Today, Connor Pearcy is an average 7-year-old boy. He loves dinosaurs, Pokemon and tetherball — so much so that the Pearcy family home has a tetherball setup in its backyard.

But until about two years ago, Connor also had something most kids don’t: A tumor just below his left knee that resisted chemotherapy and surgery and forced Connor’s family to consider amputating his leg.

Connor’s tumor was wiped out by a drug called larotrectinib, a unique new medicine that on Monday was approved by the FDA to treat a huge variety of soft tissue tumors.

Larotrectinib was developed by Connecticut-based Loxo Oncology and has been tested at sites around the U.S. for several years. It is approved to treat both children and adults whose cancer expresses a specific genetic change that occurs in an estimated 2,500 people each year.

It has the potential to wipe out stubborn cancers with few to no negative side effects, making it a unique entrant into the cancer treatment world.

Seattle Children’s Hospital has operated the largest arm of the drug’s pediatric trial, enrolling more patients than any other site, including Connor.

“Just giving larotrectinib alone, we can physically see with our own eyes — and then also confirm with doing diagnostic imaging— that tumors are really melting away,” said Dr. Katie Albert. Albert is a pediatric physician and researcher who has overseen Seattle Children’s arm of the larotrectinib trial. She said the drug could have big ramifications for children like Connor.

“We really are pretty dramatically, potentially, changing the standard of care for these patients,” Albert said.

FDA granted accelerated approval to larotrectinib for treatment of certain patients with solid tumors with NTRK gene fusion without a known acquired resistance mutation. See drug label for details: https://t.co/uVflcDQN1N — FDA Oncology (@FDAOncology) November 26, 2018

Most patients who have the kinds of tumors that larotrectinib treats currently need surgery, radiation, chemotherapy or a combination of the three to tackle the disease.

That was the case for Connor, who was born with his tumor. His doctors weren’t sure if it was a benign growth or a cancerous one that could spread. Amy Pearcy, a physician and Connor’s mother, said he had chemotherapy for the first year of his life followed by surgery to attempt to remove it.

“It’s a lot to deal with, probably more for us than for him,” Pearcy said. Despite best efforts, nothing significantly impacted Connor’s tumor.

Then the Pearcy family discovered that Connor might qualify for the larotrectinib trial. Larotrectinib works by targeting a kind of DNA change that happens in many different kinds of tumors.

“That change is what we call a translocation or a gene fusion, which is where chromosomes break apart and then come back together,” Albert explained. Specifically, the drug targets changes in NTRK genes.

Connor’s doctor ran a genetic test on his tumor and found that he did have the genetic marker that the drug targets. He started the treatment just a few weeks after his first day of Kindergarten, just a simple pill. Pearcy said he didn’t like the aftertaste of the medication, so he washed it down with a piece of string cheese.

“The first MRI they did after he started, which was two months later, showed an 80 percent reduction in the total volume of the tumor,” Pearcy said. That was the first time anything had shrunk Connor’s tumor so dramatically.

“By the second MRI, four months later, it was completely undetectable by MRI,” she said. Although it’s hard to tell if his tumor is completely gone, his MRI scans have appeared clean since that point, almost two years ago.

Connor stopped taking the medication shortly before starting school in the fall, and so far his tumor has not returned.

We can physically see with our own eyes — and then also confirm with doing diagnostic imaging— that tumors are really melting away.

“It’s been an enormous relief,” Pearcy said, although “there’s always going to be the thought that it could come back.” Connor was one of the first patients to stop taking the medication, so the Pearcy family doesn’t have a good idea of how likely it is that his tumor may come back.

Pearcy also said Connor didn’t experience any side effects from the treatment, which Albert said is common for patients.

“Most of what we see in our patients actually aren’t symptoms, they’re just mild laboratory abnormalities that don’t’ affect patients at all in their day-to-day life,” she said. “In terms of symptoms that we have seen, they tend to be very mild,” like constipation.

Larotrectinib is a case study in a new generation of cancer treatments that target specific biological markers of cancer cells, at once making them more effective and reducing the harmful side effects common to treatments like chemotherapy.

“That’s really the focus of all research in cancer therapeutics now, really focusing on the biology and trying to develop agents that can target these changes in the cancer genome,” Albert said.