Carbon nanotubes are promising drug delivery platforms that can be functionalized with a variety of biomolecules, such as antibodies, proteins, or DNA. This allows for specific, targeted payload delivery to particular tissues, organs, or cells.
Carbon nanotubes can easily penetrate cells, delivering drugs directly to the cytoplasm or nucleus. Drug delivery systems improve the pharmacological and therapeutic profile, and efficacy of the drug and lower the occurrence of off-targets.
Nanotubes bundles viewed through an electron microscope. Image Credit: Dimarion / Shutterstock
Carbon nanotubes are large molecules, made from a repeating pattern of sp2 hybridized carbon atoms in a hexagonal arrangement, wrapped into a cylinder of approximately 2.5–100 nm in diameter. Carbon nanotubes are single or multi-walled depending on the number of carbon sheets rolled together, while the ends are capped by a hemi-spherical carbon arrangement as seen in fullerenes.
Carbon nanotubes are poorly soluble in aqueous media, and need to be functionalized or modified before use as drug-delivery vehicles.
Functionalization of Carbon Nanotubes
Primary method of functionalizing carbon nanotubes includes oxidation by strong acids, resulting in reduction in their length while generating exposed carboxylic groups, improving dispersibility in aqueous solutions.
A second method is based on addition reactions to the external wall of the carbon nanotube, based on the 1,3-dipolar cycloaddition of azomethine ylides and heating in DMF in the presence of α-amino acids and an aldehyde.
Other functionalization methods include attaching one or more drugs intended to function and constituting a single, powerful payload which can be triggered to release in the target location. Also, recognition units, able to encourage the uptake of the carbon nanotube by the target cells; and imaging agents, such as a fluorescent probe, to allow the carbon nanotube to be tracked within the body.
Cellular Uptake of Carbon Nanotubes
Functionalized carbon nanotubes are easily internalized by cells through passive and endocytosis-independent mechanisms. Nanotubes conform to a perpendicular position with the cell membrane during uptake, perforating and diffusing through the lipid bilayer to enter the cytoplasm. Other methods of cell penetration are under experimentation, including the use of magnetic carbon nanotubes that ‘spear’ a target cell by the use of an external rotating magnetic field.
The size, charge, and hydrophobicity of the functional groups present on the surface of the carbon nanotubes play a large role in the ability of nanotubes to cross cell membranes.
Administration of Carbon Nanotubes
Carbon nanotubes have so far been delivered to patients by subcutaneous, abdominal, and intravenous routes. Shorter carbon nanotubes are absorbed into the body more easily through oral routes, passing through columnar cells of the intestines. Carbon nanotubes delivered subcutaneously tend to linger in the area of injection, before slowly diffusing away from the region and passing through the lymph system. This is potentially useful for targeting metastatic cancer cells that migrate in this manner.
Intravenously delivered carbon nanotubes quickly distribute to many internal organs. The retention time in the body is highly dependent on the size and surface chemistry of the tube before elimination via the kidneys and liver. Functionalization with molecules such as polyethylene glycol can improve the retention time of carbon nanotubes.
The pharmacological and toxicological profile of carbon nanotubes requires further investigation before they can be used in clinical settings.