Hepatocarcinogenicity in the male B6C3F1 mouse following a lifetime exposure to dichloroacetic acid in the drinking water: dose-response determination and modes of action.

Male B6C3F, mice were exposed to dichloroacetic acid (DCA) in the drinking water in order to establish a dose response for the induction of hepatocellular cancer and to examine several modes of action for the carcinogenic process. Groups of animals were exposed to control, 0.05, 0.5, 1, 2, or 3.5 g/L DCA in the drinking water for 90-100 wk. Mean daily doses (MDD) of 8, 84, 168, 315, and 429 mg/kg/d of DCA were calculated. The prevalence (percent of animals) with hepatocellular carcinoma (HC) was significantly increased in the 1-g/L (71%), 2-g/L (95%), and 3.5-g/L (100%) treatment groups when compared to the control (26%). HC multiplicity (tumors/animal) was significantly increased by all DCA treatments-0.05 g/L (0.58), 0.5 g/L (0.68), 1 g/L (1.29), 2 g/L (2.47), and 3.5 g/L (2.90)-compared to the control group (0.28). Based upon HC multiplicity, a no-observed-effect level (NOEL) for hepatocarcinogenicity could not be determined. Hepatic peroxisome proliferation was significantly increased only for 3.5 g/L DCA treatment at 26 wk. and did not correlate with the liver tumor response. The severity of hepatotoxicity increased with DCA concentration. Below 1 g/L, hepatotoxicity was mild and transient as demonstrated by the severity indices and serum lactate dehydrogenase activity. An analysis of generalized hepatocyte proliferation reflected the mild hepatotoxicity and demonstrated no significant treatment effects on the labeling index of hepatocytes outside proliferative lesions. Consequently, the induction of liver cancer by DCA does not appear to be conditional upon peroxisome induction or chemically sustained cell proliferation. Hepatotoxicity, especially at the higher doses, may exert an important influence on the carcinogenic process.