title: The effect of enzyme inhibition on the metabolism and activation of tacrine by human liver microsomes.
abstract: 1. Tacrine (1,2,3,4-tetrahydro-9-aminoacridine-hydrochloride: THA) underwent metabolism in vitro by a panel (n = 12) of human liver microsomes genotyped for CYP2D6, in the presence of NADPH, to both protein-reactive and stable metabolites. 2. There was considerable variation in the extent of THA metabolism amongst human livers. Protein-reactive metabolite formation showed a 10-fold variation (0.6 +/- 0.1%-5.2 +/- 0.8% of incubated radioactivity mg-1 protein) whilst stable metabolites showed a 3-fold variation (24.3 +/- 1.7%-78.6 +/- 2.6% of incubated radioactivity). 3. Using cytochrome P450 isoform specific inhibitors CYP1A2 was identified as the major enzyme involved in all routes of THA metabolism. 4. There was a high correlation between aromatic and alicyclic hydroxylation (r = 0.92, P < 0.0001) consistent with these biotransformations being catalysed by the same enzymes. 5. Enoxacin (ENOX), cimetidine (CIM) and chloroquine (CQ) inhibited THA metabolism by a preferential decrease in the bioactivation to protein-reactive, and hence potentially toxic, species. The inhibitory potency of ENOX and CIM was increased significantly upon pre-incubation with microsomes and NADPH. 6. Covalent binding correlated with 7-OH-THA formation before (r = 0.792, P < 0.0001) and after (r = 0.73, P < 0.0001) inhibition by CIM, consistent with a two-step mechanism in the formation of protein-reactive metabolite(s) via a 7-OH intermediate. 7. The use of enzyme inhibitors may provide a useful tool for examining the relationship between the metabolism and toxicity of THA in vivo.
authors.affiliation: Department of Pharmacology and Therapeutics, University of Liverpool, UK.
authors.forename: V
authors.initials: V
authors.lastname: Spaldin
authors.forename: S
authors.initials: S
authors.lastname: Madden
authors.forename: W F
authors.initials: WF
authors.lastname: Pool
authors.forename: T F
authors.initials: TF
authors.lastname: Woolf
authors.forename: B K
authors.initials: BK
authors.lastname: Park
mesh_terms: D000328:Adult; D002097:C-Reactive Protein / Q000096:biosynthesis; D002648:Child; D002738:Chloroquine / Q000494:pharmacology; D002927:Cimetidine / Q000494:pharmacology; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics; D015365:Enoxacin / Q000494:pharmacology; D005260:Female; D006801:Humans; D006900:Hydroxylation; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000235:genetics; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D008875:Middle Aged; D009249:NADP / Q000378:metabolism; D013619:Tacrine / Q000378:metabolism* / Q000493:pharmacokinetics
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D007527:Isoenzymes; D015365:Enoxacin; D013619:Tacrine; D009249:NADP; D002927:Cimetidine; D002738:Chloroquine; D002097:C-Reactive Protein; D003577:Cytochrome P-450 Enzyme System