A genetically modified pig kidney functioned in a human patient for 271 days before being removed and replaced with a human donor organ, marking the longest recorded survival of an animal-to-human kidney transplant and the first successful transition from a pig kidney to a human transplant.
Tim Andrews received the pig kidney at Massachusetts General Hospital in Boston in January 2025, when he was 66. He had previously required dialysis for two years because of kidney failure. The transplanted organ allowed him to live without dialysis for nine months before irreversible damage forced doctors to remove it.
In January this year, Andrews received a human kidney that was still functioning properly seven months later, according to the medical team that performed the procedure and reported its findings in The Lancet. The sequence points to a possible future in which genetically modified animal organs could serve as a temporary lifeline for patients while they wait for a suitable human donor.
“The scarcity of organs is the biggest crisis we have right now in the field of transplantation,” said Leonardo Riella, medical director of the kidney transplant programme at Massachusetts General Hospital and lead author of the study.
Riella said transplantation remained the best treatment for patients with end-stage kidney disease, but there were simply not enough human organs available in time. Xenotransplantation — the transfer of cells, tissues or organs between animal species and humans — could initially provide a bridge that allows patients to leave dialysis while awaiting a human kidney, he said, and could potentially become a definitive treatment if its long-term safety and durability are established.
The scale of the shortage is substantial. In 2025, 53,343 patients in the European Union were on waiting lists for organ transplants. In the United States, the number exceeded 100,000 despite the country having 25 per cent fewer people than the EU. While dialysis can sustain patients with kidney failure, it does not fully replace the organ’s functions and can progressively reduce quality of life.
Scientists have attempted animal-to-human transplantation for decades, with earlier experiments involving organs from pigs, sheep, baboons and chimpanzees producing survival periods ranging from hours to months. In 1964, Edyth Parker died one day before completing nine months with a chimpanzee kidney — a threshold Andrews reached six decades later.
The central obstacle is immune rejection, which is particularly intense between different species. Advances in genetic editing, including CRISPR, have changed the prospects for overcoming that barrier.
For Andrews’ transplant, the US company eGenesis created a genetically modified pig with 69 changes to its DNA. Some modifications were intended to prevent immediate rejection when the organ came into contact with human blood, while others introduced human genes designed to regulate inflammation, blood clotting and immune activation. Researchers also deactivated porcine endogenous retroviruses, inherited fragments of pig DNA that could potentially cause infections in humans.
The kidney began producing urine during the operation and generated between four and five litres a day during the following week, freeing Andrews from dialysis. Two weeks later, however, T cells began attacking the organ in a typical rejection episode, which doctors controlled with immune-suppressing drugs.
Andrews subsequently experienced several serious health problems, including a leg infection and angina requiring another heart stent, but the pig kidney continued functioning. After six months, doctors reduced his immunosuppressive medication to treat a bacterial infection. Inflammation of the kidney’s blood vessels then caused clots to accumulate, reducing its ability to filter blood.
The damage appeared to involve macrophages and natural killer cells, components of the innate immune system, rather than antibodies. By day 271, the damage was irreversible and surgeons removed the pig kidney.
More than two months later, Andrews received a human kidney from a donor with the rare characteristics he required. Beatriz Domínguez-Gil, director of Spain’s National Transplant Organisation, said the case raised the possibility of using animal kidneys as a “bridge” while patients wait for a human organ, although she stressed that the approach remains experimental.
The researchers caution that the study involved only one patient and had no comparison group, meaning no definitive conclusions can yet be drawn. Future work will focus on understanding the role of natural killer cells in rejection, improving immunosuppression and developing drugs to protect kidney blood vessels over the long term.
The broader goal remains unchanged: refining genetically modified pig organs until they can reliably save human lives.
