Cancer and occupational exposure to pesticides: an umbrella review.

PURPOSE: The aim was to identify the scope of the epidemiology literature reviewed regarding the risk of cancer as related to occupational exposure to pesticides and to compare regulatory toxicity results where feasible. METHODS: Review studies of breast, lung, prostate, non-Hodgkin lymphoma, and colorectal cancer were identified from the published literature from 2010 to 2020 using a priori inclusion and exclusion criteria. Epidemiology observations were first assessed and then compared against carcinogenicity profiles derived from regulatory toxicology studies. RESULTS: Several active ingredients were associated with specific cancer but overall, there was neither strong nor consistent epidemiologic data supportive of a positive association between pesticide exposure in occupational settings and cancer. Authors noted common themes related to the heterogeneity of exposure, study design, control for confounders, and the challenge to collect these data reliably and validly with an adequate sample size. Toxicology studies in laboratory animals that assessed carcinogenic potential did not reveal cancer outcomes that were concordant with reported epidemiologic findings. CONCLUSIONS: Farming and pesticides represent diverse exposures that are difficult to quantify in epidemiologic studies. Going forward, investigators will need creative and novel approaches for exposure assessment. Integration of epidemiologic and toxicological studies with attention to biological plausibility, mode of toxicological action and relevance to humans will increase the ability to better assess associations between pesticides and cancer.

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

1,3-Dichloropropene (1,3-D; CAS #542-75-6) is a fumigant used for preplant treatment of soil to control parasitic nematodes and manage soil borne diseases for numerous fruit, vegetable, field and tree and vine crops across diverse global agricultural areas. In the USA, 1,3-D has historically been classified by the U.S. EPA as likely to be carcinogenic to humans via both oral and inhalation routes. This classification for the oral route was primarily based upon increases in multiple tumor types observed in National Toxicology Program (NTP) cancer bioassays in rats and mice, while the classification for the inhalation route was based upon increased benign bronchioloalveolar adenomas in a mouse study conducted by The Dow Chemical Company. Based on U.S. EPA standard risk assessment methodologies, a low-dose linear extrapolation approach has been used to estimate risks to humans. Furthermore, genotoxicity associated with 1,3-D was historically considered a potential mode of action (MOA) for its tumorigenicity. New information is available and additional studies have been conducted that reveal a different picture of the tumorigenic potential of 1,3-D. These data and information include: (1) initial cancer studies by the NTP were conducted on an antiquated form of 1,3-D which contained a known mutagen/carcinogen, epichlorohydrin, as a stabilizer while current 1,3-D fumigants use epoxidized soybean oil (ESO) as the stabilizer; (2) results from two additional oral rodent cancer bioassays conducted on the modern form of 1,3-D became available and these two studies reveal a lack of carcinogenicity; (3) a newly conducted Big Blue study in F344 rats via the oral route further confirms that 1,3-D is not an in vivo genotoxicant; and (4) a newly conducted repeat dose inhalation toxicokinetic (TK) study shows that linear dose proportionality is observed below 30 ppm, which demonstrates the non-relevance of 60 ppm 1,3-D-induced benign lung tumors in mice for human health assessment. This weight of evidence review is organized as follows: (a) the TK of 1,3-D are presented because of relevant considerations when evaluating test doses/concentrations and reported findings of tumorigenicity; (b) the genotoxicity profile of 1,3-D is presented, including a contemporary study in order to put a possible genotoxicity MOA into perspective; (c) the six available bioassays are reviewed followed by (d) scientifically supported points of departure (PODs) and evaluation of human exposure for use in risk assessment. Through this assessment, all available data support the conclusion that 1,3-D is not a tumorigen at doses below 12.5 mg/kg bw/day via the oral route or at doses below 30 ppm via the inhalation route. These findings and clearly identified PODs show that a linear low dose extrapolation approach is not appropriate and a threshold-based risk assessment for 1,3-D is human health protective. Finally, in 2019, the Cancer Assessment Review Committee (CARC) reevaluated the carcinogenic potential of 1,3-D. In accordance with the EPA's Final Guidelines for Carcinogen Risk Assessment, the CARC classified 1,3-D (Telone) as "Suggestive Evidence of Carcinogenic Potential based on the presence of liver tumors by the oral route in male rats only." Given this finding, EPA stated that "quantification of human cancer risk is not required. The CARC recommends using a non-linear approach (i.e. reference dose (RfD)) that will adequately account for all chronic toxicity including carcinogenicity, that could result from exposure to 1,3-dichloropropene."

Correction to: Letter to the editor regarding "safety of safety evaluation of pesticides: developmental neurotoxicity of chlorpyrifos and chlorpyrifos-methyl" by Mie et al. (environmental health. 2018. 17:77).

Following publication of the original article.

Food and feed safety of DAS-444Ø6-6 herbicide-tolerant soybean.

DAS-444Ø6-6 soybean was genetically engineered (GE) to withstand applications of three different herbicides. Tolerance to glufosinate and glyphosate is achieved through expression of the phosphinothricin acetyltransferase (PAT) and double-mutated maize 5-enolpyruvyl shikimate-3-phosphate synthase (2mEPSPS) enzymes, respectively. These proteins are expressed in currently commercialized crops and represent no novel risk. Tolerance to 2,4-dichlorophenoxyacetic acid (2,4-D) is achieved through expression of the aryloxyalkanoate dioxygenase 12 (AAD-12) enzyme, which is novel in crops. The safety of the AAD-12 protein and DAS-444Ø6-6 event was assessed for food and feed safety based on the weight of evidence and found to be as safe as non-GE soybean.

No treatment-related effects with aryloxyalkanoate dioxygenase-12 in three 28-day mouse toxicity studies.

The aryloxyalkanoate dioxygenase-12 (AAD-12) protein is expressed in genetically modified soybean events DAS-68416-4 and DAS-444Ø6-6. Expression of the AAD-12 protein in soybeans confers tolerance to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) providing an additional herbicide choice to farmers. This enzyme acts by catalyzing the degradation of 2,4-D into herbicidally inactive metabolites. To meet evolving interpretation of regulations in the European Union, three separate 28-day repeat-dose oral mouse studies were conducted at increasing doses of up to 1100 mg AAD-12 protein/kg bw/day. No treatment-related effects were seen in any of these three studies.

Use of a probabilistic PBPK/PD model to calculate Data Derived Extrapolation Factors for chlorpyrifos.

A physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model combined with Monte Carlo analysis of inter-individual variation was used to assess the effects of the insecticide, chlorpyrifos and its active metabolite, chlorpyrifos oxon in humans. The PBPK/PD model has previously been validated and used to describe physiological changes in typical individuals as they grow from birth to adulthood. This model was updated to include physiological and metabolic changes that occur with pregnancy. The model was then used to assess the impact of inter-individual variability in physiology and biochemistry on predictions of internal dose metrics and quantitatively assess the impact of major sources of parameter uncertainty and biological diversity on the pharmacodynamics of red blood cell acetylcholinesterase inhibition. These metrics were determined in potentially sensitive populations of infants, adult women, pregnant women, and a combined population of adult men and women. The parameters primarily responsible for inter-individual variation in RBC acetylcholinesterase inhibition were related to metabolic clearance of CPF and CPF-oxon. Data Derived Extrapolation Factors that address intra-species physiology and biochemistry to replace uncertainty factors with quantitative differences in metrics were developed in these same populations. The DDEFs were less than 4 for all populations. These data and modeling approach will be useful in ongoing and future human health risk assessments for CPF and could be used for other chemicals with potential human exposure.

FutureTox III: Bridges for Translation.

Future Tox III, a Society of Toxicology Contemporary Concepts in Toxicology workshop, was held in November 2015. Building upon Future Tox I and II, Future Tox III was focused on developing the high throughput risk assessment paradigm and taking the science of in vitro data and in silico models forward to explore the question-what progress is being made to address challenges in implementing the emerging big-data toolbox for risk assessment and regulatory decision-making. This article reports on the outcome of the workshop including 2 examples of where advancements in predictive toxicology approaches are being applied within Federal agencies, where opportunities remain within the exposome and AOP domains, and how collectively the toxicology community across multiple sectors can continue to bridge the translation from historical approaches to Tox21 implementation relative to risk assessment and regulatory decision-making.

Toward Good Read-Across Practice (GRAP) guidance.

Grouping of substances and utilizing read-across of data within those groups represents an important data gap filling technique for chemical safety assessments. Categories/analogue groups are typically developed based on structural similarity and, increasingly often, also on mechanistic (biological) similarity. While read-across can play a key role in complying with legislations such as the European REACH regulation, the lack of consensus regarding the extent and type of evidence necessary to support it often hampers its successful application and acceptance by regulatory authorities. Despite a potentially broad user community, expertise is still concentrated across a handful of organizations and individuals. In order to facilitate the effective use of read-across, this document aims to summarize the state-of-the-art, summarizes insights learned from reviewing ECHA published decisions as far as the relative successes/pitfalls surrounding read-across under REACH and compile the relevant activities and guidance documents. Special emphasis is given to the available existing tools and approaches, an analysis of ECHA's published final decisions associated with all levels of compliance checks and testing proposals, the consideration and expression of uncertainty, the use of biological support data and the impact of the ECHA Read-Across Assessment Framework (RAAF) published in 2015.

Implementing a framework for integrating toxicokinetics into human health risk assessment for agrochemicals.

A strategic and comprehensive program in which toxicokinetic (TK) measurements are made for all agrochemicals undergoing toxicity testing (both new compounds and compounds already registered for use) is described. This approach provides the data to more accurately assess the toxicokinetics of agrochemicals and their metabolites in laboratory animals and humans. Having this knowledge provides the ability to conduct more insightful toxicity studies, refine and interpret exposure assessments and reduce uncertainty in risk assessments. By developing a better understanding of TK across species, including humans via in vitro metabolism studies, any differences across species in TK can be identified early and the most relevant species can be selected for toxicity tests. It also provides the ability to identify any non-linearities in TK as a function of dose, which in turn can be used to identify a kinetically derived maximum dose (KMD) and avoid dosing inappropriately outside of the kinetic linear range. Measuring TK in key life stages also helps to identify changes in ADME parameters from in utero to adults. A robust TK database can also be used to set internal concentration based "Reference Concentrations" and Biomonitoring Equivalents (BE), and support selection of Chemical Specific Adjustment Factors (CSAF). All of these factors support the reduction of uncertainty throughout the entire risk assessment process. This paper outlines how a TK research strategy can be integrated into new agrochemical toxicity testing programs, together with a proposed Framework for future use.

Improving weight of evidence approaches to chemical evaluations.

Federal and other regulatory agencies often use or claim to use a weight of evidence (WoE) approach in chemical evaluation. Their approaches to the use of WoE, however, differ significantly, rely heavily on subjective professional judgment, and merit improvement. We review uses of WoE approaches in key articles in the peer-reviewed scientific literature, and find significant variations. We find that a hypothesis-based WoE approach, developed by Lorenz Rhomberg et al., can provide a stronger scientific basis for chemical assessment while improving transparency and preserving the appropriate scope of professional judgment. Their approach, while still evolving, relies on the explicit specification of the hypothesized basis for using the information at hand to infer the ability of an agent to cause human health impacts or, more broadly, affect other endpoints of concern. We describe and endorse such a hypothesis-based WoE approach to chemical evaluation.

Derivation of human Biomonitoring Guidance Values for chlorpyrifos using a physiologically based pharmacokinetic and pharmacodynamic model of cholinesterase inhibition.

A number of biomonitoring surveys have been performed for chlorpyrifos (CPF) and its metabolite (3,5,6-trichloro-2-pyridinol, TCPy); however, there is no available guidance on how to interpret these data in a health risk assessment context. To address this gap, Biomonitoring Guidance Values (BGVs) are developed using a physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model. The PBPK/PD model is used to predict the impact of age and human variability on the relationship between an early marker of cholinesterase (ChE) inhibition in the peripheral and central nervous systems [10% red blood cell (RBC) ChE inhibition] and levels of systemic biomarkers. Since the PBPK/PD model characterizes variation of sensitivity to CPF in humans, interspecies and intraspecies uncertainty factors are not needed. Derived BGVs represent the concentration of blood CPF and urinary TCPy associated with 95% of the population having less than or equal to 10% RBC ChE inhibition. Blood BGV values for CPF in adults and infants are 6100 ng/L and 4200 ng/L, respectively. Urinary TCPy BGVs for adults and infants are 2100 µg/L and 520 µg/L, respectively. The reported biomonitoring data are more than 150-fold lower than the BGVs suggesting that current US population exposures to CPF are well below levels associated with any adverse health effect.

Lessons learned, challenges, and opportunities: the U.S. Endocrine Disruptor Screening Program.

In 1996, the U.S. Congress passed the Food Quality Protection Act and amended the Safe Drinking Water Act (SDWA) requiring the U.S. Environmental Protection Agency (EPA) to implement a screening program to investigate the potential of pesticide chemicals and drinking water contaminants to adversely affect endocrine pathways. Consequently, the EPA launched the Endocrine Disruptor Screening Program (EDSP) to develop and validate estrogen, androgen, and thyroid (EAT) pathway screening assays and to produce standardized and harmonized test guidelines for regulatory application. In 2009, the EPA issued the first set of test orders for EDSP screening and a total of 50 pesticide actives and 2 inert ingredients have been evaluated using the battery of EDSP Tier 1 screening assays (i.e., five in vitro assays and six in vivo assays). To provide a framework for retrospective analysis of the data generated and to collect the insight of multiple stakeholders involved in the testing, more than 240 scientists from government, industry, academia, and non-profit organizations recently participated in a workshop titled "Lessons Learned, Challenges, and Opportunities: The U.S. Endocrine Disruptor Screening Program." The workshop focused on the science and experience to date and was organized into three focal sessions: (a) Performance of the EDSP Tier 1 Screening Assays for Estrogen, Androgen, and Thyroid Pathways; (b) Practical Applications of Tier 1 Data; and (c) Indications and Opportunities for Future Endocrine Testing. A number of key learnings and recommendations related to future EDSP evaluations emanated from the collective sessions.

Chlorpyrifos: weight of evidence evaluation of potential interaction with the estrogen, androgen, or thyroid pathways.

Chlorpyrifos was selected for EPA's Endocrine Disruptor Screening Program (EDSP) based on widespread use and potential for human and environmental exposures. The purpose of the program is to screen chemicals for their potential to interact with the estrogen, androgen, or thyroid pathways. A battery of 11 assays was completed for chlorpyrifos in accordance with test guidelines developed for EDSP Tier 1 screening. To determine potential endocrine activity, a weight-of-evidence (WoE) evaluation was completed for chlorpyrifos, which included the integration of EDSP assay results with data from regulatory guideline studies and the published literature. This WoE approach was based on the OECD conceptual framework for testing and assessment of potential endocrine-disrupting chemicals and consisted of a systematic evaluation of data, progressing from simple to complex across multiple levels of biological organization. The conclusion of the WoE evaluation is that chlorpyrifos demonstrates no potential to interact with the estrogen, androgen, or thyroid pathways at doses below the dose levels that inhibit cholinesterase. Therefore, regulatory exposure limits for chlorpyrifos, which are based on cholinesterase inhibition, are sufficient to protect against potential endocrine alterations. Based on the results of this WoE evaluation, there is no scientific justification for pursuing additional endocrine testing for chlorpyrifos.

Acetylcholinesterase inhibition dose-response modeling for chlorpyrifos and chlorpyrifos-oxon.

This paper evaluates new data for cholinesterase inhibition with chlorpyrifos (CPF). Marty et al. (2012) recently conducted a CPF cholinesterase inhibition study in rats that included testing of males and females, dosing by gavage or diet, administration in corn oil or milk, and with pups and adults. Additionally, the study included cholinesterase inhibition testing for CPF-oxon, the active moiety that inhibits cholinesterase. The study included 5-6 dose groups with eight animals/sex/group for most of the tests. This paper provides a benchmark dose (BMD) analysis of the data from Marty et al. (2012), including a BMD meta-analysis that includes CPF cholinesterase inhibition data from different assays within the Marty et al. (2012) study and, in one case, from another study. From the meta-analysis, the recommended BMD(10)s, based on brain acetylcholinesterase inhibition, are 1.7 mg/kg/day (BMDL10 = 1.3mg/kg/day) for acute doses to children and adults, and 0.67 mg/kg/day (BMDL10 = 0.53 mg/kg/day) for repeat doses to children and adults. At the dose levels considered in this analysis, there was no evidence of a difference in responses between males and females, corn oil versus milk administration, or pups versus adults. The data on pups versus adults show that an extra safety factor to protect the young is not needed for CPF. CPF data from the literature suggest that brain cholinesterase inhibition is the most appropriate metric for cholinesterase inhibition risk assessment.

Identification and characterization of adverse effects in 21st century toxicology.

The practice of toxicology is changing rapidly, as demonstrated by the response to the 2007 NRC report on "Toxicity Testing in the 21(st) Century." New assays are being developed to replace animal testing; yet the use of data from these assays in decision making is not clear. A Health and Environmental Sciences Institute committee held a May 2011 workshop to discuss approaches to identifying adverse effects in the context of the NRC report. Scientists from industry, government, academia, and NGOs discussed two case studies and explored how information from new, high data content assays developed for screening can be used to differentiate adverse effects from adaptive responses. The terms "adverse effect" and "adaptive response" were defined, as well as two new terms, the relevant pathways of toxicological concern (RPTCs) and relevant responses for regulation (RRRs). RPTCs are biochemical pathways associated with adverse events and need to be elucidated before they are used in regulatory decision making. RRRs are endpoints that are the basis for risk assessment and may or may not be at the level of pathways. Workshop participants discussed the criteria for determining whether, at the RPTC level, an effect is potentially adverse or potentially indicative of adaptability, and how the use of prototypical, data-rich compounds could lead to a greater understanding of RPTCs and their use as RRRs. Also discussed was the use of RPTCs in a weight-of-evidence approach to risk assessment. Inclusion of data at this level could decrease uncertainty in risk assessments but will require the use of detailed dosimetry and consideration of exposure context and the time and dose continuum to yield scientifically based decisions. The results of this project point to the need for an extensive effort to characterize RPTCs and their use in risk assessment to make the vision of the 2007 NRC report a reality.

Acute and 28-day repeated dose toxicology studies in mice with aryloxyalkanoate dioxygenase (AAD-1) protein expressed in 2,4-D tolerant DAS-40278-9 maize.

DAS-40278-9 maize (corn) plants have been genetically modified by the insertion of the aad-1 gene (aryloxyalkanoate dioxygenase), which confers tolerance to 2,4-dichlorophenoxyacetic acid (2,4-D) and aryloxyphenoxypropionate (AOPP) acetyl coenzyme A carboxylase (ACCase) inhibitors ("fop" herbicides) to enable the effective use of these herbicides on maize. The aad-1 gene, derived from Sphingobium herbicidovorans, encodes the aryloxyalkanoate dioxygenase (AAD-1) enzyme. As part of the safety assessment of the AAD-1 protein expressed in maize, acute and repeated dose mammalian toxicology studies were conducted. AAD-1 protein (heterologously produced) was orally administered to mice at a dose of 2000mg/kg, and no acute lethality or adverse effects were observed. Similarly, no adverse effects were observed in mice in a 28-day repeated-dose dietary toxicity study that incorporated the AAD-1 protein into diets at concentrations up to 1000-fold greater than the highest estimate of human exposure to maize. These results support the conclusion that the AAD-1 protein, as expressed in biotechnology derived DAS-40278-9 maize, represents a negligible risk to human health.

Performance of broiler chickens fed diets containing DAS-68416-4 soybean meal.

Broiler chickens are a fast growing monogastric animal commonly used to evaluate the equivalence between transgenic and non-transgenic grains as part of the human safety assessment process. While commonly viewed like other livestock feeding trials, such studies are performed with transgenic crops with input traits (that are not designed to improve nutrition) to aid regulatory authorities in evaluating safety. Studies of this type are actually more similar to toxicology studies in purpose, with sensitive endpoints like growth used to detect metabolic perturbations. DAS-68416-4 soybean expresses the aryloxyalkanoate dioxygenase-12 (AAD-12) enzyme which inactivates 2,4-diclorophenoxyacetic acid (2,4-D) and provides DAS-68416-4 soybeans tolerance to this herbicide. DAS-68416-4 also expresses the phosphinothricin acetyltransferase (PAT) enzyme from Streptomyces viridochromogenes which confers tolerance to glufosinate-ammonium herbicides. A 6-week broiler study was conducted with diets containing toasted DAS-68416-4 soybean meal (40, 36, and 32% in starter, grower and finisher diets, respectively) to evaluate nutritional wholesomeness and safety compared with conventional comparators. Toasting soybean meal is required to inactivate endogenous antinutrients making soybean suitable for consumption by monogastric animals like broiler chickens. Toasting was found to denature both the AAD-12 and PAT proteins rendering them non-detectable by enzyme linked immunosorbent assays. Broiler growth and performance parameters were measured over a 6-week period of exposure to diets containing different sources of toasted soybean meal, and results indicate that DAS-68416-4 soybean is nutritionally equivalent to non-transgenic soybean.