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Stent Clot Rate Higher than Thought

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Duke Health News 919-660-1306

STOCKHOLM, SWEDEN - A new analysis conducted by Duke
University Medical Center cardiologists concludes that the
incidence of potentially life-threatening clots forming inside
stents -- tiny mesh girders designed to prop open newly cleared
arteries -- is almost three times higher than previously
reported.

The rate is much higher, they said, because earlier clinical
trials that pegged the rate at about 1 percent enrolled a
highly selective group of patients at high-volume academic
centers. They said these patients are not truly representative
of the mix of patients routinely treated by cardiologists in
their practices.

The use of stents has taken off in the past decade, and it
is estimated that up to 80 percent of the nearly 600,000
angioplasty procedures performed each year in the United States
also involve the placement of a stent.

The major problem encountered by cardiologists using stents
is the formation of a blood clot within the stent. To address
this problem, various agents have been developed to keep
platelets in the blood from clumping together, which has
reduced the rates of clotting, or thrombosis, and increased the
success rates of stent procedures.

"The conventional wisdom has been that in this modern era of
anti-platelet therapy, the risk of stent thrombosis has been
around 1 percent," said Dr. Thaddeus Tolleson, a cardiology
fellow at the Duke Clinical Research Institute (DCRI). "When we
looked at a different set of data, which we felt was more
representative of what cardiologists actually see every day,
the rate of stent thrombosis approached 3.5 percent.

"Stent thrombosis is a rare event, but when it happens, it
is quite dramatic," Tolleson continued. "They usually occur
within the first two weeks of a stent placement and typically
involve a large infarction or sudden death. They are infrequent
but catastrophic."

Tolleson prepared the results of the DCRI analysis for
presentation Monday (Sept. 3) at the annual meeting of the
European Society of Cardiology.

In the early trials of anti-platelet therapy during
angioplasty, sicker patients with more severe coronary artery
disease were typically excluded. For this reason, the Duke
researchers combined the data from two clinical trials
involving anti-platelet therapies designed expressly for these
patients with acute coronary syndromes.

Specifically, they used data gathered from the SYMPHONY
(Sibrafiban versus aspirin to Yield Maximum Protection from
ischemic Heart events post-acute coronary syndromes) and Second
SYMPHONY trials. Both trials, which compared the effectiveness
of aspirin to a new class of "super aspirin" to prevent
recurrent heart attacks, enrolled 15,904 patients at 716
hospitals in 35 countries. Of those patients, 4,641 went on to
receive an angioplasty procedure followed by a stent
placement.

"Using a group of patients who were diagnosed with acute
coronary syndromes, the overall rate of stent thrombosis was
3.5 percent -- 2.1 percent in the SYMPHONY trial and 4.7
percent in Second SYMPHONY," Tolleson said. "These are the
types of patients who were not included in earlier trials, but
who can make up a significant percentage of a cardiologist's
practice."

In addition, the data from the two trials showed that those
who are at the highest risk for having stent thrombosis are
patients with diabetes, who are elderly, who are female, or
those who have had a prior heart attack or angioplasty
procedure.

Ironically, it is the actual placement of the stent itself
that seems to ultimately lead to thrombosis. When the stent is
expanded within the artery, it inevitably causes irritation of
the endothelial lining of the artery. The body responds to this
as it would to any other injury ? platelets arrive at the scene
to initiate the healing process, which usually takes two to
four weeks. However, in some patients, this natural response
can provoke clots large enough block the stent.

"We will still continue to put stents into our patients with
acute coronary syndromes, but as a result of this study, we
will do it with a higher awareness that thrombosis is not as
rare as we once thought," said Dr. Kristin Newby, a
cardiologist at the DCRI and senior author of the study. "Will
these findings change practice? Not immediately, but it should
stimulate further studies on new treatments ? both before and
after stent placement ? to lower these rates."

The new approaches could include giving patients
anti-platelet agents for longer periods either before or after
the procedure, as well as the development of a new class of
stents impregnated with anti-clotting agents or even radiation,
which has been shown to stem the proliferation of endothelial
cells.

Newby was the principal investigator for both the SYMPHONY
and Second SYMPHONY trials, which were funded by F. Hoffmann-La
Roche, Ltd., Basel, Switzerland. The DCRI supported Tolleson's
analysis of the data.

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