One Way to Increase the Supply of Kidneys
Animal organs could save people’s lives, but which patients should participate in trials?
- By
- June 10, 2025
- CBR - Health Care
More than 100,000 men, women, and children are on the organ transplant waiting list in the United States, and most of them—nearly 90,000, as of March 2025—need a kidney. People wait years to receive a deceased-donor kidney, and only a third of those waiting ever receive one. The rest become too sick to survive a transplant, or die.
One way to increase the supply of kidneys is through xenotransplantation, the use of animal organs in human patients. Doctors have so far only transplanted genetically modified pig kidneys into critically ill or brain-dead human patients, but the results have been encouraging enough that, under Food and Drug Administration approval, the experimental treatment will be offered to more patients starting this year, according to Chicago Booth’s Bariş Ata and a team of researchers, who have developed a model that identifies which patients should be prioritized to participate in these human clinical trials. The team also includes New York University’s Robert A. Montgomery, Booth PhD student Yucel Naz Ozyoruk, NYU’s Brendan Parent, and University of Colorado’s Jesse D. Schold.
“Organ shortage remains the greatest unmet need in transplantation” despite decades of effort in the medical community, they write. The United Network for Organ Sharing, a nonprofit regulated by the federal government, arranges for human organs to be recovered, tested, packaged, and transported to hospitals for transplant. As kidneys become available from donors who have died, they are allocated to a recipient on the basis of criteria such as blood type, medical urgency, and time spent on the waiting list. The average wait time to receive a deceased-donor kidney is about four years, according to UNOS.
But pigs are a promising source of kidneys for several reasons, the researchers explain. Human and pig kidneys are similar in size, placement, development, and function. Doctors can easily edit pig genes to account for the interspecies incompatibilities that might affect a transplant. Montgomery, one of the researchers on the study team and a pioneer in the field, performed the first transplant of a kidney from a gene-edited pig in 2021.
The National Kidney Registry estimates that a deceased-donor kidney from a human typically remains viable for 10–15 years after it is transplanted into another person, but no one yet knows exactly how long a xenokidney will last. In one study, a monkey survived for more than two years with a gene-edited pig kidney.
Pig kidney now, human one later
The research finds that more patients would survive longer if they were to receive a pig kidney but be prioritized for a human kidney if the first transplanted organ were to fail.
The researchers built a model that considers the risks and benefits of xenotransplantation. For example, patients have to be healthy enough to undergo the procedure, Ata says, explaining that a doctor wouldn’t offer a xenotransplant to someone with a life expectancy of five years if the model were to put the life expectancy of a xenokidney at two years.
The model analyzed patient and donor demographics, health and treatment histories, and long-term patient and organ transplant survival outcomes from the United States Renal Data System and the Scientific Registry of Transplant Recipients. The researchers fed it all available data from kidney transplant candidates, donors, and recipients between October 1987 and September 2021.
The ideal candidate for the clinical trials is aged 50–65, has diabetes, and has been waiting more than three years for a kidney, according to the model. Within that group, people with blood types B and O are particularly likely to have longer wait times—because B is a rarer blood type, and a person with type-O blood can only receive an organ from other type-O individuals. While people with diabetes who are older than 65 and have been waitlisted at least three years could also benefit from xenotransplantation, younger people might be better equipped to undergo surgery and endure the immunosuppressive drugs required to maintain the xenokidney and better suited to rejoin the waitlist if the xenograft fails, Ata says.
Someone waiting for a deceased-donor kidney might be wary of leaving the UNOS waitlist to receive a xenotransplant, so the researchers also modeled how different incentive offers might affect patient participation. According to the analysis, patients would be unmoved by an offer to remain on the kidney waitlist but be marked as “inactive,” even if that means they continue to accrue time and keep their place in line should they need another kidney. However, granting trial participants the same priority given to people who have previously donated a kidney, or moving patients with a failing pig kidney to the top of the list to receive a human kidney, would increase the pool of trial participants, according to the research. These two incentive offers would require policy action from UNOS, Ata says.
Identifying a big group of eligible patients is critical for studying the viability of xenotransplantation, the researchers say, since various medical, logistical, and geographic constraints could further shrink the pool of candidates. If the trials demonstrate that xenotransplantation is successful, they note, patients with kidney failure could one day elect to receive a xenokidney instead of going on dialysis, an intensive treatment that typically requires visiting a medical facility three times a week. “Right now, we know nothing,” says Ata of the potential of xenotransplantation. “One day it could be like getting a human kidney. This could open a whole range of possibilities.”
Bariş Ata, Robert A. Montgomery, Yucel Naz Ozyoruk, Brendan Parent, and Jesse D. Schold, “Patient Selection for Xenotransplant Human Clinical Trials: A Data-Driven Approach,” Transplantation, April 2025.
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