Determination of linezolid in plasma by reversed-phase high-performance liquid chromatography

An HPLC-UV method was developed for assay of linezolid in dog, rat, mouse, and rabbit plasma. Linezolid and the internal standard were extracted on a solid phase cartridge (SPE) and separated on a reversed-phase column (C8, 4.6x150 mm, 5 microm) with 20% acetonitrile in water as mobile phase. The SPE quantitatively recovered linezolid and the internal standard from plasma samples. The chromatographic peak height ratio or peak area ratio based on UV absorbency at 251 nm was used for quantitative analysis. The assay procedures were simple and the assay was specific and had adequate precision and accuracy. Calibration standards with concentrations over the range of 0.01 20 microg/ml were validated for routine sample analysis to support the pharmacokinetic and toxicology studies with linezolid in dog, rat, mouse, and rabbit. Analysis of quality control samples showed the coefficients of variation were usually <10% and the measured and theoretical concentrations differed by <10% in most assays. Linezolid in the plasma samples was stable when stored at below -20 degrees C for at least 63 days, at room temperature (22-23 degrees C) for up to 24 h, and after three freeze-thaw cycles. This HPLC method has been successfully used in multiple laboratories to assay plasma samples from pharmacokinetic and toxicology studies with linezolid in the animal species.