OBJECTIVE Sirolimus (RAPA) is a new immunosuppressive drug currently in Phase III clinical trials in combination with cyclosporine A (CsA). The toxicity profiles for CsA and RAPA are only partially overlapping, with RAPA toxicity consisting primarily of hyperlipidemia and myelodepression but without the nephrotoxicity, neurotoxicity, and hepatotoxicity, which are seen with CsA. Patients in the clinical trial are being monitored using HPLC or LC/MS/MS assays; there is no immunoassay for RAPA reported to date. We have previously reported a radioreceptor assay (RRA) for RAPA, which has an excellent correlation with the HPLC assay (r = 0.997). The RRA has several advantages including excellent precision, sensitivity, rapid turnaround time, and a one-step extraction procedure. We report the evaluation of blood samples from patients who were exhibiting RAPA toxicity and comparison of the RRA results with the HPLC results. METHODS EDTA whole blood specimens (n = 42) were obtained from six renal transplant recipients taking RAPA and CsA and exhibiting decreased platelet counts. Thirty-two samples from patients without decreased platelet counts were also received. The samples were analyzed with the RRA and the results were compared to those obtained with the HPLC assay. RESULTS By HPLC, the results ranged from 3.2-72.6 micrograms/L RAPA with 43% of the results > or = 30 micrograms/L. With the RRA, the range was 7.7-83.0 micrograms/L RAPA equivalents, with 60% of the results > or = 30 micrograms/L. The RRA results are distinctly higher than the HPLC results all along the range. The correlation between the two assays was 0.861, with a slope of 0.966 and a Y-intercept of 11.1. CONCLUSIONS Since the RRA is consistently higher than HPLC concentration in patients with decreased platelet counts, but correlates well in patients with no signs of toxicity, the RRA may be useful for monitoring patients for toxicity, by giving a better indication of increasing degree of immunosuppression than the HPLC assay.