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BACKGROUND: Matrix effects in HPLC-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS)3 can cause differences in the ionization of an internal standard (IS) compared with the analyte of interest. Unless sample cleanup or chromatographic conditions eliminate or minimize ion suppression or enhancement, variability in interpatient matrices may cause erroneous results. A stable isotope-labeled IS can be used to minimize analytical interpatient variation.
METHODS: We used protein precipitation and HPLC-ESI-MS/MS to quantify sirolimus (SIR) with both desmethoxyrapamycin (DMR) and deuterium-labeled sirolimus (SIR-d^sub 3^) as the IS to analyze a range of whole-blood and extraction-matrix samples, and to estimate recovery, matrix effects, process efficiency, and interpatient variation. We also analyzed a series of blood samples from 72 patients taking SIR, including external proficiency-testing samples, with these ISs.
RESULTS: The range of interpatient assay imprecision (CV) values for the SIR assay was consistently lower with SIR-d^sub 3^ (2.7%-5.7%) than with DMR (7.6%-9.7%). The results obtained with the 2 different ISs for the patient samples showed a linear relationship, but the results were higher with DMR as the IS than with SIR-d^sub 3^.
CONCLUSIONS: The use of SIR-d^sub 3^ as the IS in the high-throughput HPLC-ESI-MS/MS assay of SIR yielded improved results compared with the use of DMR. SIR-d^sub 3^ appears to be less affected by differences in the ionization of SIR and its IS caused by the variability of interpatient matrices. The IS-related difference in SIR estimation needs further investigation.
Matrix effects on electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis have been well documented (1-3). Unless sample cleanup or chromatographic conditions mmimize ion suppression or enhancement, variation in interpatient matrices has the potential to cause erroneous results. High-throughput "rapid analysis" methods commonly used for analyzing immunosuppressants and other drugs by HPLC-MS/MS can be adversely affected by limited chromatographic retention and by limited separation of analytes from coeluting interfering substances (4 ).
The preferred choices for an internal standard (IS) are (a) versions of the analyte labeled with a stable isotope, (b) analogs of the analyte, or (c) another related molecule that closely matches the chromatographic and ionization characteristics of the analyte of interest (5, 6). Until recently, the lack of isotope-labeled versions of many commonly analyzed therapeutic drugs required the use of a "best alternative" IS.
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