Chronic Inhalation Study Findings as a Basis for Proposing a New Occupational Dust Exposure Limit

This article briefly reviews the history of the current threshold limit value and the permissible exposure limit for occupational dusts derived originally from the Nuisance Dust standard. Chronic inhalation studies of appropriate design are described in terms of their key findings and potential impact on the dust standards, especially modern, experimental studies which have been affected by the problem of dust overloading. Many studies show that dust overloading produces a significant and progressive retardation of macrophage-mediated dust removal leading to various dysfunctional and pathologic changes which confound the interpretation of chronic toxicity study findings. Assuming that there may be a human counterpart to this condition at the same milligram dust per gram lung concentration, extrapolation modeling can be used to show that the current occupational dust limits do not protect worker lungs from this eventuality. Several bases for substantially reducing the current occupational standards are discussed.

Retrospective evaluation of low long-term efficacy of antiepileptic drugs and ketogenic diet in 39 patients with CDKL5-related epilepsy

OBJECTIVE

Mutations in the CDKL5 gene cause an early-onset epileptic encephalopathy. To date, little is known about effective antiepileptic treatment in this disorder.

METHOD

Accordingly, the aim of this retrospective study was to explore the role of different antiepileptic drugs (AEDs) and the ketogenic diet (KD) in the treatment of this rare genetic disorder. We evaluated the efficacy in 39 patients with CDKL5 mutations at 3, 6 and 12 months after the introduction of each treatment. One patient was lost to follow-up after 6 and 12 months.

RESULTS

The responder rate (>50% reduction in seizure frequency) to at least one AED or KD was 69% (27/39) after 3 months, 45% (17/38) after 6 months and 24% (9/38) after 12 months. The highest rate of seizure reduction after 3 months was reported for FBM (3/3), VGB (8/25), CLB (4/17), VPA (7/34), steroids (5/26), LTG (5/23) and ZNS (2/11). Twelve patients (31%) experienced a seizure aggravation to at least one AED. Most patients showed some but only initial response to various AEDs with different modes of actions.

SIGNIFICANCE

Considering both age-related and spontaneous fluctuation in seizure frequency and the unknown impact of many AEDs or KD on cognition, our data may help defining realistic treatment goals and avoiding overtreatment in patients with CDKL5 mutations. There is a strong need to develop new treatment strategies for patients with this rare mutation.

Exclusive conservation of mitochondrial group II intron nad4i548 among liverworts and its use for phylogenetic studies in this ancient plant clade

Liverworts occupy a pivotal position in land plant (embryophyte) phylogeny as the presumed earliest-branching major clade, sister to all other land plants, including the mosses, hornworts, lycophytes, monilophytes and seed plants. Molecular support for this earliest dichotomy in land plant phylogeny comes from strikingly different occurrences of introns in mitochondrial genes distinguishing liverworts from all other embryophytes. Exceptionally, however, the nad5 gene--the mitochondrial locus hitherto used most widely to elucidate early land plant phylogeny--carries a group I type intron that is shared between liverworts and mosses. We here explored whether a group II intron, the other major type of organellar intron, would similarly be conserved in position across the entire diversity of extant liverworts and could be of use for phylogenetic analyses in this supposedly most ancient embryophyte clade. To this end, we investigated the nad4 gene as a candidate locus possibly featuring different introns in liverworts as opposed to the non-liverwort embryophyte (NLE) lineage. We indeed found group II intron nad4i548 universally conserved in a wide phylogenetic sampling of 55 liverwort taxa, confirming clade specificity and surprising evolutionary stability of plant mitochondrial introns. As expected, intron nad4i548g2 carries phylogenetic information in its variable sequences, which confirms and extends previous cladistic insights on liverwort evolution. We integrate the new nad4 data with those of the previously established mitochondrial nad5 and the chloroplast rbcL and rps4 genes and present a phylogeny based on the fused datasets. Notably, the phylogenetic analyses suggest a reconsideration of previous phylogenetic and taxonomic assignments for the genera Calycularia and Mylia and resolve a sister group relationship of Ptilidiales and Porellales.

Whole-genome linkage scan for epilepsy-related photosensitivity: a mega-analysis

Photoparoxysmal response (PPR) is considered to be a risk factor for idiopathic generalised epilepsy (IGE) and it has a strong genetic basis. Two genome-wide linkage studies have been published before and they identified loci for PPR at 6p21, 7q32, 13q13, 13q31 and 16p13. Here we combine these studies, augmented with additional families, in a mega-analysis of 100 families. Non-parametric linkage analysis identified three suggestive peaks for photosensitivity, two of which are novel (5q35.3 and 8q21.13) and one has been found before (16p13.3). We found no evidence for linkage at four previously detected loci (6p21, 7q32, 13q13 and 13q31). Our results suggest that the different family data sets are not linked to a shared locus. Detailed analysis showed that the peak at 16p13 was mainly supported by a single subset of families, while the peaks at 5q35 and 8q21 had weak support from multiple subsets. Family studies clearly support the role of PPR as a risk factor for IGE. This mega-analysis shows that distinct loci seem to be linked to subsets of PPR-positive families that may differ in subtle clinical phenotypes or geographic origin. Further linkage studies of PPR should therefore include in-depth phenotyping to make appropriate subsets and increase genetic homogeneity.

KCNQ2 and KCNQ3 mutations contribute to different idiopathic epilepsy syndromes

OBJECTIVE

To explore the involvement of M-type potassium channels KCNQ2, Q3, and Q5 in the pathogenesis of common idiopathic epilepsies.

METHODS

Sequence analysis of the KCNQ2, Q3, and Q5 coding regions was performed in a screening sample consisting of 58 nuclear families with rolandic epilepsy. Subsequently, an association study was conducted for all discovered variants in a case-control sample comprising 459 German patients with idiopathic generalized epilepsy (IGE) and 462 population controls.

RESULTS

An in-frame deletion of codon 116 in KCNQ2 (p.Lys116del) and a missense mutation in KCNQ3 (p.Glu299Lys) were detected in two index cases exhibiting rolandic epilepsy and benign neonatal convulsions. Both mutations resulted in reduced potassium current amplitude in Xenopus oocytes. Mutation analysis of families with rolandic epilepsy without neonatal seizures discovered three novel missense variations (KCNQ2 p.Ile592Met, KCNQ3 p.Ala381Val, KCNQ3 p.Pro574Ser). The KCNQ2 p.Ile592Met variant displayed a significant reduction of potassium current amplitude in Xenopus oocytes and was present only once in 552 controls. Both missense variants identified in KCNQ3 (p.Ala381Val and p.Pro574Ser) were present in all affected family members and did not occur in controls, but did not show obvious functional abnormalities. The KCNQ3 missense variant p.Pro574Ser was also detected in 8 of 455 IGE patients but not in 454 controls (p = 0.008). In KCNQ2, a silent single nucleotide polymorphism (rs1801545) was found overrepresented in both epilepsy samples (IGE, p = 0.004).

CONCLUSION

Sequence variations of the KCNQ2 and KCNQ3 genes may contribute to the etiology of common idiopathic epilepsy syndromes.

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.

Survey of the biological effects of refractory ceramic fibres: overload and its possible consequences

This paper summarizes the biological effects of refractory ceramic fibres (RCFs). RCFs are aluminosilicate glass insulation wools with similar chemical properties to other synthetic vitreous fibres (SVFs) or 'man-made vitreous fibres' (MMVFs). There is concern that RCFs could be significantly more pathogenic than other SVFs. This paper critically reviews the data on which this perception is based. Morbidity studies on workers in RCF manufacturing indicated that, in the United states, RCF exposure was associated with an increased incidence of pleural plaques and in both the united states and Europe with statistically significant changes in some measures of lung function (though not at present exposure levels). No interstitial fibrosis was found. An ongoing mortality study of limited statistical power has failed to indicate any increased incidence of lung cancer or mesothelioma. Findings in several early animal studies led to a large series of inhalation studies where rats exposed to high levels of RCF developed fibrosis and tumours but not those exposed to other SVFs. Similarly hamsters exposed to one sample (RCF1) developed mesothelioma. Subsequent analyses of the data indicated that the RCF used in these experiments had a significantly greater proportion of non-fibrous particles than those present in the other types of SVFs tested or in workplace air. Short-term studies indicated that pulmonary overload occurred at the same as RCF tissue burdens as those in the long-term animal bioassay. When RCFs were prepared in the same way as the other SVFs, a sample resulted with a more representative ratio of particles to fibres; this sample did not produce overload in short-term tests. SVFs have various abilities to persist in the lung tissue and thus accumulate to varying degrees. It is suggested that biopersistence is a key property. While RCFs are among the more persistent they are similar to many other fibre types. The scientific and regulatory implications of these findings are examined.

Partial response to biotin therapy in a patient with holocarboxylase synthetase deficiency: clinical, biochemical, and molecular genetic aspects

We report the clinical course and biochemical findings of a 10-year-old, mentally retarded girl with late-onset holocarboxylase synthetase (HCS, gene symbol HLCS) deficiency and only partial response to biotin. On treatment, even with an unusually high dose of 200mg/day, activities of the biotin-dependent mitochondrial carboxylases in lymphocytes remained below 50% of the mean control values. Not only urinary 3-hydroxyisovaleric acid excretion has been persistently elevated, but also plasma and, with even higher concentrations, cerebrospinal fluid 3-hydroxyisovaleric acid have not normalized. The unusual and insufficient response of this patient to biotin treatment can be explained by the effect of the combination of the common HLCS allele IVS10 +5 g>a on one chromosome and a truncating mutation on the other. This case illustrates mechanisms involved in the genotype-phenotype correlation that unequivocally exists in HCS deficiency.

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.

Effects of nonfibrous particles on ceramic fiber (RCF1) toxicity in rats

In previous investigations a reference test sample of prepared ceramic fibers called RCF1 induced lung tumors in a 2-yr inhalation study in rats. It was hypothesized that nonfibrous particles in RCF1 may have played a significant role. The objective of the present study was to compare lung retention and biological effects of another sample of ceramic fibers, called RCF1a, to the original RCF1. The main difference between these 2 samples was the content of nonfibrous particles: 25% of the mass of RCF1 versus 2% for RCF1a. These nonfibrous particles were chemically identical to the fibers. Female Wistar rats were exposed 6 h/day, 5 days/wk for 3 wk to either RCF1a or RCF1 fiber aerosol at a concentration of about 125 fibers (>20 microm long)/ml. Because of differences in the nonfibrous particle contents, the average gravimetric aerosol concentration differed between the two samples (RCF1, 51.2 mg/m(3); RCF1a, 25.8 mg/m(3)). The posttreatment observation period was 12 mo. Biological effects measured include the clearance function of alveolar macrophages (clearance of fibers and tracer particles), and inflammation and its persistence during the recovery period. Alveolar clearance of tracer particles ((46)Sc(2)O(3)) was barely retarded after RCF1a exposure (80 days clearance half-time compared to 60 days in controls). After RCF1 exposure, however, a severe retardation of clearance was observed (1200 vs. 66 days). In both groups, differential cell counts on pulmonary lavage showed a significant increase of polymorphonuclear leukocytes (PMNs) (about 15%) and lymphocytes 3 days after the end of exposure. The PMN influx persisted longer after exposure to RCF1 than RCF1a. The conclusion of the study is that the particle fraction of RCF1 significantly enhanced any adverse effects. This clearly demonstrates the importance of the physical characteristics of the test material for the degree of toxic effects to be expected. The presence of nonfibrous particulates can enhance the effects on the lung of a mixture of fibrous and nonfibrous particulates following exposure.

Volatile constituents in mosses (Musci)

The essential oils of mosses of the genera Mnium, Plagiomnium, Homalia, Plagiothecium and Taxiphyllum (Musci) have been investigated by gas chromatography and mass spectrometry. The new sesquiterpenes (+)-10-epi-muurola-4,11-diene and 10,11-dihxdro-alpha-cuparenone were isolated by preparative gas chromatography and identified as major constituents of the hydrodistillation products of Mnium hornmum (Hedw.) using NMR and mass spectrometry. In addition, (+)-dauca-8,11-diene and two new butenolides, 3,4,5-trimethyl-5-pentyl-5H-furan-2-one and 3.4-dimethyl-5-pentyl-5H-furan-2-one were identified as constituents in Plagiomnium undulatum (Hedw.) T. Kop. Although the amounts of volatiles present in the investigated mosses are generally smaller than in liverworts, the spectrum of terpenoid compounds is similar. The investigated mosses also generate aliphatic compounds of greater abundance and structural variety.

Chronic pulmonary effects of respirable methylene diphenyl diisocyanate (MDI) aerosol in rats: combination of findings from two bioassays

Two independent bioassays are available which have examined the potential carcinogenicity of monomeric and polymeric methylene diphenyl diisocyanate (MDI) following long-term inhalation exposure in rats. These studies are not directly comparable, however, due to differences in design and conduct of the in-life phase, and differences in nomenclature used for some of the histopathological findings. This paper presents a definitive overview ofthe pulmonary toxicity of MDI developed following a thorough review of both investigations. As part of this process, the test materials and the designs of the studies were compared, and an in-depth review of lung lesions was conducted by an independent reviewing pathologist. This included the re-examination of the original lung slides, supported by an analysis of the exposure regimens, the results of which were used to develop an accurate profile of the doses received by the animals in the two studies. Histopathological findings were then combined with this information to give an overall dose-response curve for both studies as a whole. The range of total inhalation exposures to MDI was calculated as 559, 1972, 2881, 6001, 17,575 and 17,728 mgh/m3. Major pulmonary effects included increased lung weights together with bronchiolo-alveolar adenomas and hyperplasia, and interstitial fibrosis which occurred consistently in both studies, indicating a very similar qualitative response of the lungs to polymeric and monomeric MDI. The quantitative response of the lung was clearly dose-related in each study, and when the studies were considered as a whole a reasonable overall dose-response relationship was apparent for major lung lesions. Lung tumours (in low incidences) only occurred at the highest dose level in both studies (17,575 and 17,728 mgh/m3). For inflammatory and other non-neoplastic pulmonary changes, the lowest dose examined (559 mgh/m3) was regarded as a no-observed-adverse-effect-level for both polymeric and monomeric MDI. It was concluded that the results of the two studies could be combined to serve as a basis for human risk assessment of MDI.

Pacifigorgianes and tamariscene as constituents of Frullania tamarisci and Valeriana officinalis

The sesquiterpenoid constituents of the essential oils from the liverworts Frullania tamarisci, Frullania fragilifolia and of the angiosperm Valeriana officinalis were investigated. Tamariscene, a compound with a new sesquiterpene skeleton, valerena-4,7(11)-diene and five new pacifigorgiadienes, namely pacifigorgia-1,10-diene, pacifigorgia-1(6),10-diene, pacifigorgia-1(9),10-diene, pacifigorgia-2,10-diene, and pacifigorgia-2(10),11-diene were isolated and identified. Structure elucidation was carried out by NMR spectroscopy and chemical correlations to establish absolute configurations. Compounds present in both the essential oils of the Frullania species and Valeriana officinalis were enantiomeric to each other. A plausible biogenetic relationship between the pacifigorgiane, valerenane and tamariscane skeletons is postulated. Pacifigorgia-6,11-diene, not yet detected in nature, was generated by dehydration and rearrangement of natural (-)-tamariscol.

The mitochondrial nad2 gene as a novel marker locus for phylogenetic analysis of early land plants: a comparative analysis in mosses

The mitochondrial nad2 gene is established as a novel marker locus for phylogenetic analyses among early land plants. The potential of this gene for phylogenetic resolution was checked with a broad taxon sampling of 42 mosses (Bryopsida, including the enigmatic genus Takakia) to allow both a comparative analysis with the recently explored nad5 gene and the fusion of independent data sets. The mitochondrial gene sequences provide valuable phylogenetic information on the relationships of classically defined orders and their respective monophylies. The more rapidly diverging sequences of a group I intron in nad5 and of a group II intron in nad2 add information for fine resolution. Although both genes provide phylogenetic information in the same taxonomic range (above family level), the combined sequence alignment results in an approximate doubling in the number of nodes with significant bootstrap support (>90). According to our data, Buxbaumiales are a paraphyletic taxon in a key position between the earliest branching taxa (Sphagnales, Takakiales, Andreaeales, Polytrichales, and Tetraphidales) and all other orders, possibly to be placed in the subclass Bryidae. A dichotomy in the latter recalls two previously suggested superorders Hypnanae and Dicrananae. Both genes independently question the monophyly of the orders Dicranales and Neckerales and reject the inclusion of the genera Schistostega, Timmia, and Encalypta among Eubryales.

Asbestos as reference material for fibre-induced cancer

The objective of this paper is to review published data on the carcinogenicity of asbestos fibres with regard to the elucidation of a potential risk originating from exposure to man-made vitreous fibres (MMVF). Steps in the comparison of the two fibre classes are characterization of the fibres, pulmonary deposition, biodurability and biopersistence and a review of the cancer risk from asbestos fibres after inhalation in rats and humans. Various dust samples of chrysotile, crocidolite, and amosite were used as reference materials in studies with experimental animals. These fibres are normally thinner and shorter than MMVF. These differences in dimensions cause differences in the deposition in the airways. In addition, significant dissimilarities exist in the deposition pattern between rats and humans. Data from biopersistence studies show that focusing only on fibres longer than 20 microm and using weighted half-time for a characterization of risk may be misleading. Inhalation experiments with rats need fibre exposure concentrations over 100 times higher to match the lung cancer risk of asbestos workers, and about 1,000 times higher to reach the same mesothelioma risk. Also, the striking difference between the low lung burden of amphibole fibres of asbestos workers with mesothelioma and the more than 1,000 times higher lung burden of rats with a low mesothelioma risk demonstrates the low sensitivity of the inhalation test model for the carcinogenic potency even of crocidolite fibres. It can be concluded that the rat inhalation model is also not sensitive enough to predict the cancer risk of other fibre types for humans.

Inhalation tolerance study for p-aramid respirable fiber-shaped particulates (RFP) in rats

This study was designed to assess the lung clearance function in rats after subchronic exposure to p-aramid respirable fiber-shaped particulates (RFP). Male Wistar rats were exposed 6 hrs/day, 5 days/week for 3 months to 50, 200, and 800 RFP/ml measured by scanning electron microscopy (SEM). Recovery effects were followed up through 9 months postexposure. The retention of RFP (length > 5 microm) was about 25 x 10(6) RFPs per lung in the low dose group after 3 months of exposure. The corresponding values in the medium and high dose groups amounted to overproportionally higher values of 122 x 10(6) and 576 x 10(6) RFPs per lung, respectively. A decrease in the length of the retained RFPs over the 9-month recovery period was observed, indicating a breakage of long fibrils. Alveolar clearance half-times measured by gamma tracers indicated a dust overloading of lungs for the high dose group at 0 and 3 months postexposure. Bronchoalveolar lavage parameters revealed that p-aramid RFPs induced pronounced inflammatory effects in the high and medium dose groups. Histopathologically, slight fibrotic and hyperplastic lesions were observed in the medium and high dose groups directly after the end of exposure. The findings at the 3-month postexposure interval resulted in a reduction of inflammatory changes in the medium and high dose groups compared to the sacrifices upon cessation of exposure. No histopathologic effects were detected in the low dose group. In the high dose group the maximum functionally tolerated dose was exceeded. The No Observed Adverse Effect Level (NOAEL) of RFP was 50 RFP/ml as measured by SEM.

Modulation of MAO activity by imidazoline and guanidine derivatives

I2-binding sites (I2-BS) are attributed to be a regulative site on monoamine oxidase (MAO). The in vivo and in vitro effects of various imidazoline and guanidine derivatives on MAO activity and on mitochondrial respiration were studied. Substances with high affinity for I2-BS (antazoline, idazoxan, and cirazoline: IC50 = 20.3, 33.8, and 43.4 microM) had a stronger inhibitory effect on MAO activity than did I1-ligands (efaroxan, rilmenidine, clonidine, and moxonidine: IC50 = 277, 801, 1,224, and > 10,000 microM). Substances with the highest inhibitory effects were BDF8082 (IC50 = 1.7 microM) and 2-(2-benzofuranyl)-2-imidazoline (BFI; IC50 = 4.0 microM). The enzyme is inhibited noncompetitively and is reversible, because its activity is completely or partially restored after dialysis. Agmatine, the putative endogenous ligand for IBS, also decreased MAO activity (IC50 = 168 microM), whereas its precursor, L-arginine, and its metabolite, putrescine, had no effects. In vitro inhibition of MAO and mitochondrial respiration by the IBS-ligands tested could not be correlated, suggesting no link between the function of the inner and outer mitochondrial membrane. MAO activity in vivo was significantly reduced only by pargyline (-95%), BDF8082 (-68%), BFI (-43%), and 1-(m-chlorophenyl)-biguanide (-28%). Catecholamine content of livers obtained from animals treated with different IBS-ligands was consequently increased. In conclusion, the strong inhibitory effects of I2 selective imidazoline ligands confirm the existence of I2-BS as a regulatory site on MAO.

Investigations on health-related properties of two sepiolite samples

Published i.p. injection studies have shown different biological behavior of different sepiolite samples. There was no evidence for carcinogenic potential of sepiolite from Vicalvaro, Spain, whereas a high tumor incidence was reported for sepiolite from Finland. The low biological activity of the sepiolite from Vicalvaro, compared to the Finnish sample, could be caused by low in vivo persistence or by the short length of the fibers, or both. In this study a further sepiolite sample, obtained as a commercial sample originating from China, was investigated. This sample contained a higher fraction of fibers longer than 5 microns, comparable to the Finnish sepiolite sample. The fraction of fibers with a length > 5 microns was 0.12 and 2.2% for the Vicalvaro and Chinese sepiolite, respectively. For the fiber fraction longer than 8 microns, the corresponding values were 0.0045 and 0.82%. The in vivo persistence of the sepiolite samples from China and Vicalvaro was analyzed after intratracheal instillation of 2 mg in female Wistar rats. Fiber retention in the lungs was analyzed by transmission electron microscopy at different sacrifice dates up to 12 months after application. For the Vicalvaro sepiolite, a splitting of fiber bundles was found during retention time in the lung. Therefore, no half-time of the fiber clearance could be calculated from the number of fibers. The decrease of the calculated retained fiber mass was faster for the Vicalvaro sepiolite (T1/2 = 89 days) compared to the Chinese sepiolite (T1/2 = 129 days). For 2 or 3 rats per group, at sacrifice date 12 months after i.p. injection, the lung was investigated by histopathology. The main difference between both treatment groups was a more pronounced fibrotic response in the Chinese sepiolite-treated rats compared to those treated with Vicalvaro sepiolite. It is concluded that both the higher fraction of long sepiolite fibers and the slower elimination rate of the fiber mass in the Chinese sample were important factors for the different biological reaction in comparison with Vicalvaro sepiolite.

Significance of the biodurability of man-made vitreous fibers to risk assessment

It is generally agreed that the biodurability of man-made vitreous fibers is a major factor for the characterization of potential health effects. As there is currently no standardization of experimental protocols to determine biodurabilty, the results of the clearance assays have not been used up to now for regulatory purposes. Methods used to analyze biodurability in animal models are short-term inhalational exposure and intratracheal instillation of rat respirable fibers. Both test methods have strengths and limitations for regulatory purposes. We outline recommended procedures for standardized biodurability assays that can be used to compare different fiber types. In animal experiments, biodurability is difficult to separate from biopersistence, as mucociliary and macrophage-mediated clearance occur simultaneously with dissolution and disintegration. For intratracheal instillation, a sized rat respirable sample must be used. Precautions should be taken to prevent aggregation of fibers in the lungs. Although from a scientific point of view questions remain about quantifying the influence of fiber length, diameter, dose, and exposure route, consistent data on the biodurability of vitreous glass fibers are available which may be used for regulatory purposes.

Biopersistence of man-made vitreous fibres

Methods for the determination of biodurability of man-made vitreous fibres are reviewed. For mineral wools the first step was the preparation of respirable fibre fractions. Fibres were administered to rats by inhalation or by intratracheal instillation. After serial sacrifice their lungs were digested by low-temperature ashing or by hypochlorite. The total number of fibres per lung and the distributions of length and diameter were analysed by electron microscopy. This resulted in a bivariate distribution of fibres at the various sacrifice dates. If the logarithm of the number of fibres decreased approximately linearly with time after exposure then the elimination kinetics of fibres can be characterized by a half-time. The half-times were compared between various experiments with rats exposed to mineral wool samples. In summary good agreement was found for the elimination of fibres after long-term inhalation and intratracheal instillation whereas shorter half-times were found after short-term inhalation.

Phagocytosis and chemotaxis of rat alveolar macrophages after a combined or separate exposure to ozone and carbon black

Male Wistar rats were treated by ozone or carbon black (CB) alone as well as in combination. Intratracheal instillation with various amounts of CB was followed either by an acute 7-day or subchronic 2-month ozone exposure (0.5 ppm). Two functional parameters were investigated in alveolar macrophages from bronchoalveolar lavagates, the phagocytotic capacity and the chemotactic migration capability. In the phagocytosis assay, the percentage of phagocytizing macrophages decreased significantly in the CB-exposed groups whereas the ozone groups remained close to or at the control level after 7 days and 2 months of exposure, respectively. The number of ingested particles per macrophage and the formation of superoxide anion radicals were not changed after a 7-day exposure to ozone compared to the control group but were increased after a 2-month ozone exposure. However, a reduction was found in the CB groups. A stimulating effect of ozone was observed in the combined groups. Chemotactic migration was generally retarded in the CB-treated groups. From the results it can be concluded that ozone is able to stimulate the phagocytotic and chemotactic activity of alveolar macrophages whereas CB impairs these functions.

Investigation on the biodurability of chemically different stone wool fibres

The biodurability is one of the essential factors for a carcinogenic potential of mineral fibres. The in vivo solubility of commercial fibre products can be influenced by modifications of the chemical composition. Two types of experimental stone wool samples with new chemical composition were compared to a commercial stone wool sample. Sized fractions of these samples with median lengths of 7.1, 9.3 and 6.7 microns, respectively, and median diameters of 0.76, 1.02 and 0.63 microns, respectively, were intratracheally instilled into female Wistar rats with a single dose of 2 mg in 0.3 ml. 5 animals per group were sacrificed after 2 days, 1, 3, 6, 12 and 18 months. After low-temperature ashing of the lungs about 1,000 fibres per group and sacrifice date were analysed in SEM for length and diameter. The number of fibres in the total lung was calculated. An analysis of fibre number of different length and diameter fractions was used to estimate whether dissolution, breakage or mechanical clearance is responsible for the elimination of fibres from the lung. Results indicate that the breakage of fibres with length above 20 microns and the dissolution of fibres was faster in the experimental stone wool samples compared to the commercial sample.

Comparative investigations of the biodurability of mineral fibers in the rat lung

The biodurability of various glass fibers, rockwool, and ceramic fibers was examined in rat lungs and compared with natural mineral fibers. Experiments were based on studies that have shown that the biodurability of fibers is one of the essential factors of the carcinogenic potency of these materials. Sized fractions of fibers were instilled intratracheally into Wistar rats. The evenness of distribution of fibers in the lung was checked by scanning electron microscopy (SEM) or careful examination of the fiber suspension before treatment. After serial sacrifices up to 24 months after treatment, the fibers were analyzed by SEM following low temperature ashing of the lungs. Parameters measured included number of fibers, diameter, and length distribution at the various sacrifice dates, so that analyses could be made of the elimination kinetics of fibers from the lung in relation to fiber length (FL). Size selective plots of the fiber elimination correlated with fiber diameters enables the mechanism of the fiber elimination (dissolution, fiber breakage, physical clearance) to be interpreted. The half-time of fiber elimination from the lung ranges from about 10 days for wollastonite to more than 300 days for crocidolite. The biodurability of man-made vitreous fibers (MMVF) is between these values and is dependent on the chemical composition of the fibers and the diameter and length distribution. Results indicate that the in vivo durability of glass fibers is considerably longer than expected from extrapolation of published data on their in vitro dissolution rates.

Investigation on the durability of man-made vitreous fibers in rat lungs

Two types of sized stonewool with median lengths of 6.7 and 10.1 microns and median diameters of 0.63 and 0.85 microns, and crocidolite with fibers of median length of 4.8 microns and median diameter of 0.18 microns were instilled intratracheally into female Wistar rats. A single dose of 2 mg in 0.3 ml saline was used for the stonewool samples and 0.1 mg in 0.3 ml saline for crocidolite. The evenness of distribution of fibers in the lung was checked by scanning electron microscopy (SEM). Five animals per group were sacrificed after 2 days, 1, 3, 6, and 12 months. After low-temperature ashing of the lungs about 200 fibers per animal were analyzed by SEM for length and diameter. The number and mass of fibers in the total lung were calculated. For the stonewool samples the decrease in the number of fibers in the lung ash followed approximately first order kinetics resulting in half-times of 90 and 120 days. The analysis of fiber number and diameter of different length fractions was used to estimate the contribution of three processes of fiber elimination: transport by macrophages for short fibers, breakage of fibers, and dissolution of fibers. (The process of transport by macrophages was found fastest for fibers with length < 2.5 microns). For the elimination of critical fibers with length > 5 microns, the breakage and dissolution were the most important processes. The breakage of fibers was predominant for one of the stonewool samples. The preferential type of the mechanism of fiber elimination is dependent on chemical composition and size distribution.

Investigation of the biodurability of wollastonite and xonotlite

The in vivo durability of wollastonite materials, coated and uncoated, and of xonotlite was tested. Wollastonite is an anhydrous natural silicate and xonotlite is a hydrated synthetic calcium silicate. UICC crocidolite was used as a positive control with high durability. Using a dry-sizing technique, fractions from the stock materials were prepared according to the definition of "thoracic particulate mass" and "respirable particulate mass" of the American Conference of Governmental Industrial Hygienists. Fibers were instilled intratracheally into female Wistar rats, and the evenness of their distribution in the lung was checked by scanning electron microscopy (SEM). After serial sacrifices at 2 and 14 days, 1, 3, and 6 months, and low temperature ashing of the lung, the fibers were analyzed by SEM. The number and size distribution of fibers were investigated. The total number of crocidolite fibers decreased with a half-time of 240 days, but the number of fibers > 5 microns in length was unchanged after 6 months. The elimination kinetics of wollastonite fibers from the lung were relatively fast, with half-times of 15 to 21 days. The coating of wollastonite in Wollastocoat had no effect on this elimination process. For the thoracic fraction of wollastonite, the elimination from the lung was as fast as for the respirable particulate fraction. The elimination kinetics of xonotlite from the lung was very fast. This material consisted of single crystals of acicular morphology with a median length of 1.3 micron and of agglomerates of these crystals. More than 99% of single crystals and about 85 to 89% of the agglomerates were already eliminated 2 days after instillation.

Irreversible pulmonary changes induced in rat lung by dust overload

The objective of this study was to investigate whether the effects of dust overload are reversible upon cessation of subchronic exposure to test toner. Female rats were exposed 6 hr/day, 5 days/week for 3 months to a test toner at 0, 10, and 40 mg/m3. The retained quantity of test toner in the lungs at the end of exposure was 0.4 and 3.0 mg for the low and high exposure groups, respectively. Fifteen months later, the corresponding values were 0.12 and 2.65 mg in the lungs. Alveolar clearance of tracer aerosols as well as cytologic and enzymatic parameters in the bronchoalveolar fluid was investigated at the end of exposure and subsequently up to 15 months later. The alveolar clearance of 59Fe2O3, 51Cr-polystyrene, and 85Sr-polystyrene tracer aerosols was slightly retarded at the low and substantially impaired at the high exposure level. At the low exposure level, there was some recovery in the clearance behavior up to 6 months after exposure. In contrast, at the high exposure level there was no indication of a reversal of the impaired clearance. For the beta-glucuronidase activity and the number of polymorphonuclear cells, the pattern of the effects was similar to the effects on the half-time tracer particle clearance. In conclusion, the dust overload at a lung burden of 3 mg test toner in rats was persistent for at least 15 months after termination of exposure.

Atypical malignant mesotheliomas with osseous and cartilaginous differentiation after intraperitoneal injection of various types of mineral fibres in rats

The histopathological appearance of malignant mesotheliomas with osseous and cartilaginous differentiation is described in detail. Bone and cartilage occurred in mixed and sarcomatoid mesotheliomas which were induced by intraperitoneal injection of various types of asbestos fibres (asbestos cement, crocidolite, UICC-amosite, UICC-chrysotile B, Calidria-chrysotile). Bone or cartilage were found in 32.7% of mixed mesothelioma and in 12.8% of sarcomatoid mesotheliomas.

Lung clearance and retention of toner, utilizing a tracer technique, during chronic inhalation exposure in rats

Male and female F-344 rats were exposed to 6 hr/day, 5 days/week for up to 24 months to a special test toner at 0, 1, 4, and 16 mg/m3, TiO2 at 5 mg/m3, or SiO2 at 1 mg/m3 by the inhalation route. 59Fe-labeled iron oxide and 85Sr-labeled polystyrene particles were periodically inhaled by the nose-only route and used to measure alveolar clearance rates during the course of the study. This method was used to describe a maximum functionally tolerated dose (MFTD). Pulmonary retention of toner and control materials (TiO2 and SiO2) was measured after 3, 9, 15, 21, and 24 months of exposure. The quantity of all three materials retained in the lungs and lung-associated lymph nodes increased with exposure duration and level. The final pulmonary burdens of toner at the three exposure levels were 0.22, 1.73, and 15.6 mg/lung, respectively. Alveolar clearance of both tracers was substantially impaired at the toner high-exposure level, and moderately slowed at the toner middle-exposure level. The excessive quantity of toner retained and the substantially retarded clearance in the toner high-exposure group are indicative of "lung overloading." Alveolar clearance of 85Sr-polystyrene particles was slightly slowed in the TiO2-exposed group and substantially impaired in the SiO2-exposed group. The alveolar clearance of the unexposed rats decreased about 30% during the study, a change ascribed to aging. For a general description of the toxicokinetics of the various dusts, a semiempirical kinetic model was developed, which could generally be useful for the extrapolation of lung retention of insoluble particles from a subchronic to a chronic inhalation study. Both the maximum tolerated dose (MTD) and the MFTD were exceeded at the toner high-exposure level during the study in rats.

Pulmonary response to toner upon chronic inhalation exposure in rats

A chronic inhalation study of a test toner was conducted by exposure of groups of F-344 rats for 6 hr/day, 5 days/week for 24 months. The test toner was a special Xerox 9000 type xerographic toner, enriched in respirable-sized particles compared to commercial toner, such that it was about 35% respirable according to the ACGIH criteria. The target test aerosol exposure concentrations were 0, 1.0 (low), 4.0 (medium), and 16.0 (high) mg/m3. Titanium dioxide (5 mg/m3) and crystalline silicon dioxide (1 mg/m3), used as negative and positive controls for fibrogenicity, were also evaluated. Inhalation of the test toner or the control materials showed no signs of overt toxicity. Body weight, clinical chemistry values, food consumption, and organ weights were normal in the toner- and TiO2-exposed groups, except for a 40% increase in lung weight in the toner high-exposure group. 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-exposure group. The incidence of primary lung tumors was comparable among the three toner-exposed groups and the TiO2-exposed, and air-only controls, as well as consistent with historical background levels. A mild to moderate degree of lung fibrosis was observed in 92% of the rats in the toner high-exposure group, and a minimal to mild degree of fibrosis was noted in 22% of the animals in the toner middle-exposure group. The pulmonary changes in the toner high-exposure group were smaller in magnitude than those found in the crystalline silica-exposed group. The comparative fibrogenic potency of TiO2, toner, and SiO2 was estimated to be 1:5:418 using a dosimetric model and assuming a common mechanistic basis. There were no pulmonary changes of any type at the toner low-exposure level, which is most relevant in regard to potential human exposures. The lung alterations in the toner high-exposure group are interpreted in terms of "lung overloading," a generic response of the respiratory system to saturation of its detoxification capacity. The maximum tolerated dose (MTD) criterion was met at the toner high (16 mg/m3)-exposure level.

[The persistence of chemically different glass fibers in rat lungs]

The half-times of glass fibre samples of different chemical composition and size was analysed in the rat lung. Up to two years after intratracheal instillation the number of fibres in the ashed lungs was determined by SEM. Half-times of fibre clearance of 40 to 240 days were calculated.

Reversibility of clearance impairment after subchronic test toner inhalation

In a chronic study of a test toner in F-344 rats an impairment of alveolar clearance and increased toner retention were observed at the high and, to a lesser degree, at the middle exposure level throughout the study. The objectives of the present study were: 1. To investigate the nature and the time course of test toner and surrogate tracer alveolar clearance and toner retention behaviour following a subchronic exposure interval. 2. To investigate the dependence of the response upon the pulmonary burden of the test toner.

Lung response to test toner upon 2-year inhalation exposure in rats

SPF F-344 rats were exposed 6 h/day 5 days/wk for up to 24 months to a special test toner at 0, 1, 4 and 16 mg/m3 or TiO2 at 5 mg/m3, or SiO2 at 1 mg/m3, by the inhalation route. The animals were kept for an additional 6 weeks in filtered air. Surviving animals were sacrificed at 25.5 months after start of exposure. Life-span and causes of death were independent of treatment and in accordance with published values. No evidence for systemic toxicity or any upper-respiratory system effects were found in the toner-exposed groups. The incidence of lung tumors was comparable in the control, 3 toner and TiO2-exposed groups. An incidence of 18%, combined benign and malignant tumors was observed in the quartz-treated rats. A slight to moderate degree of fibrosis was observed at the toner high exposure level in all animals, while a very slight degree of fibrosis was noted in 20% of the animals at the toner middle (4 mg/m3) exposure level. The fibrogenic potency of the test toner was calculated to be comparable to TiO2. No pulmonary changes were seen at the toner low (1 mg/m3) and environmentally most relevant exposure level.