Estimates of Cancer Potency of 2,3,7,8-Tetrachlorodibenzo(p)dioxin Using Linear and Nonlinear Dose-Response Modeling and Toxicokinetics

Linear and nonlinear toxicity criteria were derived for 2,3,7,8-tetrachlorodibenzo(p)dioxin (TCDD) using the recent National Toxicology Program rat cancer bioassay. Dose-response relationships were assessed for combined liver tumors based on lifetime average liver concentrations (LALCs) estimated with a toxicokinetic model. Rat LALC estimates at the 1% point of departure (POD) were obtained with benchmark dose (BMD) modeling to yield the BMD01 in terms of LALC. The same toxicokinetic model was used to back-extrapolate the human-equivalent external dose (HED). A linear cancer slope factor (CSF) with a value of 1 x 10(-4) per pg/kg/day was calculated as the ratio between the benchmark response rate and the HED at the lower confidence limit of the benchmark dose (BMDL)(01). A nonlinear reference dose (RfD) with a value of 100 pg/kg/day was developed from the BMD01 value by applying uncertainty factors to rat internal and human external doses. The RfD was 100 times higher than the 10(-4) risk-specific dose (RSD) based on the linear CSF. For comparison, BMD01 and BMDL01 values were developed for key events in the tumor promotion mode of action (MOA) of TCDD. This MOA involves dysregulation of the normal function of the aryl hydrocarbon receptor and its associated biological processes and results in pathologies that drive tumor promotion and progression. The BMD01 values for key events were consistent with the timing of the key events within the MOA and provided support for the choices of the 1% tumor rate as a POD and dichotomous Hill model for representing receptor-mediated carcinogenicity. Because a threshold toxicity criterion most accurately reflects the MOA, the RfD for TCDD with a value of 100 pg/kg/day is considered appropriate for regulatory purposes, consistent with a 2006 NRC panel's recommendation to develop a threshold-based cancer potency factor for TCDD and with the methodology in U.S. Environmental Protection Agency's Cancer Guidelines.