WASHINGTON, D.C.: A Massachusetts hospital has marked a breakthrough in transplant medicine, announcing that a second New Hampshire man has received a pig kidney and is recovering well, just as U.S. regulators clear the way for full-scale clinical trials.

Bill Stewart, a 54-year-old athletic trainer from Dover, underwent the operation on June 14 at Massachusetts General Hospital. "I really wanted to contribute to the science of it," he told The Associated Press, calling himself "a little bit of a science nerd."

Stewart follows another patient from the state, Tim Andrews, who has lived more than seven months with a pig kidney, the longest known survival for such a transplant. The previous record for a gene-edited pig organ was 130 days.

The Food and Drug Administration has now approved eGenesis, a biotech firm, to begin a clinical study involving 30 dialysis patients aged 50 or older who are on the transplant list. A rival company, United Therapeutics, is preparing a similar trial.

"Right now we have a bottleneck" in human organ supply, said Dr. Leonardo Riella, a Mass General kidney specialist who will help lead the new trial. More than 100,000 Americans are waiting for a transplant, most for a kidney, and thousands die each year before one becomes available.

Scientists have turned to genetically engineered pigs, modifying their organs to look more "humanlike" and less likely to trigger the body's immune system. Early efforts — two hearts and two kidneys transplanted into very ill patients — were short-lived. More recent procedures, including Stewart's and Andrews's, are focused on relatively healthier candidates.

Stewart's kidneys had failed due to high blood pressure, and after two years on dialysis with no matching donor in sight, he applied for the experimental procedure. He even reached out to Andrews before making his decision. "Worst case scenario, they can always take it out," he said.

Since surgery, Stewart has begun easing back into work and recently returned to his old dialysis clinic to show patients what may be possible. "I'm doing all right and maybe kind of give some people some hope," he said.

Riella noted that both Stewart and Andrews needed adjustments to their anti-rejection drugs, but said even temporary success could transform care. "A year, hopefully longer than that – that's already a huge advantage," he said, explaining that pig kidneys could give patients precious time off dialysis while waiting for a human organ.

The field of xenotransplantation is still in its infancy, but researchers say the results so far — and now the launch of clinical trials — suggest that animal-to-human organ transplants may one day shift from experimental to routine.