A small company called Loxo Oncology thinks it can treat every cancer patient that harbors a specific, unusual, genetic mutation. The problem: finding them.

The company’s drug, which could be on the market as early as next year, treats cancers with a mutation in a gene called TRK. That could be welcome news for patients desperate for a precision strike on their tumors. But the mutation is so rare—and gene testing is still uncommon enough—that the question is how many patients who need it will get the drug.

Drugs like Loxo’s gained attention last month, when the U.S. Food and Drug Administration, for the first time, gave a green light for doctors to prescribe another cancer drug, Keytruda, based on specific genetic features of cancers rather where in the body where the disease originated. 

The drug being developed by Connecticut-based Loxo could be the next such targeted medicine in line for possible FDA approval.

But to prescribe such drugs, doctors will have to know the genetic makeup of a tumor—and so far in the U.S. only 12 percent of late stage metastatic cancer patients are getting tests that screen broadly for genetic mutations, according to Cambridge, Massachusetts-based genetic testing company Foundation Medicine. 

Mutations in TRK, a gene that provides instructions for making a protein necessary for the development and survival of certain nerve cells, was first discovered as the culprit behind some colon cancers in 1982. Scientists wouldn’t find out until 2013 and 2014 that TRK mutations occur in other types of cancer, including of the breast, lung, and skin.

In 2015, Loxo  began the first clinical trial for a cancer drug targeting TRK in 17 different types of tumors . “It doesn’t matter what the context is, the drug seems to be working in the same way because the biology is comparable,” says Loxo CEO Josh Bilenker.

The company started treating patients in spring 2015 after partner medical centers used genetic testing to identify a handful of people with TRK mutations. It took 15 types of tests and two years to enroll 55 patients.

This past weekend at the American Society of Clinical Oncology annual meeting in Chicago, Loxo announced striking results—though in a small number of patients—for its lead drug, larotrectinib, which is designed to turn off the signaling pathway that allows TRK fusion cancers to grow. In 50 adults and children with more than a dozen different types of cancer, the drug shrank tumors in 38 patients, or 76 percent. In 30 of those patients still on the drug, their tumor growth halted completely and no new tumors formed after 12 months of treatment.

Bilenker says many of the patients enrolled in Loxo’s trials have already tried other treatments that have failed. The drug, taken twice daily in pill form or as a drinkable liquid for children, has so far been tested on patients ranging from one month old to 80 years old, Bilenker says.

Still, TRK mutations occur in a small number of cancer patients—anywhere from 1,500 to 5,000 in the U.S. annually, according to Loxo’s estimates.

Loxo plans to seek FDA approval in both adult and pediatric patients by the end of the year. If approved, Bilenker says, the company’s commercialization strategy will rely on creating awareness about this uncommon mutation. They’ll educate cancer doctors about what types of genetic tests are able to spot this abnormality and what to do with positive results, he says. 

To find patients, Loxo will need to convince more doctors to order comprehensive tests that screen multiple genes at once, including TRK. These advanced genetic tests, which can cost $5,000 or more, are offered by companies like Foundation Medicine, Caris Life Sciences, and Cancer Genetics. The problem is, insurers still consider the tests “experimental” and don’t routinely cover them, meaning patients are often stuck picking up the bill.

Dan Rhodes, CEO at Strata Oncology, a tumor sequencing company, says genetic sequencing is “absolutely the bottleneck” in finding patients who will benefit from targeted cancer drugs. His company is giving away free testing to cancer patients to help match them to experimental treatments being studied in clinical trials .

With Keytruda, there’s some good news, says Rhodes. The genetic attributes it targets occur frequently in patients with Lynch syndrome, an inherited disorder that increases the risk of several cancers, particularly colorectal cancer. Genetic testing in families with a history of Lynch syndrome has been common for several years, so Lynch syndrome patients already know they have the genetic mutation that’s treatable with Keytruda.

Robert Doebele, an oncologist and TRK researcher at Colorado State University, says his worry is that a lot of physicians aren’t ordering comprehensive genetic testing for cancer patients.

“These drugs appear to be very effective for certain types of mutations, but sometimes it’s hard to convince oncologists to test for a mutation that might only occur in 1 percent of their patients,” he says. “My sense is that they’re worried about their patients getting a $5,000 bill.”

Given the low chance of patients having the TRK gene, it’s unclear whether insurers will begin covering genetic testing if Loxo's larotrectinib is approved.

Bilenker hopes his company’s drug will become a success story that helps get insurers on board to cover advanced genetic testing in more patients. “This could be a poster child for what the future looks like.”