Abstract Abstract 1025 Introduction: Infantile acute lymphoblastic leukemia (ALL) is a rare leukemia subtype in infants with poor prognosis. Its outcome has gradually improved due to the development of treatment, including intensive chemotherapy or hematopoietic stem cell transplantation (SCT). However, prognosis of relapsed patients is extremely poor, which prompt us to establish the new treatment strategy for them. To establish the future treatment strategy for the relapsed cases, we here investigated the incidence of relapse, treatment, prognosis, and possible risk factors of relapsed cases with infantile ALL treated by the Japanese Pediatric Leukemia /Lymphoma Study Group (JPLSG) MLL03 study. Subjects: All relapsed cases with infantile ALL who underwent the MLL03 study between April 2004 and June 2009 were eligible for the study. The questionnaires were sent to all participants, asking the relapsed patients with infantile ALL by the MLL03 study. The questionnaires consisted of the time and site of relapse, treatment after relapse, presence or absence of remission prior to the second SCT, conditioning regimen for the second SCT, and age, white blood cell count, and presence or absence of central nervous system (CNS) infiltration at initial onset. Results: Total 63 patients were treated in this study period (mean follow-up, 31.8 months). Among them, 24 patients relapsed at any site after first complete remission (38%). These include 7 boys and 17 girls. Age at onset was less than 6 months in 19, and 6–12 months in 5. The overall survival (OS) rate of 24 patients was 60.2 ± 11.1% at 3 years; 44.6 ± 12.7% at 4 years, and 23.5 ± 13.4% at 5 years. Twenty-one of 24 patients (87.5%) relapsed after initial SCT; only three patients (12.5%) relapsed before SCT. Two patients relapsed early within 6 months after initial treatment terminally died. All four patients who relapsed at extramedullary site except CNS have survived, suggesting better prognosis. Patients diagnosed at age of less than 6 months also showed poor prognosis after relapse (5 year OS rate was 14.9 ± 13.0%). Presence or absence of remission prior to the second SCT was not a significant prognostic factor (28.3 ± 21.8% versus 15.6 ± 14.2%). Discussion: In the JPLSG MLL03 study, 38% of patients with infantile ALL relapsed. Only four patients relapsed prior to the initial SCT, indicating that the aim of the MLL03 study, to reduce the early relapse rate by performing the early SCT, could have been achieved. On the other hand, 87.5% of the patients relapsed after initial SCT in this study. Most of them were less than 6 months at onset. Several reasons for the high relapse rate after SCT should be considered, which include low graft-versus-leukemia (GVL) effect for infantile ALL, remained minimal residual disease (MRD) before SCT, or inappropriate conditioning regimen in this study. Since we have analyzed MRD before and after SCT in each patient treated by the MLL03 study, one reason could be resolved. In addition, the treatment strategy to induce GVL effect for infantile ALL should be established and we should reconsider the treatment strategy. Finally, appropriate treatment strategy for relapsed cases should be organized in the future. Disclosures: No relevant conflicts of interest to declare.

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