Allogeneic Hematopoietic Stem Cell Transplantation (alloHSCT) for Children and Young Adults with T-Cell Acute Lymphoblastic Leukemia (T-ALL) Treated at Investigator Discretion: A Report from Children's Oncology Group (COG) AALL0434

Introduction: Salvage therapies in children and young adults with relapsed T-ALL are successful in Patients and Methods: From 2007-2014, AALL0434 enrolled 1,895 patients (1,596 T-ALL) and included a 2 x 2 pseudo-factorial randomization using a COG-modified BFM regimen. Patients were randomized to receive escalating dose methotrexate without leucovorin rescue plus pegaspargase (CMTX) or High Dose MTX (HDMTX) with leucovorin rescue in a single Interim Maintenance phase, with or without six 5-day courses of nelarabine. No patients were risk stratified on the basis of ETP. Participants with ≥ 25% marrow blasts on day 29 of induction (IF) were assigned to receive HDMTX with nelarabine. All patients with intermediate risk (IR), high risk (HR) or IF received cranial irradiation. Results: Among 1385 eligible, evaluable T-ALL patients for whom survey information was available, 333 (24%) were taken off therapy during induction prior to randomization. Thirty-two (9.6%) of these 333 subsequently underwent alloHSCT (19 with M1, 9 with M2 and 4 with unknown Day 29 marrow status). At the end of induction, 1052 patients were risk stratified and consented to randomization. Of these, 43 patients (4.3%) were subsequently taken off protocol therapy for alloHSCT [0 low risk (LR), 2 IR, 21 HR and 20 IF]. For the 75 patients who were taken off protocol therapy during or after induction and underwent alloHSCT, time to transplantation from end induction ranged from 1 to 9 months (median 5) for the 20 IF patients, and from 1 to 30 months (median 4) for the 55 patients who achieved CR1. A variety of conditioning regimens were used, but most (80%) included total body irradiation. Five types of alloHSCTs were employed: matched unrelated in 37 (50%), matched related in 28 (38%), mismatched unrelated in 7 (10%), and mismatched related and unknown (1 each (1.4%)). Stem cells sources were bone marrow in 50 (67%), umbilical cord blood in 14 (19%) and peripheral blood in 11 (15%). Multivariate analyses on all 75 patients (adjusting for treatment arm and risk group, and using time-dependent covariates for time to HSCT) showed worse outcomes for those who received alloHSCT versus chemotherapy [Hazard ratio 3.46 (95% Confidence Interval 1.39 - 8.60); P = 0.008]. For IF patients, there was no difference in outcome for those receiving chemotherapy versus alloHSCT [Hazard Ratio 0.75 (95% CI 0.27-2.14); P = 0.60]. Centrally determined ETP status was available for 1123 (81%) of the subjects (Table), which were categorized as ETP (11%), Near ETP (17%) and Not ETP (72%) (Wood, BL: Blood 2014: [21]1). Most patients were not taken off therapy during induction, but among those who were and had ETP/Near ETP, 21 of 28 (75%) received alloHSCT versus 71 of 251 (28%) who received chemotherapy alone (Fischer9s exact test, P Conclusions: Our findings showed that only ~5% of participants in AALL0434 received alloHSCT, and that alloHSCT was employed in about half of those patients with IF. There was no EFS advantage for patients who failed induction and subsequently received alloHSCT. Patients were taken off protocol therapy and transplanted for a variety of reasons, which introduced heterogeneity. However, compared to those who received alloHSCT off therapy, we found that survival advantages favored those who received chemotherapy, regardless of ETP status. Disclosures No relevant conflicts of interest to declare.