Inclusion Criteria:

Patients must meet all of the following criteria to be eligible to
participate in the study.

Patients must be ≥1 and ≤ 21 years of age when enrolled onto this study.

- Diagnosis (Patient must have all of the following)

- Patients must have a diagnosis of relapsed acute myelogenous leukemia (AML) or
acute lymphoblastic leukemia (ALL)

- Patient must have an M1 marrow based upon a recovered marrow with less than 5%
blasts by conventional morphology

- Patient must have minimal residual disease (MRD) detected by either
multidimensional or conventional flow cytometry greater than 0.1% and less than
5% following any re-induction attempt

- Patients must have a central nervous system (CNS) disease status of 1

- Patient must have an absolute neutrophil count (ANC) >500/μL off cytokine support for
at least 24 hours and platelets >50,000 K/μL without platelet transfusion in the past
seven days

- Karnofsky > 50% for patients > 10 years of age and Lansky > 50% for patients ≤ 10
years of age.

- Patient must have adequate venous access.

- Patients must have fully recovered from the acute toxic effects of all prior
chemotherapy, immunotherapy, or radiotherapy prior to entering this study.

- At least 21 days must have elapsed from prior chemotherapy, at least 7 days must have
elapsed since receiving biological therapy.

- It must be at least 90 days from any higher dose cytarabine therapy (>1 gm/ m2/day).

- Patients must have a normal calculated creatinine clearance

- Patient must have

1. Conjugated (direct) serum bilirubin ≤ 1.5 x upper limit of normal (ULN) for age.

2. Alanine transaminase (ALT) ≤ 2.5 × ULN for age.

3. Alkaline phosphatase ≤ 2.5 × ULN for age.

4. Serum amylase ≤ 1.5 ULN for age.

5. Serum Lipase is ≤ ULN for age.

- Patient must have a shortening fraction > 28% or an ejection fraction > 50%.

- Female patients of childbearing potential must have a negative urine or serum
pregnancy test confirmed 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.

- Patient must agree to submission of blood and bone marrow for assessment of minimal
residual disease (MRD).

- All patients and/or their parents or legal guardians must sign a written informed
consent.

Exclusion Criteria:Patients who meet any of the following criteria will be excluded from
participating in the study.

- Patients with previous hematopoietic stem cell transplant (HSCT) within previous six
months.

- Patients who have had prior treatment with clofarabine.

- Patients with CNS2 or CNS 3 disease or bulky chloromatous disease.

- Patients with Down Syndrome.

- Patients with a previous history of veno-occlusive disease (VOD) or findings
consistent with a diagnosis of VOD.

- Patients with a systemic fungal, bacterial, viral, or other infection not controlled.

- Use of investigational agents within 30 days of planned treatment on this protocol.

- Patient is receiving or plans to receive concomitant chemotherapy, radiation therapy,
immunotherapy or other anti-cancer therapy other than is specified in the protocol.

- Pregnant or lactating patients.

- 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.

- Have had a diagnosis of another malignancy, unless the patient has been disease-free
for at least 3 years following the completion of curative intent therapy with the
following exceptions:

1. Patients with treated non-melanoma skin cancer, in situ carcinoma, or cervical
intra-epithelial neoplasia, regardless of the disease-free duration, are
eligible for this study if definitive treatment for the condition has been
completed.

2. Patients with organ-confined prostate cancer with no evidence of recurrent or
progressive disease based on prostate-specific antigen (PSA) values are also
eligible for this study if hormonal therapy has been initiated or a radical
prostatectomy has been performed.