An in vitro approach for comparative interspecies metabolism of agrochemicals.

The metabolism and elimination of a xenobiotic has a direct bearing on its potential to cause toxicity in an organism. The confidence with which data from safety studies can be extrapolated to humans depends, among other factors, upon knowing whether humans are systemically exposed to the same chemical entities (i.e. a parent compound and its metabolites) as the laboratory animals used to study toxicity. Ideally, to understand a metabolite in terms of safety, both the chemical structure and the systemic exposure would need to be determined. However, as systemic exposure data (i.e. blood concentration/time data of test material or metabolites) in humans will not be available for agrochemicals, an in vitro approach must be taken. This paper outlines an in vitro experimental approach for evaluating interspecies metabolic comparisons between humans and animal species used in safety studies. The aim is to ensure, where possible, that all potential human metabolites are also present in the species used in the safety studies. If a metabolite is only observed in human in vitro samples and is not present in a metabolic pathway defined in the toxicological species already, the toxicological relevance of this metabolite must be evaluated.

Relevance weighting of tier 1 endocrine screening endpoints by rank order.

Weight of evidence (WoE) approaches are recommended for interpreting various toxicological data, but few systematic and transparent procedures exist. A hypothesis-based WoE framework was recently published focusing on the U.S. EPA's Tier 1 Endocrine Screening Battery (ESB) as an example. The framework recommends weighting each experimental endpoint according to its relevance for deciding eight hypotheses addressed by the ESB. Here we present detailed rationale for weighting the ESB endpoints according to three rank ordered categories and an interpretive process for using the rankings to reach WoE determinations. Rank 1 was assigned to in vivo endpoints that characterize the fundamental physiological actions for androgen, estrogen, and thyroid activities. Rank 1 endpoints are specific and sensitive for the hypothesis, interpretable without ancillary data, and rarely confounded by artifacts or nonspecific activity. Rank 2 endpoints are specific and interpretable for the hypothesis but less informative than Rank 1, often due to oversensitivity, inclusion of narrowly context-dependent components of the hormonal system (e.g., in vitro endpoints), or confounding by nonspecific activity. Rank 3 endpoints are relevant for the hypothesis but only corroborative of Ranks 1 and 2 endpoints. Rank 3 includes many apical in vivo endpoints that can be affected by systemic toxicity and nonhormonal activity. Although these relevance weight rankings (WREL ) necessarily involve professional judgment, their a priori derivation enhances transparency and renders WoE determinations amenable to methodological scrutiny according to basic scientific premises, characteristics that cannot be assured by processes in which the rationale for decisions is provided post hoc.

Hypothesis-driven weight of evidence framework for evaluating data within the US EPA's Endocrine Disruptor Screening Program.

"Weight of Evidence" (WoE) approaches are often used to critically examine, prioritize, and integrate results from different types of studies to reach general conclusions. For assessing hormonally active agents, WoE evaluations are necessary to assess screening assays that identify potential interactions with components of the endocrine system, long-term reproductive and developmental toxicity tests that define adverse effects, mode of action studies aimed at identifying toxicological pathways underlying adverse effects, and toxicity, exposure and pharmacokinetic data to characterize potential risks. We describe a hypothesis-driven WoE approach for hormonally active agents and illustrate the approach by constructing hypotheses for testing the premise that a substance interacts as an agonist or antagonist with components of estrogen, androgen, or thyroid pathways or with components of the aromatase or steroidogenic enzyme systems for evaluating data within the US EPA's Endocrine Disruptor Screening Program. Published recommendations are used to evaluate data validity for testing each hypothesis and quantitative weightings are proposed to reflect two data parameters. Relevance weightings should be derived for each endpoint to reflect the degree to which it probes each specific hypothesis. Response weightings should be derived based on assay results from the test substance compared to the range of responses produced in the assay by the appropriate prototype hormone and positive and negative controls. Overall WoE scores should be derived based on response and relevance weightings and a WoE narrative developed to clearly describe the final determinations.