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Pig Organs Transplants May Soon be Possible - 2002-02-19

Scientists are gradually removing technical barriers to the use of pig organs to replace ailing human organs. They say the transplants might be possible in five to seven years. But more work must be done to make pig parts compatible with people.

Transferring animal organs to people is considered a solution to the worldwide shortage of organ donors. Pig organs are similar to those of humans, particularly hearts and kidneys. But there have been no experimental animal transplants to people for nearly a decade because technical barriers remain, as Harvard Medical School surgeon David Cooper told the American Association for the Advancement of Science convention in Boston.

"One problem is the science. We are not ready to do pig transplants in humans because we haven't solved the problems of the science," Dr. Cooper says.

A major hurdle remains transplant rejection. Pigs have a sugar molecule on the surface of their blood vessel cells similar to one in bacteria, so antibodies in the human immune system attack it.

In experiments in monkeys and apes, scientists have extended the time to rejection from hours to about one month by inserting into pigs a human gene that blocks the antibody attack against the sugar, and then giving the organ recipient drugs to suppress the antibodies. We have that protective gene to shield our tissue from our own antibodies when they attack bacteria. But Dr. Cooper says surgeons are stymied in their effort to prolong survival beyond a month or so.

"We seem to have reached a brick wall here. Any combination of the drugs we have available does not take us much further than that period of time," Dr. Cooper says.

In a potential breakthrough, two U.S. biotechnology companies have recently created cloned pigs lacking one of two copies of a gene that produces the offending sugar. The next step is to breed offspring that lack both copies so the antibodies have no target to attack.

"I strongly believe it will be a major step forward. Now, I don't think that's going to be the only problem. There are certainly data suggesting there are going to be other problems after that," Dr. Cooper says.

Another worry has been whether transplanted pig organs will transfer pig diseases. All pig cells contain a retrovirus harmless to the animals, but researchers do not know if it would also be harmless to people. Scientists have gotten around this by identifying a strain of pigs that cannot pass the virus to humans.

Another Harvard University researcher, Dr. Fritz Bach, says the expectation of further technical advances offers hope that pig organs can be put into people routinely well within a decade.

"If the answer is, yes, we've overcome the critical issues, then we will be able to transplant. If the effort is made to benefit from the multiple approaches we can think of, five to seven years, I think, is a reasonable time to say, yes, we can move ahead," Dr. Bach says.

Dr. Cooper agrees. But the two men strongly disagree on a related ethical issue. Fritz Bach worries about the possibility of transmitting as yet unknown pig viruses and calls on governments to impose a moratorium on animal organ transplants while the public debates whether it is safe to proceed with them.

"The public cannot decide, but if they are the ones that are going to take the potential risk, the only ethical approach is to get input from that public based on good information and consider that input very much," Dr. Bach says.

But David Cooper argues that regulatory agencies should make the decision based only on the science involved and not get mired in a public discussion about hypothetical risks.

"If you have solved all the known problems, if you are worried about unknown problems, you will never make any progress in any branch of science," Dr. Cooper says.

Canada, the Netherlands, and some other countries are listening to citizen groups before deciding whether to commit to clinical trials of animal organ transplants, and the Council of Europe has proposed this policy.