Tim Andrews, a pensioner from New Hampshire, had suffered from kidney failure for years. Dialysis could not stop the steady decline in his health. “I will probably die before I get to the point where I can have a human transplant,” Mr Andrews says. Then he read about Richard Slayman, who in 2024 had received a “xenotransplant”: an organ taken from another species.
Mr. Andrews contacted the medical team at Mass General Hospital (MGH), the Massachusetts hospital that performed the operation. On January 25, surgeons at MGH gave him a kidney from a genetically modified pig. His new organ lasted 271 days – a record.
Xenotransplantation is the fusion energy of medicine. As the well-worn joke says, this is the future—and always will be. But that punchline seems useless now. Both Mr. Andrews and Mr. Slayman, being very ill, received their organs on exceptionally compassionate grounds (Mr. Slayman later died of causes unrelated to his new organ). But in September the Food and Drug Administration, a US medical regulator, gave Egenesis, the company that provided Mr Andrews’ kidney, permission to begin full-scale clinical trials of pig kidneys – an early step towards wider medical use. United Therapeutics, which produces pig organs through its subsidiary Revivicor, was given permission to do so in February. Both Egenesis and United Therapeutics hope to begin transplanting organs soon.
Technology offers great hopes. The Global Observatory on Donation and Transplantation, run by the World Health Organization and the Spanish Transplant Organization, believes that less than 10% of people who need a transplant worldwide receive one. Even in rich countries, demand exceeds supply: About 13 people per day in the US die while on the waiting list. The shortage fuels a black market, in which patients pay thousands of dollars for organs of dubious origin.
Pigs can reduce that deficit. They are easy to breed and their organs are of almost the right size. Being mammals, they are physically somewhat similar to humans – while being distant enough cousins ​​to minimize the ethical concerns that could derail efforts to use apes or monkeys. But things have progressed slowly. Jeffrey Platt, a surgeon at the University of Michigan, published the first experiment of transplanting pig organs into monkeys in 1995.
Much of the recent progress has been due to a new gene-editing technology called CRISPR, the pioneers of which won the Nobel Prize in 2020. CRISPR allows scientists to more easily edit the pig genome to make pig organs more tolerable for humans. One of the biggest problems with any type of transplant is rejection, in which the recipient’s immune system recognizes the new organ as foreign and attacks it. Preventing rejection in human-to-human transplants often requires lifelong use of immunosuppressive drugs, which have side effects and leave recipients vulnerable to infection. This problem is exacerbated when the organ comes from a completely different species.
Revivicor and Egenesis—as well as CloneOrgan, a Chinese company that has produced organs for transplant into some human patients—make their gene-edited donor pigs in the same way. Scientists took skin cells from adult pigs and disabled three or four genes that cause violent immune responses in humans. Then they insert six or seven human genes, which help prevent rejection, as well as problems related to blood clotting and inflammation.
Next, the edited cells are used to create cloned pigs by removing the cells’ nuclei, which contain their DNA, and putting them into porcine egg cells. Once these eggs are fertilized, and implanted into a surrogate pig, the result is a gene-edited piglet. Although companies sometimes add additional edits to distinguish themselves, the ten-edited pig has become the basic organ-donor formula.
The resulting organs function well in monkeys. However, the results in people are less clear. Mr Andrews, whose kidney was provided by Egenesis, was managing the rejection issues quite well with medications. But his new kidney was losing function and it had to be removed on October 23. Mr Andrews is now back on dialysis, and is on the waiting list for a human transplant. He is the second recipient whose new kidney has failed. Tovana Looney, who received her kidney from Revivicor in November 2024, had her kidney removed after her body rejected it — the result of doctors reducing her dosage of immunosuppressant drugs to allow her body to fight an unrelated infection.
Egenesis boss Mike Curtis thinks the upcoming trials will teach the field a lot. Gene editing and improvements in both post-transplant care may be possible. He has high hopes for Egenesis’s collaboration with Alleydon, a company that is testing a new anti-rejection drug called tegoprubaart that they hope will have fewer side effects than existing drugs. Revivicor, for its part, is testing a drug called ravulizumab that is already prescribed for autoimmune disorders.
That’ll do, pig
Scientists are also looking at other organs. Revivicor has performed two pig-heart transplants so far. It also has permission to test its “euthymokidney”, which combines pig kidneys with porcine thymus tissue. The thymus is a gland that helps train the immune system to detect threats while leaving the body’s own cells alone. Revivicor scientists hope that a combination will encourage the transplant recipient’s immune system to tolerate the new kidney.
Meanwhile, Egenesis has received approval to test the pig-liver perfusion system. Unlike a full transplant, it places the organ outside the patient’s body, although connected to his circulatory system. With the help of an organ-preservation device developed by Oxford University spin-out OrganOx, there is hope that a pig’s liver could keep a patient alive until a human organ is ready. In March and August, Chinese research teams associated with CloneOrgan said they had transplanted a pig’s liver and lungs into two brain-dead people, as well as a kidney into a living patient. Another Chinese group injected pig liver into a living patient earlier this month.
Now that Mr Andrews’ kidney has been removed, the only person in the US living with a pig kidney is Bill Stewart, an athletics coach who received the kidney in June. But Mr. Stewart won’t be alone for long: MGH is scheduled to perform a third transplant later this year.
Correction (October 29, 2025): This excerpt has been updated to reflect that the FDA approval was given to United Therapeutics rather than its subsidiary Revivicor.
