title: Evaluation of tizanidine as a marker of canine CYP1A2 activity.
abstract: The dog CYP1A2 enzyme is likely an important contributor to the metabolism of veterinary drugs. Dog CYP1A2 is expressed in liver, plus it is inducible and polymorphic, creating the potential for intersubject differences in pharmacokinetics. Hence, the ability to probe dog CYP1A2 activity and inhibition is relevant toward veterinary drug development and drug-drug interaction assessment. Previous studies have relied on human probes with questionable specificity for CYP1A2, so it was hypothesized that recombinant CYP1A2 could be used to find a specific CYP1A2 substrate. Intrinsic clearance experiments demonstrated that tizanidine was a substrate of CYP1A2. Profiling of tizanidine metabolites generated by CYP1A2 identified the imidazole metabolite that was detectable in dog plasma. The imidazole metabolite was subsequently used to evaluate tizanidine as a CYP1A2 probe. Co-administration of the CYP1A inhibitor enrofloxacin with tizanidine significantly decreased (30%; n = 3) the formation of the imidazole metabolite vs. control experiments. As enrofloxacin is a weak inhibitor, further studies are required to confirm the sensitivity of tizanidine as an in vivo probe. However, tizanidine may be a more selective CYP1A2 probe than phenacetin when conducting in vitro studies due to the presence of other phenacetin-metabolizing enzymes in dog liver microsomes.
authors.affiliation: Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN, USA.
authors.forename: C W
authors.initials: CW
authors.lastname: Locuson
authors.affiliation: Department of Surgery, Vanderbilt University School of Medicine, Nashville, TN, USA.
authors.forename: P
authors.initials: P
authors.lastname: Williams
authors.affiliation: Department of Surgery, Vanderbilt University School of Medicine, Nashville, TN, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Adcock
authors.affiliation: Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN, USA.
authors.forename: J S
authors.initials: JS
authors.lastname: Daniels
mesh_terms: D058647:Adrenergic alpha-2 Receptor Agonists / Q000493:pharmacokinetics*; D000818:Animals; D001571:Benzoflavones / Q000493:pharmacokinetics; D003000:Clonidine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D019388:Cytochrome P-450 CYP1A2 / Q000235:genetics / Q000378:metabolism*; D065609:Cytochrome P-450 CYP1A2 Inhibitors / Q000493:pharmacokinetics*; D004285:Dogs / Q000097:blood / Q000378:metabolism*; D000077422:Enrofloxacin; D005260:Female; D024841:Fluoroquinolones / Q000493:pharmacokinetics; D008862:Microsomes, Liver / Q000378:metabolism; D015335:Molecular Probes; D010615:Phenacetin; D013379:Substrate Specificity; D013806:Theophylline / Q000031:analogs & derivatives / Q000493:pharmacokinetics
chemical_list: D058647:Adrenergic alpha-2 Receptor Agonists; D001571:Benzoflavones; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D024841:Fluoroquinolones; D015335:Molecular Probes; D000077422:Enrofloxacin; C011512:alpha-naphthoflavone; C023754:tizanidine; D013806:Theophylline; C050131:furafylline; D019388:Cytochrome P-450 CYP1A2; D010615:Phenacetin; D003000:Clonidine