Reza J. Rasoulpour

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.

Pronamide: Weight of evidence for potential estrogen, androgen or thyroid effects.

Based on the exposure potential to humans and environment, pronamide was one of 52 chemicals on the first list evaluated under US EPAs Endocrine Disruptor Screening Program (EDSP). The purpose of EDSP is to screen chemicals for their potential to interact with estrogen-, androgen-, or thyroid-signaling pathways. A battery of 11 Tier 1 assays was completed for pronamide in accordance with EDSP test guidelines. In addition, Other Scientifically Relevant Information, which included existing data from regulatory guideline studies and published literature, was used in a weight-of-evidence (WoE) evaluation of potential endocrine activity. The WoE conclusion is that pronamide does not interact directly with estrogen, androgen, or thyroid receptors or post-receptor events. Across in vivo studies, the liver is consistently and reproducibly the target organ for pronamides effects. Pronamide activates hepatocytic nuclear receptors (including constitutive androstane receptor), induces hepatic enzymes, produces hepatocellular hypertrophy and increases liver weights. These changes are coupled with increased metabolic activity and a subsequent increased metabolism and/or clearance of both steroid and thyroid hormones. Thus, while pronamide alters some endocrine-sensitive endpoints in EDSP Tier 1 assays, effects on liver metabolism likely explain altered hormone levels and indirect endocrine changes.

Utilizing relative potency factors (RPF) and threshold of toxicological concern (TTC) concepts to assess hazard and human risk assessment profiles of environmental metabolites: A case study

There is currently no standard paradigm for hazard and human risk assessment of environmental metabolites for agrochemicals. Using an actual case study, solutions to challenges faced are described and used to propose a generic concept to address risk posed by metabolites to human safety. A novel approach - built on the foundation of predicted human exposures to metabolites in various compartments (such as food and water), the threshold of toxicological concern (TTC) and the concept of comparative toxicity - was developed for environmental metabolites of a new chemical, sulfoxaflor (X11422208). The ultimate aim was to address the human safety of the metabolites with the minimum number of in vivo studies, while at the same time, ensuring that human safety would be considered addressed on a global regulatory scale. The third component, comparative toxicity, was primarily designed to determine whether the metabolites had the same or similar toxicity profiles to their parent molecule, and also to one another. The ultimate goal was to establish whether the metabolites had the potential to cause key effects - such as cancer and developmental toxicity, based on mode-of-action (MoA) studies - and to develop a relative potency factor (RPF) compared to the parent molecule. Collectively, the work presented here describes the toxicology programme developed for sulfoxaflor and its metabolites, and how it might be used to address similar future challenges aimed at determining the relevance of the metabolites from a human hazard and risk perspective. Sulfoxaflor produced eight environmental metabolites at varying concentrations in various compartments - soil, water, crops and livestock. The MoA for the primary effects of the parent molecule were elucidated in detail and a series of in silico, in vitro, and/or in vivo experiments were conducted on the environmental metabolites to assess relative potency of their toxicity profiles when compared to the parent. The primary metabolite, X11719474 found in soil, crops and, potentially, at low concentrations, in groundwater, was the most extensively studied, with genetic, acute, short-term rat and dog, rodent reproductive and developmental toxicity studies, and MoA studies conducted. These data supported that the toxicity profile for X11719474 was limited to liver effects via the same MoA as the parent and, overall, X11719474 was significantly less toxic than parent. Subsequently, the comparative toxicology programme was extended to cover all metabolites of sulfoxaflor. Based on structure (i.e., similarity of metabolite structures to one another), toxic effects in comparison with parent (i.e., consistency of the toxicity profiles and confidence in terms of ability to read across), residue compartment (e.g., crop, soil, water) and predicted level of exposure, fewer studies were required for establishing safety of these metabolites compared to X11719474. For example, for some metabolites with very low predicted environmental concentrations only genotoxicity testing was required. For some metabolites with low predicted concentrations, for example only present in liver, a TTC approach was utilized. This strategy of comparative assessment utilizing MoA data, relative potency, hazard characterization, read-across, predicted exposure and TTC provided a robust database, which minimized animal use, comprehensively assessed the hazard and human risk presented by these metabolites.

Integration of novel approaches demonstrates simultaneous metabolic inactivation and CAR-mediated hepatocarcinogenesis of a nitrification inhibitor

Graphical abstract

Mode of action and human relevance of pronamide-induced rat thyroid tumors.

Pronamide, a selective, systemic, pre- and post-emergence herbicide, caused an increased incidence of thyroid follicular cell adenomas in a rat carcinogenicity study. Thyroid tumors, as well as liver and pituitary changes, were limited only to the high-dose group. The evidence for and against specific potential modes of action (MoAs) for rat thyroid follicular cell adenomas and their relevance to humans is discussed. Pronamide is not mutagenic and therefore, direct DNA reactivity is not relevant as a MoA. The hypothesized MoA for this effect is altered homeostasis of the hypothalamic-pituitary-thyroid (HPT) axis mediated by the induction of hepatic enzymes, including uridine diphosphate glucuronosyltransferase (UGT). Evaluation of data from a series of regulatory guideline and MoA studies aimed at identifying the causative and associated key events supported a UGT-mediated MoA in the development of thyroid follicular tumors. This MoA for pronamide-induced thyroid tumors in rats, which involves increased thyroid hormone metabolism/clearance, altered thyroid hormone homeostasis and HPT stimulation is not considered relevant to humans based on quantitative species differences, making rats markedly more sensitive than humans to thyroid perturbations.