Response to Osimitz and Droege, 2021

We appreciate the attention and interest that our recent review, “Examining the developmental toxicity of piperonyl butoxide as a Sonic hedgehog pathway inhibitor” (Rivera-González et al., 2020), has received, including the additional perspective provided by Osimitz and Droege (2021) in their letter to the editor of Chemosphere. The authors do not comment on the technical details of our original review publication but instead extrapolate from previously published and re-presented detection values and call attention to selected findings and conclusions which, in some cases, are drawn from secondary sources without publicly available experimental methodology and primary data. These additions are welcomed and, in our view, not contradictory to the central points raised in our review that synthesized findings from peer-reviewed scientific literature.

As an example, Osimitz and Droege highlight the induction of cytochrome P450 (CYP450) enzymes by piperonyl butoxide (PBO) in mammals as an apparent contrast to evidence presented in our review indicating that PBO inhibits oxidative enzyme activity and drug metabolism in mammals, a finding supported by numerous independent research publications (Fine and Molloy, 1964; Anders, 1968; Fujii et al., 1968; Jaffe et al., 1968; Jaffe and Neumeyer, 1970; Franklin, 1972; Friedman et al., 1972; Benchaoui and McKellar, 1996; Xu et al., 2021). However, these activities are not mutually exclusive, and evidence from multiple studies support a biphasic effect in which CYP450 inhibition by PBO is followed by induction (Matthews et al., 1970; Skrinjarić-Spoljar et al., 1971; Philpot and Hodgson, 1972; Franklin, 1976; Hodgson and Levi, 1999). Demonstration of dynamic modulation of oxidative metabolism by PBO in mammalian models supports further investigation in human-relevant experimental systems.

Our review examined the developmental toxicity of PBO in the context of its newly recognized mechanism as an inhibitor of Sonic Hedgehog (Shh) signaling, a key pathway in embryonic and fetal development. Rather than being caused by a single factor, developmental outcomes associated with Shh signaling disruption are thought to result from complex interactions among genetic predisposition and multiple environmental influences during critical periods of susceptibility (Petryk et al., 2015; Krauss and Hong, 2016; Lovely et al., 2017; Beames and Lipinski, 2020). PBO was recently found to cause malformations in mice and zebrafish by disrupting Shh signaling during early development (Wang et al., 2012; Everson et al., 2019; Everson et al., 2020), making it one of many identified environmental Shh signaling inhibitors (Rimkus et al., 2016; Bao et al., 2018; Ghirga et al., 2018). In light of the widespread use of PBO, its modulation of Shh signaling and CYP450 enzyme activity, and the potential for co-exposure to multiple Shh pathway inhibitors, additional research is warranted to evaluate the impact of this pesticide synergist and its potential contribution to multifactorial and etiologically complex developmental outcomes.