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Chronic exposure to toxaphene resulted in an increase in liver tumors in B6C3F1 mice. This study was performed to investigate the mode of action of toxaphene induced mouse liver tumors. Following an initial 14 day dietary dose range-finding study in male mice, a mechanistic study (0, 3, 32, and 320ppm toxaphene in diet for 7, 14, and 28 days of treatment) was performed to examine the potential mechanisms of toxaphene induced mouse liver tumors. Toxaphene induced a significant increase in expression of constitutive androstane receptor (CAR) target genes (Cyp2b10, Cyp3a11) at 32 and 320ppm toxaphene. aryl hydrocarbon receptor (AhR) target genes (Cyp1a1 and Cyp1a2) were slightly increased in expression at the highest toxaphene dose (320 ppm). No increase in peroxisome proliferator-activated receptor alpha activity or related genes was seen following toxaphene treatment. Lipid peroxidation was seen following treatment with 320ppm toxaphene. These changes correlated with increases in hepatic DNA synthesis. To confirm the role of CAR in this mode of action, CAR knockout mice (CAR-/-) treated with toxaphene confirmed that the induction of CAR responsive genes seen in wild-type mice was abolished following treatment with toxaphene for 14 days. These findings, taken together with previously reported studies, support the mode of action of toxaphene induced mouse liver tumors is through a nongenotoxic mechanism involving primarily a CAR-mediated processes that results in an increase in cell proliferation in the liver, promotes the clonal expansion of preneoplastic lesions leading to adenoma formation. [ABSTRACT FROM AUTHOR]

This reevaluation of the current U.S. EPA cancer potency factor for toxaphene is based upon a review of toxaphene carcinogenesis bioassays in mice conducted by Litton Bionetics (unpublished report, 1978) and the National Cancer Institute (NCI) (Technical Report Series No. 37, conducted by Gulf South Research Institute, 1979). The mechanistic data available for toxaphene, including consideration of the potential of the compound to induce genotoxicity, was examined with an emphasis on whether this information supports a change in the cancer potency factor. If a quantitative dose-response assessment for toxaphene is to be performed, the data from both the NCI and Litton cancer bioassays should be used. Additionally, liver tumor results from female mice, rather than male mice, should be used for estimating potential human cancer risk because the background rate of liver tumors in females is lower and less variable than that exhibited by males. An ED10 was estimated as the point of departure. The mechanistic data were not sufficient to fully support a margin of exposure approach. Therefore, we believe that applying a linear extrapolation from the ED10 to the origin is an appropriate means to estimate risk at low doses. This is a highly conservative approach and, when it is applied, we conclude that the current EPA cancer potency factor should be reduced from 1.1 (mg/kg/day)–1 to 0.1 (mg/kg/day)–1. [ABSTRACT FROM PUBLISHER]