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

We have developed a HPLC method which allows the determination of chlorzoxa zone and its hydroxy metabolite in rat liver microsomes and in human plasma . We found that dehalogenated chlorzoxazone or 2-benzoxazolinone was a conv enient and stable internal standard. Proteins were precipitated with dilute d perchloric acid and the supernatant was extracted with ethyl acetate. Com plete 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 mu g/ml of hydroxychlo rzoxazone and 5.8% at 1 mu g/ml of chlorzoxazone in human plasma. The repro ducibility of the method has been demonstrated for a large number of sample s over a long period. (C) 1998 Elsevier Science B.V. All rights reserved.