(CNN) -- Scientists continue to make progress with research on how well pig organs might perform in humans, and now they're ready for the next step: larger-scale clinical trials.
eGenesis, one of the biotech companies developing gene-edited pigs whose organs are transplanted into humans, said Monday that the US Food and Drug Administration has given the green light to human trials involving kidneys from its pigs.
"It's a very optimistic period," said Paul Conway, chair of policy and global affairs for the American Association of Kidney Patients, the country's largest kidney patient advocacy group.
eGenesis' pigs are genetically modified so that their organs are more compatible with human recipients. Key to this effort is using a tool called CRISPR to knock out the gene responsible for a carbohydrate known as alpha gal. Without that, a human body would reject a pig organ almost immediately.
There have been a handful of such procedures, called xenotransplants, performed in the US, including pig kidney transplants at NYU Langone and pig heart transplants at the University of Maryland School of Medicine. In all of those cases, the patients had run out of other options, and the organs were transplanted not through a standard clinical trial, but under special rules that permit compassionate use of experimental therapies for people in especially dire situations.
On Monday, doctors at Massachusetts General Hospital announced their third experimental pig kidney transplant. The recipient, Bill Stewart, had the procedure June 14 and his since returned home and even gone back to work.
"There's so few of us who have done this, and they're writing the protocol as we go, so to speak," said the 54-year-old Dover, New Hampshire, resident. "But I'm feeling good."
Stewart had a family history of high blood pressure, and although he was fairly active, he had hypertension throughout most of his adult life. High blood pressure can constrict the vessels around the kidney and reduce its ability to function.
After some testing with a nephrologist, he said, doctors found that his kidneys weren't functioning as well as they could, and they eventually dropped to just 10% to 15% of their normal function. It was at that point he was put on dialysis.
Over 100,000 people in the US are waiting for an organ donation, and 86% of those on the list need a kidney.
The average wait time for a kidney is three to five years at most centers, but Stewart had worse luck: He has type O blood, and studies have found that people with that type can wait as long as 10 years to receive a donor kidney.
Stewart, an athletic trainer and physical therapist, underwent dialysis three days a week for more than two years. The five-year mortality rate for people on dialysis is higher than 50%.
Stewart is gradually returning to his two jobs: one with a nonprofit working with people with physical disabilities and another helping run Massachusetts General Hospital's sport medicine program.
"It's been a fantastic thing," Stewart said of returning to work. "I think mentally, that was a nice kind of hurdle to cross ... being able to see everybody and have some conversations and just kind of get back in the rhythm a little bit."
But he's most excited about getting back outside. Stewart and his wife often hiked and kayaked on the weekends before he got too sick to continue.
"My energy level is still nowhere near where I want it to be, but just to be outside and have the procedure behind me and know that there's hopefully, a bright future ahead," that's been great.
The first patient to receive a pig kidney at Mass General was 62-year-old Rick Slayman in March 2024. Slayman became the world's first living recipient of a genetically edited pig kidney but died two months later due to cardiac issues unrelated to the transplanted organ.
Tim Andrews, then 67, received a pig kidney at Mass General in January, and it continues to support him. He is currently the longest-living pig kidney recipient in the world.
Dr. Robert Montgomery and his team at NYU Langone have also performed pig kidney transplants. In April 2024, they gave 54-year-old Lisa Pisano both a mechanical heart pump and a pig kidney for her heart failure and end-stage kidney disease. However, the kidney was removed in May after it was determined that it was "no longer contributing enough to justify continuing the immunosuppression regimen," Montgomery said at the time. Pisano died in July 2024.
In November 2024, Montgomery and his team performed another xenotransplant on Towana Looney. The pig kidney allowed the 53-year-old grandmother to live dialysis-free for 130 days, but it was removed after an unrelated infection reduced her renal function. She has since returned to dialysis.
There have also been two efforts to transplant genetically modified pig hearts into living recipients by teams at the University of Maryland School of Medicine.
The pig kidneys at Mass General all came from eGenesis. Another company, United Therapeutics, provided the gene-edited pig organs to the patients at NYU Langone and the University of Maryland, and it expects to begin its own human trials this year.
Experts are optimistic as the trials kick off a new phase of these experimental procedures.
In the US, 37 million adults have chronic kidney disease, and about 800,000 of them have end-stage kidney failure.
Conway said a survey of members of the American Association of Kidney Patients found that if an FDA-approved pig kidney were available, more than 70% of of those surveyed would say agree to the transplant.
The trials will give researchers a chance to better understand the work in a broader population.
"The one-offs, or the single-patient studies, are super helpful to kind of understand where we are in a rough sense ... but the big question is, how does this perform in a variety of different patients?" said Mike Curtis, president and CEO of eGenesis. "The only way to answer how this is going to behave in a multitude of patients is to run a larger study."
United Therapeutics plans to transplant the first patients in its trial this year, ultimately studying up to 50 patients. EGenesis' Curtis said his company hopes to treat the first patient before the end of this year and ultimately transplant 33 patients over the course of the next two and a half years.
The initial transplant recipients -- such as Slayman and the two people at the University of Maryland who received pig hearts -- involved people who had significant underlying health problems.
"This next study allows us to study this in more patients and get a better sense of how generally applicable this technology is going to be," Curtis said.
Researching this technology in people who are healthier, who are not so far along in the course of renal disease, will also be key, said Dr. Leonardo Riella, a transplant nephrologist at Massachusetts General Hospital who helped perform all three xenotransplants there.
Trials with healthier patients will truly test the durability of the organs, Riella said. "We don't see all the other complications that, unfortunately, dialysis can cause over time and can confuse as and the interpretation of if the transplant works."
Montgomery, who is a heart transplant recipient himself, has been working toward this moment. He pioneered work in xenotransplantation by first transplanting organs into people who were brain-dead.
"We performed the first gene edited pig-to-human kidney transplant on September 25th, 2021. I don't think any of us thought there would be two FDA approved trials beginning less than 4 years after that landmark event," Montgomery wrote in an email. "I have felt for a long time that if we could just get a pig kidney into a human, we would make it work. I had no doubt about that."
Stewart said he also had little doubt about xenotransplantation: "Once I heard about it, I was really kind of committed to sort of seeing it through."
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