New OECD Rules for Endocrine Disruptors
Safety Assessment
Mary Parker

New OECD Rules for Endocrine Disruptors

Updated test guidelines lead to confusion and extra cost, but better results

The Organization for Economic Cooperation and Development (OECD) was founded in 1961 to promote international economic growth and facilitate trade. Part of their mission has been to develop internationally recognized guidelines to balance commercial interests with environmental costs through their Environment Directorate. Beginning from scratch in 1971, the OECD’s Environment, Health, and Safety (EHS) programme has developed guidelines to protect both human health and natural ecology, including rigorous testing for new commercial chemicals.

“In 1998, the OECD initiated a high-priority project aimed at revising existing test guidelines and developing new test guidelines for screening of potential endocrine disruptors,” said Manon Beekhuijzen, Section Head Developmental and Reproductive Toxicology, from Charles River’s Den Bosch facility in The Netherlands. Endpoints suitable for the detection of endocrine disrupting activity of test chemicals were added to the existing OECD 407 test guideline (Repeated Dose 28-Day Oral Toxicity Study in Rodents) in 2008 and more recently to the OECD 443 test guideline (Extended One-Generation Reproductive Toxicity Study) added in 2011.

OECD’s review included updates to the combined repeat dose and reproductive/developmental toxicity screening test guidelines (OECD 421 and OECD 422) in 2015, which resulted in the addition of endocrine disruptor relevant endpoints where exposure to test chemicals encompasses sensitive stages of reproduction and pre- and postnatal development.  These included the addition of hormonally regulated parameters such as estrous cyclicity in adult females, anogenital distance and nipple/areolae retention in offspring, and serum thyroid hormone levels in parental animals and rat pups.  In addition, the OECD 422 test guideline includes the weight and histopathological examination of endocrine-sensitive tissues.

“These endpoints are important for assessing the endocrine disrupting potential of test chemicals, but they also impact study design and present a number of practical, technical, and interpretive challenges,” said Eddie Sloter, Principal Scientific Advisor at Charles River’s Ashland, Ohio facility. These OECD 421 and 422 studies are now more labor-intensive and require a longer duration to complete the in-life phase. Potential endocrine findings should be interpreted using a weight of evidence approach based on biological relevance and plausibility and within the context of the performing laboratory’s historical control data, and not solely on the basis of statistical significance. It is important to be able to differentiate between findings that are consistent with a true pattern of endocrine disruption and those that may be indirect or non-specific signals of toxicity.   

A webinar on evaluating the endocrine disruption potential of an active substance.

According to a paper written by Beekhuijzen and others for the journal Reproductive Toxicology, these updated test guidelines are an improvement on previous versions, and are crucial for determining the endocrine effects of new chemicals.1 However, she was able to determine some aspects that could use additional revision. For example, test guideline OECD 443 discusses the standardization requirements for food and bedding for test rats, while OECD 421/422 make no mention of this requirement. The guidelines also differ slightly on other shared test requirements, such as the weighing and preservation of thyroid and mammary glands, when blood tests should be done on rat pups, and how to interpret potential thyroid hormone effects.2 These inconsistencies are due to OECD’s continued evaluation and refinement of these guidelines, and are unavoidable, but they can present issues for chemical testers. The OECD’s review is currently continuing with test guideline 414, which may add endocrine disruptor relevant endpoints to its rules regarding the testing of potential endocrine disrupting effects of chemicals on embryo-fetal development.

Sloter and Beekhuijzen are among the Charles River scientists who are evaluating these test guidelines line by line in order to streamline the process for commercial clients. By applying scientific common sense to the incomplete test guideline edits, the testing process can be made smoother and more efficient.


  1. Manon Beekhuijzen, Francois van Otterdijk, Willemien Wieland, Miranda van Tuyl, Robert Pels Rijcken, Birgit Peter, Harry Emmen. Update of OECD DART guidelines with endocrine disruptor relevant endpoints: Practical considerations. Reproductive Toxicology. 64 (2016) 64-71.                     
  2. Manon Beekhuijzen, Steffen Schneider, Narinder Barraclough, Nina Hallmark, Alan Hoberman, Sheri Lordi, Mary Moxon, Deborah Perks, Aldert H., Piersma, Susan L. Makris. The urgency for optimization and harmonization of thyroid hormone analyses and their interpretation in developmental and reproductive toxicology studies. Reproductive Toxicology (2018) pii: S0890-6238(17)30573-7. doi: 10.1016/j.reprotox.2018.04.017.