Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed acute leukemia according to the following criteria:

- Acute lymphoblastic leukemia (ALL) meeting any of the following criteria:

- Second relapse

- Refractory to induction therapy for first relapse

- Third or subsequent relapse

- Acute myeloid leukemia (AML) meeting any of the following criteria:

- Second or subsequent relapse

- Refractory to an induction therapy for first relapse

- Without a curative treatment option

- Other type of acute leukemia meeting any of the following criteria:

- First or subsequent relapse

- Refractory to induction therapy

- Not eligible for any therapy of higher curative potential

- No chronic myeloid leukemia (CML)

- Patients in relapse must have ≥ 5% blasts in the bone marrow

- Patients with refractory disease following induction must have ≥ 20% blasts in the
bone marrow

- No evidence of CNS disease

PATIENT CHARACTERISTICS:

- Karnofsky performance status (PS) 50-100% OR Lansky PS 50-100%

- Life expectancy ≥ 8 weeks

- Serum bilirubin < 1.5 times upper limit of normal (ULN)

- ALT or AST < 2.5 times ULN (5 times ULN if due to leukemic infiltration of the liver)

- Creatinine clearance ≥ 60 mL/min

- Not pregnant or nursing

- Negative pregnancy test

- Fertile female patients must use 2 of the following combined forms of contraception
(oral, injected, or implanted hormonal contraception and condom OR intra-uterine
device and condom OR diaphragm with spermicidal gel and condom) before, during, and
for 6 months after completion of study therapy

- Male patients must use 1 form of highly effective contraception (condom plus
spermicidal gel) during and for 6 months after completion of study therapy

- Men with pregnant or lactating partners should be advised to use barrier-method
contraception (condom plus spermicidal gel)

- No serological positivity for hepatitis B, hepatitis C, or HIV

- No congenital heart disease, with the exception of patent foramen ovale or small
muscular ventricular septal deficit (within the first year of life)

- No uncontrolled arterial hypertension (defined as a systolic blood pressure [BP]
and/or diastolic BP ≥ 95th percentile for age and height)

- No fractional shortening of ≤ 29% on echocardiogram

- No active graft-vs-host disease

- No current non-malignant systemic disease considered high medical risk, including any
of the following:

- Active uncontrolled infection

- Unstable or uncompensated respiratory or cardiac condition that makes study
participation undesirable

- No other condition that, in the Investigator's opinion, would not make the patient a
good candidate for the clinical trial

PRIOR CONCURRENT THERAPY:

- Recovered from toxicity of prior therapy, including toxicity following hematopoietic
stem cell transplantation

- Alopecia or certain grade 1 toxicities allowed at the discretion of the
Investigator

- A maximum of 2 days of hydroxycarbamide 10-20 mg/kg/day (or according to local
practice) in patients with AML and hyperleukocytosis allowed

- At least 7 days since prior investigational drugs (except antibodies for which a
4-week window must be observed)

- At least 7 days since prior protein kinase inhibitors and intrathecal therapy

- Concurrent intrathecal therapy allowed from course 2 onwards in patients with
ALL

- At least 14 days since prior cytotoxic therapy, including vincristine and other
anti-neoplastics

- No prior major thoracic or abdominal surgery from which the patient has not yet
recovered

- No prior aurora kinase inhibitor

- No concurrent steroid therapy

- Multikinase inhibitor AT9283 administration may be commenced once steroids have
started; however, steroids may not be started once multikinase inhibitor AT9283
has started

- Up to 5 days of prior oral dexamethasone (6 mg/m^2) for patients with ALL
experiencing a rapid rise in blast count allowed

- No other concurrent interventional clinical study

- Participation in an observational study allowed

- No other concurrent anticancer therapy or investigational drugs