Originally Published: 16 AUG 23 14:17 ET

Updated: 17 AUG 23 01:49 ET

By Deidre McPhillips, CNN

(CNN) — New advancements in transplanting pig kidneys to humans, detailed by two separate research teams on Wednesday, mark key steps forward in the evolving field of xenotransplantation, the use of non-human tissues or organs to treat medical conditions in humans.

Researchers from the University of Alabama at Birmingham Heersink School of Medicine found that transplanted kidneys not only produced urine, they provided the “life-sustaining kidney function” of filtering waste, according to a research letter published in JAMA Surgery.

And in a news conference about an ongoing study, a team from New York University Langone Health highlighted the longer-term success of a transplant.

Both research teams used genetically modified pig kidneys that were transplanted into recipients experiencing brain death in what is considered pre-clinical human research.

Healthy kidneys filter a waste product called creatinine out of the blood. A measure of serum creatinine levels can help determine how well this process is working.

Other studies have demonstrated that this can occur when pig kidneys are transplanted in non-human primates. But creatinine comes from a chemical that supplies energy to muscles, and the amount can vary by muscle mass – and average adult humans are much bigger than other primates.

“Being able to show that these pig kidneys not only made urine but were also able to clear creatinine – and do that for an adult human – is absolutely critical and important before moving into a living person,” said Dr. Jayme Locke, director of UAB’s Comprehensive Transplant Institute and lead author of the new research.

“The goal of transplantation is not to just make urine. The kidneys have to be able to function and clear toxic substances from the body, and we were able to show that.”

The UAB study was conducted in a 52-year-old man who lived with hypertension and stage 2 chronic kidney disease. He was not named at the request of his family.

Before the transplant, serum creatinine levels were well above normal. But they were cut in half within 24 hours of the transplant and normalized by 48 hours, maintaining a normal range through the remainder of the study – which lasted seven days from beginning to end.

In human-to-human transplants, kidneys from living donors tend to work better and more quickly than those from deceased donors, Locke said, and the pig kidneys behaved much more like a live donor transplant.

“What that means is, these are great kidneys, and they’re going to provide really great function for living persons, hopefully in the not-too-distant future,” Locke said.

The research team from NYU Langone has not published the findings from its ongoing work but shared updates Wednesday.

The team has been monitoring pig kidney transplants in a brain-dead decedent – a man named Maurice Miller, known as Mo, who died of a brain tumor – for nearly two months. Some immunosuppressive drugs were used, but the pig’s thymus was also transplanted to help protect the kidneys from being attacked by the human immune system.

There has been “no evidence of rejection and normal renal function and clearance of toxins,” said Dr. Robert Montgomery, director of the NYU Langone Transplant Institute and chair of the surgery department. “The pig kidney appears to replace all of the important tasks that the human kidney manages.”

Researchers say that more work is needed, including studies in living human recipients, to establish whether pig kidney transplants could be a bridge or destination therapy for people with end-stage kidney disease, but they are hopeful about the progress being made.

“We’re gaining critical evidence about how well pig kidneys work in the human environment,” said Dr. Adam Griesemer, surgical director of the NYU Langone Pediatric Liver Transplant Program and the Living Donor Transplant Program, said at the news conference Wednesday.

“Over the last 20 years, we’ve gained a lot of information about how pig kidneys work to replace the functions in primates. But the critical question – ‘Will those data be translated exactly to the human recipients?’ – was unknown. And for the first time, we’re being able to supply that information. So hopefully this also give some assurance to the FDA regarding the safety of initiating phase one clinical trials.”

The vast majority of people waiting for an organ transplant need a kidney. About 89,000 people are on the waiting list, according to data from the US Department of Health and Human Services’ Organ Procurement and Transplantation Network.