A biologically based risk assessment for vinyl acetate-induced cancer and noncancer inhalation toxicity

PBPK Modeling as an Alternative Method of Interspecies Extrapolation that Reduces the Use of Animals: A Systematic Review

INTRODUCTION

Physiologically based pharmacokinetic (PBPK) modeling is a computational approach that simulates the anatomical structure of the studied species and presents the organs and tissues as compartments interconnected by arterial and venous blood flows.

AIM

The aim of this systematic review was to analyze the published articles focused on the development of PBPK models for interspecies extrapolation in the disposition of drugs and health risk assessment, presenting to this modeling an alternative to reduce the use of animals.

METHODS

For this purpose, a systematic search was performed in PubMed using the following search terms: "PBPK" and "Interspecies extrapolation". The revision was performed according to PRISMA guidelines.

RESULTS

In the analysis of the articles, it was found that rats and mice are the most commonly used animal models in the PBPK models; however, most of the physiological and physicochemical information used in the reviewed studies were obtained from previous publications. Additionally, most of the PBPK models were developed to extrapolate pharmacokinetic parameters to humans and the main application of the models was for toxicity testing.

CONCLUSION

PBPK modeling is an alternative that allows the integration of in vitro and in silico data as well as parameters reported in the literature to predict the pharmacokinetics of chemical substances, reducing in large quantity the use of animals that are required in traditional studies.

Development of a Purity Certified Reference Material for Vinyl Acetate

Vinyl acetate is a restricted substance in food products. The quantification of the organic impurities in vinyl acetate is a major problem due to its activity, instability, and volatility. In this paper, while using the mass balance method to determine the purity of vinyl acetate, an improved method was established for the determination of the content of three impurities in vinyl acetate reference material, and the GC-FID peak area normalization for vinyl acetate was calibrated. The three trace organic impurities were identified by gas chromatography tandem high-resolution mass spectrometry to be methyl acetate, ethyl acetate, and vinyl propionate. The content and relative correction factors for the three organic impurities were measured. The purity of vinyl acetate determined by the mass balance method was 99.90% with an expanded uncertainty of 0.30%, and the total content of organic impurities was 0.08% with a relative correction factor of 1.23%. The vinyl acetate reference material has been approved as a national certified reference material in China as GBW (E) 062710.

Respiratory functions and health risk assessment in inhalational exposure to vinyl acetate in the process of carpet manufacturing using Monte Carlo simulations

Vinyl acetate (VA) is a volatile compound and the main compound of the carpenter's glue. VA causes upper respiratory tract irritation, cough, and hoarseness in occupational exposure. As Iran is one of the biggest carpet producers in the world, this study was carried out to determine the inhalational health risk for employees exposed to VA. To the best of our knowledge, this was the first health risk assessment and the first evaluation of the lung functions and respiratory symptoms in employees exposed to VA. In the six finishing shops of carpet manufacturing industry in Kashan city, Iran the cross-sectional studies were conducted in 2022. The subjects comprised of forty male employees exposed to VA and of forty non-exposed employees in the reference group. VA analyses in the workers' breathing zones were performed based on the National Institute for Occupational Safety and Health (NIOSH) 1453 Method. VA concentrations were measured using Gas Chromatography-Mass Spectrometry (GC-MS). Inhalational risk assessment to VA was performed using the United States Environmental Protection Agency method and the Monte Carlo simulations. Respiratory functions were determined using the spirometry indices. In the exposed employees, considerably higher prevalence rates of pulmonary symptoms were observed in comparison with the control group. Statistical analysis showed a remarkable difference between lung function parameters measured in the case and the control groups. The VA Hazard Quotient (HQ) values for all working posts, except the quality control unit, were > 1 indicating the substantial inhalational non-cancerogenic risk. The sensitivity analysis revealed that the VA concentrations and exposure time had the most significant contribution in the uncertainty assessment. Therefore, it is recommended to decrease exposure to VA concentrations and to reduce the working time of exposed employees.

A Review of Stable Isotope Labeling and Mass Spectrometry Methods to Distinguish Exogenous from Endogenous DNA Adducts and Improve Dose-Response Assessments

Cancer remains the second most frequent cause of death in human populations worldwide, which has been reflected in the emphasis placed on management of risk from environmental chemicals considered to be potential human carcinogens. The formation of DNA adducts has been considered as one of the key events of cancer, and persistence and/or failure of repair of these adducts may lead to mutation, thus initiating cancer. Some chemical carcinogens can produce DNA adducts, and DNA adducts have been used as biomarkers of exposure. However, DNA adducts of various types are also produced endogenously in the course of normal metabolism. Since both endogenous physiological processes and exogenous exposure to xenobiotics can cause DNA adducts, the differentiation of the sources of DNA adducts can be highly informative for cancer risk assessment. This review summarizes a highly applicable methodology, termed stable isotope labeling and mass spectrometry (SILMS), that is superior to previous methods, as it not only provides absolute quantitation of DNA adducts but also differentiates the exogenous and endogenous origins of DNA adducts. SILMS uses stable isotope-labeled substances for exposure, followed by DNA adduct measurement with highly sensitive mass spectrometry. Herein, the utilities and advantage of SILMS have been demonstrated by the rich data sets generated over the last two decades in improving the risk assessment of chemicals with DNA adducts being induced by both endogenous and exogenous sources, such as formaldehyde, vinyl acetate, vinyl chloride, and ethylene oxide.

Molecular Dosimetry of DNA Adducts in Rats Exposed to Vinyl Acetate Monomer

Vinyl acetate monomer (VAM) is heavily used to synthesize polymers. Previous studies have shown that inhaled VAM, being metabolized to acetaldehyde, may form DNA adducts including N²-ethylidene-deoxyguanosine (N²-EtD-dG), which may subsequently cause mutations and contribute to its carcinogenesis. Currently, there is little knowledge on the molecular dosimetry between VAM exposure and DNA adducts under dosages relevant to human exposure. In this study, 0.02, 0.1, 1, 10, 50, 200, and 600 ppm VAM were exposed to rats by inhalation for 14 days (6 h/day). The use of [¹³C₂]-VAM allows unambiguous differentiation and quantification of the exogenous and endogenous N²-EtD-dG by highly sensitive LC-MS/MS. Our data indicate that VAM-induced exogenous DNA adducts were formed in a non-linear manner. Exogenous DNA adducts were only detected in the nasal epithelium of rats exposed to 10, 50, 200, and 600 ppm VAM, whereas endogenous adducts were found in all nasal and other tissues analyzed. In addition, ratios of exogenous/endogenous DNA adducts were less than 1 with the dose up to 50 ppm, indicating that endogenous DNA adducts are predominant at low VAM concentrations. Moreover, differential dose-response in terms of exogenous DNA adduct formation were observed between nasal respiratory and olfactory epithelium. Furthermore, the lack of exogenous DNA adducts in distant tissues, including peripheral blood mononuclear cells, liver, brain, and bone marrow, indicates that VAM and/or its metabolite do not distribute systemically to cause DNA damage in distant tissues. Together, these results provided new molecular dosimetry to improve science-based cancer risk assessments of VAM.

Human Fecal Metabolome Reflects Differences in Body Mass Index, Physical Fitness, and Blood Lipoproteins in Healthy Older Adults

This study investigated how body mass index (BMI), physical fitness, and blood plasma lipoprotein levels are related to the fecal metabolome in older adults. The fecal metabolome data were acquired using proton nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry on 163 healthy older adults (65-80 years old, 80 females and 83 males). Overweight and obese subjects (BMI ≥ 27) showed higher levels of fecal amino acids (AAs) (valine, alanine, and phenylalanine) compared to normal-weight subjects (BMI ≤ 23.5). Adults classified in the high-fitness group displayed slightly lower concentrations of fecal short-chain fatty acids, propionic acid, and AAs (methionine, leucine, glutamic acid, and threonine) compared to the low-fitness group. Subjects with lower levels of cholesterol in low-density lipoprotein particles (LDLchol, ≤2.6 mmol/L) displayed higher fecal levels of valine, glutamic acid, phenylalanine, and lactic acid, while subjects with a higher level of cholesterol in high-density lipoprotein particles (HDLchol, ≥2.1 mmol/L) showed lower fecal concentration of isovaleric acid. The results from this study suggest that the human fecal metabolome, which primarily represents undigested food waste and metabolites produced by the gut microbiome, carries important information about human health and should be closely integrated to other omics data for a better understanding of the role of the gut microbiome and diet on human health and metabolism.

Evaluating the Environmental Impacts of Personal Protective Equipment Use by the General Population during the COVID-19 Pandemic: A Case Study of Lombardy (Northern Italy)

The diffusion of Coronavirus disease (COVID-19) impacted the whole world, changing the life habits of billions of people. These changes caused an abundant increase in personal protective equipment (PPE, e.g., masks and gloves) use by the general population, which can become a concerning issue of plastic pollution. This study aims to evaluate the negative effects of the abundant PPE use following the COVID-19 diffusion using the test site of the Lombardy region, an area highly affected by the pandemic. Population data were retrieved from national databases, and the COVID-19 national guidelines were considered to estimate the total use of PPEs during 2020. Then, the quantity of waste derived from their use was evaluated based on the weight of PPEs. As well, possible scenarios for 2021 were proposed based on 2020 estimations. The results suggested different negative effects of the diffusion of PPEs both on waste management and on the environment: The abundant increase in PPEs-derived waste caused an increase in terms of costs for management, and the potential direct spreading in the environment of these materials (especially masks) poses a serious threat for an increase in microplastics in water bodies. Following this evaluation, a careful choice regarding COVID-19 measures of containment should be performed especially by the general population, avoiding contagion diffusion and reducing the possible environmental impact derived from disposable PPE use.

Risk assessment of predicted serum concentrations of bisphenol A in children and adults following treatment with dental composite restoratives, dental sealants, or orthodontic adhesives using physiologically based pharmacokinetic modeling

Bisphenol A (BPA) is a chemical used to manufacture bisphenol A glycidyl methacrylate (BisGMA). BisGMA has been used for decades in dental composite restoratives, sealants, and adhesives. Based on published studies, exposure to low concentrations of BPA are possible from dental and orthodontic devices. The serum BPA concentrations arising from such devices and oral doses were predicted using a PBPK model in children and adult females based on 1) published extraction data for cured and uncured 3M ESPE Filtek Supreme Ultra Flowable, 3M ESPE Filtek Bulk Fill Restorative, and 3M ESPE Clinpro Sealant and 2) published 20% ethanol/water and water rinsate data following orthodontic application with 3M ESPE Transbond MIP Primer and 3M ESPE Transbond XT Adhesive. Predicted oral exposure to BPA arising from these dental and orthodontic devices is low (median <10 ng/treatment) and predicted serum BPA concentrations were also low (<10^-4 nM). Even the maximum predicted exposure in this study (533.2 ng/treatment) yields a margin of exposure of 7.5 relative to the EFSA t-TDI (4 μg/kg-day) and is only 2.8% of the daily BPA exposure for the US population in a 58-kg woman (15,660 ng/day). Therefore, the exposure to BPA arising from the 3M ESPE dental and orthodontic devices evaluated in this study is negligible relative to daily BPA exposure in the general population and these potential BPA sources do not constitute a risk to patients.

Bridging inhaled aerosol dosimetry to physiologically based pharmacokinetic modeling for toxicological assessment: nicotine delivery systems and beyond

One of the challenges for toxicological assessment of inhaled aerosols is to accurately predict their deposited and absorbed dose. Transport, evolution, and deposition of liquid aerosols are driven by complex processes dominated by convection-diffusion that depend on various factors related to physics and chemistry. These factors include the physicochemical properties of the pure substance of interest and associated mixtures, the physical and chemical properties of the aerosols generated, the interplay between different factors during transportation and deposition, and the subject-specific inhalation topography. Several inhalation-based physiologically based pharmacokinetic (PBPK) models have been developed, but the applicability of these models for aerosols has yet to be verified. Nicotine is among several substances that are often delivered via the pulmonary route, with varied kinetics depending upon the route of exposure. This was used as an opportunity to review and discuss the current knowledge and state-of-the-art tools combining aerosol dosimetry predictions with PBPK modeling efforts. A validated tool could then be used to perform for toxicological assessment of other inhaled therapeutic substances. The Science Panel from the Alliance of Risk Assessment have convened at the "Beyond Science and Decisions: From Problem Formulation to Dose-Response Assessment" workshop to evaluate modeling approaches and address derivation of exposure-internal dose estimations for inhaled aerosols containing nicotine or other substances. The discussion involved PBPK model evaluation criteria, challenges, and choices that arise in such a model design, development, and application as a computational tool for use in human toxicological assessments.

Ex vivo real-time monitoring of volatile metabolites resulting from nasal odorant metabolism

Odorant-metabolizing enzymes are critically involved in the clearance of odorant molecules from the environment of the nasal neuro-olfactory tissue to maintain the sensitivity of olfactory detection. Odorant metabolism may also generate metabolites in situ, the characterization and function of which in olfaction remain largely unknown. Here, we engineered and validated an ex vivo method to measure odorant metabolism in real-time. Glassware containing an explant of rat olfactory mucosa was continuously flushed with an odorant flow and was coupled to a proton transfer reaction-mass spectrometer for volatile compound analysis. Focusing on carboxylic esters and diketone odorants, we recorded the metabolic uptake of odorants by the mucosa, concomitantly with the release of volatile odorant metabolites in the headspace. These results significantly change the picture of real-time in situ odorant metabolism and represent a new step forward in the investigation of the function of odorant metabolites in the peripheral olfactory process. Our method allows the systematic identification of odorant metabolites using a validated animal model and permits the screening of olfactory endogenously produced chemosensory molecules.

Alternative approaches for acute inhalation toxicity testing to address global regulatory and non-regulatory data requirements: An international workshop report

Inhalation toxicity testing, which provides the basis for hazard labeling and risk management of chemicals with potential exposure to the respiratory tract, has traditionally been conducted using animals. Significant research efforts have been directed at the development of mechanistically based, non-animal testing approaches that hold promise to provide human-relevant data and an enhanced understanding of toxicity mechanisms. A September 2016 workshop, "Alternative Approaches for Acute Inhalation Toxicity Testing to Address Global Regulatory and Non-Regulatory Data Requirements", explored current testing requirements and ongoing efforts to achieve global regulatory acceptance for non-animal testing approaches. The importance of using integrated approaches that combine existing data with in vitro and/or computational approaches to generate new data was discussed. Approaches were also proposed to develop a strategy for identifying and overcoming obstacles to replacing animal tests. Attendees noted the importance of dosimetry considerations and of understanding mechanisms of acute toxicity, which could be facilitated by the development of adverse outcome pathways. Recommendations were made to (1) develop a database of existing acute inhalation toxicity data; (2) prepare a state-of-the-science review of dosimetry determinants, mechanisms of toxicity, and existing approaches to assess acute inhalation toxicity; (3) identify and optimize in silico models; and (4) develop a decision tree/testing strategy, considering physicochemical properties and dosimetry, and conduct proof-of-concept testing. Working groups have been established to implement these recommendations.

Comparison of realistic and idealized breathing patterns in computational models of airflow and vapor dosimetry in the rodent upper respiratory tract

CONTEXT

Computational fluid dynamics (CFD) simulations of airflows coupled with physiologically based pharmacokinetic (PBPK) modeling of respiratory tissue doses of airborne materials have traditionally used either steady-state inhalation or a sinusoidal approximation of the breathing cycle for airflow simulations despite their differences from normal breathing patterns.

OBJECTIVE

Evaluate the impact of realistic breathing patterns, including sniffing, on predicted nasal tissue concentrations of a reactive vapor that targets the nose in rats as a case study.

MATERIALS AND METHODS

Whole-body plethysmography measurements from a free-breathing rat were used to produce profiles of normal breathing, sniffing and combinations of both as flow inputs to CFD/PBPK simulations of acetaldehyde exposure.

RESULTS

For the normal measured ventilation profile, modest reductions in time- and tissue depth-dependent areas under the curve (AUC) acetaldehyde concentrations were predicted in the wet squamous, respiratory and transitional epithelium along the main airflow path, while corresponding increases were predicted in the olfactory epithelium, especially the most distal regions of the ethmoid turbinates, versus the idealized profile. The higher amplitude/frequency sniffing profile produced greater AUC increases over the idealized profile in the olfactory epithelium, especially in the posterior region.

CONCLUSIONS

The differences in tissue AUCs at known lesion-forming regions for acetaldehyde between normal and idealized profiles were minimal, suggesting that sinusoidal profiles may be used for this chemical and exposure concentration. However, depending upon the chemical, exposure system and concentration and the time spent sniffing, the use of realistic breathing profiles, including sniffing, could become an important modulator for local tissue dose predictions.

Implications of acetaldehyde-derived DNA adducts for understanding alcohol-related carcinogenesis

Among various potential mechanisms that could explain alcohol carcinogenicity, the metabolism of ethanol to acetaldehyde represents an obvious possible mechanism, at least in some tissues. The fundamental principle of genotoxic carcinogenesis is the formation of mutagenic DNA adducts in proliferating cells. If not repaired, these adducts can result in mutations during DNA replication, which are passed on to cells during mitosis. Consistent with a genotoxic mechanism, acetaldehyde does react with DNA to form a variety of different types of DNA adducts. In this chapter we will focus more specifically on N2-ethylidene-deoxyguanosine (N2-ethylidene-dG), the major DNA adduct formed from the reaction of acetaldehyde with DNA and specifically highlight recent data on the measurement of this DNA adduct in the human body after alcohol exposure. Because results are of particular biological relevance for alcohol-related cancer of the upper aerodigestive tract (UADT), we will also discuss the histology and cytology of the UADT, with the goal of placing the adduct data in the relevant cellular context for mechanistic interpretation. Furthermore, we will discuss the sources and concentrations of acetaldehyde and ethanol in different cell types during alcohol consumption in humans. Finally, in the last part of the chapter, we will critically evaluate the concept of carcinogenic levels of acetaldehyde, which has been raised in the literature, and discuss how data from acetaldehyde genotoxicity are and can be utilized in physiologically based models to evaluate exposure risk.

A hybrid CFD-PBPK model for naphthalene in rat and human with IVIVE for nasal tissue metabolism and cross-species dosimetry

A PBPK model for naphthalene in the rat and human that incorporates a hybrid CFD-PBPK description of the upper respiratory tract was developed to support cross-species dosimetry comparisons of naphthalene concentrations and tissue normalized rate of metabolism in the nasal respiratory and olfactory epithelium, lung and liver. In vitro measurements of metabolic rates from microsomal incubations published for rat and monkey (surrogate for human) were scaled to the specific tissue based on the tissue microsomal content and volume of tissue. The model reproduces time courses for naphthalene blood concentrations from intravenous and inhalation exposures in rats and upper respiratory tract extraction data in both naïve rats and rats pre-treated to inhibit nasal metabolism. This naphthalene model was applied to estimate human equivalent inhalation concentrations (HECs) corresponding to several NOAELs or LOAELs for the non-cancer effects of naphthalene in rats. Two approaches for cross-species extrapolation were compared: (1) equivalence based on tissue naphthalene concentration and (2) equivalence based on amount metabolized per minute (normalized to tissue volume). At the NOAEL of 0.1 ppm, the regional gas dosimetry ratio (RGDR) based on naphthalene concentration was 0.18 for the dorsal olfactory region; however, the RGDR rises to 5.4 when based on the normalized amount metabolized due to the lower of expression of CYP isozymes in the nasal epithelium of primates and humans. The resulting HEC is 0.12 ppm (0.63 mg/m(3)) continuous exposure at the rat NOAEL of 0.1 ppm (6 h/day, 5 days/week).

Nonlinear responses for chromosome and gene level effects induced by vinyl acetate monomer and its metabolite, acetaldehyde in TK6 cells

Vinyl acetate monomer (VAM) produced rat nasal tumors at concentrations in the hundreds of parts per million. However, VAM is weakly genotoxic in vitro and shows no genotoxicity in vivo. A European Union Risk Assessment concluded that VAM's hydrolysis to acetaldehyde (AA), via carboxylesterase, is a critical key event in VAM's carcinogenic potential. In the following study, we observed increases in micronuclei (MN) and thymidine kinase (Tk) mutants that were dependent on the ability of TK6 cell culture conditions to rapidly hydrolyze VAM to AA. Heat-inactivated horse serum demonstrated a high capacity to hydrolyze VAM to AA; this activity was highly correlated with a concomitant increase in MN. In contrast, heat-inactivated fetal bovine serum (FBS) did not hydrolyze VAM and no increase in MN was observed. AA's ability to induce MN was not impacted by either serum since it directly forms Schiff bases with DNA and proteins. Increased mutant frequency at the Tk locus was similarly mitigated when AA formation was not sufficiently rapid, such as incubating VAM in the presence of FBS for 4 hr. Interestingly, neither VAM nor AA induced mutations at the HPRT locus. Finally, cytotoxicity paralleled genotoxicity demonstrating that a small degree of cytotoxicity occurred prior to increases in MN. These results established 0.25 mM as a consistent concentration where genotoxicity first occurred for both VAM and AA provided VAM is hydrolyzed to AA. This information further informs significant key events related to the mode of action of VAM-induced nasal mucosal tumors in rats.

Nasal dosimetry of inspired naphthalene vapor in the male and female B6C3F1 mouse

Naphthalene vapor is a nasal cytotoxicant in the rat and mouse but is a nasal carcinogen in only the rat. Inhalation dosimetry is a critical aspect of the inhalation toxicology of inspired vapors and may contribute to the species differences in the nasal response. To define the nasal dosimetry of naphthalene in the B6C3F1 male and female mouse, uptake of naphthalene vapor was measured in the surgically isolated upper respiratory tract (URT) at inspiratory flow rates of 25 or 50 ml/min. Uptake was measured at multiple concentrations (0.5, 3, 10, 30 ppm) in controls and mice treated with the cytochrome P450 inhibitor 5-phenyl-1-pentyne. In both sexes, URT uptake efficiency was strongly concentration dependent averaging 90% at 0.5 ppm compared to 50% at 30 ppm (25 ml/min flow rate), indicating saturable processes were involved. Both uptake efficiency and the concentration dependence of uptake were significantly diminished by 5-phenyl-1-pentyne indicating inspired naphthalene vapor is extensively metabolized in the mouse nose with saturation of metabolism occurring at the higher concentrations. A hybrid computational fluid dynamic physiologically based pharmacokinetic model was developed for nasal dosimetry. This model accurately predicted the observed URT uptake efficiencies. Overall, the high URT uptake efficiency of naphthalene in the mouse nose indicates the absence of a tumorigenic response is not attributable to low delivered dose rates in this species.

A comprehensive toxicological evaluation of three adhesives using experimental cigarettes

CONTEXT

Adhesives are used in several different manufacturing operations in the production of cigarettes. The use of new, "high-speed-manufacture" adhesives (e.g. vinyl acetate based) could affect the smoke chemistry and toxicology of cigarettes, compared with older "low-speed-manufacture" adhesives (e.g. starch based).

OBJECTIVE

This study was conducted to determine whether the inclusion of different levels of three adhesives (ethylene vinyl acetate, polyvinyl acetate and starch) in experimental cigarettes results in different smoke chemistry and toxicological responses in in vitro and in vivo assays.

MATERIALS AND METHODS

A battery of tests (analytical chemistry, in vitro and in vivo assays) was used to compare the chemistry and toxicology of smoke from experimental cigarettes made with different combinations of the three adhesives. Varying levels of the different side-seam adhesives, as well as the transfer of adhesives from packaging materials, were tested.

RESULTS

There were differences in some mainstream cigarette smoke constituents as a function of the level of adhesive added to experimental cigarettes and between the tested adhesives. None of these differences translated into statistically significant differences in the in vitro or in vivo assays.

CONCLUSION

The use of newer "high-speed-manufacture" vinyl acetate-based adhesives in cigarettes does not produce toxicological profiles that prevent the adhesives from replacing the older "low-speed-manufacture" adhesives (such as starch).

Magnetic resonance imaging and computational fluid dynamics (CFD) simulations of rabbit nasal airflows for the development of hybrid CFD/PBPK models

The percentages of total airflows over the nasal respiratory and olfactory epithelium of female rabbits were calculated from computational fluid dynamics (CFD) simulations of steady-state inhalation. These airflow calculations, along with nasal airway geometry determinations, are critical parameters for hybrid CFD/physiologically based pharmacokinetic models that describe the nasal dosimetry of water-soluble or reactive gases and vapors in rabbits. CFD simulations were based upon three-dimensional computational meshes derived from magnetic resonance images of three adult female New Zealand White (NZW) rabbits. In the anterior portion of the nose, the maxillary turbinates of rabbits are considerably more complex than comparable regions in rats, mice, monkeys, or humans. This leads to a greater surface area to volume ratio in this region and thus the potential for increased extraction of water soluble or reactive gases and vapors in the anterior portion of the nose compared to many other species. Although there was considerable interanimal variability in the fine structures of the nasal turbinates and airflows in the anterior portions of the nose, there was remarkable consistency between rabbits in the percentage of total inspired airflows that reached the ethmoid turbinate region (approximately 50%) that is presumably lined with olfactory epithelium. These latter results (airflows reaching the ethmoid turbinate region) were higher than previous published estimates for the male F344 rat (19%) and human (7%). These differences in regional airflows can have significant implications in interspecies extrapolations of nasal dosimetry.

Proceedings of the Hydrogen Sulfide Health Research and Risk Assessment Symposium October 31-November 2, 2000

The Hydrogen Sulfide Health Research and Risk Assessment Symposium came about for several reasons: (1) increased interest by the U.S. Environmental Protection Agency (EPA) and several state agencies in regulating hydrogen sulfide (H2S); (2) uncertainty about ambient exposure to H2S; (3) confusion and disagreement in the literature about possible health effects at low-level exposures; and (4) presentation of results of a series of recent animal bioassays. The American Petroleum Institute (API) proposed this symposium and the EPA became an early co-sponsor, with the Chemical Industry Institute of Toxicology (CIIT) and the American Forest & Paper Association (AF&PA) contributing expertise and funding assistance. The topics covered in this symposium included Animal Research, Human Research, Mode-of-Action and Dosimetry Issues, Environmental Exposure and Monitoring, Assessment and Regulatory Issues, and closed with a panel discussion. The overall goals of the symposium were to: gather together experts in H2S health effects research and individuals from governmental agencies charged with protecting the public health, provide a venue for reporting of recent research findings, identify gaps in the current information, and outline new research directions and promote research collaboration. During the course of the symposium, presenters provided comprehensive reviews of the state of knowledge for each topic. Several new research proposals discussed at the symposium have subsequently been initiated. This report provides a summary of the talks, poster presentations, and panel discussions that occurred at the Hydrogen Sulfide Health and Risk Assessment Symposium.

Approaches for applications of physiologically based pharmacokinetic models in risk assessment

Physiologically based pharmacokinetic (PBPK) models are particularly useful for simulating exposures to environmental toxicants for which, unlike pharmaceuticals, there is often little or no human data available to estimate the internal dose of a putative toxic moiety in a target tissue or an appropriate surrogate. This article reviews the current state of knowledge and approaches for application of PBPK models in the process of deriving reference dose, reference concentration, and cancer risk estimates. Examples drawn from previous U.S. Environmental Protection Agency (EPA) risk assessments and human health risk assessments in peer-reviewed literature illustrate the ways and means of using PBPK models to quantify the pharmacokinetic component of the interspecies and intraspecies uncertainty factors as well as to conduct route to route, high dose to low dose and duration extrapolations. The choice of the appropriate dose metric is key to the use of the PBPK models for the various applications in risk assessment. Issues related to whether uncertainty factors are most appropriately applied before or after derivation of human equivalent dose (or concentration) continue to be explored. Scientific progress in the understanding of life stage and genetic differences in dosimetry and their impacts on variability in susceptibility, as well as ongoing development of analytical methods to characterize uncertainty in PBPK models, will make their use in risk assessment increasingly likely. As such, it is anticipated that when PBPK models are used to express adverse tissue responses in terms of the internal target tissue dose of the toxic moiety rather than the external concentration, the scientific basis of, and confidence in, risk assessments will be enhanced.

A PBPK model for evaluating the impact of aldehyde dehydrogenase polymorphisms on comparative rat and human nasal tissue acetaldehyde dosimetry

Acetaldehyde is an important intermediate in the chemical synthesis and normal oxidative metabolism of several industrially important compounds, including ethanol, ethyl acetate, and vinyl acetate. Chronic inhalation of acetaldehyde leads to degeneration of the olfactory and respiratory epithelium in rats at concentrations > 50 ppm (90 day exposure) and respiratory and olfactory nasal tumors at concentrations > or = 750 ppm, the lowest concentration tested in the 2-yr chronic bioassay. Differences in the anatomy and biochemistry of the rodent and human nose, including polymorphisms in human high-affinity acetaldehyde dehydrogenase (ALDH2), are important considerations for interspecies extrapolations in the risk assessment of acetaldehyde. A physiologically based pharmacokinetic model of rat and human nasal tissues was constructed for acetaldehyde to support a dosimetry-based risk assessment for acetaldehyde (Dorman et al., 2008). The rodent model was developed using published metabolic constants and calibrated using upper-respiratory-tract acetaldehyde extraction data. The human nasal model incorporates previously published tissue volumes, blood flows, and acetaldehyde metabolic constants. ALDH2 polymorphisms were represented in the human model as reduced rates of acetaldehyde metabolism. Steady-state dorsal olfactory epithelial tissue acetaldehyde concentrations in the rat were predicted to be 409, 6287, and 12,634 microM at noncytotoxic (50 ppm), and cytotoxic/tumorigenic exposure concentrations (750 and 1500 ppm), respectively. The human equivalent concentration (HEC) of the rat no-observed-adverse-effect level (NOAEL) of 50 ppm, based on steady-state acetaldehyde concentrations from continual exposures, was 67 ppm. Respiratory and olfactory epithelial tissue acetaldehyde and H(+) (pH) concentrations were largely linear functions of exposure in both species. The impact of presumed ALDH2 polymorphisms on human olfactory tissue concentrations was negligible; the high-affinity, low-capacity ALDH2 does not contribute significantly to acetaldehyde metabolism in the nasal tissues. The human equivalent acetaldehyde concentration for homozygous low activity was 66 ppm, 1.5% lower than for the homozygous full activity phenotype. The rat and human acetaldehyde PBPK models developed here can also be used as a bridge between acetaldehyde dose-response and mode-of-action data as well as between similar databases for other acetaldehyde-producing nasal toxicants.

Application of physiological computational fluid dynamics models to predict interspecies nasal dosimetry of inhaled acrolein

Acrolein is a highly soluble and reactive aldehyde and is a potent upper-respiratory-tract irritant. Acrolein-induced nasal lesions in rodents include olfactory epithelial atrophy and inflammation, epithelial hyperplasia, and squamous metaplasia of the respiratory epithelium. Nasal uptake of inhaled acrolein in rats is moderate to high, and depends on inspiratory flow rate, exposure duration, and concentration. In this study, anatomically accurate three-dimensional computational fluid dynamics (CFD) models were used to simulate steady-state inspiratory airflow and to quantitatively predict acrolein tissue dose in rat and human nasal passages. A multilayered epithelial structure was included in the CFD models to incorporate clearance of inhaled acrolein by diffusion, blood flow, and first-order and saturable metabolic pathways. Kinetic parameters for these pathways were initially estimated by fitting a pharmacokinetic model with a similar epithelial structure to time-averaged acrolein nasal extraction data and were then further adjusted using the CFD model. Predicted air:tissue flux from the rat nasal CFD model compared well with the distribution of acrolein-induced nasal lesions from a subchronic acrolein inhalation study. These correlations were used to estimate a tissue dose-based no-observed-adverse-effect level (NOAEL) for inhaled acrolein. A human nasal CFD model was used to extrapolate effects in laboratory animals to human exposure conditions on the basis of localized tissue dose and tissue responses. Assuming that equivalent tissue dose will induce similar effects across species, a NOAEL human equivalent concentration for inhaled acrolein was estimated to be 8 ppb.

Basal gene expression in male and female Sprague-Dawley rat nasal respiratory and olfactory epithelium

The nasal epithelium is an important target site for chemically induced toxicity and carcinogenicity. Experimental studies show that site-specific lesions can arise within the nasal respiratory or olfactory epithelium following the inhalation of certain chemicals. Moreover, gender differences in epithelial response are also reported. To better understand and predict gender differences in response of the nasal epithelium to inhaled xenobiotics, gene expression profiles from naive male and female Sprague-Dawley rats were constructed. Epithelial cells were manually collected from the nasal septum, naso- and maxillo-turbinates, and ethmoid turbinates of nine male and nine female rats. Gene expression analysis was performed using the Affymetrix Rat Genome 430 2.0 microarray. Surprisingly, there were few gender differences in gene expression. Gene ontology enrichment analysis identified several functional categories, including xenobiotic metabolism, cell cycle, apoptosis, and ion channel/transport, with significantly different expression between tissue types. These baseline data will contribute to our understanding of the normal physiology and selectivity of the nasal epithelial cells' response to inhaled environmental toxicants.

Sensory irritation response in rats: modeling, analysis and validation

Inhaled gases can cause respiratory depression by irritating (stimulating) nerves in the nasal cavity. Respiratory depression, in turn, decreases the rate of delivery of those gases to the stimulated nerves, potentially leading to a complex feedback response. In order to better understand how the nervous system responds to such chemicals, a mathematical model is created to describe how the presence of irritants affects respiration in the rat. The ordinary differential equation model describes the dosimetry of these reactive gases in the respiratory tract, with particular focus on the physiology of the upper respiratory tract, and on the neurological control of respiration rate due to signaling from the irritant-responsive nerves in the nasal cavity. The ventilation equation is altered to account for an apparent change in dynamics between the initial ventilation decrease and the recovery to steady state as seen in formaldehyde exposure data. Further, the model is evaluated and improved through optimization of particular parameters to describe formaldehyde-induced respiratory response data and through sensitivity analysis. The model predicts the formaldehyde data well, and hence the model is thought to be a reasonable description of the physiological system of sensory irritation. The model is also expected to translate well to other irritants.

Uncertainties in the CIIT model for formaldehyde-induced carcinogenicity in the rat: a limited sensitivity analysis-I

Scientists at the CIIT Centers for Health Research (Conolly et al., 2000, 2003; Kimbell et al., 2001a, 2001b) developed a two-stage clonal expansion model of formaldehyde-induced nasal cancers in the F344 rat that made extensive use of mechanistic information. An inference of their modeling approach was that formaldehyde-induced tumorigenicity could be optimally explained without the role of formaldehyde's mutagenic action. In this article, we examine the strength of this result and modify select features to examine the sensitivity of the predicted dose response to select assumptions. We implement solutions to the two-stage cancer model that are valid for nonhomogeneous models (i.e., models with time-dependent parameters), thus accounting for time dependence in variables. In this reimplementation, we examine the sensitivity of model predictions to pooling historical and concurrent control data, and to lumping sacrificed animals in which tumors were discovered incidentally with those in which death was caused by the tumors. We found the CIIT model results were not significantly altered with the nonhomogeneous solutions. Dose-response predictions below the range of exposures where tumors occurred in the bioassays were highly sensitive to the choice of control data. In the range of exposures where tumors were observed, the model attributed up to 74% of the added tumor probability to formaldehyde's mutagenic action when our reanalysis restricted the use of the National Toxicology Program (NTP) historical control data to only those obtained from inhalation exposures. Model results were insensitive to hourly or daily temporal variations in DNA protein cross-link (DPX) concentration, a surrogate for the dose-metric linked to formaldehyde-induced mutations, prompting us to utilize weekly averages for this quantity. Various other biological and mathematical uncertainties in the model have been retained unmodified in this analysis. These include model specification of initiated cell division and death rates, and uncertainty and variability in the dose response for cell replication rates, issues that will be considered in a future paper.

A physiological toxicokinetic model for inhaled propylene oxide in rat and human with special emphasis on the nose

Chronic exposure to high concentrations of PO induced inflammation in the respiratory nasal mucosa (RNM) of rodents and, for concentrations >or= 300 ppm, caused nasal tumors. Considering the nose to be the most relevant target organ for PO-induced tumorigenicity, we developed a physiological toxicokinetic model for PO in rats and humans. It includes compartments for arterial, venous, and pulmonary blood, liver, muscle, fat, richly perfused tissues, lung, and nose. It simulates inhalation of PO, its distribution into tissues by blood flow, and its elimination by exhalation and metabolism. In nose, lung, and liver of rats, PO conjugation with glutathione (GSH), PO-induced GSH depletion, and formation of PO adducts to DNA are described. Also modeled are PO adducts to hemoglobin of rats and humans. Required partition coefficients and metabolic parameters were derived experimentally or from publications. In rats, simulated PO concentrations in blood and GSH levels in tissues agreed with measured data. If compared with reported values, levels of adducts with hemoglobin were underpredicted up to a factor of about 2. Adducts with DNA differed up to a factor of 3. Hemoglobin adducts predicted for PO-exposed workers were 1.5-1.9 times higher than the reported ones. Considering identical conditions of PO exposure, similar PO concentrations in RNM were modeled for rats and humans. Also, PO concentrations in blood, about 1/30th of those in RNM, were similar in both species. Since the model was evaluated on all available data in rats and humans, we consider it to be useful for estimating the risk from inhalation exposure to PO.

Interspecies dose extrapolation for inhaled dimethyl sulfate: a PBPK model-based analysis using nasal cavity N7-methylguanine adducts

Dimethyl sulfate (DMS) is a volatile sulfuric acid ester used principally as a methylating agent in a wide variety of industrial applications. DMS reacts with organic macromolecules by a SN2 mechanism. The weight of experimental evidence suggests that DMS possesses genotoxic and carcinogenic potential. Inhalation studies have shown that repeated exposure to DMS leads to tumors in the nasal cavity and lower respiratory tract in both rats and mice. Here we present a quantitative assessment for cross-species dose extrapolation for inhaled DMS using a physiologically based pharmacokinetic (PBPK) model. The model is designed to simulate N7-methylguanine (N7 mG) DNA adduct levels in the nasal mucosa following DMS exposure in rats and humans. This model was parameterized and predictions were tested by comparison against experimentally measured N7 mG DNA adduct levels in rat nasal mucosa following inhalation exposure to DMS. The model-based interspecies dose comparison, using N7 mG adduct levels in the nasal respiratory tissue as the appropriate dose metrics, predicts a dose rate seven times higher in rats compared to humans.

Development of a physiologically based pharmacokinetic model for propylene glycol monomethyl ether and its acetate in rats and humans

Propylene glycol monomethyl ether (PM), along with its acetate, is the most widely used of the propylene glycol ether family of solvents. The most common toxic effects of PM observed in animal studies include sedation, very slight alpha(2u)-globulin mediated nephropathy (male rats only) and hepatomegally at high exposures (typically > 1000 ppm). Sedation in animal studies usually resolves within a few exposures to 3000 ppm (the highest concentration used in subchronic and chronic inhalation studies) due to the induction of metabolizing enzymes. Data from a variety of pharmacokinetic and mechanistic studies have been incorporated into a PBPK model for PM and its acetate in rats and mice. Published controlled exposure and workplace biomonitoring studies have also been included for comparisons of the internal dosimetry of PM and its acetate between laboratory animals and humans. PM acetate is rapidly hydrolyzed to PM, which is further metabolized to either glucuronide or sulfate conjugates (minor pathways) or propylene glycol (major pathway). In vitro half-lives for PM acetate range from 14 to 36 min depending upon the tissue and species. In vivo half-lives are considerably faster, reflecting the total contributions of esterases in the blood and tissues of the body, and are on the order of just a few minutes. Thus, very little PM acetate is found in vivo and, other than potential portal of entry irritation, the toxicity of PM acetate is related to PM. Regardless of the source for PM (either PM or its acetate), rats were predicted to have a higher Cmax and AUC for PM in blood than humans, especially at concentrations greater than the current ACGIH TLV of 100 ppm. This would indicate that the major systemic effects of PM would be expected to be less severe in humans than rats at comparable inhalation exposures.

Validation of human physiologically based pharmacokinetic model for vinyl acetate against human nasal dosimetry data

Vinyl acetate has been shown to induce nasal lesions in rodents in inhalation bioassays. A physiologically based pharmacokinetic (PBPK) model for vinyl acetate has been used in human risk assessment, but previous in vivo validation was conducted only in rats. Controlled human exposures to vinyl acetate were conducted to provide validation data for the application of the model in humans. Five volunteers were exposed to 1, 5, and 10 ppm 13C1,13C2 vinyl acetate via inhalation. A probe inserted into the nasopharyngeal region sampled both 13C1,13C2 vinyl acetate and the major metabolite 13C1,13C2 acetaldehyde during rest and light exercise. Nasopharyngeal air concentrations were analyzed in real time by ion trap mass spectrometry (MS/MS). Experimental concentrations of both vinyl acetate and acetaldehyde were then compared to predicted concentrations calculated from the previously published human model. Model predictions of vinyl acetate nasal extraction compared favorably with measured values of vinyl acetate, as did predictions of nasopharyngeal acetaldehyde when compared to measured acetaldehyde. The results showed that the current PBPK model structure and parameterization are appropriate for vinyl acetate. These analyses were conducted from 1 to 10 ppm vinyl acetate, a range relevant to workplace exposure standards but which would not be expected to saturate vinyl acetate metabolism. Risk assessment based on this model further concluded that 24 h per day exposures up to 1 ppm do not present concern regarding cancer or non-cancer toxicity. Validation of the vinyl acetate human PBPK model provides support for these conclusions.

Physiologically-based pharmacokinetic and toxicokinetic models in cancer risk assessment

Physiologically-based pharmacokinetic (PBPK) and toxicokinetic models are increasingly being used for the conduct of high dose to low dose and interspecies extrapolations required in cancer risk assessment. These models, by simulating tissue dose of toxic chemicals, help address the uncertainty associated with the default approaches for interspecies and high dose to low dose extrapolations. The applicability of PBPK models in cancer risk assessment has been demonstrated with a number of chemicals (e.g., acrylonitrile, 2-butoxyethanol, chloroform, 1,4-dioxane, methyl chloroform, methylene chloride, styrene, trichloroethylene, tetrachloroethylene, vinyl chloride, vinyl acetate). Recent advances in PBPK modeling facilitate the consideration of population distribution of parameter values, age-dependent changes in physiology and metabolism, multi-route exposures as well as multichemical interactions for application in cancer risk assessment. Whereas the average values for various input parameters have been used to evaluate the age-dependency of tissue dose, the Markov Chain Monte Carlo technique can be applied to address variability and uncertainty in parameter estimates, thus facilitating a more accurate estimation of cancer risk in the population. The PBPK models also uniquely facilitate the simulation of tissue dose, and thereby cancer risks, associated with multi-route and multichemical exposure situations. Overall, the recent advances reviewed in this article point to the continued enhancement of the scientific basis and applicability of PBPK models in cancer risk assessment.

The impact of cytochrome P450 2E1-dependent metabolic variance on a risk-relevant pharmacokinetic outcome in humans

Risk assessments include assumptions about sensitive subpopulations, such as the fraction of the general population that is sensitive and the extent that biochemical or physiological attributes influence sensitivity. Uncertainty factors (UF) account for both pharmacokinetic (PK) and pharmacodynamic (PD) components, allowing the inclusion of risk-relevant information to replace default assumptions about PK and PD variance (uncertainty). Large numbers of human organ donor samples and recent advances in methods to extrapolate in vitro enzyme expression and activity data to the intact human enable the investigation of the impact of PK variability on human susceptibility. The hepatotoxicity of trichloroethylene (TCE) is mediated by acid metabolites formed by cytochrome P450 2E1 (CYP2E1) oxidation, and differences in the CYP2E1 expression are hypothesized to affect susceptibility to TCE's liver injury. This study was designed specifically to examine the contribution of statistically quantified variance in enzyme content and activity on the risk of hepatotoxic injury among adult humans. We combined data sets describing (1) the microsomal protein content of human liver, (2) the CYP2E1 content of human liver microsomal protein, and (3) the in vitro Vmax for TCE oxidation by humans. The 5th and 95th percentiles of the resulting distribution (TCE oxidized per minute per gram liver) differed by approximately sixfold. These values were converted to mg TCE oxidized/h/kg body mass and incorporated in a human PBPK model. Simulations of 8-hour inhalation exposure to 50 ppm and oral exposure to 5 micro g TCE/L in 2 L drinking water showed that the amount of TCE oxidized in the liver differs by 2% or less under extreme values of CYP2E1 expression and activity (here, selected as the 5th and 95th percentiles of the resulting distribution). This indicates that differences in enzyme expression and TCE oxidation among the central 90% of the adult human population account for approximately 2% of the difference in production of the risk-relevant PK outcome for TCE-mediated liver injury. Integration of in vitro metabolism information into physiological models may reduce the uncertainties associated with risk contributions of differences in enzyme expression and the UF that represent PK variability.

Vinyl acetate decreases intracellular pH in rat nasal epithelial cells

Vinyl acetate is a synthetic organic ester that has been shown to produce nasal tumors in rats following exposure to 600 ppm in air. The proposed mechanism of action involves the metabolism of vinyl acetate by carboxylesterases and the production of protons leading to cellular acidification. While vinyl acetate-induced decreases in intracellular pH (pHi) have been demonstrated in rat hepatocytes, comparable data from nasal epithelial cells do not exist. Using an in vitro assay system, we have determined the effects of vinyl acetate exposure on pHi in respiratory and olfactory nasal epithelial cells from rats. The respiratory and olfactory epithelial cells were isolated from dissected maxillo- and ethmoturbinates by enzyme digestion. The cells were plated; loaded with the pH-sensitive dye, carboxyseminaphthorhodafluor-1 (SNARF-1); and observed using confocal microscopy. Individual cellular analysis demonstrated that both respiratory and olfactory epithelial cells responded to vinyl acetate exposures with a dose-dependent decrease in pHi. Changes occurred at 100 microM but reached a plateau above 250 microM. Maximal decreases in pHi were 0.3 pH unit in respiratory epithelial cells. While pHi values were normally distributed for the respiratory epithelial cells, the olfactory epithelial cells demonstrated a bimodal distribution, indicating at least two populations of cells, with only one population of cells responding to vinyl acetate. Acidification in these cells did not plateau but continued to increase at 1000 microM. Bis(p-nitrophenyl)phosphate (BNPP), a carboxylesterase inhibitor, was able to attenuate the vinyl acetate-induced decrease in pHi. Data obtained from the isolated cells were validated using tissue explants. These results are consistent with the proposed mode of action for vinyl acetate and supply further data for developing appropriate risk assessments for vinyl acetate exposure.

Challenging dogma: thresholds for genotoxic carcinogens? The case of vinyl acetate

Although many questions remain unanswered, the general principle of the sequence of events leading to cancer after exposure to genotoxic carcinogens has become increasingly clear. This helps to understand the parameters that influence the shape of the dose-effect curve for carcinogenesis, including metabolic activation and inactivation of carcinogens, DNA repair, cell cycle control, apoptosis, and control by the immune system. A linear dose-response relationship with no observable threshold seems to be a conservative but adequate description for the carcinogenic activity of many genotoxic carcinogens, such as aflatoxin B1, the tobacco-specific nitrosoketone NNK, and probably N,N-diethylnitrosamine. However, extrapolation models connecting the high-level risk to the zero intercept have clearly resulted in overestimations of risk. Vinyl acetate is an example that is discussed extensively in this review. At extremely high and toxic doses, vinyl acetate is carcinogenic in rats and mice and causes chromosomal aberrations. In tissues of contact, vinyl acetate is converted to acetic acid and acetaldehyde. Only when threshold levels are achieved do critical steps in the mechanism ultimately leading to cancer become active, namely pH reduction in exposed cells of more than 0.15 units leading to cytotoxicity, damage to DNA, and regenerative proliferation. Consistent with the known exposure to endogenous acetic acid and acetaldehyde, tissues sustain a certain level of exposure without adverse effects. Physiological modeling shows that the conditions necessary for carcinogenesis are in place only when threshold levels of vinyl acetate are exceeded. The example of vinyl acetate underlines the importance of toxicological research that unequivocally identifies genotoxic carcinogens acting through a threshold process.

Differentiating between local cytotoxicity, mitogenesis, and genotoxicity in carcinogen risk assessments: the case of vinyl acetate

Understanding the mode of action of carcinogens is critical to scientifically assessing exposure-related risk. Regulatory hazard classification schemes and dose-response assessment paradigms generally require basic knowledge of genotoxic potential to guide decisions on which scheme or paradigm is most appropriate. Although convention suggests that classification and dose-response assessment of genotoxic chemicals should be assessed using conservative assumptions of no threshold, several examples, such as vinyl acetate, exist that challenge this assumption. Vinyl acetate is carcinogenic at portals of entry (nasal cavity and upper gastrointestinal tract). Local metabolism of vinyl acetate produces DNA-reactive acetaldehyde but also produces acetic acid and protons, which contribute to intracellular acidification, cytotoxicity and cell proliferation. This paper reviews their relative contributions to the overall mode of action. Elevated cellular proliferation, well understood to be a risk factor for carcinogenesis, is observed at concentrations associated with tumor formation. Cytotoxicity and compensatory tissue regeneration is one pathway for stimulating cellular proliferation while intracellular acidification is a mitogenic stimulus. Both of these pathways may be operative in nasal tissues while mitogenic proliferation alone appears to be induced in the upper gastrointestinal tract. Using a physiologically-based pharmacokinetic model, quantitative relationships between critical tissue dosimeters and tissue responses are developed to assess the relative importance of genotoxicity and cell proliferation in the overall mode of action of vinyl acetate. This approach supports the concept that intracellular acidification is the sentinel response that precedes cytotoxicity and cellular proliferation. Secondarily, the carcinogenic potential of vinyl acetate is expressed only when tissue exposure to acetaldehyde is high and when cellular proliferation is simultaneously elevated. This mode of action suggests that exposure levels that do not increase intracellular acidification beyond homeostatic bounds will be adequately protective of adverse downstream responses including cancer. These mechanistic insights provide the scientific basis for a cancer classification that incorporates thresholds for cytotoxic and/or mitogenic cell proliferation secondary to intracellular acidification.

Physiologically-based kinetic modeling of vapours toxic to the respiratory tract

The respiratory tract is frequently identified as a site of toxicity for inhaled xenobiotic chemicals. Usually, these observations come from controlled animal studies. For these studies to be of quantitative value to human health risk assessment, species-specific factors governing dosimetry of inhaled substances must be taken into account. Toxicokinetics of vapours in the respiratory tract are defined by absorption, distribution, metabolism, and excretion, as they are in other tissues; however, these concepts take on new dimensions when considering respiratory tract toxicants, especially those that elicit portal of entry effects by directly interacting with the tissue lining the respiratory tract. Species-specific factors related to anatomy, physiology and biochemistry govern inter-species extrapolation of toxicokinetics. This article discusses critical factors of respiratory tract kinetics that should be considered when developing physiological-based toxicokinetic (PBTK) models for inhaled vapours. Important considerations such as impact of regional airflow-delivery, water solubility, reactivity, and rates of local biotransformation on respiratory tract tissue dosimetry are highlighted. These factors can be accounted for only to a limited extent when using default approaches to extrapolate dosimetry of inhaled substances across species. On the other hand, PBTK modeling has the flexibility to accommodate many of the critical determinants of respiratory tract toxicity. PBTK models can also help identify the most critical toxicokinetic data necessary to replace defaults. PBTK approaches have led to more informed estimates of human target tissue dose, and therefore human health risk, especially where these risk assessments have been based on extrapolation of animal dosimetry studies. Experience derived from the development of more intensive case studies have, in turn, enabled simplified approaches to the use of PBTK modeling for respiratory tract toxicants. Whether simplified or highly complex, PBTK modeling approaches are proven to be of great utility to risk assesors interested in applying quantitative information to informed risk assessment evaluations.

Toxicokinetic modeling and its applications in chemical risk assessment

In recent years physiologically based pharmacokinetic (PBPK) modeling has found frequent application in risk assessments where PBPK models serve as important adjuncts to studies on modes of action of xenobiotics. In this regard, studies on mode of action provide insight into both the sites/mechanisms of action and the form of the xenobiotic associated with toxic responses. Validated PBPK models permit calculation of tissue doses of xenobiotics and metabolites for a variety of conditions, i.e. at low-doses, in different animal species, and in different members of a human population. In this manner, these PBPK models support the low-dose and interspecies extrapolations that are important components of current risk assessment methodologies. PBPK models are sometimes referred to as physiological toxicokinetic (PT) models to emphasize their application with compounds causing toxic responses. Pharmacokinetic (PK) modeling in general has a rich history. Data-based PK compartmental models were developed in the 1930's when only primitive tools were available for solving sets of differential equations. These models were expanded in the 1960's and 1970's to accommodate new observations on dose-dependent elimination and flow-limited metabolism. The application of clearance concepts brought many new insights about the disposition of drugs in the body. In the 1970's PBPK/PT models were developed to evaluate metabolism of volatile compounds of occupational importance, and, for the first time, dose-dependent processes in toxicology were included in PBPK models in order to assess the conditions under which saturation of metabolic and elimination processes lead to non-linear dose response relationships. In the 1980's insights from chemical engineers and occupational toxicology were combined to develop PBPK/PT models to support risk assessment with methylene chloride and other solvents. The 1990's witnessed explosive growth in risk assessment applications of PBPK/PT models and in applying sensitivity and variability methods to evaluate model performance. Some of the compounds examined in detail include butadiene, styrene, glycol ethers, dioxins and organic esters/aids. This paper outlines the history of PBPK/PT modeling, emphasizes more recent applications of PBPK/TK models in health risk assessment, and discusses the risk assessment perspective provided by modern uses of these modeling approaches.

Toxicokinetics and physiologically based toxicokinetics in toxicology and risk assessment

Toxicokinetics is the study of kinetics of absorption, distribution, metabolism, and excretion of a xenobiotic under the conditions of toxicity evaluation. Conventional toxicokinetics uses the hypothetical compartments, and the model is composed of rate equations that describe the time course of drug and chemical disposition. The utility of toxicokinetics in toxicity evaluation and interpretation of animal toxicology data is emerging as an important tool in product discovery and development. With implementation of the International Conference on Harmonization (ICH) guidelines on systemic exposure and dose selection, toxicokinetics have been integrated in routine toxicity evaluations. Although traditional compartmental/noncompartmental models are generally adequate for assessing systemic exposure, they are unable to the predict time course of drug disposition in target tissues and often fail to relate systemic drug levels to a biological response. Physiologically based toxicokinetic (PB-TK) models address this deficiency of traditional compartmental models. PB-TK models are the kinetic models of the uptake and disposition of chemicals based on rates of biochemical reactions, physiological and anatomical characteristics. These models, when developed appropriately, can predict target organ drug distribution in different species under variety of conditions. This minireview discusses the basic principles, and applications of traditional compartmental toxicokinetic and physiologically based toxicokinetics (PB-TK) models in drug development and risk assessment. Special emphasis will be placed on discussion related to interpretation of the ICH guidelines related to toxicokinetics and the utility of toxicokinetics data in dose selection for toxicity and carcinogenicity studies. The utility of PB-TK models in risk assessment of methylene chloride, vinyl chloride, retinoic acid, dioxin, and inhaled organic esters is discussed.

Histochemical localisation of carboxylesterase activity in rat and mouse oral cavity mucosa

Vinyl acetate (VA) is widely used within the chemical industry, in the manufacture of polyvinyl alcohol, and as polyvinyl acetate emulsions in latex paints, adhesives, paper and paper board coatings. Chronic oral exposure of rodents to high concentrations of VA induces tumours within the oral cavity. Carboxylesterase-dependent hydrolysis of VA is thought to be critical in the development of nasal tumours following inhalation exposure of animals to VA. Therefore, carboxylesterase activity was determined histochemically in the oral cavities of male F344 rats and BDF mice in order to explore the potential role of carboxylesterase-dependent hydrolysis of VA in the development of oral tumours. Following fixation in 10% neutral buffered formalin heads were decalcified in neutral saturated EDTA, embedded in resin, sectioned at six levels (three each for the upper and lower jaws), and carboxylesterase activity revealed in the tissue using alpha-naphthyl butyrate as substrate. The localisation of carboxylesterase activity in freshly dissected rat oral tissue was compared to that of the resin sections and found to be identical, thus validating the decalcification process. A similar pattern of carboxylesterase activity was observed for the two species. Staining was low in areas surrounding the teeth, and medium/high in the buccal mucosa, the central/posterior upper palate and those regions of the lower jaw not proximal to the teeth. In general the intensity of staining was greater in sections from the rat compared to those from the mouse. By comparison, carboxylesterase activity was considerably higher in mouse nasal olfactory epithelium than in any of the oral tissues. Thus the mucosa of the oral cavity has the potential to hydrolyse VA to its metabolites, acetic acid and acetaldehyde, and the presence of carboxylesterases at this site is consistent with, and may be an important determining factor in, the development of oral cavity tumours following exposure to VA.

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.

Regional distribution and kinetics of vinyl acetate hydrolysis in the oral cavity of the rat and mouse

Vinyl acetate is an ester that is used to make polyvinyl acetate based polymers that are used in the manufacture of latex paints and adhesives. Chronic oral exposure to high concentrations of vinyl acetate induces oral tumors in rodents. Since carboxylesterase-dependent hydrolysis of VA to acetic acid and acetaldehyde has been implicated in the nasal inhalation carcinogenesis of this ester, the potential for oral mucosa of the F344 rat and BDF mouse to hydrolyze VA was examined. Homogenates were prepared by scraping the mucosa from four regions of the oral cavity: dorsal interior (all tissues interior to the teeth), dorsal tongue surface, ventral interior (sublingual area and lower interior tissues) and exterior (all tissues exterior to the teeth). The oral cavity was rinsed once with saline prior to dissection to determine if oral secretions possessed metabolic capacity. Aliquots of the homogenates or rinse fluid were incubated for 30 min with varying concentrations of vinyl acetate (0.05-10 mM), and the production of acetaldehyde was quantitated by HPLC. All tissue regions possessed VA hydrolysis activity. In both species the hydrolysis activity was greatest in the dorsal interior region (Vmax of 90 and 6 nmol/min in the rat and mouse, respectively, Km values of 0.5 and 0.9 mM). Activity in the other oral regions was 2-15-fold lower. Activity was observed in the rinse fluid, but was 20-fold or more lower than the dorsal interior region. Finally, solutions of vinyl acetate were placed in the mouth of anesthetized rats for 10 min and then analyzed for acetaldehyde concentrations. Acetaldehyde was detected in these solutions providing evidence that metabolism of this ester occurs in vivo in oral tissues.

Dosimetry modeling of inhaled formaldehyde: binning nasal flux predictions for quantitative risk assessment

Interspecies extrapolations of tissue dose and tumor response have been a significant source of uncertainty in formaldehyde cancer risk assessment. The ability to account for species-specific variation of dose within the nasal passages would reduce this uncertainty. Three-dimensional, anatomically realistic, computational fluid dynamics (CFD) models of nasal airflow and formaldehyde gas transport in the F344 rat, rhesus monkey, and human were used to predict local patterns of wall mass flux (pmol/[mm(2)-h-ppm]). The nasal surface of each species was partitioned by flux into smaller regions (flux bins), each characterized by surface area and an average flux value. Rat and monkey flux bins were predicted for steady-state inspiratory airflow rates corresponding to the estimated minute volume for each species. Human flux bins were predicted for steady-state inspiratory airflow at 7.4, 15, 18, 25.8, 31.8, and 37 l/min and were extrapolated to 46 and 50 l/min. Flux values higher than half the maximum flux value (flux median) were predicted for nearly 20% of human nasal surfaces at 15 l/min, whereas only 5% of rat and less than 1% of monkey nasal surfaces were associated with fluxes higher than flux medians at 0.576 l/min and 4.8 l/min, respectively. Human nasal flux patterns shifted distally and uptake percentage decreased as inspiratory flow rate increased. Flux binning captures anatomical effects on flux and is thereby a basis for describing the effects of anatomy and airflow on local tissue disposition and distributions of tissue response. Formaldehyde risk models that incorporate flux binning derived from anatomically realistic CFD models will have significantly reduced uncertainty compared with risk estimates based on default methods.

Mode of action and tissue dosimetry in current and future risk assessments

Two fundamental concepts have emerged to organize contemporary approaches to chemical risk assessment - mode of action and tissue dosimetry. Mode of action specifies the nature of the interactions between the chemical and the body that lead to toxic responses and should, under optimal circumstances, also specify the form of the tissue dose that leads to these effects. This paper highlights recent development of biologically based dose response (BBDR) models for specific toxic endpoints that use knowledge on mode of action to specify measures of dose. These dose measures then are used to support low dose and interspecies extrapolations. We first focus on a series of dose response models developed for several compounds that produce nasal toxicity. These examples demonstrate a range of model structures from simple dosimetry models (methylmethacrylate) to linkage of dosimetry with specific biological processes involved in carcinogenesis (formaldehyde). Two BBDR models with dioxin illustrate the organization of biological and dosimetry information into specific testable hypotheses that could distinguish these different models and lead to a more uniform approach to risk assessment for this compound. A final section discusses the impact of molecular biology and the genomic revolution in relation to development of BBDR models for specific toxic endpoints.

Health risks associated with inhaled nasal toxicants

Health risks of inhaled nasal toxicants were reviewed with emphasis on chemically induced nasal lesions in humans, sensory irritation, olfactory and trigeminal nerve toxicity, nasal immunopathology and carcinogenesis, nasal responses to chemical mixtures, in vitro models, and nasal dosimetry- and metabolism-based extrapolation of nasal data in animals to humans. Conspicuous findings in humans are the effects of outdoor air pollution on the nasal mucosa, and tobacco smoking as a risk factor for sinonasal squamous cell carcinoma. Objective methods in humans to discriminate between sensory irritation and olfactory stimulation and between adaptation and habituation have been introduced successfully, providing more relevant information than sensory irritation studies in animals. Against the background of chemoperception as a dominant window of the brain on the outside world, nasal neurotoxicology is rapidly developing, focusing on olfactory and trigeminal nerve toxicity. Better insight in the processes underlying neurogenic inflammation may increase our knowledge of the causes of the various chemical sensitivity syndromes. Nasal immunotoxicology is extremely complex, which is mainly due to the pivotal role of nasal lymphoid tissue in the defense of the middle ear, eye, and oral cavity against antigenic substances, and the important function of the nasal passages in brain drainage in rats. The crucial role of tissue damage and reactive epithelial hyperproliferation in nasal carcinogenesis has become overwhelmingly clear as demonstrated by the recently developed biologically based model for predicting formaldehyde nasal cancer risk in humans. The evidence of carcinogenicity of inhaled complex mixtures in experimental animals is very limited, while there is ample evidence that occupational exposure to mixtures such as wood, leather, or textile dust or chromium- and nickel-containing materials is associated with increased risk of nasal cancer. It is remarkable that these mixtures are aerosols, suggesting that their "particulate nature" may be a major factor in their potential to induce nasal cancer. Studies in rats have been conducted with defined mixtures of nasal irritants such as aldehydes, using a model for competitive agonism to predict the outcome of such mixed exposures. When exposure levels in a mixture of nasal cytotoxicants were equal to or below the "No-Observed-Adverse-Effect-Levels" (NOAELs) of the individual chemicals, neither additivity nor potentiation was found, indicating that the NOAEL of the "most risky chemical" in the mixture would also be the NOAEL of the mixture. In vitro models are increasingly being used to study mechanisms of nasal toxicity. However, considering the complexity of the nasal cavity and the many factors that contribute to nasal toxicity, it is unlikely that in vitro experiments ever will be substitutes for in vivo inhalation studies. It is widely recognized that a strategic approach should be available for the interpretation of nasal effects in experimental animals with regard to potential human health risk. Mapping of nasal lesions combined with airflow-driven dosimetry and knowledge about local metabolism is a solid basis for extrapolation of animal data to humans. However, more research is needed to better understand factors that determine the susceptibility of human and animal tissues to nasal toxicants, in particular nasal carcinogens.

Mass transport analysis: inhalation rfc methods framework for interspecies dosimetric adjustment

In 1994, the U.S. Environmental Protection Agency introduced dosimetry modeling into the methods used to derive an inhalation reference concentration (RfC). The type of dosimetric adjustment factor (DAF) applied had to span the range of physicochemical characteristics of the gases listed on the Clean Air Act Amendments in 1991 as hazardous air pollutants (HAPs) and accommodate differences in available data with respect to their toxicokinetic properties. A framework was proposed that allowed for a hierarchy of dosimetry model structures, from optimal to rudimentary, and a category scheme that provided for limiting model structures based on physicochemical and toxicokinetic properties. These limiting cases were developed from restricting consideration to specific properties relying on an understanding of the generalized system based on mass transport theory. Physiochemical characteristics included the solubility and reactivity (e.g., propensity to dissociate, oxidize, or serve as a metabolic substrate) of the gas and were used as major determinants of absorption. Dosimetric adjustments were developed to evaluate portal of entry (POE) effects as well as remote (systemic) effects relevant to the toxicokinetic properties of the gas of interest. The gas categorization scheme consisted of defining three gas categories: (1) gases that are highly soluble and/or reactive, absorbing primarily in the extrathoracic airways; (2) gases that are moderately soluble and/or reactive, absorbing throughout the airways, as well as accumulating in the bloodstream; and (3) gases that have a low water solubility and are lipid soluble such that they are primarily absorbed in the pulmonary region and likely to act systemically. This article presents the framework and the mass transport theory behind the RfC method. Comparison to compartmental approaches and considerations for future development are also discussed.