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Sirolimus (rapamycin, Rapamune) is a macrolide antibiotic, isolated from Streptomyces hygroscopicus, that suppresses T-and B-cell proliferation by inhibiting the protein kinase mTOR. It has no effects on calcineurin and, therefore, can be used in addition to cyclosporine or tacrolimus, or as a substitute for these drugs. Adverse effects of sirolimus are concentration-dependent, making therapeutic drug monitoring essential.

Preferred therapeutic ranges may vary by transplant type, protocol, and concomitant medications. Most individuals display optimal response to sirolimus with trough whole blood levels 4 to 20 ng/mL. Some patients may require higher trough levels between 20 and 30 ng/mL.

The pharmacokinetic interaction between sirolimus and cyclosporine or tacrolimus increases both therapeutic immunosuppression and the toxicity. Therefore, lower doses are required with combined use. When given with cyclosporine or tacrolimus, the therapeutic range for sirolimus is generally between 4 and 12 ng/dL.

Trough sirolimus concentrations are generally measured every 5 days. Specimen requirement is one 5 mL EDTA (lavender top) tube of blood. The recommended therapeutic range applies to trough specimens drawn immediately before a dose. Blood drawn at other times will yield higher results. Therapeutic range measured by immunoassay on an Abbott Architect is 4.5 – 28.0 ng/mL. Results by liquid chromatography with detection by liquid chromatography/tandem mass spectrometry are approximately 30% less than by immunoassay.

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