Quantitative Adverse Outcome Pathways and Their Application to Predictive Toxicology

Abstract

A quantitative adverse outcome pathway (qAOP) consists of one or more biologically based, computational models describing key event relationships linking a molecular initiating event (MIE) to an adverse outcome. A qAOP provides quantitative, dose-response, and time-course predictions that can support regulatory decision-making. Herein we describe several facets of qAOPs, including (a) motivation for development, (b) technical considerations, (c) evaluation of confidence, and (d) potential applications. The qAOP used as an illustrative example for these points describes the linkage between inhibition of cytochrome P450 19A aromatase (the MIE) and population-level decreases in the fathead minnow (FHM; Pimephales promelas). The qAOP consists of three linked computational models for the following: (a) the hypothalamic-pitutitary-gonadal axis in female FHMs, where aromatase inhibition decreases the conversion of testosterone to 17β-estradiol (E2), thereby reducing E2-dependent vitellogenin (VTG; egg yolk protein precursor) synthesis, (b) VTG-dependent egg development and spawning (fecundity), and (c) fecundity-dependent population trajectory. While development of the example qAOP was based on experiments with FHMs exposed to the aromatase inhibitor fadrozole, we also show how a toxic equivalence (TEQ) calculation allows use of the qAOP to predict effects of another, untested aromatase inhibitor, iprodione. While qAOP development can be resource-intensive, the quantitative predictions obtained, and TEQ-based application to multiple chemicals, may be sufficient to justify the cost for some applications in regulatory decision-making.

Figure 1.

The aromatase inhibition qAOP. This qAOP consists of 3 linked models: fathead minnow HPG axis (9), fathead minnow oocyte growth dynamics (12), and fathead minnow population dynamics (14). The HPG axis model predicts plasma VTG concentration as a function of inhibition of aromatase. The oocyte model takes plasma VTG as its input and predicts fecundity (egg production). The population dynamics model takes fecundity as its input and predicts population dynamics.

Figure 2.

Fadrozole 21 day, continuous exposure study. HPG axis model (9) predictions after 21 days of continuous exposure to fadrozole at concentrations of 0, 1.4, 7.3, and 57 μg/L (41). The time sequence data are plotted for plasma E2 (A) and plasma VTG (B). Boxplots of averaged fecundity (12) are plotted against fadrozole concentration (C). Fathead minnow population size (14) resulting from exposure to fadrozole in comparison to control (D). Data in Figs. 2A and2B are shown as mean ± SEM.

Figure 3.

Response-response predictions. With the qAOP (9, 12, 14) at steady state, response-response predictions are plotted. The MIE of aromatase inhibition is plotted against fadrozole concentration (A). The KE of plasma E2 concentration is plotted against percent aromatase inhibition (B). The KE of plasma VTG is plotted against plasma E2 level (C). The KE of average fecundity is plotted against plasma VTG concentration (D). The adverse outcome of fathead minnow population size is plotted as a function of fecundity (E).

Figure 4.

Read-across of response-response plots to illustrate rapid evaluation of the qAOP-predicted effects of aromatase inhibition on key events and adverse outcome (9, 12, 14). In this example, exposure to 2.3 μM fadrozole causes 50% inhibition of aromatase (A), which in turn results in a decrease of plasma E2 to 0.012 μM (B), which in turn results in a decrease of plasma VTG to about 20 μM (C), which in turn results in a decrease of average fecundity to about 4 eggs/day (D), which in turn results in a decrease of fathead minnow population to about 5% of carrying capacity (E).

Figure 5.

Use of the qAOP (9, 12, 14) to semi-quantitatively predict a BMD for iprodione. A 20% decrease in fathead minnow population was considered to be a possible endpoint of regulatory interest. Read across (Fig. 4) of the qAOP was then used to identify a predicted plasma E2, continuous exposure to iprodione, as fadrozole equivalents, to identify a BMD assoicated with the 20% population decline (0.016 μM). The current plot, obtained with the HPG axis model, indicates that this level of plasma E2 is associcated with a continuous exposure to iprodione of between 6 and 7 μg/L.