Inosine monophosphate dehydrogenase activity depends on plasma concentrations of mycophenolic acid and its glucuronides in kidney transplant recipients

Liquid chromatographic methods in the determination of inosine monophosphate dehydrogenase enzyme activity: a review

Inosine monophosphate dehydrogenase (IMPDH) is a crucial enzyme involved in the de novo synthesis of purine nucleotides. IMPDH activity is used to evaluate the pharmacodynamics/pharmacokinetics of immunosuppressant drugs such as mycophenolic acid and thiopurines. These drugs are often used to prevent organ transplant rejection and as steroid-sparing agents in autoinflammatory diseases such as inflammatory bowel disease and rheumatoid arthritis. Numerous analytical techniques have been employed to evaluate IMPDH activity in biological matrices. However, hyphenated LC techniques were most widely used in the literature. This review focuses on hyphenated LC methods used to measure IMPDH activity and provides detailed insight into the sample preparation techniques, chromatographic conditions, enzymatic assay conditions, detectors and normalization factors employed in those methods.

Inosine monophosphate dehydrogenase activity and mycophenolate pharmacokinetics in children with nephrotic syndrome treated with mycophenolate mofetil

Some studies have shown that the area under the concentration-time curve (AUC) of mycophenolic acid (MPA) should be higher for children with nephrotic syndrome (NS) than after renal transplantation. The pharmacodynamic aspect of MPA, the activity of inosine monophosphate dehydrogenase (IMPDH), has not been studied in children with NS. The study included 21 children (4-16 years) with NS treated with mycophenolate mofetil. MPA and its glucuronide plasma concentrations were determined using validated high-performance liquid chromatography (HPLC-UV). The separate HPLC-UV method was applied for IMPDH activity determination. The variability was expressed by the coefficient of variation (CV). IMPDH activity and MPA concentration (C_trough ) before the morning dose amounted to 29.95 μmol·s^-1 ·mol^-1 AMP (range, 6.71-98.60 μmol·s^-1 ·mol^-1 AMP) and 1.72 μg/mL (range, 0.39-4.34 μg/mL), respectively, whereas the area under the effect-time curve from 0 to 4 h and MPA AUC_0-4 were 130.36 μmol·s^-1 ·mol^-1 AMP∙h (range, 23.58-306.57 μmol·s^-1 ·mol^-1 AMP∙h) and 24.63 μg·h/mL (range, 12.21-67.48 μg·h/mL), respectively. IMPDH activity decreased concomitantly with MPA concentration increase, however, the variability of the pharmacodynamic parameters was greater than of the pharmacokinetics. The median degree of maximum IMPDH inhibition was 61%. MPA C_trough and predicted AUC were lower than in our previous study. Only a few MPA pharmacokinetic parameters correlated with the pharmacodynamics. IMPDH activity did not correlate with children's age and did not differ between boys and girls. MPA clearance was the highest in younger children (median 10.54 L/m² /h) and cholesterol correlated negatively with children's age (r=-0.659, p=0.003). IMPDH minimum activity and the degree of maximum IMPDH inhibition were similar to those obtained in renal transplant recipients. IMPDH activity does not undergo developmental or gender-specific regulation in children with NS. MPA underexposure might be more frequent in younger children, especially with high cholesterol and triglycerides levels due to high MPA clearance.

Inosine 5'-Monophosphate Dehydrogenase Activity for the Longitudinal Monitoring of Mycophenolic Acid Treatment in Kidney Allograft Recipients

BACKGROUND

Mycophenolic acid (MPA) is a standard immunosuppressant in organ transplantation. A simple monitoring biomarker for MPA treatment has not been established so far. Here, we describe inosine 5'-monophosphate dehydrogenase (IMPDH) monitoring in erythrocytes and its application to kidney allograft recipients.

METHODS

IMPDH activity measurements were performed using a high-performance liquid chromatography assay. Based on 4203 IMPDH measurements from 1021 patients, we retrospectively explored the dynamics early after treatment start. In addition, we analyzed the influence of clinically relevant variables on IMPDH activity in a multivariate model using data from 711 stable patients. Associations between IMPDH activity and clinical events were evaluated in hospitalized patients.

RESULTS

We found that IMPDH activity reflects MPA exposure after 8 weeks of constant dosing. In addition to dosage, body mass index, renal function, and coimmunosuppression affected IMPDH activity. Significantly lower IMPDH activities were found in patients with biopsy-proven acute rejection as compared to patients without rejection (median [interquartile range]: 696 [358-1484] versus 1265 [867-1618] pmol xanthosine-5'-monophosphate/h/mg hemoglobin, P < 0.001). The highest IMPDH activities were observed in hospitalized patients with clinically evident MPA toxicity as compared to patients with hospitalization not related to MPA treatment (1548 [1021-2270] versus 1072 [707-1439] pmol xanthosine-5'-monophosphate/h/mg hemoglobin; P < 0.001). Receiver operating characteristic curve analyses underlined the usefulness of IMPDH to predict rejection episodes (area, 0.662; confidence interval, 0.584-0.740; P < 0.001) and MPA-associated adverse events (area, 0.632; confidence interval, 0.581-0.683; P < 0.001), respectively.

CONCLUSIONS

IMPDH measurement in erythrocytes is a novel and useful strategy for the longitudinal monitoring of MPA treatment.

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 inosine 5'-monophosphate dehydrogenase activity in red blood cells of thiopurine-treated patients using HPLC

Thiopurine drugs are commonly used in immune diseases and to a lesser extent, in transplant rejection prophylaxis: however interindividual variability in drug response and in the occurrence of adverse events is observed. Genetic variation in thiopurine S-methyltransferase (TPMT) doesn't completely explain the occurrence of all adverse events and drug response variability. The potential implication of other enzymes involved in thiopurine metabolism, such as ITPA, has been investigated over the last decade but little data is available on inosine 5'-monophosphate dehydrogenase (IMPDH) in patients treated with thiopurine drugs. The authors reported a HPLC method to determine IMPDH activity in the red blood cells (RBCs) of thiopurine-treated patients. IMPDH activity was evaluated by enzymatic conversion of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP). The XMP formed was analyzed on a Luna^® NH₂ stationary phase, a weak anion exchange phase that exhibits both ionic and hydrophobic properties. XMP was eluted below 15min. Intra-assay and inter-assay precisions were below 9% for RBCs supplemented with 2, 40 and 80μmol/L of XMP. IMPDH activity was measured in adults without thiopurine treatment as well as in adult and paediatric patients treated with thiopurines. A wide interindividual variability in IMPDH activity in RBCs was observed. No difference in IMPDH activity was found between untreated subjects and adult and paediatric patients on thiopurine therapy (median value 11.8, 7.9 and 7.7nmol XPM/g Hb/h respectively). The method described is useful in the determination of IMPDH phenotype from patients on thiopurine therapy and in the investigation of the potential relationship between IMPDH activity in RBCs and the occurrence of adverse events and drug response variability.

Mycophenolic acid exposure and complement fraction C3 influence inosine 5'-monophosphate dehydrogenase activity in systemic lupus erythematosus

Background Mycophenolate mofetil has recently been reported to be effective against systemic lupus erythematosus. The influence of the pharmacokinetics of mycophenolic acid, the active form of mycophenolate mofetil and the major inactive mycophenolic acid phenolic glucuronide on the activity of the target enzyme inosine 5'-monophosphate dehydrogenase, is expected to be revealed. The aim of this study was to identify the factors associated with inosine 5'-monophosphate dehydrogenase activity in systemic lupus erythematosus patients. Methods Fifty systemic lupus erythematosus patients in remission maintenance phase (29 received mycophenolate mofetil [MMF+] and 21 did not [MMF-]) were enrolled. Median and interquartile range of dose of mycophenolate mofetil were 1500 and 1000-1500 mg/day, respectively. Stepwise multiple linear regression analysis was performed to assess the dependence between inosine 5'-monophosphate dehydrogenase activity and 25 predictor values including predose plasma concentrations of free mycophenolic acid and mycophenolic acid phenolic glucuronide. Results Median and interquartile range of predose total plasma concentrations of mycophenolic acid and mycophenolic acid phenolic glucuronide were 2.73 and 1.43-5.73 and 25.5 and 13.1-54.7  µg/mL, respectively. Predose inosine 5'-monophosphate dehydrogenase activity was significantly higher in MMF+ than MMF- patients (median 38.3 and 20.6 nmoL xanthosine 5'-monophosphate/g haemoglobin/h, P<0.01). The plasma concentration of free mycophenolic acid phenolic glucuronide, complement fraction C3 and body weight were significant predictors accounting for interindividual variability in the inosine 5'-monophosphate dehydrogenase activity (adjusted R²= 0.52, P < 0.01) in a multivariate analysis. Conclusions Predose inosine 5'-monophosphate dehydrogenase activity was higher in systemic lupus erythematosus patients receiving mycophenolate mofetil therapy. Inosine 5'-monophosphate dehydrogenase activity may be determined by mycophenolic acid exposure and complement fraction C3 in systemic lupus erythematosus patients.

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.

[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.