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WHEN GENE THERAPY ARRIVED
ON THE BIOMEDICAL SCENE IN THE EARLY 1990S, IT SEEMED THAT THE REMEDY
TO ALL GENETIC DISEASES WAS NIGH:
If a defective gene was causing a problem, slip in a new, improved
one and with this bit of molecular wizardry, the problem would be
solved. But the field, launched with such high hopes, has foundered
in recent years. After a decade of research and clinical trials,
not a single gene therapy treatment has been approved by the Food
and Drug Administration for routine use in patients.
But a team of Stanford researchers has
several reasons to believe that their gene therapy procedure will
succeed where others have failed. Hemophilia, the disease that they
are focusing on, was deliberately selected by Mark Kay, MD, PhD,
as the condition most likely to be treatable by gene therapy. Patients
with severe hemophilia will benefit tremendously if an introduced
gene can induce even modest amounts of the clotting factor that
is missing in their blood. For many other diseases, a new gene introduced
by gene therapy must replace the entire amount of a missing protein
to make a real difference.
"At 1 percent clotting factor,
it changes from
severe disease
to moderate,
which really increases quality
of life of the individual," says Kay.
Kay's high expectations are supported
by promising animal studies showing that introducing a new gene
for clotting factor significantly reduces the number of bleeding
episodes in mice and dogs with hemophilia. "We have demonstrated
in two different species -- large and small animals -- that we can
make a difference," says Kay, who conducted the studies with collaborators
at the University of North Carolina, Chapel Hill, and Cell Genesys
of Foster City, Calif.
Technical advances such as the development
of alternative gene delivery vectors have also given the entire
gene therapy field a boost towards success. The virus that Kay is
using to ferry the new gene into patients' cells is less aggravating
to the human immune system than the viruses used in earlier gene
therapy efforts.
Now Kay, who directs Stanford's program
in human gene therapy; Bertil Glader, MD, PhD, professor of pediatrics
and chief of the division of pediatric hematology/oncology; and
other Stanford colleagues are teaming up with researchers at the
Children's Hospital of Philadelphia for a clinical trial that seeks
to confirm the safety of the procedure in humans.
Hemophilia is a genetic disease that
leaves affected individuals at risk of spontaneous
and life-threatening bleeds. Unlike the blood of most people, which
quickly gels when it spills from blood vessels, the blood of hemophilia
patients fails to clot -- a result of nonexistent or abnormally
low levels of clotting factor. For these patients, minor injury
can cause copious bleeding, and any kind of trauma, including surgery
and dental extractions, can be a life-threatening concern. Those
with the severest form of the disease are also subject to a myriad
of spontaneous bleeds. Bleeding into the joints, often with no obvious
cause, is a common problem that leads to crippling degenerative
arthritis at a young age.
NINETY PERCENT OF PEOPLE
WITH HEMOPHILIA ARE AFFLICTED WITH HEMOPHILIA A, CAUSED BY INSUFFICIENT
CLOTTING FACTOR VIII, WHILE THE REMAINDER HAVE HEMOPHILIA B, CAUSED
BY A DEFICIENCY IN CLOTTING FACTOR IX. Because
the genes for both factors are carried on the X chromosome, a defect
in either gene holds grave consequences for males, since males have
only a single X chromosome per cell. Females, with a backup X chromosome,
can be carriers but rarely suffer from serious forms of the disease.
Kay and his colleagues are currently
focusing on hemophilia B because the gene for factor IX is smaller
than that for factor VIII, making some of the technical aspects
of the gene therapy protocol easier to manage. However, they hope
to eventually expand their research to develop a therapy suited
to the more common form of the disease. Like many biomedical researchers,
the team first began its experiments with mice, infusing the human
gene for factor IX into the liver of mice with hemophilia B. The
mice began producing the clotting factor and their bleeding disorder
disappeared. They showed no adverse side effects and survived one
to two years after treatment -- the normal life span
for a mouse. Untreated mice usually fall victim to spontaneous fatal
hemorrhage at two to three months of age. With
such a successful beginning, Kay and his team quickly began testing
their protocol in a larger animal that would be a more appropriate
model for humans. Like people, dogs also fall
prey to the bleeding disease. Kay's collaborator, Timothy Nichols,
MD, and members of his lab in the Department of Pathology and Laboratory
Medicine at the University of North Carolina infused two hemophiliac
dogs with the factor IX gene. The dogs, too, began producing the
clotting factor once the foreign gene was ensconced in their livers.
Due to their cautious approach, the
researchers knew that the small dose that the dogs had received
would not boost the level of clotting factor in their blood to normal
levels. Production of factor IX reached a level of 1 percent of
the normal amount found in healthy dogs' blood. Yet experience with
human patients had taught physicians that even this tiny amount
makes a big difference in clinical symptoms.
"At 1 percent clotting factor, it changes
from severe disease to moderate, which really increases quality
of life of the individual," says Kay.
The researchers were pleased to see
a significant improvement in the dogs' health. In the eight months
following treatment, both dogs' blood clotted more quickly and a
single spontaneous bleed was recorded in one of the dogs, compared
with an average of five bleeds per year in untreated hemophilia
B dogs. Now, 18 months after treatment, the dogs are still doing
well.
Following such promising results, the
research team published a paper in the January 1999 issue of Nature
Medicine that described their study, and set about gaining permission
for a clinical trial. Kay, colleagues at Stanford, Children's Hospital
of Philadelphia and Avigen Inc., a biotechnology company in Alameda,
Calif., have now launched a phase I clinical trial to test the safety
of their gene therapy procedure. Four patients
have been treated to date, three in Philadelphia and one at Stanford.
Kay expects the fifth patient to be treated in January 2000 at Stanford.
Though the leap from success in animals
to success in humans has eluded gene therapy researchers so far,
Kay is optimistic that his technique will allow gene therapy for
hemophilia to realize this transition. "The clinical condition in
mice and dogs is very similar to that in humans," says Kay. "This
is something that we've worked out in the lab that has a high potential
of working in people."
Hemophilia is a disease that is a prime
candidate for treatment by gene therapy, according to Glader, who
is co-principal investigator, with Kay, of the hemophilia gene therapy
clinical trial. If the procedure can induce the production of merely
1 percent of the amount of clotting factor found in healthy people,
the bleeding episodes experienced by people with severe
hemophilia can be significantly curtailed, Glader explains. A patient with
even this modest increase in clotting factor would be classified
as having moderate hemophilia and would be expected to escape the
frequent spontaneous bleeding episodes associated with the severe
form of the disease, says Glader.
IT IS ALSO EASY TO GAUGE
THE EFFECTS OF THERAPY IN HEMOPHILIA PATIENTS.
"We can monitor bleeding episodes and we can
measure the coagulation factor in the blood," says Glader. The success,
or otherwise, of treatment can also be measured by how frequently
each patient requires infusions of concentrated clotting factor
compared with past use, he says.
The nine patients in the trial, in three
groups of three, will receive a low, medium or high dose of the
clotting-factor gene. The protocol also calls for each patient to
be treated no less than one month apart so each patient's reaction
to the therapy can be carefully monitored before the next patient
is treated. "Once we determine the safety of this, we will look
at a larger population of patients," says Glader.
The researchers agree that it is too
early to tell if the first Stanford patient is responding to the
therapy, but they can say that it has not caused him any problems.
Most important, he has not developed inhibitors, which are antibodies
against the clotting-factor protein. Twenty percent of patients
with hemophilia A and 5 percent of those with B develop inhibitors
because of clotting-factor transfusions necessary to halt bleeding
episodes. Because the factor is not produced naturally in people
with severe hemophilia, their immune systems can mistake the factor
as a foreign invader and make antibodies to destroy it. Once a patient
has developed inhibitors, no further clotting factor can be given
and alternative treatments must be found.
"Managing patients with inhibitors is
very difficult because the factor concentrates normally used to
treat bleeds don't work," says Glader. According to Glader, most
people who develop inhibitors usually do so when first treated during
childhood, but the researchers are wary that any new treatment regime
might generate an inadvertent immune response. Both Kay and Glader
are relieved that, so far, there is no sign of the dreaded inhibitors
in any of the trial patients.
With the success of the animal studies
and the favorable early results in the human trial, Kay and his
team are undeniably enthusiastic. Glader notes that many people
with hemophilia seem to be very much in favor of the trial and eager
to participate. "Individuals with hemophilia suffer throughout their
lives with bleeding episodes, debilitating problems and the need
to inject themselves with clotting-factor precipitates on an ongoing
basis," says Glader. "Gene therapy offers a new hope."
"This is a one-time treatment that we
predict will be enough to treat the individual life long," says
Kay. SM
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