Pig hearts have been transplanted into baboons – a development that could pave the way for humans to receive porcine organs in future.
Researchers from Germany, Sweden and Switzerland said two Anubis baboons had survived for six months, while another two lived for at least three months.
Previously, the longest a baboon had survived after such a procedure was 57 days.
Since then, genetic modifications have been made to the hearts and a new transplant technique has been developed.
The pigs were modified so that they produced a human version of two proteins which block an immune response in alien cells.
It was also ensured that they generated thrombomodulin, which prevents blood from clotting after surgery.
The research team also stopped preserving donor hearts in cold storage and kept them at 8C instead.
Fluids containing oxygen, hormones, red blood cells and nutrients were circulated through them.
The baboons were given drugs to stop the pig heart growing too big, and to lower their blood pressure to match that of the pigs.
The transplant procedure was refined during three trials involving 14 baboons.
One of the five baboons in the final experiment had to be put down after developing a blood clot.
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The transplantation of an organ between two different species – known as xenotransplantation – is thought to be one way of overcoming a shortage of organs for humans needing a transplant.
The scientists' findings were published in the Nature journal.