John Balbus, M.D., M.P.H., is Chief Health Scientist.

We and many others have made analogies between nanoparticles and asbestos.  The purpose of the analogy has generally been to emphasize the long latency that can occur between exposure to toxic materials and the development and subsequent recognition of disease arising from that exposure.  And, of course, the enormous legal and financial burden of failing to adequately consider risks before allowing widespread exposure.  But a new study suggests that the analogy may be even stronger than we thought:  It may extend to the capacity to cause mesothelioma, the rare form of cancer associated with exposure to asbestos.

In a recent study published in the Journal of Toxicological Sciences, researchers at Japan’s National Institute of Health Sciences injected commercial multi-walled carbon nanotubes (MWCNTs) into the peritoneal cavities of mice that were bred to be especially susceptible to mesothelioma.  (The peritoneal cavity is the space between the abdominal organs and abdominal wall that is lined with mesothelial cells, which can give rise to mesotheliomas).

They compared the mice’s response to injected MWCNTs to the response to crocidolite asbestos (the form of asbestos most strongly associated with mesothelioma), and also to fullerenes.  They found that the potency of MWCNTs in causing mesotheliomas in these mice was at least as high as the asbestos.  In contrast, fullerenes did not cause any mesotheliomas.

The physicochemical characteristics of MWCNTs are similar in many ways to asbestos.  Both are rigid, rod-like shapes with a high length to width (aspect) ratio and lengths that can extend to 5 microns or longer. Both are biopersistent and contain iron.  Given that all of these characteristics collectively contribute to the carcinogenicity of asbestos, this study suggests these same characteristics may cause MWCNTs to be carcinogenic as well.

This study doesn’t prove that inhaling MWCNTs causes mesothelioma; it would first have to be shown that inhaled MWCNTs can make their way through the lung to contact mesothelial cells, and then persist there long enough to initiate carcinogenesis in less susceptible animals.  Asbestos does this, of course, and there’s no obvious reason why MWCNTs should behave differently.  More pieces of the puzzle need to be filled to demonstrate the actual degree of risk, but this is clearly a large red flag.

Because MWCNTs, unlike asbestos, are deliberately engineered, it’s possible that characteristics like iron content and fiber length may be controlled to render the tubes less or even non-carcinogenic.  But with the wide variations noted for single-walled carbon nanotubes (see our earlier post), manufacturers of MWCNTs would have to demonstrate the ability to manufacture product consistently to specifications, and users of MWCNTs would need to be wary of variations from producer to producer.  In the meantime, researchers developing and using MWCNTs, especially for applications that could yield long-term exposures, such as incorporation into hip prostheses, need to proceed with extreme caution and carefully assess the potential for carcinogenicity from their devices.