The study will examine the safety profile of SGN-CD123A. The study will test increasing doses of SGN-CD123A given every 3 weeks to patients.

This study is designed to evaluate the safety, tolerability, and preliminary estimate of antitumor activity of SGN-CD123A. The study will be conducted in 2 parts:

1. Part A is the dose-escalation portion of the trial, designed to identify the maximum tolerated dose (MTD) of SGN-CD123A
2. Part B is the dose-expansion portion of the trial, designed to evaluate SGN-CD123A in patients with differing CD123 expression levels

Dose-escalation in Part A will be conducted using a 3+3 study design. Patients with CD123-detectable AML will be enrolled in cohorts at escalating doses of study drug and will receive up to 2 induction cycles of SGN-CD123A treatment at an assigned dose level in 3-week cycles.

After completion of dose-escalation, patients will be enrolled in Part B of the study. Patients enrolled in Part B will receive up to 2 induction cycles of SGN-CD123A treatment at a dose level and frequency determined by results in Part A.

For both Part A and Part B, a third induction cycle may be permitted with the approval of the study medical monitor. If a patient achieves a complete remission or complete remission with incomplete hematologic recovery, optional post-remission cycles of SGN-CD123A may be administered.

Inclusion Criteria:

• Relapsed/refractory acute myeloid leukemia following at least 2 but no more than 3 prior regimens
• Patients may be eligible after only 1 previous regimen if in a high risk category
• Adequate baseline renal and hepatic function
• Eastern Cooperative Oncology Group Status of 0 or 1
• CD123-detectable leukemia

Exclusion Criteria:

• Cerebral/meningeal disease related to underlying malignancy
• Promyelocytic leukemia
• History of clinically significant pulmonary fibrosis or documented diffusing capacity of the lung for carbon monoxide <50% predicted
• Prior hematopoietic stem cell transplant
• Antileukemia or experimental treatment within 4 weeks of study drug (other than hydroxyurea or 6-mercaptopurine)
• Cardio or cerebral vascular event within 6 months