US surgeons have successfully implanted a heart from a genetically modified animal into a human patient, a first of its kind procedure, the University of Maryland Medical School said on Monday.
The surgery took place on Friday and demonstrates for the first time that an animal heart, in this case that of a pig, can survive in a human without immediate rejection, the medical school said.
The patient, David Bennett, 57, had been deemed ineligible for human transplant. The Maryland resident is being carefully monitored to determine how the new organ performs.
"It was either die or do this transplant. I want to live. I know it's a shot in the dark, but it's my last choice,"Mr Bennett said a day before the surgery.
"I look forward to getting out of bed after I recover."
He has spent the past several months bedridden on a life support machine.
The US Food and Drug Administration granted emergency authorisation for the operation on New Year's Eve, as a last ditch effort for a patient who was unsuitable for conventional transplant.
"This was a breakthrough surgery and brings us one step closer to solving the organ shortage crisis," said Bartley Griffith, who surgically transplanted the heart.
"We are proceeding cautiously, but we are also optimistic that this first-in-the-world surgery will provide an important new option for patients in the future."
Mr Bennett's donor belonged to a herd that had undergone a genetic editing procedure to get rid of a gene that produces a particular sugar, which would otherwise have caused a strong immune response and led to organ rejection.
The editing was performed by biotech firm Revivicor, which also supplied the animal used in a breakthrough kidney transplant on a brain-dead patient in New York in October.
The donated organ was kept in a machine to preserve it ahead of surgery, and the team also used a new drug along with conventional anti-rejection drugs to suppress the immune system and prevent it rejecting the organ.
It is an experimental compound made by Kiniksa Pharmaceuticals.
About 110,000 Americans are currently waiting for an organ transplant, and more than 6,000 patients die each year before getting one, according to official figures.
To meet demand, doctors have long been interested in so-called xenotransplantation, or cross-species organ donation, with experiments tracing back to the 17th century.
Early research focused on harvesting organs from primates. A baboon heart was transplanted into a newborn known as "Baby Fae" in 1984, but she survived only 20 days.
Today, pig heart valves are widely used in humans, and pig skin is grafted on human burn victims.