Citation:

Conolly, R., G. Ankley, W. Cheng, M. Mayo, D. Miller, E. Perkins, Dan Villeneuve, AND K. Watanabe. Quantitative (q)AOP for aromatase inhibition as case study to advance qAOP development practices. SOT Annual Meeting, Baltimore, Maryland, March 12 - 16, 2017.

Impact/Purpose:

The Agency needs fast, robust methods for estimating exposures, identifying hazards, and predicting health risks. This work describes development of a quantitative adverse outcome pathway (qAOP) that provides a capability for rapid, preliminary screening for health risk. In a qAOP, each quantitative key event relationship is derived from supporting laboratory studies characterized by use of multiple doses and time points. With this level of laboratory support integrated into its structure, the qAOP can be used to generate “response-response” (RR) plots showing how each key event changes as a function of its immediately upstream key event. Once the RR plots are established, high throughput, in vitro assays (e.g. ToxCast) can be used to estimate the potency of novel chemicals to activate the first stage of the qAOP (the molecular initiating event). The RR plots then allow rapid, (semi) quantitative read-across of how the various key events and ultimately the adverse outcome are affected by the chemical-specific activation of the MIE. Thus, the resource-intensive development of the qAOP produces a capability for high throughput screening of risk of adverse outcome. When this screening indicates a potential for high risk, follow-on targeted testing would be indicated. This work directly supports Goal 4 of the EPA’s 2014 – 2018 Strategic Plan: Ensuring Safety of Chemicals and Preventing Pollution.

Description:

Here we describe how “read across” of a quantitative adverse outcome pathway (qAOP) developed with data for one chemical can be used to screen impacts of other chemicals. We developed a qAOP starting with inhibition of CYP19A (aromatase) in fathead minnows (FHM) as the molecular initiating event (MIE) and ending with a decline in population as the adverse outcome (AO). Plasma estradiol (E2), plasma vitellogenin (VTG), and egg production (fecundity) are intermediate key events (KEs). VTG is an egg yolk precursor essential to fecundity. Data to identify quantitative KE relationships were obtained from concentration-response and time-course experiments in which FHM were exposed to the model aromatase inhibitor fadrozole. These experiments identified compensatory/adaptive changes, including induction of aromatase and alterations in VTG dynamics, which were coded into the KE relationships. While the qAOP can predict time-dependent changes in KEs and the AO associated with compensation, simulation of continuous exposures of at least 4 days allows the qAOP to reach a new, albeit stressed, steady state. Concentration-response simulations run to steady state support development of “response-response” (RR) plots where changes in the MIE, KE, and AO are plotted relative to each other. These RR plots allow rapid, semi-quantitative evaluation of effects on the AO without having to run the qAOP in the computer. Read-across of the RR plots shows that, at steady state, FHM exposure to 2.3 g/L fadrozole was predicted to inhibit aromatase activity by 50%, reduce plasma E2 by 30%, plasma VTG by about 80%, average fecundity by about 80%, and FHM population after 5 years by 90%. Using toxicity equivalent calculations to convert 1 ug/L of the environmentally relevant fungicide iprodione into an equivalent concentration of fadrozole, read-across was predicted to result in negligible inhibition of aromatase and, subsequently, no effects on plasma E2, plasma VTG, average fecundity or FHM population status. These results illustrate how resource-intensive development of a qAOP provides a capability for rapid, quantitative screening of aromatase inhibitors for effects on the AO in FHM. This abstract does not necessarily reflect any specific policy of the US EPA or the US ACE.

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