Chilling to subzero temperatures greatly increased survival time of rats’ organs outside the body
Wally Reeves, K. Uygun/Harvard Univ., Martin Yarmush
“Supercold but unfrozen” is the idea behind a new procedure that tripled the time a rat liver can survive outside the body. If the procedure works similarly well for humans, the method may grant life-saving transplants a passport for traveling across the world, scientist report June 29 in Nature Medicine.
Organs quickly stop working once removed from the body. To stay viable, donated organs are stored inside plastic bags filled with a liquid preservative and kept on ice. But even with this procedure, a human liver lasts only 12 hours, a time frame that instills geographical limits between organ donors and recipients. Patients waiting for a liver in New York are 20 times as likely to receive one as those in Washington state or Kentucky. If the technique works for humans, it could introduce parity, meaning an organ donated in Houston could help a patient in Honolulu or even Hong Kong.
Prolonging the time frame could save lives, but attempts at long-term preservation have floundered. Most researchers have tried freezing the organs, but the process damages cells and blood vessels, says study coleader Korkut Uygun, a chemical engineer at Harvard University. “We wanted to reduce the temperature but avoid freezing at all costs,” he says. “We didn't think that we could bring the cells back from a frozen state, given that's been tried for 20 years now with no success.”
So rather than freeze, the team supercooled organs, a process whereby tissue chills to subzero temperatures without its water turning to ice. The key was finding the right cryopreservatives — ones that could maintain cellular architecture without being toxic. The team settled on two compounds: 3-O-methyl-D-glucose, which prevents ice from forming inside cells, and polyethylene glycol, which does the same for the exterior. Uygun is seeking Food and Drug Administration approval for use of the chemicals as cryopreservatives in humans.
Another problem the researchers faced was that disconnecting an organ from its nutrient supply can initiate a cascade of injury. Further damage can occur when the tissue is reintroduced to oxygen as it is transplanted into the recipient. To overcome this hurdle, the researchers relied on an experimental technique called machine perfusion that circulates fluids in an organ to simulate regular nutrient flow. The researchers performed machine perfusion during organ cooling, while adding one of the cryopreservatives. Once the organ reached –6° Celsius, the team soaked it in the other preservative. The scientists used machine perfusion again during thawing to prep the organ for transplantation.
A rat liver usually last up to 24 hours outside the body. Supercooled rat livers were viable even after being stored for three to four days. Every rat that received a 3-day-old supercooled liver transplant survived for at least a month, but survival dropped to 58 percent for rat recipients of 4-day-old livers. Each component of the new procedure — the preservatives, supercooling and machine perfusion — was essential, as recipient survival rates dropped to 0 when the researchers removed or altered any step.
“There is more potential in cold preservation than we had previously realized,” says Peter Friend, director of the Oxford Transplant Centre in England. Rats are a long way from humans, Friend adds, but by his estimation, the study is impressive for showing that transplant storage at subzero temperatures is possible. “These methods enable us to go colder than we have before,” Friend says.
Editor's Note: This story was updated July 24, 2014 to correct how long rat livers survive outside the body, and to clarify how long human livers can do the same.
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