Biologically bounded risk assessment for receptor-mediated nongenotoxic carcinogens.

We have developed a biologically bounded marginal effect model for use in risk assessment of human exposure to receptor-mediated nongenotoxic carcinogens. Schematically this model can be reduced to four components: CI, the absence of an observable biological response; CII, observable biochemical responses but no observable pathology; CIII, observable pathology; and CIV, both observable pathology and lethality. The inflection point in the marginal response curve between CI and CII is defined as the biologically evaluated scientifically tested no observable effect level (BESTNOEL). We demonstrate the utility of this approach by applying it to the well-studied nongenotoxic carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Using a well-developed mechanistic understanding of the initial interactions of TCDD with the cell, we justify the selection of the minimal effective dose for CYP1A1 mRNA induction as the BESTNOEL. With allowance for variation in human sensitivity to TCDD, the BESTNOEL is assigned a human liver tissue burden of approximately 0.25-25 ppt and an allowable daily intake level in the range of 15-1500 fg/kg/day. In the future, the BESTNOEL can help establish a lower boundary for acceptable extrapolation when using either statistical or biologically based attributable risk models.