2,4-D

Response to the NRCD Petition

• Title

• Executive Summary

• Introduction

• Not an Endrocrine Disruptor

• Neurodevelopmental

• Mutagenic Effects

• Exposures

• Conclusion

• References

Mutagenic and Genotoxic Effects

NRDC alleges that EPA “disregarded a number of studies that highlight the mutagenicity and genotoxicity of 2,4-D.” (NRDC Pet. at 7). NRDC claims EPA only considered data on the “pure substance” or active ingredient. The NRDC petition then goes on to cite various studies as supportive of its views.

In fact, the test data submitted in support of 2,4-D reregistration evaluated 2,4-D acid plus eight different 2,4-D derivatives, e.g., 2,4-D dimethylamine salt and 2,4-D 2-ethylhexyl ester. Each of the derivatives was subjected to a battery of mutagenicity tests leading to a total of 28 studies submitted for reregistration, all of which were negative (non-mutagenic). NRDC omits mention of any of these studies or the three published papers reviewing them. The two papers by Charles, et al.(1999a; 1999b) and a paper by Gollapudi, et al. (1999) state, “There was no evidence of genotoxicity for potential for 2,4-D acid, or any of its derivatives in these assays.” Importantly, the 28 studies submitted for reregistration met stringent regulatory testing guidelines and complied with EPA’s Good Laboratory Practices (GLP) Standard as required by the Code of Federal Regulations 40CFR, Part 160.

Furthermore, EPA does acknowledge that some positive mutagenicity studies occur in the published literature on 2,4-D. However, the weight of the evidence overwhelmingly supports a conclusion of minimal or no concern for mammalian mutagenicity from 2,4-D exposure. It must be remembered that the 1988 EPA Registration Standard required new mutagenicity studies for all 2,4-D forms because the existing studies did not meet current testing guidelines or GLP requirements. A strong and substantial set of well documented studies is now available to support a diligent EPA risk assessment and overall risk management conclusion.

A 2002 review published by Garabrant and Philbert also provides another weight of evidence type conclusion: “Despite several thorough in vitro and in vivo animal studies, no experimental evidence exists supporting the theory that 2,4-D or any of its salts and esters damages DNA under physiologic conditions.” (Garabrant and Philbert, 2002)

Several inherent characteristics of 2,4-D suggest that there is a very low potential for it to cause mutagenic effects:

The half-life of 2,4-D in humans is less than 12 hours
2,4-D does not metabolize or transform
2,4-D is excreted unchanged
2,4-D does not accumulate

Among the studies cited by NRDC, Madrigal-Bujaidar et al reported an increased frequency of sister chromatid exchange in bone marrow and spermatogonial cells of mice exposed in vivo to 100 mg/kg of 2,4-D. The mid and high doses of 100 and 200 mg/kg in this study are markedly greater than the renal clearance threshold for 2,4-D in the rodent and are massively larger than realistic exposures for humans. However, even at these very high doses, effects were weak. The study’s low dose of 50 mg/kg having no reported effect is also at or above the renal clearance threshold and above realistic exposure levels. (Madrigal-Bujaidar et al 2001)

Holland et al evaluated in vivo and in vitro 2,4-D exposures and showed a 12-15% increase in replicative index, but no biomarker of genotoxicity was found. (Holland et al 2002) EPA reviewed this study and cited it in the 2,4-D RED. It is considered negative for mutagencity.

Figgs et al conducted a longitudinal study of pesticide applicators and found that urine concentrations of 2,4-D increased logarithmically as spraying time increased. (Figgs et al. 2000) Most parameters evaluated showed no effect, but the authors hypothesized that a finding of increasing lymphocyte replicative index could be attributed to the dose of 2,4-D. It is clear that this study had limitations such as the small number of subjects and an untested design, to name a few. The authors self-described the study using the terms “preliminary”, “pilot investigation”, and “the first to show a relationship”. Going on, they said, “The link between 2,4-D function and lymphocyte replicative index is unclear.” And, “Whatever the mechanism, our reports and other recent investigations do not support a genotoxic pathway.”

In the science of toxicology, substantial evidence of mutagenicity is usually considered to suggest a potential for carcinogenicity. For this reason, lifetime animal bioassays are the gold standard for more definitive determinations. With 2,4-D having a very rich toxicological database, the hallmarks of classical mutagenicity and carcinogenicity can be considered with rare conviction. Not only are there 28 high quality mutagenicity studies, but four modern carcinogenicity studies have been conducted on 2,4-D including two studies in the rat and two studies in the mouse. All of these GLP studies were evaluated by EPA and reported in the 2,4-D RED. The weight of evidence across the rodent bioassays is that 2,4-D is not carcinogenic, which is corroborative of the negative mutagenic evidence. The occasional contradictory studies in the mutagenicity database ultimately point to the plentiful rodent bioassay studies for guidance on the carcinogenic potential of 2,4-D.

Next: Exposures