Radioreceptor assay for sirolimus

Objectives: To develop a radioreceptor assay (RRA) for sirolimus (rapamycin, RAPA). Methods: A direct methanol extraction was used to prepare 45 patient samples for the RRA. Results were compared to the results obtained previously using high-performance liquid chromatography (HPLC). Between-run precision, recovery, and drug interference studies were also performed. Results: The RRA is sensitive to 1.0 μg/L RAPA equivalents in whole blood. Comparison with HPLC yielded a correlation coefficient for 45 patient samples of 0.977. Between-run precision at 2.5, 7.5, 12.5, and 20 μg/L showed coefficients of variation (CVs) of 12.9, 9.2, 8.5, and 5.9%, respectively. Recoveries from the extraction procedure were 93% at 7.5 μg/L and 103% at 12.5 μg/L. Drug interference studies showed no interference in the RRA by cyclosporine (CsA), dexamethasone, prednisone, or methotrexate. Conclusion: We have demonstrated that the RRA for RAPA correlates well with HPLC, and has excellent precision and recovery. The procedure is far less time-consuming and complex than HPLC and has potential for automation.