Inclusion Criteria:

- Signed informed consent

- Age 18 years or more

- Histologically diagnosed acute myeloid leukemia (AML) or acute lymphocytic leukemia
(ALL) by WHO classification

- Refractory or relapsed disease (requiring at least 5% leukemic blasts in the bone
marrow, regardless of the presence of other features such as new or recurrent
dysplastic changes or extramedullary disease) according to the following definitions:

AML Relapsed (defined as ≥5% leukemic blasts in the bone marrow) after receiving up to 2
prior induction regimens, (i.e., first or second relapse); Refractory (defined as ≥5%
leukemic blasts in the bone marrow) to not more than 1 prior induction regimen (defined as
failure to achieve a CR or CRp following induction therapy), (i.e., up to 1 induction
failure).

ALL Relapsed/refractory (defined as ≥5% leukemic blasts in the bone marrow) after
receiving 1 or more prior induction regimens, (i.e., any number of relapses)

- ECOG performance status of 0-2

- At least 2 weeks from administration of prior anti-leukemia therapy unless subject
has progressed while receiving targeted therapy on a continuous dosing schedule

- No remaining clinically significant toxicities from prior chemotherapy of grade 2 or
greater

- Women of child-bearing potential (i.e., women who are pre-menopausal or not
surgically sterile) must be willing to use an acceptable contraceptive method
(abstinence, oral contraceptive or double barrier device) for the duration of the
study and for 30 days following the last dose of study drug, and must have a negative
urine or serum pregnancy test within 2 weeks prior to beginning treatment on this
trial

- Sexually active men must be willing to use an acceptable contraceptive method for the
duration of time on study and for 30 days following the last dose of study drug

- Clinical laboratory values within the following ranges unless considered due to
leukemic organ involvement: Serum creatinine 2.0 mg/dl; Total bilirubin 1.5x the
upper limit of normal unless considered due to Gilbert's syndrome; Alanine
aminotransferase (ALT), or aspartate aminotransferase (AST) 3x the upper limit of
normal

- Willingness to provide at least one pre-PR104 leukemia sample (e.g., bone marrow or
peripheral blood) for analysis of AKR1C3.

Exclusion Criteria:

- Pregnant and nursing subjects

- Uncontrolled intercurrent illness including, but not limited to uncontrolled
infection, symptomatic congestive heart failure, cardiac arrhythmia, or psychiatric
illness/social situations that would limit compliance with study requirements

- Active heart disease including myocardial infarction within previous 3 months,
symptomatic coronary artery disease, arrhythmias not controlled by medication, or
uncontrolled congestive heart failure

- Another active concomitant malignancy likely to effect any of the primary or
secondary outcome measures in the current study

- Subjects receiving any other standard or investigational treatment for their
hematologic malignancy (other than hydroxyurea). Subjects with CNS leukemia are
eligible and may receive concurrent standard intrathecal chemotherapy.