More than 1,200 patients on waiting lists for heart or lung transplants die in the United States each year, and thousands more are lost worldwide.
Yet a leading international transplantation medical group on Friday says no attempt to implant pig organs in human patients should be made anytime in the near future.
While many critics of cross-species transplantation have focused on the risk that some commonplace disease of pigs or other donor animals might jump between species with such a transplant, the committee set up by the International Society for Heart and Lung Transplantation has even more doubts.
It's widely expected that pig hearts will be the best first candidates for cross-species transplant, but scientists on the panel said a number of hurdles need to be cleared first.
"We would require sufficient evidence from animal studies to assure us that the donor organ could meet the physiological needs of a human recipient, and that the methods used to prevent rejection would not otherwise compromise the patient's health and quality of life," said Dr. David Cooper, a surgeon at Massachusetts General Hospital and Harvard Medical School who co-chaired the committee.
Genetic differences between humans and pigs are so great that no amount of antirejection drugs seems adequate to keep a pig heart unmolested by the human immune system. But several strategies, including "humanizing" the pig organs by genetic modification that inserts key proteins into the pig, are showing promise.
Even if the organ is not rejected, the reviewers said it's not clear whether a pig heart - or lungs - are big enough or robust enough to sustain a human body for a long time. It's also uncertain how a pig heart might function and grow in the human metabolic environment.
There also are concerns that any adaptations the body made to accommodate a pig heart might preclude the later transplant of a more viable human heart.
Specifically, the authors recommended that if a clinical trial were intended to provide long-term support for a patient, laboratory evidence would be needed to show the transplanted heart could support a good quality of life for at least six months.
So far, the longest that a nonhuman primate has survived supported by a pig heart is 39 days, which scientists still claim is encouraging.
There has never been an attempt to transplant animal lungs into humans, but animal hearts have been transplanted experimentally and sporadically as far back as 1964. The most famous was Baby Far, who lived for 20 days with a baboon heart in 1984.
The panel said that animal organs or artificial organs "appear to represent the most likely potential solutions (to organ shortages) in the near term." It added that an implantable artificial lung seems unlikely to be developed in the foreseeable future.
Assuming further that the issue of animal-derived infections from transplants is resolved, the scientists said a trial xenotransplant should only be considered for someone who is excluded from the waiting list for a human heart or is unlikely to survive the wait for a human heart and does not qualify for a mechanical heart support.
"It's important to understand that at present, by far the best option for patients with heart and lung failure is transplantation with human organs," said Dr. Anne Keogh, an Australian surgeon who's president of the transplant society and committee co-chair.
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