(Circulation. 1998;97:125-126.)
© 1998 American Heart Association, Inc.
Meeting Highlights
Part II: 70th Scientific Sessions of the American Heart Association
Ruth SoRelle, Circulation Newswriter
Gene therapy seems to be moving past the
"proof of principle" phase with two new treatments described at the
70th Scientific Sessions of the American Heart Association meeting held
in Orlando, Fla November 9 through November 12, 1997. "It is an
opening door," said Valentin Fuster, MD, incoming American Heart
Association President and director of the
Cardiovascular Institute at Mount Sinai Medical Center
in New York City. "It's not definitive, but it's certainly very
exciting."
In each case, researchers used genes to tinker with or get around
disease-causing problems rather than striking directly at the cause of
the disorder itself. These kinds of "incremental" gene therapies
will probably become more common in the next few years as the field
itself matures.
The two groups took very separate attacks on the problem of occlusion
in limbs, or limb ischemia, which affects between 100 000 and
200 000 people in the United States each year. One was an in vivo
experiment injecting genes directly into muscles to encourage the
growth of collateral blood vessels. The second was ex vivo and involved
bathing a vein graft in a solution containing an
oligodeoxynucleotide (ODN) that is a transcription factor
decoy to block gene activity.
Clinical researchers at St. Elizabeth's Medical Center in Boston,
Mass, attempted to stimulate the growth of collateral blood vessels in
the patients' legs that had been occluded by atherosclerotic lesions.
In a technique that Jeffrey Isner, MD, of St. Elizabeth's and Tufts
Medical School called "therapeutic angiogenesis," the researchers
inserted the gene for vascular endothelial growth
factor (VEGF) directly into the muscles of the patients' legs. His
hope was that VEGF would encourage the growth of blood vessels that
would provide flow to the ischemic areas of the leg.
Patients could not be candidates for bypass therapy or other types of
treatment, said Dr Isner. Seven patients had ulcers that would not
heal, and three had chronic pain at rest. In six cases, amputation
below the knee had been recommended.
Isner and his colleagues, led by Iris Baumgartner, MD, injected the
naked plasmid DNA with the gene into 10 limbs of 9 patients. Two
injections were given, 4 weeks apart. When they were evaluated 2 to 8
months later, 8 of the treated limbs had improved, 1 was unchanged, and
1 had gotten worse. In the limbs that had improved, Isner and
colleagues measured an increase in the ankle–brachial pressure index
from 0.33 to 0.47. That increase was similar to that seen when surgery
or angioplasty is effective, said Isner.
The researchers evaluated the effectiveness of their therapies by
diagnostic angiogram, magnetic resonance angiography,
measurements of the ankle-brachial pressure, and, when possible,
exercise testing. Magnetic resonance imaging demonstrated improved
blood flow in 8 of the 10 legs, said Dr Isner. Angiograms indicated
evidence of newly visible blood vessels in 7 of the limbs. In addition,
blood pressures measured at the ankle increased after the treatment,
said Dr Isner. Leg ulcers in some of the patients improved or were
cured. Other patients reported less pain in their limbs. In 5
individuals tested, the speed of walking improved. Only 1 patient
underwent an amputation. That patient already had significant gangrene
of one foot, said Dr Isner. One patient reported no improvement.
In most patients, blood vessel growth appeared within 2 to 3 weeks of
treatment, said Dr Isner. "The duration of the gene expression is
only 2 to 3 weeks. That is only what we need to grow the blood
vessels."
Dr Elizabeth Nabel, Departments of Internal Medicine, Biological
Chemistry, and Pathology, Howard Hughes Medical Institute, University
of Michigan, Ann Arbor, said the patient's body might only need the
expression of the gene for a few weeks to a month to achieve the
desired results.
"It remains to determine which population of patients will best be
treated with this," said Dr Isner. He hopes to apply the therapy to
patients with coronary artery disease once these trials are
finished.
Dr Isner said his studies indicated that there is no evidence of
abnormal blood growth elsewhere in the body, despite the use of
angiogenesis. "All clinical evidence is that there is no systemic
distribution of the gene," he said.
Dr Isner estimated that no more than 1% of cells actually took up the
naked DNA. For that reason, he said, only a few good cells secreting
VEGF are needed to encourage the therapeutic effect. R. Sanders
Williams, MD, chief of the cardiology division at the
University of Texas Southwestern Medical School in Dallas, said the
findings are interesting and are 1 of 10 or 12 approaches currently
being considered. "I think it will take us a long time to sort out
what is really the best thing clinically," he said.
Dr Isner agreed that the results are still preliminary, but he said,
"I hope the ability to grow new blood vessels can be another
therapeutic option." He hopes that the treatment might be applicable
in other problems, including coronary artery disease. Dr Isner
does not promise that therapeutic angiogenesis will replace
coronary artery bypass grafting or angioplasty in the near
future. "Ultimately, if one can show that this does work effectively
in the heart, one could imagine that one could defer the initial
surgical or angioplasty treatment," he said.
In the second therapy, Michael Mann, MD, an instructor in medicine at
Harvard Medical School, and Victor J. Dzau, chairman of medicine at
Brigham and Women's Hospital at Harvard, described a genetic therapy
aimed at preventing neointimal hyperplasia in grafted leg
vessels.
"Bypass grafts fail routinely," said Dr Dzau. "In a patient aged
60 who gets a triple bypass graft, the probability of one or more
failing by age 65 is high. The question then is `Will we do
angioplasty or stent this?' " He and Dr Mann hope their treatment
will obviate that need by reducing the growth of new cells in the
grafted vein.
Their strategy seeks to redesign the venous walls so that they respond
to pressure in much the same way that arteries do. "If you take a
vein and put it into the arterial side, the vein is a very
thin structure acutely exposed to arterial pressure. It
gets injured because of the tremendous pressure and the wall
tension," he said. "In order for this vein to function as a bypass
graft, it will have to adapt quickly by thickening."
However, he said, the typical thickening procedure results in
neointimal hyperplasia that promotes accelerated
atherosclerosis in the grafted vein. He and coworkers
wondered what would happen "if we were able to block the cells from
undergoing cell-cycle division. Would we not therefore force these
smooth muscle cells to undergo hypertrophy, lay down a
matrix, and make a vessel that looks closer to an artery?"
By changing the adaptive biology of the grafted vein, "we end up with
a vessel that has some of the mechanical characteristics that, to some
extent, are some of the biological characteristics of an artery," he
said. Although he does not yet claim success in the attempt, he said
the laboratory results indicate that morphologically, the vein looks
like an artery.
They did this by recognizing a transcriptional "maestro" called E2F
that binds and activates genes in the cell cycle. "So what we
did is synthesize a double-stranded DNA that copies the important
sequences," said Dr Dzau. These short ODN sequences are homologous to
key cell-cycle–gene promoter regions and act as transcription factor
decoys that inhibit activation of genes that are key to formation of
the neointimal hyperplasia, said Dr Dzau.
The decoy can compete for binding and sequester the transcriptional
factor so that this factor (E2F) can bind the various genes and
therefore cannot activate them, said Dr Dzau. The veins were
bathed in an ODN-containing solution. Subsequent study showed that the
DNA was present in a majority of cells, although it did not
incorporate into the nucleus. Animal studies indicated that the effect
of preventing neointimal hyperplasia can last as long as 6
months, said Dr Dzau.
Dr Mann said, "One of the unique aspects of this type of gene therapy
is that, unlike most gene therapies, we are actually attacking a
problem that is quite common." The first five patients treated under
the protocol were "open-label," he said, meaning that physicians
knew they had received veins treated with the ODN sequences. One of the
first five was a technical failure, said Dr Dzau. The other four seem
to be doing well. "The first five patients, however, allowed us to
assess whether we could perform the genetic manipulation in the
operating room," said Dr Mann. That proved feasible, he said.
A subsequent 41 patients were enrolled in a placebo-controlled,
double-blind trial of the technique, and no results are forthcoming
from that study as of yet. "I believe this is the first truly
double-blinded study of gene therapy," said Dr Dzau.
Like Dr Isner, Drs Mann and Dzau believe their technique may have
application in coronary artery disease, in the treatment of
bypass grafts. Dr Mann said discussions of using the technique in such
patients are ongoing, and he hopes that a clinical trial can start
within the next few months.
