Rat lung tumors induced by exposure to selected poorly soluble nonfibrous particles

A Weight-of-the-Evidence Approach for Evaluating, in Lieu of Animal Studies, the Potential of a Novel Polysaccharide Polymer to Produce Lung Overload

The United States Environmental Protection Agency (EPA) is concerned about the respiratory effects caused by respirable particles of water-insoluble high molecular weight polymers. The EPA has proposed a tiered approach to evaluate polymer lung overload, a kinetic event. Kinetic polymer lung overload in itself is not necessarily adverse, however, inhalation of respirable particulate matter can have adverse effects (i.e., inflammation, fibrosis, etc.). If Tier I testing demonstrates that particles may reach the distal lung (i.e., a non-negligible amount of respirable particles/droplets ≤10 μm in diameter and lack of biosolubility), then animal inhalation testing in Tiers II-IV would be requested. In silico, in chemico, and in vitro alternatives should be considered versus in vivo testing for animal welfare purposes. An in chemico measure of biosolubility was used to demonstrate that a novel α-1,3-glucan polysaccharide, made by enzymatic polymerization of glucose from sucrose, is biosoluble and fits a simple exponential decay model with a half-life on the order of 66 days. The multiple-path particle dosimetry (MPPD) in silico model was used to predict lung burden for the novel α-1,3-glucan polysaccharide. MPPD was validated with measurements in rats exposed to a toner particulate and showed good agreement with lung burden measurements. A simulated 24 month rat exposure yielded 10-20 times less lung burden for the polysaccharide compared to the toner at equivalent exposure concentrations. The MPPD model was refined to include biosolubility data for the polysaccharide polymer. Data for amorphous silica were used to validate the clearance model, and the model incorporating dissolution predicted the amorphous silica lung burden within 20% of measured values. Human equivalent concentrations (HECs) were calculated for each toner rat exposure concentration. HECs were also determined for the polysaccharide at exposure concentrations yielding the same predicted internal doses as the toner. The in vitro, in chemico and in silico studies described here for the novel polysaccharide provide a useful weight of evidence approach in the absence of animal studies for the evaluation of polymer substances where polymer lung overload may be a concern.

An SDS-PAGE based method for the quantification of carbon black in biological samples

Carbon black (CB) has a wide range of industrial applications and recently has been used as the basic model for environmental health studies on airborne particulate matters (PM). Exposure characterization of CB is always the first and most important step towards a better understanding of its effects on human health. However, efforts were largely limited by the lack of valid methods capable of quantifying CB in biological samples. Here, we developed a new method based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) for CB quantification in bio-samples, with a detection limit of 4.3 ng. The method is so economical and convenient that it can be performed in most biology labs. The application of the method was successfully demonstrated in three different cell models (mouse macrophage cells (Raw264.7), human epithelial cells (A549) and mouse mesenchymal stem cells (MSC)) and the results showed that the uptake rates decrease in the order of MSC > Raw264.7 > A549. The surprisingly highest uptake rate of MSC deserves further investigation. The novel method provides a complementary quantitative tool to the use of conventional methods such as radioactive and fluorescent labeling and may facilitate related toxicological studies.

Ultrafine CB-induced small airway obstruction in CB-exposed workers and mice

The potential threat of superfine carbon black (CB) particles to human health has received attention, but there are few human toxicological data available. The purpose of this study was to investigate the relationships between serum CC16 and SP-A with small airway related pulmonary functions in CB workers. Ninety-nine male CB packers and 115 non-CB-exposed healthy male workers were recruited. Serum CC16 and/or SP-A and pulmonary function tests were evaluated, and the relationship between them were also analyzed. To further assess pulmonary damage induced by CB particles in target organs, an animal inhalation exposure study was conducted. Male C57BL/6 mice were exposed to 15 and 30 mg/m³ CB for 6 h per day for 28 days. Levels of CC16 and SP-A were evaluated by ELISA and immunohistochemical staining (IHC). The results showed a 20% decreased in median CC16 and a 15% increase in median SP-A among CB-exposed workers. FEV1%, FEV/FVC, MMEF%, FEF25%, and FEF75% were also decreased in CB-exposed workers (P < 0.05). A significant positive correlation was observed between serum CC16 concentration and FEV1/FVC, although a negative correlation was found between serum SP-A concentration and FEV1/FVC. Serum CC16 was significantly reduced by 72% in mice with high CB-exposure, and serum SPA was 1.65x and 1.17x higher than CB-unexposed control mice in low and high CB-exposed mice, respectively. Lung CC16 and serum CC16 levels were positively correlated in mice (P = 0.024). Long-term exposure to ultrafine CB particles is associated with a decrease in CC16 and an increase in SP-A in the peripheral blood of CB-exposed workers. In conclusion, superfine CB particles have the potential to cause small airway obstruction.

Relevance of the rat lung tumor response to particle overload for human risk assessment-Update and interpretation of new data since ILSI 2000

The relevance of particle-overload related lung tumors in rats for human risk assessment following chronic inhalation exposures to poorly soluble particulates (PSP) has been a controversial issue for more than three decades. In 1998, an ILSI (International Life Sciences) Working Group of health scientists was convened to address this issue of applicability of experimental study findings of lung neoplasms in rats for lifetime-exposed production workers to PSPs. A full consensus view was not reached by the Workshop participants, although it was generally acknowledged that the findings of lung tumors in rats following chronic inhalation, particle-overload PSP exposures occurred only in rats and no other tested species; and that there was an absence of lung cancers in PSP-exposed production workers. Since the publication of the ILSI Workshop report in 2000, there have been important new data published on the human relevance issue. A thorough and comprehensive review of the health effects literature on poorly soluble particles/lung overload was undertaken and published by an ECETOC (European Centre for Ecotoxicology and Toxicology of Chemicals) Task Force in 2013. One of the significant conclusions derived from that technical report was that the rat is unique amongst all species in developing lung tumors under chronic inhalation overload exposures to PSPs. Accordingly, the objective of this review is to provide important insights on the fundamental differences in pulmonary responses between experimentally-exposed rats, other experimental species and occupationally-exposed humans. Briefly, five central factors are described by the following issues. Focusing on these five interrelated/convergent factors clearly demonstrate an inappropriateness in concluding that the findings of lung tumors in rats exposed chronically to high concentrations of PSPs are accurate representations of the risks of lung cancer in PSP-exposed production workers. The most plausible conclusion that can be reached is that results from chronic particle-overload inhalation studies with PSPs in rats have no relevance for determining lung cancer risks in production workers exposed for a working lifetime to these poorly soluble particulate-types.

Translational toxicology in setting occupational exposure limits for dusts and hazard classification - a critical evaluation of a recent approach to translate dust overload findings from rats to humans

BACKGROUND

We analyze the scientific basis and methodology used by the German MAK Commission in their recommendations for exposure limits and carcinogen classification of "granular biopersistent particles without known specific toxicity" (GBS). These recommendations are under review at the European Union level. We examine the scientific assumptions in an attempt to reproduce the results. MAK's human equivalent concentrations (HECs) are based on a particle mass and on a volumetric model in which results from rat inhalation studies are translated to derive occupational exposure limits (OELs) and a carcinogen classification.

METHODS

We followed the methods as proposed by the MAK Commission and Pauluhn 2011. We also examined key assumptions in the metrics, such as surface area of the human lung, deposition fractions of inhaled dusts, human clearance rates; and risk of lung cancer among workers, presumed to have some potential for lung overload, the physiological condition in rats associated with an increase in lung cancer risk.

RESULTS

The MAK recommendations on exposure limits for GBS have numerous incorrect assumptions that adversely affect the final results. The procedures to derive the respirable occupational exposure limit (OEL) could not be reproduced, a finding raising considerable scientific uncertainty about the reliability of the recommendations. Moreover, the scientific basis of using the rat model is confounded by the fact that rats and humans show different cellular responses to inhaled particles as demonstrated by bronchoalveolar lavage (BAL) studies in both species.

CONCLUSION

Classifying all GBS as carcinogenic to humans based on rat inhalation studies in which lung overload leads to chronic inflammation and cancer is inappropriate. Studies of workers, who have been exposed to relevant levels of dust, have not indicated an increase in lung cancer risk. Using the methods proposed by the MAK, we were unable to reproduce the OEL for GBS recommended by the Commission, but identified substantial errors in the models. Considerable shortcomings in the use of lung surface area, clearance rates, deposition fractions; as well as using the mass and volumetric metrics as opposed to the particle surface area metric limit the scientific reliability of the proposed GBS OEL and carcinogen classification.

Response to Morfeld (2013): Second commentary to Gebel 2012-established use of cancer potency indices and biological plausibility

The evaluations in Gebel (Arch Toxicol 86(7):995-1007, 2012) were carried out according to established procedures in regulatory toxicology. The variability in the available data was taken into account. The quality of the underlying data set should not be overestimated. The relevant conclusion in Gebel (Arch Toxicol 86(7):995-1007, 2012) is that the difference in carcinogenic potency comparing nanosized to microsized respirable granular biodurable particles without known significant specific toxicity (GBP) is low and lower than previously estimated.

Evaluation of immunohistochemical markers to detect the genotoxic mode of action of fine and ultrafine dusts in rat lungs

Data on local genotoxicity after particle exposure are crucial to resolve mechanistic aspects such as the impact of chronic inflammation, types of DNA damage, and their role in lung carcinogenesis. We established immunohistochemical methods to quantify the DNA damage markers poly(ADP-ribose) (PAR), phosphorylated H2AX (γ-H2AX), 8-hydroxyguanosine (8-OH-dG), and 8-oxoguanine DNA glycosylase (OGG1) in paraffin-embedded tissue from particle-exposed rats. The study was based on lungs from a subchronic study that was part of an already published carcinogenicity study where rats had been intratracheally instilled with saline, quartz DQ12, amorphous silica (Aerosil(®) 150), or carbon black (Printex(®) 90) at monthly intervals for 3 months. Lung sections were stained immunohistochemically and markers were quantified in alveolar lining cells. Local genotoxicity was then correlated with already defined endpoints, i.e. mean inflammation score, bronchoalveolar lavage parameters, and carcinogenicity. Genotoxicity was most pronounced in quartz DQ12-treated rats, where all genotoxicity markers gave statistically significant positive results, indicating considerable genotoxic stress such as occurrence of DNA double-strand breaks (DSB), and oxidative damage with subsequent repair activity. Genotoxicity was less pronounced for Printex(®) 90, but significant increases in γ-H2AX- and 8-OH-dG-positive nuclei and OGG1-positive cytoplasm were nevertheless detected. In contrast, Aerosil(®) 150 significantly enhanced only 8-OH-dG-positive nuclei and oxidative damage-related repair activity (OGG1) in cytoplasm. In the present study, γ-H2AX was the most sensitive genotoxicity marker, differentiating best between the three types of particles. The mean number of 8-OH-dG-positive nuclei, however, correlated best with the mean inflammation score at the same time point. This methodological approach enables integration of local genotoxicity testing in subchronic inhalation studies and makes immunohistochemical detection, in particular of γ-H2AX and 8-hydroxyguanine, a very promising approach for local genotoxicity testing in lungs, with prognostic value for the long-term outcome of particle exposure.

Relationship of pulmonary toxicity and carcinogenicity of fine and ultrafine granular dusts in a rat bioassay

The current carcinogenicity study with female rats focused on the toxicity and carcinogenicity of intratracheally instilled fine and ultrafine granular dusts. The positive control, crystalline silica, elicited the greatest magnitude and progression of pulmonary inflammatory reactions, fibrosis and the highest incidence of primary lung tumors (39.6%). Addition of poly-2-vinylpyridine-N-oxide decreased inflammatory responses, fibrosis, and the incidence of pulmonary tumors induced by crystalline quartz to 21.4%. After repeated instillation of soluble, ultrafine amorphous silica (15 mg) a statistically significant tumor response (9.4%) was observed, although, the inflammatory response in the lung was not as persistently severe as in rats treated with carbon black. Instillation of ultrafine carbon black (5 mg) caused a lung tumor incidence of 15%. In contrast to a preceding study using a dose of 66 mg coal dust, lung tumors were not detected after exposure to the same coal dust at a dose of 10 mg in this study. Pulmonary inflammatory responses to coal dust were very low indicating a mechanistic threshold for the development of lung tumors connected with particle related chronic inflammation. The animals treated with ultrafine carbon black and ultrafine amorphous silica showed significantly more severe lesions in non-cancerous endpoints when compared to animals treated with fine coal dust. Furthermore, carbon black treated rats showed more severe non-cancerous lung lesions than amorphous silica treated rats. Our data show a relationship between tumor frequencies and increasing scores when using a qualitative scoring system for specific non-cancerous endpoints such as inflammation, fibrosis, epithelial hyperplasia, and squamous metaplasia.

Nanotoxicology--a pathologist's perspective

Advances in chemistry and engineering have created a new technology, nanotechnology, involving the tiniest known manufactured products. These products have a rapidly increasing market share and appear poised to revolutionize engineering, cosmetics, and medicine. Unfortunately, nanotoxicology, the study of nanoparticulate health effects, lags behind advances in nanotechnology. Over the past decade, existing literature on ultrafine particles and respirable durable fibers has been supplemented by studies of first-generation nanotechnology products. These studies suggest that nanosizing increases the toxicity of many particulates. First, as size decreases, surface area increases, thereby speeding up dissolution of soluble particulates and exposing more of the reactive surface of durable but reactive particulates. Second, nanosizing facilitates movement of particulates across cellular and intracellular barriers. Third, nanosizing allows particulates to interact with, and sometimes even hybridize with, subcellular structures, including in some cases microtubules and DNA. Finally, nanosizing of some particulates, increases pathologic and physiologic responses, including inflammation, fibrosis, allergic responses, genotoxicity, and carcinogenicity, and may alter cardiovascular and lymphatic function. Knowing how the size and physiochemical properties of nanoparticulates affect bioactivity is important in assuring that the exciting new products of nanotechnology are used safely. This review provides an introduction to the pathology and toxicology of nanoparticulates.

Pulmonary oxidative stress, inflammation, and dysregulated iron homeostasis in rat models of cardiovascular disease

Underlying cardiovascular disease (CVD) is a risk factor for the exacerbation of air pollution health effects. Pulmonary oxidative stress, inflammation, and altered iron (Fe) homeostasis secondary to CVD may influence mammalian susceptibility to air pollutants. Rodent models of CVD are increasingly used to examine mechanisms of variation in susceptibility. Baseline cardiac and pulmonary disease was characterized in healthy normotensive Wistar Kyoto (WKY) rats, cardiovascular compromised spontaneously hypertensive rats (SHR), and spontaneously hypertensive heart failure (SHHF) rats. Blood pressure, heart rate, and breathing frequencies were measured in rats 11 to 12 wk of age, followed by necropsy at 14 to 15 wk of age. Blood pressure and heart rate were increased in SHR and SHHF relative to WKY rats (SHR > SHHF > WKY). Increased breathing frequency in SHHF and SHR (SHR > SHHF > WKY) resulted in greater minute volume relative to WKY. Bronchoalveolar lavage fluid (BALF) protein and neutrophils were higher in SHHF and SHR relative to WKY (SHHF >> SHR > WKY). Lung ascorbate and glutathione levels were low in SHHF rats. BALF Fe-binding capacity was decreased in SHHF relative to WKY rats and was associated with increased transferrin (Trf) and ferritin. However, lung ferritin was lower and Trf was higher in SHHF relative to WKY or SHR rats. mRNA for markers of inflammation and oxidative stress (macrophage inflammatory protein [MIP]-2, interleukin [IL]-1alpha, and heme oxygenase [HO]-1) were greater in SHHF and SHR relative to WKY rats. Trf mRNA rose in SHR but not SHHF relative to WKY rats, whereas transferrin receptors 1 and 2 mRNA was lower in SHHF rats. Four of 12 WKY rats exhibited cardiac hypertrophy despite normal blood pressure, while demonstrating some of the pulmonary complications noted earlier. This study demonstrates that SHHF rats display greater underlying pulmonary complications such as oxidative stress, inflammation, and impaired Fe homeostasis than WKY or SHR rats, which may play a role in SHHF rats' increased susceptibility to air pollution.

Pulmonary applications and toxicity of engineered nanoparticles

Because of their unique physicochemical properties, engineered nanoparticles have the potential to significantly impact respiratory research and medicine by means of improving imaging capability and drug delivery, among other applications. These same properties, however, present potential safety concerns, and there is accumulating evidence to suggest that nanoparticles may exert adverse effects on pulmonary structure and function. The respiratory system is susceptible to injury resulting from inhalation of gases, aerosols, and particles, and also from systemic delivery of drugs, chemicals, and other compounds to the lungs via direct cardiac output to the pulmonary arteries. As such, it is a prime target for the possible toxic effects of engineered nanoparticles. The purpose of this article is to provide an overview of the potential usefulness of nanoparticles and nanotechnology in respiratory research and medicine and to highlight important issues and recent data pertaining to nanoparticle-related pulmonary toxicity.

Comparative pathology of environmental lung disease: an overview

Environmental factors play a major role in a majority of lung diseases. Asthma, chronic obstructive pulmonary disease (COPD), lung cancer, and many interstitial lung diseases are influenced or caused by environmental factors. Animals and humans may respond differently to the same agent, and a study of the comparative pathology between the two is useful for optimizing animal models of environmental lung disease and for evaluating their predictive value in carcinogenicity studies. This overview describes the most common nonneoplastic pathologic pulmonary responses to inhaled environmental agents in the human and contrasts them with the responses observed in rats exposed to the same agents. We show both similarities and difference in response to the same agents; furthermore, both species have unique responses to some agents (for example, progressive massive fibrosis in the human and proliferative squamous lesions in the rat). Quantitative analysis of the grades of response to three environmental particulate dusts revealed differences between the 2 species at the cellular level. Specifically, acute intra-alveolar inflammation, alveolar epithelial hyperplasia, and alveolar lipoproteinosis were all greater in rats than in humans exposed to the same agents. These differences may account for differences between the 2 species in carcinogenic response to nonfibrous particulates.

The role of fiber durability/biopersistence of silica-based synthetic vitreous fibers and their influence on toxicology

This work summarizes what is known about the role of fiber durability/biopersistence of silica-based synthetic vitreous fibers (SVFs) and their influence on toxicology. The article describes the key processes leading from exposure to biological effect, including exposure, pulmonary deposition, clearance by various mechanisms, accumulation in the lung, and finally possible biological effects. The dose-dimension-durability paradigm is used to explain the key determinants of SVF toxicology. In particular, the key role played by the durability/biopersistence of long (>20microm) fibers is highlighted. Relevant literature on the prediction of in-vitro dissolution rates from chemical composition is summarized. Data from in-vitro and in-vivo durability/biopersistence tests show that these measures are highly correlated for long fibers. Both durability and biopersistence are correlated with the outcome of chronic inhalation bioassays. A schematic approach is presented for the design and testing of new SVFs with lower biopersistence.

Classification of man-made vitreous fibers: Comments on the revaluation by an IARC working group

In 2001, an IARC working group revaluated the carcinogenic risks of man-made vitreous fibers (MMVF). Compared with the IARC evaluation in 1987, the overall evaluations of insulation glass wool, rock (stone) wool, and slag wool were changed from Group 2B to Group 3. These changes ensued from an alteration in the evidence for cancer in humans and in experimental animals: Instead of "sufficient," the evidence for cancer in experimental animals is now looked upon as "limited" if there is a carcinogenic response after intraperitoneal injection but not after recently conducted inhalation experiments. For these studies, it is argued that they did properly address the technological limitations of earlier inhalation experiments. For Maxim and McConnell [Maxim L.D., McConnell E.E., 2001. Interspecies comparisons of the toxicity of asbestos and synthetic vitreous fibers: a weight-of-the-evidence approach. Regul. Toxicol. Pharmacol. 33, 319-342], well-conducted inhalation studies are very sensitive and rats may be more sensitive than humans in detecting the carcinogenic potential of MMVF. However, their arguments are highly questionable. The explanations of the IARC working group for preferring the newer inhalation studies are not sufficiently supported by the published data. Having in mind the higher sensitivity of humans compared to rats after inhalation of asbestos, more emphasis should have been given to the carcinogenic response after intraperitoneal injection.

Signal transduction pathways relevant for neoplastic effects of fibrous and non-fibrous particles

Apart from their genotoxic effects, both fibrous and non-fibrous particles are known to induce signalling pathways involved in the development of malignant lung diseases. Different direct effects of particles as well as indirect cellular effects are believed to induce changes in apoptosis or proliferation in target cells. Signalling events, e.g. the induction of mitogen-activated protein kinase (MAPK) cascades resulting in the activation of the transcription factor AP-1, as well as the induction of the transcription factor NFkappaB which mainly mediates the expression of pro-inflammatory genes are discussed. There is some insight into the molecular mechanisms eliciting these pathways. Therefore, this review aims to give an overview on signalling pathways as well as initial events including effects of reactive oxygen and nitrogen species, membrane receptors and particle uptake.

Toxicity of indium arsenide, gallium arsenide, and aluminium gallium arsenide

Gallium arsenide (GaAs), indium arsenide (InAs), and aluminium gallium arsenide (AlGaAs) are semiconductor applications. Although the increased use of these materials has raised concerns about occupational exposure to them, there is little information regarding the adverse health effects to workers arising from exposure to these particles. However, available data indicate these semiconductor materials can be toxic in animals. Although acute and chronic toxicity of the lung, reproductive organs, and kidney are associated with exposure to these semiconductor materials, in particular, chronic toxicity should pay much attention owing to low solubility of these materials. Between InAs, GaAs, and AlGaAs, InAs was the most toxic material to the lung followed by GaAs and AlGaAs when given intratracheally. This was probably due to difference in the toxicity of the counter-element of arsenic in semiconductor materials, such as indium, gallium, or aluminium, and not arsenic itself. It appeared that indium, gallium, or aluminium was toxic when released from the particles, though the physical character of the particles also contributes to toxic effect. Although there is no evidence of the carcinogenicity of InAs or AlGaAs, GaAs and InP, which are semiconductor materials, showed the clear evidence of carcinogenic potential. It is necessary to pay much greater attention to the human exposure of semiconductor materials.

Approaches to carcinogenic risk assessment for polycyclic aromatic hydrocarbons: a UK perspective

This paper reviews the approaches to carcinogenic risk assessment of polycyclic aromatic hydrocarbons (PAHs) in air pollution with emphasis on high potency PAHs such as dibenzo[a,l]pyrene (DB[a,l]P). The potency of DB[a,l]P may be 100-fold greater than benzo[a]pyrene (B[a]P); thus the B[a]P surrogate approach currently used to monitor for compliance with UK air pollution standards may not be appropriate. It is suggested that an approach based on potency equivalency factors (PEFs) could be developed to include highly potent PAHs provided an appropriate reference data set for relevant PAHs using a route acceptable for inhalation risk assessment is selected. Available data suggest that intratracheal administration of low doses of PAHs to rats is likely to simulate the kinetics of inhalation exposure to PAHs in a feasible manner. The use of a measure of total DNA adducts as an endpoint, which correlates well with lung tumourigenicity, would provide surrogate data for setting PEFs without the need for long-term bioassays in rodents. Further, dose-response studies using intratracheal administration of a range of PAHs singly and in combination to assess additivity are required to develop a PEF system for inhalation PEFs derived from DNA adduct measurements.

Inhaled particles and lung cancer, part B: paradigms and risk assessment

Poorly soluble particles of low toxicity (PSP), such as CB, TiO(2) and coal mine dust, have been demonstrated to cause lung cancer in rodents, being most pronounced in rats. Adequate epidemiologic studies do not clearly indicate increased lung cancer rates in humans exposed to such particles. This has caused controversial positions in regulatory decisions on PSP on different levels. The present review discusses the current paradigms in rodent particle carcinogenicity, i.e., (i) role of particle overload and of persistent inflammation and (ii) fibrosis as an intermediate step in particle-induced lung cancer with regard to human risk assessment. Fibrosis, which is usually considered a precursor of lung cancer in humans, was not related to lung tumors in an animal study using 6 different particles, each at 3 dosages. Lung tumors after both inhalation and intratracheal instillation of PSP are related to particle surface dose, which forwards hazard assessment at surface-based nonoverload concentrations and a standard setting using surface as an exposure metric. The scarce data available on humans do not support the overload concept but suggest a role for persistent lung inflammation. Differences in antioxidant protection between different rodent species correlate with susceptibility to PSP-induced carcinogenicity and support the need for detailed studies on antioxidant response in humans. Apart from such bridging studies, further focus is also needed on surface chemistry and modifications in relation to their adverse biologic effects.

Inhalation toxicity of mineral particles: critical appraisal of endpoints and study design

Many of the mineral particles that are of concern in regard to lung toxicity are poorly soluble particles (PSPs). They include biopersistent mineral fibers and dusts containing crystalline silica. The preparation of well-defined test particles of respirable size range and their characterization are an essential step that may require more time and effort than the toxicity study itself. For toxicity studies with mineral particles, an investigation of the toxicokinetics is recommended. Such an investigation will yield information that will help to interpret the results if dust overload conditions occur. For mineral particles such as crystalline silica and mineral fibers, an important endpoint is their potential carcinogenicity. The following parameters are important for the design of chronic toxicity studies, and for the prediction of severe chronic effects: lung retention of inhaled materials for assessing the accumulation of particles, persistent inflammation in lungs, persistent proliferation of epithelial lung cells, progressive fibrogenicity, and genotoxicity in the lung cells. These endpoints should indicate whether the materials investigated are of concern in the health effects on exposed humans, and in the effects of the mineral particles for which chronic studies may be required. In addition, this paper focuses on the effects of PSPs combined with fibers, and on the strategies for investigating the potential carcinogenicity of quartz-containing dusts.

The relevance of the rat lung response to particle overload for human risk assessment: a workshop consensus report

On 23-24 March 1998, the International Life Sciences Institute (ILSI) Risk Science Institute convened a workshop entitled "Relevance of the Rat Lung Response to Particle Overload for Human Risk Assessment." The workshop addressed the numerous study reports of lung tumors in rats resulting from chronic inhalation exposures to poorly soluble, nonfibrous particles of low acute toxicity and not directly genotoxic. These poorly soluble particles, indicated by the acronym PSPs (e.g., carbon black, coal dust, diesel soot, nonasbestiform talc, and titanium dioxide), elicit tumors in rats when deposition overwhelms the clearance mechanisms of the lung resulting in a condition referred to as "overload." These PSPs have been shown not to induce tumors in mice and hamsters, and the available data in humans are consistently negative. The objectives were twofold: (1) to provide guidance for risk assessment on the interpretation of neoplastic and nonneoplastic responses of the rat lung to PSPs; and (2) to identify important data gaps in our understanding of the lung responses of rats and other species to PSPs. Utilizing the five critical reviews of relevant literature that follow herein and the combined expertise and experience of the 30 workshop participants, a number of questions were addressed. The consensus views of the workshop participants are presented in this report. Because it is still not known with certainty whether high lung burdens of PSPs can lead to lung cancer in humans via mechanisms similar to those of the rat, in the absence of mechanistic data to the contrary it must be assumed that the rat model can identify potential carcinogenic hazards to humans. Since the apparent responsiveness of the rat model at overload is dependent on coexistent chronic active inflammation and cell proliferation, at lower lung doses where chronic active inflammation and cell proliferation are not present, no lung cancer hazard is anticipated.