Simple, rapid 125I-labeled cyclosporine double antibody/polyethylene glycol radioimmunoassay used in a pediatric cardiac transplant program

We modified the Sandoz cyclosporine radioimmunoassay because of our need for frequent clinical monitoring of cyclosporine drug levels in allo-and xenograft pediatric cardiac transplant patients. With application of a commercially available [¹²⁵I]cyclosporine label in place of [³H]cyclosporine and a second antibody/polyethylene glycol (PEG) method of separation in place of charcoal separation, we simplified and enhanced the speed and precision of assay performance. Studies of 140 whole blood samples comparing this new method to the [3H]cyclosporine radioimmunoassay (RIA) method of Berk and colleagues yielded a coefficient of correlation of 0.96 (p < 0.00001) with means of 626 and 667 ng/ml for [³H]RIA and [¹²⁵I]RIA, respectively, and a regression equation of y = 28 + 1.02x. The major advantages are that (a) total assay time is reduced to ~1 h; (b) [¹²⁵I]cyclosporine label is used, avoiding the problems associated with liquid scintillation counting; and (c) precision is enhanced by separating bound and free fractions with second antibody/PEG. These modifications should provide for greater ease of assay performance and improved clinical utility of cyclosporine monitoring not only in the pediatric but also in the adult transplant patient.