Off-target resistance to larotrectinib in two patients with NTRK fusion-positive pediatric solid tumors.

Neurotrophic tyrosine receptor kinase (NTRK) gene fusions have been identified in various pediatric malignancies.1Amatu A. Sartore-Bianchi A. Siena S. NTRK gene fusions as novel targets of cancer therapy across multiple tumour types.ESMO Open. 2016; 1e000023Abstract Full Text Full Text PDF Scopus (406) Google Scholar Larotrectinib is a highly selective small-molecule inhibitor targeting all three TRK proteins. Although some trials have demonstrated impressive responses to larotrectinib,2Drilon A. Laetsch T.W. Kummar S. et al.Efficacy of larotrectinib in TRK fusion-positive cancers in adults and children.N Engl J Med. 2018; 378: 731-739Crossref PubMed Scopus (1730) Google Scholar,3Laetsch T.W. DuBois S.G. Mascarenhas L. et al.Larotrectinib for paediatric solid tumours harbouring NTRK gene fusions: phase 1 results from a multicentre, open-label, phase 1/2 study.Lancet Oncol. 2018; 19: 705-714Abstract Full Text Full Text PDF PubMed Scopus (380) Google Scholar most cancers develop resistance to it over time. Herein, we report two rare cases of pediatric solid tumors that were resistant to larotrectinib via an off-target mechanism for the first time. The first patient (Figure 1A) was a 5-year-old female with a large mass located in the left temporal parietal lobe, which was diagnosed as an infant-type hemispheric glioma. Despite surgery, radiotherapy, and chemotherapy, the tumor progressed. Since a TP53–NTRK1 fusion was identified by whole-exome sequencing (WES), the patient was treated with larotrectinib. An overall 65.8% reduction in the sum of products of perpendicular diameters of enhancing lesions was achieved according to the Response Assessment in Neuro-Oncology criteria. Unfortunately, the disease progressed after 10 cycles of treatment. Targeted gene panel sequencing revealed EGFR amplification and CDKN2A/B homozygous deletion in resistant tumors. Subsequently, she received entrectinib combined with programmed cell death protein 1 inhibitor and anlotinib, but did not benefit from it. Finally, the patient died of pneumonia, 43 months after diagnosis. The second patient (Figure 1B) was an 11-year-old male presenting with an abdominal mass. Following surgery, a low-grade spindle cell neoplasm with NTRK1 rearrangement was confirmed. The tumor progressed despite surgery, chemotherapy, and apatinib and anlotinib treatments. Based on the LMNA/NTRK1 rearrangement detected by WES, he began larotrectinib treatment. He demonstrated a rapid clinical response, with an overall 63.8% reduction in target lesion diameters according to the RECIST 1.1. Nevertheless, the tumor progressed after 20 cycles of larotrectinib treatment, and WES of resistant tumor revealed emerging BRAF/CREB3L2 rearrangement (Figure 1C), intergenic/BRAF rearrangement (Figure 1D), BRAF-exon9_exon18 amplification, and CDKN2A/B homozygous deletion. The family declined the recommended treatment of larotrectinib combined with mitogen-activated protein kinase kinase or cyclin-dependent kinase 4/cyclin-dependent kinase 6 inhibitors due to lack of funds. Consequently, the patient received a second-generation TRK inhibitor treatment. Unfortunately, this treatment had no therapeutic effect. The patient died 6 months later, 48 months after diagnosis. Resistance to TRK inhibitors is driven by both on-target and off-target mechanisms.4Cocco E. Scaltriti M. Drilon A. NTRK fusion-positive cancers and TRK inhibitor therapy.Nat Rev Clin Oncol. 2018; 15: 731-747Crossref PubMed Scopus (831) Google Scholar The on-target mechanisms are mediated by amino acid mutations in the NTRK kinase domain, resulting in the spatial blockade of drug binding, which may be optimized by second-generation TRK inhibitors. The off-target mechanisms are mediated by alterations in bypass signaling pathways, of which mitogen-activated protein kinase (MAPK) pathway-activating genomic alterations have been demonstrated in adults.5Cocco E. Schram A.M. Kulick A. et al.Resistance to TRK inhibition mediated by convergent MAPK pathway activation.Nat Med. 2019; 25: 1422-1427Crossref PubMed Scopus (124) Google Scholar These resistance mechanisms were consistent with our findings, including EGFR amplification detected in the first case and three distinct BRAF alterations detected in the second case; however, these factors were not detected before treatment with larotrectinib. Hence, we inferred that the reactivation of the MAPK pathway was the primary cause of larotrectinib resistance in these two cases. These novel genetic alterations enrich the known understanding of MAPK pathway-mediated off-target resistance mechanism, which have not been reported previously. We thank the children and children’s parents for allowing us to present their cases. This work was supported by the National Key Research and Development Program of China [grant number 2022YFC2705005], the National Natural Science Foundation of China [grant number 82203303], the Basic and Applied Basic Research Foundation of Guangdong Province [grant number 2021A1515110234], and the Medical Science and Technology Research Fund Project of Guangdong Province [grant number A2022079].