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Monitoring Immunosuppressive Drugs: Has It a Future?

The one thing that can be said with certainty on this topic is that drug monitoring of immunosuppressive drugs has a past. It has an amazingly well-documented history, which describes both the methodologic problems associated with the measurement of the drugs and the difficulties encountered in the clinical interpretation of the data. Drug monitoring is widely practiced, especially for cyclosporin and tacrolimus, and in some parts of the world it has become a regulatory requirement for the prescription of sirolimus. It is, however, widely criticized by clinicians as failing to portend important clinical events. Although mean data from distinct patient populations may be relatively easy to interpret, individual patient data are more elusive in their clinical message. Monitoring has developed in an ad hoc fashion, and the results have been heavily biased by methodologic differences for the measurement of some drugs. There have been no randomized studies to examine the potential benefit of monitoring against patients treated without monitoring, and it would be a brave center that undertook such studies for either cyclosporin or tacrolimus today. A study in which patients are to be randomized to assess the value of monitoring mycophenolic acid started in 2003 and is the first study to use such an approach for immunosuppressive drug monitoring. For the newer drugs, sirolimus and everolimus, the early clinical studies were based on concentration-controlled dosing using validated analytic techniques. These studies have yielded valuable information on which to base concentration–effect modeling. In the case of sirolimus the model examined the concentrations of 2 drugs, sirolimus and cyclosporin, in relation to their influence on graft rejection. This article examines some of the factors that limit the impact of therapeutic drug monitoring and how we might enhance the value of monitoring for this important group of drugs.