A drug-eluting stent that vanishes
when its work is done is a step closer to market thanks
to a successful clinical trial. The first ever clinical
data on the device appeared in the March 15 issue of
The Lancet and the results are very promising.
"The procedure was a success in
all 30 patients. Device success was 94%," report the
European and New Zealand researchers. Only one patient
suffered a myocardial infarction and there was no late
stent thromboses, or blockages, after one year of followup
— a significant development given the problems
that have plagued other attempts at producing these
stents.
Several pharmaceutical companies
are working on bioabsorbable stents, but the bioabsorbable
everolimus-eluting stent (BVS) is the first to produce
such positive results. Researchers developed it out
of a polymer, giving it a poly-L-lactic acid backbone,
and coated it with another polymer, the poly-D,L-lactic
acid, that contains and controls the release of everolimus,
an anti-proliferative drug.
STENT
REVOLUTION
Stents have been used in coronary angioplasties to keep
cholesterol-clogged arteries open since the mid-1990s.
"It's a metal mesh mounted on a balloon and inserted
in the artery. The stent stays in place after the balloon
collapses and provides scaffolding so the artery doesn't
recoil," explains Dr Jean-Philippe Pelletier, interventional
cardiologist at the McGill University Health Centre.
Once inside the body, the normal
healing process leads to scar tissue forming over the
stents, shielding them from the immune system and preventing
a reaction. But sometimes, too much scar tissue forms
leading to restenosis, another blockage of the artery
— that sends patients back into surgery.
Enter drug-eluting stents. These
stents are coated with meds that prevent restenosis
from happening. "Drugs are released at a certain rate
so they allow for some healing, but not so much that
the artery becomes blocked," says Dr Pelletier.
THIRD
TIME'S A CHARM
But there was another problem: scientists continued
to grapple with the fact that the need for stents in
general is only temporary — to tide the blood vessel
over until it remembers how to function properly on
its own.
In fact, having it stuck there
permanently has caused problems in some patients. The
polymers holding the drug in place caused an inflammatory
reaction, so with the stent staying in place, these
patients end up with chronic inflammation, says Dr Pelletier.
A scientific epiphany led Japanese
researchers to develop the first bioabsorbable stent,
which they presented to the world in 2000. The allure
of bioabsorbable stents is easy to see. "When you implant
them, you get the benefit of scaffolding — allowing
the artery enough time to heal — then the stent
would melt away, so you'd have no inflammation," says
Dr Pelletier. But the Japanese stent wasn't drug-coated
and had an unacceptably high rate of restenosis, so
it was dropped.
Then last year came the magnesium
alloy stent. It was metallic and bioabsorbable. However,
restenosis reared its ugly head again and within the
year, nearly half the patients needed revascularization.
DISAPPEARING
ACT
The BVS team figured out a way to effectively solve
that problem. Using the same polymer as the Japanese,
they added on the anti-proliferative drug to create
a super-stent. But before this stent makes it to broad
use, there are some kinks to iron out.
One is the fast absorption of the
stent. "A stent made with a lactic acid material starts
to be reabsorbed by the body as soon as you put it in,
so its strength decreases," says Dr Pelletier. So there's
a chance the artery might recoil.
Another concern is that the drug
inside the stent could prevent healing longer than the
stent takes to disintegrate. "If the stent starts to
break in pieces, they can fly off and lodge somewhere
distally in the artery," Dr Pelletier adds.
It will take more research to find
a balance between a stent material that degrades at
an acceptable rate and allow medication to do its work,
he says. "But these are the first steps of a new technology
that's very promising."
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