Abstract
A rapid and simple turbulent flow liquid chromatography (TFC-LC) method implementing positive heated electrospray ionization (HESI) for the accurate and precise determination of methotrexate (MTX), 7-hydroxy methotrexate (7-OH MTX), and 4-amino-4-deoxy-N(10)-methylpteroic acid (DAMPA) concentrations in serum was developed. MTX was isolated from serum samples (100μL) after protein precipitation with methanol containing formic acid and internal standard (MTX-D3) followed by centrifugation. The supernatant was injected into the turbulent flow liquid chromatography which is followed by electrospray positive ionization tandem mass spectrometry (TFC-LC-MS/MS) and quantified using a six-point calibration curve. For MTX and DAMPA the assays were linear from 10 to 1000nmol/L and for 7-OH MTX from 20 to 2000nmol/L. Dilutions of 10, 100 and 1000-fold were validated giving a clinically reportable range of 10nmol/L to 5×10(5)nmol/L. Within-day and between-day precisions at concentrations spanning the analytical measurement ranges were less than 10% for all three analytes. MTX, DAMPA and 7-OH MTX were sufficiently stable under all relevant analytical conditions. No significant matrix effect was observed during the method validation. The TFC-LC-MS/MS MTX method was also compared with three other clinically validated MTX assays: a dihydrofolate reductase (DHFR) inhibition assay, an immunoassay based on fluorescence polarization and a previously developed LC-MS/MS assay.