Patient and patient-derived xenograft data on Larotrectinib in a pediatric B-cell ALL patient. (A) Treatment course and cerebrospinal fluid (CSF) cell count over time. The day of first relapse post-HSCT (day 50 post-HSCT) was set as day 0. (B) Larotrectinib plasma and CSF concentrations. (C) Preclinical efficacy of larotrectinib initiated at 2 time points (day +1 postinjection [“laro early”] and day +8 postinjection [“laro late”]) in an NSG xenograft model (n = 5 mice per group). Images of spleens, spleen volumes, infiltration of bone marrow (BM) and spleens (SP) by flow cytometry, and CNS infiltration by morphology. **P < .0001.