Clinical response to larotrectinib in adult Philadelphia chromosome-like ALL with cryptic ETV6-NTRK3 rearrangement.

Philadelphia chromosome (Ph)-like acute lymphoblastic leukemia (ALL) is a subtype of Ph-negative ALL that molecularly resembles Ph-positive ALL. It shares the adverse prognosis of Ph-positive ALL, but lacks the BCR-ABL1 fusion oncogene. Instead, Ph-like ALL is associated with alternative mutations in signaling pathways. We describe a case of Ph-like ALL that harbored 2 genomic alterations, which activated signaling, an NRASGly12Asp mutation, and an ETV6-NTRK3 rearrangement. Initially, the NRAS mutation was detected at high frequency, whereas the gene fusion was only detectable with a targeted next-generation sequencing-based fusion assay, but not by fluorescence in situ hybridization analysis. The disease failed to respond to multiagent chemotherapy but investigational CD19-directed chimeric antigen receptor T-cell therapy resulted in a complete remission. However, the leukemia relapsed after 6 weeks. Intriguingly, the NRAS mutation was extinguished during the chimeric antigen receptor T-cell therapy and did not contribute to the relapse, which was instead associated with a rise in ETV6-NTRK3. The relapsed leukemia progressed with further chemo- and immunotherapy but was controlled for 6 weeks with substantial leukemic cytoreduction using the TRK inhibitor larotrectinib. Unfortunately, recovery of normal hematopoiesis was only marginal and the patient eventually succumbed to infections. These results demonstrate that larotrectinib has clinical activity in ETV6-NTRK3-associated Ph-like ALL.