Use of 1251-Labeled-Histamine-Cyclosporin C for MonitoringSerum CyclosporineConcentrationsin TransplantationPatients

We synthesized an 1l-IabeIed-histamine-cycIosporin C tracer, to obviate the use of tntiated tracer in radloimmunoassay of cyclosporine. With this tracer, the assay results varied linearly with concentration up to at least 800 Mg/L. The within-assayCV was 6.6% at 39 p.g/L, 4.2% at 100 ig/L, and 7.0% at 300 1i.g/L(n = 15). The between-assayCV was 10.0, 6.4, and 7.8% for the same respectiveconcentrations. Comparison with an assay involvingtritiatedtracer () showedgood agreementof results:y = 3.81 + 0.927x(r 0.975, n = 604). Analytical recovery ranged from 100 to 106%. We also compared another commerciallyavailable radioiodinatedtracer (“125loclocyclosporin”; Immunonuclear Corp.). Our tracer appeared to be more specific for cyclosponne, as determined by assaying chromatographicfractionsof bile extract from a patient being treated with cyclosporine. Resultswith use of our tracer comparedfavorably with those obtained with the tntiated tracer, and our assay has the advantagesof gamma countingvs liquid-scintillation counting.