An investigation into the formation of stable, protein-reactive and cytotoxic metabolites from tacrine in vitro: Studies with human and rat liver microsomes

Abstract

Tacrine (1,2,3,4-tetrahydro-9-aminoacridine hydrochloride; THA) is known to undergo extensive oxidative metabolism to a variety of mono- and dihydroxylated metabolites in animals and humans. The potential for tacrine to undergo metabolism to stable, protein-reactive and cytotoxic metabolites has been investigated in incubations with human and rat liver microsomes. Using lymphocytes as sensitive markers to quantify cytotoxicity, THA (50 μM) underwent NADPH-dependent bioactivation to a cytotoxic metabolite(s). NADPH-dependent cytotoxicity in the presence of rat and human micro-somes was 9.8 ± 3.1% (P < 0.05 cf. -NADPH control) and 6.2 ± 2.0% (P < 0.05 cf. -NADPH control), respectively. Stable and protein-reactive metabolites were also formed in microsomes from both species. These accounted for 28.2 ± 12.7% and 1.22 ± 0.79% of incubated radioactivity in human microsomes and 6.4 ± 2.2% and 0.4 ± 0.1% of incubated radioactivity in rat microsomes. In microsomes pooled from six human livers the NADPH-dependent cytotoxicity was 9.4 ± 1.1%. Formation of stable and protein-reactive metabolites accounted for 29.2 ± 2.3% and 1.2 ± 1.0% of incubated radioactivity. Reduced glutathione (500 μM) completely blocked NADPH-dependent cytotoxicity and inhibited protein-reactive metabolite formation by 60% (P < 0.05). Ascorbic acid (500 μM) inhibited the generation of cytotoxic and protein-reactive metabolites by 75% (P < 0.05) and 35% (P < 0.05), respectively. Cyclohexene oxide was without effect. Human serum albumin was found to protect the lymphocytes against toxicity. In microsomes prepared from the livers of four donors known to have been smokers there were no significant differences in the generation of metabolites from THA compared with microsomes prepared from livers of non-smokers. Enoxacin, a specific inhibitor of cytochrome P450 1A2 significantly inhibited all routes of THA metabolism. We have therefore demonstrated that THA may be oxidatively metabolized to stable, protein-reactive and cytotoxic metabolites in human and rat liver microsomes. A number of inhibitors may affect these processes, whilst inhibition by enoxacin indicates a role for cytochrome P450 1A2 in THA metabolism.