Statistical procedures to test for linearity and estimate threshold doses for tumor induction with nonlinear dose-response relationships in bioassays for carcinogenicity

Sublinear shapes of the dose-response curve in the low-dose range of toxicity testing are often postulated to be indicative of a no-effect threshold. We present statistical procedures to test sublinear dose responses in bioassays for carcinogenicity against the hypothesis of linearity and estimate a lower confidence limit for the dose at the postulated breakpoint. First, a control tumor incidence of 0 is assumed. Tumor incidence at dose 1 is allowed to range from 0 to 4 tumor-bearing animals (TBAs) in groups of 50 animals, dose 2 is assumed to result in a tumor incidence of 5-25 TBAs. The null hypothesis of a linear dose response is tested by (i) the likelihood ratio (LR) test and (ii) the minimum chi(2) (MC) method. Validation by simulation showed the MC method to be more conservative than the LR test. At the 5% level with MC, the following observed numbers of TBAs for the dose sequence 0-1-2 resulted in rejection of the hypothesis of linearity: 0-0-6, 0-1-10, 0-2-13, 0-3-16, 0-4-18. Second, the analysis was adapted to allow for a control tumor incidence of 0-4 TBAs/50 and a tumor incidence of 0-10 TBAs/50 at dose 1, and the minimum number of TBAs at dose 2 to reject linearity at the 5% level was calculated. Third, a program is made available to analyze data derived from protocols that include nonstandard dose span and group size. Internet access to the respective statistics software and source file is provided. Examples for nasal tumor induction by formaldehyde and for the induction of renal adenocarcinoma by ochratoxin A are shown. The proposed analysis may be useful to test sublinear sections of the dose response for the possibility of a threshold for carcinogens and to define dose levels that could be used as a starting point for setting exposure standards.     

Screening for in Vivo (Anti)estrogenic and (Anti)androgenic Activities of Technical and Formulated Deltamethrin

Concern over the possibility of hormonal systems being disrupted by chemicals in the environment has led to the development of assays to detect such chemicals. Technical and formulated deltamethrin, a widely used synthetic pyrethroid pesticide, was screened for (anti)estrogenic and (anti)androgenic activities by means of two major in vivo short-term screening tests: the uterotrophic and Hershberger assays. The results reported here indicate the absence of in vivo (anti)estrogenic and (anti)androgenic activities for both technical and formulated deltamethrin at the dose levels tested (2.0 and 4.0 mg/kg/day). However, further tests, including in vitro screening assays, are required to characterize fully the lack of endocrine action of these chemicals.     

Dermal absorption in rhesus monkeys of polychlorinated biphenyls from soil contaminated with Aroclor 1260

Human health risk assessments involving contaminated soil include dermal absorption as a potential pathway contributing to the total exposure burden. For PCB-contaminated soil, the U.S. Environmental Protection Agency uses a dermal absorption factor of 14%, based on a 1993 study of dermal absorption in rhesus monkeys. The current study examined several parameters that can influence the dermal absorption of lipophilic hydrocarbons, including soil organic content, particle size, skin residence time, and contaminant "aging" in the soil. Four groups of four female rhesus monkeys each were exposed to radiolabeled Aroclor 1260 either intravenously (100% absorption) or dermally with PCB-spiked soil. Groups exposed for 12 or 24 h to PCBs aged in soil exhibited percutaneous absorption values of 3.43 +/- 0.35 and 4.26 +/- 0.52%, respectively, while a group exposed for 24 h to soil freshly spiked with PCBs exhibited a dermal absorption value of 4.07 +/- 0.46%. Evidence strongly suggests that the factor most responsible for modulating the percutaneous absorption of highly lipophilic compounds from soil is its organic content. The base soil used in the current study with Aroclor 1260 had an organic content of 5-6% (< or =2 mm particle fraction), a value typical for U.S. soil. The organic content of the soil applied to the skin was 8.7% (<150 microm particle fraction), a value that contrasts sharply with the soil containing 0.9% organics used in the 1993 study with Aroclors 1242 and 1254 that produced a dermal absorption value of 14% for PCBs.     

Prediction of local irritant effects after repeated dermal and respiratory exposure to chemicals

Health risks resulting from occupational exposure to chemicals are controlled by the establishment of acceptable dermal and respiratory exposure levels. Due to a lack of route-specific toxicity data, acceptable levels are frequently established by means of route-to-route extrapolation. A pitfall in route-to-route extrapolation is the occurrence of local effects. Often, the results of acute irritation studies are used to assess the likelihood of the occurrence of local effects also following repeated exposure and thereby the validity of route-to-route extrapolation. We questioned this working practice and considered whether local effects observed in a given study are of any predictive value with respect to the occurrence of local effects after repeated exposure. Our database analysis indicates that substances inducing skin and/or eye irritation frequently induce local effects after repeated respiratory exposure. In contrast, observations made in any type of study show little or no positive predictive value for the occurrence of local effects after repeated dermal exposure. Notably, the absence of any indication of local effects in any type of study does not exclude the occurrence of local effects on repeated dermal or respiratory exposure. We conclude that the presumed reliability of route-to-route extrapolation in the absence of route-specific toxicity data can be questioned.     

Effects of Low Levels of Ciprofloxacin on a Chemostat Model of the Human Colonic Microflora

To study the utility of an in vitro model system for assessing the effect of low concentrations of a fluoroquinolone (FQ) drug on the ecology of the human intestinal microflora, chemostats containing human fecal flora were exposed to 0.43, 4.3, and 43microg of ciprofloxacin (CI) per milliliter. Prior to and during drug exposure, we assayed short-chain fatty acids (SCFA), bacterial populations, and the relative levels of susceptibility of these populations to CI and trovafloxacin (TV), a newer related FQ with increased activity against anaerobes. The degree to which CI affected the chemostat ecology was measured statistically by comparing observed data with the corresponding predicted "no effect" level. No changes in total SCFA were observed; only butyrate was significantly higher at the intermediate and high-dose levels. Enterococci counts and the levels of susceptibility to CI among enterococci were also unaffected. Escherichia coli counts decreased in a dose-dependent manner. Susceptibility levels in E. coli followed no interpretable pattern. Bacteroides fragilis group (BfG) counts decreased significantly following exposure to 43 and 4.3microg/mL CI. Ciprofloxacin susceptibility among the BfG in these chemostats was not determined because the BfG counts were too low (less than 30 colonies per plate) when undiluted chemostat samples were plated. However, within 2 days of exposure to 0.43microg/mL CI, the percentage of BfG resistant to 4microg/mL CI increased to over 95%. Before exposure, all BfG were susceptible to both CI (2microg/mL) and TV (0.25microg/mL). All BfG isolated during exposure were resistant to both CI (4microg/mL) and TV (2microg/mL). Resistance selection in the BfG was unexpected as the MIC(90) of CI for B. fragilis is 8microg/mL. Since the average colon flora is about 20% B. fragilis and other bacteroides, CI may impact the human gut flora even at subtherapeutic levels.     

ICCVAM Evaluation of the Murine Local Lymph Node Assay: II. Conclusions and Recommendations of an Independent Scientific Peer Review Panel

The validation status of the murine local lymph node assay (LLNA), a method for assessing the allergic contact dermatitis potential of chemicals, was evaluated by an independent peer review panel (Panel) convened by the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM). The LLNA measures lymphocyte proliferation using incorporation of radioactive thymidine or iododeoxyuridine into cells of the draining lymph nodes of mice topically exposed to a test article. The Panel concluded that the assay performed as well as currently accepted guinea pig methods [guinea pig maximization test (GPMT)/Buehler assay (BA)] for the hazard identification of strong to moderate chemical sensitizing agents, but that it might not correctly identify all weak sensitizers or metals (potential false negative response) or all strong irritants (potential false positive response). The Panel concluded also that the LLNA involves less pain and distress than conventional guinea pig methods. The Panel unanimously recommended the LLNA as a stand-alone alternative for contact sensitization hazard assessment, provided that certain protocol modifications were made. These included collection of individual, rather than pooled, animal response data; the inclusion of a concurrent positive control; and consideration of dose–response information and statistical analyses. A standardized LLNA protocol is provided.     

Consumer exposure to fragrance ingredients: providing estimates for safety evaluation

To fully apply already published procedures for the safety evaluation of fragrance ingredients, it is necessary to estimate exposure through different routes and leading to different potential endpoints. Worst-case scenario calculations indicate that deposition on the surface of the skin following use of cosmetics represents the major route of exposure to fragrance ingredients when conservative estimates for evaporation, rinsing, and other forms of product removal are employed. Hydroalcoholic perfumes and colognes deliver the highest dose after single product use. Surveys of formulas used in this type of product allow the calculation of average maximum or upper 97.5th percentile concentration of the ingredient in formulas. With this type of exaggeration, the use of estimates of "typical" cosmetic use can be maximized to take account of excessive consumption patterns for both short-term and long-term exposure estimates. In the latter case, multiple product use must be considered. Short-term exposure (single product doses) of an ingredient found at an average maximum use level of P% in fragrances is taken to be 0.2 x P% or 3P microg/cm(2). Using upper 97.5th percentile concentrations (P(97.5)) of individual ingredients in fragrances, the long-term exposure is taken to be P(97.5) x 2,547 microg/kg body wt/day. The estimates of long-term exposure incorporate a number of highly conservative assumptions (e.g., over a long period, every product used will contain a fragrance with this ingredient at this high (P(97.5)) level).     

Lack of Binding to Isolated Estrogen or Androgen Receptors, and Inactivity in the Immature Rat Uterotrophic Assay, of the Ultraviolet Sunscreen Filters Tinosorb M-Active and Tinosorb S

The presence of structurally diverse chemicals as contaminants in the environment has led to concerns regarding their possible endocrine disturbing effects. Recently, some ultraviolet absorbing components of sunscreen preparations have given positive responses in assays monitoring estrogen-like activity both in vitro and in vivo. Consequently, two recently developed sunscreen components, Tinosorb M-active and Tinosorb S, were evaluated using the in vitro estrogen and androgen receptor competitive binding assays. Neither compound gave a positive response in either of the assays, consistent with the large molecular dimensions of each chemical disfavoring binding to the hormone receptors. Both of the chemicals were inactive in immature rat uterotrophic assays conducted using the subcutaneous route of administration. It is concluded that neither of these agents possess intrinsic estrogenic/antiestrogenic or androgenic/antiandrogenic activity. The several positive control chemicals evaluated gave the expected positive responses in the assays used.     

Benefit and Risk of Organic Ultraviolet Filters

Modern sunscreen products provide broad-spectrum UV protection and may contain one or several UV filters. A modern UV filter should be heat and photostable, water resistant, nontoxic, and easy to formulate. Identification of a substance that meets these criteria is as difficult as discovering a new drug; hundreds of new molecules are synthesized and screened before a lead candidate is identified. The most important aspect in the development of a new UV filter is its safety. In our laboratories, the safety of new ultraviolet filters is assessed by an initial in vitro screen including photostability, cytotoxicity, photocytotoxicity, genotoxicity, and photogenotoxicity tests. These tests are performed in mammalian, yeast, and bacterial cell systems. Skin penetration potential is measured in vitro using human skin or, when required by regulations, in vivo. Because modern sunscreens are selected on the basis of their retention on and in the stratum corneum and are formulated as poorly penetrating emulsions, they generally have very low to negligible penetration rates. The safety and efficacy of UV filters are regulated and approved by national and international health authorities. Safety standards in the European Union, United States, or Japan stipulate that new filters pass a stringent toxicological safety evaluation prior to approval. The safety dossier of a new UV filter resembles that of a new drug and includes acute toxicity, irritation, sensitization, phototoxicity, photosensitization, subchronic and chronic toxicity, reproductive toxicity, genotoxicity, photogenotoxicity, carcinogenicity, and, in the United States, photocarcinogenicity testing. The margin of safety of new UV filters for application to humans is estimated by comparing the potential human systemic exposure with the no-effect level from in vivo toxicity studies. Only substances with a safe toxicological profile and a margin of safety of at least 100-fold are approved for human use. Finally, prior to marketing, new UV filters undergo stringent human testing to confirm their efficacy as well as the absence of irritation, sensitization, photoirritation, and photosensitization potential in man. UV filters not only protect against acute skin injury, such as sunburn, but also against long-term and chronic skin damage, including cellular DNA damage, photoinduced immune suppression, and, by extension, skin cancer. The protection provided by modern sunscreens against UV-induced skin cancer was shown in animal photocarcinogenicity studies and confirmed by numerous in vitro, animal, and human investigations: UV filters protect the p53 tumor suppressor gene from damage and prevent UV-induced immune suppression. Recent studies suggest that sunscreens protect against precursor lesions of skin cancer, such as actinic keratoses. Additional benefits of ultraviolet filters include prevention of photodermatoses, such as polymorphic light eruption, and, possibly, photoaging. Modern sunscreens are safe for children and adults. Percutaneous penetration and irritation rates of topically applied substances in children and adults are similar. The principal protective measure is to keep children out of the sun and/or to cover them with protective clothes; however, sunscreens are a safe and effective and often the only feasible defense of children against UV radiation. In conclusion, sunscreens are safe protective devices that undergo stringent safety and efficacy evaluation.     

PCBs and Neurodevelopmental Effects in Michigan Children: An Evaluation of Exposure and Dose Characterization

Despite the fact that PCB levels in the general environment have continued to decline over the past decade, concern for potential neurodevelopmental deficits from in utero exposure to these compounds remains unabated. In fact, some regulatory and scientific bodies have concluded that the evidence suggesting that prenatal exposure to PCBs may lead to neurodevelopmental deficits is one of the greatest public health concerns surrounding PCBs. The primary basis for the concern that low-level in utero exposure to PCBs causes neurodevelopmental deficits in children is a series of reports on a cohort of Michigan children presumably exposed to PCBs as a result of their mother's consumption of Great Lakes fish. These children, known collectively as the Jacobson cohort, have been followed from birth to 11 years of age. The investigators following these children concluded that they have demonstrated persistent neurodevelopmental effects in this cohort attributable solely to PCBs. However, a detailed analysis of the cohort's exposure characterization, particularly in the initial reports, reveals considerable uncertainty as to the actual exposure status of mothers characterized as "fish eaters" and their offspring. Failure to adequately characterize the PCB exposure of these mothers, or their children, precludes any causal association between in utero exposure to PCBs and neurodevelopmental deficits.     

Toxicological studies on Laccase from Myceliophthora thermophila expressed in Aspergillus oryzae

The bioindustrially produced enzyme laccase can be used in different technical and food applications to facilitate processes. It can be added to different oral care products such as mouthwash, toothpaste, mints, and gums to prevent halitosis. Laccase, produced by submerged fermentation of Aspergillus oryzae, containing a gene originating from Myceliophthora thermophila, was subject to a series of toxicological tests to document its safety in use. It was not found to be mutagenic in the Salmonella typhimurium reverse mutation assay, nor did it cause chromosomal aberrations in cultured human lymphocytes. No evidence of inhalation toxicity or skin and eye irritation was found. There was no evidence of possible skin sensitization in a human skin sensitization test when Laccase was tested at 10% (w/v): thus Laccase would appear to have a low skin sensitization potential. Oral administration to rats of up to 10.0 mL/kg body wt/day (equivalent to a total organic solids dosage of 1.72 g/kg body wt/day) for 13 weeks did not cause any adverse effect.     

Scientific criteria used for the development of occupational exposure limits for metals and other mining-related chemicals

The scientific approaches employed by selected internationally recognized organizations in developing occupational exposure limits (OELs) for metals and other mining-related chemicals were surveyed, and differences and commonalities were identified. The analysis identified an overriding need to increase transparency in current OEL documentation. OEL documentation should adhere to good risk characterization principles and should identify (1) the methodology used and scientific judgments made; (2) the data used as the basis for the OEL calculation; and (3) the uncertainties and overall confidence in the OEL derivation. At least within a single organization, a consistent approach should be used to derive OELs. Opportunities for harmonization of scientific criteria were noted, including (1) consideration of severity in identification of the point of departure; (2) definition of the minimum data set; (3) approaches for interspecies extrapolation; (4) identification of default uncertainty factors for developing OELs; and (5) approaches for consideration of speciation and essentiality of metals. Potential research approaches to provide the fundamental data needed to address each individual scientific criterion are described. Increased harmonization of scientific criteria will ultimately lead to OEL derivation approaches rooted in the best science and will facilitate greater pooling of resources among organizations that establish OELs and improved protection of worker health.     

Interspecies Comparisons of the Toxicity of Asbestos and Synthetic Vitreous Fibers: A Weight-of-the-Evidence Approach

This analysis reviews the available literature on interspecies comparisons of the toxicity of asbestos and synthetic vitreous fibers (SVFs). This topic is of substantial practical importance because most quantitative risk analyses on the effects of inhalation of SVFs are based upon extrapolation of data from rodent inhalation studies. Available information on interspecies comparisons for both dosimetry (the relation between exposure concentration and fiber lung burden) and potency (the relation between lung burden and disease) is summarized. Dosimetry models indicate that, on a normalized basis, fiber deposition and clearance rates are lower in humans than rats. Potency is less well understood than dosimetry, in part because the source of relevant human data is asbestos studies, which are adequate to demonstrate hazard, but are problematic in other regards. There are significant interspecies differences between the mouse, hamster, rat, and human. The available evidence suggests that the rat is preferable as a model for the human. Rats develop fibrosis at comparable lung burdens [10(6) long (> 20 microm length) fibers per gram of dry lung] to those in humans. This analysis concludes that, on a weight-of-evidence basis, there is no reason to conclude that humans are more sensitive to fibers than rats with respect to the development of lung cancer.     

The Costly Illusion of Regulating Unknowable Risks

Uncommon but alarming man-made toxic episodes have occasioned regulation to ensure the safety of novel and commonly used substances and foremost to control agents that may cause cancer. However, the safety or harm of novel entities has no durable record of human use, and hence is virtually unknown. Compelled to act, regulators have chosen animal tests to forecast human cancer risks. To this end, animal data are filtered through a series of preconceived assumptions that are presumed to overcome a host of human/animal differences of biology, exposure, and statistics-differences that in reality are insurmountable. Asked for authoritative opinion, the National Academy of Sciences repeatedly found this regulatory approach without factual or scientific justification and, by implication, arbitrary and irrational. On these grounds, such a regulatory process appears vulnerable to scientific, legal, and constitutional challenges. Although it cannot provide credible assurance that an agent may be safe or harmful, the process has been a prime reason for an annual regulatory burden that exceeds the combined after-tax profits of all U.S. industrial activity. It is argued here that better use of national resources would come from a rational approach based on trade-off considerations, where a substance would be regulated depending on its socio-economic utility and on scientific evidence of proved relevance to human safety.     

Evaluation of risk assessment guideline levels for the chemical warfare agents mustard,GB, and VX

The U.S. Army has estimated acute lethality guideline levels for inhalation of the chemical warfare agents mustard, GB, and VX. These levels are expressed as dosages measured in milligram-minutes per cubic meter (mg-min/m(3)). The National Advisory Council has also proposed acute emergency guideline levels (AEGLs) for the agents. The AEGLs are threshold exposure limits for the general public for mild effects, serious adverse effects, and lethality. They are expressed as air concentrations (in units of mg/m(3)) and are applicable to emergency exposure periods ranging from 10 min to 8 h. The report discusses strengths and deficiencies in the levels, important parameters (i.e., exposure time, breathing rate) that need to be explicitly addressed in deriving the guideline levels, and possible impacts that could result from using AEGLs instead of guideline dosages in future assessments.     

Indices of fiber biopersistence and carcinogen classification for synthetic vitreous fibers (SVFs)

It is generally accepted that the biopersistence of a synthetic vitreous fiber (SVF) is an important determinant of its biological activity. Experimental protocols have been developed to measure the biopersistence of an SVF from short-term inhalation experiments with rats. Clearance kinetics of long (>20 microm) fibers (those believed to have greatest biological activity) have been approximated by one- or two-pool models. Several measures or indices of biopersistence have been proposed in the literature of which three, the weighted half-time (WT(1/2)), the time required to clear 90% of long fibers (T(0.9)), and the so-called slow-phase half-time (T(2)), have been investigated in some detail. This paper considers both one- and two-pool models for long fiber clearance, characterizes the properties of these candidate indices of fiber biopersistence, identifies measures with potentially superior statistical properties, suggests possible cutoff values based on the relation between biopersistence and the outcome of chronic bioassays, and offers comments on the selection of efficient experimental designs. This analysis concludes that WT(1/2) and T(0.9) are highly correlated, are efficient predictors of the outcome of chronic bioassays, and have reasonable statistical properties. T(2), although perhaps attractive in principle, suffers from some statistical shortcomings when estimated using present experimental protocols. The WT(1/2) is shown to be directly proportional to the cumulative exposure (fiber days) after the cessation of exposure and also the mean residence time of these fibers in the lung.     

ICCVAM Evaluation of the Murine Local Lymph Node Assay: III. Data Analyses Completed by the National Toxicology Program Interagency Center for the Evaluation of Alternative Toxicological Methods

To evaluate the reliability of the murine local lymph node assay (LLNA), a test for allergic contact dermatitis activity, the inter- and intralaboratory consistency statistics (h and k, respectively) were calculated for validation studies testing multiple chemicals. The analysis indicated the absence of excessive variability in the dose calculated to induce a threefold or greater increase in the stimulation index (SI). To assess the appropriateness of using an SI of 3 as the decision criteria for identifying a sensitizing compound, LLNA results based on SI values of 2.0, 2.5, 3.0, 3.5, and 4.0 were compared with guinea pig or human results. The results supported the use of an SI of 3 as the decision criteria. Assay performance was determined by comparing LLNA results to results obtained for guinea pigs or humans. The accuracy of the LLNA was 89% when compared with results from the guinea pig maximization test (GPMT)/Buehler assay (BA). The performance of the LLNA and the GPMT/BA was similar when each was compared to human maximization test results plus substances included as human patch test allergens. The LLNA offered advantages over the GPMT in respect to both the time required to conduct the test and the assay cost.     

The utility of the guppy (Poecilia reticulata) and medaka (Oryzias latipes) in evaluation of chemicals for carcinogenicity.

There has been considerable interest in the use of small fish models for detecting potential environmental carcinogens. In this study, both guppies (Poecilia reticulata) and medaka (Oryzias latipes) were exposed in the aquaria water to three known rodent carcinogens for up to 16 months. Nitromethane, which caused mammary gland tumors by inhalation exposure in female rats, harderian gland and lung tumors in male and female mice, and liver tumors in female mice by inhalation, failed to increase tumors in either guppies or medaka. Propanediol, which when given in the feed was a multisite carcinogen in both sexes of rats and mice, caused increased liver tumors in male guppies and male medaka. There was reduced survival in female guppies and no increased tumors in female medaka. 1,2,3-Trichloropropane, which when administered by oral gavage was a multisite carcinogen in both sexes of rats and mice, caused an increased incidence of tumors in the liver of both male and female guppies and medaka and in the gallbladder of male and female medaka. The results of this study demonstrate that for these three chemicals, under these specific exposure conditions, the fish appear less sensitive and have a narrower spectrum of tissues affected than rodents. These results suggest that fish models are of limited utility in screening unknown chemicals for potential carcinogenicity.     

Impact of physiologically based pharmacokinetic modeling on benchmark dose calculations for TCDD-induced biochemical responses

In risk assessment, noncancer risk is currently estimated using a no observed adverse effect level (NOAEL) from an experimental dose-response study, divided by uncertainty factors, to estimate a presumably safe level of human exposure. A benchmark dose approach, in which an effective dose (ED) resulting in a specified percentage increase over background for effects is estimated by empirical modeling, has been proposed as a replacement for the NOAEL methodology. The aim of this analysis is to compare methods for estimation of body burden resulting in a 1 or 10% maximum increase over background (BB(01) or BB(10)) for biochemical responses following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in female Sprague-Dawley rats. In one method, an ED resulting in a prespecified increase in response over background was estimated using average daily doses and an empirical Hill model. The ED was then converted to an equivalent body burden by a simple kinetic model assuming steady-state conditions, half-life of TCDD in the rat, and 100% absorption of TCDD. Alternatively, a mechanistic physiologically based pharmacokinetic (PBPK) model of TCDD in the rat was used to predict body burdens for administered doses. These PBPK-modeled body burdens were then used directly by the Hill model to calculate a BB(01) or BB(10). In general, the body burden values derived from EDs were within five-fold of BB(01) or BB(10) calculated from the PBPK model. BB(01) and BB(10) values from both methods were within two orders of magnitude of current human general population exposure to all dioxin-like compounds.     

Evaluation of the adequacy of published studies of low-dose effects of bisphenol A on the rodent prostate for use in human risk assessment

Studies conducted in our laboratories and by others found no consistent correlation between prostate size, prostate pathology, or the development of prostate cancer under a variety of experimental conditions. Furthermore, an evaluation of eight published studies that were conducted in mice and rats following in utero exposure by oral treatment of dams with low levels of bisphenol A (BPA) and that focused on the prostate identified several discrepancies that affect their adequacy for use in human risk assessment. For example, there was inadequate reporting of the purity of BPA and the animal supplier used, and housing of offspring was not the same among the studies. In addition, there were differences between studies with mice and rats in exposure regimen, route of exposure, and numbers of dams or pups used per BPA dose group. Poor inter- and intraspecies correlation (i.e., mouse to rat or between mouse or rat strains) further complicates the ability to use results from these studies to predict potential prostate effects in humans. Thus, we conclude that a finding of increased prostate weight in rodent studies with perinatal exposure in the absence of associated pathologic and/or functional changes is meaningless and not indicative of a potential adverse effect in humans.