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

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

霉酚酸暴露量临床评估

霉酚酸(Mycophenolateacid,MPA)可非竞争性地结合次黄嘌呤单核苷酸脱氢酶(Inosinemonophosphatedehydrogenase,IMPDH),抑制淋巴细胞增殖。临床将其前体药物霉酚酸酯作为免疫抑制剂,广泛应用于器官移植患者。由于霉酚酸治疗窗窄,影响其药物代谢因素较多,药动学个体差异较大,临床应用时必须监测霉酚酸血药浓度,将暴露量维持在有效范围内。本文就临床评估霉酚酸暴露量的方法做一综述,以期为临床合理用药提供参考。     

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

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

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

Purpose

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

Methods

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

Results

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

Conclusions

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

Therapeutic drug monitoring of mycophenolate mofetil in transplantation

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

Effect of mycophenolate mofetil on IMP dehydrogenase after the first dose and after long-term treatment in renal transplant recipients

OBJECTIVE: Mycophenolate mofetil (MMF) is routinely used as an immunosuppressant in a fixed daily dose regimen although it shows marked fluctuations in pharmacokinetics, and despite the fact that in regard to the active metabolite, mycophenolic acid (MPA), there is a well-known association between the pharmacokinetic parameters and clinical outcome. METHOD: In order to determine the time course and the variability in cellular target of MPA after renal transplantation, we investigated the pharmacodynamic response in 8 patients receiving 1 g MMF for the first time prior to renal transplantation and in 8 stable renal transplant patients maintained on long-term MMF therapy (1 g b.i.d.) for more than 1 year. The pharmacodynamic response was measured using inosine 5'-monophosphate dehydrogenase (IMPDH) activity in peripheral mononuclear cells. MPA plasma concentrations were measured in parallel, IMPDH activity in 89 healthy blood donors was used as a control. RESULTS: We observed a high interindividual variability in IMPDH activity in the 89 untreated healthy volunteers (4.0 - 32.9 nmol/h/mg protein), in 8 patients on dialysis (5.3 - 18.9 nmol/h/mg protein) and in 8 renal transplant patients under long-term MMF treatment (2.3 - 14.4 nmol/h/mg protein). The mean AUC0-12h for mycophenolic acid was 2-fold higher in patients receiving long-term treatment with MMF (62.2 +/- 16.6 mg x h/ml) compared to dialysis patients receiving 1 g MMF for the first time (31.5 +/- 15.6 mg x h/ml). Despite this pharmacokinetic difference there were no statistically significant differences in the cellular pharmacodynamic response. Minimal IMPDH activity (1.62 +/- 1.23 vs. 1.77 +/- 1.49 nmol/h/mg protein) and maximal IMPDH inhibition (87.5 +/- 0.08 vs. 77.4 +/- 18.8%) during the dosing interval were similar. CONCLUSIONS: The considerable interindividual variability in the pharmacokinetics of MMF as well as in the drug target support the use of pharmacodynamic drug monitoring to optimize MMF dosing and to reduce the risk of graft rejection and side effects.     

Individualization of drug dosage--past,present and future

The development of drug immunoassays led to the introduction of therapeutic drug monitoring and the use of population pharmacokinetic (PK) methodology to develop guidelines for individualizing drug dosage. Recognition of pharmacodynamic (PD) variability and identification of PK parameters that best correlate with clinical response has occurred in several areas including oncology, infection and immunosuppression. In the future, drugs will be selected to suit the genetic characteristics that determine the likelihood of a response, while individualization of dosage will consider genetic information on drug metabolism.     

Review of the antiproliferative properties of mycophenolate mofetil in non-immune cells

Mycophenolate mofetil (MMF), the prodrug ofmycophenolic acid (MPA), is a selective, non-competitive and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH) and of the type II isoform in particular. IMPDH is the rate-limiting enzyme in the de novo biosynthesis of guanosine nucleotides. MMF strongly inhibits both T- and B lymphocyte proliferation and has been used in the prevention of acute and chronic allograft rejection since the mid 1990s. Recent evidence, however, suggests that MMF is also capable of inhibiting the proliferation of non-immune cells. In various cell lines, i.e. smooth muscle cells, renal tubular cells and mesangial cells, MPA reduced or even abrogated proliferation in response to proliferative stimuli. Furthermore, data from our own laboratory demonstrate a dose-dependent inhibition of dermal fibroblast proliferation by MPA. In animal studies, MMF ameliorated renal lesions in immune-mediated disease, i.e. in the anti-thy 1.1 model and experimental lupus nephritis, but was also effective in non-immune-mediated renal damage in the rat remnant-kidney model. These observations prompted several investigators to study the effects of MMF in proliferating (renal) disease of non-immune origin in humans. MMF significantly reduced proteinuria in minimal-change disease and focal segmental glomerulosclerosis. In addition, MMF showed beneficial effects in the treatment of chronic allograft nephropathy and calcineurin inhibitor toxicity through reduction of immune- and non-immune-mediated renal damage. MMF is well tolerated and has proven to be a relatively safe drug causing only minor bone marrow suppression. Taken together, there is a growing body of evidence pointing to therapeutic applications of MMF other than immunosuppression, in particular the prevention of fibrosis.     

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

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

Optimal immunosuppressive therapy based on pharmacokinetics and pharmacodynamics of antimetabolites in clinical practice

An immunosuppressive antimetabolite, mycophenolate mofetil (MMF), has been widely used in combination with a calcineurin inhibitor for organ transplantation and autoimmune diseases. A fixed dosing of MMF often causes bone marrow toxicity or cytomegalovirus antigenemia under the optimal dosing of calcineurin inhibitors. Pharmacokinetic characteristics of MMF and its relation to the degree of immune suppression have not been fully clarified in clinical practice. This review summarizes our achievements on pharmacokinetic disposition of mycophenolic acid (MPA) and inosine 5'-monophosphate dehydrogenase (IMPDH) activity in patients with kidney transplantation and with lupus nephritis. Contribution of enterohepatic recirculation to plasma disposition of MPA in lupus nephritis patients was similar to that in tacrolimus-treated kidney transplant recipients. MPA pharmacokinetics in lupus nephritis was characterized by high MPA clearance most likely due to better renal function. In addition, concomitant metal cation decreased MPA concentration in patients receiving tacrolimus but not cyclosporine. This interaction may depend on amount of biliary-excreted MPA glucuronide. Renal clearance of MPA was higher in cyclosporine- than tacrolimus-treated patients. Its ratio to creatinine clearance was much higher than unbound fraction of MPA in each calcineurin inhibitor treatment. These kinetic data revealed the presence of renal tubular secretion in the urinary excretion process. In multivariate analysis, the plasma disposition of MPA and its glucuronides affected IMPDH activity in erythrocytes. The IMPDH activity might be a useful marker reflecting a long-term exposure by MPA. Our findings in this review would contribute to optimal dosing of MMF in immunosuppressive regimen including a calcineurin inhibitor.     

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 (AUC_0–12) is associated with increased incidence of biopsy-proven acute rejection although AUC_0–12 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.     

Therapeutic drug monitoring of mycophenolic acid: does it improve patient outcome?

Treatment with the immunosuppressive agent mycophenolate mofetil (MMF) decreases the risk of rejection after renal transplantation and improves graft survival compared with azathioprine. The exposure to the active metabolite mycophenolic acid (MPA) is correlated to the risk of developing acute rejection. The interpatient variability in exposure of MPA is wide relative to the proposed therapeutic window of the MPA AUC(0 12) (30 - 60 mg.h/l). The pharmacokinetics of MPA are influenced by patient characteristics such as gender, time after transplantation, serum albumin concentration, renal function, comedication and pharmacogenetic factors. Therapeutic drug monitoring is likely to reduce inter-patient variability. Limited sampling strategies are used to predict the full AUC(0 12). Three prospective randomised studies compared concentration controlled MMF therapy to a fixed-dose regimen. Preliminary outcomes of these studies showed conflicting results and longer follow up is needed to further clarify the role of therapeutic drug monitoring in increasing the therapeutic potential of MMF.     

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Novel mycophenolic adenine bis(phosphonate)s as potential immunosuppressants.

Mycophenolic acid (MPA) is the most potent and specific inhibitor of inosine monophosphate dehydrogenase (IMPDH). This compound was reported to bind the NAD site of IMPDH and mimic the binding of nicotinamide moiety of nicotinamide adenine dicnucleotide. We linked MPA derivatives with the adenine moiety of NAD through a methylenebis(phonphonate) birdge to form novel mycophenolic adenine dinucleotides (MADs) which resemble well the intact natural cofactor. The MAD analogues differ by the length of the side chain (linker) between the aromatic ring of mycophenolic derivative and the beta-phosphorus atom of the adenosine bis(phosphonate) moiety. Regardless of the linker size, MADs were found to be potent inhibitors of human IMPDH type I and type II with Ki's = 0.25-0.52 microM, an order of magnitude less potent than MPA itself (Ki = 0.01-0.04 microM). The growth of K562 cells was inhibited by MPA (IC50 = 0.03 microM) and the MAD analogues (IC50 = 0.01-1.15 microM) with a similar potency. Accordingly, a suppression of alloantigen- induced proliferation of human lymphocytes by the MAD analogues at concentration of 10-20 microM was equally effective as that observed for MPA. In contrast to MPA, MAD analogues were found to be resistant to glucuronidation in vitro. Since therapeutic potential of MPA is limited by its undesirable glucuronidation, the glucuronidation- resistant MAD analogues may be superior immunosuppressants if they are not glucuronidated in vivo.     

Mycophenolic acid inhibits inosine 5'-monophosphate dehydrogenase and suppresses immunoglobulin and cytokine production of B cells

Mycophenolic acid (MPA) reversibly inhibits inosine 5'-monophosphate dehydrogenase (IMPDH), an enzyme involved in the de novo synthesis of guanine nucleotides. Previously, mycophenolate mofetil (MMF), the pro-drug of MPA, was shown to exert beneficial effects on the systemic lupus erythematosus (SLE)-like disease in MRLlpr/lpr mice. In this study, MPA's immunomodulating effects in vitro on the B cell hybridoma MAR 18.5 were investigated. The cells were exposed for MPA at either 1 or 10 microM for 24 h, and the levels of immunoglobulins, cytokines and lactate dehydrogensase in supernatants were measured. The frequency of immunoglobulin producing cells and the proliferation and viability of the cells was also investigated. MPA exposure reduced the frequency of immunoglobulin producing cells, decreased the levels of immunoglobulins and cytokines in the supernatants, and decreased the cell proliferation. MPA was slightly cytotoxic as indicated by increased lactate dehydrogenase (LDH) levels and reduced viability. All MPA-induced effects were totally reversed by the addition of guanosine to the cultures. Thus, since activated B lymphocytes play a central role in lupus and our results show that B cells are targets for MPA, we propose that direct effects on B cells may be an important mechanism for the ameliorating effects of MMF in SLE.     

药物动力学和药效动力学结合在抗菌药物研发和临床治疗中的应用

面临耐药性致病菌的逐年增加,新型抗菌药物亟待开发。众所周知,新药开发是一个艰难和复杂的过程。制定合理给药方案是药物开发的重要课题,而以制定抗菌药物的合理给药方案最富挑战性。近二十年来,便宜快捷的体外动力学实验和动物体内感染模型以及药效学数据处理方法不断完善,其价值在抗菌新药开发上被不断验证和肯定,弥补了临床试验所无法获得的信息。近十年来临床群体药代动力学模型和蒙地卡罗模拟结合临床前实验确定的靶值,致病菌MIC分布,利用电脑模拟比较不同给药方案的中靶率,已成为临床三期试验确定最佳给药方案行之有效的方法和针对耐药菌抗菌药物临床剂量再评价和进一步调整的科学依据。可以预期,在今后的抗菌素新药的开发上,人们将更有把握从临床前的实验数据来预计临床最佳给药方案,并优化临床试验的设计,达到既快速又经济的目的。     

Pharmacological profiles of mycophenolate mofetil (CellCept), a new immunosuppressive agent

Mycophenolate mofetil (MMF, CellCept), a semisynthetic derivative of mycophenolic acid (MPA) produced by a fungus, is an inhibitor of the inosine monophosphate dehydrogenase (IMPDH) enzyme (IC50 = 25 nM) that catalyzes the synthesis of guanosine monophosphate (GMP) from inosine. GMP is an essential nucleoside for purine synthesis during cell division. As T and B-lymphocytes almost exclusively use the de novo pathway of purine synthesis, these cells are particularly sensitive to the inhibitory action of MMF. It has a mechanism of action distinct from cyclosporine and tacrolimus. Although MMF does not affect cytokine production, by inhibiting the rate-limiting enzyme IMPDH in the de novo synthesis of purines, it inhibits the proliferation of T and B-lymphocytes, the production of antibodies, and the generation of cytotoxic T lymphocytes. Reversal of acute allograft rejection and increased survival of kidney, heart and bone marrow cell allograft has been shown in several animal studies. Moreover, it was suggested that MMF combined with CsA prevented the acute rejection, and approximately half of the animals became long-term survivors. The Ministry of Health and Welfare approved MMF in 1999 for use for rejection treatment in renal transplantation based on several prospective, randomized and blind efficacy trials.     

Mycophenolate mofetil and its mechanisms of action

Mycophenolate mofetil (MMF, CellCept(R)) is a prodrug of mycophenolic acid (MPA), an inhibitor of inosine monophosphate dehydrogenase (IMPDH). This is the rate-limiting enzyme in de novo synthesis of guanosine nucleotides. T- and B-lymphocytes are more dependent on this pathway than other cell types are. Moreover, MPA is a fivefold more potent inhibitor of the type II isoform of IMPDH, which is expressed in activated lymphocytes, than of the type I isoform of IMPDH, which is expressed in most cell types. MPA has therefore a more potent cytostatic effect on lymphocytes than on other cell types. This is the principal mechanism by which MPA exerts immunosuppressive effects. Three other mechanisms may also contribute to the efficacy of MPA in preventing allograft rejection and other applications. First, MPA can induce apoptosis of activated T-lymphocytes, which may eliminate clones of cells responding to antigenic stimulation. Second, by depleting guanosine nucleotides, MPA suppresses glycosylation and the expression of some adhesion molecules, thereby decreasing the recruitment of lymphocytes and monocytes into sites of inflammation and graft rejection. Third, by depleting guanosine nucleotides MPA also depletes tetrahydrobiopterin, a co-factor for the inducible form of nitric oxide synthase (iNOS). MPA therefore suppresses the production by iNOS of NO, and consequent tissue damage mediated by peroxynitrite. CellCept(R) suppresses T-lymphocytic responses to allogeneic cells and other antigens. The drug also suppresses primary, but not secondary, antibody responses. The efficacy of regimes including CellCept(R) in preventing allograft rejection, and in the treatment of rejection, is now firmly established. CellCept(R) is also efficacious in several experimental animal models of chronic rejection, and it is hoped that the drug will have the same effect in humans.     

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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