Evaluation of the New AxSYM Cyclosporine Assay: Comparison with TDx Monoclonal Whole Blood and Emit Cyclosporine Assays

Abstract

Cyclosporine (CsA) is a cyclic undecapeptide that has potent immunosuppressive activity, but its narrow therapeutic range and variable pharmacokinetics in humans make monitoring of CsA mandatory (1). The generally accepted whole-blood therapeutic ranges of CsA (with specific assay) are 100–200 μg/L for renal transplant patients and 150–250 μg/L for cardiac, hepatic, and pancreatic transplant patients during the maintenance phase (2)(3), whereas slightly higher ranges are recommended during the induction phase. The CsA metabolites arising from oxidative pathways (4) cross-react in immunoassays (5)(6), leading to results higher than those by HPLC. Trough concentrations of metabolite AM1, the major metabolite in human blood (1)(4), can exceed those of the parent drug (1). For metabolites AM1, AM9, and AM4N, immunosuppressive activity is <10% of that observed for CsA (7)(8), and at least in rats, various CsA metabolites are not nephrotoxic (9). Thus CsA itself, rather than its metabolites, is the major pharmacologically active substance (10), and specific analytical methods are recommended (3)(11)(12). HPLC is specific but time-consuming and labor-intensive; it also displays the highest between-center CV (1). Three major immunoassays based on different CsA monoclonal antibodies were developed by Abbott, IncStar, and Syva Laboratories: a fluorescence polarization immunoassay (FPIA; for the TDx analyzer), a Cyclo-Trac RIA, and an Emit, respectively (13)(14). All of these immunoassays yield slightly higher concentrations than does HPLC, with FPIA concentrations high by 20–30% (15). The results can be summarized as HPLC < Emit < RIA < TDx. Very recently, a new monoclonal CsA FPIA assay was introduced by Abbott Laboratories for the AxSYM analyzer; it was aimed at reducing metabolite cross-reactivity further and, therefore, at correlating better with HPLC. We first evaluated the analytical performances of this new assay by comparing it with the …