Two US biotech companies say the Food and Drug Administration has cleared them to conduct clinical trials of their gene-edited pig kidneys for human transplants.

United Therapeutics along with another company, eGenesis, have been working since 2021 on experiments implanting pig kidneys into humans: initially brain-dead patients and more recently living recipients.

Advocates hope the approach will help address the severe organ shortage. More than 100,000 people in the United States are awaiting transplants, including over 90,000 in need of kidneys.

United Therapeutics?s approval, announced Monday, allows the company to advance its technology toward a licensed product if the trial succeeds.

The study authorization was hailed as a ?significant step forward in our relentless mission to expand the availability of transplantable organs,? by Leigh Peterson, the company?s executive vice president.

The trial will initially enroll six patients with end-stage renal disease before expanding to as many as 50, United Therapeutics said in a statement. The first transplant is expected in mid-2025.

Meanwhile, rival eGenesis said it had received FDA approval in December for a separate three-patient kidney study.

?The study will evaluate patients with kidney failure who are listed for a transplant but who face a low probability of receiving a deceased donor offer within a five-year timeframe,? the company said.

Xenotransplantation ? transplanting organs from one species to another ? has been a tantalizing yet elusive goal for science.

Early experiments in primates faltered, but advances in gene editing and immune system management have brought the field closer to reality.

Pigs have emerged as ideal donors: they grow quickly, produce large litters, and are already part of the human food supply.

United Therapeutics said trial patients would be monitored for life, assessing survival rates, kidney function, and the risk of zoonotic infections ? diseases that jump from animals to humans.

Currently, there is only one living human recipient of a pig organ: Towana Looney, a 53-year-old from Alabama who received a United Therapeutics kidney on November 25, 2024.

She is also the longest-surviving recipient, having lived with a pig kidney for 71 days as of Tuesday. David Bennett of Maryland received a pig heart in 2022 and survived 60 days. (AFP)

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