RADIORECEPTOR ASSAY FOR QUANTIFYING FK-506 IMMUNOSUPPRESSANT IN WHOLE-BLOOD

We describe a quantitative radioreceptor assay (RRA) for quantifying FK-506 in whole blood. FK-506 extracted from whole blood with a cyclohexyl-sorbent column competes with [H-3]dihydro-FK-506 for binding to a partially purified preparation of FK-506 binding protein (FK-BP). Free and protein-bound FK-506 are separated on LH 20 Sephadex chromatographic columns. We compared the results of this method with those of an enzyme immunoassay that uses a monoclonal antibody: r = 0.97, S(y\x) = 0.039. Between-day precisions (CV) at FK-506 concentrations of 8 and 17-mu-g/L were 9.2% and 8.2%, respectively. Within-run precisions were 5.9%, 8.1%, and 9.4%, respectively, at 4, 8, and 15-mu-g/L. Analytical recovery, evaluated at 5, 10, 15, 20, and 25-mu-g/L for FK-506 added to whole blood, ranged from 98% to 103%. The assay can reliably quantify FK-506 blood concentrations between 1.0 and 25-mu-g/L.