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A rapid and robust method for measuring methotrexate (MTX) and its two primary metabolites, 7-hydroxymethotrexate (7-OHMTX) and 2,4-diamino-N-10-methylpteroic acid (DAMPA), was developed for use in pharmacokinetic studies of plasma and cerebrospinal fluid samples collected from infants with malignant brain tumors. Sample aliquots (100 mu L) were prepared for bioanalysis of MTX and metabolites using a Waters Oasis HLB microelution solid-phase extraction (SPE) plate. Chromatography was performed using a Phenomenex Synergi Polar-RP 4 mu 75 x 2.0 mm ID column heated to 40 degrees C. A rapid gradient elution on a Shimadzu HPLC system was used, with mobile phase A consisting of water/formic acid (100/0.1 v/v) and mobile phase B consisting of acetonitrile/formic acid (100/0.1 v/v). Column eluent was analyzed using AB Sciex QTRAP 5500 instrumentation in electrospray ionization mode. The ion transitions (m/z) monitored were 455.2 -> 308.1, 471.1 -> 324.1, and 326.2 -> 175.1 for MTX, 7-OHMTX, and DAMPA, respectively. The method was linear over 0.0022-5.5 mu M for MTX, 0.0085-21 mu M for 7-OHMTX, and 0.0031-7.7 mu M for DAMPA. The method was applied to the analysis of serial plasma samples obtained from infants diagnosed with malignant brain tumors receiving high-dose methotrexate and results were compared to MTX concentrations from an immunoassay based on fluorescence polarization.