Functionalized carbon nanotubes in drug design and discovery

Carbon nanotubes (CNTs) have been proposed and actively explored as multipurpose innovative carriers for drug delivery and diagnostic applications. Their versatile physicochemical features enable the covalent and noncovalent introduction of several pharmaceutically relevant entities and allow for rational design of novel candidate nanoscale constructs for drug development. CNTs can be functionalized with different functional groups to carry simultaneously several moieties for targeting, imaging, and therapy. Among the most interesting examples of such multimodal CNT constructs described in this Account is one carrying a fluorescein probe together with the antifungal drug amphotericin B or fluorescein and the antitumor agent methotrexate. The biological action of the drug in these cases is retained or, as in the case of amphotericin B constructs, enhanced, while CNTs are able to reduce the unwanted toxicity of the drug administered alone. Ammonium-functionalized CNTs can also be considered very promising vectors for gene-encoding nucleic acids. Indeed, we have formed stable complexes between cationic CNTs and plasmid DNA and demonstrated the enhancement of the gene therapeutic capacity in comparison to DNA alone. On the other hand, CNTs conjugated with antigenic peptides can be developed as a new and effective system for synthetic vaccine applications. What makes CNTs quite unique is their ability, first shown by our groups in 2004, to passively cross membranes of many different types of cells following a translocation mechanism that has been termed the nanoneedle mechanism. In that way, CNTs open innumerable possibilities for future drug discovery based on intracellular targets that have been hard to reach until today. Moreover, adequately functionalized CNTs as those shown in this Account can be rapidly eliminated from the body following systemic administration offering further encouragment for their development. CNT excretion rates and accumulation in organs and any reactivity with the immune system will determine the CNT safety profile and, consequently, any further pharmaceutical development. Caution is advised about the need for systematic data on the long-term fate of these very interesting and versatile nano-objects in correlation with the type of CNT material used. CNTs are gradually plyaing a bigger and more important role in the emerging field of nanomedicine; however, we need to guarantee that the great opportunities they offer will be translated into feasible and safe constructs to be included in drug discovery and development pipelines.