"We found that some carbon nanoparticles activate human platelets and stimulate them to aggregate or clump together," said Marek Radomski of the University of Texas. "We also demonstrate that the same nanoparticles stimulated blockage of the carotid artery in the rat model."
Radomski and colleagues tested the effect of the particles on human platelets and in a model of carotid artery thrombosis using anaesthetized rats. Fullerene molecules showed the least effect - they did not affect human platelet aggregation and had only a small effect on rat thrombosis. The mixed carbon nanoparticles had the greatest effect in both tests, followed by single-walled carbon nanotubes, multi-walled carbon nanotubes and standard urban particulate matter (which had an average size of 1.4 µm).
"This research is not a case against nanotechnology," said Radomski. "It's difficult to overestimate the importance of this amazing technology's ability to transform medicine. But it's good to assess the risk of a new technology in advance. This is a case for moving ahead in a cautious and informed way."
The mixed carbon nanoparticles, nanotubes and standard urban particulate matter activated the glycoprotein integrin receptor on the platelets, which is "vital to their aggregation". The researchers say that this seems to be the underlying mechanism for the nanoparticles' effects, but that each nanoparticle employed a different molecular pathway to activate the receptor.
"Medical evidence has been accumulating mainly from epidemiological studies that exposure of humans to particulate matter, and to very small particles, increases the risk of cardiovascular disease," said Radomski. "The mechanisms of that risk are not well known. Clot formation is my research interest and we wanted to look at the effect of nanoparticles - both the pollutants caused by combustion and engineered nanoparticles that might be used in various nanomedical devices such as improved drug-delivery systems."
The researchers also believe that nanotubes mimic the molecular bridges involved in platelet interactions while fullerene molecules don't. As a result, fullerenes may have an advantage for use in targeted drug delivery or imaging.
The researchers reported their work in the British Journal of Pharmacology.