Childhood relapsed acute lymphoblastic leukemia: Biology and recent treatment progress.

Acute lymphoblastic leukemia (ALL) is the most frequent cancer in children. Despite remarkable improvement in the prognosis of childhood ALL over the past few decades, the treatment of relapsed ALL is still challenging. The prognosis of first ALL relapse is associated with time of relapse after initial therapy, sites of relapse, and immunophenotype. More recently, response to treatment, which is evaluated by assessment of minimal residual disease (MRD), has been found to be clinically significant in relapsed ALL as well as in the initially diagnosed disease. Utilizing these factors, risk-oriented treatment stratification for first ALL relapse has been established. In the standard-risk group for first ALL relapse, intensification of conventional ALL-type therapy can provide a cure in approximately 70% of patients. It is important to assess MRD after reinduction therapy to determine the indications for stem cell transplantation in the standard-risk group. In contrast, no standardized therapy has been established for the high-risk group, which accounts for more than half of relapsed ALL patients. Recent studies have shed light on the clonal origin of relapsed ALL, which usually exists as a minor subclone at the time of initial diagnosis. Clonal selection and evolution take place during chemotherapy, resulting in distinct genetic and epigenetic characteristics of relapsed ALL, some of which are linked to drug resistance, a common and problematic feature of ALL after relapse. To overcome resistance to standard ALL-type therapy, and considering the heterogeneous biological background of high-risk relapsed ALL, innovative therapies using new agents are necessary.