Measurement of cyclosporine A by a specific radioimmunoassay with a monoclonal antibody and 125I tracer.

We developed a sensitive radioimmunoassay (CYCLO-Trac SP) that specifically measures cyclosporine A in serum, plasma and whole blood of transplant patients. The specific monoclonal antibody was from Sandoz and the tracer was an 125I derivative of cyclosporine C. The assay is performed at room temperature for 1 h followed by a 20 min centrifugation. The sensitivities of the assays are 2.6 ng/mL and 8.7 ng/mL for the serum/plasma assay and the whole blood assay, respectively. Within-run and between-run CVs for both types of assays using cyclosporine concentrations of 80 and 58 ng/mL (serum) and 186 and 199 ng/mL (whole blood) were less than 5% and 9%, respectively. Averaged recovery of serum/plasma and whole blood assays at various levels ranged from 93% to 115%. Interferences by bilirubin, triglyceride, cholesterol, hemoglobin, OKT-3, azathioprine, methylprednisolone and 20 other drugs were insignificant. Multicenter proficiency studies showed an excellent correlation between the CYCLO-Trac SP and the specific 3H-Sandimmune assay from Sandoz: whole blood assay (r = 0.998) and serum assay (r = 0.997).