(Bloomberg) -- The young boy was running out of time. His brain tumor was growing so fast that he had trouble putting words together. All standard cancer treatments had failed.

Then last summer the child started taking an experimental pill that targeted a rare genetic mutation found inside his tumor. Within months, the malignant growth started to shrink and his cognitive symptoms eased. Soon the tumor had all but disappeared. Today he’s still on the drug but back at school and doing well.

The child is one of the first to benefit from an experimental treatment from Roche Holding AG called entrectinib that is showing promise in some of the most difficult-to-treat child tumors, including those that grow on the brain. His progress was described by one of the doctors involved in his treatment.

While the drug targets only a minuscule subset of child cancers, so far all 12 patients whose tumors harbored the mutations targeted by the drug have responded to treatment, according to results released Wednesday by the American Society of Clinical Oncology. The study of 29 patients was released in advance of the group’s annual meeting starting on May 31.

“It’s stunning,” said Giles Robinson, a pediatric brain tumor oncologist at St. Jude Children’s Research Hospital in Memphis, Tennessee. He’s the lead author of the study, which was conducted at several hospitals across the U.S. “It speaks of the potency of targeted therapy when you have the target and the right drug.”

The results are preliminary and it’s too soon to tell how long the effects will last, but many of the young patients have been on treatment for more than 1O months. The drug’s potential side effects including weight gain, fatigue and taste disturbances.

Precision Medicine

The Roche data is “highly encouraging,” said Edward Kim, a oncologist at the Levine Cancer Institute in Charlotte, North Carolina, who wasn’t involved in the study. “That is how we dream precision medicine will work.”

The drug targets genetic alterations called fusions in three genes -- NTRK, ROS1 and ALK -- that drive aberrant cellular growth. In the trial, none of the 16 children whose tumors didn’t have the mutations responded to the drug, according to the results.

Robinson estimates that 1% to 2% of solid tumors in children might harbor the target mutations.

The Roche trial is part of a broader trend toward testing new targeted treatments in kids as well as adults. Historically, children have often been excluded from cancer trials of experimental drugs, which tended to focus on the much larger adult market. But now researchers are realizing that some of the same gene mutations driving cancerous growth in adults are also found in kids.

In addition, a federal law that will go into effect next year requires companies to test new cancer drugs in kids if the medicine is directed at a molecular target that’s also found in child tumors. It’s already having a big impact, said Robinson. Now, at big meetings like ASCO, companies “are coming out and seeking us,” he said.

The Roche drug is now under review by the U.S. Food and Drug Administration for both pediatric and adult use. The agency is scheduled to render a decision by August, according to Roche. If approved, Roche’s drug will compete with Vitrakvi from Bayer AG, which also targets NTRK and was approved last year for adults and children.

Bayer’s drug also has shown good results in children. In an analysis also to be presented at ASCO, Bayer said 94% of 34 children with NTRK mutations in its trials had responded to the drug, including 12 whose tumors completely disappeared.

Bayer’s drug costs $32,800 for a 30-day supply of capsules, according to a spokeswoman. Dosing of the liquid oral formulation used in children can cost between $11,000 and $32,800, based on the patient’s size.

To contact the reporter on this story: Robert Langreth in New York at rlangreth@bloomberg.net

To contact the editors responsible for this story: Drew Armstrong at darmstrong17@bloomberg.net, Mark Schoifet

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