Wyeth notified healthcare professionals of changes to the Rapamune Prescribing Information regarding changes in the performance of an immunoassay used for therapeutic drug monitoring (TDM) of sirolimus. The TDM results reported from the assay are both assay and laboratory-dependent. In addition, the results may change over time. Therefore, adjustment to the targeted therapeutic range must be made with a detailed knowledge of the site-specific assay used.
Sirolimus whole blood concentrations can be measured by either chromatographic or immunoassay methodologies. These two methodologies are not directly interchangeable and the measured sirolimus whole blood concentrations depend on the type of assay used. As such, if different assays are used in monitoring a single patient, the dose of Rapamune might be adjusted improperly with potential consequences, such as allograft rejection if drug exposure is too low or toxic side effects if exposure is too high.
Wyeth has advised healthcare providers involved in the management of patients taking Rapamune to determine: 1) which assay is being used in their laboratory(ies); 2) if there is any change to the assay used; 3) if there is a change to the laboratory’s reference range and/or a subsequent change to the institution’s or referring center’s recommended range for sirolimus. With this information, target levels can be appropriately adjusted in order to achieve optimal clinical results.
It is critical that the clinician caring for a patient on sirolimus maintain communication with their laboratory to determine whether the assay used for measuring sirolimus concentrations has been changed.