Sensitive high-performance liquid chromatographic determination of chlorzoxazone and 6-hydroxychlorzoxazone in plasma

A rapid and sensitive high-performance liquid chromatographic assay was developed for the quantitation of chlorzoxazone and its major metabolite 6-hydroxychlorozoxazone in plasma. These compounds, as well as the internal standard 5-fluorobenzoxazolone, were extracted from plasma (0.5 ml) using C18 solid-phase extraction columns. The extracts were analyzed on a 10-microns Waters C18 muBond-apak column with a mobile phase of acetonitrile-tetrahydrofuran-0.1 M ammonium acetate (22.5:5.5:72). The assay utilized ultraviolet detection (283 nm) which provided sensitivity and specificity sufficient to simultaneously quantify > or = 100 ng/ml chlorozoxazone and 6-hydroxychlorzoxazone in plasma. The mean correlation coefficient of the multi-level standard curves for each compound was 0.996 or greater over a concentration range of 100-3000 ng/ml. The inter-day and intra-day coefficients of variation were < 6%. This method has been used by our laboratory to provide the unattended overnight analysis of chlorozoxazone and 6-hydroxychlorzoxazone in plasma samples obtained from human study subjects.