Many of the 72,000 Americans awaiting organ transplants may be turning to swine for hearts, kidneys, and other organs. A small biotech concern that has been researching "xenotransplantation" -- the field of transplanting organs from other species into humans -- claims to have achieved a milestone that may result in tests of pig organ transplants to humans within the next two years.

A scientist from Biotransplant revealed Monday that a pig virus known as the porcine endogenous retrovirus, which is transmitted when pig tissue is transplanted to humans, has been genetically altered so that it cannot reproduce in humans, effectively rendering the disease harmless. Dr. Clive Patience, principal scientist at Biotransplant, disclosed his findings Monday in Rome at the International Congress of the Transplantation Society.

Biotransplant has a development agreement with Novartis, whereby the Swiss drug company shoulders some development costs in exchange for the rights to market the animal's organs. Biotransplant CEO Elliot Lebowitz declined to say what percentage of revenues would go to Biotransplant, but he claims commercialization has the "potential for generating $100 million in annual cash flow" for the 70-person company.

However, don't expect those numbers anytime soon. Earlier studies have shown no ill effects on human health from the transmission of pig viruses, but their potential for harm has been one reason xenotransplantation isn't ready for prime time. An even larger stumbling block has been the human immune system's rejection of organs from other species.

INBREEDING WAS BENEFICIAL?

Dr. Patience tested the virus from nine of Biotransplant's inbred pigs on human kidney cells and discovered how the virus was unable to reproduce in human cells, rendering it harmless. Kidney cells are ordinarily the most receptive host for the virus to reproduce. The finding was a welcome but unintended consequence of inbreeding.

"This is a nice advance," says Dr. Daniel Salomon, an expert on experimental medicine at Scripps Research Institute. "But I would like to see this confirmed in an independent laboratory before I go to the bank with it."

He notes there are still other issues, such as the possibility the retrovirus will mutate into a variety that reproduces after a transplant. Another potential problem for Biotransplant, he notes, is that its discovery could make it easier for other scientists to figure out ways of breeding their own pig herds with the same attributes without violating Biotransplant's patents.

Biotransplant developed its own "inbred herd of miniature swine," says Mr. Lebowitz. The animals reach a maximum weight of about 250 pounds and have organs that are similar in size to human organs.

PAST PIG USES

Although entire organ transplants pose immune-system rejection issues, pig tissue has been transplanted to humans for several years, particularly in Eastern bloc countries. Pig skin is grafted onto burn victims and pig cells have been used in dialysis for patients with liver and kidney failure. A study by Novartis tracked down 160 of these patients and found none of them has suffered infections from the virus.

The organ-rejection issue is still the biggest stumbling block. Biotransplant has been transplanting pig kidneys into baboons, says Mr. Lebowitz. But the baboon's immune systems reject the organs. When Biotransplant started the experiments, the animals lived for only a few minutes. But now through techniques designed to appease the immune system by introducing bone marrow first, the animals are able to live a month, he says.

Scientists have transplanted pig cells into people's brains with apparently no negative impact from the virus. But the brain is shielded more from the immune system.

Dr. Salomon says a federal committee earlier this year wanted animals to have a median survival time of six months before testing should be considered on humans.

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