This Phase I/II, multi-centered, non-randomized, trial is designed to determine a safe dose of PT-523 for subjects, and to make preliminary evaluations on the activity of PT-523 as therapy in subjects with refractory leukemia.

Inclusion Criteria:

• Relapsed/refractory leukemias including acute non-lymphocytic leukemia (AML), acute lymphocytic leukemia (ALL), chronic lymphocytic leukemia (CLL), and chronic myelogenous leukemia in blastic phase (CML-BP) for which no standard therapies are anticipated to result in a durable remission; or poor-risk myelodysplasia (MDS) patients to include refractory anemia with excess blasts (RAEB) or excess blasts in transformation (RAEBT) and chronic myelomonocytic leukemia (CMML) who are either relapsed/refractory or who refuse/are not eligible for frontline therapy.
• Only patients with refractory ALL or CML in lymphoid blastic phase are eligible for phase II.
• ECOG performance status 0 - 2.
• Adequate organ function and bone marrow reserve.
• At least 4 weeks must have elapsed from the time of major surgery.
• Use of appropriate contraceptive method.
• Signed patient informed consent.

Exclusion Criteria:

• Known human immunodeficiency virus (HIV).
• Active heart disease including myocardial infarction within previous 3 months, symptomatic coronary artery disease, arrhythmias not controlled by medication, or uncontrolled congestive heart failure.
• Patients receiving any other standard or investigational treatment for their leukemia other than corticosteriods, hydroxyurea, and 6-mercaptopurine.
• Any medical condition which, in the opinion of the investigator, places the patient at an unacceptably high risk for toxicities.