A system developed by investigators at the Massachusetts General Hospital (MGH) Center for Engineering in Medicine allowed successful transplantation of rat livers after preservation for as long as four days, more than tripling the length of time organs currently can be preserved. The team describes their protocol – which combines below-freezing temperatures with the use of two protective solutions and machine perfusion of the organ – in a Nature Medicine paper receiving advance online publication.
"To our knowledge, this is the longest preservation time with subsequent successful transplantation achieved to date," says Korkut Uygun, PhD, of the MGH Center for Engineering in Medicine (MGH-CEM), co-senior author of the report. "If we can do this with human organs, we could share organs globally, helping to alleviate the worldwide organ shortage."
Once the supply of oxygen and nutrients is cut off from any organ, it begins to deteriorate. Since the 1980s, donor organs have been preserved at temperatures at or just above freezing (0˚ Celsius or 32˚ Fahrenheit) in a solution developed at the University of Wisconsin (UW solution), which reduces metabolism and organ deterioration ten-fold for up to 12 hours. Extending that preservation time, the authors note, could increase both the distance a donor organ could safely be transported and the amount of time available to prepare a recipient for the operation.
Keeping an organ at below-freezing temperatures, a process called supercooling, could extend preservation time by further slowing metabolism, it also could damage the organ in several ways. To reduce those risks the MGH-CEM protocol involves the use of two protective solutions – polyethylene glycol (PEG), which protects cell membranes, and a glucose derivative called 3-OMG, which is taken into liver cells.
After removal from donor animals, the livers were attached to a machine perfusion system – in essence, an 'artificial body' that supports basic organ function – where they were first loaded with 3-OMG and then flushed with a combination of UW and PEG solutions while being cooled to 4˚C (40˚ F). The organs were then submerged in UW/PEG solution and stored at -6˚C (21˚F) for either 72 or 96 hours, after which the temperature was gradually increased back to 4˚C. The organs were then machine perfused with UW/PEG solution at room temperature for three hours before being transplanted into healthy rats.
All of the animals that received organs supercooled for 72 hours were healthy at the end of the three-month study follow-up period. Although only 58 percent of animals receiving organs supercooled for 96 hours survived for three months, analysis of several factors done while the organs were being rewarmed could distinguish between the organs that were and were not successfully transplanted.
"This ability to assess the livers prior to transplantation allows us to determine whether the supercooled organ is still good enough for transplantation," explains study co-author Bote Bruinsma, MSc, of the MGH-CEM. "Even among the livers preserved for four days, if we had only used those in which oxygen uptake, bile production and the flow of perfusion solution were good, we would have achieved 100 percent survival."
While much work needs to be done before this approach can be applied to human patients, extending how long an organ can safely be preserved may eventually allow the use of organs currently deemed unsuitable for transplant, notes Martin Yarmush, MD, PhD, founding director of MGH-CEM and co-senior author of the paper. "By reducing the damage that can occur during preservation and transportation, our supercooling protocol may permit use of livers currently considered marginal – something we will be investigating – which could further reduce the long waiting lists for transplants." Yarmush and Uygun are both on the faculty of Harvard Medical School.