BIOMEDICAL APPLICATIONS IV: CARBON NANOTUBE-NUCLEIC ACID COMPLEXES FOR BIOSENSORS, GENE DELIVERY AND SELECTIVE CANCER THERAPY - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 133

5.4.2 CNT-DNA complexes for cancer therapy
As clearly reported throughout this chapter, CNT-DNA complexes present
several interesting properties which could be applied not only in biochemical
sensors or gene therapy but also in cancer treatment.
A common strategy consists of incorporating folic acid on a system in order
to target tumour tissues and trigger selective cancer cell death. As regards
this aspect, Ko et al. reported an interesting example, by assembling folic acid
and DNA into a multifunctional system. 90 The starting material consisted of
an ssDNA sequence of 52 bases, with four palindromic segments. 91 Repeated
units of ssDNA self-assembled to originate a DNA nanotube (NT) with a
diameter of 50-200 nm and a maximum length of 40 μm. To reiterate, no
CNTs were complexed with DNA, but only oligonucleotides were assembled
into a tubular structure. In order to test such supramolecular architecture for
drug delivery, the assemble was non-covalently functionalised with both a
luorescent dye (which represented the drug prototype) and folic acid, so as to
target cancer cells on the basis of their high expression of folic acid receptors
(FR). More precisely, the free amino groups in DNA were conjugated with
folic acid residues in the form of N -hydroxysuccinimidic (NHS) esters, thus
providing a dual-functionalised DNA-NT. Interestingly, extensive washing and
treatment with DNAse I did not reduce the luorescence of internalised DNA-
NT, although its morphology seemed to be affected by the uptake process.
In other words, it is possible that the internalisation inside cells required
shortening of DNA-NT or changing of the supramolecular structure, but this
could not be ascertained because of the limit of the sensitivity (1 μm) of the
microscope. However, no sign of cytotoxicity was conirmed even four days
after cell incubation with the bulky sample. An important observation was
the absence of luorescence when the dye and the folic acid moiety were
not in the same complex and instead formed metastable, single-stranded
structures, thus conirming that the self-assembled the DNA-NT composite
was a structural requirement for the internalisation process. Finally, it was
noticed that the amount of luorescence was directly correlated with the
composition of folic acid, the time of incubation and the concentration of DNA,
although there seemed to exist a plateau in terms of both DNA (not speciied
throughout the article) and folic acid (10%) content.
In a different approach, 700-1,100 nm NIR light was adopted to deliver
DNA inside the nucleus of cancer cells, through the strategic use of CNTs as
local heaters. 92 Biological systems are known to be completely transparent
within this spectral window, while SWCNTs show intense optical absorbance
because of the electronic transitions between the irst or second van Hove
singularities of the nanotubes. 93,94 Such difference can be used to transport