For nine months, a genetically modified pig kidney kept a 66-year-old man alive without dialysis while he waited for a human donor organ.
Tim Andrews received the experimental kidney at Massachusetts General Hospital in January 2025 after spending more than two years on dialysis because of end-stage kidney disease. The kidney continued working for 271 days, the longest reported period of dialysis-free survival after a pig kidney transplant in a living person.
The kidney eventually had to be removed after doctors found damage to its blood vessels. But by then, Andrews had reached another milestone: a human kidney became available, allowing him to undergo a conventional transplant.
His unusual journey offers a glimpse of how animal organs could one day help people survive the long wait for a human kidney.
Why Andrews Needed Another Kidney
Andrews' kidneys had failed to the point where they could no longer adequately filter waste and excess fluid from his blood.
That left him dependent on dialysis, in which a machine performs much of the filtering work that healthy kidneys normally handle. He had already been receiving the treatment for more than two years, and the strain was affecting his daily life. He also had an O blood type, which can make finding a compatible donor kidney more difficult.
A human kidney was ultimately the goal. But there was no guarantee one would become available when he needed it.
That is where the experimental transplant came in.
The Kidney Came From A Genetically Modified Pig
The organ Andrews received came from a pig whose genome had been extensively altered before the kidney was removed.
Scientists made 69 genetic edits to the donor pig. The changes were intended to make its organs more compatible with humans by removing genes that could trigger dangerous immune reactions and adding human genes that could help the organ function in its new host.
Researchers also inactivated pig retroviruses in the donor animal to reduce the risk of those viruses infecting the recipient.
The approach is known as xenotransplantation, meaning an organ is transplanted between different species.
That difference creates one of the biggest obstacles in the field. The human immune system is designed to recognise foreign material and attack it. A pig kidney therefore cannot simply be placed inside a person and expected to work normally without careful genetic engineering and immune-suppressing treatment.
Andrews' transplant was an attempt to overcome some of those barriers.
The Pig Kidney Started Doing Its Job
After the operation, the kidney began functioning, and Andrews was able to stop dialysis.
That was more than a technical achievement.
For someone who has spent years attached to a dialysis machine, being able to live without regular treatment can change how much time and energy is consumed by kidney failure.
The organ continued functioning for months.
By July 2025, Andrews had already become the longest-living recipient of a genetically modified pig kidney, reaching 137 days after his operation.
But the kidney's performance was not permanent.
Around six months after transplantation, doctors detected problems involving the kidney's blood vessels. The damage progressed until the organ could no longer function adequately, and surgeons eventually removed it after 271 days.
For xenotransplantation researchers, however, the fact that the organ had functioned for that long was significant.
Previous attempts had lasted much less time, so extending the period to nine months gave researchers more information about what happens when a genetically modified pig organ remains inside a human body for an extended period.
Then A Human Kidney Became Available
Once the pig kidney was removed, Andrews returned to dialysis.
He did not have to wait indefinitely.
A human donor kidney became available, and he subsequently received the organ in a conventional transplant. The sequence meant the experimental pig kidney had been followed by the type of transplant doctors ultimately wanted for him.
That makes his case particularly unusual.
Xenotransplantation is often discussed as a potential way to provide organs when human donors are unavailable. But Andrews' experience also raises another possibility: a genetically modified animal kidney could potentially keep some patients stable while they remain on a transplant waiting list.
The pig kidney did not need to function for the rest of his life to make a meaningful difference. It kept his kidneys' essential filtering work going for nine months before he was able to receive a human organ.
Why Scientists Are Turning To Pigs
The shortage of donor organs is one of the biggest problems facing transplantation.
There are far more people who need kidneys than there are suitable human organs available. Some patients spend years on dialysis waiting for a transplant, while others become too sick to receive one.
Pigs are being studied because their organs are similar enough to human organs in size and function to make transplantation possible, while genetic engineering can be used to alter some of the biological differences that cause rejection.
Researchers have been working on this problem for decades, but successful transplantation into living humans is still in its early stages.
Massachusetts General Hospital performed the world's first genetically edited pig kidney transplant into a living human in 2024. Andrews' procedure was the hospital's second such transplant.
Each patient gives scientists another opportunity to understand why some pig organs survive longer than others and what needs to change before the approach could be used more widely.
Nine Months Is A Milestone, Not A Solution
Andrews' experience does not show that pig kidneys are ready to replace human donor kidneys.
It was a single experimental transplant, performed under the FDA's expanded-access pathway for patients with serious or life-threatening conditions who have limited alternatives. Researchers still need to understand how to prevent immune reactions, blood-vessel damage, and other complications over much longer periods.
But the nine-month record changes the conversation.
A pig kidney that functions for days or weeks would offer limited help to someone facing years of kidney failure. One that can keep working for months could give doctors considerably more time.
For Andrews, those months ultimately ended with a human kidney.
For researchers, they provided something just as valuable: evidence that a genetically modified animal organ can perform the work of a human kidney in a living person for far longer than previous attempts.
The next challenge is figuring out how to make that performance last.
