The first person to receive a heart transplant from a pig has died two months after the ground-breaking surgery.
David Bennett, died on Tuesday at the University of Maryland Medical Centre in the US.
British experts have watched the case with interest, but have said that there is “still some way to go” before this sort of treatment becomes an everyday reality.
One professor of stem cell biology said that it is not yet clear whether Mr Bennett died because he was too frail or whether the pig heart was not compatible.
The team of US doctors did not give an exact cause of death, saying only that Mr Bennett’s condition had begun deteriorating several days ago.
Commenting on the case, a spokesperson for NHS Blood and Transplant said: “Our thoughts are with the family of Mr Bennet at this time.
“Xenotransplantation is still in very early stages of development.
“While research has moved forward our knowledge of this area of transplant medicine, it has been widely recognised that there is still some way to go before this is an everyday reality.
“We are grateful to all of those who work so hard to improve knowledge and understanding of transplantation techniques and patients and families who support valuable research which carries the possibility to save and improve the lives of many others still waiting.”
Chris Denning, professor of stem cell biology at the University of Nottingham, said: “I don’t think the patient’s death is much of a surprise – it is the first attempt of this transplant type and it was never going to be easy.
“What will be interesting is why the patient died. I note in the press release that there was no apparent immune rejection, which is a big step.
“The hospital did say the patient was very frail to start with, so it might be that he just wasn’t strong enough.”
Prof Denning added: “Although anatomy of the pig heart and human heart is similar, it is not the same. The extra pressures that the human heart has to produce to counter gravity and pump blood around a biped is much greater than that of a pig heart, and so the pig heart may have simply not been strong enough to survive in a human circulatory system.
“So, at this stage on the information that has been provided, cause of death could be anywhere from the patient being too frail or a pig heart not being compatible with the human circulatory system.”
Mr Bennett is said to have thought that medics were joking when the operation was initially proposed, but agreed to try the experimental procedure as a last-ditch attempt to save his life.
The 57-year-old was not eligible to be put on the transplant list – he was bed-ridden and using a heart-lung bypass machine to stay alive.
The pig was genetically engineered so its organs could survive in a human body.
Cross-species transplantations offer the prospect of an unlimited supply of organs and cells for transplantation and could potentially resolve issues over shortages of donors.
Many attempts at such transplants – or xenotransplantation – have failed, largely because patients’ bodies rapidly rejected the animal organ.
Doctors said that the transplant, which took place on January 7, shows that a heart from a genetically modified animal can function in the human body without immediate rejection.
Last month, the hospital released a video of Mr Bennett watching the Super Bowl from his hospital bed.
And even after Mr Bennett’s death, medics have said they “remain optimistic” for future clinical trials.
“We are devastated by the loss of Mr Bennett. He proved to be a brave and noble patient who fought all the way to the end,” Dr Bartley Griffith, who performed Mr Bennett’s surgery, said in a statement.
Muhammad Mohiuddin, professor of surgery and scientific director of the cardiac xenotransplantation program at University of Maryland School of Medicine, added: “We have gained invaluable insights learning that the genetically modified pig heart can function well within the human body while the immune system is adequately suppressed.
“We remain optimistic and plan on continuing our work in future clinical trials.”
Mr Bennett’s son, David Bennett Jnr, said: “We are grateful for every innovative moment, every crazy dream, every sleepless night that went into this historic effort.
“We hope this story can be the beginning of hope and not the end.”