TRK fusion cancer is a rare malignancy seen in a wide variety of adult and childhood tumor types. Among pediatric malignancies, infantile fibrosarcoma and congenital mesoblastic nephroma are rare, but have high NTRK gene fusion frequency. Other sarcomas and pediatric high-grade gliomas, for example, are less rare but have low NTRK gene fusion frequency. Larotrectinib, a first-in-class and the only selective TRK inhibitor, has high potency against the 3 NTRK genes that encode the neurotrophin receptors. It is highly selective and has limited inhibition of the other kinases. The US Food and Drug Administration approved larotrectinib for the treatment of patients with solid tumors harboring NTRK fusions. Cornelis Martinus van Tilburg, MD, of the Hopp Children’s Cancer Center, Heidelberg University Hospital, and German Cancer Research Center in Heidelberg, Germany, presented the findings.
Investigators enrolled 38 children and adolescents younger than 18 years from the SCOUT (NCT02637687) and NAVIGATE (NCT02576431) studies of larotrectinib who had non-central nervous system (CNS) TRK fusion cancers. Not all patients had the recommended phase 2 dose, Dr. van Tilburg pointed out, but most did. Hence, 29 of the 38 patients received the 100 mg/m2 twice-daily, phase 2 dose until progression, withdrawal, or unacceptable toxicity.
Patients were young, with a median age of 2.3 years (range, 0.1 to 14.0 years). Almost two-thirds (61%) had prior surgery, 11% had prior radiotherapy, and 68% had prior systemic therapy. For 12 patients, larotrectinib was their first systemic therapy. The predominant tumor types were infantile fibrosarcoma (47%) and other soft tissue sarcoma (42%). And 47% of patients had NTRK3 fusions with ETV6, most of which were infantile fibrosarcoma.
Efficacy
Thirty-four patients were evaluable, and 32 had a reduction in tumor size, for an ORR of 94%, CR of 35%, and PR of 59%. Two patients with infantile fibrosarcoma had pathologic CRs—after treatment, no fibroid tissue in the tumors could be found. Median time to response was 1.8 months, median duration of treatment was 10.24 months, and 33 of 38 patients (87%) remained on treatment or underwent surgery with curative intent. As of the data cutoff of July 30, 2018, the secondary endpoints were not yet reached. However, 84% of responders were estimated to have a response duration of a year or more, and progression-free and overall survival looked very promising, according to Dr. van Tilburg.
Adverse events were primarily grades 1 and 2. The grades 3 and 4 treatment-related adverse events were quite few and consisted of increased alanine aminotransferase, decreased neutrophil count, and nausea. Longer follow-up of the patient safety profile is required, particularly since NTRK has multiple roles in neurodevelopment. The investigators recommended that routine testing for NTRK gene fusions in pediatric patients with cancer be conducted in appropriate clinical contexts.
In a discussion after the presentation, Daniel Alexander Morgenstern, MB BChir, PhD, of Great Ormond Street Hospital, London, UK, said that in many ways, the NTRK inhibitors have become the new poster child for precision oncology in pediatrics because of “these really spectacular results” with larotrectinib [and entrectinib]. One of the questions he raised regarding larotrectinib was the issue of CNS penetration, since patients with CNS cancer were not enrolled in the trial and preclinical data suggest limited CNS penetration for larotrectinib.
SOURCE: van Tilburg CM, et al. J Clin Oncol 37, 2019 (suppl; abstr 10010).
The studies were funded by Loxo Oncology, Inc., and Bayer AG.
Disclosures relevant to this presentation include consulting or advisory roles for Bayer for Drs. van Tilburg and Morgenstern. A few coauthors also had consulting/advisory roles or research funding from various companies, including Loxo and Bayer.