NANOPARTICLES 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 surrounding tissue.