Significance of mycophenolate monitoring in liver transplant recipients: toward the cut-off level

Therapeutic drug monitoring in kidney and liver transplantation: current advances and future directions

INTRODUCTION

Immunosuppressive drugs (ISD) present a narrow therapeutic window and extremely high inter- and intra-individual pharmacokinetic variability, which complicates their use in solid organ transplant recipients. In order to find a narrow appropriate equilibrium for each patient with the aim of maintaining clinical efficacy and reducing the risk of adverse drug reactions, a complex both clinical and biological monitoring is required, in particular through the use of therapeutic drug monitoring (TDM).

AREA COVERED

This review provides an overview of the available information on the relationship between exposure to immunosuppressive drugs and their efficacy and/or toxicity in kidney and liver transplantation. The aim of the review is to describe the pharmacodynamic/pharmacokinetic relationship that exists for immunosuppressive drugs, to summarize the studies that assess the value of TDM for these drugs in clinical practice, and to present the target and monitoring strategies aimed at optimizing patient immunosuppression, which could help to take a step forward in the field of solid organ transplant patient care.

EXPERT OPINION

To improve the care of transplant patients, several TDM innovations can be pursued by investigators. Among these, the development of microsampling methods for TDM or the combination of pharmacodynamic biomarkers with ISD exposure measurements appear to be relevant strategies.

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.

Prognostic value of endogenous and exogenous metabolites in liver transplantation

Liver transplantation has been widely accepted as an effective intervention for end-stage liver diseases and early hepatocellular carcinomas. However, a variety of postoperative complications and adverse reactions have baffled medical staff and patients. Currently, transplantation monitoring relies primarily on nonspecific biochemical tests, whereas diagnosis of multiple complications depends on invasive pathological examination. Therefore, a noninvasive monitoring method with high selectivity and specificity is desperately needed. This review summarized the potential of endogenous small-molecule metabolites as biomarkers for assessing graft function, ischemia-reperfusion injury and liver rejection. Exogenous metabolites, mainly those immunosuppressive agents with high intra- and inter-individual variability, were also discussed for transplantation monitoring.

Effect of mycophenolic acid on inosine monophosphate dehydrogenase (IMPDH) activity in liver transplant patients

BACKGROUND

Due to the development of immunosuppressants, the focus in transplanted patients has shifted from short-term to long-term survival as well as a better adjustment of these drugs in order to prevent over- and under-immunosuppression. Mycophenolic acid (MPA) is a noncompetitive inhibitor of inosine monophosphate dehydrogenase (IMPDH) and approved for prophylaxis of acute rejection after kidney, heart, and liver transplantation, where it has become a part of the standard therapy. Targeting inosine monophosphate IMPDH activity as a surrogate pharmacodynamic marker of MPA-induced immunosuppression may allow a more accurate assessment of efficacy and aid in limiting toxicity in liver transplanted patients.

AIM

Assess IMPDH-inhibition in liver transplant recipients and its impact on biliary/infectious complications, acute cellular rejection (ACR) and liver dependent survival.

METHODS

This observational cohort study comprises 117 liver transplanted patients that were treated with mycophenolate mofetil (MMF) for at least 3 months. Blood samples (BS) were collected and MPA serum level and IMPDH activity were measured before (t(0)), 30minutes (t(30)) and 2h after (t(120)) MMF morning dose administration. Regarding MPA, we assessed the area under the curve (AUC). Patients were prospectively followed up for one year and assessed for infectious and biliary complications, episodes of ACR and liver dependent survival.

RESULTS

The MPA levels showed a broad interindividual variability at t(0) (2.0±1.8ng/ml), t(30) (12.7±9.0ng/ml) and t(120) (7.5±4.3ng/ml). Corresponding IMPDH activity was at t(o) (23.2±9.5 nmol/h/mg), at t(30) (16.3±8.8 nmol/h/mg) and t(120) (18.2±8.7 nmol/h/mg). With regard to MPA level we found no correlation with infectious or biliary complications within the follow-up period. Patients with baseline IMPDH(a) below the median had significant more viral infections (6 (10.2%) vs. 17 (29.3%); P=0.009) with especially more cytomegalovirus (CMV) infections (1 (3.4%) vs. 6 (21.4%); P=0.03)). Furthermore, patients with baseline IMPDH(a) above the median developed more often non-anastomotic biliary strictures (8 (13.6%) vs. 1 (1.7%), P=0.03). We found the group reaching the combined clinical endpoint of death and re-transplantation showing significantly lower MPA baseline values (t(0) 0.9±0.7 vs. 2.1±1.8μg/ml Mann-Whitney-U: P=0.02). We calculated a simplified MPA(AUC) with the MPA level at baseline, 30 and 120minutes after MPA administration. Whereas we found no differences with regard to baseline characteristics at entry into the study patients with MPA (AUC) below the median experienced significantly more often the combined clinical endpoint (12.1% (7/58) vs. 0.0% (0/57); P=0.002) and had a reduced actuarial re-transplantation-free survival (1.0 year vs. 0.58 years; Log-rank: P=0.007) during the prospective one-year follow-up period. In univariate and multivariate analysis including gender, age, BMI, ACR, MPA (AUC) and IMPDH(a) only BMI, MPA (AUC) and IMPDH(a) were independently associated with reduced actuarial re-transplantation-free survival.

CONCLUSION

MPA-levels and IMPDH-activity in liver transplanted patients allows individual risk assessment. Patients with higher IMPDH inhibition acquire more often viral infections. Insufficient IMPDH inhibition is associated with development of non-anastomotic bile duct strictures and reduced re-transplantation-free survival.

Clinical mycophenolic acid monitoring in liver transplant recipients

In liver transplantation, the efficacy of mycophenolate mofetil (MMF) has been confirmed in clinical trials and studies. However, therapeutic drug monitoring for mycophenolic acid (MPA) has not been fully accepted in liver transplantation as no long-term prospective study of concentration controlled vs fixed-dose prescribing of MMF has been done. This review addressed MPA measurement, pharmacokinetic variability and reasons of this variation, exposure related to acute rejection and MMF-associated side effects in liver transplant recipients. Limited sampling strategies to predict MPA area under the concentration-time curve have also been described, and the value of clinical use needs to be investigated in future. The published data suggested that a fixed-dosage MMF regimen might not be suitable and monitoring of MPA exposure seems helpful in various clinical settings of liver transplantation.