Relevance Weighting of Tier 1 Endocrine Screening Endpoints by Rank Order

Abstract

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 (W REL ) 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.

Weight-of-the-evidence evaluation of 2,4-D potential for interactions with the estrogen, androgen and thyroid pathways and steroidogenesis

“…Notably, there were no effects signifying increased male VTG, which as noted is considered a rank 1, i.e. relatively specific and sensitive endpoint for assessing potential estrogenicity (Borgert et al 2014). Data from an acceptable reproductive toxicity study in quail (Mitchell et al 2000) show no effects potentially related to an estrogen pathway interaction at any dietary concentration.…”

“…In a recent paper, Borgert et al 2014 proposed the following ranking scheme for evaluating the endpoints assessed in the EDSP Tier 1 Tests for relevance, sensitivity and specificity to testing hypothesized endocrine pathway interactions:…”

“…High quality in vitro and in vivo data from the published literature and ToxCast TM screening results are also considered important in developing the WoE and are discussed in the text. The only type of in vivo assay considered to provide a clear rank 1 result for estrogenic pathway activity are: 1) the uterotrophic assay (none available for 2,4-D), in which statistically significant and marked uterine weight increases are considered a reliable and predictive endpoint of estrogenicity, and the male VTG endpoint in the FSTRA (available for 2,4-D), which when increased appears closely associated with estrogenicity (Borgert et al 2014). The VTG endpoint for the FSTRA for 2,4-D was negative.…”

“…. †Marino et al 2010 decreased fecundity observed only at high (limit dose) dose; non-specific finding; equivocal relationship to pathway interaction All studies: NNo evidence of pathway interaction LPotential evidence of pathway interaction at limit dose only -Endpoint not evaluated #Decreased relative to control Endpoint scores in parentheses based on Borgert et al (2014) (1) specific and sensitive to the hypothesis (2) potentially sensitive for the hypothesis; stronger if correlated with Rank 1 data (3) relevant, but useful only if corroborating Rank (1) or (2) endpoints but incubated eggs at two different temperatures, gonadal histopathology was evaluated. No effects of 2,4-D were found, in contrast to estradiol.…”

“…‡Jeffries et al 1995 Ovary weight decrease at 150 mg/kg/day high dose terminal sacrifice attributable to body weight loss, no correlating histopathological findings. ¶Decreased incidence of mammary gland hyperplasia at 150 mg/kg/day terminal sacrifice; likely attributable to body weight loss All studies NNo evidence of potential interaction OFinding over the threshold for renal saturation "Increased relative to control #Decreased relative to control -Endpoint not evaluated Endpoint scores in parentheses based on Borgert et al 2014 (modified in some cases based on context or strength of response) (1) specific and sensitive to the hypothesis (2) potentially sensitive for the hypothesis; stronger if correlated with Rank 1 data (3) relevant, but useful only if corroborating Rank (1) or (2) endpoints loss. Note that mammary histopathology in the chronic rat study is ranked a 3 for sensitivity (as opposed to 2 for this endpoint in subchronic studies) because the high background incidence of mammary tumors in the F344 rat strain is a potential confounder.…”