TRK inhibitor shows ‘striking’ activity, durability across diverse adult and pediatric cancers

CHICAGO – Larotrectinib, an oral inhibitor of tropomyosin receptor kinase (TRK), has durable efficacy across diverse adult and pediatric cancers that harbor a genetic aberration known as TRK fusion, finds an analysis of three trials reported at the annual meeting of the American Society of Clinical Oncology.

Fusion of a TRK gene with an unrelated gene leads to uncontrolled signaling in the TRK pathway, potentially causing tumor growth and addiction to this input, lead author David Hyman, MD, chief of early drug development at Memorial Sloan Kettering Cancer Center in New York explained in a press briefing.

^{Susan London/Frontline Medical News}

Dr. David Hyman

Dr. Hyman and his colleagues analyzed data from 55 patients having 17 discrete types of advanced cancer harboring TRK fusions who were treated with larotrectinib in phase I and II trials. Results showed an overall response rate of 76%, and the large majority of responses were still ongoing at 12 months.

“I believe these data support larotrectinib as a potential new standard of care for these patients,” he said. “However, I want to emphasize that really recognizing this benefit in the community will require that we test patients more universally for the presence of TRK fusions or other tumor-agnostic biomarkers, such as microsatellite instability.”

On the basis of these promising data, the drug’s manufacturer, Loxo Oncology, plans to submit a New Drug Application to the Food and Drug Administration later this year or early next year. Larotrectinib has already been granted both orphan drug designation (for drugs used to treat rare conditions) and breakthrough therapy designation (for drugs used to treat serious conditions showing greater efficacy than available therapies).

A randomized trial pitting larotrectinib against other therapies is unlikely given the low prevalence of TRK fusions, the lack of treatment options for the fairly heavily pretreated trial patients, and the drug’s impressive performance, according to Dr. Hyman.

“The efficacy is so striking that it really exceeds almost any existing standard of care for solid tumors,” he elaborated. “There is hardly any chemotherapy or targeted therapy that has a response rate or durability that looks like larotrectinib in these patients.”

Expert perspective

The data for larotrectinib “really bring us into a new era where treatment is truly based on mutation, not location,” said Sumanta Kumar Pal, MD, a medical oncologist at City of Hope, in Duarte, Calif. “When I was in training, which was not too long ago, it really would have been a pipe dream to think that we could have treated cancers independent of their site of origin. … With the data presented by Dr. Hyman for larotrectinib, we may now be poised to treat many cancers in a manner that is agnostic of their site of origin and that is instead based on molecular criteria.

^{Susan London/Frontline Medical News}

Dr. Sumanta Kumar Pal

TRK testing

Several next-generation sequencing–based tests already available clinically can pick up TRK fusions, Dr. Hyman pointed out. “But it is important for the ordering physician to understand whether the tests they are ordering includes fusion detection and, if it’s an option, to select it. Otherwise, they will not find TRK fusions.

“The list price for these tests is in the kind of low thousands of dollars, which equates essentially to a PET scan for the cancer patient,” he noted. In cancers where sequential single-gene testing is already being done as standard of care, there is “minimal” incremental cost of instead using comprehensive testing that would detect TRK fusions.

Oncologists should be aware that obtaining test results can take weeks, Dr. Hyman stressed. “My personal opinion is that this [testing] should be more broadly adopted and should be adopted at a point in the patient’s treatment … [so that they] don’t become too sick, as we see in our own experience as well, and don’t have an opportunity to be treated even when the test results come back positive. So I would generally advocate early testing.”

Study details

For the study, which was funded by Loxo Oncology, the investigators analyzed data from three trials in which patients with advanced TRK fusion–positive solid cancers received larotrectinib (LOXO-101): a phase I trial among 8 adult patients, a phase I/II trial among 12 pediatric patients (SCOUT), and a phase II “basket” trial among 35 adult and adolescent patients (NAVIGATE).

“I want to emphasize that these patients were identified by local testing,” Dr. Hyman noted. “We did not perform central screening to find the TRK fusions, and in fact, 50 different laboratories identified the 55 patients. So this in a sense really represents the real-world identification of these patients.”

In an integrated analysis, the overall rate of confirmed response as assessed by investigators was 76%, with complete response in 12% of patients and partial response in 64%. Two patients had such deep tumor regression that they experienced downstaging enabling them to undergo potentially curative surgery. Efficacy was consistent regardless of tumor type, which TRK gene was affected, and the fusion partner gene.

Median time to response was 1.8 months. “This is actually just a reflection of when the first scan was obtained. But in the clinic, patients reported dramatic improvement of their symptoms within days of beginning therapy,” Dr. Hyman said.

With a median follow-up of 5.8 months, the median duration of response was not yet reached. Fully 79% of responses were still ongoing at 12 months. Median progression-free survival was likewise not reached; the 12-month rate was 63%.

The leading treatment-emergent adverse events were fatigue (38%), dizziness (27%), nausea (26%), and anemia (26%). “This is an extremely well tolerated therapy with only 13% of patients requiring any form of dose modification and not a single patient discontinuing due to adverse events,” he said.

It is unclear why some patients had apparent primary resistance to larotrectinib, but their TRK fusion test results may have been incorrect, Dr. Hyman speculated. Six patients developed acquired resistance to larotrectinib; five of them were found to have an identical resistance mutation, and two went on to receive and have a response to LOXO-195, a next-generation TRK inhibitor that appears to retain activity in the presence of this mutation (Cancer Discov. 2017 June 3. doi: 10.1158/2159-8290.CD-17-0507).

Dr. Hyman disclosed that he has a consulting or advisory role with Atara Biotherapeutics, Chugai Pharma, and CytomX Therapeutics, and that he receives research funding from AstraZeneca and Puma Biotechnology.