Murine Model of Leukemia Relapse to Induction Chemotherapy for Acute Lymphoblastic Leukemia.

Acute lymphoblastic leukemia (ALL) is a neoplasm of immature lymphoid precursor cells and one of the most common childhood malignancies. Although cure rates in pediatric ALL approach 90%, relapse and drug resistance remain challenges. In this context, studying cancer biology in vivo is essential, and patient-derived xenografts (PDX) using murine models are widely used for this purpose. However, PDXs from relapsed samples - or compared to diagnostic ones - do not fully assess the mechanisms behind relapse initiation and progression. This protocol relies on generating B-ALL PDX relapses in mice, allowing collection of leukemic cells throughout the process of acquiring drug resistance. Animals engrafted with B-ALL PDX cells are treated with a combination of four drugs commonly used in ALL therapy, representing distinct mechanisms of action: dexamethasone, vincristine, L-asparaginase, and daunorubicin. After a 4-week remission induction phase, animals are monitored for relapses during a resting period. The cycle of treatment and relapse is repeated until the time to relapse shortens, serving as a surrogate for increased in vivo drug resistance. To explore mechanisms underlying relapse and resistance, leukemic cells are collected before and after each treatment cycle for phenotypic and genetic analysis. To validate the model, different pediatric ALL samples were tested, including both good and poor responders, as assessed by the presence of blasts in peripheral blood after 7 days of corticosteroid therapy and by minimal residual disease (MRD) at days 15, 33, and 78/96. The results observed in mice were consistent with patient data, confirming the model's accuracy in mimicking the clinical response to chemotherapy. This approach enables the dynamic study of relapse and drug resistance in B-ALL, supporting mechanistic investigations and the development of novel therapeutic strategies.