Determination of linezolid in human plasma by LC-MS-MS

A rapid, sensitive and selective LC-atmospheric pressure-chemical ionizatio n-MS-MS method for the determination of the new antimicrobial agent, linezo lid, in human plasma using selected reaction monitoring (SRM) was developed . Linezolid and the internal standard were extracted from the biological sa mples by solid phase extraction (SPE) and analyzed on a reversed-phase Shim Pack CLC-CN, C18 column with the mobile phase of acetonitrile and 20 mM am monium acetate solution (4 + 1 v/v). Detection was accomplished using an LC Q(TM) mass spectrometer (Finnigan), which was programmed in positive MS-MS mode to permit measurement of the fragment ions of linezolid and internal s tandard at m/z 296.2 and 223.2, respectively. The assay run-time was less t han 3.5 min. Quantitative analysis was based on peak area ratio of linezoli d to the internal standard. Calibration plots were established over the con centration range of 0.1-20 mug ml(-1) of linezolid with the lowest detectio n limit of 0.05 mug ml(-1) using 10 mul sample volume. The SPE technique qu antitatively recovered linezolid and the internal standard from the plasma samples at a percentage range of 89.1-93.7%. Determination of control sampl es of linezolid in plasma validated the LC-MS-MS-SRM method. Intra-assay an d inter-assay precision were in the range of 5.1-11.4% relative standard de viation, whereas the intra- and inter-accuracy were in the range of 97.5-11 4.0% of the nominal concentrations of linezolid added. The data confirmed t hat the plasma samples of linezolid were stable at room temperature and whe n stored at -20 degreesC for at least 10 d. The developed LC-MS-MS-SRM meth od is recommended for the determination of linezolid in human plasma.