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