Pharmacoeconomics of therapeutic drug monitoring in transplantation

Immunosuppressive drug monitoring - what to use in clinical practice today to improve renal graft outcome

Therapeutic drug monitoring (TDM) of immunosuppressive therapy is becoming an increasingly complex matter as the number of compounds and their respective combinations are continuously expanding. Unfortunately, in clinical practice, monitoring predose trough blood concentrations is often not sufficient for guiding optimal long-term dosing of these drugs. The excellent short-term results obtained nowadays in renal transplantation confer a misleading feeling of safety despite the fact that long-term results have not substantially improved, definitely not to a point where longer graft survival could counteract the increasing need for transplant organs and less toxicity and side-effects could ameliorate patient survival. It is therefore a challenging task to try to tailor immunosuppressive drug therapy to the individual patient profile and this in a time-dependent manner. For the majority of currently used immunosuppressive drugs, measurement of total drug exposure by determination of the dose-interval area under the concentration curve (AUC) seems to provide more useful information for clinicians in terms of concentration-exposure and exposure-response as well as reproducibility. To simplify this laborious way of measuring drug exposure, several validated abbreviated AUC profiles, accurately predicting the dose-interval AUC, have been put forward. Together with an increasing knowledge of the time-related pharmacokinetic behaviour of immunosuppressive drug and their metabolites, studies are focusing on how to apply abbreviated AUC sampling methods in clinical transplantation, taking into account the numerous factors affecting drug pharmacokinetics. Eventually, TDM using abbreviated AUC profiles has to be prospectively tested against classic methods of drug monitoring in terms of cost-effectiveness, feasibility and clinical relevance with the ultimate goal of improving patient and graft survival.

Therapeutic monitoring of immunosuppressant drugs. Where are we?

The emergence of specific immunosuppressive drugs (cyclosporine, tacrolimus, mycophenolate mofetil and sirolimus) during the last two decades has contributed dramatically to the success of organ transplantation. However, optimum balance between therapeutic efficacy and the occurrence of side effects has been a real challenge for physicians, mainly due to inter- and intra-patient variability arising from pharmacokinetic, pharmacogenetic and pharmacodynamic individual properties. Therapeutic drug monitoring, defined as the measurement and interpretation of concentrations of these drugs in biological fluids, with as a final objective the prediction of organ responses, became an integral part of transplant protocols. New analytical techniques became available with different performances in terms of specificity and sensitivity. In addition, there has been progress in understanding the mechanisms of action of these drugs that have implications for the development of better monitoring strategies and for their coprescription. The purpose of this review is to examine the current strategies in use for the therapeutic drug monitoring of immunosuppressant drugs and to discuss some of the factors that impinge on the monitoring of these drugs.

Therapeutic drug monitoring: is it cost-effective?

During the last 30 years, therapeutic drug monitoring has evolved from an abstract consideration to a routine intervention. In this increasingly cost-conscious environment, questions are raised about the cost-effectiveness of therapeutic drug monitoring. This review presents the state of therapeutic drug monitoring today, predictions for its near future and the status of economic analyses of the intervention.

Immunosuppressive therapy for paediatric transplant patients: pharmacokinetic considerations

Immunosuppressive therapy in paediatric transplant recipients is changing as a consequence of the increasing number of available immunosuppressive agents. Generic and other new formulations are now emerging onto the market, clinical experience is growing, and it is expected that clinicians should tailor immunosuppressive protocols to individual patients by optimising dosages and drugs according to the maturation and clinical status of the child. Most information about the clinical pharmacokinetics of immunosuppressive drugs in paediatrics is centred on cyclosporin, tacrolimus and mycophenolate mofetil in renal and liver transplant recipients; data regarding other immunosuppressants and transplant types are limited. Although the clinical pharmacokinetics of these drugs in paediatric transplant recipients are still under investigation, it is evident that the pharmacokinetic parameters observed in adults may not be applicable to children, especially in younger age groups. In general, patients younger than 5 years old show higher clearance rates irrespective of the organ transplanted or drug used. Another important factor that frequently affects clearance in this patient population is the post-transplant time. In accordance with these findings, and in contrast with the usual under-dosage in children, the need for higher dosages in younger recipients and during the early post-transplant period seems evident. To achieve the best compromise between prevention of rejection and toxicity, dosage individualisation is required and this can be achieved through therapeutic drug monitoring (TDM). This approach is particularly useful to ensure the cost-effective management of paediatric transplant recipients in whom the pharmacokinetic behaviour, target concentrations for clinical use and optimal dosage strategies of a particular drug may not yet be well defined. Although TDM may be a tool for improving immunosuppressive therapy, there is little information concerning its positive contribution to clinical events, including outcomes, for paediatric patients. Substantial information to support the use of TDM exists for cyclosporin and, to a lesser extent, for tacrolimus, but a diversity of options affects their implementation in the clinical setting. The role of TDM in therapy with mycophenolate mofetil and sirolimus has yet to be defined regarding both methods and clinical indications. Pharmacodynamic monitoring appears more suited to other immunosuppressants such as azathioprine, corticosteroids and monoclonal or polyclonal antibodies. If coupled with pharmacokinetic measurements, such monitoring would allow earlier and more precise optimisation of therapy. Very few population pharmacokinetic studies have been carried out in paediatric transplant patients. This type of study is needed so that techniques such as Bayesian forecasting can be applied to optimise immunosuppressive therapy in paediatric transplant patients.

Therapeutic drug monitoring: do the improved outcomes justify the costs?

In the 30 years that therapeutic drug monitoring (TDM) has moved from an abstract consideration to a routine intervention, issues remain over justifying the benefits in the light of the ever-increasing competition for budgetary resources. Resolving the issues is constrained by various methodological concerns. These include considerations such as: (i) the changed environment of knowledge and practice during the generation in which TDM has been used and evaluated; (ii) the predominance of studies using system-related rather than patient-centred outcomes; (iii) using a timeframe for analysis that is too short; (iv) a lack of rigour in many of the pharmacoeconomic analyses; and (v) excessive use of a site-specific rather than a societal perspective. Current observation suggests that the greatest benefit of TDM accrues from targeted or specialty populations: those with severely decompensated renal function, those at the extremes of age, and those using immunosuppressive, some antineoplastic, some psychotherapeutic and some anticonvulsant drugs. In these situations, safe and humane practice considers TDM a necessity without respect to cost. But for many routine situations with drugs for which TDM has commonly been used in the past, present reliance on the intervention may have become excessive in the light of today's knowledge base of practitioners.

Therapeutic drug monitoring for immunosuppressants

BACKGROUND

Immunosuppressants have significantly increased patient survival, e.g. in renal transplant up to 90% for the first year.

METHODS

Four immunosuppressants are used for clinical applications in the United States: cyclosporine (CsA) (Sandimmune and Neoral), FK 506-tacrolimus (ProGraf), mycophenolic mofetil (CellCept)--the prodrug for the mycophenolic acid (MPA), and rapamycin (RAPA) (Sirolimus). For CsA and FK 506, the rationale for monitoring is due to the variable pharmacokinetics, acute infection, dosage adjustment, non-compliance check, and for long-term maintenance therapy. Targeted whole blood concentrations ranges are: for CsA, 100-400 ng/ml depending on the methods, therapy and organs; and for FK 506, 5-20 ng/ml. For MPA, drug bioavailability--the plasma area-under-curve up to 12 h of 32.2-60.6 mg h/l was correlated to the biopsy-proven rejection rate of <10%. Monitoring is advocated for liver and renal transplants, for pediatrics, and for checking for non-compliance. RAPA monitoring is useful to check for variable pharmacokinetics, for non-compliance and others. The therapeutic range is tentatively targeted for 5-15 ng/ml. Monitoring methodologies are: for CsA, immunoassays such as fluorescence polarization immunoassay, and liquid chromatography (LC); for FK 506, microparticle enzyme immunoassay (MEIA); for MPA, enzyme multiplied immunoassay and LC; and for RAPA, MEIA, LC and LC-mass spectrometry. Proficiency survey programs for CsA and FK 506 are available from the US and Europe.

CONCLUSIONS

Monitoring of immunosuppressants has become an essential adjunct to the drug therapy for organ transplant patients.