might help to improve MMF therapy.     

Influence of UGT1A8 and UGT2B7 genetic polymorphisms on mycophenolic acid pharmacokinetics in Japanese renal transplant recipients

Objective UGT1A8 and UGT2B7 are important uridine diphosphate-glucuronosyltransferase isoforms for the glucuronidation of mycophenolic acid (MPA). The aim of this investigation was to elucidate MPA pharmacokinetics in UGT1A8 and UGT2B7 genotypes in Japanese renal transplant recipients. Methods Seventy-two recipients received repeated doses of mycophenolate mofetil and tacrolimus. On day 28 after renal transplantation, plasma MPA concentrations were measured for the next 24 h using high-performance liquid chromatography. UGT1A8*2 (A¹⁷³G) and UGT2B7*2 (Y²⁶⁸) were detected using a PCR-RFLP-based procedure. Results There were no significant differences in daytime and nighttime pharmacokinetics of MPA between UGT1A8 or UGT2B7 genotypes. The mean daytime dose-adjusted AUC_0–12 of MPA in UGT1A8*1/*1, *1/*2 and *2/*2 were 2.47, 2.33 and 2.57 ng·h/ml/mg/kg (P = 0.7711), and the mean nighttime AUC_0–12 were 2.15, 2.00 and 2.08 ng·h/ml/mg/kg (P = 0.4656). The mean daytime and nighttime dose-adjusted AUC_0–12 of MPA in UGT2B7*1/*1, *1/*2 and *2/*2 were 2.61, 2.24 and 2.03 ng·h/ml/mg/kg and 2.18, 1.94, and 1.45 ng·h/ml/mg/kg, respectively (P = 0.3475 and 0.2575). The mean nighttime C_max, t_max, and AUC_6–12/AUC_0–12 ratio (enterohepatic circulation and recirculation ratio) of MPA in all UGT1A8 and UGT2B7 genotypes were lower, longer, and higher, respectively, than the daytime values. Conclusions Both UGT1A8 and UGT2B7 allelic variants seem not to affect Japanese interindividual variability for plasma MPA concentration. Regardless of UGT1A8 and UGT2B7 genetic polymorphisms, the absorption of MPA through enterohepatic recirculation is higher at night.     

Inosine monophosphate dehydrogenase variability in renal transplant patients on long-term mycophenolate mofetil therapy

AIMS

Long-term mycophenolate mofetil (MMF) therapy may induce inosine 5′-monophosphate dehydrogenase (IMPDH) activity in peripheral blood mononuclear cells (PBMCs), thus decreasing MMF immunosuppressive properties. Pharmacodynamic monitoring was used to investigate whether biological activity is altered after long-term therapy.

METHODS

IMPDH activity was measured in PBMC samples from 54 stable kidney transplant patients, already on MMF (for at least 3 months), before (t₀) and 2 h after (t₂) MMF morning dose administration; levels were monitored for up to 15 months, together with total mycophenolic acid (MPA) and free MPA concentrations.

RESULTS

During the 15 months'' monitoring, t₀ IMPDH activity in transplant recipients increased from 5.9 ± 3.7 nmol h⁻¹ mg⁻¹[95% confidence interval (CI) 4.9, 6.9] to 9.0 ± 3.9 nmol h⁻¹ mg⁻¹ (95% CI 7.2, 10.8), with an intra- and interpatient variability of 28% and 42%. Five patients experienced acute rejection during the follow-up: t₀ IMPDH activity was increased during rejection vs. nonrejection, and the trend was significantly higher in rejecting than in nonrejecting subjects for the whole monitoring period.

CONCLUSIONS

Even though a correlation has been found between IMPDH activity and rejection, its efficacy as a predictive tool in long-term transplant outcomes may be affected by high interpatient variability; on the other hand, continuous monitoring of the IMPDH trend could make an effective prognostic parameter of rejection. Other trials also including pre-transplant data on both IMPDH expression and activity are warranted to better assess their role as biomarkers for MPA effect in clinical practice.     

Pharmacokinetic and pharmacodynamic analysis of enteric-coated mycophenolate sodium: limited sampling strategies and clinical outcome in renal transplant patients

AIMS

Pharmacokinetic (PK) and pharmacodynamic (PD) monitoring strategies and clinical outcome were evaluated in enteric-coated mycophenolate sodium (EC-MPS)-treated renal allograft recipients.

METHODS

PK [mycophenolic acid (MPA)] and PD [inosine monophosphate dehydrogenase (IMPDH) activity] data were analysed in 66 EC-MPS and ciclosporin A (CsA)-treated renal allograft recipients. Adverse events were considered in a follow-up period of 12 weeks.

RESULTS

Analyses confirmed a limited sampling strategy (LSS) consisting of PK and PD data at predose, 1, 2, 3 and 4 h after oral intake as an appropriate sampling method (MPA r²= 0.812; IMPDH r²= 0.833). MPA AUC_0–12 of patients with early biopsy-proven acute rejection was significantly lower compared with patients without a rejection (median MPA AUC_0–12 28 µg*h ml⁻¹ (7–45) vs. 40 µg*h ml⁻¹ (16–130), P < 0.01), MPA AUC_0–12 of patients with recurrent infections was significantly higher compared with patients without infections (median MPA AUC_0–12 65 µg*h ml⁻¹ (range 37–130) vs. 37 µg*h ml⁻¹ (range 7–120), P < 0.005). Low 12-h IMPDH enzyme activity curve (AEC_0–12) was associated with an increased frequency of gastrointestinal side-effects (median IMPDH AEC_0–12 43 nmol*h mg⁻¹ protein h⁻¹[range 12–67) vs. 75 nmol*h mg⁻¹ protein h⁻¹ (range 15–371), P < 0.01].

CONCLUSIONS

Despite highly variable absorption data, an appropriate LSS might be estimated by MPA AUC_0–4 and IMPDH AEC_0–4 in renal transplant patients treated with EC-MPS and CsA. Regarding adverse events, the suggested MPA-target AUC_0–12 from 30 to 60 µg*h ml⁻¹ seems to be appropriate in renal allograft recipients.     

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

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

Monitoring of inosine monophosphate dehydrogenase activity as a biomarker for mycophenolic acid effect: potential clinical implications

Mycophenolic acid (MPA) is a reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH) and, in combination with other immunosuppressive drugs, effectively inhibits rejection in solid organ transplant recipients. MPA has a relatively narrow therapeutic window and exhibits wide inter- and intrapatient pharmacokinetic (PK) variability. This has stimulated the use of therapeutic drug monitoring as a strategy to tailor the MPA exposure to each patient's individual needs. Despite increasing therapeutic drug monitoring use, PK-assisted dosing is not universally adopted in part because of MPA's complex PK behavior. Targeting inosine monophosphate IMPDH activity as a surrogate pharmacodynamic (PD) marker of MPA-induced immunosuppression may allow for increased precision when used in an integrated PK-PD fashion, providing a more accurate assessment of efficacy and aid in limiting toxicity. IMPDH activity displays wide interpatient variability but relatively small intrapatient variability even after long-term administration of MPA. The advent of calcineurin and corticosteroid-sparing regimens necessitates more patient-specific PK-PD parameters, which can be used throughout the posttransplant period to optimize MPA exposure and immediate and long-term graft and patient outcomes. Quantification of IMPDH posttransplant may serve as a stable, surrogate PD marker of MPA-induced immunosuppression when combined with current PK and monitoring strategies.     

Therapeutic drug monitoring of mycophenolate in adult solid organ transplant patients: an update

Introduction: Mycophenolate (MPA) therapeutic drug monitoring (TDM) in adult solid organ transplant recipients was summarized extensively in consensus reports published between 2009 and 2011. Thus, this review provides an update on the science of MPA TDM over the past 5 years.

Areas covered: PubMed and Google Scholar (January 2010-January 2016) were searched; relevant articles from bibliographies of identified articles were extracted for further review. New evidence on TDM-guided dosing in MPA efficacy and toxicity and best approaches for estimating MPA area-under-the-curve for TDM were retrieved.

Expert opinion: Since 2011, little advancement in consensus on MPA TDM has been established for any type of solid organ transplant. Lack of systematic studies validating or further defining MPA’s target range suggests that routine TDM is still unwarranted.

Accurate, precise, and user-friendly limited sampling strategies (LSSs) are available in specific patient populations taking mycophenolate mofetil but not enteric-coated mycophenolate sodium. In absence of outcome data, routine use of LSSs in MPA TDM still cannot be recommended.

Further research should attempt to define factors that modulate MPA’s pharmacokinetics to elucidate their impact on utility of TDM. Future studies should also validate LSSs in larger patient populations and demonstrate benefits of LSSs in improving patient outcomes.     

Enteric-coated mycophenolate sodium for transplant immunosuppression.

PURPOSE: The pharmacology, pharmacokinetics, drug interactions, clinical efficacy, adverse effects, monitoring, and dosage and administration of enteric-coated (EC) mycophenolate sodium are reviewed. SUMMARY: EC mycophenolate sodium is the EC salt form of mycophenolic acid (MPA), the active component of the pro-drug, mycophenolate mofetil. EC mycophenolate sodium was developed to reduce the upper-gastrointestinal (GI) effects of mycophenolate mofetil. Unlike oral mycophenolate mofetil, which releases MPA in the stomach, EC mycophenolate sodium releases MPA in the small intestine. The absolute bioavailability of EC mycophenolate sodium is 72%. MPA undergoes hepatic metabolism by glucuronyl transferase to the inactive mycophenolic acid glucuronide (MPAG), the predominant metabolite. The majority of an administered dose of EC mycophenolate sodium is found as MPAG in the urine. The mean terminal half-life of MPA ranges from 8 to 16 hours. EC mycophenolate sodium and mycophenolate mofetil have equivalent mechanisms of action and drug interaction profiles. Thus far, EC mycophenolate sodium has demonstrated similar efficacy and safety to mycophenolate mofetil in two Phase III clinical trials of adult renal transplant recipients. One study demonstrated improved health-related quality of life in patients switched from mycophenolate mofetil to EC mycophenolate sodium. Ongoing Phase IV studies are trying to further determine advantages of the EC product. CONCLUSION: EC mycophenolate sodium is a safe and effective immunosuppressive agent approved for use in the prevention of acute rejection after renal transplantation. It offers an excellent addition to the current armamentarium of immunosuppressive drugs for transplant immunosuppression.     

Inosine monophosphate dehydrogenase activity in paediatrics: age-related regulation and response to mycophenolic acid

Purpose

Since many drug targets and metabolizing enzymes are developmentally regulated, we investigated a potential comparable regulation of inosine 5’-monophosphate dehydrogenase (IMPDH) activity that has recently been advocated as a pharmacodynamic biomarker of mycophenolic acid (MPA) effects in the paediatric population. Since the field of pharmacodynamic monitoring of MPA is evolving, we also analyzed the response of IMPDH activity on MPA in children vs adolescents after renal transplantation.

Methods

We analyzed IMPDH activity in peripheral blood mononuclear cells (PBMCs) in 79 healthy children aged 2.0–17.9?years in comparison to 106 healthy adults. Pharmacokinetic/pharmacodynamic profiles of MPA and IMPDH over 6 or 12?h after mycophenolate mofetil dosing were performed in 17 paediatric renal transplant recipients. IMPDH activity was measured by HPLC and normalized to the adenosine monophosphate (AMP) content of the cells, MPA plasma concentrations were measured by HPLC.

Results

Inosine 5’-monophosphate dehydrogenase activity displayed a high inter-individual variability (coefficient of variation 40.2%) throughout the entire age range studied. Median IMPDH did not differ significantly in healthy pre-school children (82 [range, 42–184] μmol/s/mol AMP), school-age children (61 [30–153]), adolescents (83 [43–154]) and healthy adults (83 [26–215]). Similar to adults, IMPDH activity in children and adolescents was inversely correlated with MPA plasma concentration.

Conclusions

In conclusion, our data do not show a pronounced developmental regulation of IMPDH activity in PBMCs in the paediatric population and there is a comparable inhibition of IMPDH activity by MPA in children and adolescents after renal transplantation.     

Therapeutic drug monitoring of mycophenolate mofetil in transplantation

A roundtable meeting to discuss the use of therapeutic drug monitoring (TDM) to guide immunosuppression with mycophenolate mofetil was held in New York in December 2004. Existing recommendations for the initial months after transplantation were updated. After ensuring adequate levels of mycophenolic acid (MPA, the active metabolite of mycophenolate mofetil) immediately after transplantation, optimal efficacy may require only a few dose adjustments, because intrapatient variability in exposure seems low. Recommendations based on current knowledge were made for posttransplantation sampling time points and for target MPA concentrations. Algorithms for estimating MPA exposure using limited sampling strategies were presented, and a new assay for MPA discussed. It was agreed that because of interpatient variability and the influence of concomitant immunosuppressants, TDM might help optimize outcomes, especially in patients at higher risk of rejection. The value of TDM in the general transplant population will be assessed from large, ongoing, randomized studies.     

Associations of UDP-glucuronosyltransferases polymorphisms with mycophenolate mofetil pharmacokinetics in Chinese renal transplant patients

Aim:

To evaluate the effects of UDP-glucuronosyltransferases (UGTs) polymorphisms on the pharmacokinetics of the immunosuppressant mycophenolate mofetil (MMF) in Chinese renal transplant recipients.

Methods:

A total of 127 renal transplant patients receiving MMF were genotyped for polymorphisms in UGT1A9 −1818T>C, I399C>T, −118T9/10, −440C>T, −331T>C, UGT2B7 IVS1+985A>G, 211G>T, −900A>G, UGT1A8 518C>G and UGT1A7 622T>C. The plasma concentrations of the MMF active moiety mycophenolic acid (MPA) and main metabolite 7-O-MPA-glucuronide (MPAG) were analyzed using HPLC. Univariate and multivariate analyses were used to assess the effects of UGT-related gene polymorphisms on MPA pharmacokinetics.

Results:

The dose-adjusted MPA AUC_{0–12 h} of the patients with the UGT2B7 IVS1+985AG genotype was 48% higher than that of the patients with the IVS1+985AA genotype, which could explain 11.2% of the inter-individual variation in MPA pharmacokinetics. The dose-adjusted MPAG AUC_{0–12 h} of the patients with the UGT1A7 622CC and UGT1A9 −440CT/−331TC genotypes, respectively, was significantly higher than that of the patients with 622T homozygotes and −440C/−331T homozygotes. Furthermore, the genotypes UGT1A9 −1818T>C and UGT1A8 518C>G were associated with a low dose-adjusted MPAG AUC_{0–12 h}.

Conclusion:

The UGT2B7 11+985A>G genotype is associated with the pharmacokinetics of MPA in Chinese renal transplant patients, which demonstrates the usefulness of this SNP for individualizing MMF dosing.     

Population pharmacogenetic pharmacokinetic modeling for flip-flop phenomenon of enteric-coated mycophenolate sodium in kidney transplant recipients

Purpose

Enteric-coated mycophenolate sodium (EC-MPS) is effective and safe in preventing rejection after transplantation and is mainly transported by ABCs and OATPs and metabolized by UGTs. The genetic polymorphisms affect the inter-individual variation in drug disposition and elimination. The aims of this study were to develop a population pharmacokinetic (PK) model and to evaluate the influence of genetic and clinical factors on the PK of mycophenolic acid (MPA) in Korean renal transplant recipients.

Methods

Population analysis of EC-MPS was performed using non-linear mixed effects modeling (NONMEM). After clinical and genetic factors were evaluated using a stepwise covariate method, we selected clinically relevant covariates considering covariate effects. The final model was validated by bootstrap and visual predictive check. At last, we performed the model-based simulations in order to explore an optimal dose to achieve target area under the curve (AUC) in hypothetical scenarios.

Results

From 166 plasma concentrations (n=34), a time-lagged two-compartment with a flip-flop model best describes the PK of MPA. The covariate analysis identified lower creatinine clearance (CLcr) and SLCO1B1 variant genotype were correlated with lower MPA clearance, on the contrary, UGT1A9 variant had decreased distribution of MPA, contributing to lower absorption. When considering to UGT1A9, SLCO1B1 genotypes, and renal function, the new recommended dose of 540 mg twice daily resulted in a higher success of achieving the target AUC_0-12h in the 30-60 mg.h/L.

Conclusions

CLcr, UGT1A9 and SLCO1B1 genotypes seem to be promising parameters to predict the pharmacokinetics with flip-flop phenomenon of EC-MPS in transplant recipient having stable renal function. This model on clinical practice may help prevent overexposure and achieve a proper AUC in the Korean population.     

Within-patient variability of mycophenolic acid exposure: therapeutic drug monitoring from a clinical point of view

Exposure to mycophenolic acid (MPA) is highly variable among patients on standard dose mycophenolate mofetil (MMF) therapy. In addition, MPA exposure increases with time posttransplant and exposure is predictive for the development of acute rejection. Consequently, therapeutic drug monitoring (TDM) of MPA may improve clinical outcome, although a large within-patient variability could be a limitation. This study was designed to analyze the extent of within-patient variability of MPA exposure for area-under-the-curve (AUC0-12) and pre-dose concentrations (C0). For 9 occasions during the first 5 months after transplantation, AUC0-12 and C0 values from 45 renal transplant recipients, all using cyclosporine and corticosteroids, were divided into quartiles. When AUC0-12 or C0 changed 1, 2, or 3 quartiles within a patient from one occasion to the next, a score of respectively 1, 2, or 3 points was assigned. Doing this for all 8 between occasion intervals, the maximal score for within-patient variability could be 8 x 3 = 24 per patient. For AUC0-12, the median overall score was 3.4 of maximal 24. For C0 measurements, this score was significantly higher: 6.0 (P < 0.001). The higher overall score for C0 was explained by more quartile changes during the first weeks after transplantation. It is concluded that within-patient variability for MPA exposure is low in kidney transplant recipients during the first 5 months after transplantation. In the first weeks after transplantation, within-patient variability is larger for C0 than for AUC0-12.     

Mycophenolic acid trough level monitoring in solid organ transplant recipients treated with mycophenolate mofetil: association with clinical outcome

ABSTRACT

Background: Mycophenolate mofetil (MMF) is widely and successfully used in immunosuppressive regimens for the prophylaxis of organ rejection following transplantation. Conventionally, it is administered at a fixed dose without serial measurements of plasma concentrations of mycophenolic acid (MPA), the active metabolite. Recently, there has been an increased interest in therapeutic drug monitoring (TDM) of MMF therapy to optimize the benefit/risk index of the drug. Predose trough samples of MPA are considered most convenient and economic, thereby allowing an increased use of TDM in the transplant setting. However, the added value of TDM for MMF therapy is still under debate.

Objective: This paper reviews (based on a systematic PubMed and EMBASE search, 1995–June 2006) the current evidence of the usefulness and clinical relevance of MPA trough level monitoring during MMF therapy in solid organ transplantation.

Findings and conclusions: Based on data available in the public domain, the contribution of MPA trough level monitoring during MMF therapy in solid organ transplant recipients remains unproven. Available studies have limitations and report conflicting results. There is a lack of prospective randomized trials, particularly in pediatric renal transplant recipients and in cardiac and liver transplantation. While there is a suggestion that there may be a relationship between efficacy and MPA trough levels, the majority of studies showed no correlation between MPA plasma concentrations and adverse effects. Based on current evidence, the adherence to presently recommended target ranges for MPA troughs in solid organ transplantation cannot assure an improved clinical outcome with MMF therapy. Whether MPA trough level monitoring leads to improved efficacy and less toxicity is currently subject to a large randomized trial; final results are eagerly awaited.     

Effect of diabetes mellitus on mycophenolate sodium pharmacokinetics and inosine monophosphate dehydrogenase activity in stable kidney transplant recipients

Effect of diabetes mellitus on mycophenolic acid (MPA) pharmacokinetics and catalytic activity of inosine monophosphate dehydrogenase (IMPDH) was investigated in maintenance kidney transplant recipients. Demographically matched diabetic (n=9) and nondiabetic (n=9) patients were included in a 12-hour open-label, steady-state study after oral administration of enteric-coated mycophenolate sodium. Concentrations of total MPA and free MPA, MPA-glucuronide, and acyl-MPA-glucuronide were measured and oral acetaminophen absorption was used as a marker for gastric-emptying rate. Median (range) of MPA area under the curve(0-12) was 36.7 (range, 16.4-116.4) mg*h/L in diabetic and 48.2 (range, 34.9-80.1) mg*h/L in nondiabetic patients (P=0.49). All other primary pharmacokinetic parameters, including time to maximum concentration, for total or unbound MPA as well as MPA metabolites were comparable. In contrast, IMPDH activity was 17.5+/-2.8 versus 46.6+/-2.5 nmol XMP/h/microg protein in diabetics and nondiabetics, respectively (P<0.0001) and was significantly lower in the diabetics irrespective of concomitant therapy with cyclosporine or tacrolimus. This study demonstrated that diabetes does not alter MPA pharmacokinetics when administered as enteric-coated mycophenolate sodium; however, IMPDH activity appeared to be significantly lower in patients with diabetes independent of the unbound or total concentrations of MPA. Further investigations are warranted to investigate the regulation of IMPDH enzyme in patients with diabetes.     

Therapeutic drug monitoring of mycophenolate mofetil and enteric-coated mycophenolate sodium in patients with systemic lupus erythematosus

Objective: Mycophenolic acid (MPA), the active form of mycophenolate mofetil (MMF) and enteric-coated mycophenolate sodium (EC-MPS), is used to treat systemic lupus erythematosus (SLE). MMF and EC-MPS pharmacokinetics were examined to devise guidance for therapeutic drug monitoring (TDM) for SLE patients with normal renal function.

Research design and methods: This observational study included 21 patients receiving MMF (1000 mg twice daily) and 14 taking EC-MPS (720 mg twice daily). MPA AUC between 0 and 12 h (AUC_0–12h), C_max, T_max, and 12-h trough concentrations (C_12h) were determined.

Results: Means of dose-normalized MMF– or EC-MPS–MPA C_max were 64.6 ± 25 and 61.4 ± 27.1 h mg/l, respectively. MPA T_max for EC-MPS was longer and more variable than for MMF. MMF-MPA AUC_0–12h and C_12h were correlated (r = 0.78, p = 0.0001), but EC-MPS–MPA C_max and single concentrations were weakly correlated. A limited-sampling strategy (LSS) combining C_max and C_12h gave satisfactory predictive performance to estimate MPA AUC_0–12h after EC-MPS administration.

Conclusions: For TDM in SLE patients with GFR >?60 ml/min/1.73 m², C_12h after MMF ingestion could predict MPA AUC_0–12h, while an LSS around T_max should be used for patients on EC-MPS.     

Review: metabolism of immunosuppressant drugs

Pharmacokinetic concepts provide a basis for individualization of drug therapy to optimize outcomes of the critical-dose drugs cyclosporine (CsA), tacrolimus (TRL), sirolimus (SRL), and mycophenolate mofetil (MMF). The therapeutic range of a drug-defined as the concentrations at which the desired pharmacologic effect is produced without adverse effects in most patients-is difficult to achieve given the significant inter-and intrapatient variability of the effects of a given concentration of therapeutic agents. Because of the highly variable rates of absorption of immunosuppressive agents and clinical responses to given concentrations in transplant recipients, individualization of drug regimens by using therapeutic drug monitoring (TDM) is essential to optimize pharmacotherapy Assessing proclivity for acute rejection episodes in transplant recipients currently is attempted by estimating drug exposure using the area under the time-concentration curve (AUC) for MMF and the average concentration (Cav, the quotient of the AUC and the dosing interval) for CsA. These studies have revealed that low oral bioavailability was a more important predictor of rejection than was a rapid clearance rate. In addition, the degree of intra-individual variability of AUC values correlated with the development of chronic rejection in renal transplant recipients. Similarly, TDM of MMF requires AUC determinations. Low mycophenolic acid (MPA) exposure, as estimated by the AUC, demonstrates a significant association with an increased risk of an acute renal transplant rejection episode. The AUC0-2 estimate of MPA shows good agreement with the 12-hr AUC estimate from samples obtained during the entire dosing interval. In contrast, trough levels are utilized during treatment with TRL or SRL, potent new immunosuppressive agents that display a pleiotropic array of side effects. Standard body measures, including weight and body mass index, poorly predict the concentration of SRL in whole blood. Large inter- and intra-individual differences displayed in patients also could not be predicted by demographic features or by laboratory parameters. When SRL is given with other immunosuppressive agents such as CsA, which shares with SRL mutual microsomal metabolism by the cytochrome P450 3A system, pharmacokinetic interactions occur, especially when the agents are administered concomitantly. Because of the critical-dose nature of most of the recent generation of immunosuppressive agents, therapeutic drug monitoring is becoming increasingly important in the selection of doses and treatment regimens.     

Pharmacodynamics of mycophenolic acid in CD4+ cells: a single-dose study of IMPDH and purine nucleotide responses in healthy individuals

Mycophenolate mofetil is used in rejection prophylaxis after allograft transplantation. The highly variable pharmacokinetics and pharmacodynamics (PD) of the active moiety mycophenolic acid (MPA) render this drug attractive for therapeutic monitoring. The aim of this study was to characterize the exposure-response relationship for MPA to guide future strategies for individualized therapy based on PD monitoring. A single-dose (100, 250, 500, and 1000 mg mycophenolate mofetil) crossover exposure-response study of MPA PD in CD4 cells was performed in 5 healthy individuals. The activity of inosine 5'-monophosphate dehydrogenase (IMPDH) at time 0 ranged from 1.2 to 7.2 pmol per 10 cells/min. IMPDH was strongly inhibited by MPA; MPA EC50 (concentration required for 50% inhibition) of 2.3 mg/L was determined by a pooled data analysis. Decreased IMPDH gene expression was associated with the exposure to MPA. There were no immediate reductions of guanine nucleotides. On the contrary, a trend toward increased guanosine triphosphate was observed. IMPDH activity AUC0-12h approached maximum reduction at MPA AUC0-12h 22 mg x h/L (corresponding to the 500 mg dose), whereas plasma concentrations exceeding approximately 6 mg/L did not further increase the IMPDH inhibition. The results suggest that guanine nucleotides in circulating lymphocytes may not serve as immediate response biomarkers to MPA. Strategies for preventing over- or underexposure to MPA may be developed by means of IMPDH activity combined with MPA concentration measurement.     

高效液相色谱法同时测定霉酚酸及其葡糖苷酸的血浆浓度及临床应用

目的建立同时检测人血浆中霉酚酸（MPA）及其葡糖苷酸代谢物浓度的高效液相色谱法（HPLC）,并将其应用于肾移植术后患者霉酚酸酯（MMF）的治疗药物监测。方法以蛋白沉淀剂甲醇-5％硫酸锌（70：30,V：V）溶液进行血浆样品药物提取;内标物为萘普生;色谱柱为C18（4．6mm×250mm,5μm）;流动相为甲醇-水（含0．1％三氟乙酸）（65：35,V：V）;流速0．8mE／rain;检测波长215nm。结果MPA与其葡糖苷酸代谢物MPAG的线性范围分别为0．1—50mg／L及0．5—200mg／L,r2〉0．999;绝对回收率分别为63．95％～83．76％及57．14％-66．19％;两者日间及日内的相对标准偏差（RSD）均小于10％。6例。肾移植术后早期患者服用1．5g/d霉酚酸酯胶囊达药物稳态后,MPA及MPAG的Tmax分别为（1．08±0．74）h及（2．58±1．24）h,Cmax分别为（21．33±8．61）mg／L及（106．98±31．91）mg／L,AUC0-t分别为（58．73±16．26）及（833．32±215．03）mg·L^-1·h^-1。结论HPLC检测方法准确、灵敏、特异性好,并成功应用在。肾移植术后早期患者的治疗药物监测（TDM）中。     

Influence of uridine diphosphate (UDP)-glucuronosyltransferases and ABCC2 genetic polymorphisms on the pharmacokinetics of mycophenolic acid and its metabolites in Chinese renal transplant recipients

The aim was to investigate the effect of UGT1A9, UGT1A8, UGT2B7 and ABCC2 polymorphism on the pharmacokinetics of mycophenolic acid (MPA) and its metabolites phenolic glucuronide (MPAG) and acyl glucuronide (AcMPAG) in Chinese renal transplant recipients. Single nucleotide polymorphisms (SNP) in UGT1A9-118(dT)₉/₁₀, UGT1A9 T-440C/C-331T, UGT1A8*3, UGT2B7 G211T, UGT2B7 C802T, ABCC2 C-24T, and ABCC2 G1249A were detected. A total of 46 recipients were enrolled in the pharmacokinetics study at day 30 after kidney transplantation. Differences in the MPA pharmacokinetic profiles confirmed large inter-patient variation of MPA exposure. A statistical significant increase in the dose-adjusted AUC_6–12 level of MPA was found in patients bearing the -118(dT)₁₀ allele of the UGT1A9 gene (T₉ = 7.34 ± 4.11 mg h ml^?1 g^?1; T₉/T₁₀ = 11.54 ± 7.62 mg h ml^?1 g^?1; and T₁₀ = 11.89 ±8.76 mg h ml^?1 g^?1, p = 0.041). A similar trend was also observed for the dose-adjusted AUC_0–12 and AUC_6–12 of MPAG. Patients carrying the heterozygous mutant alleles of ABCC2 G1249A exhibited higher AUC_6–12/D of AcMPAG than those with wild-type genotype (p = 0.016). The other SNPs that were genotyped did not cause any significant variation in MPA and MPAG pharmacokinetic parameters. In conclusion, the enterohepatic recirculation of MPA in the patients seems to be more extensive in UGT1A9-118(dT)₁₀ allele carriers, and the exposure of AcMPAG is higher in patients carrying ABCC2 G1249A genotype than those with wild-type genotype.