The calcineurin inhibitor cyclosporin A (CsA) is used as a primary immunosuppressant in solid organ transplantation and for the treatment of many autoimmune diseases. Individualization of therapy is required because CsA has high inter- and intrapatient pharmacokinetic variability, the potential for drug interactions, a narrow therapeutic index, and important compliance issues (1). The use of the CsA formulation Neoral[R] has been associated with a shift in monitoring from trough (predose) to 2-h postdose (C2) whole blood samples (2,3). To provide efficient CsA therapeutic drug-monitoring, a rapid turnaround of results, high selectivity, and high sensitivity are required (4). These requirements have led to the development of several high-throughput HPLC-mass spectrometry (HPLC-MS) methods (4-9). For quantification purposes, internal standardization is used in all methods. Currently, there is no consensus on the preferred internal standard for CsA measurement. To date, the internal standards that have been used can be classified into 3 types: isotope-labeled CsA (10, 11), a structural analog (8,12), or a structurally unrelated compound (4, 5). The aim of this study was to evaluate the analytical performance of these 3 classes of internal standards with deuterium-labeled CsA ([CsA-d.sub.12]), cyclosporin D (CsD), and ascomycin for the quantification of CsA by a high-throughput HPLC-MS method.
