By Elizabeth Cohen, Senior Medical Correspondent
(CNN) -- See Momo. See Momo run. See Momo run faster, farther and with far more vigor, energy and youthfulness, his owners say, now that he's taking a drug meant for humans with cancer.
"It's been remarkable," Paola Anderson said as she watched Momo, her 13-year-old white Pomsky, run around the backyard, keeping up with dogs a third his age.
The drug is called rapamycin. After nearly a decade of research showing that it makes mice live 60 percent longer, scientists are trying it out as an anti-aging drug in dogs and humans.
Rapamycin was discovered nearly 50 years ago in soil collected from Easter Island in the South Pacific and studied in a Canadian lab, and it's the most promising drug to fight aging that Arlan Richardson has ever seen.
A professor at the Reynolds Oklahoma Center on Aging, Richardson has been doing this kind of research for 40 years.
"It's the best bet we have," he said.
Now, scientists are moving forward and testing the drug in dogs.
Researchers at the University of Washington's Dog Aging Project gave rapamycin to 16 dogs and imaged their hearts.
"It started to function better. It started to look like a more youthful heart," said Matt Kaeberlein, co-director of the Dog Aging Project, who has presented this research at conferences but hasn't yet published it.
Those dogs took rapamycin for only 10 weeks. Here's what happened to Momo and his "brother," Sherman, who took it for much longer.
For many years, Momo and Sherman were regular visitors to the Laguna Pet Spa in Laguna Hills, California, getting baths and haircuts.
Then, on Christmas Eve 2010, their owner dropped them off and never came back.
Anderson, who runs the spa, said she called the owner, but her phone had been disconnected.
Anderson was horrified but not shocked. This was during Southern California's housing loan crisis, and this owner wasn't the first to abandon a dog to her care.
But this was more than a foreclosure. Sherman was sick, very sick.
On Christmas Day, the tiny 8-year-old Pomeranian was vomiting and howling in pain. Anderson rushed him to the emergency room. It was an attack of acute pancreatitis.
Anderson nursed him back to health and became, in her words, "mom" to Sherman and Momo.
The dogs had another mom, too: Anderson's partner, Sarah Godfrey, who was then living in Northern California but moved a few years later to Laguna Hills to live with Anderson and "the boys."
All was well until one day in May 2015, when Sherman fell over. He'd had a stroke.
"They gave him two weeks to live, unless he had surgery," Anderson remembers. Even with an operation, he had only a 20 percent chance of surviving.
Anderson and Godfrey were wary of surgery for a fragile dog who was already 13 years old; that's equivalent to 68 years old for a human being, according to the American Kennel Club.
Over the years, the couple had sought help from an herbalist for people problems, and now they desperately turned to him for help with their pooch problem. The herbalist did some research and came up with a possible treatment: rapamycin.
Excited but also skeptical, Anderson and Godfrey went online and ran across Kaeberlein and the Aging Dog Project, which was recruiting canine subjects for the rapamycin research.
They begged Kaeberlein to enroll Sherman, but the answer was no. Dogs had to be healthy and over 40 pounds. Sherman was neither.
The couple calculated their next step.
"We knew we could go to Mexico and get rapamycin or order it online, but we wanted to be guided by a veterinarian, by a professional," Anderson said.
That proved to tougher than they thought.
Five vets refused to prescribe the drug. Finally, a sixth vet agreed to prescribe rapamycin, but only after consulting with Kaeberlein to determine the best dose for Sherman.
By this point, a month after his stroke, Sherman was so weak, he had to be fed by hand and carried everywhere.
But rapamycin changed all that, Anderson and Godfrey said.
"The third day after taking rapamycin, he could eat on his own. By the seventh day, he was walking on his own," Anderson said.
Sixteen months later, the dog who had been given two months to live is still alive, and while clearly old, he's still active and able to run around the yard.
That got the moms thinking about Momo. He wasn't sick like Sherman, but at 13, he was getting old and achy and losing stamina. The couple decided to try rapamycin on him, too.
"Why not have your dog live longer if you can?" Godfrey said.
She said that within days of taking the drug, Momo was able to run for hours, whereas before, just a 30-minute walk would tire him out. On a hot summer day when CNN visited, he was able to keep up with Anderson's parents' dogs, who are four and five years old.
Anderson and Godfrey couldn't be happier.
"We call Sherman and Momo our rapamycin babies," Godfrey said.
Take a look at the labels for Rapamune, made by Pfizer, and Afinitor (PDF), made by Novartis, two drugs that are essentially the same as rapamycin and are used to treat cancer patients and organ transplant recipients. The list of things that can go wrong is long and horrifying: cancer, diabetes, infections and more.
"You have to be concerned about these side effects," Kaeberlein said. But that hasn't stopped him from doing research on the drug.
First, Kaeberlein thinks the side effects seen in cancer patients and transplant recipients might not be because of rapamycin per se but because those patients were very sick to begin with, because they were taking a whole host of other drugs as well, or both.
Secondly, he uses a much lower dose of the drug on his healthy dogs compared with the dose used on sick people.
Richardson, the aging expert at the University of Oklahoma, agrees with Kaeberlein. He's so convinced that he gave a low dose of rapamycin to his own dog, coincidentally named MoMo.
MoMo had a heart problem, and Richardson said it stabilized after he started taking rapamycin. He said the Tibetan terrier looks and acts younger than his 14 years, which would be around 80 or 90 in human years.
Plus, he said, there have been no side effects.
"We've been doing blood chemistries on her the whole time, and there's nothing bad," Richardson said.
The researcher was quick to note that one dog's experience did not constitute proven scientific data -- but he added that he's given rapamycin to tiny monkeys called marmosets and hasn't see any negative side effects for them, either.
But what about humans?
Rapamycin has had very limited testing in healthy humans. Novartis gave rapamycin to 218 elderly volunteers, and it enhanced their response to the flu vaccine by 20 percent.
The results "raise the possibility that (rapamycin) may have beneficial effects" on the decline in immune function that occurs naturally as we get older, the study authors wrote.
They reported that the side effects of rapamycin were "relatively well-tolerated." Severe side effects, they wrote, occurred at a "similar" rate as those experienced by the patients in the study who took a placebo, or a sugar pill.
Of the 53 patients on the lowest dose of rapamycin, 22 suffered some side effect, most commonly mouth sores.
Dr. Monica Mita, who's done her own research with drugs like rapamycin, said she thinks the side effects can be managed.
"It's really a matter of using the right dose and keeping an eye on the patients," said Mita, co-director of experimental therapeutics at Cedars-Sinai hospital in Los Angeles.
Kaeberlein, the professor of pathology at the University of Washington, is nervous about this article.
He wants readers to know that Momo and Sherman's story is a tale of two dogs -- that's it -- and not scientific evidence in any way, shape or form, especially considering the placebo effect, meaning Momo and Sherman's owners might subconsciously be seeing what they want to see because they believe so much in rapamycin and fought so hard for it.
He doesn't want Momo and Sherman's seeming success to "encourage dog owners to go off to their veterinarians and demand rapamycin."
And Kaeberlein, who's also president of the American Aging Association, is nervous for another reason, too.
He dislikes the term "anti-aging," as it conjures up images of snake oil salesmen peddling the fountain of youth.
Instead, he prefers to think in terms of treatments that will delay the onset of diseases of aging, such as dementia or heart disease. In mice, rapamycin has been shown to slow these two types of declines, as well as several others.
Over the next year, Kaeberlein will be studying rapamycin in a much larger group of dogs: about 150, compared with the 16 he studied earlier.
He said other groups are looking at doing more aging studies in rapamycin in humans, too.
It's been a long journey for the compound discovered more than half a century ago in the dirt of a South Pacific island.
"The rapamycin story is one of the most surprising, enticing, satisfying and unique stories in the history of medicine," Mita wrote in a medical journal five years ago. "And the end is not near."
CNN's John Bonifield contributed to this story.