Advances in Colorectal Cancer
New drugs show promise, but more research needs to be done.
Antiangiogenesis, at Last? continued...
Avastin is a monoclonal antibody, which is a manufactured
version of the natural antibodies that the body uses to defend itself against
foreign substances. It's designed to block the effects of the vascular
endothelial growth factor (VEGF), a substance in the blood that helps these
tumors grow new blood vessels.
Because of its specific target, Avastin also has few side
effects, especially when compared with the toxic effects of traditional
Researchers still have a lot to learn about the drug. The
recent trial of Avastin was only in people with advanced colorectal cancer that
had spread elsewhere in the body. The next step is to use Avastin in people
with earlier stages of the disease, where the chances of curing it should be
higher. Researchers are conducting trials now, Mooney says.
While the success of an angiogenesis inhibitor is exciting,
Avastin has not been successful in treating other kinds of cancer.
"We know from an unsuccessful breast cancer trial that
Avastin is not a magic bullet," says Helen Chen, MD, senior investigator in
the Investigational Drug Branch at the National Cancer Institute. "At this
time, it's hard to predict which patients with which cancer will benefit most.
It's important to wait for the clinical trials to come out before we use
Avastin in practice."
Erbitux, a new drug recently approved by the FDA for metastatic
colorectal cancer, has also made news. When used in combination with the
chemotherapy drug Camptosar, a study showed that Erbitux shrank the tumors in
23% of the people who had metastatic colorectal cancer and had exhausted other
chemotherapy treatment options; it also slowed the tumor's growth by about four
months. On its own, Erbitux shrank tumors by 11% and delayed the growth of the
tumor by one-and-a-half months.
Like Avastin, Erbitux is a monoclonal antibody. It also blocks
the effects of a growth factor, although a different one called epidermal
growth factor (EGF), which encourages the development of cancer cells. Unlike
traditional chemotherapy, in which toxic drugs don't discriminate between the
tumor and healthy cells, Erbitux and Avastin are targeted and cause fewer side
It's important to understand that Erbitux did not lengthen the
lives of the people in the study. So the results may seem like modest success
at best, and you may wonder at the benefit of shrinking a tumor if it doesn't
help someone live longer.
But Mooney points out that this wasn't the trial's purpose.
"The results may seem disappointing, but the study wasn't
designed to see if [Erbitux] helps people live longer," she says. Instead,
the purpose was to see if the drug worked well enough to merit further trials,
which it did.
According to Mooney and Chen, further trials are now under way
testing the full potential benefits of the drug. As with the current Avastin
trials, the next step is to try Erbitux in people with less advanced colorectal
cancer and in combination with other medications.