Review Article
Toxicity issues in the application of carbon nanotubes to biological systems

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Abstract

Carbon nanotubes (CNTs) have recently emerged as a new option for possible use in methodologies of cancer treatment, bioengineering, and gene therapy. This review analyzes the potential, through possible toxicologic implications, of CNTs in nanomedicine. Generally, proven success in other fields may not translate to the use of CNTs in medicine for reasons including inconsistent data on cytotoxicity and limited control over functionalized-CNT behavior, both of which restrict predictability. Additionally, the lack of a centralized toxicity database limits comparison between research results. To better understand these problems, we seek insight from currently published toxicity studies, with data suggesting postexposure regeneration, resistance, and mechanisms of injury in cells, due to CNTs.

From the Clinical Editor

Carbon nanotubes (CNTs) have recently emerged as a new option for cancer treatment, bioengineering, and gene therapy. Inconsistent data on cytotoxicity and limited control over functionalized-CNT behavior currently restrict predictability of such applications.

Section snippets

Modifying CNTs for use in medicine

As CNTs are intrinsically not water soluble, modification through chemical functionalization using suitable dispersants and surfactants can enhance solubility to the range of g/mL4 and is essential for their controlled dispersion. For example, constituent polar molecules can render CNTs soluble, whereas nonpolar moieties make CNTs immiscible. Such processes have proved especially important in that nonsolubilized CNTs have been found to cause cell death in culture.17, 18, 19

Additionally, the

Toxicity at the level of fabrication and functionalization

Each CNT could be intrinsically different due to limitations on the fabrication of structurally identical CNTs with minimal impurities.33 Subtle variations in local and overall charge, catalyst residue (typically Fe, Co, and Ni), and length of individual nanotubes are three representative issues that preclude precise use of CNTs in the biomedical sciences.

Mechanisms of interaction

Although much progress has been made in understanding how CNTs traverse the lipid membrane of a given cell type, the details of the proposed mechanisms are still debated. Such considerations are important in that the failure to understand the uptake mechanisms of nanoscale materials and their influence on toxicity could create another level of unpredictability.45 To date, two major mechanisms have been widely considered: (1) endocytosis/phagocytosis56 and (2) nanopenetration (Figure 3).

In vitro toxicity: Dosage, tolerance, and regeneration

As with most biomedical research, in vitro tests for toxicity pave the way for future applications in vivo. However, for CNT application, it seems that in vivo studies have been attempted while many questions from in vitro studies have been left unanswered (eg, issues on overall safe CNT dosage for a particular task and the sensitivity of particular cell lines). The probing of such issues is relevant as there may not be a linear relationship between mass and toxicity tolerance.

In vivo toxicity: new insights

Currently, insights into how CNTs behave in the human body (Figure 4) are obtained through recent studies of systemic toxicity, as representatively illustrated in Table 2. To date, the number of studies suggesting CNTs to be nontoxic in vivo outnumbers those proposing otherwise. For example, doses of 20 μg diethylene triamine pentaacetic acid (DTPA)-MWCNT/μL phosphate buffer saline (PBS) and 20 μg DTPA-SWCNT/μL PBS were administered in different mice intravenously with no acute toxicity

Pharmacokinetic profile

The map of CNT pharmacokinetics and biodistribution seems to be developing rapidly. The majority of intravenously injected CNTs in mice mainly seem to be emptied in the urine, with far less found in the liver, spleen, and lungs.74 However, other studies indicate the liver and spleen to be the main sites of CNT accumulation.79 Studies have also found CNT deposits mostly in the excretory systems like the bladder, kidneys, and intestines, in feces,81 and again in the kidneys.69 A contributing

Testing protocol: An unforeseen obstacle

The crux of the experimental method is the process of testing, but what implications will it have on our current understanding of CNT toxicity if some results could arise from internally invalid tests? The argument for increasing testing confidence stems from an in vitro study83 where it was revealed that common amino acids and vitamins (eg, phenylalanine and folate) passively adsorb onto CNTs. Concentrations of SWCNTs as low as 0.01 to 0.1 mg/mL culture was able to deplete 2 nM of folic acid

Conclusions

CNTs have been proposed for use in medicine chiefly for (1) their ability to be functionalized, both covalently and noncovalently, with various moieties, and (2) their aspect ratio and geometry, which enables penetration into the cell. In our survey of existing work and methods, we have found that considerably more fundamental and applied research must be carried out before the viability of CNTs can be realized. One of the major issues affecting nanotube biomedical application is that of charge

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    The authors gratefully acknowledge financial support from the National Science Foundation (grant no. ECS-05-08514) and from the Office of Naval Research (award no. N00014-06-1-0234).

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