The use of biochemical and molecular parameters to estimate dose-response relationships at low levels of exposure.

Biomarkers based on alterations in molecular and biochemical parameters may be useful in chemical risk assessment for establishing the presence of an exposure, ranking relative risks among exposed individuals, and estimating risks at low levels of exposure. Because it is unlikely that the relation between toxic responses and the degree of alteration in the biomarker is equivalent at all doses, quantification of risks at low levels is not necessarily more accurate using these biomarkers for extrapolation. The application of response biomarkers for risk evaluation at low levels of exposure is discussed in relation to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a compound that causes induction of cytochromes CYP1A1 and CYP1A2 in liver and other tissues. CYP1A1 induction in liver increases monotonically with TCDD dosage; however, several of the dose-response curves for hepatic effects of TCDD are U-shaped. The U-shaped dose-response curve for hepatic tumor promotion appears to result because the integrated toxicologic response depends on multiple underlying processes--mitosuppression, toxicity, and cell proliferation--each of which has a different dose-response relationship with respect to TCDD. Although dose-response relationships for the biomarkers are not expected to duplicate the complex shapes seen with the integrated responses, measurements and pharmacodynamic modeling of the changes in these molecular and biochemical parameters can still be useful for obtaining an upperbound risk estimate at low levels of exposure.