October 23, 2019

expert reaction to study looking at chemical piperonyl butoxide (PBO) and birth defects in mice

A study, published in Environmental Health Perspectives, reports a link between the common chemical piperonyl butoxide (PBO) and birth defects in mice.

Prof Daniel Nebert, Professor Emeritus of Environmental Health, University of Cincinnati, said:

“The lowest dose in which these birth defects in mice were seen is 66 mg/kg, which translates to a 4.6-gram oral dose administered to a 150-lb (70 kg) pregnant woman.  So as is always the case with these studies, dose is everything, and gives us reason to be reassured here.  This dose probably exceeds any human reality concern by an order of at least 1 million.”

Dr Peter Jenkinson, Managing Director of CEHTRA Group and of CEHTRA Limited (Consultancy for Environmental & Human Toxicology and Risk Assessment), said:

“The authors state on page 15 “Because many PBO-containing products are applied as aerosol sprays or directly to skin, human exposure to PBO likely also occurs through inhalation and dermal absorption. With reported logP values between 4.60-4.95, PBO is relatively lipophilic and has the potential for bioaccumulation. However, the pharmacokinetics of PBO and its concentration range in human serum have not been rigorously examined.”  In fact there are easily accessible pharmacokinetic data in both rats and humans on PBO, and these can be seen in the REACH disseminated dossier for this substance: https://echa.europa.eu/registration-dossier/-/registered-dossier/10134/7/2/2.  The REACH dossier concludes that, in an orally dosed study in rats, that the majority of the substance was recovered in the faeces and the rest in the urine.  Also that PBO was extensively metabolised and the data indicate that bioaccumulation of PBO or its degradants in tissues is unlikely following oral administration of 14C PBO.  In a human clinical trial dermal absorption was less than 3% of the applied dose either in isopropyl alcohol or water, and there was no evidence to indicate that dermally applied PBO accumulated in the skin.  The authors could have done a little more research to confirm their assumption about the properties of PBO and the existing knowledge of its pharmacokinetics.

“In general, toxic effects in the pregnant female can indirectly have an impact on the developing foetus.  The authors here make no mention of any toxic effects observed in the female mice dosed with PBO, but it is not clear whether this is because there were no effects or because they just do not report them.  The absence of such data, whether positive or negative, is a deficiency in the report.  It is also noted that 2 of the 5 females in the high dose group had no live foetuses, which indicates either maternal toxicity, or a problem with the females or the male/s used for impregnation.  In any case, it reduces the already small group sizes used in the study.

“The authors state on page 6 “The few existing studies of the potential developmental toxicity of PBO were conducted before PBO’s inhibitory action on the Shh pathway was known and were therefore untargeted (Daiss and Edwards 2006).”  The authors claim that such studies were untargeted does not necessarily mean that they can be ignored.  In the REACH disseminated dossier (https://echa.europa.eu/registration-dossier/-/registered-dossier/10134/7/9/1) there is a summary of a high quality 2-generation reproductive toxicity, dietary exposure study (nominal exposures of 30, 100 and 500 mg/kg/day) from 1986.  There were no significant effects on any parameter except for reduced food consumption, and consequently also in bodyweight, in the high dose group in both parental and offspring animals.  No gross malformations were observed that were related to treatment with PBO.  There is also a high quality developmental toxicity study from 1991.  Rats were dosed by oral gavage on gestation days 6 to 15 (including the sensitive period identified as critical for the effects seen in this report) with 200, 500 or 1000 mg/kg/day.  Maternal toxicity was observed at the high dose but no embryotoxic or teratogenic effects were observed in the foetuses, even though foetuses were examined for external malformations including cleft palate, so it is unlikely that any malformations such as interocular distance or upper lip length would have been missed.  Note that the brain development is also examined in great detail in such studies and no effects were observed.

“I also note that this substance was removed from the EU CoRAP list earlier this year https://echa.europa.eu/information-on-chemicals/evaluation/community-rolling-action-plan/corap-table/-/dislist/details/0b0236e1807ea6a1 indicating that the EU has no serious concerns about the safety of PBO.

“In conclusion, it is not clear why the authors saw the effects described in their report when such effects were not observed in high quality, regulatory standard, studies, but the contradictory data indicates that the relevance to humans and human exposure levels may be considered to be low.  Note that 1800 mg/kg is almost the same as the maximum recommended dose level of 2000 mg/kg used in a standard acute oral toxicity (LD50) study and is clearly many, many orders of magnitude greater than any likely human exposure.”

‘Developmental toxicity of piperonyl butoxide exposure targeting Sonic Hedgehog signaling during forebrain and face development in the mouse’ by Joshua L. Everson et al. was published in Environmental Health Perspectives at 14:00 UK time on Wednesday 23 October 2019.

Declared interests

Prof Daniel Nebert: “No interests to declare.”

Dr Peter Jenkinson: “I have no interests to declare.”