Styrene lung cancer risk assessment: an alternative evaluation of human lung cancer risk assuming mouse lung tumors are potentially human relevant and operating by a threshold-based non-genotoxic mode of action.

Rodent inhalation studies indicate styrene is a mouse lung-specific carcinogen. Mode-of-action (MOA) analyses indicate that the lung tumors cannot be excluded as weakly quantitatively relevant to humans due to shared oxidative metabolites detected in rodents and humans. However, styrene also is not genotoxic following in vivo dosing. The objective of this review was to characterize occupational and general population cancer risks by conservatively assuming mouse lung tumors were relevant to humans but operating by a non-genotoxic MOA. Inhalation cancer values reference concentrations for respective occupational and general population exposures (RfCcar-occup and RfCcar-genpop) were derived from initial benchmark dose (BMD) modeling of mouse inhalation tumor dose-response data. An overall lowest BMDL10 of 4.7 ppm was modeled for lung tumors, which was further duration- and dose-adjusted by physiologically based pharmacokinetic (PBPK) modeling to derive RfCcar-occup/genpop values of 6.2 ppm and 0.8 ppm, respectively. With the exception of open-mold fiber reinforced composite workers not using personal protective equipment (PPE), the RfCcar-occup/genpop values are greater than typical occupational and general population human exposures, thus indicating styrene exposures represent a low potential for human lung cancer risk. Consistent with this conclusion, a review of styrene occupational epidemiology did not support a conclusion of an association between styrene exposure and lung cancer occurrence, and further supports a conclusion that the conservatively derived RfCcar-occup is lung cancer protective.

Derivation of no significant risk levels for three lower acrylates: Conclusions and recommendations from an expert panel.

A panel of toxicology, mode of action (MOA), and cancer risk assessment experts was engaged to derive no-significant-risk-levels (NSRLs) for three lower acrylates: methyl acrylate (MA), ethyl acrylate (EA), and 2-ethylhexyl acrylate (2EHA) using the best available science, data, and methods. The review was structured as a five-round, modified Delphi format, a systematic process for collecting independent and deliberative input from panel members, and it included several procedural elements to reduce potential sources of bias and groupthink. Input from the panel for key decisions in the dose-response assessments resulted in NSRL values of 530 µg/day (330-800 µg/day), 640 µg/day (280-670 µg/day), and 1700 µg/day (1300-2700 µg/day) for MA, EA, and 2EHA, respectively. Novel to this approach were the use of nonneoplastic lesions reported at point of contact where tumors have been reported in laboratory rodents, along with nonlinear extrapolation to low doses (uncertainty factor approach) based upon panel recommendations. Confidence in these values is considered medium to high for exposures applied to the routes of exposure tested (inhalation for MA and EA, dermal for 2EHA), but confidence is considered lower when applied to other routes of exposure.

Cancer weight of evidence for three lower acrylates: Conclusions and recommendations from an expert panel.

An international panel of experts was engaged to assess the cancer weight of evidence (WOE) for three lower acrylates: methyl acrylate, ethyl acrylate, and 2-ethylhexyl acrylate. The review was structured as a three-round, modified Delphi format, a systematic process for collecting independent and deliberative input from panel members, and it included procedural elements to reduce bias and groupthink. Based upon the available science, the panel concluded: (1) The MOA for point of contact tumors observed in rodent cancer bioassays that is best supported by available data involves increased cell replication by cytotoxicity and regenerative proliferation; (2) The WOE supports a cancer classification of "Not likely to be carcinogenic to humans" a conclusion that is more in line with an IARC classification of Group 3 rather than Group 2 B; (3) Quantitative cancer potency values based on rodent tumor data are not required for these chemicals; and (4) Human health risk assessment for these chemicals should instead rely on non-cancer, precursor endpoints observed at the point of contact (e.g., hyperplasia). The degree of consensus (consensus scores of 0.84-0.91 out of a maximum score of 1) and degree of confidence (7.7-8.7 out of a maximum score of 10) in the WOE conclusions is considered high.

Ethylene oxide review: characterization of total exposure via endogenous and exogenous pathways and their implications to risk assessment and risk management.

This review is intended to provide risk assessors and risk managers with a better understanding of issues associated with total exposures of human populations to ethylene oxide from endogenous and exogenous pathways. Biomonitoring of human populations and lab animals exposed to ethylene oxide has relied upon the detection of hemoglobin adducts such as 2-hydroxyethylvaline (HEV), which provides a useful measure of total exposure to ethylene oxide from all pathways. Recent biomonitoring data from CDC provide an excellent characterization of total exposure to ethylene oxide to the general U.S. population by demographic factors such as age, gender, and race as well as smoking habit, which might be comparable to previous measurements reported for humans and lab animals. The biochemical pathways including gastrointestinal (production by bacteria) and systemic (enzymatic production) pathways by which endogenous ethylene is generated and converted to ethylene oxide are described. The relative importance of endogenous pathways and exogenous pathways via ambient air or tobacco smoke was quantified based upon available data to characterize their relative importance to total exposure. Considerable variation was noted for HEV measurements in human populations, and important sources of variation for all pathways are discussed. Issues related to risk assessment and risk management of human populations exposed to ethylene oxide are provided within the context of characterizing total exposure, and data needs for supporting future risk assessment identified.

Evaluation of potential health effects associated with occupational and environmental exposure to styrene - an update.

The potential chronic health risks of occupational and environmental exposure to styrene were evaluated to update health hazard and exposure information developed since the Harvard Center for Risk Analysis risk assessment for styrene was performed in 2002. The updated hazard assessment of styrene's health effects indicates human cancers and ototoxicity remain potential concerns. However, mechanistic research on mouse lung tumors demonstrates these tumors are mouse-specific and of low relevance to human cancer risk. The updated toxicity database supports toxicity reference levels of 20 ppm (equates to 400 mg urinary metabolites mandelic acid + phenylglyoxylic acid/g creatinine) for worker inhalation exposure and 3.7 ppm and 2.5 mg/kg bw/day, respectively, for general population inhalation and oral exposure. No cancer risk value estimates are proposed given the established lack of relevance of mouse lung tumors and inconsistent epidemiology evidence. The updated exposure assessment supports inhalation and ingestion routes as important. The updated risk assessment found estimated risks within acceptable ranges for all age groups of the general population and workers with occupational exposures in non-fiber-reinforced polymer composites industries and fiber-reinforced polymer composites (FRP) workers using closed-mold operations or open-mold operations with respiratory protection. Only FRP workers using open-mold operations not using respiratory protection have risk exceedances for styrene and should be considered for risk management measures. In addition, given the reported interaction of styrene exposure with noise, noise reduction to sustain levels below 85 dB(A) needs be in place.

Assessment of the genotoxicity of trichloroethylene in the in vivo micronucleus assay by inhalation exposure.

The in vivo genotoxic potential of trichloroethylene (TCE) was evaluated by examining the incidence of micronucleated polychromatic erythrocytes (MN-PCEs) in the bone marrow. Groups of male CD rats were exposed by inhalation to targeted concentrations of 0 (negative control), 50, 500, 2500 or 5000 ppm for 6 consecutive hours on a single day. The exposure concentrations were selected to overlap those employed by a published study that reported a 2- to 3-fold increase in the frequency of micronuclei in male rats following a single inhalation exposure to 5, 500 and 5000 ppm TCE for 6h but not following repeated exposure to similar concentrations. In addition, any treatment-related findings were assessed in the context of potential TCE-induced hypothermia. Clinical signs consistent with marked TCE-induced sedation were observed in rats exposed to 5000 ppm and subsequently three rats died prior to the end of the 6h exposure period. No remarkable changes in body temperature were observed in surviving animals monitored with transponders before and after exposures. There were no statistically significant increases in the frequencies of MN-PCEs in groups treated with the test material as compared to the negative controls. The positive control animals showed a significant increase in the frequency of MN-PCEs and a decrease in the relative proportion of PCEs among erythrocytes as compared to the negative control animals. There were no statistically significant differences in the per cent PCEs in groups treated with the test material. As no increase in the incidence of micronuclei was observed in any of the TCE exposure groups, kinetochore analyses were not performed. Under the experimental conditions used, TCE was considered to be negative in the rat bone marrow micronucleus test.

Paraquat: model for oxidant-initiated toxicity.

Paraquat, a quaternary ammonium bipyridyl herbicide, produces degenerative lesions in the lung after systemic administration to man and animals. The pulmonary toxicity of paraquat resembles in several ways the toxicity of several other lung toxins, including oxygen, nitrofurantoin and bleomycin. Although a definitive mechanism of toxicity of paraquat has not been delineated, a cyclic single electron reduction/oxidation of the parent molecule is a critical mechanistic event. The redox cycling of paraquat has two potentially important consequences relevant to the development of toxicity: generation of "activated oxygen" (e.g., superoxide anion, hydrogen peroxide, hydroxyl radical) which is highly reactive to cellular macromolecules; and/or oxidation of reducing equivalents (e.g., NADPH, reduced glutathione) necessary for normal cell function. Paraquat-induced pulmonary toxicity, therefore, is a potentially useful model for evaluation of oxidant mechanisms of toxicity. Furthermore, characterization of the consequences of intracellular redox cycling of xenobiotics will no doubt provide basic information regarding the role of this phenomena in the development of chemical toxicity.

Paraquat toxicity: proposed mechanism of action involving lipid peroxidation.

The purpose of this study was to investigate the hypothesis that paraquat pulmonary toxicity results from cyclic reduction-oxidation of paraquat with sequential generation of superoxide radicals and singlet oxygen and initiation of lipid peroxidation. In vitro mouse lung microsomes catalyzed an NADPH-dependent, single-electron reduction of paraquat. Incubation of paraquat with NADPH, NADPH-cytochrome c reductase, and purified microsomal lipid increased malondialdehyde production is a concentration dependent manner. Addition of either superoxide dismutase or a single oxygen trapping agent 1,3-dipheylisobenzo furan inhibited paraquat stimulated lipid peroxidation. In vivo, pretreatment of mice with phenobarbital decreased paraquat toxicity, possibly by competing for electrons which might otherwise reduce paraquat. In contrast, paraquat toxicity in mice was increased by exposure to 100% oxygen and by deficiencies of the antioxidants selenium, vitamin E, or reduced glutahione (GSH). Paraquat, given IP to mice, at 30 mg/kg, decreased concentrations of the water-soluble antioxidant GSH in liver and lipid soluble antioxidants in lung. Oxygen-tolerant rats, which hae increased activities of pulmonary enzymes which combat lipid peroxidation, were also tolerant to lethal doses of paraquat as indicated by an increased paraquat LT50. Furthermore, rats chronically exposed to 100 ppm paraquat in the water had elevated pulmonary activities of glucose-6-phosphate dehydrogenase and GSH reductase. These results were consistent with the hypothesis that lipid peroxidation is involved in the toxicity of paraquat.

Developmental toxicity studies in rats and rabbits on 2,4-dichlorophenoxyacetic acid and its forms.

The potential for 2,4-D and its salts and esters to induce developmental toxicity was investigated in rats (8 studies) and rabbits (7 studies). Maternal toxicity associated with exposure was dependent on the dose level expressed as 2,4-D acid equivalents. The severity of the maternal effect was correlated to the 2,4-D acid-equivalent dose, with increasing dose levels that exceeded renal clearance causing increasingly more severe maternal effects. In both species, maternal body weight effects began to be manifested at dose levels of 30 mg 2,4-D acid equivalent/kg/day. At higher dose levels (50-75 mg/kg/day in rats and 75-90 mg/kg/day in rabbits), body weights and feed consumption were more severely affected. At dose levels > or =90 mg/kg/day in rats, clinical signs of toxicity (ataxia, muscular stiffness, and decreased motor activity) and mortality were noted. The no-observed-adverse-effect level (NOAEL) for maternal toxicity in both species across the family of 2,4-D salts and esters was approximately 10 mg/kg/day. Significantly decreased fetal body weights and increased fetal variations were seen in rats only at maternally toxic dose levels in excess of 90 mg/kg/day acid equivalent. At maternally toxic doses in rabbits, embryonal and fetal development were essentially unaffected. There were no effect on maternal reproductive measures such as litter size, resorption rates, or fetal body weights, and there was no evidence of teratogenic activity. In summary, equivalent toxicity of the salts and esters is consistent with rapid and complete metabolic conversion to 2,4-D acid. No adverse fetal effects were noted at dose levels that did not also produce evidence of maternal toxicity or exceed renal clearance of 2,4-D indicating that the developing rat and rabbit fetus were not uniquely sensitive to 2,4-D and its forms.

The relationship of carbon disulfide metabolism to development of toxicity.

The metabolism of CS2 proceeds by two distinct metabolic pathways, direct reaction with amine or thiol functions of cellular constituents, and microsomal oxidation to reactive intermediates that covalently bind to cell macromolecules. Reaction with amines or thiols may alter protein function by two primary mechanisms: formation of dithiocarbamate metabolites capable of inactivating metalloenzymes by chelation of metal ions such as copper and zinc, and direct reaction with such functional groups on proteins. Both of these mechanisms may contribute to CS2-induced neurotoxicity. Formation of reactive intermediates catalyzed by microsomal metabolism is clearly important in mediating CS2-induced hepatotoxicity. Its role in catalyzing neurotoxicity is less certain, however, since the mixed function oxidase activity of the central and peripheral systems has not been well characterized.

The morphology of carbon disulfide neurotoxicity.

The morphology of carbon disulfide induced peripheral neuropathy was studied in rats exposed to three concentrations of carbon disulfide by inhalation for 90 days. Rats exposed to 800 ppm developed neurofilamentous axonal swellings in the distal portions of long fibers, including the dorsal ascending sensory and corticospinal tracts of the spinal cord. In peripheral nerve the predominant effect was seen at the level of the posterior tibial nerve. Teased fiber preparations of the muscular branch of the posterior tibial nerve showed numerous paranodal and internodal swellings as well as Wallerian degeneration. Ultrastructurally the swellings were characterized by neurofilament accumulations, decreased numbers of microtubules and thin myelin. Other features included segregation of axoplasmic organelles and cytoskeletal components, intrusion of Schwann cell processes into the axoplasm, Schwann cells with increased cytoplasmic contents, and Schwann cell proliferation around many swollen and demyelinated axons. These features draw important parallels between the morphology of carbon disulfide neuropathy and the neurofilamentous neuropathies induced by hexacarbons and beta,beta' iminodipropionitrile (IDPN).

Dose-dependent metabolism and dose setting in chronic studies.

Because of the expense involved in conducting chronic studies, limited numbers of animals and dose groups are used. This has given rise to the practice of including as one of the dose groups the "Maximum Tolerated Dose" (MTD). This dose is operationally defined as the highest dose which can be administered to animals without adversely affecting their survival through effects other than cancer. Since many detoxification systems in animals are capacity-limited, they frequently become saturated in MTD studies. This may lead to difficulties in interpreting the results of MTD studies, particularly when it is necessary to estimate the hazard for human populations whose exposure is typically much lower than the MTD. For this reason, it is important to characterize the dose-dependency of absorption, distribution, metabolism, and elimination (pharmacokinetics) of test substances prior to the initiation of a chronic study. This provides a basis for determining the number and spacing of doses to be used in a chronic study. If the appropriate information is collected it may also be possible to develop a physiologically based pharmacokinetic model which facilitates extrapolation of the toxicity results between different species and routes of administration as well as between high and low doses. For instance, methylene chloride and vinyl chloride are predominantly metabolized by saturable oxidative pathway(s) at low exposure concentrations. In each case, the oxidative pathway saturates at exposures much lower than the MTD. Knowledge of the pharmacokinetic behavior of these substances provided a basis for appropriately interpreting the chronic studies which have been conducted with these materials.

The effect of buthionine sulfoximine, an inhibitor of glutathione synthesis, on hepatic drug metabolism in the male mouse.

Buthionine sulfoximine significantly reduced the hepatic non-protein sulfhydryl (NPSH) content of male B6C3F1 mice within 2 h after intraperitoneal (i.p.) injection. This treatment did not affect the activity of several hepatic microsomal and cytosolic enzymes responsible for xenobiotic metabolism. Pretreatment of mice with buthionine sulfoximine (2 mmol/kg) increased the hepatotoxicity of chloroform, but did not affect the hepatotoxicity of carbon tetrachloride. These findings suggest that buthionine sulfoximine can be a useful agent for studying the role of glutathione (GSH) in hepatic biotransformation of xenobiotics.

Mode of action considerations in the use of transgenic animals for mutagenicity and carcinogenicity evaluations.

Genetically altered rodent models can be useful in facilitating the extrapolation of results from animal carcinogenicity studies to human risk assessment by contributing mode of action data. Transgenic mutation models make it possible to analyze mutations in vivo in any tissue of interest. Validation studies using genotoxic and epigenetic carcinogens indicated a good correlation between mutation induction and the tumor target tissues and have provided data on mode of tumorigenic action. However, carcinogenesis is a complex process and mutation induction in a given tissue does not always lead to tumors in that tissue. Genetically altered animal models such as the p53 +/- mouse can be useful in differentiating genotoxic carcinogens from those operating by non-genotoxic mechanisms. An understanding of the tumor responses of these short-term alternative transgenic and knockout mice to epigenetic events such as tissue injury and enzyme induction at high maximum tolerated doses will eventually increase our level of confidence in these animal models for hazard evaluation and mechanistic studies.

Role of testicular versus epididymal toxicity in the induction of cytotoxic damage in Fischer-344 rat sperm by methyl chloride.

Exposure of male Fischer-344 (F-344) rats to methyl chloride (MeCl) results in testicular and epididymal toxicity and the induction of both pre- and postimplantation embryonic loss; the preimplantation loss is caused by cytotoxic damage to sperm that leads to failure of fertilization (Toxicol Appl Pharmacol 1986; 86:124-130). The present study examined whether the cytotoxicity of MeCl to sperm is due to the testicular or epididymal toxicity of MeCl. Groups of 18 males were exposed to 3000 ppm MeCl 6 h/day for 5 days, with and without concurrent treatment with the anti-inflammatory agent 3-amino-1-[m-(trifluoromethyl)phenyl]-2-pyrazoline (BW755C; 10 mg/kg, i.p. 1 h pre- and postexposure); BW755C was used to inhibit the epididymal toxicity of MeCl. Control groups were untreated or injected as described above with BW755C. Six males from each group were killed weekly for 3 weeks. Toxic effects of MeCl on the testis were demonstrated by decreased relative organ weight (week 3), testicular histopathology (weeks 1-3) and decreased daily sperm production (weeks 1-3); these effects were not prevented by BW755C. In both the MeCl and the MeCl + BW755C treatment groups, tubules devoid of sperm were observed in regions 4 and 5 of the epididymis at week 2, and in regions 6A and 6B at week 3. Sperm were present in the vas deferens of both groups at week 3 in decreased numbers and had decreased motility and more frequent morphologic abnormalities compared to untreated controls. In conjunction with known epididymal transit times for F-344 rat sperm, these data indicate that the induction of preimplantation loss by MeCl at weeks 2 and 3 postexposure is likely to result from cytotoxic effects on sperm located in the testes at the time of exposure.

Evaluation of the genotoxicity of 2,4-dichlorophenoxyacetic acid and its derivatives in mammalian cell cultures.

2,4-dichlorophenoxyacetic acid and its derivatives (collectively known as 2,4-D) are herbicides used to control a wide variety of broadleaf and woody plants. The genetic toxicity of an ester (2,4-D 2-butoxyethylester) and two salts (2,4-D isopropylamine and 2,4-D triisopropanolamine) was investigated in cultured mammalian cells. The end points used were the induction of chromosomal aberrations in primary cultures of rat lymphocytes and forward mutations at the HGPRT locus of Chinese hamster ovary cells. There was no evidence of genotoxicity for the test materials in the experimental systems used. These results were consistent with the general lack of genotoxic potential for 2,4-D in a number of other test systems.

Ames assays and unscheduled DNA synthesis assays on 2, 4-dichlorophenoxyacetic acid and its derivatives.

2,4-dichlorophenoxyacetic acid and several of its derivatives (collectively known as 2,4-D) are herbicides used to control a wide variety of broadleaf and woody plants. The genetic toxicity in vitro of 2,4-D and seven of its salts and esters were examined by employing gene mutation in bacteria (Ames test) and induction of DNA damage and repair in rat hepatocytes. In addition, an in vivo unscheduled DNA synthesis (UDS) assay was performed on 2,4-D. There were no indications of genotoxic potential for 2,4-D acid, or any of its derivatives, in these assays. These results are consistent with the reported lack of carcinogenic potential for 2,4-D in both mice and rats.

In vivo micronucleus assays on 2,4-dichlorophenoxyacetic acid and its derivatives.

The potential for 2,4-D and seven of its salts and esters to induce cytogenetic abnormalities in mammalian cells in vivo was investigated in the mouse bone marrow micronucleus test. All the test materials were administered to male and female mice by oral gavage and the frequencies of micronucleated polychromatic erythrocytes (MN-PCE) in the bone marrow were determined at intervals of 24, 48 and 72 h following dosing. There were no significant increases in the incidence of MN-PCE in the treated mice at any of the bone marrow sampling times. These results are consistent with the reported lack of in vitro genetic toxicity for these materials in various in vitro genotoxicity assays as well as the absence of carcinogenic potential for 2,4-D in both mice and rats.

Perspectives on the mechanism of action of the splenic toxicity of aniline and structurally-related compounds.

Aniline and several structurally-related aromatic amines produce spleen tumours in rats given high doses of compound in 2-year bioassay studies. Evaluation of the pathogenesis of the splenic lesions and characterization of the disposition of radiolabelled aniline in animals suggests that the spleen tumours may be a secondary response resulting from chemically-mediated erythrocyte toxicity. It is proposed that compound-derived toxicity to erythrocytes results in scavenging of damaged red blood cells by the spleen, initiating a series of events which may contribute to the development of spleen tumours. These events potentially include (i) specific accumulation of the parent compound or toxic metabolite(s) carried to the spleen by erythrocytes; (ii) deposition of erythrocytic debris, particularly iron, which may catalyse tissue-damaging free-radical reactions; and (iii) induction of splenic hyperplasia resulting from erythrocyte overload. Linkage of the splenic tumorigenicity of these aromatic amines to an initial toxic event in the erythrocyte suggests that the carcinogenicity of such compounds may be determined by a definable threshold dose, i.e. the events leading to the carcinogenicity are not initiated until the capacity of the red blood cell to cope with the toxic insult is exceeded.