Atrazine and Breast Cancer: A Framework Assessment of the Toxicological and Epidemiological Evidence

James W. Simpkins 3 James A. Swenberg 0 Noel Weiss 2 5 David Brusick 4 J. Charles Eldridge jj James T. Stevens jj Robert J. Handa jjj Russell C. Hovey jjjj Tony M. Plant 1 Timothy P. Pastoor 3 Charles B. Breckenridge 3 0 Environmental Sciences and Engineering, University of North Carolina , Chapel Hill, North Carolina 27599 1 Department of Obstetrics, Gynecology and Reproductive Sciences, University of Pittsburgh School of Medicine, Magee Women's Research Institute , Pittsburgh, Pennsylvania 15213 2 Department of Epidemiology, University of Washington , Seattle, Washington 98195 3 Department of Pharmacology and Neurosciences, University of North Texas Health Science Center , Fort Worth, Texas 76107 4 123 Moody Creek Road, Bumpass, Virginia 23024 5 The Fred Hutchinson Cancer Research Center , Seattle, Washington 98109 The causal relationship between atrazine exposure and the occurrence of breast cancer in women was evaluated using the framework developed by Adami et al. (2011) wherein biological plausibility and epidemiological evidence were combined to conclude that a causal relationship between atrazine exposure and breast cancer is ''unlikely''. Carcinogenicity studies in female Sprague-Dawley (SD) but not Fischer-344 rats indicate that high doses of atrazine caused a decreased latency and an increased incidence of combined adenocarcinoma and fibroadenoma mammary tumors. There were no effects of atrazine on any other tumor type in male or female SD or Fischer-344 rats or in three strains of mice. Seven key events that precede tumor expression in female SD rats were identified. Atrazine induces mammary tumors in aging female SD rats by suppressing the luteinizing hormone surge, thereby supporting a state of persistent estrus and prolonged exposure to endogenous estrogen and prolactin. This endocrine mode of action has low biological plausibility for women because women who undergo reproductive senescence have low rather than elevated levels of estrogen and prolactin. Four alternative modes of action (genotoxicity, estrogenicity, upregulation of aromatase gene expression or delayed mammary gland development) were considered and none could account for the tumor response in SD rats. Epidemiological studies provide no support for a causal relationship between atrazine exposure and breast cancer. This conclusion is consistent with International Agency for Research on Cancer's classification of atrazine as ''unclassifiable as to carcinogenicity'' and the United States Environmental Protection Agency's classification of atrazine as ''not likely to be carcinogenic.'' - Toxicology is rapidly being transformed from a descriptive science to one capable of prediction. Central to this progress is the understanding of key genomic, proteomic, biochemical, physiological, and pathological events on the pathway from chemical exposure to the expression of toxicity. The succession of key events following initial exposure, through intermediate states and ultimately to measurable adverse outcomes has been called a mode of action (Meek et al., 2003; Sonich-Mullin et al., 2001; USEPA, 2005). When the sequence of events is understood at a fundamental level of chemical-cell molecular interaction, the mode of action becomes a mechanism of toxicity. Mechanisms of toxicity are often postulated but in fact are rarely established. In the expression of toxicity, it is not uncommon for different facets of toxicity to be triggered at different dose levels (Slikker et al., 2004a,b) or through different mechanisms. Furthermore, differences between species with respect to absorption, distribution, metabolism, elimination, and target-specific susceptibility may render the extrapolation from in vitro models or even from in vivo animal data to humans difficult. To accommodate the complexity of evaluating a mode of action, systematic approaches have been developed to evaluate the relevance of findings in animal studies to humans (Meek et al., 2003). Three key questions are: (1) Is there sufficient evidence in animal studies to propose a mode of action? (2) Is that mode of action operative in humans? and (3) Is the mode of action relevant to humans after considering differences between species with respect to pharmacokinetic and toxicodynamic factors operative at expected levels of human exposures? Ultimately, the question of whether humans display toxicity following exposure to the chemical through ingestion, inhalation, or dermal contact can best be ascertained from observational epidemiology. Although there is a long history of interpreting epidemiology studies, only recently have methods been developed to systematically integrate animal data with data from observational epidemiology (Adami et al., 2011; European Center for Ecotoxicology and Toxicology of Chemicals, 2009; Swaen, 2006). This paper presents a case study utilizing the methodology described by Adami et al. (2011) wherein toxicological and epidemiological evidence were combined in a systematic framework to conclude whether a causal relationship exists between atrazine exposure and breast cancer in humans. Mode of action research using animal models (Brusick, 1994; Cooper et al., 2007; Eldridge and Wetzel, 2008; Stevens et al., 1994; Yi, Simpkins, and Breckenridge, in preparation) and epidemiology studies on breast cancer in women were evaluated. Breast cancer was selected for this case study because there were many mode of action research studies and a number of epidemiological studies on breast cancer that have been previously evaluated by regulatory authorities as part of a comprehensive cancer risk assessment (Australian Pesticides and Veterinary Medicines Authority [APVMA], 2004, 2008; Food and Agricultural Organization of the United Nations and the World Health Organization [FAO/WHO], 2009; USEPA, 2003b, 2006). A detailed review of the association between triazine exposure and any cancer has also just recently been published (Sathiakumar et al., 2011). MATERIALS AND METHODS The five-step method outlined by Adami et al. (2011) was followed including (1) collection of all relevant studies, (2) assessment of quality, (3) evaluation of the weight of evidence, (4) assignment of a scalable conclusion, and (5) placement on a causal relationship grid. All relevant toxicological and epidemiological studies were identified and study quality was assessed according to guidelines for evaluating toxicological (USEPA, 1993, 2001, 2003a) and epidemiology studies (London Principles, 1995: von Elm, 2007). Studies that characterize the effect of atrazine on the latency and the incidence of adenocarcinoma and fibroadenoma in the mammary glands of female Sprague-Dawley (SD) rats were summarized (Cooper et al., 2007; Eldridge and Wetzel, 2008; Stevens et al., 1994, 1999). Studies relevant to the proposed mode of action underlying the effect of atrazine on mammary tumors were evaluated and a concordance analysis was conducted. The framework analysis als (...truncated)