Studies on the effects of organic anions on the susceptibility of male CD-1 mice to acetaminophen (APAP)-induced hepatotoxicity

Acetaminophen (APAP) is a widely used analgesic and antipyretic drug that under high doses can cause severe hepatic damage. Biliary excretion of APAP is prominent and can reflect changes in bioactivation and detoxification pathways. Little is known about the mechanisms by which APAP and its metabolites are transported into bile. Recent studies showed that co-administration of the organic anion indocyanine green (ICG) inhibits the biliary excretion of APAP through competition for excretion pathways. However, the toxicological implications of such changes were not studied in those experiments. In theory, hepatic retention of a toxic compound should increase liver damage. Nevertheless, toxicity studies in mice receiving ICG followed by a toxic dose of APAP demonstrated that ICG has a protective effect against APAP-liver damage. The nature of this paradoxical result was studied in this dissertation work. Multidrug associated protein 2 (Mrp2) is responsible for the biliary secretion of a variety of organic anions such as ICG, reduced and oxidized glutathione (GSH and GSSG, respectively). Hepatic retention of thiols due to competition for biliary excretion with ICG and APAP metabolites could explain the observed protection. However, co-administration of ICG and APAP did not alter hepatobiliary GSH-homeostasis, indicating that changes in hepatic GSH are not responsible for the reduction in APAP-toxicity produced by ICG. In addition, toxicity studies using transport deficient (TR−) rats showed that lack of Mrp2 function is not a main contributor in the observed protection. ICG decreases bile flow rate significantly. Cholestasis induced by bile duct ligation in rats also protects against APAP toxicity. Collectively, these results suggest that a cholestatic liver is more resistant to APAP injury. Studies examining the effect of cholestatic and non-cholestatic organic anions in APAP-hepatotoxicity showed that only organic anions reducing bile flow protect against APAP. This indicates that cholestasis is an important determinant of the changes in susceptibility to APAP toxicity produced by ICG. Furthermore, ICG also protects against carbon tetrachloride (CCl4) hepatotoxicity. This chemical undergoes minimal biliary excretion, which supports the role of cholestasis in this protection, since reduction in bile flow would result in retention of compound(s) with hepatoprotective properties.