Pulmonary effects and clearance after long-term inhalation of potassium octatitanate whiskers in rats

Carcinogenic effect of potassium octatitanate (POT) fibers in the lung and pleura of male Fischer 344 rats after intrapulmonary administration

Background

Potassium octatitanate fibers (K₂O•8TiO₂, POT fibers) are used as an asbestos substitute. Their physical characteristics suggest that respirable POT fibers are likely to be carcinogenic in the lung and pleura. However, previous 2-year inhalation studies reported that respired POT fibers had little or no carcinogenic potential. In the present study ten-week old male F344 rats were left untreated or were administered vehicle, 0.25 or 0.5 mg rutile-type nano TiO₂ (r-nTiO₂), 0.25 or 0.5 mg POT fibers, or 0.5 mg MWCNT-7 by intra-tracheal intra-pulmonary spraying (TIPS), and then observed for 2 years.

Results

There were no differences between the r-nTiO₂ and control groups. The incidence of bronchiolo-alveolar cell hyperplasia was significantly increased in the groups treated with 0.50 mg POT and 0.50 mg MWCNT-7. The overall incidence of lung tumors, however, was not increased in either the POT or MWCNT-7 treated groups. Notably, the carcinomas that developed in the POT and MWCNT-7 treated rats were accompanied by proliferative fibrous connective tissue while the carcinomas that developed in the untreated rats and the r-nTiO₂ treated rats were not (carcinomas did not develop in the vehicle control rats). In addition, the carcinoma that developed in the rat treated with 0.25 mg POT was a squamous cell carcinoma, a tumor that develops spontaneously in about 1 per 1700 rats. The incidence of mesothelial cell hyperplasia was 4/17, 7/16, and 10/14 and the incidence of malignant mesothelioma was 3/17, 1/16, and 2/14 in the 0.25 mg POT, 0.5 mg POT, and MWCNT-7 treated groups, respectively. Neither mesothelial cell hyperplasia nor mesothelioma developed in control rats or the rats treated with r-nTiO₂. Since the incidence of spontaneously occurring malignant mesothelioma in rats is extremely low, approximately 1 per 1000 animals (Japan Bioassay Research Center [JBRC] historical control data), the development of multiple malignant mesotheliomas in the POT and MWCNT-7 treated groups was biologically significant.

Conclusion

The incidence of pleural mesotheliomas in male F344 rats administered POT fibers and MWCNT-7 was significantly higher than the JBRC historical control data, indicating that the incidence of pleural mesothelioma in the groups administered POT fibers and MWCNT-7 fibers via the airway using TIPS was biologically significant. The incidence of type II epithelial cell hyperplasia and the histology of the carcinomas that developed in the POT treated rats also indicates that respirable POT fibers are highly likely to be carcinogenic in the lungs of male F344 rats.

Electronic supplementary material

The online version of this article (10.1186/s12989-019-0316-2) contains supplementary material, which is available to authorized users.

Potassium octatitanate fibers induce persistent lung and pleural injury and are possibly carcinogenic in male Fischer 344 rats

Potassium octatitanate fibers (K₂O·8TiO₂, POT fibers) are widely used as an alternative to asbestos. We investigated the pulmonary and pleural toxicity of POT fibers with reference to 2 non‐fibrous titanium dioxide nanoparticles (nTiO₂), photoreactive anatase (a‐nTiO₂) and inert rutile (r‐nTiO₂). Ten‐week‐old male F344 rats were given 0.5 mL of 250 μg/mL suspensions of POT fibers, a‐nTiO₂, or r‐nTiO₂, 8 times (1 mg/rat) over a 15‐day period by trans‐tracheal intrapulmonary spraying (TIPS). Rats were killed at 6 hours and at 4 weeks after the last TIPS dose. Alveolar macrophages were significantly increased in all treatment groups at 6 hours and at 4 weeks. At week 4, a‐nTiO₂ and r‐nTiO₂ were largely cleared from the lung whereas a major fraction of POT fibers were not cleared. In the bronchoalveolar lavage, alkaline phosphatase activity was elevated in all treatment groups, and lactate dehydrogenase (LDH) activity was elevated in the a‐nTiO₂ and POT groups. In lung tissue, oxidative stress index and proliferating cell nuclear antigen (PCNA) index were elevated in the a‐nTiO₂ and POT groups, and there was a significant elevation in C‐C motif chemokine ligand 2 (CCL2) mRNA and protein in the POT group. In pleural cavity lavage, total protein was elevated in all 3 treatment groups, and LDH activity was elevated in the a‐nTiO₂ and POT groups. Importantly, the PCNA index of the visceral mesothelium was increased in the POT group. Overall, POT fibers had greater biopersistence, induced higher expression of CCL2, and provoked a stronger tissue response than a‐nTiO₂ or r‐nTiO₂.

Pulmonary Effects and Biopersistence of Deposited Silicon Carbide Whisker After 1-Year Inhalation in Rats

Silicon carbide whisker (SiCW) is a durable asbestos substitute. To clarify the biopersistence, the changes of geometrical configurations of deposited whiskers, and potential carcinogenicity and fibrogenicity, 42 Wistar male rats were exposed to SiCW for 6 h/day, 5 days/wk for 1 yr by inhalation. The mass median aerodynamic diameter (geometric standard deviation, GSD), the geometric mean fiber diameter (GSD), and the geometric mean fiber length (GSD) were 2.4 microm (2.4), 0.5 microm (1.5) and 2.8 microm (2.3), respectively. The daily average exposure concentrations were 2.6 +/- 0.4 mg/m3 (98 +/- 19 fibers/ml) and the rats were sacrificed at 6 days and 3, 6, and 12 mo after the exposure. The SiCW amount deposited in each rat lung at 6 days after the exposure, determined by an x-ray diffraction method, was 5.3 +/- 1.4 mg. The biological half-time (BHT) was 16 mo calculated from the deposited SiCW at 6 days and 3, 6, and 12 mo. The BHT was more prolonged than normal physiological clearance. The diameter of SiCW in the lung at each sacrificed time during 12 mo of clearance after the inhalation did not change. However, as for the length of SiCW in the lung, longer SiCW tended to be retained in the lung as the clearance time increased, especially after 6 mo. The histopathological examination revealed bronchoalveolar hyperplasia (BAH) in 2 rats at 1 yr after the exposure and severe fibrotic changes around aggregated SiCW.

Histopathological Changes in Rat Lung Following Intratracheal Instillation of Silicon Carbide Whiskers and Potassium Octatitanate Whiskers

We evaluated the pattern of pulmonary inflammation for the assessment of the biological hazards of two man-made mineral fibers. Rats were exposed by intratracheal instillation to a 2 mg dose of each of two kinds of man-made mineral fibers (PT1, potassium octatitanate whisker; and SiCW, silicon carbide whisker), or three kinds of comparable respirable particles (crystalline silica, crocidolite asbestos, and titanium dioxide, TiO(2)). The lung tissue was evaluated at 3 day, 1 wk, and 1, 3 and 6 mo after exposure. Digital images taken of the lung sections were examined by morphometric point counting method (PCM). PT1 and SiCW showed a similar inflammatory pattern, which contains temporal inflammation such as moderate alveolitis within 1 wk after the exposure, and in later phase aggregation foci of instilled fibers. Differences in repair patterns of these two man-made mineral fibers showed that the toxicity of these two fibers is less toxic than for crocidolite or crystalline silica. Although SiCW showed a higher inflammation score than TiO(2) within 1 mo after instillation, the inflammation scores and fibrotic changes of PT1 and SiCW were not significant as TiO(2) at 3 mo and 6 mo in this study. Careful use should be recommended when these materials are used in the workplace.

The Effect of Lung Burden on Biopersistence and Pulmonary Effects in Rats Exposed to Potassium Octatitanate Whiskers by Intratracheal Instillation

In our previous inhalation studies on health effects of the asbestos substitute, potassium octatitanate whiskers (POW), we showed that an excess amount of POW deposition in the rat lung increased biopersistence resulting in fibrotic changes. The critical deposition amount which induced the higher biopersistence was estimated to lie between 1.5 mg and 2.4 mg. In order to find the exact amount, the relationship between the lung POW burden and biopersistence was investigated by the intratracheal instillation method. The chemical formula of POW is K2Ti8O17 and the geometric mean fiber diameter (geometric standard deviation, GSD) and geometric mean fiber length (GSD) are 0.35 microm (1.6) and 4.4 microm (2.7), respectively. Rats were intratracheally instilled with 0.5 mg, 1.0 mg, 2.0 mg or 5.0 mg of POW and sacrificed at 1 day and 1, 3, 6 and 12 months after the instillation. The POW amount in each lung was chemically analyzed by ICP-AES after microwave digestion and the biological half time (BHT) of each POW dose was calculated. The BHTs of each group were 10, 15, 20 and 42 months for 0.5, 1.0, 2.0 and 5.0 mg of POW, respectively, and BHT showed a linear dose-dependent increase, but without a threshold within the range of 0.5 mg to 5.0 mg, which was recognized in our earlier inhalation studies. In the histopathological photograph just after the instillation, many macrophages, which had phagocytized many more fibers, existed around the bronchiole compared with the earlier inhalation study at almost the same deposited amount. The relationship between POW amount and biopersistence in this intratracheal instillation study was different from that of our previous inhalation studies, probably due to the unnatural method of the fiber introduction to the lung, which in turn led to a different fiber distribution. It is suggested that an intratracheal instillation study is not an appropriate method for estimating excess deposition amounts of POW and an inhalation study will be needed. However, this intratracheal instillation study clarified that the clearance of POW was delayed as compared with previous inhalation studies at similar deposition amounts and this result has importance for the hazard assessment of dusts in animal experiments.

The Effect of Lung Burden on Biopersistence and Pulmonary Effects in Rats Exposed to Potassium Octatitanate Whiskers by Inhalation

The effect of lung burden on biopersistence and histopathological changes caused by potassium octatitanate whiskers (POW) which is one of the asbestos substitutes were investigated for 1-yr and 4-wk inhalation periods. In the 1-yr inhalation experiment, male Wistar rats were exposed to POW (TW) for 6 h/d, 5 d/wk under the same conditions as a previous study of POW (PT1, JFM fiber) which is made by different manufacturer. The exposure concentration was 1.9 +/- 0.7 mg/m(3) and the mass median aerodynamic diameter (MMAD) and geometric standard deviation (GSD) in the chamber were 1.6 microm and 2.9. Rats were sacrificed at 3 d and 1 yr after the inhalation experiment and TW deposits in the lungs were determined by ICP-AES. Lung burden at 3 d and 1 yr after the inhalation was 2.39 +/- 0.50 mg and 1.37 +/- 0.96 mg, respectively, the deposition fraction was 8.1% and biological half time (BHT) was 15 months. Aggregated dust cells and mild fibrotic changes around these dust cells were observed in the exposed rat lung. These results were almost the same as those obtained in the previous 1-yr PT1 study. In the 4-wk inhalation experiment, to investigate the effect of lung burden on biopersistence and histopathological change, male Wistar rats were exposed to PT1. The exposure concentration was 102 +/- 21 mg/m(3), MMAD (GSD), the geometiric mean length and diameter (GSD) of the PT1 in the chamber were 1.6 microm (3.0), 2.2 microm (1.8) and 0.33 microm (1.5), respectively. Rats were sacrificed at 3 d, 1 wk, and 1, 3, 6 and 12 months after the inhalation experiment. The lung burden of POW at 3 d after 4 wk inhalation was 1.49 +/- 0.19 mg, which was close to the estimated amount of overload. The BHT of the total mass (4.1 months) was not prolonged, but aggregated dust cells were observed in the subpleural region and around the bronchioles and mild fibrotic changes were observed only around the dust cells at one year after the 4-wk inhalation. It is considered that the excessive exposure which impairs the function of alveolar macrophage mediated clearance may cause the aggregation of dust cells and fibrotic changes.

Research needs to improve risk assessment of fiber toxicity

Risk characterization of exposure to toxic compounds requires information on the intrinsic toxic properties, including toxic mechanism and toxicokinetics, on dose response at the most critical targets for identification of the NOEL or for extrapolation from high to low dose, and on human exposure. Abundant information is available on the intrinsic properties of MMMF, on the three D's (dose, dimension, durability) and on the toxic mechanisms. However, only a few of these studies provide information on the dose response of the effects or of the mechanisms investigated. Moreover, in many cases single high doses exceeding the MTD have been applied and are difficult to interpret for lower exposure scenarios. Risk characterization is further hampered by the still open question whether MMMF are directly genotoxic or induce secondary genotoxicity via inflammation. Finally, there is disagreement about the relevance of animal studies on MMMF for humans and thus about the most rational extrapolation of the dose response of toxic effects observed in animals to man. These deficits are briefly described and discussed from a toxicological point of view.