Ex vivo assessment of immunosuppression in undiluted whole blood from pigsdosed with tacrolimus (FK506)

To assess the duration of immunosuppression in FK506-dosed pigs, an undilut ed whole blood assay was established to measure reactivities of T cells in their physiological milieu. PMA and ionomycin were shown to induce IL-2 pro duction in swine blood. The IC50 of FK506 in inhibiting IL-2 production in whole blood and isolated PBMC stimulated with PMA and ionomycin measured 1. 2 and 0.04 nM, respectively. These data underscore the influence of red blo od cells and plasma proteins on drug potency. IL-2 levels were determined i n blood drawn immediately before and 1, 24, 48, and 72 h after iv dosing. F or pigs dosed with 0.05 mg/kg, 50% recovery of IL-2 production was observed at 16 h and 100% at 35 h after dosing. For pigs dosed with 0.15 mg/kg; 50% recovery was observed at 38 h and 100% at 72 h. Blood concentrations of FK 506 at 50 and 100% recovery of IL-2 production measured 10.8 and 2.2 nM for pigs dosed with 0.05 mg/kg and 6.1 and 1.1 nM:for pigs dosed with 0.15 mg/ kg, respectively. These concentrations are severalfold higher than predicte d from the IC,, of FK506 for inhibiting IL-2 production in the whole blood assay. These data suggest that the true potency of FK506 in blood after dos ing is influenced by additional factors, which could include plasma protein binding, the presence of active or interfering metabolites, serum interfer ing factors, and sequestration of drug in blood cells. Our results demonstr ate the utility of an undiluted whole blood assay for assessing the duratio n of immunosuppression in drug-dosed animals and emphasize the importance o f assessing drug potency in the whole blood environment ex vivo. (C) 1999 A cademic Press.