Single-Walled Carbon Nanotubes: Cause for Reflection

Speaker:

Moein Moghimi Professor, Faculty of Pharmaceutical Sciences, Copenhagen University

Co-Author(s):

S. Moein Moghimi1, A. Christy Hunter2, Islam Hamad2, Kenneth J. Rutt2

The complement system is one of the body’s most ancient defense strategies, which serves as an important effector arm of both innate and acquired immunity. Three consequences ensue when nanoparticulates trigger complement system in the blood. These are priming of the nanoparticle surface by opsonic complement fragments (eg, C3b, iC3b) for recognition and clearance by phagocytic cells, generation of anaphylatoxins and chemoattractants (eg, C3a, C5a), which may subsequently initiate pseudoallergy-related anaphylaxis in sensitive individuals, and formation of the membrane attack complex (C5b-9) through the activity of C5 convertases. Additionally, multiprotein C5b-9 complexes have the capacity to elicit non-lytic stimulatory responses from vascular endothelial cells and modulate endothelial regulation of haemostasis and inflammatory cell recruitment.
Single-walled carbon nanotubes (SWNTs) have received considerable attention as promising materials for a wide range of experimental diagnostic and therapeutic applications following intravenous injection, particularly in cancer scenarios. Nanotube stability in the blood can be enhanced by surface functionalization with poly(ethylene glycol)-phospholipid (PEG-PL) conjugates. Such surface modification procedures confer longevity to SWNTs in the systemic circulation; a property most likely arises from the steric hindrance of the projected long PEG chains to nanotube–macrophage interaction, which is similar to what has been reported with PEGylated liposomes. PEGylated SWNTs, through prolonged circulation times in the blood, can ultimately target elements of tumour vasculature following conjugation of targeting ligands to the distal end of the projected PEG chains bearing a reactive functional group and therefore act as experimental cancer nanomedicines.
Here we report the interaction between long circulating PEGylated SWNTs and the human complement system. AminoPEG5000-distearoylphosphatidylethanolamine  (aminoPEG5000-DSPE) and methoxyPEG5000-DSPE coated as-grown HIPco SWNTs activated complement in undiluted normal human serum as reflected in significant rises in C4d and SC5b-9 levels, but not the alternative pathway split-product Bb, thus indicating activation exclusively through C4 cleavage. Studies in C2-depleted human serum confirmed that PEGylated nanotube-mediated elevation of SC5b-9 was C4b2a convertase-dependent. With the aid of monoclonal antibodies against C1s and human serum depleted from C1q, nanotube-mediated complement activation in C1q-depleted serum was also shown to be independent of classical pathway. Nanotube-mediated C4d elevation in C1q-depleted serum, however, was inhibited by N-acetylglucosamine, Futhan and anti-MASP-2 antibodies; these observations are indicative of C4 cleavage via MASP-2 activation. Intravenous injection of PEGylated nanotubes in some rats was associated with a significant rise in plasma thromboxane B2 levels, which is measure of in vivo nanotube-mediated complement activation. The clinical implications of these observations will be discussed.

Hamad, I. et al. (2008) Mol. Immunol. doi: 10.1016/j.molimm.2008.05.020 (in press); Moghimi, S. M. et al. (2006) FASEB J. 20, E2057–E2067; Moghimi, S. M. et al. (2005) FASEB J. 19, 311–330.