- Nelarabine has shown impressive single agent clinical activity in T-cell acute lymphoblastic leukemia (T-ALL), but has been associated with significant neurotoxicities in heavily pre-treated patients. We showed previously that it was safe to add nelarabine to a BFM-86 chemotherapy backbone (AALL00P2). Children's Oncology Group (COG) AALL0434 is a Phase III study designed to test the safety and efficacy of nelarabine when incorporated into a COG augmented BFM-based regimen, which increases exposure to agents with potential neurotoxicity compared to the historical AALL00P2 regimen.AALL0434 included a safety phase to assess nelarabine toxicity. Patients with high-risk (HR) T-ALL were randomized to receive Capizzi-style escalating methotrexate (MTX) plus pegaspargase or high dose (HD) MTX with/without six five-days courses of nelarabine. We report results from 94 patients who participated in the initial safety phase of the study.There were no differences in the incidence of peripheral motor neuropathies, sensory neuropathies or central neurotoxicities among those randomized to the nelarabine (n = 47) and non-nelarabine arms (n = 47).The addition of nelarabine to COG-augmented BFM chemotherapy regimen is safe and feasible. The ongoing AALL0434 Efficacy Phase will determine whether the addition of nelarabine treatment improves outcome for patients with T-ALL. Pediatr Blood Cancer 2015;62:1176-1183. © 2015 Wiley Periodicals, Inc.© 2015 Wiley Periodicals, Inc.