Determination of plasma trimetrexate levels using gas chromatography-mass spectrometry with selected-ion monitoring.

Trimetrexate is a potent inhibitor of dihydrofolate reductase and has demonstrated significant antitumor activity against murine and human cell lines both in vitro and against several murine transplanted tumors. The importance of antifolate concentration and exposure time in determining toxic and therapeutic effects necessitates an assay of suitable sensitivity, accuracy and specificity for investigation of trimetrexate pharmacokinetics. This paper describes a gas chromatographic-mass spectrometric (GC-MS) procedure using selected-ion monitoring (SIM) for the determination of plasma trimetrexate levels. Using the C18 Bond-Elut extraction columns, the drug and internal standard are removed from plasma, derivatized to their bis(trimethylsilyl) derivatives and analysed by GC-SIM-MS. The reproducibility of the daily standard curves had coefficients of variation ranging from 4.9 to 11.4%. The precision of the assay yielded a coefficient of variation ranging from 5.6 to 10.1%, and the concentration means for the seeded control samples were found to be within -3.7 to +0.7% of the theoretical values for trimetrexate. No interfering peaks have been observed in application of the procedure on patient samples. The minimum detectable level under the conditions described was 0.005-0.014 microM trimetrexate.