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The effect of enzyme inhibition on the metabolism and activation of tacrine by human liver microsomes.
Author(s) -
Spaldin V,
Madden S,
Pool WF,
Woolf TF,
Park BK
Publication year - 1994
Publication title -
british journal of clinical pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.216
H-Index - 146
eISSN - 1365-2125
pISSN - 0306-5251
DOI - 10.1111/j.1365-2125.1994.tb04316.x
Subject(s) - microsome , metabolite , biochemistry , metabolism , hydroxylation , cytochrome p450 , enzyme , chemistry , cyp1a2 , drug metabolism , tacrine , pharmacology , biology , acetylcholinesterase
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.