With human livers and cell lines in extremely short supply because of a donor shortage, researchers working to extend the lives of liver-failure patients have turned to pig livers in the past decade as a potential short-term method for detoxifying human blood. Such treatment could make a life-or-death difference, allowing a patient to regenerate his or her own liver or find a transplant.
Since 1997, some 200 people have been treated worldwide with whole pig livers, by a costly and cumbersome method in which a patients’ blood is passed through a freshly extracted pig liver. Although some people have been kept alive for a day or two in this manner, the pig livers generally die two to six hours into the procedure, killed by human antibodies. And the process is controversial because of concerns that patients could contract a pig virus or micro-organism.
That’s why researchers have turned to pig liver cells, called porcine hepatocytes, in an effort to create a new class of liver-dialysis machines. Now the Mayo Clinic in Rochester, MN, is announcing important strides toward creating such a device. Its machine keeps clusters of pig cells alive far longer than the typical lifespan of a whole dissected organ. And the human blood is separated before entering the machine, reducing the number of white blood cells that would otherwise attack the helpful pig cells.
[For images of the liver-dialysis machine and its process, click here.]
Although human clinical trials are at least two years off, principal investigator Scott L. Nyberg says his machine, called the Spheroid Reservoir Bioartificial Liver, was tested with success in late 2005 in a pre-clinical study on dogs with drug-induced liver failure.
“The dogs treated with our machine lived longer and did not develop signs of brain swelling and brain death, while the untreated control dogs did develop these manifestations of liver failure,” says Nyberg, who is also a transplant surgeon. While the early study involved only six animals, the results were encouraging enough that Nyberg’s team is refining its two prototype devices for more testing in dogs and primates. Their ultimate goal: a device that can keep a patient in liver failure alive for weeks, even months.
If all goes well, the group might do Phase I clinical testing in humans with acute liver failure in 2008. “We want to extend the lifespan of the cells and duration of the treatment,” Nyberg says, recalling a case in which a teenage patient died 14 hours before a donor liver became available. “A device like this could have kept her alive for just that one more day.”
The Mayo liver device looks vaguely like a fish tank set on a cantilevering metal platform. The reservoir is filled with a highly oxygenated liquid medium into which Nyberg deposits up to 500 grams of live pig hepatocytes. Blood from the patient first courses through membranes that separate red cells and plasma from the larger white blood cells.
The plasma and smaller blood cells continue on their circuit into a tube immersed in the liquid suspension of clustered pig cells. The pore size of the tube’s membrane allows blood to flow in and out of the hollow fiber while the hepatocytes remove bile, ammonia, urea, and other impurities. The pore size also blocks the hepatocytes and any pig cell debris from entering the patient’s blood.
The machine uses a rocking motion – 15 seesaws per minute – to bathe the liver cells in nutrients so they can survive longer and function better. In the dog tests, Nyberg says, the cells remained fully active in the reservoir for 48 straight hours of blood detoxification. He says he’s kept pig liver cells metabolically viable in the device for up to a month, and sees no reason why humans could not be kept alive while attached to the device for at least that long.
For those achievements alone, Nyberg and laboratory director Bruce Amiot deserve plaudits, says Dr. Mehmet Toner, a professor of biomedical engineering and surgery at Harvard Medical School who specializes in liver tissue preservation. “Since the oxygenization of the liver cells is a key problem in preserving them in an external device, this approach is a good step forward,” he said. “Nyberg has always been very good in his controls and studies, and porcine cells are definitely better than preserved human cell lines as we now stand.”
Toner explains that a few research efforts in liver-dialysis have used cryogenically preserved human cells, but those cells lose most of their liver-cleansing attributes in the freezing process. Live human cells are very hard to come by because of a lack of donors.
Another crucial aspect of Nyberg’s device is that it encourages the pig liver cells to form rapidly into aggregates called spheroids. Unlike isolated liver cells, which lose functionality as they flatten out over time, spheroids of liver cells, which roughly resemble a microscopic soccer ball, perform vital liver functions at a far higher metabolic rate. “Creating a machine that can keep a very large number of liver cells alive and biochemically active is essential,” Nyberg says, “because only primary hepatocytes isolated straight from a liver – human or animal – have been shown to perform all the necessary purification activities simultaneously.”
Nyberg’s machine, if it succeeds, would fill a huge technology void. Today, there are no active FDA-approved tests of any external liver device in the United States, according to the American Society for Artificial Internal Organs. Meanwhile, about 17,000 Americans are on waiting lists for liver transplants, with fewer than 5,000 livers becoming available annually. Some 40,000 Americans die every year of liver ailments.
Of course Nyberg’s is not the only pig-cell device in the works. Several other corporate and academic labs are working on devices, but Toner says Nyberg’s effort stands out for the length of cell life and the machine’s overall capacity to cleanse blood. “There is a vital niche application for the porcine device in the sense that it keeps the patient alive while treatment is sought or while human donors become available,” he adds.
Meanwhile, other researchers are trying to create genetically engineered pig livers that could be transplanted into humans, but that remains far-off and controversial.
What all researchers agree on, though, is that pig tissue is safer than tissue of other animals because humans have relatively few diseases in common with pigs.
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