Toward a bioartificial liver: Buying time, boosting hope
A Mayo Clinic researcher is collaborating with others to regenerate human liver cells, using animals as incubators. Two institutions are working to find a way to keep patients alive by using the right mouse and the right pig in the right way.
Scott L. Nyberg, M.D., Ph.D., a Mayo Clinic transplant surgeon and researcher, is trying to make a practical bioartificial liver.
If your liver fails for more than a few days, you die.
Liver failure has many causes — trauma from accidents, drug abuse or diseases such as hepatitis. One common cause is drug overdose, either through years of maintenance medications or accidentally overdosing on common painkillers, such as acetaminophen. If people with liver failure could hold out for just a few days in the hospital, they would survive because their liver would spontaneously recover — the organ can recharge if given enough time. Yet in the intervening period, these patients often succumb to the toxic buildup of chemicals the liver can't filter.
Despite more than 50 years of research on liver disease, there is still virtually no help, no stopgap solution. The need for a bioartificial liver is great.
Plus, the liver is huge — it's the largest organ in the body — so you need a lot of artificial liver to do the job. Scott L. Nyberg, M.D., Ph.D., has spent his research career thinking big to try to create a device that contains a very large number of liver cells (hepatocytes) to perform the function of a normal liver. He envisions capturing available healthy liver cells from a patient, injecting them into animals to grow large supplies and ultimately returning them to the same patient.
"In the United States, there is not an approved device for such treatment. There is no liver dialysis," Dr. Nyberg says. "It's either drug treatment or a liver transplant, if a person is eligible. There is a great demand for a liver-support device. Almost every week I get calls from people around the United States asking if my device is ready for use. The answer is not yet."
Two problems confront Dr. Nyberg: One, creating a device that can hold large numbers of viable liver cells and putting them in contact with the blood of the patient in such a way that the blood is filtered and cleaned. And two, growing very large numbers of liver cells to put in the device.
His peers point to his brilliant work in both areas. And by collaborating with the Mayo Clinic Division of Engineering and its chairperson, Kevin Bennet, major innovative advances in both problem areas have been accomplished. Probably the more challenging problem has been to grow the liver cells. Living cells can't be reused — they have to be generated fresh and fast for every patient. So Dr. Nyberg has become the master of large-scale liver cell production. He can take a piece of human liver, extract the liver cell progenitors and grow them to very large numbers faster than anyone. But this technology still falls short.
Dr. Nyberg hopes to solve the cell problem by using pigs whose livers have been replaced by human liver cells.
It's based on a concept already proved using mice. There is a type of mouse whose liver can be destroyed by changing its diet because of a very specific genetic defect. However, scientists can rescue that mouse from certain death by giving it some normal liver stem cells before changing its diet. As the mouse loses its liver cells, the normal stem cells undergo multiple cycles of division and regenerate the organ. The mouse essentially grows a new liver with normal cells. If the mouse is given human stem cells, it regenerates a new liver made of human liver cells. Yet mice are too small to provide enough human liver cells for a bioartificial liver. The pig is another matter.
Dr. Nyberg has been collaborating with the group of Markus Grompe M.D., at Oregon Health & Science University, which developed the mouse model. Together, they managed to create a pig with this same genetic defect. Their next task is to see if they can get human liver cells to take over when the pig's liver is destroyed. If that works, it could lead to the long-awaited solution for patients with liver failure.
The device and how it works
The spheroid liver cell (hepatocyte). Millions of these are needed to approximate the function of the natural human liver.
The current spheroid reservoir designed to keep specialized liver cells viable and patients stable.
The processing cycle of the bioartificial liver.
The device that would contain the transplanted cells is called the Mayo Clinic Spheroid Reservoir Bioartificial Liver. A 2005 version showed encouraging results. Through collaboration with the Division of Engineering, an optimized bioreactor has been created. Preliminary animal testing of the latest system is expected to start in early 2012.
Dr. Nyberg began pursuing a device during his residency at the University of Minnesota. His mentor there, surgeon John Najarian, M.D., urged him to put his engineering background to work. Nyberg holds a bachelor's degree in chemical engineering from the Massachusetts Institute of Technology and a doctorate in biomedical engineering from the University of Minnesota. It was, Dr. Nyberg says, a perfect challenge.
A big hurdle in liver research has been the organ's multitasking complexity. The reddish-brown liver filters blood coming from the digestive tract. It detoxifies chemicals and metabolizes medications. It also secretes bile into the intestines and produces proteins for a number of the body's housekeeping needs.
What makes Dr. Nyberg's device unique is its spheroid reservoir, which provides a nurturing environment for liver cells, and a multishelf rocker that maintains cell circulation and allows for the culturing of additional hepatocytes. The device, Dr. Nyberg notes, also allows for the liver's important production of albumin, the main protein in blood plasma, and for ureagenesis, the liver's pathway for detoxification of ammonia from the blood.
Previous versions of bioartificial devices supported 50 to 70 grams of hepatocytes — only about 5 to 7 percent of the liver's mass. Early efforts often relied on porcine hepatocytes, which, like human versions, detoxify ammonia. But pig cells cause allergic reactions, are prone to rejection and have some different metabolic pathways from human cells. By growing human hepatocytes in bioengineered mice and pigs, these problems might be eliminated, Dr. Nyberg says.
"Human hepatocytes would produce human proteins and peptides, reducing the risk of an allergic response in a patient, especially if the patient is treated on more than one occasion with a bioartificial liver," says scientist Bruce Amiot, president of Brami Biomedical Inc. of Minneapolis and technical adviser to Dr. Nyberg's team.
The risk of allergic reaction to pig hepatocytes is greatest in patients with chronic liver disease, and that group represents about 90 percent of liver disease patients, he says.
"The Mayo Clinic device is capable of supporting 200 to 400 grams of hepatocytes," says Amiot, who holds two U.S. patents related to cell-culturing systems.
"While there are several advantages to our version of a bioartificial liver," he says, "in my mind, the principal improvement is the large number of viable, highly metabolically active hepatocytes that we can put in the system versus what has been previously tried."
The immediate goal, Dr. Nyberg says, is to develop the first U.S.-approved liver-dialysis device for use over long periods of time. Conceivably, he says, a patient's liver might be given time to regenerate itself by continual exposure to healthy hepatocytes. If not, then the patient would have more time to wait for a transplant.
The Mayo Clinic team is growing human hepatocytes in bioengineered pigs in collaboration with Dr. Grompe and an Oregon-based start-up company called Yecuris that Grompe co-founded, as well as with Texas Children's Hospital in Houston. In the journal Hepatology in 2011, the eight collaborators documented that their approach is a viable model system.
The collaborators now are breeding male and female pigs that carry this targeted gene defect to produce a herd of pigs to serve as incubators for human hepatocytes.
"After I first saw Dr. Grompe give a talk on this process in mice, I asked him if he had tried this in pigs," Dr. Nyberg recalls. "He said he'd like to, but that he hadn't figured out how to do it. That led to our partnership to develop a pig that has the same gene defect as in the mice."
The herd should be ready for study in early 2012. These human cells could then be used as the source of liver function in a bioartificial liver or used for cell transplantation into the patient.
"We must prove we can do this first in animals before we can do it in humans," he says. "It's not going to happen tomorrow, but it could happen."