Chronic Inhalation Studies In Rats Research Articles

The parallels between particle induced lung overload and particle induced periprosthetic osteolysis (PPOL).

Background: When particles deposit for instance in the lung after inhalation or in the hip joint after local release from a hip implant material they can initiate a defense response. Even though these particles originate from inert materials such as polyethylene (PE) or titanium, they may cause harm when reaching high local doses and overwhelming local defense mechanisms. Main body: This paper describes the parallels between adverse outcome pathways (AOP) and particle properties in lung overload and periprosthetic osteolysis (PPOL). It is noted that in both outcomes in different organs , the macrophage and cytokine orchestrated persistent inflammation is the common driver of events, in the bone leading to loss of bone density and structure, and in the lung leading to fibrosis and cancer. Most evidence on lung overload and its AOP is derived from chronic inhalation studies in rats, and the relevance to man is questioned. In PPOL, the paradigms and metrics are based on human clinical data, with additional insights generated from in vitro and animal studies. In both organ pathologies the total volume of particle deposition has been used to set threshold values for the onset of pathological alterations. The estimated clinical threshold for PPOL of 130 mg/ml is much higher than the amount to cause lung overload in the rat (10 mg/ml),although the threshold in PPOL is not necessarily synonymous to particle overload. Conclusions: The paradigms developed in two very different areas of particle response in the human body have major similarities in their AOP. Connecting the clinical evidence in PPOL to lung overload challenges relevance of rat inhalation studies to the human lung cancer hazard. .

The parallels between particle induced lung overload and particle induced periprosthetic osteolysis (PPOL)

Background: When particles deposit for instance in the lung after inhalation or in the hip joint after local release from a hip implant material they can initiate a defense response. Even though these particles originate from inert materials such as polyethylene (PE) or titanium, they may cause harm when reaching high local doses and overwhelming local defense mechanisms. Main body: This paper describes the parallels between adverse outcome pathways (AOP) and particle properties in lung overload and periprosthetic osteolysis (PPOL). It is noted that in both outcomes in different organs , the macrophage and cytokine orchestrated persistent inflammation is the common driver of events, in the bone leading to loss of bone density and structure, and in the lung leading to fibrosis and cancer. Most evidence on lung overload and its AOP is derived from chronic inhalation studies in rats, and the relevance to man is questioned. In PPOL, the paradigms and metrics are based on human clinical data, with additional insights generated from in vitro and animal studies. In both organ pathologies the total volume of particle deposition has been used to set threshold values for the onset of pathological alterations. The estimated clinical threshold for PPOL of 130 mg/ml is much higher than the amount to cause lung overload in the rat (10 mg/ml). Conclusions: The paradigms developed in two very different areas of particle response in the human body have major similarities in their AOP. Connecting the clinical evidence in PPOL to lung overload challenges current paradigms and the human relevance of rat inhalation studies.

Genetic toxicity and carcinogenicity studies of glutaraldehyde?a review

Glutaraldehyde is a high production volume chemical that is highly reactive and has many medical and industrial uses. The majority of human exposures are via inhalation, but the exposure is not widespread. It has been extensively tested for genetic activity in vitro and in vivo, and there is disagreement in the literature with regard to glutaraldehyde's genetic activity. Glutaraldehyde produced DNA damage in bacteria and some cultured mammalian cell systems. In vitro, it was mutagenic in Salmonella and E. coli, produced inconsistent positive responses in mammalian cells, weak and inconsistent responses in chromosome aberration and SCE studies, and did not induce transformation in cultured SHE cells. In vivo, inhalation of glutaraldehyde induced cell proliferation in nasal tissue in rats and mice, but DNA damage and UDS were not induced at these sites in rats. Chromosome aberrations in bone marrow cells were reported in only one of eight studies using rats and mice, micronuclei were not induced in bone marrow cells of mice, and dominant lethal mutations were not induced in mice. Glutaraldehyde did not induce cell transformation in SHE cells in vitro. Bone marrow hyperplasia and low, but statistically significant, levels of leukemia were seen in one chronic drinking water study in rats, but not in a chronic inhalation study in rats or two chronic inhalation studies in mice.

A RETROSPECTIVE REVIEW OF THE CARCINOGENICITY OF REFRACTORY CERAMIC FIBER IN TWO CHRONIC FISCHER 344 RAT INHALATION STUDIES: An Assessment of the MTD and Implications for Risk Assessment

The purpose of this article is to review previous chronic inhalation studies in rats with refractory ceramic fiber (RCF), the mathematical modeling efforts to describe the deposition, clearance, and retention of RCF fiber in the rat and human, and the concept of ''overload,'' and to assess the possibility that the maximum tolerated dose (MTD) was exceeded. Lastly, based on recent biopersistence and pulmonary clearance studies of several investigators with a particulate-free RCF, we examine the potential impact on the chronic RCF rat bioassay of coexposure to both RCF particulate and RCF fibers. The review concludes, inter alia, that RCF particulate coexposure probably had a major impact on the observed chronic adverse effects, that the MTD was probably exceeded at the highest exposure concentration of 30 mg/m3 in the rat bioassay, and that inclusion of the highest dose in the risk assessment process may overstate human health risk if a linear rather than nonlinear model is used.

PULMONARY RESPONSE TO TONER, TiO2 AND CRYSTALLINE SILICA UPON CHRONIC INHALATION EXPOSURE IN SYRIAN GOLDEN HAMSTERS

A chronic inhalation study of a test toner, TiO2, and crystalline silica was conducted by exposure of groups of Syrian golden hamsters (strain: Han:AURA) for 6 h/day, 5 days/ wk for 18 mo. Subsequently, the animals inhaled clean air only for an additional period of 5 mo. The target test aerosol exposure concentrations were 1.5, 6, and 24 mg/3m for the test toner, 40 mg/m3 for TiO2, and 3 mg/m3 for SiO2. The latter two materials were used as negative and positive controls for fibrogenicity. The aerosol concentrations were changed to 4, 16, and 64 mg/m3 for toner (referred to as toner low, toner medium, and toner high) and 30 mg/m3 for TiO2 after 5 mo, in order to achieve the desired lung burdens. Inhalation of the test toner or the control materials showed no signs of overt toxicity. Body weight, hematology and clinical chemistry values, food consumption, and organ weights were normal in the toner-, TiO2-, and SiO2-exposed groups, except for relative lung weights with elevations of up to 56%, 98%, and 98% for the toner high, the TiO2, and the SiO2 group at the end of the study. Small increases in the number of segmented neutrophils and decreases in the mean percentage of lymphocytes were observed in the same groups, indicating a mild inflammatory reaction in the lung. All of the changes in the toner-exposed groups were restricted to the lungs or associated lymph nodes. A chronic inflammatory response was evident from the bronchoalveolar lavage parameters for the toner high-, the TiO2-, and the SiO2-exposed groups. Primary lung tumors were not found in the control or toner-exposed groups. One small-sized bronchioloalveolar adenoma was observed in a female of the TiO2exposed group and one in a female of the SiO2-exposed group. A very slight to slight degree of interstitial fibrosis was observed after exposure to toner medium (in 49% of hamsters), to toner high (83%), to TiO2(82%), and SiO2(91%). Two squamous-cell carcinomas of the forestomach were observed in the male toner high group as well as in the female SiO2 group. Upon closer examination, an increased incidence of fore stomach papillomas was detected in all particle-exposed groups, but there was no doseresponse relationship among the three toner-exposed groups. Several of the observed lesions randomly distributed in most of the exposure groups, such as the neuroendocrine tumors (larynx and trachea), heart thrombosis, and forestomach papilloma(s), are not considered to be relevant for human risk assessment. The maximum tolerated dose (MTD) criterion was exceeded at the toner high and the TiO exposure concentra2 tions. The results of the present investigation coupled with the previously documented chronic inhalation studies in rats lead to the firm conclusion that inhalation of toner does not represent a carcinogenic hazard.

Lung Particle Overload: Implications for Occupational Exposures to Particles

Chronic pulmonary inflammation, pulmonary fibrosis, and even lung tumors developed in a number of chronic inhalation studies in rats with highly insoluble nonfibrous particles of low cytotoxicity. Concerns were expressed that these responses are due to excessive particulate lung burdens, and the term "particle overload" was coined to characterize these conditions. The hallmark of the particle-overloaded lung is an impairment of alveolar macrophage (AM)-mediated lung clearance which has been demonstrated in all species tested so far and which eventually leads to accumulation of excessive lung burdens. Experimental evidence suggests that the volume of the particles phagocytized by AM: is most critical for causing their impaired clearance function, and that the condition of lung overload is reached once the retained lung particle burden reaches a level equivalent to a volume of approximately 1 μl/g of lung. Cytotoxic particles also cause impaired AM clearance function, yet at a much lower lung burden which does not qualify as particle overload. Significant species differences exist with respect to the induction of adverse chronic effects in response to lung overload; i.e., mice and hamsters are less prone to developing chronic inflammation and pulmonary fibrosis, and lung tumors have been observed only in rat studies. Lung tumors or fibrosis in the rats were seen only at lung burdens having caused impaired particle clearance, and a threshold dose for the adverse chronic effects can be postulated which is defined by a lung particle burden not causing impairment of clearance. Thus, the lung tumors observed in chronic rat studies at very high particulate exposure concentrations may not be relevant for human extrapolation to low-exposure concentrations. Evidence in humans suggests that particle-overloaded lungs, e.g., in coal workers, respond with fibrosis, but no increased incidence of lung tumors has been found in this group. However, it cannot be excluded that other types of chronically inhaled particles may have a carcinogenic potential in the human lung if accumulating to very high lung burdens. More research is needed for a detailed understanding of the basic mechanism leading to nonfibrous particle-induced tumorigenesis in the lungs of different species. Altered particle accumulation and retention kinetics and chronic inflammation in the overloaded lung indicate that the maximum tolerated dose (MTD) has been exceeded. No specific guidelines for inhalation studies defining the MTD have been established; general guidelines are not necessarily applicable for chronic inhalation studies. One suggestion is to define the MTD in chronic particle inhalation studies as a maximum functionally tolerated dose (MFTD) based on a functional parameter of lung particle clearance. However, other endpoints based on an evaluation of alveolar inflammation and lung histology should be included as well. For the estimation of the MTD or MFTD a range-finding study of sufficient length, preferably 3 months, would be required. It is important to realize, however, that with increasing chronicity of exposure and increasing age of the animals, a shift in the dose-response relationship may occur. One important conclusion from our understanding of lung particle overload is that occupational exposure limits should be set to prevent impaired AM-mediated lung clearance which will-avert the accumulation of high pulmonary dust burdens; conceivably, this will also prevent the subsequent induction of adverse chronic effects including inflammation, fibrosis, and tumors since these particle-induced tumors are likely secondary to continued inflammation, tissue damage, and cell proliferation. Extrapolation from rat studies of such exposure limits predicts that present occupational standards for highly insoluble particles of low cytotoxicity may not prevent lung overload in humans and should be reevaluated.

Deposition, Clearance, and Shortening of Kevlar Para-aramid Fibrils in Acute, Subchronic, and Chronic Inhalation Studies in Rats

Deposition, Clearance, and Shortening of Kevlar Para-aramid Fibrils in Acute, Subchronic, and Chronic Inhalation Studies in Rats. Kelly, D. P., Merriman, E. A., Kennedy, G. L., Jr., and Lee, K. P. (1993). Fundam. Appl. Toxicol. 21, 345-354.The deposition and clearance of lung-deposited Kevlar para-aramid fibrils (subfibers) have been investigated as part of a subchronic and chronic inhalation toxicity testing program. Fibrils recovered from lung tissue in para-aramid-exposed Sprague-Dawley rats were microscopically counted and measured after exposures to airborne fibrils which were about 12 μm median length (ML) and <0.3 μm median diameter. In each of three studies lung-recovered fibrils were progressively shorter with increasing residence time in the lungs. Twenty-eight days after a single 6-hr exposure at 400 respirable fibrils per cubic centimeter (f/cm3) the ML of recovered fibrils decreased to about 5 μm. Twenty-four months after a 3-week exposure to 25 or 400 f/cm3, fibrils reached about 2 μm ML. After 2 years of continuous exposure at 2.5, 25, or 100 f/cm3 or 1 year exposure plus 1 year recovery at 400 f/cm3, fibril ML approached 4 μm. In the 2-year study, the lung-fiber accumulation rate/exposure concentration was similar for the three highest concentrations and was about 3× greater than that seen at 2.5 f/cm3, indicating that concentrations of about 25 f/cm3 or more may overwhelm clearance mechanisms. Time required for fibrils to be reduced to <5 μm in the lung was markedly less at lower exposure concentration and shorter exposure time. The primary shortening mechanism is proposed to be long fibril cutting by enzymatic attack at fibril defects. However, length-selective fibril deposition and clearance may contribute to shortening in the first few days after exposure. The enzymatic cutting hypothesis is supported by measured increases in numbers of short fibers following cessation of exposures, continued shortening of the fibril length distribution up to 2 years following exposure, and in vitro fibril shortening after 3 months in a proteolytic enzyme preparation. The conclusion is that para-aramid fibrils are less durable in the lungs of rats than expected from the known chemical resistance of commercial yarn. These data suggest that at the low para-aramid fibril exposures found in the workplace, this fibril-shortening mechanism may limit the residence time of long fibers in the lungs of exposed workers. In addition, associated cascade impactor aerodynamic measurements indicate that due to their ribbon shape and curly nature, para-aramid fibrils behave aerodynamically larger than straight fibers.

Deposition, Clearance, and Shortening of Kevlar Para-aramid Fibrils in Acute, Subchronic, and Chronic Inhalation Studies in Rats

The deposition and clearance of lung-deposited Kevlar paraaramid fibrils (subfibers) have been investigated as part of a subchronic and chronic inhalation toxicity testing program. Fibrils recovered from lung tissue in para-aramid-exposed Sprague-Dawley rats were microscopically counted and measured after exposures to airborne fibrils which were about 12 μm median length (ML) and <0.3 μm median diameter. In each of three studies lung-recovered fibrils were progressively shorter with increasing residence time in the lungs. Twenty-eight days after a single 6-hr exposure at 400 respirable fibrils per cubic centimeter (f/cm3) the ML of recovered fibrils decreased to about 5 μm. Twenty-four months after a 3-week exposure to 25 or 400 (f/cm3) fibrils reached about 2 μm ML. After 2 years of continuous exposure at 2.5, 25, or 100 f/cm3 or 1 year exposure plus 1 year recovery at 400 f/cm fibril ML approached 4 μm. In the 2-year study, the lung-fiber accumulation rate/exposure concentration was similar for the three highest concentrations and was about 3× greater than that seen at 2.5 f/cm3 indicating that concentrations of about 25 f/cm or more may overwhelm clearance mechanisms. Time required for fibrils to be reduced to <5 μm in the lung was markedly less at lower exposure concentration and shorter exposure time. The primary shortening mechanism is proposed to be long fibril cutting by enzymatic attack at fibril defects. However, length-selective fibril deposition and clearance may contribute to shortening in the first few days after exposure. The enzymatic cutting hypothesis is supported by measured increases in numbers of short fibers following cessation of exposures, continued shortening of the fibril length distribution up to 2 years following exposure, and in vitro fibril shortening after 3 months in a proteolytic enzyme preparation. The conclusion is that para-aramid fibrils are less durable in the lungs of rats than expected from the known chemical resistance of commercial yarn. These data suggest that at the low para-aramid fibril exposures found in the workplace, this fibril-shortening mechanism may limit the residence time of long fibers in the lungs of exposed workers. In addition, associated cascade impactor aerodynamic measurements indicate that due to their ribbon shape and curly nature, para-aramid fibrils behave aerodynamically larger than straight fibers.

Particle Overload in Toxicological Studies: Friend or Foe?

ABSTRACT The overloading of particle clearance is an important issue in the design and interpretation of inhalation toxicological studies. This issue is particularly important in chronic inhalation bioassays in rats, in which overloading is associated with inflammation, epithelial proliferation, and fibrosis, which may amplify carcinogenic responses or, as suggested by some, even induce cancer regardless of the inhaled material. At present, the key issue is whether or not data from exposures causing overload in animals are useful for predicting health effects in man. A review of reports of chronic inhalation studies in rats exposed to a spectrum of materials suggests that not all exposures resulting in overloading cause cancer, and that the cancer incidences from exposures causing overloading appear to reflect the relative carcinogenic potentials of the test materials. Data from such exposures, however, do little to establish the exposure-response relationship at lower doses most typically relevant to hum...

Chronic inhalation study in rats, using cigarettes containing different amounts of cytrell tobacco supplement

The paper describes a chronic (18-month) inhalation study in rats, comparing respiratory tract lesions produced by exposures to cigaretts with different inclusion of Cytrel®, a tobacco supplement. Histopathological examinations indicated lower responses in animals exposed to Cytrel-containing cigarettes than in those exposed to all-tobacco cigarettes, despite similar total deliveries of smoke. The decrease in response was approximately proportional to the level of Cytrel inclusion.

Experimental data on the neurotoxicity of methyl-ethyl-ketone (MEK)

A severe potentiating effects of methyl-ethyl-ketone (MEK) on the peripheral and central neurotoxicity of n-hexane could be demonstrated in a chronic inhalation study in rats.