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The potent immunosuppressant agent sirolimus (rapamycin; Wyeth-Ayerst) is undergoing clinical trials in solid-organ transplantation (1)(2). Evidence suggests that therapeutic drug monitoring of trough sirolimus concentrations may be beneficial for two fundamental reasons: (a) a strong correlation between the trough concentration and the area under the concentration-time curve has been shown; and (b) a strong correlation between the evidence of toxicity and trough concentrations substantially >15 μg/L has been reported (3). To date, there have been only HPLC methods available to measure sirolimus in human whole blood. These include HPLC with mass spectrometric detection (4)(5) and HPLC with ultraviolet detection (6)(7)(8)(9). As an alternative, a semiautomated method utilizing the IMx analyzer (Abbott Diagnostics) that incorporates microparticle enzyme immunoassay (MEIA) technology has been developed. There are two versions of the MEIA for sirolimus in whole blood, the prototype version (Mode 1A) and the recently manufactured premarket version (Mode 1C). Both of these assays are undergoing evaluation based on their analytical performance and clinical use. This study compares the analytical performance of the prototype MEIA (Mode 1A) with HPLC-tandem mass spectrometry (HPLC-MS) within one center through evaluation of interference with endogenous and exogenous compounds, imprecision, recovery, and the quantification range. In addition, blood samples from renal transplant patients receiving sirolimus therapy are compared using both methods. Throughout this study, HPLC-MS was performed as per our reported method (4) and MEIA according to manufacturer’s instructions. All patient specimens were stored at −75 °C until analysis. Investigation into potential interferences in HPLC-MS and MEIA in relation to endogenous compounds was determined based on 209 samples collected into EDTA tubes from 23 renal transplant recipients …

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® 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)(5)(6)(7)(8)(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-d12), cyclosporin D (CsD), and ascomycin for the quantification of CsA by a high-throughput HPLC-MS method. The HPLC-MS method in this study is a modification based on the previously reported assay by Keevil et al. (5). Whole blood samples (50 μL) were treated with 0.1 mol/L zinc sulfate (200 μL) followed by acetonitrile (500 μL) containing internal standard (50 μg/L). Because of interference from CsA-d12 in the CsD mass transition, 2 extractions were performed and analyzed: 1 with …