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

Pharmacogenomic considerations for immunosuppressive therapy

Immunosuppression, the art of suppressing the endogenous immune system to allow organ transplantation or treatment of autoimmune disease, is a clinico-pharmacological field that has markedly developed over the past three decades with the advent of highly potent and rationally targeted immunosuppressive agents. Pharmacogenomics, the art of providing tailored pharmacological therapy with the highest therapeutic index based on the genomic composition of the individual, is a science that has rapidly developed over the past decade, along with the advances in the human genome project and in biotechnology. Pharmacogenomics of immunosuppression is the combined art of tailoring specific immunosuppressive drug therapy to specific immune-mediated clinical entities which require immunosuppression, with optimum matching of the drug to the individual's genomic makeup. Timely and judicious application of pharmacogenomics to clinical immunosuppression should direct the clinician to the best immunosuppressive drug for any given clinical condition, and markedly increase its efficacy as well as decreasing the incidence of side effects and toxicity, thereby decreasing morbidity and prolonging survival. Is this a description of an ongoing clinical evolution in immunosuppression or a prediction of future events? The promises of pharmacogenomics of immunosuppression are high, yet the availability and/or application and/or realization of the promises of this highly specialized clinical science are very slow to come.

Mycophenolate mofetil for solid organ transplantation: does the evidence support the need for clinical pharmacokinetic monitoring?

The need for clinical pharmacokinetic monitoring (CPM) of the immunosuppressant mycophenolate mofetil (MMF) has been debated. Using a previously developed algorithm, the authors reviewed the evidence to support or refute the utility of CPM of MMF. First, MMF has proven efficacy for prevention of organ rejection in renal and cardiac transplant populations. In addition, the pharmacologically active form of MMF, mycophenolic acid (MPA), can be measured readily in plasma, and relationships between the incidence of rejection and MPA predose concentrations and MPA area under the curve (AUC) have been reported. A lower limit of the therapeutic range (MPA predose concentrations >1.55 microg/mL, as measured by enzyme multiplied immunoassay technique [EMIT], or MPA AUC >30 or 40 microg. h/mL, as measured by high-performance liquid chromatography [HPLC]) has been suggested to prevent rejection in renal allograft patients. Similarly, in cardiac transplant patients, decreased incidences of organ rejection have been reported in patients with MPA concentrations >2 or 3 microg/mL (using EMIT) and total AUC values >42.8 microg. h/mL (using HPLC). However, the relationship between pharmacokinetic parameters and adverse events in renal and cardiac transplant patients remains unclear. Due to the nature of antirejection therapy, the pharmacologic response of MMF is not readily assessable, and therapy is life-long. MPA pharmacokinetics exhibit large inter- and intrapatient variability and may be altered in specific patient populations due to changes in protein binding, concomitant disease states, or interactions with concurrent immunosuppressants. Therefore, on the basis of current evidence, CPM can provide more information regarding efficacy of MMF than clinical judgment alone in select patient populations. However, further randomized, prospective trials are required to clarify unresolved issues. Specifically, an upper limit of the therapeutic range, above which the risk of side effects is increased, needs to be elucidated for MMF therapy. Other future directions for research include determining a practical limited sampling strategy for MPA AUC; clarifying the relationship between free MPA concentrations, efficacy, and toxicity; and defining the pharmacodynamic relationship between activity of inosine monophosphate dehydrogenase (the enzyme inhibited by MPA) and risk of rejection or adverse effects.

Therapeutic monitoring of mycophenolate mofetil in organ transplant recipients: is it necessary?

Adequate immunosuppression minimising the risk of organ rejection with acceptable tolerability of the used drugs is a crucial step in organ transplantation. The primary goal is to maintain a consistent time-dependent target concentration by tailoring individual dosage leading to the best efficacy and tolerability combination. The use of therapeutic drug monitoring (TDM) to optimise immunosuppressive therapy is routinely employed for maintenance drugs such as cyclosporin and tacrolimus. The question whether therapeutic monitoring of mycophenolic acid (MPA) in organ transplant recipients treated with mycophenolate mofetil is necessary is not definitely answered. The correlation of mycophenolic acid pharmacokinetic parameters with efficacy and toxicity makes the therapeutic monitoring of this drug promising. However, further studies are mandatory to draw the best guidelines in order to achieve higher levels of evidence that MPA-TDM may improve patient outcome.

Adverse gastrointestinal effects of mycophenolate mofetil: aetiology, incidence and management

Mycophenolate mofetil (MMF) is a relatively new immunosuppressive drug. It inhibits inosine monophosphate dehydrogenase, a key enzyme in the de novo pathway of purine synthesis, and thus causes lymphocyte-selective immunosuppression. Large clinical trials have revealed the efficacy of MMF in the prevention of allograft rejection when administered together with cyclosporin or tacrolimus and corticosteroids. Although the adverse effect profile of MMF is comparatively benign, gastrointestinal adverse effects are a major concern. These effects are partially explained by the increased immune suppression, by the mode of action and by interactions, particularly with other immunosuppressants. The aetiology of the rarest gastrointestinal adverse effects is still not completely clear. Therapy depends upon the clinical gravity of the adverse effects and is therefore a case of waiting and ob- serving. An adjustment of dosage of immunosuppressants according to the clinical situation and, particularly in the case of MMF, spreading the total dosage over more than 2 daily doses are often sufficient. Should adverse effects persist for a longer period of time and be of a more serious nature, a comprehensive invasive diagnostic process is necessary, including endoscopy and biopsy and the search for opportunistic infections. In this case, dosage reduction or the complete withdrawal of MMF seems to be unavoidable. Severe gastrointestinal complications with MMF are rare, but when they do occur they may require extensive diagnosis and treatment. In the future, therapeutic drug monitoring and, where necessary, pharmacological modifications of MMF could lead to a further reduction of adverse effects with an equal or even increased efficacy.

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

BACKGROUND

The immunosuppressive activity of mycophenolate mofetil (MMF) is based on the reversible inhibition of inosine 5'-monophosphate dehydrogenase (IMPDH) by mycophenolic acid (MPA). It was the aim of this study to develop a nonradioactive method for specific measurement of IMPDH activity in isolated peripheral mononuclear cells (MNC).

METHODS

The procedure is based on the incubation of lysed MNC with inosine 5'-monophosphate (IMP) followed by direct chromatographic determination of produced xanthosine 5'-monophosphate (XMP). IMPDH activity was measured in MNC of MMF-treated patients and nontreated volunteers.

RESULTS

The within-run (n = 10) and between-run (n = 20) coefficients of variation (CV) for IMPDH activity were < 8% and < 10%, respectively. IMPDH activity in 60 healthy volunteers (19-63 yr) ranged from 4.72 to 32.92 nmol/h/mg protein (mean = 18.39 +/- 6.24). The IC(50) for in vitro inhibition of IMPDH activity was about 2 to 3 microg/L. Application of a single dose of 1 g MMF in dialysis patients resulted in a significant inhibition (by 47-95%; p < 0.05) of IMPDH activity in lysed MNC.

CONCLUSIONS

The proposed assay specifically and reliably measures IMPDH activity in MNC. The procedure is applicable to evaluate pharmacodynamic activity in MMF-treated patients. The observed interindividual variability of IMPDH activity may reflect pharmacodynamic differences in MMF-treated patients.

Mycophenolate mofetil: suggested guidelines for use in kidney transplantation

Mycophenolate mofetil (MMF) is an immunosuppressive drug designed to inhibit inosine monophosphate dehydrogenase (IMPDH). IMPDH is a key enzyme in the de novo purine synthesis of lymphocytes. It is crucially important for proliferative responses of human T and B lymphocytes. The inhibition of IMPDH thus leads to selective lymphocyte suppression. After successful use in various in vitro and animal models, MMF was brought to clinical trial in patients undergoing transplantation. The drug is rapidly and completely absorbed following oral administration. Pilot studies of administration with cyclosporin and corticosteroids suggested a significant reduction in the incidence of organ rejection at dosages of 1 to 3 g/day. As a result of these studies, 3 pivotal randomised double-blind multicentre trials, involving nearly 1500 patients, were designed to investigate the effects of addition of MMF to different standard immunosuppressive protocols on the prevention of acute renal allograft rejection. After 6 months, the rates of biopsy-proven rejection were significantly reduced in patients receiving MMF. In combination with cyclosporin and corticosteroids, the adverse effect profile resembled that of azathioprine. Most adverse effects were associated with the gastrointestinal tract, the blood system and opportunistic infections. MMF offers improved immunosuppressive therapy following renal and probably other solid organ transplantation. MMF has been licensed since 1995 for the prevention of acute renal allograft rejection in most countries. It has been used in different combinations of immunosuppressive drugs and in various dosages and regimens.

Genetics of drug response to immunosuppressive treatment and prospects for personalized therapy

The use of immunosuppressive agents in the treatment of transplant rejection and autoimmune disorders is gaining momentum, with significant improvements of both graft and patient survival. The individual response to drugs, however, is variable and unexpected toxicity, or impaired activity might be seen, as a result of molecular determinants that eventually dictate how the individual will respond to immunosuppressive agents. This review addresses a number of issues related to pharmacogenetics, and discusses how this approach might be used to improve the clinical efficacy and tolerability of therapeutic options for the management of organ transplantation and autoimmune disorders in the next decade.

Pharmacodynamic monitoring of mycophenolate mofetil

The immunosuppressive activity of Mycophenolate Mofetil (MMF) is based on the reversible inhibition of inosine-5'-monophosphate dehydrogenase (IMPDH) by mycophenolic acid. Pharmacodynamic monitoring by measurement of IMPDH activity reflects directly the biological response to MMF. For measurement of IMPDH activity in peripheral mononuclear cells we established a modified non-radioactive procedure, based on the incubation of cell lysates with inosine-5'-monophosphate and the chromatographic quantification of produced xanthosine-5'-monophosphate by isocratic ion-pair reversed phase HPLC. The between-run precision and within-run precision were 7% and 5%, respectively. We determined the time course of IMPDH activity in five patients after 1 g MMF and in five healthy subjects without administration of MMF. Additionally, IMPDH activity was determined in a population study of 40 healthy volunteers. In healthy volunteers, we observed a wide range of IMPDH activity (4.7-32.9 nmol/h/mg) with only weak diurnal variation. All patients receiving MMF had a significant reduction of IMPDH activity (65-100%) after administration of the drug. Inhibition persisted for up to 6 hours, and after 11 hours IMPDH activity returned to predose activities. The interindividual variability of IMPDH activity may account for pharmacodynamic differences in MMF-treated patients. Based on pharmacodynamic monitoring better dosing strategies for MMF-treated patients may evolve.

Pharmacodynamics of immunosuppressive drugs

The inability to measure the effects of immunosuppressive drugs on immune cells in vivo has always severely limited preclinical drug development, the design and interpretation of clinical trials and the optimal clinical use of this drug class in transplantation. Now, new technologies using microliter samples of whole blood and exploiting the specificity, sensitivity and versatility of flow cytometry have been developed. These novel techniques not only are illuminating the 'black box' that has obscured the pharmacodynamic effects of immunosuppressants but also are uncovering new mechanisms of action of these drugs. Pharmacodynamic assays measure biologically relevant events in vivo, since changes in lymphocyte functions in blood collected from immunosuppressed graft recipients faithfully reflect histopathologic events within allograft tissue.

Assessment of Mycophenolic Acid-induced Immunosuppression: A New Approach

Background: Mycophenolic acid (MPA), a metabolite of mycophenolate mofetil (MMF), is an immunosuppressive agent that inhibits inosine monophosphate dehydrogenase (IMPDH), a key enzyme in the ex novo synthesis of GTP. We measured IMPDH activity in peripheral blood mononuclear cells (PBMCs) from MMF-treated patients to evaluate the efficacy of MMF in individual patients.

Methods: IMPDH activity was measured by 3H released from [2,8-3H]IMP that had been formed in the cells from added [2,8-3H]hypoxanthine in PBMCs of 35 renal transplant recipients treated with cyclosporin A and corticoids plus MMF: 2 g (n = 10), 1.5 g (n = 7), 1 g (n = 10), or 0 g (n = 8) per day. An alternative method, based on the capacity of the patients’ sera to inhibit spontaneous proliferation of the CEM cell line, was also analyzed.

Results: The IMPDH activity of PBMCs in transplanted patients was highly variable. For the method based on CEM cell line proliferation: (a) cell proliferation was inhibited only in MMF-treated patients; (b) there was a clear postdose increase in inhibition; (c) inhibition was not affected by other immunosuppressants in vitro or in vivo; (d) inhibition from predose to predose sample was correlated; and (e) when the MMF dosage was &lt;20 mg · kg−1 · day−1, two groups of patients were identified, one that maintained a high inhibitory capacity in all dose intervals, and one with periods of low inhibitory capacity.

Conclusions: Measurement of the inhibition of CEM cell line proliferation by sera from MMF-treated patients may be useful for evaluating the relative efficacy of MMF treatment in individual patients, especially those receiving low doses of MMF.