Abstract
Immunoassays of cyclosporin A (CsA) have been routinely used to measure CsG. We investigated the cross-reactivities of CsG and its metabolites, as well as the proportion CsG constitutes in relation to total drug measured, for six CsG metabolites (GM1, GM9, GM4N, GM1c, GM1c9, GM19) in the following CsA assays: Sandimmune selective RIA (SS), Sandimmune nonselective RIA (NS), Cyclotrac SP-RIA (CT), fluorescence polarization immunoassay (FPIA), and enzyme immunoassay (EMIT). The cross-reactivity of CsG in these assays was as follows: SS, FPIA, CT, approximately 100%; NS, approximately 40%; EMIT, < 2%. The cross-reactivities of CsG metabolites were investigated in all assays except EMIT and varied among metabolites and assays. The most significant variance was found with the NS assay, where most of the metabolites exhibited cross-reactivities of > 40%. In contrast, in the SS, FPIA, and CT assays, cross-reactivities of < 5% were observed for most of the metabolites. The ranking of cross-reactivities of CsG metabolites in the assays is SS = CT < FPIA < NS. The degree of cross-reactivity did not change significantly when the SS, CT, and FPIA assays were calibrated with CsG instead of CsA--whether parent CsG was present or not. The data suggest that the SS, CT, and FPIA methods would be suitable for the routine monitoring of CsG.
A Modified Fluorescence Polarization Immunoassay Method Incorporating Fat Emulsion(FE-FPIA) to Determine Cyclosporin A Concentrations in Rat Skin.