New exposure biomarkers as tools for breast cancer epidemiology, biomonitoring, and prevention: a systematic approach based on animal evidence

BACKGROUND

Exposure to chemicals that cause rodent mammary gland tumors is common, but few studies have evaluated potential breast cancer risks of these chemicals in humans.

OBJECTIVE

The goal of this review was to identify and bring together the needed tools to facilitate the measurement of biomarkers of exposure to potential breast carcinogens in breast cancer studies and biomonitoring.

METHODS

We conducted a structured literature search to identify measurement methods for exposure biomarkers for 102 chemicals that cause rodent mammary tumors. To evaluate concordance, we compared human and animal evidence for agents identified as plausibly linked to breast cancer in major reviews. To facilitate future application of exposure biomarkers, we compiled information about relevant cohort studies.

RESULTS

Exposure biomarkers have been developed for nearly three-quarters of these rodent mammary carcinogens. Analytical methods have been published for 73 of the chemicals. Some of the remaining chemicals could be measured using modified versions of existing methods for related chemicals. In humans, biomarkers of exposure have been measured for 62 chemicals, and for 45 in a nonoccupationally exposed population. The Centers for Disease Control and Prevention has measured 23 in the U.S. population. Seventy-five of the rodent mammary carcinogens fall into 17 groups, based on exposure potential, carcinogenicity, and structural similarity. Carcinogenicity in humans and rodents is generally consistent, although comparisons are limited because few agents have been studied in humans. We identified 44 cohort studies, with a total of > 3.5 million women enrolled, that have recorded breast cancer incidence and stored biological samples.

CONCLUSIONS

Exposure measurement methods and cohort study resources are available to expand biomonitoring and epidemiology related to breast cancer etiology and prevention.

Benzene-induced cancers: abridged history and occupational health impact

Benzene-induced cancer in humans was first reported in the late 1920s. Carcinogenesis findings in animals were not reported conclusively until 1979. Industry exploited this "discrepancy" to discredit the use of animal bioassays as surrogates for human exposure experience. The cardinal reason for the delay between first recognizing leukemia in humans and sought-after neoplasia in animals centers on poor design and conduct of experimental studies. The first evidence of carcinogenicity in animals manifested as malignant tumors of the zymbal glands (sebaceous glands in the ear canal) of rats, and industry attempted to discount this as being irrelevant to humans, as this organ is vestigial and not present per se in humans. Nonetheless, shortly thereafter benzene was shown to be carcinogenic to multiple organ sites in both sexes of multiple strains and multiple species of laboratory animals exposed via various routes. This paper presents a condensed history of the benzene bioassay story with mention of benzene-associated human cancers.

Environmental pollutants, diet, physical activity, body size, and breast cancer: where do we stand in research to identify opportunities for prevention?

Breast cancer is the most common invasive cancer in women worldwide and the leading cause of death in US women in mid-life. Treatment has adverse effects, adding to the importance of finding modifiable risk factors. At the invitation of Susan G. Komen for the Cure, we reviewed studies of breast cancer and environmental pollutants, diet (assessed prospectively), body size, and physical activity, and animal studies that identify chemicals as potential mammary carcinogens. Databases developed in the review include information on 216 chemicals that increased mammary gland tumors in animal studies and 450 epidemiologic studies (accessible at www.silentspring.org/sciencereview and www.komen.org/environment). Exposure to potential mammary carcinogens is widespread from chemicals found in consumer products, air and drinking water pollution, food, and women's workplaces. Epidemiologic studies have included only a small number of chemicals identified as mammary carcinogens or as hormone disruptors, which may have implications for breast cancer; however, evidence is emerging for associations between breast cancer and polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and organic solvents. Prospective diet studies have not revealed consistent associations with breast cancer. Improved exposure assessment methods will help advance future human studies of both diet and environmental pollutants. Studies of physical activity show that it is protective. In the same vein as evidence-based medicine, messages for patients, policymakers, and the public should support decision-making based on the strength of current evidence; such messages might address exposure reduction for some pollutants. Investments in research on environmental factors in breast cancer have potentially large public health benefits.

Chemicals causing mammary gland tumors in animals signal new directions for epidemiology, chemicals testing, and risk assessment for breast cancer prevention

Identifying chemical carcinogens in animal studies is currently the primary means of anticipating cancer effects in humans. Animal studies to evaluate potential chemical carcinogenicity are particularly important for breast cancer because environmental and occupational epidemiologic research is sparse. Chemicals that increased mammary gland tumors in animal studies were compiled from the International Agency for Research on Cancer (IARC), the U.S. National Toxicology Program (NTP), and other sources. Summary assessments of the carcinogenic potential for each chemical and potentially exposed populations were also compiled. In all, 216 chemicals were identified that have been associated with increases in mammary gland tumors in at least 1 study. These include industrial chemicals, chlorinated solvents, products of combustion, pesticides, dyes, radiation, drinking water disinfection byproducts, pharmaceuticals and hormones, natural products, and research chemicals. Twenty-nine are produced in the U.S. at >1 million pounds/year; 35 are air pollutants, 25 have involved occupational exposures to >5000 women, and 73 have been present in consumer products or as contaminants of food. Thus, exposure is widespread. Nearly all of the chemicals were mutagenic and most caused tumors in multiple organs and species; these characteristics are generally believed to indicate likely carcinogenicity in humans. To our knowledge, this is the most comprehensive list developed of animal mammary gland carcinogens and, along with associated data, is publicly available at URL: www.silentspring.org/sciencereview and at URL: www.komen.org/environment. Valuable information from cancer bioassays is not well utilized in risk assessment and regulatory processes, suggesting a need to strengthen chemicals testing and risk assessment as tools for breast cancer prevention.