Duke Surgeon: Use of Common Clotting Agent Should Be Restricted
DURHAM, N.C. – A substance derived from cow blood and used
to control bleeding in more than 500,000 surgeries each year
appears to stimulate an abnormal immune response that puts
patients at greater risk of suffering from complications,
especially if that agent is used in subsequent operations,
according to Duke University Medical Center investigators.
Furthermore, the investigators recently discovered that this
substance, known as bovine thrombin, also causes a syndrome in
mice similar to that of lupus, a disease common in humans. The
results of their latest studies were published in the November
issue of the American Journal of
Based on the results of studies in humans and recent
experiments in mice used to study this phenomenon, the Duke
researchers believe that bovine thrombin should be restricted
to use in only life-saving surgical procedures.
Thrombin is a potent enzyme that acts at the end of the
complex cascade of events leading to the clotting of blood.
Because it acts quickly, surgeons have been applying it
topically during operations to control bleeding, or "oozing,"
at the site of an incision or suture inside the body.
However, in 1977, the Food and Drug Administration (FDA)
revoked the licenses for all commercially prepared human
thrombin preparations due to the high rates of hepatitis B
resulting from virally contaminated blood used in these
preparations. Although development and widespread use of these
agents continued in Europe, cows became the main source of
thrombin in the U.S. Although the FDA ban on human thrombin
preparations was lifted in 1998, there are still two
preparations of bovine thrombin available.
"Since the 1970s, we have been using bovine thrombin without
understanding its possible adverse effects," said lead
investigator Dr. Jeffrey Lawson, a Duke surgeon and biochemist.
"Because, in most cases, bovine thrombin is used more as a
convenience for the surgeon than a benefit for the patient, and
because our studies suggest that it can cause a serious
abnormal immune response, we recommend that its use be
restricted to life-saving procedures."
The main problem with bovine thrombin, Lawson explained, is
that while the molecule itself is quite similar to human
thrombin, there are subtle differences that make it immediately
recognized as foreign by the immune system. For example, one
possible key to the immune response is a carbohydrate known as
galactose-alpha 1-3-galactose (alpha-Gal). This carbohydrate is
believed to be one of the main barriers to successful
xenotransplantation (the transplantation of animal organs into
humans) and is found in all mammals except apes, Old World
monkeys and humans. So, according to Lawson, when bovine
thrombin, with its alpha-Gal, enters the human body, the
alpha-Gal acts like a trip-wire that alerts the immune system,
causing it to leap into action.
"In essence, using bovine thrombin is like performing a
small xenotransplant," Lawson said. "We are taking foreign
proteins from another species, and putting it into humans. When
something foreign enters the body, the immune system responds
by creating antibodies which home in on the invader and destroy
it. We have found that patients who receive bovine thrombin
create antibodies to it, making the body sensitized to it."
The Duke research suggests that the first exposure doesn't
usually create immediate problems, Lawson said. However, the
real danger comes when the patient is exposed to bovine
thrombin a second time.
"When someone whose immune system is already sensitized to
bovine thrombin encounters it again, the body remembers and
generates more antibodies," Lawson said. He compared the
reaction to what happens in a vaccination, where a patient is
presented with a weakened form of bacteria, so that if the real
bacteria is encountered later, the body mounts a larger and
more effective attack against it.
"As a result of this over-stimulation from subsequent
exposures to bovine thrombin, the antibodies also attack human
proteins which appear to correlate with post-operative
complications including bleeding and clotting problems," he
This assault on one's own tissue is known as an autoimmune
response – the body literally attacks itself.
In January, the team reported in the Annals of Surgery the
results of a study of 150 patients undergoing heart surgery.
They found that more than 90 percent of those patients
developed antibodies against the therapy, and that 30 percent
of those patients developed antibodies that reacted to human
proteins. Further, patients who had evidence of preoperative
antibodies to bovine thrombin were at increased risk for
complications following re-exposure during surgery.
Although this report suggested that exposure to bovine
thrombin led to these complications, it was difficult to prove
that these complications were directly caused by bovine
thrombin. For example, many of the patients who undergo heart
surgery have other diseases, which could complicate this
analysis. Therefore, to prove that bovine thrombin alone can
provoke this autoimmunity, the Duke researchers created a mouse
In the current study, the Duke team tested both of the
commercially produced and FDA-approved formulations of bovine
The researchers used a line of mice that do not possess
alpha-Gal and subjected them to a series of experiments. These
mice are known as "knockout" mice because a specific gene has
been deleted from their genetic make-up. Lawson found that in
all cases, the knockout mice exposed to bovine thrombin showed
an immune response to bovine thrombin that was similar to that
observed in humans, while the untreated mice didn't.
Interestingly, and quite to their surprise, the researchers
also noticed that the mice exposed to bovine thrombin also
developed an autoimmune syndrome that was almost identical to
that of lupus. As in humans who have lupus, female mice
developed this syndrome more than males, and mice that
developed the syndrome developed antibodies against DNA and
kidney problems, both hallmarks of the disease.
Not only did the results of this study confirm that bovine
thrombin alone can cause autoimmunity, they may provide insight
into a possible cause of a disease that has baffled physicians
for years. "We believe that there are properties of bovine
thrombin that are not due to the foreign nature of the protein,
which could contribute to the understanding of the cause of
lupus," Lawson said.
Despite this unexpected discovery, Lawson is focusing
primarily on the population of patients who have been exposed
to bovine thrombin. "In general, there are millions of
Americans out there who are sensitized to bovine thrombin, and
surely many of them will need re-operations. The primary focus
of our work is to determine what is safe to use in these
Changing over to newer agents faces hurdles, Lawson said,
because bovine thrombin is cheap, readily available and
effective, and is often used solely out of habit. Further, many
of the substitutes for bovine thrombin, such as human thrombin,
have not been tested. However, as the evidence of its adverse
effects mount, Lawson believes surgeons will stop using it.
"Based on the results of our studies, I believe that if
these preparations were to be submitted today for FDA review,
they would be denied," said Lawson.
Grants from the American Heart Association, Duke's
department of surgery, the Fannie Rippel Foundation, Baxter
Healthcare and the Lupus Foundation of America supported the
Members of the Duke team include Jonathan Schoenecker,
Rachel Johnson, Aaron Lesher, Jarrod Day, Stephanie Love, Dr.
Maureane Hoffman, Dr. Thomas Ortel and Dr. William Parker.