Carbon nanotubes were originally the third allotrope of carbon after fullerenes discovered in 1991. It has a one-dimensional quantum material with a special structure: the axial dimension is micrometers, the radial dimension is nanometers, and both ends are basically closed. Carbon nanotubes have high biocompatibility and drug-carrying capacity, and allow chemical modification and functional processing, and exhibit high cell specificity. Therefore, they have received general attention in gene carriers and drug carriers. For example, chemically modified carbon nanotubes can be connected to nucleotides, proteins, and some natural or synthetic anticancer drugs. Existing studies have shown that the drug connected to the carbon nanotube has better biological activity than its free form. They can be more effectively taken up by the cell, can achieve controlled release of the drug, enhance cytotoxicity, and increase the residence time in the body And extend the half-life, etc.
CNTs are generally divided into two categories: single-walled carbon nanotubes (SWCNTs) composed of single cylindrical graphene and multi-walled carbon nanotubes (MWCNTs) composed of multilayer graphene sheets. The cylindrical structure of CNTs is in contact with cells at multiple points, which is conducive to carrying drugs for transmembrane penetration, and effectively promotes drugs to pass through the blood-cerebrospinal fluid barrier. Compared with traditional drug delivery carriers such as liposomes, its ultra-high specific surface area greatly increases the drug loading, can carry a large number of different molecular types and anti-tumor drugs with structures that are easily captured by target cells, and can be mediated by the mechanism of endocytosis or endocytosis independent mechanism (membrane fusion, diffusion or direct pore transport) enters the cell and avoids being discharged by the cell pump.
The Mechanism of Cellular Uptake of CNTs
CNTs have a nanometer size, can be taken up by different types of cells, and their unique tubular structure effectively increases the surface area. These advantages make it a molecular carrier that has attracted much attention today. In recent years, a large number of studies have shown that there are three different mechanisms for cellular uptake of CNTs, namely endocytosis, phagocytosis, and translocation directly through the cell membrane. Endocytosis is mainly the cellular uptake of macromolecular substances (such as proteins, antibodies, etc.), which reach the endosome or lysosome through the formation of small vesicles. The principle of phagocytosis is similar to endocytosis. The difference is that phagocytosis takes in larger particles (such as bacteria with a diameter of less than 1μm) and is transported by phagocytes. It has also been reported that CNTs modified by polycations can be similar to cell-penetrating peptides in potential and morphology, and can be directly translocated on mammalian cell membranes.
Studies have found that the factors that affect the uptake mechanism are the length of the nanotubes, the degree of aggregation and their characterization. Different characterizations and different types of effector cells in the preparation of CNTs determine different uptake mechanisms. The essential characteristics of functional groups covalently or non-covalently bound to the surface of CNTs and the size of CNTs have an important influence on the interaction between cells and them. CNTs can be taken up by mammalian cells and prokaryotic cells through two different energy-dependent and non-energy-dependent mechanisms.
Single-walled carbon nanotubes (SWNTs) can bind to different proteins, enter endosomes through endocytosis, and then be transported to lysosomes by epithelial cells or mesenchymal cells, or form phagosomes through phagocytosis by dedicated phagocytes . The phagosome combines with the lysosome to digest the carried material. For example, the fluorescently-labeled protein is combined with biotin-functionalized oxidized SWCNTs and then enters the cell through endocytosis. The biological macromolecules on the surface of carboxylated or amidated CNTs, such as proteins, antibodies, and DNA, can form small bundles and enter cells through energy-dependent endocytosis. Longer multi-walled carbon nanotubes (MWNTs) can form coiled or bundled structures on their own and hinder cell uptake. Therefore, shorter MWCNTs can penetrate cell membranes more efficiently and enter cells more efficiently than longer ones. The cellular uptake mechanism of SWCNTs and MWCNTs bundles is different, which indicates that the size of nanotubes affects the way cells take them up. SWCNTs enter cells more by direct penetration, while MWCNTs enter cells more by endocytosis. Since the biological functions of phagocytes and non-phagocytes are different, the types of effector cells are different, and the mechanism of cellular uptake is also different. Studies have reported that functionalized CNTs can be taken up by cells of different species, such as non-mammalian fungi and yeast cells. In addition, the surface of CNTs is coated with different substances to produce different effects. SWCNTs coated with collagen, proteins, polymers, pegylated lipids, etc. can be endocytosed by cells. Regardless of whether ssDNA and CNTs are covalently or non-covalently bound, they can be transferred to the nucleus by HeLa cells through endocytosis. In this process, the cell recognizes the coating material on the surface of CNTs instead of the CNTs skeleton. These macromolecular coatings modify the inherent characteristics of CNTs by "hiding" the structure of CNTs. Regardless of whether the functional groups coated on the surface of CNTs are negatively charged, positively charged or neutral, they can be taken up by cells. The effects of different functional groups on the surface of CNTs can be tested by flow cytometry, laser confocal and treatments that inhibit energy-dependent internalization.
Carbon Nanotube Delivery System
Small Molecule Drug Delivery
Compared with traditional DDS such as liposomes and dendritic drug carriers, SWNTs have a larger surface area per unit weight and therefore have a greater drug loading capacity. Moreover, CNTs are stable in nature and flexible in structure, which can extend the cycle time of loaded drugs and improve bioavailability. Many studies have reported that CNTs combined with various anti-cancer drugs, such as cisplatin, doxorubicin, taxane, methotrexate, gemcitabine, epirubicin, paclitaxel, etc., can improve the availability of anti-cancer drugs and enhance Anti-cancer effect, etc. In addition, the combination of CNTs and the antifungal drug Amphotericin B reduces the toxic effect on other cells and improves its antifungal effect.
Figure 1. Application of carbon nanotube delivery system.
Shi Kam et al. combined biotin with oxidized CNT to construct a fluorescent streptavidin complex. Biotin can be easily combined with CNT through the amide bond. Since biotin and streptavidin can specifically bind to each other, the application of the biotin-streptavidin system is more convenient. They also studied the ability of human acute myeloid leukemia (HL60) cells and human T lymphocytes (Jurkat) to take up protein-laden CNTs through endocytosis. And found that this pathway does not depend on protein connection and functionalization. Research by Kam et al. confirmed that SWCNTs can enhance the cell internalization of streptavidin, protein A, bovine serum albumin (BSA) and cytochrome c. Oral peptide drug therapy has poor efficacy due to factors such as easy degradation by enzymes and poor intestinal absorption capacity, and CNTs can effectively improve this situation. Erythropoietin is a peptide drug used to treat specific types of anemia and can regulate red blood cell production. Venkatesan et al. constructed a CNTs transport system to enable oral administration of erythropoietin.
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