Reevaluation of the cancer potency factor of toxaphene: Recommendations from a peer review panel

This reevaluation of the current U.S. EPA cancer potency factor for toxaphe ne is based upon a review of toxaphene carcinogenesis bioassays in mice con ducted by Litton Bionetics (unpublished report, 1978) and the National Canc er Institute (NCI) (Technical Report Series No. 37, conducted by Gulf South Research Institute, 1979). The mechanistic data available for toxaphene, i ncluding consideration of the potential of the compound to induce genotoxic ity, was examined with an emphasis on whether this information supports a c hange in the cancer potency factor. If a quantitative dose-response assessm ent for toxaphene is to be performed, the data from both the NCI and Litton cancer bioassays should be used. Additionally, liver tumor results from fe male mice, rather than male mice, should be used for estimating potential h uman cancer risk because the background rate of liver tumors in females is lower and less variable than that exhibited by males. An ED,. was estimated as the point of departure. The mechanistic data were not sufficient to ful ly support a margin of exposure approach. Therefore, we believe that applyi ng a linear extrapolation from the ED,. to the origin is an appropriate mea ns to estimate risk at low doses. This is a highly conservative approach an d, when it is applied, we conclude that the current EPA cancer potency fact or should be reduced from 1.1 (mg/kg/day)(-1) to 0.1 (mg/kg/day)(-1).