FK506 measurement: comparison of different analytical methods

In this study, we used solid-phase extraction with Sep-Pak and a liquid-liquid extraction with methylene chloride as two primary methods of extracting FK506 from plasma. The extracts were either analyzed directly by enzyme-linked immunosorbent assay (ELISA) or subjected to high-performance liquid chromatography (HPLC) separation and different fractions were analyzed by ELISA. Serial blood samples were obtained from four kidney transplant patients and four patients who underwent liver transplantation, from day 1 until day 30-35 posttransplantation. There was no significant difference in the FK506 plasma concentration as measured by all four methods in normal transplant patients. However, in liver transplant patients, the solid-phase extraction method gave higher FK506 concentrations than the methylene chloride extraction during the early postoperative period. The concentrations measured after methylene chloride extraction were higher than that after HPLC-ELISA. This higher FK506 concentration measured by direct ELISA could be attributed to possible cross-reacting metabolites that were present in the plasma of patients with abnormal liver functions. Once liver function returns to normal, all four methods give identical plasma concentrations for FK506.