Skin irritation, basal epithelial cell proliferation, and carcinogenicity evaluations of a representative specialty acrylate and methacrylate.

Specialty acrylates and methacrylates (SAM) comprise a large family of industrial monomers. In the late 1980s, the United States EPA and the industry SAM Panel collaborated to evaluate the potential effects, particularly carcinogenesis, of this family of chemicals. As part of this arrangement, the SAM Panel, with EPA input and approval, conducted four studies with a representative acrylate, triethyleneglycol diacrylate (TREGDA), and methacrylate, triethyleneglycol dimethacrylate (TREGDMA). All studies used unoccluded skin application to male mice as follows: Study 1, evaluation of skin irritation compared to cell proliferation in the basal epithelium (BE) following 7 or 14 days of treatment; Study 2, 14-day dose range-finding study; Study 3, 90-day subchronic toxicity study; and Study 4, chronic bioassays employing the EPAs draft guidelines for dermal chronic bioassays. BE cell proliferation was determined in subchronic and carcinogenicity studies (Studies 1, 3, and 4). Organ weight changes (Studies 3 and 4) and increased mortality (Study 4) were observed for the highest dose of TREGDMA. However, there was no related histopathology. Both chemicals induced cell proliferation (7 days through 78 weeks) that correlated with acute and chronic inflammation of the skin. No skin tumors were observed in this study. TREGDA resulted in skin lesions at doses approximately 20-fold lower than TREGDMA. Most of the skin lesions showed similar patterns of microscopic cutaneous alteration suggestive of nonspecific irritation for both chemicals. However, the high concentration TREGDA group in the 78-week study also had evidence of epidermal cell necrosis. In contrast to earlier studies with acrylates, dose selection was based on careful examination of skin irritation and cell proliferation to avoid excessive skin damage. Under these conditions, TREGDA and TREGDMA were not carcinogenic.

A hybrid computational fluid dynamics and physiologically based pharmacokinetic model for comparison of predicted tissue concentrations of acrylic acid and other vapors in the rat and human nasal cavities following inhalation exposure.

To assist in interspecies dosimetry comparisons for risk assessment of the nasal effects of organic acids, a hybrid computational fluid dynamics (CFD) and physiologically based pharmacokinetic (PBPK) dosimetry model was constructed to estimate the regional tissue dose of inhaled vapors in the rat and human nasal cavity. Application to a specific vapor would involve the incorporation of the chemical-specific reactivity, metabolism, partition coefficients, and diffusivity (in both air and tissue phases) of the vapor. This report describes the structure of the CFD-PBPK model and its application to a representative acidic vapor, acrylic acid, for interspecies tissue concentration comparisons to assist in risk assessment. By using the results from a series of short-term in vivo studies combined with computer modeling, regional nasal tissue dose estimates were developed and comparisons of tissue doses between species were conducted. To make these comparisons, the assumption was made that the susceptibilities of human and rat olfactory epithelium to the cytotoxic effects of organic acids were similar, based on similar histological structure and common mode of action considerations. Interspecies differences in response were therefore assumed to be driven primarily by differences in nasal tissue concentrations that result from regional differences in nasal air flow patterns relative to the species-specific distribution of olfactory epithelium in the nasal cavity. The results of simulations with the seven-compartment CFD-PBPK model suggested that the olfactory epithelium of the human nasal cavity would be exposed to tissue concentrations of acrylic acid similar to that of the rat nasal cavity when the exposure conditions are the same. Similar analysis of CFD data and CFD-PBPK model simulations with a simpler one-compartment model of the whole nasal cavities of rats and humans provides comparable results to averaging over the compartments of the seven-compartment model. These results indicate that the general structure of the hybrid CFD-PBPK model applied in this assessment would be useful for target tissue dosimetry and interspecies dose comparisons for a wide variety of vapors. Because of its flexibility, this CFD-PBPK model is envisioned to be a platform for the construction of case-specific inhalation dosimetry models to simulate in vivo exposures that do not involve significant histopathological damage to the nasal cavity.

Application of a hybrid CFD-PBPK nasal dosimetry model in an inhalation risk assessment: an example with acrylic acid.

The available inhalation toxicity information for acrylic acid (AA) suggests that lesions to the nasal cavity, specifically olfactory degeneration, are the most sensitive end point for developing a reference concentration (RfC). Advances in physiologically based pharmacokinetic (PBPK) modeling, specifically the incorporation of computational fluid dynamic (CFD) models, now make it possible to estimate the flux of inhaled chemicals within the nasal cavity of experimental species, specifically rats. The focus of this investigation was to apply an existing CFD-PBPK hybrid model in the estimation of an RfC to determine the impact of incorporation of this new modeling technique into the risk assessment process. Information provided in the literature on the toxicity and mode of action for AA was used to determine the risk assessment approach. A comparison of the approach used for the current U.S. Environmental Protection Agency (U.S. EPA) RfC with the approach using the CFD-PBPK hybrid model was also conducted. The application of the CFD-PBPK hybrid model in a risk assessment for AA resulted in an RfC of 79 ppb, assuming a minute ventilation of 13.8 l/min (20 m(3)/day) in humans. This value differs substantially from the RfC of 0.37 ppb estimated for AA by the U.S. EPA before the PBPK modeling advances became available. The difference in these two RfCs arises from many factors, with the main difference being the species selected (mouse vs. rat). The choice to conduct the evaluation using the rat was based on the availability of dosimetry data in this species. Once these data are available in the mouse, an assessment should be conducted using this information. Additional differences included the methods used for estimating the target tissue concentration, the uncertainty factors (UFs) applied, and the application of duration and uncertainty adjustments to the internal target tissue dose rather than the external exposure concentration.

Induction of the mitochondrial permeability transition in vitro by short-chain carboxylic acids.

We recently reported that acrylic acid (AA) induces the MPT in vitro, which we suggested might be a critical event in the acute inflammatory and hyperplastic response of the olfactory epithelium. The purpose of the present investigation was to determine if induction of the MPT is a general response to short-chain carboxylic acids or if there are critical physical chemical parameters for this response. Freshly isolated rat liver mitochondria were incubated in the presence of varying concentrations of selected carboxylic acids. All of the acids that we tested caused a concentration-dependent induction of the MPT, which was blocked by cyclosporine A. Although the C4 carboxylic acids were slightly more potent than the C5 acids, there was no correlation with the degree of saturation, the octanol/water coefficient (log P), or the dissociation constant (pK(a)) of the acids that we tested. We conclude that induction of the MPT in vitro is a general response to short-chain carboxylic acids having a pK(a) of 4 to 5.

Lessons learned in applying the U.S. EPA proposed cancer guidelines to specific compounds.

An expert panel was convened to evaluate the U.S. Environmental Protection Agency's "Proposed Guidelines for Carcinogen Risk Assessment" through their application to data sets for chloroform (CHCl3) and dichloroacetic acid (DCA). The panel also commented on perceived strengths and limitations encountered in applying the guidelines to these specific compounds. This latter aspect of the panel's activities is the focus of this perspective. The panel was very enthusiastic about the evolution of these proposed guidelines, which represent a major step forward from earlier EPA guidance on cancer-risk assessment. These new guidelines provide the latitude to consider diverse scientific data and allow considerable flexibility in dose-response assessments, depending on the chemical's mode of action. They serve as a very useful template for incorporating state-of-the-art science into carcinogen risk assessments. In addition, the new guidelines promote harmonization of methodologies for cancer- and noncancer-risk assessments. While new guidance on the qualitative decisions ensuing from the determination of mode of action is relatively straightforward, the description of the quantitative implementation of various risk-assessment options requires additional development. Specific areas needing clarification include: (1) the decision criteria for judging the adequacy of the weight of evidence for any particular mode of action; (2) the role of mode of action in guiding development of toxicokinetic, biologically based or case-specific models; (3) the manner in which mode of action and other technical considerations provide guidance on margin-of-exposure calculations; (4) the relative roles of the risk manager versus the risk assessor in evaluating the margin of exposure; and (5 ) the influence of mode of action in harmonizing cancer and noncancer risk assessment methodologies. These points are elaborated as recommendations for improvements to any revisions. In general, the incorporation of examples of quantitative assessments for specific chemicals would strengthen the guidelines. Clearly, any revisions should retain the emphasis present in these draft guidelines on flexibility in the use of scientific information with individual compounds, while simultaneously improving the description of the processes by which these mode-of-action data are organized and interpreted.

Dosimetric adjustment factors for methyl methacrylate derived from a steady-state analysis of a physiologically based clearance-extraction model.

Cells within the epithelial lining of the nasal cavity metabolize a variety of low-molecular-weight, volatile xenobiotics. In common with terminology developed for other metabolizing organs, the nose extracts these chemicals from the airstream, thereby clearing some portion of the total nasal airflow. In this article, a physiologically based clearance-extraction (PBCE) model of nasal metabolism is used to predict extraction for steady-state conditions. This model, developed by simplification of existing physiologically based pharmacokinetic (PBPK) nasal models, has three tissue regions in two flow paths. A dorsal flow stream sequentially passes over a small area of respiratory epithelium and then over the entire olfactory epithelial surface within the nose. A ventral airstream, consisting of most of the total flow, passes over the larger portion (>80%) of the respiratory epithelium. Each underlying tissue stack has a mucus layer, an epithelial tissue compartment, and a blood exchange region. Metabolism may occur in any of the subcompartments within the tissue stacks. The model, solved directly for a steady-state condition, specifies the volumetric airflow over each stack. Computational fluid dynamic (CFD) solutions for the rat and human for the case with no liquid-phase resistance provided a maximum value for regional extraction, E(max)'. Equivalent air-to-liquid phase permeation coefficients (also referred to as the air-phase mass transfer coefficient) were calculated based on these E(max)' values. The PBCE model was applied to assess expected species differences in nasal extraction and in localized tissue metabolism of methyl methacrylate (MMA) in rats and in humans. Model estimates of tissue dose of MMA metabolites (in micromol metabolized/h/ml tissue) in both species were used to evaluate the dosimetric adjustment factor (DAF) that should be applied in reference concentration (RfC) calculations for MMA. For human ventilation rates equivalent to light exercise, the DAF was estimated to be 3.02 at 28.4 ppm, the benchmark concentration for nasal lesions. Depending on specific assumptions about distribution of esterase activities in human tissues, the range of DAF values was 1.56-8.00. The DAF for heavy exercise with a ventilation rate of 42 L/min was still 2.98. Estimated DAFs were concentration dependent, varying between 2.4 and 4.76 in the inhaled concentration range from 1 and 400 ppm. Present default methods utilize a DAF of 0.145. These steady-state calculations with this PBCE model should be useful in risk assessment calculations for a variety of vapors and gases that are converted to toxic metabolites in cells in the respiratory tract.

Application of a hybrid computational fluid dynamics and physiologically based inhalation model for interspecies dosimetry extrapolation of acidic vapors in the upper airways.

This study provides a scientific basis for interspecies extrapolation of nasal olfactory irritants from rodents to humans. By using a series of short-term in vivo studies, in vitro studies with nasal explants, and computer modeling, regional nasal tissue dose estimates were made and comparisons of tissue doses between species were conducted. To make these comparisons, this study assumes that human and rodent olfactory epithelium have similar susceptibility to the cytotoxic effects of organic acids based on similar histological structure and common mode of action considerations. Interspecies differences in susceptibility to the toxic effects of acidic vapors are therefore assumed to be driven primarily by differences in nasal tissue concentrations that result from regional differences in nasal air flow patterns relative to the species-specific distribution of olfactory epithelium in the nasal cavity. The acute, subchronic, and in vitro studies have demonstrated that the nasal olfactory epithelium is the most sensitive tissue to the effects of inhalation exposure to organic acids and that the sustentacular cells are the most sensitive cell type of this epithelium. A hybrid computational fluid dynamics (CFD) and physiologically based pharmacokinetic (PBPK) dosimetry model was constructed to estimate the regional tissue dose of organic acids in the rodent and human nasal cavity. The CFD-PBPK model simulations indicate that the olfactory epithelium of the human nasal cavity is exposed to two- to threefold lower tissue concentrations of a representative inhaled organic acid vapor, acrylic acid, than the olfactory epithelium of the rodent nasal cavity when the exposure conditions are the same. The magnitude of this difference varies somewhat with the specific exposure scenario that is simulated. The increased olfactory tissue dose in rats relative to humans may be attributed to the large rodent olfactory surface area (greater than 50% of the nasal cavity) and its highly susceptible location (particularly, a projection of olfactory epithelium extending anteriorly in the dorsal meatus region). In contrast, human olfactory epithelium occupies a much smaller surface area (less than 5% of the nasal cavity), and it is in a much less accessible dorsal posterior location. In addition, CFD simulations indicate that human olfactory epithelium is poorly ventilated relative to rodent olfactory epithelium. These studies suggest that the human olfactory epithelium is protected from irritating acidic vapors significantly better than rat olfactory epithelium due to substantive differences in nasal anatomy and nasal air flow. Furthermore, the general structure of the hybrid CFD-PBPK model used for this study appears to be useful for target tissue dosimetry and interspecies dose comparisons for a wide range of inhaled vapors.

A CFD-PBPK hybrid model for simulating gas and vapor uptake in the rat nose.

In laboratory studies of rodents, the inhalation of organic vapors often results in preferential damage to olfactory epithelium. Such focal lesion formation may be due either wholly or in part to a corresponding nonuniformity in the spatial distribution of vapor uptake within the nasal cavities. As a tool for determining this dose distribution, a mathematical model based on a combination of computational fluid dynamics (CFD) and physiologically based pharmacokinetic (PBPK) modeling was developed for simulating toxicant vapor uptake in the rat nose. The nasal airways were subdivided into four distinct meatuses selected such that each contained a major air flow stream. Each meatus was further divided into four serial regions attached to separate tissue stacks containing mucus, epithelial, and subepithelial compartments. Values for the gas-phase mass transfer coefficients and gas flows in the 16 airway regions were determined by a solution of the Navier-Stokes and convection-diffusion equations using commercially available CFD software. These values were then input to a PBPK simulation of toxicant transport through the 16 tissue stacks. The model was validated by using overall uptake data from rodent inhalation studies for three "unreactive" vapors that were either completely inert (i.e., acetone), reversibly ionized in aqueous media (i.e., acrylic acid), or prevented from being metabolized by an enzyme inhibitor (i.e., isoamyl alcohol). A sensitivity analysis revealed that accurate values of the mass transfer coefficient were not necessary to simulate regional concentrations and uptake of unreactive vapors in the rat nose, but reliable estimates of diffusion coefficients in tissue were crucial for accurate simulations.

Implementation of EPA Revised Cancer Assessment Guidelines: Incorporation of Mechanistic and Pharmacokinetic Data.

A workshop entitled "Implementation of EPA Revised Cancer Assessment Guidelines: Incorporation of Mechanistic and Pharmacokinetic Data" was held in Anaheim, California, in 1996 at the 35th Annual Meeting of the Society of Toxicology (SOT). This workshop was jointly sponsored by the Carcinogenesis, Risk Assessment, and Veterinary Specialty Sections of the SOT. The thrust of the workshop was to discuss the scientific basis for the revisions to the EPA Guidelines for cancer assessment and EPA's plans for their implementation. This is the first revision to the original EPA guidelines which have been in use by EPA since 1986. The principal revisions are intended to provide a framework for an increased ability to incorporate biological data into the risk assessment process. Two cases were presented, for chloroform and triclioroethylene, that demonstrated the use of the revised guidelines for specific cancer risk assessments. Using these new guidelines, nonlinear margin of exposure analyses were proposed for these chemicals instead of the linearized multistage model previously used by the EPA as the default method. The workshop participants generally applauded the planned revisions to the EPA guidelines. For the most part, they considered that the revised guidelines represented a positive step which should allow for and encourage the use of biological information in the conduct of cancer risk assessments. Several participants cautioned however that the major problem with cancer risk assessments would continue to be the inadequacy of available data on which to conduct more scientific risk assessments.

Summary of panel discussion on the 'advantages/limitations/uncertainties in the use of physiologically based pharmacokinetic and pharmacodynamic models in hazard identification and risk assessment of toxic substances'.

A panel of scientists discussed a variety of issues related to the development of physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) models for toxicological risk assessment. The panel concluded that although there are a variety of potential technical problems associated with the use of these models for hazard identification and risk assessment, PBPK/PD modeling represents an important technical advance in risk assessment methodology that should continue to be developed and applied. In addition to the technical issues that were addressed, the necessity of providing additional education for toxicologists in the skills necessary for the development and evaluation of PBPK/PD models was stressed.

Applying simulation modeling to problems in toxicology and risk assessment--a short perspective.

The goals of this perspective have been to examine areas where quantitative simulation models may be useful in toxicology and related risk assessment fields and to offer suggestions for preparing manuscripts that describe these models. If developments in other disciplines serve as a bell-wether, the use of mathematical models in toxicology will continue to increase, partly, at least, because the new generations of scientists are being trained in an electronic environment where computation of all kinds is learned at an early age. Undoubtedly, however, the utility of these models will be directly tied to the skills of investigators in accurately describing models in their research papers. These publications should convey descriptions of both the insights obtained and the opportunities provided by these models to integrate existing data bases and suggest new and useful experiments. We hope these comments serve to facilitate the expansion of good modeling practices as applied to toxicological problems.

The regional hydrolysis of ethyl acrylate to acrylic acid in the rat nasal cavity.

Cytotoxicity is primarily limited to the olfactory epithelium of the dorsal meatus region of the nasal cavity of rodents following inhalation exposure to acrylic monomers. To investigate the biochemical basis for this effect, three regions of the Fischer F344N rat nasal cavity were evaluated for carboxylesterase activity for the representative acrylic ester, ethyl acrylate. Prior studies have indicated that the rodent olfactory epithelium is sensitive to the cytotoxic effects of short chain organic acids. In this study, no regional difference in carboxylesterase activity was observed between sensitive and non-sensitive regions of olfactory epithelium. Respiratory epithelium (resistant to cytotoxicity) was found to be have a much lower rate of carboxylesterase activity than olfactory epithelium. These results suggest that the regional distribution of cytotoxicity observed in the rat nasal cavity at high concentrations of inhaled acrylic monomers may be due in part to the amount of released organic acid following deposition. However, the observation of the same esterase activity in sensitive and nonsensitive olfactory regions suggests that nasal air flow patterns and regional deposition may also be critical factors.

Quantitation of an epithelial S-phase response in the rat forestomach and glandular stomach following gavage dosing with ethyl acrylate.

Gavage dosing of the irritant, ethyl acrylate (EA), has been found to induce hyperplasia in the rat forestomach, but no signs of toxicity in the glandular stomach or in organs remote from the site of dosing. To quantitatively describe this effect as a background for subsequent modeling studies, pulse measurements of the number of S-phase cells were made following a single gavage dose of EA. The time-course of the S-phase response in the forestomach epithelium following a high dose (200 mg/kg or a 4% solution in corn oil) indicated that the number of S-phase nuclei was decreased relative to control animals immediately following gavage dosing with a minimum at 6 hr, but that the number of S-phase nuclei increased significantly above control values by 20 hr and remained significantly elevated until at least 48 hr following the gavage dose. A single-dose dose-response study with gavage doses of 0, 2, 10, 20, 50, 100, or 200 mg/kg EA and S-phase analysis at 24 hr following gavage dosing indicated that a significant increase in S-phase nuclei was evident at doses of 20 mg/kg or higher. Dosing with EA for 2 weeks at dose levels of 0, 10, 50, or 200 mg/kg caused a prolonged elevation of S-phase nuclei only at the 200 mg/kg dose level during the 24 hr following the last gavage dose. Lower doses did not induce a significant increase in the S-phase nuclei. In contrast to the forestomach, the S-phase response of the glandular stomach was transient following a single 200 mg/kg gavage dose, and only a marginal response was observed following multiple 200 mg/kg doses. No effects were observed at lower doses. Comparison of these results to prior determinations of the effect of EA on the concentration of nonprotein sulfhydryls (primarily glutathione) in the forestomach and glandular stomach indicate a correlation of the stimulation in S-phase activity in the forestomach with the repletion and overshoot of tissue nonprotein sulfhydryl levels.

Metabolism of acrylic acid to carbon dioxide in mouse tissues.

Acrylic acid (AA) is acutely irritating at sites of initial contact but causes little systemic toxicity probably due to its rapid metabolism to CO2 and acetyl-CoA via a secondary pathway of propionic acid catabolism. In this study, the rate of AA oxidation in 13 tissues of C3H mice was measured by incubating tissue slices with [1-14C]AA and collecting, 14CO2. Oxidation of AA followed pseudo-Michaelis-Menten kinetics in the liver, kidney, and skin. Pseudo-Km values were similar among these tissues and averaged 0.67 mM. The maximal rate of AA oxidation in kidney, liver, and skin was 2890 +/- 436 (mean +/- SE, N = 3), 616 +/- 62, and 47.9 +/- 5.8 nmol/hr/g, respectively. The remaining organs oxidized AA at rates less than 40% of the rate in liver. Rates of metabolism in tissues from male and female mice were similar. 3-Hydroxypropionic acid was the only metabolite detected by high-performance liquid chromatographic analysis following incubation of tissues with [1-14C]AA. Kidney and liver also oxidized [2,3-14C]AA and [1-14C]acetate well, thus providing for the complete metabolism of AA carbons to CO2. These results demonstrate that the rate of AA metabolism varies significantly among mouse tissues and suggest that the kidneys and liver are major sites of detoxification of AA.

Limiting the uncertainty in risk assessment by the development of physiologically based pharmacokinetic and pharmacodynamic models.

Analysis of the default cancer risk assessment methodology suggests that the confidence interval usually associated with the prediction of an upper bound on risk underestimates the uncertainty in the risk estimate. This underestimate of uncertainty is based on the use of a large number of policy decisions or professional judgements that are incorporated into the methodology as exact values with no estimate of error. An alternative approach is to develop a comprehensive biologically based risk assessment that provides scientific data to substitute for many of the policy decisions of the default methodology.

Rate and route of oxidation of acrylic acid to carbon dioxide in rat liver.

Results of in vivo metabolism studies with acrylic acid (AA) have indicated that 60-80% of the administered dose is excreted as CO2 within 2-8 hr of oral dosing of rats; however, the pathway of AA metabolism to CO2 in mammals has not been determined. To define this route, rat hepatocytes were isolated and incubated with [1-14C]AA in a sealed vial modified to trap evolved 14CO2. Rapid oxidation of AA to CO2 was observed. Similar incubations conducted with rat liver homogenates fortified with ATP, ADP, coenzyme A, carnitine, and malate also resulted in oxidation of AA. Mitochondria isolated from liver homogenates were incubated with AA under the same conditions and yielded higher rates of AA oxidation than homogenates. Addition of equimolar amounts of propionic acid, 3-hydroxypropionic acid, or 3-mercaptopropionic acid significantly inhibited the oxidation of AA by mitochondria. HPLC analysis of the mitochondrial incubation mixtures indicated that a single major metabolite, which coeluted with 3-hydroxypropionate, accumulated in the solution. The results indicate that AA is rapidly incorporated into a mitochondrial pathway for propionic acid catabolism that results in the release of CO2 and possible bioincorporation as acetate. This pathway appears to be the principal route of detoxification of AA in mammals.

A physiologically based pharmacokinetic and pharmacodynamic model to describe the oral dosing of rats with ethyl acrylate and its implications for risk assessment.

A physiologically based pharmacokinetic and pharmacodynamic model has been developed to describe the absorption, distribution, and metabolism of orally dosed ethyl acrylate. The model describes the metabolism of ethyl acrylate in 14 tissues based on in vitro metabolic studies conducted with tissue homogenates. The routes of metabolism included in the model are carboxylesterase-catalyzed ester hydrolysis, conjugation with glutathione, and binding to protein. To adequately describe the rate and extent of glutathione depletion following gavage dosing, the steady-state rate of glutathione synthesis in the organs of interest was included. In vivo validation of the model was conducted by comparing the predictions of the model to the results of a variety of gavage dosing experiments with ethyl acrylate, including (1) the time course of glutathione depletion in a variety of tissues up to 98 hr following dosing at three dose levels, (2) the rate and extent of radiolabeled carbon dioxide excretion, and (3) protein binding in the forestomach. The very rapid metabolism predicted by the model was consistent with the observation that ethyl acrylate was metabolized too rapidly in vivo to be detected by common analytical techniques for tissue metabolite analysis. The validation data indicated that the model provides a reasonable description of the pharmacokinetics and the pharmacodynamic response of specific rat tissues following gavage dosing of ethyl acrylate. A dose surrogate, or measure of delivered dose, for ethyl acrylate was calculated and correlated with the incidence and severity of contact site toxicity (edema, inflammation, ulceration, and hyperplasia). The model provides a quantitative tool for evaluating exposure scenarios for their potential to induce contact-site toxicity, and it provides a quantitative approach for understanding the lack of toxicity in tissues remote from the dosing site.

The histopathological and biochemical response of the stomach of male F344/N rats following two weeks of oral dosing with ethyl acrylate.

Male F344/N rats were dosed with ethyl acrylate (EA) either by daily gavage or in the drinking water for 2 weeks. The gavage dose levels were 0, 2, 10, 20, 50, 100, and 200 mg/kg; the drinking water dose concentrations were 0, 200, 1,000, 2,000, and 4,000 ppm (corresponding to 0, 23, 99, 197, and 369 mg/kg/day, respectively). In those animals dosed by gavage, irritation of the forestomach increased in incidence and severity over the 20-200 mg/kg dose range. In those animals dosed with EA in the drinking water, a much lower incidence of forestomach irritation and less severe lesions were observed at corresponding dose levels. No lesions were observed in the glandular stomach from either of the 2 modes of oral administration. Following 2 weeks of gavage dosing with EA, the total non-protein sulfhydryl (NPSH) content of the forestomach and glandular stomach, and the NPSH concentration of the liver were determined 2-24 hr after the last gavage dose. Animals dosed at 200 mg/kg reached approximately 11% of the initial NPSH content in the forestomach at 6 hr after dosing. NPSH depletion of this magnitude has been associated with cytotoxicity of other tissues in other studies. By contrast, either the glandular stomach nor liver were depleted of NPSH to levels generally associated with toxicity. These observations are consistent with the conclusion that bolus dosing of EA induces severe depletion of critical cellular thiols in the forestomach with toxic consequences, but not in the glandular stomach or liver. Changing the mode of oral administration for EA to continued small doses in the drinking water allowed efficient detoxification and did not induce sulfhydryl depletion or comparable forestomach toxicity at the same daily body burden.