Fenbendazole is a member of the benzimidazole carbamate class of drugs commonly used as anthelmintics (Velík et al., 2004
; Villar et al., 2007
). Due to strain and species differences in sensitivity, the dose of fenbendazole used therapeutically varies (Hunter et al., 2007
; Ramp et al., 2010
; Short et al., 1988
). In mice, fenbendazole is typically delivered in food at a dose of 8–12 mg/kg for 7 days. The present studies demonstrate that treatment of mice with this clinical dose regimen of fenbendazole exacerbates the hepatotoxicity of acetaminophen resulting in significant mortality within 24 h. These findings are novel and suggest that there should be more controlled use of this anthelmintic agent in laboratory rodent colonies.
In mice, glucuronidation is the major pathway by which acetaminophen is detoxified (Gregus et al., 1988
; Laine et al., 2009
). Consistent with previous reports (Gardner et al., 2010
; Yoon et al., 2006
), we found high levels of acetaminophen-glucuronide, as well as free acetaminophen in serum within 30 min of acetaminophen administration, which rapidly declined toward control. Our findings that there were no differences in the response of control, and fenbendazole-treated mice suggest that the rate of acetaminophen clearance via glucuronidation does not contribute to exaggerated hepatotoxicity.
Previous studies in mice and rats showed no effects of fenbendazole on total hepatic cyp450 content or on its activity following therapeutic dosing for 5 days (Dalvi et al., 1991
). In accord with these findings, we observed no significant differences between control and fenbendazole-treated mice in the activities of cyp2e1 or cyp3a, enzymes key in the metabolism of acetaminophen to NAPQI (Moyer et al., 2011
). These data demonstrate that the major metabolic pathways involved in the generation of this cytotoxic acetaminophen metabolite are unaffected by fenbendazole. In contrast, increased cyp1a2, cyp2b1, and cyp4a1 activity has been noted in liver microsomes from rats fed a fenbendazole-containing diet for 8 weeks (Shoda et al., 1999
). Similar increases in cyp1a2 were reported in pigs and rabbits fed a diet-containing fenbendazole for 5 days or a related derivative, oxfendazole for 10 days (Gleizes et al., 1991
; Savlík et al., 2006
). Interestingly, we found that cyp1a2 activity was suppressed in liver microsomes isolated from mice fed the fenbendazole-containing diet for 7 days, relative to control diet. Differences between our findings and previous reports may be due to animal strain or species differences and/or the dosing schedule. Cyp1a2 is constitutively expressed at relatively high levels in rodent livers and reportedly plays a role in metabolizing endogenous substrates such as estradiol, as well as a wide variety of xenobiotics, including toxic doses of acetaminophen to NAPQI (Laine et al., 2009
; Omiecinski et al., 1999
; Snawder et al., 1994
; Villar et al., 2007
). The fact that hepatotoxicity is increased in fenbendazole fed mice, despite reduced levels of cyp1a2, indicate that this isozyme is not key to acetaminophen metabolism in this model. Metabolites of fenbendazole, including oxfendazole and 4′-hydroxyfenbendazole are potent inhibitors of the cyp1a enzyme family in rat microsomes in vitro
(Murray et al., 1992
). Decreased levels of cyp1a2 in mice treated with fenbendazole may be due to increased production of fenbendazole metabolites.
GSH is known to be present at high levels in hepatocytes (Yuan and Kaplowitz, 2009
). In addition to scavenging reactive oxygen species, GSH plays a key role in the detoxification of NAPQI (Jaeschke, 1990
). In accord with previous studies (Chiu et al., 2003
; Gardner et al., 2010
), we found that acetaminophen intoxication resulted in a rapid (within 1 h) decline in reduced GSH levels in livers of mice fed the control diet. Similar results were noted in mice fed the fenbendazole-containing diet. Whereas by 12 h post-acetaminophen, GSH levels were at control levels in mice fed the standard diet, they remained suppressed in mice fed the fenbendazole-containing diet. GSH levels were also suppressed in control mice fed fenbendazole. Fenbendazole is known to undergo sulfoxidation to generate oxfendazole, which can act as a carbamylating agent for reduced GSH (Schuphan et al., 1981
; Short et al., 1988
). This leads to the formation of oxfendazole GSH conjugates, effectively depleting GSH from the tissue (Schuphan et al., 1981
). Oxfendazole and fenbendazole have also been reported to upregulate glutathione peroxidase and glutathione-S
-transferase in rats (Dewa et al., 2007
); this may also contribute to persistent GSH depletion.
In summary, the present studies demonstrate that fenbendazole exacerbates the hepatotoxicity of acetaminophen in part by reducing hepatic GSH levels. At present, it is not known if there are other components of the xenobiotic clearance system, such as drug transporters, that are altered by fenbendazole. Fenbendazole treatment of rodents has been shown to be an effective method for controlling nematode infestations. However, in view of its ability to suppress hepatic GSH, as well as cyp1a2 enzyme activity, care should be taken in utilizing this anthelmintic drug in rodent studies.