AN INVESTIGATION INTO THE FORMATION OF STABLE, PROTEIN-REACTIVE AND CYTOTOXIC METABOLITES FROM TACRINE IN-VITRO - STUDIES WITH HUMAN AND RAT-LIVER MICROSOMES

Citation
S. Madden et al., AN INVESTIGATION INTO THE FORMATION OF STABLE, PROTEIN-REACTIVE AND CYTOTOXIC METABOLITES FROM TACRINE IN-VITRO - STUDIES WITH HUMAN AND RAT-LIVER MICROSOMES, Biochemical pharmacology, 46(1), 1993, pp. 13-20
Citations number
38
Categorie Soggetti
Pharmacology & Pharmacy",Biology
Journal title
ISSN journal
00062952
Volume
46
Issue
1
Year of publication
1993
Pages
13 - 20
Database
ISI
SICI code
0006-2952(1993)46:1<13:AIITFO>2.0.ZU;2-L
Abstract
Tacrine (1,2,3,4-tetrahydro-9-aminoacridine hydrochloride; THA) is kno wn to undergo extensive oxidative metabolism to a variety of mono- and dihydroxylated metabolites in animals and humans. The potential for t acrine to undergo metabolism to stable, protein-reactive and cytotoxic metabolites has been investigated in incubations with human and rat l iver microsomes. Using lymphocytes as sensitive markers to quantify cy totoxicity, THA (50 muM) underwent NADPH-dependent bioactivation to a cytotoxic metabolite(s). NADPH-dependent cytotoxicity in the presence of rat and human microsomes was 9.8 +/- 3.1% (P < 0.05 cf. -NADPH cont rol) and 6.2 +/- 2.0% (P < 0.05 cf. -NADPH control), respectively: Sta ble and protein-reactive metabolites were also formed in microsomes fr om 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 microsome s pooled from six human livers the NADPH-dependent cytotoxicity was 9. 4 +/- 1.1%. Formation of stable and protein-reactive metabolites accou nted for 29.2 +/- 2.3% and 1.2 +/- 1.0% of incubated radioactivity. Re duced glutathione (500 muM) completely blocked NADPH-dependent cytotox icity and inhibited protein-reactive metabolite formation by 60% (P < 0.05). Ascorbic acid (500 muM) inhibited the generation of cytotoxic a nd protein-reactive metabolites by 75% (P < 0.05) and 35% (P < 0.05), respectively. Cyclohexene oxide was without effect. Human serum albumi n was found to protect the lymphocytes against toxicity. In microsomes prepared from the livers of four donors known to have been smokers th ere were no significant differences in the generation of metabolites f rom THA compared with microsomes prepared from livers of non-smokers. Enoxacin, a specific inhibitor of cytochrome P450 1A2 significantly in hibited all routes of THA metabolism. We have therefore demonstrated t hat 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 i ndicates a role for cytochrome P450 1A2 in THA metabolism.