Induction of cytokine release by the acyl glucuronide of mycophenolic acid: a link to side effects?

Genetic polymorphisms influence mycophenolate mofetil-related adverse events in pediatric heart transplant patients

BACKGROUND

Mycophenolate mofetil (MMF) is an effective and commonly used immunosuppressant but has frequent adverse events. Genetic polymorphisms may contribute to variability in MMF efficacy and related complications. In this study we explore the distribution frequencies of common single nucleotide polymorphisms (SNPs) of IMPDH1, IMPDH2 and ABCC2 and investigate whether these SNPs influence MMF adverse events in 59 pediatric heart recipients.

METHODS

Genotypes were assessed by TaqMan analysis of: ABCC2 rs717620; IMPDH2 rs11706052; and IMPDH1 rs2288553, rs2288549, rs2278293, rs2278294 and rs2228075. Gastrointestinal (GI) intolerance was defined as diarrhea, vomiting, nausea or abdominal pain requiring dose-holding for >48 hours or MMF discontinuation. Bone marrow toxicity was evaluated using Common Terminology Criteria for Adverse Events Version 3 (CTCAE).

RESULTS

GI intolerance occurred in 21 patients, and 21 had bone marrow toxicity. The ABCC2 rs717620 A variant was significantly associated with GI intolerance leading to drug discontinuation (p < 0.001); the IMPDH1 rs2278294 A variant and rs2228075 A variant were also associated with greater GI intolerance (p = 0.029 and p = 0.002, respectively). The IMPDH2 rs11706052 G variant was associated with more frequent neutropenia requiring dose-holding (p = 0.046).

CONCLUSIONS

In this small sample of pediatric heart transplant patients receiving MMF, ABCC2, IMPDH1 and IMPDH2 SNPs were associated with MMF GI intolerance and bone marrow toxicity. Thus, genetic polymorphisms may directly influence MMF adverse events.

The acyl glucuronide metabolite of mycophenolic acid induces tubulin polymerization in vitro

OBJECTIVES

The acyl glucuronide (AcMPAG) of mycophenolic acid (MPA) forms covalent protein adducts and possesses antiproliferative properties independent of IMPDH inhibition. The underlying mechanism is unknown. Disorganized tubulin polymerization prevents cell cycle progression. We investigated whether AcMPAG interacts with tubulin polymerization.

DESIGN AND METHODS

AcMPAG (1.0-100 microM) was incubated with bovine tubulin in the presence of GTP. Polymerization was followed at 340 nm. The time until onset and the extent of polymerization were determined. MPA (100 microM), phenolic glucuronide MPAG (100 microM), and paclitaxel (10 microM) served as controls.

RESULTS

MPAG was without effect. The AcMPAG effect on tubulin polymerization was dose dependent and significantly stronger (about 2.5-fold) than that of MPA (n=4; p<0.05), but weaker than paclitaxel.

CONCLUSIONS

MPA and AcMPAG can induce tubulin polymerization in the presence of GTP with AcMPAG being significantly stronger. This property of AcMPAG may contribute to its IMPDH independent antiproliferative effect.

The pharmacokinetics of mycophenolate mofetil in renal transplant recipients receiving standard-dose or low-dose cyclosporine, low-dose tacrolimus or low-dose sirolimus: the Symphony pharmacokinetic substudy

BACKGROUND

Exposure to mycophenolic acid (MPA), the primary active metabolite of mycophenolate mofetil (MMF), is correlated with therapeutic efficacy of MMF but varies depending on the concomitantly administered immunosuppressive drugs.

METHODS

A 3-month pharmacokinetic substudy of the prospective, randomized, multicentre, open-label Symphony study was performed. Eighty-three adult renal transplant patients received standard-dose cyclosporine, MMF 2 g/day and corticosteroids, or daclizumab induction, MMF 2 g/day and corticosteroids plus low-dose cyclosporine, low-dose tacrolimus or low-dose sirolimus. The area under the concentration-time curve (AUC(0-12)) of MPA and its metabolites between treatment groups was compared. Pharmacokinetic sampling was performed before MMF administration and at 20, 40, 75 min; 2, 3, 6, 8, 10 and 12 h post-dose on Day 7 and Months 1 and 3.

RESULTS

Compared with standard-dose cyclosporine, patients receiving low-dose tacrolimus or low-dose sirolimus had significantly higher AUC(0-12) values for MPA at Day 7 and Month 1 and for free MPA at Day 7, and significantly lower AUC(0-12) values for 7-O-MPA-glucuronide (MPAG) at Month 1 and for acyl-glucuronide at Months 1 and 3 (P < 0.05). AUC(0-12) of MPA and free MPA was significantly greater with low-dose tacrolimus and low-dose sirolimus than with low-dose cyclosporine in the first month (P < 0.05). The ratio of MPA to MPAG exposure was significantly higher in the three low-dose groups than in the standard-dose cyclosporine group (P < 0.05).

CONCLUSIONS

Standard- and low-dose cyclosporine reduces the exposure of MPA and free MPA compared to low-dose tacrolimus or low-dose sirolimus in patients given the same dose of MMF.

AcylMPAG plasma concentrations and mycophenolic acid-related side effects in patients undergoing renal transplantation are not related to the UGT2B7-840G>A gene polymorphism

Mycophenolic acid (MPA) is metabolized primarily by glucuronidation to form the biologically inactive 7-O-glucuronide conjugate (MPAG), which is the major urinary excretion product. MPA is also converted to acyl-glucuronide metabolite (AcylMPAG), which has been suggested to be involved in the generation of MPA-related adverse events such as diarrhea or leucopenia. This conversion of MPA to AcylMPAG is catalyzed by UDP-glucuronosyltransferase 2B7 (UGT2B7). We studied the impact of the -840G>A polymorphisms in the UGT2B7 gene on the pharmacokinetics of AcylMPAG. We also investigated whether the plasma concentrations of AcylMPAG are correlated with MPA-related toxicity to further evaluate its potential clinical significance. In a randomized, controlled trial, comparing fixed-dose mycophenolate mofetil (MMF) with concentration-controlled MMF therapy, patients undergoing renal transplantation were treated with a calcineurin inhibitor, MMF, and corticosteroids. Informed consent was obtained from 332 patients for genotyping. In all patients, blood samples were drawn (three samples within the first 2 hours after administration) on Day 3, Day 10, Week 4, and Months 3, 6, and 12 to measure MPA and AcylMPAG plasma concentrations. The pharmacokinetics of AcylMPAG were correlated with the -840G>A single nucleotide polymorphism (SNP) in the UGT2B7 gene. Heterozygosity for the -840G>A SNP in the UGT2B7 gene was found in 145 patients (145 of 332 [44%]) and 93 (93 of 332 [28%]) patients were homozygous for the -840G>A allele. No difference was found in the dose-normalized AcylMPAG trough (C0) levels and dose-normalized AcylMPAG areas under the concentration-time curve (AUCs) at each visit between carriers and noncarriers of the -840G>A SNP. Also, metabolic ratios, expressed as AcylMPAG/MPA and AcylMPAG/MPAG, were not related to UGT2B7 genotype. The dose-normalized AcylMPAG-C0 and AcylMPAG AUC were higher in the cyclosporine-treated group compared with the tacrolimus-treated patients at each visit. There was no difference in AcylMPAG concentrations (trough or AUC) or AcylMPAG/MPAG ratio between patients with compared with patients without diarrhea. None of the -840G>A UGT2B7 SNPs was disproportionately present among the patients with diarrhea. There was a higher incidence of diarrhea in tacrolimus-treated patients [26 of 163 (16.0%)] compared with cyclosporine-treated individuals [five of 51 (9.8%)], although AcylMPAG concentrations were lower in tacrolimus-treated patients. In this study, we have found no influence of the -840G>A UGT2B7 SNP on AcylMPAG exposure in patients undergoing renal transplantation. There also was no association between this variant genotype and the incidence of diarrhea or leucopenia, two adverse events for which a role for AcylMPAG has been suggested.

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.

Liquid chromatographic determination of mycophenolic acid and its metabolites in human kidney transplant plasma: pharmacokinetic application

BACKGROUND AND OBJECTIVE

Difference in the hydrophilic properties of mycophenolic acid metabolites makes it technically difficult to simultaneously determine their plasma levels in one analytical run. Therapeutic drug monitoring (TDM) for MPA ensures adequate MPA exposure levels to both prevent rejection and avoid related toxicity. One measure limitation for TDM for MPA is the availability of simple, rapid and reproducible method for determination of MPA derivatives.

METHOD

Herein we report a single method to measure MPA and its metabolites using a gradient elution system in less than 10 min. We further tested applicability of our method in both stable and unstable renal transplant recipients with a wide range of levels.

RESULTS

Intra- and inter-day imprecision were less than 8% and 10%, respectively. Accuracy of the estimated concentrations ranges from 90% to 108%.

CONCLUSION

Collectively these data show that the new method is reasonably accurate and precise for the simultaneous determination of MPA and its metabolites in human plasma.

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.

Influence of SLCO1B1, 1B3, 2B1 and ABCC2 genetic polymorphisms on mycophenolic acid pharmacokinetics in Japanese renal transplant recipients

OBJECTIVE

We investigated the association between mycophenolic acid (MPA) pharmacokinetics and organic anion-transporting polypeptide (OATP/SLCO)1B1, 1B3, 2B1 and multidrug resistance-association protein 2 (MRP2/ABCC2) genetic polymorphisms and diarrhea.

METHODS

Eighty-seven renal allograft recipients were given repeated doses of mycophenolate mofetil every 12 h at a designated time (09:00 and 21:00). The pharmacokinetics of MPA were analyzed on day 28 posttransplantation.

RESULTS

The dose-adjusted area under the cuve (AUC)(6-12) of MPA, an estimate of enterohepatic recirculation, was greater in SLCO1B3 T334G GG (or G699A AA) carriers than in TT carriers (or G699A GG) (40 vs. 25 ng h/mL per milligram, respectively, P = 0.0497). None of the polymorphism of SLCO1B1, SLCO2B1, or ABCC2 C-24T were associated with MPA pharmacokinetics or diarrhea. However, the oral clearance of MPA in recipients having both the SLCO1B3 T334G GG genotype and the ABCC2 C-24T T allele was significantly lower than in patients having both the SLCO1B3 T334G TT and ABCC2 C-24T CC genotypes (0.15 vs. 0.18 L/h per kilogram, respectively, P = 0.0010).

CONCLUSIONS

MPA excretion into bile in patients with SLCO1B3 T334G GG (or G699A AA) was higher than in those with T334G TT (or G699A GG), probably resulting in a higher AUC(6-12) value of MPA. MPA uptake into hepatocytes and excretion into bile at first pass may be greater in SLCO1B3 T334G GG carriers than in TT carriers. In addition, the ABCC2 C-24T polymorphism also seems to be associated with enhanced enterohepatic circulation of MPA. The SLCO1B3 and ABCC2 transporters rather than uridine diphosphate-glucuronosyltransferase (UGT) may partly affect interindividual variety in plasma MPA concentration.

Influence of Renal Insufficiency on Pharmacokinetics of ACYL-Glucuronide Metabolite of Mycophenolic Acid in Renal Transplant Patients

The acyl glucuronide of mycophenolic acid (AcMPAG) is a metabolite with in vitro immunosuppressive activity. The chemical properties of acyl glucuronides have been associated with the toxicity of some drugs. The aim of our study was to analyze the influence of renal insufficiency on the pharmacokinetics of AcMPAG. Areas under the 12-hour curve (AUC(0-12h)) of MPA, glucuronide of MPA (MPAG), and AcMPAG were determined by high performance liquid chromatography performed in 20 renal transplantation patients under treatment with mycophenolate mofetil (MMF), cyclosporine, and steroids. They were divided between a group with preserved renal function (group I, mean creatinine clearance [Clcr] of 105 +/- 7 mL/min) and one with advanced renal insufficiency (group II, mean Clcr of 27 +/- 5 mL/min). There was no difference in MMF dose or MPA-AUC(0-12h) values between groups. Mean predose levels of AcMPAG-C0 and AcMPAG-AUC(0-12h) were much higher in group II than in group I (0.5 +/- 2 vs 1.6 +/- 1 microg/mL and 12 +/- 2 vs. 32 +/- 19 microg*h/mL respectively, P < .005). The present data suggested that AcMPAG, a metabolite with immunosuppressive activity that may be related to toxic effects of MPA, is renally eliminated. Its levels can significantly rise in patients with renal insufficiency. Although further studies with more patients are required to determine the role of AcMPAG in MPA toxicity, we believe that this accumulation may be of clinical relevance.

Individualization of mycophenolate mofetil dose in renal transplant recipients

The immunosuppressive agent mycophenolate mofetil has been successfully used over the past 10 years to prevent acute allograft rejection after renal transplantation. It has mainly been administered as a fixed dose of mycophenolate mofetil 1000 mg b.i.d. The pharmacokinetics of mycophenolic acid, the active moiety of the prodrug mycophenolate mofetil, show large between-patient variability, and exposure to mycophenolic acid correlates with the risk for acute rejection. This suggests that already excellent clinical results can be further improved by mycophenolate mofetil dose individualization. This review discusses different arguments in favour of individualization of mycophenolate mofetil dose, as well as strategies for managing mycophenolate mofetil therapy individualization, including pharmacokinetic and pharmacodynamic monitoring and dose individualization based on pharmacogenetic information. It is expected that pharmacokinetic monitoring of mycophenolic acid will offer the most effective and feasible tool for mycophenolate mofetil dose individualization.

Pharmacokinetics of mycophenolate mofetil in hematopoietic stem cell transplant recipients

Mycophenolate mofetil (MMF), a prodrug of mycophenolic acid (MPA), is increasingly used in the prophylaxis of graft-versus-host disease (GVHD) after hematopoietic stem cell transplantation (HCT). Few pharmacokinetic data are available about the use of MMF for this indication. This case series aimed at analyzing the pharmacokinetics of MMF in a population of HCT recipients representative for everyday practice. From 15 HCT recipients, serial plasma samples were taken after twice-daily oral intake of MMF. Plasma concentrations of total MPA and its glucuronide metabolites, as well as free MPA, were quantified. Median apparent oral MPA clearance (CL/F), apparent half-life, and total MPA area under the curve for hours 0 to 12 (AUC0-12, normalized to 1000 mg MMF) were, respectively, 56 L/h (range: 29-98 L/h), 2.3 hours (range: 0.8-5.7 hours), and 18.0 mg*h/L (range: 10-35 mg*h/L). Total MPA concentrations were below 2 mg/L 8 hours after MMF administration, indicating reduced enterohepatic recirculation. Median free MPA AUC0-12 (normalized to 1000 mg MMF) was 224 microg*h/L (range: 56-411 microg*h/L). Because of high CL/F, total MPA exposure in HCT recipients is low and apparent half-life is short in comparison with reference values from renal transplantation. Exposure may be improved in HCT recipients by higher or more frequent MMF dosing.

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.

Targeted inhibition of glucuronidation markedly improves drug efficacy in mice - a model

Finding UDP-glucuronosyltransferases (UGT) require protein kinase C-mediated phosphorylation is important information that allows manipulation of this critical system. UGTs glucuronidate numerous aromatic-like chemicals derived from metabolites, diet, environment and, inadvertently, therapeutics to reduce toxicities. As UGTs are inactivated by downregulating PKCs with reversibly-acting dietary curcumin, we determined the impact of gastro-intestinal glucuronidation on free-drug uptake and efficacy using immunosuppressant, mycophenolic acid (MPA), in mice. Expressed in COS-1 cells, mouse GI-distributed Ugt1a1 glucuronidates curcumin and MPA and undergoes irreversibly and reversibly dephosphorylation by PKC-specific inhibitor calphostin-C and general-kinase inhibitor curcumin, respectively, with parallel effects on activity. Moreover, oral curcumin-administration to mice reversibly inhibited glucuronidation in GI-tissues. Finally, successive oral administration of curcumin and MPA to antigen-treated mice increased serum free MPA and immunosuppression up to 9-fold. Results indicate targeted inhibition of GI glucuronidation in mice markedly improved free-chemical uptake and efficacy using MPA as a model.

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.

A comparison of the effect of ciclosporin and sirolimus on the pharmokinetics of mycophenolate in renal transplant patients

AIM

To compare the pharmacokinetics of mycophenolic acid when given with either ciclosporin or sirolimus, and investigate in vitro the potential effect of ciclosporin, sirolimus, tacrolimus and everolimus on mycophenolic acid metabolism.

METHODS

In renal transplant patients given mycophenolate mofetil in combination with ciclosporin (n = 19) or sirolimus (n = 12), concentration-time profiles of mycophenolic acid, mycophenolic-acid-phenyl-glucuronide, mycophenolic-acid-acyl-glucuronide and mycophenolic-acid-phenyl-glucoside were determined at one month post-transplant. The effect of immunosuppressive drugs on mycophenolic acid glucuronidation and glycosylation was investigated in vitro using human liver microsomes.

RESULTS

The mean mycophenolic acid AUC(0-9 h) in the sirolimus group was 44.9 mg h(-1) L(-1) (95% CI: 34.7-55.1), vs. 30.5 mg h(-1) L(-1) (95% CI: 25.4-35.6) in the ciclosporin group, corresponding to 1.5-fold dose-normalized difference (95% CI: 1.1-1.9; P < 0.05). In addition, the metabolite/mycophenolic acid AUC(0-9 h) ratios were significantly higher in patients cotreated with ciclosporin than with sirolimus, giving values of 1.8-fold (95% CI: 1.3-2.3; P = 0.0009), 2.6-fold (95% CI: 2.0-3.3; P < 0.0001) and 4.3-fold (95% CI: 2.6-6.0; P = 0.0016) for mycophenolic-acid-phenyl-glucuronide, mycophenolic-acid-acyl-glucuronide and mycophenolic-acid-phenyl-glucoside, respectively. In vitro, none of the immunosuppressive drugs tested inhibited mycophenolic acid metabolism.

CONCLUSION

Patients taking mycophenolate mofetil and sirolimus experience a higher exposure to mycophenolic acid and a lower exposure to mycophenolic acid metabolites than those being treated with mycophenolate mofetil and ciclosporin. This interaction is probably not caused by inhibition of mycophenolic acid glucuronidation or glycosylation, but is more likely to be due to the influence of ciclosporin on the excretion of mycophenolic acid metabolites into bile.

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.

Gender-related differences in mycophenolate mofetil-induced gastrointestinal toxicity in rats

Mycophenolate mofetil (MMF), the prodrug of mycophenolic acid (MPA), is included in current combination immunosuppressive regimens following organ transplant. Treatment with MMF often results in dose-limiting gastrointestinal (GI) side effects. The underlying mechanisms responsible for these side effects are not fully understood, but exposure of the intestinal epithelia to MPA during enterohepatic recycling may be involved. The present study demonstrated that female rats are more susceptible to MMF-induced GI toxicity than male rats. Female Sprague-Dawley rats treated chronically with an oral dose of 50 mg of MPA equivalents/kg/day experienced greater GI toxicity than male rats, as measured by diarrhea grade and weight loss. Intestinal microsomes harvested from the upper jejunum of female rats had approximately 3-fold lower MPA glucuronidation rates compared with male rats. In the remaining areas of the small and large intestine, there was also a trend toward decreased glucuronidation in the female rats. The area under the plasma concentration-time curve (AUC) for MPA following an oral dose of 50 mg of MPA equivalents/kg was roughly similar between genders, whereas the AUC for mycophenolic acid phenolic glucuronide (MPAG) was significantly lower in female rats. Female rats also excreted half of the biliary MPAG as male rats. The greater susceptibility of female rats to MMF-induced gastrointestinal toxicity, despite diminished intestinal MPA exposure via reduced biliary excretion of MPAG, may result from reduced protection of enterocytes by in situ glucuronidation. Likewise, susceptibility to MMF-induced GI toxicity in humans may also result from variable intestinal glucuronidation due to UDP glucuronosyltransferase polymorphisms or differential expression.

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

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.

Intestinal tract injury by drugs: Importance of metabolite delivery by yellow bile road

Drug secretion into bile is typically considered a safe route of clearance. However, biliary delivery of some drugs or their reactive metabolites to the intestinal tract evokes adverse consequences due to direct toxic actions or indirect disruption of intestinal homeostasis. Biliary concentration of the chemotherapy agent 5-fluorodeoxyuridine (FUDR) and other compounds is associated with bile duct damage while enterohepatic cycling of antibiotics contributes to the disruptions of gut flora that produce diarrhea. The goal of this review is to describe key evidence that biliary delivery is an important factor in the intestinal injury caused by representative drugs. Emphasis will be given to 3 widely used drugs whose reactive metabolites are plausible causes of small intestinal injury, namely the nonsteroidal anti-inflammatory drug (NSAID) diclofenac, the immunosuppressant mycophenolic acid (MPA), and the chemotherapy agent irinotecan. Capsule endoscopy and other sensitive diagnostic techniques have documented a previously unappreciated, high prevalence of small intestinal injury among NSAID users. Clinical use of MPA and irinotecan is frequently associated such severe intestinal injury that dosage must be reduced. Observations from clinical and experimental studies have defined key events in the pathogenesis of these drugs, including roles for multidrug resistance-associated protein 2 (MRP2) and other transporters in biliary secretion and adduction of enterocyte proteins by reactive acyl glucuronide metabolites as a likely mechanism for intestinal injury. New strategies for minimizing the adverse intestinal consequences of irinotecan chemotherapy illustrate how basic information about key events in the biliary secretion of drugs and the nature of their proximate toxicants can lead to safer protocols for drugs.

Therapeutic Drug Monitoring of Mycophenolic Acid in Solid Organ Transplant Patients Treated With Mycophenolate Mofetil: Review of the Literature

Mycophenolate mofetil (MMF) has conventionally been administered at a fixed dose without routinely monitoring blood levels of mycophenolic acid (MPA), the active metabolite. The contribution of therapeutic drug monitoring (TDM) during MMF therapy remains controversial. A literature review was performed to explore the usefulness of TDM for MPA in solid organ transplantation. In addition, emphasis was placed on the potential clinical benefits and limitations of TDM for MPA. Available studies have limitations and report conflicting results. Although early after transplantation MPA area under the curve might have predictive value for the risk of acute rejection, predose levels appear less reliable. With regard to MPA toxicity, most studies showed no correlation between MPA pharmacokinetics and adverse effects. TDM is hampered by several factors such as the considerable intra-subject variability of MPA pharmacokinetics and the increasing number of different drug combinations. Proposed target ranges are restricted to the early posttransplant period when MMF is used in combination with cyclosporine. The current review of the literature indicates no clear support for a substantial clinical benefit of TDM and more data from prospective randomized trials are needed.

Multidrug Resistance Protein 2 Genetic Polymorphisms Influence Mycophenolic Acid Exposure in Renal Allograft Recipients

BACKGROUND

Mycophenolic acid (MPA) is glucuronidated by uridine diphosphate-glucuronosyltransferases (UGTs) to its pharmacologically inactive 7-O-glucuronide metabolite (MPAG). MPAG is excreted into the bile via the multidrug resistance-associated protein 2 (MRP2/ABCC2), which is essential for enterohepatic (re)circulation (EHC) of MPA(G).

METHODS

The objective of this study was to determine the relationship between single nucleotide polymorphisms (SNPs) in the MRP2 (G-1549A, G-1023A, A-1019G, C-24, G1249A, C3972T and G4544A) and UGT1A9 (C-2152T, T-275AandT98C) genes and MPA pharmacokinetics in 95 renal allograft recipients at days 7, 42, 90, and 360 after transplantation. In addition to mycophenolate mofetil, all patients received tacrolimus and corticosteroids as immunosuppression.

RESULTS

At day seven after transplantation, in the absence of the MRP2 C-24T SNP, mild liver dysfunction was associated with significantly lower MPA dose-interval exposure and higher MPA oral clearance, while liver dysfunction did not affect MPA pharmacokinetics in patients with the MRP2 C-24T variant. A similar effect is noted for the C-3972T variant, which is in linkage disequilibrium with C-24T. At later time points after transplantation the MRP2 C-24T SNP was associated with significantly higher dose-corrected MPA trough levels. Patients with the MRP2 C-24T variant had significantly more diarrhea in the first year after transplantation.

CONCLUSIONS

The MRP2 C-24T and C-3972T polymorphisms protect renal transplant recipients from a decrease in MPA exposure associated with mild liver dysfunction. Furthermore, this study suggests that the C-24T SNP is associated with a lower oral clearance of MPA in steady-state conditions.

Influence of nonsynonymous polymorphisms of UGT1A8 and UGT2B7 metabolizing enzymes on the formation of phenolic and acyl glucuronides of mycophenolic acid

Mycophenolic acid (MPA) is the active metabolite of mycophenolate mofetil (MMF), a standard immunosuppressive drug approved for clinical use in the prevention of acute allograft rejection after organ transplantation. This study examines the role of the genetic variants of UDP-glucuronosyltransferase (UGT) 1A8 and 2B7 enzymes involved in the formation of the primary metabolite of MPA, the inactive phenolic glucuronide (MPAG), and the reactive acyl glucuronide (AcMPAG). The first exon of UGT1A8 was first resequenced in the region encoding for the substrate binding domain in 254 Caucasians and 41 African Americans. Eight nonsynonymous changes were observed and led to the following amino acid substitutions: S43L, H53N, S126G, A144V, A173G, A231T, T240A, and C277Y. Thirteen haplotypes were inferred, comprising only two previously described alleles, namely, UGT1A8*2 (A173G) and UGT1A8*3 (C277Y). Upon stable expression in human embryonic kidney 293 cells, the UGT1A8*3 (C277Y), *5 (G173A240), *7 (A231T), *8 (S43L), and *9 (N53G) proteins were associated with the most profound decreases in the formation of MPAG and AcMPAG, indicating that these amino acids are critical for substrate binding and enzyme function. Altogether, the low-activity UGT1A8 enzymes are carried by 2.8 to 4.8% of the population. The variant of the UGT2B7 protein (UGT2B7*2 Y268), the main enzyme involved in the formation of AcMPAG, demonstrated a catalytic efficiency comparable with that of UGT2B7*1 (H268). In conclusion, although the common UGT2B7*2 variant is predicted to have limited impact, several UGT1A8 variants identified may potentially account for the large interindividual variance in MMF pharmacokinetics and deserve further clinical investigations.

Pharmacogenetic differences and drug-drug interactions in immunosuppressive therapy

With the advent of new immunosuppressants and formulations, the elucidation of molecular targets and the evolution of therapeutic drug monitoring, the field of organ transplantation has witnessed significant reductions in acute rejection rates, prolonged graft survival and improved patient outcome. Nonetheless, challenges persist in the use of immunosuppressive medications. Marked interindividual variability remains in drug concentrations and drug response. As medications with narrow therapeutic indices, variations in immunosuppressant concentrations can result in acute toxicity or transplant rejection. Recent studies have begun to identify factors that contribute to this variability with the promise of tailoring immunosuppressive regimens to the individual patient. These advances have uncovered differences in genetic composition in drug-metabolising enzymes, drug transporters and drug targets. This review focuses on commonly used maintenance immunosuppressants (including cyclosporin, mycophenolate mofetil, tacrolimus, sirolimus, everolimus, azathioprine and corticosteroids), examines current studies on pharmacogenetic differences in drug-metabolising enzymes, drug transporters and drug targets and addresses common drug-drug interactions with immunosuppressant therapies. The potential role of drug-metabolising enzymes in contributing to these drug-drug interactions is briefly considered.

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

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

Quantification by liquid chromatography tandem mass spectrometry of mycophenolic acid and its phenol and acyl glucuronide metabolites

BACKGROUND

We developed and validated a rapid and reliable liquid chromatography-tandem mass spectrometry (LC-MS/MS) procedure for the quantification of mycophenolic acid (MPA) and its phenol glucuronide (MPAG) and acyl glucuronide (AcMPAG) metabolites.

METHODS

We performed protein precipitation on all samples (calibrators, quality controls, and patient samples) and then subjected them to online solid-phase extraction followed by reversed-phase liquid chromatography for 4.0 min. The carboxybutoxy ether of MPA (MPAC) was used as the internal calibrator. The separated compounds (MPA, MPAG, AcMPAG, and MPAC) were detected by electrospray ionization-coupled MS/MS. We compared LC-MS/MS results with results for the same samples obtained with a validated HPLC procedure with an ultraviolet detector.

RESULTS

Comparison with the validated HPLC-ultraviolet procedure demonstrated good agreement. The Passing-Bablok regression was y = 0.968x - 0.058 for MPA, y = 1.08x - 1.697 for MPAG, and y = 0.952x + 0.076 for AcMPAG. Assay imprecision showed a CV <10% at 3 concentrations for each compound. The lower limit of quantification was 0.1 mg/L for MPA, 1.0 mg/L for MPAG, and 0.05 mg/L for AcMPAG. The mean analytical recovery was 90%-110%. The assay was linear from 0.1 to 50 mg/L for MPA (r = 0.9987), from 1 to 500 mg/L for MPAG (r = 0.9999), and from 0.05 to 10 mg/L for AcMPAG (r = 0.9988). Quantification of the compounds was not affected by in-source fragmentation or ion suppression.

CONCLUSION

The LC-MS/MS assay described here is valid and reliable for the quantification of total MPA, MPAG, and AcMPAG in serum.

High-performance liquid chromatography method for the determination of mycophenolic acid and its acyl and phenol glucuronide metabolites in human plasma

Measuring the concentration of the pharmacologically active metabolite of mycophenolic acid (MPA), acyl-MPAG (AcMPAG), in addition to the pharmacologically inactive phenol glucuronide metabolite (MPAG) may prove useful in the therapeutic drug monitoring of MPA. A simple high-performance liquid chromatography method with ultraviolet detection (HPLC-UV) was established for simultaneous determination of MPA, AcMPAG, and MPAG in human plasma. The method utilizes 2 internal standards (IS), phenolphthalein glucuronic acid (PGA) for MPAG and a carboxy butoxy derivative of MPA (MPAC) for AcMPAG and MPA. The method consists of solid-phase extraction of the analytes followed by analysis over a Zorbax Rx C8 column (150 x 4.6 mm, 5 mum) at 254 nm. The analytes were separated with a gradient mixture of methanol and 0.1% phosphoric acid over a run time of 14 minutes at a flow rate of 1 mL/min. The assay was linear in the concentration range from 0.2 to 50 mg/L for MPA, 0.5 to 25 mg/L for AcMPAG, and 2 to 500 mg/L for MPAG. The mean +/- SD interday accuracy and %CV for MPA were 100.3 +/- 5.7 and 5.7%, for AcMPAG, 102.6 +/- 5.7 and 5.6%, and for MPAG 100.5 +/- 5.3 and 5.3%, respectively. The average +/- SD of MPA, MPAG, and AcMPAG maximum concentrations (Cmax) in 23 kidney transplant recipients on 500 or 1000 mg twice daily mycophenolate mofetil were 11.77 +/- 9.43, 88.15 +/- 46.4, and 3.01 +/- 1.73 mg/L, respectively, and the predose trough (Cmin morning) concentrations were 2.24 +/- 3.11, 55.44 +/- 29.55, and 1.42 +/- 0.74 mg/L, respectively. The method described is robust, sensitive, reproducible, and will be useful in therapeutic drug monitoring or pharmacokinetic studies of MPA.

Role of Mrp2 in the hepatic disposition of mycophenolic acid and its glucuronide metabolites: effect of cyclosporine

Mycophenolic acid (MPA) is part of the immunosuppressant therapy for transplant recipients. This study examines the role of the canalicular transporter, Mrp2, and the effect of cyclosporin A (CsA), on the biliary secretion of the ether (MPAGe) and acyl (MPAGa) glucuronides of MPA. Isolated livers from Wistar rats (n = 6), or Wistar TR- rats (n = 6) were perfused with MPA (5 mg/l). A third group of Wistar rats (n = 6) was perfused with MPA and CsA (250 microg/l). There was no difference in the half-life, hepatic extraction ratio (E(H)), clearance or partial clearance of MPA to MPAGe, but there was a difference in partial clearance to MPAGa between control and CsA groups (0.9 +/- 0.4 versus 0.5 +/- 0.1 ml/min). TR- rats had a lower E(H) (0.59 +/- 0.30 versus 0.95 +/- 0.30), a lower clearance (18 +/- 8 versus 29 +/- 7 ml/min), and a longer half-life (19.5 +/- 10.3 versus 10.1 +/- 2.4 min) than controls. Compared to controls, MPAGe and MPAGa biliary excretion was reduced by 99% and 71.8%, respectively, in TR- rats, and 17.5% and 53.8%, respectively, in the MPA-CsA group. The biliary excretion of MPAGe is mediated by Mrp2, whereas that of MPAGa seems to depend on both Mrp2 and another unidentified canalicular transporter. Although CsA can inhibit Mrp2, our data suggest that it may also inhibit the hepatic glucuronidation of MPA in Wistar rats.

Glucuronidation in therapeutic drug monitoring

BACKGROUND

Glucuronidation is a major drug-metabolizing reaction in humans. A pharmacological effect of glucuronide metabolites is frequently neglected and the value of therapeutic drug monitoring has been questioned. However, this may not always be true.

METHODS

In this review the impact of glucuronidation on therapeutic drug monitoring has been evaluated on the basis of a literature search and experience from the own laboratory.

RESULTS

The potential role of monitoring glucuronide metabolite concentrations to optimize therapeutic outcome is addressed on the basis of selected examples of drugs which are metabolized to biologically active/reactive glucuronides. Furthermore indirect effects of glucuronide metabolites on parent drug pharmacokinetics are presented. In addition, factors that may modulate the disposition of these metabolites (e.g. genetic polymorphisms, disease processes, age, and drug-drug interactions) are briefly mentioned and their relevance for the clinical situation is critically discussed.

CONCLUSION

Glucuronide metabolites can have indirect as well as direct pharmacological or toxicological effects. Although convincing evidence to support the introduction of glucuronide monitoring into clinical practice is currently missing, measurement of glucuronide concentrations may be advantageous in specific situations. If the glucuronide metabolite has an indirect effect on the pharmacokinetics of the parent compound, monitoring of the parent drug may be considered. Furthermore pharmacogenetic approaches considering uridine diphosphate (UDP) glucuronosyltransferases polymorphisms may become useful in the future to optimize therapy with drugs subject to glucuronidation.

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.

The Rationale for and Limitations of Therapeutic Drug Monitoring for Mycophenolate Mofetil in Transplantation

The addition of mycophenolate mofetil (MMF) to calcineurin inhibitor-based regimens reduces the incidence of acute rejection after kidney transplantation. The interpatient variability, changes over time of pharmacokinetic parameters, and the potential for drug interactions make the systemic exposure of mycophenolic acid (MPA) unpredictable at a fixed-dose regimen. An increase in plasma concentration of MPA significantly correlates with a decreased likelihood of an acute rejection after kidney or heart transplantation; therefore, a strategy of therapeutic drug monitoring for MMF therapy could improve outcome. Two large randomized, multicenter, prospective trials investigating the added value of therapeutic drug monitoring for MPA, by comparing fixed-dose treatment with concentration-controlled MMF treatment in kidney transplant recipients, are currently ongoing. More data are needed to fully establish the meaning of the reported prognostic value of preoperative inosine monophosphate dehydrogenase (IMPDH) activity, and longitudinal studies monitoring IMPDH activity after transplantation are eagerly awaited.

[Relationship between dose of mycophenolate mofetil and the occurrence of cytomegalovirus infection and diarrhea in renal transplant recipients]

To establish guidelines for avoiding the side effects of mycophenolate mofetil (MMF) in renal transplant recipients with tacrolimus (TAC)-based immunosuppression, the relationship between the daily dose of MMF and the occurrence of side effects was analyzed in this study. The frequency of side effects was investigated retrospectively in 28 renal transplant recipients treated with immunosuppression (men 14 : women 14, age: 33.0+/-12.4 years, weight: 50.9+/-10.7 kg). Cytomegalovirus (CMV) infection and diarrhea were the most frequent side effects in the early transplant phase (from transplantation to 3-month biopsy) in the recipients. In 18 recipients, excluding the recipients with risk factors for CMV infection (ABO-incompatible transplantation, donor (+)/recipient (-) CMV serostatus, etc.), no significant correlation was shown between the daily dose of MMF and the occurrence of CMV infection in the two-sample t-test. On the other hand, the daily dose in the diarrhea group (33.2+/-4.3 mg/kg/day, n = 5) was significantly higher than that in the no-diarrhea group at 30 days (28.4+/-3.7 mg/kg/day, n = 23, p < 0.05) and 90 days (25.7+/-4.4 mg/kg/day, n = 21, p < 0.005) after transplantation, respectively. The receiver-operating characteristic (ROC) curve also revealed that the risk of diarrhea increased with a daily MMF dose higher than 30 mg/kg/day. In conclusion, to decrease the risk of diarrhea in the early transplant phase in renal transplant recipients with TAC-based immunosuppression, the daily dose of MMF should not be more than 30 mg/kg/day.

Immunosuppressive drug monitoring - what to use in clinical practice today to improve renal graft outcome

Therapeutic drug monitoring (TDM) of immunosuppressive therapy is becoming an increasingly complex matter as the number of compounds and their respective combinations are continuously expanding. Unfortunately, in clinical practice, monitoring predose trough blood concentrations is often not sufficient for guiding optimal long-term dosing of these drugs. The excellent short-term results obtained nowadays in renal transplantation confer a misleading feeling of safety despite the fact that long-term results have not substantially improved, definitely not to a point where longer graft survival could counteract the increasing need for transplant organs and less toxicity and side-effects could ameliorate patient survival. It is therefore a challenging task to try to tailor immunosuppressive drug therapy to the individual patient profile and this in a time-dependent manner. For the majority of currently used immunosuppressive drugs, measurement of total drug exposure by determination of the dose-interval area under the concentration curve (AUC) seems to provide more useful information for clinicians in terms of concentration-exposure and exposure-response as well as reproducibility. To simplify this laborious way of measuring drug exposure, several validated abbreviated AUC profiles, accurately predicting the dose-interval AUC, have been put forward. Together with an increasing knowledge of the time-related pharmacokinetic behaviour of immunosuppressive drug and their metabolites, studies are focusing on how to apply abbreviated AUC sampling methods in clinical transplantation, taking into account the numerous factors affecting drug pharmacokinetics. Eventually, TDM using abbreviated AUC profiles has to be prospectively tested against classic methods of drug monitoring in terms of cost-effectiveness, feasibility and clinical relevance with the ultimate goal of improving patient and graft survival.

Formation and protein binding of the acyl glucuronide of a leukotriene B4 antagonist (SB-209247): relation to species differences in hepatotoxicity

SB-209247 [(E)-3-[6-[[(2,6-dichlorophenyl)-thio]methyl]-3-(2-phenylethoxy)-2-pyridinyl]-2-propenoic acid], an anti-inflammatory leukotriene B4 receptor antagonist, was associated in beagle dogs but not male rats with an inflammatory hepatopathy. It also produced a concentration-dependent (10-1000 microM) but equal leakage of enzymes from dog and rat precision-cut liver slices. The hepatic metabolism of SB-209247 was investigated with reference to the formation of reactive acyl glucuronides. [14C]SB-209247 (100 micromol/kg) administered i.v. to anesthetized male rats was eliminated by biliary excretion of the acyl glucuronides of the drug and its sulfoxide. After 5 h, 1.03 +/- 0.14% (mean +/- S.E.M., n = 4) of the dose was bound irreversibly to liver tissue. The sulfoxide glucuronide underwent pH-dependent rearrangement in bile more rapidly than did the SB-209247 conjugate. [14C]SB-209247 was metabolized by sulfoxidation and glucuronidation in rat and dog hepatocytes, and approximately 1 to 2% of [14C]SB-209247 (100 microM) became irreversibly bound to cellular material. [14C]SB-209247 sulfoxide and glucuronide were the only metabolites produced by dog, rat, and human liver microsomes in the presence of NADPH and UDP-glucuronic acid (UDPGA), respectively. V(max) values for [14C]SB-209247 glucuronidation by dog, rat, and human microsomes were 2.6 +/- 0.1, 1.2 +/- 0.1, and 0.4 +/- 0.0 nmol/min/mg protein, respectively. Hepatic microsomes from all three species catalyzed UDPGA-dependent but not NADPH-dependent irreversible binding of [14C]SB-209247 (100-250 microM) to microsomal protein. Although a reactive acyl glucuronide was formed by microsomes from every species, the binding did not differ between species. Therefore, neither the acute cellular injury nor glucuronidation-driven irreversible protein binding in vitro is predictive of the drug-induced hepatopathy.

Elucidation of the mechanism of inhibition of cyclooxygenases by acyl-coenzyme A and acylglucuronic conjugates of ketoprofen

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit the cyclooxygenase (COX) isoforms which accounts for their clinical effects. The differential inhibition of COX-1 and COX-2 is not sufficient to explain the absence of a correlation between in vitro and in vivo effects, especially for 2-aryl-propionates, thus indicating the participation of metabolites. Conjugates to glucuronic acid and to coenzyme-A are mainly produced, and have been shown to be chemically reactive. Therefore, we studied the interaction of the ketoprofen metabolites with the COX enzymes. After incubation with bovine pulmonary artery endothelial cells (BPAEC), COX-1 was inhibited stereoselectively by S-ketoprofen acylglucuronide, and more significantly by CoA-thioester. After washing-out the medium, COX-1 activity was essentially recovered, indicating a reversible inhibition. In LPS-stimulated J774.2 cells, COX activity (mainly inducible COX-2) was inhibited reversibly and stereospecifically by S-ketoprofen glucuronide, whereas it disappeared totally and was not recovered after incubation with CoA-thioester. Correspondingly, inhibition of purified COX-2 with this compound was observed to be rapid and irreversible. Using an anti-ketoprofen antibody, COX immunoprecipitated from cells exhibited adduct formation for COX-2 but not for COX-1. This was observed after incubation with CoA-thioester, and, surprisingly, also with glucuronide. Molecular docking gave support to explain this discrepancy: the glucuronide was found to establish a strong interaction with Y115 located in the membrane binding domain, whereas the thioester was preferentially bound to the active site of the enzyme. Overall, our results suggest a contribution of CoA-thioester metabolites of carboxylic NSAIDs to their pharmacological action by irreversibly and selectively inhibiting COX-2.

Identification of the UDP-glucuronosyltransferase isoforms involved in mycophenolic acid phase II metabolism

Mycophenolic acid (MPA), the active metabolite of the immunosuppressant mycophenolate mofetil is primarily metabolized by glucuronidation. The nature of UDP-glucuronosyltransferases (UGTs) involved in this pathway is still debated. The present study aimed at identifying unambiguously the UGT isoforms involved in the production of MPA-phenyl-glucuronide (MPAG) and MPA-acylglucuronide (AcMPAG). A liquid chromatography-tandem mass spectrometry method allowing the identification and determination of the metabolites of mycophenolic acid was developed. The metabolites were characterized in urine and plasma samples from renal transplant patients under mycophenolate mofetil therapy and in vitro after incubation of mycophenolic acid with human liver (HLM), kidney (HKM), or intestinal microsomes (HIM). The UGT isoforms involved in MPAG or AcMPAG production were investigated using induced rat liver microsomes, heterologously expressed UGT (Supersomes), and chemical-selective inhibition of HLM, HKM, and HIM. The three microsomal preparations produced MPAG, AcMPAG, and two mycophenolate glucosides. Among the 10 UGT isoforms tested, UGT 1A9 was the most efficient for MPAG synthesis with a K(m) of 0.16 mM, close to that observed for HLM (0.18 mM). According to the chemical inhibition experiments, UGT 1A9 is apparently responsible for 55%, 75%, and 50% of MPAG production by the liver, kidney, and intestinal mucosa, respectively. Although UGT 2B7 was the only isoform producing AcMPAG in a significant amount, the selective inhibitor azidothymidine only moderately reduced this production (approximately -25%). In conclusion, UGT 1A9 and 2B7 were clearly identified as the main UGT isoforms involved in mycophenolic acid glucuronidation, presumably due to their high hepatic and renal expression.

Crohn's-like changes in the colon due to mycophenolate?

OBJECTIVE

Mycophenolate mofetil (MMF) is used for prevention of allograft rejection in kidney transplant patients. A subset of patients suffers from chronic diarrhoea of unknown origin. The aim of the study was to investigate the effect of MMF on the colonic mucosa.

MATERIALS AND METHODS

Colonic mucosal biopsies from 24 kidney transplant patients receiving MMF and presenting with chronic diarrhoea were analysed using routine stainings and immunohistochemistry for Ki67 and E-cadherin. Results were compared with a control group of 19 kidney transplant patients not receiving MMF. In all patients routine clinical and laboratory investigations were performed in order to explain the diarrhoea.

RESULTS

In 11 patients, the diarrhoea seemed to be of infectious origin. Furthermore, 19/24 of MMF-patients showed characteristic histological alterations of the mucosa that were Crohn's disease-like: discontinuous crypt architectural distortion, increased epithelial mucin secretion, mildly active inflammation and focal presence of dilated and inflamed crypts. Ki67 staining was abnormal in 6/24 MMF patients but also in 4/19 control patients. E-Cadherin staining was normal in most MMF and control patients.

CONCLUSIONS

Diarrhoea following MMF treatment is frequently infectious in origin and associated with morphological changes with a Crohn's-like pattern in the colonic mucosa in a subset of patients. MMF does not induce major alteration in the proliferative compartment of colonic epithelium. The diarrhoea is not associated with altered E-cadherin expression in the colonic epithelium.

cDNA Microarray Analysis Reveals New Candidate Genes Possibly Linked to Side Effects Under Mycophenolate Mofetil Therapy

BACKGROUND

Mycophenolate mofetil (MMF) the prodrug of mycophenolic acid is usually well tolerated. Side effects such as anemia and diarrhea occur in approximately 10%-15% of patients. The aim of this study was to examine in a rat model the effect of MMF on gene expression in liver and gut to identify target genes with possible relevance to MMF side effects.

METHODS

Twelve Wistar rats were treated with 40 mg/kg body weight MMF orally for 21 days. Controls (n=9) received vehicle only. RNA was extracted from liver, jejunum, ileum, and colon and transcribed into cDNA. Regulated genes were identified in liver by DNA microarray experiments. Gene regulation was verified in liver and gut using quantitative real-time PCR on the LightCycler instrument. Transcription elongation factor 2 served as reference gene.

RESULTS

Microarray analysis revealed that major alpha-hemoglobin, polymeric immunoglobulin receptor, catalase, and CCAAT/enhancer protein alpha gene expression were down-regulated in livers of MMF-treated rats 10-, 5.5-, 4-, and 5-fold, respectively. These findings could be confirmed through quantitative real-time PCR analysis of gene expression in liver, ileum, jejunum, and colon.

CONCLUSION

Using microarray analysis and a rat model four candidate genes which may be functionally linked to side effects (major alpha-hemoglobin-->anaemia; polymeric immunoglobulin receptor-->protection of mucosa; catalase and CCAAT/enhancer protein alpha-->oxidative stress) of MMF therapy were identified.

Human UDP-glucuronosyltransferases show atypical metabolism of mycophenolic acid and inhibition by curcumin

Although the promising immunosuppressant, mycophenolic acid (MPA), has many desirable properties and is widely prescribed for organ transplant recipients, its high oral dosage requirement is not understood. Whereas previous Northern blot analysis by Basu and colleagues (2004) located the mRNAs encoding MPA primary metabolizers, UDP-glucuronosyltransferases (UGTs) 1A7, 1A8, 1A9, and 1A10, in human gastrointestinal (GI) tissues, in situ hybridization analysis of mRNAs found that the isozymes were restricted to the mucosal layer of various GI organs. Concomitantly, MPA was glucuronidated by microsomes isolated from normal adjoining specimens. Microsomal studies showed the highest relative rates of metabolism in esophagus, ileum, duodenum, colon, and stomach at pH 6.4; only esophagus showed high pH 7.6 activity. Properties of the recombinant UGTs indicate that MPA is metabolized with pH 6.4 or 7.6 optimum. Activity for 1A7 and 1A9 increased with increasing concentrations up to 2.4 mM, with parallel production of both ether- and acylglucuronides; similarly, 1A8 and 1A10 reached plateaus at 1.6 mM, producing both glucuronides. K(m) values were 250 to 550 microM. Between 400 and 1600 microM MPA, isozymes generated between 15 and 42% of the acylglucuronides. In effect, high K(m) values (MPA) are associated with high concentrations to achieve saturation kinetics. Finally, transient inhibition of UGTs in human LS180 colon cells and mouse duodenum by the dietary agent, curcumin, has implications for in vivo pretreatment to reduce MPA glucuronidation to increase the therapeutic index.

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.