Amphibian studies to investigate the endocrine-disrupting properties of chemicals through the thyroid modality: a comparison of their statistical power

Amphibians are the current model species for investigating the endocrine-disrupting (ED) properties through the thyroid modality in non-mammalian species. A recurrent question in the European Union (EU) regulatory endocrine assessment of pesticide active substances (2018/605) is whether the positive results from an in vivo screening test, that is, Amphibian Metamorphosis Assay (AMA), can be considered sufficient to conclude on the ED properties of a pesticide active substance or whether the larval amphibian growth and developmental assay (LAGDA) is a necessary step to further clarify the concerns identified in the AMA. Another one is the consideration of the extended AMA (EAMA). To further clarify some of the uncertainties around the use of the LAGDA and to help further consideration of the EAMA in a regulatory context, the statistical power of the three test designs was tested for all the parameters entailed to be measured in the respective study design (except for thyroid histopathology) by using data from real experimental studies. Our findings showed that the statistical power of the EAMA is in line with other Organisation for Economic Co-operation and Development standardized tests, that is, AMA, LAGDA. Our results also confirmed that the LAGDA is more powerful to detect effects on relevant parameters, that is, time to reach metamorphosis, compared to other in vivo tests. However, the difference in power was small, questioning its contribution to an overall weight of evidence already supporting the identification of a substance as an ED. These findings should be considered only in the context of hazard-based endocrine assessment of active substances (i.e., EU regulatory ED assessment of pesticide active substances, 2018/65), while they may not be fully applicable for a risk assessment-based approach.