Induction of inosine monophosphate dehydrogenase activity after long-term treatment with mycophenolate mofetil

Effect of hypoalbuminemia on drug pharmacokinetics

Hypoalbuminemia, defined as serum albumin levels below 35 g/L, is common in patients with conditions such as nephrotic syndrome, cirrhosis, or sepsis. This review examines the impact of hypoalbuminemia on the pharmacokinetics of selected drugs-such as antibiotics, immunosuppressants, antifungals, or anticonvulsants-emphasizing its role in drug efficacy and safety. Albumin is the main drug transporter and key binding protein, which influences the free drug concentration and drug activity. The review includes all studies available in the scientific literature found in the PubMed, Scopus, and Cochrane databases. The paper emphasizes the importance of therapeutic drug monitoring (TDM) in patients with hypoalbuminemia to avoid subtherapeutic or toxic drug levels. Many drugs need for dose adjustments to achieve therapeutic levels, especially in critically ill patients. The results of studies emphasize the need for individualized dosing regimens based on TDM to optimize drug therapy in patients with hypoalbuminemia. Our review is the first article to summarize the influence of hypoalbuminemia on the pharmacokinetic parameters of drugs and may be a useful tool for clinicians in their daily work.

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.

Personalized Therapy for Mycophenolate: Consensus Report by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology

ABSTRACT

When mycophenolic acid (MPA) was originally marketed for immunosuppressive therapy, fixed doses were recommended by the manufacturer. Awareness of the potential for a more personalized dosing has led to development of methods to estimate MPA area under the curve based on the measurement of drug concentrations in only a few samples. This approach is feasible in the clinical routine and has proven successful in terms of correlation with outcome. However, the search for superior correlates has continued, and numerous studies in search of biomarkers that could better predict the perfect dosage for the individual patient have been published. As it was considered timely for an updated and comprehensive presentation of consensus on the status for personalized treatment with MPA, this report was prepared following an initiative from members of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT). Topics included are the criteria for analytics, methods to estimate exposure including pharmacometrics, the potential influence of pharmacogenetics, development of biomarkers, and the practical aspects of implementation of target concentration intervention. For selected topics with sufficient evidence, such as the application of limited sampling strategies for MPA area under the curve, graded recommendations on target ranges are presented. To provide a comprehensive review, this report also includes updates on the status of potential biomarkers including those which may be promising but with a low level of evidence. In view of the fact that there are very few new immunosuppressive drugs under development for the transplant field, it is likely that MPA will continue to be prescribed on a large scale in the upcoming years. Discontinuation of therapy due to adverse effects is relatively common, increasing the risk for late rejections, which may contribute to graft loss. Therefore, the continued search for innovative methods to better personalize MPA dosage is warranted.

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.

Clinical Pharmacokinetic and Pharmacodynamic Monitoring for Mycophenolate Mofetil

The use of mycophenolate mofetil (MMF), in combination with cyclosporine (CsA) or tacrolimus (FK) and corticosteroids, has been shown to improve clinical outcomes through significant reduction in the incidence of acute rejection in solid organ transplant patients. A fixed oral dosing regimen of 1 or 1.5 g MMF twice daily received Food and Drug Administration approval in 1995 with no recommendations for concentration monitoring at that time. Subsequent evidence has generated substantial debate on the need of clinical monitoring for MMF. This article summarizes the rationale, evidence, and approaches of clinical monitoring for MMF. Mycophenolic acid (MPA), the active moiety of MMF, noncompetitively inhibits the enzyme inosine monophosphate dehydrogenase (IMPDH), which is the target enzyme for MPA. Pharmacokinetic monitoring, by use of MPA predose or MPA area under the concentration-time curve (AUC) values, and pharmacodynamic monitoring by analysis of inhibition of IMPDH have been evaluated in organ transplant patients. The possibility of drug interactions between other immunosuppressive agents has also received attention recently. The clinical implications of drug interactions are discussed in this article.

New Antiproliferative Immunosuppressive Agents

Since the early 1980s, the combination of cyclosporine, azathioprine, and prednisone has been the mainstay tripledrug immunosuppressive regimen used in transplantation. However, advances in drug research, design, and development have allowed for the introduction of new agents that have greatly increased the number of immunosuppressive agents available for use in transplant recipients. Particularly, the newer antiproliferative immunosuppressive drugs (agents that directly inhibit the proliferation of T and B lymphocytes) have had an important impact on patient outcomes posttransplant. These agents are mycophenolate mofetil and sirolimus.

A Rationale for Age-Adapted Immunosuppression in Organ Transplantation

Demographic changes are associated with a steady increase of older patients with end-stage organ failure in need for transplantation. As a result, the majority of transplant recipients are currently older than 50 years, and organs from elderly donors are more frequently used. Nevertheless, the benefit of transplantation in older patients is well recognized, whereas the most frequent causes of death among older recipients are potentially linked to side effects of their immunosuppressants.Immunosenescence is a physiological part of aging linked to higher rates of diabetes, bacterial infections, and malignancies representing the major causes of death in older patients. These age-related changes impact older transplant candidates and may have significant implications for an age-adapted immunosuppression. For instance, immunosenescence is linked to lower rates of acute rejections in older recipients, whereas the engraftment of older organs has been associated with higher rejection rates. Moreover, new-onset diabetes mellitus after transplantation is more frequent in the elderly, potentially related to corticosteroids, calcineurin inhibitors, and mechanistic target of rapamycin inhibitors.This review presents current knowledge for an age-adapted immunosuppression based on both, experimental and clinical studies in and beyond transplantation. Recommendations of maintenance and induction therapy may help to improve graft function and to design future clinical trials in the elderly.

Impact of Genetic Polymorphisms on 6-Thioguanine Nucleotide Levels and Toxicity in Pediatric Patients with IBD Treated with Azathioprine

BACKGROUND

Thiopurine-related toxicity results in discontinuation of therapy in up to 30% of patients with inflammatory bowel disease. Although thiopurine S-methyltransferase (TPMT) is implicated in toxicity, not all toxicity can be attributed to TPMT polymorphisms. We investigated the effects of polymorphisms of genes involved in thiopurine and folate metabolism pathways on 6-thioguanine nucleotide levels and toxicity.

METHODS

Retrospective clinical data and blood samples were collected from 132 pediatric patients with inflammatory bowel disease treated with azathioprine. Eighty-seven genetic polymorphisms of 30 genes were screened using the MassARRAY system, and 70 polymorphisms of 28 genes were selected for further analysis.

RESULTS

TPMT genotype (P < 0.001), concurrent use of mesalazine (P = 0.006), ABCC5 (rs2293001) (P < 0.001), ITPA (rs2236206 and rs8362) (P = 0.010 and P = 0.003), and ABCB1 (rs2032582) (P = 0.028) were all associated with the ratio of 6-thioguanine nucleotides to azathioprine dose. ADK (rs10824095) (P = 0.004, odds ratio [OR] = 6.220), SLC29A1 (rs747199) (P = 0.016, OR = 5.681), and TYMS (rs34743033) (P = 0.045, OR = 3.846) were associated with neutropenia. ABCC1 (rs2074087) (P = 0.022, OR = 3.406), IMPDH1 (rs2278294) (P = 0.027, OR = 0.276), and IMPDH2 (rs11706052) (P = 0.034, OR = 3.639) had a significant impact on lymphopenia.

CONCLUSIONS

This study describes genetic polymorphisms in genes whose products may affect pharmacokinetics and which may predict the relative likelihood of benefit or risk from thiopurine treatment. These findings may serve as a basis for personalized thiopurine therapy in pediatric patients with inflammatory bowel disease, although our data need to be validated in further studies.

Inosine monophosphate dehydrogenase expression and activity are significantly lower in kidney transplant recipients with diabetes mellitus

BACKGROUND

Inosine 5'-monophosphate dehydrogenase (IMPDH) is a target of the immunosuppressive drug, mycophenolic acid (MPA). A 12-hour clinical pharmacokinetic and pharmacodynamic study was conducted to compare IMPDH1 and IMPDH2 gene expression, IMPDHI and IMPDHII protein levels, and enzyme activity between kidney transplant recipients with respect to diabetes status.

METHODS

Nondiabetic (ND, n = 11) and diabetic (D, n = 9) kidney transplant recipients and on nontransplant nondiabetic (n = 10) and diabetic (n = 10) volunteers were included in the study.

RESULTS

Area under the effect curve values for gene expression: IMPDH1 [ND: 22.1 (13.8-31.3) versus D: 4.5 (2.3-6.5), P < 0.001] and IMPDH2 [ND: 15.3 (11.0-21.7) versus D: 6.1 (4.6-8.6), P < 0.001], protein level: IMPDHI [ND: 1.0 (0.5-1.3) versus 0.5 (0.4-0.7), P = 0.002] and IMPDHII [ND: 1.0 (0.6-1.6) versus D: 0.7 (0.6-0.8) P < 0.001] and enzyme activity [ND: 180 (105-245) versus D: 29.9 (15.3-35.6) µmole·s(-1)·mole(-1) adenosine monophosphate, P < 0.001] was significantly lower in transplant recipients with diabetes. Similar results were observed in nontransplanted volunteers. Kinetic studies of MPA-mediated suppression of IMPDH activity in nontransplanted individuals revealed an approximately 2.5-fold lower half-maximum effective concentration (EC50) for diabetic as compared with nondiabetic [ND: 50.2 (49.8-50.7) versus D: 15.8 (15.6-16.3) nmole/L, P = 0.004] volunteers. This difference was not related to several IMPDH gene variants.

CONCLUSIONS

This study indicates a significantly lower IMPDH gene expression, protein level, and enzyme activity in diabetic patients. Further clinical studies in a larger number of patients are warranted to verify whether MPA dosing must be optimized for kidney transplant recipients with diabetes mellitus.

Monitoring of inosine monophosphate dehydrogenase activity and expression during the early period of mycophenolate mofetil therapy in de novo renal transplant patients

Measurement of inosine-monophosphate dehydrogenase (IMPDH) activity or gene expression was used as a further approach in pharmacokinetics (PK)/pharmacodynamic (PD)-guided mycophenolate mofetil (MMF) therapy. Forty-four de novo kidney transplant patients were enrolled; 35 of these completed the study, and were followed for 24 weeks for clinical status, PK parameters, IMPDH activity and IMPDH1/2 gene expression. IMPDH activity and expression were measured in peripheral blood mononuclear cells before transplant and at week 2,4,12 and 24, drawn before (t0) and 2 h (t2 h) after MMF administration. No significant correlation was found between IMPDH activity/expression and PK parameters. For both genes, significant enhancement in t2 h expression was observed, then decreases towards week 24 with a trend following steroid dosages. Seven patients experienced acute rejection (AR) and exhibited significantly higher pre-transplant expression of both IMPDH1 (median 3.42 vs. 0.84; p=0.0025), and IMPDH2 genes (135 vs. 104; p=0.0218) with respect to non-rejecting patients. A significant association was also found between pre-transplant IMPDH1 mRNA and haematological complications (p=0.032). This study suggests that high steroid dosages may influence IMPDH1/2 expression, hampering their use as a PD biomarker, particularly during the early post-transplant period. The measurement of pre-transplant levels of IMPDH1/2 may contribute to prediction of individual drug responsiveness to improve the clinical management of patients in MMF therapy.

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.

Mycophenolic acid response biomarkers: a cell line model system-based genome-wide screen

Mycophenolic acid (MPA) is commonly used to treat patients with solid organ transplants during maintenance immunosuppressive therapy. Response to MPA varies widely, both for efficacy and drug-induced toxicity. A portion of this variation can be explained by pharmacokinetic and pharmacodynamic factors, including genetic variation in MPA-metabolizing UDP-glucuronyltransferase isoforms and the MPA targets, inosine monophosphate dehydrogenase 1 and 2. However, much of the variation in MPA response presently remains unexplained. We set out to determine whether there might be additional genes that modify response to MPA by performing a genome-wide association study between basal gene mRNA expression profiles and an MPA cytotoxicity phenotype using a 271 human lymphoblastoid cell line model system to identify and functionally validate genes that might contribute to variation in MPA response. Our association study identified 41 gene expression probe sets, corresponding to 35 genes, that were associated with MPA cytotoxicity as a drug response phenotype (p<1×10(-6)). Follow-up siRNA-mediated knockdown-based functional validation identified four of these candidate genes, C17orf108, CYBRD1, NASP, and RRM2, whose knockdown shifted the MPA cytotoxicity curves in the direction predicted by the association analysis. These studies have identified novel candidate genes that may contribute to variation in response to MPA therapy and, as a result, may help make it possible to move toward more highly individualized MPA-based immunosuppressive therapy.

Polymorphisms in type I and II inosine monophosphate dehydrogenase genes and association with clinical outcome in patients on mycophenolate mofetil

BACKGROUND

Type I and II inosine monophosphate dehydrogenases (IMPDH) are the targets of mycophenolic acid (MPA), a widely used immunosuppressant. The aims of this study were: to check the presence of controversial polymorphisms in the IMPDH II gene; to look for new ones; and to investigate potential associations between the most frequent SNPs in both IMPDH genes and clinical outcome in renal transplant recipients.

METHODS

The DNA and clinical data of 456 patients from two clinical trials were collected. We sequenced the IMPDH II gene in 80 patients and we genotyped the 456 patients' DNA for the IMPDH II rs4974081, rs11706052, 787C>T and the IMPDH I rs2278293 and rs2278294 SNPs, all of which were earlier reported to be potentially involved in MPA treatment related outcome. We investigated the associations of biopsy proven acute rejection (BPAR), leucopenia, cytomegalovirus infections and other infections with these IMPDH polymorphisms, as well as with demographic, biological and treatment data using multivariate analysis.

RESULTS

Many IMPDH II variant alleles referenced in Genbank were not detected and no new polymorphisms were identified. In the whole group of 456 patients, the IMPDH I rs2278294 SNP was associated with a lower risk of BPAR and a higher risk of leucopenia over the first year post-transplantation. No other IMPDH I or IMPDH II polymorphism was significantly associated with any clinical outcome. Interestingly, calcineurin inhibitor and MPA exposures below the therapeutic range increased the risk of BPAR. Cytomegalovirus infection was the factor most closely linked with leucopenia, whereas tacrolimus was associated with fewer infections than cyclosporine.

CONCLUSION

IMPDH II genotyping may not improve MPA treatment outcome over the first year post-transplantation, in contrast to MPA and calcineurine inhibitor therapeutic drug monitoring and IMPDH I genotyping.

Target enzyme activity as a biomarker for immunosuppression

Pharmacokinetic drug monitoring has been used for many years to relate immunosuppressant dose to drug exposure in vivo. However, this conventional therapeutic drug monitoring of blood immunosuppressant levels may not necessarily predict the pharmacologic effects on immune cells. The direct determination of target enzyme activity (eg, calcineurin activity, inosine-5'-monophospahte dehydrogenase [IMPDH] activity, p70S6 kinase) may help to better assess the individual response to the immunosuppressant. However, its use is limited by the difficulties of the assay systems, which did not allow yet the prospective assessment of these enzymes in larger patient cohorts with the establishment of validated pharmacodynamic drug monitoring. The most progress regarding a robust and reproducible test system has been achieved with the determination of IMPDH activity as a specific pharmacodynamic parameter of mycophenolic acid activity. This recently validated and standardized assay allows the investigation of IMPDH activity in larger clinical studies. Although the determination of target enzyme activity, eg, by the determination of IMPDH activity, holds promise for a more individualized therapy in transplant medicine, more studies are needed to prospectively validate this approach.

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

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

* Mycophenolic acid (MPA) is a potent, selective and reversible inhibitor of inosine 5'-monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme for de novo guanosine triphosphate biosynthesis. * The large IMPDH interindividual variability could be responsible for the differences in therapeutic effects and side-effects observed with MPA. * Induction of IMPDH activity has been observed in whole blood during immunosuppressive therapy.

WHAT THIS STUDY ADDS

* Our data were acquired in long-term mycophenolate mofetil-treated renal transplant recipients on different combinations of immunosuppressive agents (ciclosporin, tacrolimus, sirolimus) and with different treatment duration (up to 8.8 years post transplant). * The increasing trend in IMPDH activity that we observed throughout our 12-month observation period was significantly higher in rejecting than in nonrejecting subjects.

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 (t(0)) and 2 h after (t(2)) 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, t(0) 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: t(0) 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.

Improved assay for the nonradioactive determination of inosine 5'-monophosphate dehydrogenase activity in peripheral blood mononuclear cells

Mycophenolic acid (MPA) inhibits the enzyme inosine 5'-monophosphate dehydrogenase (IMPDH). Thus, the measurement of IMPDH activity could serve as a specific pharmacodynamic (PD) tool for monitoring MPA therapy. At present, however, monitoring of pharmacokinetic parameters is preferred over that of PD parameters because, in general, PD assays are labor-intensive and poorly reproducible. Currently, cell count or protein concentration is widely accepted as methods to normalize enzyme activity. In the present study, we have attempted to further improve a method for the determination of IMPDH activity to increase the robustness and reproducibility of the IMPDH activity assay itself, without making the assay more labor-intensive. Therefore, several aspects of the IMPDH method were investigated regarding their influence on the reproducibility and also modified to increase the feasibility and consistency of the assay. The isolation of peripheral blood mononuclear cells (PBMCs) of whole blood samples was found to be the most variable step. Normalization on cell count is labor-intensive and at the same time has a poor reproducibility. Determination of the protein content in cell extracts is impaired by contamination with extracellular proteins and non-PBMCs. Alternatively, the intracellular substance adenosine monophosphate (AMP) was investigated to normalize the newly generated xanthosine monophosphate. Among various subject groups, no significant differences in mean AMP concentration were found. To simplify the procedure, PBMCs were diluted to a fixed volume after isolation from sample of whole blood, and the IMPDH activity was normalized to the AMP concentration quantified in the same high-performance liquid chromatography run as xanthosine monophosphate was quantified. The within-run and total imprecision (coefficient of variation) ranged from 4.2% to 10.6% and from 6.6% to 11.9%, respectively. In conclusion, the modified method described here for the measurement of IMPDH activity can be used reliably in multicenter trials and in longitudinal studies to evaluate the additional value of any PD monitoring among a diversity of patients treated with MPA.

Interpatient variability in IMPDH activity in MMF-treated renal transplant patients is correlated with IMPDH type II 3757T > C polymorphism

OBJECTIVES

The active metabolite of mycophenolate mofetil (MMF), mycophenolic acid, inhibits the activity of the target enzyme inosine monophosphate dehydrogenase (IMPDH). The aim of this study was to correlate eight different single nucleotide polymorphisms of the IMPDH type II gene to the activity of the IMPDH enzyme to explain between-patient differences in IMPDH activity.

METHODS AND RESULTS

In a prospective study, we measured IMPDH activity, mycophenolic acid plasma concentrations, and eight polymorphisms of IMPDH type II in de novo kidney transplant recipients, 6 days posttransplantation while on MMF treatment. Polymorphisms in the IMPDH type II gene were only observed for the IMPDH type II 3757T > C (rs11706052) single nucleotide polymorphism. Ten of 101 patients (10%) were heterozygous and two of 101 patients (2%) homozygous for IMPDH type II 3757T > C. The allele frequency was 6.9%. The IMPDH activity over 12 h (AUC(act)) was 49% higher for patients with an IMPDH type II 3757C variant [n = 12 vs. n = 68; 336 (95% confidence interval: 216-521) vs. 227 (95% confidence interval: 198-260) hmicromol/s/mol adenosine monophosphate; P = 0.04]. The IMPDH activity measured before transplantation (Act(pre-Tx)) was not significantly different between IMPDH type II 3757TT wild-type and variant carrier patients (P = 0.99).

CONCLUSION

We report that the IMPDH type II 3757T > C polymorphism is associated with an increased IMPDH activity in MMF-treated renal transplant patients. This polymorphism explains 8.0% of the interpatient variability in IMPDH activity.

Mycophenolate pharmacokinetics and pharmacodynamics in belatacept treated renal allograft recipients - a pilot study

BACKGROUND

Mycophenolic acid (MPA) is widely used as part of immunosuppressive regimens following allograft transplantation. The large pharmacokinetic (PK) and pharmacodynamic (PD) variability and narrow therapeutic range of MPA provide a potential for therapeutic drug monitoring. The objective of this pilot study was to investigate the MPA PK and PD relation in combination with belatacept (2nd generation CTLA4-Ig) or cyclosporine (CsA).

METHODS

Seven renal allograft recipients were randomized to either belatacept (n = 4) or cyclosporine (n = 3) based immunosuppression. Samples for MPA PK and PD evaluations were collected predose and at 1, 2 and 13 weeks posttransplant. Plasma concentrations of MPA were determined by HPLC-UV. Activity of inosine monophosphate dehydrogenase (IMPDH) and the expressions of two IMPDH isoforms were measured in CD4+ cells by HPLC-UV and real-time reverse-transcription PCR, respectively. Subsets of T cells were characterized by flow cytometry.

RESULTS

The MPA exposure tended to be higher among belatacept patients than in CsA patients at week 1 (P = 0.057). Further, MPA concentrations (AUC0-9 h and C0) increased with time in both groups and were higher at week 13 than at week 2 (P = 0.031, n = 6). In contrast to the postdose reductions of IMPDH activity observed early posttransplant, IMPDH activity within both treatment groups was elevated throughout the dosing interval at week 13. Transient postdose increments were also observed for IMPDH1 expression, starting at week 1. Higher MPA exposure was associated with larger elevations of IMPDH1 (r = 0.81, P = 0.023, n = 7 for MPA and IMPDH1 AUC0-9 h at week 1). The maximum IMPDH1 expression was 52 (13-177)% higher at week 13 compared to week 1 (P = 0.031, n = 6). One patient showed lower MPA exposure with time and did neither display elevations of IMPDH activity nor IMPDH1 expression. No difference was observed in T cell subsets between treatment groups.

CONCLUSION

The significant influence of MPA on IMPDH1 expression, possibly mediated through reduced guanine nucleotide levels, could explain the elevations of IMPDH activity within dosing intervals at week 13. The present regulation of IMPDH in CD4+ cells should be considered when interpreting measurements of IMPDH inhibition.

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

BACKGROUND

The immunosuppressive effect of mycophenolic acid (MPA) is essentially attributed to IMPDH II inhibition, which leads to a reduction of lymphocyte proliferation. We investigated the action of the MPA metabolites MPA-phenyl-glucuronide (MPAG) and MPA-acyl-glucuronide (AcMPAG) on recombinant human IMPDH II (rhIMPDH II), as well as their passage into lymphocytes in vitro.

METHODS

We measured rhIMPDH II activity spectrophotometrically through the initial velocity of NADH formation, leading to the computation of the kinetic parameters K(m), IC(50), and K(i) (Michaelis constant, half-maximal inhibition concentration, and inhibition constant). We measured intracellular and extracellular concentrations of MPA, MPAG, and AcMPAG after incubation of Jurkat lymphoma cells with each compound separately, using liquid chromatography-tandem mass spectrometry.

RESULTS

MPA and AcMPAG showed an inhibition of rhIMPDH II (IC(50) 25.6 microg/L and 301.7 microg/L, respectively; the K(i) of MPA for NAD and IMP was 50.8 and 57.7 nmol/L, respectively; and that of AcMPAG for NAD and IMP was 382.0 and 511.0 nmol/L. MPAG had no significant effect on the enzyme. AcMPAG apparently acts by the same uncompetitive inhibition mechanism as MPA, with a 12-fold higher IC(50) and an 8-10 times higher K(i). When coincubated with MPA, AcMPAG activity was negligible at pharmacological concentrations. Furthermore, after 6-h incubation at their respective maximum concentration (C(max)), MPA was 10 times more concentrated in Jurkat cells than AcMPAG.

CONCLUSIONS

AcMPAG is a weaker inhibitor of rhIMPDH II than MPA and is less concentrated in lymphocytes in vitro, suggesting that it would not be pharmacologically active in vivo and might not need to be monitored in MPA-treated patients.

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.

Expression of IMPDH1 is regulated in response to mycophenolate concentration

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes de novo guanine nucleotide synthesis. Mycophenolic acid (MPA) exerts immunosuppressive effects by inhibiting IMPDH. The aim of this study was to investigate gene expressions of two IMPDH isoforms, during in vivo exposure to MPA. Healthy volunteers (n=5) were given single doses of 100, 250, 500 and 1000 mg mycophenolate mofetil (MMF). Blood was sampled pre-dose and at 1, 2, 4, 6, 8, 12, and 24 h post-dose. The expressions of IMPDH 1 and 2 were quantified in CD4+ cells and whole blood by real-time reverse transcription-PCR. Following MMF doses of 500 mg, the expression of IMPDH 1 and 2 in CD4+ cells was reduced 39% (P=0.043) and 10% (P=0.043), respectively. Smaller reductions (ns) were observed after 1000 mg MMF. Similar trends were demonstrated for whole blood. The largest reductions of IMPDH1 occurred at MPA AUC(0-12 h) of 20 mg h/L. Below this, increasing MPA exposure correlated with larger reductions of IMPDH1 expression (CD4+ cells: r=-0.82, P<0.001, and whole blood: r=-0.50, P=0.04, n=17), while higher MPA exposure seemed to be associated with smaller reductions of expression (CD4+ cells: r=0.42, ns, and whole blood: r=0.77, P=0.039, n=8). The concentration-dependent modulation of IMPDH 1 and 2 expressions by MPA might impact IMPDH activity. Knowledge of the regulation of the two IMPDH isoenzymes in vivo by MPA is of importance considering pharmacodynamic monitoring and optimization of MPA treatment.

Expression of IMPDH1 and IMPDH2 after transplantation and initiation of immunosuppression

BACKGROUND

Mycophenolic acid (MPA) mediates immunosuppressive effects by inhibiting inosine monophosphate dehydrogenase (IMPDH). Induction of IMPDH activity has been observed in whole blood and erythrocyte samples during immunosuppressive therapy. Information concerning the mechanisms for increased IMPDH activity is limited and the potential implications of induction have been debated.

METHODS

Whole blood, CD4+ cell, and reticulocyte samples were collected from 30 renal transplant patients pre- and posttransplantation. The expressions of two IMPDH isoforms, type 1 and 2, were analyzed by real-time reverse-transcription polymerase chain reaction and quantified using a housekeeping gene index. The IMPDH activity was determined by ultraviolet high-performance liquid chromatography.

RESULTS

Transplantation and the initiation of immunosuppressive therapy was associated with increased IMPDH1 (50-88%, P<0.0005) and decreased IMPDH2 (42-56%, P<0.0005) expression. In CD4+ cells, however, IMPDH2 increased (15%, P=0.009). These changes are probably related to glucocorticoid effects. Two weeks posttransplant, MPA-treated patients displayed elevated IMPDH 1 and 2 in reticulocytes, suggesting enzyme induction in these cells during prolonged MPA therapy. Patients with acute rejection during follow-up demonstrated higher IMPDH2 expression in CD4+ cells pretransplant than nonrejecting patients (median expression 1.26 vs. 0.87 respectively, P=0.017).

CONCLUSIONS

Knowledge of changes in IMPDH 1 and 2 expression after transplantation and initiation of immunosuppression is important considering the action of MPA on IMPDH and the potential for pharmacodynamic monitoring of MPA by measuring IMPDH activity. The expression of IMPDH2 in CD4+ cells pretransplant may be an indicator of immune activation.

Mycophenolate mofetil therapy for refractory inflammatory bowel disease

BACKGROUND

Mycophenolate mofetil (MMF) is an immunomodulatory drug, and its use in inflammatory bowel disease has previously been reported. The aim of this study was to review the Leeds Colitis Clinic experience of the safety and efficacy of MMF in treating patients with refractory Crohn's disease (CD) and ulcerative colitis (UC). This is an extension of a previously published study from our center with a longer follow-up period and approximately twice the number of patients.

METHODS

A retrospective analysis was performed of the records of all patients treated with MMF for inflammatory bowel disease over a 5-year period.

RESULTS

Of 70 patients identified, 67 had previously been treated with azathioprine unsuccessfully. Seventeen of the 70 patients had been successfully maintained in remission with MMF for an average duration of 33 months. Treatment with MMF was discontinued for 53 patients, 17 because of side effects and 36 because they had not responded to the treatment.

CONCLUSIONS

In our series, 17 patients (24.3%) had a sustained steroid-free remission with MMF therapy. Nineteen patients (27%) experienced side effects, of which 17 (24.3% of the total group) had to discontinue therapy. An additional 36 (51.4%) required an escalation in medical therapy or surgery because of failure of the MMF therapy. MMF may have a role in the treatment of refractory inflammatory bowel disease, especially in patients who have previously failed standard therapies such as azathioprine.

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.

Expression of inosine monophosphate dehydrogenase type I and type II after mycophenolate mofetil treatment: a 2-year follow-up in kidney transplantation

The objective of the study was to evaluate the effect of mycophenolate mofetil (MMF) on the regulation of inosine monophosphate dehydrogenase (IMPDH) during the first 2 years after renal transplantation. Twelve patients were enrolled, and 10-h time-course evaluations of the effects of MMF were regularly performed during the study. IMPDH activity and gene expression were measured in whole blood and in mononuclear cells, respectively. Type I IMPDH (IMPDH-I) mRNA was increased during the first 3 months following transplantation and reached its maximal level during acute rejection episodes, whereas type II IMPDH mRNA was stable. Furthermore, although no alteration in the predose samples was observed, patients with prolonged MMF treatment exhibited an increase in the induction potency of both IMPDH activity and gene expression. In vitro experiments confirmed that IMPDH-I is inducible, but preferentially in monocytes than in lymphocytes. This finding suggests that the measurement of IMPDH mRNAs may provide reliable information to predict acute rejection.

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.

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.

Real-Time PCR Determination of IMPDH1 and IMPDH2 Expression in Blood Cells

Background: Inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in de novo guanine nucleotide synthesis and is implicated in cell cycle control. Inhibition of this enzyme is associated with immunosuppressive, antiviral, and antitumor activity. IMPDH basal activity increases after initiation of immunosuppressive therapy.

Methods: A real-time reverse-transcription PCR assay was developed and validated for mRNA quantification of the 2 human IMPDH isoforms. Target gene expressions were normalized to the geometric mean of 3 housekeeping genes. Assay utility was tested by analyzing patient samples and cultured cells exposed to immunosuppressive drugs such as the IMPDH inhibitor mycophenolic acid.

Results: The assay was linear over 6 logs of cDNA input and demonstrated specific quantification of IMPDH1 and IMPDH2 expression in cultured cells and patient samples. Limits of detection and quantification were 10 and 103 copies of cDNA per reaction, respectively. Within-run and total between-day CVs were &lt;15% for normalized expression. Changes in IMPDH1 and 2 expression were observed in patient samples after initiation of an immunosuppressive regimen that included calcineurin inhibitors, mycophenolate mofetil, and steroids.

Conclusions: This assay can be used to study the regulation of IMPDH expression and the involvement of the enzymes in immunological and malignant proliferative conditions. This may contribute to the processes of drug development and to the establishment of monitoring strategies for treatment effect and disease activity.

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.

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.

Activation of the purine salvage pathway in mononuclear cells of cardiac recipients treated with mycophenolate mofetil

BACKGROUND

The objective of this study was to investigate purine nucleotide metabolism in peripheral blood mononuclear cells (PBMC) of cardiac transplant recipients switched from azathioprine to mycophenolate mofetil (MMF).

METHODS

Concentrations of guanosine 5'triphosphate (GTP) and adenosine 5'triphosphate (ATP), the activities of inosine monophosphate dehydrogenase (IMPDH), guanine phosphoribosyltransferase (GPRT), and hypoxanthine phosphoribosyltransferase (HPRT) were determined in PBMC of 27 cardiac transplant recipients before switch to MMF and 3, 6, and 12 months thereafter.

RESULTS

There was no difference in the activities of IMPDH and salvage pathway enzymes GPRT and HRPT as well as in intracellular GTP and ATP concentrations between the patients before switch to MMF and healthy controls. The GTP and ATP concentrations in PBMC of cardiac recipients did not change during the entire observation period. Although the MPA trough level remained similar, IMPDH activity declined from 897 to 316 pmol/10(6)PBMC/h 3 months after MMF onset, was almost completely inhibited after 6 months, and partially restored to 143 pmol/10(6)PBMC/h 12 months after switch to MMF. In contrast, GPRT activity increased after 3, 6, and 12 months of MMF therapy and HPRT activity 3 and 6 months after switch to MMF.

CONCLUSIONS

We demonstrated for the first time an induction of salvage pathway enzyme activities in PBMC under MMF therapy. This probably accounts for the maintenance of intracellular purine nucleotide pools and prevents the GTP depletion.

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.

Enteric-coated mycophenolate sodium for transplant immunosuppression

PURPOSE

The pharmacology, pharmacokinetics, drug interactions, clinical efficacy, adverse effects, monitoring, and dosage and administration of enteric-coated (EC) mycophenolate sodium are reviewed.

SUMMARY

EC mycophenolate sodium is the EC salt form of mycophenolic acid (MPA), the active component of the pro-drug, mycophenolate mofetil. EC mycophenolate sodium was developed to reduce the upper-gastrointestinal (GI) effects of mycophenolate mofetil. Unlike oral mycophenolate mofetil, which releases MPA in the stomach, EC mycophenolate sodium releases MPA in the small intestine. The absolute bioavailability of EC mycophenolate sodium is 72%. MPA undergoes hepatic metabolism by glucuronyl transferase to the inactive mycophenolic acid glucuronide (MPAG), the predominant metabolite. The majority of an administered dose of EC mycophenolate sodium is found as MPAG in the urine. The mean terminal half-life of MPA ranges from 8 to 16 hours. EC mycophenolate sodium and mycophenolate mofetil have equivalent mechanisms of action and drug interaction profiles. Thus far, EC mycophenolate sodium has demonstrated similar efficacy and safety to mycophenolate mofetil in two Phase III clinical trials of adult renal transplant recipients. One study demonstrated improved health-related quality of life in patients switched from mycophenolate mofetil to EC mycophenolate sodium. Ongoing Phase IV studies are trying to further determine advantages of the EC product.

CONCLUSION

EC mycophenolate sodium is a safe and effective immunosuppressive agent approved for use in the prevention of acute rejection after renal transplantation. It offers an excellent addition to the current armamentarium of immunosuppressive drugs for transplant immunosuppression.

Pharmacodynamics in the development of new immunosuppressive drugs

Over the past 10-20 years a number of immunosuppressive drugs, such as cyclosporine A, tacrolimus, sirolimus, or mycophenolate mofetil have been approved for clinical use and have been highly successful in preventing or delaying graft rejection. Nevertheless, there is an incessant need for better and safer drugs to improve short-term and long-term outcomes following transplantation. A number of low-molecular-weight molecules that interfere with immune cell functions are in development. These include molecules that inhibit the janus protein tyrosine kinase JAK3, compounds that alter lymphocyte trafficking (the sphingosine-1-phosphate receptor antagonist FTY720), and new malononitrilamides (FK778). All seem to show promising therapeutic potential. Among the biologic agents, there are high expectations for antibodies or recombinant chimeric molecules targeting costimulatory surface molecules or pathways involved in the migration of immune cells. The list of such targets includes the ligand pairs CD28:B7, CD154:CD40, LFA-1:ICAM-1, ICOS:B7RP-1, and VLA-4:VCAM-1. However, the clinical development of drugs for transplantation has proved to be difficult, complex, and time consuming. Therefore, newly emerging drug candidates will also demand better methods for monitoring their efficacy as well as their side effects in vivo. Pharmacokinetics (PK) and pharmacodynamics (PD) are complementary approaches used to select drugs on the basis of their in vivo efficacy as well as safety. Whereas PK monitors the handling of the drug by the body, PD focuses on the biologic effect of the drug on its target. Therefore, PD studies of in vivo efficacy are useful for clinical decisions to determine the optimal dose and type of immunosuppressant. At the preclinical stage, PD is aimed at accelerating the selection of lead compounds via PD-controlled trials in animals. Moreover, PD can help to discover new mechanisms of action for a drug or a drug candidate. However, its full potential has not been used, mainly because of laborious and time-consuming methodology. This review focuses on established and novel PD/PK approaches to assess immunosuppressive compounds in the context of new evolving drugs or drug combinations.

Pharmacokinetics and Pharmacodynamics of Low Dose Mycophenolate Mofetil in HIV-Infected Patients Treated with Abacavir, Efavirenz and Nelfinavir

BACKGROUND

The use of mycophenolate mofetil in combination with highly active antiretroviral therapy (HAART) has been proposed in order to inhibit HIV replication. Due to the low doses involved, pharmacokinetic-pharmacodynamic monitoring is recommended.

OBJECTIVE

The aim of this study was to characterise the pharmacokinetic and pharmacodynamic monitoring of low doses of mycophenolate mofetil (0.25 g twice daily) in HIV-infected patients treated with HAART and after programmed discontinuation of HAART, in order to assess whether low doses of this immunosuppressive agent provide a biological effect.

METHODS

Mycophenolic acid (MPA) plasma levels (assessed by high-performance liquid chromatography) and the capacity of patients' sera to inhibit CEM cell line proliferation (assessed by (3)H-thymidine uptake) were measured post-dose at 0, 20, 40 minutes and 1, 2, 4, 6, 8, 10 and 12 hours in nine HIV-infected patients treated with a combination of abacavir, nelfinavir and efavirenz (HAART) and mycophenolate mofetil 0.25 g twice daily at days 7, 28, 120 and 150 (30 days without HAART) after the treatment initiation. A control group of eight patients was treated with HAART alone.

RESULTS

In the 35 post-dose curves analysed, no differences were found in MPA levels between days 7, 28, 120 and 150: area under the plasma concentration-time curve - mean value 15.3 mg . h/L, range 10.4-24.4 mg . h/L; minimum plasma concentration - mean value 0.60 mg/L, range 0.20-4.67 mg/L; maximum plasma concentration mean value 2.60 mg/L, range 0.94-7.98 mg/L. Pretreatment patients' sera did not inhibit CEM proliferation. Post-treatment patients' sera inhibited CEM proliferation to <40% in 25 of 35 curves at 0 hours (six of nine patients), in 34 of 35 curves at 1 hour, in 32 of 35 curves at 2 hours, in 22 of 35 curves at 4 hours, and in 8 of 35 curves at 12 hours. The MPA level versus CEM proliferation inhibition had a concentration that produces 50% of the maximum drug effect (EC(50)) of 0.33 mg/L. Viral load at day 150 was >200 copies/mL in all control patients and in three of nine patients receiving mycophenolate mofetil. These three patients were the only ones repeatedly unable to inhibit pre-dose CEM proliferation to <40%.

CONCLUSIONS

Mycophenolate mofetil pharmacokinetic profiles in HIV patients under HAART are not significantly different from those found in transplant patients. Sera from the majority of patients receiving low doses of mycophenolate mofetil inhibited lymphocyte proliferation during most of the inter-dose interval, despite low MPA plasma levels. For some patients, higher doses may be necessary: the capacity of sera to inhibit CEM proliferation may help to identify these patients.

Pharmacogenetics and pharmacogenomics of immunosuppressive agents: perspective for individualized therapy

Immunosuppressive therapy has markedly improved over the past years with the advent of highly potent and rationally targeted immunosuppressive agents. Since these drugs are characterized by a narrow therapeutic index, major efforts have been carried out to define therapeutic windows based on the blood levels of each immunosuppressant, and relating those concentrations to clinical events. Although pharmacokinetic-based approaches are currently used as useful tools to guide drug dosing, they present several limitations. Pharmacogenomics – a science that studies the inherited basis of differences between individual responses to drugs in order to identify the best dose and therapy for each patient – might represent a complementary support. Preliminary studies that have focused on polymorphisms of genes encoding enzymes involved in drug metabolism, drug distribution, and pharmacological target, have shown promising results. Indeed, pharmacogenomics holds promise for improvement in the ability to individualize pharmacological therapy based on the patient’s genetic profile.

An exploratory study on pharmacogenetics of inosine-monophosphate dehydrogenase II in peripheral mononuclear cells from liver-transplant recipients

Mycophenolate mofetil (MMF) is an immunosuppressant used for the prophylaxis of rejection in renal, pancreas, and liver transplantation. It inhibits the inducible isoform of the enzyme inosine-monophosphate dehydrogenase (IMPDH II) via its active metabolite mycophenolic acid (MPA). IMPDH II is necessary for de novo purine synthesis in activated lymphocytes. The aims of the present study were to evaluate the feasibility of a real-time polymerase chain reaction (PCR) quantitative assessment of IMPDH II gene expression in liver transplant recipients as well as to provide a preliminary evaluation of possible correlations with drug tolerability. RNA was extracted from peripheral blood mononuclear cells of liver recipients after at least 6 months of MMF administration. IMPDH II gene expression was assessed using quantitative, real-time PCR and normalized using glyceraldheyde-3-phosphate dehydrogenase (GAPDH). Finally, adverse events associated with MMF administration were recorded. Real-time PCR quantitation of IMPDH II gene expression was reliable, sensitive, and specific. The intrapatient variability for both IMPDH II and GAPDH assays was lower than 0.6% in all patients. The results demonstrated a wide interpatient variability, with the mean value +/- standard deviation of 0.949 +/- 0.525 (95% confidence interval, 0.669-1.229) and a median value of 0.797. Patients with treatment-related toxicities displayed a trend to a higher level of IMPDH II expression than those without toxicity (mean, 1.126 vs 0.771). In conclusion, pharmacogenetic analysis of IMPDH II may represent a novel approach to MMF therapeutic monitoring.

Mycophenolate mofetil treatment following renal transplantation decreases GTP concentrations in mononuclear leucocytes

MMF (mycophenolate mofetil) has been proven to provide an effective immunosuppression by non-competitive selective reversible inhibition of IMPDH (inosine monophosphate dehydrogenase), the enzyme playing a crucial role in GTP biosynthesis. However, the exact metabolic changes induced by inhibition of IMPDH in target cells of the immune system have been the subject of recent debate. The aim of the present study was to evaluate whether MMF treatment produced sustained changes in the guanosine nucleotide pool of MNLs (mononuclear leucocytes) in vivo. Sixty-two renal failure patients were divided into three groups: chronic renal failure patients undergoing haemodialysis (CRF-HD; n=20) and two groups of patients after renal transplantation, the first on AZA (azathioprine; TN-AZA; n=23) and the second treated with MMF (TN-MMF; n=19). In addition, MNLs from 25 healthy subjects were analysed as controls. Anion-exchange HPLC was used to quantify purine and pyrimidine nucleotides in MNLs. We report a significant decrease in GTP and the total MNL guanine nucleotide pool in the TN-MMF group (P<0.05) compared with control, CRF-HD and TN-AZA groups, although no significant differences were found between any of the other groups. Adenine nucleotide concentrations in MNLs were decreased in the TN-AZA group, but not in the TN-MMF group compared with the CRF-HD group and controls. There were no differences in CTP concentrations, but UTP concentrations were decreased in the CRF-HD, TN-AZA and TN-MMF groups compared with controls. MMF caused a significant and sustained decrease in the guanine nucleotide pool in MNLs from renal transplant recipients. This decrease contrasts with the elevation in GTP reported in erythrocytes of MMF-treated patients.

Mycophenolate mofetil, an inhibitor of inosine monophosphate dehydrogenase, causes a paradoxical elevation of GTP in erythrocytes of renal transplant patients

The immunosuppressant MMF (mycophenolate mofetil) has increasingly replaced AZA (azathioprine) in renal transplantation. MMF is a prodrug of MPA (mycophenolic acid), which inhibits lymphocyte IMPDH (inosine monophosphate dehydrogenase), thereby drastically decreasing GTP concentrations essential to lymphocyte proliferation in vitro and in vivo. Erythrocyte GTP concentrations are commonly elevated in severe renal disease, but normalize following successful engraftment. Consequently, elevated GTP in renal transplant recipients might signal impending loss of immunosuppression and graft failure. In the present study, we compared erythrocyte nucleotides and plasma metabolites in two groups of 25 patients after renal transplantation, both receiving prednisolone and cyclosporin A, but one group receiving MMF and the other AZA. No patients had recent allograft biopsy evidence of rejection. Erythrocyte GTP concentrations at MMF commencement were 50.4+/-23.4 micromol/l. An increase occurred during the first 3 months after transplant when MMF was used de novo, stabilizing at 146.7+/-62.9 micromol/l after 4 months. This was significantly higher (P=2.5 x 10(-6)) than erythrocyte GTP (40.4+/-15.9 micromol/l) in the AZA group, which was essentially unchanged from values immediately after successful transplantation. The effect of MMF on erythrocyte GTP levels was reversible, since GTP levels fell when MMF therapy was terminated. The results demonstrate paradoxically high GTP concentrations in erythrocytes of renal transplant patients receiving MMF. MPA may stabilize reticulocyte IMPDH, allowing the protein to persist during erythropoiesis. This behaviour is in marked contrast with the decrease in GTP levels seen in white blood cells of patients on chronic MMF therapy.

Calcineurin activity as a functional index of immunosuppression after allogeneic stem-cell transplantation1

BACKGROUND

The authors have previously shown that mononuclear cells derived from patients with resistant chronic graft-versus-host-disease (GVHD) express high calcineurin (CN) activity, suggesting that in vitro assessment of CN activity may be a useful index to estimate the degree of immunosuppression afforded by cyclosporine A (CsA). The goal of this study was to assess CN activity during the first 2 months after allogeneic stem-cell transplantation (SCT) and to correlate its evolution with the occurrence of acute GVHD.

METHODS

Thirty-one allogeneic SCT recipients were enrolled during a 21-month period. All received GVHD prophylaxis with CsA (2 mg/kg/day) and methotrexate (on days 1, 3, and 6). CN activity was measured before transplant, and then once weekly, for at least 2 months.

RESULTS

Eighteen patients developed acute grade II or higher GVHD at a median time of 22.5 days and were treated with steroids. CN activity was significantly increased in these 18 patients when compared with 13 patients who did not develop GVHD. Analysis involving the receiver operating characteristic curve demonstrated that acute grade II or higher GVHD can be predicted with a sensitivity of 89% and a specificity of 54% with the use of a cutoff value of 28 pmol RII/mg proteins/min of CN activity.

CONCLUSIONS

CN activity appears to be a promising therapeutic test to predict acute GVHD after allogeneic SCT. This functional assessment of the in vivo efficacy of CsA opens new insights for CsA dose adjustment-in particular, the administration of its most efficient dose instead of its maximal tolerated dose, as is currently performed.

Regulation of inosine monophosphate dehydrogenase type I and type II isoforms in human lymphocytes

The enzyme inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in the de novo biosynthesis of guanine nucleotides. Inhibition of IMPDH leads to immunosuppression by decreasing guanine nucleotides that are required for the proliferation of lymphocytes. IMPDH activity is mediated by two highly conserved isoforms, type I and type II. We have characterized the mRNA and protein expression of the two isoforms in a variety of human tissues, peripheral blood mononuclear cells (PBMCs), and selected cell lines to investigate their regulation. Type I mRNA was expressed in most tissues with high expression in PBMCs and low expression in thymus. IMPDH type II transcript was also detected in most tissues with low expression in spleen and PBMCs. In PBMCs, induction of both type I and type II mRNAs was observed within 12 hr of mitogenic stimulation. Using type-selective IMPDH antibodies, an increase in the levels of type I and type II proteins was observed after mitogenic stimulation. The effect of two IMPDH inhibitors, MPA and VX-497, was investigated on the expression of type I and type II isoforms. VX-497 is an orally bioavailable, potent and reversible inhibitor of IMPDH, with broad applicability in many viral and immune system-mediated diseases. MPA and VX-497 inhibit both isoforms of IMPDH in vitro. Prolonged treatment of lymphocytes with either VX-497 or MPA did not lead to an increase in type I or type II IMPDH protein levels. These results are discussed in the context of IMPDH being a target for immunosuppressive, anti-viral and anti-cancer therapy.

From pharmacokinetics to pharmacogenomics: a new approach to tailor immunosuppressive therapy

One of the main tasks in the management of organ transplantation is the optimization of immunosuppressive therapy, in order to provide therapeutic efficacy limiting drug-related toxicity. In the past years major efforts have been carried out to define therapeutic windows based on blood/plasma levels of each immunosuppressant relating those concentrations to drug dosing and clinical events. Although this traditional approach is able to identify environmental and nongenetic factors that can influence drug exposure during the course of treatment, it presents limitations. Therefore, complementary strategies are advocated. The advent of the genomic era gives birth to pharmacogenomics, a science that studies how the genome as a whole, including single genes as well as gene-to-gene interactions, may affect the action of a drug. This science is of particular importance for drugs characterized by a narrow therapeutic index, such as the immunosuppressants. Preliminary studies focused on polymorphisms of genes encoding for enzymes actively involved in drug metabolism, drug transport and pharmacological target. Pharmacogenomics holds promise for improvement in the ability to individualize immunosuppressive therapy based on the patient's genetic profile, and can be viewed as a support to traditional therapeutic drug monitoring. However, the clinical applicability of this approach is still to be proven.

Mycophenolate mofetil for Crohn's disease: short-term efficacy and long-term outcome

AIM

To assess the long-term efficacy of the antimetabolite agent mycophenolate mofetil in patients with Crohn's disease.

METHODS

Twenty patients with complicated Crohn's disease were treated with mycophenolate mofetil, 1 g b.d., for up to 7 years. Twelve patients were intolerant to azathioprine, seven were resistant to azathioprine and one had a history of mesalazine-induced pancreatitis. The response to mycophenolate mofetil was determined by calculation of the Harvey-Bradshaw index, the ability to taper steroids and the grading of fistula activity.

RESULTS

After 6 months, 11 of the 20 patients had responded. Seven of the 11 responders relapsed after a median of 18 months, three have an ongoing response at month 17, 19 and 82, and one discontinued mycophenolate mofetil owing to toxicity. After initial treatment failure, mycophenolate mofetil was continued in 12 of 17 patients for a further 2-41 months without inducing a stable remission. Mycophenolate mofetil was of benefit in nine of the 12 patients intolerant to azathioprine and in two of the seven patients resistant to azathioprine. Perianal fistulas improved in seven of eight patients; five of the seven subsequently deteriorated, but only one due to reactivated perianal disease.

CONCLUSIONS

Mycophenolate mofetil was initially effective in a sizeable fraction of patients with complicated Crohn's disease, but relapse within 18 months was common. Nevertheless, mycophenolate mofetil could be a useful alternative in patients intolerant to azathioprine.

Pharmacodynamic approach to immunosuppressive therapies using calcineurin inhibitors and mycophenolate mofetil

BACKGROUND

Graft survival depends on adequate immunosuppression. To evaluate the effect on the immune system of immunosuppressive therapies using calcineurin inhibitors (CNIs), several pharmacodynamic indices have been proposed to complement pharmacokinetic data. In this preliminary study we compared some of these parameters during combined immunosuppressant therapies.

METHODS

We treated 65 stable renal transplant recipients with cyclosporin A (CsA; n = 16), tacrolimus (TRL; n = 10); CsA + mycophenolate mofetil (MMF; n = 14); TRL + MMF (n = 13), and MMF (n = 12). Twelve nontreated healthy controls were also included. Calcineurin activity (CNA) in peripheral blood mononuclear cells was measured using (32)P-labeled peptide. Interleukin-2 (IL-2) and interferon-gamma production in phytohemagglutinin-activated whole blood were measured at 0 and 2 h postdose. The areas under the curves, c(min), c(max), and concentration at 2 h (c(2 h)) were also measured.

RESULTS

We found no differences in CNA between groups receiving CNIs alone or combined with MMF [median (25th-75th percentiles)]: CsA(2 h), 3.87 (3.00-6.85)% alkaline phosphatase (AP); CsA+MMF(2 h), 3.90 (1.78-5.19)% AP; TRL(2 h), 5.68 (3.02-16.00)% AP; TRL+MMF(2 h), 11.80 (4.05-14.63)% AP. In vitro IL-2 production was significantly lower in the groups receiving combined therapy than in groups receiving CNIs alone [median (25th-75th percentiles)]: CsA(2 h), 276.52 (190.41-385.25) ng/L; CsA+MMF(2 h), 166.48 (81.06-377.01) ng/L (P <0.001); TRL(2 h), 249.34 (127.48-363.50) ng/L; TRL+ MMF(2 h), 122.13 (51.02-180.00) ng/L (P <0.001). The correlations (r) between c(2 h) and CNA 2 h postdose were as follows: CsA, r = -0.74; CsA+MMF, r = -0.84; TRL, r = -0.70; TRL+ MMF, r = -0.70 (P <0.001 in all cases).

CONCLUSIONS

The measurement of CNA may be of help in following the effect on the immune system of CNI treatments, even in combined therapies, but does not reflect the additional effect of MMF. In contrast, IL-2 in vitro production reflects the effect of both MMF and CNIs.