Use of fixed ratios in mixture studies, in vitro evidence of issues

Authors: Shappell, Nancy

Submitted to: Integrated Environmental Assessment and Management
Publication Type: Other
Publication Acceptance Date: 3/1/2018
Publication Date: 5/1/2018
Citation: Shappell, N.W. 2018. Use of fixed ratios in mixture studies, in vitro evidence of issues. Integrated Environmental Assessment and Management. 14(3):420-422. https://doi.org/10.1002/ieam.4038.
DOI: https://doi.org/10.1002/ieam.4038

Interpretive Summary: .

Technical Abstract: In attempts to investigate the effects of chemicals in a way that more accurately reflects the reality of environmental exposure, scientists have moved from evaluating single chemical exposure to mixtures. The logic behind this approach is undeniable, as species of concern are rarely exposed to only a single compound with potential detrimental effects. The problem arises from a standard approach taken in mixture experiments, where a number of test chemicals are prepared as a mixture, each present at a fixed concentration. These mixtures are then “dosed” at various dilutions of the mixture. Results of these experiments are typically used to support the theory of chemical additivity, meaning that sum of the mixtures is equal to the sum of the activity of each chemical when tested separately. In fact, this experimental design only assesses the effects of increasing mixture concentrations. Instead by testing mixtures two at a time, holding the concentration of one chemical constant while varying the concentration of the other chemical, the true effect of one chemical's presence on another chemical's effect can be demonstrated. In the investigation of endocrine disrupting estrogens present in agricultural animal waste, our laboratory found that adding a weak estrogen at low concentrations in the presence of a strong estrogen, in fact resulted in a decrease in the estrogenic response. This is in contrast to the concept of additivity, where the additional weak estrogen's presence should have increased the total estrogenic response. The explanation for the reduction of proliferation could be as simple as competition for receptor binding, or more complicated, possibly involving a feedback. The issue with fixed ratio mixtures was investigated using estrogenic endocrine disruptors, but it may also prove relevant to evaluate the use of fixed ratio mixtures in the study of metal effects on biological systems.