But Folkman and a small group of believers were undeterred.
Despite scant funding, their understanding inched forward. Tumors put
into rabbits’ eyes grew in the iris but not in the cornea, which
has no blood vessels. Through windows cut into the shells of fertilized
chicken eggs, scientists watched blood vessels sprout toward tumors. Yet
while evidence mounted to support his basic hypothesis—that angiogenesis
is crucial to tumor growth—progress in isolating a chemical that
promoted angiogenesis was excruciatingly slow. Folkman’s laboratory
finally isolated a tumor protein that could stimulate angiogenesis and
published its findings in Science in 1984.
While Folkman focused on malignant tumors, others
were increasingly interested in isolating the growth factors at work in
benign human tissue—to learn how organs develop as a step toward
bioengineering new ones. For Napoleone Ferrara, an Italian-born obstetrician/gynecologist,
this quest led to the pituitary glands of cattle, whose follicular cells
caused blood vessel cells to multiply profusely.
Hired in 1988 by the biotechnology company Genentech
in San Francisco to develop a drug to hasten labor, Ferrara was most passionate
about this other research. He spent nights and weekends grinding cow pituitaries
into a soup containing thousands of proteins. Early the next year, he
finally isolated a protein with an amino acid sequence that matched that
of no known substance. Ferrara called the protein vascular endothelial
growth factor, or VEGF. Further testing determined that it was crucial
for tumor angiogenesis, and scientists now know that more than half of
the 220 kinds of human cancer produce VEGF to signal endothelial cells
to start sprouting blood vessels to nourish tumors.
Meanwhile, Folkman and his colleagues were searching
for substances that would turn off angiogenesis. Their discovery of the
first, interferon alpha, was published in Science in 1980, followed by
tetrahydrocortisol, in 1985, and ultimately, by nine other antiangiogenic
compounds. Yet in the early 1990s, despite the lab’s groundbreaking
findings, drug manufacturers had no interest in making angiogenesis inhibitors.
Finally, EntreMed, a year-old biotechnology company, agreed to produce
endostatin, angiostatin, thalidomide and 2-methoxyestradiol for clinical
use. (Thalidomide and 2-methoxyestradiol are now in clinical trials for
cancer.)
In Folkman’s lab, the job of finding an antiangiogenic compound
fell to a persistent researcher, Michael O’Reilly. After months
of experimenting, O’Reilly found a lung cancer that would kill a
mouse in three weeks. Yet the tumor’s metastases remained dormant
until he excised the tumor. That essential characteristic corresponded
to the phenomenon Folkman had long observed, that tiny metastases would
grow only after all visible evidence of cancer had been cut out.
Folkman suspected that the tumor was emitting both
angiogenesis inhibitors and stimulators, and that the stimulator was cleared
out of the bloodstream rapidly while the inhibitor lingered. Once the
tumor was removed, he surmised, the inhibitor disappeared from circulation,
which permitted remote metastases in other organs to begin forming blood
vessels.
After a year of analyzing vats of mouse urine, O’Reilly and Folkman
finally discovered a substance that appeared to inhibit angiogenesis.
It was a fragment of a protein molecule called plasminogen, produced by
the liver to regulate blood clotting. Folkman and O’Reilly called
the fragment angiostatin.
In 1994, O’Reilly planted the deadly lung tumors on the backs of
mice. Three weeks later he removed the tumors and gave one group of mice
daily injections of angiostatin, while giving a control group a saline
solution. When O’Reilly killed the mice and examined their lungs,
those on angiostatin were free of cancer, while the controls were riddled
with it.
A year later O’Reilly found a second inhibiting molecule: endostatin.
This time he gave daily doses to mice while the primary tumor was in place.
The cancers disappeared, but they returned when he halted the endostatin.
Combining angiostatin and endostatin worked better, though the tumors
still returned after the drugs were halted.
Ferrara, meanwhile, was proceeding with his own animal
studies. After locating the gene that produces VEGF, he created an antibody
that shrank tumors in mice by as much as 95%. Two years of additional
animal testing later, Genentech created Avastin, a monoclonal antibody
that binds to VEGF and blocks tumor angiogenesis.
Trials on patients with advanced breast and colorectal
cancer began in 1997. The breast cancer trials failed; although tumor
size shrank somewhat, survival time didn’t increase. “We were
all very gloomy,” says Ferrara. But trials involving metastatic
colorectal cancer prolonged lives by as much as 50%. In February 2004,
the FDA approved Avastin for treating colon cancer, and the drug racked
up almost $1 billion in sales in 2005. |
