Current opinions on therapeutic drug monitoring of immunosuppressive drugs.

The pharmacokinetics of the immunosuppressive drugs cyclosporine, tacrolimus, mycophenolate mofetil (MMF), and sirolimus are complex and unpredictable. A narrow therapeutic index unique to each patient, as well as variable absorption, distribution, and elimination, are characteristics of these drugs. Therapeutic drug monitoring plays a key role in helping clinicians maintain blood and plasma levels of immunosuppressive drugs within their respective therapeutic ranges. Variation in concentrations outside the narrow therapeutic ranges can result in adverse clinical outcomes. Therapeutic drug monitoring ensures that concentrations are not too high or too low, thereby reducing the risks of toxicity or rejection, respectively. Therapeutic monitoring of immunosuppressive drugs has been based on several choices of assay and biologic fluid (i.e., whole blood, plasma) appropriate for a particular drug. High-performance liquid chromatography (HPLC) remains the gold standard among assay methods used to monitor immunosuppressive drugs. Although HPLC is the assay of choice for cyclosporine, newer monoclonal assays are suitable as well for routine monitoring. HPLC is also widely used for therapeutic drug monitoring of mycophenolic acid, the active metabolite of MMF, and an immunoassay (used in European centers) has been developed. Therapeutic drug monitoring of tacrolimus has been improved with the recent development of assays with greater sensitivity and specificity for tacrolimus than those previously available. No commercial assays are currently available for the therapeutic monitoring of sirolimus. It is also important to identify a specific pharmacokinetic parameter for each individual drug, whether it is trough or area under the concentration-time curve, that may be most useful as a tool for optimal therapeutic drug monitoring in clinical practice. With an increased understanding of the pharmacokinetics of immunosuppressive drugs, therapeutic drug monitoring guidelines will be more clearly defined to ensure the safe and effective management of transplant recipients.