In vitro assessment of FK 506 immunosuppressive activity in transplant patients.

FK 506 has established itself as a promising immunosuppressive drug in organ transplantation and in the treatment of autoimmune disease. Therapeutic monitoring of plasma concentrations of FK 506 is essential to ensure appropriate dosage for adequate immunosuppression and to minimize potential side effects. Routine monitoring of FK 506 plasma concentration is performed with an enzyme-linked immunoassay (ELISA) previously developed by Tamura et al1 and modified by Cadoff et al.2 Plasma FK 506 levels also have been measured with an in vitro bioassay.3 This assay is based on the inhibition of the alloantigen driven proliferation of cloned alloreactive T cells. These activated lymphocytes show a narrow sensitivity range to FK 506 and the IC50 is 0.07 to 0.12 nmol/L. In liver allograft recipients. the FK 506 levels as determined by bioassay are consistently lower than those measured by ELISA (Fig 1). These results suggest that the plasma may contain biologically less active FK 506 metabolites, which can be detected by ELISA. Fig 1 FK 506 levels in plasma measured by the bioassay and ELISA in a liver transplant recipient.