The US National Toxicology Program’s findings contradict the conclusions of the cancer agency IARC. But there are problems with their argument

Tests by the US National Toxicology Program (NTP) have found that some complete commercial formulations of glyphosate-based herbicides ("GBHs") are genotoxic (damage DNA), but that glyphosate on its own ("glyphosate technical") is not genotoxic. Furthermore, the NTP says neither glyphosate nor its formulations cause oxidative stress.

These findings are significant because in 2015 the International Agency for Research on Cancer (IARC) classified glyphosate as a probable human carcinogen, naming genotoxicity and oxidative stress as two main mechanisms by which exposure to glyphosate and glyphosate-based herbicides can lead to cancer.

In contrast, the US Environmental Protection Agency (EPA) says glyphosate is not carcinogenic, a view that will gain support from the NTP’s findings.

Good news for Bayer-Monsanto?

The NTP’s findings, albeit they are presented as preliminary, are likely to be welcomed by Bayer-Monsanto, which this year faces the beginning of a new round of court cases brought by sufferers from Non-Hodgkin’s lymphoma who believe that their cancers were caused by exposure to the company’s glyphosate-based herbicides. The first three cases resulted in massive damages being awarded against Monsanto/Bayer, though all are under appeal. The scientific basis for the court cases was provided to a significant extent by IARC’s review of the evidence leading to its conclusion that glyphosate is a Class 2A carcinogen (probable human carcinogen).

On the other hand, these lawsuits are ultimately about exposure to Monsanto’s Roundup and other similar glyphosate-based formulated products and NTP admits some of these formulated products can be genotoxic. But as the companies don’t disclose the ingredients in the formulations and these haven’t been well tested for different types of toxicity, identifying the chemical culprits for someone’s cancer will not be possible as long as the current state of secrecy persists.

Other chemicals blamed for DNA damage

Regarding genotoxicity, the NTP researchers say the danger with the formulations probably lies with added ingredients that include chemicals called surfactants, which enable the glyphosate to be absorbed into plants, plus the weedkilling chemicals diquat dibromide, mesotrione and metolachlor, which are added to some glyphosate products.

An article by Carey Gillam in The Guardian tells more about the genotoxicity findings: “The scientists have not been able to specifically study the surfactants used in the products because manufacturers do not have to disclose what is considered to be confidential business information. Product labels typically refer to surfactants only as ‘other ingredients’. NTP scientists cannot say if the genotoxic effects are due to the other herbicides mixed in with glyphosate or the surfactants.”

The NTP scientists summarize their findings as follows: “These results suggest that while glyphosate alone lacks genotoxic activity, the genotoxicity of GBFs [glyphosate based formulations] may require further evaluation.”

Studies find both glyphosate alone and formulations cause genotoxicity and oxidative stress

The problem with the NTP’s results is that they conflict with those of numerous other studies, which have found just glyphosate (technical grade, i.e. at a sufficient level of purity for commercial use), as well as some formulations, to be genotoxic and to cause oxidative stress.

In 2019 Dr Charles Benbrook published a review of the studies that led the US EPA and IARC to form opposing opinions of glyphosate's toxicity.

Dr Benbrook found a significant number of studies concluding that glyphosate technical (grade), as well as the formulations, was genotoxic. Only one of these was a study produced purely for regulatory purposes (the type of study sponsored by the pesticide industry); the others were peer-reviewed studies published in the scientific literature (the highest bench mark).

He wrote, "Of the total 104 assays on glyphosate technical, one regulatory assay and 35 published assays reported positive evidence of a genotoxic response, for a total of 36 positive assays in the case of glyphosate technical. Thus, for glyphosate technical just 2% of the regulatory assays and 67% of the assays published in peer-reviewed journals reported positive results... In the case of formulated GBHs, 43 regulatory assays and 65 assays from published literature were cited by EPA and/or IARC, for a total of 108. Of these 108, none of the regulatory assays reported evidence of a positive genotoxic response, while 49 published assays did (75%)... in its evaluation of glyphosate genotoxicity, the EPA took fully into account the results of only 23% of the assays considered by IARC."

In other words, all but one of the regulatory studies sponsored by the pesticide industry concluded that glyphosate technical was not genotoxic, and the EPA relied largely on these studies. IARC, on the other hand, also considered [or only considered? If the latter, omit ‘also’] published studies from the peer-reviewed literature, of which 67% concluded that glyphosate technical was genotoxic.

Many of the studies identified in Dr Benbrook’s review found genotoxicity from glyphosate and its formulations identified oxidative stress as a mechanism. It was these studies that led IARC to conclude that “Strong evidence exists that glyphosate, AMPA [glyphosate’s metabolite], and glyphosate-based formulations can induce oxidative stress.”

In sum, rather than providing a definitive view on the potential of glyphosate and its formulations to cause genotoxicity and oxidative stress, the NTP seems to have simply added to a large body of data characterized by conflicting results. And crucially, the NTP’s results do not disprove those of the numerous other studies that reached opposite conclusions.

Rush to make results public

Surprisingly, the NTP researchers chose to make their results public not through the usual route of peer-reviewed publication, but via posters displayed at conferences. According to Gillam’s article, the researchers don’t expect their work to appear in a peer-reviewed journal until the middle of the year.

Why the hurry to get the results out now? We may never know. But the NTP’s posters could be viewed as Exhibit No. 1 for Monsanto/Bayer’s defence attorneys in upcoming court cases. The key wording is: “Based on this data set, it is unlikely that any genotoxic activity of GBFs is due to glyphosate” and “Gly[phosate] and its formulations did not induce DNA damage and oxidative stress”.

There may have been other benefits for the NTP in releasing its results via posters rather than a scientific publication: you don’t have to address results from other studies that conflict with your own; and you can omit potentially revealing details such as the names of which specific types of genotoxicity assays you performed.

Why do the NTP’s results differ from those of other studies?

In the absence of a detailed scientific publication in a journal, it’s difficult to identify exactly why the NTP’s results differ from thosethat found glyphosate technical genotoxic. But based on the limited information in the NTP’s posters, there are a couple of possible reasons.

The NTP tested just three different cell lines. However, Dr Michael Antoniou, a leading researcher in the field of glyphosate and GBH toxicology based at King’s College London, said a more complete picture of the genotoxicity of glyphosate and its formulations would have been gained by testing a larger number of cell lines. He said, “It is not possible to generalize the results from one cell line to another because they have varying sensitivity to different chemicals. You need to test the chemicals on a large range of cell types to give an idea of the genotoxic potential to the whole organism. If only a few or even just one cell type in our bodies is found to be prone to glyphosate-induced genotoxicity, which could lead to cancer, that is reason enough to avoid exposure to these substances.”  

It’s also possible that the NTP researchers didn’t use the most sensitive assays. While they don’t name all the assays they used, they do mention the Multiflow® DNA Damage assay. Dr Antoniou said, “It’s possible that the assays they conducted revealed some but not all types of DNA damage. It would have been useful to combine the Multiflow DNA Damage assay with the comet assay, which gives a broader readout of more different types of DNA damage.”

Until the full details of the study are known, we simply don’t know whether the researchers used the comet assay.

Oxidative stress

One mechanism by which chemicals can cause DNA damage is through oxidative stress. This involves production of reactive oxygen molecules (free-radicals or oxygen-containing molecules such as hydroxyl groups and hydrogen peroxide), which then attack and damage DNA. In their first poster presentation in March 2019, the NTP scientists stated that neither glyphosate nor the tested GHFs caused oxidative stress in the two cell lines they used in their experiments. They argue that since glyphosate did not cause oxidative stress then it is highly unlikely to cause DNA damage.

However, as in the case of the NTP’s genotoxicity evaluation of glyphosate and GBFs, there are limitations in the experiments performed that do not allow these observations to be generalized to all cell types in our bodies.

First, the NTP scientists used only one measure of oxidative stress (hydrogen peroxide production) and thus they could have missed oxidative stress caused by other molecules (e.g. oxygen free radicals, hydroxyl groups).

Second, the capacity of different cell types to deal with an oxidative stress insult varies, with some proving more robust than others. It is thus possible that the two cell lines that the NTP scientists employed in their assays (human liver HepaRG cells, human keratinocyte (skin type) HaCaT cells) are capable of maintaining balance in the face of such a challenge and thus scored negative. But this does not mean that all cell types will behave in the same manner.

Surprisingly, the NTP scientists do not present data from any assays to measure oxidative stress in their second poster, which appeared in September 2019, reporting results from testing a completely different type cell line (TK6 B-type blood cell).

These limitations in the NTP experiments may offer some explanation as to why their observations are at odds with those of others in the peer-reviewed scientific literature.

Crucial gap in NTP research

There’s also an important gap in the NTP’s research, judging from the limited information on the posters. The researchers state that “it is unlikely that any genotoxic activity of GBFs is due to glyphosate” and that the genotoxic effects are due to the other herbicides mixed in with glyphosate or the added ingredients.

But it seems that the researchers have not addressed the possibility that the genotoxic effects could be due to glyphosate combined with other ingredients. A way to test this possibility would have been to include an “added ingredients only” control, without the glyphosate.

The researchers couldn’t do this because pesticide companies do not declare what the added ingredients are. But until that test is done, we can’t rule out the possibility that even in the NTP experiments, ‘glyphosate plus x’ was genotoxic. It’s therefore unclear on what basis the NTP researchers are claiming that glyphosate is “unlikely” to be genotoxic.

Given that so many other studies have found glyphosate to be genotoxic, it seems probable that a combination of glyphosate with other ingredients added to make it more toxic would have a genotoxic effect.

Again, we await the publication of the full results before we can conclude on this issue. As long as the added ingredients in glyphosate herbicide formulations remain commercial secrets, researchers will not be able to identify precisely which ingredients are causing the genotoxic effects. So it seems that in the US at least, glyphosate-based herbicides will remain on the market for the indefinite future, helped by the ‘unlikely to be genotoxic’ verdict bestowed by the NTP’s as-yet unpublished experiments.

We don’t doubt that the NTP researchers carried out their particular tests as well as they can be done, but the question is whether they did the most informative tests.

Meanwhile, given their belief that the genotoxic effects of some glyphosate herbicide formulations are due to other ingredients than the glyphosate, it is their clear public duty to demand a change in the law to require pesticide manufacturers to disclose all the ingredients in their formulations and to require regulators to assess the biological effects of these chemical cocktails as an integral part of the pesticide safety evaluation.


Report: Claire Robinson