A predictive data-driven framework for endocrine prioritization: a triazole fungicide case study

Abstract

The US Environmental Protection Agency Endocrine Disruptor Screening Program (EDSP) is a tiered screening approach to determine the potential for a chemical to interact with estrogen, androgen, or thyroid hormone systems and/or perturb steroidogenesis. Use of high-throughput screening (HTS) to predict hazard and exposure is shifting the EDSP approach to (1) prioritization of chemicals for further screening; and (2) targeted use of EDSP Tier 1 assays to inform specific data needs. In this work, toxicology data for three triazole fungicides (triadimefon, propiconazole, and myclobutanil) were evaluated, including HTS results, EDSP Tier 1 screening (and other scientifically relevant information), and EPA guideline mammalian toxicology study data. The endocrine-related bioactivity predictions from HTS and information that satisfied the EDSP Tier 1 requirements were qualitatively concordant. Current limitations in the available HTS battery for thyroid and steroidogenesis pathways were mitigated by inclusion of guideline toxicology studies in this analysis. Similar margins (3-5 orders of magnitude) were observed between HTS-predicted human bioactivity and exposure values and between in vivo mammalian bioactivity and EPA chronic human exposure estimates for these products' registered uses. Combined HTS hazard and human exposure predictions suggest low priority for higher-tiered endocrine testing of these triazoles. Comparison with the mammalian toxicology database indicated that this HTS-based prioritization would have been protective for any potential in vivo effects that form the basis of current risk assessment for these chemicals. This example demonstrates an effective, human health protective roadmap for EDSP evaluation of pesticide active ingredients via prioritization using HTS and guideline toxicology information.

Keywords: Endocrine Disruptor Screening Program; ExpoCast; Tox21; ToxCast; high-throughput screening; myclobutanil; propiconazole; triadimefon.

Figure 1.

Number of HTS assays available for the three triazoles within each endocrine activity hypothesis. The assay endpoints available by estrogen (E), anti-estrogen (AE), androgen (A), anti-androgen (AA), steroidogenesis (S), thyroid receptor (TR), anti-thyroid receptor (ATR) and hepatic catabolism related to thyroid hormone (Liver). Not all triazoles were tested in all assays, leading to minor discrepancies between triazoles for the number of assay endpoints available.

Figure 2.

Triadimefon: Parallel comparison of predicted bioactivity and exposure and US EPA chronic exposure estimates and in vivo bioactivity. Annotated to show Bins 1–2. See text, Table 6, and Supplemental File 2 for additional details on AC₅₀ by assay and bin. Dotted line = cytotoxicity caution flag (1.68 μM or 5.13 mkd). LPEAD, lowest potentially endocrine active dose (= 1800 ppm or 100 mkd for triadimefon).

Figure 3.

Propiconazole: Parallel comparison of predicted bioactivity and exposure and EPA chronic exposure estimates and in vivo bioactivity. Annotated to show Bins 1–8. See text, Table 7, and Supplemental File 2 for additional details on AC₅₀ by assay and bin. Dotted line = cytotoxicity caution flag (4.6 μM or 4.15 mkd). LPEAD, lowest potentially endocrine active dose (= 2500 ppm or 200.4 mkd for propiconazole).

Figure 4.

Myclobutanil: parallel comparison of predicted bioactivity and exposure and EPA chronic exposure estimates and in vivo bioactivity. Annotated to show Bins 1–4. See text, Table 8, and Supplemental File 2 for additional details on AC₅₀ by assay and bin. Dotted line = cytotoxicity caution flag (4.69 μM or 5.13 mkd). LPEAD, lowest potentially endocrine active dose (= 200 ppm or 9.84–12.86 mkd for myclobutanil). For graphing purposes, the midpoint of the interval between 9.84 and 12.86, or 11.35 mkd, was used as the LPEAD.

Figure 5.

Proposed roadmap for EDSP evaluation of pesticide active ingredients (PAIs). For PAIs, the available high-throughput (HT) hazard and exposure information would be considered first, followed by evaluation of the available guideline toxicology study information before making any priority decisions. Priority decisions could include decisions to collect additional hazard or exposure information, or to make a risk assessment (RA) decision based on adequate available data or suitable margins of separation between predicted exposure and bioactivity.