Letter to the editor regarding “safety of safety evaluation of pesticides: developmental neurotoxicity of chlorpyrifos and chlorpyrifos-methyl” by Mie et al. (environmental health. 2018. 17:77)

A commentary in Environmental Health titled “Safety of Safety Evaluation of Pesticides: developmental neurotoxicity of chlorpyrifos and chlorpyrifos-methyl” by Mie et al. [15] erroneously suggests that developmental neurotoxicity studies conducted for chlorpyrifos and chlorpyrifos-methyl and then submitted to regulatory authorities were misleading and impaired the authorities’ ability to perform a valid evaluation. We will address the specific technical points, assertions and assumptions individually and demonstrate why the chlorpyrifos study - conducted more than 20 years ago - was reviewed and accepted by global regulatory authorities, including the U.S. Environmental Protection Agency, Australian Pesticides and Veterinary Medicines Authority, Health Canada Pest Management Regulatory Agency and the European Union. Labeled uses of chlorpyrifos rest on five decades of experience in use, health surveillance of manufacturing workers and applicators, and more than 4000 studies and reports examining the product in terms of health, safety and the environment. We will also respond to the allegations made about the chlorpyrifos-methyl study which was conducted more recently. Brain weight Mie et al. [15] challenge the chlorpyrifos DNT study by stating that “For high dose pups, the test laboratory reported a significant reduction of total brain weight and the dimensions of several brain regions on PND 11, but not on PND 65. The test lab argued that these observed effects do not indicate DNT…To support this interpretation, the test laboratory calculated that the average effect on all brain regions is similar to the effect on brain weight…Accordingly, the EPA has identified this analytical approach as an inappropriate and inconclusive manipulation of the data, but a correction was apparently not requested from the pesticide producer submitting the report.” The methodology used in the DNT studies was sound. The highest dose tested in the chlorpyrifos DNT study produced an appropriate level of maternal toxicity, verifying the use of adequate dose levels in this study. The toxicity observed (reduction in bodyweight gain and food consumption, adverse clinical observations, reduction in cholinesterase levels) did not interfere with the production of offspring but did result in a reduction in pup body weight and as noted by Mie et al. [15], a reduction in brain weight and the size of several brain regions in PND 11 pups but not in PND 65 adult rats. In addition to the reduction in weight and measurements of brain areas, the study design included a functional evaluation (i.e., behavioral assessments) in the offspring post weaning and a neurohistopathological assessment of the brain at two ages (i.e., PND 11 and 65) by a board certified neuropathologist. In the chlorpyrifos study, effects in the high dose pups were only observed on the weights and brain area measurements and these effects were not apparent when the pups grew up (at PND 65). The neurohistopathology of the CNS and peripheral nervous system and behavioral assessments were unaffected at the highest dose tested. When results such as observed in this study are produced, there could be several explanations. What the “testing laboratory was arguing” was supported by previously published articles [5, 6, 10] that demonstrated effects on brain weight and the size of various areas of the brain, without any other evidence of damage to the nervous system, is often due to undernutrition. This undernutrition would be expected in offspring of mothers who were growth retarded and food deprived during gestation. In addition to the literature references, a calculation of the % reduction in brain weight to the % reduction in the size of various brain regions was conducted to further support this theory of undernutrition causing the effects in the highest dose group. The real difference in opinion from the evaluation of the study results was in the interpretation of statistically significant reductions in the size of selected regions of the brain in the lower dose groups on PND 11. These differences were not apparent in the middle dose group of the adult rats (PND 65), the tissues were histologically normal at all doses, and there were no functional deficits at any dose, so it was argued that no developmental neurotoxicity was occurring at any dose level but clearly none was occurring at the middle dose level.