Inclusion Criteria:

- Histologically confirmed diagnosis of one of the following:

- Acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL) meeting 1 of
the following criteria:

- Refractory to initial treatment (stratum 1)

- Recurrent disease after prior high-dose chemotherapy with or without stem
cell support (stratum 1)

- High-risk refractory disease defined as failed ≥ 2 regimens for remission
induction (i.e., twice induction failure) (stratum 2)

- High-risk relaspsed disease defined as disease in second or greater bone
marrow relapse (stratum 2)

- Chronic myelogenous leukemia in blast crisis (stratum 1)

- Myeloid or lymphoid blast crisis that did not respond to or progressed
after prior high-dose imatinib mesylate (600-800 mg/day for ≥ 2 weeks)

- No acute promyelocytic leukemia

- Ineligible for or unwilling to undergo potentially curative allogeneic or autologous
stem cell transplantation

- Patients with relapsed AML that is refractory to re-induction therapy comprising
an active, intensive salvage regimen are eligible

- CNS involvement allowed provided there are no residual leukemic cells in the
cerebrospinal fluid after intrathecal chemotherapy or radiotherapy

- Performance status - ECOG ≥ 2 for patients > 10 years of age

- Performance status - Lansky 50-100% for patients ≤ 10 years of age

- At least 8 weeks

- Bilirubin ≤ 2 times upper limit of normal (ULN)* (unless due to Gilbert's syndrome)

- ALT and AST ≤ 5 times ULN*

- Creatinine ≤ 2.0 mg/dL* (stratum 1)

- Creatinine > 1.3 times ULN (stratum 2)

- LVEF ≥ 40% by echocardiogram or MUGA (stratum 1)

- Shortening fraction ≥ 28% by echocardiogram (stratum 2)

- No symptomatic congestive heart failure

- No unstable angina pectoris

- No cardiac arrhythmia

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception

- HIV negative

- No history of allergy to study drug

- No active infection requiring IV antibiotics

- No serious medical or psychiatric illness that would preclude giving informed consent
or limit survival

- No other uncontrolled illness

- See Disease Characteristics

- Recovered from all prior immunotherapy treatment-related toxicity (stratum 2)

- More than 8 weeks since prior biological agents (e.g., monoclonal antibodies)
(stratum 2)

- See Disease Characteristics

- Recovered from all prior chemotherapy treatment-related toxicity (stratum 2)

- More than 24 hours since prior hydroxyurea (for patients who do not have highly
proliferative disease)*

- More than 2 weeks since other prior chemotherapy (6 weeks for nitrosourea or
mitomycin)

- No other concurrent chemotherapy

- Prior hydrea and/or steroids allowed (stratum 2)

- No concurrent hormones, except steroids for adrenal failure or infusional toxicity
(i.e., cytokine release syndrome) or hormones for non-disease-related conditions
(e.g., insulin for diabetes)

- See Disease Characteristics

- Recovered from all prior radiotherapy treatment-related toxicity (stratum 2)

- More than 2 weeks since prior radiotherapy

- No concurrent palliative radiotherapy

- Post stem cell transplant allowed provided completion ≥ 4 months prior to study entry
and no evidence of active acute or chronic graft vs host disease (stratum 2)

- No other concurrent investigational agents

- No concurrent chronic systemic anticoagulant therapy for a medical condition (e.g.,
deep vein thrombosis or atrial fibrillation)

- Concurrent heparin allowed to maintain central line patency (i.e., catheter
flush)

- No other concurrent anticancer therapy