Early BCR::ABL1 Reduction as a Predictor of Deep Molecular Response in Pediatric Chronic-Phase Chronic Myeloid...

Background: Tyrosine kinase inhibitors (TKIs) have transformed the prognosis of chronic myeloid leukemia (CML), but pediatric patients face unique challenges due to prolonged exposure. Early molecular response (EMR, BCR::ABL1 ≤ 10% at 3 months) is a recognized predictor of favorable outcomes in adults and has been correlated with improved responses in children. However, its relationship with achieving deep molecular remission (DMR, BCR::ABL1 ≤ 0.01%) in pediatric CML remains unclear. Methods: We performed a single-center, retrospective analysis of 103 pediatric patients with chronic-phase CML treated with frontline TKIs. Among them, 88 were evaluable for molecular response. BCR::ABL1 transcript levels were quantified by real-time quantitative PCR on the International Scale, and molecular responses were assessed. Associations between early molecular dynamics and long-term outcomes were evaluated using Kaplan-Meier and cumulative incidence analyses. Results: At 3 months, 64.8% achieved EMR. Early responders had significantly higher MMR rates at 12 months (80.8% vs. 5.6%; p = 0.00018) and DMR at 24 months (70.4% vs. 42.2%; p = 0.029). The ≥0.45-log reduction in BCR::ABL1 transcripts at 3 months predicted shorter times to MMR (median 11 vs. 29 months) and DMR (18 vs. 50 months), as well as higher overall MMR (p = 0.011) and DMR (p = 0.014) incidences. Bone marrow fibrosis correlated with inferior molecular outcomes (p = 0.017 for MMR). Conclusions: Early BCR::ABL1 decline kinetics independently predict molecular depth in pediatric CML. Quantitative early transcript reduction may guide risk-adapted management and optimize long-term TKI strategies in children.