SciPi 145: Peer review of the genotoxicity, toxicokinetics, and carcinogenicity of a pesticide
Since the carcinogenicity of 1,3-D has been in question for more than 30 years, and every one of the 8 lifetime bioassays to date is subject to some degree of controversy, would it not be helpful to conduct a state-of-the-art bioassay, by inhalation (the predominant human exposure route), using a current commercially-available formulation, that could resolve these doubts one way or the other? (for example, with larger sample sizes at doses of 5 and 20 ppm, to have sufficient power to detect small excesses if they exist)
Results
(3 Answers)
Expert 11
There are two oral studies with the currently available formulation both using rats and mice. The only tumor response in these studies was in male rats in one study applying doses that induced a 19 % reduction in bw gain. The study used application with diet that resulted in much lower Cmax as compared to a negative gavage study with identical daily doses. Lung tumor incidences were not changed in the oral studies supporting a portal of entry effect as outlined in the white paper.
The inhalation studies showed an increased incidence of lung adenomas in mice as the only tumor response with a high incidence of this tumor type present in controls. Highly variable incidences of lung tumors in control mice are evident from the Dow database and from a number of other sources. Increased adenoma incidences in mice have been seen after inhalation of a number of other chemicals (e.g styrene, naphthalene, and trichloroethene) and mode of action studies with these chemicals suggest a high susceptibility of the target cells in the lung of mice to chemical-induced toxicity and GSH depletion. This toxicity and resulting hyperplasia were identified as key steps in lung tumor induction by these compounds. Moreover, there are anatomical differences between mice and rats/humans with respect to the target cells as elucidated for styrene and other chemicals supporting the conclusion that any tumor tumor response induced by thes MoA is not relevant to humans. The observed GSH depletion in mice after inhalation of 1,3-D may not be sufficient to induce toxicity and hyperplasia ?
In my opinion, the arguments regarding variability of control incidence and absence of lung toxicity of 1,3-D in the studies, which is key for lung tumor induction in mice by several non-genotoxic chemicals, are supporting the conclusion that the lung tumors are not treatment related.
I am not convinced that the argument regarding non-linear kinetics can be used to conclude that the tumors seen are present at doses above a KMD have no relevance based on kinetic analysis. Blood kinetics show non-linearity for parent material, but the blood kinetics may not reflect exposure of the target tissue lung. These are expected to be driven by air concentrations of 1,3-D and should thus be linear. The non-linearity of blood levels of 1,3-D after inhalation would only be relevant if systemic toxicity is to be considered.
Any new study, even with higher sample sizes, will still have the issue of variable background of lung tumors in mice and difficulties in data interpretation.
Expert 9
Any scientific study may be criticized based on the design, methods, interpretation of results, etc. As long as we accept this reality, I don't agree that all the bioasays are "subject to controversy". This is certainly true of some of the studies such as those that tested the product containing the carcinogen/mutagen contaminant. However, some studies appear to have been well designed and conducted. Therefore, the results are not controversial. I do not agree with the need for additional studies to answer this question.
Expert 6
I do not think that another study with 1,3-D by inhalation route is necessary. Results of the available studies are clear to me.
The new formulation of 1,3-D is not genotoxic. In a review by Zeiger, 2005, it has been concluded that “the results in the studies that had an appropriate study design and used 1,3-D that did not contain epichlorohydrin should be used. These studies were uniformly negative.” and “1,3-D is not an in vivo genotoxicant”.
Moreover, two inhalation carcinogenesis studies were conducted ( Stott, 1987, in mice, and Lomax 1987 in rats). In the mouse inhalation study, a statistically identified increase in benign lung tumors (bronchioloalveolar adenomas) was observed in male mice only exposed to high-concentration (60 ppm )of 1,3-D (22/50 vs. 9/50 in controls). In the rat inhalation study, there were no statistically identified increases in any tumor incidence in rats exposed by inhalation to 1,3-D. All microscopic tumor observations were considered by
pathologists to be incidental and/or age-related changes.
In addition, no treatment-related lung tumors were identified in the 4 oral carcinogenicity studies, 1,3-D inhalation-induced lung tumors appears to be a portal-of-entry effect.
Moreover, toxicokinetic studies indicate that oral and inhalation administration of 1,3-D in rodents and humans are similar in terms of uptake, detoxification and formations of major metabolites. Following rapid uptake, 1,3-D is primarily metabolized and detoxified by conjugation with GSH to form mercapturic acid metabolites. Therefore, the available studies in mice and rats indicate that 1,3D new formulation (without epichlorohydrin) is not carcinogenic by inhalation route, and no new studies are necessary.
Expert 6
04/17/2019 19:23I guess we all agree that the new formulation of Telone II is not genotoxic. Only 2 studies have shown some evidence of carcinogenesis, which are the Scott study (1995) (Increased incidences of hepatocellular adenoma were observed in both high dose males and females and in mid-dose males) and the inhalation study in B6C3F1 Mice, in which statistically identified increase in benign lung tumors (bronchioloalveolar adenomas) was observed in male mice only exposed to high-concentration (60 ppm )1,3-D (22/50 vs. 9/50 in controls).
Carcinogens nowadays must be characterized according to their mode and mechanism of action. What would be the mechanism of action of 1,3D for carcinogenesis? Any evidence?
Expert 11
04/18/2019 02:59For lung, the mouse is very susceptible to GSH-depletion, but the MoA for other chemicals causing lung tumors in mice require cytotoxicity and hyperplasia. Based on absence of toxicity in the mouse lung and no lung response in the rat inhalation study, my conclusion is that the lung tumors are not treatment related. Issue with Scott et al, are much lower Cmax as compared to Kelly. Still struggle a little with the indication of a dose-response in Scott ? Mechanisms could be GSH-depletion at high doses/concentrations ? Agree that it is highly unlikely that direct genotoxicity plays a role in the responses.
Expert 2
04/18/2019 10:09Most of these responses (those in the prior round and some of the comments to date) sidestep the original question by focusing on genotoxicity. The original question proposed a larger-N bioassay (not necessarily a "mega-mouse" study by any means) so that we wouldn't have to make qualitative ("increases above controls aren't really increases because in some years the control rate would have been greater...") and quantitative ("at this dose there were no statistically significant tumors") pronouncements with low power to make them.
A larger study would allow for slightly lower doses to be used, and for the uncertainty in historical control rates to make far less potential difference.
Expert 2
04/18/2019 10:19(separating this from my immediately prior comment, for emphasis)
I remain puzzled by the complete lack of attention, both in the White Paper and in this exercise to date, of one particular positive bioassay finding that seems quite important: the striking and dose-related increase (above a ZERO control rate) of transitional cell carcinomas of the bladder in female mice (NTP 1985-- 16% at 25 ppm and 42% at 50 ppm).
Can anyone explain why we should be completely ignoring these? I know-- "epichlorohydrin." EXCEPT that (1) epichlorohydrin has never to my knowledge been implicated in bladder carcinogenesis; and (2) if you believe the back-of-the-envelope risk assessment (see user-37600 [Expert 2] comment on the WOE topic 5.5 elsewhere on this site) using EPA's cancer potency (slope) factor for epichlorohydrin, it's virtually impossible for 1 PERCENT impurity to cause more than 1 or 2 tumors per 50 rodents at ANY site, let alone one where the impurity does not seem to be active.
Various sections of the White Paper, and various commenters, have at least coherent theories as to why we should ignore the liver adenomas, the forestomach tumors, and the inhalation lung adenomas-- but I've seen no attempt-- other than silence-- to explain away the bladder tumors...
Expert 8
04/18/2019 11:49The WOE from reviewers suggests that the extant bioassays provide sufficient data to support a hazard WOE. Rather than require more cancer bioassays, I would recommend more cogent and logical explication of the most probable MOAs.
Expert 9
04/18/2019 19:47User-37600 [Expert 2] raises a valid point about the transitional cell carcinomas in female B6C3F1 mice in the NTP gavage study. The tumor incidences of 0, 16 and 42% in control, low and high-dose mice when viewed in isolation are impressive and suggest a carcinogenic effect in the bladder. However,
we need to look at these tumor incidences in the context of all the other relevant data. The presence of 1% epichlorohydrin in the test article is important. User-37600 [Expert 2] states that this 1% impurity is unlikely to have caused this effect based on EPA's data and on the absence of any prior information that epichlorohydrin could produce this effect. These are valid points. However, we have no information on the potential interaction between this impurity and 1,3-D. Is a synergistic effect possible? We don't know. Secondly, The same doses of 1,3-D were given to B6C3F1 mice by gavage in the Redmond, 1995 study with no suggestion of an effect on the bladder. Thirdly, Stott, 1987 reported no bladder tumors in their inhalation exposure study, a route that is more relevant to human exposure to 1,3-D. Finally, there is no evidence of bladder tumors in any of the other bioassays conducted in CD-1 mice mice and in F344 or CD rats. Therefore, looking at all the relevant data, the weight of evidence is that 1.3-D did not cause bladder tumors in multiple oral and inhalation bioassays with the exception of one study in which a potentially confounding impurity was present. Given this data base, I have to conclude that this one result was an outlier that could not be reproduced in multiple additional studies.
Expert 11
04/19/2019 00:54My point is that with the variable incidences of lung tumors in mice, it will always be difficult to obtain reliable results no matter how large the groups are. I think that more information on the composition of the materials tested including impurities other then epichlorohydrin may help explain some of the contradictions. I also agree that presence of epichlorohydrin may only explain the forestomach tumors. My conclusion that the NTP study should be set aside is based on issues with study design and conduct. The bladder tumors are somewhat puzzling and there seems to be dose-response. I tend to agree with user 750802 [Expert 9] regarding consitency of the overall database on carcinogenicity
Expert 4
04/22/2019 13:32I agree with the comment from user-2441 that the existing bioassays provide sufficient data to support a hazard WOE, and that a more cogent and logical explication of the most probable MOAs would likely shed more light on the issue
Expert 6
04/24/2019 08:34I think we should not include the 2 NTP studies in our discussions, because of the contamination with epichlorohydrin. I would like to know the composition of TeloneII used for the other studies, and also the composition of DD92 used for some studies.