Significant excess risks of bladder and colon cancers were observed in the Agricultural Health Study among applicators exposed to the heterocyclic aromatic amine herbicide imazethapyr. For bladder cancer, participants in the highest exposure category of imazethapyr had a 137% higher risk than nonexposed pesticide applicators. For colon cancer, detailed analysis by subsite revealed that imazethapyr use was significantly associated with a 173% increased risk of proximal cancers, but not with distal or rectal cancers.
Although bladder cancer risk has been reported to be elevated in some agricultural populations,15
this is the first report of a significant increase in bladder cancer specifically linked to the pesticide imazethapyr. While there is the possibility that this could be a chance finding, other aromatic amines are well established in the etiology of bladder cancer. Significant excess bladder cancer risks have been observed among those employed in aromatic amine manufacture, dyestuff manufacture and use, rubber manufacture, painting, aluminum industry, leather industry and truck driving.16,17
Many of these excess risks are related to two specific aromatic amines, benzidine and 2-naphthylamine. Among workers exposed to these compounds, the time between first exposure and death due to bladder cancer ranged from 12 to 41 years suggesting that the current duration of exposure to imazethapyr, which was first available in 1989, may be adequate for cancer development.18
Farmers generally have significantly lower risk of bladder cancer and colon cancer than the general population, possibly due to lower rates of smoking and increased levels of physical activity; however, studies of colon cancer and pesticides have been inconsistent.19-24
Previous analyses in the AHS cohort have linked increased risks of colon cancer with exposure to dicamba25
Evidence linking HCA exposure to colon cancer comes from the dietary literature. Meat cooked at high temperature results in increased formation of HCA compounds and increased intake of well-cooked meat has been positively associated with colon cancer risk.26-28
Inside the body, HCAs are activated to carcinogenic intermediates via xenobiotic metabolizing enzymes. A similar mode of action may be at work for imazethapyr metabolism but no human metabolism data are available for this compound.
We also found that the excess risk of colon cancer observed was largely due to cancers occurring in the proximal colon. There are well described differences in the incidence and risk factors for proximal and distal colon cancers.29,30
Although no studies have identified HCA pesticide exposure as a risk factor for proximal colon cancer, a body of evidence suggests that proximal cancers are associated with certain molecular events that may be related to pesticide exposure. Proximal cancers are more often associated with microsatellite instability, an accumulation of errors at microsatellite loci, whereas distal cancers tend to exhibit chromosomal instability manifested by aneuploidy and loss of heterozygosity.29,30
Microsatellite instability results from the loss of DNA mismatch repair due to altered methylation and subsequent silencing of MHL1.31,32
Pesticide exposure has also been linked to altered methylation in several animal studies.33-35
Thus, it is plausible that imazethapyr exposure, through altered DNA methylation mechanisms, may be linked to excess proximal versus distal colon cancers. However, there is also the possibility that these findings are due to chance given the small number of cases observed.
The AHS has several unique strengths. The study population is large and frequently exposed to pesticides. Comprehensive histories of pesticide use in terms of duration, frequency, and intensity of exposure were collected for a variety of different pesticides prior to the onset of cancer. Information on a number of other potential confounders including other occupational and lifestyle factors (i.e. smoking) was also collected. The participation rates at recruitment (82%) and follow-up (less than 2% lost to follow up) were very high. Self-reported pesticide use information has been found to be reliable in this cohort.36,37
However, it has been noted that 5% of users of imazethapyr have inaccurate information with respect to duration of use, or years of use. To address this issue we performed a sensitivity analysis excluding those subjects with total years that exceeded the number of years since first registration and found no difference is risk estimates. As in many occupational settings, applicators are not exposed to just one chemical agent; however, we were able to control for potential confounding from other pesticides by both adjusting for use of highly correlated pesticides and by using the lowest exposed groups as the referent for comparison. In addition, we were able to use detailed smoking history information to finely control for smoking status as this is an independent risk factor for bladder cancer in the AHS cohort.
A few limitations are worth noting. Although some exposure misclassification is inevitable, exposure information was obtained prior to onset of cancer and thus, misclassification is likely to be nondifferential with a resulting bias toward the null. While we did observe an increase in risk in the highest exposure category for leukemia, this result was not significant and has only few exposed cases, thus we are unable to make any definitive conclusions about potential risk. Similarly, we were unable to evaluate certain cancers due to small numbers of exposed cases. Continued follow-up of the cohort will aide in following up any suggestive findings for leukemia as well as evaluations for cancers that we could not assess at this time. Additionally, this analysis consists of predominantly white males potentially limiting generalizability to women and other race/ethnic groups.
Since imazethapyr first became available in the U.S. in 1989 consideration must be given to the relatively short duration of its exposure. Out of the 41 exposed bladder cancer cases, 37 were diagnosed in 1998 or later, approximately ten years from imazethapyr’s registration on the market. While this is still less than the 15-20 year latency expected for the development of solid tumors, further analyses of days of use, years of use, and intensity of use modeled separately all indicated that greater frequency, duration and intensity were associated with increased risks of cancer of the bladder and the colon lending consistency to the findings. We additionally explored the latency question by exploring an analysis where cases of bladder cancer diagnosed within approximately ten years of imazethapyr registration were excluded, thus we only included cases diagnosed in 1999 or later. Results from this sensitivity analysis indicate a similar and slightly larger magnitude of effect for imazethapyr on bladder cancer risk, RR in the highest exposure group compared with never users = 3.15 95% CI: 1.48, 6.69. An alternative explanation, however, could be that imazethapyr is acting as a promoter of the malignant phenotype rather than as a true initiator. Given that we observe an increased association with days per year as well as intensity for both cancers this promoting role could be plausible. To further explore this we attempted to consider a stratified analysis by smoking status for bladder cancer as this is a major established risk factor for this cancer. Due to small numbers, we were not able to due a true stratified analysis by smoking status (never, former, current), but when we excluded current smokers from analyses, the effect for imazethapyr and increased bladder cancer persisted in direction and magnitude thus still implicating this chemical as a potential initiator of carcinogenesis. Continued follow-up of the cohort will aide in further investigation of this alternative hypothesis.
In conclusion, we found a significantly increased risk of cancers of the bladder and colon among applicators using the aromatic amine pesticide, imazethapyr. However, there is still no biologic or experimental evidence that this pesticide is carcinogenic. These findings provide new evidence for the possible role of a widely used heterocyclic aromatic amine compound in the etiology of these cancers. Since a large portion of the world’s work force is estimated to be farmers, most of whom are regularly exposed to pesticides, this newly emerging class of HCA compounds deserve further examination in biologic and epidemiologic fields; the continued follow of this cohort to accrue more total cases and more exposed cases is necessary.
Novelty Statement: While several other aromatic amine compounds are well documented risk factors for bladder and colon cancer, we believe that our findings provide new evidence that exposure to an aromatic amine pesticide, imazethapyr, may be an overlooked exposure in the etiology of these cancers. We were able to use highly detailed exposure information on occupational pesticide exposure among pesticide applicators in the U.S. Agricultural Health study, which is one of the most comprehensive databases for this information worldwide.