Larotrectinib for Newly Diagnosed Infantile Fibrosarcoma and Other Pediatric Fusion-Positive Solid Tumors (Children's Oncology Group ADVL1823).

Purpose: The TRK inhibitor larotrectinib is US Food and Drug Administration approved for fusion-positive solid tumors that lack a satisfactory alternative or have progressed after treatment but has not been systematically studied as a frontline therapy with a defined duration of treatment. ADVL1823 evaluated larotrectinib in patients with newly diagnosed fusion-positive solid tumors with response-adapted duration of therapy and local control.

Methods: Patients received larotrectinib twice daily in 28-day cycles for a predefined duration of treatment, ranging from 6 to 26 cycles depending on response to therapy and surgical resectability. The primary end point was the objective response rate (ORR) within six cycles in patients with infantile fibrosarcoma (IFS); patients with other histologic diagnoses were analyzed in a separate cohort. Secondary objectives included event-free survival (EFS) and overall survival (OS).

Results: Thirty-three patients were enrolled: 18 with IFS and 15 with other solid tumors. The ORR within six cycles was 94% (17/18; 95% adjusted CI, 72.7 to 98.6) among children with IFS and 60% (9/15; 95% CI, 32.3 to 83.7) among children with other solid tumors. Six percent (2/33; 95% CI, 0.7 to 22.2) patients developed progressive disease while on therapy. Two-year EFS and OS among these groups were 82.2% (95% CI, 54.3 to 93.9) and 93.8% (95% CI, 63.2 to 99.1) for IFS and 80% (95% CI, 50.0 to 93.1) and 93.3% (95% CI, 61.3 to 99.0) for other solid tumors, respectively. Patients undergoing surgical resection of their tumor had prolonged EFS, with only 1 of 16 such patients experiencing disease progression. Four of 33 patients had dose-limiting toxicities.

Conclusion: Larotrectinib is highly active in patients with newly diagnosed fusion-positive solid tumors. Larotrectinib should be a frontline option for patients with IFS and other fusion-positive solid tumors. Local control with surgical resection remains important in the treatment of patients with IFS.