IN CANCER THERAPY
It is possible to destroy tumor cells by a variety of toxic chemicals or localized heating. In both cases a major issue is delivering the fatal reagent to the cancer cells and avoiding nearby healthy tissue. When using toxic chemical reagents, the reagent must be not only delivered specifically to the target cells but also prevented from diffusing out of the cancer cells. Both related objectives may be achieved by using hollow nanoparticles to carry the reagent. Nanoparticles may be targeted to tumors by adding specific receptors or reactive groups to the outside of the nanoparticles. These are chosen to recognize proteins that are solely or predominantly displayed on the surface of cancer cells. It is hoped that such nanoparticles will be safe to give by mouth.
Diffusion is more difficult
to deal with, but may be limited to some extent by designing nanoparticles for
slow release of the reagent. A clever alternative is to produce the toxic agent
inside the nanoparticle after it has entered the cancer cell. Photodynamic
cancer therapy involves generating singlet oxygen by using a laser to irradiate
a photosensitive dye. The singlet oxygen is highly reactive and in particular
destroys biological membranes via oxidation of lipids. After diffusing out of
the nanoparticle, the toxic oxygen reacts so fast that it never leaves the
cancer cell (Fig. 7.11).
Nanoparticles may also be
used to kill cancer cells by localized heating. In one approach nanoparticles
with a magnetic core are used. An alternating magnetic field is used to supply
energy and heats the nanoparticle to a temperature lethal to mammalian cells.
Another approach uses metal nanoshells. These consist of a core, often silica,
surrounded by a thin metal layer, such as gold. Varying the size of the core
and thickness of the metal layer allows such nanoparticles to be tuned to absorb
from any region of the spectrum from UV through the visible to the IR. Because
living tissue absorbs least in the near infrared, the nanoparticles are
designed to absorb radiant energy in this region of the spectrum. This results
in external near infrared being specifically absorbed and heating the