A 384-well solid-phase extraction for LC/MS/MS determination of methotrexate and its 7-hydroxy metabolite in human...

A solid-phase extraction procedure, in a 384-well format, has been developed for methotrexate and its primary metabolite, 7-hydroxymethotrexate, in human urine and plasma. This format has not been utilized previously for solid-phase extraction of drugs from biological fluids. The 384-well plates contained a C-18 stationary phase bonded to silica particles which are incorporated into a glass-fiber membrane. Methotrexate and 7-hydroxymethotrexate have been quantified across the curve range of 1 to 50 microg/mL and 50 to 1000 ng/mL, respectively, in urine and from 5 to 250 ng/mL and 5 to 100 ng/mL, respectively, in plasma. Both analytes are quantified by linear regression using 20-microL sample aliquots. Experiments to evaluate the influence of particle size, elution volume, and injection volume on signal intensity were conducted and are reported, along with the results of experiments examining cross contamination between wells. Recovery was determined to be > or = 95% from urine. Results from a run of 384 samples analyzed over a 14-h period indicate that 384-well SPE can be successfully utilized to increase analytical run sizes and sample throughput for LC/MS/MS determination of small drug molecules in biological samples.