THE EFFECT OF ENZYME-INHIBITION ON THE METABOLISM AND ACTIVATION OF TACRINE BY HUMAN LIVER-MICROSOMES

1 Tacrine (1,2,3,4-tetrahydro-9-aminoacridine-hydrochloride: THA) unde rwent metabolism in vitro by a panel (n = 12) of human liver microsome s genotyped for CYP2D6, in the presence of NADPH, to both protein-reac tive and stable melabolites. 2 There was considerable variation in the extent of THA metabolism amongst human livers. Protein-reactive metab olite formation showed a 10-fold variation (0.6 +/- 0.1%-5.2 +/- 0.8% of incubated radioactivity mg(-1) protein) whilst stable metabolites s howed a 3-fold variation (24.3 +/- 1.7%-78.6 +/- 2.6% of incubated rad ioactivity). 3 Using cytochrome P450 isoform specific inhibitors CYP1A 2 was identified as the major enzyme involved in all routes of THA met abolism. 4 There was a high correlation between aromatic and alicyclic hydroxylation (r = 0.92, P < 0.0001) consistent with these biotransfo rmations being catalysed by the same enzymes. 5 Enoxacin (ENOX), cimet idine (CIM) and chloroquine (CQ) inhibited THA metabolism by a prefere ntial decrease in the bioactivation to protein-reactive, and hence pot entially toxic, species. The inhibitory potency of ENOX and CIM was in creased 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, cons istent 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 m etabolism and toxicity of THA in vivo.