Trial of Nelarabine, Etoposide and Cyclophosphamide in Relapsed T-cell ALL and T-cell LL

Inclusion Criteria: * Patients to be enrolled in the dose-escalation portion of this study must have T-cell ALL or T-cell lymphoblastic lymphoma (LL) in first relapse or must have failed primary induction chemotherapy (ie, never attained a complete remission following an initial course of standard therapy for T-ALL or T-LL). Patients to be enrolled in the cohort expansion portion of this study (ie, those treated at the recommended phase 2 dose) must have T-cell ALL in first relapse or must have failed primary induction chemotherapy (ie, never attained a complete remission following an initial course of standard therapy for T-ALL). T-LL patients are not eligible for the cohort expansion phase. * Patients with T-cell ALL must have greater than 25% blasts in the bone marrow with or without extramedullary disease. * Patients with T-cell LL must have recurrent disease, documented by clinical or radiographic criteria, as well as histologic verification of the malignancy at original diagnosis. Patients with T-cell LL enrolled in the phase I dose-escalation study are not required to have measurable disease; however, patients enrolled in the phase II cohort expansion at the MTD must have measurable disease. * Patients may have CNS 1 or CNS 2 disease but not CNS 3. * ECOG 0-2 or Karnofsky ≥ 50% for patients \> 16 years of age; Lansky ≥ 50% for patients ≤16 years of age. * Patients may be enrolled on study regardless of the timing of prior Intrathecal therapy; however, they MAY NOT BEGIN TREATMENT ON THIS PROTOCOL UNTIL A MINIMUM OF 7 DAYS HAS ELAPSED SINCE PRIOR INTRATHECAL THERAPY. * At least 6 weeks must have elapsed since administration of nitrosureas. * At least 12 weeks must have elapsed since administration of craniospinal or hemipelvic radiation. * Female patients of childbearing potential must have a negative urine or serum pregnancy test confirmed within 2 weeks prior to enrollment. * Female patients with infants must agree not to breastfeed their infants while on this study. * Male and female patients of child-bearing potential must agree to use an effective method of contraception approved by the investigator during the study and for a minimum of 6 months after study treatment. * Adequate renal function defined as serum creatinine ≤ 1.5x upper limit of normal (ULN) for age. If the serum creatinine is above these values, the calculated creatinine clearance or radioisotope GFR must be ≥ 70 mL/min/1.73m2. * Total bilirubin ≤ 1.5x ULN for age. If the total bilirubin is elevated, patient will still be eligible if the conjugated (direct) serum bilirubin ≤ ULN for age. * ALT ≤ 5x ULN of normal for age. * Adequate cardiac function defined as shortening fraction of ≥ 27% by echocardiogram or ejection fraction ≥ 45% by gated radionuclide study. * No evidence of dyspnea at rest * No exercise intolerance * A pulse oximetry ≥ 94% at sea level (≥ 90% at altitude ≥ 5000 feet) if there is clinical indication for determination. * Patients and/or their parents or legal guardians must be capable of understanding the investigational nature, potential risks and benefits of the study. All patients and/or their parents or legal guardians must sign a written informed consent. Exclusion Criteria: * Patients with Down syndrome are excluded. * Patients with pre-existing Grade 2 (or greater) peripheral motor or sensory neurotoxicity per the CTCAE 3.0 as determined by the treating physician or a neurologist. * Patients with a history of prior veno-occlusive disease (VOD) or findings consistent with a diagnosis of VOD, defined as: conjugated serum bilirubin \>1.4 mg/dL AND unexplained weight gain greater than 10% of baseline weight or ascites AND hepatomegaly or right upper quadrant pain without another explanation, OR reversal of portal vein flow on ultrasound, OR pathological confirmation of VOD on liver biopsy. * Previous hematopoetic stem cell transplantation. * Patients with a prior seizure disorder requiring anti-convulsant therapy are not eligible to receive nelarabine. For the purposes of this study, this includes any patient that has received anticonvulsant therapy to prevent/treat seizures in the prior two years. * Positive blood culture within 48 hours of study enrollment. * Fever above 38.2 within 48 hours of study enrollment with clinical signs of infection. * Plan to administer non-protocol chemotherapy, radiation therapy, or immunotherapy during the study period. * Any significant concurrent disease, illness, psychiatric disorder or social issue that would compromise patient safety or compliance, interfere with consent, study participation, follow up, or interpretation of study results.