Medical physicists at the University
of Virginia have created a novel way to kill tumor cells using nanoparticles
and light. The technique, devised by Wensha Yang, an instructor in radiation
oncology at the University of Virginia, and colleagues Ke Sheng, Paul W. Read,
James M. Larner, and Brian P. Helmke, employs quantum dots. Quantum dots are
semiconductor nanostructures, 25 billionths of a meter in diameter, which can
confine electrons in three dimensions and emit light when exposed to ultraviolet
Yang and his colleagues realized that quantum dots also give off light when
exposed to megavoltage x-rays, such as those used in cancer radiotherapy. That
property, the scientists realized, makes quantum dots an ideal mediator in therapies
employing light-activated compounds to treat cancer.
A compound called Photofrin is the only photosensitizer currently approved
by the FDA. Photofrin is absorbed by cancer cells and, upon exposure to light,
becomes active and kills cells. It is currently used to treat certain kinds
of shallowly located tumors, but Yang and his colleagues realized that combing
Photofrin with quantum dots could create an efficient method to kill even deeply
seated cancer cells. Upon exposure to high doses of radiation, the dots become
luminescent and emit light; that light triggers the cancer-killing activity
of the Photofrin. In theory, the process, which so far has been studied only
in cancer cells grown in culture, could work on tumors located too deep within
the body to be reached by an external light source.
To prevent normal tissues from being affected by the treatment, the toxicity
of the quantum dot-Photofrin conjugate is only activated when radiation is applied.
Also, the area to be treated is targeted with conformal radiation, which is
delivered with high precision within the three-dimensional contours of the tumor,
with minimal spillover to surrounding healthy tissues. As a result, Yang says,
"the toxicity of the drug is substantially lower in the lower radiation
dose area" outside the boundaries of the tumor. In tests on human lung
carcinoma cells, the process resulted in a 2-6 times lower tumor cell survival
compared to radiation alone, but with minimal toxicity to nearby cells.