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 Pathology.
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 said.
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 model.
In the current study, the Duke team tested both of the commercially produced and FDA-approved formulations of bovine thrombin.
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 patients."
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 research.
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.