Weight of evidence analysis of the tumorigenic potential of 1,3-dichloropropene supports a threshold-based risk assessment

Assessing the genotoxicity of N-nitrosodiethylamine with three in vivo endpoints in male Big Blue® transgenic and wild-type C57BL/6N mice

The detection of N-nitrosamines in drug products has raised global regulatory interest in recent years due to the carcinogenic potential of some nitrosamines in animals and a need to identify a testing strategy has emerged. Ideally, methods used would allow for the use of quantitative analysis of dose-response data from in vivo genotoxicity assays to determine a compound-specific acceptable intake for novel nitrosamines without sufficient carcinogenicity data. In a previous study we compared the dose-response relationships of N-nitrosodiethylamine (NDEA) in three in vivo genotoxicity endpoints in rats. Here we report a comparison of NDEA's genotoxicity profile in mice. Big Blue® mice were administered NDEA at doses of 0.001, 0.01, 0.1, 1 and 3 mg/kg/day by oral gavage for 28 days followed by 3 days of expression. Statistically significant increases in the NDEA induced mutations were detected by both the transgenic rodent mutation assay (TGR) using the cII endpoint and by duplex sequencing in the liver but not bone marrow of mice. In addition, administration of NDEA for two consecutive days in male C57BL/6N mice caused elevated DNA damage levels in the liver as measured by % tail DNA in comet assay. The benchmark dose (BMD) analysis shows a BMDL50 of 0.03, 0.04 and 0.72 mg/kg/day for TGR, duplex sequencing and comet endpoints, respectively. Overall, this study demonstrated a similar genotoxicity profile of NDEA between mice and rats and provides a reference that can be used to compare the potential potency of other novel nitrosamines for the induction of gene mutations.

Increasing confidence in new approach methodologies for inhalation risk assessment with multiple end point assays using 5-day repeated exposure to 1,3-dichloropropene

New Approach Methodologies (NAMs) are being widely used to reduce, refine, and replace, animal use in studying toxicology. For respiratory toxicology, this includes both in silico and in vitro alternatives to replace traditional in vivo inhalation studies. 1,3-Dichloropropene (1,3-DCP) is a volatile organic compound that is widely used in agriculture as a pre-planting fumigant. Short-term exposure of humans to 1,3-DCP can result in mucous membrane irritation, chest pain, headache, and dizziness. In our previous work, we exposed differentiated cells representing different parts of the respiratory epithelium to 1,3-DCP vapor, measured cytotoxicity, and did In Vitro to In Vivo Extrapolation (IVIVE). We have extended our previous study with 1,3-DCP vapors by conducting transcriptomics on acutely exposed nasal cultures and have implemented a separate 5-day repeated exposure with multiple endpoints to gain further molecular insight into our model. MucilAir™ Nasal cell culture models, representing the nasal epithelium, were exposed to six sub-cytotoxic concentrations of 1,3-DCP vapor at the air-liquid interface, and the nasal cultures were analyzed by different methodologies, including histology, transcriptomics, and glutathione (GSH) -depletion assays. We observed the dose-dependent effect of 1,3-DCP in terms of differential gene expression, change in cellular morphology from pseudostratified columnar epithelium to squamous epithelium, and depletion of GSH in MucilAir™ nasal cultures. The MucilAir™ nasal cultures were also exposed to 3 concentrations of 1,3-DCP using repeated exposure 4 h per day for 5 days and the histological analyses indicated changes in cellular morphology and a decrease in ciliated bodies and an increase in apoptotic bodies, with increasing concentrations of 1,3-DCP. Altogether, our results suggest that sub-cytotoxic exposures to 1,3-DCP lead to several molecular and cellular perturbations, providing significant insight into the mode-of-action (MoA) of 1,3-DCP using an innovative NAM model.

In vitro and in vivo assessments of the genotoxic potential of 3-chloroallyl alcohol

3-Chloroallyl alcohol (3-CAA) can be found in the environment following the application of plant protection products. 3-CAA is formed in groundwater following the injection of 1,3-dichloropropene, a fumigant used to control nematodes. 3-CAA is also formed, in leafy crops, as a glycoside conjugate following application of the herbicide, clethodim. Human exposure may occur from groundwater used as drinking water or through dietary consumption. To characterize 3-CAA's potential to cause genotoxicity in mammals, in vitro and in vivo studies were conducted. 3-CAA was negative in an Ames test and positive in a mouse lymphoma forward mutation assay. 3-CAA was negative in an acute in vivo CD-1 mouse bone marrow micronucleus assay when administered up to a dose level of 125 mg/kg/day for two consecutive days. In a combined gene mutation assay and erythrocyte micronucleus assay, using transgenic Big Blue® Fischer 344 rats, 3-CAA was administered via drinking water at targeted dose levels of 0, 10, 30, and 100 mg/kg/day for 29 days. Peripheral blood samples, collected at the end of treatment, were analyzed for micronucleus induction in reticulocytes using flow cytometry. Liver and bone marrow samples, collected 2 days after the termination of the treatment, were analyzed for the induction of mutations at the cII locus. 3-CAA did not induce an increase in mutant frequency or micronuclei under the experimental conditions. In conclusion, the mutagenic response observed in the in vitro mouse lymphoma assay is not confirmed in the whole animal. 3-CAA is not considered to pose a mutagenic risk.

An Ecologic Study of the Association between 1,3-Dichloropropene and Pancreatic Cancer

BACKGROUND

1,3-Dichloropropene (1,3-D) is a soil fumigant that is used to protect fruit, vegetable, field, tree, and vine crops from nematode infestation and soil borne diseases. It is a commonly use pesticide, is applied by either direct injection into the soil or drip irrigation and is highlight volatile. Though currently classified as "Suggestive Evidence of Carcinogenic Potential", the literature in animal-based studies has inconsistent results and there is limited research among a human population with one study only among the California population. The purpose of the current analysis is to conduct a state-level analysis of the association between 1,3-D and pancreatic cancer mortality.

METHODS

Data for this ecological study were derived from death certificate data (for pancreatic mortality) from 1999 to 2020 and United States Geologic Survey National Water-Quality Assessment project for years 1992-2016 (1,3-D use). A negative binomial regression adjusted for selected lifestyle risk factors of pancreatic cancer (i.e., obesity, alcohol use, and smoking prevalence) estimated rate ratios (RRs) and associated 95% confidence intervals (CIs)for the association between 1,3-D quartiles and pancreatic cancer mortality rate. Models lagged in five-year increments to account for the induction period of pancreatic cancer.

RESULTS

Overall, there was no association between 1,3-D quartile and pancreatic cancer mortality rate; however, limiting the analyses to states reporting 1,3-D use for at least 20 years, the highest quartile of 1,3-D use was associated with an 11% increase in the pancreatic cancer mortality rate in the five-year lagged model (RR 1.11, 95% CI 1.06-1.16). This association was consistent across the other lag periods.

CONCLUSIONS

Accounting for lifestyle factors associated with pancreatic cancer risk, there is a significantly increase rate of pancreatic cancer mortality among states that have the highest quartile of 1,3-D use and have been using 1,3-D for a long-term period.

Investigation of Relationships Between the Geospatial Distribution of Cancer Incidence and Estimated Pesticide Use in the U.S. West

The objective of the study was to evaluate the potential geospatial relationship between agricultural pesticide use and two cancer metrics (pediatric cancer incidence and total cancer incidence) across each of the 11 contiguous states in the Western United States at state and county resolution. The pesticide usage data were collected from the U.S. Geological Survey Pesticide National Synthesis Project database, while cancer data for each state were compiled from the National Cancer Institute State Cancer Profiles. At the state spatial scale, this study identified a significant positive association between the total mass of fumigants and pediatric cancer incidence, and also between the mass of one fumigant in particular, metam, and total cancer incidence (P-value < 0.05). At the county scale, the relationship of all cancer incidence to pesticide usage was evaluated using a multilevel model including pesticide mass and pesticide mass tertiles. Low pediatric cancer rates in many counties precluded this type of evaluation in association with pesticide usage. At the county scale, the multilevel model using fumigant mass, fumigant mass tertiles, county, and state predicted the total cancer incidence (R-squared = 0.95, NSE = 0.91, and Sum of square of residuals [SSR] = 8.22). Moreover, this study identified significant associations between total fumigant mass, high and medium tertiles of fumigant mass, total pesticide mass, and high tertiles of pesticide mass relative to total cancer incidence across counties. Fumigant application rate was shown to be important relative to the incidence of total cancer and pediatric cancer, at both state and county scales.

Peer review of a cancer weight of evidence assessment based on updated toxicokinetics, genotoxicity, and carcinogenicity data for 1,3-dichloropropene using a blinded, virtual panel of experts

A cancer weight of evidence (WOE) analysis based on updated toxicokinetics, genotoxicity, and carcinogenicity data for 1,3-dichloropropene was peer reviewed by a panel of experts. Historically, 1,3-dichloropropene has been classified in the U.S. as "likely to be carcinogenic to humans" via oral and inhalation exposure routes based upon the results of rodent cancer bioassays conducted in the 1980s. Contemporary studies led the authors of the WOE analysis to conclude that the currently manufactured form of 1,3-dichloropropene is not mutagenic and not carcinogenic below certain doses, pointing to a threshold-based approach for cancer risk assessment. SciPinion conducted a peer review of the WOE analysis using methods for assembling and managing blinded expert panels that maximize expertise while minimizing potential selection/participation bias. The process was implemented through a web-based application that poses a series of questions soliciting the experts' scientific opinions and observations about specific topics. The goal of the peer review was to have experts provide conclusions about the WOE for carcinogenicity classification of 1,3-dichloropropene, identify potential data gaps, and evaluate the validity of a threshold-based risk assessment for 1,3-dichloropropene. Based on a robust peer review of the current scientific information, a cancer WOE classification of "not likely to be carcinogenic to humans" is best supported for 1,3-dichloropropene. This conclusion is reached with a high degree of consensus (consensus score = 0.92) across expert panel members.