Effects of cytochrome P450 2E1 modulators on the pharmacokinetics of chlorzoxazone and 6-hydroxychlorzoxazone in rats

Life Sci. 1996;58(18):1575-85. doi: 10.1016/0024-3205(96)00132-4.


A previously observed correlation between the rate of 6-hydroxylation of chlorzoxazone (CZX), a potent skeletal muscle relaxant, and cytochrome P450 2E1 activity in vitro led to the postulation that this drug may be used as a non-invasive probe for P450 2E1 activity in vivo. In this study, comparative pharmacokinetics of CZX and 6-hydroxychlorzoxazone (OH-CZX) were conducted in rats pretreated with an inhibitor or inducer of P450 2E1. After administration of CZX (150 mumol/kg, i.v.) to rats, blood samples were taken at different time points and the plasma concentrations of CZX and OH-CZX were determined by HPLC. The concentrations for CZX and OH-CZX over time were simultaneously fitted to a model of first-order elimination of CZX and first-order formation and elimination of OH-CZX using the computer program PCNONLIN to give pharmacokinetic parameters. Diallyl sulfide, a P450 2E1 inhibitor, at an oral dose of 50 or 200 mg/kg 12 hr prior to the CZX dose markedly inhibited the hydroxylation of CZX. Pretreatment with ethanol (15% in the drinking water for six days), a condition known to induce P450 2E1, slightly enhanced the formation of OH-CZX. To observe possible involvement of enzymes other than P450 2E1 in CZX metabolism, dexamethasone and phenobarbital were also used. Pretreatment with dexamethasone (50 mg/kg, i.p. daily for four days) did not cause changes in CZX and OH-CZX pharmacokinetics. Pretreatment with phenobarbital (75 mg/kg, i.p. daily for three days) enhanced CZX metabolism slightly. Our results suggest that P450 2E1 plays a major role in CZX hydroxylation in rats, but other factors may also be involved in the metabolism in vivo.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Allyl Compounds*
  • Animals
  • Chlorzoxazone / analogs & derivatives*
  • Chlorzoxazone / metabolism
  • Chlorzoxazone / pharmacokinetics*
  • Cytochrome P-450 CYP2E1
  • Cytochrome P-450 Enzyme Inhibitors*
  • Cytochrome P-450 Enzyme System / biosynthesis*
  • Dexamethasone / pharmacology
  • Enzyme Induction / drug effects
  • Enzyme Inhibitors / pharmacology
  • Ethanol / pharmacology
  • Hydroxylation
  • Male
  • Muscle Relaxants, Central / pharmacokinetics*
  • Oxidoreductases, N-Demethylating / antagonists & inhibitors*
  • Oxidoreductases, N-Demethylating / biosynthesis*
  • Phenobarbital / pharmacology
  • Rats
  • Rats, Sprague-Dawley
  • Sulfides / pharmacology


  • Allyl Compounds
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Muscle Relaxants, Central
  • Sulfides
  • 6-hydroxychlorzoxazone
  • Ethanol
  • allyl sulfide
  • Dexamethasone
  • Cytochrome P-450 Enzyme System
  • Cytochrome P-450 CYP2E1
  • Oxidoreductases, N-Demethylating
  • Chlorzoxazone
  • Phenobarbital