FDA for the first time has approved an experiment that involves temporarily transplanting pig tissue into humans, a move that experts say could lead to further developments in animal-to-human donations, Emily Mullin reports for Medium's "OneZero."

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A first-of-its-kind trial

Each year in the United States, the waiting list for organ transplants grows and outpaces the number of available donations. And due to strict donor criteria, skin is also difficult to acquire, Mullin reports.

The new FDA-approved trial will focus on skin transplanted from pigs to humans.

Usually, second and third-degree burns are treated with human skin grafts to protect the wounds and help fight off infection. But researchers at biotech company XenoTherapeutics have genetically engineered miniature pigs to allow for use in humans. The skin, called Xeno-Skin, is made from living pig tissue.

So far, the Xeno-Skin has been transplanted to one patient who received a five-by-five centimeter piece of pig skin as well as human skin grafts. Five days after the transplant, doctors removed both skin grafts and replaced them with permanent grafts from the patient's thigh. The donor pig skin healed as well as the human skin graft, according to doctors involved in the trial. The patient also did not experience any adverse reactions to the graft. 

Now, as part of the FDA-approved trial, Xeno-Skin will be transplanted as temporary skin grafts to five more burn victims at Massachusetts General Hospital. The grafts will be removed once the patients' skin grows back.

The experiment is the first of its kind to be approved by FDA, Mullin reports.

XenoTherapeutics is also developing nerves in the genetically engineered pigs that could be transplanted into people with nerve damage from accidents. The clinical trial for the new treatment will likely begin next year, Mullin reports.

In the past, donations from animals to humans, also called xenotransplantation, have almost always failed, Mullin writes, with common issues including patients experiencing an immune response to the donation or the transmission of an animal virus to the human recipient. Surgeon Keith Reemtsma in the 1960s transplanted chimpanzee kidneys into humans, but most of the transplants were rejected or became infected within a few weeks. And in 1984, a baby called "Baby Fae," received a heart transplant from a baboon, but died within a month.

After transplants from apes and monkeys failed, doctors started to consider pigs as potential human donrs, Mullin reports. "The reasons are largely practical," Mullin writes. "For one, pigs take only months to grow to a size suitable for organ donation, unlike monkeys, which require 10 to 15 years. Pig organs are also closer in size to human ones."

When it comes to skin transplants, while dead, processed pig skin is already used for wound dressing, living pig tissue had never been used in humans, Mullin reports. To prevent rejection, scientists used genetic engineering to eliminate molecules found in pigs that would trigger an immune system reaction in humans. Researchers also used the gene-editing tool CRISPR to remove a group of viruses found in pigs that are considered an infection risk for human donation recipients.

What about other transplants?

While temporary pig skin grafts are much more low risk than transplanting major pig organs into humans permanently, the successful skin graft transplant further demonstrates that "the risk" of xenotransplants "is manageable," according to Megan Sykes, director of the Center for Translational Immunology at Columbia University. "The fact that the FDA allowed this [pig skin] trial reflects that thought as well."

While xenotransplants are often used as bridge transplants, or temporary transplants that are designed to keep patients alive for a period of time before they receive a transplant from a human donor, Muhammad Mohiuddin, director of the cardiac transplantation program at the University of Maryland School of Medicine who has led research on pig transplants, believes that organ xenotransplantation might evolve enough to become a longer term solution. "[O]nce we have an ideal pig available and it survives for a longer period of time, there will not be a need to replace that organ," he said.

But convincing the public that pig transplants are safe might prove to be more difficult, he added. "If you tell someone, 'We want to put a pig heart in you,' there would probably be quite an uproar," Mohiuddin said. "If this trial is successful, that will pave the way for other types of transplantations, like kidneys, hearts, lungs, or livers."

While these types of transplants might not happen for another few decades, Ardehali said the field is getting closer. "Are we about to embark on a new chapter? I don't know the answer to that. This requires a significant leap of faith" (Mullin, "OneZero," Medium, 11/11).