ToxSci Advance Access published online on October 12, 2009
Toxicological Sciences, doi:10.1093/toxsci/kfp247
Subchronic 13-week Inhalation Exposure of Rats to Multiwalled Carbon Nanotubes: Toxic Effects are determined by Density of Agglomerate Structures, not fibrillar Structures
Institute of Toxicology, Bayer Schering Pharma, 42096 Wuppertal, Germany
Correspondence: Prof. Dr. Jürgen Pauluhn, Department of Inhalation Toxicology, Bayer Schering Pharma, Building no. 514, 42096 Wuppertal, Germany, Tel. ++49 (202) 363909, Fax. ++49 (202) 364589, email: juergen.pauluhn{at}bayerhealthcare.com
Received July 21, 2009; revision received September 6, 2009; accepted October 6, 2009
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Abstract |
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Wistar rats were nose-only exposed to Multi-walled Carbon Nanotubes (MWCNT, Baytubes®) in a subchronic 13-week inhalation study. The focus of study was on respiratory tract and systemic toxicity, including analysis of MWCNT biokinetics in the lungs and lung-associated lymph nodes (LALNs). The time-course and concentration-dependence of pulmonary effects were examined by bronchoalveolar lavage (BAL), and histopathology up to 6 months postexposure. Particular emphasis was directed to the comparative characterization of MWCNT structures prior to and after micronization and dry powder dispersion into inhalation chambers. These determinations were complemented by additional analyses in digested BAL-cells. Animals were exposed on 6 hrs/day, five days per week for 13 consecutive weeks to 0, 0.1, 0.4, 1.5, and 6 mg/m3. The subchronic exposure to respirable solid aerosols of MWCNT was tolerated without effects suggestive of systemic toxicity. Kinetic analyses demonstrated a markedly delayed clearance of MWCNT from lungs at overload conditions. Translocation into LALNs occurred at 1.5 and 6 mg/m3 and required at least 13 weeks of study to become detectable. At these exposure levels the lung and LALN weights were significantly increased. Sustained elevations in BAL PMNs and soluble collagen occurred at these concentrations with borderline effects at 0.4 mg/m3. Histopathology revealed principal exposure-related lesions at 0.4 mg/m3 and above in the upper respiratory tract (goblet cell hyper- and/or metaplasia, eosinophilic globules, focal turbinate remodeling) and the lower respiratory tract (inflammatory changes in the bronchiolo-alveolar region, increased interstitial collagen staining). Granulomatous changes and a time-dependent increase of a bronchiolio-alveolar hyperplasia occurred at 6 mg/m3. All endpoints examined were unremarkable at 0.1 mg/m3 (NOAEL). In summary, this study demonstrates that the induced pathological changes are consistent with overload-related phenomena. Hence, the etiopathological sequence of inflammatory events caused by this type of MWCNT appears to be related to the high displacement volume of the low-density MWCNT-assemblage structure rather than to any yet ill-defined intrinsic toxic property. Thus, the hypothesis of study is verified, namely common denominators between carbon black and MWCNT do exist.
Key Words: Nanoparticles; Multi Walled Carbon Nanotubes; repeated inhalation exposure; disposition; respirability; clearance; aggregates; pulmonary and extrapulmonary toxicity; volume displacement overload.