Non-radioactive determination of inosine 5'-monophosphate dehydro-genase (IMPDH) in peripheral mononuclear cells

Pharmacodynamic evaluation of the first dose of mycophenolate mofetil before kidney transplantation

BACKGROUND AND OBJECTIVES

The effect of mycophenolate mofetil (MMF) on T cell function has not been evaluated in patients undergoing kidney transplantation. The aim of this study was to assess the effect of 1g of MMF on T cell function, that is, intralymphocyte cytokine expression, T cell activation (CD25 and CD71), and T cell proliferation, as well as inosine monophosphate dehydrogenase (IMPDH) activity, to better understand the relationship between pharmacokinetic and pharmacodynamic markers in patients receiving the first dose of MMF before kidney transplantation.

PATIENTS

Twenty-four patients undergoing a kidney transplantation from a living donor were enrolled in this study.

RESULTS

Compared with baseline (before MMF intake), T cell proliferation (93%), IMPDH activity (74%), CD25 (46%), and CD71 (50%) expression significantly decreased during the first hour after MMF intake, in parallel to the rise in MPA concentration. Thereafter, all pharmacodynamic markers, except IMPDH activity, returned back to baseline level. There was a complex inverse relationship between pharmacokinetic and pharmacodynamic markers. The inhibition of T cell proliferation was highly correlated to IMPDH activity, but also to T cell activation markers.

CONCLUSION

The administration of MMF to patients is associated not only with a dramatic decrease in both T cell proliferation and IMPDH activity, but also with in a decrease in CD25 and CD71 expression.

Variation in Leukocyte Subset Concentrations Affects Calcineurin Activity Measurement: Implications for Pharmacodynamic Monitoring Strategies

Background: In renal transplantation patients, therapeutic drug monitoring of the calcineurin (CN) inhibitor cyclosporin A (CsA) is mandatory because of the drug’s narrow therapeutic index. Pharmacodynamic monitoring of CN inhibition therapy could provide a tool to define and maintain the therapeutic efficacy of CsA therapy. We investigated the effect of variation in cell counts of leukocyte subsets on leukocyte CN activity measurement in renal transplant recipients.

Methods: We measured leukocyte CN activity, whole blood CsA concentrations, and leukocyte subset cell counts in 25 renal transplant recipients. Blood was collected before graft implantation and CsA therapy, 1 day before transplantation when CsA therapy was already started, and 5 days after transplantation. Monocyte, granulocyte, CD4+ T-cell, CD8+ T-cell, B-cell, and natural killer–cell CN activities and CsA inhibition sensitivities were determined in vitro by a spectrophotometric CN assay.

Results: Leukocyte CN activity was inhibited after drug intake. Inter- and intrapatient variation in leukocyte subset cell counts resulted in variation of sample composition. The mean (SD) CN activity varied among leukocyte cell subsets, ranging from 650 (230) to 166 (26) pmol/min/106 cells for monocytes and CD4+ T cells, respectively. CsA half maximal inhibitory concentration (IC50) values ranged from 15 to 78 μg/L for monocytes and B cells, respectively.

Conclusion: Inter- and intraindividual leukocyte subset cell count variation can affect measured CN activity independent of CsA concentration. Cell-specific activity and drug sensitivity should be considered for sample validation to optimize method specificity when pharmacodynamic monitoring strategies are applied in a clinical setting.

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.

IMPDH1 gene polymorphisms and association with acute rejection in renal transplant patients

Inosine 5'-monophosphate dehydrogenase 1 (IMPDH1) catalyzes the rate-limiting step of the de novo pathway for purine synthesis and is a major target of the immunosuppressive drug mycophenolic acid (MPA). Few variants of the IMPDH1 gene have been reported. The objective of this study was to identify and characterize IMPDH1 variants to determine whether genetic variation contributes to differences in MPA response and toxicity in transplant patients. Seventeen genetic variants were identified in the IMPDH1 gene with allele frequencies ranging from 0.2 to 42.7%. In this study, 191 kidney transplant patients who received mycophenolate mofetil were genotyped for IMPDH1. Two single-nucleotide polymorphisms, rs2278293 and rs2278294, were significantly associated with the incidence of biopsy-proven acute rejection in the first year post-transplantation. Future studies of the multifactorial nature of acute rejection must consider IMPDH1 polymorphisms in MPA-treated patients.

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.

IMPDH activity in thiopurine-treated patients with inflammatory bowel disease - relation to TPMT activity and metabolite concentrations

AIMS

Azathioprine and 6-mercaptopurine are steroid-sparing drugs used in inflammatory bowel disease (IBD). The polymorphic enzyme thiopurine S-methyltransferase (TPMT) is of importance for thiopurine metabolism and occurrence of adverse events. The role of other thiopurine-metabolizing enzymes is less well known. This study investigated the role of inosine-5'-monophosphate dehydrogenase (IMPDH), which is a key enzyme in the de novo synthesis of guanine nucleotides and also strategically positioned in the metabolic pathway of thiopurines.

METHODS

IMPDH was measured in 100 healthy blood donors. IMPDH, TPMT and metabolite concentrations were studied in 50 patients with IBD on stable thiopurine therapy. IMPDH activity was measured in peripheral blood mononuclear cells. TPMT activity, 6-methylthioinosine 5'-monophosphate (meTIMP) and 6-thioguanine nucleotide (6-TGN) concentrations were measured in red blood cells, which is the current practice in clinical monitoring of thiopurines. Enzyme activities were related to metabolite concentrations and clinical characteristics.

RESULTS

A wide range of IMPDH activity was observed both in healthy blood donors (median 13.1, range 4.7-24.2 nmol mg(-1) protein h(-1)) and IBD patients (median 14.0, range 7.0-21.7). There was a negative correlation between IMPDH activity and dose-normalized meTIMP concentrations (r(s) = -0.31, P = 0.03), but no evident correlation to 6-TGN concentration or the meTIMP/6-TGN ratio. There were no significant correlations between TPMT activity and metabolite concentrations.

CONCLUSION

Even though the meTIMP concentrations correlated inversely to the IMPDH activity, the role of IMPDH in balancing the formation of methylated and phosphorylated metabolites was not evident. Taken together, the results give cause to question established opinions about thiopurine metabolism.

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.

A novel variant L263F in human inosine 5'-monophosphate dehydrogenase 2 is associated with diminished enzyme activity

BACKGROUND AND OBJECTIVE

Inosine 5'-monophosphate dehydrogenase 2 is required for purine synthesis in activated lymphocytes. Variants in the IMPDH2 gene may account for the large inter-individual variability in baseline enzyme activity, immunosuppressive efficacy and side effects in transplant recipients receiving mycophenolic acid. Therefore, the objective of this study was to identify and functionally characterize IMPDH2 variants.

METHODS

DNA samples from 152 solid organ transplant patients were screened at exons and exon/intron junctions of the IMPDH2 genes by PCR amplification followed by bidirectional direct DNA sequencing. Genetic variant was constructed by site-directed mutagenesis and transformed to an inosine 5'-monophosphate dehydrogenase-deficient strain of Escherichia coli h712. Proteins were purified to homogeneity and the enzymatic activity was measured by reduced nicotinamide adenine dinucleotide production.

RESULTS

Nine genetic variants were identified in the IMPDH2 gene, with frequencies of the rarer alleles ranging from 0.5 to 10.2%. A novel nonsynonymous variant L263F was identified, and the kinetic assay demonstrated that the inosine 5'-monophosphate dehydrogenase activity of L263F variant was decreased to 10% of the wild-type. The Ki for mycophenolic acid inhibition of the L263F variant was comparable with the wild-type, and the variant Km for inosine 5'-monophosphate and nicotinamide adenine dinucleotide did not change significantly.

CONCLUSIONS

IMPDH2 has low genetic diversity, but the nonsynonymous variant L263F has a significant impact on inosine 5'-monophosphate dehydrogenase activity. This novel functional variant may be one of the factors contributing to the inter-individual difference of baseline inosine 5'-monophosphate dehydrogenase activity as well as drug efficacy and adverse events in transplant patients.

Pharmacokinetics and pharmacodynamics of mycophenolic acid after enteric-coated mycophenolate versus mycophenolate mofetil in patients with progressive IgA nephritis

Mycophenolic acid can be administered orally using mycophenolate mofetil or enteric-coated mycophenolate. In renal transplant patients on immunosuppressant combination therapy, the overall mycophenolic acid exposure after oral dosing with mycophenolate mofetil and enteric-coated mycophenolate is similar. This study compared pharmacokinetics and pharmacodynamics of mycophenolic acid after equivalent doses of enteric-coated mycophenolate (360 mg twice daily) or mycophenolate mofetil (500 mg twice daily) in 7 patients with progressive IgA nephritis (glomerular filtration rate 20-35 mL/min) using a randomized crossover design. The pharmacokinetics of mycophenolic acid concentrations and pharmacodynamics (using inosine 5'-monophosphate dehydrogenase activity as a bio-marker) were sequentially monitored for 12 hours. After enteric-coated mycophenolate treatment, the mycophenolic acid peak concentration (Cmax = 12.8 vs 6.0 microg/mL, P < .05) and the overall exposure were significantly higher (AUC = 60.9 vs 40.7 microg.h/mL, P < .05), and the apparent clearance was significantly lower (CL/F = 7.9 vs 10.7 L/h, P < .05) as compared to that after mycophenolate mofetil. Paradoxically, inosine 5'-monophosphate dehydrogenase activity was not significantly different. In conclusion, the steady-state mycophenolic acid exposure was higher during treatment with enteric-coated mycophenolate as compared to mycophenolate mofetil, which might be explained by more extensive enterohepatic recycling of mycophenolic acid after administration of enteric-coated mycophenolate, whereas inosine 5'-monophosphate dehydrogenase suppression was not different.

Pharmacokinetic and pharmacodynamic comparison of enteric-coated mycophenolate sodium and mycophenolate mofetil in maintenance renal transplant patients

The aim of this single-center crossover substudy was to assess pharmacokinetics and pharmacodynamics [inosine 5'-monophosphate dehydrogenase (IMPDH) activity] of enteric-coated mycophenolate sodium (EC-MPS) and mycophenolate mofetil (MMF) at steady-state conditions. Stable maintenance renal transplant patients on 1 g MMF b.i.d. participating in a double-blind, multicenter study, were randomized to receive EC-MPS (720 mg b.i.d.) or continue receiving MMF (1000 mg b.i.d.) for 12 months. Thereafter, all patients (n = 18) received 720 mg EC-MPS b.i.d. Area under the plasma mycophenolic acid (MPA) concentration-time curve with EC-MPS (57.4 +/- 15.0 microg h/mL) fulfilled bioequivalence criteria (geometric mean 0.98 (90% CI: 0.87-1.11) compared to MMF (58.4 +/- 14.1 microg h/mL). Consistent with the delayed release characteristics of EC-MPS, peak MPA concentration (geometric mean 0.89; 90% CI: 0.70-1.13) occurred approximately 0.5 h later (p < 0.05) and predose MPA levels (geometric mean 2.10; 90% CI: 1.51-2.91) were higher and more variable, not fulfilling bioequivalence criteria. IMPDH activity inversely followed MPA concentrations and was inhibited to a similar degree (approximately 85%) by both formulations. The calculated value for 50% IMPDH inhibition was identical for both drugs. In conclusion, equimolar doses of EC-MPS and MMF produce equivalent MPA exposure, while the delayed release formulation of EC-MPS exhibits more variable predose levels and T(max). Overall, IMPDH activity reflected MPA pharmacokinetics.

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.

Conversion From Mycophenolate Mofetil to Enteric-Coated Mycophenolate Sodium in Maintenance Renal Transplant Recipients Receiving Tacrolimus: Clinical, Pharmacokinetic, and Pharmacodynamic Outcomes

BACKGROUND

Mycophenolic acid (MPA) pharmacokinetics using the mycophenolate mofetil (MMF) formulation are known to differ between patients receiving tacrolimus or cyclosporine, but only limited data exist concerning concomitant use of tacrolimus and enteric-coated mycophenolate sodium (EC-MPS).

METHODS

In this six-month, multicenter, open-label, single-arm trial, 63 maintenance renal transplant patients receiving tacrolimus were converted from mycophenolate mofetil (MMF) to EC-MPS.

RESULTS

MPA concentration-time profiles in 21 patients showed that MPA exposure was similar with MMF or EC-MPS (mean area under the curve 39.9+/-11.6 microg x h/mL versus 43.7+/-17.4 microg x h/mL at day 14 post-conversion). Median time to peak concentration was 0.5 hr with MMF and 1.5 hr with EC-MPS. Inosine monophosphate dehydrogenase (IMPDH) activity was almost identical: area under the enzyme activity time curve (AEC) was 124.2+/-32.0 nmol x h/mg prot/h with MMF and 130.3+/-36.6 nmol x h/mg prot/h with EC-MPS at 14 days post-conversion; average daytime IMPDH activity was 10.3+/-2.7 nmol/h/mg protein and 10.9+/-2.7 nmol/h/mg protein, respectively. Maximal daytime inhibition of IMPDH activity was 67% with MMF and 62% with EC-MPS at day 14. One patient (1.6%) experienced mild biopsy-proven acute rejection. No graft losses or deaths occurred. Renal function remained stable (mean calculated creatinine clearance 70.6+/-26.8 mL/min with MMF and 68.8+/-25.4 mL/min six months post-conversion). Adverse events or infections with a suspected relation to EC-MPS occurred in 12 patients (19%). Four patients discontinued EC-MPS due to adverse events or infections.

CONCLUSIONS

MMF and EC-MPS are associated with similar MPA exposure and equivalent pharmacodynamic effect. Conversion of tacrolimus-treated maintenance renal transplant patients from MMF to EC-MPS is safe and well-tolerated and does not compromise therapeutic efficacy.

Determination of inosine monophosphate dehydrogenase activity in human CD4+ cells isolated from whole blood during mycophenolic acid therapy

Inosine 5'-monophosphate dehydrogenase (IMPDH) is an established target in immunosuppression following organ transplantation. In lymphocytes, reversible inhibition of this enzyme by mycophenolic acid (MPA) results in reduced production of guanine and deoxyguanine nucleotides and thereby retarded proliferation of activated cells. In order to examine MPA pharmacodynamics in renal allograft recipients, the authors have developed an assay for the determination of IMPDH activity in CD4+ cells directly isolated from a small blood volume. Paramagnetic beads coated with anti-CD4 antibodies were utilized for the cell isolation. The intracellular MPA concentration was restored by incubating the cells in microfiltrated plasma from the original sample. Inosine 5'-monophosphate (IMP; substrate) and nicotine adenine dinucleotide (NAD; co-factor) were added to cell lysates, and IMPDH activity was quantified as the xanthosine 5'-monophosphate (XMP) production rate (pmol/10 cells/min) determined by liquid chromatography after hydrolytic cleavage to xanthine. The reaction kinetics were saturated with IMP and NAD concentrations of 1.79 micromol/L and 0.38 micromol/L, respectively. The production rate was linear in the interval 0.13 to 8.7 pmol XMP/min. Total interseries CVs based on seven replicates at each MPA concentration 0, 2.2, and 8.6 microg/mL were 25%, 16%, and 13%, respectively. When a single 1 gram mycophenolate mofetil dose was administered to a healthy individual, the measured IMPDH activity was 13% of predose value at the MPA peak concentration. The present assay allows reliable determination of IMPDH activity in CD4+ cells during MPA exposure, reducing the potential influence of sample preparation on the measured enzyme activity to a minimum. The assay may be applied to assess MPA pharmacodynamics during immunosuppressive treatment, maintaining the influence of intracellular MPA on the IMPDH activity.

Effect of Mycophenolate Mofetil Monotherapy on T-Cell Functions and Inosine Monophosphate Dehydrogenase Activity in Patients Undergoing a Kidney Transplantation

The aim of this study was to assess the effects of 1 g of mycophenolate mofetil (MMF) on T-cell function and inosine monophosphate dehydrogenase (IMPDH) activity among patients undergoing kidney transplantation. Five patients undergoing renal transplantation from a living donor were enrolled in this study. Compared to baseline (before MMF intake), CD25 and CD71 expression were significantly decreased during the first hour following MMF intake. T-cell proliferation and IMPDH activity also decreased dramatically. Thereafter, all biomarker levels increased over time. At 4 hours, CD25 and CD71 levels, as well as IMPDH activity, returned to almost baseline values, whereas T-cell proliferation remained below baseline. Intracytoplasmic IL-2 expression remained unchanged after MMF ingestions. In conclusion, administration of 1 g of MMF was associated with a transient decrease in CD25 expression in addition to a temporary dramatic decrease in both T-cell proliferation and IMPDH activity.

Inosine monophosphate dehydrogenase activity in renal allograft recipients during mycophenolate treatment

OBJECTIVE

Mycophenolic acid (MPA) exerts its immunosuppression by inhibiting inosine 5'-monophosphate dehydrogenase (IMPDH), depleting activated lymphocytes of guanine nucleotides and retarding their proliferation. An optimal strategy for monitoring has not been established for mycophenolate mofetil (MMF) in renal transplantation, and clinical investigations of the pharmacokinetic-pharmacodynamic relationship are warranted.

MATERIAL AND METHODS

Mycophenolic acid pharmacokinetics and whole blood cell IMPDH activity were investigated in two separate groups of renal allograft recipients. One group was studied within the 12-h dose interval, while the second group was examined by pre-dose samples pre-transplant and then repeatedly during 8 weeks post-transplant.

RESULTS

An inverse relationship between plasma MPA and IMPDH activity within the dose interval was demonstrated. Minimum IMPDH activity was a median 8 % of values pre-MMF dose, coinciding with the MPA peak. Six hours post-dose, IMPDH activity had returned to pre-dose values. Patients receiving MMF had a 4.5-fold higher pre-dose enzyme activity than transplanted patients without MMF. During the 8 weeks post-transplant, the median MPA trough concentration was fairly stable. Following an initial decrease during the first 4 days post-transplant, IMPDH activity gradually increased during the 40 days post-transplant, reaching 5-fold the pre-transplant values.

CONCLUSIONS

Provided that the changes in IMPDH activity in whole blood cells predict the clinical effect, these pharmacokinetic-pharmacodynamic findings may prove useful in the attempts to identify optimal timing and range for the monitoring of mycophenolate in renal transplantation. The question of whether MPA concentrations or measurements of IMPDH activity per se will be the optimal way of monitoring this immunosuppressant remains open and will only be answered by prospective clinical testing.

Pre-transplant inosine monophosphate dehydrogenase activity is associated with clinical outcome after renal transplantation

Mycophenolate mofetil (MMF), an inhibitor of inosine monophosphate dehydrogenase (IMPDH) activity, is usually administered as a standard dose of 1 g b.i.d. after renal transplantation. Because MMF dose reductions are associated with inferior outcome, we investigated pre-transplant IMPDH activity, MMF dose reductions and outcome. IMPDH activity was determined in isolated peripheral mononuclear cells immediately prior to renal transplantation. We observed considerable inter-individual variability in pre-transplant IMPDH activity (9.35 +/- 4.22 nmol/mg/h). Thirty of 48 patients (62.5%) with standard MMF dose (1 g b.i.d.) had dose reductions within 3 years post-transplant; these patients also had significantly lower IMPDH activity. The area under the receiver-operating characteristics curve (AUC-ROC) for prediction of dose reduction within 6 months post-transplant was 0.75 (95% CI, 0.61-0.89; p < 0.004). IMPDH activity above the cut-off value, MMF dose reduction and age of recipient were significant contributors for the occurrence of acute rejection in the multivariate logistic regression. Patients with high IMPDH activity and MMF dose reduction had the highest rejection rate (81.8% vs. 36.4%; p < 0.01).

CONCLUSION

Patients with low IMPDH activity experienced more complications of MMF therapy. High pre-transplant IMPDH activity and MMF dose reductions were associated with rejection. Determination of IMPDH activity prior to transplantation may help to improve MMF therapy after renal transplantation.