RADIORECEPTOR ASSAY FOR SIROLIMUS IN PATIENTS WITH DECREASED PLATELETCOUNTS

Objective: Sirollmus (RAPA) is a new immunosuppressive drug currently in Phase III clinical trials in combination with cyclosporine A (CsA). The toxicity profiles for CsA and PAPA are only partially overlapping , with RAPA toxicity consisting primarily of hyperlipidemia and myelod epression but without the nephrotoxicity, neurotoxicity, and hepatotox icity, which are seen with CsA. Patients in the clinical trial are bei ng monitored using HPLC or LC/MS/MS assays; there is no immunoassay fo r PAPA reported to date. We have previously reported a radioreceptor a ssay (RRA) for PAPA, 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 extracti on procedure. We report the evaluation of blood samples from patients who were exhibiting RAPA toxicity and comparison of the RRA results wi th 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 patient s without decreased platelet counts were also received. The samples we re analyzed with the RRA and the results were compared to those obtain ed with the HPLC assay. Results: By HPLC, the results ranged from 3.2- 72.6 mu g/L RAPA with 43% of the results greater than or equal to 30 m u g/L. With the RRA, the range was 7.7-83.0 mu g/L RAPA equivalents, w ith 60% of the results greater than or equal to 30 mu g/L. The RRA res ults are distinctly higher than the HPLC results all along the range. The correlation between the two assays was 0.861, with a slope of 0.96 6 and a Y-intercept of 11.1. Conclusion: Since the RRA is consistently higher than HPLC concentration in patients with decreased platelet co unts, but correlates well in patients with no signs of toxicity, the R RA may be useful for monitoring patients for toxicity, by giving a bet ter indication of increasing degree of immunosuppression than the HPLC assay.