Estimation of chlorzoxazone hydroxylase activity in liver microsomes and of the plasma pharmacokinetics of chlorzoxazone by the same high-performance liquid chromatographic method.

We have developed a HPLC method which allows the determination of chlorzoxazone and its hydroxy metabolite in rat liver microsomes and in human plasma. We found that dehalogenated chlorzoxazone or 2-benzoxazolinone was a convenient and stable internal standard. Proteins were precipitated with diluted perchloric acid and the supernatant was extracted with ethyl acetate. Complete resolution of the peaks was achieved within 20 min with a Spherisorb ODS-1 column. The inter-day R.S.D.s were 6.5% at 0.5 microgram/ml of hydroxychlorzoxazone and 5.8% at 1 microgram/ml of chlorzoxazone in human plasma. The reproducibility of the method has been demonstrated for a large number of samples over a long period.