Methyl methacrylate toxicity in rat nasal epithelium: investigation of the time course of lesion development and recovery from short term vapour inhalation

Comprehensive analysis of chronic rodent inhalation toxicity studies for methyl acrylate with attention to test conditions exceeding a maximum tolerated concentration

MA is a chemical intermediate, manufactured and processed within closed systems. While so far available subacute to chronic inhalation toxicity studies performed in compliance with OECD TG principles gave no indication of any carcinogenic potential for MA, a recent 2-year inhalation study with F344/DuCrlCrlj rats published in 2017 by the JBRC showed a statistically significant increase of squamous cell carcinoma in the nasal cavity of male rats at the highest tested concentration of 160 ppm. However, the results of the different studies in total indicate that this high concentration exceeded the MTC. As MA has a low potential for genotoxic and mutagenic activity, the increased tumour incidence can be attributed to a non-genotoxic mechanism, namely to a strong inflammatory response observed in this study. Together with mechanistic and epidemiologic data for other compounds related to nasal carcinogenesis via this mode of action, it can be concluded that the relevance of this increased tumour incidence in male rats for humans is questionable. Also, a long-term exposure to higher concentrations of MA is highly unlikely to be reached in the environment or at workplaces. Therefore, a risk for humans including cancer hazard is considered implausible.

No acute effects of an exposure to 50 ppm methyl methacrylate on the upper airways

PURPOSE

The German MAK value of methyl methacrylate has been fixed at 50 ppm. The aim of this study was to evaluate possible acute effects of an exposure to 50 ppm methyl methacrylate on the upper airways of human subjects.

METHODS

Twenty healthy subjects were exposed to 50 ppm methyl methacrylate and to air (sham) in an exposure chamber for 4 h according to a crossover design. Symptoms were assessed by the SPES questionnaire. Olfactory thresholds for n-butanol and mucociliary transport time were measured before and after exposure. Concentrations of interleukin 1ß and interleukin 8 were determined in nasal secretions taken after exposure. mRNA levels of interleukins 1ß, 6 and 8, tumor necrosis factor α, granulocyte-macrophage colony-stimulating factor, monocyte chemotactic protein 1, and cyclooxygenases 1 and 2 were measured in nasal epithelial cells, obtained after exposure. Possible effects were investigated by semiparametric and parametric crossover analyses.

RESULTS

The score of the item "irritation to the nose" was slightly elevated following exposure to methyl methacrylate (p ≤ 0.01). Olfactory functioning was not impaired. Mucociliary transport time did not change. Neither concentrations of interleukins in nasal secretions nor mRNA levels were elevated.

CONCLUSION

Only minor irritating effects on the nose were observed. The acute exposure to 50 ppm methyl methacrylate did not cause any adverse effects. However, the results cannot be extrapolated to chronic exposure.

Sensory irritation as a basis for setting occupational exposure limits

There is a need of guidance on how local irritancy data should be incorporated into risk assessment procedures, particularly with respect to the derivation of occupational exposure limits (OELs). Therefore, a board of experts from German committees in charge of the derivation of OELs discussed the major challenges of this particular end point for regulatory toxicology. As a result, this overview deals with the question of integrating results of local toxicity at the eyes and the upper respiratory tract (URT). Part 1 describes the morphology and physiology of the relevant target sites, i.e., the outer eye, nasal cavity, and larynx/pharynx in humans. Special emphasis is placed on sensory innervation, species differences between humans and rodents, and possible effects of obnoxious odor in humans. Based on this physiological basis, Part 2 describes a conceptual model for the causation of adverse health effects at these targets that is composed of two pathways. The first, “sensory irritation” pathway is initiated by the interaction of local irritants with receptors of the nervous system (e.g., trigeminal nerve endings) and a downstream cascade of reflexes and defense mechanisms (e.g., eyeblinks, coughing). While the first stages of this pathway are thought to be completely reversible, high or prolonged exposure can lead to neurogenic inflammation and subsequently tissue damage. The second, “tissue irritation” pathway starts with the interaction of the local irritant with the epithelial cell layers of the eyes and the URT. Adaptive changes are the first response on that pathway followed by inflammation and irreversible damages. Regardless of these initial steps, at high concentrations and prolonged exposures, the two pathways converge to the adverse effect of morphologically and biochemically ascertainable changes. Experimental exposure studies with human volunteers provide the empirical basis for effects along the sensory irritation pathway and thus, “sensory NOAEC_human” can be derived. In contrast, inhalation studies with rodents investigate the second pathway that yields an “irritative NOAEC_animal.” Usually the data for both pathways is not available and extrapolation across species is necessary. Part 3 comprises an empirical approach for the derivation of a default factor for interspecies differences. Therefore, from those substances under discussion in German scientific and regulatory bodies, 19 substances were identified known to be human irritants with available human and animal data. The evaluation started with three substances: ethyl acrylate, formaldehyde, and methyl methacrylate. For these substances, appropriate chronic animal and a controlled human exposure studies were available. The comparison of the sensory NOAEC_human with the irritative NOAEC_animal (chronic) resulted in an interspecies extrapolation factor (iEF) of 3 for extrapolating animal data concerning local sensory irritating effects. The adequacy of this iEF was confirmed by its application to additional substances with lower data density (acetaldehyde, ammonia, n-butyl acetate, hydrogen sulfide, and 2-ethylhexanol). Thus, extrapolating from animal studies, an iEF of 3 should be applied for local sensory irritants without reliable human data, unless individual data argue for a substance-specific approach.

Hypothesis-based weight-of-evidence evaluation of methyl methacrylate olfactory effects in humans and derivation of an occupational exposure level

Over 40 years of scientific evidence indicates that methyl methacrylate (MMA) causes olfactory effects in rodents that are relevant to humans. More recent scientific studies have focused on understanding the apparent lack of species concordance between the rodent and human studies. Toxicokinetic studies and a physiologically based pharmacokinetic (PBPK) model describing inhalation dosimetry of MMA in the upper respiratory tract (URT) of rats and humans point to differences in nasal morphology and biochemistry that could explain and reconcile these differences as species-specific manifestations of a common toxicological process. We have applied the hypothesis-based weight-of-evidence (HBWoE) approach to evaluate the concordance of the available data and the hypothesis that the observed difference in sensitivity between rats and humans may be the expected result of physiological and biochemical differences. Our WoE analysis indicates that when the several lines of evidence (i.e., animal, human, mode-of-action, and toxicokinetics data) are integrated, they inform interpretation of one another and, overall, support use of the human data for derivation of an MMA occupational exposure level (OEL) of 50 ppm.

Methyl methacrylate and respiratory sensitization: a critical review

Methyl methacrylate (MMA) is a respiratory irritant and dermal sensitizer that has been associated with occupational asthma in a small number of case reports. Those reports have raised concern that it might be a respiratory sensitizer. To better understand that possibility, we reviewed the in silico, in chemico, in vitro, and in vivo toxicology literature, and also epidemiologic and occupational medicine reports related to the respiratory effects of MMA. Numerous in silico and in chemico studies indicate that MMA is unlikely to be a respiratory sensitizer. The few in vitro studies suggest that MMA has generally weak effects. In vivo studies have documented contact skin sensitization, nonspecific cytotoxicity, and weakly positive responses on local lymph node assay; guinea pig and mouse inhalation sensitization tests have not been performed. Cohort and cross-sectional worker studies reported irritation of eyes, nose, and upper respiratory tract associated with short-term peaks exposures, but little evidence for respiratory sensitization or asthma. Nineteen case reports described asthma, laryngitis, or hypersensitivity pneumonitis in MMA-exposed workers; however, exposures were either not well described or involved mixtures containing more reactive respiratory sensitizers and irritants. The weight of evidence, both experimental and observational, argues that MMA is not a respiratory sensitizer.

Subchronic, reproductive, and developmental toxicity of a fluorotelomer-based urethane polymeric product

A commercial fluorotelomer-based urethane polymeric dispersion, consisting of polymer, surfactant, and water, was evaluated in subchronic, reproduction, and developmental toxicity studies. The dispersion was administered daily by gavage to rats at dosages of 0, 50, 250, or 1000 mg polymer/kg/day or with 70 mg/kg/day of the sulfonate surfactant. Dose levels of 0, 50, 250, or 1000 mg polymer/kg/day were also used for the reproductive and developmental studies. Nasal olfactory epithelial degeneration and necrosis occurred in all dose groups in the 90-day study. Nasal adhesions were observed only in rats administered surfactant alone. Liver-enzyme alterations at 250 and 1000 mg/kg were considered to be potentially adverse effects. The subchronic no-observed-adverse-effects level (NOAEL) was 50 mg/kg. For the reproduction study, rats were dosed for 10 weeks prior to cohabitation and throughout mating, gestation, and lactation. There were no effects on reproductive function in males or females at any dosage. Thyroid weight was decreased in the 250 and 1000 mg/kg day F(1) groups unaccompanied by microscopic effects. In the developmental toxicity study, female rats were dosed from gestation days 6-20; there was no test-substance-related embryolethality, nor was there any dose-related increase in either fetal malformations. Fetal weight was minimally decreased at 1000 mg/kg/day in the presence of slight maternal toxicity; the NOAEL for developmental parameters was 250 mg/kg/day. The polymeric product was not a specific developmental or reproductive toxin.

mRNA-induction and cytokine release during in vitro exposure of human nasal respiratory epithelia to methyl methacrylate

BACKGROUND

Methyl methacrylate (MMA) has been reported to cause histopathological changes in rodent nasal epithelium after inhalation challenges. Data in humans are lacking.

METHODS

In this in vitro design 22 primary cell cultures taken from inferior turbinate tissue of healthy individuals were exposed to MMA concentrations of 50 ppm (German MAK-value) and 200 ppm. mRNA expression and cytokine release of inflammatory mediators were quantified after 4h and after 24h. Controls were exposed to synthetic air. Q-PCR analysis was performed for TNF-alpha, IL-1beta, IL-6, IL-8, MCP-1, GMCSF, Cox-1 and Cox-2. ELISA assays were performed from culture supernatants for TNF-alpha, IL-1beta, IL-6, IL-8, MCP-1 and GMCSF.

RESULTS

Acute inductions of mRNA after 4h were observed for TNF-alpha, IL-1beta, IL-6, IL-8 and MCP-1 at 50 ppm. ELISA analysis of the described parameters did not reveal any significant upregulations at both concentrations after both 4h and 24h.

CONCLUSIONS

The obtained data suggest that exposure of human respiratory epithelia in vitro to 50 ppm and to 200 ppm of MMA does not induce lasting upregulation of the inflammatory mediators measured in this study. The exposure limit of 50 ppm appears safe following these results obtained from human respiratory epithelia.

The nose revisited: a brief review of the comparative structure, function, and toxicologic pathology of the nasal epithelium

The nose is a very complex organ with multiple functions that include not only olfaction, but also the conditioning (e.g., humidifying, warming, and filtering) of inhaled air. The nose is also a "scrubbing tower" that removes inhaled chemicals that may be harmful to the more sensitive tissues in the lower tracheobronchial airways and pulmonary parenchyma. Because the nasal airway may also be a prime target for many inhaled toxicants, it is important to understand the comparative aspects of nasal structure and function among laboratory animals commonly used in inhalation toxicology studies, and how nasal tissues and cells in these mammalian species may respond to inhaled toxicants. The surface epithelium lining the nasal passages is often the first tissue in the nose to be directly injured by inhaled toxicants. Five morphologically and functionally distinct epithelia line the mammalian nasal passages--olfactory, respiratory, squamous, transitional, and lymphoepithelial--and each nasal epithelium may be injured by an inhaled toxicant. Toxicant-induced epithelial lesions in the nasal passages of laboratory animals (and humans) are often site-specific and dependent on the intranasal regional dose of the inhaled chemical and the sensitivity of the nasal epithelial tissue to the specific chemical. In this brief review, we present examples of nonneoplastic epithelial lesions (e.g., cell death, hyperplasia, metaplasia) caused by single or repeated exposure to various inhaled chemical toxicants. In addition, we provide examples of how nasal maps may be used to record the character, magnitude and distribution of toxicant-induced epithelial injury in the nasal airways of laboratory animals. Intranasal mapping of nasal histopathology (or molecular and biochemical alterations to the nasal mucosa) may be used along with innovative dosimetric models to determine dose/response relationships and to understand if site-specific lesions are driven primarily by airflow, by tissue sensitivity, or by another mechanism of toxicity. The present review provides a brief overview of comparative nasal structure, function and toxicologic pathology of the mammalian nasal epithelium and a brief discussion on how data from animal toxicology studies have been used to estimate the risk of inhaled chemicals to human health.

Assessment of methyl methacrylate vapor toxicity on the rat tracheal epithelium

Methyl methacrylate (MMA) is a monomer that is polymerized into resin by light and heat, producing a clear, resistant, durable and relatively inert plastic material. Because of these characteristics, MMA is largely used in Medicine as bone cement and in Dentistry, in dental braces and prostheses, thus generating continuous interest in its toxicity. Experimental and clinical studies have documented that monomers may cause a wide range of adverse health effects. The most important occupational exposure route of MMA is by inhalation. This study aims to evaluate the toxicity of MMA to the tracheal epithelium, according to the time of exposure. For this purpose, two experimental groups of rats were exposed to MMA by inhalation under poor ventilation: one group (n = 36) was exposed permanently, and the other (n = 36) was exposed during 8 hours per day, without water and food supply during the exposure period. A control group (n = 8) received normal air supply. Twelve animals of each study group were sacrificed after 5, 8 and 10 days of exposure together with two or four control animals. Twenty-nine (80.5%) of the rats continuously exposed to MMA developed inflammation on the tracheal epithelium, as well as 58.33% (n = 21) of those exposed 8 h/day and 87.5% (n = 7) of the control rats. No association was observed between the inflammatory process and MMA exposure; no significant alterations in the tracheal epithelium thickness were observed. Further studies on longer exposure times and analysis of other parameters will have to be conducted to exclude the possibility of tracheal damage by vapors of MMA.

Development of methodology for the three-dimensional modelling of the metabolic capacity of the rat nasal cavity using glutathione S-transferase M1 as an example

A variety of chemicals induce site-specific lesions in the rodent nasal cavity. In order to explore the reasons for this site-selectivity, methodology for (a) creation of a 3-dimensional (3D) model of a rat nasal cavity, and (b) mapping of semiquantitative data onto the model has been developed. The head of a rat was fixed, decalcified, step-sectioned (every 100 microm) and stained with hematoxylin and eosin. Digital images of the sections were optically captured, and a KS400 image analysis system (Imaging Associates, Thame, Oxford, UK), attached to a standard personal computer, was used to align adjacent images and reconstruct the series in 3D. The final model was anatomically correct, and could be rotated in any plane and manipulated to display individual internal structures. The spatial localization of a glutathione S-transferase (rGSTM1, previously known as GST 3-3) within this model was investigated using immunohistochemistry. Step sections (every 400 microm) were stained, analyzed by imaging densitometry, and the results for the stained regions within the nasal cavity divided into 4 grades representing high to low expression of rGSTM1. The data was mapped onto the 3D model and showed that the highest expression of this enzyme was in the central regions of the nasal cavity at the transition between respiratory and olfactory epithelia. This methodology will allow investigation of the relationship between the in situ localization of bioactivating and detoxifying enzyme systems and the site-specificity of nasal lesions.

Three-dimensional mapping of the lesions induced by beta-beta'-iminodiproprionitrile, methyl iodide and methyl methacrylate in the rat nasal cavity

The nasal cavity is an important target organ for toxicity, and many chemicals induce site-specific lesions in this region. The factors responsible for this site-selectivity have not been unequivocally identified, but probably include regional dosimetry and bioactivation. The purpose of this study was to map, in 3 dimensions, the lesions induced by beta-beta'-iminodipropionitrile (IDPN), methyl iodide (MeI) and methyl methacrylate (MMA) in the rat nasal cavity. Animals were administered IDPN (150 mg/kg, IP) or exposed via inhalation to MeI (100 ppm, 2 hours) or MMA (400 ppm, 4 hours) and sacrificed after 24 hours. Heads were decalcified, step-sections (1 every 400 microm) cut and stained, and the severity of the epithelial lesion graded as mild (vacuolation and pyknosis), moderate (undulation and mild stripping), or marked (complete stripping). These grades were mapped onto a 3D-model of a rat nasal cavity using the KS400 imaging system (Imaging Associates, Thame, UK). Despite the different routes of exposure the lesions induced by the 3 compounds had very similar distributions, predominantly affecting the dorsal-medial aspects of the ethmoturbinates and, in the case of MMA, the organ of Rodolfo Masera. These results suggest that, with these chemicals, local bioactivation plays a more important role than dosimetry in determining lesion distribution.

Antioxidant status of the rat nasal cavity

Despite extensive interest in the rodent nasal cavity as a target organ for toxicity, there is very limited information regarding nasal defenses against oxidative stress and xenobiotic-derived oxidants. Using immunohistochemistry, we have examined the distribution of Cu,Zn and Mn superoxide dismutase (SOD), catalase, glutathione (GSH) peroxidase, and DT-diaphorase in rat nasal tissues. In addition, we have determined the concentrations of ascorbate and alpha-tocopherol and the activities of SOD (combined Cu,Zn and Mn forms), catalase, GSH peroxidase, GSH reductase, and DT-diaphorase in nasal respiratory epithelium (RE), olfactory epithelium (OE), and in lung. Immunohistochemistry demonstrated that all four enzymes were similarly distributed, with the greatest staining intensity in dorsal-medial regions of the nasal cavity. In respiratory epithelium, ciliated columnar cells and subepithelial glands stained positively, while in olfactory tissue the enzymes were detected in the sustentacular cells and Bowman's glands. With the exception of SOD, enzyme activities were higher in RE than OE, while concentrations of ascorbate and alpha-tocopherol were higher in OE than RE. With the exception of catalase, nasal activities were either higher than or comparable to those of the lung. Thus, the rat nasal cavity appears to be well protected against oxidative damage.

Physiologically based pharmacokinetic (PBPK) models for nasal tissue dosimetry of organic esters: assessing the state-of-knowledge and risk assessment applications with methyl methacrylate and vinyl acetate

Mathematical models have been developed to describe nasal epithelial tissue dosimetry with two compounds, vinyl acetate (VA) and methyl methacrylate (MMA), that cause toxicity in these tissues These models couple computational fluid dynamics (CFD) calculations that map airflow patterns within the nose with physiologically based pharmacokinetic (PBPK) models that integrate diffusion, metabolism, and tissue interactions of these compounds. Dose metrics estimated in these models for MMA and VA, respectively, were rates of MMA metabolism per volume of tissue and alterations in pH in target tissues associated with VA hydrolysis and metabolism. In this article, four scientists who have contributed significantly to development of these models describe the many similarities and relatively few differences between the MMA and VA models. Some differences arise naturally because of differences in target tissues, in the calculated measures of tissue dose, and in the modes of action for highly extracted vapors (VA) compared with poorly extracted vapors (MMA). A difference in the approach used to estimate metabolic parameters from human tissues provides insights into interindividual extrapolation and identifies opportunities for studies with human nasal tissues to enhance current risk assessments. In general, the differences in model structure for these two esters were essential for describing the biology of the observed responses and in accounting for the different measures of target tissue dose. This article is intended to serve as a guide for understanding issues of optimum model structure and optimal data sources for these nasal tissue dosimetry models. We also hope that it leads to greater international acceptance of these hybrid CFD/PBPK modeling approaches for improving risk assessment for many nasal toxicants. In general, these models predict either equivalent (VA) or lower (MMA) nasal tissue doses in humans compared with tissue doses at equivalent exposure concentrations in rats.

Use of a hybrid computational fluid dynamics and physiologically based inhalation model for interspecies dosimetry comparisons of ester vapors

Numerous inhalation studies have demonstrated that exposure to high concentrations of a wide range of volatile acids and esters results in cytotoxicity to the nasal olfactory epithelium. Previously, 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. This study extends this methodology to a representative volatile organic ester, ethyl acrylate (EA). An in vitro exposure of explants of rat olfactory epithelium to EA with and without an esterase inhibitor demonstrated that the organic acid, acrylic acid, released by nasal esterases is primarily responsible for the olfactory cytotoxicity. Estimates of the steady-state concentration of acrylic acid in olfactory tissue were made for the rat nasal cavity by using data from a series of short-term in vivo studies and from the results of CFD-PBPK computer modeling. Appropriate parameterization of the CFD-PBPK model for the human nasal cavity and to accommodate human systemic anatomy, metabolism, and physiology allowed interspecies dose comparisons. The CFD-PBPK model simulations indicate that the olfactory epithelium of the human nasal cavity is exposed to at least 18-fold lower tissue concentrations of acid released from EA than the olfactory epithelium of the rat nasal cavity under the same exposure conditions. The magnitude of this difference varies with the specific exposure scenario that is simulated and with the specific dataset of human esterase activity used for the simulations. The increased olfactory tissue dose in rats relative to humans may be attributed to both the vulnerable location of the rodent olfactory tissue (comprising greater than 50% of the nasal cavity) and the high concentration of rat olfactory esterase activity (comparable to liver esterase activity) relative to human olfactory tissue. These studies suggest that the human olfactory epithelium is protected from vapors of organic esters significantly better than rat olfactory epithelium due to substantive differences in nasal anatomy, nasal and systemic metabolism, systemic physiology, and air flow. Although the accumulation of acrylic acid in the nasal tissues may be a primary concern for nasal irritation and human risk assessment, acute animal inhalation studies to evaluate lethality (LD50-type studies) conducted at very high vapor concentrations of ethyl acrylate indicated that a different mechanism is primarily responsible for mortality. The rodent studies demonstrated that systemic tissue nonprotein sulfhydryl depletion is a primary cause of death at exposure concentrations more than two orders of magnitude above the concentrations that induce nasal irritation. The CFD-PBPK model adequately simulated the severe depletion of glutathione in systemic tissues (e.g., liver and lung) associated with acute inhalation exposures in the 500-1000 ppm range. These results indicate that the CFD-PBPK model can simulate both the low-dose nasal tissue dosimetry associated with irritation and the high-dose systemic tissue dosimetry associated with mortality. In addition, the comparison of simulation results for ethyl acetate and acetone to nasal deposition data suggests that the CFD-PBPK model has general utility as a tool for dosimetry estimates for a wide range of other esters and slowly metabolized vapors.