Anti-Cancer Nanoparticles:
Scientists at The Scripps Research Institute Design Gene-Tipped
Tumor Regressor "Smartbombs"
By Jason Socrates
Bardi
A group of researchers from The Scripps Research Institute
(TSRI) have demonstrated what, in principle, could be a new
way of treating cancer and several other diseases where angiogenesis
occurs. Angiogenesis, the formation and differentiation of
new blood vessels, is a crucial process in cancer, and, when
blocked, improves a patient's prognosis.
In cancer-related angiogenesis, tumors develop their own
blood supplies by causing cells that line blood vessels to
proliferate, forming new vessels and bringing more blood to
the tumor. The increased oxygen and nutrients the tumors receive
allows them to grow and enables certain "metastatic" cells
to leave the tumor, enter the bloodstream, migrate to other
tissues of the body, and establish more tumors.
In an article appearing in the latest issue of the journal
Science, the TSRI investigators combined a gene that
shuts off angiogenesis with a 50 to100 nanometer-sized particle
that selectively targets the cells that form new blood vessels
in cancer tumors. This approach combines gene delivery with
specific vascular targeting thereby effectively disrupting
the blood supply of tumors without influencing the normal
blood vessels or any other tissue.
This anti-cancer nanoparticle is like a smart bomb that
delivers its multiple warhead genetic payload into endothelial
cells that proliferate during angiogenesis—which is the
medical equivalent of cutting off all the supply routes to
destroy the tumor. Once angiogenesis is stopped, the tumor
cells starve, and the tumor is ultimately destroyed.
Anti-angiogenics have been known of and studied for many
years, but this anti-cancer nanoparticle is a new type of
anti-angiogenic. Unlike other, "systemic" angiogenesis blockers,
which become diffused throughout the blood steam upon injection,
the nanoparticle-targeting vehicle directs itself to areas
of the body where the tumors exist and where local vascular
cells are expanding to form new blood vessels. The nanoparticle
homes in on these cells and drops off multiple copies of a
gene that effectively blocks angiogenesis and kills tumors.
"We saw strong regression of large tumors in every system
we looked at," says TSRI Immunology Professor David Cheresh,
who led the study.
How Cancer Hijacks Blood Vessels
Angiogenesis, the process where blood vessels are formed
and differentiated, is the tumor's way of responding to hypoxia,
a deficiency of oxygen-rich blood reaching its proliferating
cells.
As the cells of a tumor grow too fast to be fed by their
existing blood supply, they are starved of oxygen. They respond
by exhibiting a hypoxic response—turning on genes that
produce growth factor proteins, which tell the body to make
new local blood vessels and bring more oxygen-rich blood.
The body responds when endothelial cells—the major
cell type lining the vasculature—receive the growth factors
and begin to proliferate, producing more blood vessels. Once
these new blood vessels are made, they bring vital oxygen-rich
blood to the tumor. This increased blood supply allows the
cells to survive oxygen deprivation, allows the tumor to grow,
and even allows cancerous cells to escape into the bloodstream
and migrate to other tissues (the process known as metastasis).
If blocking angiogenesis can kill tumors, the only question
that remains is: How?
Stopping the Hijacking
The problem with tumor angiogenesis is that tumors typically
produce over 20 different growth factors that are capable
of inducing angiogenesis. Blocking only a few of these growth
factors has a minimal effect on tumor angiogenesis and tumor
growth. Therefore, to produce an effective antiangiogenic
drug, one must block a general component of the angiogenic
cascade—some crucial mediator of angiogenesis that gets
turned on by multiple growth factors. Block this general component
and you will block angiogenesis.
This general component proved to be the protein Raf-1 kinase.
"If you could block Raf-1 kinase, you could block the growth
factors' signaling," says Cheresh. "That should block all
the initiators of angiogenesis."
The problem was how to target Raf-1 only in angiogenic endothelial
cells. But Cheresh already had the solution to this in hand.
Several years ago, he discovered a specific "integrin" receptor
protein that is displayed on only newly sprouting endothelial
cells such as those on tumor-associated blood vessels.
"This receptor is called avb3,"
says Cheresh. "It goes from essentially zero to high levels
very quickly."
By specifically targeting avb3,
it is possible to target only those endothelial cells involved
in angiogenesis-like those that line vessels bringing blood
to tumors.
Cheresh and his colleagues had spent several years studying
the role avb3
played in angiogenesis. It turned out that avb3
was capable of regulating a number of intercellular enzymes,
called kinases, that appeared to play crucial roles in angiogenesis.
One of these kinases was Raf-1.
Cheresh and his colleagues designed a dominant negative
mutant form of the Raf gene that was defective and
when delivered into endothelial cells would shut down the
normal Raf-1 kinase activity—effectively shutting
off angiogenesis.
The only question that remained was how to combine the therapeutic
effect of the mutant Raf gene with the angiogenic cell-specific
targeting of the avb3.
A Gene Warhead and its Delivery Vehicle
Interestingly, the solution was provided by viruses, certain
types of which had evolved the ability to use avb3
integrins to gain entry into cells.
Following the lead of the viruses, Cheresh and his collaborators
designed a "cationic nanoparticle" that contained polymerized,
positively charged fat molecules containing an organic ligand
that mimicked the binding site for the viral protein receptor
for avb3.
The positive charges on the nanoparticles allowed them to
stick genes to their surfaces, since DNA is negatively charged.
These gene-loaded nanoparticles then direct themselves to
endothelial cells displaying the avb3
receptor, where they gain entry and deliver their genetic
payload.
In the current study, the TSRI investigators first report
how they successfully delivered nanoparticles with "reporter"
genes—such as those encoding for luciferase or green
fluorescent protein, proteins that glow like the tail of a
firefly. These reporter genes allowed dramatic demonstrations
of the specific targeting of the nanoparticles to tumors.
(The tumors glowed green under a microscope).
Cheresh and his colleagues then combined the nanoparticle
with the mutant Raf gene and tested whether they could
regress tumors in vivo, and they found the technique
worked. Everywhere there were metastatic lesions in the lung
or liver, the Raf gene eliminated them.
The next step, says Cheresh, is to refine these particles,
identify other gene targets, and utilize them in diseases
characterized by abnormal neovascularization—like heart
disease, stroke, rheumatoid arthritis, and certain types of
blindness in elderly patients (age-related macular degeneration)
and in patients with diabetes (diabetic retinopathy).
The research article "Tumor Regression by Targeted Gene
Delivery to the Neovasculature" is authored by John D. Hood,
Mark Bednarski, Ricardo Frausto, Samira Guccione, Ralph A.
Reisfeld, Rong Xiang, and David A. Cheresh and appears in
the June 28, 2002 issue of the journal Science.
The research was supported by the National Institutes of
Health and by a grant from Merck KGAa.
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