Interpreting Estrogen Screening Assays in the Context of Potency and Human Exposure Relative to Natural Exposures to Phytoestrogens

Abstract

While the Environmental Protection Agency and the Organization for Economic Cooperation and Development have developed validated in vitro and in vivo screening assays to measure interaction of substances with estrogen, androgen and thyroid pathway components, to date, methods to contextualize such results in terms of potencies and actual human exposures are lacking. To place endocrine screening results in the context of potency and human exposure, we propose a method that entails (1) calculating a benchmark dose for a response measured in an endocrine screen; (2) estimating the human urinary concentration (biomonitoring equivalent, BE) expected to correspond to this dose (BE BMD ); (3) deriving the exposure:activity ratio (EAR) by comparing actual urinary values from human biomonitoring studies (e.g., National Health and Nutrition Examination Survey (NHANES)) to the BE BMD . Using OECD uterotrophic assay validation studies and NHANES results, we calculated EARs for genistein (EAR GEN = 6.6 × 10 −4 ) and bisphenol A (EAR BPA = 8.8 × 10 −7 ). The EAR GEN is more than 700-fold greater than the EAR BPA . Not only can these methods be applied to additional endocrine assays and compounds, they can contribute to weight of evidence decisions regarding the need for additional endocrine screening and testing-substances with low EARs may not warrant additional testing.

Employing Dietary Comparators to Perform Risk Assessments for Anti-Androgens Without Using Animal Data

“…Exposure: activity profiling was performed using a similar approach to that proposed for estrogenic responses (Becker et al , 2014, 2015). First, a suitable comparator was identified from the dietary components tested in the assay as a substance which showed a specific effect on the AR signaling pathway.…”

“…One way to address these questions is to put margins of exposure derived from in vitro only risk assessments into context against margins of exposure for comparator substances with the same mode of action. One method that has been proposed for estrogen agonists is to calculate the exposure: activity ratios (EARs) for test substances, and directly compare these with the EAR of a comparator with a history of dietary exposure (Becker et al , 2014, 2015). In that approach for estrogen agonists, the phytoestrogen genistein was selected as the comparator.…”

An exposure:activity profiling method for interpreting high-throughput screening data for estrogenic activity—Proof of concept

“…For estimating activity values from in vivo animal data, Becker et al (2014) used a BMD value from the uterotrophic assay to obtain the corresponding biomonitoring equivalent (BE) value; thus, they expressed an external dose as a urinary concentration. This enabled comparison of the human exposure values (mg/L in urine from human biomonitoring studies) to the concentration in urine corresponding to the uterotrophic BMD.…”

“…To calculate a REEAQ value as in Becker et al (2014), two EAR values are required: one for the chemical of interest and the other for the reference chemical, in this case, GEN. For each of these EAR values, estimates of exposure and activity are needed. All four of the values required (two for each EAR) should be expressed in the same units and represent a comparable biological quantity, e.g., dose or tissue concentration.…”

“…For chemicals other than GEN, IVIVE was required because the estrogen receptor-related bioactivity of interest were expressed as in vitro concentrations; Supplementary Information SF-1 shows a schematic for comparison of the methodology in Becker et al (2014), Wetmore et al (2012), and also that used here.…”