Effect of mycophenolate acyl-glucuronide on human recombinant type 2 inosine monophosphate dehydrogenase

The effects of food on the pharmacokinetics of mycophenolate mofetil in healthy horses

Additional immunomodulatory treatment is needed for the management of immune-mediated disease in horses. Mycophenolate mofetil (MMF) is an immunomodulatory agent used in human and veterinary medicine for the prevention of graft rejection and the management of autoimmune diseases. Few studies exist investigating the pharmacokinetics of MMF in horses. The aim of this study was to evaluate the pharmacokinetics of a single dose of MMF in healthy horses in the fed vs. fasted state. Six healthy Standardbred mares were administered MMF 10 mg/kg by a nasogastric (NG) tube in a fed and fasted state. A six-day washout period was performed between the two doses. No statistically significant differences in mycophenolic acid (MPA) concentrations were seen at any time point apart from 8 h, when plasma metabolite concentrations were significantly higher in the fasted state compared to the fed state (p = .038). Evidence of enterohepatic recirculation was seen only in the fasted state; this did not yield clinical differences in horses administered a single-dose administration but may be significant in horses receiving long-term MMF treatment.

Noncompartmental pharmacokinetics of three intravenous mycophenolate mofetil concentrations in healthy Standardbred mares

BACKGROUND

Mycophenolate mofetil (MMF) is the prodrug of mycophenolic acid (MPA) which acts as an immunosuppressive agent. During the biotransformation of MMF to MPA, additional metabolites including MPA phenol glucuronide (MPAG), MPA acyl glucuronide (AcMPAG) and MPA phenol glucoside (MPG) are formed.

OBJECTIVE

To define the noncompartmental pharmacokinetic (PK) parameters of three single doses of intravenous (i.v.) MMF and its downstream metabolites in healthy horses.

ANIMALS

Six healthy Standardbred mares.

MATERIALS AND METHODS

Generic MMF (Par Pharmaceuticals; Chestnut Ridge, NY, USA) was reconstituted and administered as a single i.v. bolus at 1.0 mg/kg, 5.0 mg/kg and 10.0 mg/kg with an eight day washout between treatments. Blood samples were collected immediately before MMF administration and over 24 h. A liquid chromatography-tandem mass spectrometry assay was developed following FDA guidance to determine plasma MMF, MPA, MPAG, AcMPAG and MPG concentrations. Plasma concentrations were analysed independently, followed by calculation of geometric mean and coefficient of variation.

RESULTS

Noncompartmental PK parameters were determined for MMF and all metabolites at all doses. MMF was rapidly converted to MPA in all horses. Each incremental dose of MMF resulted in increases in C_max and AUC_inf _obs for MPA and the three additional metabolites. Within the 10-fold dose range, the increase in C_max and AUC_inf _obs for MMF and its metabolites was nonlinear.

CONCLUSIONS AND CLINICAL RELEVANCE

Horses biotransform MMF into MPA, MPAG, AcMPAG and MPG via the glucuronidation and glucosidation clearance pathways. Equine reference PK profiles for MPA and the metabolites, MPAG, AcMPAG and MPG were established.

Association of ABCC2 Haplotypes to Mycophenolic Acid Pharmacokinetics in Stable Kidney Transplant Recipients

Mycophenolic acid exhibits significant interpatient pharmacokinetic variability attributed to factors including race, sex, concurrent medications, and enterohepatic circulation of the mycophenolic acid glucuronide metabolite to mycophenolic acid. This conversion by enterohepatic circulation is mediated by the multidrug resistance-associated protein 2, encoded by ABCC2. This study investigated ABCC2 haplotype associations with mycophenolic acid pharmacokinetics in 147 stable kidney transplant recipients receiving mycophenolic acid in combination with calcineurin inhibitors. The role of the ABCC2 genotypes -24C>T (rs717620), 1249C>T (rs2273697), and 3972C>T (rs3740066) were evaluated in prospective, cross-sectional pharmacokinetic studies of stable recipients receiving mycophenolic acid and either tacrolimus or cyclosporine. Haplotype phenotypic associations with mycophenolic acid pharmacokinetic parameters were computed using THESIAS (v. 3.1). Four ABCC2 haplotypes with estimated frequencies greater than 10% were identified (H1:CGC [wild type], H9:CGT, H2:CAC, H12:TGT). There were no differences in haplotype frequencies by either race or sex. There were significant associations of pharmacokinetic parameters with ABCC2 haplotypes for mycophenolic acid clearance (L/h), mycophenolic acid AUC_0-12h (mg·h/L), and the ratio of mycophenolic acid glucuronide to mycophenolic acid AUC_0-12h . The wild-type haplotype ABCC2 CGC had greater mycophenolic acid AUC_0-12h (P = .017), slower clearance (P = .013), and lower mycophenolic acid glucuronide to mycophenolic acid AUC_0-12h ratio (P = .047) compared with the reduced function ABCC2 haplotype CGT. These differences were most pronounced among patients receiving tacrolimus cotreatment. No phenotypic associations were found with the cyclosporine-mycophenolic acid regimen. Variation in ABCC2 haplotypes contributes to subtherapeutic mycophenolic acid exposure and influences interpatient variability in pharmacokinetic phenotypes based on concurrent calcineurin inhibitor treatment.

Influence of SLCO1B1 521T>C, UGT2B7 802C>T and IMPDH1 -106G>A Genetic Polymorphisms on Mycophenolic Acid Levels and Adverse Reactions in Chinese Autoimmune Disease Patients

Introduction

Mycophenolate mofetil (MMF), a new type of immunosuppressant, has emerged as a frontline agent for treating autoimmune diseases. Mycophenolic acid (MPA) is an active metabolite of MMF. MPA exposure varies greatly among individuals, which may lead to adverse drug reactions such as gastrointestinal side effects, infection, and leukopenia. Genetic factors play an important role in the variation of MPA levels and its side effects. Although many published studies have focused on MMF use in patients after organ transplant, studies that examine the use of MMF in patients with autoimmune diseases are still lacking.

Methods

This study will not only explore the genetic factors affecting MPA levels and adverse reactions but also investigate the relationships between UGT1A9 −118(dT)9/10, UGT1A9 - 1818T>C, UGT2B7 802C>T, SLCO1B1 521T>C, SLCO1B3 334T>G, IMPDH1 −106G>A and MPA trough concentration (MPA C₀), along with adverse reactions among Chinese patients with autoimmune diseases. A total of 120 patients with autoimmune diseases were recruited. The MPA trough concentration was detected using the enzyme multiplied immunoassay technique (EMIT). Genotyping was performed using a real-time polymerase chain reaction (PCR) system and validated allelic discrimination assays. Clinical data were collected for the determination of side effects.

Results

SLCO1B1 521T>C demonstrated a significant association with MPA C₀/d (p=0.003), in which patients with the CC type showed a higher MPA C₀/d than patients with the TT type (p=0.001) or the CT type (p=0.000). No significant differences were found in MPA C₀/d among the other SNPs. IMPDH1 −106G>A was found to be significantly related to infections (p=0.006). Subgroup analysis revealed that UGT2B7 802C>T was significantly related to Pneumocystis carinii pneumonia infection (p=0.036), while SLCO1B1 521T>C was associated with anemia (p=0.029).

Conclusion

For Chinese autoimmune disease patients, SLCO1B1 521T>C was correlated with MPA C₀/d and anemia. IMPDH1 −106G>A was significantly related to infections. UGT2B7 802C>T was significantly related to Pneumocystis carinii pneumonia infection.

Therapeutic drug monitoring of mycophenolate mofetil in pediatric patients: novel techniques and current opinion

Introduction: Mycophenolate mofetil (MMF) is an ester prodrug of the immunosuppressant mycophenolic acid (MPA) and is recommended and widely used for maintenance immunosuppressive therapy in solid organ and stem-cell transplantation as well as in immunological kidney diseases. MPA is a potent, reversible, noncompetitive inhibitor of the inosine monophosphate dehydrogenase (IMPDH), a crucial enzyme in the de novo purine synthesis in T- and B-lymphocytes, thereby inhibiting cell-mediated immunity and antibody formation. The use of therapeutic drug monitoring (TDM) of MMF is still controversial as outcome data of clinical trials are equivocal. Areas covered: This review covers in great depth the existing literature on TDM of MMF in the field of pediatric (kidney) transplantation. In addition, the relevance of TDM in immunological kidney diseases, in particular childhood nephrotic syndrome is highlighted. Expert opinion: TDM of MMF has the potential to optimize therapy in pediatric transplantation as well as in nephrotic syndrome. Limited sampling strategies to estimate MPA exposure increase its feasibility. Future perspectives rather encompass approaches reflecting total immunosuppressive load than single drug TDM.

Determination of mycophenolic acid in human plasma by ultra-performance liquid chromatography-tandem mass spectrometry and its pharmacokinetic application in kidney transplant patients

To implement and validate an analytical method by ultra-performance liquid chromatography-tandem mass spectrometry (UPLC MS/MS) to quantify mycophenolic acid (MPA) in kidney transplant patients. Quantification of MPA was performed in an ACQUITY UPLC H Class system coupled to a Xevo TQD detector and it was extracted from plasma samples by protein precipitation. The chromatographic separation was achieved through an ACQUITY HSS C₁₈ SB column with 0.1% formic acid and acetonitrile (60:40 vol/vol) as mobile phase. The pharmacokinetic parameters were calculated by non-compartmental analysis of MPA plasma concentrations from 10 kidney transplant patients. The linear range for MPA quantification was 0.2-30 mg/L with a limit of detection of 0.07 mg/L; the mean extraction recovery was 99.99%. The mean intra- and inter-day variability were 2.98% and 3.4% with a percentage of deviation of 8.4% and 6.6%, respectively. Mean maximal concentration of 10 mg/L at 1.5 h, area under the concentration-time curve of 36.8 mg·h/L, elimination half-life of 3.9 h, clearance of 0.32 L/h/kg and volume of distribution of 1.65 L/kg were obtained from MPA pharmacokinetics profiles. A simple, fast and reliable UPLC-MS/MS method to quantify MPA in plasma was validated and has been applied for pharmacokinetic analysis in kidney transplant patients.

Impact of SLCO1B3 polymorphisms on clinical outcomes in lung allograft recipients receiving mycophenolic acid

Single-nucleotide polymorphisms (SNPs) in genes involved in mycophenolic acid (MPA) metabolism have been shown to contribute to variable MPA exposure, but their clinical effects are unclear. We aimed to determine if SNPs in key genes in MPA metabolism affect outcomes after lung transplantation. We performed a retrospective cohort study of 275 lung transplant recipients, 228 receiving mycophenolic acid and a control group of 47 receiving azathioprine. Six SNPs known to regulate MPA exposure from the SLCO, UGT and MRP2 families were genotyped. Primary outcome was 1-year survival. Secondary outcomes were 3-year survival, nonminimal (≥A2 or B2) acute rejection, and chronic lung allograft dysfunction (CLAD). Statistical analyses included time-to-event Kaplan-Meier with log-rank test and Cox regression modeling. We found that SLCO1B3 SNPs rs4149117 and rs7311358 were associated with decreased 1-year survival [rs7311358 HR 7.76 (1.37-44.04), p = 0.021; rs4149117 HR 7.28 (1.27-41.78), p = 0.026], increased risk for nonminimal acute rejection [rs4149117 TT334/T334G: OR 2.01 (1.06-3.81), p = 0.031; rs7311358 GG699/G699A: OR 2.18 (1.13-4.21) p = 0.019] and lower survival through 3 years for MPA patients but not for azathioprine patients. MPA carriers of either SLCO1B3 SNP had shorter survival after CLAD diagnosis (rs4149117 p = 0.048, rs7311358 p = 0.023). For the MPA patients, Cox regression modeling demonstrated that both SNPs remained independent risk factors for death. We conclude that hypofunctional SNPs in the SLCO1B3 gene are associated with an increased risk for acute rejection and allograft failure in lung transplant recipients treated with MPA.

IMPDH2 promotes colorectal cancer progression through activation of the PI3K/AKT/mTOR and PI3K/AKT/FOXO1 signaling pathways

Background

Inosine 5′-monophosphate dehydrogenase type II (IMPDH2) was originally identified as an oncogene in several human cancers. However, the clinical significance and biological role of IMPDH2 remain poorly understood in colorectal cancer (CRC).

Methods

Quantitative real-time polymerase chain reaction (qPCR), western blotting analysis, the Cancer Genome Atlas (TCGA) data mining and immunohistochemistry were employed to examine IMPDH2 expression in CRC cell lines and tissues. A series of in-vivo and in-vitro assays were performed to demonstrate the function of IMPDH2 and its possible mechanisms in CRC.

Results

IMPDH2 was upregulated in CRC cells and tissues at both mRNA and protein level. High IMPDH2 expression was closely associated with T stage, lymph node state, distant metastasis, lymphovascular invasion and clinical stage, and significantly correlated with poor survival of CRC patients. Further study revealed that overexpression of IMPDH2 significantly promoted the proliferation, invasion, migration and epithelial-mesenchymal transition (EMT) of CRC cells in vitro and accelerated xenograft tumour growth in nude mice. On the contrary, knockdown of IMPDH2 achieved the opposite effect. Gene set enrichment analysis (GSEA) showed that the gene set related to cell cycle was linked to upregulation of IMPDH2 expression. Our study verified that overexpressing IMPDH2 could promote G1/S phase cell cycle transition through activation of PI3K/AKT/mTOR and PI3K/AKT/FOXO1 pathways and facilitate cell invasion, migration and EMT by regulating PI3K/AKT/mTOR pathway.

Conclusions

These results suggest that IMPDH2 plays an important role in the development and progression of human CRC and may serve as a novel prognostic biomarker and therapeutic target for CRC.

Safety of oral and intravenous mycophenolate mofetil in healthy cats

Objectives The aim of this study was to evaluate the safety and clinical effects of intravenous (IV) and oral mycophenolate mofetil (MMF) in healthy cats. Methods A total of 24 healthy adult cats weighing >3.5 kg were either given IV MMF (over a 2 h infusion) or oral MMF. The dosages used were as follows: 5 mg/kg IV once (n = 2), 10 mg/kg q12h IV for 1 day (n = 1), 20 mg/kg q12h IV for 1 day (n = 6) and 10 mg/kg q12h IV for 3 days (n = 5). Blood was collected from each cat at intervals of up to 12 h from the last dose for analysis purposes. Oral MMF was given at 10 mg/kg q12h for 7 days (n = 3), 15 mg/kg q12h for 7 days (n = 3) and 15 mg/kg q8h for 7 days (n = 4). Results Side effects to MMF were minimal. There was no anorexia or vomiting noted in any of the cats during or after IV medication administration. Only 4/14 cats had diarrhea from 12-48 h after IV administration. There was hyporexia in 1/10 cats given oral MMF and no vomiting noted. In 5/10 cats given oral MMF, there was diarrhea between days 2 and 7 of the study. Conclusions and relevance Cats tolerate MMF at an IV dose of 10 mg/kg q12h for 3 days and an oral dose ⩽15 mg/kg q12h for up to 7 days. There seems to be a dose-dependent incidence of gastrointestinal side effects. MMF may be a useful alternative immunosuppressant to be considered for use in some cats.

Comparative metabolism of mycophenolic acid by glucuronic acid and glucose conjugation in human, dog, and cat liver microsomes

Use of the immunosuppressant mycophenolic acid (MPA) in cats is limited because MPA elimination depends on glucuronidation, which is deficient in cats. We evaluated formation of major (phenol glucuronide) and minor (acyl glucuronide, phenol glucoside, and acyl glucoside) MPA metabolites using liver microsomes from 16 cats, 26 dogs, and 48 humans. All MPA metabolites were formed by human liver microsomes, while dog and cat liver microsomes formed both MPA glucuronides, but only one MPA glucoside (phenol glucoside). Intrinsic clearance (CLint) of MPA for phenol glucuronidation by cat liver microsomes was only 15-17% that of dog and human liver microsomes. However, CLint for acyl glucuronide and phenol glucoside formation in cat liver microsomes was similar to or greater than that for dog and human liver microsomes. While total MPA conjugation CLint was generally similar for cat liver microsomes compared with dog and human liver microsomes, relative contributions of each pathway varied between species with phenol glucuronidation predominating in dog and human liver microsomes and phenol glucosidation predominating in cat liver microsomes. MPA conjugation variation between cat liver microsomes was threefold for total conjugation and for phenol glucosidation, sixfold for phenol glucuronidation, and 11-fold for acyl glucuronidation. Our results indicate that total MPA conjugation is quantitatively similar between liver microsomes from cats, dogs, and humans despite large differences in the conjugation pathways that are utilized by these species.

Use of pharmacogenomics in pediatric renal transplant recipients

Transplant recipients receive potent immunosuppressive drugs in order to prevent graft rejection. Therapeutic drug monitoring is the current approach to guide the dosing of calcineurin inhibitors, mammalian target of rapamycin inhibitors (mTORi) and mofetil mycophenolate. Target concentrations used in pediatric patients are extrapolated from adult studies. Gene polymorphisms in metabolizing enzymes and drug transporters such as cytochromes CYP3A4 and CYP3A5, UDP-glucuronosyl transferase, and P-glycoprotein are known to influence the pharmacokinetics and dose requirements of immunosuppressants. The implications of pharmacogenomics in this patient population is discussed. Genetic information can help with achieving target concentrations in the early post-transplant period, avoiding adverse drug reactions and drug-drug interactions. Evidence about genetic studies and transplant outcomes is revised.

Associations between polymorphisms in target, metabolism, or transport proteins of mycophenolate sodium and therapeutic or adverse effects in kidney transplant patients

OBJECTIVES

Different associations between single nucleotide polymorphisms (SNPs) in cellular target, metabolism enzymes or transport proteins, and biopsy-proven acute rejection (BPAR) or adverse events have been reported in transplant patients receiving mycophenolate mofetil. This work aimed to study these in patients on enteric-coated mycophenolate sodium (EC-MPS).

PATIENTS AND METHODS

The study included 189 renal transplant patients from the DOMINOS trial. Fifteen SNPs in IMPDH2, IMPDH1, ABCC2, SLCO1B3, UGT1A8, UGT1A9, UGT2B7, CYP2C8, HUS1, and IL12A were genotyped in all patients. Associations between SNPs and the first event of BPAR or diarrhea were investigated using multivariate logistic regressions. Associations between SNPs and leukopenia or anemia at nine different visits between days 0 and 190 after transplantation were studied using time-dependent Cox proportional hazards regression models.

RESULTS

Multivariate analyses showed that the CYP2C8 rs11572076 wild-type genotype was associated significantly with a lower risk of leukopenia [GG vs. GA: hazard ratio (95% confidence interval) 0.14 (0.03, 0.59), P=0.00783]. Higher EC-MPS doses and the UGT2B7 c.-840 G>A variant allele were associated with an increased risk of anemia [EC-MPS per unit dose increase: 1.004 (1.003, 1.005), P<0.0001; UGT2B7 GA vs. AA: 1.65 (1.12, 2.43), P=0.01043; GG vs. AA: 1.88 (1.23, 2.88), P=0.00343]. However, no significant association was found between any of the SNPs studied and diarrhea or BPAR.

CONCLUSION

Two pharmacogenetic associations reported previously with mycophenolate mofetil were found in a population of 189 renal transplant patients treated with EC-MPS.

Pharmacology and toxicology of mycophenolate in organ transplant recipients: an update

This review aims to provide an update of the literature on the pharmacology and toxicology of mycophenolate in solid organ transplant recipients. Mycophenolate is now the antimetabolite of choice in immunosuppressant regimens in transplant recipients. The active drug moiety mycophenolic acid (MPA) is available as an ester pro-drug and an enteric-coated sodium salt. MPA is a competitive, selective and reversible inhibitor of inosine-5'-monophosphate dehydrogenase (IMPDH), an important rate-limiting enzyme in purine synthesis. MPA suppresses T and B lymphocyte proliferation; it also decreases expression of glycoproteins and adhesion molecules responsible for recruiting monocytes and lymphocytes to sites of inflammation and graft rejection; and may destroy activated lymphocytes by induction of a necrotic signal. Improved long-term allograft survival has been demonstrated for MPA and may be due to inhibition of monocyte chemoattractant protein 1 or fibroblast proliferation. Recent research also suggested a differential effect of mycophenolate on the regulatory T cell/helper T cell balance which could potentially encourage immune tolerance. Lower exposure to calcineurin inhibitors (renal sparing) appears to be possible with concomitant use of MPA in renal transplant recipients without undue risk of rejection. MPA displays large between- and within-subject pharmacokinetic variability. At least three studies have now reported that MPA exhibits nonlinear pharmacokinetics, with bioavailability decreasing significantly with increasing doses, perhaps due to saturable absorption processes or saturable enterohepatic recirculation. The role of therapeutic drug monitoring (TDM) is still controversial and the ability of routine MPA TDM to improve long-term graft survival and patient outcomes is largely unknown. MPA monitoring may be more important in high-immunological recipients, those on calcineurin-inhibitor-sparing regimens and in whom unexpected rejection or infections have occurred. The majority of pharmacodynamic data on MPA has been obtained in patients receiving MMF therapy in the first year after kidney transplantation. Low MPA area under the concentration time from 0 to 12 h post-dose (AUC0-12) is associated with increased incidence of biopsy-proven acute rejection although AUC0-12 optimal cut-off values vary across study populations. IMPDH monitoring to identify individuals at increased risk of rejection shows some promise but is still in the experimental stage. A relationship between MPA exposure and adverse events was identified in some but not all studies. Genetic variants within genes involved in MPA metabolism (UGT1A9, UGT1A8, UGT2B7), cellular transportation (SLCOB1, SLCO1B3, ABCC2) and targets (IMPDH) have been reported to effect MPA pharmacokinetics and/or response in some studies; however, larger studies across different ethnic groups that take into account genetic linkage and drug interactions that can alter a patient's phenotype are needed before any clinical recommendations based on patient genotype can be formulated. There is little data on the pharmacology and toxicology of MPA in older and paediatric transplant recipients.

Characterization of the novel Trypanosoma brucei inosine 5'-monophosphate dehydrogenase

There is an alarming rate of human African trypanosomiasis recrudescence in many parts of sub-Saharan Africa. Yet, the disease has no successful chemotherapy. Trypanosoma lacks the enzymatic machinery for the de novo synthesis of purine nucleotides, and is critically dependent on salvage mechanisms. Inosine 5'-monophosphate dehydrogenase (IMPDH) is responsible for the rate-limiting step in guanine nucleotide metabolism. Here, we characterize recombinant Trypanosoma brucei IMPDH (TbIMPDH) to investigate the enzymatic differences between TbIMPDH and host IMPDH. Size-exclusion chromatography and analytical ultracentrifugation sedimentation velocity experiments reveal that TbIMPDH forms a heptamer, different from type 1 and 2 mammalian tetrameric IMPDHs. Kinetic analysis reveals calculated K m values of 30 and 1300 μ m for IMP and NAD, respectively. The obtained K m value of TbIMPDH for NAD is approximately 20-200-fold higher than that of mammalian enzymes and indicative of a different NAD binding mode between trypanosomal and mammalian IMPDHs. Inhibition studies show K i values of 3·2 μ m, 21 nM and 3·3 nM for ribavirin 5'-monophosphate, mycophenolic acid and mizoribine 5'-monophosphate, respectively. Our results show that TbIMPDH is different from its mammalian counterpart and thus may be a good target for further studies on anti-trypanosomal drugs.

Inosine 5'-monophosphate dehydrogenase activity as a biomarker in the field of transplantation

Inosine 5'monophosphate dehydrogenase (IMPDH) is the rate limiting enzyme in the de novo synthesis of guanine nucleotides. The direct determination of target enzyme activity as a biomarker of mycophenolic acid (MPA) may help to estimate better the individual response to the immunosuppressant. However, the assessment of the clinical utility of this approach is limited by the diversity of the assay systems, which has not yet allowed the prospective assessment of this enzyme in larger patient cohorts. A recently validated and standardized assay allows the investigation of IMPDH activity in larger clinical studies. Although descriptive results from observational studies hold promise for a more individualized therapy in transplant medicine, more studies are needed to prospectively validate this approach.

The influence of pharmacogenetics and cofactors on clinical outcomes in kidney transplantation

INTRODUCTION

Immunosuppressive drugs have a narrow therapeutic range and large inter-individual response variability. This has prompted pharmacogenetic studies, mostly with regard to their dose-concentration relationships, but also about proteins involved in their pharmacodynamics. Some polymorphisms of genes involved in their disposition pathways were shown to affect their dose-concentration relationships. The impact of pharmacogenetics on tissue distribution and the resulting clinical effects have less often been studied. More importantly, a few single nucleotide polymorphisms seem to have a significant impact on the incidence of acute rejection or the adverse effects of immunosuppressants. Environmental factors often interact with such genotype-phenotype relationships.

AREAS COVERED

This article reviews the impact of genetic polymorphisms of the metabolic enzymes, membrane transporters and target proteins of mycophenolic acid, calcineurin inhibitors and mammalian target of rapamycin inhibitors on clinical outcomes in kidney transplantation.

EXPERT OPINION

The current level of evidence is not yet high enough to recommend pharmacogenetic personalization of immunosuppressive regimens in transplant recipients. The prevention of cellular toxicity associated with local metabolism or transport, which cannot be addressed by routine monitoring, is worth investigating further.

The evolution of population pharmacokinetic models to describe the enterohepatic recycling of mycophenolic acid in solid organ transplantation and autoimmune disease

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

Acyl glucuronides: the good, the bad and the ugly

Acyl glucuronidation is the major metabolic conjugation reaction of most carboxylic acid drugs in mammals. The physiological consequences of this biotransformation have been investigated incompletely but include effects on drug metabolism, protein binding, distribution and clearance that impact upon pharmacological and toxicological outcomes. In marked contrast, the exceptional but widely disparate chemical reactivity of acyl glucuronides has attracted far greater attention. Specifically, the complex transacylation and glycation reactions with proteins have provoked much inconclusive debate over the safety of drugs metabolised to acyl glucuronides. It has been hypothesised that these covalent modifications could initiate idiosyncratic adverse drug reactions. However, despite a large body of in vitro data on the reactions of acyl glucuronides with protein, evidence for adduct formation from acyl glucuronides in vivo is limited and potentially ambiguous. The causal connection of protein adduction to adverse drug reactions remains uncertain. This review has assessed the intrinsic reactivity, metabolic stability and pharmacokinetic properties of acyl glucuronides in the context of physiological, pharmacological and toxicological perspectives. Although numerous experiments have characterised the reactions of acyl glucuronides with proteins, these might be attenuated substantially in vivo by rapid clearance of the conjugates. Consequently, to delineate a relationship between acyl glucuronide formation and toxicological phenomena, detailed pharmacokinetic analysis of systemic exposure to the acyl glucuronide should be undertaken adjacent to determining protein adduct concentrations in vivo. Further investigation is required to ascertain whether acyl glucuronide clearance is sufficient to prevent covalent modification of endogenous proteins and consequentially a potential immunological response.

Pharmacokinetic optimization of immunosuppressive therapy in thoracic transplantation: part II

Part I of this article, which appeared in the previous issue of the Journal, reviewed calcineurin inhibitors--ciclosporin and tacrolimus. In part II, we review the pharmacokinetics and therapeutic drug monitoring of mycophenolate and mammalian target of rapamycin inhibitors--sirolimus and everolimus--in thoracic transplantation, and we provide an overall discussion and suggest various areas for future study.

Investigation of the crossreactivity of mycophenolic acid glucuronide metabolites and of mycophenolate mofetil in the Cedia MPA assay

The immunosuppressant mycophenolic acid (MPA) used for solid organ transplantation is predominantly metabolized to a pharmacologically inactive phenolic glucuronide (MPAG) and, to a lesser extent, to the pharmacologically active acyl glucuronide (AcMPAG). The recently introduced CEDIA Mycophenolic Acid Assay from Microgenics has been reported to overestimate MPA in clinical samples and crossreactivity with AcMPAG has been suspected. A detailed investigation of the crossreactivity of AcMPAG and the prodrug mycophenolate mofetil (MMF) in the CEDIA assay is presented using pure substances. In addition, MPA concentrations in plasma were compared with a validated high-performance liquid chromatography-ultraviolet method. Plasma samples from kidney (KTx, n = 50), heart (HTx, n = 50), and liver (LTx, n = 50) transplant recipients were analyzed by the CEDIA (MPA) and a high-performance liquid chromatography-ultraviolet method (MMF, MPA, MPAG, AcMPAG). Crossreactivity of MMF (0.93-46.3 mg/L), MPAG (50-1000 mg/L), and AcMPAG (0.5-10 mg/L) was investigated using spiked drug-free plasma. Method comparison was performed using Bland & Altman and Passing & Bablok analysis. The method bias was correlated to AcMPAG concentrations using Spearman's rank correlation. Crossreactivity with AcMPAG and MMF was concentration-dependent and reached 215% and 143%, respectively. There was no crossreactivity with MPAG. The CEDIA assay showed a mean positive bias of 36.3% in patient samples. The mean bias was lowest with HTx samples (15%), 41.7% with KTx samples, and highest with LTx samples (52.3%). There was a positive correlation between the method bias and AcMPAG concentrations (r = 0.829; P < 0.001). No MMF was detected in patient samples. The CEDIA overestimates MPA concentrations on average by 36%. This bias is mainly the result of AcMPAG as previously observed with the EMIT MPA assay. It should be considered that the putative therapeutic range for MPA with the CEDIA assay will be higher than the range using high-performance liquid chromatography.

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

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

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.